CTDI, DLP, SSDE, and why a single CTDI is not the patient's dose.
For Radiologists · radiographers · physicists·8 min
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You will be able to
01Say what CTDI vol averages, and what it hides.
02Turn CTDI into SSDE with a size factor.
03Name average glandular dose as the mammography quantity.
Chapter 01
A number on the console
CTDI vol is a standardised measurement in a phantom, not a person. It averages centre and periphery over a slice, then folds in pitch. DLP multiplies by scan length. Both are useful for audits. Neither is organ dose. A small patient in a large-phantom CTDI is under-dosed on paper and over-dosed in truth — that is why SSDE exists.
Quote CTDI with phantom size, kV, and collimation.
Simulation
Weighted and volumetric CTDI — Change the numbers; the scene follows.
Variables
CTDI_w = 10.6667 mGy
CTDIvol=31CTDIc+32CTDIp
Keep thisQuote CTDI with phantom size, kV, and collimation.
SSDE multiplies CTDI vol by a factor from water-equivalent diameter. A 16 cm paediatric body is not a 32 cm adult. Conversion factors from AAPM 204 and 220 are the language of a dose report that a radiologist can defend.
Simulation
Size-specific dose estimate SSDE — Change the numbers; the scene follows.
Variables
f = 1.3255
Keep thisIf you only report CTDI, you have not reported this patient.
Mammography quotes average glandular dose. Compression, kV, target and filter, and glandularity all move it. The paddle thins the breast so scatter falls and AGD falls with it. That is not comfort theatre. It is dosimetry.
Simulation
Mean glandular dose (Dance) — Change the numbers; the scene follows.
Variables
AGD = 1.464 mGy
Keep thisAGD is the quantity. ESD is not a substitute.