MRI Hot TopicsNeuroimaging at 3T

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Neuroimaging at 3TJohn Grinstead, Staff Scientist

IntroductionNeuroimaging holds tremendous promise bymoving to 3 Tesla since brain imaging has relativelylimited difficulties with motion and flow artifacts.The brain’s architecture and function are extremelycomplex, and increasing the detail with whichwe can image the brain will provide tremendousbenefit for understanding its structure. 3T opensup these possibilities. The benefit of higher SNRat 3T is well known, and includes the options ofincreasing the imaging matrix, using thinner slices,or reducing averaging. (See Figures 1A and 1B.)

To fully achieve these benefits and make thetransition from 1.5 T successful, the technicalchallenges such as RF technology and SAR must be overcome.

Currently, in addition to the standard CP head coil, theTrio has an optional 8-channel iPAT-compatible head

array coil for high resolution and high speed brainimaging, and a 13 Element/8 Channel iPAT-compatibleneurovascular array coil for examinations of thecarotids as well as the complete head and neckvasculature. (See Figure 2.) Allegra systems are

currently being upgraded to 4 channel and 8 channelhead coils. Sophisticated pulse sequences utilizingintegrated parallel acquisition techniques (iPAT)and SAR reduction techniques like hyperechoesand variable flip angle turbo-spin echo have beendeveloped by Siemens.

iPAT and 3TThe Siemens solution to parallel imaging, iPAT, notonly includes a choice of iPAT algorithms (mSENSEand GRAPPA) but also has several benefits at 3T.Siemens is the only vendor that can provide asystem that images with up to 32 receiverchannels and has built and delivered head coilsthat can simultaneously utilize more than 8receivers. With the integrated auto-calibration theentire exam is simply faster. Using iPAT to reducethe echo train length in single-shot EPI techniquescan greatly reduce susceptibility artifacts that aretypically twice as large at 3T compared to 1.5T.(See Figure 3.) In cases where susceptibility is nota significant problem, such as in turbo spin-echo

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Figure 1A: High resolutionimages—Optical nerveimaging with 2D TSE with0.3x0.4x3mm resolution.Courtesy of University ofUtah, USA

Figure 1B: Showsmeasurements of theoptic nerve—magnified.

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Figure 2: Head neckce-MRA with neurovascular coil.Courtesy of Dr. Paul Finn, UCLA

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Figure 3: Reducedsusceptibility artifactswith iPAT. Single-shot EPIdiffusion, TR/TE=2600/75ms , Matrix 164x256,(GRAPPA) PAT 2

sequences, iPAT can be used to lower the SAR byreducing the total number of RF pulses needed.(See Figure 4.) Best of all, although parallelimaging will reduce the image SNR (by a factorproportional to the square root of the scan timesavings), at 3 Tesla you inherently have more SNRto play with so it is often not the limiting factor.This opens up a range of available applications

Figure 4: 50% reduction inSAR with iPAT. 2D Dark-FluidT2 with PAT2.

TSE, will potentially allow up to a factor of 6reduction in SAR. Both are optimized for 3Dimaging with T2 weighting as well as dark fluidcontrast, and are suitable for imaging of the entirehead at high spatial resolution. (See Figure 5).

Accurate SAR prediction, online monitoring,and a detailed SAR history for each patient letyou optimize each sequence. SAR limitations

are a factor of 4 more prohibitive at 3 Tesla,relative to 1.5T, and are of concern with RF-intensesequences, such as turbo spin-echo. High turbo-factors can limit the achievable number of slices orthe minimum achievable TR (which will forcea longer scan time), or require reducing therefocusing flip angle below 180º. The best wayto solve this problem is through optimal pulsesequence design. Siemens uses hyperechoes toreduce SAR and to realize more of the advantagesof 3T imaging.

that are not feasible at lower field strengths suchas fast complete examinations or highly detailedexams in an equivalent time.

SARRF deposition at 3T is a critical issue, which,if not properly managed, can lead to limitedprotocols and poor image quality. One of themost innovative solutions utilizes hyperechoes,which use a special train of RF pulses which actsimilarly as the echo train of a TSE sequence,but with up to an 80% reduction in SAR. Thisbenefit is best with higher echo trains. Anotherrelated solution, using SPACE (Sampling Perfectionwith Application Optimized Contrasts usingdifferent flip angle Evolutions) variable flip angle

Figure 5: 3D Dark Fluid T2 with variable flip angle TSE—SPACE—89% reduction of SAR compared to standard TSE.

Contrast Enhanced MRAngiography (ce-MRA)Superior gradient performance coupled with iPATparallel imaging allows ultrashort TR/TE examswith high SNR. Siemens is a world leader ingradient technology. This is of particular benefitin angiographic applications in which the smallestpossible TE is desired to reduce spin dephasingcaused by field inhomogeneities and acceleration.(See Figure 8.)

Diffusion and Perfusion Simply speaking, diffusion imaging is a signalstarved technique. In diffusion imaging, the SNR isinversely proportional to the strength of diffusionencoding (the b-value), and at very high b-valuesSNR approaches zero. Signal averaging is helpfulbut even subtle motion (including blood pulsation)

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7A 7B

Figure 8: Head-neck ce-MRA tothe circle. Courtesy ofDr. Paul Finn, UCLA.

Parameters Sequence/TR/TE/Protocol Time Slice Thickness/Matrix/PAT Clinical use

Localizer 00:08 GRE/20/5/10/256 To plan next studies

T1 MPRAGE SAG 02:01 TFL/1200/2.02/5/256/PAT2/TI1100 Depicts anatomy

T2 TSE TRA FS 01:00 TSE/4960/99/5/256/PAT2/Fatsat Depicts pathology

T2 TSE Dark-Fluid TRA 01:48 TSE/9000/99/5/256/PAT2/TI 2500 For periventricular and intra-ventricular lesions

Diffusion EPI TRA 01:00 EP2D_Diff/4000/83/5/128/PAT2 Useful in evaluation ofdisease conditions likestroke or tumor.

TOTAL time 05:57 min

Six-Minute Brain Examination

Figure 6: syngo MR for6-minute brainexamination.

Routine NeuroimagingThe increased signal at 3 Tesla allows theacquisition of ultra-fast neuro-anatomical images.A 6 minute brain examination illustrates the point.

MR Angiography

The routine 3D ToF MR angio sequence yieldsexcellent results with good visualization of secondaryand tertiary blood vessels. The increased T1 at 3Talso leads to improved background suppression intime-of-flight techniques. (See Figures 7A and 7B.)

Figure 7A: MIP of 3D ToF MR Angiogram, 0.8 mm Slices,72 partitions, 512 matrix, TA 4:50 min. Figure 7B: Surface ShadedDisplay of a 3D ToF MR Angiogram.

can cause complications. Imaging at 3 Tesla withshorter echo times than are possible at 1.5 Tallow for stronger diffusion encoding withacceptable SNR. (See Figures 9A and 9B, diffusionstroke case.)

Perfusion imaging uses susceptibility changesinduced by a passing bolus of contrast. Theseeffects increase with field strength, again allowingone to either get a larger susceptibility change, oruse less contrast medium compared to 1.5T. MRperfusion imaging is considered an important stepand plays a critical role in the clinical managementof patients.

fMRILow BOLD contrast-to-noise ratio has long beenthe limiting factor in functional MR experiments,requiring large voxel sizes and long experimentswith multiple or repeating paradigms. Thequadratic increase in the BOLD effect at 3Timproves the precision of fMRI experiments,while there is simultaneous benefit from thestandard increase in image SNR. This is especiallyimportant in single trial or event-related paradigmswhere one needs the largest possible BOLDresponse with minimal background noise.The superior functional data at 3T improves thereal-time viewing of statistical maps using theSiemens online fMRI display, allowing real-timevisualization of the activation maps. (See Figure 10.)

Faster and stronger gradients allow extremely highbandwidth scans for a closer echo spacing and ashorter echo time. This reduces the amount ofsusceptibility induced image distortions andreduces susceptibility induced signal voids. iPATcan also be used to reduce the echo train, furtherreducing susceptibility artifacts. Siemens willpotentially exploit higher iPAT factors when current8-channel technology is upgraded for the Trio to32 independent channels.

ConclusionSiemens is a world leader in MR technology andis at the forefront of the new ultra high field market.High quality complete diagnostic neuro exams withMAGNETOM Trio are quick and produce excellentimages. With the developments of new techniquesand strategies to image at ultra high field thediagnostic information will only get better. Ourresearch into field strengths beyond 3 Tesla furtherdemonstrates our commitment to developinghigh-end technology for improved neuroimaging.

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Figure 9: (9A) DWI, (9B) ADC Map in a case of stroke. Courtesy ofHong Kong Sanatorium and Hospital.

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Figure 10: Real-time fMRI for accurate results.

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