Image registration for targeted MRI-guided transperineal prostate biopsy

Abstract

Purpose: To develop and evaluate image registration methodology for automated re-identification of tumor-suspicious foci from preprocedural MR exams during MR-guided transperineal prostate core biopsy.

Materials and methods: A hierarchical approach for automated registration between planning and intra-procedural T2-weighted prostate MRI was developed and evaluated on the images acquired during 10 consecutive MR-guided biopsies. Registration accuracy was quantified at image-based landmarks and by evaluating spatial overlap for the manually segmented prostate and sub-structures. Registration reliability was evaluated by simulating initial mis-registration and analyzing the convergence behavior. Registration precision was characterized at the planned biopsy targets.

Results: The total computation time was compatible with a clinical setting, being at most 2 min. Deformable registration led to a significant improvement in spatial overlap of the prostate and peripheral zone contours compared with both rigid and affine registration. Average in-slice landmark registration error was 1.3 ± 0.5 mm. Experiments simulating initial mis-registration resulted in an estimated average capture range of 6 mm and an average in-slice registration precision of ±0.3 mm.

Conclusion: Our registration approach requires minimum user interaction and is compatible with the time constraints of our interventional clinical workflow. The initial evaluation shows acceptable accuracy, reliability and consistency of the method.

Figure 1

Registration results for patients 2 (top row) and 5 (bottom row). Images on the left (a and c) show an intra-procedural axial T2W image, images on the right (b and d) show the same slice of the registered pre-procedure T2W MRI. Yellow contour corresponds to the outline of the prostate gland segmented in the diagnostic image and warped to the intra-procedural image space by means of registration. Images for Patient 2 show brachytherapy seeds artifacts, as the patient previously underwent radiation therapy.

Figure 2

Summary of the LRE and DSC for the affine registration results with various values of the initial misalignment for the registration case with the smallest capture range (patient 2, n=500 registration experiments). (a) Binned histogram of the maximum LRE as a function of the initial misalignment magnitude. As the initial misalignment magnitude exceeds 4 mm, it becomes more likely for the registration to converge to a local minimum. (b) Histogram of the DSC values over the 500 experiments. Registration success was concluded when DSC exceeded 0.8 so that the results corresponding to the local minima (n=129) are discarded. (c) Histogram of the DSC values for the registration experiments results deemed successful.