Diagnostic Planar and SPECT Imaging
Planar and SPECT fundamentals
Planar imaging projects three-dimensional activity onto a two-dimensional detector; SPECT reconstructs tomographic slices from multiple projections. Know the trade-offs among collimator, matrix size, pixel size, count density, and reconstruction filter. Attenuation correction, scatter correction, and motion correction improve SPECT, especially for cardiac perfusion and bone. Common planar studies include bone scan, V/Q lung scan, hepatobiliary scan, GI bleeding scan, renal scan, thyroid/parathyroid imaging, and sentinel node lymphoscintigraphy.
Interpretation traps
Questions often test pattern recognition plus artifacts. A three-phase bone scan separates flow, blood pool, and delayed uptake; increased flow and blood pool with delayed uptake suggests infection or complex regional pain syndrome, while a photopenic lesion can be aggressive or benign. A “superscan” shows diffusely increased skeletal uptake with absent renal activity. In V/Q scanning, a mismatch-normal ventilation with absent perfusion-raises PE probability, whereas matched defects suggest pneumonia or atelectasis. Beware attenuation artifacts, patient motion, injection-site activity, and downscatter from high-energy isotopes.
Worked example
On a perfusion lung scan, a wedge-shaped defect is seen in the right lower lobe with normal ventilation. This is a mismatch and is interpreted as intermediate-to-high probability for pulmonary embolism, depending on clinical context and prior probability. Rule of thumb: match the finding to the physiologic mechanism-perfusion defects without airway disease are vascular; matched defects are more likely parenchymal.