Prostate cancer diagnosis is dependent on a number of tests. Most of these tests are performed by your GP at you prostate check-up. Multiparametric MRI is another method that can be used as part of the screening and diagnosis of prostate cancer.
Researchers have experimented with MRI of the prostate since the 1980s. However, recent technological advancements have allowed MRI to clearly visualise the prostate gland, which can aid in prostate cancer detection.
Multiparametric MRI provides detailed anatomical and functional information which is unavailable from traditional ultrasound imaging.
Radiologists can use multiparametric MRI to:
- Identify the location of a tumor
- Measure the extent of a tumor
- Estimate the Gleason score of a tumor
- Determine whether a tumor has spread beyond the prostate gland
What’s special about Multiparametric MRI?
A multiparametric MRI exam consists of three separate imaging techniques (three ‘parameters’):
- T2-weighted imaging
- Diffusion-weighted imaging
- Dynamic contrast-enhanced imaging
A T2-weighted imaging sequence provides anatomic information about the prostate gland, offering detailed images of the gland and its distinct zones.
On T2-weighted images, cancers in the peripheral zone of the prostate (where most cancers are situated) typically appear as areas of enhancement or bright spots against a dark background.
Cancers in the transition zone are more difficult to detect but cancers in this zone are uncommon.
T2-weighted imaging also allows the opportunity to examine the seminal vesicles and the bladder wall to determine if a tumour has spread beyond the prostate.
Diffusion-weighted imaging (DWI) measures the motion of water molecules within the prostate to provide useful functional data about cancers.
This sequence produces an ADC value for different areas of the prostate gland.
ADC values measure the degree of motion through different tissues. Lower ADC values appear in cancerous tissue, and also correlate with a higher more worrying Gleason score.
Dynamic Contrast-enhanced Imaging
During dynamic contrast-enhanced (DCE) imaging, a contrast agent (gandolinium) is used to evaluate blood flow through the prostate.
Cancerous tissue absorbs the contrast agent more quickly than healthy tissue, which is apparent on DCE images.
The role of DCE imaging is secondary to T2-Weighted Imaging and DWI, but it can help to detect small, yet significant cancers missed by the other two techniques.
Multiparametric MRI Interpretation and prostate cancer diagnosis
Multiparametric MRI exams are interpreted according to the Prostate Imaging Reporting and Data System (PI-RADS).
This is a classification system that uses a 5-point scale to standardise assessment of exams.
A PI-RADS assessment indicates the likelihood of intermediate – and high-risk cancers based on findings from the three multiparametric MRI sequences.
- Very unlikely that clinically significant cancer is present – PI-RADS 1
- Unlikely that clinically significant cancer is present – PI-RADS 2
- Uncertain whether clinically significant cancer is present –PI-RADS 3
- Likely that clinically significant cancer is present – PI-RADS 4
- Highly likely that clinically significant cancer is present – PI-RADS 5
For results of PI-RADS 4 or 5, prostate biopsy is always indicated.
Results of PI-RADS 1 or 2, a recommendation for biopsy is unlikely, but other factors should be considered.
PI-RADS 3 results may require a biopsy depending on patient history, local preferences and preferred standard of care.
Should an MRI be performed before a biopsy?
Firstly, some men may be so reassured by a negative MRI that they decide not to have a biopsy at all. We know that a very high quality MRI (ideally at 3T) is, if negative, more reassuring about the absence of tumour than a negative biopsy. They may decide to go no further, and to have both DRE and their PSA checked regularly, and ideally another MRI at an interval. If they do decide on biopsy, and that is negative also, they are very unlikely indeed to have a cancer that will harm them.
Secondly, most significant tumours are visible on MRI, so that a biopsy (or biopsies) can be accurately targeted to the suspicious area. This minimises the risk of missing tumours that lie in difficult to reach places (around 10% of significant cancers are completely missed by standard biopsy because of where they lie) and it also helps to be sure that the sample is representative: sometimes random biopsies just shave the edge of a large tumour, underestimating how much there is.
Thirdly. If a small amount of tumour is detected, MRI can check that this is not the edge of a large amount, or that there is a larger tumour in the front of the prostate. Active surveillance is unsuccessful in some men precisely because of such undetected tumours, most of which can be seen with MRI.
Finally, the finding of a significant cancer means that staging is required to detect spread outside the prostate. Radiologists who have looked at MRI images both before and after biopsy have no doubt that they are degraded for several months by the effects of bleeding from the biopsy: the best quality staging scan is undoubtedly one done before any of this has occurred – before the biopsy.
3T or 1.5 T MRI. Which is better?
The main magnet in an MRI scanner is a superconducting magnet, which is always on. Typically, the strength of the magnetic field produced by these magnets is 1.5 to 3 T (Tesla). To give you an idea of how powerful these scanners are, 1 Tesla is roughly 30 thousand times stronger than the Earth’s magnetic field.
Most scanners in clinical use in Ireland are 1.5 T, although 3 T magnets are now becoming more common. Both 1.5 T and 3T magnets give excellent views of the prostate.
While1.5T are absolutely acceptable, 3T scanners offer certain advantages. These include
Because the 3T magnet is so powerful, the procedure does not require the invasive endo-rectal coil that 1.5T MRI machines require. Patient comfort – and willingness to undergo the imaging procedure – is dramatically improved scanning times are considerably shorter.
Smaller tumours that might be missed on a 1.5T scanner can be seen on the more powerful 3T
What the journals say
January 19th 2017
1 in 4 men with suspected prostate cancer could avoid unnecessary biopsy if given an MRI scan first.
Giving men with suspected prostate cancer an MRI scan could improve prostate cancer diagnosis and save those who do not have aggressive cancers from having an unnecessary biopsy, according to a study published in The Lancet.
The study estimates that adding the extra test could help one in four (27%) men avoid an unnecessary biopsy and reduce the number of men who are over-diagnosed – diagnosed with a cancer that does not go on to cause any harm during their lifetime – by 5%.
British Medical Journal
Multiparametric MRI followed by targeted prostate biopsy for men with suspected prostate cancer: a clinical decision analysis
To compare the diagnostic outcomes of the current approach of trans rectal ultrasound (TRUS)-guided biopsy in men with suspected prostate cancer to an alternative approach using Multiparametric MRI (mpMRI), followed by MRI-targeted biopsy if positive.
Design Clinical decision analysis was used to synthesise data from recently emerging evidence in a format that is relevant for clinical decision making.
In 1000 men, mpMRI followed by MRI-targeted biopsy ‘clinically dominates’ TRUS-guided biopsy. It results in fewer expected biopsies (600 vs 1000), more men being correctly identified as having clinically significant cancer (320 vs 250), and fewer men being falsely identified (20 vs 50).
The mpMRI-based strategy dominated TRUS-guided biopsy in 86% of the simulations in the probabilistic sensitivity analysis.
Conclusions Our analysis suggests that mpMRI followed by MRI-targeted biopsy is likely to result in fewer and better biopsies than TRUS-guided biopsy.