Left ventricular mass quantitaton using single-phase cardiac magnetic resonance imaging

Left Ventricular Mass Quantitation Using Single-Phase Cardiac Magnetic Resonance Imaging Gerard Aurigemma, MD, Ashley Davidoff, MD, Kevin Silver, MD, and John Boehmer, MD, with the technical assistance of Anthony Serio, CNMT, and Nancy Pattison, RT Magnetic resonance hnaging (MRI) has been used to measure left ventricular (LV) mass in animals with superior accuracy. However, its use in cardiac patients has been limited by the long total scan tbnes neces&Hed by imaging the heart at endd&stole at each of 8 to 10 slke locations. Recent canine studies showed that LV mass may be determined accurately, with considerable timesatings, by use of sequential images throughout the cardiac cycle (&gle-phase MRI). Twenty normal subjects underwent Wn-echo MRI to determine the relationship between l.V mass computed from ringle-phase MRI and resuRs obtained from the more time-consuming end-diastolic MRI (which was used as ths reference standard for this study). The left ventrkle was spanned with 2 interleaved seties of 5 short-axk 1 cm thkk slkes. 5 hnages, evenly spaced througheut the cardiac cycle, were obtained at each dke location in all subjscts. LV mass ranged from 86 to 198 g. Although end-diastolic LV mass exceeded dngle-phase results by an average of S g (p <0.002), there was a close correlation between the 2 (slope = 0.99; r = 0.96). Although LV mass derived from end-diastolic images exceedsd dngkphase results, this difference is unlikely to be clinically significant and is small compared with the standard error of echocardiographic methods. Furthermore, when the order in which dngle-phase images were selected was reversed, there was improved agreement with end-diastolic MRI. Thus, the close correlation between dngle-phase and end-diastolic resuits indicater that ringle-phase MRI may be a practkal, time-effkient method to determine LV mass in humans with normal LV shape. (Am J Cardiol1992;70:259-262).

R

ecent studies documentedthe ability of magnetic resonanceimaging (MRI) to quantitate left ventricular (LV) mass with superior accuracy in both excised hearts’ and experimental animals in viv~.*-~ Previously reported spin-echo MRI LV mass calculations were obtained using Simpson’s rule reconstruction of end-diastolic images obtained at each slice location.4y5However, the total scan time neededto ob tain end-diastolic spin-echo images at each of 8 to 10 slice locations in subjects with slow heart rates may range from 45 to 60 minutes and therefore make MRI determination of LV massimpractical for many cardiac patients. Recent in vivo canine studies showedthat LV massmay be determined accurately when images from different phasesof the cardiac cycle are used,6calling into question the necessityof imaging each slice location at end-diastole. The use of a multislice, singlephase MRI protocol (with an image at each sequential slice location obtained at a different phaseof the cardiac cycle) may therefore permit accurate LV massdetermination in cardiac patients with dramatically shortenedt& tal MRI time. Therefore, singlephase MRI would potentially be a practical strategy for determining LV mass. Twenty normal subjects underwent multiphase, multislice MRI to determine the relationship between LV masscomputed from single-phaseMRI and results obtained when end-diastolic images are used. We then compared LV mass determinations (using Simpson’s rule) obtained from end-diastolic imageswith those of a simulated single-phase MRI protocol obtained by se letting sequential images from the multiphase, multislice matrix.

MEWODS MagneUe B imaging tsdnklue: Twenty normal subjects underwent gated cardiac MRI using spin-echotechnique on a 1.5 Telsa Signa system (General Electric, Milwaukee, Wisconsin). Echo time was 20 ms, and repetition time was equal to the RR interval; the image matrix was 128 X 256, and field of view was 24,32 or 40 cm, dependingon subject’ssize. An ungated coronal localizing series established the thoracic landmarks in each case. MRI along the true cardiac From the CardiovascularDivision, Departmentsof Medicine and Radi- short axis was performed by double oblique angulation ology, University of MassachusettsMedical Center, and the Central MassachusettsMagnetic Imaging Center, Worcester, Massachusetts. using the method de-scribedby Clark et al7 (Figure 1). The entire LV volume was encompassedwith 2 sets This study wassupportedin part by Grant 6-32779from the Biomedical ResearchSupport Grant Program, National Institutes of Health, Be- of 5 interleaved 1 cm thick sliceswith a 1 cm interslice thesda, Maryland. Manuscript received December 11, 1991; revised distance using multiphase/multislice MRI. Thus, after manuscript receivedand acceptedMarch 23,1992. 2 acquisitions the entire left ventricle was imaged in Address for reprints: Gerard Aurigemma, MD, Cardiology Division, University of MassachusettsMedical Center, 55 Lake Avenue contiguous 1 cm thick slices. Usually, 8 to 10 slices (mean 9.2) were neededto image the left ventricle. ImNorth, Worcester,Massachusetts01655. MRI QUANTITATION OF LEFT VENTRICULAR MASS

259

ages were obtained at evenly spacedintervals throughout the cardiac cycle (Figure 2). Five images were obtained at each slice location for a total of 50 images in each subject. The end-diastolic image at each slice location was obtained at 4 ms from the R wave. Total MRI time comprisedapproximately 5 minutes for the coronal series and from 16 to 25 minutes for each short-axis series,using 2 excitations for each acquisition. Thus, total scan time for subjects in this seriesvaried from ap proximately 40 to 55 minutes. pknimetry: Images were interactively analyzed by planimetry from film hard copy by an experiencedoperator using a commercially available personalcomputerbased program. For each study, window and contrast levels were set at the time of scanning to optimize the myocardial/cavity interface. A video camera displayed imagesfrom the backlit hard copy on a television screen for planimetry. For each image demonstrating myocardium, the cavity area and myocardial area were ob tained in triplicate by planimetry using a mouse. Mus-

cle volume for each slice was the product of myocardial area and slice thickness. Likewise, cavity volume was the product of cavity area and slice thickness. Myocardial and cavity volume were computed by Simpson’s rule reconstruction: V = Z AiT, where V = total volume (cavity or myocardial); A = area by planimetry; T = slice thickness; and i = slice number. Myocardial volume was multiplied by specific gravity (assumedto be 1.05 g/ml) to estimate myocardial mass. lktmlationwithca~phase(Figures2and3):

For each subject, LV masswas computed by Simpson’s rule reconstruction from both end-diastolic and singlephase images (Figure 2). For this method, the image was chosen at each slice location to simulate a single phaseMRI protocol and thus correspondedto the shortest trigger delay for the first slice location, the next longest delay for the secondlocation and the longest delay for the fifth slice. The sequencewas repeated for each image displaying myocardium in the secondacquisition. Single-phaseLV mass was calculated twice. First, the images were chosen with the most apical slice at the shortest trigger delay (Figure 2). The entire procedure was repeated with images selectedin the reverse order (reverse phase) with the shortest trigger delay image beginning with the most basal slice location, which showed myocardium, and proceeding sequentially. StaWksr LV mass from single and reverse-phase image reconstruction methods was compared with re sults of end-diastolic images by linear regression.Mean values for LV mass obtained by various methods were compared by analysis of variance for repeated measures,with p <0.05 consideredstatistically significant. RESULTS

IMAGE SELECTION: APEX TO BASE TRIGGER

DELAY

(ms)

SLICE 4

139

274

409

544

4

139

274

409

544

4

139

274

409

544

LV massranged from 86 to 198 g (Table I). Diastolic exceededsingle-phaseLV massby an averageof 5 g (125 vs 121; p <0.002) when the shortest trigger delay image was at the most apical slice location (“apex to base”; Figure 4). Diastolic and single-phaseLV mass correlated closely (r = 0.96; slope = 0.99; SEE 9 g over the range of values 86 to 198 g). However, when images were chosenwith the most basal image at the shortest trigger delay (“reverse phase”; Figure 5), there was IMAGE SELECTION: BASE TO APEX

4

139

274

4

139

274

diastolic

260

409

TRIGGER

DELAY

(ms)

SLICE

r

I

1

4

139

274

409

544

2

4

139

274

409

544

3

4

139

274

409

544

4

4

139

274

409

544

5

4

139

274

409

544

544

phase

THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 70

JULY 15, 1992

closer agreement (125 g for end-diastolic and 126 g for reversephase;p = not significant) with an equally close correlation (r = 0.97; slope = 0.94; SEE 8 g over the range of values 91 to 198 g). DISCUSSION LV hypertrophy has been shown to be an independent predictor of cardiovascular morbidity and mortality.*-i i Furthermore, recent echocardiographic studies established that the risk of cardiovascular morbidity and mortality varies, with LV mass as a continuous variable.8J0 However, echocardiographic determination of LV mass has important limitations; in ideal circumstances,the standard error of the technique is 130 g,12 rendering it insensitive to small changes in LV mass produced by clinical intervention. Moreover, M-mode echocardiographic methods necessitate geometric assumptions concerning LV shape, which limit its use in normally shapedventricles. Finally, the technical quality of this study may be inadequate for LV massdetermination in 120% of subjects9 MRI is an attractive method for LV volume and mass quantitation, becausethe entire LV volume may be encompassedtomographically, permitting use of Simpson’srule reconstruction. BecauseMRI techniques provide images with excellent contrast betweenmyocardium and the blood-filled cavity, operator planimetry is feasible,2J3and computerized volumetric quantitation may be possible in the future.’ The validation studies have shown an excellent correlation between MRI LV massand anatomic weight, in both the normaP3J4 and abnormal6 left ventricle. However, the use of spin-echo MRI for LV mass determination in humans has been liited by the long total scan times, becauseLV massdeterminations have

SINGLE

PHASE

r

TABLE

I Left Ventricular

AgeW & Sex 35F 32F 24M 29M 38M 25M 32M 37M 26M 33F 28M 29M 33M 29M 29F 28M 29F 30M 31M 27M Mean

1

Mass(g)

BSA

Single-Phase

Reverse-Phase

End-Diastolic

1.5 1.6 1.6 2.0 1.7

86 95 112 153

91

91

99 123 148

95

101 139 139

99 136 142

92

103

110 134

1.9

121

124

110 145 144 101 127

1.7

98

100

104

1.9

114

117

114

2.0 2.0 1.6 1.6

122

132

127

1.9

1.9 2.1 1.8

1.7 2.0 2.0 2.0

129

131

126

91 98

103 106

103

126 101

142 103

109 145 100

131 176

156 172

151 173

192

190

198

121

126*

125

l p <0.002 Left ventricular mass for 20 normal subjects grouped according reconstruction. Group means are shown. BSA = body surface area.

to method of image

usually been obtained from end-diastolic images obtained at each slice location.4JJ5Acquisition of end-diastolic imagesat eachof 8 slice locations using the multiphase/multi&e approach we outlined requires 245 minutes of total imaging time, assuming 2 signal averagesand an averageheart rate of 70 beats/min. Therefore, strategies aimed at reducing total scan time without sacrificing volumetric accuracy would be desirable.

REVERSE PHASE v DIASTOLIC

v DIASTOLIC

n=20

0

LV MASS-DIASTOLIC

(g)

I

SO

LV MASS

100

DIASTOLIC

150

200

(g)

MRI QUANTITATION OF LEFT VENTRICULAR MASS

261

The in vivo canine study of Shapiro et al6 demonstrated that LV mass may be determined with equivalent accuracy using either end-diastolic (r = .94; SEE 9 g) or end-systolic (r = 0.97; SEE = 7 g) frames. That study also demonstratedthat a standard multislice/single-phasealgorithm may be used to estimate LV mass with an accuracy equivalent to that of end-diastolic MRI. Therefore, we performed the present study to investigate the correlation between LV mass computed from end-diastolic images and from selected images from the multiphase/multislic matrix that simulated a singlephase protocol. The results indicate that in normal hearts, LV mass determined from end-diastolic images correlates well with results obtained from the simulated single-phase protocol. The timesavings associatedwith the use of a single-phaseapproach is considerable.This timesavings results directly from shortened acquisition times, because setup time (typically 5 to 10 minutes) would be similar for both single-phaseand multiphase MRI. For a subject with a heart rate of 75 beats/mm, 2 multislice/multiphase acquisitions (with 2 signal averages) comprising 10 sliceswould need 34 minutes. In contrast, 2 multi&e/single-phase acquisitions for the same subject (using 2 signal averages) would require approximately 8 minutes, representing a 26minute savings in acquisition time; total scantime (including setup) would be approximately 20 minutes. End-diastolic LV mass slightly (but significantly) exceededsingle-phaseLV masswhen images were chosenin an apex-to-baseorientation. This difference is unlikely to be clinically significant and is small compared with the SEE associatedwith M-mode echocardiography.12 Furthermore, when LV mass using “reverse phase” (basetoapex) reconstruction is compared with end-diastolic images, a smaller discrepancy is observed. The close agreement between end-diastolic and singlephase results may be explained by the considerable time-averaging that occurs for each image when the spin-echotechnique is used. Moreover, the imageswere obtained at evenly spacedintervals throughout the cardiac cycle; with this approach, 3 of 5 images are obtained in diastole. Although it is possiblethat the greater diastolic myocardial blood contentt6J7 may lead to slightly higher diastolic LV mass,it is unlikely that this technique is sensitive enough to detect this difference. We also observedthat when imagesare reconstructed with the most basal image obtained at end-diastole, the resulting LV mass more closely approximates enddiastolic results than when single-phaseimagesare chosen in the opposite order. Although the explanation for these findings is not completely clear, the likely mechanism may be related to the through-plane motion of the left ventricle. Recent work showedthat there is consid-

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erable long-axis shortening in the normally functioning left ventricle, which appearsto be greatest in basal systolic segments6 Such base-to-apexmovement may be expectedto result in an underestimation of LV masson short-axis single-phaseimages, becausemyocardium at more basal levels of the ventricle would have moved out of the imaging plane on systolic images and been “replaced” by tissue from the mitral annulus, left atrium or aorta. To the extent that the apex-to-basedirection of reconstruction included more systolic basal slices, this method would have provided a slightly smaller value for total LV massthan would either end-diastolic or singlephaseMRI with a base-to-apexdirection of reconstruction. REFERENCES 1. Amigemma GP, Reichek NR, Venuaopal R. Trivedi S, and Herman G. resonanceimaging: in vitro validation. km J C&d& Imaging 1991;5:257:263. 2. Maddahi J, CruesJ, BermanDS, Mericle J, BecerraA, Garcia EV, Henderson R, Bradley W. Noninvasive quantification of left ventricular myocardial massby gated proton nuclear magneticresonanceimaging. J Am Call Cardiol1987;lO: 682-692. 3. Keller A, PeshockR, Malloy C, Buja LM, Nunnally R, Parkey R, Willerson J.

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IS. Katz J, Milliien MC, Stray-GundersenJ, Buja LM, Parkey RW, Mitchell JH, Peshok RM. Estimation of human myocardial mass with MR imaging. Radiology 1988;169:495-498. 16. GaaschWH, Bernard S. The effect of acute changesin coronary blood flow on left ventricular end-diastolic waU thickness.Cirnrlation 1977~56593-598. 17. Iwasaki T, Sinak JL, Hoffman EA, Robb RA, Harris LD, Badn RC, Ritman EL. Mass of the left ventricular myocardiumestimatedwith the dynamic spatial reconstmctor.Am J Physiol 1984;246zH138-H142.

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