Volumetric MR-Guided High-Intensity Focused Ultrasound with Direct Skin Cooling for the Treatment of Symptomatic Uterine Fibroids: Proof-of-Concept Study

Comparison of computer simulations and clinical treatment results of magnetic resonance-guided focused ultrasound surgery (MRgFUS) of uterine fibroids

PURPOSE

Magnetic resonance-guided focused ultrasound surgery (MRgFUS) can be used to noninvasively treat symptomatic uterine fibroids by heating with focused ultrasound sonications while monitoring the temperature with magnetic resonance (MR) thermometry. While prior studies have compared focused ultrasound simulations to clinical results, studies involving uterine fibroids remain scarce. In our study, we perform such a comparison to assess the suitability of simulations for treatment planning.

METHODS

Sonications (N = 67) were simulated retrospectively using acoustic and thermal models based on the Rayleigh integral and Pennes bioheat equation followed by MR-thermometry simulation in seven patients who underwent MRgFUS treatment for uterine fibroids. The spatial accuracy of simulated focus location was assessed by evaluating displacements of the centers of mass of the thermal dose distributions between simulated and treatment MR thermometry slices. Temperature-time curves and sizes of 240 equivalent minutes at 43°C (240EM43 ) volumes between treatment and simulation were compared.

RESULTS

The simulated focus location showed errors of 2.7 ± 4.1, -0.7 ± 2.0, and 1.3 ± 1.2 mm (mean ± SD) in the anterior-posterior, foot-head, and right-left directions for a fibroid absorption coefficient of 4.9 Np m-1  MHz-1 and perfusion parameter of 1.89 kg m-3 s-1 . Linear regression of 240EM43 volumes of 67 sonications of patient treatments and simulations utilizing these parameters yielded a slope of 1.04 and a correlation coefficient of 0.54. The temperature rise ratio of simulation to treatment near the end of sonication was 0.47 ± 0.22, 1.28 ± 0.60, and 1.49 ± 0.71 for 66 sonications simulated utilizing fibroid absorption coefficient of 1.2, 4.9, and 8.6 Np m-1  MHz-1 , respectively, and the aforementioned perfusion value. The impact of perfusion on peak temperature rise is minimal between 1.89 and 10 kg m-3 s-1 , but became more substantial when utilizing a value of 100 kg m-3 s-1 .

CONCLUSIONS

The results of this study suggest that perfusion, while in some cases having a substantial impact on thermal dose volumes, has less impact than ultrasound absorption for predicting peak temperature elevation at least when using perfusion parameter values up to 10 kg m-3 s-1 for this particular array geometry, frequencies, and tissue target which is good for clinicians to be aware of. The results suggest that simulations show promise in treatment planning, particularly in terms of spatial accuracy. However, in order to use simulations to predict temperature rise due to a sonication, knowledge of the patient-specific tissue parameters, in particular the absorption coefficient is important. Currently, spatially varying patient-specific tissue parameter values are not available during treatment, so simulations can only be used for planning purposes to estimate sonication performance on average.

Ultrastructural Analysis of Volumetric Histotripsy Bio-effects in Large Human Hematomas

Large-volume soft tissue hematomas are a serious clinical problem, which, if untreated, can have severe consequences. Current treatments are associated with significant pain and discomfort. It has been reported that in an in vitro bovine hematoma model, pulsed high-intensity focused ultrasound (HIFU) ablation, termed histotripsy, can be used to rapidly and non-invasively liquefy the hematoma through localized bubble activity, enabling fine-needle aspiration. The goals of this study were to evaluate the efficiency and speed of volumetric histotripsy liquefaction using a large in vitro human hematoma model. Large human hematoma phantoms (85 cc) were formed by recalcifying blood anticoagulated with citrate phosphate dextrose/saline-adenine-glucose-mannitol solution. Typical boiling histotripsy pulses (10 or 2 ms) or hybrid histotripsy pulses using higher-amplitude and shorter pulses (0.4 ms) were delivered at 1% duty cycle while continuously translating the HIFU focus location. Histotripsy exposures were performed under ultrasound guidance with a 1.5-MHz transducer (8-cm aperture, F# = 0.75). The volume of liquefied lesions was determined by ultrasound imaging and gross inspection. Untreated hematoma samples and samples of the liquefied lesions aspirated using a fine needle were analyzed cytologically and ultrastructurally with scanning electron microscopy. All exposures resulted in uniform liquid-filled voids with sharp edges; liquefaction speed was higher for exposures with shorter pulses and higher shock amplitudes at the focus (up to 0.32, 0.68 and 2.62 mL/min for 10-, 2- and 0.4-ms pulses, respectively). Cytological and ultrastructural observations revealed completely homogenized blood cells and fibrin fragments in the lysate. Most of the fibrin fragments were less than 20 μm in length, but a number of fragments were up to 150 μm. The lysate with residual debris of that size would potentially be amenable to fine-needle aspiration without risk for needle clogging in clinical implementation.

AAPM Task Group 241: A medical physicist's guide to MRI-guided focused ultrasound body systems

Magnetic resonance-guided focused ultrasound (MRgFUS) is a completely non-invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

A Computational Study on Temperature Variations in MRgFUS Treatments Using PRF Thermometry Techniques and Optical Probes

Structural and metabolic imaging are fundamental for diagnosis, treatment and follow-up in oncology. Beyond the well-established diagnostic imaging applications, ultrasounds are currently emerging in the clinical practice as a noninvasive technology for therapy. Indeed, the sound waves can be used to increase the temperature inside the target solid tumors, leading to apoptosis or necrosis of neoplastic tissues. The Magnetic resonance-guided focused ultrasound surgery (MRgFUS) technology represents a valid application of this ultrasound property, mainly used in oncology and neurology. In this paper; patient safety during MRgFUS treatments was investigated by a series of experiments in a tissue-mimicking phantom and performing ex vivo skin samples, to promptly identify unwanted temperature rises. The acquired MR images, used to evaluate the temperature in the treated areas, were analyzed to compare classical proton resonance frequency (PRF) shift techniques and referenceless thermometry methods to accurately assess the temperature variations. We exploited radial basis function (RBF) neural networks for referenceless thermometry and compared the results against interferometric optical fiber measurements. The experimental measurements were obtained using a set of interferometric optical fibers aimed at quantifying temperature variations directly in the sonication areas. The temperature increases during the treatment were not accurately detected by MRI-based referenceless thermometry methods, and more sensitive measurement systems, such as optical fibers, would be required. In-depth studies about these aspects are needed to monitor temperature and improve safety during MRgFUS treatments.

Design and evaluation of an open-source, conformable skin-cooling system for body magnetic resonance guided focused ultrasound treatments

PURPOSE

Magnetic resonance guided focused ultrasound (MRgFUS) treatment of tumors uses inter-sonication delays to allow heat to dissipate from the skin and other near-field tissues. Despite inter-sonication delays, treatment of tumors close to the skin risks skin burns. This work has designed and evaluated an open-source, conformable, skin-cooling system for body MRgFUS treatments to reduce skin burns and enable ablation closer to the skin.

METHODS

A MR-compatible skin cooling system is described that features a conformable skin-cooling pad assembly with feedback control allowing continuous flow and pressure maintenance during the procedure. System performance was evaluated with hydrophone, phantom and in vivo porcine studies. Sonications were performed 10 and 5 mm from the skin surface under both control and forced convective skin-cooling conditions. 3D MR temperature imaging was acquired in real time and the accumulated thermal dose volume was measured. Gross analysis of the skin post-sonication was further performed. Device conformability was demonstrated at several body locations.

RESULTS

Hydrophone studies demonstrated no beam aberration, but a 5-12% reduction of the peak pressure due to the presence of the skin-cooling pad assembly in the acoustic near field. Phantom evaluation demonstrated there is no MR temperature imaging precision reduction or any other artifacts present due to the coolant flow during MRgFUS sonication. The porcine studies demonstrated skin burns were reduced in size or eliminated when compared to the control condition.

CONCLUSION

An open-source design of an MRgFUS active skin cooling system demonstrates device conformability with a reduction of skin burns while ablating superficial tissues.

Are Current Technical Exclusion Criteria for Clinical Trials of Magnetic Resonance-Guided High-Intensity Focused Ultrasound Too Restrictive?: Early Experiences at a Pediatric Hospital

Certain technical criteria must be met to ensure the treatment safety of magnetic resonance-guided high-intensity focused ultrasound. We retrospectively reviewed how our enrollment criteria were applied from 2014 to 2017 in a clinical trial of magnetic resonance-guided high-intensity focused ultrasound ablation of recurrent malignant and locally aggressive benign solid tumors. Among the 36 screened patients between 2014 and 2017, more than one-third were excluded for technical exclusion criteria such as the anatomic location and proximity to prosthetics. Overall, patients were difficult to accrue for this trial, given the incidence of these tumors. To increase potential accrual, screening exclusion criteria could be more generalized and centered on the ability to achieve an acceptable treatment safety margin, rather than specifically excluding on the basis of general anatomic areas.

Uterine Myomas: Extravascular Treatment

Uterine fibroids are common benign tumors that affect the female reproductive tract. They are responsible for considerable morbidity and deterioration of life quality. The main advantages offered by mini invasive techniques are low grade of invasiveness and short times of hospitalization. The most diffuse technique is uterine artery embolization (UAE). Common concerns with UAE include postprocedural pain, postembolization syndrome, and risk of infection. Image-guided thermal ablation techniques like radiofrequency ablation, percutaneous microwave ablation, and imaging-guided high-intensity focused ultrasound were introduced to overcome the side effects related to UAE and surgery. The aim of this review is to briefly analyze the ablative procedures and their role in the management of symptomatic fibroids, and to describe the safety profile and outcomes of these modalities.

Ultrasound Hyperthermia Technology for Radiosensitization

Hyperthermia therapy (HT) raises tissue temperature to 40-45°C for up to 60 min. Hyperthermia is one of the most potent sensitizers of radiation therapy (RT). Ultrasound-mediated HT for radiosensitization has been used clinically since the 1960s. Recently, magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU), which has been approved by the United States Food and Drug Administration for thermal ablation therapy, has been adapted for HT. With emerging clinical trials using MRgHIFU HT for radiosensitization, there is a pressing need to review the ultrasound HT technology. The objective of this review is to overview existing HT technology, summarize available ultrasound HT devices, evaluate clinical studies combining ultrasound HT with RT and discuss challenges and future directions.

Acoustic field characterization of a clinical magnetic resonance-guided high-intensity focused ultrasound system inside the magnet bore

PURPOSE

With the expanding clinical application of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), acoustic field characterization of MR-HIFU systems is needed for facilitating regulatory approval and ensuring consistent and safe power output of HIFU transducers. However, the established acoustic field measurement techniques typically use equipment that cannot be used in a magnetic resonance imaging (MRI) suite, thus posing a challenge to the development and execution of HIFU acoustic field characterization techniques. In this study, we developed and characterized a technique for HIFU acoustic field calibration within the MRI magnet bore, and validated the technique with standard hydrophone measurements outside of the MRI suite.

METHODS

A clinical Philips MR-HIFU system (Sonalleve V2, Philips, Vantaa, Finland) was used to assess the proposed technique. A fiber-optic hydrophone with a long fiber was inserted through a 24-gauge angiocatheter and fixed inside a water tank that was placed on the HIFU patient table above the acoustic window. The long fiber allowed the hydrophone control unit to be placed outside of the magnet room. The location of the fiber tip was traced on MR images, and the HIFU focal point was positioned at the fiber tip using the MR-HIFU therapy planning software. To perform acoustic field mapping inside the magnet, the HIFU focus was positioned relative to the fiber tip using an MRI-compatible 5-axis robotic transducer positioning system embedded in the HIFU patient table. To perform validation measurements of the acoustic fields, the HIFU table was moved out of the MRI suite, and a standard laboratory hydrophone measurement setup was used to perform acoustic field measurements outside the magnetic field.

RESULTS

The pressure field scans along and across the acoustic beam path obtained inside the MRI bore were in good agreement with those obtained outside of the MRI suite. At the HIFU focus with varying nominal acoustic powers of 10-500 W, the peak positive pressure and peak negative pressure measured inside the magnet bore were 3.87-68.67 MPa and 3.56-12.06 MPa, respectively, while outside the MRI suite the corresponding pressures were 3.27-67.32 MPa and 3.06-12.39 MPa, respectively. There was no statistically significant difference (P > 0.05) between measurements inside the magnet bore and outside the MRI suite for the p⁺ and p^- at any acoustic power level. The spatial-peak pulse-average intensities (I_SPPA ) for these powers were 312-17816 W/cm² and 220-15698 W/cm² for measurements inside and outside the magnet room, respectively. In addition, when the scanning step size of the HIFU focus was increased from 100 μm to 500 μm, the execution time for scanning a 4 × 4 mm² area decreased from 210 min to 10 min, the peak positive pressure decreased by 14%, the peak negative pressure decreased by 5%, and the lateral full width at half maximum dimension of pressure profiles increased from 1.15 mm to 1.55 mm.

CONCLUSIONS

The proposed hydrophone measurement technique offers a convenient and reliable method for characterizing the acoustic fields of clinical MR-HIFU systems inside the magnet bore. The technique was validated for use by measurements outside the MRI suite using a standard hydrophone calibration technique. This technique can be a useful tool in MR-HIFU quality assurance and acoustic field assessment.

Moderate-to-deep sedation technique, using propofol and ketamine, allowing synchronised breathing for magnetic resonance high-intensity focused ultrasound (MR-HIFU) treatment for uterine fibroids: a pilot study

Background

Magnetic resonance high-intensity focused ultrasound (MR-HIFU) treatment for uterine fibroids is rapidly gaining popularity as a treatment modality. This procedure is generally uncomfortable, painful, and requires minimal or absence of movement and an MR-HIFU synchronised breathing pattern of the patient. Procedural sedation and analgesia protocols have become the standard practice in interventional radiology departments worldwide. The aim of this study was to explore if a sedation regimen with low-dose propofol and ketamine performed by trained non-medical sedation practitioners could result in relief of discomfort for the patient and in adequate working conditions for MR-HIFU treatment for uterine fibroids.

Methods

In this study, conducted from August 2013 until November 2014, 20 patients were subjected to MR-HIFU treatment of uterine fibroids. Patients were deeply sedated using intravenous propofol and esketamine according to a standardised hospital protocol to allow synchronisation of the breathing pattern to the MR-HIFU. The quality of sedation for MR-HIFU and complications were recorded and analysed. The side effects of the sedation technique, the propofol and esketamine consumption rate, the duration of recovery, and patient satisfaction after 24 h were examined.

Results

A total of 20 female patients (mean age 42.4 [range 32–53] years) were enrolled. Mean propofol/esketamine dose was 1309 mg/39.5 mg (range 692–1970 mg/ 23.6–87.9 mg). Mean procedure time was 269 min (range 140–295 min). Application of the sedation protocol resulted in a regular breathing pattern, which could be synchronised with the MR-HIFU procedures without delay. The required treatment was completed in all cases. There were no major adverse events. Hypoxemia (oxygen desaturation <92%) and hallucinations were not observed.

Conclusions

The use of a specific combination of IV propofol and esketamine for procedural sedation and analgesia reduced the discomfort and pain during MR-guided HIFU treatments of uterine fibroids. The resulting regular breathing pattern allowed for easy synchronisation of the MR-HIFU procedure. Based on our results, esketamine and propofol sedation performed by trained non-medical sedation practitioners is feasible and safe, has a low risk of major adverse events, and has a short recovery time, avoiding a session of general anaesthesia.

Electronic supplementary material

The online version of this article (doi:10.1186/s40349-017-0088-9) contains supplementary material, which is available to authorized users.

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

Effective preclinical research is a vital component in the development of MRI-guided focused ultrasound (MRgFUS) and its translation to clinic. In this review, we seek to outline the challenges at hand for effective preclinical research, survey different solutions, and underline best practices. Furthermore, we summarize efforts to build and characterize dedicated preclinical MRgFUS equipment, including lab prototypes and available commercial products. Finally, we discuss constraints and considerations specific to using clinical MRgFUS equipment in preclinical research. Specifically, we examine additional hardware that has been used to adapt clinical MRgFUS equipment to better position, constrain, and image preclinical subjects, as well as software solutions that have been used to extend the potential and capabilities of clinical devices.

Pediatric Sarcomas Are Targetable by MR-Guided High Intensity Focused Ultrasound (MR-HIFU): Anatomical Distribution and Radiological Characteristics

BACKGROUND

Despite intensive therapy, children with metastatic and recurrent sarcoma or neuroblastoma have a poor prognosis. Magnetic resonance guided high intensity focused ultrasound (MR-HIFU) is a noninvasive technique allowing the delivery of targeted ultrasound energy under MR imaging guidance. MR-HIFU may be used to ablate tumors without ionizing radiation or target chemotherapy using hyperthermia. Here, we evaluated the anatomic locations of tumors to assess the technical feasibility of MR-HIFU therapy for children with solid tumors.

PROCEDURE

Patients with sarcoma or neuroblastoma with available cross-sectional imaging were studied. Tumors were classified based on the location and surrounding structures within the ultrasound beam path as (i) not targetable, (ii) completely or partially targetable with the currently available MR-HIFU system, and (iii) potentially targetable if a respiratory motion compensation technique was used.

RESULTS

Of the 121 patients with sarcoma and 61 patients with neuroblastoma, 64% and 25% of primary tumors were targetable at diagnosis, respectively. Less than 20% of metastases at diagnosis or relapse were targetable for both sarcoma and neuroblastoma. Most targetable lesions were located in extremities or in the pelvis. Respiratory motion compensation may increase the percentage of targetable tumors by 4% for sarcomas and 10% for neuroblastoma.

CONCLUSIONS

Many pediatric sarcomas are localized at diagnosis and are targetable by current MR-HIFU technology. Some children with neuroblastoma have bony tumors targetable by MR-HIFU at relapse, but few newly diagnosed children with neuroblastoma have tumors amenable to MR-HIFU therapy. Clinical trials of MR-HIFU should focus on patients with anatomically targetable tumors.

High-intensity focused ultrasound therapy in combination with gemcitabine for unresectable pancreatic carcinoma

Objective

To investigate the therapeutic effect and safety of high-intensity focused ultrasound (HIFU) therapy combined with gemcitabine in treating unresectable pancreatic carcinoma.

Methods

The 45 patients suffering from pancreatic carcinoma were randomized into two groups. The patients in the experimental group (n=23) received HIFU in combination with gemcitabine and those in the control group (n=22) received gemcitabine alone. The effect and clinical benefit rates in the two groups were compared. The median survival time and 6-month and 12-month survival rates were calculated by Kaplan–Meier method and log-rank test.

Results

The median survival time and 6-month survival rate were significantly higher in the experimental group than in the control group (8.91 months vs 5.53 months, 73.9% vs 40.9%, respectively P<0.05), but 12-month survival rate was not statistically different between the two groups (13.0% vs 4.5%, P>0.05). The clinical benefit rates in the experimental group and the control group were 69.6% and 36.3%, respectively (P<0.05). The pain remission rate in the experimental group was significantly higher than that in the control group (65.2% vs 31.8%, P<0.05).

Conclusion

HIFU in combination with gemcitabine is better than gemcitabine alone. This combinatorial therapy may become a better and effective treatment for unresectable pancreatic carcinoma.

T2-based temperature monitoring in abdominal fat during MR-guided focused ultrasound treatment of patients with uterine fibroids

Background

Near-field heating is a potential problem in focused ultrasound treatments, as it can result in thermal injury to skin, subcutaneous fat, and other tissues. Our goals were to determine if T2-based temperature mapping could be used reliably to measure near-field heating in adipose tissue and whether it is practical to perform such mapping during focused ultrasound treatments.

Methods

We investigated the dependence of T2 on temperature in ex vivo adipose tissue at 3T using a double-echo fast spin echo (FSE) sequence. We implemented and evaluated the T2-based temperature mapping technique in the adipose tissue of two healthy volunteers. Finally, we applied the technique during magnetic resonance-guided focused ultrasound (MRgFUS) treatments to measure near-field heating in eight patients with uterine fibroids.

Results

Calibration experiments in porcine adipose tissue determined a temperature coefficient of 6.16 ms/°C during heating and 5.37 ms/°C during cooling. The volunteer experiments demonstrated a strong correlation between the skin temperature and T2-based temperature measurements in the fat layer. During the treatments of patients with uterine fibroids, we observed a measurable change in the T2 of fat tissue within the path of the ultrasound beam and a temperature increase of up to 15 °C with sustained heating of more than 10 °C.

Conclusions

Our results demonstrate the feasibility and importance of monitoring near-field heating in fatty tissues. The implementation of near-field monitoring between sonications can shorten treatments by reducing the cooling time. It can help improve safety by avoiding excessive heating in the near field.