Chaim S. Lotan

Left ventricular volume measurement using cardiac axis nuclear magnetic resonance imaging. Validation by calibrated ventricular angiography.

Proton nuclear magnetic resonance (NMR) imaging has the potential to serially assess left ventricular (LV) volumes with optimal accuracy because it is a high-resolution, three-dimensional, noninvasive modality. Previous NMR studies to assess LV volumes have been suboptimal, as they have used either planes aligned with the axes of the body, which are compromised by partial volume effects, or spin-echo techniques that have been time-consuming to acquire and analyze. Accordingly, for LV volume measurement, we developed a gradient-echo (cine) NMR strategy that uses two orthogonal planes intersecting along the intrinsic long axis of the heart (two-chamber and four-chamber). This approach was validated against calibrated contrast biplane LV cineangiography (CATH) and also compared with a previously reported short-axis spin-echo NMR method. Twenty-one patients underwent CATH and NMR (long-axis, n = 21; short-axis, n = 14) within a 3-day interval. Although both long- and short-axis NMR LV volumes and ejection fractions correlated well with CATH (r greater than 0.90, p less than 0.001 in all), end-diastolic volumes by both long-axis (161 +/- 85 ml) and short-axis (151 +/- 81 ml) NMR were systematically less than those by CATH (182 +/- 85 ml) (p less than 0.05). Consequently, ejection fractions by long-axis (48 +/- 17%) and short-axis (49 +/- 17%) NMR consistently underestimated those by CATH (54 +/- 16%, p less than 0.05). End-systolic volumes by long-axis (94 +/- 71 ml) and short-axis (87 +/- 72 ml) NMR were not significantly different from those by CATH (92 +/- 69 ml). Both NMR techniques had low intraobserver and interobserver variation (less than 11%); however, short-axis spin-echo NMR involved longer acquisition/reconstruction (35 versus 18 minutes) and analysis (25 versus 10 minutes) times. We conclude that both short-axis spin-echo and long-axis gradient-echo NMR approaches reliably estimate LV volumes. Currently, the long-axis strategy appears more practical for clinical use because the scan and analysis times are relatively short.

Assessment of postreperfusion myocardial hemorrhage using proton NMR imaging at 1.5 T.

BackgroundIntramyocardial hemorrhage occurs frequently after reperfusion of acute myocardial infarction. However, its significance has not yet been established, mainly because of the lack ofmethods for detecting such hemorrhage. The following ex vivo study was carried out to assess the potential of nuclear magnetic resonance (NMR) imaging to detect and quantitate postreperfusion intramyocardial hemorrhage. Methods and ResultsSixteen adult mongrel dogs underwent 3 hours of coronary occlusion followed by 1 hour of reperfusion, and three dogs underwent 4 hours of occlusion without reperfusion. Radiolabeled microspheres and 51Cr-labeled red blood cells were used to assess flow and evaluate the extent of hemorrhage. These results were later compared with both NMR and histology. Spin-echo NMR imaging was performed on the excised hearts using a 1.5-T system. Macroscopic assessment of the sliced myocardium revealed the existence of hemorrhage in 14 of the 16 dogs that underwent reperfusion but in none of those with occlusion only. In all 16 dogs with reperfusion, zones of increased signal intensity (SI) ratio (1.68±0.41 compared with control, p<0.05) were seen in regions relating to the distribution of the occluded coronary artery, whereas in 13 of the 16 dogs, areas of decreased SI within the zone of increased SI ratio (0.81±0.16 compared with control, p<0.05) were also seen, corresponding to regions with macroscopic hemorrhage. In contrast, in the three dogs without reperfusion, no macroscopic hemorrhage was observed, and likewise, no NMR zones of reduced SI were detected. Hemorrhage size by NMR (decreased SI zones), correlated well with hemorrhage size calculated from tissue slices (r=0.96, SEE=0.92%, p<0.0l), or by 51Cr labeling (r=0.78, SEE=1.5, p=0.1). In the reperfusion group, T2 relaxation times in the infarcted hemorrhagic zone (58±9 msec) were significantly lower than the infarcted zones without hemorrhage (98±13 msec, p<0.001). In contrast, when compared with control (964±72 msec), T1 relaxation times were significantly increased in both infarct zones, either with (1,284±176 msec) or without (1,266±103 msec) hemorrhage. The selective shortening of T2 relaxation times in the hemorrhagic regions is consistent with the paramagnetic effects of deoxyhemoglobin. ConclusionsNMR imaging may provide a noninvasive approach for the detection and quantitation of intramyocardial hemorrhage. This observation may provide a means to further characterize pathological processes associated with acute myocardial infarction and assess the role of myocardial hemorrhage after reperfusion therapy.

Interstudy reproducibility of biplane cine nuclear magnetic resonance measurements of left ventricular function

Cine nuclear magnetic resonance (NMR) imaging, as a noninvasive and high-resolution imaging modality, has been shown to be reliable for determining absolute left ventricular (LV) volumes and ejection fraction. A relatively new gradient echo cine NMR approach using 2 orthogonal long-axis planes (2- and 4-chamber) aligned with the true axes of the left ventricle has been previously developed and validated against radiographic biplane LV cineangiography. The aim of the present investigation was to determine the reproducibility of this more rapid cine NMR approach for the measurement of LV volumes and ejection fraction. Eighteen normal subjects underwent 2 cine NMR studies, on different days, using a 1.5-tesla clinical imaging system. Studies were analyzed on-line and blindly by 2 independent observers. Intraobserver error was also determined in a blinded manner. Mean values of measurements determined by this method in this group of normal subjects were end-diastolic volume (120 +/- 20 ml), end-systolic volume (39 +/- 9 ml) and ejection fraction (67 +/- 4%). Paired analysis of data revealed no significant bias between interstudy, interobserver or intraobserver measurements, except for interobserver end-diastolic volume, where the first observer measurements were slightly elevated (5.6 +/- 7.8 ml) compared with the second. This resulted in a small difference in ejection fraction (1.7 +/- 2.3%) between observers. The absolute variation between measurements (square root of variance components) was low for all interstudy, interobserver and intraobserver comparisons: end-diastolic volume was less than +/- 6.7 ml, end-systolic volume less than +/- 3.5 ml and ejection fraction less than +/- 2.4%.(ABSTRACT TRUNCATED AT 250 WORDS)

The value of cine nuclear magnetic resonance imaging for assessing regional ventricular function

Previous nuclear magnetic resonance (NMR) imaging studies to assess left ventricular function have used multiple axial planes, which are compromised by partial volume effects and are time consuming to acquire and analyze. Accordingly, an imaging approach using cine NMR and planes aligned with the true cardiac axes of the left ventricle was developed in views comparable with left ventricular cineangiography. Cine NMR imaging was used to assess regional wall motion and was validated by comparison with biplane left ventricular cineangiography. Fifty-nine patients underwent cineangiographic and NMR studies within 72 h. A poor quality NMR study precluded analysis in 4. leaving a study group of 55 patients (mean age 58 +/- 12: 17 women). Cine NMR movie loops were acquired in two long-axis planes: 1) right anterior oblique plane, parallel to the septum, and 2) four chamber orthogonal plane, perpendicular to the septum (this view is comparable to the angiographic left anterior oblique view). To assess regional wall motion, the left ventricle in both cine NMR and cineangiographic images was divided into five segments and graded on a five point grading scale from 3 for normal through 0 for akinesia and -1 for dyskinesia. Regional wall thickening was used qualitatively to aid in the analysis of wall motion. For the 275 segments compared in the right anterior oblique view, agreement was within one grade in 263 (95.6%) of 275 segments, whereas absolute agreement was observed in 171 (62%) of 275 segments. In the left anterior oblique view, of 200 segments evaluated, agreement within one grade was achieved in 184 segments (92%) and agreement was complete in 132 (66%).(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid assessment of aortic regurgitation and left ventricular function using cine nuclear magnetic resonance imaging and the proximal convergence zone

In patients with aortic regurgitation (AR), knowledge of the severity of AR, and the degree of left ventricular (LV) dysfunction are important for optimal management. Previous nuclear magnetic resonance (NMR) studies to assess these parameters used multiple tomographic planes that are time-consuming to obtain and analyze, and thus not cost-effective. In addition, these studies assessed the severity of AR by looking simply at the size of the regurgitant jet, a parameter that relates only poorly to regurgitant volume. The present study evaluates a rapid, single-plane, cine NMR approach (scan time < 10 minutes), and a new grading system for AR that is based on the presence, size and persistence of not only the regurgitant jet, but also the zone of proximal signal loss. Compared with color Doppler echocardiography (n = 42), the NMR approach detected AR with a specificity of 100% and a sensitivity of 95%. NMR regurgitant jet area correlated well with color Doppler regurgitant jet area (n = 20; r = 0.81; p < 0.01), but did not discriminate well between all grades of AR as compared with x-ray contrast aortography (n = 14). Using the new NMR grading criteria, AR grade by NMR was in accordance with aortographic grade in 12 patients, differing by only 1 grade in the remaining 2 patients. NMR grade was in accordance within 1 grade of Doppler in all patients compared (n = 20). LV volumes and ejection fraction using this single-plane approach correlated well with a previously validated, NMR biplane approach (r > 0.87; n = 18).(ABSTRACT TRUNCATED AT 250 WORDS)

Amiloride in ouabain-induced acidification, inotropy and arrhythmia: 23Na & 31P NMR in perfused hearts

The increase in intracellular sodium (Nai), resulting from inhibition of the Na/K ATPase by cardiac glycosides, is known to increase calcium influx via Na(+)-Ca2+ exchange, and thereby increase contractility. This increase in intracellular Ca2+ has been related to the development of intracellular acidification and enhanced activity of the Na(+)-H+ exchanger as a measure by the cell to prevent further acidification. Thus, the efflux of the H+ ions results in an additional increase in Nai. This may subsequently lead to an increased rate of Ca2+ influx and therefore to the potentiation of the effects of cardiac glycosides. To assess the role of Na(+)-H+ exchange in the mechanism of ouabain action in the beating heart we used amiloride, a known inhibitor of Na(+)-H+ exchange. Isolated rat hearts were perfused with either ouabain (50 microM) alone (n = 8, Group I), amiloride (1.0 mM) + ouabain (50 microM) (n = 8, Group II), or amiloride (1.0 mM) alone as a control group (n = 4, Group III). 23Na and 31P NMR spectroscopy were used to assess the changes in Nai and intracellular pH (pHi), respectively, while simultaneous and continuous monitoring of left ventricular pressure was carried out. Perfusion with both ouabain alone (Group I) or ouabain + amiloride (Group II), resulted in a time dependent increase in Nai levels, reaching (within 25 mins) a maximum of 200 +/- 7% of control in Group I, and 170 +/- 10% of control in Group II. Concurrently, a mild but significant decrease in pHi was observed in both groups. This decrease, however, was significantly higher in Group II compared to Group I (0.34 pH units vs. 0.19 pH units, respectively; P less than 0.05), suggesting that inhibition of Na(+)-H+ exchange by amiloride limits the recovery from ouabain-induced intracellular acidification. While developed pressure gradually increased in Group I to a maximum of 268 +/- 52% of control, the addition of amiloride in Group II substantially reduced the positive inotropic effect. Ventricular fibrillation (VF) developed in three of the eight hearts in Group I within 10-13 mins after the addition of ouabain. Interestingly, the rate of Nai increase in hearts that sustained VF was significantly higher compared to those without VF (mean slope 10.1 +/- 2.11 vs. 3.9 +/- 1.0, respectively; P less than 0.0001). Ventricular fibrillation did not develop in Group II or III.(ABSTRACT TRUNCATED AT 400 WORDS)

Cine NMR: An Emerging Technology

Nuclear magnetic resonance (NMR) imaging, sometimes called magnetic resonance imaging or MRI, is a relatively new noninvasive modality which has rapidly gained widespread popularity. As with any new imaging technique, clinical efficacy is largely dependent upon the examination time. This is particularly true with NMR imaging where conventional acquisition times are prolonged and the cost of instrumentation as well as maintenance is expensive. Therefore, an important focus of new strategies has been directed towards reduction of examination time.