No premedication equals fewer staff, more patients

Coronary CT angiography is commonly performed using beta blockers to slow the heart rate and reduce motion artifacts in the images, thereby improving diagnostic accuracy. Oral metoprolol is often prescribed one hour prior to the procedure. Intravenous metoprolol may be given immediately prior to the procedure as an alternative or if oral beta blocker alone did not achieve adequate heart rate control.

Despite premedication, not all patients achieve a target heart rate of 60 bpm or less. Moreover, not all patients can safely be given beta blockers. Contraindications include severe congestive heart failure, hypotension, heart block, bronchospastic disease, pheochromcytoma, and known sensitivity to beta blockers. While not all patients require heart rate control, many do, and this requires the monitoring and assistance of a nurse, decreasing CT scanner throughput.

The advent of dual-source CT technology eliminates the need for beta blockers, simplifying scheduling while improving patient throughput and comfort.

Recent publications have shown diagnostic accuracy of DSCT to be similar to that of single-source 64-slice CT but without the need for heart rate control. While arrhythmia may still be a problem, it tends to be less severe on DSCT than with single-source technology. For these reasons, we have used beta blockers only rarely since the installation of a DSCT system, reserving their use for obese patients with heart rates greater than 60 bpm.

CLINICAL USE

The dual-source scanner is located in the Emory Clinic, an outpatient facility physically connected to the 576-bed Emory Hospital. Most of the patients currently scanned for CTA have atypical chest pain. We will shortly begin imaging chest pain patients from the emergency room.

Normally, only a half-rotation scan is used to reconstruct coronary CTA. This provides a temporal resolution of 83 msec on a dual-source CT. It is possible, however, to use the full rotation for image reconstruction, gaining a theoretical 40% improvement in image noise without additional radiation exposure. This is our protocol for obese patients. The penalty is temporal resolution, which then becomes 165 msec and requires the use of beta blockers (Figure 2).

In addition to coronary CTA, DSCT is advantageous for coronary artery calcium screening. Prospective ECG-trigger sequential imaging is normally recommended for calcium screening to minimize radiation exposure. With spiral retrospective gating, it is possible to minimize partial volume effects by using overlapping slices at the expense of greater radiation dose. That dose penalty can be minimized, however, through the use of ECG-pulse modulation. Normally, systolic phases have 20% of the tube current to reduce the dose and permit low-quality image reconstruction if necessary during systole. Radiation dose approaching prospective ECG-triggered sequential imaging is achievable by turning the current off during systole and choosing a narrow reconstruction window with increased current. This does not permit systolic reconstruction but does allow for some flexibility in the reconstruction time.

It is uncommon to use beta blockers for coronary artery calcium screening, even on single-source MSCT scanners. Motion artifact is frequently seen in prospective ECG-triggered sequential images. This leads to errors in the coronary calcium score. Through-plane motion tends to result in lower scores, whereas in-plane motion increases the score by spreading the calcium over a larger area. The time in cardiac cycle for the image is fixed at the time of acquisition with prospective ECG-triggered sequential images, whereas it is freely selectable in the spiral retrospectively gated mode.

It is possible to obtain an estimate of the best phase by reconstructing a series of low-resolution images in 2% RR interval increments and assigning a motion score to each phase. Software to do this automatically is available on the DSCT scanner. In this way, motion artifacts may be minimized. Because the images are obtained with spiral technique, partial volume errors are also reduced. Consequently, DSCT permits more accurate coronary artery calcium scores than can be achieved on single-source MSCT with comparable dose.

Dr. Stillman is director of the division of cardiothoracic imaging in the radiology department at Emory University Hospital.