Gating: Keep It Regular

Beat Rejection Software: All current computer systems offer beat rejection software and the ability to set a range of acceptable beats per window (usually as a percentage) for acquisition. This window is based on the patient’s R-to-R interval. The window setting (usually from 10% to 100%) represents the variation in heart rate deemed to be acceptable. For example, the duration of a heartbeat in a patient with a heart rate of 72 bpm is 0.8 s. Therefore, a 20% window for gated acquisition would result in acceptance of all beats between 0.72 and 0.88 s. Any beats outside this range will be rejected. As the window percentage increases, a greater number of irregular beats are accepted within the R-to-R interval, negatively affecting the accuracy of the LVEF when images are processed.

Heart Rhythm Assessment: Patients with sinus rhythm are ideal candidates for gated SPECT MPI. Patients with mild tachy- or bradyarrhythmias are also acceptable candidates. However, severe arrhythmias (atrial fibrillation, frequent premature ventricular contractions, or certain types of heart block) result in a poor-quality gated acquisition that may affect the accuracy of LVEF and observed wall motion abnormalities.

Data Storage: Fortunately, many SPECT systems are able to store the gated and raw perfusion data separately. This separate storage is helpful when issues arise with the gated data and prevents jeopardizing the perfusion data collected.

Lead Attachment: In prepping the patient for image gating, the technologist must ensure that the leads are securely attached. Although the medical professional who preps the patient for the stress test usually performs an excellent job of attaching the leads, the stress test itself (particularly if it is exercise stress) can affect the attachment.

Lead Placement: Gated acquisition requires 3 leads: left arm, right arm, and left leg. The arm leads are placed on the upper chest, and the leg lead is placed on the left lower rib area. If this placement is suboptimal, it may be helpful to place the arm lead on the patient’s inner wrist, similarly to the placement for a standard 12-lead electrocardiograph. The goal is to optimize placement so that the electrocardiograph tracing can track similarly to a standard 12-lead electrocardiograph, with a pronounced (higher-voltage) monophasic or reversed S-wave QRS complex and a relatively lower-voltage T and P wave (Fig. 1).

System Detection of Cardiac Cycle: The goal is to ensure that the system is tracing the R-wave or reversed S-wave QRS complex, because this interval is what must be measured to provide an accurate LVEF for interpretation. Patient body habitus will sometimes affect where leads should be placed to acquire the ideal electrocardiograph pattern. The gating equipment supplied with different camera systems usually provides several lead configurations and voltage adjustments to assist in finding the optimal QRS complex, allowing the system to properly gate the study. If it is difficult to find an acceptable R-to-R interval on a particular patient’s electrocardiograph, the technologist might have to use other standard areas for lead placement. Trying multiple or different lead locations is usually a good start (e.g., putting the upper leads on the patient’s arms or inner wrists). If gating parameters cannot be changed on the system, one can ask the manufacturer for help in fine-tuning the acquisition set-up.

Failed Gating: Under certain circumstances—such as in a patient who has arrhythmias during the study—gated images cannot be acquired. In these cases, it is important to tell the interpreting physician about all the correction techniques that were attempted for gating. Most physicians who routinely interpret MPI studies are aware of the challenges that inaccurate data present and that such data will only compromise the results.