For a gamma camera scan the patient is given a tracer, usually technetium-99m bound to a compound that the organ being studied takes up. After it has had time to reach the organ, the gamma camera is placed close to the body and records an image of the distribution of the tracer.Doctors compare the image with the normal pattern:• a hot spot is an area of unusually high uptake, for example in a bone scan where a tumour or fracture has increased bone activity
• a cold spot is an area of unusually low uptake, for example heart muscle with a poor blood supply, or dead tissue after a heart attackImages taken one after another show how the tracer moves with time, so the camera can measure how well an organ functions, for example how quickly the kidneys remove the tracer from the blood. X-rays and CAT scans mainly show structure; gamma camera images show function.The resolution of a gamma camera is only about 1 cm, poorer than an X-ray image, so gamma images are often combined with CAT images.Worked example: the count rate from a kidney falls from 6000 to 1500 counts per minute over 30 minutes. 6000 → 3000 → 1500 is two halvings, so the count rate halves every 30 / 2 = 15 minutes. A healthy kidney clears the tracer quickly; a much longer time than normal suggests the kidney is not working well.
• a cold spot is an area of unusually low uptake, for example heart muscle with a poor blood supply, or dead tissue after a heart attackImages taken one after another show how the tracer moves with time, so the camera can measure how well an organ functions, for example how quickly the kidneys remove the tracer from the blood. X-rays and CAT scans mainly show structure; gamma camera images show function.The resolution of a gamma camera is only about 1 cm, poorer than an X-ray image, so gamma images are often combined with CAT images.Worked example: the count rate from a kidney falls from 6000 to 1500 counts per minute over 30 minutes. 6000 → 3000 → 1500 is two halvings, so the count rate halves every 30 / 2 = 15 minutes. A healthy kidney clears the tracer quickly; a much longer time than normal suggests the kidney is not working well.