Nuclear Medicine (SPECT + SPECT/CT)

Functional imaging using gamma cameras that detect photons emitted by injected radiotracers. Most common clinical tracer: Tc-99m (technetium-99m, ~6 hr half-life, 140 keV gamma). Other tracers: Tl-201 (thallium, cardiac perfusion), I-131 (thyroid), I-123 (DaTscan neuro), In-111 (ProstaScint, WBC scans), Ga-67 (legacy infection/inflammation).

Distinct from PET/CT — different detector physics (direct gamma detection vs positron coincidence), different tracer library, lower cost, broader availability.

Modality variants

• Planar gamma camera — single-position 2D imaging (bone scans, thyroid, lung V/Q).
• SPECT (Single-Photon Emission Computed Tomography) — camera rotates around the patient, reconstructs 3D functional images.
• SPECT/CT — SPECT + integrated CT for anatomic correlation + attenuation correction. Clinical standard for most oncology + parathyroid + cardiac attenuation-corrected perfusion since ~2010.

Physics

1. Inject radiotracer. Tracer distributes according to physiology (Tc-99m MDP to bone, sestamibi to cardiac tissue, MAA to pulmonary capillaries, etc.).
2. Gamma photons exit the body. Collimator selects photons traveling in specific directions (limits to ~1 in 10,000 photons detected — collimator inefficiency is fundamental).
3. Scintillator crystal (NaI(Tl) typical; CZT semiconductor on newer cardiac cameras) converts gammas to visible light.
4. Photomultiplier tube array converts light to electrical signal; position logic locates each event.
5. Events binned by angle form projections; reconstruction produces 3D images.

Key specs

• Crystal type — NaI(Tl) (standard), CZT (cadmium zinc telluride, newer cardiac-dedicated)
• Field of view — large-FOV (~40×54 cm) for whole-body, small-FOV for dedicated cardiac
• Camera heads — single-head (rare today), dual-head (standard), triple-head (cardiac-specialty)
• Collimator set — LEHR (low-energy high-resolution, Tc-99m), LEAP (low-energy all-purpose), MEGP (medium-energy), HEGP (high-energy, I-131)
• Energy resolution — 9–10% FWHM at 140 keV typical for NaI(Tl)
• Intrinsic spatial resolution — ~3.5–4 mm for NaI; submillimeter-reportable for CZT
• CT integration — 6 / 16 / 64-slice on hybrid systems

Clinical applications

• Cardiac SPECT (stress/rest perfusion)
• Bone scan
• Thyroid (uptake, scan, I-131 therapy)
• Parathyroid (sestamibi)
• Pulmonary V/Q (ventilation/perfusion)
• DaTscan (neurology, Parkinson vs essential tremor)
• WBC (labeled white blood cell, infection localization)

Systems

• GE Discovery NM family
• Siemens Symbia family
• Philips BrightView (discontinued, still deployed)
• Digirad (cardiac-dedicated, alternative architecture)

Service reality

• Crystal integrity — NaI(Tl) is hygroscopic. Any seal breach → crystal clouding → ruined image quality. Replacement is a factory event.
• PMT gain drift — uniformity calibration shifts. Monthly flood QC catches it; tuning resolves most drift.
• Center-of-rotation (COR) — SPECT reconstruction quality depends on it. COR calibration every 6 months minimum.
• Collimator damage — drops deform the lead foil. Inspect after every swap.

Regulatory

• NRC or Agreement State license required to hold the radiotracers
• RSO + authorized users + documented safety program
• ACR Nuclear Medicine accreditation for reimbursement
• Annual medical physicist survey per ACR

Related

• PET/CT (hybrid molecular imaging, different physics)
• PET/MR
• Nuclear licensing