4.1 Principle of CT scan

Computed tomography (CT) produces cross-sectional images (slices) of the body by measuring how much a thin, rotating X-ray beam is attenuated along hundreds of thousands of paths through the patient and then calculating, with a computer, the attenuation of every small volume element in the slice. The idea was developed by Godfrey Hounsfield, whose first clinical scanner was used in 1971, building on the mathematics of Allan Cormack; the two shared the 1979 Nobel Prize. Unlike a plain radiograph, which superimposes all structures on one image, CT separates them, so differences in tissue density of well under one percent can be seen. Example: the radiology department of Moi Teaching and Referral Hospital in Eldoret relies on its multislice CT for head injury cases referred from across the North Rift.

Basic principle: attenuation of X-rays

When a narrow beam of X-rays of intensity I₀ passes through a thickness x of uniform material, the transmitted intensity I is:

I = I₀ × e^(−μx)

where μ is the linear attenuation coefficient of the material (per cm). The body is not uniform, so along each ray path the detector measures the sum of all the attenuation coefficients the ray passes through:

ln(I₀ ÷ I) = μ₁Δx + μ₂Δx + μ₃Δx + ... (a projection value)

One rotation of the tube gives a set of such projection values from many angles. The computer then solves for the μ value of each voxel (volume element) in the slice, a process called image reconstruction (topic 4.3).

CT numbers (Hounsfield units)

The calculated μ of each voxel is expressed on a scale relative to water, the CT number or Hounsfield unit (HU):

CT number = 1000 × (μ tissue − μ water) ÷ (μ water − μ air)

Because μ of air is close to zero, this is approximately 1000 × (μ tissue − μ water) ÷ μ water.

Each pixel's CT number is mapped to a grey level for display (see 4.6), so that bone appears white, air black and soft tissues in shades of grey.

Main components of a CT scanner

Scanner generations

• First generation: A pencil beam and a single detector, translated across the patient then rotated by 1 degree; about 5 minutes per slice (EMI head scanner).
• Second generation: A narrow fan beam and a small row of detectors, still translate and rotate; faster, about 20 seconds per slice.
• Third generation: A wide fan beam covering the whole patient with a curved detector array; tube and detectors rotate together (rotate-rotate). This is the design of almost all modern scanners.
• Fourth generation: A stationary full ring of detectors with only the tube rotating (rotate-stationary); now rarely produced.
• Electron beam CT: No moving parts; an electron beam is steered onto tungsten target rings, giving very fast cardiac scans.

Helical and multislice CT

In helical (spiral) CT, slip rings allow the tube to rotate continuously while the table moves, so the beam traces a helix through the patient and a whole region is scanned in one breath-hold. Multislice (multidetector) scanners have many detector rows (16, 64, 128, 256 or more) and acquire many slices per rotation, with rotation times of about 0.25 to 0.5 seconds. Pitch describes how fast the table moves:

Pitch = table travel per rotation ÷ total collimated beam width

Example: table travel 40 mm per rotation, beam width 64 × 0.625 mm = 40 mm, so pitch = 40 ÷ 40 = 1. A pitch above 1 lowers the dose and scan time; a pitch below 1 overlaps the data and improves image quality at a higher dose.

Why CT matters to the maintenance technician

Every part of the chain, from tube output and beam filtration to detector response and the data system, affects the CT numbers and the image. The technician must understand the principle so that faults such as ring artefacts (a bad detector channel), wrong CT numbers (calibration drift) or noise (low tube output) can be traced to the right component. CT is also a high-dose examination, so correct functioning of collimation, dose display and interlocks is part of radiation safety under the Kenya Nuclear Regulatory Authority's licensing conditions.