The CTDIvol value you see on the CT console before scanning is a summary of that protocol's dose output. But what exactly does this number measure? Short answer: CTDI is an index that summarizes the dose the scanner produces in a standard phantom for a scan — not the actual dose to a patient. This article walks through the CTDI family (CTDI100 → CTDIw → CTDIvol) step by step. For the broader picture: Dose in CT.
What is CTDI?
CTDI (Computed Tomography Dose Index) defines the dose delivered in a single axial rotation, divided by the beam width.1 Its purpose from the start was to make the dose output of different CT scanners comparable.1 CTDI is not a single number but a family of related concepts: the measured CTDI100, the weighted CTDIw, and the volume CTDIvol.
How is it measured?
Measurement uses a 100 mm cylindrical ("pencil") ionization chamber in a standard PMMA phantom: 16 cm diameter for the head, 32 cm for the body.1 The pencil chamber is placed in turn in the center hole and the peripheral holes. CTDI100 is the integral of the dose distribution over ±50 mm along the z-axis.1 The center and peripheral measurements are combined with a ⅓–⅔ weighting to give CTDIw, representing the average dose in the slice:1
CTDIvol and pitch
In helical (spiral) scanning, dose is inversely related to pitch: pitch is the table travel per full gantry rotation divided by the nominal beam width.1 So CTDIw converts to the volume CTDIvol as:1
As pitch rises (the table moves faster), CTDIvol drops. This is the value shown on the console; the scanner displays a factory-measured value over the kV range, scaled by mAs and pitch.1 Multiplying CTDIvol by the scan length (L) gives the DLP.
Is it patient dose?
No — and this is the most commonly confused point. CTDIvol is a dose index, not designed as a patient dose.1 It is measured in a standard PMMA phantom; it does not represent a real patient's size, shape or organ dose. Even if two patients are scanned at the same CTDIvol, the actual dose to a thin and a large patient differs. For a size-aware correction, SSDE (size-specific dose estimate) is used; detail is in Dose in CT.
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. §11 (CT): CTDI ve CTDI100 — 100 mm 'kalem' (pencil) iyon odası, 16 cm (baş) / 32 cm (gövde) PMMA fantom (Şekil 11-7, Denklem 11-8); CTDIw = ⅓·merkez + ⅔·periferi (Denklem 11-9); pitch tanımı ve doz ∝ 1/pitch (Denklem 11-10); CTDIvol = CTDIw/pitch (Denklem 11-11); CTDIvol'ün bir doz indeksi olduğu, hasta dozu olarak tasarlanmadığı (s.388–390). Sayfa numaraları bu baskıya aittir.
- İlişkili: BT'de Doz (CTDI, DLP, SSDE) · DLP Nedir? · BT Görüntüleme Parametreleri