Modelling the temperature evolution of bone under high intensity focused ultrasound

Update on musculoskeletal applications of magnetic resonance-guided focused ultrasound

Magnetic resonance-guided focused ultrasound (MRgFUS) is a noninvasive, incisionless, radiation-free technology used to ablate tissue deep within the body. This technique has gained increased popularity following FDA approval for treatment of pain related to bone metastases and limited approval for treatment of osteoid osteoma. MRgFUS delivers superior visualization of soft tissue targets in unlimited imaging planes and precision in targeting and delivery of thermal dose which is all provided during real-time monitoring using MR thermometry. This paper provides an overview of the common musculoskeletal applications of MRgFUS along with updates on clinical outcomes and discussion of future applications.

High Intensity Focused Ultrasound for Treatment of Bone Malignancies-20 Years of History

High Intensity Focused Ultrasound (HIFU) is the only non-invasive method for percutaneous thermal ablation of tissue, with treatments typically performed either under magnetic resonance imaging or ultrasound guidance. Since this method allows efficient heating of bony structures, it has found not only early use in treatment of bone pain, but also in local treatment of malignant bone tumors. This review of 20 years of published studies shows that HIFU is a very efficient method for rapid pain relief, can provide local tumor control and has a very patient-friendly safety profile.

Synthetic CT for the planning of MR-HIFU treatment of bone metastases in pelvic and femoral bones: a feasibility study

Objectives

Visualization of the bone distribution is an important prerequisite for MRI-guided high-intensity focused ultrasound (MRI-HIFU) treatment planning of bone metastases. In this context, we evaluated MRI-based synthetic CT (sCT) imaging for the visualization of cortical bone.

Methods

MR and CT images of nine patients with pelvic and femoral metastases were retrospectively analyzed in this study. The metastatic lesions were osteolytic, osteoblastic or mixed. sCT were generated from pre-treatment or treatment MR images using a UNet-like neural network. sCT was qualitatively and quantitatively compared to CT in the bone (pelvis or femur) containing the metastasis and in a region of interest placed on the metastasis itself, through mean absolute difference (MAD), mean difference (MD), Dice similarity coefficient (DSC), and root mean square surface distance (RMSD).

Results

The dataset consisted of 3 osteolytic, 4 osteoblastic and 2 mixed metastases. For most patients, the general morphology of the bone was well represented in the sCT images and osteolytic, osteoblastic and mixed lesions could be discriminated. Despite an average timespan between MR and CT acquisitions of 61 days, in bone, the average (± standard deviation) MAD was 116 ± 26 HU, MD − 14 ± 66 HU, DSC 0.85 ± 0.05, and RMSD 2.05 ± 0.48 mm and, in the lesion, MAD was 132 ± 62 HU, MD − 31 ± 106 HU, DSC 0.75 ± 0.2, and RMSD 2.73 ± 2.28 mm.

Conclusions

Synthetic CT images adequately depicted the cancellous and cortical bone distribution in the different lesion types, which shows its potential for MRI-HIFU treatment planning.

Key Points

• Synthetic computed tomography was able to depict bone distribution in metastatic lesions.

• Synthetic computed tomography images intrinsically aligned with treatment MR images may have the potential to facilitate MR-HIFU treatment planning of bone metastases, by combining visualization of soft tissues and cancellous and cortical bone.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00330-022-08568-y.

[Diffusion-weighted magnetic resonance imaging instead of contrast-enhanced imaging for evaluating immediate therapeutic efficacy of high-intensity focused ultrasound ablation of adenomyosis]

OBJECTIVE

To explore the feasibility of diffusion-weighted magnetic resonance imaging (DWI) instead of contrast-enhanced (CE) imaging for evaluation of the immediate therapeutic efficacy of high-intensity focused ultrasound (HIFU) ablation for treatment of adenomyosis.

METHODS

We retrospectively analyzed the data of 29 patients aged 40.5 ± 5.4 years under going HIFU treatment for adenomyosis in our hospital between December, 2017 and July, 2020. The patients received MRI examination both before and within 24 h after the operation. Two observers analyzed the morphology of the ablation area on DWI and classified the lesions into type 1 (spot-like or no obvious signal intensity changes), type 2 (patchy signal intensity changes) and type 3 (ring-like signal intensity changes). The inter- and intra-observer reliability of morphological assessment was assessed using kappa test. The volume of necrotic tissues following the ablation was measured with both DWI and CE imaging, and the consistency of the measurements and the inter- and intra-observer reliability of DWI-based measurements were evaluated using Bland-Altman plot tests.

RESULTS

The median volume of necrotic tissues was 36.9 cm3 (range 16.4-65.5 cm3) following ablation of the 29 lesions. DWI findings identified 24-25 (82.8%-86.2%) lesions with type 2 or 3 signal changes following the ablation with measurable necrotic tissue volume. On DW images, the Kappa value of intra- and inter-observer reliability for morphological classification of the ablation area reached 0.798 (P=0.000) and 0.933 (P= 0.000), respectively. Bland-Altman plots showed a good consistency of the necrotic volumes measured by CE and at DWI with a strong inter- and intra-observer reliability of DWI-based volume measurements (with only one point outside the range of 95% limits of agreement). The remaining 4 or 5 lesions, which presented with type 1 signal changes following ablation, were all small in size (the largest was only 18.61 cm3) and showed poor therapeutic responses to the ablation.

CONCLUSION

DWI-based morphological assessment and necrotic tissue volume measurement can replace CE imaging for assessment of the immediate therapeutic efficacy of HIFU ablation for treatment of adenomyosis.

Basics in Magnetic Resonance guided Focused Ultrasound: technical basis and clinical application. A brief overview

First applications of high focused ultrasound as intracranial ablative therapy were firstly described in early 50’. Since then, the technological innovations have shown an increasingly safe and effective face of this technique. And in the last few years, Magnetic Resonance (MR) guided Focused Ultrasound (gFUS) has become a valid minimally invasive technique in the treatment of several diseases, from bone tumors to symptomatic uterine fibroids or essential tremors. MR guidance, through the tomographic view, offers the advantage of an accurate target detection and treatment planning. Moreover, real-time monitoring sequences allow to avoid non-target ablation. An adequate knowledge of FUS is essential to understand its clinical effectiveness. Therefore, this brief review aims to debate the physical characteristics of US and the main fields of clinical application.

Heating technology for malignant tumors: a review

The therapeutic application of heat is very effective in cancer treatment. Both hyperthermia, i.e., heating to 39-45 °C to induce sensitization to radiotherapy and chemotherapy, and thermal ablation, where temperatures beyond 50 °C destroy tumor cells directly are frequently applied in the clinic. Achievement of an effective treatment requires high quality heating equipment, precise thermal dosimetry, and adequate quality assurance. Several types of devices, antennas and heating or power delivery systems have been proposed and developed in recent decades. These vary considerably in technique, heating depth, ability to focus, and in the size of the heating focus. Clinically used heating techniques involve electromagnetic and ultrasonic heating, hyperthermic perfusion and conductive heating. Depending on clinical objectives and available technology, thermal therapies can be subdivided into three broad categories: local, locoregional, or whole body heating. Clinically used local heating techniques include interstitial hyperthermia and ablation, high intensity focused ultrasound (HIFU), scanned focused ultrasound (SFUS), electroporation, nanoparticle heating, intraluminal heating and superficial heating. Locoregional heating techniques include phased array systems, capacitive systems and isolated perfusion. Whole body techniques focus on prevention of heat loss supplemented with energy deposition in the body, e.g., by infrared radiation. This review presents an overview of clinical hyperthermia and ablation devices used for local, locoregional, and whole body therapy. Proven and experimental clinical applications of thermal ablation and hyperthermia are listed. Methods for temperature measurement and the role of treatment planning to control treatments are discussed briefly, as well as future perspectives for heating technology for the treatment of tumors.

Vein wall shrinkage induced by thermal coagulation with high-intensity-focused ultrasound: numerical modeling and in vivo experiments in sheep

BACKGROUND

Varicose veins are a common disease that may significantly affect quality of life. Different approaches are currently used in clinical practice to treat this pathology.

MATERIALS AND METHODS

In thermal therapy (radiofrequency or laser therapy), the vein is directly heated to a high temperature to induce vein wall coagulation, and the heat induces denaturation of the intramural collagen, which results macroscopically in vein shrinkage. Thermal vein shrinkage is a physical indicator of the efficiency of endovenous treatment. High-intensity focused ultrasound (HIFU) is a noninvasive technique that can thermally coagulate vein walls and induce vein shrinkage. In this study, we evaluated the vein shrinkage induced in vivo by extracorporeal HIFU ablation of sheep veins: six lateral saphenous veins (3.4mm mean diameter) were sonicated for 8 s with 3MHz continuous waves. Ultrasound imaging was performed before and immediately post-HIFU to quantify the HIFU-induced shrinkage.

RESULTS

Luminal constriction was observed in 100% (6/6) of the treated veins. The immediate findings showed a mean diameter constriction of 53%. The experimental HIFU-induced shrinkage data were used to validate a numerical model developed to predict the thermally induced vein contraction during HIFU treatment.

CONCLUSIONS

This model is based on the use of the k-wave library and published contraction rates of vessels immersed in hot water baths. The simulation results agreed well with those of in vivo experiments, showing a mean percent difference of 5%. The numerical model could thus be a valuable tool for optimizing ultrasound parameters as functions of the vein diameter, and future clinical trials are anticipated.

Modeling Focused-Ultrasound Response for Non-Invasive Treatment Using Machine Learning

The interactions between body tissues and a focused ultrasound beam can be evaluated using various numerical models. Among these, the Rayleigh-Sommerfeld and angular spectrum methods are considered to be the most effective in terms of accuracy. However, they are computationally expensive, which is one of the underlying issues of most computational models. Typically, evaluations using these models require a significant amount of time (hours to days) if realistic scenarios such as tissue inhomogeneity or non-linearity are considered. This study aims to address this issue by developing a rapid estimation model for ultrasound therapy using a machine learning algorithm. Several machine learning models were trained on a very-large dataset (19,227 simulations), and the performance of these models were evaluated with metrics such as Root Mean Squared Error (RMSE), R-squared (R²), Akaike Information Criterion (AIC), and Bayesian Information Criterion (BIC). The resulted random forest provides superior accuracy with an R² value of 0.997, an RMSE of 0.0123, an AIC of -82.56, and a BIC of -81.65 on an external test dataset. The results indicate the efficacy of the random forest-based model for the focused ultrasound response, and practical adoption of this approach will improve the therapeutic planning process by minimizing simulation time.

Mild hyperthermia by MR-guided focused ultrasound in an ex vivo model of osteolytic bone tumour: optimization of the spatio-temporal control of the delivered temperature

BACKGROUND

Magnetic resonance guided focused ultrasound was suggested for the induction of deep localized hyperthermia adjuvant to radiation- or chemotherapy. In this study we are aiming to validate an experimental model for the induction of uniform temperature elevation in osteolytic bone tumours, using the natural acoustic window provided by the cortical breakthrough.

MATERIALS AND METHODS

Experiments were conducted on ex vivo lamb shank by mimicking osteolytic bone tumours. The cortical breakthrough was exploited to induce hyperthermia inside the medullar cavity by delivering acoustic energy from a phased array HIFU transducer. MR thermometry data was acquired intra-operatory using the proton resonance frequency shift (PRFS) method. Active temperature control was achieved via a closed-loop predictive controller set at 6 °C above the baseline. Several beam geometries with respect to the cortical breakthrough were investigated. Numerical simulations were used to further explain the observed phenomena. Thermal safety of bone heating was assessed by cross-correlating MR thermometry data with the measurements from a fluoroptic temperature sensor inserted in the cortical bone.

RESULTS

Numerical simulations and MR thermometry confirmed the feasibility of spatio-temporal uniform hyperthermia (± 0.5 °C) inside the medullar cavity using a fixed focal point sonication. This result was obtained by the combination of several factors: an optimal positioning of the focal spot in the plane of the cortical breakthrough, the direct absorption of the HIFU beam at the focal spot, the "acoustic oven effect" yielded by the beam interaction with the bone, and a predictive temperature controller. The fluoroptical sensor data revealed no heating risks for the bone and adjacent tissues and were in good agreement with the PRFS thermometry from measurable voxels adjacent to the periosteum.

CONCLUSION

To our knowledge, this is the first study demonstrating the feasibility of MR-guided focused ultrasound hyperthermia inside the medullar cavity of bones affected by osteolytic tumours. Our results are considered a promising step for combining adjuvant mild hyperthermia to external beam radiation therapy for sustained pain relief in patients with symptomatic bone metastases.

Modeling the interference between shear and longitudinal waves under high intensity focused ultrasound propagation in bone

Magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) is a noninvasive thermal technique that enables rapid heating of a specific area in the human body. Its clinical relevance has been proven for the treatments of soft tissue tumors, like uterine fibroids, and for the treatments of solid tumors in bone. In MR-HIFU treatment, MR-thermometry is used to monitor the temperature evolution in soft tissue. However, this technique is currently unavailable for bone tissue. Computer models can play a key role in the accurate prediction and monitoring of temperature. Here, we present a computer ray tracing model that calculates the heat production density in the focal region. This model accounts for both the propagation of shear waves and the interference between longitudinal and shear waves. The model was first compared with a finite element approach which solves the Helmholtz equation in soft tissue and the frequency-domain wave equation in bone. To obtain the temperature evolution in the focal region, the heat equation was solved using the heat production density generated by the raytracer as a heat source. Then, we investigated the role of the interaction between shear and longitudinal waves in terms of dissipated power and temperature output. The results of our model were in agreement with the results obtained by solving the Helmholtz equation and the frequency-domain wave equation, both in soft tissue and bone. Our results suggest that it is imperative to include both shear waves and their interference with longitudinal waves in the model when simulating high intensity focused ultrasound propagation in solids. In fact, when modeling HIFU treatments, omitting the interference between shear and longitudinal waves leads to an over-estimation of the temperature increase in the tissues.

Chest wall hernia induced by high intensity focused ultrasound treatment of unresectable massive hepatocellular carcinoma: A case report

In the present study, a case of unresectable massive hepatocellular carcinoma (HCC) treated with high intensity focused ultrasound (HIFU) alone is reported. Although the treatment induced chest wall hernia, its efficacy in treating the HCC was demonstrated. The medical records of a patient with an unresectable massive tumor that was effectively treated with serial HIFU ablation were retrospectively studied. Chest wall hernia was detected as a complication of the HIFU treatment, which has not been reported thus far in the literature. The patient has survived for 44 months since the first diagnosis in September 2010. Treatment resulted in partial remission of the tumor, pain relief, decreased levels of alpha-fetoprotein and chest wall hernia, as a complication. Therefore, HIFU may be an effective approach for the treatment of unresectable HCC, although it may occasionally cause complications.