Physics, Techniques and Safety in Radiology
X-ray production and image quality
Electrons accelerated across a tube potential (kVp) strike a tungsten target, producing bremsstrahlung and characteristic radiation. Raising kVp increases beam energy and penetration, reduces photoelectric absorption and subject contrast, and increases scatter. Filtration removes low-energy photons and hardens the beam; the half-value layer quantifies beam quality. mAs controls photon quantity and therefore noise.
CT
Pitch equals table feed per rotation divided by collimator width. Pitch above 1 reduces dose and may increase noise or artefacts. CTDIvol reflects dose per unit volume in a standard phantom; DLP equals CTDIvol multiplied by scan length; effective dose equals DLP multiplied by a region-specific conversion factor (k) and is expressed in millisieverts. Iterative reconstruction lowers dose while preserving noise texture.
Radiation biology
Deterministic effects have thresholds (skin erythema, cataract). Stochastic effects (cancer, hereditary) are probabilistic with no proven threshold. Justification and optimisation underpin ICRP guidance and the ALARA principle.
MRI and ultrasound
MRI safety zones I to IV; static field, gradient and radiofrequency hazards; specific absorption rate limits; gadolinium-associated nephrogenic systemic fibrosis. Ultrasound: thermal and mechanical indices, ALARA.
Traps
CTDIvol is not patient dose; effective dose is a population-averaged risk metric, not an individual dose. Doubling distance quarters dose (inverse square law). Higher kVp does not simply mean a lower dose at constant mAs.
Rule of thumb
Effective dose (mSv) = DLP (mGy·cm) × k, with k approximately 0.015 for chest and 0.02 for abdomen.