1988, The British Journal of Radiology, 61, 456-462 Magnetic resonance velocity mapping in aortic dissection By *H. G. Bogren, MD, S. R. Underwood, MA, MRCP, D. N. Firmin, BSc, MPhil, R. H. Mohiaddin, MB, MSc, R. H. Klipstein, BA, MB, R. S. 0. Rees, MA, FRCP, FRCR and D. B. Longmore, FRCS Magnetic Resonance Unit, The National Heart and Chest Hospitals, 30 Britten St, London SW3 6NN (Received July 1987 and in revised form November 1987) Abstract. We describe three patients with chronic aortic dissection in whom both spin-echo magnetic resonance imaging (MRI) and cine field-echo imaging were performed. The field even-echo rephasing (FEER) sequence showed the intimal flaps much more clearly than the spin-echo sequence and provided a distinction between thrombus and static blood. Velocity mapping allowed flow measurements in the true and false lumens. The management of the three patients was based upon the information provided by MRI. It is suggested that MRI may avoid invasive investigation and be the method of choice in haemodynamically stable patients with aortic dissection provided that the FEER sequence is used. Magnetic resonance imaging has been used successfully to detect aortic dissection, to assess its extent and to identify involvement of the major aortic branches (Amparo et al, 1985; Geisinger et al, 1985; Goldman et al, 1986). It has compared very favourably with computed X-ray tomography and with invasive arterio- graphy and it has been suggested that magnetic resonance imaging may obviate the need for arterio- graphy in dissections involving only the descending aorta (Geisinger et al, 1985). In dissections involving the ascending aorta, these early studies concluded that a disadvantage of magnetic resonance imaging was the difficulty in assessing the competence of the aortic valve and the state of the coronary arteries, although aortic regurgitation can now be detected and measured by this method (Underwood et al, 1986, 1987a). Another problem was that it was difficult to distinguish blood from thrombus in the false lumen, although velocity- dependent phase shifts have been used in one patient to identify flow in the false lumen (Dinsmore et al, 1986). Additional information is available using a field-echo sequence with even-echo rephasing (FEER) and we have previously described a method of velocity mapping which uses this sequence and which we have shown to produce accurate flow measurements (Nayler et al, 1986; Firmin et al, 1987; Klipstein et al, 1987). We now describe three patients with aortic dissection in whom the use of the FEER sequence with velocity mapping aided both the anatomical diagnosis and the assessment of function. Methods We used a Picker International Vista MR 2055 magnetic resonance scanner operating at 0.5T. Electro- cardiographically gated images were acquired using a *On sabbatical leave from Department of Radiology, School of Medicine, UC Davis, California, USA. spin-echo sequence with time to repeat equal to the duration of the cardiac cycle and with time to echo 40 ms. Two repetitions of the sequence were averaged for each of 128 phase-encoding steps, and there were 256 pixels in the direction of frequency encoding. The images were interpolated to a 512x512 matrix. Slice thickness was 10 mm with afieldof view between 30 cm and 45 cm, as appropriate. Velocity mapping was performed using a field-echo sequence with even-echo rephasing and the same imaging parameters as the spin-echo images (Nayler et al, 1986). This sequence produces high signal intensity from blood moving at all velocities commonly found in the cardiovascular system, except in the presence of turbulence, when signal is lost. Lower signal intensity is obtained from other cardiac structures. Gradient modifications encoded the velocity in a selected direction with respect to the imaging plane so that phase reconstruction produced a velocity map. The phase images were subtracted from reference phase images without gradient modification in order to compensate for phase changes produced by field inhomogeneity. A reduced excitation pulse angle (40°) allowed repetition every 40 ms to provide a cine display with up to 16 frames per cardiac cycle, depending on cycle length. Computed X-ray tomography was carried out using an Elscint 2002 scanner during and immediately after intravenous injection of 150 ml of iso-osmolar contrast medium. Contiguous 10 mm sections were acquired and exposure time was 5 s per image. Case reports Case 1 A 60-year-old man was involved in a road traffic accident without obvious external injury. Later the same day he developed severe pain in the neck, jaw and central chest. On admission to hospital he had signs of aortic regurgitation with cardiac tamponade and unequal arm pulses. Echocardiography 456 The British Journal of Radiology, June 1988
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1988, The British Journal of Radiology, 61, 456-462
Magnetic resonance velocity mapping in aortic dissectionBy *H. G. Bogren, MD, S. R. Underwood, MA, MRCP, D. N. Firmin, BSc, MPhil,R. H. Mohiaddin, MB, MSc, R. H. Klipstein, BA, MB, R. S. 0. Rees, MA, FRCP, FRCR andD. B. Longmore, FRCS
Magnetic Resonance Unit, The National Heart and Chest Hospitals, 30 Britten St, London SW3 6NN
(Received July 1987 and in revised form November 1987)
Abstract. We describe three patients with chronic aortic dissection in whom both spin-echo magnetic resonance imaging (MRI)and cine field-echo imaging were performed. The field even-echo rephasing (FEER) sequence showed the intimal flaps much moreclearly than the spin-echo sequence and provided a distinction between thrombus and static blood. Velocity mapping allowedflow measurements in the true and false lumens. The management of the three patients was based upon the information providedby MRI. It is suggested that MRI may avoid invasive investigation and be the method of choice in haemodynamically stablepatients with aortic dissection provided that the FEER sequence is used.
Magnetic resonance imaging has been used successfullyto detect aortic dissection, to assess its extent and toidentify involvement of the major aortic branches(Amparo et al, 1985; Geisinger et al, 1985; Goldmanet al, 1986). It has compared very favourably withcomputed X-ray tomography and with invasive arterio-graphy and it has been suggested that magneticresonance imaging may obviate the need for arterio-graphy in dissections involving only the descendingaorta (Geisinger et al, 1985). In dissections involvingthe ascending aorta, these early studies concluded that adisadvantage of magnetic resonance imaging was thedifficulty in assessing the competence of the aortic valveand the state of the coronary arteries, although aorticregurgitation can now be detected and measured by thismethod (Underwood et al, 1986, 1987a). Anotherproblem was that it was difficult to distinguish bloodfrom thrombus in the false lumen, although velocity-dependent phase shifts have been used in one patient toidentify flow in the false lumen (Dinsmore et al, 1986).
Additional information is available using a field-echosequence with even-echo rephasing (FEER) and wehave previously described a method of velocity mappingwhich uses this sequence and which we have shown toproduce accurate flow measurements (Nayler et al,1986; Firmin et al, 1987; Klipstein et al, 1987). We nowdescribe three patients with aortic dissection in whomthe use of the FEER sequence with velocity mappingaided both the anatomical diagnosis and the assessmentof function.
MethodsWe used a Picker International Vista MR 2055
magnetic resonance scanner operating at 0.5T. Electro-cardiographically gated images were acquired using a
*On sabbatical leave from Department of Radiology, Schoolof Medicine, UC Davis, California, USA.
spin-echo sequence with time to repeat equal to theduration of the cardiac cycle and with time to echo40 ms. Two repetitions of the sequence were averagedfor each of 128 phase-encoding steps, and there were256 pixels in the direction of frequency encoding. Theimages were interpolated to a 512x512 matrix. Slicethickness was 10 mm with a field of view between 30 cmand 45 cm, as appropriate.
Velocity mapping was performed using a field-echosequence with even-echo rephasing and the sameimaging parameters as the spin-echo images (Nayleret al, 1986). This sequence produces high signal intensityfrom blood moving at all velocities commonly found inthe cardiovascular system, except in the presence ofturbulence, when signal is lost. Lower signal intensity isobtained from other cardiac structures. Gradientmodifications encoded the velocity in a selecteddirection with respect to the imaging plane so thatphase reconstruction produced a velocity map. Thephase images were subtracted from reference phaseimages without gradient modification in order tocompensate for phase changes produced by fieldinhomogeneity. A reduced excitation pulse angle (40°)allowed repetition every 40 ms to provide a cine displaywith up to 16 frames per cardiac cycle, depending oncycle length.
Computed X-ray tomography was carried out usingan Elscint 2002 scanner during and immediately afterintravenous injection of 150 ml of iso-osmolar contrastmedium. Contiguous 10 mm sections were acquired andexposure time was 5 s per image.
Case reportsCase 1
A 60-year-old man was involved in a road traffic accidentwithout obvious external injury. Later the same day hedeveloped severe pain in the neck, jaw and central chest. Onadmission to hospital he had signs of aortic regurgitation withcardiac tamponade and unequal arm pulses. Echocardiography
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Figure 1. Case 1. (a) Transverse spin-echo image of the aortic arch showing a dissection dividing the aorta into true (straightarrow) and false lumens (curved arrow). The false lumen impinges upon the trachea (T). (b), (c) Contiguous coronal spin-echoimages (RPA, right pulmonary artery): the systolic image (b) shows some signal within the false lumen because of slow flow, andthe diastolic image (c) shows a much higher signal intensity which could be mistaken for thrombus. The magnitude FEER image(not illustrated) showed high signal intensity from blood without thrombus in both lumens, (d) The velocity map shows highvelocity (white) in the true lumen but virtually zero velocity (grey) in the false lumen.
showed a dilated ascending aorta with an intimal flap. Atemergency surgery the ascending aorta was replaced by aDacron graft with resuspension of the aortic valve andreimplantation of the coronary arteries. Post-operatively,partial palsy of the right vocal cord was noted. Two monthsafter surgery the patient presented with stridor and a completeleft vocal cord palsy. An aortogram showed persistentdissection but the study was incomplete because of patientcoughing. Computed X-ray tomography showed dissectionbeyond the distal anastomosis of the graft and also possibleperiaortic clot. It was uncertain whether this could account forthe vocal cord palsy and magnetic resonance imaging wasrequested.
Multislice spin-echo magnetic resonance images wereacquired in transverse, coronal and left anterior oblique planesin the chest, and in transverse and coronal planes in theabdomen. Cine velocity mapping was performed in the aorticarch and the abdominal aorta.
A dissection was found extending from the distalanastomosis of the ascending aortic graft for the whole lengthof the aorta and into the left external iliac artery. The arch wasdilated and the false lumen appeared to impinge upon thetrachea causing the left laryngeal nerve palsy (Fig. la). It wasuncertain from the spin-echo images whether the false lumen inthe arch was thrombosed (Figs lb, c) but the magnitudereconstruction using the FEER sequence showed high signal
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(c)
Figure 2. Case 1. Coronal images of the abdominal aorta. The spin-echo image (a) shows no clear evidence of dissection, while theamplitude FEER image (b) and the velocity maps (c, d) show a dissection with forward flow (black) in both lumens in systole(c) but retrograde flow (white) in the false lumen in diastole (d).
intensity, indicating fluid blood as opposed to the lower signalintensity of thrombus. The velocity maps showed that flow inthe false lumen was very slow (Fig. Id). The spin-echo imagesof the abdominal aorta did not allow a confident diagnosis ofdissection, but the FEER images did. Velocity mappingshowed antegrade flow in both lumens in systole butretrograde flow in diastole in the false lumen (Fig. 2). The rightrenal artery arose from the true lumen while the left was notvisualized and probably arose from the false lumen.
Because of the magnetic resonance findings, surgery wasinitially planned, but his clinical condition improved and he iscurrently being treated conservatively.
Case 2A 57-year-old man was admitted to hospital with chest pain
which settled spontaneously without a definite diagnosis. Three
weeks later he was readmitted with shortness of breath.Echocardiography showed a pericardial effusion whichproduced blood-stained fluid on aspiration. Computed X-raytomography confirmed the pericardial effusion and showed adilated ascending aorta but no evidence of dissection. Anaortogram showed aortic dissection involving only theascending aorta.
Multislice magnetic resonance images were acquired intransverse, coronal and sagittal planes and cine velocitymapping was performed in a transverse plane through theascending aorta. The multislice images showed the ascendingaorta to be dilated and an intimal flap was seen in severalimages, with a large false lumen anteriorly (Figs 3a, b). Thecine flow images showed the dissection much more clearly,however, with no evidence of thrombus in either lumen(Fig. 3c). Rapid forward flow was seen in the true lumen in
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(c)
Figure 3. Case 2. (a) Systolic and (b) diastolic transverse spin-echo images showing a dilated ascending aorta with an intimal flapseparating true (T) from false (F) lumens. The flap is barely seen in (a) but is better seen in (b) because of the signal from staticblood in the false lumen, (c) The amplitude FEER image shows the dissection much more clearly, (d) The velocity image showshigh velocity in the true lumen (straight arrow) and zero velocity in the false lumen (curved arrow). Caudal velocities are seen inthe superior vena cava and descending aorta (black).
systole with the normal circulating pattern in diastole. Thefalse lumen had virtually no flow (Fig. 3d), and the time courseof flow in each lumen is shown in Fig. 4. The diagnosis wasconfirmed at surgery.
Case 3A 78-year-old man with a long history of hypertension was
admitted to hospital with epigastric pain which rapidly settled.A chest radiograph showed fusiform dilatation of thedescending thoracic aorta and echocardiography showed a
mildly dilated ascending aorta. Magnetic resonance imagingwas requested to clarify the anatomy and to exclude dissection.
Multislice magnetic resonance images were acquired intransverse and coronal planes in the thorax. Cine velocitymapping was performed in a transverse plane through whichboth ascending and descending aortas passed. A dissectionconfined to the descending aorta was found. The false lumenhad a homogeneous appearance throughout the cardiac cycleon the spin-echo images while the true lumen showed no signalin systole and a varying signal in diastole, giving an
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Time (ms)
Figure 4. Case 2. Flow in the true and false ascending aorticlumens throughout the cardiac cycle.
Figure 5. Case 3. (a) Systolic and (b) diastolic spin-echo imagesshowing dissection of the descending aorta with true (doublearrow) and false lumens (single arrow). The appearance of thefalse lumen suggests that it is thrombosed but this appearancecould also be due to static blood, (c) The FEER image showslow signal intensity and (d) the velocity map shows zerovelocity in the false lumen, which suggest that it is thrombosed.(AA, ascending aorta; SVC, superior vena cava; LPA, leftpulmonary artery.)
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inhomogeneous appearance (Figs 5a, b). In the field-echoimages the false lumen had a very low signal intensity,implying that it was thrombosed and velocity mapping showedno movement (Figs 5c, d). Confirmation of these findings wasnot obtained but similar findings have been observed in anexperimental clot and in patients with atrial thrombi in ourlaboratory (Underwood et al, 1987b).
DiscussionThe amplitude and phase reconstructions using the
FEER sequence provide different information. Theamplitude reconstruction shows anatomy but withdifferent contrast to spin-echo images, since bloodalways gives a high signal intensity unless it is turbulent.The phase reconstruction is a map of velocities in achosen direction at all points in the imaging plane.
In the first two patients, the amplitude reconstructionof the FEER sequence showed the thoracic dissectionmore clearly than the conventional spin-echo images,and the abdominal dissection in the first patient wouldnot have been seen from the spin-echo images alone.The FEER images showed the intimal flap as a thin linewith no signal, separating the areas of high signalintensity within the true and false lumens. In spin-echoimages, moving blood gives no signal and a thin intimalflap may be more difficult to see, although at certainparts of the cardiac cycle there may be differentvelocities within the lumens and hence different signalto help the diagnosis.
A consequence of high signal intensity from blood inthe FEER sequence is that thrombus and blood caneasily be distinguished, whereas using a spin-echosequence they may appear the same. The ability to mapvelocities and to measure flow aids this distinction. Inthe third patient, the FEER images suggested thrombusin the false lumen whereas the spin-echo images wouldhave been compatible with either thrombus or staticblood. The FEER sequence with velocity mappingtherefore seems to alleviate some of the concernexpressed by Amparo et al (1985), Geisinger et al(1985) and Barentsz et al (1987).
Velocity mapping provides information that isdifficult to obtain using any other technique (Hayashiet al, 1974). It was particularly valuable in the firstpatient, in whom there was antegrade flow in the falselumen of the abdominal aorta in systole but retrogradeflow in diastole. This implies that some abdominalorgans were supplied by the false lumen, which was fedfrom a distal re-entry site. The coeliac axis, the superiormesenteric artery and the left renal artery were seen toarise from the true lumen, but the right renal artery wasmore difficult to identify and it probably arose from thefalse lumen.
In this patient the decision to operate was made onthe strength of the magnetic resonance findings. Inretrospect, it can be concluded that if magneticresonance had been performed before aortography andcomputed X-ray tomography, these examinations couldhave been avoided. The same is true in the secondpatient, in whom magnetic resonance yielded the sameinformation as the aortogram. In the third case the
diagnosis of dissection was made by magnetic resonanceafter echocardiography had been inconclusive andfurther examinations were not contemplated.
We suggest that magnetic resonance should beperformed early in the investigation of patients withsubacute or chronic aortic dissection, since moreinvasive studies may be avoided. Echocardiography willusually precede magnetic resonance but, unless veryurgent surgery is required, magnetic resonance is asensible next step to confirm the diagnosis and to assessits extent. We have no experience with acute dissectionsbut see no reason to alter this policy provided that thepatient is haemodynamically stable.
It has been suggested by Amparo et al (1985) thatmagnetic resonance is not able to evaluate thecompetence of the aortic valve nor show the status ofthe coronary arteries, so that cardiac catheterization isnecessary in patients with dissection. However, we arenow able to assess regurgitation either by demonstra-ting the turbulent regurgitant jet using a cine FEERsequence (Underwood et al, 1987a) or by measuring theregurgitant fraction from left and right ventricularstroke volume measurements (Underwood et al, 1986).Coronary arteriography is not always necessary pre-operatively. In our first case, for instance, the rightcoronary artery was reimplanted into the aortic graftwithout pre-operative coronary arteriography. Inanother case, not reported here, coronary arteriographyshowed severe disease but surgery was postponed untilafter the repair of the dissection. This strengthens ourbelief that magnetic resonance is valuable inhaemodynamically stable patients with aortic dissectionand that invasive investigation can be avoided.Magnetic resonance will also provide images of theheart for evaluation and a more complete studyincluding left ventricular volume and ejection fraction.
AcknowledgmentsWe gratefully acknowledge financial support from the Board
of Governors of the National Heart and Chest Hospitals, theCoronary Artery Disease Research Association (CORDA), theLeague of Friends of the Brompton Hospital, The ViscountRoyston Trust, the Halley Stewart Trust, the Charles WolfsonCharitable Foundation, and GEC/Picker International. Wealso thank Sister Elizabeth Burman and Mr Karl Lotey fortheir help.
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