Effects of tomographic table attenuation on prone and supine cardiac imaging

The purpose of this study was to determine the attenuation characteristics of tomographic imaging tables and to evaluate these effects on prone and supine imaging of the heart, using a cardiac phantom. Methods: Table attenuation as a function of imaging angle was measured in seven different types of tomographic imaging tables, by imaging ^99mTc and ²⁰¹Tl line sources placed at various distances above the center-line of the table. Three imaging tables exhibiting low, medium and high attenuation were further studied using a cardiac phantom filled with ²⁰¹Tl to see the effects of attenuation on myocardial image quality. Studies were performed under three conditions: no table, table above phantom (prone imaging) and table beneath phantom (supine imaging). Circumferential count profiles were generated from short-axis slices of the myocardium. Results: Most SPECT tables attenuated only 5%-10% (^99mTc) and 8%-12% (²⁰¹Tl) at an angle of 180° (face of detector parallel to table). At angles of 90° and 270°, however, table attenuation was highly dependent on the tables' design and material content. From the cardiac phantom studies, short-axis circumferential count profiles showed no significant difference for low-attenuation tables and a slight difference (<5%) in the septal region for medium-attenuation tables. In prone imaging, high-attenuation tables caused a reduction in counts in the antero- lateral wall (>10%) and in the septum (>5%). Conclusion: Table attenuation characteristics should be considered when comparing studies from different systems and in establishing a normal database for ²⁰¹Tl polar maps. High- attenuation tables should be used with caution in cardiac studies.