Resources/CT fundamentals
Dose metrics
CTDIvol measures dose intensity — how much dose the protocol delivers per slice of a standard phantom. DLP multiplies that by scan length to express the total for the exam. Read both after every scan; optimization begins with knowing your numbers.
The two numbers on the console
| Metric | Unit | Meaning |
|---|---|---|
| CTDIvol | mGy | Dose intensity of the protocol, independent of scan length |
| DLP | mGy·cm | CTDIvol × scan length — the whole-exam figure |
A routine adult chest CT commonly sits around a CTDIvol of 5–10 mGy with a DLP of a few hundred mGy·cm; national diagnostic reference levels give you the benchmark for your protocol and patient size. If your numbers run consistently above reference, the protocol — not the patient — is usually the reason.
What moves them
| Change | Effect on dose | Effect on image |
|---|---|---|
| mAs −50% | Dose −50% | Noise up ~41% (1/√mAs) |
| kVp 120→100 | Dose down sharply | More contrast, more noise; size-dependent |
| Pitch up | Dose down (per-slice) | Possible helical artifacts, less overlap |
| Scan length +5 cm | DLP up proportionally | No image benefit if outside the indication |
Optimization as a habit
ALARA is not a slide — it is the loop of reading your CTDIvol and DLP after every scan, comparing against the reference level, and knowing which parameter you would trim next. Learners who build that loop in the simulator carry it to every patient they ever scan.
Try it in ScanFlo: Run the chest protocol at 200 mAs, then again at 100 mAs. Compare noise in the liver on the mediastinal window, and compare the CTDIvol readouts. Decide: was the extra dose buying you anything diagnostic?