Know The Difference Between In-bore MRI and Fusion

Word has gotten out to prostate cancer patients about MRI-guided focal treatments. It all started with prostate biopsies. Patients have learned that traditional 12-core biopsies guided by transrectal ultrasound (TRUS) are basically blind and random because ultrasound can’t detect or show the actual tumors. Demand for in-bore MRI guided targeted biopsies began to increase because they were proven more accurate, with fewer needles. Then, when focal laser ablation (FLA) could be done under in-bore MRI guidance, the real time high resolution images plus thermometric monitoring made FLA incredibly accurate and precise. Meanwhile, HIFU device manufacturers were developing ways to perform in-bore HIFU in order to gain the benefits of real time tumor visualization and monitoring.

Now there is another type of treatment guidance called MRI-ultrasound fusion. I have previously written about fusion-guided biopsies, so I will focus on fusion-guided treatment here.

What is MRI-ultrasound fusion?

As the name implies, two different types of imaging (MRI and ultrasound) are merged or “fused” together. Many clinical articles and treatment centers call this “MRI-guidance” but this is misleading! The MRI images are not live. They were previously captured. This is a significant distinction: live ultrasound can show the prostate from different views, it can show needles or probes inserted into the gland, and it can even register temperature-induced changes in tissue up to a certain point – but ultrasound cannot define the tumor within the gland!

Here’s what’s involved in fusion. Step One: per a urologist’s prescription, the patient visits a radiology center to have an in-bore multiparametric MRI of the prostate. The images are captured and sent to the urologist. Step Two: the patient returns to the urologist, who has an ultrasound device equipped with special fusion software. With the MRI images uploaded into the device, the urologist does a real-time transrectal ultrasound of the patient’s prostate. Keep in mind that during the MRI, the patient was on his back and had no wand in his rectum; now, during the ultrasound, the patient is on his side with his knees pulled up and a wand inserted – thus, different positioning. Step Three: the urologist manually inputs the prostate outlines from the live ultrasound images into the computer; he does the same with the previously captured MRI prostate and tumor outlines. The software then assigns “coordinates” to the inputs in order to “marry” the MRI and ultrasound outlines coordinate by coordinate. This is called image registration or co-registration. The result is a hybrid blend of the static MRI and tumor with the live ultrasound. Step Four: on the monitor screen, the software produces a synthetic 3D image showing a schematic of the prostate and its tumor (size and location). This image is not identical to a real time, live mpMRI. There will be slight variations or distortions due to things like different positioning, patient movement and even breathing, or slightly inaccurate segmentation.

When it comes time for the fusion-guided treatment (e.g. focal HIFU or FLA) the patient is anesthetized and an ultrasound wand inserted in the rectum. Again, the segmented outlines from the previously captured MRI showing the tumor, and the outlines from the real time ultrasound that does NOT show the tumor are inputted. This new synthetic image is used to map and plan the ablation delivery. However, since the ultrasound cannot show the tumor in real time during the treatment, the doctor is using his knowledge based on the previous MRI and the current synthetic image to treat the tumor. It this still as accurate as in-bore MRI guidance, monitoring and verification?

The answer is no. The clue lies in published biopsy comparison data. “The major advantage of the in-bore technique is that it offers the most accurate targeting of the MRI-identified lesions and reduced detection of insignificant tumors.”[i] In other words, no contest. Fusion-guided treatments can’t replace in-bore, real time MRI-guided ablation because they can’t assure the same accurate precision – and therefore fusion guidance can’t deliver results with the same high degree of confidence.

Misleading labels

Patients should not be told that they are having an “MRI-guided biopsy” or an “MRI-guided ablation” just because the fusion software has an MRI component. The patient will be nowhere near MRI equipment during fusion-guided treatment! When it comes to the prostate gland nestled deeply in the pelvic bed, in-bore, real time MRI simply outperforms real time ultrasound. The artificial blend of the two cannot offer the real time “eyesight” of the MRI.

To draw an analogy from a TV set, you might think of fusion like this. Imagine that you own a wide screen high definition flat panel TV in your family room, and an old black-and-white TV in your basement rec room. You take a color photo of a nature scene from your high-def TV, print an enlargement, and tape it to the screen of your basement set. You would hardly bring guests downstairs in hopes of entertaining them with a still photo, no matter how lovely – you’d much rather they join you for a real-time, high resolution program.

So, to return to in-bore MRI vs. fusion, imagine two patients scheduled for focal prostate cancer treatment. Their prostates are the same size, they both have a Gleason 3+4 tumor in the right peripheral zone, each man has an expert doctor who has treated over 600 cases. With all else being equal, what is the single factor that could make the difference between complete tumor destruction vs. missing a tiny amount of cancer? It would be which imaging technology is guiding and monitoring the procedure. Which would you want? It should be obvious. In-bore MRI is the safer, wiser choice.

Life presents us with many cost/benefits dilemmas, and this is often (and sadly) the case for prostate cancer patients when it comes to treatment decisions. Not everyone will be able to bring about their desired best-case choice. However, mpMRI can make every component of detection, diagnosis and focal treatment a better-case choice than the fusion alternative.


[i]Rothwax J, Geoarge A, Wood B, Pinto P. Multiparametric MRI in biopsy guidance for prostate cancer: fusion-guided. BioMed Res Intl. 2014;2014, Artice ID 429171, 7 pages. https://www.hindawi.com/journals/bmri/2014/439171/