Noninvasive angiography with magnetic resonance is demonstrated. Signal arising in all structures except vessels that carry pulsatile flow is eliminated by means of velocity-dependent phase contrast, electrocardiographic gating, and image subtraction. Background structures become in effect transparent, enabling the three-dimensional vascular tree to be imaged by projection to a two-dimensional image plane. Image acquisition and processing are accomplished with entirely conventional two-dimensional Fourier transform magnetic resonance imaging techniques.
Electrocardiographically gated magnetic resonance images were acquired in 100 patients with a variety of cardiac diagnoses in planes oriented to intrinsic axes of the heart and aorta. The technique used combines patient positioning and alteration of magnetic gradient angle. Images in these planes appear to have advantages over conventional orthogonal images, both for display of cardiovascular anatomy and for evaluation of cardiac size and function.
The combination of gated cardiac imaging with the magnetic resonance parameters of flow void and tissue characterization forms the basis for a powerful noninvasive technique for imaging the cardiovascular system. Images constructed in planes orthogonal to the axis of the heart allow cardiac structures to be reproducibly detected in serial studies and measured. The data acquisition techniques used at Massachusetts General Hospital are presented, along with examples of their application in a variety of cardiac diseases.
MRI offers the best soft-tissue contrast of any radiographic modality. It provides better delineation of soft-tissue tumors and bone tumors than CT, but several different pulse sequences may be needed to make optimum use of this potential. MRI is also well suited to evaluation of abnormalities of the bone marrow, as it is not subject to the dense bone artifact that may occur with CT. Changes in signal intensity may be used to gain some appreciation of blood flow in the major vessels. Examples of each of these applications are discussed.
In order to establish normal values for left heart dimensions by magnetic resonance imaging (MRI), electrocardiographically gated MRI was performed in 16 normal asymptomatic subjects using a whole-body 0.6-T superconducting magnet with a spin-echo pulse sequence and an echo delay (TE) of 30 msec. Images were oriented along the long and short axes of the left ventricle in planes similar to two-dimensional echocardiograms (2DE). Comparable 2DE images were obtained for validation of the MRI measurements.
An atrial septal mass was identified by echocardiogram in a patient with multiple subcutaneous lipomas. In order to differentiate the benign condition of lipomatous hypertrophy from myxoma, thrombus and other tumors, nuclear magnetic resonance and computed tomographic imaging were performed. Both techniques identified the adipose nature of the tissue noninvasively, consistent with the diagnosis of lipomatous hypertrophy. Pathologically demonstrated lipomatous hypertrophy in a postmortem heart was similarly characterized by nuclear magnetic resonance imaging.
Projective MR images of vascular anatomy and flow are performed at 0.14 T by using phase contrast to suppress the signal contribution of the stationary background. The source of the contrast is the distinctive phase evolution of moving protons under the influence of the read-out gradient of a conventional two-dimensional Fourier transform (2D FT) spin-echo pulse sequence. By using short echo times, small phase shifts may be obtained. When phase shifts are less than about 45 degrees, the phase contrast assumes a simple and useful form.
Experimental canine studies have demonstrated the potential of magnetic resonance imaging (MRI) for detecting and characterizing acute myocardial infarction (AMI) in humans. Accordingly, electrocardiographic-gated spin-echo MR images of the left ventricular short axis were obtained in 34 patients a mean of 11 +/- 6 days (range 3 to 30) after AMI. This imaging technique allowed division of the left ventricle into segments corresponding to the left ventricular segments on angiography.
