Clinical imaging: Imaging principles, modalities, safety and requesting

🩻 Imaging principles, modalities, safety and requesting - page 1 of the Clinical Imaging chapter. Most hospital patients have at least one imaging test, and doctors at every level request them, interpret them, act on them and explain them. The skill isn't just reading images - it's choosing the right test, using radiation and contrast safely, making sure urgent findings are acted on, and managing incidental findings sensibly. Imaging for specific body systems is covered on the following pages.
Your four questions for every imaging request: What clinical question am I asking - and will the answer change management? Which test answers it best, with the least harm? Is it safe (pregnancy, kidney function, contrast allergy, implants)? Who will act on the result, and how quickly?

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Imaging - Overview

• Main modalities:
  • Ionising radiation - plain radiographs (X-rays), CT, fluoroscopy, nuclear medicine and PET, mammography, DEXA.
  • Non-ionising - ultrasound and MRI.
• Choosing a test - balance diagnostic accuracy, availability, speed, radiation dose, contrast risk, cost and patient factors (claustrophobia, obesity, mobility, pregnancy, children).
• Royal College of Radiologists iRefer - evidence-based referral guidelines for which test to use for each clinical problem (often built into electronic requesting).
• Radiology team - radiologists (consultant doctors who report and perform interventions), radiographers (acquire images; some report plain films and perform ultrasound), sonographers, nuclear medicine technologists, medical physicists, radiology nurses.
• Interventional radiology - image-guided minimally invasive procedures (biopsy, drainage, embolisation, stenting - see page 6).
• Demand - imaging demand in the NHS rises every year (CT and MRI fastest); reporting backlogs make good requesting and appropriate use essential.
• Digital systems - PACS (picture archiving), RIS (radiology information system), image sharing between trusts; AI tools increasingly support detection (e.g. lung nodules, fractures, stroke) - always under clinician oversight.

Plain Radiographs

• How they work - X-rays pass through the body; dense tissues absorb more and appear white (radiopaque); air absorbs least and appears black (radiolucent).
• Five densities (white to black) - metal, bone/calcium, soft tissue/fluid, fat, air.
• Silhouette sign - loss of a normal boundary between two structures of similar density in contact (e.g. consolidation abutting the heart border) - localises disease.
• Projection - PA (X-rays enter from the back - standard chest film; heart not magnified) vs AP (portable, supine - heart appears larger); lateral views add depth.
• Two views at right angles are needed for most bone injuries.
• Strengths - quick, cheap, widely available, low dose; first-line for chest, bone injury, some abdominal problems.
• Limitations - 2D superimposition; poor soft-tissue detail; many conditions are invisible early (e.g. occult fractures, early pneumonia, PE, early bone metastases).
• Mammography - low-dose breast X-ray - NHS Breast Screening Programme (women 50–71, every 3 years); diagnostic mammography in symptomatic clinics.
• DEXA - dual-energy X-ray absorptiometry - bone mineral density (very low dose) - T-score −2.5 or less = osteoporosis.

Computed Tomography

• How it works - a rotating X-ray tube and detectors create cross-sectional images; reconstructed in any plane (axial, coronal, sagittal) and 3D.
• Hounsfield units (HU) - measure density: air −1,000, fat about −100, water 0, soft tissue 20–60, acute blood 50–80, bone over 400 (up to 1,000+), metal very high.
• Windows - adjust brightness and contrast to view lung, soft tissue, bone or brain.
• Contrast:
  • IV iodinated contrast - highlights vessels and enhancing tissues; timing matters - arterial phase (CT angiography, bleeding, tumours), portal venous phase (routine abdomen), delayed (urinary tract - CT urogram).
  • Oral contrast - opacifies bowel (selected abdominal CT).
  • Non-contrast - CT head for haemorrhage, CT KUB for stones, HRCT lungs.
• Strengths - fast (seconds), widely available 24/7, excellent for trauma, acute abdomen, stroke, PE, cancer staging, lungs and bone detail.
• Limitations - higher radiation dose than plain films; contrast risks; less soft-tissue contrast than MRI (brain, spinal cord, pelvis, joints).
• Special CT - CT pulmonary angiography (CTPA), CT coronary angiography, CT colonography, CT perfusion (stroke), cone-beam CT (dental, interventional), low-dose CT (lung cancer screening, myeloma, kidney stones), dual-energy CT (gout, stone composition).

Magnetic Resonance Imaging

• How it works - a strong magnet (usually 1.5 or 3 Tesla) aligns hydrogen nuclei (protons); radiofrequency pulses disturb them, and the signals emitted as they relax build the image. No ionising radiation.
• Key sequences:
  • T1-weighted - fat bright, water (CSF) dark - anatomy; with gadolinium - enhancing lesions (tumour, inflammation).
  • T2-weighted - water bright (CSF, oedema, cysts) - most pathology appears bright.
  • FLAIR - T2 with CSF signal suppressed - periventricular lesions (MS), oedema.
  • STIR / fat-suppressed - bone marrow oedema (occult fractures, osteomyelitis).
  • DWI (diffusion-weighted) - restricted diffusion (bright on DWI, dark on ADC) - acute infarct, abscess, cellular tumours, CJD.
  • Susceptibility (SWI, gradient echo) - blood products, microbleeds, calcium.
  • MR angiography and venography; MRCP (bile ducts); cardiac MRI; MR enterography.
• Strengths - best soft-tissue contrast - brain, spinal cord, MSK (ligaments, cartilage, marrow), pelvis (prostate, rectum, uterus), liver, heart; no radiation (good for children, pregnancy, repeated imaging).
• Limitations - slow (20–60 minutes), noisy, claustrophobic, motion artefact, less available, safety screening needed (implants, metal), less good for lung parenchyma and cortical bone.
• Whole-body MRI - myeloma staging, cancer predisposition (Li–Fraumeni surveillance).

Ultrasound and Point-of-care Ultrasound

• How it works - high-frequency sound waves reflect at tissue interfaces; real-time images. No ionising radiation.
• Appearance - fluid anechoic (black); solid tissue echogenic (grey); bone, calcium and stones very bright with acoustic shadowing; air scatters the beam (bowel gas obscures structures).
• Doppler - blood flow direction and velocity - DVT, carotid stenosis, testicular torsion, arterial disease, liver vessels, fetal and placental flow.
• Uses - gallbladder and biliary tree, liver, kidneys and bladder, aorta (AAA), pelvis (gynaecology and obstetrics), thyroid and neck lumps, testes, soft tissues, joints and tendons, breast (with mammography), neonatal brain and hips, appendicitis and intussusception in children, guidance for procedures (cannulation, aspiration, biopsy, drains, nerve blocks).
• Strengths - safe, cheap, portable, real-time, repeatable; first line in children and pregnancy.
• Limitations - operator-dependent; limited by obesity and bowel gas; can't see through bone or air (lungs - except specific signs).
• Point-of-care ultrasound (POCUS) - by clinicians at the bedside to answer focused questions:
  • eFAST (trauma) - free fluid (haemoperitoneum, haemopericardium), pneumothorax.
  • Focused echo in cardiac arrest and shock - pericardial effusion and tamponade, RV dilatation (PE), LV function, IVC.
  • Lung ultrasound - pneumothorax (absent lung sliding), B-lines (pulmonary oedema), effusion, consolidation.
  • Aorta (AAA), DVT (two-point compression), bladder volume, guided procedures.
  • Needs accredited training and governance (e.g. RCEM, FUSIC - Intensive Care Society).

Nuclear Medicine and PET-CT

• Principle - a radioactive tracer (radiopharmaceutical) is given (IV, oral or inhaled); a gamma camera or PET scanner images its distribution - shows function and metabolism rather than anatomy.
• Common studies:

  Study	Uses
  Bone scan (Tc-99m MDP)	Bone metastases (prostate, breast), occult fractures, osteomyelitis, Paget's; "cold" in myeloma (use MRI or CT)
  V/Q scan	PE - especially in pregnancy, contrast allergy, renal impairment, with a normal chest X-ray
  Myocardial perfusion scan	Inducible ischaemia (SPECT); viability
  Thyroid uptake scan	Hyperthyroidism - Graves' (diffuse uptake), toxic nodule ("hot"), thyroiditis (low uptake)
  Renal (DMSA, MAG3)	Renal scarring (DMSA - children after UTI), split function, obstruction and drainage (MAG3)
  DaTscan	Parkinsonism vs essential tremor; dementia with Lewy bodies (reduced uptake)
  Parathyroid (sestamibi)	Localising parathyroid adenoma before surgery
  Sentinel lymph node	Breast cancer and melanoma staging
  Gastric emptying, HIDA, Meckel's scan	Gastroparesis; gallbladder function; ectopic gastric mucosa (GI bleeding in children)

• PET-CT - usually ¹⁸F-FDG (glucose analogue) taken up by metabolically active tissue:
  • Uses - cancer staging and response (lymphoma, lung cancer before radical treatment, oesophageal, head and neck, melanoma), unknown primary, pyrexia of unknown origin, large-vessel vasculitis, infection (prosthetic grafts), dementia (FDG brain).
  • Pitfalls - false positives in infection and inflammation (sarcoid, TB), brown fat, recent surgery; false negatives in small or low-grade tumours; high blood glucose reduces quality (fasting; diabetes management).
  • Other PET tracers - PSMA PET (prostate cancer), DOTATATE (neuroendocrine tumours), amyloid PET (research and selected dementia assessment).
• Therapeutic nuclear medicine - radioiodine (I-131 - Graves', toxic nodules, thyroid cancer), lutetium-177 (neuroendocrine, PSMA), radium-223 (bone metastases) - radiation protection advice after treatment.

Fluoroscopy and Contrast Studies

• Fluoroscopy - real-time X-ray imaging - used for contrast studies and guiding procedures.
• Contrast studies:
  • Contrast swallow - dysphagia (achalasia - "bird's beak"; pharyngeal pouch), post-operative leaks, suspected oesophageal perforation (use water-soluble contrast - barium causes mediastinitis; CT is now often first line).
  • Water-soluble contrast follow-through - adhesional small bowel obstruction (diagnostic and may be therapeutic - Gastrografin).
  • Contrast enema - anastomotic leaks, Hirschsprung's (children), sigmoid volvulus; largely replaced by CT and colonoscopy for diagnosis.
  • Videofluoroscopy - swallowing assessment with speech and language therapy (aspiration).
  • Micturating cystourethrogram (MCUG) - vesicoureteric reflux in children.
  • Hysterosalpingogram - tubal patency (fertility).
  • ERCP and PTC - biliary imaging with therapy.
• Interventional fluoroscopy - angiography, stent and line placement, joint injections, vertebroplasty, orthopaedic and cardiac procedures (cardiac catheterisation).
• Dose - can be high with prolonged screening (skin injury in long interventional procedures) - dose monitoring.

Radiation Dose and Risk

• Effective dose (millisieverts, mSv) estimates whole-body cancer risk. UK natural background radiation - about 2.7 mSv a year (radon is the largest part).
• Typical effective doses (approximate - vary with technique and patient size):

  Examination	Dose (mSv)	Equivalent background
  Chest X-ray (PA)	About 0.015–0.02	A few days
  Limb or joint X-ray	Under 0.01	Under 1.5 days
  Abdominal X-ray	About 0.4–0.7	About 2–3 months
  Lumbar spine X-ray	About 0.7–1	About 4 months
  Mammogram	About 0.5	About 2 months
  CT head	About 1.5–2	About 8 months
  CT chest or CTPA	About 3–7	1–2.5 years
  CT abdomen and pelvis	About 8–10	About 3 years
  Bone scan	About 3–4	About 1.5 years
  FDG PET-CT	About 10–25	4–9 years
  Ultrasound, MRI	0	None

• Risks:
  • Stochastic effects (probability rises with dose, no threshold) - cancer; lifetime risk from a single CT abdomen is roughly 1 in 2,000 - small compared with the baseline lifetime cancer risk (about 1 in 2), but adds up with repeated scans.
  • Deterministic effects (above a threshold) - skin burns, hair loss, cataracts - mainly in prolonged interventional procedures and radiotherapy.
  • Higher risk in children (more sensitive tissues, longer life), young women (breast), and with repeated imaging.
• Principles - justification (benefit must outweigh risk) and optimisation (ALARP - as low as reasonably practicable); use non-ionising tests where they answer the question; avoid repeating recent scans; check previous imaging.
• Communicating risk - compare with background radiation; emphasise the risk of missing a diagnosis.

IR(ME)R - the Law on Medical Radiation

• Ionising Radiation (Medical Exposure) Regulations 2017 - UK law protecting patients from unnecessary medical radiation; inspected by the CQC (England).
• Duty holders:
  • Referrer - a registered healthcare professional entitled (by the employer) to request exposures - must supply adequate clinical information.
  • Practitioner - usually a radiologist (or radiographer under guidelines) - justifies each exposure (benefit outweighs risk).
  • Operator - performs the practical aspects (radiographer).
  • Employer - provides procedures, training, audit and dose reference levels.
• Requirements:
  • Every exposure must be justified and authorised.
  • Pregnancy enquiry for people of childbearing potential (typically aged 12–55) before examinations of the abdomen and pelvis (and high-dose procedures).
  • Patients must be given information about benefits and risks.
  • Breastfeeding enquiry before nuclear medicine.
  • Significant accidental or unintended exposures (e.g. wrong patient, wrong body part, unnecessary repeat) must be investigated and reported (CQC).
• Practical point - vague requests ("?pathology", "pain") may be rejected; good clinical information is a legal duty, not a courtesy.
• Wrong-patient imaging - use positive patient identification (name, date of birth, address) before every exposure.

Imaging in Pregnancy and Breastfeeding

• Never withhold necessary imaging from a pregnant woman - the risk of missing serious disease (PE, appendicitis, trauma) usually far outweighs the radiation risk.
• Prefer ultrasound and MRI where they answer the question.
• Fetal doses - tests away from the abdomen (chest X-ray, CT head, CTPA, extremities) give negligible fetal dose; CT abdomen and pelvis gives about 10–25 mGy - well below the threshold for deterministic effects (about 100 mGy) but a small increase in childhood cancer risk.
• Suspected PE in pregnancy (RCOG) - leg ultrasound if leg symptoms; chest X-ray; then V/Q (perfusion) scan or CTPA - CTPA gives a higher dose to maternal breast tissue, V/Q a slightly higher (still very low) fetal dose; local policy; start LMWH while waiting.
• MRI in pregnancy - considered safe at 1.5T in all trimesters when clinically needed (often avoided in the first trimester if it can wait); avoid gadolinium unless essential (crosses the placenta).
• Iodinated contrast in pregnancy - can be given if needed; the newborn's thyroid function is checked (routine newborn blood spot screening).
• Breastfeeding - no need to stop after iodinated or gadolinium contrast (tiny amounts in milk); some nuclear medicine tracers need a temporary pause (expressing and discarding milk) - check with nuclear medicine.
• Inadvertent exposure in early pregnancy - calculate fetal dose (medical physics); reassure that doses from diagnostic imaging are almost never a reason for termination.

Iodinated Contrast - Kidneys, Metformin and Reactions

• Post-contrast acute kidney injury:
  • Risk with modern IV contrast is much lower than once thought; main risk factors - eGFR under 30 (especially under 30 with diabetes), AKI, dehydration, heart failure, large or repeated doses, intra-arterial contrast.
  • Prevention (RCR, NICE NG148) - check recent eGFR in at-risk patients; consider alternatives; IV hydration (e.g. 0.9% saline or sodium bicarbonate) for high-risk patients; temporarily stop nephrotoxic drugs (consider holding ACE inhibitors and ARBs in eGFR under 40 per local policy); use the lowest dose; check creatinine 48–72 hours afterwards if high risk.
  • Don't withhold urgent contrast scans (e.g. CTPA, CT angiography for stroke or bleeding, trauma) because of kidney function - the benefit outweighs the risk.
• Metformin (RCR 2022) - no need to stop if eGFR is over 30 and kidney function is stable; stop for 48 hours after contrast if eGFR under 30 or AKI (lactic acidosis risk), restarting when kidney function is stable.
• Contrast reactions - mostly non-IgE (direct mast cell activation):
  • Mild (common) - nausea, warmth, itch, a few hives - observe; antihistamine.
  • Moderate - widespread urticaria, bronchospasm, facial swelling - treat.
  • Severe (rare - about 1 in 10,000–40,000) - anaphylaxis - IM adrenaline and standard treatment (see Immunology chapter).
  • Previous reaction - use a different contrast agent, a non-contrast or alternative test where possible; premedication with steroids has limited evidence; have resuscitation equipment ready.
  • "Iodine" or shellfish allergy is not a contraindication (iodine itself isn't the allergen).
• Delayed reactions - rash 1 hour to 1 week later - usually mild.
• Thyroid - iodinated contrast can precipitate thyrotoxicosis in untreated hyperthyroidism or multinodular goitre, and delays radioiodine treatment or uptake scans (avoid for about 6–8 weeks before).
• Extravasation - swelling and pain at the cannula site - elevate, cold compress, monitor for compartment syndrome.

Gadolinium and MRI Safety

• Gadolinium-based contrast agents (MRI):
  • Nephrogenic systemic fibrosis - rare, severe skin and organ fibrosis in severe CKD (eGFR under 30) or dialysis with older linear agents - now very rare with macrocyclic agents (use the lowest dose; dialysis soon after in dialysis patients).
  • Gadolinium retention in the brain and bone with repeated doses (linear agents) - clinical significance uncertain; macrocyclic agents preferred.
  • Allergic-type reactions - rare.
• MRI safety - the magnet is always on:
  • Projectile effect - ferromagnetic objects (oxygen cylinders, scissors, trolleys) are pulled into the scanner - strict access zones and screening.
  • Implants and devices - classified as MR safe, MR conditional (safe under specific conditions) or MR unsafe:
    • Pacemakers and ICDs - many modern devices are MR conditional - scanning with cardiology/physics protocols and device reprogramming; older devices and abandoned leads may be unsafe.
    • Cochlear implants, neurostimulators, insulin pumps and some drug pumps, older ferromagnetic aneurysm clips, some heart valves (most are safe), programmable shunts (may need resetting).
    • Metallic foreign bodies - especially intraocular metal (metalworkers - orbital X-ray first), shrapnel, bullets.
    • Most orthopaedic implants, joint replacements, coronary stents, sternal wires and surgical clips are safe or conditional (may cause artefact).
  • Heating and burns - from wires, ECG leads, tattoos, medication patches with metal backing (remove).
  • Noise - ear protection; claustrophobia - open or wide-bore scanners, sedation (oral benzodiazepine) or general anaesthesia (children).
  • Pregnancy - see above.
• Complete an MRI safety questionnaire - including for unconscious patients (check records, X-rays for implants, family history).

Requesting Imaging Well

• A good request includes:
  • Correct patient identifiers, location, contact details of the referrer.
  • The clinical question - "?PE", "?small bowel obstruction", "?scaphoid fracture" - and what will change management.
  • Relevant clinical details - symptoms, duration, examination findings, key results (D-dimer, CRP, eGFR), previous imaging and surgery, cancer history.
  • Safety information - pregnancy status, kidney function, diabetes and metformin, contrast allergy, implants (MRI), anticoagulants and platelets (interventions), mobility and infection status.
  • Urgency - and why.
• Use iRefer and local pathways; discuss complex or urgent requests with the radiologist directly.
• Don't request imaging that won't change management - e.g. plain X-ray for non-specific low back pain, skull X-ray for head injury, AXR for most abdominal pain, routine pre-operative chest X-rays.
• Check previous imaging before requesting - avoid duplication; ask for comparison with outside images.
• Prepare the patient - fasting (for ultrasound of the gallbladder, some CT), full bladder (pelvic ultrasound), cannula (contrast), removing metal (MRI), consent (interventions), explaining the test.
• Radiologist's perspective - the more specific the question, the more useful the report; "please assess" doesn't help.

💡 Clinical Pearl: Write the request as if the radiologist has never met the patient - because they haven't. "62M, sudden pleuritic chest pain, HR 110, SpO₂ 92%, Wells 6, eGFR 78, ?PE" gets a better, faster answer than "SOB ?cause".

Reports, Urgent Findings and Communication

• The requester is responsible for reading and acting on the report - even if the patient has been discharged or moved; electronic acknowledgement systems help.
• Structure of a report - clinical details, technique, comparison, findings, conclusion (impression and recommendations).
• Hedged language - "cannot be excluded", "likely", "suggest correlation" - clarify with the radiologist if the answer matters.
• Critical and urgent findings (RCR standards) - radiologists communicate directly (telephone) and urgently:
  • Critical - e.g. tension pneumothorax, misplaced NG tube in the lung, intracranial haemorrhage, aortic dissection, free gas, large PE, cord compression.
  • Urgent - e.g. new suspected cancer, unexpected fracture.
  • Unexpected significant findings - clear alerts and failsafe systems for follow-up.
• Missed results are a recognised patient safety risk (e.g. a suspected cancer on a chest X-ray done in the ED) - use failsafe pathways; tell patients about results and follow-up plans; include in discharge summaries.
• Discrepancies - junior or ED interpretation differs from the formal report - check reports on discharged patients and recall if needed.
• Radiology–clinical meetings - multidisciplinary team meetings (MDTs) review imaging in cancer and complex cases.
• Patients can access reports (NHS App) - sometimes before seeing the clinician; explain findings clearly and promptly.

Incidental Findings

• Incidentalomas - unexpected findings unrelated to the clinical question - very common with CT and MRI.
• Balance - find the few significant lesions without causing harm (anxiety, more radiation, biopsies, cost) from investigating benign findings; follow guideline-based pathways.
• Common examples:

  Finding	Approach
  Pulmonary nodule	BTS 2015 - under 5 mm (or under 80 mm³) - no follow-up; 5–8 mm - CT surveillance (volumetry); 8 mm or more (or 300 mm³) - Brock risk model, then PET-CT or biopsy if high risk; consider age, smoking and comorbidity
  Adrenal incidentaloma	Under 10 HU on non-contrast CT - lipid-rich adenoma (benign); all need hormone screening (overnight dexamethasone suppression test; metanephrines if not clearly adenoma; aldosterone–renin ratio if hypertensive or low potassium); over 4 cm or suspicious features - endocrine surgery MDT
  Thyroid nodule	Ultrasound with U-classification (U1–U5 - BTA); FNA for U3–U5; incidental FDG-avid nodules need assessment (higher cancer risk)
  Renal cyst	Bosniak classification - I and II benign; IIF follow-up; III and IV - likely malignant (urology)
  Liver lesion	Simple cysts and typical haemangiomas - no follow-up; indeterminate - MRI; cirrhosis - LI-RADS (HCC risk)
  Pancreatic cyst	Often IPMN - MRI surveillance per European guidelines; worrying features - EUS and surgical review
  Small AAA	Refer to vascular or surveillance pathway (see page 3)
  Vertebral fracture	Often unreported - consider osteoporosis assessment (FRAX, DEXA)
  Coronary calcification	On chest CT - consider cardiovascular risk assessment and statin

• Tell the patient - explain the finding and the plan; make sure follow-up is booked and someone owns it (not left for "the GP to arrange" without clear information).

Imaging Modalities at a Glance

Modality	Radiation	Best for	Limitations
X-ray	Low	Chest, fractures, joints, tubes and lines	2D; poor soft tissue; insensitive early
CT	Moderate–high	Trauma, acute abdomen, stroke, PE, lungs, staging, bone detail	Dose; contrast risk
MRI	None	Brain, spine, MSK, pelvis, liver, heart	Slow, claustrophobic, implant safety, availability
Ultrasound	None	Gallbladder, kidneys, aorta, pelvis, pregnancy, children, DVT, guiding procedures	Operator-dependent; gas, bone and obesity
Nuclear medicine	Low–moderate	Function - bone, V/Q, thyroid, renal, cardiac perfusion	Poor anatomical detail; time
PET-CT	High	Cancer staging, PUO, vasculitis	False positives with inflammation; dose; cost
Fluoroscopy	Variable	Contrast studies, guided interventions	Dose with long procedures

Spot the Diagnosis

Scenario	Best answer
Bright on T2, dark on T1, in a round lesion in the kidney	Fluid-filled cyst
Bright on DWI, dark on ADC, 2 hours after stroke onset	Acute infarct (restricted diffusion)
Lesion of −90 HU on CT	Fat (e.g. lipoma, angiomyolipoma, dermoid)
Adrenal mass of 6 HU on non-contrast CT	Lipid-rich adenoma - still needs hormone screening
Bright structure with acoustic shadowing in the gallbladder on ultrasound	Gallstone
Bone scan normal in a man with lytic lesions and anaemia	Myeloma ("cold" on bone scan)
FDG-avid mediastinal nodes in a woman with erythema nodosum	Sarcoidosis - PET false positive for cancer
Metalworker needs MRI brain	Orbital X-ray first (intraocular metal)
"Iodine allergy" from shellfish before CT	Not a contraindication to iodinated contrast
On metformin, eGFR 25, needs contrast CT	Stop metformin for 48 hours after
Pregnant woman with suspected PE and normal chest X-ray	V/Q or CTPA - don't withhold imaging
Breastfeeding mother after contrast CT	No need to stop breastfeeding
Dialysis patient needs gadolinium MRI	Macrocyclic agent, lowest dose, dialysis soon after
6 mm incidental lung nodule in a smoker	CT surveillance (BTS)
Bosniak IV renal cyst	Likely malignant - urology
Wrong patient X-rayed because of a name mix-up	IR(ME)R significant unintended exposure - report

Exam-style Scenarios

1. A 68-year-old man with type 2 diabetes on metformin needs a contrast CT abdomen for suspected cancer. eGFR is 26.
Answer: High risk of post-contrast AKI; metformin must be withheld.

• Discuss whether a non-contrast CT, MRI or ultrasound would answer the question; if contrast CT is needed, proceed.
• IV hydration before and after per local protocol; minimise contrast dose; review nephrotoxic drugs.
• Stop metformin for 48 hours after the scan and restart once creatinine is stable (check at 48–72 hours).

2. A 29-year-old woman, 24 weeks pregnant, has pleuritic chest pain and breathlessness. She is worried about radiation.
Answer: Suspected PE in pregnancy - imaging must not be withheld.

• Start LMWH while awaiting imaging; chest X-ray (negligible fetal dose); leg Doppler if leg symptoms.
• Then V/Q or CTPA (local policy) - both give very low fetal doses; explain the small risks (breast dose with CTPA, fetal dose with V/Q) and the much greater risk of untreated PE.
• Document the discussion and her choice.

3. A 50-year-old man needs an urgent MRI spine for suspected cauda equina syndrome. He has a pacemaker fitted 8 years ago.
Answer: Check MRI compatibility of the device urgently.

• Identify the device and leads (records, pacing clinic) - if MR conditional, scan under the cardiology/physics protocol with device reprogramming.
• If MR unsafe or unknown - CT myelogram or CT as an alternative; discuss with radiology and spinal surgeons.
• Don't delay - cauda equina is a surgical emergency.

4. A CT abdomen for renal colic reports a 2.5 cm left adrenal mass measuring 8 HU.
Answer: Adrenal incidentaloma - likely lipid-rich adenoma.

• No further imaging needed for an under 4 cm, under 10 HU lesion (ESE guidance).
• Hormone screening - overnight dexamethasone suppression test; plasma metanephrines (if not clearly benign); aldosterone–renin ratio if hypertensive or hypokalaemic.
• Explain the finding to the patient and refer to endocrinology if any hormone excess.

5. A junior doctor requests a CT head with the details "confused - ?cause". The radiologist rejects the request.
Answer: Inadequate justification under IR(ME)R.

• Provide adequate clinical information - onset, focal neurology, fall or head injury, anticoagulants, GCS, previous imaging.
• State the clinical question (e.g. "?subdural haematoma - fall 3 days ago on apixaban, new confusion").
• Consider whether imaging will change management (delirium workup first).

Key Points

• X-ray, CT, fluoroscopy and nuclear medicine use ionising radiation; ultrasound and MRI don't.
• CT HU - air −1,000, fat −100, water 0, acute blood 50–80, bone over 400.
• MRI - T1 fat bright; T2 water bright; FLAIR suppresses CSF; DWI shows acute infarcts.
• Chest X-ray about 0.02 mSv (days of background); CT abdomen about 10 mSv (about 3 years); UK background about 2.7 mSv a year.
• IR(ME)R 2017 - every exposure must be justified; referrers must give adequate clinical information; pregnancy checks.
• Don't withhold necessary imaging in pregnancy; prefer ultrasound and MRI; avoid gadolinium.
• Post-contrast AKI risk mainly with eGFR under 30 or AKI - hydrate; stop metformin for 48 hours if eGFR under 30 or AKI.
• Shellfish or "iodine" allergy isn't a contrast contraindication; previous contrast reaction - change agent and be prepared.
• MRI - always screen for implants and metal; many pacemakers are MR conditional.
• The requester must read and act on every report; critical findings are phoned through.
• Manage incidental findings by guideline - BTS nodules, adrenal (under 10 HU, hormone screen), Bosniak cysts.

Guidelines and Further Reading

• Royal College of Radiologists: iRefer - making the best use of clinical radiology
• The Ionising Radiation (Medical Exposure) Regulations 2017 (IR(ME)R); CQC IR(ME)R guidance
• UK Health Security Agency: Patient dose information and radiation risk
• Royal College of Radiologists: Guidance on contrast agents (2022 - including metformin); standards for the communication of critical, urgent and unexpected significant findings
• NICE NG148: Acute kidney injury (contrast)
• Medicines and Healthcare products Regulatory Agency: Safety guidelines for MRI equipment in clinical use
• BTS: Guidelines for the investigation and management of pulmonary nodules (2015); ESE: Adrenal incidentaloma guideline (2023)
• BNF - for current doses