Physics and Instrumentation
Physics and Instrumentation in CT tests x-ray production, attenuation, detector geometry, pitch, and reconstruction. You need to connect tube settings to dose and image quality rather than memorize isolated numbers.
Core rules
- kVp controls photon energy and penetrability; mAs controls photon quantity and noise. Increasing kVp raises dose and lowers contrast, but improves penetration in large patients.
- CT numbers use water as zero: HU = 1,000 × (μ_tissue − μ_water) / μ_water. Air is about −1,000 HU, bone is high positive, water is 0.
- Pitch = table travel per rotation / total collimation. Pitch >1 reduces dose and scan time; pitch <1 increases dose and oversampling.
- Third-generation scanners use a rotate-rotate geometry with a detector array; FBP and iterative reconstruction convert projection data into images.
Worked example
A tissue has linear attenuation 0.19 cm⁻¹; water is 0.20 cm⁻¹. HU = 1,000 × (0.19 − 0.20)/0.20 = −50 HU. The negative value means the tissue is less attenuating than water, not that it is air. If the same tissue were compared with a different water calibration, the number would shift, so CT numbers are relative.
Traps
- Pitch is not the same as table speed; it is normalized to beam width.
- Partial volume averaging can make a small high-density object appear lower in HU.
- Beam hardening and scatter degrade CT numbers and can create streaks.
- More mAs reduces noise but does not fix poor contrast timing or motion.