CONGENITAL HEART DISEASE

Comparison of Magnetic Resonance Imaging with Cross-Sectional Echocardiography in the Assessment of Left Ventricular Mass in Children Without Heart Disease and in

Aortic lsthmic Coarctation Michael Vogel, MD, Heiko Stern, MD, Richard Bauer, MD, and Konrad BOhlmeyer, MD

Although left ventricular (LV) mass may be impor- tant to judge effects of left-sided cardiac obstruc- tion or hypertension, reproducible noninvasively determined normal data in the pediatric age group are scarce. To validate cross-sectional echocardio- graphic LV mass determination, our data were compared with LV mass assessed by magnetic resonance imaging (MRI). MRI was considered to be a good reference method because there is usu- ally no problem in defining endo- and epicardial borders with MRI. LV mass was assessed in 14 children aged 5.3 years (10 days to 14.7 years) with a mean body surface area of 0.78 m* (range 0.25 to 1.61). With cross-sectional echocardiogra- phy the epicardial and endocardial volumes were calculated using a Simpsons rule algorithm in the apical 2- and 4-chamber view. The difference be- tween epi- and endocardial volumes was multi- plied by 1.05 to yield the mass. Mass was as- sessed with MRI using a multislice technique; the area of each myocardial slice was calculated and multiplied with the slice thickness, and the resul- tant slice volumes were added to obtain the myo- cardial volume. On cross-sectional echocardiogra- phy, the mass was 55 g (range 12 to 126) or 64 g/m* (range 46 to 79); on MRI it was 60 g (range 33 to 87) or 69 g/m* (range 46 to 89). Regression analysis yielded an r value of 0.98 with a standard error of the estimate of 5.7 g or a 10% difference. In older children, LV mass determined by MRI was bigger than the one derived by echocardiogra- phy. It is concluded that cross-sectional echocardi- ography can reliably assess LV myocardial mass in pediatric patients.

(Am J Cardiol lSS2;69:941-944)

From the Department of Pediatric Cardiology, Deutsches Herzzentrum Miinchen, Miinchen, Germany. This study was presented in part at the annual scientific session of the American College of Cardiology, March 3 to 7, 199 1, in Atlanta, Georgia. Manuscript received September 26, 1991; revised manuscript received and accepted December 4, 199 1.

Address for reprints: Michael Vogel, MD, Kinderkardiologie am Deutschen Herzzentrum Mtinchen, Lothstrasse 11, 8000 Mitnchen 2, Germany.

L eft ventricular (LV) mass may be important in the follow-up of patients with left-sided cardiac ob- struction’ and may play a role in epidemiologic

studies of childhood hypertension.2 Surgical decision re- garding timing of surgery in complete transposition of the great arteries3,4 or tricuspid atresia5 may be in- fluenced by LV myocardial mass. We had previously published normal values of LV myocardial mass in 95 control subjects with normal hearts ranging from new- borns to adolescents.6 To further validate the echocar- diographically determined data, we examined 14 chil- dren by cross-sectional echocardiography and the nucle- ar magnetic resonance imaging (MRI) technique.

METHODS Patients: The patients either had a structurally nor-

mal heart (n = 7) or had undergone repair of an aortic coarctation (n = 7) 2 to 3 years before the study. All of the latter group had arm systolic blood pressure within the 95th percentile7 and none had a systolic pressure gradient between ascending and descending aorta of >20 mm Hg determined by Doppler echocardiography. The echocardiographic study had been performed in all patients on the same day as the MRI study.

Echocardiography: Commercially available equip- ment consisting of an Ultramark 9 sector scanner by Advanced Technology Laboratories or a Hewlett-Pack- ard 1000 sector scanner was used for the echocardio- grams, which were obtained with either a 5 or 7.5 mHz transducer. The images were recorded on M-inch video- casette tape. For all children the standard apical 4- and 2-chamber views were used for analysis of LV mass. Using an offline computer (Kontron 200) the epicardi- um and subsequently the endocardium were manually traced on the electronic drawing board with a light pen in the 2 imaging planes. The entire ventricular septum was considered to be part of the left ventricle. The epi- cardial and endocardial volumes were calculated by di- viding the LV shape into slices of 2 mm and adding up the single slice volumes applying the geometric formula of Simpson.8 After the biplane epicardial and endocar- dial volumes had been measured, LV mass was calculat- ed from the difference between epicardial and endocar- dial LV volume times the specific gravity of the heart ( 1.050).9 LV mass volume indexi was calculated as LV

LEFT VENTRICULAR MASS BY MRI AND ECHOCARDIOGRAPHY 941

TABLE I Clinical Data and Data on Left Ventricular Mass Determined by MRI or Echocardiography

Pt.

1 2 3 4 5 6 7 8 9

10 11 12 13 14

Age (yr) Heart & Sex Disease

1OdaysM 0 0.8 F 0 1.0 M 0 6.0 F 0 6.6 M 0 8.1 M 0

14.7 M 0 1.6 M C of A 2.2 M C of A 2.3 M C of A 3.3 M C of A 6.4 F C of A 8.9 M C of A

10.9 M C of A

Mass by MRI

(!a

12.6 21.6 30.5 38.5 46.2 95.0

142.8 36.0 35.8 35.5 48.0 55.6 89.8

115.5

Mass by Echo. (g)

11.5 20.6 26.8 38.5 52.0 82.4

126.4 32.3 42.3 33.3 35.0 54.6 80.2 98.0

C of A = coarctatian of the aorta; Echo. = echocardlography; MRI = magnetic resonance imaging; 0 = none.

muscle mass at end-diastole divided by LV end-diastolic volume. For the measurement of LV endocardial and epicardial volumes, 3 consecutive systolic and diastolic echocardiographic frames were analyzed and the mean of these 3 measurements calculated.

Magnetic resonawe imaging technique: MRI was performed by use of a Philips Gyroscan S 15 with a 1.5 Tesla magnet. Uncooperative patients were sedated with chloralhydrate, 70 to 80 mg/kg body weight. Chil- dren weighing Cl8 kg were examined in the so-called head coil, which has a 32 cm (12.8 inches) diameter; children weighing >18 kg were examined in the so- called body coil, which has a 50 cm (20 inches) diame- ter. Imaging was triggered to the patients electrocardio- gram, and a double echo technique with echo times of 30 and 60 ms was used. Acquisition and reconstruction matrix had 256 X 256 pixels. To shorten study time, a multisclice technique was used. Thus, images were ob-

tained at multiple points in the cardiac cycle. The slice thickness was 3 to 5 mm. Tl weighted images were ac- quired in 3 orthogonal planes. In the axial tomograms the myocardial cross-sectional area (A) was assessed in n = k slices and multiplied with slice thickness (th). Myocardial volume (mV) was calculated as:

k mV = Sum x A x th

n=l

Myocardial mass was obtained by multiplying myocar- dial volume by 1.05.9

Statistical analysis: Data are given as mean f stan- dard deviation. Linear regression analysis was used to determine correlation between LV mass assessed by dif- ferent imaging modes. For comparison of data a paired students t test was performed. A p value <0.05 was con- sidered to indicate a significant change.

RESULTS The results of the echocardiographic and MRI mea-

surements of LV mass are listed in Table I for each individual child. With cross-sectional echocardiography the mass was 55 g (range 12 to 126) or 64 g/m2 (range 46 to 79), and with MRI it was 60 g (range 33 to 87) or 69 g/m2 (range 46 to 89). Figure 1 shows the cor- relation curve between echocardiographic and MRI mass assessment. The standard error of estimate was 5.7 g or 10%.

DISCUSSION Noninvasive a ssessment of left ventricular mass:

Several clinical or epidemiologic studies have used serial LV myocardial mass measurements.2J 1,12 A repeatable, noninvasive method of LV mass determination thus seems desirable. The surface electrocardiogram, al- though highly specific, lacks sensitivity. l 3 Noninvasive imaging modes for assessing LV mass include, among others,14J5 M-model6 or cross-sectional echocardiogra- phy6 and MRI.17-19 We did not examine accuracy of

0 .Y&, 0 20 40 60 60 100 120 140

LV MYOCARDIAL MASS BY CROSS SECTIONAL ECHO (gr)

FIGURE 1. Condtbn cuye between ehea&grephii (ECHO) and mlypletic resonance lmaglnp (MRI) lell v~trkular (LV) MU -.

942 THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 69 APRIL 1. 1992

M-mode determination of LV mass in this study. Data from published reports comparing LV mass at autopsy with M-mode echocardiographically determined mass have shown a variable correlation.16*20~21 In a previous study in normal children, we found a good reproducibil- ity of cross-sectional echocardiographic LV mass deter- mination@ when assessing interobserver variability of measuring LV mass offline from a videotape. So far, all echocardiographic studies have been performed by one person (MV), but we appreciate that variability of cross-sectional echocardiographic mass determination may be substantially higher if image acquisition is per- formed by different observers. Cross-sectional imaging of the heart may be difficult in obese and adolescent patients, and accurate LV mass determination by cross- sectional echocardiography may not be possible.** MRI may become the golden standard of LV mass determi- nation because there is usually little difficulty in defin- ing endo- and epicardial borders (if errors by blood flow signals are excluded by appropriate techniques as dou- ble-echo or presaturation techniques). Usually there is no difficulty in imaging the cardiac apex, and obesity of a patient does not interfere with image quality in MRI. Another advantage of MRI may be that mass determi- nation is not influenced by the geometric complexity of the heart.r8 The echocardiographic study used a Simp- sons rule algorithm to calculate epi- and endocardial volume. Simpsons rule is based on the assumption of an elliptical shape, the minor and major axis of which are defined by 2 rectangular projections. The above-men- tioned formula used to calculate volumes in the MRI study is independent of the real shape, since the area of each tomographic slice is manually outlined. Further studies with diseased hearts will show whether this theo- retical advantage has practical implications for the ac- curacy of mass determination with MRI versus echo- cardiography.

VaMdation of magnetic resonance imaging method: The accuracy of the MRI technique had been assessed in an experimental study using 20 water-filled balloons of different irregular shapes containing volumes ranging from 50 to 300 ml. The correlation between measured volume and MRI-estimated volume was 0.99 (SEE 2.7%~).*~ Although to date we have no autopsy data on any of our children, whose mass had been determined by MRI or cross-sectional echocardiography, these data are available in published reports and have shown good correlation between MRI-estimated mass and the true heart weight from human cadavers.18

Limitations of magnetic resonance imaging: In most in vivo MRI studies there was an overestimation of ventricular mass.17-19 In our clinical study MRI mass was usually higher than mass determinded by cross-sec- tional echocardiography. This possible overestimation of myocardial mass by MRI may be related to partial vol- ume effects and the presence of intracavitary flow sig- n& 18,193

MRI studies in most pediatric patients aged <6 years and in some older children require sedation, and anxiety has been well documented in patients undergo- ing MRI studies. 25 Imaging time was longer than that used for echocardiographic assessment of the heart.

A multislice technique, which was used because of time constraints in our study, does not allow for com- parison of end-systolic and end-diastolic mass (which is another valid method of studies determining mass) be- cause the cardiac mass does not change during the car- diac cycle.lO Accuracy of LV mass measurement by MRI can be improved by use of multicycled double an- gulated scans. With this technique slices perpendicular to the main axis of the left ventricle and triggered at the same time as the cardiac cycle are used. We had previ- ously shown that the mean error between the multislice technique used in this study and the multicycled scans is 13%. However, the latter method has the disadvantage of a significantly longer imaging time.

Correlation of echocardiography and magnetic res- onance imaging: The correlation between both methods of LV mass determination has been satisfactory. From our relatively small proband number we get the impres- sion that this correlation is better in infants and smaller children than in adolescents. The applicability of cross- sectional echocardiographic mass measurements may be limited to newborns, infants and younger children. Fur- ther studies need to be done to define more exactly the influence of age and body composition on the accuracy of cross-sectional echocardiographic mass determina- tion.

Acknowledgment: We thank Helen von Bibra, MD, for letting us use her Hewlett-Packard sector scanner for part of the study.

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