Study of the efficacy of respiratory gating in myocardial SPECT using the new 4D NCAT phantom

Respiratory motion can cause artifacts in myocardial SPECT images, which can lead to the misdiagnosis of cardiac diseases. One method to correct for respiratory artifacts is through respiratory gating. We study the effectiveness of respiratory gating through a simulation study using the newly developed NURBS-based cardiac-torso (NCAT) phantom. The organ shapes in the NCAT phantom are formed using non-uniform rational b-splines (NURBS) and are based on detailed human image data. With its basis on actual human data, the NCAT phantom realistically simulates human anatomy and motions such as the cardiac and respiratory motions. With the NCAT software, we generated 128 phantoms over one respiratory cycle (5 seconds per cycle) with the diaphragm and heart set to move a total of 4 cm from end-inspiration to end-expiration. The heart was set to beat at a rate of 1 beat per second resulting in a total of five heart cycles. We divide the respiratory cycle into different numbers of respiratory gates (16, 8, and 4) by averaging the phantoms. For each gate, we generate its projection data using an analytical projection algorithm simulating the effects of attenuation, scatter, and detector response. We then reconstruct the projections using an iterative OS-EM algorithm compensating for the three effects. The reconstructed images for each gating method were examined for artifacts due to the respiratory motion during that gate. We found that respiratory artifacts are significantly reduced if the respiratory motion of the heart that occurs during a gating time period is 1 cm or less. We conclude that respiratory gating is an effective method for reducing effects due to respiration. The timing of the gates is dependent on the extent of the heart´s motion during respiration.