--- title: "Computed tomography, CT vs X-ray, biological effects" subject: "Biomedical Instrumentation" chapter: 5 key: "semester_7/biomedical_instrumentation:computed-tomography-ct-xray" source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/computed-tomography-ct-xray site: BCT Engineering --- # Computed tomography, CT vs X-ray, biological effects Biomedical Instrumentation · Chapter 5: Non- Invasive Diagnostic Instruments - Asked in: 2075 Chaitra, 10 marks - Video: [What is Computed Tomography (CT) and how does it work?](https://www.youtube.com/watch?v=0OdNI3lSgLc) — Siemens Healthineers - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/computed-tomography-ct-xray - Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-5 ## Question 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. ## 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. ## Answer **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): 1. **Translate-rotate (pencil beam):** the source and detector translate across the patient, then rotate by 1°, repeating up to 180°. 2. **Translate-rotate with fan beam:** a few detectors, fewer rotations. 3. **Rotate-rotate:** a wide fan beam covers the whole body; the source and a detector array rotate together, giving fast scans (a few seconds). 4. **Rotate-fixed (stationary detector ring):** a ring of 600 to 4800 detectors; only the tube rotates. 5. **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 ```text [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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a 'profile' in CT? A. The set of intensity readings across the beam at one angle (a projection) B. The final reconstructed image C. The patient's outline D. The X-ray tube voltage setting Answer: A. A profile is a projection: detector readings across the beam at one angle. (Key point: Profile) 2. What gives better resolution in CT reconstruction? A. Fewer projections and wide collimation B. Many projections, narrow slice collimation, many small detectors and a finer matrix C. Using a lower matrix such as 64 x 64 D. Only increasing the tube current Answer: B. More profiles, thin collimation, many small detectors and a finer matrix (512 x 512) improve resolution. (Key point: Better resolution) 3. Which is a correct difference between CT and conventional X-ray? A. CT gives overlapped projections, X-ray gives slices B. CT gives cross-sectional slices without overlap and excellent soft-tissue contrast, but at higher dose and cost C. CT uses film, X-ray uses electronic detectors D. CT has a lower dose than X-ray Answer: B. CT uses electronic detectors and computer reconstruction for slices; dose and cost are higher. (Key point: CT vs X-ray) 4. Why are X-rays biologically harmful? A. They heat the body only B. They are ionising radiation that produces free radicals and damages DNA C. They are non-ionising and act only on bone D. They only affect the skin surface by pressure Answer: B. X-rays ionise water into free radicals and break DNA, causing somatic and genetic effects. (Key point: Biological effects) ## Related in Biomedical Instrumentation - [MRI components, image detection, MRI vs CT](https://www.bctengineer.com/question.md?key=semester_7%2Fbiomedical_instrumentation%3Amri-components-vs-ct) — Biomedical Instrumentation, chapter 5 ## Related in other subjects - [Biological treatment principles; grit vs sedimentation](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-vs-sedimentation-differences) — Sanitary Engineering, chapter 7 - [Computer image processing with example](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Acomputer-image-processing) — Multimedia System, chapter 3 - [Digital vs analog communication; DCS block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-vs-analog-dcs-block) — Communication Systems, chapter 1 - [Conventional vs non-conventional sources with schematic](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconventional-vs-nonconventional-sources-schematic) — Power System Analysis I, chapter 1