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What Is Pitch? (CT Pitch)

Pitch is one of helical CT's most fundamental yet most confused parameters: it defines how far the table travels per tube rotation relative to the beam width. A single number affects scan speed, dose and image continuity. What is pitch, what does dose ∝ 1/pitch mean, and how is it linked to CTDIvol? Concise, grounded in Bushberg.

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

In helical CT the tube rotates, the table slides, and a helical (spiral) trajectory is traced around the patient. A single number sets how "tight or loose" that helix is: the pitch. Small pitch means a tight (overlapping) helix; large pitch a loose one. This simple ratio directly affects scan time, dose and image continuity. Broader picture: CT Imaging Parameters.

What is pitch?

As Bushberg defines it, pitch is the distance the table travels during one full gantry rotation (360°) divided by the nominal beam width (nT):1

pitch = (table travel per 360°, mm) / (nominal beam width nT, mm)

The intuitive reading: at pitch = 1 the table advances by exactly one beam width per rotation — slices are contiguous. At pitch < 1 the table advances less and successive rotations overlap (more data, more dose). At pitch > 1 the table advances faster than the beam width — the scan speeds up and dose falls.

Pitch · tightness of the helical pathpitch < 1 (overlap)pitch = 1 (contiguous)pitch > 1 (spaced)↑ dosebalance↓ dose, ↑ speed
As pitch rises the helix loosens: pitch<1 overlaps (more dose), pitch=1 is contiguous, pitch>1 is sparser (faster, less dose).1

Pitch and dose

In helical CT, dose is inversely proportional to pitch:1

dose ∝ 1 / pitch

The reason is intuitive: with small pitch the same region is irradiated more times (overlapping), so dose rises; with large pitch less radiation falls on the same region. This relationship appears directly in the CTDIvol formula:1

CTDIvol = CTDIw / pitch

So with all else fixed, doubling the pitch halves CTDIvol (and dose).

High or low?

There is no single "right" pitch; it depends on the task. High pitch scans faster (less prone to motion artifact, less dose) but may trade off z-axis sampling and image continuity. Low pitch gives denser sampling and continuity (e.g. cardiac or high-resolution work) but raises dose. The right choice is the balance that answers the clinical question at the lowest dose — i.e. ALARA.

In a nutshell
Pitch = table travel / beam width. dose ∝ 1/pitch and CTDIvol = CTDIw/pitch. High pitch = fast, low dose; low pitch = dense sampling, high dose. The choice depends on the task and ALARA.
Related articles
What Is CTDI? · What Is DLP? · CT Imaging Parameters · Dose in CT

References

  1. Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. §11 (CT): pitch = gantrinin tam bir dönüşünde (360°) masa ilerlemesi / nominal ışın genişliği (nT) (Denklem 11-10); sarmal BT'de doz ∝ 1/pitch; CTDIvol = CTDIw / pitch (Denklem 11-11) (s.389). Sayfa numaraları bu baskıya aittir.
  2. İlişkili: CTDI Nedir? · BT Görüntüleme Parametreleri · BT'de Doz
Note: This content is for education; for clinical decisions or regulatory compliance, consult a qualified medical physicist and current regulations.

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