Optimization of magnetic resonance imaging parameters for left ventricular wall motion studies at 0.5 T.

Magnetic resonance cine imaging of left ventricular wall motion at rest or during stress may be used to assess myocardial function, infarction and viability, or reversible ischaemia. Whilst interpretation of the cines rests critically on image quality, there is little in the literature which systematically examines the optimal imaging parameters for such wall motion studies at rest or during stress. This study was designed to examine several imaging parameters for cine optimization using a conventional 0.5 T scanner. Gradient echo imaging was performed in two groups of volunteers with varying echo times and flip angles. The period between excitations was 80 ms (simulating a resting heart rate) in one group, and 40 ms (simulating tachycardia during stress) in the other group. Short axis imaging yielded the highest contrast between blood and myocardium for both repetition times (rest p = 0.02; stress p < 0.001) compared with the long axes, because of magnetic saturation of blood moving slowly in-plane. Contrast was higher at end-diastole than end-systole for the long axes (rest p < 0.0001; stress p < 0.0002), but not significantly different in the short axis. Increasing the echo time and flip angle resulted in increased signal but eventually caused motion artefact and magnetic saturation of blood. The optimal parameters were an echo time of 14 ms and a 45 degrees flip angle for resting heart rates, with the flip angle falling to between 35 degrees and 45 degrees for tachycardia. The choice of imaging parameters is therefore a compromise between improved signal and unwanted artefacts, although the latter are less evident in the short axis plane, which yields the best contrast results because of high blood inflow effects.

Diagnostic and prognostic value of dipyridamole and dobutamine stress echocardiography in patients with Q-wave acute myocardial infarction.

The aim of this study was to compare dipyridamole and dobutamine stress echocardiography, performed early in patients with acute myocardial infarction (AMI) to evaluate residual ischemia, viability, and prognosis. Fifty patients (mean age 55 +/- 9 years, 47 men, 3 women) with AMI, all treated with thrombolytic therapy, underwent standard dipyridamole and dobutamine tests, within the fifth day of the event. Wall motion score index and the 16 segments model were used to evaluate contractility. Forty-seven patients underwent coronary angiography within the tenth day of the event. The mean follow-up was 24 +/- 12 months. No side effects occurred during both tests. Both dipyridamole and dobutamine tests were positive for ischemia, in 32 and 33 of 47 patients, respectively (sensitivity 73% and 75%; specificity 67% and 67%); these tests induced an improvement of contractility in 23 and 38 of 139 abnormal segments at baseline, respectively (sensitivity 52% and 86%; specificity 100% and 100%). Cardiac events occurred in 26 of 50 patients, 22 with a positive dipyridamole test and 21 with positive dobutamine test. Thus, both tests were feasible, safe, and useful to evaluate residual ischemia, viability, and prognosis. No significant differences were found in sensitivity and specificity between tests.

Assessment of magnetic resonance velocity mapping of global ventricular function during dobutamine infusion in coronary artery disease.

BACKGROUND: Magnetic resonance imaging (MRI) is a versatile technique for examination of the cardiovascular system but only recently has assessment of myocardial ischaemia in coronary artery disease (CAD) become possible, for example by demonstrating abnormalities of regional ventricular contraction during stress. Global ventricular function during stress was assessed by MRI of aortic flow, which has not been previously attempted. DESIGN: Variables measured by MRI reflecting the effect of ischaemia on global ventricular function during dobutamine stress were correlated with thallium-201 myocardial perfusion tomography. PATIENTS: 10 normal controls and 25 patients with CAD. SETTING: Tertiary cardiac referral centre. METHODS: Novel MRI sequences and analysis systems were used to measure the following variables during staged dobutamine infusion to 20 micrograms/kg/min: stroke volume, cardiac output, cardiac power output, peak flow, peak flow acceleration, aortic back flow, and flow wave velocity. Heart rate, blood pressure, double product, and maximum tolerated dobutamine dose were also measured. Multiple regression analysis was used to compare changes during stress with 201TI tomography. RESULTS: All parameters except for stroke volume and diastolic blood pressure increased in the controls. In the patients with CAD a significant relation was shown between the extent of reversible ischaemia and the change in peak flow acceleration (P < 0.00001), peak flow (P = 0.002), cardiac power output (P = 0.036), maximum dobutamine dose (P = 0.039), and systolic blood pressure (P = 0.04). Peak flow acceleration accounted for 58.4% of the variation in reversible ischaemia, and after allowing for this, only cardiac power output remained independently predictive adding a further 4.2% to the model (adjusted r2 = 0.626). A decrease in peak flow acceleration with an increase in dobutamine infusion indicated moderate or severe ischaemia (chi 2 = 10.2, P = 0.017). CONCLUSION: MRI may be used to assess variables of aortic flow during stress, which includes acceleration with high temporal resolution. Peak flow acceleration was the most sensitive indicator of the effect of ischaemia on global ventricular function.

Magnetic resonance jet velocity mapping in mitral and aortic valve stenosis.

BACKGROUND: Magnetic resonance (MR) phase-shift velocity mapping is an established method for measurement of nonturbulent intravascular flow. Shortening the echo time of the MR sequence to 3.6 msec allowed application of the technique to turbulent jet flow. The objective of this study was validation of MR jet velocity mapping in patients with cardiac valve stenosis. METHODS AND RESULTS: We used a 0.5-T Picker MR machine to measure peak poststenotic jet velocity in 15 consecutive patients recruited with known valve disease (six mitral stenosis, three of these restudied after valvoplasty, and 11 aortic stenosis). On the same day as the MR study, these patients underwent independent Doppler echocardiographic measurement of peak jet velocity. The results of 10 further MR investigations of aortic stenosis are also reported and compared with Doppler studies performed within 6 months. Of the 29 MR studies, 28 (97%) produced interpretable velocity maps, the one failure being attributed to misplacement of the imaging slice in a case of severe aortic stenosis. Agreement between MR and Doppler measurements of peak jet velocity in the recruited group was as follows: n = 18; range, 1.4-6.1 m/sec; mean, 3 m/sec; mean of differences (MR-Doppler), 0.23 m/sec; standard deviation of differences, 0.49 m/sec. CONCLUSIONS: In vivo MR peak jet velocity measurements agree well with those made by Doppler ultrasound. The technique, which is not subject to restricted windows of access and has potential for further refinements, could contribute to improved evaluation of stenoses, especially at locations where ultrasonic access is limited.

Magnetic resonance imaging during dobutamine stress in coronary artery disease.

Cine magnetic resonance imaging (MRI) provides a tomographic method of assessing regional ventricular function in any desired plane. It has not been possible to obtain adequate images during dynamic exercise, and this has limited its value in patients with coronary artery disease (CAD). Therefore, an infusion of dobutamine was used to study 25 patients with exertional chest pain and abnormal exercise electrocardiograms. Areas of abnormal wall motion were compared with areas of abnormal myocardial perfusion imaged by dobutamine thallium emission tomography and with coronary arteriography. Twenty-two patients had significant CAD. Twenty-one (96%) of these patients had reversible myocardial ischemia shown by dobutamine thallium tomography, and 20 (91%) had reversible wall motion abnormalities shown by dobutamine MRI. Comparison of abnormal segments of perfusion and wall motion showed 96% agreement at rest, 90% agreement during stress, and 91% agreement for the assessment of functional reversibility. The normalized magnetic resonance signal intensity of the ischemic segments showed a small but significant reduction when compared with that of normal segments (-67 units [9.2%]; p less than 0.05). Dobutamine infusion was well-tolerated, despite causing chest discomfort in 24 patients (96%). Nine patients (36%) developed a minor dysrhythmia that was usually ventricular premature complexes, but this did not limit infusion, and other side effects were mild. The short plasma half-life of dobutamine makes it ideal as a stress agent for imaging techniques (such as MRI), and these results suggest that it is more effective in the provocation of wall motion abnormalities than is dipyridamole in patients with CAD.

MR phase-shift velocity mapping of mitral and pulmonary venous flow.

Mitral and pulmonary venous flows are important indexes in the evaluation of left ventricular diastolic function and in the assessment of mitral valve disease. We used MR phase-shift velocity mapping to measure mitral and pulmonary venous flow velocity in 10 healthy volunteers and mitral flow velocity in 5 patients with mitral valve stenosis. Normal mitral flow shows two positive peaks: one during early ventricular diastole and the other during atrial contraction. Peak mitral flow velocity (mean +/- SD) in early diastole was 68 +/- 12 cm/s and during atrial contraction 39 +/- 10 cm/s. The ratio of peak mitral flow velocity in early diastole to that during atrial contraction was 1.9 +/- 0.6. In patients with mitral valve stenosis, the initial high flow velocity persisted through diastole. Peak mitral flow velocity of patients with mitral valve stenosis correlated well with values obtained from Doppler echocardiography. Pulmonary venous flow showed two positive peaks: one during ventricular systole and the other in ventricular diastole. A small backflow during atrial contraction was noticed. Peak systolic velocity in the right lower pulmonary vein was 47 +/- 11 cm/s, peak diastolic velocity was 40 +/- 9 cm/s, and peak backflow velocity was 14 +/- 3 cm/s. Magnetic resonance velocity mapping is a noninvasive technique for the evaluation of time-related flow velocity patterns and for quantitative measurement of mitral and pulmonary venous blood flow velocity.

Usefulness of the dipyridamole-Doppler test for diagnosis of coronary artery disease.

Two-dimensional and Doppler echocardiographic studies and a hemodynamic investigation were performed during dipyridamole testing in 42 subjects (13 control subjects and 29 patients with coronary artery disease [CAD]), to evaluate the ability of dipyridamole Doppler echocardiography in identifying patients with ischemic left ventricular dysfunction. In the control group, after dipyridamole infusion, Doppler-derived parameters increased significantly from baseline (p less than 0.001). In patients with CAD, peak flow velocity, flow velocity integral and stroke volume failed to increase after dipyridamole infusion (0.89 +/- 0.21 to 0.85 +/- 0.18 m/s, difference not significant; 14 +/- 3 to 12 +/- 4 cm, difference not significant, and 56 +/- 13 to 50 +/- 14 ml/beat, p less than 0.05, respectively). Heart rate, rate pressure product, systemic vascular resistance and mean right atrial pressure had similar variations in the 2 groups. Changes in the 3 Doppler-derived parameters are closely related to the variations of peak positive dP/dt, stroke volume (thermodilution) and left ventricular end-diastolic pressure and are closely related to the coronary angiography jeopardy score and to the appearance of wall motion abnormalities. Thus, by combining Doppler and 2-dimensional echocardiography, dipyridamole-induced myocardial ischemia may be detected in a high percentage of CAD patients, providing a sensitive tool for identifying patients with high-risk coronary artery anatomy.