Effects of Non-radioactive Material around Radioactive Material on PET Image Quality.

PURPOSE: Subcutaneous fat is a non-radioactive material surrounding the radioactive material. We developed a phantom, and examined the effect of subcutaneous fat on PET image quality. METHODS: We created a cylindrical nonradioactive mimic of subcutaneous fat, placed it around a cylindrical phantom in up to three layers with each layer having a thickness of 20 mm to reproduce the obesity caused by subcutaneous fat. In the cylindrical phantom, hot spheres and cold spheres were arranged. The radioactivity concentration ratio between the hot spheres and B.G. was 4:1. The radioactivity concentration of B.G. was changed as follows: 1.33, 2.65, 4.00, and 5.30 kBq/mL. 3D-PET images were collected during 10 minutes. RESULTS: When the thickness of the mimicked subcutaneous fat increased from 0 mm to 60 mm, noise equivalent count decreased by 58.9‒60.9% at each radioactivity concentration. On the other hand, the percentage of background variability increased 2.2‒5.2 times. Mimic subcutaneous fat did not decrease the percentage contrast of the hot spheres, and did not affect the cold spheres. CONCLUSION: Subcutaneous fat decreases the noise equivalent count and increases the percentage of background variability, which degrades PET image quality.