The effect of distance on target to background ratios in quantitative nuclear medicine.

Objectives In any medical field, quality studies are key to good patient care. Medical professionals strive to achieve the highest quality images or study possible. In nuclear medicine, it is beneficial for the target tissue or organ to have a high uptake of radioactive tracer in comparison to the background tissues. This results in a clearer image. Quantitative nuclear medicine deals with drawing regions of interest around an area to determine the count rate. The purpose of this study was to see if increasing collimator distance from a “patient” could alter the ratio of counts in target tissue to background tissue.

Methods Two 50mL saline bags were filled with different amounts of Tc-99m-tetrofosmin and placed in conjunction to represent a target organ and background tissue uptake. Images were taken at fifteen different distances. A region of interest (ROI) was drawn in the target area and a ROI of the exact same size was placed in the background tissue. A ratio was created for each increasing distance to see if distance could alter the number of counts in the target area verses the background area.

Results As the distance increased, the image quality decreased significantly and total count loss was apparent. The count loss in the target area was proportional to the background area. The ratio of counts in the target area to background area was not affected by distance.

Conclusions Distance between the sources affected both the target area and background area equivalently. Collimator distance appears to have less effect on quantitative nuclear medicine than diagnostic imaging, as the image quality was decreased significantly.