Physics, Instrumentation and Radiation Safety
The physics, instrumentation and radiation safety domain tests whether you can connect X-ray tube physics to image quality and to the dose a patient receives. Expect calculations using the inverse square law, half-value layer and CT dose indices, plus practical MRI and ultrasound safety.
Photon production and beam quality
- Bremsstrahlung produces the continuous spectrum; characteristic radiation produces discrete K-shell peaks.
- kVp controls beam energy and penetration; mAs controls photon number and dose approximately linearly.
- Filtration removes low-energy photons, raises the half-value layer and lowers skin dose without changing the useful beam much.
Dose metrics and instrumentation
- Absorbed dose is in gray (Gy); equivalent and effective doses are in sievert (Sv).
- CTDIvol is scanner output measured in a phantom; DLP = CTDIvol × scan length. Effective dose is approximately DLP × k, with k about 0.014-0.015 mSv/(mGy·cm) for adult abdomen.
- Pitch below 1 overlaps slices and raises dose for the same anatomy.
Worked example
An abdominal CT has DLP 600 mGy·cm. Effective dose = 600 × 0.015 = 9 mSv, roughly 3-4 years of natural background (about 2.4 mSv/year). If pitch is reduced from 1.2 to 0.8, dose rises by 1.2/0.8 = 1.5-fold to about 13.5 mSv, so only reduce pitch when fine detail genuinely requires it.
Traps
- Treating CTDIvol as the patient's dose rather than a phantom index.
- Forgetting that a lead apron does not shield against the primary beam.
- Quoting MRI zone IV as safe for ferromagnetic implants; it is the scanner room and the highest-risk zone.