Donald B. Longmore

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.

Pulmonary artery distensibility and blood flow patterns: A magnetic resonance study of normal subjects and of patients with pulmonary arterial hypertension

Abstract Pulmonary artery distensibility was studied with spin-echo magnetic resonance imaging in 20 normal subjects of variable age and in four patients with pulmonary arterial hypertension. The distensibility was found to be significantly lower (8%) in patients with pulmonary arterial hypertension than it was in normal subjects (23%). No age-related difference occurred. Magnetic resonance velocity mapping of the pulmonary artery blood flow was performed in 26 normal subjects—11 had mapping in the mid pulmonary artery, 15 had mapping in the distal pulmonary artery, and mapping in the four patients with pulmonary arterial hypertension was in the mid pulmonary artery. The pulmonary artery flow volume was compared with aortic flow and left ventricular stroke volume and a very good correlation was found. A retrograde flow of 2% occurred in the normal subjects serving to close the pulmonic valve. Antegrade plug flow occurred in most normal subjects but varied among individuals. There were also other variations in the flow pattern among normal individuals. All patients with pulmonary arterial hypertension had a markedly irregular ante- and retrograde flow and a large retrograde flow (average 26%). Magnetic resonance imaging offers a noninvasive way to evaluate pulmonary arterial hypertension as well as to quantitate pulmonary and aortic flows in, for example, left-to-right shunts.

The function of the aorta in ischemic heart disease: A magnetic resonance and angiographic study of aortic compliance and blood flow patterns

Regional compliance of the ascending aorta, aortic arch, and the descending aorta was measured in 70 normal subjects at varying ages, in 17 patients with coronary artery disease (10 coronary artery disease patients, 3 with syndrome X), and in 13 trained athletes using magnetic resonance imaging. Ascending aortic compliance was measured angiographically in 22 patients with documented coronary artery disease and in 11 patients with syndrome X. Magnetic resonance velocity mapping was used in six patients with documented coronary artery disease and in three patients with syndrome X to study two-dimensional velocity profiles in the proximal and mid-ascending aorta and to quantify both forward and reverse flow. The measurements were compared with earlier published measurements from 24 normal subjects. It was found that patients with ischemic heart disease or syndrome X had decreased or no measurable aortic compliance and that they had significantly reduced or abnormal ascending aortic reverse flow likely to cause reduced coronary artery flow. A new theory is advanced that decreased myocardial perfusion leading to ischemic heart disease has two sources: (1) insufficient blood flow into the coronary artery inlet due to abnormal aortic function and independent of coronary artery stenosis and (2) local coronary artery stenosis. Observations supporting the theory are presented.

Quantitation of antegrade and retrograde blood flow in the human aorta by magnetic resonance velocity mapping

Magnetic resonance velocity mapping was used in 24 normal subjects to study two-dimensional velocity profiles in the proximal and mid-ascending aorta, and to quantify both forward and reverse flow. The aortic flow measurements were validated by comparison with left ventricular stroke volume in all subjects and by comparison with pulmonary flow measurements in 12. Agreement was good with standard errors of the estimate of 7.8 and 7.1 ml, and correlation coefficients of 0.93 and 0.95, respectively. Systolic velocity maps were similar in the proximal aorta and the mid-ascending aorta, with maximum early systolic flow along the left posterior wall. Toward the end of systole and throughout diastole, a channel of reverse flow developed in the same region in the mid-ascending aorta, but in the proximal aorta it split to enter the sinuses of Valsalva, predominantly the left and the right coronary sinuses. Mean percentage ratio of retrograde-to-antegrade flow was 6.3%, with the majority of retrograde flow occurring in early diastole. The findings suggest that the retrograde flow is related to coronary artery flow and it is possible that aortic disease, which is known to influence aortic flow patterns, may also influence coronary flow.

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.

Application of flow measurements by magnetic resonance velocity mapping to congenital heart disease

Abstract Measurement of flow in the systemic and pulmonary circulations is one of the main reasons for performing cardiac catheterization in patients with congenital heart disease. We have previously reported a magnetic resonance method of flow measurement that involves the acquisition of velocity maps 1,2 using a field-echo sequence with even echo rephasing (FEER). 3,4 It depends on the fact that a magnetic resonance signal has both amplitude and phase and that phase can be encoded to give a measure of flow velocity. Stationary tissues have zero phase and are shown as mid-gray, but flowing blood has a phase shift, which is proportional to its velocity in a defined direction and is displayed as a darker or lighter shade.

Comparison of magnetic resonance imaging with echocardiography and radionuclide angiography in assessing cardiac function and anatomy following mustard's operation for transposition of the great arteries

The Mustard operation in infancy and childhood has successfully palliated many patients with transposition of the great arteries who have now survived to adulthood. Right ventricular dysfunction and tricuspid regurgitation are important determinants of late morbidity and mortality. The value of noninvasive magnetic resonance imaging (MRI) in the assessment of cardiac function and anatomy 9 to 20 years after this procedure has been investigated, and compared with findings on echocardiography, radionuclide ventriculography and angiography in 17 adult patients. Ejection fractions measured by MRI were higher compared with radionuclide ventriculography. The correlation for the left ventricle was closer (r = 0.75) than for the right ventricle (r = 0.49). Tricuspid regurgitation was assessed by Doppler echocardiography and by MRI using the right/left ventricular stroke volume ratio. The mean stroke volume ratio in those with Doppler evidence of tricuspid regurgitation was 1.6:1 compared to 1.1:1 in those without, and this difference reached significance (p less than 0.01). The anatomy of the great arteries was clearly visible in all patients. Five patients had a residual ventricular septal defect which, with the exception of 1 small defect, was easily visualized. The intraatrial baffle was best seen in transverse slices, and the systemic venous connection showed as a relatively narrow channel lying in the posterior part of the cavity. In general, baffle anatomy was easier to assess on 2-dimensional echocardiography.(ABSTRACT TRUNCATED AT 250 WORDS)

MR imaging of age-related dimensional changes of thoracic aorta.

Changes in the dimensions of the thoracic aorta with age were studied in 70 healthy volunteers between the ages of 10 and 83 years. Spin echo images were acquired at end-diastole in three oblique planes through the ascending aorta, aortic arch, and descending aorta. A double oblique image through the whole of the thoracic aorta was also acquired. Measurements of aortic cross-sectional area and length were corrected for body surface area, and normal magnetic resonance standards were established. The direct correlation that aortic dimensions have with age is likely to be due to loss of elasticity. The ratio between areas of the ascending aorta and the aortic arch is directly related to age whereas the ratio between the aortic arch and the descending aorta is inversely related to age.

Evaluation of Fontan's operation by magnetic resonance imaging

Abstract Fontan-type procedures for tricuspid atresia 1–3 and the modifications used for other complex cyanotic cardiac malformations 4,5 should be judged by the complications and long-term state of patients. 6 The most important complication is insidious obstruction. Early recognition of this is vital since it can lead to arrhythmias and loss of atrial and ventricular function. At catheterization attention must be paid to small gradients as low as 2 or 3 mm Hg. A reliable noninvasive method for assessing the atriopulmonary connection as well as the cardiac function applicable to outpatients is necessary for proper management. Magnetic resonance imaging (MRI) has been used as a noninvasive method of assessing congenital heart disease, both pre- and postoperatively. 7–10 This study assesses its value for demonstrating the anatomy of the atriopulmonary connection and recognizing obstruction compared to 2-dimensional echocardiography and the findings at catheterization and operation.

Magnetic resonance morphological, chemical shift and flow imaging in peripheral vascular disease

We have used magnetic resonance imaging to study the aorto-iliac region in 13 patients with peripheral disease. Five healthy volunteers were studied for comparison. Magnetic resonance spin-echo imaging, chemical shift imaging to determine lipid content of atheroma, phase-shift velocity mapping and quantitative flow studies were obtained and the findings compared with radiological angiograms. The velocity profiles study showed an increased velocity at the site of a stenosis in eight patients who had iliac artery disease. Quantitative flow measurements made in both iliac arteries and the aorta in five patients and five volunteers showed a flow ratio in both iliac arteries less than 0.85 in patients with a stenosis of one iliac artery and a ratio greater than 0.85 in the volunteers. In one patient studied before and after angioplasty, flow improved post-angioplasty. The flow curve showed a characteristic distortion in diseased vessels compared with healthy vessels. In the chemical shift images of aortic atheroma five were classified as fibrous and three were lipid rich. This preliminary study showed the potential of magnetic resonance to assess non-invasively the morphology, composition and the haemodynamic significance of atheroma. This could be important in the study of the progression of peripheral vascular disease and its response to pharmacological and surgical intervention and in the planning treatment of lesions.