Coronary Arterial Imaging

1

What are some noninvasive methods of coronary artery imaging?

Imaging the coronary arteries is an important part of evaluating ischemic heart disease. Traditionally, this imaging has been performed using cardiac catheterization, an invasive procedure that requires arterial puncture. Newer technologies promise noninvasive, cross-sectional imaging of coronary artery patency and offer the potential to characterize coronary artery plaque. Coronary magnetic resonance imaging (MRI), using bright blood, dark blood, and contrast-enhanced techniques, can image vessel anatomy and vessel patency and determine lipid content in coronary plaques. Coronary computed tomography angiography (CTA) can image vessel anatomy and vessel patency, potentially with a higher spatial resolution than MRI, and measure coronary arterial calcification.

2

What is coronary CTA, and how does it differ from routine chest CT?

Coronary CTA is a contrast-enhanced CT study optimized for visualization of the coronary arteries ( Figure 14-1 ). On a regular CT scan of the chest, the heart appears blurred because of its motion, so coronary CT requires synchronization of image acquisition to the electrocardiogram (ECG) signal. The challenge of imaging the coronary arteries is compounded by the fact that these vessels are very small and move a large amount in every cardiac cycle. The left main coronary artery measures a maximum of 5 mm in diameter, and for much of their course the other arteries are only about 2 mm in diameter; therefore the resolution of the CT scanner must be in the submillimeter range. The right coronary artery can move up to 5 cm in each cardiac cycle—imaging a 2 mm structure that moves 5 cm in each direction every second is challenging. To acquire high-quality images, the following techniques are employed: ECG gating, multidetector image acquisition, fast gantry rotation, and, in some scanners, dual x-ray tubes.

3

How long does a cardiac CT acquisition take?

To acquire an image, the gantry of the CT scanner must rotate 180 degrees around the patient. With a rotation speed of 330 msec, a modern scanner acquires enough data to make an image in 165 msec, which is the temporal resolution. Some scanners have two x-ray tubes, which permit image acquisition using only 90 degrees of gantry rotation, improving the temporal resolution to 83 msec. This temporal resolution, however, is still quite slow compared to digital subtraction angiography (DSA), which has a temporal resolution of 7 to 10 msec, and therefore gating is still required. The total acquisition time will depend on the width of the CT scanner detector; wide detector (e.g., 320 slice) machines can acquire images of the entire heart during a single heartbeat, while smaller (e.g., 64 slice) detector scanners will take 4 to 5 heartbeats.

4

What is gating?

Gating is the process of synchronizing the CT acquisition to the cardiac cycle. ECG leads are placed on the patient and connected to the scanner. Images can be acquired either prospectively or retrospectively. The aim is to leverage the temporal resolution of the scanner to maximal advantage by acquiring data during phases in the cardiac cycle when cardiac motion is least; this is usually in late diastole.

In retrospective gating , the patient is scanned in a continuous helical fashion. The x-ray tube is on throughout the cardiac cycle, although tube current modulation may be used to decrease the radiation dose during phases of the cardiac cycle where motion is most prominent (e.g., systole). Images are then reconstructed from data acquired during specific portions of the cardiac cycle. Advantages include the ability to reconstruct images from any phase of the cardiac cycle, which may help in evaluation of a coronary artery segment that is blurred during the preferred late diastolic phase. In addition, if images are generated from all phases of the cardiac cycle, functional information is available regarding ejection fraction, valve motion, and wall motion. Retrospective gating is, however, accompanied by a relative radiation dose penalty compared with prospective gating of about 2 to 3 : 1.

In prospective gating (also called prospective triggering), the x-ray tube is turned on only during a specific phase of the cardiac cycle, usually in late diastole when cardiac motion is at a minimum. During the remainder of the cycle, the table moves, and the patient is repositioned for the next set of images, which are taken during the same phase of the next heartbeat. Prospective gating exposes the patient to a lower radiation dose than retrospective gating. Strict heart rate control is required for a prospective scan, however, because if there is motion blur on an image, there are no data available from other points in the cardiac cycle to attempt reconstruction at a different phase.

5

Can coronary CT imaging be performed at any heart rate?

To minimize motion and acquire images of diagnostic quality, a slow rate and regular cardiac rhythm are highly desirable. A rate of 60 beats/minute or less is ideal because it allows for effective use of prospective gating, minimizing radiation dose. Administration of oral beta-blockers is a safe method of achieving this heart rate in most patients. Contraindications include severe asthma or congestive obstructive pulmonary disease, heart failure, and heart block. In the setting of acute chest pain, cocaine use is also a contraindication. As technology continues to advance, CT scanners with improved temporal resolution and motion reduction postprocessing have increased the upper heart rate limit for a quality study.

6

What are the indications for CT coronary calcium scoring, and how is it performed?

CT for coronary calcium scoring is appropriate for asymptomatic intermediate coronary artery disease (CAD) risk patients, and for the specific subset of low-risk patients in whom a family history of premature CAD is present. Intermediate risk is defined as a 10-year risk of cardiovascular events between 10% and 20% using Framingham risk factors.

Coronary calcium scoring is performed using a noncontrast, prospectively ECG-gated CT scan of the heart. It is a very safe test because no contrast material is required and radiation doses are negligible (around 1 mSv, compared to an average annual background radiation dose of 3 mSv).

7

What is the diagnostic performance of CT coronary calcium scoring?

Coronary calcium scoring allows for detection and quantification of subclinical coronary atherosclerosis. It has been shown to identify at-risk patients with a higher predictive accuracy than conventional risk factor assessment. Moreover, the Multi-Ethnic Study of Atherosclerosis (MESA) trial of 6809 patients demonstrated that coronary calcium score was the most accurate tool for predicting long-term cardiac events. Compared to patients with no coronary calcium, hard event rates in patients with scores from 101 to 300 were increased 7.73-fold, and 9.67-fold among those with a score above 300. Three other large trials have confirmed the benefits of imaging subclinical coronary atherosclerosis for improved risk prediction, including in elderly patients.

8

What are the indications, contraindications, typical scanning protocols, advantages, and disadvantages for coronary CTA versus magnetic resonance angiography (MRA)? Which patients can benefit from coronary CTA?

The largest group of referrals for coronary CTA is patients with atypical or low-risk chest pain. In these patients, a negative study prevents a chain of cardiac investigations such as stress tests and cardiac catheterization. These patients may present to the emergency department or may be scanned on an outpatient basis. Chest pain patients with a high risk of acute coronary syndrome are not typically evaluated by coronary CTA because the probability of requiring intervention is high enough to justify catheterization in many cases. Evaluation for aberrant coronary artery origin and course is also a good indication for coronary CTA, as is further evaluation of a patient with an equivocal stress test. In addition, evaluation of coronary bypass graft patency is a common indication. General contraindications for CTA are allergy to iodinated contrast media, renal insufficiency, and pregnancy; specific contraindications to coronary CTA would also include dysrhythmia and tachycardia, particularly if the patient is unable to receive beta-blockers because of a history of asthma or recent cocaine use.

The advantages of coronary CTA over coronary MRA include lower cost, ability to identify calcified versus noncalcified plaque, and higher spatial resolution which can be obtained in isotropic fashion and a much shorter scan time. The main disadvantages are the use of ionizing radiation and iodinated contrast material. A typical protocol for coronary CTA utilizes prospective ECG triggering during a dual-phase injection of 60 to 100 ml of iodinated contrast.

The most common indication for coronary MRA is for evaluation of coronary artery anomalies. The utility of coronary MRA in assessing coronary artery stenosis is limited. Although software techniques and hardware have substantially changed over the years, MRI systems have not been able to match results from the various generations of CT systems, except in cases with high coronary calcium score in which coronary CTA may be of limited use. On the other hand, coronary MRA has the advantage of no exposure to ionizing radiation or potentially nephrotoxic contrast agents. Contraindications to MRA include claustrophobia and presence of implanted medical devices such as pacemakers. Typically, coronary MRA is obtained during free breathing using respiratory navigator gating. Coronary MRA at 1.5 T is generally performed without gadolinium-based contrast material using a steady state free precession technique, while 3T protocols typically use a nonbalanced gradient echo technique after contrast administration.