Prostate Magnetic Resonance Imaging (MRI)

Prostate Magnetic Resonance Imaging (MRI) 

Michael S. Leapman, M.D. 12/2015 

What is a Prostate MRI? 

Magnetic resonance imaging (MRI) is a method for visualizing soft tissue structures in the body. Images are generated by MRI scanners using magnetic fields instead of x-rays or other forms of radiation. 

When applied to imaging of the prostate, MRI offers advantages in identifying potentially significant aggressive prostate cancers and recent advances in technology also facilitate directlesion targeted biopsy.  

What is a Fusion Biopsy? 

Fusion MRI-ultrasound biopsy is a technique that overlays previously acquired MRI images of the prostate with real-time transrectal ultrasonography (TRUS) to allow clinicians to directly target suspicious areas identified on MRI. It combines ability to target in real-time ultrasound with the more detailed MR images to allow for a more accurate diagnosis of prostate cancer. The technique involves acquisition of the MR images that are reviewed by experienced radiologists to identify areas of interest for targeting the biopsy. Your provider has the ability to perform direct biopsy of the regions of special concern, in addition to sampling of other areas within the prostate.  

Why might my physician recommend that I have a prostate MRI? 

Prostate MRI is a highly useful staging method for prostate cancer that has been studied widely. 

Elevated PSA (before first biopsy): Several studies have evaluated the ability of MRI and subsequent targeted MRI-ultrasound fusion biopsyto identify cancers within the prostate. Such techniques appear to offer improved detection rates for high-grade cancer compared with systematic sampling alone. MRI does not identify all high-grade cancers, and a negative MRI does not obviate the need for a biopsy 

 Following the diagnosis of prostate cancer:Prostate MRI can be used after the diagnosis of prostate cancer as a staging tool to confirm the stage of disease, size and location of lesions within the prostate that may be relevant to treatment planning for both surgery or radiation therapy.  

During active surveillance: For men with low-risk prostate cancers managed with active surveillance, MRI may be performed at the initiation of surveillance to rule out the presence of higher-grade elements not detected by a TRUS prostate biopsy, and also may be performed periodically in some cases to evaluate changes within the prostate.  

Following treatment: MR imaging of the prostate or prostate bed can be utilized following surgery or radiation therapy in men with suspicion of recurrence to identify potential areas of residual disease. These may be useful in subsequent treatment planning.  

What does “3T” or “1.5T” refer to? 

T (Tesla) refers to the strength of the magnetic field, and higher field strengths improve imaging resolution and the speed of imaging. For imaging of the prostate, field strengths of 3T offer significant advantages in image quality. 

How long will the MRI take? 

The length of the MRI study take between 45 minutes and one hour, including placement of an intravenous (IV) line, placement of the endorectal coil (a balloon placed inside the rectum to improve image resolution), and MR imaging.  

Why does the MRI require intravenous contrast? 

Intravenous contrast (Gadolinium DTPA) is used in prostate MRI because it improves the localization of prostate cancer and because prostate cancer can be better characterized on images acquired after the administration of contrast. 

Advantages of an endorectal coil? 

The endorectal coil is a disposable, flexible receiver which is surrounded by an inflatable latex balloon. Use of the endorectal coil offers significant improvements in image quality, yielding favorable detection rates of lesions within the prostate.  

Is there a risk of radiation exposure with MRI? 

Because MRI utilizes magnetic fields, there is no exposure to ionizing radiation. 

What is a “multi-parametric” prostate MRI? 

In order to assess multiple characteristics of the prostate, several sequences are typically performed within the same study, referred to as multi-parametric MRI (mpMRI), which is the preferred study for accurate assessment. Experienced radiologists will review the MRI and grade each sequence and generate an overall suspicion score. These sequences frequently include: 

  • T2: an anatomic view of the prostate and surrounding structures.  This sequence is particularly important for staging of the primary tumor. 
  • Diffusion Weighted Imaging (DWI): a measure of 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 (DCE) images that are obtained after intravenous contrast is administered. Areas within the prostate that show rapid uptake of contrast are more likely to represent prostate cancer.  
  • MR Spectroscopy: an imaging sequence that evaluates the metabolic content within the prostate, specifically examining the relative concentrations of citrate and choline that can help characterize prostate cancers.  

The overall suspicion score (1-5) is known as the PIRADS score and is currently defined as: 

PIRADS 1 – Very low (clinically significant cancer is highly unlikely to be present) 

PIRADS 2 – Low (clinically significant cancer is unlikely to be present) 

PIRADS 3 – Intermediate (the presence of clinically significant cancer is equivocal) 

PIRADS 4 – High (clinically significant cancer is likely to be present) 

PIRADS 5 – Very high (clinically significant cancer is highly likely to be present) 

It is important to note this is not a linear scale. The diagnostic accuracy of this system is still under investigation 

Novel enhancements to MRI  - including next-generation spectroscopy and incorporation of novel targeting agents - are being actively studied at UCSF.