Cross-sectional Imaging: CT, MRI and Ultrasound Fundamentals
This domain tests basic principles, safety and image interpretation concepts in CT, MRI and ultrasound. It is not a full specialist reporting examination, but it expects correct terminology and safety awareness.
Core rules
- CT uses Hounsfield units: water is 0, air about -1000, dense bone high positive. Windowing changes display, not raw data.
- CT pitch relates table movement per rotation to beam width. Higher pitch can reduce dose but may affect image quality and coverage.
- MRI contrast depends on T1, T2 and proton density; T1 shows fat bright and fluid dark; T2 shows fluid bright.
- MRI safety screening is mandatory for implants, pacemakers, metal and pregnancy. Know the zones and never assume an implant is safe.
- Ultrasound frequency trades penetration for resolution: higher frequency gives better resolution but less depth. Doppler assesses flow.
Worked example
A CT protocol uses a pitch of 1.5. The reasoning is that pitch above 1 means the table advances more than the beam width per rotation, reducing overlap and often dose, but too high a pitch can create gaps or noise depending on scanner capability. Pitch must be adjusted with kVp, mA and reconstruction, not changed in isolation.
Traps
Do not confuse CT window level with actual tissue density. Do not say all MRI implants are safe because they are non-ferrous. Do not use the highest ultrasound frequency for deep abdominal scanning. Do not ignore acoustic shadowing or artifact.