Until recently, step-and-shoot techniques were not feasible, mainly because limited detector coverage required multiple consecutive axial scans to cover the entire heart, and the delay between scans made the overall scan too long for breath-hold contrast-enhanced imaging. Such techniques also require:

• Precise, high-speed on-off switching of the X-ray beam, synchronized to table movement and location.
• Precise synchronization of the X-ray to the ECG signal to ensure both maximum dose reduction and continuity of the acquired data between steps.
• Advanced algorithms to reconstruct high-quality, thin-slice axial images.

Figure 1 - Illustration of SnapShot Pulse acquisition

The main requirement for the SnapShot Pulse technique is that data acquisition occurs at the right heart phase to guarantee high-quality arterial imaging. The system decides when to turn the X-rays on in a given heartbeat before the ECG signal of the next R-wave appears. Therefore, it is critical to predict the R-wave’s timing accurately and to have enough extra data to accommodate variations in heart rate.

SnapShot Pulse accomplishes this by monitoring the ECG signal in real time, analyzing heart rate changes, and adapting the timing of scans accordingly. The technique uses a statistical model based on the length and variability of previous heart cycles to time the next axial scan.

The overall scan time must be as short as possible: The contrast bolus must be caught at the time of maximum and steady enhancement to ensure high visibility of small vessels and to minimize density changes from one axial scan to the next. Short scans also help reduce variations in heart rate during a study.4

The LightSpeed VCT XT scanner is ideal for the SnapShot Pulse mode because its detector covers a wide 40-mm slab in one rotation. This means only three or four such slabs are needed to cover the length of a typical heart (see Figure 1). Ideally, one slab is completed in every heartbeat, and that is possible at low heart rates. At higher rates, it is necessary to skip one beat between slab acquisitions, and the overall exam lasts five or seven heartbeats.

SnapShot Pulse scanning is enabled by fast and precise movements of the table. The scanner uses an advanced system to control both the X-ray beam and the table. As soon as the X-rays are turned off in one axial scan, the table moves rapidly yet smoothly to the next position. The patient is then in position for the X-rays to be turned on again at the right time in the ECG signal.

High-quality image reconstruction is essential for SnapShot Pulse scanning. CCTA requires very thin slices and relatively high current to achieve the best diagnostic results. For the LightSpeed VCT XT scanner, individual 0.625-mm images must be reconstructed from the 64 detector rows in the 40-mm-wide detector array. GE-exclusive algorithms enable accurate 3D volume reconstructions and ensure uniform performance across the detector array.

The method has been validated and documented in scientific research and publications.5

Figure 2 - Retrospective gating helical vs SnapShot Pulse acquisition

For years, clinicians and CT scanner manufacturers have agreed on the need for new protocols to reduce dosage in CCTA without sacrificing image quality and diagnostic value. The main reason for the high dose in CCTA is that conventional helical scan protocols use very low pitch factors to ensure continuous data availability for reconstruction of images at each location along the heart at the required heart phase.

In the process, the X-ray beam irradiates each location along the scan path three to five times, even though, in most cases, a large portion of the information acquired during the scan is not used. In reality, as documented by two studies using the GE LightSpeed VCT scanner, CCTA exams using reconstruction from a single heart phase provide the needed diagnostic images.1,2

The logical solution is to confine X-ray exposure as much as possible to a single heart phase, so that the necessary data is captured while the overall X-ray on-time is shortened, and total radiation exposure is thus reduced.

Several variations on this approach have been tried. One of the most successful is a retrospective gated helical acquisition technique called ECG-driven X-ray beam current modulation. This method, available for most high-end CT scanners, uses high tube current for a small range of heart phases to ensure low noise and high image quality during those phases, while reducing current to about 20 percent of the maximum during the remaining phases. Dose is reduced by 30 to 50 percent3.

The GE SnapShot Pulse technique is a natural extension of this method. It uses prospectively triggered axial step-andshoot scans in which X-rays are turned on only during the required heart phase and turned off completely at all other times (see Figure 2). Figure 2 - Retrospective gating helical vs SnapShot Pulse acquisition

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Figure 3 - SnapShot Pulse performance chart

GE conducted laboratory tests using various phantoms to measure image-quality parameters including slice sensitivity profile, in-plane resolution, noise, low-contrast resolution, and image uniformity with simulated heart rates from 40 to 65 bpm.6 The results were then compared to those of helical coronary CCTA exams. Quality parameters with SnapShot Pulse compared favorably, while the X-ray dose with SnapShot Pulse was markedly lower (see Figure 3).

Figure 6 - Representative images from study

SnapShot Pulse cardiac scanning takes advantage of the latest technologies and algorithms on the GE LightSpeed VCT XT scanner to achieve significant dose reductions in CCTA imaging with image quality as good as or better than that of conventional helical scans. Clinical studies show that SnapShot Pulse is an acceptable alternative to helical scans when only one heart phase is required, which reduces radiation exposure to 1 to 5 mSv, equivalent to only four to twenty months of exposure to average natural background radiation.7

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