This study demonstrated that the TFC can provide active intra-procedural fusion image correction automatically after simulated motion in a model pelvis. Inaccuracies in targeting pre-identified prostate locations are often multi-factorial, and may be related to prostate motion or deformation, patient movement, or imaging challenges, including impaired segmentation, image registration or mismatch, and offset of imaging planes [7, 25]. The TRUS plane for acquisition, registration, and manual intra-procedural alignment correction is slightly offset from that of the MRI, adding to the challenge . Current FBx platforms and software systems inadequately compensate for motion of the prostate, thus limiting the accuracy of this technology, even when the prostate is mostly stationary and static. Additionally, registration methods and techniques are widely variable and non-standardised across many commercial platforms, contributing toward variable outcomes from those centres performing or reporting image-guided prostate procedures . Standardisation of rigid and elastic registration combined with dynamic referencing of the prostate might reduce the operator variability and the inaccuracies induced by subjectivity, experience, and variable intra-procedural corrections of misaligned fusion images.
Within the TFC, six degrees of freedom tracking sensors induce a weak current in the presence of a rapidly changing magnetic field, enabling spatial localisation on imaging (same principle of electromagnetic tracking in existing FBx systems) [18, 28]. The TFC resides within the urethra, a non-linear central pathway. This urethral path is less altered by prostate motion (especially rotational) or deformation, than other areas of the prostate, and can act as a standardisation tool or “home reference” for correcting fusion image mismatch during a procedure or surgery. Although speculative, it is possible that FBx can perhaps then be more accurately performed in the presence of prostate motion, since the motion of the urethra (and TFC within) can provide real-time referencing information, even with poor image visualisation. The fusion image mismatch can thus automatically be accounted for, and a correction may be computed and applied to restore the initial registration in real time.
During image-guided prostate procedures, the patient often moves involuntarily due to pain, discomfort, and/or pressure related to the insertion of needles or TRUS transducer. Moreover, a TRUS transducer can cause distortion of the contour of the prostate during the FBx. Finally, focal blood, local anaesthetic, bowel gas, dissection, bladder filling, or respiratory motion of the patient may cause shifting of the prostate, especially when the patient is in the prone position . The TFC has the ability to maintain appropriate MRI/US fusion image alignment after simulated organ/patient motion in a catheterisable pelvic phantom model. All three methods of evaluation (DSC, OD, SE) supported the potential benefit of the TFC.
The TFC core concept of active intra-procedural fusion image correction may potentially be applied to other procedures, settings, or organs with additional hardware components and software . The TFC ability to maintain accurate image alignment after motion could not only benefit existing procedures using fusion imaging (FBx, trans-perineal FBx, focal PCa ablations), but also potentially enable fusion guidance within robotic platforms [8, 12, 15, 16, 19,20,21,22,23,24, 29,30,31]. In addition to aid in alignment after simulated prostate motion, this study also indicated that the TFC may aid in more accurate image alignment while the patient remains static. It is possible this finding may stem from the fact that there is some degree of organ motion or deformation when the US transducer is introduced in the rectum, either during the initial US sweep or at any other point during image-guided procedures. The adjustments provided by the TFC may automatically correct for this subtle organ motion or deformation. Our data indicated increased fusion image alignment when the TFC correction was available and no simulated motion was induced, reporting significantly smaller OD in this scenario. Reported DSC scores showed a similar trend. Although the TFC SE was favoured when compared to no TFC SE when static, the results were not statistically significant.
Standardisation and reproducibility of current image-guided prostate procedures have been less than ideal [5, 9, 10]. Accordingly, there is an urgent need for hardware, systems, and methods for maintaining accurate registration between pre-procedural and intra-procedural images. Image-guided prostate procedures require operator experience for reliable registration of mpMRI/US images and for accurate targeting of pre-identified areas [8, 17, 32]. The TFC may help with the issue of prostate motion by providing a standardised/semi-automated method of maintaining the original registration. Moreover, computer-assisted biopsy systems may facilitate more widespread and standardised use of FBx, even by novice operators who may be uncomfortable manipulating dedicated navigation software [25, 32]. On the other hand, some intrinsic limitations are still present such as the general contraindications for MRI (e.g., the claustrophobia and the presence of a magnetic field or of the pacemakers) and the current failure rate of TRUS-guided biopsy [12, 33,34,35,36]. Additional research may explore the extent to which the TFC aids in fusion image alignment when the prostate is static, specifically among novice FBx users.
In our study, fusion image alignment with and without the TFC was compared in three ways with similar results and one method physically targeted region of interests with needles. However, there were several limitations of this study. The physical targeting of identified structures with needles damaged the phantom model somewhat and limited the number of SEs that could be measured on one custom anthropomorphic phantom. There was also no easy and effective way to blind the operator to the presence or absence of the TFC and artificial motion without crippling the study. It remains to be seen whether (and in which scenarios) this solution might prove worthy of the cost and risks associated with an additional catheterisation procedure. Also speculative is the eventual utility for addressing intra-operative prostate organ motion during robotic prostatectomy, which suffers even greater dynamic referencing challenges and shortcomings than FBx. The TFC merits evaluation in a clinical trial setting for biopsy, ablation, and robotic prostatectomy.
In conclusion, fusion-guided prostate procedures have proven benefits. However, fusion biopsy is not yet standardised and suffers from variability and challenges in maintaining accurate image alignment as well as a dearth of tools for facilitated and semi-automated registration. Challenges remain in maintaining registration integrity, procedural standardisation, and reproducibility that may limit the benefits or adoption of fusion guidance. A custom TFC maintained accurate fusion image alignment after movement and provided significantly better fusion image alignment when static. The inherent challenges of maintaining fusion image alignment may be addressed by this additional smart device, in the presence of an electromagnetic field generator. Although yet to be proven, a TFC may help address clinical needs related to standardisation and correction of intra-procedural prostate motion during fusion-guided prostate procedures such as biopsy, ablation, or robotic prostatectomy.