CT Scan (Computerized Tomography) — How It Works, Benefits & Recovery — Procedure Guide, Recovery & Risks | MyMedicPlus

Computerised tomography (CT) uses a rotating X-ray tube and an opposing detector array to acquire multiple cross-sectional images (slices) of the body in seconds, reconstructed by computer into detailed 2D and 3D images. Modern multidetector CT (MDCT) scanners with 64–320 detector rows acquire the entire chest or abdomen in under 10 seconds. CT provides superior visualisation of bone, blood vessels, solid organs, and soft tissues compared to plain X-ray, and is significantly faster than MRI. Iodinated intravenous contrast agent enhances blood vessels, tumours, and areas of inflammation, enabling phase-specific imaging. CT is the primary cross-sectional imaging modality in most emergency and oncological settings worldwide. Over 80 million CT scans are performed annually in the United States alone; the NHS performs approximately 6 million per year in England. The technology was invented by Sir Godfrey Hounsfield in 1972 at EMI Laboratories, earning him the Nobel Prize in Physiology or Medicine in 1979. Tissue attenuation is measured in Hounsfield units (HU): water is 0 HU, air –1,000 HU, compact bone +1,000 HU, and fat approximately –100 HU. These standardised density values allow quantitative tissue characterisation and comparison between scans. CT is performed and reported by radiologists and radiographers working in hospital radiology departments, imaging centres, or dedicated cancer diagnostic units.

CT is indicated across a wide range of clinical settings. Suspected pulmonary embolism (CT pulmonary angiography, CTPA) is one of the most common emergency indications; acute stroke assessment (CT head for haemorrhage; CT perfusion for ischaemia treatment planning); trauma assessment, where whole-body CT reduces missed injury rates compared with selective imaging; cancer staging and monitoring chemotherapy or surgical response; acute abdominal pain including suspected appendicitis, bowel obstruction, diverticulitis, and pancreatitis; aortic aneurysm detection and surveillance; CT coronary angiography (CTCA) for coronary artery disease evaluation in low-to-intermediate risk chest pain; CT-guided biopsy and drainage of abscesses and fluid collections; and pre-operative surgical planning for complex procedures. Referral indications follow clinical guidelines including NICE, ACR Appropriateness Criteria, and Royal College of Radiologists iRefer guidance to ensure that CT is appropriate given its radiation dose. CT is not appropriate as a screening tool for unselected populations due to radiation exposure and high rates of incidental findings requiring investigation.

The patient lies on a motorised table that moves through the doughnut-shaped CT gantry bore. No claustrophobia comparable to MRI occurs as the bore is wide (70–90 cm) and short. A peripheral intravenous cannula is inserted for contrast administration. Iodinated contrast (typically 60–120 mL of non-ionic low-osmolar contrast at 3–4 mL/s) is injected by a power injector; the patient experiences transient warmth, a metallic taste, and flushing lasting 10–20 seconds — these are normal and not allergic reactions. Actual CT scanning takes 5–30 seconds per phase. Most abdominal CT scans include arterial phase (25–35 seconds after injection), portal venous phase (70–80 seconds), and sometimes a delayed phase (3–5 minutes) for specific indications such as urological or hepatocellular carcinoma imaging. The radiographer communicates breath-hold instructions via intercom. Oral contrast (dilute gastrografin or water) is given for selected gastrointestinal indications 45–60 minutes before scanning. A CT radiographer positions the patient and performs the scan; a consultant radiologist formally reports the images, usually with the aid of computer-aided detection software. Total appointment time including preparation, contrast injection, and scanning is typically 15–45 minutes for most clinical indications. Dual-source CT scanners use two X-ray tubes offset by 95 degrees to enable cardiac CT at very high temporal resolution (66–75 milliseconds), reducing motion artifact in tachycardic patients.

CT provides excellent diagnostic performance across multiple clinical settings. Sensitivity exceeds 95% for pulmonary embolism (CTPA), 90–95% for acute appendicitis, and approaches 100% for detecting solid organ injury in major trauma. CT coronary angiography has a negative predictive value of 99% for excluding significant coronary artery disease, making it ideal for ruling out coronary disease in low-to-intermediate risk chest pain presentations. CT-guided biopsy achieves a diagnostic yield of 90–95% for solid lesions accessible via a percutaneous route. Modern low-dose CT protocols using iterative reconstruction algorithms (ASIR, IRIS, iDose) have substantially reduced radiation dose — a chest CT now delivers 1–3 mSv, enabling repeated scanning in follow-up. CT is faster than MRI by an order of magnitude (seconds versus 30–90 minutes), available 24 hours per day in most acute hospitals, and is not contraindicated by metallic implants, pacemakers, or claustrophobia as MRI is. Dual-energy CT provides additional tissue characterisation — differentiating uric acid from calcium urinary stones, quantifying fat content in liver lesions, and virtual non-contrast reconstructions from a single contrast acquisition.

Ionising radiation: a typical abdominal CT delivers 8–10 mSv effective dose (equivalent to approximately 3–4 years of background radiation). A single scan carries an estimated lifetime attributable cancer risk increase of approximately 1 in 1,000–2,000. This small risk is almost always outweighed by the diagnostic benefit of an accurate diagnosis. Cumulative CT dose across multiple examinations is tracked by radiology departments. CT is avoided in pregnancy unless the clinical situation is life-threatening; the fetal dose from a CT abdomen is approximately 20–25 mGy, well below the threshold associated with deterministic fetal effects. Contrast reactions: mild reactions (nausea, urticaria) occur in 0.5–3% of intravenous contrast administrations; severe anaphylactoid reactions requiring treatment (bronchospasm, hypotension) in approximately 0.04%; fatal reactions in approximately 1 in 170,000 doses. Pre-medication with corticosteroids and antihistamines is used for patients with prior reactions. Contrast-induced nephropathy: clinically significant post-contrast acute kidney injury is rare in patients with eGFR above 30 mL/min/1.73m² when standard hydration is maintained; risk is substantially increased below this threshold, requiring alternatives or dose minimisation protocols.

No recovery period is required after non-contrast CT. After intravenous contrast administration, patients are advised to drink at least 500 mL of water over the following 2–4 hours to promote renal contrast clearance. Patients taking metformin for type 2 diabetes should withhold it for 48 hours after iodinated contrast if eGFR is below 60 mL/min/1.73m², due to the theoretical risk of metformin-associated lactic acidosis caused by contrast-related renal impairment. Driving is permitted immediately after non-sedated CT. Breastfeeding mothers who receive iodinated contrast can resume breastfeeding immediately — the iodine load excreted into breast milk is minimal. Incidental findings on CT — so-called incidentalomas including adrenal nodules, small pulmonary nodules, and hepatic haemangiomas — require follow-up imaging per published guidelines (Fleischner Society for pulmonary nodules, ACR for incidental abdominal findings). The referring clinician communicates significant CT findings and follow-up plans to the patient. Results are formally reported by a consultant radiologist within 24 hours for routine requests, and immediately for emergency and urgent scans.