Physics and Instrumentation
Core concepts
CT physics questions test the x-ray tube, filtration, collimation, detector arrays, data acquisition, image reconstruction, and dose metrics. Understand how kVp and mAs affect penetration, noise, contrast, and dose; how pitch relates table speed to beam width; and how voxel size, reconstruction kernel, and slice thickness influence spatial and contrast resolution. Know Hounsfield units, window width and level, helical versus axial scanning, multidetector geometry, and the purpose of iterative reconstruction. Also review CTDIvol, dose-length product, and the relationship between pitch and radiation dose.
Traps
Higher kVp generally increases penetration and reduces noise but lowers soft-tissue contrast and raises dose. Increasing mAs raises dose and reduces noise, with noise changing approximately as the inverse square root of mAs. Pitch greater than 1 reduces dose but can increase noise and artifact; pitch less than 1 increases dose. Window width controls displayed contrast; window level controls brightness. Do not confuse spatial resolution with contrast resolution, or CTDIvol with patient dose. Isocenter positioning matters because off-center patients can receive higher surface dose and suffer out-of-field artifacts.
Worked example / rule of thumb
Pitch = table feed per gantry rotation divided by total beam collimation. If the table moves 30 mm per rotation and the collimation is 40 mm, pitch = 30/40 = 0.75. Because pitch is below 1, slices overlap and dose increases relative to pitch 1. For Hounsfield units, HU = [(μ tissue − μ water) / μ water] × 1000; water is 0 HU and air is approximately −1000 HU.
Dose and quality
Optimize rather than simply minimize: use appropriate kVp, automatic exposure control, and iterative reconstruction while preserving diagnostic quality.