CT PRINCIPLES

What is the basic principle of CT image formation

X-rays measure tissue attenuation from multiple angles; a computer reconstructs cross-sectional images.

What does a CT detector measure

X-ray photons transmitted through the patient after attenuation.

What determines how strongly a tissue attenuates x-rays

Mainly tissue density, atomic number and photon energy.

What is a Hounsfield unit (HU)

A numerical representation of tissue attenuation relative to water.

What are the reference HU values for water and air

Water = 0 HU; air = −1000 HU.

What are useful approximate HU values to know

Fat ≈ −100; soft tissue ≈ +20–80; bone = hundreds to thousands HU.

What does window width (WW) control

The range of HU displayed and therefore displayed contrast.

What happens when WW is narrowed

Fewer HU displayed → fewer gray shades → higher displayed contrast.

What happens when WW is widened

More HU displayed → more gray shades → lower displayed contrast.

What does window level (WL) represent

The centre HU of the displayed window.

How are the upper and lower window limits calculated

Upper = WL + ½WW; lower = WL − ½WW.

What is spatial resolution

Ability to distinguish small, closely spaced structures as separate.

What is low-contrast resolution

Ability to distinguish tissues with small differences in attenuation.

What is temporal resolution

Ability to accurately image structures that change or move rapidly.

What primarily improves CT temporal resolution

Faster data acquisition, especially faster gantry rotation.

What does MTF measure

How well an imaging system preserves contrast at different spatial frequencies.

What does image noise represent in CT

Random variation in pixel HU values that reduces low-contrast visibility.

What is the relationship between photons and CT noise

More detected photons → less quantum noise; fewer photons → more noise.

What is pitch in helical CT

Table travel per rotation ÷ total x-ray beam width.

What generally happens when pitch increases

Faster coverage and usually lower dose, with less sampling/overlap.

What is a voxel

A 3D volume element represented by a pixel on the reconstructed CT image.

What causes partial-volume averaging

Different tissues occupy one voxel and their attenuation values are averaged.

How can partial-volume averaging be reduced

Use thinner slices/smaller voxels.

What is CT aliasing

An artifact caused by insufficient sampling of the object/projection data.

What is beam hardening

Preferential absorption of low-energy photons, increasing mean beam energy and causing artifacts.

What is MPR

Reformatting volumetric CT data into other planes such as coronal or sagittal.

What is segmentation

Isolating selected tissues or structures from a CT dataset.

What is surface rendering

Threshold-based 3D display of selected structure surfaces.

What does edge enhancement do

Emphasizes abrupt pixel-value changes to increase apparent boundary sharpness.

What is raw CT data?

Detector measurements acquired before image reconstruction.

What is a CT reconstruction kernel?

Processing that determines image characteristics such as sharpness and noise.

Sharp vs smooth CT reconstruction kernel?

Sharp: ↑ spatial resolution/noise. Smooth: ↓ noise, ↑ low-contrast visibility.

What is MIP?

Displays the highest-attenuation voxel along each viewing ray.

What is iterative reconstruction mainly used for?

Reduces image noise, allowing acceptable image quality at lower dose.