Infographics / CT Physics

Effective dose from DLP (E = k x DLP)

Two panels. Panel one: the fundamental definition, effective dose E equals the sum over irradiated tissues of tissue weighting factor w-T times equivalent dose H-T; example tissues lung, breast and colon each have w-T about 0.12. Panel two: the practical CT estimate, CTDIvol in mGy times scan length L gives DLP in mGy per cm, then times a conversion coefficient k gives effective dose E in mSv, so k equals E divided by DLP. A bar chart shows k by body region: head about 0.002 (lowest), chest about 0.014, abdomen about 0.015, pelvis about 0.015 to 0.02 (highest). Notes state DLP uses CTDIvol not CTDIw, estimates effective dose not organ absorbed dose, and that k depends on body region and scanner design.

Effective dose: from tissue weighting factors to the DLP conversion shortcut Two panels. Panel one: the fundamental definition, effective dose E equals the sum over irradiated tissues of tissue weighting factor w-T times equivalent dose H-T; example tissues lung, breast and colon each have w-T about 0.12. Panel two: the practical CT estimate, CTDIvol in mGy times scan length L gives DLP in mGy per cm, then times a conversion coefficient k gives effective dose E in mSv, so k equals E divided by DLP. A bar chart shows k by body region: head about 0.002 (lowest), chest about 0.014, abdomen about 0.015, pelvis about 0.015 to 0.02 (highest). Notes state DLP uses CTDIvol not CTDIw, estimates effective dose not organ absorbed dose, and that k depends on body region and scanner design. Effective dose: the definition and the DLP shortcut 1 · Fundamental definition — sum over the irradiated tissues E  =  Σ ( wT × HT ) tissue T wT HT (equiv. dose) wT · HT lung 0.12 Hlung 0.12 Hlung breast 0.12 Hbreast 0.12 Hbreast colon 0.12 Hcolon 0.12 Hcolon + all other organs (each × its own wT) E (mSv) Σ wT = 1 over the whole body. Tissues with high wT contribute more to E. 2 · Practical CT estimate — from scanner output to effective dose CTDIvol (mGy) × length L DLP = CTDIvol × L  (mGy·cm) × k E = k × DLP (mSv) k = E / DLP  (mSv per mGy·cm) — a region-specific coefficient, not E × DLP. DLP uses CTDIvol (includes pitch), not CTDIw; it estimates E, not organ absorbed dose. k also depends on scanner design (beam quality, filtration, geometry) — not universal. Conversion coefficient k by body region (adult, approximate): head ~0.002  lowest chest ~0.014 abdomen ~0.015 pelvis ~0.015–0.02  highest (gonads, bladder) More radiosensitive / high-wT organs in the beam → larger k → more E for the same DLP.
Practise CT PhysicsDownload SVG

More in CT Physics

Practice the Physics paper

FRCRX is a Part 1 Physics question bank: true/false items with explanations, timed mocks and topic analytics.

Start question practiceSee plans