Prospectively gated CTA dramatically reduces dose

By reducing radiation exposure while maintaining image quality, technique could make coronary CTA viable as screening tool

Initial clinical studies of coronary angiography with 64-slice CT show that it is even more accurate than 16-slice CCTA. Sensitivity, specificity, and positive and negative predictive values are all excellent when compared with coronary catheterization in detecting 50% or greater stenoses.1,2 With the help of a prospectively gated technique, 64-slice systems may soon also dramatically reduce radiation dose incurred during CCTA.

Like its predecessors, 64-slice CCTA involves a radiation dose of 7 to 20 mSv, even after the routine use of ECG-driven tube current modulation. The dose may exceed this range if the exam performed requires sizable coverage in the z-axis such as bypass graft or triple rule-out cases. Although the radiation exposure associated with CCTA is comparable to or even less than nuclear studies,3 the ALARA principle suggests that CT scanner manufacturers and clinicians continuously seek ways to reduce radiation exposure with CCTA.

Most published CCTA studies, especially those involving patients with low heart rates, report that a limited reconstruction phase range can be used for diagnosis. This observation is the design principle behind ECG-driven tube current modulation as well. Prospectively gated CCTA, which dramatically reduces radiation exposure with no compromise in image quality or diagnostic accuracy, is soon to be a clinical reality. Our preliminary experience with the technique demonstrates up to a 70% reduction in radiation exposure for patients with heart rates below 65 bpm.

GATING TECHNIQUES

Multislice CCTA has always been performed using retrospective gating, which occurs when the CT scanner collects anatomical data continuously and an ECG monitor separately collects temporal data such as cardiac rhythm. It is only after the scan is complete that the reconstruction engine generates images for a selected set of cardiac phases, usually defined as the R-R interval, from the anatomical scan data using the temporal data. The reconstruction phases are selected so that they will produce images with the best temporal and spatial resolution possible. Because this is performed retrospectively, the x-ray beam is on continuously throughout the scan, resulting in deliberate oversampling of the tissue. This in turn results in an increase in radiation exposure.

Prospective gating has been used for years to produce coronary artery calcium scores. During the exam, we are not attempting to obtain images of flowing intravascular contrast, and therefore there are no timing constraints. We simply acquire as many sequential step-and-shoot scans as needed to cover the heart. The lesions in question, coronary artery calcifications, are stationary in the wall of the coronary arteries.

Recent advances in CT technology have made prospectively gated CCTA possible:

- 4-cm detector coverage with a 64 x 0.625-mm slice profile to enable covering the whole heart with thin slices in three or four scans;

- a sufficiently powerful x-ray tube with faster gantry speeds to maintain image noise at acceptable levels; and

- fast table and x-ray tube control to adapt the scan acquisition to real-time ECG information and enable completion of the exam while maintaining peak arterial contrast enhancement.

In addition, advanced conebeam reconstruction algorithms generate consistently high quality images in the axial scanning mode.

With prospective gating, the temporal information gathered by the ECG monitor is used in real-time to turn the x-ray beam on and off during the user-selected phases in the cardiac cycle and to quickly move the patient to the next scanning position. Radiation exposure is markedly diminished as there is no oversampling of the tissue (Figure 1).

Phantom studies suggest that up to a 70% reduction in radiation exposure is possible without any compromise in image quality. One drawback of the technique is that images are acquired during a limited, preselected phase range. If patients have a high or irregular heart rhythm, the best time to visualize their coronary arteries may be during a different phase in the R-R interval. These patients need to be identified before the exam and instead have a retrospectively gated CCTA.

HANDS-ON EXPERIENCE

Our study involved 31 male patients (women were not included to avoid x-ray exposure to breasts), aged 38 to 60. They were scanned using prospective gating followed by retrospective gating. Patients provided their informed consent of the additional x-ray exposure, which was approved by an outside institutional review board. Each patient received 100 mg of metoprolol orally one hour before the study with a goal of achieving a heart rate of < 65 bpm. Patients' heart rates during scans ranged from 38 bpm to 67 bpm, with an average of 54 bpm. Heart rate variations during scanning ranged from 0 bpm to 9 bpm with an average of 3 bpm.

Technical parameters for the techniques are listed in Table 1. The major differences exist in the phases of the cardiac cycle during which patients were radiated, and slight differences arose in the mAs used during the scans. Contrast administration among patients differed only slightly, with an additional 10 cc used during prospectively gated CCTA (Table 2).

All patients were scanned using a GE LightSpeed VCT 64-channel scanner; contrast was injected using a Medrad Stellant injector. Images of the prospectively gated CCTA exams were acquired using GE's SnapShot Cine option and were compared with retrospectively gated CCTA images to verify diagnostic accuracy.

About half of the patients required scanning at three locations and over five heartbeats to cover the entire heart; the other half required four locations and seven heartbeats. Total scan times varied from 4.5 seconds to 10 seconds with an average of 6.7 seconds. Diagnostic images of all 15 American Heart Association coronary artery segments were obtained using both gating techniques in all patients.

The retrospectively gated CCTA images were processed and interpreted immediately. Prospectively gated CCTA images were reconstructed and given a segment-by-segment rating.

A comparison of image quality and diagnostic confidence was performed by the same radiologist to ensure consistency.

A few initial cases showed a slight increase in image noise relative to the helical counterpart, prompting the addition of 50 mAs to the subsequent prospectively gated studies to equalize noise measurements. The expert radiologist reviewing the cases rated the image quality as excellent, with pathology considered identical using the two separate gating techniques. No pathology was missed when using the same or the higher mA setting (Figures 2 through 5). Most important, x-ray exposure was reduced by 50% to 70% using prospective gating compared with retrospective gating with ECG-driven tube current modulation.

Additional improvements are anticipated in the short term once the technique is perfected, possibly using an overlay of different filters resulting in improved x-ray beam homogeneity.

Prospectively gated CCTA dramatically decreases radiation exposure. In patients with heart rates < 65 bpm, it can routinely replace retrospectively gated CCTA. This level of heart rate control is easily achieved using one dose of oral beta blockers.

With the reduced radiation exposure made possible with prospective gating, combined with short-term optimization of the technique, we may be approaching dose levels that could make CCTA a viable screening tool.

Dr. Dowe is chief operating officer and medical director of Atlantic Medical Imaging in Galloway, NJ. He is a consultant to GE Healthcare and MedRad and has also received grants and research support from both of these companies as well as Bracco.

References

1. Leschka S, Alkadhi H, Plass A, et al. Accuracy of MSCT coronary angiography with 64-slice technology: first experience. Eur Heart J 2005;26(15):1482-1487.

2. Mollett NR, Cademartiri F, van Miegham CA, et al. High resolution spiral computed tomography coronary angiography in patients referred for diagnostic conventional coronary angiography. Circulation 2005;112(15):2318-2323.

3.Perisnakis K, Theocharopoulos N, Karkavitasis N, Damilakin T. Patient effective radiation dose and associated risk from transmission scans using Gd-153 line sources in cardiac SPECT studies. Health Phys 2002;83(1):66-74.