Sperling Prostate Center

MRI/Ultrasound Fusion May Come with a Pitfall Called Registration Error

I came across a study written by a team of German urology/radiology research team. The authors are all physicians, and their report compares three commercial MRI/ultrasound fusion technologies: BioJet (D&K Technologies), UroNav (Invivo Corp.) and Trinity (KOELIS).[i] Their comparison study occupies the intellectual space where technology and medicine meet, since fusion literally integrates the brainpower of clinicians into engineered devices. It therefore seems apt that a German medical team, trained and experienced in a culture renowned for dedication to engineering precision, would conduct a head-to-head analysis of three fusion systems.

Fusion technology aims to bring the radiologic benefits of prostate multiparametric MRI (mpMRI) into urologic practice, where a pre-recorded MRI scan showing suspicious areas is merged with real-time transrectal ultrasound (TRUS) that can’t identify these areas. The point-by-point merging process is called registration. A National Institutes of Health paper tells us, “Registration is the often tedious process of bringing separate imaging modalities (MRI and US) into spatial alignment for ease of viewing, either side by side or blended on top of each other using fusion software.”[ii] There are two types:

  • Rigid registration – The urologist manually overlays the MRI and ultrasound images on a monitor, rotating them for the best overall alignment. This is the simplest and least time-consuming way to match the real-time ultrasound with the prostate anatomy shown on MRI.
  • Elastic registration – A software algorithm is used to compensate for the differences in prostate shape between the recorded MRI and the live ultrasound. The doctor may initially perform alignment as for rigid registration, but when the semiautomated software algorithm is applies, it “de-forms” (that is, stretches and molds) the identified prostate margin or shape from the MRI to the ultrasound (or vice versa, depending on the program’s design). According to the NIH paper, “This is done to maximize fusion image alignment and create what seems like a more precise depiction of the lesion superimposed on US but it may in fact warp or distort the true anatomy in the process.”[iii] On the other hand, “As elastic registration compensates for possible deformation caused by the introduction of an ultrasound probe for example, it is expected that it would perform better than rigid registration.”[iv]

Results of the German comparison study

The German team set out to explore the usability and diagnostic accuracy of three fusion devices. The first two, BioJet and UroNav, employ rigid registration, while Trinity uses elastic registration. The study enrolled 60 prostate biopsy patients of whom 20 had BioJet, 20 had UroNav, and 20 had Trinity fusion biopsies. The four urologists in the study rated the systems for usability and diagnostic accuracy.

Their results were based on their findings: The rigid registration systems were more user-friendly, with shorter biopsy procedure times than the elastic system. Both types had similar detection rates (13 PCa diagnoses for each of the rigid systems, 14 for the elastic) with no significant differences regarding PCa grade.

Interestingly, three years prior to the German study, a Dutch academic group recognized for their work with mpMRI reviewed published literature on both rigid and elastic registration in terms of detecting clinically significant prostate cancer (csPCa). They reported: “No significant difference was identified between rigid and elastic registration for MRI-TRUS fusion-guided biopsy in the detection of csPCa; however, both techniques detected more csPCa than TRUS-guided biopsy alone.”[v]

An important critique

It thus appears that neither rigid nor elastic registration has a clear advantage over the other. However, a very telling critique out of the National Institutes of Health suggests that both methods are subject to registration errors, especially by less experienced urologists. The researchers tested fusion registration accuracy using prostate phantom (i.e., a life size prostate model with four small gold markers placed within it to simulate PCa lesions). The metal markers were detectable by both MRI and ultrasound.

The MRI scans of the phantom, with regions of interest labeled, were then provided to a novice and experienced user of fusion guidance. Their task was to register the MRI and ultrasound scans of the prostate phantom using both rigid and elastic systems. What they found is what we’ve been consistently concerned over in our blogs about registration error. “A total of 224 registration error measurements were recorded.”[vi] To us, this is dangerous.

The authors note that elastic registration did not significantly improve registration error over rigid, and in fact had a higher rate of error for lesions closer to the periphery. Not surprisingly, the more experienced user had a lower error rate than the novice user. Their bottom line can’t be ignored: “…elastic registration algorithms cannot serve as a replacement for attention to detail during the registration process and anatomical landmarks indicating accurate registration when beginning the [biopsy] procedure and before targeting each region of interest.”[vii]

An important note: Cata, et al. (2021) conducted a critical review of 37 prospective studies of both fusion-guided and in-bore MRI guided prostate biopsy. They concluded, “In-Bore MRI targeted biopsy performs better than Fusion biopsy especially in cases of apical lesions.”[viii] Enough said. Fusion is a kind of middleman that may indeed bring mpMRI into a urologist’s suite, but with a pitfall: registration error that a human must be very careful to compensate for—especially when dealing with cancer.

NOTE: This content is solely for purposes of information and does not substitute for diagnostic or medical advice. Talk to your doctor if you are experiencing pelvic pain, or have any other health concerns or questions of a personal medical nature.

[i] Sokolakis I, Pyrgidis N, Koneval L, Krebs M et al. Usability and diagnostic accuracy of different MRI/ultrasound-guided fusion biopsy systems for the detection of clinically significant and insignificant prostate cancer: a prospective cohort study. World J Urol. 2021 Jun 17.
[ii] Hale GR, Czarniecki M, Cheng A, Bloom JB et al. Comparison of Elastic and Rigid Registration during Magnetic Resonance Imaging/Ultrasound Fusion-Guided Prostate Biopsy: A Multi-Operator Phantom Study. J Urol. 2018 Nov;200(5):1114-1121.
[iii] Ibid.
[iv] Venderink W, de Rooij M, Sedelaar JPM, Huisman J, Futterer J. Elastic Versus Rigid Image Registration in Magnetic Resonance Imaging-transrectal Ultrasound Fusion Prostate Biopsy: A Systematic Review and Meta-analysis. Eur Urol Focus. 2018 Mar;4(2):219-227.
[v] Ibid.
[vi] Hale et al. Ibid.
[vii] Hale et al. Ibid.
[viii]Cata ED, Andras J, Telecan T, Tamas-Azora A et al. MRI-targeted prostate biopsy: the next step forward! Med Pharm Rep. 2021 Apr;94(2):145-157.


About Dr. Dan Sperling

Dan Sperling, MD, DABR, is a board certified radiologist who is globally recognized as a leader in multiparametric MRI for the detection and diagnosis of a range of disease conditions. As Medical Director of the Sperling Prostate Center, Sperling Medical Group and Sperling Neurosurgery Associates, he and his team are on the leading edge of significant change in medical practice. He is the co-author of the new patient book Redefining Prostate Cancer, and is a contributing author on over 25 published studies. For more information, contact the Sperling Prostate Center.

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