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Respiratory motion can cause artifacts in myocardial single photon emission computed tomography (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 four-dimensional (4-D) NURBS-based cardiac-torso (NCAT) phantom. The organ shapes in the 4-D NCAT phantom are formed using nonuniform rational b-splines (NURBS) and are based on detailed human image data. With its basis on actual human data, the 4-D NCAT phantom realistically simulates human anatomy and motions such as the cardiac and respiratory motions. With the 4-D NCAT phantom, we generated 128 phantoms over one respiratory cycle (5 s 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 with a normal contractile motion at a rate of I 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 summing 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 respiratory gates for reduced image artifacts is dependent on the extent of the heart's motion during respiration. Index Terms-Biomedical image processing, biomedical nuclear imaging, image analysis, motion compensation, respiratory system, simulation software, single photon emission computed tomography (SPECT).