X-ray, CT and nuclear medicine use ionising radiation; ultrasound and MRI do not. Every exposure is weighed against benefit and kept As Low As Reasonably Achievable (ALARA).
How it's asked: Units (gray, sievert, becquerel), dose limits for workers and the public, the law of Bergonié and Tribondeau, and the most radiosensitive cells and cell-cycle phase.
Why this is true
Ionising radiation knocks electrons off atoms, mostly creating free radicals from water that break DNA. Cells that divide often have less time to repair DNA before copying it, so they are the most radiosensitive — Bergonié and Tribondeau's law. Exposure falls with the inverse square of distance, so stepping back from the source cuts the dose far more than people expect; time, distance and shielding are the three levers of protection.
Key points
Choosing a modality
| Modality | Ionising? | Best for |
|---|---|---|
| X-ray | Yes | Bones, chest, first look |
| CT | Yes | Acute bleed, trauma, bone detail, lungs |
| Ultrasound | No | Gallbladder, pregnancy, FAST, vessels |
| MRI | No | Soft tissue, brain, spinal cord, joints |
| Nuclear medicine | Yes | Function — thyroid, bone, kidney, PET |
Common traps
- Sievert, not gray, compares risk across different radiation types and organs.
- Lens cataract is deterministic — it needs a threshold dose; cancer is stochastic.
Mnemonic
Protection = Time, Distance, Shielding — and distance follows the inverse square law.
Clinical case
A radiographer doubles her distance from the X-ray tube during fluoroscopy.
High-yield
Gy absorbed, Sv effective, Bq activity. Worker 20 mSv/yr, public 1 mSv/yr. Lymphocyte most sensitive; G2–M most sensitive phase. Cataract deterministic, cancer stochastic.
Quick check
Q1.The SI unit of effective dose is:
Q2.The most radiosensitive phase of the cell cycle is: