Martijn Hoogenboom

Mechanical high-intensity focused ultrasound destruction of soft tissue: working mechanisms and physiologic effects

The best known method of high-intensity focused ultrasound is thermal ablation, but interest in non-thermal, mechanical destruction is increasing. The advantages of mechanical ablation are that thermal protein denaturation remains limited and less damage is created to the surrounding tissue by thermal diffusion. The two main techniques for mechanical fragmentation of tissue are histotripsy and boiling histotripsy. These techniques can be used for complete liquefaction of tumor tissue into submicron fragments, after which the fragmented tissue can be easily removed by natural (immunologic) responses. Interestingly it seems that there is a correlation between the degree of destruction and tissue specific characteristics based on the treatment settings used. In this review article, the technical aspects of these two techniques are described, and an overview of the in vivo pathologic and immunologic responses is provided.

Why and Where do We Miss Significant Prostate Cancer with Multi-parametric Magnetic Resonance Imaging followed by Magnetic Resonance-guided and Transrectal Ultrasound-guided Biopsy in Biopsy-naïve Men?

BACKGROUND Knowledge of significant prostate (sPCa) locations being missed with magnetic resonance (MR)- and transrectal ultrasound (TRUS)-guided biopsy (Bx) may help to improve these techniques. OBJECTIVE To identify the location of sPCa lesions being missed with MR- and TRUS-Bx. DESIGN, SETTING, AND PARTICIPANTS In a referral center, 223 consecutive Bx-naive men with elevated prostate specific antigen level and/or abnormal digital rectal examination were included. Histopathologically-proven cancer locations, Gleason score, and tumor length were determined. INTERVENTION All patients underwent multi-parametric MRI and 12-core systematic TRUS-Bx. MR-Bx was performed in all patients with suspicion of PCa on multi-parametric MRI (n=142). OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS Cancer locations were compared between MR- and TRUS-Bx. Proportions were expressed as percentages, and the corresponding 95% confidence intervals were calculated. RESULTS AND LIMITATIONS In total, 191 lesions were found in 108 patients with sPCa. From these lesion 74% (141/191) were defined as sPCa on either MR- or TRUS-Bx. MR-Bx detected 74% (105/141) of these lesions and 61% (86/141) with TRUS-Bx. TRUS-Bx detected more lesions compared with MR-Bx (140 vs 109). However, these lesions were often low risk (39%). Significant lesions missed with MR-Bx most often had involvement of dorsolateral (58%) and apical (37%) segments and missed segments with TRUS-Bx were located anteriorly (79%), anterior midprostate (50%), and anterior apex (23%). CONCLUSIONS Both techniques have difficulties in detecting apical lesions. MR-Bx most often missed cancer with involvement of the dorsolateral part (58%) and TRUS-Bx with involvement of the anterior part (79%). PATIENT SUMMARY Both biopsy techniques miss cancer in specific locations within the prostate. Identification of these lesions may help to improve these techniques.

Thermal and mechanical high-intensity focused ultrasound: perspectives on tumor ablation, immune effects and combination strategies

Tumor ablation technologies, such as radiofrequency-, cryo- or high-intensity focused ultrasound (HIFU) ablation will destroy tumor tissue in a minimally invasive manner. Ablation generates large volumes of tumor debris in situ, releasing multiple bio-molecules like tumor antigens and damage-associated molecular patterns. To initiate an adaptive antitumor immune response, antigen-presenting cells need to take up tumor antigens and, following activation, present them to immune effector cells. The impact of the type of tumor ablation on the precise nature, availability and suitability of the tumor debris for immune response induction, however, is poorly understood. In this review, we focus on immune effects after HIFU-mediated ablation and compare these to findings using other ablation technologies. HIFU can be used both for thermal and mechanical destruction of tissue, inducing coagulative necrosis or subcellular fragmentation, respectively. Preclinical and clinical results of HIFU tumor ablation show increased infiltration and activation of CD4+ and CD8+ T cells. As previously observed for other types of tumor ablation technologies, however, this ablation-induced enhanced infiltration alone appears insufficient to generate consistent protective antitumor immunity. Therapies combining ablation with immune stimulation are therefore expected to be key to boost HIFU-induced immune effects and to achieve systemic, long-lasting, antitumor immunity.

In vivo MR guided boiling histotripsy in a mouse tumor model evaluated by MRI and histopathology

Boiling histotripsy (BH) is a new high intensity focused ultrasound (HIFU) ablation technique to mechanically fragmentize soft tissue into submicrometer fragments. So far, ultrasound has been used for BH treatment guidance and evaluation. The in vivo histopathological effects of this treatment are largely unknown. Here, we report on an MR guided BH method to treat subcutaneous tumors in a mouse model. The treatment effects of BH were evaluated one hour and four days later with MRI and histopathology, and compared with the effects of thermal HIFU (T‐HIFU). The lesions caused by BH were easily detected with T2w imaging as a hyper‐intense signal area with a hypo‐intense rim. Histopathological evaluation showed that the targeted tissue was completely disintegrated and that a narrow transition zone (<200 µm) containing many apoptotic cells was present between disintegrated and vital tumor tissue. A high level of agreement was found between T2w imaging and H&E stained sections, making T2w imaging a suitable method for treatment evaluation during or directly after BH. After T‐HIFU, contrast enhanced imaging was required for adequate detection of the ablation zone. On histopathology, an ablation zone with concentric layers was seen after T‐HIFU. In line with histopathology, contrast enhanced MRI revealed that after BH or T‐HIFU perfusion within the lesion was absent, while after BH in the transition zone some micro‐hemorrhaging appeared. Four days after BH, the transition zone with apoptotic cells was histologically no longer detectable, corresponding to the absence of a hypo‐intense rim around the lesion in T2w images. This study demonstrates the first results of in vivo BH on mouse tumor using MRI for treatment guidance and evaluation and opens the way for more detailed investigation of the in vivo effects of BH. Copyright

Ex vivo MRI evaluation of prostate cancer: Localization and margin status prediction of prostate cancer in fresh radical prostatectomy specimens

To investigate the ability of high field ex vivo magnetic resonance imaging (MRI) to localize prostate cancer (PCa) and to predict the margin status in fresh radical prostatectomy (RP) specimens using histology as the reference standard.

Photoacoustic and high frequency ultrasound imaging of mechanical and thermal HIFU ablation

The possibility of focusing high ultrasound intensities HIFU opened the door for promoting ultrasound as a non-invasive therapeutic method. HIFU is capable of producing both thermal and mechanical effects on tissue. The thermal effect relies on temperature increase and has been introduced to the clinic a decade ago. While mechanical effect such as boiling histotripsy induces extremely high pressures to pulverize targeted tissue and is still confined to laboratories due to the lack of adequate imaging techniques to better understand its in-vivo pathological and immunological effects. In this work, we investigate if high-frequency ultrasound (US) and photoacoustics (PA) can be a potential combination to evaluate the effect of boiling histotripsy invivo. Furthermore, we compare its outcome to thermal HIFU ablation.

Impact of MR-guided boiling histotripsy in distinct murine tumor models

Interest in mechanical high intensity focused ultrasound (HIFU) ablation is rapidly growing. Boiling histotripsy (BH) is applied for mechanical fragmentation of soft tissue into submicron fragments with limited temperature increase using the shock wave and cavitation effects of HIFU. Research on BH has been largely limited to ex vivo experiments. As a consequence, the in vivo pathology after BH treatment and the relation to preexistent tissue characteristics are not well understood. This study reports on in vivo MR guided BH treatment, either with 100 or 200 pulses per focal spot, in three different subcutaneous mouse tumor models: a soft-tissue melanoma (B16OVA), a compact growing thymoma (EL4), and a highly vascularized neuroblastoma (9464D). Extensive treatment evaluation was performed using MR imaging followed by histopathology 2h after treatment. T2 weighted MRI allowed direct in vivo visualization of the BH lesions in all tumor models. The 100-pulse treated area in the B16OVA tumors was larger than the predicted treatment volume (500±10%). For the more compact growing EL4 and 9464D tumors this was 95±13% and 55±33%, respectively. Histopathology after the 100-pulse treatment revealed completely disintegrated lesions in the treated area with sharp borders in the compact EL4 and 9464D tumors, while for B16OVA tumors the lesion contained a mixture of discohesive (partly viable) clusters of cells, micro-vessel remainings, and tumor cell debris. The treatment of B16OVA with 200 pulses increased the fragmentation of tumor tissue. In all tumor types only micro-hemorrhages were detected after ablation (slightly higher after 200-pulse treatment for the highly vascularized 9464D tumors). Collagen staining revealed that the collagen fibers were to a greater or lesser extent still intact and partly clotted together near the lesion border in all tumor models. In conclusion, this study reveals effective mechanical fragmentation of different tumor types using BH without major hemorrhages. However, treatment settings may need to be adjusted to the tissue characteristics for optimal tissue fragmentation.

Reconstruction of nonlinear ultrasound field of an annular therapeutic array from acoustic holograms of its individual elements

Acoustic holography method has been shown to provide accurate reconstruction of 3D ultrasound fields generated by various medical transducers including multi-element arrays as well as to set a boundary condition for nonlinear field modeling at high pressure levels. Here an approach of measuring holograms of individual array elements for modeling of an entire array field is proposed and tested for a 3 MHz 16-element annular array (48 mm diameter and 35 mm radius of curvature). The array is a part of a high intensity focused ultrasound system with magnetic resonance guidance used for developing thermal and mechanical methods of tissue ablation in mouse tumors. The holograms measured separately for each array element were combined together to obtain a boundary condition for the array with all operating elements. Modeling results were compared to low-amplitude beam scans and good agreement was demonstrated. Then, nonlinear field simulations were performed at increasing power outputs based on the 3D Westervelt equation. It was shown that the transducer is capable to produce focal waveforms with 140 MPa shock amplitude at 110 W acoustic power and thus is well suited for evaluating shock-based ablation therapies in small animals.

A software tool for advanced MRgFUS prostate therapy planning and follow up

US guided HIFU/FUS ablation for the therapy of prostate cancer is a clinical established method, while MR guided HIFU/FUS applications for prostate recently started clinical evaluation. Even if MRI examination is an excellent diagnostic tool for prostate cancer, it is a time consuming procedure and not practicable within an MRgFUS therapy session. The aim of our ongoing work is to develop software to support therapy planning and post-therapy follow-up for MRgFUS on localized prostate cancer, based on multi-parametric MR protocols.The clinical workflow of diagnosis, therapy and follow-up of MR guided FUS on prostate cancer was deeply analyzed. Based on this, the image processing workflow was designed and all necessary components, e.g. GUI, viewer, registration tools etc. were defined and implemented. The software bases on MeVisLab with several implemented C++ modules for the image processing tasks.The developed software, called LTC (Local Therapy Control) will register and visualize automatically all image...

Pathology and immune effects of magnetic resonance imaging-guided Boiling Histotripsy in murine tumor models

In situ tumor ablation techniques are successfully applied for the destruction of local, often inoperable tumor masses. Following ablation tumor antigens become instantly available for immune cells, but systemic abscopal effects have only occasionally been reported after ablation monotherapy. Which ablation technique combines optimal local destruction with effective antigen release for induction of anti-tumor immunity is largely unknown. We study non-invasive MRI-guided high intensity focused ultrasound-ablation (MRgHiFU) in murine tumor models for local destruction by heating or mechanical disruption using Boiling histotripsy (BH). BH mechanically fragmentizes soft tissue into submicron fragments that are absorbed as part of a physiological healing response. BH treatment was performed using a MR compatible animal HIFU system (Image Guided Therapy, Pessac, France) with a 3 MHz transducer (focal spot size 0.5 x 0.5 x 2.0 mm). A 7T animal MR scanner was used for treatment guidance and evaluation. Here, we w...