Final answer:
In nuclear medicine imaging, 67-Ga citrate's target-to-background ratio increases as it concentrates in tissues with rapid cell growth such as tumours, making it useful for cancer imaging. After an initial injection, the patient's scan is delayed to allow gallium to build up in the targeted areas over time.
Step-by-step explanation:
In imaging with 67-Ga citrate, the target-to-background ratio increases over time due to the selective accumulation of gallium in areas with rapid cell growth, such as tumours and sites of infection. Gallium-67 acts as a radiotracer in nuclear medicine imaging, and it is produced by proton bombardment on zinc-66. After administration, the patient usually undergoes a whole-body scan after several days to allow for sufficient accumulation of gallium in the target tissues, which enhances the contrast and visual differentiation between the abnormal and normal tissues. Specifically, the reason the target-to-background ratio increases is that gallium-67 gets concentrated in cancerous tissues where there is rapid cell turnover. As gallium is cleared from non-target tissues over time, the contrast between the target areas (cancer cells) and the background normal tissues becomes more apparent, facilitating the diagnosis and assessment of malignancies using imaging techniques such as the gamma camera.