17.5 Clinical Imaging: Choosing Tests, Radiation Safety & Interpreting X-rays
Key Takeaways
Under IR(ME)R 2017, every exposure to ionising radiation must be justified, and people who could be pregnant are asked about pregnancy before abdominal or pelvic imaging.
A CT of the abdomen and pelvis gives roughly the radiation dose of several years of natural background radiation, whereas a chest X-ray gives about a few days' worth.
MRI is contraindicated by non-MRI-conditional pacemakers and metallic foreign bodies in the eye, and gadolinium is used cautiously in severe renal impairment.
A nasogastric tube position is checked first by aspirate pH of 5.5 or below; if this fails, a chest X-ray must show the tube below the diaphragm before any feed or medicine is given.
On a chest X-ray, pulmonary oedema shows alveolar shadowing, Kerley B lines, cardiomegaly, upper lobe diversion and effusions.
Clinical Imaging: Choosing Tests, Radiation Safety & Interpreting X-rays
Clinical imaging is a new area of practice in the 2025 MLA content map. Doctors must choose appropriate imaging, request it safely, recognise key abnormalities and act on results.
Radiation Protection
- The Ionising Radiation (Medical Exposure) Regulations 2017 (IR(ME)R) require every medical exposure to be justified (benefit outweighs risk) and optimised (as low as reasonably practicable). The referrer must give enough clinical information for a practitioner (usually a radiologist or radiographer) to justify the request.
- Pregnancy checks: people of childbearing potential (typically aged 12–55) are asked whether they could be pregnant before examinations that irradiate the abdomen or pelvis; if they might be, the request is reconsidered or modified, although an urgent clinically necessary scan should not be withheld.
- Approximate effective doses (UK natural background is about 2.7 mSv a year):
| Examination | Approximate Dose | Equivalent Background Radiation |
|---|---|---|
| Chest X-ray | About 0.015–0.02 mSv | A few days |
| CT head | About 2 mSv | About 1 year |
| CT chest | About 6 mSv | About 2–3 years |
| CT abdomen and pelvis | About 10 mSv | About 3–4 years |
| Ultrasound and MRI | No ionising radiation | — |
Children are more sensitive to radiation, so ultrasound and MRI are preferred where they answer the question.
Contrast & MRI Safety
- Iodinated contrast (CT): check renal function; people with eGFR below about 30, or AKI, are at higher risk of contrast-associated AKI (consider alternatives and IV hydration per local policy); review metformin around contrast in significant renal impairment; ask about previous contrast reactions (anaphylaxis needs adrenaline).
- MRI: screen for pacemakers and defibrillators (many are MRI-conditional, but non-conditional devices are a contraindication), cochlear implants, aneurysm clips, neurostimulators and metallic foreign bodies in the eye (welders may need an orbital X-ray first). Gadolinium is used cautiously in severe renal impairment because of nephrogenic systemic fibrosis.
Choosing the Right First Test
| Clinical Question | First-Line Imaging |
|---|---|
| Suspected acute stroke | Non-contrast CT head with CT angiography |
| Suspected subarachnoid haemorrhage | CT head (very sensitive within 6 hours of onset); lumbar puncture after 12 hours if CT is negative and the history is suggestive |
| Head injury meeting NICE criteria | CT head (section 1.5) |
| Suspected pulmonary embolism | CT pulmonary angiography (V/Q scan if contrast is contraindicated or to reduce breast radiation in young women) |
| Suspected DVT | Compression duplex ultrasound |
| Renal colic | Non-contrast CT KUB (ultrasound in pregnancy and children) |
| Right upper quadrant pain, gallstones | Abdominal ultrasound |
| Suspected bowel obstruction or perforation | CT abdomen and pelvis (erect chest X-ray for free gas) |
| Suspected appendicitis | Clinical diagnosis; ultrasound in children and young women; CT in older adults if uncertain |
| Cauda equina or cord compression | Emergency MRI |
| Suspected scaphoid or hip fracture with normal X-ray | MRI (or CT) |
| Suspected osteomyelitis | MRI |
| Pregnancy | Ultrasound or MRI where possible; essential imaging with ionising radiation is not withheld |
A Systematic Approach to the Chest X-ray
- Details and quality: patient, date, projection (PA standard; AP films magnify the heart), rotation (spinous processes midway between clavicle heads), inspiration (5–6 anterior ribs above the diaphragm), exposure.
- Airway: trachea central or deviated.
- Breathing: lung fields, zones and pleura; look for consolidation (with air bronchograms), collapse (volume loss, shifted fissures; left lower lobe collapse gives a "sail sign" behind the heart), masses, pneumothorax (visible pleural edge with no lung markings beyond), and effusion (blunted costophrenic angle, meniscus).
- Circulation: heart size (cardiothoracic ratio above 0.5 on a PA film suggests cardiomegaly), mediastinal width, hila.
- Diaphragm: free gas under the diaphragm on an erect film indicates perforation.
- Everything else: bones (fractures, lytic lesions), soft tissues, lines and tubes, and review areas (apices, behind the heart, below the diaphragm).
Pulmonary oedema shows Alveolar oedema (bat's wing shadowing), B (Kerley B) lines, Cardiomegaly, Dilated upper lobe vessels (upper lobe diversion) and pleural Effusions.
Nasogastric Tube Position
A misplaced nasogastric tube used for feeding or medicines is an NHS Never Event. Check position by aspirate pH of 5.5 or below first; if no aspirate is obtained or the pH is above 5.5, request a chest X-ray, which must show the tube in the midline, bisecting the carina, crossing the diaphragm and with its tip below the left hemidiaphragm. Never use the "whoosh" test or litmus paper.
The Abdominal X-ray
Its role is limited (CT is usually better), but it helps with suspected obstruction, toxic megacolon and some foreign bodies. Small bowel shows central loops with valvulae conniventes crossing the full width (dilated above 3 cm); large bowel shows peripheral haustra that do not cross fully (dilated above 6 cm, caecum above 9 cm). A "coffee bean" sign suggests sigmoid volvulus. Rigler's sign (both sides of the bowel wall visible) indicates free intraperitoneal gas.
Acting on Results
The doctor who requests an investigation is responsible for reviewing and acting on the result, or ensuring someone else does. Urgent unexpected findings (for example a new lung mass on a chest X-ray done for another reason) must be communicated, followed up and documented, and the patient informed.
A nasogastric tube has been inserted for feeding in a patient after a stroke. No aspirate can be obtained. What is the most appropriate way to confirm its position before feeding?
Inject 20 mL of air and listen over the stomach for a whoosh
Request a chest X-ray to confirm the tip is below the diaphragm
Start the feed slowly at 10 mL per hour and watch closely for coughing
Test any fluid in the tube with blue litmus paper for acidity
A 26-year-old woman has a 2-day history of right iliac fossa pain. Her pregnancy test is negative. Appendicitis is suspected but the clinical picture is uncertain. Which imaging test is most appropriate first?
Plain abdominal X-ray
MRI of the lumbar spine
CT abdomen and pelvis with contrast
Abdominal and pelvic ultrasound
A patient's chest X-ray shows bilateral perihilar alveolar shadowing, Kerley B lines, an enlarged cardiac shadow, prominent upper lobe vessels and small bilateral effusions. What is the most likely diagnosis?
Bilateral bacterial pneumonia
Bilateral tension pneumothoraces
Cardiogenic pulmonary oedema
Pulmonary sarcoidosis (stage 2)
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