Instrumentation and Image Production, Including PET and CT
Core concepts
A gamma camera uses a collimator, a NaI(Tl) crystal, and photomultiplier tubes to convert scintillation light into position and energy signals. The energy window is set around the photopeak-commonly about 140 keV for Tc-99m-and must balance scatter rejection against count sensitivity. Collimator choice trades spatial resolution for sensitivity. Matrix size, field of view, and zoom determine pixel size, while count density affects noise. SPECT reconstructs projections into tomographic slices; PET detects annihilation photons at 511 keV in coincidence and commonly uses CT for attenuation correction and anatomic localization. Time-of-flight PET improves localization of annihilation events.
Traps
Do not equate smaller pixels with better intrinsic resolution; smaller pixels improve sampling but increase noise if counts are fixed. A wide energy window admits scatter, while a narrow window loses counts. In PET/CT, misregistration between PET and CT can create false attenuation-correction artifacts. Know that CT provides attenuation maps and anatomy, not PET tracer uptake. SUV is affected by body weight, injected dose, uptake time, and reconstruction, so compare cautiously.
Worked example
A camera has a 400 mm field of view and a 128 × 128 matrix. Pixel size is 400 mm ÷ 128 = 3.125 mm. A 256 × 256 matrix gives 1.5625 mm pixels, but the image may look noisier unless counts are sufficient. Rule of thumb: optimize pixel size, collimator, and count density together; never judge resolution from matrix size alone.