Phase 1: Foundations of Imaging Physics, Radiobiology and Basic Pathology Correlation
What Phase 1 tests
Phase 1 asks whether you understand why an image looks the way it does. Core concepts: X-ray production and beam quality (kVp controls penetration and subject contrast; mAs controls quantity), the photoelectric versus Compton interaction and how each shapes contrast and noise, and detector/DR physics including the detective quantum efficiency. For CT you must be fluent in Hounsfield unit definition, window width/level, pitch and its effect on dose and z-axis resolution, and the trade-off between slice thickness and partial-volume averaging. For MRI, know T1/T2/PD contrast mechanisms, the effect of TR and TE, and the safety rules (projectile risk, heating, gadolinium and renal function). Ultrasound basics: frequency versus resolution versus penetration, and artefacts such as posterior acoustic enhancement and shadowing. Radiobiology: the linear-quadratic model, alpha/beta ratios, fractionation, and the ALARA/dose-reference-level framework. Pathology correlation is limited but real: recognise how oedema, fat, calcification, haemorrhage and fibrosis alter attenuation or signal.
Traps
Questions love the inverse relationship: increasing kVp lowers subject contrast but reduces dose; increasing pitch lowers dose but can degrade z-resolution. Another trap is confusing spatial resolution with contrast resolution. In MRI, remember that fat is bright on T1 and (usually) bright on fast spin-echo T2, so 'fat is dark on T2' is a classic distractor. In radiobiology, do not mix up alpha/beta of tumour (high, ~10 Gy) with late-responding tissue (low, ~3 Gy).
Rule of thumb
For any physics stem, ask: which variable is being held constant? Most errors come from ignoring the fixed parameter. Worked example: if kVp rises from 70 to 90 at fixed mAs, expect lower contrast, greater penetration and lower patient dose per exposure.