CT Scan — Purpose, Procedure & Normal Values | MyMedicPlus

A computed tomography (CT) scan — also called a CAT scan — uses a rapidly rotating X-ray tube and an array of detectors to acquire hundreds of cross-sectional images (slices) of the body in seconds. Unlike a conventional planar X-ray, which superposes all structures onto a single projection, CT reconstruction algorithms — typically filtered back-projection or iterative reconstruction — generate detailed anatomical images in axial, coronal, and sagittal planes with excellent spatial resolution. Density values of tissues are expressed in Hounsfield units (HU): air is approximately −1,000 HU, fat −100 to −50 HU, water 0 HU, soft tissue 20–80 HU, and compact bone 700–3,000 HU. Modern multi-detector CT (MDCT) scanners with 64–320 detector rows complete chest, abdominal, and pelvic studies in under 10 seconds of actual scan time, enabling cardiac and vascular CT angiography with sub-millimetre spatial resolution. Intravenous iodinated contrast medium (typically 80–120 mL of non-ionic contrast injected at 3–5 mL/second) markedly enhances blood vessels, organ parenchyma, and tumour vascularity, improving lesion detection and characterisation. CT is performed by radiographers and reported by radiologists in hospital radiology departments, imaging centres, and emergency settings worldwide.

CT is ordered across a broad range of acute and elective clinical scenarios. In emergency medicine: suspected stroke (ischaemic or haemorrhagic), acute subarachnoid haemorrhage, pulmonary embolism (CT pulmonary angiography), aortic dissection or aneurysm rupture, traumatic brain injury, thoracoabdominal trauma, and suspected appendicitis or bowel obstruction. In oncology: primary tumour detection, local staging, lymph node assessment, distant metastasis detection, treatment response evaluation, and post-treatment surveillance. In cardiology: CT coronary angiography (CTCA) for non-invasive assessment of coronary artery stenosis, calcium scoring for cardiovascular risk stratification. In nephrology and urology: kidney stones (non-contrast CT KUB), renal masses, bladder cancer staging. CT-guided interventional procedures include core needle biopsy of deep lesions, drainage catheter placement, and radiofrequency ablation planning. Radiotherapy planning uses CT images to delineate target volumes and calculate dose distribution.

For a CT scan without intravenous contrast dye, no preparation is required; you may eat and drink normally. For a CT with intravenous iodinated contrast, fast for 4–6 hours before the scan to reduce the risk of nausea and aspiration if you feel unwell after contrast injection. Inform your doctor and radiographer of a previous allergic reaction to iodinated contrast (hives, bronchospasm, or anaphylaxis), impaired renal function (serum creatinine or eGFR should be checked beforehand — contrast is generally avoided if eGFR <30 mL/min/1.73m²), type 2 diabetes treated with metformin (metformin is routinely held for 48 hours post-contrast to avoid the rare risk of metformin-associated lactic acidosis), thyroid disease or planned radioiodine therapy. Remove all metal objects before the scan: jewellery, piercings, hair clips, belt buckles, and hearing aids. Wear comfortable clothing; a hospital gown is provided for torso or abdominal scans. Arrange transport if conscious sedation is administered. Drink the oral contrast solution provided (if bowel opacification is required) 1–2 hours before your appointment.

You lie on a motorised examination table that slides smoothly through the large aperture (gantry) of the CT machine — the machine is open on both ends and not a fully enclosed tunnel, unlike an MRI scanner. An IV cannula is inserted into a forearm vein before the scan if contrast is required. The X-ray tube and detector array rotate around you silently at high speed, acquiring multiple images per rotation. For chest or abdominal CT, you will be asked to hold your breath for 10–20 seconds during each acquisition to eliminate respiratory motion blur; instructions are given via the scanner's intercom. When intravenous contrast is injected, you may experience a brief warm flushing sensation throughout your body, a transient metallic taste in the mouth, and an urge to urinate — these are normal sensations that pass within 30–60 seconds and do not indicate an allergic reaction. The actual scan time is typically 10–20 seconds per body region; however, total appointment time including preparation, positioning, and cannula removal is 20–30 minutes. You will be observed for 15–20 minutes after contrast injection before discharge.

A consultant radiologist analyses all acquired CT images on a high-resolution diagnostic workstation and issues a structured written report. Routine studies are reported within 24–48 hours; urgent or emergency CT requests in an acute hospital setting are verbally communicated to the referring clinician within minutes to 1 hour. The radiology report describes the anatomical structures visualised, any abnormalities identified — including their size (measured in three planes), location, attenuation in Hounsfield units, enhancement characteristics on contrast phases, and morphological features — and a differential diagnosis when appropriate. Standardised reporting systems improve clinical communication: Lung-RADS (1–4) for pulmonary nodules on lung cancer screening CT, Liver Imaging Reporting and Data System (LI-RADS) for hepatic lesions in cirrhotic patients, and TNM classification for cancer staging (tumour size, nodal involvement, metastasis). Hounsfield unit measurements characterise tissues: renal cysts are near 0 HU (water density); renal cell carcinomas enhance above 20 HU on contrast; pleural effusions are 0–20 HU. Your referring clinician will discuss the radiologist's report with you and recommend further investigations, treatment, or follow-up imaging as appropriate.

CT delivers ionising radiation, and doses vary by body region: head CT approximately 2 mSv; chest CT 4–7 mSv; abdomen/pelvis CT 8–15 mSv; compared to the natural annual background radiation of approximately 2.4 mSv. The estimated lifetime excess risk of radiation-induced cancer from a single abdominal CT is approximately 1 in 2,000 for a 40-year-old — small but not negligible, underscoring the importance of clinical justification for every scan. Paediatric patients are significantly more radiosensitive; dose-reduction protocols and substitution with ultrasound or MRI are prioritised wherever clinically appropriate. Iodinated contrast medium risks include allergic reactions: mild reactions (nausea, urticaria, itching) in 1–3% of patients; severe anaphylaxis in approximately 1 in 10,000 injections. Patients with prior severe contrast reactions require pre-medication (corticosteroids plus antihistamines) or substitution with a non-contrast modality. Contrast-induced acute kidney injury (CI-AKI) is a concern in patients with pre-existing renal impairment — adequate pre-hydration, iso-osmolar contrast agents, and minimised contrast volumes reduce this risk. CT is contraindicated in pregnancy unless the clinical benefit clearly outweighs radiation risk to the foetus; ultrasound or MRI should be the first-line modality in pregnant patients. CT cannot match MRI for soft tissue contrast in brain, spinal cord, cartilage, or musculotendinous structures.