Magnetic Resonance–Guided Shielding of Prefocal Acoustic Obstacles in Focused Ultrasound Therapy: Application to Intercostal Ablation in Liver

Magnetic Resonance Acoustic Radiation Force Imaging (MR-ARFI)

This review covers the theoretical background, pulse sequence considerations, practical implementations, and multitudes of applications of magnetic resonance acoustic radiation force imaging (MR-ARFI) described to date. MR-ARFI is an approach to encode tissue displacement caused by the acoustic radiation force of a focused ultrasound field into the phase of a MR image. The displacement encoding is done with motion encoding gradients (MEG) which have traditionally been added to spin echo-type and gradient recalled echo-type pulse sequences. Many different types of MEG (monopolar, bipolar, tripolar etc.) have been described and pros and cons are discussed. We further review studies investigating the safety of MR-ARFI, as well as approaches to simulate the MR-ARFI displacement. Lastly, MR-ARFI applications such as for focal spot localization, tissue stiffness interrogation following thermal ablation, trans-skull aberration correction, and simultaneous MR-ARFI and MR thermometry are discussed. EVIDENCE LEVEL: N/A TECHNICAL EFFICACY: Stage 1.

Computer-Aided Intra-Operatory Positioning of an MRgHIFU Applicator Dedicated to Abdominal Thermal Therapy Using Particle Swarm Optimization

PURPOSE

Transducer positioning for liver ablation by magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU) is challenging due to the presence of air-filled organs or bones on the beam path. This paper presents a software tool developed to optimize the positioning of a HIFU transducer dedicated to abdominal thermal therapy, to maximize the treatment's efficiency while minimizing the near-field risk.

METHODS

A software tool was developed to determine the theoretical optimal position (TOP) of the transducer based on the minimization of a cost function using the particle swarm optimization (PSO). After an initialization phase and a manual segmentation of the abdomen of 5 pigs, the program randomly generates particles with 2 degrees of freedom and iteratively minimizes the cost function of the particles considering 3 parameters weighted according to their criticality. New particles are generated around the best position obtained at the previous step and the process is repeated until the optimal position of the transducer is reached. MR imaging data from in vivo HIFU ablation in pig livers was used for ground truth comparison between the TOP and the experimental position (EP).

RESULTS

As compared to the manual EP, the rotation difference with the TOP was on average -3.1 ± 7.1° and the distance difference was on average -7.1 ± 5.4 mm. The computational time to suggest the TOP was 20s. The software tool is modulable and demonstrated consistency and robustness when repeating the calculation and changing the initial position of the transducer.

In Vivo Thermal Ablation of Deep Intrahepatic Targets Using a Super-Convergent MRgHIFU Applicator and a Pseudo-Tumor Model

BACKGROUND

HIFU ablation of liver malignancies is particularly challenging due to respiratory motion, high tissue perfusion and the presence of the rib cage. Based on our previous development of a super-convergent phased-array transducer, we aimed to further investigate, in vivo, its applicability to deep intrahepatic targets.

METHODS

In a series of six pigs, a pseudo-tumor model was used as target, visible both on intra-operatory MRI and post-mortem gross pathology. The transcostal MRgHIFU ablation was prescribed coplanar with the pseudo-tumor, either axial or sagittal, but deliberately shifted 7 to 18 mm to the side. No specific means of protection of the ribs were implemented. Post-treatment MRI follow-up was performed at D7, followed by animal necropsy and gross pathology of the liver.

RESULTS

The pseudo-tumor was clearly identified on T1w MR imaging and subsequently allowed the MRgHIFU planning. The peak temperature at the focal point ranged from 58-87 °C. Gross pathology confirmed the presence of the pseudo-tumor and the well-delineated MRgHIFU ablation at the expected locations.

CONCLUSIONS

The specific design of the transducer enabled a reliable workflow. It demonstrated a good safety profile for in vivo transcostal MRgHIFU ablation of deep-liver targets, graded as challenging for standard surgery.

Assessing Enhanced Acoustic Absorption From Sonosensitive Perfluorocarbon Emulsion With Magnetic Resonance-Guided High-Intensity Focused Ultrasound and a Percolated Tissue-Mimicking Flow Phantom

OBJECTIVE

Sonosensitive high-boiling point perfluorocarbon F₈TAC₁₈-PFOB emulsions previously exhibited thermal enhancement during focused ultrasound heating in ex vivo pig livers, kidneys and a laminar flow phantom. The main objectives of this study were to evaluate heating under turbulent conditions, observe perfusion effects, quantify heating in terms of acoustic absorption and model the experimental data.

METHODS

In this study, similar perfluorocarbon emulsions were circulated at incremental concentrations of 0.07, 0.13, 0.19 and 0.25% v:v through a percolated turbulent flow phantom, more representative of the biological tissue than a laminar flow phantom. The concentrations represent the droplet content in only the perfused fluid, rather than the droplet concentration throughout the entire cross-section. The temperature was measured with magnetic resonance thermometry, during focused ultrasound sonications of 67 W, 95% duty cycle and 33 s duration. These were used in Bioheat equation simulations to investigate in silico the thermal phenomena. The temperature change was compared with the control condition by circulating de-gassed and de-ionized water through the flow phantom without droplets.

RESULTS

With these 1.24 µm diameter droplets at 0.25% v:v, the acoustic absorption coefficient increased from 0.93 ± 0.05 at 0.0% v:v to 1.82 ± 0.22 m^-1 at 0.25% v:v using a 0.1 mL s^-1 flow rate. Without perfusion at 0.25% v:v, an increase was observed from 1.23 ± 0.07 m^-1 at 0.0% v:v to 1.65 ± 0.17 m^-1.

CONCLUSION

The results further support previously reported thermal enhancement with F₈TAC₁₈-PFOB emulsion, quantified the increased absorption at small concentration intervals, illustrated that the effects can be observed in a variety of visceral tissue models and provided a method to simulate untested scenarios.

Cytoreductive Surgical Treatment of Pleural Mesothelioma in a Porcine Model Using Magnetic-Resonance-Guided Focused Ultrasound Surgery (MRgFUS) and Radiofrequency Ablation (RFA)

A combination of surgery and chemotherapy is the most effective treatment available for Malignant Pleural Mesothelioma (MPM). However, both cause significant collateral damage and cannot eliminate residual microscopic disease. This investigation aimed to compare and determine the feasibility of utilizing Radiofrequency Ablation (RFA) and Magnetic-Resonance-guided Focused Ultrasound Surgery (MRgFUS) as alternative treatments for MPM. A large animal tumor model was developed in 13 Yorkshire female pigs using the MSTO211H cell line. Two pigs were initially used to determine the cyclosporine dose required for immunosuppression and tumor development. Subsequently, 11 other pigs underwent tumor development. Of these 11, 2 died during cell inoculation. Small tumor masses and adhesions were present in the other 9, indicating mesothelioma development. Five pigs then received RFA treatment, and 4 pigs received MRgFUS treatment. Tumor model development and effect of the two treatments were examined using MRI and by necropsy. RFA and MRgFUS both successfully ablated approximately the same sized area in the same treatment time. This study demonstrates that RFA and MRgFUS are feasible for tumor debulking, and while MRgFUS requires more pretreatment planning compared to RFA, MRgFUS is a completely noninvasive procedure.

A Novel Concept of a Phased-Array HIFU Transducer Optimized for MR-Guided Hepatic Ablation: Embodiment and First In-Vivo Studies

Purpose

High-intensity focused ultrasound (HIFU) is challenging in the liver due to the respiratory motion and risks of near-/far-field burns, particularly on the ribs. We implemented a novel design of a HIFU phased-array transducer, dedicated to transcostal hepatic thermo-ablation. Due to its large acoustic window and strong focusing, the transducer should perform safely for this application.

Material and Methods

The new HIFU transducer is composed of 256 elements distributed on 5 concentric segments of a specific radius (either 100, 111, or 125 mm). It has been optimally shaped to fit the abdominal wall. The shape and size of the acoustic elements were optimized for the largest emitting surface and the lowest symmetry. Calibration tests have been conducted on tissue-mimicking gels under 3-T magnetic resonance (MR) guidance. In-vivo MR-guided HIFU treatment was conducted in two pigs, aiming to create thermal ablation deep in the liver without significant side effects. Imaging follow-up was performed at D0 and D7. Sacrifice and post-mortem macroscopic examination occurred at D7, with the ablated tissue being fixed for pathology.

Results

The device showed −3-dB focusing capacities in a volume of 27 × 46 × 50 mm³ as compared with the numerical simulation volume of 18 × 48 × 60 mm³. The shape of the focal area was in millimeter-range agreement with the numerical simulations. No interference was detected between the HIFU sonication and the MR acquisition. In vivo, the temperature elevation in perivascular liver parenchyma reached 28°C above physiological temperature, within one breath-hold. The lesion was visible on Gd contrast-enhanced MRI sequences and post-mortem examination. The non-perfused volume was found in pig #1 and pig #2 of 8/11, 6/8, and 7/7 mm along the LR, AP, and HF directions, respectively. No rib burns or other near-field side effects were visually observed on post-mortem gross examination. High-resolution contrast-enhanced 3D MRI indicated a minor lesion on the sternum.

Conclusion

The performance of this new HIFU transducer has been demonstrated in vitro and in vivo. The transducer meets the requirement to perform thermal lesions in deep tissues, without the need for rib-sparing means.

PFOB sonosensitive microdroplets: determining their interaction radii with focused ultrasound using MR thermometry and a Gaussian convolution kernel computation

Purpose: Micron-sized perfluorocarbon droplet adjuvants to focused ultrasound therapies allow lower applied power, circumvent unwanted prefocal heating, and enhance thermal dose in highly perfused tissues. The heat enhancement has been shown to saturate at increasing concentrations. Experiments were performed to empirically model the saturating heating effects during focused ultrasound.Materials and methods: The measurements were made at varying concentrations using magnetic resonance thermometry and focused ultrasound by circulating droplets of mean diameter 1.9 to 2.3 µm through a perfused phantom. A simulation was performed to estimate the interaction radius size, empirically.Results: The interaction radius, representing the radius of a sphere encompassing 90% of the probability for the transformation of acoustic energy into heat deposition around a single droplet, was determined experimentally from ultrasonic absorption coefficient measurements The simulations suggest the interaction radius was approximately 12.5-fold larger than the geometrical radius of droplets, corresponding to an interaction volume on the order of 2000 larger than the geometrical volume.Conclusions: The results provide information regarding the dose-response relationship from the droplets, a measure with 15% precision of their interaction radii with focused ultrasound, and subsequent insights into the underlying physical heating mechanism.

Perfluorocarbon emulsion enhances MR-ARFI displacement and temperature in vitro: Evaluating the response with MRI, NMR, and hydrophone

Sonosensitive perfluorocarbon F₈TAC₁₈-PFOB emulsion is under development to enhance heating, increase thermal contrast, and reduce treatment times during focused ultrasound tumor ablation of highly perfused tissue. The emulsion previously showed enhanced heating during ex vivo and in vitro studies. Experiments were designed to observe the response in additional scenarios by varying focused ultrasound conditions, emulsion concentrations, and surfactants. Most notably, changes in acoustic absorption were assessed with MR-ARFI. Phantoms were developed to have thermal, elastic, and relaxometry properties similar to those of ex vivo pig tissue. The phantoms were embedded with varying amounts of F₈TAC₁₈-PFOB emulsion or lecithin-PFOB emulsion, between about 0.0-0.3% v:w, in 0.05% v:w increments. MR-ARFI measurements were performed using a FLASH-ARFI-MRT sequence to obtain simultaneous displacement and temperature measurements. A Fabry-Perot hydrophone was utilized to observe the acoustic emissions. Susceptibility-weighted imaging and relaxometry mapping were performed to observe concentration-dependent effects. ¹⁹F diffusion-ordered spectroscopy NMR was used to measure the diffusion coefficient of perfluorocarbon droplets in a water emulsion. Increased displacement and temperature were observed with higher emulsion concentration. In semi-rigid MR-ARFI phantoms, a linear response was observed with low-duty cycle MR-ARFI sonications and a mono-exponential saturating response was observed with sustained sonications. The emulsifiers did not have a significant effect on acoustic absorption in semi-rigid gels. Stable cavitation might also contribute to enhanced heating.

Evaluation of Ultrasonic Attenuation in Primary and Secondary Human Liver Tumors and Its Potential Effect on High-Intensity Focused Ultrasound Treatment

Primary and secondary liver tumors are completely different diseases but are usually treated similarly using high-intensity focused ultrasound (HIFU). However, the acoustic parameters of these tissues are not well documented. In this study, attenuation coefficients were evaluated in fresh primary (N = 8) and secondary (N = 13) human liver tumor samples recovered by hepatectomy. The average attenuation coefficients of the primary and secondary liver tumors were 0.10 ± 0.03 and 0.20 ± 0.04 Np/cm/MHz, respectively. The average attenuation coefficients of the liver tissue surrounding the primary and secondary tumors were 0.16 ± 0.07 and 0.07 ± 0.02 Np/cm/MHz, respectively. Numerical simulations performed using these values revealed that completely different HIFU ablation patterns were created in primary and secondary liver tumors using the same exposure parameters. The dimensions of a typical HIFU lesion were two times larger in secondary liver tumors than in primary tumors. HIFU treatment parameters should be set properly according to the acoustic properties of the diseased liver tissue.

Enhancement of HIFU thermal therapy in perfused tissue models using micron-sized FTAC-stabilized PFOB-core endovascular sonosensitizers

BACKGROUND

High intensity focused ultrasound (HIFU) is clinically accepted for the treatment of solid tumors but remains challenging in highly perfused tissue due to the heat sink effect. Endovascular liquid-core sonosensitizers have been previously suggested to enhance the thermal energy deposition at the focal area and to lower the near-/far-field heating. We are investigating the therapeutic potential of PFOB-FTAC micro-droplets in a perfused tissue-mimicking model and postmortem excised organs.

METHOD

A custom-made in vitro perfused tissue-mimicking model, freshly excised pig kidneys (n = 3) and liver (n = 1) were perfused and subjected to focused ultrasound generated by an MR-compatible HIFU transducer. PFOB-FTAC sonosensitizers were injected in the perfusion fluid up to 0.235% v/v ratio. Targeting and on-line PRFS thermometry were performed on a 3 T MR scanner. Assessment of the fluid perfusion was performed with pulsed color Doppler in vitro and with dynamic contrast-enhanced (DCE)-MRI in excised organs.

RESULTS

Our in vitro model of perfused tissue demonstrated re-usability. Sonosensitizer concentration and perfusion rate were tunable in situ. Differential heating under equivalent HIFU sonications demonstrated a dramatic improvement in the thermal deposition due to the sonosensitizers activity. Typically, the energy deposition was multiplied by a factor between 2.5 and 3 in perfused organs after the administration of micro-droplets, while DCE-MRI indicated an effective perfusion.

CONCLUSION

The current PFOB-FTAC micro-droplet sonosensitizers provided a large and sustained enhancement of the HIFU thermal deposition at the focal area, suggesting solutions for less technological constraints, lower risk for the near-/far- field heating. We also report a suitable experimental model for other MRgHIFU studies.

MRI-compatible breast/rib phantom for evaluating ultrasonic thermal exposures

INTRODUCTION

The target of this study was the development of a magnetic resonance imaging (MRI) compatible breast phantom for focused ultrasound which includes plastic (ABS) ribs. The objective of the current study was the evaluation of a focused ultrasound procedure using the proposed phantom that eliminates rib heating.

MATERIAL AND METHODS

The proposed phantom was evaluated using two different focused ultrasound exposures. The surrounding breast tissue was mimicked using an agar-silica-evaporation milk gel (2% w/v - 2% w/v - 40% v/v).

RESULTS

The attenuation of the ABS was similar to that of ribs. MR thermometry of focused ultrasound exposures were acquired using the breast/rib phantom. In one exposure focused ultrasound was applied with far-field targeting of the ribs. In the other exposure, the transducer was positioned laterally, thus avoiding exposure of the rib to focused ultrasound.

CONCLUSIONS

Due to growing interest in using MRI guided focused ultrasound (MRgFUS) for patients with breast cancer, the proposed breast/rib phantom can be utilized as a very useful tool for evaluating ultrasonic protocols.

Flooded Lung Generates a Suitable Acoustic Pathway for Transthoracic Application of High Intensity Focused Ultrasound in Liver

Background: In recent years, high intensity focused ultrasound (HIFU) has gained increasing clinical interest as a non-invasive method for local therapy of liver malignancies. HIFU treatment of tumours and metastases in the liver dome is limited due to the adjacent ultrasound blocking lung. One-lung flooding (OLF) enables complete sonography of lung and adjoining organs including liver. HIFU liver ablation passing through the flooded lung could enable a direct intercostal beam path and thus improve dose deposition in liver. In this study, we evaluate the feasibility of an ultrasound guided transthoracic, transpulmonary HIFU ablation of liver using OLF. Methods: After right-side lung flooding, ultrasound guided HIFU was applied transthoracic- transpulmonary into liver to create thermal lesions in three pigs. The HIFU beam was targeted five times into liver, two times at the liver surface and three times deeper into the tissue. During autopsy examinations of lung, diaphragm and liver located in the HIFU path were performed. The focal liver lesions and lung tissue out of the beam path were examined histologically. Results: Fifteen thermal liver lesions were generated by transpulmonary HIFU sonication in all targeted regions. The lesions appeared well-demarcated in grey color with a cigar-shaped configuration. The mean length and width of the superficial and deeper lesions were 15.8 mm (range: 13-18 mm) and 5.8 mm (range: 5-7 mm), and 10.9 mm (range: 9-13 mm) and 3.3 mm (range: 2-5 mm), respectively. Histopathological, all liver lesions revealed a homogeneous thermal necrosis lacking vitality. There were no signs of damage of the overlying diaphragm and lung tissue. Conclusions: Flooded lung is a suitable pathway for applying HIFU to the liver, thus enabling a transthoracic, transpulmonary approach. The enlarged acoustic window could enhance the ablation speed for targets in the hepatic dome.

Spatiotemporal control of gene expression in bone-marrow derived cells of the tumor microenvironment induced by MRI guided focused ultrasound

The tumor microenvironment is an interesting target for anticancer therapies but modifying this compartment is challenging. Here, we demonstrate the feasibility of a gene therapy strategy that combined targeting to bone marrow-derived tumor microenvironment using genetically modified bone-marrow derived cells and control of transgene expression by local hyperthermia through a thermo-inducible promoter. Chimera were obtained by engraftment of bone marrow from transgenic mice expressing reporter genes under transcriptional control of heat shock promoter and inoculated sub-cutaneously with tumors cells. Heat shocks were applied at the tumor site using a water bath or magnetic resonance guided high intensity focused ultrasound device. Reporter gene expression was followed by bioluminescence and fluorescence imaging and immunohistochemistry. Bone marrow-derived cells expressing reporter genes were identified to be mainly tumor-associated macrophages. We thus provide the proof of concept for a gene therapy strategy that allows for spatiotemporal control of transgenes expression by macrophages targeted to the tumor microenvironment.

A clinically feasible treatment protocol for magnetic resonance-guided high-intensity focused ultrasound ablation in the liver

OBJECTIVES

Magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) allows for noninvasive thermal ablation under real-time temperature imaging guidance. The purpose of this study was to assess the feasibility and safety of MR-HIFU ablation of liver tissue in a clinically acceptable setting. The experimental protocol was designed with a clinical ablation procedure of a small malignant tumor in mind; the procedures were performed within a clinically feasible time frame and care was taken to avoid adverse events. The main outcome was the size and quality of the ablated liver tissue volume on imaging and histology. Secondary outcomes were safety and treatment time.

MATERIALS AND METHODS

Healthy pigs (n = 10) under general anesthesia were positioned on a clinical MR-HIFU system, which consisted of an HIFU tabletop with a skin cooling system integrated into a 1.5-T MR scanner. A liver tissue volume was ablated with multiple sonication cells (4 × 4 × 10 mm, 450 W). Both MR thermometry and sonication were respiratory-gated using a pencil beam navigator on the diaphragm. Contrast-enhanced T1-weighted (CE-T1w) imaging was performed for treatment evaluation. Targeted total treatment time was 3 hours. The abdominal wall, liver, and adjacent organs were inspected postmortem for thermal damage. Ablated tissue volumes were processed for cell viability staining. The ablated volumes were analyzed using MR imaging, MR thermometry, and cell viability histology.

RESULTS

Eleven volume ablations were performed in 10 animals, resulting in a median nonperfused volume (NPV) on CE-T1w imaging of 1.6 mL (interquartile range [IQR], 0.8-2.3; range, 0.7-3.0). Cell viability histology showed a damaged volume of 1.5 mL (IQR, 1.1-1.8; range, 0.7-2.3). The NPV was confluent in 10 of the 11 cases. The ablated tissue volume on cell viability histology was confluent in all 9 available cases. In all cases, there was a good correspondence between the aspects of the NPV on CE-T1w and the ablated volume on cell viability histology. Two treatment-related adverse events occurred: 1 animal had a 7-mm skin burn and 1 animal showed evidence of thermal damage on the surface of the spleen. Median ablation time was 108 minutes (IQR, 101-120; range, 96-181 minutes) and median total treatment time was 180 minutes (IQR, 165-224; 130-250 minutes).

CONCLUSIONS

Our results demonstrate the feasibility and safety of MR-HIFU ablation of liver tissue volumes. The imaging data and cell viability histology show, for the first time, that confluent ablation volumes can be achieved with motion-gated ablation and MR guidance. These results were obtained using a readily available MR-HIFU system with only minor modifications, within a clinically acceptable time frame, and with only minor adverse events. This shows that this technique is sufficiently reliable and safe to initiate a clinical trial.

Spatio-temporal quantitative thermography of pre-focal interactions between high intensity focused ultrasound and the rib cage

OBJECTIVE

The aim of this paper is to quantitatively investigate the thermal effects generated by the pre-focal interactions of a HIFU beam with a rib cage, in the context of minimally invasive transcostal therapy of liver malignancies.

MATERIALS AND METHODS

HIFU sonications were produced by a phased-array MR-compatible transducer on Turkey muscle placed on a sheep thoracic cage specimen. The thoracic wall was positioned in the pre-focal zone 3.5 to 6.5 cm below the focus. Thermal monitoring was simultaneously performed using fluoroptic sensors inserted into the medullar cavity of the ribs and high resolution MR-thermometry (voxel: 1 × 1 × 5 mm3, four multi-planar slices).

RESULTS

MR-thermometry data indicated nearly isotropic distribution of the thermal energy at the ribs' surface. The temperature elevation at the focus was comparable with the pericostal temperature elevation around unprotected ribs, while being systematically inferior, by more than a factor of four on average, to the intra-medullar values. The spatial profiles of the pericostal and intra-medullar thermal build-up measurements could be smoothly connected using a Gaussian function. The dynamics of the post-sonication thermal relaxation as determined by fluoroptic measurements was demonstrated to be theoretically coherent with the experimental observations.

CONCLUSION

The experimental findings motivate further efforts for the transfer towards clinical routine of effective rib-sparing strategies for hepatic HIFU.

Sonographic analysis of the intercostal spaces for the application of high-intensity focused ultrasound therapy to the liver

OBJECTIVE

The purposes of this study were to assess the widths of the intercostal spaces of the right inferior human rib cage through which high-intensity focused ultrasound therapy would be applied for treating liver cancer and to elucidate the demographic factors associated with intercostal space width.

SUBJECTS AND METHODS

From March 2013 to June 2013, the widths of the intercostal spaces and the ribs at six areas of the right inferior rib cage (area 1, lowest intercostal space on anterior axillary line and the adjacent upper rib; area 2, second-lowest intercostal space on anterior axillary line and the adjacent upper rib; areas 3 and 4, lowest and second-lowest spaces on midaxillary line; areas 5 and 6, lowest and second-lowest spaces on posterior axillary line) were sonographically measured in 466 patients (214 men, 252 women; mean age, 53.0 years) after an abdominal sonographic examination. Demographic factors and the presence or absence of chronic liver disease were evaluated by multivariate analysis to investigate which factors influence intercostal width.

RESULTS

The width of the intercostal space was 19.7 ± 3.7 mm (range, 9-33 mm) at area 1, 18.3 ± 3.4 mm (range, 9-33 mm) at area 2, 17.4 ± 4.0 mm (range, 7-33 mm) at area 3, 15.4 ± 3.5 mm (range, 5-26 mm) at area 4, 17.2 ± 3.7 mm (range, 7-28 mm) at area 5, and 14.5 ± 3.6 mm (range, 4-26 mm) at area 6. The corresponding widths of the ribs were 15.2 ± 2.3 mm (range, 8-22 mm), 14.5 ± 2.3 mm (range, 9-22 mm), 13.2 ± 2.0 mm (range, 9-20), 14.3 ± 2.2 mm (range, 9-20 mm), 15.0 ± 2.2 mm (range, 10-22 mm), and 15.1 ± 2.3 mm (range, 8-21 mm). Only female sex was significantly associated with the narrower intercostal width at areas 1, 2, 3, and 5 (regression coefficient, 1.124-1.885; p = 0.01-0.04).

CONCLUSION

There was substantial variation in the widths of the intercostal spaces of the right inferior rib cage such that the anterior and inferior aspects of the intercostal space were relatively wider. Women had significantly narrower intercostal spaces than men.

Model-guided respiratory organ motion prediction of the liver from 2D ultrasound

With the availability of new and more accurate tumour treatment modalities such as high-intensity focused ultrasound or proton therapy, accurate target location prediction has become a key issue. Various approaches for diverse application scenarios have been proposed over the last decade. Whereas external surrogate markers such as a breathing belt work to some extent, knowledge about the internal motion of the organs inherently provides more accurate results. In this paper, we combine a population-based statistical motion model and information from 2d ultrasound sequences in order to predict the respiratory motion of the right liver lobe. For this, the motion model is fitted to a 3d exhalation breath-hold scan of the liver acquired before prediction. Anatomical landmarks tracked in the ultrasound images together with the model are then used to reconstruct the complete organ position over time. The prediction is both spatial and temporal, can be computed in real-time and is evaluated on ground truth over long time scales (5.5 min). The method is quantitatively validated on eight volunteers where the ultrasound images are synchronously acquired with 4D-MRI, which provides ground-truth motion. With an average spatial prediction accuracy of 2.4 mm, we can predict tumour locations within clinically acceptable margins.

Respiratory-gated MRgHIFU in upper abdomen using an MR-compatible in-bore digital camera

Objective. To demonstrate the technical feasibility and the potential interest of using a digital optical camera inside the MR magnet bore for monitoring the breathing cycle and subsequently gating the PRFS MR thermometry, MR-ARFI measurement, and MRgHIFU sonication in the upper abdomen. Materials and Methods. A digital camera was reengineered to remove its magnetic parts and was further equipped with a 7 m long USB cable. The system was electromagnetically shielded and operated inside the bore of a closed 3T clinical scanner. Suitable triggers were generated based on real-time motion analysis of the images produced by the camera (resolution 640 × 480 pixels, 30 fps). Respiratory-gated MR-ARFI prepared MRgHIFU ablation was performed in the kidney and liver of two sheep in vivo, under general anaesthesia and ventilator-driven forced breathing. Results. The optical device demonstrated very good MR compatibility. The current setup permitted the acquisition of motion artefact-free and high resolution MR 2D ARFI and multiplanar interleaved PRFS thermometry (average SNR 30 in liver and 56 in kidney). Microscopic histology indicated precise focal lesions with sharply delineated margins following the respiratory-gated HIFU sonications. Conclusion. The proof-of-concept for respiratory motion management in MRgHIFU using an in-bore digital camera has been validated in vivo.

An experimental model to investigate the targeting accuracy of MR-guided focused ultrasound ablation in liver

Background

Magnetic Resonance-guided High Intensity Focused Ultrasound (MRgHIFU) is a hybrid technology that aims to offer non-invasive thermal ablation of targeted tumors or other pathological tissues. Acoustic aberrations and non-linear wave propagating effects may shift the focal point significantly away from the prescribed (or, theoretical) position. It is therefore mandatory to evaluate the spatial accuracy of ablation for a given HIFU protocol and/or device. We describe here a method for producing a user-defined ballistic target as an absolute reference marker for MRgHIFU ablations.

Methods

The investigated method is based on trapping a mixture of MR contrast agent and histology stain using radiofrequency (RF) ablation causing cell death and coagulation. A dedicated RF-electrode was used for the marker fixation as follows: a RF coagulation (4 W, 15 seconds) and injection of the mixture followed by a second RF coagulation. As a result, the contrast agent/stain is encapsulated in the intercellular space. Ultrasonography imaging was performed during the procedure, while high resolution T1w 3D VIBE MR acquisition was used right after to identify the position of the ballistic marker and hence the target tissue. For some cases, after the marker fixation procedure, HIFU volumetric ablations were produced by a phased-array HIFU platform. First ex vivo experiments were followed by in vivo investigation on four rabbits in thigh muscle and six pigs in liver, with follow-up at Day 7.

Results

At the end of the procedure, no ultrasound indication of the marker’s presence could be observed, while it was clearly visible under MR and could be conveniently used to prescribe the HIFU ablation, centered on the so-created target. The marker was identified at Day 7 after treatment, immediately after animal sacrifice, after 3 weeks of post-mortem formalin fixation and during histology analysis. Its size ranged between 2.5 and 4 mm.

Conclusions

Experimental validation of this new ballistic marker method was performed for liver MRgHIFU ablation, free of any side effects (e.g. no edema around the marker, no infection, no bleeding). The study suggests that the absolute reference marker had ultrasound conspicuity below the detection threshold, was irreversible, MR-compatible and MR-detectable, while also being a well-established histology staining technique.

Magnetic resonance-guided laser-induced interstitial thermotherapy of breast cancer liver metastases and other noncolorectal cancer liver metastases: an analysis of prognostic factors for long-term survival and progression-free survival

PURPOSE

The purpose of this study was the evaluation of prognostic factors for long-term survival and progression-free survival (PFS) after treatment of noncolorectal cancer liver metastases through MR-guided laser-induced thermotherapy (LITT).

PATIENTS AND METHODS

We included 401 patients (mean age, 57.3 years) with liver metastases from different primary tumors who were treated with LITT. Long-term survival and progression-free-survival rates were evaluated using the Kaplan-Meier method. A Cox regression model tested different parameters that could be of prognostic value. The tested prognostic factors were as follows: the location of primary tumor, TNM classification, extrahepatic metastases, hepatic resection or neoadjuvant transarterial chemoembolization or systemic chemotherapy before LITT, the number of initial metastases, the volume of metastases, and the quotient of total volumes of metastases and necroses per patient.

RESULTS

The median survival was 37.6 months starting from the date of LITT. The 1-, 2-, 3-, 4-, and 5-year survival rates were 86.5%, 67.2%, 51.9%, 39.9%, and 33.4%, respectively. The median PFS was 12.2 months. The 1-, 2-, 3-, 4-, and 5-year PFS rates were 50.6%, 33.8%, 26%, 20.4%, and 17%, respectively. The initial number of metastases, the volumes of metastases, and the quotient of the volumes of metastases and necroses influenced the long-term survival and the PFS.

CONCLUSIONS

Laser-induced thermotherapy is a minimally invasive method in the treatment of hepatic metastases of noncolorectal cancer, and it shows good results in long-term survival and PFS. The initial number of metastases and their volume are the most important prognostic factors. The status of the lymph nodes, the existence of other extrahepatic metastases, the location of the primary tumor, and different neoadjuvant therapies are of nonprognostic value.

Experimental methods for improved spatial control of thermal lesions in magnetic resonance-guided focused ultrasound ablation

Magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU, or MRgFUS) is a hybrid technology that was developed to provide efficient and tolerable thermal ablation of targeted tumors or other pathologic tissues, while preserving the normal surrounding structures. Fast 3-D ablation strategies are feasible with the newly available phased-array HIFU transducers. However, unlike fixed heating sources for interstitial ablation (radiofrequency electrode, microwave applicator, infra-red laser applicator), HIFU uses propagating waves. Therefore, the main challenge is to avoid thermo-acoustical adverse effects, such as energy deposition at reflecting interfaces and thermal drift of the focal lesion toward the near field. We report here our investigations on some novel experimental solutions to solve, or at least to alleviate, these generally known tolerability problems in HIFU-based therapy. Online multiplanar MR thermometry was the main investigational tool extensively used in this study to identify the problems and to assess the efficacy of the tested solutions. We present an improved method to cancel the beam reflection at the exit window (i.e., tissue-to-air interface) by creating a multilayer protection, to dissipate the residual HIFU beam by bulk scattering. This study evaluates selective de-activation of transducer elements to reduce the collateral heating at bone surfaces in the far field, mainly during automatically controlled volumetric ablation. We also explore, using hybrid US/MR simultaneous imaging, the feasibility of using disruptive boiling at the focus, both as a far-field self-shielding technique and as an enhanced ablation strategy (i.e., boiling core controlled HIFU ablation).

The road to clinical use of high-intensity focused ultrasound for liver cancer: technical and clinical consensus

Clinical use of high-intensity focused ultrasound (HIFU) under ultrasound or MR guidance as a non-invasive method for treating tumors is rapidly increasing. Tens of thousands of patients have been treated for uterine fibroid, benign prostate hyperplasia, bone metastases, or prostate cancer. Despite the methods' clinical potential, the liver is a particularly challenging organ for HIFU treatment due to the combined effect of respiratory-induced liver motion, partial blocking by the rib cage, and high perfusion/flow. Several technical and clinical solutions have been developed by various groups during the past 15 years to compensate for these problems. A review of current unmet clinical needs is given here, as well as a consensus from a panel of experts about technical and clinical requirements for upcoming pilot and pivotal studies in order to accelerate the development and adoption of focused ultrasound for the treatment of primary and secondary liver cancer.