# High intensity focused ultrasound periosteal ablation in an animal model: potential for palliation of bone metastases

Ron Aginsky, Suzanne Leblang, Eric Miller, Brian Skoglind, Abraham Nyska, Niv Rebhun, Michael Gofeld, Jean-François Aubry, Arik Hananel

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## ► To cite this version:

Ron Aginsky, Suzanne Leblang, Eric Miller, Brian Skoglind, Abraham Nyska, et al.. High intensity focused ultrasound periosteal ablation in an animal model: potential for palliation of bone metastases. International Journal of Hyperthermia, 2025, 42 (1), pp.2543330. 10.1080/02656736.2025.2543330. hal-05335775

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# High intensity focused ultrasound periosteal ablation in an animal model: potential for palliation of bone metastases

Ron Aginsky, Suzanne Leblang, Eric Miller, Brian Skoglind, Abraham Nyska, Niv Rebhun, Michael Gofeld, Jean-Francois Aubry & Arik Hananel

**To cite this article:** Ron Aginsky, Suzanne Leblang, Eric Miller, Brian Skoglind, Abraham Nyska, Niv Rebhun, Michael Gofeld, Jean-Francois Aubry & Arik Hananel (2025) High intensity focused ultrasound periosteal ablation in an animal model: potential for palliation of bone metastases, *International Journal of Hyperthermia*, 42:1, 2543330, DOI: 10.1080/02656736.2025.2543330

**To link to this article:** https://doi.org/10.1080/02656736.2025.2543330

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© 2025 The Author(s). Published with license by Taylor & Francis Group, LLC

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# High intensity focused ultrasound periosteal ablation in an animal model: potential for palliation of bone metastases

Ron Aginskyª, Suzanne Leblangᵇ, Eric Millerª, Brian Skoglindª, Abraham Nyskaᶜ, Niv Rebhunª, Michael Gofeldd*, Jean-Francois Aubrye and Arik Hananelª

ªFUSMobile, Alpharetta, GA, USA; ᵇFocused Ultrasound Foundation, Charlottesville, VA, USA; ᶜConsultant in Toxicologic Pathology, Tel Aviv University, Tel Aviv, Israel; ᵈUnika Medical Centre, North York, Canada; ᵉPhysics for Medicine Paris, Inserm, ESPCI Paris, CNRS, PSI, Research University, Paris, France

**ABSTRACT**

**Purpose:** Radiotherapy is standard-of-care for painful bone metastases yet has limitations and associated side effects. Ablation of sensory nerves endings along the periosteum with magnetic resonance imaging guided high intensity focused ultrasound (MRgFUS) proved safe and clinically effective for pain relief in patients with bone metastases, received FDA and CE approval, but has not gained widespread adoption due to significant cost and procedural and logistical complexity. This preclinical study evaluated the safety and feasibility of a fluoroscopy-guided high intensity focused ultrasound platform to ablate a targeted region along the surface of bones.

**Methods:** Two healthy adult pigs received 6kJ to 10kJ sonifications to the femur, ileum, and ribs. Animals were followed-up for 3months. Longitudinal clinical observation and follow up magnetic resonance imaging (MRI), and computed tomography (CT) scans were performed. After sacrifice, the targeted bone and adjacent tissues were sent for histopathological evaluation to confirm thermal ablation.

**Results:** Clinical observations revealed no neurological or musculoskeletal deficits. MRI scans on day 5 demonstrated robust ablation in all targeted sites. At 12weeks, CT scans and histopathological evaluation showed complete healing of ablated regions.

**Conclusions:** Periosteal bone ablation using the Neurolyser XR in a healthy porcine model is feasible with no adverse events, and no radiological or histological evidence of lasting injury or fracture to the targeted bones.

**ARTICLE HISTORY**

Received 3 October 2024
Revised 30 June 2025
Accepted 31 July 2025

**KEYWORDS**

Thermal ablation; high intensity focused ultrasound; porcine model; bone ablation; periosteum

## Introduction

Secondary bone tumors represent clinical challenges and often cause significant pain in patients with advanced cancer. The primary palliative approach is external beam radiation therapy (EBRT), which results in effective pain management in approximately 50% to 85% of cases [1]. Nevertheless, administration of EBRT can be time-consuming and often warrants several visits to the hospital. Furthermore, pain relief is often delayed [2], (median time of 4 weeks with a range of 1-15 weeks), and approximately 50% of initial responders develop recurrent pain within 1 year [3]. Repeat EBRT is effective in only 46%–66% of cases and its use is often limited due to constraints on maximal dosage and adverse effects such as fatigue, dermatitis, enteritis, nausea, and even bone marrow suppression. Consequently, a significant proportion of patients with painful bone metastases do not benefit from EBRT resulting in the need for additional palliative modalities to alleviate pain and improve quality of life.

High intensity focused ultrasound (HIFU) has emerged as an image-guided, noninvasive, non-ionizing therapeutic modality for palliation of bone metastases-related pain [4-6]. Compared to radiation therapy, HIFU results in a more rapid pain reduction, allows repeated treatments as there is no cumulative upper dose limit, and can be performed with the concurrent delivery of chemotherapy and radiation therapy.

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Unlike thermal ablation of soft tissue tumors, for denervation, HIFU utilizes the high acoustic absorption and low thermal conductivity of bony tissue, resulting in a predictable and rapid thermal rise at the periosteum and subcortical bone where the sensory nerves and nerve endings reside [7,8].

To date, MRgFUS has been studied extensively as a treatment modality for various musculoskeletal indications, including painful bone metastases. A significant advantage of using MRI guidance for a FUS procedure is the ability to monitor treatment effect in real time using MRI thermometry for closed loop feedback. Despite these advantages and demonstrated safety and efficacy of MRgFUS, clinical adoption has been slow despite FDA and CE approval due to the high cost and complex logistics associated with MRI as the guidance system.

Common features of the MRI-based HIFU systems for the treatment of painful bone metastasis include an integrated robotic positioner, integrated cooling system, water bath, the need for MR compatibility of the therapeutic HIFU system, a predictor, planner and an extended treatment duration derived from the need to acquire MRI images. These systems operate by first acquiring MRI images of the targeted area, then marking relevant anatomy, planning a grid-based procedure plan via the device software, followed by a spot-by-spot energy delivery with MRI closed loop thermometry to evaluate each energy delivery outcome.

Additionally, there are research-stage MRI based HIFU systems [9–11], which try to avoid some of the features listed above. For example, a robotic positioner is replaced with manual positioning + optical tracker, or the water bath is replaced by a membrane containing water, which is attached to the HIFU transducer.

By comparison, the fluoroscopy-guided HIFU system is optimized for nerve ablation on bone-tissue interfaces. Targeting is performed manually by physically manipulating the HIFU device—a method which is cumbersome (if not impossible) when an MRI bore obstructs physician access to the patient. This obviates the need for an integrated robotic positioner. Further, the system is designed for ablation in a single sonication as opposed to conventional soft-tissue methods, which use multiple sonifications on a grid thereby avoid the need for a planner and predictor. A water bath is also obviated because coupling is done with a gel pad placed directly on the patient's skin. Finally, there is no need for a cooling system due to the low acoustic intensity on the skin, the usage of a low energy level and the movement of the gel pad and transducer between targets.

Replacing MRI guidance with fluoroscopy guidance results in an affordable, logistically simple, and versatile and portable HIFU device, which may encourage clinical adoption for painful bone metastases. Fluoroscopy guidance is currently used as image guidance for many interventional MSK procedures because bone is well seen on x-ray. In addition, as can be seen via in-silico modeling [12], previous pre-clinical histology and macro pathology, and clinical results including contrast-enhanced MRI imaging [13–16], the temperature rise is predictable and accurate when targeting the bone tissue interface. This eliminates the need for real-time MR thermometry, but only when targeting the bone tissue interface. For soft tissue indications such as liver or uterine fibroids, MR thermometry is still crucial.

In the present investigation, a fluoroscopy-guided HIFU device was evaluated in a porcine model to establish the safety and feasibility of periosteal ablation along various healthy bones.

## Material and methods

The study was approved by the Institutional Animal Care and Use Committee of the Assaf Harofeh Medical Center, (Tel Aviv, Israel) and performed at Assaf Harofeh GLPigs research building, (Tel Aviv, Israel).

As a nonrandomized, prospective feasibility experiment, the sample size calculation was deemed non-applicable because of its observational methodology with imaging and histological outcome reporting. Two healthy, female pigs, (Swine/Large White X Landracer), weighing 65.5 and 68.5 kg were used.

## Pre-procedure

Before the procedure, the animals fasted for 24 h and underwent baseline neurological evaluation.

At the start of the procedure the animals were intubated and placed under general anesthesia using air/oxygen/isoflurane inhalation. Their vital signs and pain-related cardiovascular responses were continuously monitored. The animals were placed in left or right decubitus position on the procedure table per selected target. Along the targeted regions, hair was removed from the overlying skin with shaving and depilatory cream (Veet® cream) and then rinsed with soap and water.
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## Device

The fluoroscopy guided HIFU device, (Neurolyser XR, FUSMobile Inc., Alpharetta, GA, USA), was coupled to a C-arm system (Siemens IOS, Erlangen, Germany).

The HIFU transducer is made of 4 concentric ring elements operating in 1 MHz. The transducer's outer diameter is 88 mm and a focal distance of 77 mm. The system uses an incoherent mode [12] for acoustic energy delivery.

The setup time of the device was approximately 10 to 15 min, which included rolling the system into the room on a cart, connecting the relevant video cable to the C-Arm and preforming a daily quality assurance (DQA) procedure to verify device readiness.

The HIFU transducer was placed on the animal's skin above a domed, 10 cm in diameter, dedicated gel pad, (FUSMobile Inc., Alpharetta, GA, USA), that conforms to the transducer curvature and enhances acoustic coupling between the transducer surface and skin (Figure 1). Potable water and ultrasound gel were applied to the skin, before placing the gel pad and additional water was applied to the top of the gel pad before placing the transducer on top of the gel pad. It is important to note that the gel pad is transparent, allowing the clinician to visually verify proper coupling with the skin and ensure there are no air bubbles between the skin and the gel pad.

## Procedure

At each sonicated location, the Neurolyser XR transducer, mounted on a mechanical arm, was positioned so the target was centered within the fluoroscopy image. Image calibration at the target plane was performed by placing a sphere with a known diameter at the same imaging plane as the target and using the C-arm system software for the calibration computation. The C-Arm software comes with an image calibration feature, whereby placing an object with a known size at the field of view, marking its length on the image, and inputting the size into the software allows measurement of distances in the image, as long as the distances are within the image viewing plane.

The targeting was done manually, and the verification was done via a mechanical aim located at the center of the transducer, composed of two concentric rings, (upper and lower). Once the transducer was aligned with the target on anteroposterior (AP) view, the rings appeared concentric, with the acoustic spot located at the center of the overlayed circles. Depth verification, using the outer shell of the Neurolyser XR applicator as seen on lateral fluoroscopy view with system overlays, visualized the beam path and incoherent spot location as a digital extension of the outer shell. The target was considered within treatment depth if the digital overlay showed the spot location overlapping the target. For more

<figure><img src="image_16.png" /><figcaption>**Figure 1.** Procedure set-up. The therapeutic transducer with the aiming accessory is attached to the cradle, which in turn is connected to a metal positioner. The imaging workstation screen in the background shows the x-ray image of the transducer on top of the animal anatomy.</figcaption></figure>
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details about the actual targeting procedure, see the Appendix and Gofeld et al. publication [13]. The incoherent mode results in an elongated spot along the central axis of the beam path [12]. Thermal ablation of the bone tissue interface is created at the intersection of the spot with the bone, thereby ablating the periosteum facing the transducer.

Some steering along the beam path is possible when using an annular array, but this approach would require precise measurements of the distance to the depth, which is difficult to acquire using a C-arm. Instead, the incoherent mode creates an elongated spot, which would create the ablation as long as it intersects with the outer cortex of the target bone. This intersection is verified by visualizing the superimposition of the system overlay on a lateral image. The result of this approach is simplified depth verification and reduction of procedure time.

The first animal underwent bilateral ablations, targeting the lateral cortices of the mid femoral shaft, lateral mid iliac bone, and posterior ribs. The second animal was treated only on the right side, at the same three locations, with the left side intact as a control.

Figure 2 shows the order of sonication.

Depending on the location, 3-5 sonifications at 2 kJ (double the standard energy level used in previous human clinical studies with the same HIFU platform to test safety margins) were performed 1 cm apart in each target area (ilium, femur, and rib) for a cumulative dosage of 6-10 kJ (Figure 3 and Table 1). As the ribs are smaller compared to the femur and iliac bones, only 3 sonifications were completed on the ribs, compared to five for the other bones. The 1-cm spacing was selected based on previous work [9] showing lesion size at this energy level is large enough (average diameter of 12 mm) to create overlap between sonication spots and creates a continuous lesion.

<figure><img src="image_18.png" /><figcaption>Figure 2. Schematic sonication group. Sonifications were made 1 cm apart.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Figure 3. Sonication groups at three different targets. Left femur (a), mid ilium (B), and right rib (C).</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Table 1.** Procedure locations and parameters.</figcaption></figure>

<table><thead><tr><td>Pig ID</td><td>Sonifications</td><td>Acoustic energy</td><td>Total acoustic energy</td><td>Target location</td></tr></thead><tbody><tr><td>12205</td><td>5</td><td>2000 J</td><td>5*2000J=10 kJ</td><td>Mid right femur</td></tr><tr><td>12205</td><td>5</td><td>2000 J</td><td>5*2000J=10 kJ</td><td>Mid right iliac bone</td></tr><tr><td>12205</td><td>3</td><td>2000 J</td><td>3*2000J=6 kJ</td><td>Posterior right 10<sup>th</sup> rib</td></tr><tr><td>12205</td><td>5</td><td>2000 J</td><td>5*2000J=10 kJ</td><td>Mid left femur</td></tr><tr><td>12205</td><td>5</td><td>2000 J</td><td>5*2000J=10 kJ</td><td>Mid left iliac bone</td></tr><tr><td>12205</td><td>3</td><td>2000 J</td><td>3*2000J=6 kJ</td><td>Posterior left 9<sup>th</sup> rib</td></tr><tr><td>12206</td><td>5</td><td>2000 J</td><td>5*2000J=10 kJ</td><td>Mid right femur</td></tr><tr><td>12206</td><td>5</td><td>2000 J</td><td>5*2000J=10 kJ</td><td>Mid right iliac bone</td></tr><tr><td>12206</td><td>3</td><td>2000 J</td><td>3*2000J=6 kJ</td><td>Posterior right 10<sup>th</sup> rib</td></tr></tbody></table>

Once energy delivery was completed for each sonication, the transducer was aimed at the next nearest sonication target in the same bone until all planned sonication at the targeted bone had been delivered. At the end of the ablation for each target, the transducer was removed, and the animal's skin was evaluated for changes.

## Post procedure

Following the procedure, the animals were monitored during anesthetic recovery and the endotracheal tube was removed after the animals regained a swallow reflex. The animals were transferred to their habitats for observation. No antibiotic treatment was provided though each animal received buprenorphine for pain at a dosage of 1.5 ml intramuscularly twice a day for 4 days following the procedure.

## Follow-up

Animals were monitored for general behavior, food and water intake, and neurological status.

On day 5, the animals were anesthetized and underwent MRI scans, (Siemens, MAGNETOM 3T, Erlangen, Germany), obtaining the following sequences: T2w fat sat (STIR, 3-mm thickness, 3.9-mm spacing, 320×320), T1 (TSE, 3-mm thickness, 3.9-mm spacing, 320×320) and T1 fat sat images with and without contrast, (TSE, 3-mm thickness, 3.9-mm spacing, 320×320). Subtraction images were generated from the T1w fat sat with and without contrast. Images were obtained in the axial plane and an additional plane orthogonal to target. Tissue ablation was defined as an area of central nonenhancement with a surrounding rim of enhancement on MRI T1 post-contrast images with fat saturation.

After 12 weeks of follow-up, the animals were again anesthetized and computed tomography (CT) scans, (Siemens, SOMATOM Force, Erlangen, Germany), were obtained, (512×512, 1-mm thickness, no gap).

After completion of CT, the animals were euthanized with intravenous (IV) infusion of KCL injection under IV anesthesia, and the treated bones and surrounding soft tissue were excised, evaluated, and placed in formaldehyde.

## Necropsy and tissue collection

For gross pathological evaluation, the pre-marked treated areas and untreated samples serving as controls were collected along with surrounding soft tissues (~1 cm). The estimated areas of ultrasonic application were marked using a suture.

Samples were fixed in 4% formaldehyde solution (10% NBF) and kept in containers at a volume ~10 times larger than the sample size. Following fixation, the tissues/bones were grossly cut using a diamond wheel saw and the bone slices were decalcified, processed, and sectioned into two (or more) SuperMega slides (1 for hematoxylin and eosin [H&E] staining and 1 unstained slide as a reserve sample). The H&E-stained slide was scanned using a high-resolution scanner.

Histopathological examination was performed by a board-certified pathologist (European College Veterinary Pathologist, ECVP).

Histopathological changes were described and scored using a semi-quantitative scale with five grades (0-4), from Grade 0 for no change through minimal (Grade 1), mild (Grade 2), moderate (Grade 3), and severe (Grade 4) [17].
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# Results

## Clinical observation

Both animals behaved normally and displayed no significant limp, neurological deficits, loss of energy or appetite during the follow-up period. No adverse events (AEs) occurred during clinical observation, extubation, or imaging studies. Post procedure pain was difficult to assess because the pigs received the preemptive medication. Skin that had been exposed to treatment remained intact with no evidence of damage.

## Imaging

MRI scans revealed prominent areas of ablation in 9/9 targeted areas, as manifested by areas of central nonenhancement with surrounding enhancement on T1 post-contrast images with fat saturation. The non-enhancement volume includes the soft tissue adjacent to the target bone with minor penetration into the bone marrow. No fractures were noted on long-term follow-up CT scanning performed at 86 days.

## Ribs

More detailed analysis demonstrated full-thickness ablation across the right rib of Pig 12205 on MRI (Figure 4A), which correlated with full-thickness sclerosis on CT (Figure 4B). In addition, there was asymmetric cortical thickening with tiny erosions along the inferior surface (Figure 4C), which on evaluation of planning images, correlated with setting the center of the sonication more inferiorly along the surface of the rib and not at the center of the bone (Figure 4D). MRI revealed that the left rib on Pig 12205 and right rib on Pig 12206 received only partial-thickness ablation and CT scanning showed no changes in those bones.

## Ilium and femur

The right ilium (Figures 5A and B and Figure 6) and femur (Figure 7) in Pig 12206 demonstrated prominent ablation at the targeted location along the bone-soft tissue interface on MRI scanning and some ablation of the underlying marrow. Additional details of radiological findings on CT and MRI are listed in Table 2.

## Pathology

Twelve weeks after the procedure, no abnormalities were observed during necropsy and harvesting of bone, including in the areas treated and surrounding sonicated tissue.

Histopathological evaluation of tissue samples from the ilium and femur showed focal new spongiosa bone formation (i.e. combined tissues from the bone trabeculae and marrow) on the periosteal side, indicating a process of positive reactive healing of bone (Figures 8 and 9).

<figure><img src="image_22.png" /><figcaption>**Figure 4.** Pig 12205 coronal MRI T1 post contrast image with full-thickness ablation across the rib (arrow) (a), coronal (B) and sagittal (C) CT reformations with asymmetric cortical thickening with tiny erosions along the inferior surface (arrow) which also correlated with more asymmetric targeting of the inferior surface of the rib on planning images (D).</figcaption></figure>
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /><figcaption>**Figure 5.** Pig 12206 axial T1 post contrast MRI image with ablation along the periosteum and minimal penetration into the adjacent marrow in the right ilium (arrow points to the ablation) (A) and axial CT slice demonstrating minimal cortical and periosteal thickening and tiny erosions (arrow) (B). In both a and B, the sonication direction was left to right. Note that in image B, the fluoroscopy software mouse pointer is visible, but should be ignored because it has no relevance to this image.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 6.** Pig 12205 T1 post contrast axial MRI scan demonstrates ablation along the periosteum of the iliac bone extending into the marrow bilaterally (arrows).</figcaption></figure>

No inflammatory or necrotic reaction was noted. Therefore, given the evidence of complete healing, the overall reaction in the bone was judged as "not adverse" according to criteria from The Society of Toxicologic Pathology (STP) and in other pivotal publications by Baldrick et al. Kerlin et al. and Pallazzi et al. [18-20].

## Discussion

The present study supports feasibility and safety of fluoroscopy-guided HIFU for periosteal ablation as a potential therapeutic modality for pain palliation due to bone metastases. All sonication sites demonstrated successful ablation at the target with localized non-enhancement of the soft tissue and the adjacent targeted bone cortex with minimal penetration into the bone marrow and peripheral enhancement as seen on MRI.

There was some variability in soft tissue ablation volume as seen on MRI, for lesions which received the same amount of energy, which is likely related to non-identical target placement between animals, variation in target depth, and thickness of the target bone. However, had this been a clinical case with
<figure><img src="image_26.png" /><figcaption>**Figure 7.** Pig 12206 T1 post contrast coronal MRI scans demonstrating abundant ablation along the periosteum in the right femur and adjacent soft tissues (green arrow), with minimal involvement of the marrow, (darker part inside the bone).</figcaption></figure>

<table><caption>**Table 2.** Imaging results.</caption><thead><tr><th></th><th>MRI + C fat sat</th><th>Marrow ablation</th><th>Histology</th><th>CT periosteal thickening/ sclerosis</th><th>CT erosion</th><th>CT fracture</th><th>Notes</th></tr></thead><tbody><tr><td>12205 Right Rib</td><td>Ablation</td><td>Yes</td><td>None</td><td>Yes</td><td>Yes</td><td>No</td><td>Targeted lower margin of ribs with more sclerosis; full-thickness ablation through marrow</td></tr><tr><td>Iliac</td><td>Ablation</td><td>minimal</td><td>Focal new spongiosa bone formation on the periosteal side</td><td>No</td><td>No</td><td>No</td><td></td></tr><tr><td>Femur 12205 Left Rib</td><td>Ablation</td><td>Yes</td><td>None</td><td>No</td><td>No</td><td>No</td><td></td></tr><tr><td>Iliac</td><td>Ablation</td><td>Yes</td><td>None</td><td>No</td><td>No</td><td>No</td><td></td></tr><tr><td>Femur</td><td>Ablation</td><td>No</td><td>Focal new spongiosa bone formation on the periosteal side</td><td>No</td><td>No</td><td>No</td><td>Smaller area of ablation compared with other femurs</td></tr><tr><td>12206 Right Rib</td><td>Ablation</td><td>Yes</td><td>None</td><td>No</td><td>No</td><td>No</td><td></td></tr><tr><td>Iliac</td><td>Ablation</td><td>Yes</td><td>Focal new spongiosa bone formation on the periosteal side</td><td>Yes</td><td>Yes</td><td>No</td><td>Large ablation area</td></tr><tr><td>Femur</td><td>Ablation</td><td>Yes</td><td>None</td><td>Yes</td><td>Yes</td><td>No</td><td>Large ablation area</td></tr></tbody></table>

MRI, magnetic resonance imaging; CT, computed tomography.

the goal of palliating a painful lesion, it is likely to have resulted in a successful clinical outcome as the goal of the palliative therapy is denervating the periosteal layer of the bone.

In previous studies using this platform [13–15] we have seen no adverse events. When evaluating histology and imaging, the ablation-zone extended less than 1 cm into the soft tissue adjacent to the bone when sonicating at the clinical energy level of 1000 J. Although in this study the soft tissue ablation was larger due to the higher level of energy used, for clinical practice, keeping a relevant distance between sensitive areas and the target on the bone tissue interface is recommended.

For this specific work, accuracy was never evaluated because it was not the goal of this specific study. Healthy animal model with no tumors, and with arbitrary selection of targets does not allow to assess the precision. However, previous studies [13,15,16] have shown lesion formation at the targeted locations, addressing possible accuracy concerns that may arise when using this system.
<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /><figcaption>**Figure 8.** Histological section of sample from left (not treated), ilium in Pig 12206. No abnormality was detected. The green arrows indicate periosteum and bone.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Figure 9.** Histological section of sample from ilium of Pig 12206. Red arrows indicate focal new spongiosa bone formation on the periosteal side, suggesting a process of positive reactive healing of the bone. No inflammatory or necrotic reaction was noted. Green arrows indicate the normal preexisting cortex.</figcaption></figure>

There were no adverse effects in this study as well and postmortem histology was normal except for bone healing. In this study the procedure duration was about 30 min per lesion (6.5 to 8 min per sonication) which is longer than usual due to the research setting requiring documentation and technical challenges associated with an anesthetized animal. In our experience, the average time to perform 1 sonication for medial branch nerve ablation in a clinical setting using the same FUS system is 2.5 to 4 min, as measured from the start of targeting to the end of energy delivery. However, as a bone metastasis palliation procedure in a clinical setting with this platform has not been performed to date, further clinical and preclinical data are needed to establish procedure duration.

Previously published human clinical studies, evaluating MRgFUS ablation for palliation of pain from bone metastases have reported similar ablation methodology at targeted locations and reported similar post procedure MRI and CT appearance with improved pain relief [21,22]. Although an animal study
<figure><img src="image_30.png" /></figure>

could not include subjective outcomes, the similarities should be a good indication of equivalency with a likelihood of palliative outcome once this platform is used in a clinical study.

Despite the clinical success of HIFU, the adoption of MRgFUS for palliating pain from bone metastases has remained limited. A group concept mapping (GCM) study of contextual factors affecting adoption of HIFU in four European countries identified 49 such factors organized in 12 clusters: competitive treatments, physician attitudes, alignment of resources, logistics and workflow, technical disadvantages, radiation therapy as first-line treatment, aggregate knowledge and awareness, clinical effectiveness, patient preference, reimbursement, cost-effectiveness, and hospital costs [23]. A HIFU system that can be used outside the MRI suite may ameliorate many of these concerns, such as alignment of resources, logistics and workflow, technical disadvantages, patient preference, cost-effectiveness, and hospital costs.

Cost-wise the difference between a fluoroscopically guided HIFU procedure where the only disposable is a gel pad compared to a radiotherapy procedure, is expected to be significant.

There are several limitations to this report which evaluated the feasibility and safety of a fluoroscopic guided HIFU machine to induce ablation along the periosteum or normal bone. Although bone absorbs significant more ultrasound energy than soft tissue, the presence of an underlying lytic or blastic lesion may affect lesion size if the bone outer cortex is breached. However, the actual composition of the lesion is less relevant for the treatment parameters as long as the outer cortex remains intact. In addition, to confirm safety, double the amount of clinical level energy was used to create thermal lesions which could overestimate the size and quality of the ablated area. This level of energy is also about 30% to 50% higher than that used in equivalent MRgFUS procedures, but because this is a safety study, we felt that using a higher level of energy was justified. Dose escalation studies are necessary further optimize the ablation parameters for cases in which the amount of soft tissue ablation has clinical value. Although soft tissue ablation into the muscles was noted beyond the periosteum, the volume was small and resulted in no change in clinical ambulation or respiratory pattern.

In addition, as the transducer is 8.8cm wide, an area along the skin that allows for placement of the similar sized gel pad and overlying transducer is necessary. Therefore, although lesions in the pelvis, femur, and ribs are likely to be technically feasible, the upper limbs and lower parts of the lower extremities could present coupling challenges.

Based on this preclinical feasibility study, and previous clinical work [14–16] with the same fluoroscopy guided HIFU platform that now has CE and Health Canada approval for medial branch nerve ablation as a treatment for lumbar joint zygapophyseal joint pain, next steps include conducting a clinical study on a carefully selected cohort of patients. Acoustic parameters for such a future study will likely add a depth targeting overlay with suggested energy levels per depth, which is currently not part of the system.

## Conclusion

This feasibility study of fluoroscopy guided HIFU in a porcine model demonstrated successful ablation of all targets, as confirmed by post-treatment MRI scanning. No adverse events were observed following the procedure, as confirmed by clinical evaluation, histological analysis, and imaging follow-up.

## Acknowledgments

The authors thank Judith M. Orvos, ELS of Orvos Communications, LLC, for editing assistance in the preparation of the manuscript.

## Disclosure statement

AH and RA are the founders of FUSMobile. MG is consultant for FUSMobile and holds an option to buy ordinary shares. EM, BS, and NR are employees of FUSMobile and hold options to buy ordinary shares. EM and SL hold shares and options to buy ordinary shares in FUSMobile. JFA holds an option to buy ordinary shares. AN has nothing to disclose.
<figure><img src="image_31.png" /></figure>

## Funding

The funding for the work was provided by FUSMobile Inc.

## Data availability

The data from this study can be made available upon request directed to the corresponding author. Proposals will be reviewed on the basis of scientific merit, ethical review, available resources, and regulatory requirements. After approval of a proposal, the data will be made available for reuse. A steering committee will have the right to review and comment on any draft papers based on these data before publication.

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# Appendix

## Steps in target alignment

An initial anatomical image of the treatment location is taken.

The HIFU transducer is then placed on the skin in contact with a dedicated gel pad that enhances acoustic coupling.

Using the predicted targeting overlay generated by the imaging workstation, the aiming accessory is moved and tilted such that the cradle holding the therapeutic transducer (Figure 1), points to each of the desired anatomical targets. The cradle is then locked in position and targeting is confirmed with one repeat fluoroscopic imaging (Figure 2A). Once an anatomy image is acquired, there is usually a need for an additional one to two x-ray images to verify targeting because the actual positioning of the therapeutic transducer is done via computerized prediction.

Once the target has been confirmed, a lateral view is obtained at 90 degrees relative to the cradle to verify the targeting depth (Figure 2B) and the target is marked on an overlay (red lines). Confirmation of depth is only required once per anatomical location and requires an additional one to two x-ray images.

Next, using the lateral image, the C-arm is rotated back, a second AP image is obtained, and targeting is verified again.

<figure><img src="image_33.png" /><figcaption>**Figure A1.** (A) Disposable coupling gel pad. (B) Cradle. (C) Neurolyser positioner, which includes the therapeutic transducer inside the cradle along with the aiming system; central rings for target localization (black arrow) and adjacent top (white) and side (blue) camera boxes for assisting with side to side and top down aiming.</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>**Figure A2.** A) Targeting overlay as seen top down. Blue circles are the transducer inner and outer diameter. Green circles are the targeting rings, and the pink spot is the predicted targeting location. B) Targeting overlay as seen from side to side. The red triangle is the beam path. The two vertical lines are the treatment envelop, and the horizontal line is the location of the acoustic spot within the treatment envelop.</figcaption></figure>