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CT scan technical parameters kVp mAs pitch slice thickness FOV explained radiology
Here is a thorough breakdown of CT technical parameters:
CT Technical Parameters
1. kVp (Peak Kilovoltage)
kVp is the peak voltage applied across the X-ray tube, determining the energy (penetrating power) of the X-ray beam.
| kVp | Effect |
|---|---|
| Higher kVp | More penetrating beam, less noise, less contrast between tissues, higher dose |
| Lower kVp | Less penetrating, more photoelectric absorption, higher image contrast (especially with iodine contrast), but more noise and higher dose to superficial structures |
- Common values: 80, 100, 120, 140 kVp
- 120 kVp is the standard for most adult body imaging
- 80-100 kVp is preferred for contrast-enhanced scans (increases iodine attenuation) and pediatric patients
- Dose is proportional to kVp raised to the power of ~2.5-3.1 - so kVp has the greatest single effect on radiation dose
2. mA and mAs (Tube Current and Tube Current-Time Product)
- mA (milliamperes): the tube current - controls the quantity (number) of X-ray photons produced per rotation
- mAs = mA × rotation time (seconds): the total photon fluence delivered
- Effective mAs = mAs / pitch (in helical CT, pitch affects actual dose received)
| mAs | Effect |
|---|---|
| Higher mAs | More photons, less image noise, higher dose |
| Lower mAs | Fewer photons, more noise (quantum mottle), lower dose |
- Dose is directly proportional to mAs
- If mAs is halved, noise increases by a factor of √2 (~40% increase)
- Automatic Exposure Control (AEC / mA modulation) adjusts mAs based on patient size and attenuation region-by-region to optimize dose
3. Pitch (Helical CT)
Pitch = Table distance traveled per rotation ÷ Total X-ray beam width (collimation)
| Pitch value | Meaning |
|---|---|
| Pitch = 1 | Table moves exactly one beam width per rotation (no gaps, no overlap) |
| Pitch > 1 | Table moves faster than beam width - gaps in data, faster scan, less dose |
| Pitch < 1 | Overlapping data, slower scan, higher dose, better image quality |
- Typical range: 0.75 to 1.5
- Dose is inversely proportional to pitch - doubling pitch halves dose
- Higher pitch = faster scan (good for uncooperative/pediatric patients) but increased effective slice thickness and potential helical artifacts
- Cardiac CT uses very low pitch (0.2-0.4) for overlapping data
4. Slice Thickness
The thickness of each reconstructed image slice along the z-axis (longitudinal axis).
| Thin slices | Thick slices |
|---|---|
| Better z-axis (longitudinal) resolution | Less detail in z-axis |
| Less partial volume averaging | More partial volume averaging |
| More noise (for same mAs) | Less noise |
| Better for small structures, MPR, 3D reconstructions | Better for large structures, faster review |
- Typical ranges:
- Thin: 0.5 - 1.5 mm (high-resolution chest, coronary CTA)
- Standard: 3 - 5 mm (routine abdomen/pelvis)
- Thick: 7 - 10 mm (older protocols)
- Modern scanners acquire thin slices (~0.6 mm) and reconstruct thicker slices for viewing
5. FOV (Field of View)
Two types of FOV in CT:
a) Scan FOV (SFOV) / Acquisition FOV
- The diameter of the region from which data is actually collected by the detector
- Determined by the pre-patient collimator setting
- Cannot be changed after scanning
b) Display FOV (DFOV) / Reconstruction FOV
-
The diameter of the area reconstructed into the image matrix
-
Can be changed retrospectively from raw data
-
Controls pixel size: Pixel size = DFOV ÷ Matrix size (512)
- Smaller DFOV → smaller pixels → better spatial resolution (sharper image)
- Larger DFOV → larger pixels → lower spatial resolution
-
Standard body DFOV: 35-50 cm
-
Targeted DFOV (e.g., kidney, adrenal): 15-25 cm
6. Rotation Time
Time for one complete 360° rotation of the gantry.
- Typical range: 0.27 - 1.0 seconds
- Shorter rotation time = faster scan = less motion artifact
- Dose is directly proportional to rotation time
- Cardiac CT requires very fast rotation (~0.27-0.35 s) to freeze cardiac motion
7. Reconstruction Filter (Kernel)
Controls the trade-off between spatial resolution and noise in the reconstructed image.
| Kernel type | Resolution | Noise | Use |
|---|---|---|---|
| Soft/smooth kernel | Low | Low | Brain, liver, soft tissue |
| Standard kernel | Medium | Medium | Abdomen, general use |
| Bone/sharp kernel | High | High | Bone, lung HRCT, small structures |
Summary Table: Parameter-Dose-Quality Relationships
| Parameter | Increase → Dose | Increase → Noise | Increase → Resolution |
|---|---|---|---|
| kVp ↑ | Dose ↑↑↑ | Noise ↓ | Contrast resolution ↓ |
| mAs ↑ | Dose ↑ (linear) | Noise ↓ | Spatial res unchanged |
| Pitch ↑ | Dose ↓ | Noise ↑ | Z-resolution ↓ |
| Slice thickness ↑ | Dose unchanged | Noise ↓ | Z-resolution ↓ |
| DFOV ↓ | Dose unchanged | Noise unchanged | Spatial res ↑ |
| Rotation time ↑ | Dose ↑ (linear) | Noise ↓ | - |
Key Clinical Relationships to Remember
- Dose ∝ mAs × kVp^(2.5-3) × (1/pitch)
- Noise ∝ 1/√mAs - to halve noise, you must quadruple mAs (4× the dose)
- Spatial resolution is primarily determined by DFOV, matrix size, and reconstruction kernel
- For contrast-enhanced CT, lower kVp (80-100) boosts iodine conspicuity
- For obese patients, higher kVp and mAs are needed to maintain acceptable noise
- For pediatric patients, reduce kVp and mAs significantly (size-based protocols)
These parameters interact with each other and must be balanced against clinical need, patient size, and radiation dose optimization (ALARA principle).
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