Tailoring MR techniques improves image quality

November 1, 2004

Five years ago, cardiac MR imaging was limited to evaluating dysplasia, congenital heart disease, pericardial constriction, and, occasionally, valvular regurgitation. MR has expanded to assess heart failure and ischemic burden and to monitor the effects of treatment for hypertension and congestive heart failure.

It can distinguish between thrombus and myxoma lesions in the left atrium, plan ablation of atrial fibrillation, check for aberrant coronary artery origins or right ventricular dysplasia as causes of syncope, and determine coronary artery or bypass graft patency. It can plot scar distribution to assess myocardial viability, monitor the dissolution of intracardiac clots, and determine the degree of damage to the heart from chemotherapeutic agents such as adriamycin.

A rapid succession of new sequence releases has made MR increasingly practical as a cardiac imaging tool. The inversion recovery gradient-echo sequence, which was introduced in the late 1990s, opened the door to viability measurement. Steady-state free precession reduced acquisition times and improved the quality of cine images. Stress perfusion techniques allowed imagers to detect coronary artery disease through evaluation of ischemia and impaired blood delivery. Echo-planar sequences and other fast imaging strategies enabled real-time imaging during arrhythmias. Phase-contrast imaging added color Doppler-equivalent studies of blood flow, velocity time integrals, and regurgitant and shunt fractions. V-mode beam imaging added pulsed-wave Doppler measurements of velocity along specified lines. And the protocols available on current MR scanners barely touch the surface, said Dr. Justin Pearlman, director of advanced cardiovascular imaging in the departments of medicine and radiology at Dartmouth-Hitchcock Medical Center.

MR can do what other tests cannot. In fact, it is the preferred test for nonischemic cardiomyopathy to differentiate a purely idiopathic cause from a secondary etiology such as amyloidosis or hypertrophic cardiomyopathy, said Dr. Dipan J. Shah, director of the Nashville Cardiovascular Magnetic Resonance Institute and a consulting assistant professor of medicine at Duke University.

Because of MR's capability of quantifying flow measurements, it offers more precise assessments of valvular disease and holes or shunts within the heart than echocardiography or angiography, said Dr. Scott Flamm, director of MRI and cardiovascular MRI research at the Texas Heart Institute at St. Luke's Episcopal Hospital in Houston.

But taking advantage of all that MR has to offer is not practical in every examination of every cardiac patient; it would simply take too long. Radiologists need to tailor protocols to address the suspected underlying disease and customize exams to answer diagnostic questions that directly affect clinical management.

IMAGING PLANES

Radiologists use the same initial imaging orientations for all cardiac patients: scout images to determine the 3D orientation of the heart, then long-axis images (two-chamber, four-chamber-, and "five-chamber" views that include the left ventricular outflow track), as well as a complete set of short-axis cine images through the heart. Oblique views that account for the angulation of the heart as it sits in the patient's chest vary from patient to patient. Some hearts lie flat, almost horizontally within the chest, others rest straight up and down, some lean far to the left, and others point directly forward, Flamm said.

Cardiac coordinates are particularly important, according to Pearlman. Images must be oriented with respect to the 3D long axis of the left ventricle, not just a vertical and horizontal long axis, and chest axial views. From these coordinates come classic echocardiographic-like views of the chambers and short-axis views for calculating basic cardiac measurements such as the diameter of the left ventricle at peak systole and end diastole at the level of the papillary muscle tips, mean wall thickness of the left and right ventricles, pericardial thickness, end diastolic volume, left ventricular mass, stroke volume, ejection fraction for both right and left ventricles, and the dimensions of all four chambers.

Within the cardiac coordinates, the goals are to assess anatomy through a dark blood technique such as double inversion recovery, function by means of a bright blood dynamic method that produces a movie of the heart's moving parts, and tissue characterization with a delta R2 measurement or chemically selective imaging such as spectral spiral MR.

PATIENT MANAGEMENT

The sequence parameters of baseline studies frequently must be modified because patients can't hold their breath long enough. Although newer sequences reduce breath-hold problems, radiologists may have to modify the field-of-view and make other adjustments to shorten imaging time. Or they may reduce the spatial resolution of images from the typical voxel size of 1.5 to 1.8 cm down to 2 or 5 mm, Shah said.

When exam time is limited because a patient is claustrophobic, Flamm hand-injects a double dose of contrast before placing the patient in the scanner and then performs viability and cine imaging as quickly as possible.

DIAGNOSTIC QUESTIONS

In addition to these baseline studies, radiologists add extra sequences and image sections to further explore specific diagnostic concerns. When analyzing the coronary vessels, Dr. Steven Klepac, an associate professor of radiology at the University of Illinois, performs short-axis imaging to obtain functional information on ventricular volumes and ejection fractions, just as he would with echocardiography or angiography. From there, he proceeds to oblique axial imaging through the aortic root with both cine gradient-echo and straight T1-weighted cardiac-gated sequences.

While Klepac opts for 5-mm sections, Pearlman typically obtains 3-mm sections.

An analysis of right ventricular dysplasia rests on short-axis imaging plus thin sections parallel to the long axis that visualize the right ventricle. Functional long-axis ventricular images depict the apex of the right ventricle as well as the subtricuspid region, the most common areas affected by right ventricular dysplasia. Section size for imaging most adults is 6 to 8 mm or 1 cm thick. By analyzing fat and water separately and in mathematical recombinations, Pearlman can check for fat in the myocardium with chemically selective imaging.

Postcontrast imaging helps determine whether patients with possible pericarditis have acute disease on top of chronic inflammation or whether restrictive cardiomyopathy is present. For this application, Klepac uses T2-weighted short-axis and axial imaging, a combination of T1- and T2-weighted sequences, and contrast enhancement to identify amyloidosis, sarcoidosis, or other etiologies. Pearlman also observes filling curves throughout diastole or more abrupt impairment toward the end of the cycle to distinguish restrictive disease from constriction.

Combinations of gradient- and spin-echo techniques are best suited for ischemic cardiomyopathy. As in assessment of left ventricular cardiomyopathy in general, Flamm typically obtains T2-weighted spin-echo scans to look for variations or signal abnormalities from the myocardium that suggest an infiltrative process. He then focuses on abnormal signal intensity within the left ventricular myocardium. Various gradient-echo sequences, such as steady-state free precession cine studies, evaluate segmental or global function of the left ventricle and valves. T1-weighted scans with an inversion recovery pulse depict tissue viability, and phase contrast techniques provide quantification.

Imaging in the presence of cardiac masses does not focus strictly on viability. T1- and T2-weighted spin-echo images and T1-weighted spin-echo postcontrast scans look for patterns of enhancement in lesions. Dynamic gradient-echo perfusion-sensitive sequences that acquire one or more slices for each heartbeat during passage of a small bolus of gadolinium contrast examine enhancement in the myocardium and normal tissue as well as in the mass. Gradient-echo T1-weighted inversion recovery further characterizes the tissue in the mass, Flamm said.

Pearlman uses carbogen (inhaled 95% oxygen plus 5% carbon dioxide) as well as gadolinium, and hybrid T2*, T1 k-space coverage to identify blood supply patterns, including microvascular development.

Flow measurements in shunts within the heart show differences in circulation between left- and right-sided chambers as well as between pulmonary arterial and aortic systems. Quantification of the amount of blood flowing backward through the aortic valve is a characteristic measure of aortic insufficiency. Quantification sequences through the ascending aorta and short-axis cine images through the left ventricle tally the total amount of blood that is pumped out by the ventricle and the amount that enters the aorta, which ascertains the degree of mitral regurgitation, Flamm said.

To determine the ejection fraction as accurately as possible in patients whose hearts may be damaged by chemotherapy, Pearlman obtains contiguous 5-mm slices from the base to the apex of the heart and long-axis slices that correct for partial volume. Using this approach, he has achieved an accuracy within 2% of actual volume in phantom studies.

CONTRAST ADMINISTRATION

Except for assessing heart damage after chemotherapy, many cardiac studies require the administration of contrast. During delayed-enhancement inversion recovery and fast gradient-echo scans, imaging may begin five to 10 minutes after the injection of gadolinium to find bright areas of myocardial infarction or damage, determine the number and volume of infarcts, and decide whether infarcts are chronic or subacute, Klepac said.

In first-pass stress perfusion or cine testing, imaging is done as gadolinium is washing into the myocardium immediately after rapid contrast injection through a peripheral IV, Shah said.

The exact timing of imaging after injection is not as crucial with cardiac viability investigations as with vascular imaging, when scanning must occur during the arterial phase of the arrival of the contrast bolus.

Stress perfusion sequences generally begin with the infusion of gadolinium and run continuously until the contrast washes from the right heart to the left heart and into the left ventricular myocardium, which takes about 40 to 45 seconds, according to Shah.

Scanning 10 to 20 minutes after a double dose of contrast administration reveals scarring, which appears as bright areas because the negatively charged collagen holds onto the contrast agent, Pearlman said.

"If you're not in an acute setting, you are pretty much assessing scar. The rule is that if the scar is less than a third of the wall thickness, you would expect the patient to benefit from revascularization, whereas a wall with scar exceeding two-thirds of the wall thickness is unlikely to improve," he said.

That distinction helps in making clinical decisions about what needs to be fixed and whether a patient should undergo surgery or interventional catheterization.

To accomplish complex studies of viability, perfusion, left ventricular, and valvular function, Flamm injects contrast three times. For myocardial perfusion, he administers two half-doses of gadolinium, one during adenosine stress and one at rest. Viability imaging follows the administration of a third, full dose of gadolinium.

Although all currently available MR contrast agents are interstitial, a number of manufacturers are developing intravascular blood pool or near-blood pool media that leak minimally into the interstitium. The added value from purely intravascular blood-pool agents has not been fully proved, but some evidence suggests that they may be useful for perfusion and vascular imaging.

"There may be a theoretical advantage in seeing blood pool contrast come into the left ventricular capillary and the left ventricular myocardium itself as opposed to watching extravasation into the extracellular space," Shah said.

One of the limitations of current interstitial contrast agents is that they don't accomplish true first-pass perfusion through the myocardium, according to Flamm.

"Because the contrast quickly leaks out into the interstitium, some of the calculations we make to try and quantify the amount of blood flow break down," Flamm said. "But with a blood-pool agent, we may be able to come up with good quantitative measurements of blood flow."