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CTDI / CTDIvol / DLP measurement and pitch relations

A PMMA CTDI phantom cross-section with a central hole and four peripheral holes, measured by a 100 mm pencil ionisation chamber. CTDIw = one third centre plus two thirds periphery; CTDIvol = CTDIw divided by pitch; DLP = CTDIvol times scan length; effective dose = k times DLP. Higher pitch lowers CTDIvol and DLP; the head phantom is 16 cm and the body phantom 32 cm.

CT dose metrics: CTDI phantom, pencil chamber, and the CTDIw to DLP to effective-dose chain A PMMA CTDI phantom cross-section with a central hole and four peripheral holes, measured by a 100 mm pencil ionisation chamber. CTDIw = one third centre plus two thirds periphery; CTDIvol = CTDIw divided by pitch; DLP = CTDIvol times scan length; effective dose = k times DLP. Higher pitch lowers CTDIvol and DLP; the head phantom is 16 cm and the body phantom 32 cm. CT dosimetry: phantom output to patient effective dose peripheral holes (× 4) central hole 100 mm pencil ionisation chamber 100 mm, integrated per single rotation derived dose metrics CTDIw = ⅓ centre + ⅔ periphery weighted dose index — mGy CTDIvol = CTDIw ÷ pitch volume dose index — mGy DLP = CTDIvol × scan length dose–length product — mGy·cm E = k × DLP effective dose — mSv CTDIw = ⅓ × central + ⅔ × mean peripheral dose (mGy); read with a 100 mm pencil chamber per rotation. CTDIvol = CTDIw ÷ pitch, so pitch > 1 lowers CTDIvol — an inverse relation — and, at fixed length, lowers DLP. DLP = CTDIvol × scan length (mGy·cm): DLP rises with scan length and number of rotations. Head phantom 16 cm, body 32 cm; equal kVp/mAs gives higher head CTDIw. E = k × DLP is calculated, not measured.
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