Retrospective Gating versus Prospective Triggering

There are two general approaches to cardiac synchronization of the CT acquisition. One is prospective and “observes” the electrocardiogram (ECG) for a small number of heart beats (or more accurately the peak of the R-wave or R-peak) and then anticipates when the next R peak is to occur. Given the anticipated time point of the future R peak, the scanner will then only acquire x-ray projections in a prespecified phase of the cardiac cycle (usually late in diastole where the heart is most quiescent). This approach is called prospective triggering because the x-ray tube is triggered to shoot in a predefined cardiac phase. Data acquisition is in an axial fashion, and the table only moves in between heartbeats and is stationary during x-ray transmission. This approach has the advantage of having a low radiation dose to the patient, but it has a number of disadvantages. One of the major disadvantages is that typically only one dataset (or few similar ones) can be acquired in the anticipated cardiac cycle, which may not turn out to be of optimal image quality (Fig. 8-1).

FIGURE 8-1 Prospective triggering mode. This mode uses sequential axial (nonspiral) acquisitions during every other heart beat. X-rays are only emitted during a predefined cardiac window (typically diastole), and the tube is turned off during the reminder of the cardiac cycle. In the in-between heartbeats the table moves forward by the equivalent of the detector width (step-and-shoot mode).

The newer 256- and 320-slice scanners use a modified step-and-shoot mode. Because their detectors cover a large volume, in many cases the entire heart, no table motion is necessary to acquire a coronary CTA dataset. Having the tube current turned on for approximately one entire heartbeat allows acquiring a dataset for analysis of cardiac anatomy and function. Theoretically, x-ray exposure can be limited to a short segment in diastole if only coronary artery visualization, and if no information on function is desired. Multi-phase reconstruction (to improve temporal resolution) would, however, require data acquisition during several consecutive cardiac phases (Fig. 8-2).

FIGURE 8-2 Whole organ coverage computed tomography. Current 320-slice scanners have a detector width of 16 cm in z-dimension, which allows acquisition of a coronary computed tomography angiography without the need to move the table. The x-ray tube is turned on only for the duration of one heartbeat. If whole chest coverage is desired, a step-and-shoot mode with only two steps is used.

A radically different approach is retrospective gating (Fig. 8-3). Retrospective gating allows acquiring unlimited complete datasets in any phase of the cardiac cycle. This approach uses a spiral CT acquisition, in which the x-ray current remains turned on during the entire scan. The user may then in retrospect define what phase of the cardiac cycle to reconstruct. The major advantage of this approach is that the interpreter may decide to try a different phase of the cardiac cycle if the initial reconstruction demonstrates motion artifact. Another advantage is the ability to “edit” the ECG. ECG editing allows the user to select heartbeats that should not be used for reconstructions (e.g., premature ventricular contractions [PVCs]), or to correct trigger points that were not placed on an R peak by the computer algorithm. The major disadvantage of retrospective gating is the high radiation dose to the patient. For this reason a number of dose reduction strategies were developed.

FIGURE 8-3 Retrospective gating. This mode is a spiral computed tomography angiography acquisition, in which the tube emits x-rays during the entire cardiac cycles over a 5- to 20-second period (depending on z-axis coverage and scanner type). Data that were acquired during a specific cardiac phase (typically diastole) are then retrospectively selected to reconstruct the corresponding images. Any phase of the cardiac cycle (early systole to late diastole) can be reconstructed. Functional cine images can be obtained by reconstructing datasets every 10% of the cardiac cycle.

Tube Modulation

One of the most important dose reduction strategies in cardiac CT is ECG-correlated x-ray tube current modulation or short tube modulation. In this algorithm the scanner does perform a spiral acquisition using the retrospective gating method; however, it prospectively down regulates the x-ray tube current (typically in systole and usually down to approximately 20% of the maximum). This reduces the radiation dose to the patient during systole, but nevertheless allows for reconstructions of images in systole, if necessary. The penalty for reducing the tube current is a higher noise level. This approach is clearly a compromise trying to capture the advantages of both the prospective triggering and the retrospective gating acquisitions (Figure 8-4).

FIGURE 8-4 Retrospective gating with x-ray tube modulation. This mode is a variant of retrospective gating, in which the tube emits the desired amount of x-rays during diastole (or any other predefined window), but in which the tube current is substantially reduced during the remainder of the cardiac cycle. This allows optimal noise levels during diastole, but results in increased noise in systole. This technique allows reduction of the radiation dose by up to 50% if the heart rate is low.

Beta-Blockers and Nitroglycerine

With current generation of single source scanners (64-slice, 256- or 320-slice MDCT), it is advisable to slow the heart rate by administration of beta-blockers. This can be done by oral administration, intravenous application, or a combination of both. The effect not only reduces the motion of the coronary within the image acquisition window (less motion artifacts), but it also results in a lower radiation dose if ECG based tube current modulation is used. The application of oral nitroglycerine immediately before scanning results in increased diameter of the coronary arteries and substantially improves evaluability.

Breath Holding and Positioning

It is important to practice inspiratory breath holding with the patient on the CT table before scanning. This has a number of important beneficial effects. First, the patients are familiar with the breath hold instructions and are more likely to perform optimal breath holds. Second, the physician or technologist can ensure that patients perform sufficient breath holds (long enough, no slow exhalation, etc.). Third, the heart rate varies between rest and breath holding, and the beta-blocker dosage can be adjusted to the heart rate that is seen during the breath holds.

Contrast Injection

There are a number of injection protocols that can be applied to coronary CTA. It is beyond the scope of this chapter to review all these. However, there are a number of common principles that are important to review. The intravenous catheter is ideally of large bore and placed in the right antecubital fossa. The right-sided injection is somewhat preferred because of a shorter route to the heart and because it avoids the crossing of dense contrast past the left internal mammary artery (LIMA) and the great vessels via the left innominate vein. This has the potential to cause streak artifact, which, for example, can hinder the assessment of a portion of a LIMA graft.

High flow rates (5 to 7 ml/sec) are needed for adequate opacification of the coronary arteries. Larger patients generally require faster flow rates (and larger overall volumes) to achieve the same opacification as smaller patients. Dual-head injection pumps are standard and allow a saline chaser bolus to follow the contrast injection (biphasic protocols). This allows for washing out the veins and right atrium.

Some sites prefer using triphasic protocols, in which the initial injection is contrast at a fast rate, say 5 ml/sec (phase 1), followed by a slower rate of contrast (e.g., 2 ml/sec), or a mixture of saline and contrast (phase 2), and lastly followed by pure saline (phase 3). The proposed advantage of these protocols is that they result in “better” opacification of the right ventricle, while still avoiding excessive mixing artifact. Thus, evaluation of right ventricular function and ventricular septal motion is thought to be improved. However, the value of triphasic over biphasic injection protocols has not been shown today.

Image Reconstruction (Best Phase Selection)

The initial reconstruction of images of coronary CTA datasets is usually performed in mid to late diastole in which the heart is most quiescent. Late diastole is usually less desirable because the atrial kick generates rapid motion of the coronary arteries, especially the right and left circumflex coronary artery. The field of view of the axial source slices is limited to the heart to maximize in-plane resolution (12 to 15 cm). Slice thickness is 0.6 to 1 mm, and most centers use an overlap of about one third of the slice thickness. (e.g., 0.75-mm slices with 0.5-mm spacing). Typically the images are initially analyzed for presence of coronary motion artifact, and if motion blurring is found, additional reconstructions are necessary. Late systolic reconstructions may be helpful if right coronary artery (RCA) motion cannot otherwise be eliminated by reconstructing various diastolic phases. During interpretation the interpreter may need to jump back and forth between different phases because each segment of the coronary arteries may be best displayed in different phases. In the presence of stents or calcium that makes interpretation difficult, reconstructions are repeated with sharper high-contrast kernels. Additional reconstructions of a larger field of views are performed with thicker slices for a review of extra cardiac structures.