Published in UAB Insight, Spring 2007

Fast, Accurate System Revolutionizes Noninvasive Cardiac Imaging

Heart disease is the leading killer of both women and men in the United States, accounting for up to a third of all deaths each year. The financial burden also is high. In 2006 costs exceeded $140 billion, including health care, medications, and lost productivity, according to the Centers for Disease Control and Prevention. Conventional (catheter-based) coronary angiography remains the gold standard for investigation of suspected ischemic heart disease, but rapidly evolving, less invasive diagnostic imaging techniques are changing the landscape of prevention and detection and hold potential to reduce morbidity, mortality, and expense.

With continuing improvements in image detail (spatial resolution) and acquisition speed (temporal resolution) multidetector computed tomography (MDCT) is increasingly able to provide clinically useful information about the heart and its blood vessels.

UAB¡¯s Cardiac CT Program is a joint effort between the Divisions of Cardiovascular Disease and Cardiopulmonary Radiology. The cardiac CT team is headed by radiologists Satinder P. Singh, MD, and Hrudaya Nath, MD, and cardiologists Steven G. Lloyd, MD, PhD, and Himanshu Gupta, MD.

Although there are many noninvasive methods of studying the heart, MDCT offers distinct advantages. Speed is a major benefit. A 64-slice CT scan lasts 8 to 12 seconds and the entire procedure is over in 10 to 12 minutes. Other noninvasive imaging tests, including magnetic resonance imaging (MRI) and single photon emission computed tomography (SPECT), can take considerably longer. CT also avoids hardware-related contraindications of MRI, an important concern because many cardiac disease patients have cardiac pacemakers and other implanted devices.

In one breath hold, MDCT produces motion¨Cfree high-resolution images of the entire coronary tree. Once images are acquired, computer postprocessing can produce volume-rendered three- dimensional views that reveal structural contours and spatial relationships. In addition, dynamic four-dimensional reconstructions created with data from different phases of cardiac contraction can assess ventricular function, wall motion, and ejection fraction.

Requirements of Cardiac CT
High temporal resolution is crucial to limit cardiac motion artifacts produced by the beating heart. Conventional angiography captures an image in about 10 ms while 64-slice CT acquires each image in 150 to 250 ms, depending on the patient¡¯s heart rate. At least 150 ms temporal resolution is required for good visualization of small cardiac structures such as the coronary arteries ¡ª this is consistently achievable with 64-slice CT. The technology is constantly improving. In the next 5 years temporal resolutions of 50 ms may be possible.

Respiratory motion artifacts are no longer a major issue with cardiac CT. The 16-slice CT scanners reduced imaging time to 25 to 40 seconds, which is a reasonable period for healthy people to hold their breath, although it is not always feasible for elderly patients or those who have respiratory or heart disease. The 64-slice CT brings breath-hold time down to 8 to 10 seconds, which is possible for most patients.

To reduce cardiac motion artifacts, crucial for creating high-quality images of small arteries, acquisition of CT image slices must be synchronized with patients¡¯ electrocardiograms (ECG). The heart typically moves least in mid-to-late diastole. As the heart is filling, it is almost at a standstill. Three-dimensional reconstructions require all slices be obtained at the same point in the cardiac cycle during consecutive heartbeats, which is achieved using retrospective ECG gating of the CT scan images.

Slower heart rates prolong diastole and the period of minimal cardiac motion. A heart rate of 60 bpm or lower is ideal for capturing high-quality images. To reduce heart rate to desired levels, most patients require oral beta-blockers 1 to 2 days before the procedure and, if necessary, intravenously during the scan. If beta-blockers are contraindicated because of asthma or chronic obstructive pulmonary disease, the cardiac CT team can use alternative medications such as calcium channel blockers. Sublingual nitroglycerine also may be used to dilate vessels for improved visualization. In patients with faster heart rates, recently introduced dual-source CT scanners can produce images with equivalent quality to 64-slice MDCT.

MDCT protocols require the use of iodinated contrast media, which is associated with nephrotoxicity, and patients with renal insufficiency or renal failure must be carefully evaluated prior to CT procedures. Pacemakers with multiple leads also may produce artifacts that compromise imaging of coronary arteries, although this is not a major contraindication when the interpreting cardiologist or radiologist has appropriate information before the procedure. Extensive coronary artery calcification also can produce artifacts that may interfere with accurate depiction of luminal narrowing.

High spatial resolution is necessary to visualize small cardiac structures. Coronary arteries range from 3 to 4 mm at their origins to less than 1 mm distally. Spatial resolution of 64-slice CT is now in the submillimeter range, 0.4 x 0.4 mm, but CT cannot yet approach the image detail of invasive angiography, which produces resolutions of 0.2 x 0.2 mm.

Clinical Applications
Cardiac CT has a range of cardiovascular indications, which will likely expand as the technology¡¯s sensitivity and specificity increase and data from clinical trials provide evidence of efficacy.

Cardiac CT does not replace other imaging techniques, but in many cases, it can provide valuable complementary information. This is particularly true of coronary CT angiography (CTA). CTA can evaluate coronary artery stenosis and occlusion of vessels ¡Ý1.5 mm; the origin and course of anomalous coronary arteries; graft patency; complications of coronary artery stents; and can accurately exclude suspected coronary artery disease (CAD) in patients with atypical symptoms who are at intermediate risk.

A recent study found coronary CTA compares favorably with invasive angiography for detecting obstructive CAD. Of 67 participants who underwent both invasive angiography and 64-slice CTA, all vessels ¡Ý1.5 mm were adequately seen by MDCT. Invasive angiography detected 176 vessels with >50% stenosis while MDCT correctly identified 165, resulting in sensitivity, specificity, and positive and negative predictive values of 94%, 97%, 87%, and 99%, respectively (European Heart J. 2005;26:1451-1453).

Raff et al report that compared with invasive angiography for detection of stenosis >50%, segment¨Cbased sensitivity, specificity, and positive and negative predictive values of the 64-slice CT were 92%, 91%, 80%, and 97%, respectively (J Am Coll Cardiol. 2005;46:852-857).

Of the 2 million cardiac catheterizations performed annually in the United States, two thirds are for diagnosis alone. Cardiac CT does not add value for those already diagnosed with CAD. However, for undiagnosed individuals at intermediate risk with atypical chest pain, a questionable or normal SPECT, or a strong family history of heart disease, CTA provides a good assessment of the coronary arteries that can allow many patients to avoid invasive angiography.

CTA also can distinguish among causes of acute chest pain in patients who present to emergency departments (ED). A recent study found that in 103 ED patients with normal blood tests and ECGs, 64-slice CT accurately ruled out acute coronary syndrome (ACS). CT revealed the presence or absence of atherosclerotic plaque or significant stenosis. An expert panel, blinded to CT results, then determined if patients had ACS based on hospitalization data and results from 5-month follow-up. Clinicians ultimately diagnosed ACS in 14 of 103 participants. ACS is unlikely in the absence of plaque, and study authors noted patients without such plaque could be safely discharged based on CT findings alone. When plaque is present, however, CT results must be interpreted in the context of other clinical indicators before patients can be safely dismissed from the hospital (Circulation. 2006;114:2251-2260).

Each year, millions of people who present with acute chest pain are hospitalized with an undetermined diagnosis. CT is an important tool to evaluate patients who may be able to avoid the expense and discomfort of hospitalization and additional tests.

MDCT can evaluate coronary artery plaque burden. The presence of calcified plaque is an important predictor of chronic atherosclerotic changes and future coronary event risk. CT also can measure soft, noncalcified plaque, the type most vulnerable to rupture. Intravascular ultrasound accurately detects noncalcified plaque, but the test is invasive. If a patient is a smoker, hyperlipidemic, hypertensive, or has other risk factors, then evaluating calcified and noncalcified plaque burden has important predictive value. For these patients, CT is likely to play an increasingly important role in quantifying the risk of major adverse cardiac events.

Limitations
Although MDCT is less invasive than angiography, it is not without risk. Iodine-based intravenous contrast agents are used to demonstrate cardiac anatomy, especially small structures such as the lumen of the coronary arteries. Radiation exposure is another consideration. MDCT delivers two to three times more radiation than conventional angiography (10 to 15 mSv versus 3 to 7 mSv). ECG dose modulation can reduce CTA radiation exposure up to 40%, depending on the patient¡¯s baseline heart rate. The comparable radiation dose for myocardial SPECT is 20 to 22 mSv.

UAB¡¯s cardiac CT team is using MDCT for evaluation of coronary artery diseases, anomalous coronary arteries, bypass grafts, coronary stent restenosis, valvular heart disease, cardiac masses, and congenital heart disease, as well as for coronary venous and pulmonary venous mapping before ablation procedures. UAB is recruiting patients for several studies focusing on the role of cardiac CT, particularly CTA.

For more information:
Dr. Satinder Singh
Dr. Himanshu Gupta
Dr. Steven Lloyd
Dr. Hrudaya Nath
1.800.UAB.MIST
mist@uabmc.edu