6.2 Core Imaging Parameters — kVp, mAs, Pitch & Collimation

ARRT Computed Tomography CT

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1 Introduction & Exam Overview

1.1 ARRT CT Exam Facts, Format & Scoring 1.2 How to Study & Test-Taking Strategy for the CT Exam

2 Patient Assessment, Communication & Safe Handling

2.1 Pre-Scan Assessment — Scheduling, History, Lab Values & Communication 2.2 Patient Education, Consent, Ergonomics & Positioning for the Scan 2.3 Patient Monitoring, Accessory Devices, Medications & Infection Control

3 Contrast Media & Injection Procedures

3.1 Contrast Media Types, Properties & Special Considerations 3.2 Venipuncture, Administration Routes & Power Injection Techniques 3.3 Contrast Timing, Reactions & Postprocedure Care

4 CT Radiation Physics

4.1 X-ray Production & Target Interactions 4.2 X-ray Beam Characteristics & Geometry 4.3 Photon Interactions with Matter & Tissue Attenuation

5 Radiation Protection & Dose Management

5.1 Minimizing Patient Dose — Technical Parameters & Modulation 5.2 Shielding & Patient-Specific Dose Considerations (Pediatric/Adult/Pregnancy) 5.3 Dose Measurements, Reporting & Personnel Protection

6 CT Scanner Components & Imaging Parameters

6.1 CT Unit Components — Gantry, Detectors & Data Acquisition System 6.2 Core Imaging Parameters — kVp, mAs, Pitch & Collimation 6.3 Slice Thickness, Scan Planes & Field of View

7 Data Acquisition Modes & Image Reconstruction

7.1 CT Acquisition Modes — Sequential, Helical & Volumetric Scanning 7.2 Cine/Shuttle Acquisition & Dual-Energy CT 7.3 Reconstruction Algorithms — Filtered Backprojection vs. Iterative Reconstruction 7.4 Reconstruction Parameters — Slice Thickness, Interval & Interpolation

8 Postprocessing, Image Display & Informatics

8.1 Postprocessing — MPR, 3D Rendering & Quantitative Analysis 8.2 Image Display Fundamentals — Pixels, Matrix & Field of View 8.3 Windowing & Measurement Tools — W/L, W/W & Region of Interest 8.4 CT Informatics — PACS, DICOM, Networking & Downtime Procedures

9 Image Quality & Artifacts

9.1 Image Quality Metrics — Resolution, Noise & Uniformity 9.2 CT Number, QA/QC & Accreditation 9.3 Patient-Related Artifacts — Beam Hardening, Motion, Metal & Edge Gradient 9.4 Equipment Artifacts — Rings, Streaks, Tube Arcing & Cone Beam

10 Head CT Procedures

10.1 Brain & Cranium CT — Routine and Perfusion Imaging 10.2 Temporal Bone, Orbit & Sinus CT 10.3 Maxillofacial, Mandible & TMJ CT

11 Spine & Musculoskeletal CT

11.1 Spine CT — Cervical, Thoracic & Lumbosacral Protocols 11.2 CT Myelography & Postmyelogram Imaging 11.3 Extremity CT — Upper & Lower Extremity, Postarthrogram 11.4 Shoulder, Bony Pelvis & Hip CT

12 Neck & Chest CT

12.1 Neck CT — Larynx/Airway & Soft Tissue 12.2 Cardiac CT — Coronary Angiography, Calcium Scoring, Gating & TAVR 12.3 Chest CT — Mediastinum, Airway, Chest Wall & Esophagram 12.4 HRCT, Lung Nodules & Low-Dose Lung Cancer Screening

13 Abdominal CT Procedures

13.1 Multiphase Liver, Pancreas & Adrenal CT 13.2 Renal CT — Multiphase Kidney, Urogram & Stone Protocol 13.3 GI Tract CT & Spleen Imaging

14 Pelvic CT Procedures

14.1 Pelvis CT — Bladder, Cystogram & Reproductive Organs 14.2 CT Colonography & Colorectal Imaging

15 Vascular, Interventional & Trauma CT

15.1 CT Angiography & Vascular Procedures — CTA, CTV, Runoff, Dissection & PE 15.2 CT-Guided Interventional Procedures — Biopsy, Drainage & Aspiration 15.3 CT Trauma Protocols & Surgical Planning

6.2 Core Imaging Parameters — kVp, mAs, Pitch & Collimation

Key Takeaways Explain This Topic AI

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.

Last updated: July 2026

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.