Abstract The concept of interven-tional magnetic resonance imaging(MRI) is based on the integration ofdiagnostic and therapeutic proce-dures, favored by the combination ofthe excellent morphological andfunctional imaging characteristics ofMRI. The spectrum of MRI-assistedinterventions ranges from biopsiesand intraoperative guidance to ther-mal ablation modalities and vascularinterventions. The most relevant re-cently published experimental andclinical results are discussed. In the
future, interventional MRI is expect-ed to play an important role in inter-ventional radiology, minimal inva-sive therapy and guidance of surgicalprocedures. However, the associatedhigh costs require a careful evalua-tion of its potentials in order to en-sure cost-effective medical care.
Eur Radiol (2004) 14:2212–2227DOI 10.1007/s00330-004-2496-9 M A G N E T I C R E S O N A N C E
Thomas SchulzSilvia PucciniJens-Peter SchneiderThomas Kahn
Interventional and intraoperative MR: review and update of techniques and clinical experience
Introduction
Interventional magnetic resonance imaging (iMRI) hasgrown from a small special topic into a rapidly develop-ing wide field [1, 2]. Originally, it was intended only asimage guidance for common therapies and interventions.The rapid acceptance of MR guidance occurred largelybecause of MR’s multiplanar capability and accurate dis-crimination between normal and abnormal soft tissues.Moreover, the lack of ionizing radiation has become animportant issue with the increasing concerns about theradiation exposure of personnel and patients. The idea ofiMRI includes not only image guidance, even if opensurgical procedures can benefit to a great extent fromMR guidance. Evolution of iMRI comes along with thedevelopment of new minimally invasive techniques thatcannot be performed otherwise. Current research focuseson MR-compatible instrumentation, visualization tech-niques, minimal invasive thermal therapy, endovascularapplications, biopsy and intraoperative MRI. The processof developing such new techniques is multidisciplinary.A close collaboration between physicians, physicists, en-gineers and many other specialists has been necessary to
realize the interventional MRI techniques achieved sofar, and should be pursued and intensified in the future inorder to invent new solutions for minimally invasivetherapy.
The purpose of this review is to highlight the techni-cal improvements and the new technologies for interven-tional MR-guided procedures. In addition, their use forminimal invasive procedures and the application of MRtechniques for intraoperative guidance will be described.
MR systems and interventional devices
MR systems
Historically, the combination of long imaging times andthe very limited patient access characteristic of the tradi-tional closed-bore cylindrical systems has made MR anunsuitable technique to guide operative procedures. Inthe past years, the improvement of both hardware andpulse sequences has solved many of these shortcomings,enabling the development of open magnets capable ofrapid imaging. Different system configurations have
T. Schulz (✉) · S. Puccini · J.-P. SchneiderT. KahnDepartment of Diagnostic Radiology,Leipzig University Hospital,Liebigstr. 20, 04103 Leipzig, Germanye-mail:[email protected].: +49-341-9717447Fax: +49-341-9717239
been proposed, with a trade off between patient access,field of view, signal/contrast to noise ratio and costs. Theoptimal design when considering field homogeneity, andthus image quality, would be a sphere without opening[3], whereas, in contrast, the environment best suited tointervention should allow complete freedom in the pa-tient approach. A compromise between these two oppos-ing requirements has been achieved through a number ofdifferent solutions. To discuss the peculiarity of interven-tional MR systems, one has to distinguish between im-age guidance and procedural monitoring. There are manyinterventions in which the information provided by MRimaging can be used for monitoring purposes. Examplescan be found not only in traditional open surgery, e.g.,the intermittent check of the status of a tumor resection,but also in the more recent minimally invasive surgery,such as percutaneous thermal ablation, in which the tem-perature dependence of several MR parameters can beexploited to monitor the heat deposition and the resultingdamage in the target tissue. The monitoring of interven-tional procedures requires no major modification to stan-dard MR systems, since patient access is not necessarilyneeded. MR-monitored interventions have been per-formed on conventional cylindrical super conductingscanners by several authors employing different tech-niques, such as brain surgery [4], breast biopsy [5], laser-induced or radio frequency (RF) thermal ablation of sol-id tumors in the brain [6], head and neck [7] and liver[8]. During these thermoablative procedures, MRI hasbeen used for temperature monitoring. The laser applica-tors or RF electrodes are inserted under CT guidance orusing a neurosurgical stereotactic approach based on ret-rospective MR data. However, there are several con-traindications to patient transport during a surgical pro-cedure, mainly related to anesthesia and sterility prob-lems, but also to possible displacement of catheters. Twosolutions have been proposed to accommodate a high-field 1.5-T scanner in a surgical suit. The first one ex-ploits a magnet moveable on trails affixed to the ceiling[9, 10]; the second one combines a MR scanner and RF-shielded C-arm fluoroscopy unit [11]. In both cases, thesurgical table is placed outside the five Gauss line of themagnet, which is actively shielded to ensure a rapid falloff of the static magnetic field, so that surgery can beperformed as in a conventional operating room. Intermit-tent MR imaging to survey the procedure however canbe performed by sliding the patient into the magnet, orthe magnet toward the patient in the first case. Not allthe problems related to patient transport can be solved inthis way; in particular, sterility remains a challenge,since the inside of a cylindrical magnet cannot be easilydraped.
Interventional guidance implies the use of MRI duringthe manipulation of needles, catheters, etc., by the radiol-ogist or endoscopes, scalpels, etc., by the surgeon. Theseprocedures require a significant departure from traditional
diagnostic concepts and imaging systems. The simplestway to realize such guidance is to use a retrospective dataset in combination with a stereotactic system, but a fargreater emphasis has been devoted to real time or near-real time guidance. The key feature of systems capable ofinterventional guidance is the combination of patient ac-cess and imaging capability, including device tracking.Several companies have brought “open” MR systemsspecifically designed for interventional purposes onto themarket . The first one was the “double doughnut” config-uration of the Signa SP/I (General Electric Medical Sys-tems, Milwaukee, WI), with 0.5-T static field strength.This design has taken the central segment out of a superconducting cylindrical magnet, leaving a 54-cm gap be-tween the two halves, i.e., enough room for two small tomedium-sized physicians (Fig. 1a). It is the only systemcurrently available that allows complete vertical and sideaccess to the patient at the isocenter, and has been em-ployed to guide and monitor a large number of biopsiesand surgical procedures [12–17]. A hybrid imaging sys-tem combining the Signa SP/I scanner with X-ray hasbeen described [18], which should exploit both thestrengths of MRI (soft-tissue contrast, arbitrary plane se-lection) and X-ray fluoroscopy (high-resolution real-timeprojection images, clear portrayal of bony structures), al-lowing switching between the imaging modalities with-out moving the patient.
Another widespread strategy for an interventional MRscanner is based on the biplanar magnet design, in whichthe patient is positioned between flat magnetic poles.The patient can be accessed from the side, the accessfreedom depending on the number and position of thesupports separating the two poles. These systems uselow-field permanent or resistive magnets as well as su-per-conducting mid-field magnets, with field strengthsranging from 0.2 to 0.6 T. In this category, the largest pa-tient access is granted by C-arm systems, such as thoseproduced by Siemens Medical Solutions (up to 2000Magnetom Open Viva, Fig. 2, currently Magnetom Con-certo, 0.2 T) and Philips (Panorama family, 0.23 T and0.6 T, based on former product of Marconi Medical Sys-tems). A further 1.0-T Panorama system is currently awork in progress. These systems allow surgical ap-proaches analogous to C-arm X-ray fluoroscopy, i.e., an-terior or posterior interventional strategies requireoblique positioning, which may be impaired by the rela-tively limited space between the biplanar magnet poles.Other manufacturers, including Hitachi (Airis, 0.3 T andAltaire, 0.7 T) and Toshiba (Opart MR system, 0.35 T)produce biplanar systems with two supports, resulting ina slightly more restricted access to the patient. The bipla-nar concept has the advantage of a relatively homogene-ous static magnetic field, but allows less freedom in thesurgical strategy. In particular, a vertical approach isonly possible when the table is moved outside the mag-net, thus excluding MR guidance [19].
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Generally, image quality of open systems is negative-ly affected by the relatively limited field strength and ho-mogeneity, resulting in a comparative long imaging timeand lack of vascular and functional information with re-
spect to the conventional closed scanners. Nonetheless,over the years several technical improvements have beenintegrated in the open systems, facilitating the guidanceof interventional procedures. Image guidance could nothave been possible without the development of rapidgradient echo pulse sequences, which allow a wide rangeof tissue contrast in a time frame sufficient for devicetracking [20–22]. Moreover, several methods have beenproposed to further decrease imaging time, includingkeyhole imaging and wavelet encoding [23–25]. A sig-nificant innovation has been the development of frame-less stereotactic tracking systems. Such a system was in-tegrated for the first time in the “double doughnut” SignaSP/I scanner, consisting in a hand-held needle guideequipped with infrared light-emitting diodes. The posi-tion and orientation of the guide is detected by a cameraarray mounted into the top support piece of the scannerand can be used to drive scan plane acquisition (Fig. 1b).A similar system has been subsequently developed alsofor C-arm scanners [26]. Recently, additional navigationtools have been proposed, which use the stereotactic in-formation to select the image plane out of a high-resolu-tion preoperative or intraoperative three dimensionaldata set and are characterized by a higher frame rate andimage quality as compared to the system-integrated(near) real time imaging. Moreover, multimode imagefusion allows a refined surgical planning, also takinginto account functional information, such as MR angiog-raphy or functional MRI [27–29].
Given the above-described advantages and limita-tions, the ultimate design of future interventional MRsystems is still unclear. The optimal system for intra-operative guidance should combine high performancecomputing in order to exploit multimode information,real time image update during the procedure and thecomfort of a conventional operating suite. Up until now,
Fig. 1 a General Electric Medical Systems Signa SP/I scannerconsists of two very short super-conducting cylindrical magnetsseparated by a 54-cm space, allowing the physician or nurse to ac-cess the patient from the top or sides. Here, the surgeon stands be-hind the head of the patient using an MR-compatible microscope(white arrow). The scanner is sited in an operating room-type en-vironment, with electrical outlets and anesthesia gases integratedinto the magnet covers. b This system design was the first to inte-grate an optically linked frameless stereotactic tracking system(open arrow). The position and orientation of infrared light-emit-ting diodes (asterisks) on this hand-held needle guide are detectedby a camera array integrated into the top support piece of the scan-ner (reproduced from ref. [75])
Fig. 2 Example of biplanar system. Siemens Magnetom OpenViva system sited in the operating room complex at UniversityHospitals of Cleveland. Side access is possible, but the uppermagnet pole blocks direct vertical access (reproduced from ref.[19])
there is no available system completely matching allthose needs; therefore, different designs can be advocat-ed for different interventional procedures. The choice ofthe interventional system will depend on the single in-stitution’s optimal compromise among the types of in-terventions to be performed, required image quality andcosts and benefits of intraoperative MR-guided proce-dures.
Interventional and surgical instruments and devices
Advances in interventional MRI require the developmentof dedicated MR-compatible equipment, since many in-struments used in conventional surgery are made of me-tallic material and are thus unsuitable for MR-guided in-terventions, due both to safety concerns and image quali-ty degradation. Safety issues include unwanted move-ment caused by magnetic field interactions (e.g., the mis-sile effect, translational attraction and torque) and heat-ing generated by RF power deposition [30]. Materialsthat are attracted by the magnet, such as iron, cannot beused. Stainless steel can be magnetic or nonmagnetic,depending on its constituents. The nonmagnetic form canbe used; however, any stainless steel in the imaging vol-ume causes massive image disruption. Other materialsthat are not ferromagnetic, such as titanium alloys, maybe used in the imaging volume and only produce localimage distortions [31]. Theoretically, the ideal materialshould have similar magnetic susceptibility to the humanbody. This is the case, for example, for many plasticssuch as Teflon and Plexiglas, but instruments made ofthese materials cannot be made as sharp or stiff as theconventional ones and are difficult to visualize and ster-ilize [32]. The choice of new MR-compatible instru-ments and tools will additionally depend on financialconstraints, because only a few manufacturers producethese materials, and then they are made in a small num-ber of pieces compared to conventional instruments.Some applications will require the development of adhoc equipment, as in the case of the piezoelectric drillingmachine used by our group for MR-guided bone biopsies[33].
The proper visualization of the instruments greatly af-fects the accuracy and safety of MR-guided procedures.Regarded as induced artifacts, currently the best needlesare those made of titanium alloys. The artifact can be es-timated as roughly twice the size of the needle, depend-ing on the alloy components [32]. Moreover, severaluser-defined imaging parameters and trajectory decisionscan affect the visibility of the device [34]. The visibilitydepends on the sensitivity of the pulse sequence to mag-netic susceptibility effects and the angle between the in-strument and the static magnetic field. In general, arti-facts are more pronounced when using gradient echo se-quences than spin echo sequences, due to the lack of the
refocusing pulse [35]. The device trajectory should bechosen taking into account that when the instrument(needle, electrodes, etc.) is oriented parallel to the staticmagnetic field, artifacts at the device tip can obscure thetrue tip position. On the other hand, artifactual wideningis smaller with respect to non-parallel orientation, mak-ing it difficult to identify thin devices [36, 37].
Despite the various difficulties described, MR-com-patible versions of most instruments are available andcan be used successfully to carry out most procedures inan iMRI setting that otherwise normally would be per-formed elsewhere. Safe application of MR-guided tech-nique requires careful consideration of the factors affect-ing device visibility.
MRI-guided biopsy and drainage procedures
One of the most straightforward interventional applica-tions for a cross-sectional imaging technique is percuta-neous biopsy and aspiration. The tissue contrast, spatialresolution and feasibility of multiplanar imaging of MRhave obvious benefits for those procedures and makeMR the best choice for several indications compared toultrasound or CT, despite the higher costs. The lack ofionizing radiation decreases procedure-related risks tothe radiologist and patient, particularly those patients inthe pediatric or obstetric population. Compared to CT, byanalogous spatial resolution, the MR capability to per-form oblique imaging allows a greater visualization ofthe needle tract in relation to the lesion. This is a deci-sive advantage, especially in regions of complex anato-my, with many vital structures to be spared, such as inthe head and neck [38] or skull base [39]. In our experi-ence, the use of MR imaging should be recommendedfor guidance in the area of the clivus or petrous apex, viaa transsphenoidal approach. Due to the deep structuresclose to the major vessels and nervous systems, biopsiesin this region are not only technically difficult, but alsopotentially dangerous (Fig. 3).
Neurobiopsy using intraoperative MR imaging repre-sents a natural progression from the stereotactic methodin the way in which neurosurgeons perform brain biop-sy, and MR-guided brain biopsies have been reported inseveral studies [40]. Depending upon the radiologist’spreferences and experience, biopsies under MR guid-ance can be performed with free-hand methods, needleholders or the use of a frameless optically linked local-ization system as described above. Brain biopsies great-ly benefit from prospective guidance, i.e., based on anintraoperatively acquired data set and not on preopera-tive data, as in traditional stereotactic interventions.Prospective stereotaxy can provide good and acceptabletargeting accuracy in the presence of brain shift, whichmakes the technique very suitable also for open neuro-surgery [41].
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MR-assisted biopsies of suspect breast findings aregaining increasing interest, since some breast lesions canbe detected and visualized only by contrast-enhancedMRI, which rules out both conventional mammographyand ultrasound for interventional guidance. MR-guidedbreast biopsies have been reported by several authors [5,42–48], using interventional open MR scanners as wellas conventional ones. Meanwhile, the outcomes of thefirst evaluation studies on large patient collectives havebeen published, with encouraging results. In a relativelyold study, Fischer et al. reported 35 MR-guided biopsies;among them, three unsuccessful biopsies [49]. A morerecent study on 206 contrast-enhancing breast lesions us-ing vacuum biopsy had a failure rate of only 2% (4 cas-es). The authors pointed out that the biopsy failure wasimmediately detected on post-interventional images, thusavoiding false negative diagnosis [50]. According to ourgroup experience, MR-guided biopsies can be success-fully gained also by small lesions (5–17 mm in diameter)[51]. Tissue shift during needle insertion, no direct proofof correct biopsy in closed magnets and incomplete MRcompatibility of biopsy equipment are the main reasonsfor most authors to recommend MR-guided biopsy onlyfor lesions >10 mm. On the other hand, MRI is often theonly modality to visualize small (<10 mm) lesions. Weperformed MR-guided biopsy on 21 lesions; of them, 20
were successful (Fig. 4). The total procedure took about45 min: the most time-consuming step was the comfort-able patient positioning (prone) and the localization ofthe lesion, the actual intervention taking only 10–15 min.Other applications of interventional MRI in the breast in-clude the preoperative wire marking of uncertain breastlesions in order to allow the surgeon to identify the targetduring conventional or tissue-sparing surgery [48, 52].
In the abdominal region, MR-guidance has proved tobe of valuable assistance not only to perform biopsies,but also for drainage procedures, as demonstrated by thework of several groups, mostly using interventional openMR scanners [26, 53–56]. A few of such procedureshave been reported also employing conventional scan-ners [57, 58] or hybrid systems combining MRI and X-ray fluoroscopy [59]. Contrarily, MR is not a reliablemethod in chest biopsies. A lesion within the mediasti-num, chest wall or even the lung may be well displayed,but MR imaging is still unable to document pneumotho-rax formation, which occurs in approximately 20–40%of all chest biopsies [54], so that CT remains the methodto be favored for percutaneous chest procedures. MR-guided musculoskeletal biopsies have been reported inlow or midfield open systems [26, 60, 61]. MR guidanceis indicated also for the biopsy of bone marrow edema ofunknown origin, since MRI allows a unique lesion dis-
Fig. 3 Images from an inter-active MRI-guided biopsy of a lymphoma (short arrows) of the clivus. Preoperative T1-weighted axial spin-echo imag-es before (a) and after contrastmedium (b). The tumor showshomogeneous enhancement andis situated between the two in-ternal carotid arteries and thebasilar artery. The angulationand entry point of the accesspass to the tumor (c) were veri-fied using the virtual line of thetracking system, which is seenin a single-acquisition (imagingtime 3 s) sequence and (d) us-ing the MR-Track-Pointer (longarrow) filled with a dilution ofcontrast medium (reproducedfrom ref. [39])
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play and visualization [54, 62]. For the biopsy of all oth-er bone lesions, such as primary and secondary bone tu-mors, MRI does not provide any additional essential in-formation compared to CT, even if, due to the lack ofionizing radiation, it may be preferred in case of youngpatients. Finally, interventional MRI has been used toguide the placement of catheters for further minimallyinvasive therapy approaches or to optimize such onco-logic treatments as brachytherapy of the prostate [63] aswell as in the head and neck region [64].
MRI-assisted surgical procedures
The reason for applying imaging modalities to guide sur-gical procedures is the limitation of direct visualizationof anatomy. Especially within the constraint of percuta-neous interventional approaches or minimally invasivesurgical access, there are simply not enough visible cluesto appreciate the entire anatomy. By applying computertechnology, intraoperative image guidance, the so-callednavigation, based on previously acquired images is read-ily available, fulfilling the need of enhanced visibility.Nevertheless, preoperative images cannot represent the
changes occurring intraoperatively, such as unavoidableshifts and deformations of soft tissues. In addition, intra-operatively acquired images can assist the surgeon or ra-diologist in localizing the lesions and defining their mar-gins. The main application field of intraoperative MRguidance is neurosurgery, mainly for resection of braintumors [65–68], but also for the as conservative as possi-ble treatment of epilepsy [69]. MR has been advocated tominimize the size of the craniotomy necessary for lesionexposure, identify adjacent structures for maximal nor-mal tissue preservation, determine completeness of tu-mor resection and provide surveillance for intraoperativecomplications [19]. The possibility to include functionalinformation in the surgical planning, such as that arisingfrom diffusion tensor imaging and functional MRI [27],further improves the safety of the procedure. As a conse-quence of the advantages of intraoperative MRI, withneurosurgeons increasingly relying on this technique, anumber of questions arises regarding the optimal utiliza-tion and cost-effectiveness of MR-guided neurosurgery.This issue has been addressed recently through a detailedanalysis of the costs and benefits of MR-guided tumorresection compared to conventional resection [70, 71].The authors found that the hospitalization length and
Fig. 4 Patient (67 years of age)with invasive lobular carcinoma.Upper panel: coronal post-contrast (left) and subtraction(right) image (3D FLASH, 1.5-T scanner) demonstrate an irregularly shaped mass(arrow) with rim enhancementin the right breast. Lower pan-el: coronal images of the rightbreast obtained at a 0.5-T SignaSP/I scanner. Left: in the nearreal-time image (2D fast multi-planar spoiled gradient echo*sequence), the lesion (arrow)can be localized. The needlemarked by the artifact (arrow-head) is in contact with the lesion (arrow) before taking thecores (reproduced from ref. [51])
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hospital charges were lower for patients undergoing MR-guided than conventional neurosurgery. Moreover, thecosts of MR-guided surgical procedures per gained qual-ity-adjusted life years were comparable with, e.g., kneearthroplasty or coronary balloon angioplasty. Therefore,one should also bear in mind that the length of survivalof brain tumor patients most likely correlates with theextent of removal, probably even in patients with high-grade gliomas [72, 73]. Also, in patients with low-gradeglial tumors an as radical as possible surgery is regardedas the best therapeutic option, since these tumors show aclear tendency toward anaplastic change. On the otherhand, the use of MR guidance increases the radicality ofthe resection, helping to direct the surgeon to areas of re-sidual tumors that may not be apparent by visual inspec-tion due to interposed normal tissue blocking the sur-geon’s view or to the similar appearance of tumor andadjacent normal brain parenchyma [74, 75]. The experi-
ence of our group, gained through a study including 12patients, confirms that MR guidance allows a gross-totalresection of supratentorial low-grade gliomas. Beforecorticotomy, a data set of T2-weighted multislice imageswith high resolution was acquired for the first determi-nation of the tumor extent. At the point at which the surgeon considered the resection complete (as deter-mined by inspection of the operating field with the mi-croscope), the T2-weighted acquisition was repeated(Figs. 5, 6), finding significant residual tumor, i.e., morethan 10% of the original tumor volume, in eight patients(Fig. 7). Postoperatively, eight patients had no functionalneurological deficit; in one patient there was no changeof a preexisting left-side hemiparesis; two patients un-dergoing resections marginally involving the languagecortex developed a mild dysplasia that resolved within 3 months. Only in one patient with a large precentral tumor was a discrete paresis of the right hand still evi-dent 3 months after surgery.
In the head and neck region, MR guidance allowssafe transnasal surgical access to the petrous apex andclivus, as demonstrated by our own experience [76], aswell as the transsphenoidal microsurgery of pituitaryadenomas and other sellar lesions [77]. Orthopedic ap-plications include the MR-assisted arthrography of thesacro-iliac joint [78] and the fixation of spine fractures[79] (Fig. 7). Experience in the surgery of the urinarytract [80] and in the resection of skin tumors [81] hasbeen reported. MRI has proved to provide valuable as-sistance for the guidance of surgical procedures in pedi-atric patients. In this case, the possibility to avoid pro-longed exposure to ionizing radiation, as in X-ray fluo-roscopy-guided procedures, is gaining an increasing rel-evance [82, 83].
Fig. 5 Patient with astrocytoma, WHO grade II, undergoing MR-guided resection. Top left and top right: T2-weighted axial fastspin-echo images were used to determine the tumor area before re-section. Middle left and middle right: comparable slices obtainedat the point at which the neurosurgeon considered the resection tobe complete (first control) were used to calculate the residual tu-mor volume. Bottom left and bottom right: T2-weighted images atthe same position were used to measure very small areas of residu-al tumor tissue at the end of the operation before closing the crani-otomy. Reproduced from ref. [75]
Fig. 6 Volume of tumor/residual tumor detected on MR images in12 patients undergoing MR-guided resection of low-grade astrocy-toma at three different points of resection: (1) preoperatively (be-fore craniotomy/corticotomy), (2) at the point of first control con-dition (when resection was considered complete by inspection ofthe operative field) and (3) at the conclusion of surgery (beforeclosing craniotomy). Reproduced from ref. [75]
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MRI-guided vascular applications
Even if X-ray fluoroscopy still is the imaging modalityof choice to guide vascular interventions, MRI has beenreceiving increasing attention in the last years, in partic-ular because of the perception that MR angiography(MRA) has grown to a serious alternative to digital sub-traction angiography (DSA) for diagnostic vascular im-aging [84, 85]. A drawback of X-ray fluoroscopy is thepotential radiation hazard for patients and operators,since some interventions can take up to several hours. Inaddition, this method relies on the injection of iodinatedcontrast medium and thus is contraindicated for patientssuffering from allergy or with renal insufficiency.
Major issues in endovascular iMRI are the need to de-pict adequately the vessel through which the endovascu-lar device is driven and to track its position at an accept-able frame rate. Several approaches have been proposedand validated through experiments in phantoms or ani-mal models, and can be roughly divided into passive andactive methods. Passive devices cause susceptibility-based signal voids in order to be visualized in the MRimage [86], whereas active devices incorporate small re-ceiver coils, enabling fast localization of the tip positionin three dimensions [87–89]. The MR safety issue is par-ticularly critical in endovascular iMRI due to the poten-tial hazards associated with the use of long conductors inthe MR environment, since they could act as linear RFantennas and lead to dangerous local temperature in-
creases. Several solutions have been proposed, rangingfrom spatially distributing the temperature increasealong the cable [90] to developing theoretical models ofthe local tissue heating as a function of the specific ab-sorption rate (SAR) [91]. The ultimate solution, howev-er, has not been found yet. It is likely that the use of non-conducting material, and in particular of fiber optic tech-niques, plays an important role as a safe mean of datatransmission through endovascular devices [85] (Fig. 8).
Endovascular iMRI applications greatly benefit fromthe possibility to perform selective MRA, i.e., to injectselectively contrast material directly through the catheterduring the interventional procedures, as is common usein conventional X-ray guided interventions, in order tooptimally visualize the vascular anatomy and pathologyunder treatment. The technique is essentially the same asused for contrast-enhanced MRA, with the simplificationthat there is no need to synchronize bolus passage andMRA acquisition. Moreover, time-resolved MRA can begained by a reduced amount of contrast agent. The tech-nique was demonstrated both in vitro in flow phantomsand in vivo in animal models [89, 92] and humans [93,94] (Fig. 9). One of the most challenging applications ofendovascular iMRI is the stenoses treatment via percuta-neous transluminal angioplasty (PTA). The feasibility ofMR-guided PTA was demonstrated in animal models us-ing passive tracking in the abdominal aorta [95] and re-nal arteries [96]. An active tracking approach, allowingan image refresh rate of 20 frames/s, was proposed by
Fig. 7 MR-guided fixation of aspine fracture by means of fourpedicle screws. All the screwswere correctly placed in thevertebral bodies
Fig. 8 Selective, time-resolved 3D contrast-enhanced (CE) MRA ofthe superior mesenteric arterial system of a pig. a–e: MIPs of 3D CEMR angiograms that were each acquired in 0.68 s. Five of 40 datasets that were acquired in 27 s of contrast injection are shown. Con-
trast agent was administered through the tip of an active catheter thatwas tracked into the superior mesenteric artery. The displayed framescover the a early arterial, b arterial, c late arterial/early venous, d early venous and e late venous phases. Reproduced from ref. [89]
ly, the possibility to automatically select the imagingplane according to the position of the used instrument,the so-called slice tracking, can be a very helpful tool tospeed up MR-guided endovascular procedures, whichcurrently are still longer than the corresponding X-rayguided ones. Slice tracking strategies have been pro-posed by Bock et al. at a 3-Hz frame rate [104].
In our opinion, endovascular iMRI will need sometime to grow more to maturity: currently, MR guidancestill does not offer the speed and robustness of conven-tional X-ray guidance. However, the rapid developmentsin this field encourage one to expect that MR-guidedvascular interventions can evolve toward an increasinglypromising research frontier.
MRI-guided thermal ablations
The goal of MR-guided thermoablative techniques is toachieve a temperature change sufficient to produce irre-versible damage in a predetermined target volume, spar-ing or minimally damaging the surrounding tissue struc-tures. Thermal therapies can be based on temperature de-crease, like cryotherapy [105], or temperature increase,obtained by the coupling of laser [106], RF [107], micro-wave [108] or focused ultrasound (fUS) power [109].The main advantages in comparison to surgery and ra-diotherapy are the nearly unlimited possibility to repeatthe treatment and the minimal invasiveness. By all thenamed thermoablative strategies, with the exception offUS, the thermal energy is delivered through an applica-tor, to be inserted percutaneously into the target beforestarting the actual treatment, whereas by fUS therapy theplacement of the transducer is completely noninvasive.As a further advantage, fUS therapy enables an irregularconformation of the target, whereas the shape of the ther-mal lesion achieved with the other modalities is roughlyellipsoidal, i.e., potentially a better adjustment to tumorswith irregular margins. A major drawback of fUS thera-py is, however, the long treatment time, due to the needto produce multiple small lesions, even if employingphased-array transducers can speed up the application.All thermoablative methods have been available for arelatively long time. Nevertheless, a broad applicationhas been impaired by the lack of sufficient monitoring,in particular of the energy deposition in the target region.MRI has proved to be a valuable tool for the guidance ofminimally invasive thermal therapies. Prior to treatment,the accurate placement of the applicator can be assistedby MRI for other imaging modalities, such as CT andUS, MRI having the advantage of a superior soft tissuecontrast by high spatial resolution. During the thermaltherapy, MRI allows the noninvasive monitoring of thetemperature evolution [111]. In the case of fUS therapy,MRI is decisive for the planning, since the correct posi-tioning of the transducer is checked by causing a mild
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Bücker et al. [97]. MR-guided placement of endopros-thesis has been performed by several authors. A majorrequirement for such implants is their MR compatibility,so that stents to be used for iMRI preferably should bemade of nitinol or tantalum. In addition, the delivery de-vice cannot contain ferromagnetic components. In vivoresults on animal models have been reported by severalgroups, including placement of a vena cava filter [98]and coronary artery stent [99]. In the latter study, high-field (1.5-T) MR guidance was used, with a real-timeimaging technique combining steady state free preces-sion for high signal-to-noise ratio and radial k-spacesampling for motion artifact suppression (rSSFP). Also,patient studies were performed. Manke et al. evaluatedthe MR-guided stent placement in 14 iliac artery steno-ses and found the procedure to be feasible, though time-consuming, and not yet ready for clinical use [100]. Other endovascular iMRI procedures include renal em-bolization [101] and local drug delivery [102]. The useof a combined DSA-MRI setup for guiding chemoem-bolization procedures in 30 patients with liver tumorswas evaluated by Vogl et al., who concluded that addingreal-time and dynamic MRI to the conventional ap-proach improved the therapeutic capability [103]. Final-
Fig. 9 Morphometry of laser-induced damage in a liver metastasisfrom colorectal cancer and comparison with the results of theMonte-Carlo simulation of the treatment. Contrast-enhanced MRimages 1 week after treatment (right) with corresponding 5× en-larged views of the damaged region (left). The transverse MR slice(top) is in plane with the laser applicator axis (dorsoventral), andthe coronal MR slice (bottom) is perpendicular to it. The simulatedlesion is confined by the white ellipse and circle, respectively. Re-produced from ref. [122]
temperature increase, with no therapeutic effect, but suf-ficient to be detected by MRI.
Various MR thermometry techniques, all based on thetemperature dependence of a particular physical quantity,have been described in the literature. To detect localheating, the more frequently used methods rely on thedecrease of the T1-weighted signal [112] and the shift ofthe proton resonance frequency (PRF) [113, 114]. ThePRF method has the advantage of being independent oftissue type, with the exception of fat [115], so that thecalibration of the scanner to quantify relative tempera-ture changes is relatively simple. The T1-weighted signalchanges are on the contrary dependent on tissue type. Itmakes the calibration procedure very complicated, sothat this method is generally used only for a qualitativeestimate of temperature changes. On the other hand, it isless sensitive to motion and can be used also in fatty tis-sue, such as the breast [116]. During cryotherapy, thefrozen tissue appears as a complete signal void in con-ventional MRI. However, ultra-short echo times allowfrozen tissue to be imaged and to measure the local tem-perature through the assessment of R2* [117]. Onlinetemperature mapping greatly increases the safety and ef-ficacy of the thermal therapies, in particular ensuringtemperature changes high enough to have therapeutic ef-fects and avoid damage to surrounding healthy tissue.However, a more detailed knowledge about the mecha-nisms of thermal cell killing and in particular about therelationship between time/temperature exposure and theresulting irreversible damage is needed to estimate thelesion extent from MR thermometry data. First attemptsto clarify this issue have been made by a few authors forheating thermoablative procedures [118–121]. There is ageneral agreement that two parameters can be suitablefor dosimetry purposes, i.e., the Arrhenius equivalent ex-posure time at 43°C, and a tissue-dependent critical tem-perature, above which irreversible damage can be as-sumed to occur. We investigated the predictive value ofMonte Carlo simulations of the laser-induced tissue heat-ing and damage, finding a very good agreement on invitro tissue samples and also in three patients receivinglaser-induced interstitial thermotherapy (LITT) of livermetastases of colorectal cancer (Fig. 9). These resultsprovide an independent confirmation of the substantialagreement between the Arrhenius equivalent exposuretime and the critical temperature models. Moreover,Monte Carlo simulations can potentially be used to cal-culate the tissue-dependent critical temperature, givingestimates in good agreement with the experimentally de-termined values reported in the literature [122]. Con-cerning cryotherapy, the tissue damage induced by tem-perature decrease is usually interpreted in correlation tofreeze-thaw cycles and achieved local temperature. Anew interpretation scheme has been proposed recently,suggesting that the damage induced by cryotherapy cor-relates with a local cryo-induced ischemic necrosis, and
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allowing a good interpretation of the histological find-ings on sheep brain [123]
The main application targets of thermoablative proce-dures are brain [6] and liver [124, 125] tumors. Experi-mental validation of thermal therapies of brain lesionshas been described by several groups, assessing the fea-sibility of cryotherapy [126], fUS therapy [127], LITT[121] and RF therapy [128] in animal models. However,clinical experience has been reported mainly for LITT[129–132]. For brain LITT, the guidance provided by ahigh field MR scanner allows a more sensitive thermalmapping than low- and middle-field systems (Fig. 10).Leonardi et al. treated 24 patients with low- to high-grade gliomas with stereotactic LITT, finding an im-provement of functional status. In particular, five of theseven patients with low-grade astrocytomas near elo-quent areas maintained a high quality functional statusfor 11–43 months after therapy [133]. Clinical experi-ence with fUS in the brain is very limited and has been
Fig. 10 PRF-based thermal mapping during the LITT of a braintumor in a high-field scanner. The calculated temperatures are col-or-coded (scale bottom right). Images acquired a 1 min, b 4 min, c 8 min and d 10 min after starting the laser therapy show a gradu-ally increasing heat-affected zone with increasing maximum temperatures. After switching off the laser (e 2 min, f 4 min), thetemperature returns to baseline values. Reproduced from ref. [131]
tient collective seem to indicate that LITT of liver metas-tases can compete in selected cases with surgery with re-spect to the survival rate, potentially gaining an impor-tant role as a curative and not only palliative therapeuticapproach. Vogl et al. reported the outcome of 1,801LITT of liver metastases of colorectal cancer on 603 pa-tients, finding a local recurrence rate at the 6-month fol-low-up of 1.9% (9 of 474) for metastases up to 2 cm indiameter, 2.4% (13 of 539) for metastases 2.1–3.0 cm indiameter, 1.2% (4 of 327) for metastases 3.1–4.0 cm indiameter and 4.4% (13 of 294) for metastases larger than4 cm in diameter. The mean survival rate for all treatedpatients, with the calculation started on the date of diag-nosis of the metastases (which were treated with LITT),was 4.4 years [141].
Other applications of MR-guided thermal therapiesinclude LITT of head and neck tumors [142], disk de-compression [143], vascular malformations [144], gyne-cological lesions [145], the breast [146] and cryotherapyof renal lesions [147]. LITT of osteoid osteoma underMR guidance in a low-field open system has been re-ported on a small patient collective (5 cases). After the6-month follow-up, four patients were symptom free andone had a local recurrence [148]. A new trend is the useof thermal therapy as an adjuvant of gene therapy andtherapeutic agent delivery. The opening of the bloodbrain barrier has been reported by fUS [149] and LITT[150] in animal models. Local hyperthermia has been re-ported to enhance and control gene expression, thus po-tentially ensuring a more effective gene therapy [151,152].
Conclusion
Interventional MRI has gained an important position inthe field of interventional radiology, as demonstrated bythe excellent results reported by several groups as a con-clusion of many clinical studies with large patient collec-tives. In particular, MR-guided biopsies as well as MR-assisted open and minimally invasive surgery are proce-dures whose clinical feasibility, safety and efficacy havebeen proved. Besides, a number of interesting applica-tions are still in a more or less advanced experimentallevel, such as endovascular applications and selectedtopics in thermal ablation procedures, such as dosimetryissues and adjutancy to other therapy approaches, suchas drug delivery or gene therapy. The rapid developmentin these fields, however, encourages one to expect thatalso these applications can achieve a clinical relevance.
The technical improvement in the field of interven-tional MRI has been impressive, including the develop-ment of new MR systems, optimization of receiver chainand pulse sequences, software solutions enabling new vi-sualization tools, data processing, image fusion and de-velopment of MR-compatible instruments. The progress
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hampered by the requirement of pretreatment cranioto-my. Recently, a large-scale phased-array transducer ca-pable of focusing through an intact skull has been devel-oped, and a first feasibility test has been performed in aprimate model, which offers tissue depths close to that ofhumans [134].
Thermal ablation of liver tumors has received increas-ing attention in the last years, since conventional conser-vative therapies such as systemic chemotherapy or radia-tion have proved ineffective. The incidence of this pa-thology is very high: hepatocellular carcinoma is in-creasing in incidence worldwide due to the prevalence ofhepatitis B and C virual infections. Moreover, colorectalcancer is a widespread disease in all developed coun-tries, and two-thirds of these patients have liver metas-tases by the time of death. Surgical resection is consid-ered the only potentially curative approach, but only afew patients are suitable for surgery. Moreover, hepatictumors are highly recurrent, but surgery can be repeatedonly in very few instances. The most used thermal thera-pies for the treatment of liver tumors are LITT and RFtherapy, whereas the application of cryotherapy, thoughfeasible, is not widespread [135]. The first is often per-formed under MR guidance in order to exploit thermalmonitoring (Fig. 11), whereas the RF therapy is usuallyperformed under CT guidance, i.e., without thermal in-formation, due to MR-compatibility issues of the RFelectrodes, and MR guidance has been reported only inexperimental studies [136]. The feasibility of MR-guidedLITT of liver tumors has been assessed beyond doubt ina row of clinical studies [17, 137–139]. To further in-crease the safety of MR-guided LITT procedures, we de-veloped a monitoring and control system based on MRthermometry data. It provides an interface to the laser,allowing the automatic on/off switching of the laserpower according to preoperatively defined control crite-ria [140]. Early laser applicators could only be used toablate small tumors (up to ≈2 cm in diameter), whereasnewer systems are equipped with an additional coolingsheet and work beyond that limit. The recently publishedfirst results of a clinical study performed on a large pa-
Fig. 11 PRF-based thermal mapping during the LITT of a livermetastases of colorectal cancer in a 0.5-T interventional scanner(P=20 W). Irradiation time: 8 min (left) and 20 min (right)
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in this field is by far not yet at an end; however, the highassociated costs require a careful evaluation of the po-tential of MR-guided interventions in order to ensure anappropriate role of interventional MRI in the delivery ofcost-effective medical care. It should not be forgotten,
moreover, that the main goal of interventional MRI isnot to offer an alternative (better) imaging modality forthe guidance of traditional procedures, but to assist thedevelopment of new minimal invasive therapy strategiesthat cannot be performed otherwise.
7. Vogl TJ, Mack MG, Muller P, PhillipC, Bottcher H, Roggan A, Juergens M,Deimling M, Knobber D, Wust P et al(1995) Recurrent nasopharyngeal tu-mors: preliminary clinical results withinterventional MR imaging-controlledlaser-induced thermotherapy. Radiolo-gy 196:725–733
8. Vogl TJ, Muller PK, Hammerstingl R,Weinhold N, Mack MG, Philipp C,Deimling M, Beuthan J, Pegios W,Riess H et al (1995) Malignant liver tu-mors treated with MR imaging-guidedlaser-induced thermotherapy: techniqueand prospective results. Radiology196:257–265
9. Sutherland GR, Kaibara T, Louw D,Hoult DI, Tomanek B, Saunders J(1999) A mobile high-field magneticresonance system for neurosurgery. J Neurosurg 91:804–813
10. Hoult DI, Saunders JK, Sutherland GR,Sharp J, Gervin M, Kolansky HG,Kripiakevich DL, Procca A, SebastianRA, Dombay A, Rayner DL, RobertsFA, Tomanek B (2001) The engineer-ing of an interventional MRI with amovable 1.5 Tesla magnet. J Magn Reson Imaging 13:78–86
11. Truwit CL, Hall WA (2001) Intraopera-tive MR systems. High-field approach-es. Neuroimaging Clin N Am11:645–650
12. Bootz F, Keiner S, Schulz T, SchefflerB, Seifert V (2001) Magnetic reso-nance imaging-guided biopsies of thepetrous apex and petroclival region.Otol Neurotol 22:383–388
13. Tyler D, Mandybur G (1999) Interven-tional MRI-guided stereotactic aspira-tion of acute/subacute intracerebralhematomas. Stereotact Funct Neuro-surg 72:129–135
15. Dion YM, Ben El Kadi H, Boudoux C,Gourdon J, Chakfe N, Traore A,Moisan C (2000) Endovascular proce-dures under near-real-time magneticresonance imaging guidance: an exper-imental feasibility study. J Vasc Surg32:1006–1014
16. Schneider JP, Dietrich J, Lieberenz S,Schmidt F, Sorge O, Trantakis C,Seifert V, Kellermann S, Schober R,Franke P (1999) Preliminary experi-ence with interactive guided brainbiopsies using a vertically opened 0.5-TMR system. Eur Radiol 9:230–236
17. Fiedler VU, Schwarzmaier HJ, Eickmeyer F, Muller FP, Schoepp C,Verreet PR (2001) Laser-induced inter-stitial thermotherapy of liver metastas-es in an interventional 0.5 T MRI system: technique and first clinical experiences. J Magn Reson Imaging13:729–737
19. Lewin JS, Metzger A, Selman WR(2000) Intraoperative magnetic reso-nance image guidance in neurosurgery.J Magn Reson Imaging 12:512–524
20. Duerk JL, Lewin JS, Wendt M, Petersilge C (1998) Remember trueFISP? A high SNR, near 1-second im-aging method for T2-like contrast ininterventional MRI at 0.2 T. J MagnReson Imaging 8:203–208
21. Shimizu K, Mulkern RV, Oshio K, Panych LP, Yoo SS, Kikinis R, JoleszFA (1998) Rapid tip tracking with MRIby a limited projection reconstructiontechnique. J Magn Reson Imaging8:262–264
22. Chung YC, Merkle EM, Lewin JS,Shonk JR, Duerk JL (1999) Fast T2-weighted imaging by PSIF at 0.2 T forinterventional MRI. Magn Reson Med42:335–344
23. Busch M, Bornstedt A, Wendt M,Duerk JL, Lewin JS, Groenemeyer D(1998) Fast “real time” imaging withdifferent k-space update strategies for interventional procedures. J MagnReson Imaging 8:944–954
24. Gelman N, Wood ML (1998) Waveletencoding for improved SNR and retro-spective slice thickness adjustment.Magn Reson Med 39:383–391
25. Zientara GP, Panych LP, Jolesz FA(1998) Applicability and efficiency of near-optimal spatial encoding fordynamically adaptive MRI. Magn Reson Med 39:204–213
26. Lewin JS, Petersilge CA, Hatem SF,Duerk JL, Lenz G, Clampitt ME, Williams ML, Kaczynski KR, LanzieriCF, Wise AL, Haaga JR (1998) Interactive MR imaging-guided biopsyand aspiration with a modified clinicalC-arm system. Am J Roentgenol170:1593–1601
27. Moche M, Busse H, Dannenberg C,Schulz T, Schmitgen A, Trantakis C,Winkler D, Schmidt F, Kahn T (2001)Fusion of MRI, fMRI and intraopera-tive MRI data. Methods and clinicalsignificance exemplified by neurosur-gical interventions. Radiologe41:993–1000
2224
28. Gering DT, Nabavi A, Kikinis R, Hata N, O’Donnell LJ, Grimson WE,Jolesz FA, Black PM, Wells WM(2001) An integrated visualization system for surgical planning and guid-ance using image fusion and an openMR. J Magn Reson Imaging13:967–975
29. Jolesz FA, Nabavi A, Kikins R (2001)Integration of interventional MRI with computer assisted surgery. J Magn Reson Imaging 13:69–77
30. Shellock FG (2001) Metallic surgicalinstruments for interventional MRIprocedures: evaluation of MR safety. J Magn Reson Imaging 13:152–157
31. Schenck JF (1996) The role of magneticsusceptibility in magnetic resonanceimaging: MRI magnetic compatibilityof the first and second kinds. Med Phys23:815–850
33. Fritzsch D, Scholz R, Werner A, Gründer W, Winkel A, Kahn T (2002)Use of a newly developed piezoelectri-cally driven drilling machine for MR-guided bone biopsies. Rofo FortschrGeb Rontgenstr Neuen Bildgeb Verfahr174:1309–1312
34. Liu H, Hall WA, Martin AJ, Truwit CL(2001) Biopsy needle tip artifact inMR-guided neurosurgery. J Magn Re-son Imaging 13:16–22
35. Nitz WR, Reimer P (1999) Contrastmechanisms in MR imaging. Eur Radiol 9:1032–1046
36. Lewin JS, Duerk JL, Jain VR, Petersilge CA, Chao CP, Haaga JR(1996) Needle localization in MR-guided biopsy and aspiration: effects of field strength, sequence design andmagnetic field orientation. Am JRoentgenol 166:1337–1345
37. Frahm C, Gehl HB, Melchert UH,Weiss HD (1996) Visualization of magnetic resonance compatible needlesat 0.2 and 1.5 T. Cardiovasc Intervent Radiol 19:335–340
38. Lai A, Maghami E, Borges A,Bonyadilou S, Curran J, Abemayor E,Lufkin R (2003) MRI-guided access tothe retropharynx. J Magn Reson Imag-ing 17:317–322
40. Hall WA, Liu H, Martin AJ, Truwit CL(2001) Minimally invasive procedures.Interventional MR image-guided neu-robiopsy. Neuroimaging Clin N Am11:705–713
41. Liu H, Hall WA, Truwit CL (2001)Neuronavigation in interventional MRimaging. Prospective stereotaxy. Neuroimaging Clin N Am 11:695–704
42. Schneider JP, Schulz T, Ruger S, HornLC, Leinung S, Briest S, Schmidt F,Kahn T (2002) MR-guided preopera-tive localization and percutaneous corebiopsy of suspected breast lesions. Possibilities and experience on the ver-tically open 0.5-T-system. Radiologe42:33–41
43. Pfleiderer SO, Reichenbach JR, AzhariT, Marx C, Malich A, Schneider A,Vagner J, Fischer H, Kaiser WA (2003)A manipulator system for 14-gaugelarge core breast biopsies inside a high-field whole-body MR scanner. J MagnReson Imaging 17:493–498
44. Pfleiderer SO, Reichenbach JR, AzhariT, Marx C, Wurdinger S, Kaiser WA(2003) Dedicated double breast coil formagnetic resonance mammography im-aging, biopsy, and preoperative local-ization. Invest Radiol 38:1–8
45. Kuhl CK (2002) Interventional breastMRI: needle localization and corebiopsies. J Exp Clin Cancer Res21:65–68
46. Viehweg P, Heinig A, Amaya B, Alberich T, Laniado M, Heywang-Kobrunner SH (2002) MR-guided interventional breast procedures con-sidering vacuum biopsy in particular.Eur J Radiol 42:32–39
48. Thiele J, Schneider JP, Franke P,Lieberenz S, Schmidt F (1998) New method of MR-guided mammarybiopsy. Rofo Fortschr Geb RontgenstrNeuen Bildgeb Verfahr 168:374–379
49. Fischer U, Kopka L, Grabbe E (1998)Magnetic resonance guided localiza-tion and biopsy of suspicious breast lesions. Top Magn Reson Imaging9:44–59
50. Perlet C, Schneider P, Amaya B,Grosse A, Sittek H, Reiser MF, Heywang-Kobrunner SH (2002) MR-guided vacuum biopsy of 206 contrast-enhancing breast lesions. Rofo Fortschr Geb Rontgenstr NeuenBildgeb Verfahr 174:88–95
51. Schneider JP, Schulz T, Horn LC, Leinung S, Schmidt F, Kahn T (2002)MR-guided percutaneous core biopsyof small breast lesions: first experiencewith a vertically open 0.5 T scanner. J Magn Reson Imaging 15:374–385
53. Frahm C, Gehl HB, Weiss HD,Roßberg WA (1996) Technique of MR-guided core biopsy of abdominal mass-es using an open low-field scanner:feasibility and first clinical results.Rofo Fortschr Geb Rontgenstr NeuenBildgeb Verfahr 164:62–67
54. Adam G, Bücker A, Nolte-Ernstling C,Tacke JK, Günther RW (1999) Inter-ventional MR imaging: percutaneousabdominal and skeletal biopsies anddrainages of the abdomen. Eur Radiol9:1471–1478
56. Faiss S, Zeitz M, Wolf KJ, Lewin JS,Wacker FK (2003) Magnetic reso-nance-guided biliary drainage in a patient with malignant obstructivejaundice and thrombocytopenia. Endoscopy 35:89–91
57. Rofsky NM, Yang BM, Schlossberg P,Goldenberg A, Taperman LW, WeinrebJC (1998) MR-guided needle aspira-tion biopsies of hepatic masses using a closed bore magnet. J Comput AssistTomogr 22:633–637
58. Salomonowitz EK, Cejna M, Dewey C(2000) Simple and effective techniqueof guided biopsy in a closed MRI sys-tem. Abdom Imaging 25:638–642
59. Bücker A, Neuerburg JM, Adam GB,Nolte-Ernsting CCA, Hunter DW,Glowinski A, van Vaals JJ, GüntherRW (2001) MR-guided percutaneousdrainage of abdominal fluid collectionsin combination with X-ray fluorosco-py: initial clinical experience. Eur Radiol 11:670–674
61. Genant JW, Vandevenne JE, BergmanAG, Beaulieu CF, Kee ST, NorbashAM, Lang P (2002) Interventionalmusculoskeletal procedures performedby using MR imaging guidance with avertically open MR unit: assessment oftechnique and applicability. Radiology223:127–136
2225
62. Neuerburg JM, Adam G, Bücker A,Zilkens KW, Schmitz-Rode T, HunterD, van Vaals JJ, Günther RW (1998)MRI-guided biopsy of bone in a hybridsystem. J Magn Reson Imaging8:85–90
63. D’Amico AV, Cormack R, TempanyCM, Kumar S, Topulos G, Kooy HM,Coleman CN (1998) Real-time magnet-ic resonance image-guided interstitialbrachytherapy in the treatment of se-lected patients with clinically localizedprostate cancer. Int J Radiat Oncol BiolPhys 42:507–515
64. Bootz F, Schulz T, Weber A, SchefflerB, Keiner S (2001) The use of openMRI in otorhinolaryngology: initial experience. Comput Aided Surg6:297–304
65. Jolesz FA, Talos IF, Schwartz RB, Mamata H, Kacher DF, Hynynen K,McDannold N, Sivironporn P, Zao L(2002) Intraoperative magnetic reso-nance imaging and magnetic resonanceimaging-guided therapy for brain tumors. Neuroimaging Clin N Am12:665–683
66. McPherson CM, Bohinski RJ, DagnewE, Warnick RE, Tew JM (2002) Tumorresection in a shared-resource magneticresonance operating room: experienceat the University of Cincinnati. ActaNeurochir 85:39–44
67. Ray MC, Tummala RP, Kucharczyk J,Truwit CL, Maxwell RE (2001) Minimally invasive procedures. Inter-ventional MR image-guided functionalneurosurgery. Neuroimaging Clin NAm 11:715–725
68. Seifert V, Zimmermann M, TrantakisC, Vitzthum HE, Kühnel K, Raabe A,Bootz F, Schneider JP, Schmidt F, Dietrich J (1999) Open MRI-guidedneurosurgery. Acta Neurochir141:455–464
69. Buchfelder M, Ganslandt O, FahlbuschR, Nimsky C (2000) Intraoperativemagnetic resonance imaging in epilepsysurgery. J Magn Reson Imaging12:547–555
70. Hall WA, Kowalik K, Liu H, TruwitCL, Kucharczyk J (2003) Costs andbenefits of intraoperative MR-guidedbrain tumor resection. Acta NeurochirSuppl 85:137–142
71. Kucharczyk J, Hall WA, BroaddusWC, Gillies GT, Truwit CL (2001)Cost-efficacy of MR-guided neurointerventions. Neuroimaging Clin N Am 11:767–772
72. Schulder M, Carmel PW (2003) Intra-operative magnetic resonance imaging:impact on brain tumor surgery. CancerControl 10:115–124
74. Schneider JP, Trantakis C, Schulz T,Dietrich J, Kahn T (2003) Intraoperativeuse of an open mid-field MR scannerin the surgical treatment of cerebralgliomas. Z Med Phys 13:214–218
75. Schneider JP, Schulz T, Schmidt F, Dietrich J, Lieberenz S, Trantakis C,Seifert V, Kellermann S, Schober R,Schaffranietz L, Laufer M, Kahn T(2001) Gross-total surgery of supra-tentorial low-grade gliomas under intraoperative MR guidance. Am JNeuroradiol 22:89–98
76. Schulz T, Bootz F, Weber A, SchneiderJP, Weidenbach H, Heinke W, Kohler-Brock A, Schmidt F, Kahn T (2001)Value of magnetic resonance tomogra-phy for interventions in the ENT spe-cialty. Rofo Fortschr Geb RontgenstrNeuen Bildgeb Verfahr 173:430–436
77. Fahlbusch R, Ganslandt O, BuchfelderM, Schott W, Nimsky C (2001) Intra-operative magnetic resonance imagingduring transsphenoidal surgery. J Neurosurg 95:381–390
78. Ojala R, Klemola R, Karppinen J, Sequeiros RB, Tervonen O (2001)Sacro-iliac joint arthrography in lowback pain: feasibility of MRI guidance.Eur J Radiol 40:236–239
79. Verheyden P, Katscher S, Schulz T,Schmidt F, Josten C (1999) Open MRimaging in spine surgery: experimentalinvestigations and first clinical experi-ences. Eur Spine J 8:346–353
80. Wong TZ, Silverman SG, Fielding JR,Tempany CMC, Hynynen K, Jolesz FA(1998) Open-configuration MR imag-ing, intervention, and surgery of theurinary tract. Urol Clin North Am25:113–122
81. Gould SW, Agarwal T, Benoist S, PatelB, Gedroyc W, Darzi A (2002) Resec-tion of soft tissue sarcomas with intra-operative magnetic resonance guid-ance. J Magn Reson Imaging15:114–119
82. Schulz T, Bennek J, Schneider JP,Trobs RB, Trantakis C, Bootz F, ScholzR, Tannapfel A, Hirsch W, Schmidt F,Kahn T (2003) MRI-guided pediatricinterventions. Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr175:1673–1681
83. Balmer B, Bernays RL, Kollias SS,Yonekawa Y (2002) InterventionalMR-guided neuroendoscopy: a newtherapeutic option for children. JPediatr Surg 37:668–672
84. Smits HFM, Bos C, van der Weide R,Bakker CJG (1999) Interventional MR:vascular applications. Eur Radiol9:1488–1495
86. Bakker CJ, Bos C, Weinmann HJ(2001) Passive tracking of cathetersand guidewires by contrast-enhancedMR fluoroscopy. Magn Reson Med45:17–23
87. Debatin JF, Wildermuth S, von Schultess GK (1998) Intravascularinterventions with active MR tracking.In: Debatin JF, Adma G (eds) Interven-tional magnetic resonance imaging.Springer, Berlin Heidelberg New York,pp 269–282
89. Quick HH, Kühl H, Kaiser G, Hornscheidt D, Mikolajczyk KP, Aker S, Debatin JF, Ladd ME (2003)Interventional MRA using actively visualized catheters, trueFISP, and real-time image fusion. Magn ResonMed 49:129–137
90. Ladd ME, Quick HH (2000) Reductionof resonant RF heating in intravascularcatheters using coaxial chokes. MagnReson Med 37:891–897
91. Yeung CJ, Atalar E (2001) A Green’sfunction approach to local RF heatingin interventional MRI. Med Phys28:826–832
92. Omary RA, Henseler KP, Unal O, Maciolek LJ, Finn JP, Li D, NemcekAA, Vogelzang RL, Grist TM (2002)Comparison of intraarterial and iv ga-dolinium-enhanced MR angiographywith digital subtraction angiographyfor the detection of renal artery steno-sis in pigs. Am J Roentgenol178:119–123
93. Tello R, Mitchell PJ, Melhem ER,Witte D, Thomson KR (1999) Inter-ventional catheter magnetic resonanceangiography with a conventional 1.5 Tscanner: work in progress. AustralasRadiol 43:435–439
94. Bos C, Smits JH, Zijlstra JJ, der MarkWA, Blankestijn PJ, Bakker CJ,Viergever MA, Mali WP (2001) MRAof hemodialisis access grafts and fistu-lae using selective contrast injectionand flow interruption. Magn ResonMed 45:557–561
95. Godart F, Beregi JP, Nicol L, OccelliB, Vincentelli A, Daanen V, Rey C,Rousseau J (2000) MR-guided balloonangioplasty of stenosed aorta: in vivoevaluation using near-standard instru-ments and a passive tracking tech-nique. J Magn Reson Imaging12:639–644
2226
96. Le Blanche AF, Rossert J, Wassef M,Levy B, Bigot JM, Boudghene F(2000) MR-guided PTA in experimen-tal bilateral rabbit renal artery stenosisand MR angiography follow up versushistomorphometry. Cardiovasc Inter-vent Radiol 23:368–374
97. Bücker A, Adam GB, Neuerburg JM,Kinzel S, Glowinski A, Schäffter T,Rasche V, van Vaals JJ, Günther RW(2002) Simultaneous real-time visual-ization of the catheter tip and vascularanatomy for MR-guided PTA of iliacarteries in an animal model. J MagnReson Imaging 16:201–208
98. Bartels LW, Bos C, van der Weide R,Smits HF, Bakker CJ, Viergever MA(2000) Placement of an inferior venacava filter in a pig guided by high-resolution MR fluoroscopy at 1.5 T. J Magn Reson Imaging 12:599–605
99. Spüntrup E, Rübben A, Schäffter T,Manning WJ, Günther RW, BücknerA (2002) Magnetic resonance-guidedcoronary artery stent placement in aswine model. Circulation105:874–879
100. Manke C, Nitz WR, Djavidani B,Strotzer M, Lenhart M, Volk M,Feuerbach S, Link J (2001) MR imag-ing-guided stent placement in iliac arterial stenoses: a feasibility study.Radiology 219:527–534
101. Fink C, Bock M, Umathum R, Volz S,Zuehlsdorff S, Grobholz R, KauczorHU, Hallscheidt P (2004) Renal embolization: feasibility of magneticresonance-guidance using active catheter tracking and intraarterialmagnetic resonance angiography. Invest Radiol 39:111–119
102. Yang X, Atalar E, Li D, Serfaty JM,Wang D, Kumar A, Cheng L (2001)Magnetic resonance imaging permitsin vivo monitoring of catheter-basedvascular gene delivery. Circulation104:1588–1590
103. Vogl TJ, Balzer JO, Mack MG, Bett G, Oppelt A (2002) Hybrid inter-ventional MR imaging system: com-bined MR and angiography suiteswith single interactive table. Feasibility study in vascular liver tumor procedures. Eur Radiol12:1394–1400
104. Bock M, Volz S, Zuehlsdorff S,Umathum R, Fink C, Hallscheidt P,Semmler W (2003) Automatic slicetracking in interventional magneticresonance imaging. Z Med Phys13:177–182
105. Tacke J (2001) Thermal therapies in interventional MR imaging. Cryo-therapy. Neuroimaging Clin N Am11:759–765
106. Straube T, Kahn T (2001) Thermaltherapies in interventional MR imag-ing. Laser. Neuroimaging Clin N Am11:749–757
108. Chen JC, Moriarty JA, Derbyshire JA,Peters RD, Trachtenberg J, Bell SD,Arrelano R, Wright GA, HenkelmanRM, Hinks RS, Lok SY, Toy A,Kucharczyk W (2000) Prostate can-cer: MR imaging and thermometryduring microwave thermal ablation—initial experience. Radiology214:290–297
109. Jolesz FA, Hynynen K (2002) Magnetic resonance image-guided focused ultrasound surgery. Cancer J8:S100–S112
110. Jolesz FA (1998) Interventional andintraoperative MRI: a generaloverview of the field. J Magn ResonImaging 8:3–7
111. Jolesz FA, Bleier AR, Lakob P, Ruetzel W (1988) MR imaging of laser-tissue interaction. Radiology168:249–253
112. Dickinson RJ, Hall AS, Hind AJ,Young IR (1990) Measurement ofchanges in tissue temperature usingMR imaging. J Comput Assist Tomogr 14:430–436
113. de Zwart JCA, Vimeux C, DelalandeC, Canioni P, Moonen CTW (1999)Fast lipid-suppressed MR temperaturemapping with echo-shifted gradient-echo imaging and spectral-spatial excitation. Magn Reson Med42:53–59
114. Harth T, Kahn T, Rassek M, SchwabeB, Schwarzmaier HJ, Lewin JS, Mödder U (1997) Determination oflaser-induced temperature distribu-tions using echo-shifted turboFLASH.Magn Reson Med 38:238–245
115. Peters RD, Hinks RS, Henkelman RM(1998) Ex vivo tissue-type indepen-dence in proton resonance frequencyshift MR thermometry. Magn ResonMed 40:454–459
116. Hynynen K, McDannold N, MulkernRV, Jolesz FA (2000) Temperaturemonitoring in fat with MRI. MagnReson Med 43:901–904
117. Wansapura JP, Daniel BL, Pauly J,Butts K (2001) Temperature mappingof frozen tissue using eddy currentcompensated half excitation RF pulse.Magn Reson Med 46:985–992
118. Graham SJ, Chen L, Leitch M, Peters RD, Bronskill MJ, Foster FS,Henkelman RM, Plewes DB (1999)Quantifying tissue damage due to focused ultrasound heating observedby MRI. Magn Reson Med41:321–328
119. Peters RD, Chan E, Trachtenberg J,Jothy S, Kapusta L, Kucharczyk W,Henkelman RM (2000) Magnetic resonance thermometry for predictingthermal damage: an application of interstitial laser coagulation therapy in an in vivo canine prostate model.Magn Reson Med 44:873–883
120. McDannold NJ, King RL, Jolesz FA,Hynynen K (2000) Usefulness of MRimage-derived thermometry and dosimetry in determining the thresh-old for tissue damage induced by thermal surgery in rabbits. Radiology216:517–523
121. Chen L, Wansapura JP, Heit G, ButtsK (2002) Study of laser ablation inthe in vivo rabbit brain with MR thermometry. J Magn Reson Imaging16:147–152
122. Puccini S, Bär NK, Bublat M, KahnT, Busse H (2003) Simulations ofthermal tissue coagulation and its value for the planning and monitoringof laser-induced interstitial thermo-therapy. Magn Reson Med 49:351–362
123. Gründer W, Goldammer A, SchoberR, Vitzthum HE (2003) Cryotherapyof the brain—a new methodologicalapproach. Z Med Phys 13:203–207
124. MgGahan JP, Dodd GD (2000) Radiofrequency ablation of the liver:current status. Am J Radiol 176:3–16
125. Vogl TJ, Muller PK, Mack MG,Straub R, Engelmann K, Neuhaus P(1999) Liver metastases: intervention-al therapeutic techniques and results,state of the art. Eur Radiol 9:675–684
126. Tacke J, Speetzen R, Adam G, Sellhaus B, Glowinski A, Heschel I,Schäffter T, Schorn R, Großkorten-haus S, Rau G, Günther RW (2002)Experimental MR imaging-guided interstitial cryotherapy of the brain.Am J Neuroradiol 22:431–440
127. Kangasniemi M, Diederich CJ, PriceRE, Stafford RJ, Schomer DF, OlssonLE, Tyreus PD, Nau WH, Hazle JD(2002) Multiplanar MR temperature-sensitive imaging of cerebral thermaltreatment using interstitial ultrasoundapplicators in a canine model. J MagnReson Imaging 16:522–531
128. Mia Y, Ni Y, Yu J, Zhang H, MarchalG (2002) Evaluation of radiofrequen-cy ablation as an alternative for thetreatment of brain tumors in rabbits. J Neurooncol 56:119–126
129. Schwarzmaier HJ, Yaroslavsky IV,Yaroslavsky AN, Fiedler V, Ulrich F,Kahn T (1998) Treatment planningfor MRI-guided laser-induced intersti-tial thermotherapy of brain tumors—the role of blood perfusion. J MagnReson Imaging 8:121–127
2227
130. Schwabe B, Kahn T, Harth T, UlrichF, Schwarzmaier HJ (1997) Laser-induced thermal lesions in the humanbrain: short- and long-term appear-ance on MRI. J Comput Assist Tomogr 21:818–825
131. Kahn T, Harth T, Kiwit JC, Schwarzmaier HJ, Wald C, Mödder U(1998) In vivo MRI thermometry using a phase-sensitive sequence: preliminary experience during MRI-guided laser-induced interstitial thermotherapy of brain tumors. J Magn Reson Imaging 8:160–164
132. Kahn T, Schwabe B, Bettag M, HarthT, Ulrich F, Rassek M, SchwarzmaierHJ, Mödder U (1996) Mapping of thecortical motor hand area with func-tional MR imaging and MR imaging-guided laser-induced interstitial ther-motherapy of brain tumors. Work inprogress. Radiology 200:149–157
133. Leonardi MA, Lumenta CB (2002)Stereotactic guided laser-induced in-terstitial thermotherapy in gliomaswith intraoperative morphologic mon-itoring in an open MR: clinical expe-rience. Minim Invasive Neurosurg45:201–207
134. McDannold N, Moss M, Killiany R,Rosene DL, King RL, Jolesz FA,Hynynen K (2003) MRI-guided fo-cused ultrasound surgery in the brain:tests in a primate model. Magn ResonMed 49:1188–1191
138. Vogl TJ, Eichler K, Straub R, Engelmann K, Zangos S, WoitaschekD, Bottger M, Mack MG (2001) Laser-induced thermotherapy of malignant liver tumors: general prin-cipals, equipment(s), procedure(s)—side effects, complications and re-sults. Eur J Ultrasound 13:117–127
139. Dick EA, Joarder R, de Jode M, Taylor-Robinson SD, Thomas HC,Foster GR, Gedroyc WM (2003) MR-guided laser thermal ablation of primary and secondary liver tumors. Clin Radiol 58:112–120
140. Bär NK, Schulz T, Puccini S,Schirmer T, Kahn T, Busse H (2003)MR-guided laser-induced thermo-ab-lation of liver tumors: clinical experi-ences and therapy control concepts. Z Med Phys 13:209–213
141. Vogl TJ, Straub R, Eichler K, SollnerO, Mack MG (2004) Colorectal carci-noma metastases in liver: laser-in-duced interstitial thermotherapy—local tumor control rate and survivaldata. Radiology 230:450–458
142. Mack MG, Vogl TJ (2002) MR-guid-ed ablation of head and neck tumors.Magn Reson Imaging Clin N Am10:707–713
144. Wacker FK, Cholewa D, Roggan A,Schilling A, Waldschmidt J, Wolf KJ(1998) Vascular lesions in children:percutaneous MT imaging-guided in-terstitial Nd:YAG laser therapy—pre-liminary results. Radiology208:789–794
145. Law P, Gedroyc WM, Regan L (2000)Magnetic resonance-guided percutaneous laser ablation of uterinefibroids. J Magn Reson Imaging12:565–570
146. Vogl TJ, Mack MG, Straub R, EichlerK, Zangos S, Engelmann K,Hochmuth K, Ballenberger S, JacobiV, Diebold T (2002) MR-guided laser-induced thermotherapy with a cooledpower laser system: a case report of apatient with a recurrent carcinoid me-tastasis in the breast. Eur Radiol12:S101–S104
147. Shingleton WB, Sewell PE (2001)Percutaneous renal tumor cryoabla-tion with magnetic resonance imagingguidance. J Urol 165:773–776
148. Sequeiros RB, Hyvonen P, SequeirosAB, Jyrkinen L, Ojala R, Klemola R,Vaara T, Tervonen O (2003) MR im-aging-guided laser ablation of osteoidosteomas with use of optical instru-ments guidance at 0.23 T. Eur Radiol13:2309–2314
149. Hynynen K, McDannold N, Vykhodtseva N, Jolesz FA (2003)Non-invasive opening of BBB by focused ultrasound. Acta NeurochirSuppl 86:555–558
150. Sabel M, Rommel F, Kondakci M,Gorol M, Willers R, Bilzer T (2003)Locoregional opening of the rodentblood-brain barrier for paclitaxel using Nd:YAG laser-induced thermotherapy: a new concept of adjuvantglioma therapy? Lasers Surg Med33:75–80
151. Guilhon E, Quesson B, Moraud-Gaudry F, de Verneuil H, Canioni P,Salomir R, Voisin P, Moonen CT(2003) Image-guided control of transgene expression based on localhyperthermia. Mol Imaging 2:11–17
152. Huber PE, Mann MJ, Melo LG, EhsanA, Kong D, Zhang L, Rezvani M,Peschke P, Jolesz F, Dzau VJ, Hynynen K (2003) Focused ultra-sound (HIFU) induces localized enhancement of reporter gene expression in rabbit carotid artery.Gene Ther 10:1600–1607
