Beta blockers prove optional with dual-source scanners

November 1, 2007
U. Joseph Schoepf, MD
U. Joseph Schoepf, MD

During my first eight years of performing and interpreting cardiac multislice CT, up to 2006, our mantra was always that slower heart rates improve diagnostic image quality. Aside from image quality, slow heart rates also traditionally translated into lower patient radiation exposure. Our most elegant tool for radiation protection at cardiac CT was ECG-dependent tube current modulation, which applies the full nominal tube output only during diastole. This tool, unfortunately, has been limited for the longest time to patients with slow and steady heart rates.

Note: We've asked three leading cardiac imagers to detail how the dual-source CT scanner has changed the way they perform cardiac imaging. With the fastest temporal resolution out there, the dual-source scanner obviates the need for beta blockers. This has enormous workflow implications. As you'll read in the following articles, the benefits to radiology departments and imaging facilities are significant.-Ed.

During my first eight years of performing and interpreting cardiac multislice CT, up to 2006, our mantra was always that slower heart rates improve diagnostic image quality. Aside from image quality, slow heart rates also traditionally translated into lower patient radiation exposure. Our most elegant tool for radiation protection at cardiac CT was ECG-dependent tube current modulation, which applies the full nominal tube output only during diastole. This tool, unfortunately, has been limited for the longest time to patients with slow and steady heart rates.

At faster heart rates, the optimal time point for image reconstruction becomes more difficult to predict, so that use of ECG pulsing was not advisable with heart rates greater than 70 bpm. The reason was the need to maintain full flexibility for finding the optimal image reconstruction sweet spot with the least cardiac motion. With faster heart rates, particularly, this sweet spot is often found in late systole. Accordingly, to improve image quality and keep radiation low, we religiously practiced and preached the administration of beta blockers or other rate-controlling drugs for use with cardiac MSCT.

PROFOUND CHANGES

The game changed profoundly with the advent of dual-source CT (Somatom Definition, Siemens Medical Solutions). Current DSCT scanners still rotate with the same speed (i.e., 330 msec) as the last generation of single-source 64-slice scanners. The two-source/detector combinations, however, simultaneously collect a data set of 90 degrees each during one heartbeat, resulting in a temporal resolution of 83 msec compared with 165 msec for single-source scanners.

One of my first patients to have a dual-source coronary CT angiogram had a steady heart rate in the low 60s. As soon as the contrast material started flowing, however, his heart rate suddenly and unexpectedly accelerated to 140 bpm, where it stayed throughout the scan. After all reconstructions were performed, the images turned out to be completely diagnostic and esthetically appealing. We have not used beta blockers with our dual-source CT scanner since.

Evidently, some centers still advocate the use of beta blockers despite the virtues of DSCT. They may have a point. In a vanishingly small percentage of patients with extremely high and irregular heart rates, it may be possible to obtain prettier images with the use of rate controlling agents. To this date, however, I have not seen a DSCT study that would have been nondiagnostic because of high heart rates (Figure 1).

Advocates of using beta blockers with DSCT forget a small but ever more important detail: With DSCT, the relationship between heart rate and patient radiation exposure is reversed, compared with traditional single-source scanners. Because of the heart rate adaptive table speed with DSCT, scan times are significantly shorter in patients with faster heart rates as compared with beta blocker-induced slow heart rates, with very substantial savings in patient radiation dose.

Our DSCT scanner at the outpatient center for the Medical University of South Carolina is used to perform the full spectrum of CT studies, including three to eight elective coronary CTA exams each day. We plan to install a second DSCT unit in our emergency department, where we currently operate a single-source 64-slice CT scanner, for general trauma and medical emergency work as well as for an increasing volume of ECG-gated triple rule-out scans in patients with acute chest pain.

From a workflow perspective, the logistical advantages of abandoning beta blockers vastly outweigh the minor incremental improvement in image quality that may be achievable with rate control in a very small subset of patients. Before DSCT, all our cardiac CT scans had to be scheduled around the availability of our limited number of radiology nurses. We administered intravenous beta blockers with the patient on the scanner table, so that our cardiac CT slots were scheduled for 30 minutes, as opposed to our routine 20-minute slots, to allow for the time needed to bring down the heart rate. With the clinical implementation of dual-source CT, cardiac CT has become a regular, routine exam with no scheduling constraints.

With the advent of dual-source CT as the most high-tech instrument to date, we are well on our way to helping radiologists realize the simple truth that CT of the heart is just another scan-nothing special, nothing mystical, nothing scary. The way it always was. The way it should be.

Suggested reading

Schoepf UJ, Zwerner PL, Savino G, et al. How I do it: coronary CT angiography. Radiology 2007;244:48-63.

Dr. Schoepf is the director of CT R&D at Medical University of South Carolina in Charleston.