CMR is very reproducible, with a low interstudy reproducibility. For example, comparing the mean end diastolic volume obtained with CMR and with echocardiography (131 ml) in a patient shows that although they both resulted in the same volume, the standard deviation with CMR was 4.6 while the standard deviation with echocardiography was 18, about 4 times higher. A comparison of echocardiography and CMR in heart failure patients also showed that CMR had a greater reproducibility for left ventricular (LV) mass, left ventricular ejection fraction (LVEF), end diastolic volume (EDV), and end systolic volume (ESV).

Study sample sizes

Smaller samples sizes are required with CMR compared to echocardiography in clinical studies. For example, in a study to detect with 80% power and a p value of 0.05 changes in LV EDV, LV ESV, LVEF, and LV mass in which 250 patients for echocardiography was arbitrarily selected, 18%, 14%, 20%, and 3% fewer patients, respectively, would be needed with CMR.

This results in large savings in cost and effort. For example, in a study of left ventricular hypertrophy (LVH) regression to examine for a 10 gm change in mass over 6 months, the sample size needed with echo is 1010 and with CMR 28, a difference of 982 patients. AtAt an assumed per patient cost of $5000, the total cost savings is $5 million, the cost of 3 CMR scanners.

CMR is a gold standard for volumes in post-infarction patients. It is dimensionally accurate, no geometric assumptions are made, in vitro and in vivo validation is good, and is highly reproducible.

Navigator imaging and advanced technology are two of the advances in CMR coronary imaging.

Navigator imaging

This advance allows patients to breathe normally throughout the imaging process. This is because inspiration and expiration can be traced over time with the image obtained, which can then be plotted to gate the imaging of the coronaries to the ECG and to the respiratory trace. An acceptance window is then set for end expiration that determines which data to accept and which to reject.

CMR has been applied in patients with coronary anomalies and in patients with congenital heart disease in whom the incidence of coronary anomalies is about 30%, compared to 1 in 1000 or 1 in 1000 in the normal population depending on the series. This capability has proven useful in their adult congenital heart disease unit.

Navigator imaging can be used in 3-D imaging, but there are some problems because of the fairly lengthy process. A comparison of change in diaphragm position from zero over 35 minutes shows that in one set the diaphragm is drifting downward by up to 2.5 mm and in the other set it is drifting upwards by 1-1.5 mm. These movements have significant effects on the ability of the navigator to follow over time the changes in respiratory patterns. Another problem is periodic breathing with sleep and other respiratory disorders, as the inspirations and expirations become very condensed and this abnormal respiratory pattern is very difficult to capture with navigator imaging. There are problems with navigator imaging that may not be solvable with the current technology.

Breath-hold is one of the advanced techniques being investigated in 3-D magnetic resonance coronary angiography (MRCA), which requires very fast scans. This technique has been used to successfully image the right coronary artery and the left anterior descending, stenoses that compare well to x-ray coronary angiogram, and for vein graft re-surface rendering in a breath-hold of about 20 seconds.

Some of these images can be converted using imaging processing. A right coronary "fly through" for example allows one to look inside the right coronary artery and look down the wall of a 20-second acquisition. High-resolution spiral MRCA breath-hold imaging has a resolution of about 0.5 mm by 0.5 mm. Accelerated MRCA uses each of the multiple coils surrounding the patient to generate a portion of the image, which allows for acceleration while the imaging time remains the same. The image quality compared to native imaging is quite good.

MR Perfusion

Spin echo EPI perfusion technique to demonstrate perfusion has been investigated by Pennell. This very fast technique shows the myocardium as very bright and blood as black. The technique clearly shows perfusion abnormalities and has the advantage of speed. Very good quality parametric maps can be generated using spin echo EPI. In their experience the technique has been very comparable to single-photon emission computed tomography (SPECT) imaging in terms of sensitivity, specificity, and accuracy.

Tagging

Tagging may or may not have a large future in coronary disease. During diastole a grid is laid down of pre-saturation slices that extend through the heart and cut in cubes. The cubes are present, for example, at end diastole and the computer places white dots on the intersection of the cube lines, such that at end systole when they move it is possible to see how the lines have deformed into curves and the points have stretched out. Using computer analysis, it is then possible to measure contractility and movement of the points directly. In an example of anteroseptal MI the points are not moving, but in the remote infralateral region the points can be seen to be moving rather briskly with hyperdynamic motion. This can be applied in stress CMR, and compares well to thallium scans with dipyridamole and dobutamine.

Viability

CMR and FDG-PET have proven to be quite comparable, with similar rates for sensitivity, specificity, and accuracy for wall thickness and dobutamine response. There is hope this will be used in clinical practice. For example, in a dobutamine CMR image of a transmural MI, in the infralateral wall a thinned area that does not respond to dobutamine can bee seen, where the FDG uptake is abnormally low. This represents two aspects of viability by MR: thinning and lack of response to dobutamine. This compares to a fairly similar non-transmural infarct where the wall is thicker, responds to dobutamine and the FDG uptake is preserved.

Plaque characterization

MR angiography is very good. An example of renal angiography by MR shows it compares very well with the x-ray contrast in identifying a tight stenosis. Two important factors regarding the plaque are the fibrous cap and the amount of lipid within the plaque.

T2-weighted imaging is quite valuable in evaluating plaque, on which the lipid pool appears as black whereas calcium appears as black on both the T1 and T2 images. In the normal arterial wall the T2 image shows a relatively bright endothelium and adventitia.

Some early attempts to determine plaque stability or instability in vivo have begun and this work is quite promising. An example is a T2-weighted image in the aorta showing an eccentric plaque with a clear lipid pool and relatively thin fibrous cap_the sort of detail that is difficult to see on transesophogeal echo. An example of a T2-weighted image of an internal carotid atheroma was able to show an eccentric plaque, thrombus, the fibrous cap and the lipid pool, which compared well to the ex vivo specimen from the carotid endarterectomy. This has now been applied to the coronary arteries.