Image Production, Instrumentation, and Quality Assurance
Scanner components and reconstruction
A CT system includes the X-ray tube, detector array, slip rings, collimator, data acquisition system, and computer. Detector rows determine the number of simultaneous slices. Reconstruction uses filtered back projection or iterative methods; kernels are chosen for soft tissue, bone, or lung. Spatial resolution, contrast resolution, noise, and slice thickness are traded off against dose. Pitch equals table feed per rotation divided by collimation.
Artifacts
Recognize beam hardening (dark streaks between dense objects), photon starvation, motion, metal streak, ring artifact from detector calibration, partial volume averaging, and out-of-field artifact. Correct with mA modulation, thinner slices, metal artifact reduction, and proper patient centering.
Quality assurance
Daily tests include tube warm-up, CT number of water (0 ± a few HU), noise, and uniformity. Periodic tests include slice thickness, laser alignment, modulation transfer function, and dose measurements. The ACR phantom is commonly used for accreditation testing.
Worked example
For soft tissue, use window width about 400 and level about 40; for lung, width about 1500 and level about −600; for bone, width about 2000 and level about 400. Water should measure 0 HU, and air should measure approximately −1000 HU. If water reads +20 HU, suspect calibration drift and perform QA before clinical use.