Not long ago, a raised PSA usually led straight to a biopsy, with samples taken from across the prostate in the hope of finding any cancer. Today, most men in the UK have an MRI scan first. This change has made diagnosis more accurate, and for some men it removes the need for a biopsy altogether.
What mpMRI means
mpMRI stands for multiparametric magnetic resonance imaging. "Multiparametric" means the scan combines several types of image, each showing a different feature of the prostate tissue:
- T2-weighted images: show the shape and structure of the prostate in detail.
- Diffusion-weighted images: show how freely water moves through the tissue. Cancer cells are tightly packed, which restricts this movement.
- Contrast-enhanced images: use an injected dye to show blood flow, which can be increased in cancer.
Some centres now use a shorter scan without contrast, known as biparametric MRI, for suitable men.
Why MRI comes before biopsy
NICE guidance on prostate cancer recommends MRI as the first test for men with suspected prostate cancer that is confined to the gland. There are three main benefits:
- Finding significant cancers: MRI is good at showing the cancers most likely to need treatment.
- Avoiding some biopsies: if the scan shows no suspicious area, some men can safely avoid a biopsy and continue with PSA monitoring.
- Targeting biopsies: if an area looks suspicious, the biopsy can be aimed directly at it.
MRI is also less likely than a random biopsy to pick up small, low-risk cancers that may never cause harm.
The PI-RADS score
Radiologists report prostate MRI using a scale called PI-RADS, from 1 to 5:
- 1 or 2: clinically significant cancer is unlikely
- 3: the finding is uncertain
- 4 or 5: clinically significant cancer is likely
A score of 3 is often weighed alongside other information, such as PSA density, before deciding whether to biopsy.
What the scan is like
An MRI scan is painless and does not use X-rays. You lie on your back on a table that slides into the scanner, usually for 30 to 40 minutes. The machine is noisy, so you will be given ear protection. You may have a small injection of contrast dye and sometimes a medicine to reduce bowel movement, which improves the images.
Let the team know beforehand if you have a pacemaker, metal implants or any worries about enclosed spaces. Most men find the scan easier than they expected, and you can usually drive home and return to normal activities straight afterwards. The images are then reviewed by a specialist radiologist, and the report is usually ready within a few days.
The limits of MRI
MRI is very helpful but not perfect. A small number of significant cancers do not show up clearly. Our article on significant prostate cancer after a negative MRI looks at this in more detail. This is why doctors still consider PSA, family history and examination findings alongside the scan.
MRI beyond diagnosis
MRI is also used to:
- help plan surgery, showing where a tumour sits in relation to the nerves
- guide active surveillance, with repeat scans checking for change over time, so that many men with low-risk cancer can avoid repeated biopsies
- assess whether cancer may have spread outside the prostate
For some men, a PSMA PET scan is added to look for spread elsewhere. Our article on combining MRI and PSMA PET scans explains when this helps.
What happens next
After your scan, a specialist will go through the result with you. If a biopsy is recommended, you will be told which method is planned and why. Prostate Cancer UK has more information on what to expect.
Professor Prokar Dasgupta uses MRI to guide diagnosis and treatment planning at The London Clinic. You can read about his approach on our robotic prostatectomy page, or contact the clinic to arrange a consultation.