CT techniques overcome cardiac motion artifact

November 1, 2004



Although the heart and its vasculature dominate the center of a conventional chest CT scan, they have for the most part been ignored by radiologists. They tend to leave the diagnosis of cardiovascular disease to cardiologists skilled in viewing the beating heart on echocardiograms or evaluating nuclear medicine physiology studies. Faced with motion artifact on CT that blurs tissue edges, radiologists have lacked confidence to make definitive diagnoses of coronary artery stenosis or global cardiac disease.

But as multidetector technology produces sharper images, radiologists are beginning to recognize CT's potential for generating maps of the pulmonary veins, assessing the patency of stents and coronary artery bypass grafts, ruling out pulmonary embolus, tracing collateral vessel development, and even looking at wall motion defects.

Learning how to obtain high-quality images is the final hurdle. As far back as 2001, a paper published in Circulation concluded that multidetector spiral CT found high-grade coronary artery stenosis and occlusion with 91% sensitivity and 84% specificity, as long as image quality was high enough. But radiologists from the University of Erlangen-Nuremberg in Germany could evaluate only 68% of a total of 256 coronary arteries from 64 patients, and the most common reason the others could not be evaluated was the intrusion of image artifacts due to coronary motion (Circulation 2002;103;2535-2538).

The trick to imaging the heart and the great vessels is, of course, to stop cardiac motion, but not at the cost of sacrificing spatial resolution. Despite advances in electron-beam technology that acquires data so rapidly it stops motion, just as a fast camera shutter speed stops motion in a photograph, electron-beam scanners are not the answer. The technique for generating x-ray photos in an electron-beam scanner does not permit the kind of photon flux needed to acquire sharp, reliable imagery. Electron-beam scanners compensate by obtaining thicker slices, which improves the signal-to-noise ratio but reduces spatial resolution, said Dr. Lawrence Boxt, chief of cardiovascular imaging at Beth Israel Medical Center in New York City.

With MSCT scanners, the speed of orbiting governs temporal resolution. The newest generation multidetector machines have rotation times of 330 msec. Since radiologists need essentially 180degrees of data, a half-cycle tomographic image may take 165 msec, which approaches the 100-msec cutoff point for eliminating motion in the arteries, said Dr. Daniel S. Berman, director of cardiac imaging at Cedars Sinai Medical Center in Los Angeles.

Nevertheless, MSCT scanners cannot acquire data as rapidly as electron-beam machines. Even prototype 32- and 64-detector scanners still take five seconds to acquire images. When imaging a patient with a resting heart rate of 60 beats per minute, the heart will beat five times in the space of five seconds, producing motion artifact. Radiologists therefore need to add electrocardiograph gating to tie projection data gathered as the scanner revolves around the patient to ECG signals from the patient's heart, according to Boxt.

"We know the projection is acquired at a particular point in space and time. Software can shuffle all these data cards and put them in temporal as well as spatial order, so all projections from one axial slice are acquired at the same time point, thus stopping cardiac motion," he said.

Obtaining a clear ECG tracing means taking care to properly place ECG leads on a patient's chest, exactly as leads are positioned prior to MR scanning of the heart or an ECG. Preparation includes making sure the chest surface is clean, body hair has been shaved, and alcohol has removed body oil that might interfere with contact between the leads and the skin.

It may take several reconstructions in the R-R window to choose data from the best phase of the cardiac cycle when imaging coronary arteries in individual patients, said Dr. Suhny Abbara, director of cardiovascular imaging and intervention at Massachusetts General Hospital. Reconstruction at about 70% of the R-R interval often produces optimal scans when the patient's heart rate is 65 bpm. Because the right coronary artery has a high degree of motion, imaging may be done in two sweeps: one in diastole and another at 25% to 30% of the R-R interval. If the right coronary artery is not well defined in images, the data can be reconstructed at 25%, or around 250 msec after the last R peak.

Slowing the heart rate is central to obtaining homogeneous CT scans of the heart, Boxt said. A clinical trial at Beth Israel is designed to investigate whether a threshold exists beyond which the heart rate is too fast to attempt CT imaging or if there is a range of heart rates that can produce decent-quality images for evaluating particular coronary clinical questions.

Slowing the heart rate to 50 or 60 bpm will yield diagnostic-quality pictures every time, according to Boxt. Reaching this rate requires intravenous administration of a beta blocker, such as 5 mg metropolol, in the exam room or oral doses of 100 mg metropolol the evening before and morning of the scan. While beta blockers are generally safe drugs, they are contraindicated in patients with asthma or other bronchial conditions because they cause bronchoconstriction.

CONTRAST ADMINISTRATION

Contrast material administered by bolus tracks the route it takes through the heart into the lungs and back out the aorta for flow studies in the vasculature or patency within stents and bypass grafts. Continuous infusion of contrast keeps the ventricles of the heart opacified during contraction, illuminates all branches of the pulmonary arteries, and identifies aneurysms and dissections in the aorta.

Because a CT coronary arteriogram is essentially a fancy CT aortogram, the timing and bolusing of contrast administration are the same for examining the coronary arteries or ruling out an aortic dissection, Boxt said. A contrast dose of 125 mL delivered at a rate of 5 mL/sec is usually given over a short timing run. Imaging after contrast administration needs to be carefully timed to be sure all contrast material has navigated through the superior vena cava. It also must account for changes in resistance to venous return in the chest due to breath-holding.

When a patient takes a deep breath, resistance to venous return to the chest is increased, Boxt said. Rather than moving from the arm to the chest and the aorta, contrast moves into the neck, where it meets less venous resistance. Imaging too soon after contrast administration reduces the quality of arterial studies.

Radiologists imaging small structures in the heart need to predict the arrival of contrast accurately. Circulation time can be calculated by measuring the time it takes for a diluted solution of magnesium sulfate passing from the antecubital vein in the arm to cause a sensation of heat at the base of the patient's tongue, said Dr. William Stanford, a professor of radiology at the University of Iowa School of Medicine. Another option is to track the length of time needed to detect cardiogreen dye in the earlobe with a densitometer.

To be sure contrast remains in the heart throughout a functional study, the contrast circulation time may be estimated at about 12 seconds. Assuming a transit time of 12 seconds for the leading edge of contrast to reach the ventricle and another four or five seconds for contrast to fill the heart, imaging would start at the circulation time plus 10 seconds, Stanford said. In flow studies, imaging would begin at the circulation time minus six seconds to assess a blood vessel at the time of arrival and washout of the bolus.

The transit time for contrast to pass from the injection site in the antecubital fossa to the aortic route can be used to determine the amount of contrast to administer for angiography acquisitions, Abbara said. Multiplying the injection rate by the length of the scan allows Abbara to tailor bolus administration for peak opacification during scanning. The contrast injection rate is typically 4 or 5 mL/sec, and a scan of the coronary arteries takes about 20 seconds. For a patient receiving 4 mL of contrast for a 19-second scan, the dose of contrast bolus totals 76 mL.

In general, cardiac CT imaging requires higher concentrations of contrast to assure a sufficient amount of material in the coronary arteries and within the heart. To evaluate cardiac function, contrast is often infused at a concentration of 76% iodine at a dose of 75 to 100 mL and a rate of 2.3 to 2.4 mL/sec. During flow studies, particularly in CT angiography involving 3D reconstructions of bypass grafts and vessels off the heart, contrast at a concentration of 76% iodine may be given at a dose of 40 mL and a more rapid rate of 10 mL/sec.

One of the advantages of obtaining extra slices with MSCT scanners is the brevity of scanning, which reduces the amount of time for maintaining peak opacification in the arteries and consequently the dose of contrast, according to Berman.

Images of the aorta and pulmonary arteries do not have to be exceptionally bright, so contrast concentrations can be reduced to 300 mg, Stanford said.

CLINICAL APPLICATIONS

MSCT is finding increasing use in cardiac and chest imaging.

- Coronary artery stenosis. Rather than looking at surrogate measures of stenosis such as calcium, MSCT provides views of the artery itself, enabling radiologists to identify patients with stenotic lesions suitable for treatment with a stent or bypass graft. A developing body of literature indicates that CT coronary arteriography is a better negative than positive predictor of stenosis. Possible stenosis shown on a CT coronary arteriogram has only a 70% chance of being proved on gold standard invasive coronary angiography. This is better than a coin flip but not that reliable, Boxt said.

On the other hand, when CT coronary arteriography is negative, the chance that a real stenosis is present falls to 5%. Consequently, the procedure is being used primarily to exclude patients from the cath lab, such as those with chest pain syndrome but no CT evidence of occlusion or severe stenosis.

"It's an economic argument," Boxt said. "Cath labs are running at very low margins, so diagnostic coronary arteriograms, which are reimbursed pitifully, aren't worth taking up time in the cath lab. As a negative predictor, CT keeps patients who most likely don't need intervention out of the cath lab, so it can concentrate on doing as many interventions as possible."

Presentations at the 2003 Society of Computed Body Tomography and Magnetic Resonance meeting found that CT imaging of the coronaries can be done in a single breath-hold acquisition following a bolus injection of 120 to 150 mL contrast at 3.5 to 4 mL/sec. The field-of-view is small, 26 to 30 cm, and slices are 1 mm thick. Total acquisition time ranges from 45 seconds with four-slice scanners to 20 seconds with 16-slice scanners.

- Pulmonary vein mapping. Before attempting pulmonary vein ablation to produce a scar around the ostium that will interrupt misfiring myocardial cells and prevent atrial fibrillation, interventional cardiologists need a road map through complex venous anatomy. Although some radiologists obtain gated CT scans, others, such as Abbara, prefer gated studies that reverse the direction of standard coronary artery imaging.

While coronary artery CT scanning takes a top to bottom approach, pulmonary vein mapping runs from the bottom up, so less artifact-producing contrast will appear in the superior vena cava. Coronary artery scanning starts at the level of the main pulmonary artery just below the carina, but imaging of the pulmonary vein tends to be positioned higher in the chest-images cut through the aortic knob and include more of the pulmonary vein. Reconstruction for the pulmonary vein is usually done in systole, because motion is more consistent in patients with atrial fibrillation, Abbara said.

- Bypass graft patency. CT is an excellent tool for noninvasively determining whether blood is flowing freely through bypass grafts. The field-of-view for this purpose is larger than usual, extending to the aortic knob to cover the full length of bypass grafts, which often are inserted high on the axillary aorta. A timing bolus acquisition at the top of the intended scan area can provide a preview image. If the bypass graft is not in that image, the field-of-view needs to be higher, Abbara said.

- Pulmonary embolization. Timing of contrast is critical in evaluating patients with possible pulmonary emboli to assure that contrast is within the pulmonary arteries at scan acquisition. A region of interest is placed over the pulmonary artery, and when the contrast arrives, it automatically triggers the scan during a breath-hold. Images may be 1.5-, 2-, or 3-mm thick to see all the small branches of the pulmonary artery.

- Congenital heart disease. Because patients with congenital heart disease frequently have findings outside the heart, such as large collateral vessels descending from the subclavian artery, a cardiac-gated CT scan covers the entire chest, including the supra-aortic vessels, the aortic inlet, and the bottom of the heart. Such a large scan extends imaging time, prolongs the breath-hold period, and requires additional contrast material. For patients who cannot hold their breath long enough, collimation may be adjusted from 0.75 mm to 1.5 mm, which makes the scan twice as fast and cuts breath-hold time from 36 to 18 seconds.

FILLING DIAGNOSTIC GAPS

CT is emerging as a technique for evaluating problematic populations of patients. A negative CT exam in 40- or 50-year-old individuals with atypical chest pain but no elevations in cardiac enzymes, ECG changes, or wall motion abnormalities on echocardiography provides at least some objective evidence that they do not have ischemic heart disease and can be managed conservatively, Boxt said.

A CT arteriogram in individuals who have a history of cardiac disease and unusual signs and symptoms but indeterminate nuclear studies or abnormalities difficult to evaluate on echocardiography, such as an apical left ventricular anomaly, can explicitly distinguish a nuclear or echo artifact from actual stenosis, he said.

With a little more experience, radiologists should be able to characterize fatty from nonfatty plaques on vessel walls, identify ischemic zones of hypoperfusion in the myocardium, produce 3D cine movies of wall-motion defects, and turn to CT in apparent cardiac emergencies.

"The three major life-threatening diagnoses of chest discomfort that need to be ruled out in the emergency room are pulmonary embolization, aortic dissection, and acute coronary syndrome. It's possible that CT could be the one-stop shop for making that triple rule-out," Berman said.