By Xavier Garcia-Rojas, M.D., Ph.D, M.B.A.
MRI is increasingly being found to be a very good tool for detecting prostate cancer. I’d like to give a short background on how MRI works, and specifically how it relates to diagnosing disease in the prostate.
Inside the MRI is a strong magnet. The strength of the magnet is denoted in Tesla units, which is abbreviated “T.” Magnetic field strengths of 1.5T and above are considered adequate for prostate imaging, with many considering 3T scanners the highest commercial quality for prostate imaging. The purpose of the magnet is to make the body’s hydrogen atoms align along the direction of the magnetic field. The space inside the large tube, called the “bore,” is where a patient will lie down to get imaged.
Anyone who has been in an MRI knows that it is a noisy study. Those loud noises are the effect of a sequence of radio pulses emitted by the scanner, whose purpose is to change the direction and spin of the hydrogen atoms in the body. After the end of each of the radio pulses, those hydrogen atoms again snap back to align with the magnetic field and the spins change. When they do this, the body’s hydrogen atoms emit a weak radio signal themselves. This signal is detected by the scanner and transformed mathematically into a computer image.
The order and type of radio pulses created by the machine are called MRI pulse sequences. Each kind of sequence is given technical names (such as T1, T2, diffusion, etc.) that describe the parameters for how the radio signals are obtained.
The different MRI image parameters not only give you anatomic detail, they can also tell you relative difference in how much water and fat and certain other molecules (such as iron) are concentrated in different tissues. Combined with a liquid that is injected into your veins (known as “contrast”), MRI images can give information on how much blood flow goes to different tissues and how fast this blood gets there. The combination of T1, T2, diffusion, and contrast-enhanced images are what currently make the foundation of “multiparametric” prostate MRI.
Each piece of information gives hints at underlying health or disease in the prostate. It takes looking at the combination of these image parameters to come up with the diagnostic criteria that radiologists use to describe findings in the prostate.
Even with all this information and knowledge, comparing MRI centers can sometimes feel like comparing apples and oranges. There are multiple vendors of MRI equipment, and even within the vendors there are multiple versions of scanners and tools and ways of personalizing each machine. Some use rectal probes (also known as a “rectal coil”) to detect the radio signals coming from the prostate, whereas others only use external antennas (called “body coils”) that wrap around the pelvis or are part of the MRI table. A general rule of thumb is that the MRI scanners with the lower field strength of 1.5T benefit from the rectal coils, whereas 3T scanners can provide a similar picture with the external coils. The quality of these sensing coils is just as important as the quality of the rest of the machine, and it takes all these factors in concert along with an experienced and well-trained radiologist, to give you the best possible study.
The prostate gland is particularly challenging for radiologists to interpret. Worldwide experts in prostate MRI came together to create a system known as PI-RADS to standardize the reporting and interpretation of prostate MRI. PI-RADS stands for Prostate Imagine-Reporting and Data System. This system has been revised and is currently in its second version, known as PI-RADS 2. In this system, a score is given to any suspicious lesion in the prostate to denote its likelihood of being cancer. Scores range from 1 to 5, with 1 being normal to 5 being “highly suspicious for cancer.”
MRI is a powerful screening tool for men at risk for prostate cancer, but by itself is still not considered enough to diagnose prostate cancer. There are varied recommendations for follow-up of suspicious lesions found on MRI; however, most generally will recommend biopsy of lesions given higher PI-RADS scores (3 or above). It is the biopsy of the suspicious lesions that will guide further management.
In a person that has a positive biopsy, MRI is a great way to determine where and how large the cancerous lesion is in the gland, and if it has invaded the tissues beyond the gland (including the urinary bladder, the urethra, the nerves that are responsible for erectile function, the rectal wall, suspiciously enlarged pelvic lymph nodes, and even the bones in the pelvis).
Additionally, MRI is now used directly in interventional procedures, to both guide needles for biopsies of suspicious lesions in the prostate as well as to guide focal treatments with laser ablation. MR guided biopsies have shown to have a higher yield of clinically significant tumors as noted by the Felker study. I will discuss the benefits of MR guided biopsies in a future posting.
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