Breast MRI
Breast MRI is the most sensitive test for breast cancer. It uses a strong magnet, radio waves and a gadolinium contrast agent to show tumour vessels and tissue. There is no ionising radiation.
Cancers develop leaky new vessels, and they take up contrast quickly and wash it out. Dynamic imaging shows this pattern, and it separates many cancers from normal tissue.
The patient lies prone with the breasts hanging in a dedicated coil, at 1.5 or 3 tesla. The sequences are T2-weighted with and without fat suppression, T1-weighted before contrast, and dynamic T1-weighted fat suppressed images before and several times after contrast.
Gadolinium (0.1 mmol/kg) is given at 2 to 3 mL/s, and the examination takes 20 to 30 minutes. Diffusion weighted imaging adds specificity.
Sensitivity is 90 to 99% for invasive cancer and 80 to 90% for ductal carcinoma in situ. Specificity is 70 to 90%.
MRI finds an extra cancer in 10 to 20% of women who have a new diagnosis, and in 1 to 2% in the opposite breast. Used in high risk screening, it finds 10 to 20 cancers in each 1000 women screened.
Indications.
- Screening of women at high risk: a mutation in BRCA1, BRCA2 or another gene, chest irradiation before age 30, or lifetime risk of 20 to 25% or more.
- Staging of newly diagnosed cancer, especially invasive lobular cancer, dense breasts or discordant tests.
- Cancer found in an axillary node with no breast primary on other imaging.
- Response to treatment given before surgery, and planning of surgery.
- Suspected recurrence, when other tests cannot tell scar from tumour.
- Problem solving when other imaging is inconclusive.
- Implant integrity (no contrast needed, and silicone specific sequences).
- Evaluation of nipple discharge or Paget disease of the nipple when other tests are negative.
- Direction of biopsy to lesions seen only on MRI.
Contraindications and precautions.
- Cardiac pacemakers, defibrillators and some implants: only if the device is labelled safe, and with a device team present. Cerebral aneurysm clips and metal in the eye are contraindications.
- Pregnancy: gadolinium is avoided. Breast feeding: continuation is usual after the contrast.
- Renal impairment: the estimated glomerular filtration rate should be over 30 mL/min. In severe impairment, the risk of nephrogenic systemic fibrosis is weighed.
- Allergy to gadolinium: rare, with the risk of severe reaction under 1 in 10 000.
- Claustrophobia, obesity beyond the bore size, and inability to lie prone for 20 to 30 minutes limit use.
- Premenopausal women are scanned in days 7 to 14 of the cycle, when background enhancement is lowest.
- Breast feeding and hormone treatment increase background enhancement.
- Tissue expanders and metal markers cause artefacts.
Preparation and technique.
A safety questionnaire is completed. A venous line is placed. Metal is removed. The patient lies prone, with the breasts placed carefully in the coil openings and gently compressed, and arms over the head if possible.
Pre-contrast images are taken, then dynamic images are taken at 60 to 120 second intervals for 5 to 8 minutes after contrast. Subtraction images show only the enhancing tissue.
The images are reviewed with maximum intensity projections, multiplanar reconstructions and enhancement curves. Diffusion measurements (apparent diffusion coefficient) are added.
Implant studies use silicone selective sequences with no contrast.
