Multiparametric Magnetic Resonance Imaging (mpMRI)

Prostate multiparametric magnetic resonance imaging (mpMRI) has emerged as the imaging method best able to detect clinically significant prostate cancer and to guide biopsy.

The term “multiparametric” refers to the multiple MRI sequences that are analyzed to determine the diagnosis, which differs from other MRI scans in which the diagnosis is based on a single MRI sequence. In prostate mpMRI, no one MRI sequence is definitive; rather, it is the combination of positive parameters that contribute to the diagnosis.

Multiparametric MRI combines traditional anatomic imaging techniques with functional techniques to reveal detailed information about potential prostate cancers. These sequences generally include:

Anatomic imaging — Prostate cancer can be identified on T2 – Weighted (T2W) imaging based on pathologic changes within the prostate. Anatomic imaging is an excellent technique for assessing the spread of cancer outside the gland.

Diffusion-weighted imaging — Diffusion-weighted imaging (DWI) measures the movement of water molecules within prostate tissue. Areas with restricted motion (or diffusion) of water molecules are identified as suspicious.  This imaging technique has become one of the most important sequences for detecting prostate cancer.

Dynamic contrast-enhanced — Dynamic contrast-enhanced (DCE) images are obtained after intravenous contrast is administered. Areas within the prostate that show rapid uptake of contrast are more likely to represent prostate cancer. DCE can also be used to assess prostate cancer presence, spatial extent, and aggressiveness.

The Prostate Imaging Reporting and Data System (p), is employed to categorizing mpMRI findings. PI‐RADS v2 assessment uses a 5‐point scale based on the likelihood (probability) that a combination of mpMRI findings on T2W, DWI, and DCE correlates with the presence of a clinically significant cancer for each lesion in the prostate gland. The PI-RADS v2 assessment categories are defined with the following scores:

1. PI-RADS 1 — Very low (clinically significant cancer is highly unlikely to be present)
2. PI-RADS 2 — Low (clinically significant cancer is unlikely to be present)
3. PI-RADS 3 — Intermediate (the presence of clinically significant cancer is equivocal)
4. PI-RADS 4 — High (clinically significant cancer is likely to be present)
5. PI-RADS 5 — Very high (clinically significant cancer is highly likely to be present)

Compared with conventional diagnostic tools used in prostate cancer detection and diagnosing, multiparametric MRI is proving to be useful in multiple phases of a more reliable option when it comes to determining which men would benefit from biopsy. Paired with prostate-specific antigen (PSA) screening, multiparametric MRI can be used to evaluate elevated and rising PSA levels prior to biopsy. Additionally, multiparametric MRI images can be used to guide biopsy needles with greater accuracy than other imaging techniques and can provide information to assist prostate cancer staging and treatment planning.

Benefits of Multiparametric MRI

Early detection of prostate cancer — PSA, digital rectal examination and blind transrectal biopsies of the prostate have until now been the gold standard of prostate cancer diagnosis. While they have been shown to save lives, they have also been shown to detect relatively harmless, insignificant cancers. Furthermore, they sometimes miss aggressive tumors. Multiparametric MRI offers the possibility of reducing the number of unnecessary biopsies while detecting aggressive tumors in high-risk individuals. This, coupled with the appropriate use of targeted biopsy, should aid in the appropriate early detection of significant prostate cancer.

Improving the accuracy of biopsy — Multiparametric MRI offers the option of more accurate targeting of tumors in the prostate. If an MRI detects an obvious tumor, then, with sophisticated fusion technology overlaying the MRI image with the ultrasound image, it is now possible to accurately biopsy the area in question. This not only detects more significant cancers, which are those picked up by MRI, but it also avoids biopsying unnecessary, insignificant cancers, thus leading to less over-detection. MRI is not perfect in detection at this stage, particularly in certain sections of the prostate called the transition zone. Fortunately, this is the least common site for aggressive prostate cancers.

Planning surgery and radiotherapy — Multiparametric MRI, by improving the accuracy of assessing the extent of cancer of the prostate, helps us to know whether the cancer is through the capsule, whether it is has invaded the seminal vesicles or whether it has spread into the lymph glands. This can then help us target with appropriate surgery and radiotherapy the most appropriate form of treatment and dosage of radiotherapy. It is also helping us preserve the erectile nerves in surgery by giving us further confidence that the nerves are not infiltrated with cancerous tissue. Finally, it is even helpful in predicting the likelihood of incontinence after surgery by measuring the length of the urethra.

Monitoring patients on active surveillance — An increasing number of patients with low-risk cancers have their cancer simply monitored. This has traditionally involved regular biopsies and PSA readings. MRI has added an extra dimension to the monitoring of these cancers and clearly is much less invasive than biopsy. Furthermore, it helps by excluding cancers which may have been missed by the initial biopsy. It is very likely that MRI will have an increasing role and biopsies will have a decreasing role in the monitoring of these patients on active surveillance after initial diagnosis.

Focal therapy — MRI and transperineal grid-directed biopsy have allowed more accurate imaging and sampling of the prostate, allowing the possibility of ‘lumpectomy’ or focal therapy to be a real option in prostate cancer patients. New energy sources, including high-intensity focused ultrasound, focal cryoablation and focal laser therapy have allowed this to emerge as a possible treatment. Focal therapy would not be possible without accurate localization and multiparametric MRI has allowed this to occur. Focal therapy at this stage is in its early formative stage but is certainly going to have an increasing role in the management of patients who have earlier and more localized prostate cancer.