Diagnostic methods in medicine: (c) diagnosis using gamma camera

The idea

Diagnosis with a gamma camera begins by administering a radiopharmaceutical whose gamma-emitting radionuclide is chosen to accumulate in the organ of interest. For example, technetium-99m labelled to a carrier may concentrate in bone or kidneys. Gamma photons escaping the patient are detected by the camera, and the computer builds a two-dimensional map of count rate against position. A healthy organ shows a characteristic pattern of uptake; disease may appear as a cold spot where uptake is reduced, or a hot spot where uptake is increased. The clinician compares the image with normal expectations and may quantify counts in regions of interest. The technique gives functional information about physiology rather than sharp anatomical detail, and the collimator limits sensitivity, so image quality depends on tracer activity, acquisition time and patient positioning.

What earns the marks

• A gamma-emitting radiopharmaceutical is administered and is designed to accumulate in the target organ or tissue.
• Gamma photons emitted from within the patient are detected externally by the gamma camera.
• The computer forms a two-dimensional image showing the distribution of tracer uptake.
• Areas of abnormally high or low uptake are interpreted as hot spots or cold spots that may indicate disease.
• The image provides functional information about the organ, and diagnosis depends on comparing the pattern with normal uptake.

Common mistakes and how to put them right

• Thinking the gamma camera itself emits radiation into the patient.

  Put it right: The radiation comes from the administered radiopharmaceutical.

• Confusing a cold spot with a hot spot.

  Put it right: A cold spot has reduced tracer uptake, while a hot spot has increased uptake.

• Assuming the gamma camera gives detailed anatomical images like a CT scan.

  Put it right: It primarily shows functional distribution of tracer.

• Believing any gamma emitter is suitable.

  Put it right: The radionuclide must emit gamma rays of suitable energy and have a half-life long enough for imaging but short enough to limit dose.

Exam tips

• State the clinical purpose: to assess organ function by mapping tracer uptake.
• Use the terms hot spot and cold spot correctly and link each to increased or decreased uptake.
• Mention that the choice of radiopharmaceutical depends on the organ being investigated.