Effect of tomographic orbit and type of rotation on apparent myocardial activity

In myocardial perfusion imaging, the type of orbit that provides the best image quality is still the subject of debate. This study correlates the effects of angular rotation (180 degrees vs. 360 degrees), type of orbit (circular vs. body contouring) and location of the heart within the orbit, with changes in spatial resolution and consequential changes in the uniformity of short axis slices of a normal myocardial phantom.

All studies were performed on a dual-head gamma camera equipped with low energy all-purpose collimators. A myocardium was suspended in air with no scattering or attenuating material present. SPECT acquisitions were performed using circular and body-contouring orbits of various radii and with the myocardium at the maximum off-axis position.

The average uniformity of myocardial short axis slices was approximately 4% for 360 degrees circular orbits and for 180 degrees circular orbits where the myocardium was close to the centre of rotation. With body-contouring orbits, the average uniformity increased to 8% and 18% when the myocardium was located on the long and short axes of rotation, respectively. Changes in system resolution with a rotation of >3 mm increased non-uniformities in the myocardial images.

Changes in resolution associated with the angular rotation, type of orbit and myocardial location affect the apparent distribution of activity in short axis slices of the heart. The most uniform images were obtained with a 360 degrees circular orbit. Results with a 180 degrees circular orbit and both 360 degrees and 180 degrees body-contouring orbits were highly dependent on the location of the myocardium.