SPECT stands for single photon emission computed tomography, a medical imaging method that shows how organs are working inside the body. A small amount of radioactive tracer is injected, swallowed, or inhaled, and it travels to specific tissues depending on its chemistry. As the tracer decays, it emits gamma rays that can escape the body and be detected by a gamma camera.
This matters because SPECT can reveal blood flow, metabolism, bone activity, and organ function, not just anatomy.
A gamma camera uses collimators, scintillation crystals, light sensors, and electronics to locate where gamma rays came from. In SPECT, one or more detector heads rotate around the patient and collect many 2D views from different angles. A computer reconstructs these views into a 3D map of tracer concentration, similar in idea to CT reconstruction but using radiation emitted from inside the body.
The image quality depends on tracer dose, detector sensitivity, collimator design, patient motion, and the physics of radioactive decay.
Key Facts
- Radioactive decay follows N = N0(1/2)^(t/T1/2), where T1/2 is the half-life.
- Activity is the decay rate: A = λN, measured in becquerels, where 1 Bq = 1 decay/s.
- Technetium-99m is common in SPECT because it emits 140 keV gamma rays and has a half-life of about 6 h.
- A collimator allows gamma rays traveling in selected directions to reach the detector, improving position information but reducing sensitivity.
- A scintillation crystal converts incoming gamma-ray energy into flashes of visible light.
- SPECT reconstruction combines many projections taken around the patient to estimate a 3D tracer distribution.
Vocabulary
- SPECT
- SPECT is a nuclear medicine imaging method that reconstructs 3D images from gamma rays emitted by a radioactive tracer inside the patient.
- Gamma camera
- A gamma camera is a detector system that records gamma rays and converts them into signals used to form medical images.
- Radiotracer
- A radiotracer is a radioactive substance designed to travel to particular tissues or organs so their function can be imaged.
- Collimator
- A collimator is a lead or tungsten grid that blocks most angled gamma rays so the detector can estimate the direction they came from.
- Scintillation
- Scintillation is the production of tiny flashes of light when high-energy radiation deposits energy in a special crystal.
Common Mistakes to Avoid
- Thinking the gamma camera sends gamma rays into the patient is wrong because in SPECT the tracer inside the patient emits the gamma rays that the camera detects.
- Ignoring half-life when planning imaging time is wrong because tracer activity decreases with time, reducing count rate and changing image quality.
- Assuming more dose always gives a better scan is wrong because patient radiation exposure must be kept as low as reasonably achievable while still producing useful images.
- Confusing SPECT with CT is wrong because CT measures X-rays transmitted through the body, while SPECT measures gamma rays emitted from a radiotracer inside the body.
Practice Questions
- 1 Technetium-99m has a half-life of 6 h. If a patient receives an activity of 600 MBq, what activity remains after 12 h?
- 2 A detector records 240,000 gamma-ray counts during a 5 min acquisition. What is the average count rate in counts per second?
- 3 A patient moves during a SPECT scan while the detector heads rotate. Explain how this motion could affect the reconstructed 3D image and why keeping still matters.