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What Is DLP? (Dose-Length Product)

DLP (dose-length product) summarizes a CT scan's total dose output: it multiplies CTDIvol by the scan length. If CTDIvol is 'intensity per slice', DLP is 'the total over the whole scan'. How is DLP computed, why is it linked to effective dose, and what is the k-factor? Concise, grounded in Bushberg.

Intermediate · radiologists, technologists & physicists· ~7 min read· Sourced

A CT report's dose information shows two numbers: CTDIvol and DLP. The first is "dose intensity per slice", the second is "the total over the whole scan". DLP (dose-length product) summarizes a scan's total dose output by multiplying CTDIvol by the scanned length. This short article covers what DLP is, how it differs from CTDIvol, and how it links to effective dose.

What is DLP?

DLP (Dose-Length Product) is CTDIvol multiplied by the scan length (L):1

DLP = CTDIvol × L   (mGy·cm)

Its unit is mGy·cm. The logic is intuitive: CTDIvol gives the dose intensity at the slice level; multiply it by the scan length and you get the total dose output of the scan. Scanning a longer region at the same CTDIvol raises the DLP — and the total exposure.

DLP = CTDIvol × scan length (L)CTDIvol×scan length L (cm)=DLP(mGy·cm)
CTDIvol is the dose intensity per slice; multiplied by the scan length it gives DLP, the total dose output of the scan.1

Difference from CTDIvol

The two answer different questions. CTDIvol answers "how intensely does this protocol irradiate at each slice?"; DLP answers "how much irradiation was there over the whole scan?". A short head CT and a long abdomen-pelvis CT can have the same CTDIvol, but the long scan produces a larger DLP. So DLP is more informative when tracking total dose burden and diagnostic reference levels (DRLs).

Effective dose & k

DLP's most useful practical property is that it is approximately proportional to effective dose.1 The slope of that proportionality is the k-factor, which varies with the body region:1

E ≈ k × DLP
Examination k (mSv / mGy·cm)
Head 0.0021
Chest 0.014
Abdomen 0.015
Abdomen-pelvis 0.015
A caution
The effective dose computed with a k-factor is a rough estimate: it rests on population-average coefficients and does not fully reflect an individual patient's size and organ distribution. Still, it is a practical tool for comparing the relative risk of different scans. Values are from AAPM Report 96.2
Related articles
What Is CTDI? · Dose in CT (CTDI, DLP, SSDE) · Why Does Dose Matter? · The ALARA Principle

References

  1. Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. §11 (CT): DLP = CTDIvol × tarama uzunluğu L (Denklem 11-12); DLP'nin efektif dozla yaklaşık orantılı olması, eğimin k-faktörü olması ve incelemeye göre değişmesi — Tablo 11-5 (AAPM Report 96): kafa 0,0021; toraks 0,014; abdomen 0,015 mSv/(mGy·cm) (s.390). Sayfa numaraları bu baskıya aittir.
  2. AAPM Report 96. The Measurement, Reporting, and Management of Radiation Dose in CT, 2008 — DLP, CTDIvol ve k-faktörü kaynağı. aapm.org
  3. İlişkili: CTDI Nedir? · BT'de Doz (CTDI, DLP, SSDE)
Note: This content is for education; for clinical decisions or regulatory compliance, consult a qualified medical physicist and current regulations.

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