- 2075 Chaitra · 10 marks
Computed tomography, CT vs X-ray, biological effects
Short answer
Computed tomography reconstructs cross-sectional images from X-ray attenuation profiles taken at many angles, using translate-rotate or rotate-rotate scans with narrow collimation. It differs from conventional X-ray by providing slice images without overlap, using computer reconstruction. X-rays ionize tissues, causing somatic effects like cancer and cataracts, and genetic mutations.
What is Computer Tomography? How profiles are acquired for better resolution in image reconstruction technique in CT? Explain with schematic block diagram. What are the differences between CT and X-ray machine? Explain some biological effects of x-rays.
Computed Tomography (CT) is an imaging technique in which X-ray transmission measurements are taken from many angles around a cross-section of the body, and a computer reconstructs a cross-sectional (slice) image of the tissue's X-ray attenuation.
Acquisition of profiles
A profile (projection) is the set of intensity readings across the beam at one angle. Many profiles at different angles are needed; more profiles, more detector readings and thin collimation give better resolution. Methods (scanner generations):
- Translate-rotate (pencil beam): the source and detector translate across the patient, then rotate by 1°, repeating up to 180°.
- Translate-rotate with fan beam: a few detectors, fewer rotations.
- Rotate-rotate: a wide fan beam covers the whole body; the source and a detector array rotate together, giving fast scans (a few seconds).
- Rotate-fixed (stationary detector ring): a ring of 600 to 4800 detectors; only the tube rotates.
- Spiral/helical CT (later development): continuous rotation while the table moves.
Resolution improves with a large number of projections (about 180 to 1000), narrow slice collimation, many small detectors, and a finer matrix (512 × 512).
Schematic block diagram
[X-ray tube + HV gen] -> [Collimator] -> Patient -> [Detector array]
^ |
[Gantry control/ rotation] [Data acquisition
^ system (amp, ADC)]
| |
[Computer: reconstruction (filtered back projection)] <+
| |
[Display] [Storage / hard copy]
The computer solves for the absorption coefficient of each pixel, expressed as a CT number in Hounsfield units: HU = 1000 (µ − µw)/µw.
Differences between CT and X-ray
| Point | Conventional X-ray | CT |
|---|---|---|
| Image | 2D projection of overlapped structures | Cross-sectional slices, no overlap |
| Detector | Film or screen | Electronic detectors |
| Contrast | Poor for soft tissue | Excellent soft tissue contrast |
| Processing | Direct | Computer reconstruction |
| Dose | Lower | Higher |
| Cost | Low | High |
Biological effects of X-rays
- X-rays are ionising radiation. They damage cells by ionising water, which produces free radicals, and by breaking DNA.
- Somatic effects: skin burns (erythema), hair loss, cataract, blood changes (leukaemia), cancer, sterility, and radiation sickness at high doses.
- Genetic effects: mutations in germ cells that pass to offspring.
- A foetus is very sensitive. Rapidly dividing cells (bone marrow, gonads) are more sensitive.
- Protection: minimum dose, shielding (lead apron), distance, time, and dose limits (about 20 mSv/year for workers).
✓ Answer checked by Claude Opus 5.5 (high effort)
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