International Journal of Angiology 6:193-196 (1997)

Pulmonary Artery Anatomy in Congenital Heart Disease with Decreased Pulmonary Blood Flow by Magnetic Resonance Imaging Koichiro Niwa M.D., F.I.C.A., Mika Uchishiba M.D., F.I.C.A., Hiroyuki Aotsuka M.D., Shigeru Tateno M.D., Kimimasa Tobita M.D., Hiromichi Hamada M.D., Tadashi Fujiwara M.D., Kozo Matsuo M.D. Department of Cardiology and Cardiovascular Surgery, Chiba Children's Hospital and the Department of Pediatrics, School of Medicine, Chiba University, Chiba, Japan

Abstract. We performed magnetic resonance imaging (MRI) in 65 patients (ages 8 days to 17 years old) with congenital heart disease accompanied by pulmonary atresia or stenosis, who had not undergone radical or functional repair of the heart to assess the usefulness of MRI in evaluating the pulmonary artery (PA) tree and calculating the diameter of the pulmonary arteries. Imaging was performed with a superconducting magnet operating at 0.5 tesla with spin-echo sequence. MRI clearly visualized the pulmonary artery anatomy and the spatial relation between PA and great vessels in all 65 patients. In 11 of 12 patients with branch PA stenosis, MRI clearly demonstrated the sites of pulmonary stenosis. The diameter of the PAs and Nakata's PA index, as measured by MRI, were significantly correlated with those measured by PA angiography (y = 0.88X + 0.5, r = 0.97 for the right PA, y = 0.91X + 0.1, r = 0.96 for the left PA, and y = 0.74X + 33, r = 0.93 for the PA index). MRI technique in this study had low intra- and interobserver variations. In conclusion, MRI was a useful modality for noninvasive assessment of the PA in congenital heart disease with decreased pulmonary blood flow.

Introduction As cardiac surgery progressed it became important for pediatric cardiologist and surgeons to accurately evaluate pulmonary (PA) artery anatomy, the route of PAs, and PA size in complex congenital heart disease with decreased pulmonary blood flow [1,2]. Pulmonary' artery size and Nakata's PA index, calculated from PA size, were thought to be important prognostic indicators for cardiac surgery on these anomalies [3]. in recent years, magnetic resonance imaging (MRI) easily

Correspondence to: Koichiro Niwa M.D., F.I.C.A., Adult Congenital Heart Disease Center, Division of Cardiology, UCLA School of Medicine, 47123 CHS, 10833 Le Conte Avenue, Los Angeles, CA 90024-1679

and accurately visualized cardiac anatomy and the anatomy of the great vessels [4-71. There are some reports [8-13] on evaluation of PAs in congenital heart disease with decreased pulmonary blood flow. However, there are few reports on evaluation of the PA for surgical management planning [14]. In the present study, we performed MRI on patients with congenital heart disease with pulmonary atresia or stenosis, who had not yet undergone radical or functional repair of the heart, to assess the usefulness of MRI in evaluating the PA tree, measuring the diameter of the PAs, and determining the PA index.

Materials and Methods Patients Sixty-five patients, aged 8 days to 17 years old (mean 2.5 + 3.4 years), hospitalized between January 1990 and December 1994 were examined. They had not yet undergone radical or functional repair of the heart. The diagnosis were tetralogy of Fallot in 25 patients, single ventricle with pulmonary atresia or stenosis in 16, pulmonary atresia with intact ventricular septum in 8, corrected transposition of the great arteries with pulmonary atresia in 7, tricuspid atresia in 4, double outlet right ventricle with pulmonary stenosis in 3 and transposition of the great arteries with pulmonary stenosis in 2. Pulmonary artery branch stenosis was a complication in 12 patients. Discontinued PA was associated in 2 patients. Surgery on these patients included radical or functional repair in 48 patients and palliative surgery without intracardiac repair in 17. The study was approved by our hospital Review Board, and parents of all patients gave written informed consent for the angiography and MRL

MRI hnaging was performed with a commercially available superconducting magnet operating at a 0.5 tesla, images were obtained using a spin echo pulse sequence with an echo delay time of 30 ms and a pulse repetition time defined by each patient's heart rate. The slice thickness was 5 or 10 mm without gap. Four signals were averaged. Electrocardiographic (ECG) gating was obtained with the use of three chest electrodes. The acquisition matrix was 128 x 256 or 128 x 128 or lower and the field of view was 25 x 25 cm or 20 x 20 cm. Imaging on transverse, sagittal, coronal, and proper oblique planes as obtained using a

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K. Niwa et al: Pulmonary Artery Anatomy by Magnetic Resonance Imaging

Fig. 1. Various planes of measuring the pulmonary artery size in congenital heart disease with decreased pulmonary blood flow. Upper left, PA in axial plane; upper right, oblique plane of the left PA and the other oblique plane which was perpendicular to the right PA, was selected in the axial plane for measurement of bilateral PA size (white line 1 for the left PA and 2 for the right PA); bottom left, right PA size measurement in the oblique plane; bottom right, left PA measurement in the oblique plane. AO, aorta; PA, pulmonary artery; LA, left atrium; LV, left ventricle.

multisection technique. We use a cylindrical head coil with an aperture of 30 x 30 cm. All patients less than 8 years old who could not be examined in a steady state were sedated with monosodium trichlorethyl phosphate (80 mg/kg orally). Patients were monitored by pulse oximeter in the examination room, and the monitoring line was shielded from the magnetic field. Discontinuity of the PA was defined as nonconfluency of the pulmonary arteries on all planes. Pulmonary artery size was measured at the proximal site of the first pulmonary branch or at the distal site of BlalockTanssig's shunt (Fig. 1). The PA index was calculated as follows: right PA area (ram 2) + left PA area (mm2)/body surface area; PA area = 7r x (diameter/2 x f)2, f = magnification coefficient [3].

Angiocardiography Pulmonary artery anatomy and size were determined on biplane cineangiography obtained during routine diagnostic cardiac catheterization in all patients in this study. Biplane pulmonary anteriography was performed with 5-7 Fr size Belrnann angiographic catheter at 50 frames/second. For calibration, a grid of fine steel 1 x 1 cm each embedded in Plexiglas was filmed perpendicular to the anteroposterior and lateral x-ray tubes at the exact position during pulmonary angiography. Pulmonary artery size was measured by a single observer at the end-systole at the proximal site before the first pulmonary artery branch was observed. All patients who could not undergo catheterization examination in the steady state were sedated with pethidine bydrochloride (1 mg/kg, intravenously) and diazepam (0.5 mg/ kg, intravenously). Pulmonary angiography was performed within 2 days after MRI. Data from these two modalities were evaluated prospectively by different observers who had no knowledge of the results of the other studies.

Reproducibility To determine interobserver error for MRI, pulmonary artery size was analyzed by a further independent investigator. For determination of intraobserver error for both techniques, studies were reanalyzed by the same observer after at least 3 months, with the observer blinded to the original results.

Fig. 2. Visualization of the PA branch stenosis. Upper panel, MR axial plane showing the right PA stenosis (black arrowhead); lower panel, posteroanterior pulmonary angiography of a patient with corrected transposition of the great arteries. AO, aorta; P, pulmonary artery.

Statistical Analysis Pulmonary artery sizes by MRI and angiography were compared by linear regression analysis, Probability (p) values less than 0.05 were considered significant. Similarly, intraobserver and interobserver variation were analyzed by linear regression analysis using paired data from each patient.

Result M R I t i m e s r a n g e d f r o m 28 to 45 m i n u t e s . All M R I e x a m i n a t i o n s in this study w e r e d i a g n o s t i c . In all 65 patients, M R I clearly v i s u a l i z e d bilateral PAs. M o r e o v e r , M R I c o u l d also clearly v i s u a l i z e spatial r e l a t i o n s b e t w e e n s u p e r i o r v e n a c a v a (bilateral or unilateral) a n d P A s a n d t h o s e a m o n g a o r t a or s u b c l a v i a n arteries, P A s , a n d a z y g o s or h e m i a z y g o s vein. In 11 o f 12 p a t i e n t s w i t h p u l m o n a r y ' b r a n c h stenosis, M R I clearly d e m o n s t r a t e d the s t e n o t i c sites (Fig. 2). In the rem a i n i n g p a t i e n t s w i t h s e v e r e p u l m o n a r y b r a n c h stenosis, we d i a g n o s e d the c o n d i t i o n as d i s c o n t i n u o u s P A b r a n c h . In t w o p a t i e n t s w i t h d i s c o n t i n u e d P A b r a n c h , M R I v i s u a l i z e d the d i s c o n t i n u e d site o f PAs. T h e d i a m e t e r o f the P A s a n d N a k a t a ' s P A index, as m e a s u r e d b y M R I , w e r e s i g n i f i c a n t l y c o r r e l a t e d w i t h t h o s e m e a s u r e d b y cine a n g i o g r a p h y (y = 0.88x + 0.5, r = 0.97 for the r i g h t P A ; y = 0.91x + 0.1, r = 0.96 for the left PA; a n d y = 0.74x + 33, r = 0.93 in P A index) (Figs. 3, 4, a n d 5).

K. Niwa et al: PulmonaryArtery Anatomyby Magnetic ResonanceImaging

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Reproducibility Regression data between the two observers and the repeat MRI studies are as follows: The correlation coefficients for the repeat MRI studies as measured by a single observer for the right PA size, the left PA size, and PA index were 0.96, 0.93, and 0.89, respectively (p < 0.001 in all). The con-elation coefficients between two observers for the right pulmonary artery size, the left PA size, and PA index were 0.92, 0.91, and 0.87, respectively (p < 0.001 in all).

Discussion

Evaluating Pulmonao, Arteries In treating patients with complex congenital heart disease with decreased pulmonary blood flow, measurement and evaluation of both right and left PAs are important for planning proper surgical strategy. Currently, the most common means of measuring PAs arteries in children are pulmonary

angiography [15,16] and echocardiography [17-21,22]. However, these modalities have some limitations and demerits in pediatric patients. Angiography provides good delineation of the right and left PA, with high temporal resolution. However, the examination is invasive and involves radiation exposure. Therefore, the application of ventriculography for routine monitoring of cardiac function is impractical. Two-dimensional echocardiography has been used for the evaluation of the PA in children. However, it is sometimes difficult to visualize the right and left PAs in complex congenital heart disease, especially those associated with malrotation and/or malposition or severe deformity of the heart [17,18]. Computed tomography has been validated for assessing the pulmonary artery [15,23]. However, this technique requires injection of radiopaque contrast medium and uses ionizing radiation. Therefore, the application of these examinations for routine evaluation of PAs in congenital heart disease with decreased pulmonary blood flow is impractical.

Evaluating of Pulmonary Artery Anatomy by MRI MRI can easily provide multiple tomographic section of the PA even in patients with cardiac malposition. Furthermore, it is reported that in highly complicated complex congenital heart disease, MRI could easily and accurately visualize cardiovascular anatomy [4-7]. Cardiovascular surgeons require not only accurate diagnosis of PA anatomy, size, and continuity, but also spatial relation between superior vena cava (bilateral or unilateral) and PAs. This is necessary for undertaking Fontan procedure, total cavopulmonary connections, and Glenn procedure. Spatial relation between aorta or subclavian arteries and PAs and the azygos or hemiazygos vein is necessary for undertaking Blalock-Taussig's shunt. Furthermore, balloon pulmonary artery plasty was recently attempted in patients with severe pulmonary branch stenosis before and after intracardiac repair [24]. Therefore, making an accurate noninvasive diagnosis of PA anatomy

196 and continuity was increasingly important in determining further therapeutic strategies in these patients. In all 65 patients in this series, M R I could p r o v i d e the P A anatomy and the spatial relations b e t w e e n P A s and great vessels. Furthermore, in 1 1 of 12 patients with branch p u l m o n a r y stenosis and in both patients with P A discontinuity, M R I clearly demonstrated these anomalies in this study. There are other reports supporting the usefulness of M R I in evaluating the PAs in children [8-14]. Therefore, M R I is a useful modality for evaluating the PAs in c o m p l e x congenital heart disease with p u l m o n a r y atresia or stenosis.

Evaluating Pulmonary Artery Size and Index In this study, the right and left P A s in all patients could be evaluated by M R I , and correlation b e t w e e n M R I and angiography in the P A size and index was good. Furthermore, the M R I techniques in this study had low interobserver and intraobserver variations. It was reported that c o m p a r i s o n o f P A size in congenital heart disease with decreased p u l m o nary b l o o d f l o w yielded a g o o d correlation b e t w e e n M R I and angiography [11-13]. Furthermore, it was reported [14] that correlation b e t w e e n M R I and angiography in evaluating and m e a s u r i n g the P A size and index in patients with Fontan procedure was excellent. Different f r o m the evaluation o f the P A s by e c h o c a r d i o g r a p h y and angiography, M R I can evaluate the right and left P A s simultaneously f r o m the s a m e c o n t i g u o u s slices. F r o m p r e v i o u s reports and our study, M R I is a suitable, n o n i n v a s i v e m o d a l i t y for measuring P A size and calculating the P A index. In this study, there is an underestimation o f P A index. This is because we calculated P A size at the end-systole on cine angiography but we calculated P A size in a variable phase by M R I . W e used the slice thickness o f 5 or 10 m m in this study, but it could be too large a slice for visualizing and m e a s u r i n g small-size PAs. Therefore, it m i g h t be necessary to calculate the P A size at the end-systole on cine M R I with smaller slice thickness. H o w e v e r , it is s o m e t i m e s difficult to visualize the P A s clearly by cine M R I .

Conclusion M R I was a useful modality for n o n i n v a s i v e assessment o f the P A anatomy, size, and P A index in congenital heart disease with decreased p u l m o n a r y b l o o d flow.

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