Preclinical MRI-Guided Focused Ultrasound: A Review of Systems and Current Practices

Strongly Focused HIFU Transducers With Simultaneous Optical Observation for Treatment of Skin at 20 MHz

High-intensity focused ultrasound (HIFU) transducers are proposed as a new treatment modality for dermatology. The shape and size of pressure fields generated by strongly focused transducers with an f-number equal to 0.75 operating at frequencies up to 20 MHz are analyzed analytically using the Lucas-Muir model and numerically with the wide-angle Khokhlov-Zabolotskaya-Kuznetsov method. The modeling results under quasi-linear conditions are compared against measurements performed in an acoustic tank with the aid of a fiberoptic hydrophone. The size of the focal zone expressed by their depth of focus and focal diameter is found to be directly controlled by their operating frequency and f-number. Devices manufactured for an operating frequency of 20 MHz are characterized by their 6 dB depth of focus of 490 μm and focal diameter of 80.6 µm. The devices are further studied at high power levels using a polyacrylic tissue-mimicking phantom. The devices are equipped with an optical observation system allowing simultaneous treatment and observation of the skin surface. In comparison to conventional cosmetic applications of HIFU, the devices analyzed are concluded to be ideal for treatment of precisely selected and confined layers of the human skin.

Translation of focused ultrasound for blood-brain barrier opening in glioma

Survival outcomes for patients with glioblastoma multiforme (GBM) have remained poor for the past 15 years, reflecting a clear challenge in the development of more effective treatment strategies. The efficacy of systemic therapies for GBM is greatly limited by the presence of the blood-brain barrier (BBB), which prevents drug penetration and accumulation in regions of infiltrative tumour, as represented in a consistent portion of GBM lesions. Focused ultrasound (FUS) - a technique that uses low-frequency ultrasound waves to induce targeted temporary disruption of the BBB - promises to improve survival outcomes by enhancing drug delivery and accumulation to infiltrating tumour regions. In this review we discuss the current state of preclinical investigations using FUS to enhance delivery of systemic therapies to intracranial neoplasms. We highlight critical methodological inconsistencies that are hampering clinical translation of FUS and we provide guiding principles for future preclinical studies. Particularly, we focus our attention on the importance of the selection of clinically relevant animal models and to the standardization of methods for FUS delivery, which will be paramount to the successful clinical translation of this promising technology for treatment in GBM patients. We also discuss how preclinical FUS research can benefit the development of GBM immunotherapies.

Treatment of Seborrheic Keratosis by High Frequency Focused Ultrasound - An Early Experience with 11 Consecutive Cases

PURPOSE

High intensity focused ultrasound operating at 20 MHz has been demonstrated as a safe and efficient treatment modality for a range of dermatological indications. The method is potentially also applicable to removal of seborrheic keratosis.

PATIENTS AND METHODS

A total of 54 seborrheic keratoses in 11 volunteer subjects (8 women and 3 men, average age 51.5 ± 13.2 years) were treated in a single session with a medical 20 MHz high intensity focused ultrasound device developed for dermatological conditions. Handpieces with nominal focal depths of 0.8 mm below the skin surface were used to administer acoustic energy of 0.99-1.2 J/dose. An integrated dermoscope in the handpiece was used to monitor the treatment in real-time. Treatment efficacy and side-effects were assessed directly after treatment and at follow-up 4-15 weeks after treatment.

RESULTS

The treatment showed positive results in 96.3% of the cases. About 68.5% of the cases were classified as complete response and 27.8% of the cases as partial response. Two cases (3.7%) did not respond to treatment and were classified as stable condition. No subjects experienced worsening of their condition, and no treatment received the classification of progressive condition. Side effects were primarily redness in the treatment area due to superficial telangiectasia, mild scarring, and persisting and slow-healing lichen planus-like keratosis. No adverse events were observed.

CONCLUSION

HIFU is concluded to be a safe and efficient skin treatment for seborrheic keratoses. It has advantages over conventional treatments that can lead to pain during treatment and scarring after healing.

An Acoustic Measurement Library for Non-Invasive Trans-Rodent Skull Ultrasonic Focusing at High Frequency

OBJECTIVE

To investigate the feasibility of developing an acoustic measurement library for non-invasive trans-rodent skull ultrasonic focusing at high frequency.

METHODS

A fiber-optic hydrophone (FOH) was positioned at the geometric focus of a spherically-curved phased array (64 elements, 25 mm diameter, 20 mm radius of curvature). Elements were driven sequentially (3.3 MHz driving frequency) and FOH waveforms were recorded with and without intervening ex-vivo rodent skullcaps. Measurements were carried out on 15 skullcaps (Sprague-Dawley rats, 182-209 g) across 3 fixed transmission regions per specimen. An element-wise measurement library of skull-induced phase differences was constructed using mean values across all specimens for each transmission region. Library-based transcranial phase differences were compared with direct FOH-based measurements across 5 additional skullcaps not included in the library.

RESULTS

Library-based phase corrections deviated less from FOH-based trans-skull phase difference values than those calculated for the water-path case, and restored partial transcranial focal quality relative to that recovered using invasive hydrophone-based corrections. Retrospective analysis suggests comparable performance can be obtained using smaller library sizes.

CONCLUSION

An acoustic measurement library can facilitate non-invasive transcranial aberration correction in rodents at high frequency.

SIGNIFICANCE

Library-based focusing represents a practical approach for delivering high-frequency ultrasound brain treatments in small animals.

Treatment of superficial benign vascular tumors by high intensity focused ultrasound: Observations in two illustrative cases

Background

Existing therapeutic methods for reduction or removal of superficial vascular malformations and tumors have high risks of scarring and other complications that result in aesthetic appearance less favorable than the baseline. Patients are often cautioned against intervention, which can lead to psychosocial problems and low self‐esteem. Improved treatment modalities are therefore relevant from both medical and aesthetic perspectives.

Methods

Two volunteer subjects were treated with a medical 20 MHz high intensity focused ultrasound device developed for dermatological conditions. One patient was given three treatments to remove a superficial congenital hemangioma on the left middle cheek. The other patient was given a single treatment to remove seven cherry angiomas on the thighs. Handpieces with nominal focal depths of 0.8 – 1.8 mm below the skin surface were used to administer acoustic energy of 1.1 – 1.2 J/dose. An integrated dermoscope in the handpiece was used to monitor the treatment in real‐time.

Results

During treatment, blood in the capillary network of the lesions was coagulated immediately, and capillary walls were collapsed due to the thermal and mechanical effects of the high intensity focused ultrasound. During the healing phase, the areas regenerated a normal skin structure with very limited scar or dyspigmentation. At follow‐up, a clear aesthetic improvement was observed over the baseline for all treated targets with the exception of two cherry angiomas, where focal depth and/or dose coverage had not been optimal.

Conclusion

High intensity focused ultrasound is concluded to be a safe and efficient skin treatment for benign superficial vascular malformations and tumors.

Removal of Common Warts by High-Intensity Focused Ultrasound: An Introductory Observation

Therapies of common warts are cumbersome and not very effective. Recurrences are common. A new 20 MHz high-intensity focused ultrasound (HIFU) method is introduced as a new potential treatment modality. With HIFU, selected targets in the epidermis and dermis can be treated with full control of the depth and position of the ultrasound lesion and the energy applied to the target. The treatment can be monitored directly in real-time via an integrated dermoscope in the ultrasound probe. Two warts were treated with 8-10 shoulder-by-shoulder treatment doses, focal depth 1.3 mm, and 1.2 J/dose. Pretreatment ultrasound B-mode scanning had shown the thickness and depth of the warts. The treated areas developed a dry wound covered by a crust over the next 1-2 days. After 2 weeks the skin was healed, with no wart and no scar. Observation showed no reoccurrence. HIFU has future potential for treatment of common warts and flat warts, and a broad range of skin lesions being logic further candidates for targeted ablative treatment. One single treatment may suffice. It is, therefore, a new modality in dermatology with a large range of indications.

A High-Frequency Phased Array System for Transcranial Ultrasound Delivery in Small Animals

Existing systems for applying transcranial focused ultrasound (FUS) in small animals produce large focal volumes relative to the size of cerebral structures available for interrogation. The use of high ultrasonic frequencies can improve targeting specificity; however, the aberrations induced by rodent calvaria at megahertz frequencies severely distort the acoustic fields produced by single-element focused transducers. Here, we present the design, fabrication, and characterization of a high-frequency phased array system for transcranial FUS delivery in small animals. A transducer array was constructed by micromachining a spherically curved PZT-5H bowl (diameter = 25 mm, radius of curvature = 20 mm, fundamental frequency = 3.3 MHz) into 64 independent elements of equal surface area. The acoustic field generated by the phased array was measured at various target locations using a calibrated fiber-optic hydrophone, both in free-field conditions as well as through ex vivo rat skullcaps with and without hydrophone-assisted phase aberration corrections. Large field-of-view acoustic field simulations were carried out to investigate potential grating lobe formation. The focal beam size obtained when targeting the array's geometric focus was [Formula: see text] mm in water. The array can steer the FUS beam electronically over cylindrical volumes of 4.5 mm in diameter and 6 mm in height without introducing grating lobes. Insertion of a rat skullcap resulted in substantial distortion of the acoustic field ( [Formula: see text]% [Formula: see text]); however, phase corrections restored partial focal quality ( [Formula: see text]% [Formula: see text]). Using phase corrections, the array is capable of generating a trans-rat skull peak negative focal pressure of up to ~2.0 MPa, which is sufficient for microbubble-mediated blood-brain barrier permeabilization at this frequency.

High-frequency (20 MHz) high-intensity focused ultrasound: New ablative method for color-independent tattoo removal in 1-3 sessions. An open-label exploratory study

BACKGROUND

High-intensity focused ultrasound (HIFU) operating at 20 MHz is new and potentially applicable to ablative tattoo removal. The method was documented safe and rational in preclinical testing.

MATERIALS AND METHODS

High-intensity focused ultrasound was introduced to subjects when lasers and dermatome shaving had failed or caused side effects. Transducers with focal depths between 1.1 mm and 1.7 mm in the skin were used, and settings of 0.4-1.2 J/shot at pulse durations of 150 ms were applied. Tattoos were covered with synergistic "shoulder-by-shoulder" focused ultrasound shots. Effectiveness and side effects were measured.

RESULTS

Twenty-two subjects with 67 tattoos were treated. 62% benefitted (19% cleared, 43% partially cleared), and 28% had minor effect. VAS pain was 5-6 versus 7-9 with previous lasers removal. Wound healing was longer after HIFU ablation (1-3 months). 57% of subjects had no scar or minor visible changes of skin surface markings only, while 19% had moderate or major skin thickening. Hypertrophic scar or keloid scars were not observed.

DISCUSSION/CONCLUSION

High-intensity focused ultrasound was effective in removal of difficult tattoos of any color where Nd:YAG lasers had failed. The method only needs 1-3 sessions. As an ablative method, the wound healing period is longer than with laser removal and needs attention. Focused ultrasound can be used as a first-line treatment of smaller tattoos independent of color, and second line when Nd:YAG lasers have failed or caused problems. The operator shall be qualified, as with lasers.

High-frequency (20 MHz) high-intensity focused ultrasound: New Treatment of actinic keratosis, basal cell carcinoma, and Kaposi sarcoma. An open-label exploratory study

Background

Skin cancer is common, growing, challenging, and in need of progress in early‐stage treatment. 20 MHz high‐intensity focused ultrasound (HIFU) is new and applied to actinic keratosis (AK) and skin cancers for the first time. HIFU of lower frequency is already used in the treatment of internal cancers.

Materials and Methods

Eight patients with 201 AK lesions, one patient with 7 basal cell carcinomas (reoccurrences after PDT), and one patient with 7 Kaposi sarcoma lesions (4 treated with radiotherapy in the past) were given 1‐3 HIFU treatments. Twenty megahertz HIFU was dosed as 150 ms at 0.6‐1.2 J/shot applied to target lesions. Probes with different target depths were available. The preferred shot energy and focal depth in AK were 0.9 J and 1.3 mm. A “Sandwich” strategy with HIFU applied in two depths were tried in cancers. The follow‐up period was 3‐6 months.

Results

All AK cleared except 5, giving a cure rate of 97%. Post‐treatment lesion healed in 1‐2 weeks with no scar. VAS pain was from 1 to 8, and in any case less than experienced with previous PDT. In both basal cell carcinoma (BCC) and sarcoma, healing was confirmed by histological verification.

Discussion/conclusion

20 MHz HIFU was an effective and safe treatment of AK. This new treatment, applicable to any anatomical site, has promising advantages relative to PDT and has the potential to replace or supplement PDT in future. Case‐observations indicated that HIFU can be useful in skin cancers as well.

Endobronchial high-intensity ultrasound for thermal therapy of pulmonary malignancies: simulations with patient-specific lung models

Objective: This study investigates the feasibility of endobronchial ultrasound applicators for thermal ablation of lung tumors using acoustic and biothermal simulations.Methods: Endobronchial ultrasound applicators with planar (10 mm width) or tubular transducers (6 mm outer diameter (OD)) encapsulated by expandable coupling balloons (10 mm OD) are considered for treating tumors from within major airways; smaller catheter-based applicators with tubular transducers (1.7-4 mm OD) and coupling balloons (2.5-5 mm OD) are considered within deep lung airways. Parametric studies were applied to evaluate transducer configurations, tumor size and location, effects of acoustic reflection and absorption at tumor-lung parenchyma interfaces, and the utility of lung flooding for enhancing accessibility. Patient-specific anatomical lung models, with various geometries and locations of tumors, were developed for further evaluation of device performance and treatment strategies. Temperature and thermal dose distributions were calculated and reported.Results: Large endobronchial applicators with planar or tubular transducers (3-7 MHz, 5 min) can thermally ablate tumors attached to major bronchi at up to 3 cm depth, where reflection and attenuation of normal lung localize tumor heating; with lung flooding, endobronchial applicators can ablate ∼2 cm diameter tumors with up to ∼2 cm separation from the bronchial wall, without significant heating of intervening tissue. Smaller catheter-based tubular applicators can ablate tumors up to 2-3 cm in diameter from deep lung airways (5-9 MHz, 5 min).Conclusion: Simulations demonstrate the feasibility of endobronchial ultrasound applicators to deliver thermal coagulation of 2-3 cm diameter tumors adjacent to or accessible from major and deep lung airways.

High-frequency (20 MHz) high-intensity focused ultrasound system for dermal intervention: A 12-week local tolerance study in minipigs

BACKGROUND

High-intensity focused ultrasound (HIFU) operating at 20 MHz is new and applicable to skin. Details of use and instrumentation are not documented.

MATERIALS AND METHODS

A GLP compliant 12-week study of Göttingen minipigs (n = 3) was undertaken. Effects of HIFU treatment at different focal depths, energy levels and field size (single shot vs 5 × 5 multiple shots) were studied. Clinical scoring and histology of treated sites were made.

RESULTS

High-intensity focused ultrasound showed instant and initial effects with wheal and flare responses followed by delayed inflammatory reactions associated with outer skin necrosis, depending on energy dose. HIFU treatment was tunable in the range 0.3-1.5 J, ablative at higher energy level. Transducers with deeper focal points gave more profound effects, while epidermal effects were comparable. Multiple doses of 5 × 5 shots produced stronger reactions than single dose indicating that nearby applied shots were synergistic. Recovery from single doses was faster than in multidose areas. Clinical scarring at the end point was not seen despite occasional fibrous change of dermis. Findings illustrated intended therapeutic use; no special safety issues of concern were raised.

CONCLUSION

The new 20 MHz HIFU was reproducible, tunable and produced targeted effects in the outer skin, for example instant wheal and flare followed by inflammation and possibly necrosis depending on energy setting. Reactions recovered during the study with only minor findings at study end. No special safety concerns were raised. The method can be controlled and modulated, and it is ready for clinical testing of dermatological disease indications including conditions presently treated with lasers.

High-frequency (20-MHz) high-intensity focused ultrasound (HIFU) system for dermal intervention: Preclinical evaluation in skin equivalents

BACKGROUND

High-intensity focused ultrasound (HIFU) for non-invasive treatment of a range of internal pathologies including cancers of major organs and cerebral pathologies is in exponential growth. Systems, however, operate at relatively low frequencies, in the range of 200-2000 kHz as required for deep axial penetration of the body. HIFU utilizing frequencies in excess of 15 MHz has so far not been explored, but presents an opportunity to extend the HIFU modality to target specific dermal lesions and small animal research.

MATERIALS AND METHODS

A new 20-MHz HIFU system (TOOsonix ONE-R) with narrow focus corresponding to the dermis was studied in acoustic skin equivalents, for example, in a tissue-mimicking gel and in bovine liver. HIFU lesion geometry, depth, and diameter were determined. The temperature increase in the focal point was measured as a function of acoustic power and the duration of HIFU exposure.

RESULTS

The system produces highly reproducible ultrasound lesions with predictable and configurable depths of 1-2 mm, thus corresponding to the depth of the human dermis. The lesion geometry was elongated triangular and sized 0.1-0.5 mm, convergent to a focal point skin deep. Focal point temperature ranged between 40 and 90°C depending on the chosen setting. Observations were confirmed ex vivo in bovine liver and porcine muscle. Variation of acoustic power and duration of exposure produced linear effects in the range of the settings studied. Thus, effects could be adjusted within the temperature interval and spatial field relevant for clinical therapy and experimental intervention targeting the dermal layer of human skin.

CONCLUSION

The tested 20-MHz HIFU system for dermal applications fulfilled key prerequisite of narrow-field HIFU dedicated to cutaneous applications regarding reproducibility, geometry, and small size of the applied ultrasound lesions. Controlled adjustment of acoustic lesions within the temperature range 40-90°C qualifies the system for a range of non-ablative and ablative applications in dermatological therapy.

Development of custom RF coils for use in a small animal platform for magnetic resonance-guided focused ultrasound hyperthermia compatible with a clinical MRI scanner

Three different magnetic resonance imaging (MRI) coils were developed and assessed for use with an experimental platform designed to generate hyperthermia in mice using magnetic resonance-guided focused ultrasound (MRgFUS). An ergonomic animal treatment bed was integrated with MRI coils. Three different coil designs optimized for small targets were tested, and performance in targeting and conducting accurate temperature imaging was evaluated. Two transmit/receive surface coils of different diameters (4 and 7 cm) and a transmit-only/receive-only (TORO) coil were used. A software platform was developed to provide real-time targeting and temperature maps and to deliver controlled ultrasound exposure. MR thermometry was conducted on different targets, including fresh chicken breasts and mouse cadavers. Multiple experiments were performed in which tissues were targeted with high reproducibility. The TORO coil was the most resilient to temperature drift, resulting in an increase in the calculated temperature of 0.29 ± 0.12 °C, compared to 1.27 ± 0.13 °C and 0.47 ± 0.04 °C for the medium and small coils, respectively. Controlled closed-loop hyperthermia exposure was successfully performed with all three coils. Considering all assessments, the TORO coil exhibited the best overall performance for thermometry acquisition when accounting for stability, precision, temperature spread and resilience to temperature drift. B₁ maps of the three coils confirmed that the TORO coil exhibited the most homogeneous B₁ field, which explained the improved thermometry performance. The use of coils specifically designed for small targets within the proposed experimental platform allowed accurate thermometry during hyperthermia.

Focused Ultrasound-enabled Brain Tumor Liquid Biopsy

Although blood-based liquid biopsies have emerged as a promising non-invasive method to detect biomarkers in various cancers, limited progress has been made for brain tumors. One major obstacle is the blood-brain barrier (BBB), which hinders efficient passage of tumor biomarkers into the peripheral circulation. The objective of this study was to determine whether FUS in combination with microbubbles can enhance the release of biomarkers from the brain tumor to the blood circulation. Two glioblastoma tumor models (U87 and GL261), developed by intracranial injection of respective enhanced green fluorescent protein (eGFP)-transduced glioblastoma cells, were treated by FUS in the presence of systemically injected microbubbles. Effect of FUS on plasma eGFP mRNA levels was determined using quantitative polymerase chain reaction. eGFP mRNA were only detectable in the FUS-treated U87 mice and undetectable in the untreated U87 mice (maximum cycle number set to 40). This finding was replicated in GL261 mice across three different acoustic pressures. The circulating levels of eGFP mRNA were 1,500-4,800 fold higher in the FUS-treated GL261 mice than that of the untreated mice for the three acoustic pressures. This study demonstrated the feasibility of FUS-enabled brain tumor liquid biopsies in two different murine glioma models across different acoustic pressures.