Multimodality Non-rigid Image Registration for Planning, Targeting and Monitoring During CT-Guided Percutaneous Liver Tumor Cryoablation

Rationale and Objectives: To develop non-rigid image registration between pre-procedure contrast enhanced MR images and intra-procedure unenhanced CT images, to enhance tumor visualization and localization during CT-guided liver tumor cryoablation procedures. Materials and Methods: After IRB approval, a non-rigid registration (NRR) technique was evaluated with different pre-processing steps and algorithm parameters and compared to a standard rigid registration (RR) approach. The Dice Similarity Coefficient (DSC), Target Registration Error (TRE), 95% Hausdorff distance (HD) and total registration time (minutes) were compared using a two-sided Student’s t-test. The entire registration method was then applied during five CT-guided liver cryoablation cases with the intra-procedural CT data transmitted directly from the CT scanner, with both accuracy and registration time evaluated. Results: Selected optimal parameters for registration were section thickness of 5mm, cropping the field of view to 66% of its original size, manual segmentation of the liver, B-spline control grid of 5×5×5 and spatial sampling of 50,000 pixels. Mean 95% HD of 3.3mm (2.5x improvement compared to RR, p<0.05); mean DSC metric of 0.97 (13% increase); and mean TRE of 4.1mm (2.7x reduction) were measured. During the cryoablation procedure registration between the pre-procedure MR and the planning intra-procedure CT took a mean time of 10.6 minutes, the MR to targeting CT image took 4 minutes and MR to monitoring CT took 4.3 minutes. Mean registration accuracy was under 3.4mm. Conclusion: Non-rigid registration allowed improved visualization of the tumor during interventional planning, targeting and evaluation of tumor coverage by the ice ball. Future work is focused on reducing segmentation time to make the method more clinically acceptable

Variability in MRI vs. ultrasound measures of prostate volume and its impact on treatment recommendations for favorable-risk prostate cancer patients: a case series

Background: Prostate volume can affect whether patients qualify for brachytherapy (desired size ≥20 mL and ≤60 mL) and/or active surveillance (desired PSA density ≤0.15 for very low risk disease). This study examines variability in prostate volume measurements depending on imaging modality used (ultrasound versus MRI) and volume calculation technique (contouring versus ellipsoid) and quantifies the impact of this variability on treatment recommendations for men with favorable-risk prostate cancer. Methods: We examined 70 patients who presented consecutively for consideration of brachytherapy for favorable-risk prostate cancer who had volume estimates by three methods: contoured axial ultrasound slices, ultrasound ellipsoid (height × width × length × 0.523) calculation, and endorectal coil MRI (erMRI) ellipsoid calculation. Results: Average gland size by the contoured ultrasound, ellipsoid ultrasound, and erMRI methods were 33.99, 37.16, and 39.62 mLs, respectively. All pairwise comparisons between methods were statistically significant (all p < 0.015). Of the 66 patients who volumetrically qualified for brachytherapy on ellipsoid ultrasound measures, 22 (33.33%) did not qualify on ellipsoid erMRI or contoured ultrasound measures. 38 patients (54.28%) had PSA density ≤0.15 ng/dl as calculated using ellipsoid ultrasound volumes, compared to 34 (48.57%) and 38 patients (54.28%) using contoured ultrasound and ellipsoid erMRI volumes, respectively. Conclusions: The ultrasound ellipsoid and erMRI ellipsoid methods appeared to overestimate ultrasound contoured volume by an average of 9.34% and 16.57% respectively. 33.33% of those who qualified for brachytherapy based on ellipsoid ultrasound volume would be disqualified based on ultrasound contoured and/or erMRI ellipsoid volume. As treatment recommendations increasingly rely on estimates of prostate size, clinicians must consider method of volume estimation

MRI-guided percutaneous cryoablation of renal tumors: use of external manual displacement of adjacent bowel loops.

PURPOSE: We sought to investigate retrospectively the safety and effectiveness of using external hand compression to displace adjacent bowel loops during MRI-guided percutaneous cryoablation of renal tumors. MATERIALS AND METHODS: Fourteen patients (six women, eight men; mean age: 72 years) with 15 renal tumors (mean diameter: 2.4 cm; range: 1.4-4.6 cm) adjacent to bowel were treated with MRI-guided percutaneous cryoablation during which bowel was displaced manually. Bowel loop of concern was ascending colon (n=5), descending colon (n=8), descending colon and small bowel (n=1), ascending colon and small bowel (n=1). To analyze effectiveness of the maneuver, mean distance between tumor margin and bowel before and after the maneuver were compared and analyzed using paired Student\u27s t-test. Minimum distance between iceball edge and adjacent bowel with external manual displacement during freezing was also measured. Safety was assessed by analyzing post-procedural MR imaging for adjacent bowel wall thickening and focal fluid collections as well as patients\u27 clinical and imaging follow-up. RESULTS: Mean distance between tumor margin and closest adjacent bowel increased from 0.8 cm (range: 0-2 cm) before external manual compression to 2.6 cm (range: 1.6-4.1 cm) with manual displacement (p CONCLUSION: MRI-guided percutaneous cryoablation of renal tumors adjacent to bowel can be done safely and effectively using external hand compression to displace bowel loops

Percutaneous cryoablation techniques and clinical applications.

Once requiring surgery, cryoablation now can be performed percutaneously under image guidance, thanks to the development of small probes. Sonography was used to guide cryoablation performed surgically; now, computed tomography and magnetic resonance images are typically used to guide percutaneous cryoablation. Intraprocedural monitoring helps those performing the procedure to treat the tumor completely, while avoiding complications. Percutaneous cryoablation has been shown to be safe and effective for many clinical applications including kidney, liver, prostate, breast, and musculoskeletal cancers. In this article, we briefly review percutaneous cryoablation techniques and clinical applications

Interventional MRI for oncologic applications.

Interventional radiology plays an important role in the diagnosis and treatment of many oncologic disorders. Magnetic resonance imaging (MRI) has excellent soft-tissue contrast capabilities and allows visualization of anatomical details of many organs that is not possible with other imaging modalities. MRI also is multiplanar, has the ability to display tissue temperature changes, and uses no ionizing radiation. With an open configuration MRI system, the radiologist may stand alongside the patient and view images, all while performing an interventional procedure. In this article, we explain the rationale for using MRI to guide interventions, focus on technical aspects of biopsy, cryoablation, focused ultrasound, and brachytherapy, and provide a primer for the interventional radiologist wishing to use MRI to guide oncologic interventions

MRI-guided cryotherapy.

Over the last decade the focus of published research on MRI-guided cryotherapy has switched from the study of experimental models to the clinical treatment of patients. The latter reports attest to the safety and feasibility of treating lesions in the liver, kidney, and other sites throughout the body. Further, the published images and initial results speak to the utility of MRI for the task of monitoring this specific procedure. This clinical utility is a realization of the promise of the earlier experimental work that showed the clarity with which interstitial ice is seen under MRI under various pulse sequence parameters. Early adopters have taken advantage of access to the patient that is provided by low and mid-field open scanners; the near future will test the suitability of higher field systems. It has been critical that an FDA-approved cryotherapy system and suitably thin probes were customized for the MRI environment a decade ago by which percutaneous cryotherapy could be performed. There is still work to be done to expand the role of percutaneous cryotherapy, to understand various tissue responses, and to optimize visualization of therapeutic isotherms. Also, long-term outcomes need to be assessed. Overall, in a worldwide environment in which the practice of ablation is growing and an appreciation for such therapies is on the rise, the work of these recent years provides sound footing for the advances that lay ahead for clinical MRI-guided cryotherapy