Nuclear medicine is fundamentally different from every other imaging modality. Instead of sending radiation through the patient like X-ray or CT, nuclear medicine injects radioactive tracers into the patient and images the gamma rays emitted from within. This means nuclear medicine images function, not anatomy — they show how organs work, not just what they look like.
This guide covers the physics behind the gamma camera, SPECT, and PET imaging, and the key radionuclides used in modern practice.
In radiography and CT, an external X-ray source transmits radiation through the patient to form an image. In nuclear medicine, the patient emits radiation from internally administered radiopharmaceuticals. This reversal — imaging what's inside rather than what passes through — gives nuclear medicine its unique ability to assess organ function at the cellular level.
The gamma camera, invented by Hal Anger in 1958, is the workhorse of nuclear medicine. While modern SPECT and PET systems are more sophisticated, the principles of the Anger camera underpin all nuclear imaging.
The most important component for image quality. Types: parallel-hole (most common), converging, diverging, pinhole. Without a collimator, no useful image is possible.
Thallium-activated sodium iodide. High light output per gamma ray absorbed. Thickness: ¼ inch to 1 inch (3/8" most common). Thicker = more sensitive but less spatial resolution.
Typically 37–91 PMTs per detector. Larger arrays = better spatial resolution. Signal from each PMT is digitized and analyzed by the computer module.
The simplest nuclear medicine exam is planar imaging — the gamma camera is positioned over the patient in one or more static views, collecting counts for a set time. The result is a 2D projection image, similar in concept to an X-ray, where deeper structures are superimposed on shallower ones.
Common planar exams:
Single Photon Emission Computed Tomography (SPECT) extends the gamma camera from planar 2D to tomographic (cross-sectional) 3D imaging. One or more gamma camera heads rotate around the patient, acquiring projections from multiple angles that are reconstructed into cross-sectional slices — similar to how CT reconstructs from X-ray projections.
| Feature | Planar | SPECT |
|---|---|---|
| Dimensionality | 2D projection | 3D tomographic |
| Detector motion | Stationary | Rotating (180° or 360°) |
| Lesion detection | Limited (overlap obscures lesions) | Superior (separates overlapping structures) |
| Quantification | Semi-quantitative | More accurate quantification |
| Scan time | 5–15 minutes | 15–30 minutes |
Common SPECT applications: myocardial perfusion imaging (cardiac stress test), brain perfusion (dementia evaluation), bone SPECT (complex fractures, infection), and parathyroid adenoma localization.
Positron Emission Tomography (PET) uses a completely different physical principle — annihilation coincidence detection — which gives it higher sensitivity and spatial resolution than SPECT.
SPECT requires a lead collimator to reject scattered gamma rays — the collimator absorbs most gamma rays, wasting ~99.9% of emitted photons. PET uses electronic collimation (coincidence detection) — both 511 keV photons must be detected within ~10 nanoseconds of each other. This makes PET 100-1000× more sensitive than SPECT and gives better spatial resolution.
| Radionuclide | Half-Life | Gamma Energy | Common Uses |
|---|---|---|---|
| Tc-99m | 6 hours | 140 keV | ~80% of all NM procedures. Bone, cardiac, renal, lung, thyroid, infection |
| I-123 | 13 hours | 159 keV | Thyroid imaging, metaiodobenzylguanidine (MIBG) |
| I-131 | 8 days | 364 keV | Thyroid therapy, whole-body scan for thyroid cancer |
| Ga-67 | 78 hours | 93–394 keV | Infection and inflammation imaging |
| In-111 | 2.8 days | 171, 245 keV | White blood cell labeling, somatostatin receptor imaging |
| F-18 (PET) | 110 min | 511 keV (annihilation) | FDG for oncology (cancer staging), amyloid imaging (Alzheimer's) |
| Ga-68 (PET) | 68 min | 511 keV | Neuroendocrine tumor imaging (DOTATATE/DOTATOC) |
| Rb-82 (PET) | 76 sec | 511 keV | Cardiac perfusion (produced by generator) |
Technetium-99m is the workhorse of nuclear medicine, used in approximately 80% of all procedures. But Tc-99m's 6-hour half-life means it can't be stored or transported long distances. Instead, it's produced on-site using a molybdenum-99 / technetium-99m generator (commonly called a "cow" or "moly cow").
Mo-99 (half-life: 66 hours) decays to Tc-99m. The generator contains Mo-99 adsorbed on an alumina column. When a saline solution is passed through the column ("milking" the generator), the Tc-99m is eluted while the Mo-99 remains trapped. A single generator can be milked for about one week before the Mo-99 decays below useful levels.
A key quality control measure is checking for Mo-99 breakthrough — Mo-99 contaminating the Tc-99m eluate. This increases patient dose without diagnostic benefit. Regulatory limits require <0.15 kBq of Mo-99 per MBq of Tc-99m. This is checked daily using a dose calibrator equipped with a lead shield.
Nuclear medicine has more rigorous quality control than any other imaging modality because gamma cameras are sensitive to environmental changes:
Nuclear medicine technologists can pursue certification through the ARRT (NMT) or the NMTCB (Nuclear Medicine Technology Certification Board). Both require completion of an accredited NM technology program and a comprehensive exam covering radiation physics, instrumentation, radiopharmacy, clinical procedures, and radiation safety.
For the fundamentals of nuclear medicine imaging and clinical applications, see our Nuclear Medicine modality overview.