MRI Scan (Magnetic Resonance Imaging)

What It Shows

• Brain tumours — primary and secondary (metastases) with exceptional soft-tissue resolution
• Spinal cord compression, vertebral metastases, and leptomeningeal disease
• Prostate cancer — local staging, extracapsular extension, seminal vesicle invasion (mpMRI)
• Rectal cancer — mesorectal fascia involvement, lymph node status (essential for surgical planning)
• Liver lesions — differentiating haemangioma, HCC, metastases with hepatobiliary contrast
• Breast cancer — extent of disease, multifocal disease, chest wall invasion (in high-risk patients)
• Bone marrow infiltration in myeloma and lymphoma
• Diffusion-weighted imaging: cell density (cancer restriction), treatment response

How It Works

The patient lies inside a cylindrical magnet (typically 1.5T or 3T). The magnetic field aligns hydrogen protons in the body. Radiofrequency pulses temporarily knock protons out of alignment. As protons return to equilibrium (relaxation), they emit signals detected by receiver coils. Different tissues relax at different rates (T1 and T2 relaxation times), creating tissue contrast. Gadolinium-based contrast agents enhance vascularity and blood-brain barrier breakdown. Advanced sequences (DWI, DCE, MR spectroscopy) provide additional functional layers. A radiologist specialising in the relevant anatomy interprets the images.

• Brain imaging (CT misses small lesions, lacks soft-tissue contrast)
• Prostate cancer — mpMRI before biopsy (PI-RADS, MRI-guided TRUS biopsy)
• Rectal cancer — mandatory pre-operative staging
• Liver lesion characterisation when CT is inconclusive
• Assessment of bone metastases and marrow disease
• Post-treatment response in brain tumours (contrast enhancement vs pseudoprogression)
• Screening in BRCA carriers and high-risk breast cancer patients (alternating with mammography)

MRI has strong evidence across multiple oncology indications. In prostate cancer, the PROMIS trial demonstrated mpMRI has 93% sensitivity for clinically significant prostate cancer — superior to standard TRUS biopsy. The PRECISION trial showed MRI-guided biopsy detects 12% more clinically significant cancers. In rectal cancer, MRI-based CRM (circumferential resection margin) assessment is Level 1 evidence for surgical planning — it predicts local recurrence with 94% specificity. Whole-body MRI for myeloma staging is NICE-recommended, replacing skeletal surveys. Breast MRI screening in BRCA carriers detects cancers missed by mammography in ~50% of cases.

Preparation & What to Expect

Remove all metal objects (jewellery, piercings, hearing aids). Declare any metal implants, pacemakers, cochlear implants — some are MRI-incompatible. No fasting required for most scans (except liver MRI where 4-hour fast is recommended). For prostate mpMRI: bowel preparation and abstinence from ejaculation 3 days before. Gadolinium contrast requires kidney function check (eGFR). Scan duration 30–90 minutes depending on area. Claustrophobic patients may need open MRI or sedation.

• Cannot be used with many metallic implants, pacemakers, or cochlear implants (device-specific — check with manufacturer)
• Claustrophobia — 5–10% of patients cannot tolerate standard closed-bore MRI
• Motion artefact — breathing, cardiac motion, bowel peristalsis can degrade image quality
• Long scan times — uncomfortable for patients in pain or unable to lie still
• Gadolinium contrast: small risk of nephrogenic systemic fibrosis in severe kidney disease
• Expensive and limited availability compared to CT — waiting times can be long

• Absolute contraindications: non-MRI-safe pacemakers, cochlear implants, certain aneurysm clips, metallic foreign bodies in eyes
• Gadolinium deposits in brain and bone tissue with repeated use — clinical significance under investigation
• Pregnancy: MRI without contrast is generally safe after first trimester; gadolinium avoided unless essential
• Inform team of any previous surgery with implanted devices