MRI vs CT: A Clinical Decision Guide for GPs

The fundamental clinical trade-off between MRI and CT is soft-tissue contrast versus speed and accessibility.

Choose MRI when: • Superior soft-tissue contrast is clinically necessary • The patient is young or will need repeated imaging (no cumulative radiation) • Neurological detail is required (white/grey matter differentiation, cord lesions) • Joint, tendon, ligament, or cartilage pathology is suspected • Pelvic organ characterisation is needed (endometriosis, prostate staging, uterine lesions)

Choose CT when: • Speed is critical (trauma, acute chest pain, suspected PE, stroke protocol) • Bone detail is needed (complex fractures, bony metastases) • Calcification or vascular pathology is the primary question • The patient has a contraindication to MRI (implants, severe claustrophobia) • Medicare eligibility for MRI is uncertain and CT will provide sufficient diagnostic information

In Australia, Medicare eligibility for MRI is more restricted than for CT.

CT scans are Medicare-eligible for the vast majority of clinical indications with a valid GP referral. Most referred CT scans will attract at least a partial rebate.

MRI has a more restrictive eligibility schedule. Medicare eligibility for GP-referred MRI requires: • The scan must be performed on a Medicare-eligible (licensed) MRI machine • The clinical indication must fall within the Medicare Benefits Schedule (MBS) item descriptor • A GP referral covers most common indications, but some require specialist referral (e.g., cardiac MRI, whole-body MRI)

Common GP-eligible MRI indications include: • Brain and spine (headache, seizure, radiculopathy, suspected MS) • Knee, shoulder, and other joints (where X-ray and clinical exam are inconclusive) • Breast MRI for high-risk patients (specific criteria apply) • Pelvic MRI for specific gynaecological indications

If you are unsure whether an MRI indication is Medicare-eligible, specify the clinical question clearly in your referral - the radiologist and booking staff can advise. Alternatively, check the MBS online at mbsonline.gov.au.

When patients ask about radiation:

• Chest X-ray: 0.02 mSv (~2 days natural background) • CT chest: 7 mSv (~3.5 years natural background) • CT abdomen/pelvis: 8-10 mSv • CT head: 2 mSv • Full-body PET/CT: 14-25 mSv • MRI: 0 mSv (no ionising radiation)

Key clinical implications: • For patients under 40, particularly women of childbearing age or children, the ALARA principle is especially important - if MRI will provide equivalent diagnostic information, it is the preferred modality. • Cumulative dose matters in patients with cancer, chronic conditions, or those likely to need repeated imaging over months to years. • A single CT scan carries a very small absolute risk (estimated 1 in 2,000 lifetime cancer risk for a CT abdomen in a 40-year-old), but this should inform frequency, not prevent necessary imaging.

Presentation → Recommended first-line imaging

Low back pain (non-specific, <6 weeks) → X-ray lumbar spine (to exclude fracture/malignancy) Low back pain with radiculopathy or red flags → MRI lumbar spine Knee pain, suspected ligament/meniscal injury → MRI knee Shoulder pain, suspected rotator cuff → MRI or ultrasound shoulder New headache, seizure, or focal neurology → CT head (acute), then MRI brain Suspected stroke → CT head immediately; MRI if subacute Cough with haemoptysis or weight loss → CT chest Suspected pulmonary embolism → CT pulmonary angiogram (CTPA) Renal colic / flank pain → CT KUB (unenhanced) Abdominal pain, possible appendicitis → CT abdomen/pelvis Right upper quadrant pain → Ultrasound (gallstones), then CT if inconclusive Suspected breast pathology → Mammogram ± ultrasound; MRI for high-risk or dense breast Prostate staging → MRI prostate (after elevated PSA and biopsy decision) Infertility / endometriosis → Ultrasound first, MRI pelvis for surgical planning

The quality of your referral directly affects whether the correct scan is performed and whether Medicare approves it.

For MRI referrals, always include: • Specific body part and laterality • Clinical question (e.g., "Rule out meniscal tear" not just "knee pain") • Relevant history - prior imaging, previous injury, duration of symptoms • Relevant examination findings • Any contraindications you're aware of (pacemakers, implants, claustrophobia)

For CT referrals: • Body part and region (e.g., CT chest with contrast, or CT KUB without contrast) • Whether contrast is required and any relevant contraindications (contrast allergy, renal impairment, metformin use) • Clinical urgency

Avoid vague indications like "?pathology" or "for review" - these delay the booking process, may not meet Medicare criteria, and slow the radiologist's reporting.

For practical referral writing advice, see our Referral Best Practices for GPs guide.

Some imaging is better ordered after specialist assessment:

• Cardiac MRI and cardiac CT - typically ordered by a cardiologist • MRCP (magnetic resonance cholangiopancreatography) - often ordered after gastroenterology review • Whole-body MRI for cancer staging - generally ordered by oncology • PET scan - requires referral from a specialist or oncologist for Medicare approval • MRI prostate (mpMRI) - usually follows urology assessment and PSA workup

In these cases, a GP referral to the relevant specialist, rather than directly for imaging, produces better outcomes for the patient and reduces avoidable scan costs.