Cine magnetic resonance imaging and color Doppler flow mapping in infants and children with pulmonary artery bands

Cine Magnetic

Resonance Imaging and Color apping in Infants and Children with Pulmonary Artery Bands

MD,* Lilliam M. Valdes-Cruz, MD, Dean P. Berthoty, MD, Jack B. Powell, MD, John R. Hesselink, MD, Kyung J. Chung, MD, and David J. Sahn, MD

lain 8. Simpson,

Cine magnetic resonance imaging (MRI) and color Doppler flow mapping were performed in 12 ib fants and children (aged 3 to 35 months) after pulmonary artery banding to define the anatomy and physiology of the right ventricular oufflow tract and evaluate the anatomy. MRI was performed using a 1.5 Tesla magnet in the sagdtal, axial and oblique views with all patients studied in the 24 cm head coil following adequate sedation. Highresolution tine MRI was obtained in all patients and the narrowest flow diameter on tine MRI COF related well with the pressure gradient measured across the band in 11 patients at cardiac cathetelc ization or surgery (r q -0.95). Signal loss was always seen distal to the band associated with tur bulent flow as seen by color Doppler flow map ping Signal loss in tine MRI was also seen proximal to the band. The length of this proximal signal void also correlated well with the pressure gradient measured across the band (r = 0.91) and was closely matched by the zone of proximal spatial acceleration defined by digital computer analysis of color Doppler flow map images (r = 0.89), which also demonstrated low grade variance associated with the laminar accelerating flow stream. The position of the band was accurately defined by tine MRI which identified inadequate pulmonary artery banding in 2 patients confirmed subsequently at cardiac catheterization and angiography. Cine MRI and color Doppler flow mapping when used together provide high-resolution detail about the right ventricular outflow tract and pulmonary artery band anatomy and function. Comparative observations of spatial flow velocity information on color Doppler flow -From the Division of Pediatric Cardiology, University of California, San Diego, California. This study was supported in part by the Magnetic Resonance Division of General Electric Medical Systems, Milwaukce, Wisconsin, and by Grant ROl HL 43287 from the National Institutes of Health, Bethesda, Maryland. Dr. Simpson was supported by a British-American Research Fellowship of the British Heart Foundation and American Heart Association. Manuscript received August 31, 1992; revised manuscript received and accepted January 20, 1993. “Current address: Wessex Regional Cardiac Centre, Southampton General Hospital, Tremona Road, Southampton SOY 4XY, United Kingdom. Address for reprints: David J. Sahn, MD, Pediatric Cardiology, UHN60, Oregon Health Sciences University, 3181 S.W. Sam Jackson Park Road, Portland, Oregon 97201.

mapping allow important insights into the flow visualization characteristics of tine MRI. (AmJ Cardiol1993;71:1419-1428)

ven in an era stressingprimary surgical correction of most cardiac lesions in infancy, pulmonary artery banding is still a common palliative surgical procedure for infants and children with large or multiple ventricular septal defects, single ventricle or other complex cardiac lesions. Problems related to the postoperative results of pulmonary artery banding are usually associatedwith loosening (or too loose placement) of the band and/or its migration and displacement toward the branch pulmonary arteries’ producing branch pulmonary artery obstruction. Echocardiography can provide information on band anatomy,and continuous-wave Doppler ultrasound now allows estimation of the pressure gradient across the band2,3;however, they are limited by the problems of ultrasound penetration, particularly in older patients or those who have undergone repeated surgical interventions. Even in those patients where high-quality images are obtained, the branch pulmonary arteries cannot always be visualized well by echocardiography. Color Doppler flow mappingM and tine magnetic resonance imaging @4X1)7-15provide dynamic appreciation of flow in combination with structural detail. The use of either of thesetechniquesfor imaging patients with pulmonary artery banding has not beenreported.We have previously demonstratedthat color Doppler flow mapping can delineate interesting flow phenomena that occur at various sites both proximal and distal to an obstructive lesion in vitro16 and in vivo.17 It is not clear to what extent the Bow information obtained by tine MRI compares to flow velocity data obtained by color Doppler flow mapping in the clinical context. This study assessescolor Doppler flow mapping and tine MRI in infants and chi-dren with pulmonary artery banding and compares the anatomic definition and flow relationships defined by these techniques with information obtained at cardiac catheterization, angiography and surgery.We also wished to compare some of the display characteristicsof velocity, acceleration and turbulence obtained by color Doppler flow mapping and tine MRI in these patients.

E

Patient%: Fourteen magnetic resonance studies were performed in 12 patients (age range 3 to 3.5 months) MAGNETICRESONANCEIN PULMONARYARTERYBANDING 1

after pulmonary artery banding. Of the 12 patients, 1 was studied both before and after banding and 1 other was restudied after the pulmonary artery was debanded. All tine MRI studies of the pulmonary artery bands were performed 2 to 24 months after the banding procedure. Magnetic resonance imaging: MRI was performed using a General Electric Signa super-conducting magnet with a field strength of 1.5 Tesla. All patients were sedated with oral chloral hydrate (80 to 100 mg/kg) administered 30 minutes before the study, and all were imaged in the 24 cm head coil to obtain optimal image resolution. Total study time for individual patients ranged from 45 minutes to 1 hour. It was necessaryto ensure a study time 2 separateviews by the standard technique of gradient-recalled acquisition in steady state with flow compensation, and exact slice locations chosen from the gated images that best

RGURE 1. Convention;’ netic resonance image monary ae--. L--Al--

-‘-J----diographically gated magew) in a patient after pulU nut= the presence of a large

ventliculai

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displayed the pulmonary band anatomy.The flip angle was 30”, echo time 12 ms and the repetition time 22 ms per slice. A slice thickness of 5 mm was used and 2 slice locations were chosen per acquisition to optimize the temporal resolution of the technique, as the effective repetition time increaseswith the number of slice locations. All images were reconstructed to 16 frames per cardiac cycle and displayed in a tine loop format. Single stop frames in diastole and systole from a tine magnetic resonancesequencein the axial view are shown in Figure 2. Note on the diastolic image that with tine MRI, blood flow is displayed as a high-intensity signal compared with surrounding stationary, anatomic structural detail because of the continuous entry of unsaturated spins from flowing blood into the imaged slice locations. The signal is strongestfor flow directed perpendicular to the imaging plane,1s,19 All stop frame measurementsfrom the tine MRI sequenceswere made directly on screenusing the available automatically calibrated dimensional analysis package of the Signa system.Acceleration was defined as the zone of signal loss seen proximal to the pulmonary artery band, a zone that we have previously characterized as a nonturbulent, accelerating Bow zone on both tine MRI and color flow mapping.20Cursors were

FIGURE 2. Stop frames from a tine magnetic resonance imapling sequence (axial view) at emhliastole (fop panel) and peak systole (bottom panel). lhe severity of the pulmonary artery band is indicated by the discrete, narrowed flow diameter in the main pulmonary artery. During systole, there is loss of the higHtiensity signal distal to the band and into the branch pulmonary arteries, associated with tup bulent flow. Also note the signal loss proximal to the band in a zone of laminar flow acceleration. Note that the signal level in the ascending aorta which has normal flow is unchanged.

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placed visually on the image at each edge of the band surements were made in random patient order without for measurementof band diameter and for acceleration knowledge of the results of the color flow maps so as length horn the earliest position of proximal signal loss to avoid observer bias. COW Doppler fiow mappin$ Color Doppler flow mapin the right ventricular outflow tract to the center of the band. All mexarementn

of hand rliamdcr

and ?cce!pz-

tion !ength were made to the nearest0.5 mm. The mea-

mh-g ~::zz ~~~zkxzxx!

k

z,!! pk~~;

z;i;,g

;r TX%;,

SSH65A with a 3.75 MHz transducer. Color Doppler

FlGlJDE 3. A, color Doppler flow map image of the right ve* tricular outflow tract and pulmonary arteries with flow going away horn the transducer. The narrowed flow diameter indicates the site and severity of the band. Note the narrowing accelerating flow stream proximal to the band imlicated by an alias to red and the mosaic pattern of orange, green and turquoise distal to the band imlicating turbulent flow. If, Digital velocity maps for the blue, red and green components of each color pixel in A. Proximal to the band there are increasing numerical assignments of blue that alias to the highest red values, and then the red numbers decrease toward the band indicati~ proximal spatial acceleration. Also note the presence of low-grade gmeu (variance) associated with the zone of acceleration and the hitier green values distally associated with visually apparent turbulence. C, pseudo 3dimensional maps for the blue, red and @u?ep (variance) components of the pixels in A. Again, note the high Mue values aliasing to decreasing red values proximal to the band and the distribution of variance in the accelere tion zone with higher variance values distal to the band.

MAGNETICRESONANCEIN PULMONARYARTERYBANDING

flow mapping studies were performed immediately after MRI to obviate temporal changes that may affect comparison of the 2 techniques and so that effects of sedation were similar during both studies. Imaging was performed from multiple precordial positions including parastemal, subcostal and moditied apical views, but satisfactory imaging of the right ventricular outllow tract and pulmonary arteries was consistently obtained from the parastemal short-axis view. Color Doppler flow mapping was performed in the velocity-variance mode at a constant 4 KHz pulse repetition frequency and at gain settings that were just below the level that produced random noise in order to standardize the instrumentation factors as much as possible. All images were recorded on %-inch videotape for subsequentanalysis. A systolic frame that demonstrated the maximal aliasing proximal to the pulmonary artery band was chosen for analysis (Figure 3A). Digital computer analysis was performed TABLE I Clinical Characteristics

using a Sony Medical Systemsmodel 70G video-digitizing computer, using methodology we have previously described and validated.16J7Images were digitized into an g-bit RGB matrix allowing numerical velocity assignments for the red, green and blue components of each color pixel. With 3 bits available for red and green, possible numerical assignmentsranged from 0 to 7 and with 2 bits for blue, from 0 to 3. Data were then reconstructed into digital velocity maps (Figure 3B) and pseudo 3-dimensional maps (Figure 3C) for the red, blue and green components of each color pixel in the spatial distribution of the region of interest on the color Doppler flow map image. Spatial changesin velocity proximal to an obstruction can be characterizedon color flow mapping and digital computer analysis can be applied to estimate individual velocity values and spatial acceleration that have been shown to give quantitative information on pressure gradients acrossobstructive lesions.16,17

of Patients Diameter

Patient

Pressure Gradient (mm Hg)

Age (mos)

Diagnosis

1 2 3 4 5 6 7 8

VSD, coarctation Coarctation, ASD, VSD VSD VSD, TGA VSDt VSD VSD VSD, TGA

3 5 5 6 8 8 8 9

9 10 11 12

VSD, TGA VSDt Single ventricle, TGA VSD, coactation, TGAt

15 15 26 35

*At surgery. tMultipleVSDs. ASD = atrial septal defect, MRI = magnetic

Angiography (mm)

MRI (mm)

Color (mm)

MRI Acceleration Length (mm)

Color Flow Acceleration (mm)

40* 55

2.0 3.0 5.5 Inadequate 2.0 4.0

2.0 3.5 5.0 4.5 2.0 5.5 6.0 4.5

1.5 3.0 4.5 4.0 1.5 6.0 5.5 Inadequate

28 18 4 10 20 8 4 10

18 13 7 9 17 7 5 -

40 60 90 60

4.0 3.0 Inadequate 3.5

5.0 3.0 2.0 4.0

4.0 2.5 2.0 3.5

5 15 24 17

8 11 26 15

80* 65 45 50 90

resonance

imaging,

TGA = transposition

of the great arteries; VSD = ventricular

septal defect.

MRI Band Diameter (mm)

r=-0.95

8

6

FIGURE 4. Linear regression analysis comparing the diameter of the pulmonary artery band on tine magnetic resonance imaging (MRI) (ordinate) with the pressure gradient across the band (abscissa).

o0

I

I

/

I

I

I

I

I

I

10

20

30

40

50

60

70

80

90

Pressure Gradient 1422

(mmHg)

THE AMERICANJOURNALOF CARDIOLOGY VOLUME71

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Cadiac catheterization: Cardiac catheterizationwas performed with DemerolB, PhenerganB and Thorazine@sedation in 10 of the 12 patients who underwent tine MRI and color Doppler flow mapping within 6 weeks of the MRI and DoDnler studies. Although the cardiac catheterization studies were occasionally performed several weeks after the MKI and Doppler studies, repeat color Doppler flow mapping at the time of catheterizationfailed to demonstrateany significant difference from the color Doppler flow mapping studies performed at the time of MRI. The position and anatomy of the band was assessedby biplane right venhicular angiographyin the 45’ “sitting up” position, and the peak-to-peakpressuregradient acrossthe band was measured. When clear visualization of the band had been obtained at angiography,the flow diameter of the band was measured to the nearest 0.5 mm, calibrated by a grid or the known diameter of the catheters that had been used.

was 25 mm and the proximal acceleration length of signal loss on tine MRI was <5 mm (patients 3 and 7, Table I). The results obtained in individual patients are listed in Table I. Fnr

cnmyison

Aspects

RESULTS

Satisfactory high-resolution tine MN studies were obtained in all 12 patients. Pressure gradients were obtained across the pulmonary artery band in 9 of the 10 patients undergoing cardiac catheterization; in 2 other patients the gradient was measured at the time of definitive surgery.For the 11patients with pressure gradients available, the narrowest flow diameter across the band on tine MRI correlated well with the pressure gradient across the band, r = -0.95 (Figure 4). The pulmonary artery band diameter as measuredby tine MRI was not significantly different from the band diameter measured using conventional spin-echo MRT. In 2 patients, pulmonary artery banding was considered inadequate on the basis of clinical, angiographic and/or surgical data, and both these patients were identtied correctly using tine MFCI where the narrowest flow diameter at the band

with

cinp

JI~J,

++-qL!$ity

~2!~r

Doppler flow mappiilg studies were obtained in 11 of the 12 patients. In all 11, spatial flow acceleration was imaged proximal to the band, seen as increasing color intensities of blue aliasing to red and often with a further alias to blue proximal to the band (Figure 3A). Visually apparent turbulence was seen distal to the band in all 11 as a multicolor mosaic-patterned high-intensity green on digital computer analysis (Figure 3, B and C). In all patients, low-grade variance, detied as a level of <3 for green on digital computer analysis (which was nof: visually apparent on the color Doppler flow map images) was detectable on digital computer analysis proximal to the band associated with the zone of proximal spatial acceleration. of flow: magnetic

resonance

imaging

and

color Doppler flow mappi- On tine MRI, signal loss was seen during systole distal to the pulmonary artery band and signal loss penetrated into the branch pulmonary arteries in all patients. This was associatedwith the visually apparent turbulence seen distal to the band on color Doppler flow mapping and a maximal systolic velocity that was always >3 m/s on continuous-wave Doppler. However, loss of signal intensity was also seen during systole, proximal to the band (Figure 2) in a zone where flow would be expected to be accelerating laminar flow. On color Doppler flow mapping, this zone of signal loss was matched by the zone of laminar proximal flow acceleration as detected by digital computer analysis. When we measured the length of the proximal signal void on sag&al plane tine magnetic resonance images, it was related to the length of proximal acceleration on color flow mapping measured from the posi-

Proximal Accln Length - MRI (mm)

FIGURE 5. Linear regression analysis comparing the length of the proximal acceleration (Accln) zone imaged by color Doppler flow mapping (abcksa~ with the length of the proximal signal void imaged by tine megnetic resD nance imaging (MRI) (ordinate).

5

10 Proximal

15

20

25

Accln Length - Color (mm)

MAGNETICRESONANCEIN PULMONARYARTERYBANDING

3(

tion of iirst alias to the site of the band in the 11 patients with proximal acceleration visualized on highquality color Doppler flow map images (Figure 5). The length of the proximal signal void on tine MRI was also related to the pressure gradient across the band as measured at cardiac catheterization or surgery (Figure 6, top). Additionally, the minimum internal diameter of the band as visualized by MFU correlated well with the band diameter measured at angiography (Figure 6, bottom). The position of the pulmonary artery band was accurately identified in all patients compared with results

Proximal

on angiography; in 1 patient (patient 11) MRI correctly predicted the migration of a band toward the branch pulmonary arteries with obstruction to the origin of the left pulmonary artery and relatively unobstructedflow in the right branch pulmonary artery (subsequentlyconfirmed at angiography). The exact position of the band in relation to the main and branch pulmonary arteriescould not always be contidently predicted by color Doppler flow mapping, which failed to identify accurately the patient in whom the band had migrated distally, mainly because of failure to define the branch pulmonary arteries.

Accln Length - MRI (mm)

30

. r=0.91

25 20

. .

15 10 5

0

IO

20

30

Pressure MRI Band Diameter

40

50

Gradient

60

70

80

90 90

(mmHg)

(cm)

7

”

0

1

2

3

4

5

Angio Band Diameter (cm)

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6

FlGlJRE 6. Top, linear regression analysis comparing the length of the proximal acceleration (Accln) on tine magnetic resonance imaging (MRI) (otiimite) with the pressure gradient across the hand (abcksa), Bottom, linear regression analysis comparing the pulmonary artery band diameter on magnetic resonance imaging (MRI) (odinate) with the band diameter at angio@aphy (Ae@o) (akiSS@-

ISCUSSION

Both color Doppler flow mapping and tine MRI allow visualization of intracardiac flow. Although the information obtained by color Doppler flow mapping from distinct flow regions proximal and distal to obstructive lesions, such as those seen in tight venticula: out!!ow tract obstruction, has been studied in vitro,i7 this approach of characterizing anatomy and flow has not been applied to the clinical investigation of pulmonary artery banding. In addition, the relationship of color Doppler flow mapping to flow visualization by tine MRI has not been characterizedfor such a lesion. Our study has demonstratedthat MRI can provide accurate, highresolution data on the location and anatomic size of a pulmonary artery band. Cine MRI, unlike conventional spin-echo imaging, can provide additional flow information that relates well to some of the flow characteristics seen on color Doppler flow mapping in these patients. Signal loss on tine MRI was seenboth proximal and distal to the pulmonary artery band. Distal to the band, color Doppler flow mapping characterized a region of turbulent flow seen visually as a multicolored mosaic pattern and associatedwith high levels of variance on digital computer analysis. This is similar to the pattern seen in regions of turbulence using color Doppler flow mapping studies in vitroI and in other obstructive lesions studied in vivo,17 and would seemto confirm that the tine magnetic signal loss distal to pulmonary artery banding represents a region of flow turbulence.18J9 However, turbulence cannot account for the loss of the tine MRI signal seenproximal to the band, becausethis flow region proximal to an obstruction is characterized by a laminar zone of flow acceleration. Spatial flow acceleration was always seen proximal to the pulmonary artery band using digital computer analysis of color Doppler flow map images; this zone of acceleration matched the zone of signal loss on tine MRI, implying that rapid spatial flow acceleration may be another hemodynamic factor associatedwith signilicant loss of signal on tine MRI. Acceleration rather than purely highvelocity flow appearsto cause this appearance,because we have been able to produce high-velocity laminar jets in vitro that are imaged as high-signal intensity on tine MRP” but where the velocities exceed those seen in the zone of proximal acceleration in this study. Color Doppler Bow mapping also displays low-grade variance in the zone of acceleration resulting from sampling in an area of rapid spatial and temporal acceleration and from the presenceof an increasedrange of velocities sampled acrosseach pixel. This may also be the mechanism that produces loss of signal on tine MRI in this proximal acceleration zone. Because the proximal signal loss on tine MRI appearsto be related to the zone of acceleration, it is not surprising that this signal loss was found to be closely related to the pressure gradient acrossthe pulmonary artery band. We previously demonstrated that the zone of acceleration seen on color Doppler flow mapping proximal to obstructive lesions is related quantitatively to their severity.16,17 Accurate velocity quantitation is not possible by tine MRI as used

in this study. We did not have a phase-encoding sequence and there was no discrimination between ditferent flow velocities (the loss of signal results from the presence of flow acceleration). However, new applications l]sinE ye+ic

phac+=mrocll~m

mp
with qadient imaging sequencescan allow direct flow veloc& encoding to be applied to tine MRI,20-22 and these have considerable potential for providing quantitative flow velocity information in a variety of cardiac lesions. We believe that tine MRI and color Doppler flow mapping are complimentary techniques for the noninvasive assessmentof pulmonary artery banding in infants and children. Color Doppler flow mapping provides excellent spatial flow velocity information combined with structural detail in real time, and it is relatively inexpensive. Accurate estimation of the pressure gradient across the band can also be obtained using continuous-wave Doppler; these techniques alone may allow a comprehensive assessmentof many patients with pulmonary artery banding. The anatomic deli&ion provided by tine MRI, however, is considerably better than that of echocardiography in patients who are dillicult echo subjects. Where doubt exists as to the position of the band, the degree of narrowing produced by the band, or distortion of the pulmonary arteries, MRI can be more effective than echocardiography. 1. Stewart S, Harris P, Manning J. Pulmonary artery banding. An analysis of aurent risks, results and indications. .I Thorac Cardiovasc Surg 1980;80:431436. 2. Stevenson JG, Kawabori 1. Noninvasive determination of pressure gradients in children: two methods employing pulsed Doppler echocardiography. JAm Coil Cardial 1984;3:179-182. 3. Fvfe DA. Currie PJ. Seward JB. Taiik AJ. Reeder GS. Mair DD. Ha&r DJ. Continuous wave Doppler determin&ion~of the pressure g&ient across pulmonary atery bands: hcmodynamic cotrclation in 20 patients. Mayo Clin Proc 1984;59: 74b750. 4. Omoto R, Yokote Y, Takamoto S, Kyo S, Ueda K, Asano H, Namekawa K, Kaai C, Kondo Y, Koyano A. The development of real-time two-dimensional Doppler echocardiography and its clinical sigticancc in acquticd valvular diseases. With specitic reference to the evaluation of valvular regurgitation. Jpn HearrJ 1984; 25:325-340. 5. Miyatake K, Okamoto M, Kiuoshita N, Iaumi S, Owa M, l’akao S, Sakakibara H, Nimura Y. Clinical applications of a new type of real-time two-dimensional flow imaging syslem. Am J Cardiol 1984:54:857-X68. 6. Sahn DJ. Real-time two-dimensional echo Doppler flow mapping. Circulation 1985;71:849-853, 7. Fram J, Haasc A, Math& D. Rapid tlnee-dimensional MR imaging using the FLASH technique. Comput Assist Tomogr 1986; 10:363-368. 8. Haasc A, Math& D, Hanickc W. FLASH imaging: rapid NMR imaging using low flip angle pulses. J Mqnetic Res 1986;67:258-266. 9. Sechtem U, Pflugfelder P, Cassidy MC, Holt W, WolIe C, Higgins CB. Ventricular septal defect: visualization of shunt flow and determination of shunt size by tine MR imaging. AJR 1987;149:689492. 10. Sechtem U, Pflugfelder P, White RD, Gould RG, Holt W, Lipton MJ, Higgins CB. Cine MR imaging: potential for the evaluation of cardiovascular function. AJR 1987;148:239-246. 11. Utz JA, H&kens RJ, Heinsimer JA, Bashore T, Califf R, Glover G, Pelt N, Shimakawa A. Cine MR determination of left venh-icular ejection fraction. AJR 1987;148:839-843. 12. Sechtem U, Pflugfelder PW, Gould RG, Cassidy MC, Higgins CB. Measuremerit of rieht and left ventricular volumes in healthv individuals with tine MR imaging. Ridiology 1987; 163:697-702. X3. Chung KJ, Simpson IA, S&n DJ, Glass R, Hess&& JR. Cine magnetic resonance imaging in congenital heart disease. Dynnnz Cardiovasc Imai 1987;1:2 133-138. 14. Chune KJ. Simuson IA. Glass RF. S&n DJ, Hesselink JR. Cine maenetic rcsonance imaging after surgical repair in patients with transposition of the great a&ies. Ciradution lY88;77: 104-109. 16. Simpson LA, Chung KJ, Glass RF, S&n DJ, Sherman FS, Hesselink JR. Cinc magnetic resonance imaging for evaluation of anatomy and flow relationships I.

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in infants and children with coarctation of the aorta. Circulation 1988;78:142148. 16. Simpson IA, Valdes-Cmz LM, Yoganathan AP, Jimoh A, Sung HW, S&n DJ. Spatial velocity distribution and acceleration in serial subvalve hmnel and valvular obstructions: an in vitm study using Doppler color flow mapping. J Am Cd Cardial 1989;13:241-248. 17. Simpson IA, Sahn DJ, Valdes-Cruz LM, Chung KJ, Sherman FS, Swensson RE. Color Doppler flow mapping in coactation of the aorta: new observations and improved evaluation using color flow diameter and proximal acceleration as predictors of severity. Circulation 1987;77:73&744. 3.6. Bradley WG, Waluch V. Blood Flow: magnetic resonance imaging. Radio/ogy 1985;154:443-450.

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19. Souza SP, Dumoulin CL. Dynamic magnetic resonance angiography. Dymm Cardiovasc hag 1987;1:2 126-132. 20. Simpson IA, Ma&l BC, Elias W, Makes V, Valdes-Cruz L, Hesselink J, Sahn DJ, Chug K.J. Cine magnetic resonance imaging and Doppler flow mapping COTrelates of flow velocity, spatial acceleration and jet imaging: an in vitro study cabs@). Circulation 1988;78(suppl II):&589. 21. Underwood SR, Finnin DN, Klipstein RH, Rees RSO, Longmore DB. Magnetic resonance velocity mapping: clinical application of a new technique. Br Heart J 1987;57:404-412. 22. Klipstein RH, Firmin DN, Underwood SR, Rees RSO, Longmore DB. Blood flow patterns in the human aorta studied by magnetic resonance. Br Heart .l 1987; 58:316323.

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