6.2 Core Imaging Parameters — kVp, mAs, Pitch & Collimation
Key Takeaways
- kVp controls x-ray beam energy and penetration; mAs (= mA x seconds) controls photon quantity and image noise - raising mAs does not fix an underpenetrated image on a large patient.
- Pitch = table feed per rotation / total beam collimation width; pitch below 1 overlaps data and raises dose, pitch above 1 leaves gaps and lowers dose.
- Worked example: a 64 x 0.625 mm detector configuration gives a 40 mm beam width; a 60 mm table feed per rotation yields a pitch of 1.5.
- Collimation/beam width at the tube is pre-patient beam shaping and is distinct from the post-patient detector collimation covered under CT unit components.
- Patient dose scales linearly with mAs, nonlinearly (roughly squared to cubed) with kVp, and inversely with pitch.
Why This Topic Matters
Once you know what the CT hardware does, the exam moves to how a technologist controls that hardware for every single scan: kVp, mAs, pitch, and collimation/beam width are the four parameters a technologist actively selects (or that a protocol pre-selects) for essentially every acquisition. These four items sit inside "Imaging Parameters," part of the 30-question Image Formation subcategory, but they also reappear indirectly across Safety (dose optimization) and Procedures (protocol-specific parameter choices) — meaning a solid grasp here pays off across multiple domains, not just Image Production.
kVp — Beam Energy and Penetration
kVp (peak kilovoltage) is the maximum electrical potential applied across the x-ray tube, and it determines the energy and penetrating power of the x-ray beam. Common CT kVp settings run from about 70-80 kVp (pediatric, low-dose, or CT angiography protocols benefiting from higher iodine contrast) up to 120 kVp (standard adult body imaging) and 140 kVp (large or obese patients, where more penetration is needed to reach the detector with adequate signal). Raising kVp:
- Increases beam penetration through tissue (fewer photons absorbed/scattered before reaching the detector)
- Reduces image contrast somewhat and can increase beam hardening effects (low-energy photons are absorbed first, leaving a higher mean beam energy)
- Increases patient dose roughly with the square to cube of the kVp change (a nonlinear relationship, unlike mAs)
mAs — Photon Quantity and Noise
mAs (milliampere-seconds) is the product of tube current (mA) and exposure time per rotation (seconds): mAs = mA x s. It controls the quantity of photons produced, not their energy. Raising mAs:
- Increases the number of photons reaching the detector, which reduces quantized mottle (quantum noise) and produces a smoother image
- Increases patient dose in direct linear proportion — doubling mAs doubles dose
- Does not meaningfully improve beam penetration through a large patient the way raising kVp does
The critical exam distinction: kVp changes penetration and beam energy; mAs changes photon quantity and noise. A technologist scanning a 140 kg patient who only raises mAs (not kVp) may still get an underpenetrated, noisy, low-quality image because the beam itself is not energetic enough to pass through the added tissue.
Pitch — Table Speed Relative to Beam Width
Pitch is a unitless ratio: Pitch = table feed per gantry rotation (mm) / total beam collimation width (mm).
Worked example: a scanner with a 64 x 0.625 mm detector configuration has a total beam width of 64 x 0.625 = 40 mm. If the table advances 60 mm per rotation, pitch = 60 / 40 = 1.5.
- Pitch = 1: table feed exactly matches beam width — contiguous coverage, no gap or overlap.
- Pitch < 1: the table moves less than the beam width per rotation, so successive rotations overlap — this increases dose but improves z-axis sampling/resolution, useful for CT angiography and other studies needing high z-axis detail.
- Pitch > 1: the table moves more than the beam width per rotation, leaving gaps that must be filled by interpolation — this decreases dose and speeds up coverage (useful for large-volume or breath-hold-limited scans) but can reduce z-axis resolution and, if pushed too high, introduce helical/interpolation artifacts.
Collimation / Beam Width
Collimation (also called beam width) refers to the width of the primary x-ray beam as shaped at the tube (pre-patient collimation) before it reaches the patient and detector array. Total beam width is the product of the number of active detector rows and the width of each row (as in the 40 mm example above). Increasing collimation (a wider beam, using more detector rows) covers more anatomy per rotation, which shortens scan time and can reduce motion artifact and contrast bolus timing challenges — but a wider beam also increases scatter radiation reaching the detector, which can reduce image quality if scatter correction is inadequate.
Parameter Comparison Table
| Parameter | Formula/Definition | Increasing It Mainly Affects | Dose Relationship |
|---|---|---|---|
| kVp | Peak tube voltage | Beam energy/penetration, contrast, beam hardening | Nonlinear (roughly kVp²-kVp³) |
| mAs | mA x rotation time (s) | Photon quantity, image noise | Direct linear |
| Pitch | Table feed / beam width | Z-axis sampling, scan speed | Inverse (higher pitch = lower dose) |
| Collimation | Active detector rows x row width | Anatomic coverage per rotation, scatter | Higher scatter can require dose correction |
Exam Scenario
A CT coronary angiography protocol calls for a pitch of 0.2-0.3 to support accurate cardiac reconstruction, while a routine trauma pan-scan protocol on a stable adult might use a pitch near 1.0-1.5 to move quickly through a large volume. Why the difference? Cardiac imaging needs dense z-axis sampling (overlapping data) to reconstruct a moving structure without gaps, accepting the dose trade-off; a trauma pan-scan prioritizes speed and dose efficiency across a large body region.
Key Traps to Avoid
- Do not assume raising mAs fixes an underpenetrated large-patient image — that is a kVp problem, not an mAs problem.
- Do not confuse pitch direction: pitch below 1 raises dose (overlap); pitch above 1 lowers dose (gaps filled by interpolation). Many candidates reverse this.
- Remember collimation here refers to the pre-patient beam-shaping width, distinct from the detector-side (post-patient) collimation discussed as part of CT unit components in the previous section.
A helical CT scan uses a total beam collimation of 20 mm and a table feed of 30 mm per gantry rotation. What is the pitch, and what does it indicate about dose relative to pitch = 1?
A technologist is scanning a large-bodied patient and the resulting images look underpenetrated and streaky, even after increasing mAs substantially. What is the most likely correction needed?
Why does a cardiac CT angiography protocol typically use a pitch well below 1.0 rather than a pitch near 1.5?