Unlike radiography or CT, most nuclear medicine examinations detect radiation emitted after a radiopharmaceutical is administered by injection, ingestion, or inhalation. The resulting image primarily maps tracer distribution and physiology, not a purely anatomical boundary. Anatomy is still present indirectly and is often supplied directly by hybrid CT or MRI.
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. In emission imaging, detectors map photons originating from an administered radiopharmaceutical. Uptake reflects the tracer's biochemical or physiological pathway; it is not automatically specific for one disease or cell type.
The scintillation camera developed by Hal Anger in the late 1950s remains the basis of conventional planar and SPECT imaging. PET uses a different detector geometry and coincidence-localization principle.
Types include parallel-hole, converging, diverging, and pinhole. High-resolution designs generally sacrifice sensitivity; resolution also worsens as a parallel-hole collimator moves farther from the source.
Thallium-activated sodium iodide has high light output. Conventional Tc-99m cameras often use a crystal about 9.5 mm (3/8 inch) thick. Greater thickness improves absorption efficiency for higher-energy photons but can degrade intrinsic localization.
The light pattern shared across multiple PMTs supports position and energy estimation. Performance depends on the complete detector design and calibration—not simply on PMT count.
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 | Static, dynamic, or whole-body translation | Camera heads rotate through a protocol-selected arc (commonly 180° for some cardiac studies or 360° for many others) |
| Depth information | Structures overlap in projection | Reconstruction separates activity by depth, although noise, motion, and artifacts can limit detection |
| Quantification | Counts or count ratios are protocol-dependent | Absolute activity concentration is possible only with appropriate calibration and corrections |
| Acquisition duration | Varies widely with examination, activity, patient, camera, views/projections, and count target | |
Common SPECT applications: myocardial perfusion imaging (cardiac stress test), brain perfusion (dementia evaluation), bone SPECT (complex fractures, infection), and parathyroid adenoma localization.
SPECT records a series of angular projections. Filtered back projection can be used, but iterative methods such as ordered-subsets expectation maximization are now common and can model attenuation, scatter, and collimator-detector response. Inadequate counts, patient motion, truncation, incorrect center-of-rotation calibration, or attenuation-correction errors can create artifacts. CT-based attenuation correction uses a CT-derived attenuation map; it can improve localization and quantification but misregistration and metal can propagate artifacts.
Positron Emission Tomography (PET) uses annihilation coincidence detection. Avoiding a projection collimator generally gives PET substantially higher detection sensitivity than SPECT, while spatial resolution is system- and task-dependent.
SPECT localization relies on a physical collimator, which rejects most emitted photons. PET uses coincidence localization: a valid pair must satisfy scanner-specific timing and energy criteria. This greatly improves geometric detection efficiency, but scattered and random coincidences still require correction. Numerical timing windows and PET-to-SPECT sensitivity ratios are scanner- and protocol-dependent.
Conventional coincidence detection places an event somewhere on a line of response. Time-of-flight (TOF) PET uses the small arrival-time difference to estimate where along that line the event occurred; it improves signal-to-noise rather than determining an exact point. PET/CT and SPECT/CT add anatomical localization and CT-based attenuation correction. Because emission and CT data are acquired at different times and often under different breathing conditions, registration must be checked. The CT exposure may range from low-dose localization/attenuation correction to diagnostic CT, so technique and dose are protocol-dependent.
| Radionuclide | Approx. physical half-life | Principal detected photon(s) | Examples of use |
|---|---|---|---|
| Tc-99m | 6.01 h | 140.5 keV gamma | Agent-dependent: bone, myocardial perfusion, renal, lung, hepatobiliary, thyroid, and other imaging |
| I-123 | 13.2 h | 159 keV gamma | Sodium iodide thyroid imaging/uptake; I-123 iobenguane imaging |
| I-131 | 8.02 d | Beta particles (therapy) plus 364 keV principal gamma | Thyroid treatment and post-treatment/diagnostic imaging in selected settings |
| Ga-67 | 78.3 h | Multiple gammas, chiefly 93, 185, 300, and 394 keV | Selected infection, inflammation, and tumor imaging |
| In-111 | 2.80 d | 171 and 245 keV gammas | In-111-labeled leukocytes and selected labeled compounds |
| F-18 (PET) | 109.8 min | Two 511 keV annihilation photons | Agent-dependent: FDG, amyloid, PSMA, bone, and other PET imaging |
| Ga-68 (PET) | 67.7 min | Two 511 keV annihilation photons | Agent-dependent: somatostatin-receptor and PSMA PET |
| Rb-82 (PET) | 75 s | Two 511 keV annihilation photons | Generator-produced myocardial perfusion imaging |
The radionuclide identifies the decay physics; the chemical form determines biodistribution. For example, Tc-99m MDP localizes mainly through adsorption to bone mineral at sites influenced by perfusion and osteoblastic activity, Tc-99m MAA is temporarily trapped in pulmonary capillaries in proportion to regional perfusion, and F-18 FDG reflects facilitated glucose transport and phosphorylation. FDG uptake is not cancer-specific: brain, myocardium, inflammation, infection, muscle activity, and brown fat may also be avid. Interpretation therefore depends on preparation, timing, administered agent, physiology, and clinical context.
Technetium-99m is among the most widely used diagnostic radionuclides. Its short half-life favors frequent production from a molybdenum-99 / technetium-99m generator. A generator may be housed at the imaging facility or at a radiopharmacy that prepares and transports patient doses.
Mo-99 (physical half-life about 66 hours) decays to Tc-99m. In a common fission-produced generator design, molybdate is adsorbed on an alumina column and saline elutes sodium pertechnetate Tc-99m while most Mo-99 remains on the column. Useful service life and elution schedule depend on generator activity, demand, and manufacturer instructions; approximately one week is common.
Mo-99 breakthrough in Tc-99m eluate adds unnecessary exposure. In the United States, 10 CFR 35.204 requires licensees to measure the Mo-99 concentration in each generator elution used to prepare patient doses. At administration it must not exceed 0.15 microcurie Mo-99 per millicurie Tc-99m (equivalently 0.15 kBq/MBq). The measurement method and shielding follow the licensee's validated procedure and manufacturer instructions; requirements differ by jurisdiction.
Quality control schedules must follow applicable regulation, accreditation requirements, manufacturer instructions, and the facility's physicist-approved program. Typical checks include:
Optimization uses the smallest administered activity consistent with the clinical purpose and applies time, distance, and shielding to staff exposure. Patient dose depends on the radiopharmaceutical's administered activity, physical decay, biodistribution, and biological clearance; hybrid CT contributes a separate exposure. Hydration and frequent voiding can reduce dose for many renally excreted agents when clinically appropriate.
In the United States, nuclear medicine technologists may earn the ARRT(N) credential through the American Registry of Radiologic Technologists or CNMT through the Nuclear Medicine Technology Certification Board. They are separate credentials, not “NMT versus NMTCB.” ARRT's primary pathway requires an associate degree or higher and completion of an ARRT-approved educational program, plus ethics and examination requirements. NMTCB eligibility includes graduates of NMTCB-recognized nuclear medicine technology programs and any other pathways listed in its current policy. Current content specifications—not this article—control what each examination tests; state licensure is separate from certification.
For the fundamentals of nuclear medicine imaging and clinical applications, see our Nuclear Medicine modality overview.
Try these ARRT-style multiple choice questions based on this article. Click an option to check your answer — correct answers turn green, wrong ones turn red.