MR imaging-guided focused ultrasound surgery of uterine leiomyomas: a feasibility study

Early results of magnetic resonance-guided focused ultrasound surgery of adenomyosis: analysis of 20 cases

STUDY OBJECTIVE

To evaluate the thermal ablative effects of magnetic resonance-(MR) guided focused ultrasound surgery (MRgFUS) on adenomyosis and to assess improvement in clinical parameters.

DESIGN

Twenty patients with adenomyosis were treated with MRgFUS. Extensive adenomyosis (6 cases) was treated with 2 applications. Uterine volume was evaluated by MR imaging before and immediately after MRgFUS. Ablation of adenomyosis and the architecture of nonperfused areas were evaluated immediately after MRgFUS. Improvement in patient symptoms was assessed through the symptom severity score questionnaire (Canadian Task Force classification II-3).

SETTING

Department of gynecology at a Japanese general hospital.

PATIENTS

Premenopausal women at least 18 years of age with symptomatic adenomyosis.

INTERVENTIONS

Thermal ablation by MRgFUS.

MEASUREMENTS AND MAIN RESULTS

We classified the nonperfused lesions on contrast-enhanced MR images immediately after MRgFUS into 3 types: lesions with round margins (type R), serrated margins (type S), and honeycomb architecture (type H). Type R was the most common (16/20 patients). Most adenomyosis lesions could be sufficiently ablated close to the serosal surface or to the endometrium by MRgFUS. The mean uterine volume 6 months after therapy was decreased by 12.7%. Symptom severity score improved significantly during 6 months of follow-up. No serious complications were observed.

CONCLUSION

These early results indicate the safe and effective ablation of adenomyosis tissue by MRgFUS. The procedure also resulted in the improvement in clinical symptoms during the 6 months of follow-up.

Short-duration-focused ultrasound stimulation of Hsp70 expression in vivo

The development of transgenic reporter mice and advances in in vivo optical imaging have created unique opportunities to assess and analyze biological responses to thermal therapy directly in living tissues. Reporter mice incorporating the regulatory regions from the genes encoding the 70 kDa heat-shock proteins (Hsp70) and firefly luciferase (luc) as reporter genes can be used to non-invasively reveal gene activation in living tissues in response to thermal stress. High-intensity-focused ultrasound (HIFU) can deliver measured doses of acoustic energy to highly localized regions of tissue at intensities that are sufficient to stimulate Hsp70 expression. We report activation of Hsp70-luc expression using 1 s duration HIFU heating to stimulate gene expression in the skin of the transgenic reporter mouse. Hsp70 expression was tracked for 96 h following the application of 1.5 MHz continuous-wave ultrasound with spatial peak intensities ranging from 53 W cm(-2) up to 352 W cm(-2). The results indicated that peak Hsp70 expression is observed 6-48 h post-heating, with significant activity remaining at 96 h. Exposure durations were simulated using a finite-element model, and the predicted temperatures were found to be consistent with the observed Hsp70 expression patterns. Histological evaluation revealed that the thermal damage starts at the stratum corneum and extends deeper with increasing intensity. These results indicated that short-duration HIFU may be useful for inducing heat-shock expression, and that the period between treatments needs to be greater than 96 h due to the protective properties of Hsp70.

Treatment of esophageal tumors using high intensity intraluminal ultrasound: first clinical results

BACKGROUND

Esophageal tumors generally bear a poor prognosis. Radical surgery is generally the only curative method available but is not feasible in the majority of patients; palliative therapy with stent placement is generally performed. It has been demonstrated that High Intensity Ultrasound can induce rapid, complete and well-defined coagulation necrosis. Thus, for the treatment of esophageal tumors, we have designed an ultrasound applicator that uses an intraluminal approach to fill up this therapeutic gap.

METHODS

Thermal ablation is performed with water-cooled ultrasound transducers operating at a frequency of 10 MHz. Single lesions extend from the transducer surface up to 10 mm in depth when applying an intensity of 14 W/cm2 for 10s. A lumen inside the therapy applicator provides path for an endoscopic ultrasound imaging probe operating at a frequency of 12 MHz. The mechanical rotation of the applicator around its axis enables treatment of sectorial or cylindrical volumes. This method is thus particularly suitable for esophageal tumors that may develop only on a portion of the esophageal circumference. Previous experiments were conducted from bench to in vivo studies on pig esophagi.

RESULTS

Here we report clinical results obtained on four patients included in a pilot study. The treatment of esophageal tumors was performed under fluoroscopic guidance and ultrasound imaging. Objective tumor response was obtained in all cases and a complete necrosis of a tumor was obtained in one case. All patients recovered uneventfully and dysphagia improved significantly within 15 days, allowing for resuming a solid diet in three cases.

CONCLUSION

This clinical work demonstrated the efficacy of intraluminal high intensity ultrasound therapy for local tumor destruction in the esophagus.

Transcostal high-intensity-focused ultrasound: ex vivo adaptive focusing feasibility study

Ex vivo experiments have been conducted through excised pork rib with bone, cartilage, muscle and skin. The aberrating effect of the ribcage has been experimentally evaluated. Adaptive ultrasonic focusing through ribs has been studied at low power. Without any correction, the pressure fields in the focal plane were both affected by inhomogeneous attenuation and phase distortion and three main effects were observed: a mean 2 mm shift of the main lobe, a mean 1.25 mm spreading of the half width of the main lobe and up to 20 dB increase of the secondary lobe level. Thanks to time-reversal focusing, a 5 dB decrease in the secondary lobes was obtained and the ratio between the energy deposited at the target location and the total amount of energy emitted by the therapeutic array was six times higher than that without correction. Time-reversal minimizes the heating of the ribs by automatically sonicating between the ribs, as demonstrated by temperature measurements using thermocouples placed at different locations on the ribcage. It is also discussed how this aberration correction process could be achieved non-invasively for clinical application.

Current status and future potential of MRI-guided focused ultrasound surgery

The combination of the imaging abilities of magnetic resonance imaging (MRI) with the ability to delivery energy to targets deep in the body noninvasively with focused ultrasound presents a disruptive technology with the potential to significantly affect healthcare. MRI offers precise targeting, visualization, and quantification of temperature changes and the ability to immediately evaluate the treatment. By exploiting different mechanisms, focused ultrasound offers a range of therapies, ranging from thermal ablation to targeted drug delivery. This article reviews recent preclinical and tests clinical of this technology.

Method for target tracking in focused ultrasound surgery of liver using magnetic resonance filtered venography

The purpose of this work is to develop a magnetic resonance (MR) technique for guiding a focal point created in Focused Ultrasound Surgery (FUS) onto a specific target position in an abdominal organ, such as the liver, which moves and deforms with respiratory motion. The translational distance, rotational angles, and amount of expansion and contraction of the organ tissue were measured by obtaining the gravity points of the veins filtered from the sagittal, cine MR images of healthy livers during free breathing. Using the locations of the vessels at each time point, the target position at which the ultrasound focus was to be placed was estimated. In the volunteer experiments (N = 2), the lower limit of the spatial matrix dimension for delineating the veins was 128 x 128. The average displacement of the liver was 19.6 +/- 3.6 mm in superior-inferior (SI) direction and 3.1 +/- 1.4 mm in anterior-posterior (AP) direction. The deformations were 3.7 +/- 1.1 mm in SI direction and 3.0 +/- 1.2 mm in AP direction. The error between the actual and the estimated target point was 0.7 +/- 0.5 mm in SI direction, 0.6 +/- 0.4 mm in AP direction and 1.0 +/- 0.5 mm in distance, and less than 2.1 mm in all the trials. These results suggested that the proposed technique is sufficient for targeting the focus on a specific tissue location and for tracking the slice slab for thermometry to cover the region of focus.

[Long-term results after fibroid embolization]

Fibroids (leiomyoma) are the most frequent benign tumors of the uterus during female reproductive age. In the case of clinical symptoms, uterine artery embolization (UAE) is a well established treatment option for symptomatic fibroids, resulting in promising long-term results. In order to estimate long-term success after UAE, reduction of fibroids and uterus volume is less important than complete improvement and disappearance of fibroid-related symptoms. In addition to a high technical success rate and a low perioperative complication rate, UAE results in high short-term and long-term patient satisfaction (>87%) and improvement of symptoms (>81%). Candidates for UAE should be informed about potential reinterventions in terms of repeated UAE (up to 18% of cases) or surgical treatment options.

Magnetic resonance-guided focused ultrasound surgery

Uterine leiomyomas pose a significant health issue to reproductive-age women. Many women desire uterine conservation, and previously safe and efficacious therapies have been limited. Magnetic resonance-guided focused ultrasound surgery is a new noninvasive therapy that has been proven to be both safe and efficacious in the treatment of fibroids.

Magnetic resonance guided focused ultrasound surgery. Ablation of soft tissue at bone-muscle interface in a porcine model

BACKGROUND

Pain management treatments of patients with bone metastases have either efficacy problems or significant side effects. Percutaneous radiofrequency ablation has recently proved to be of palliative value. Magnetic resonance guided focused ultrasound surgery (MRgFUS) uses focused ultrasonic energy to non-invasively create a heat-coagulated lesion deep within the body in a controlled, accurate manner. The surgeon can monitor and control energy deposition in real time. This technology represents a potential treatment modality in oncological surgery. We investigated the ability of two MRgFUS methods to accurately and safely target and ablate soft tissue at its interface with bone.

MATERIALS AND METHODS

Heat-ablated lesions were created by MRgFUS at the bone-muscle interface of 15 pigs. Two different methods of energy delivery were used. Temperature rise at the target adjacent to bone was monitored by real time MR thermal images. Results were evaluated by MRI (magnetic resonance imaging), nuclear scanning and by histopathological evaluation.

RESULTS

Soft tissue lesion sizes by both methods were in the range of 1-2 cm in diameter. Targeting the focus 'behind' the bone, achieved the same result with a single sonication only. Follow up MRI and histopathological examination of all lesions showed focal damage at its interface with bone and localized damage to the outer cortex on the side closer to the targeted tissue. There was no damage to non-targeted tissue.

CONCLUSION

MRgFUS by both energy deposition methods can be used to produce controlled well-localized damage to soft tissue in close proximity to bone, with minimal collateral damage.

Feasibility of MR-temperature mapping of ultrasonic heating from a CMUT

In the last decade, high intensity focused ultrasound (HIFU) has gained popularity as a minimally invasive and noninvasive therapeutic tool for treatment of cancers, arrhythmias, and other medical conditions. HIFU therapy is often guided by magnetic resonance imaging (MRI), which provides anatomical images for therapeutic device placement, temperature maps for treatment guidance, and postoperative evaluation of the region of interest. While piezoelectric transducers are dominantly used for MR-guided HIFU, capacitive micromachined ultrasonic transducers (CMUTs) show competitive advantages, such as ease of fabrication, integration with electronics, improved efficiency, and reduction of self-heating. In this paper, we will show our first results of an unfocused CMUT transducer monitored by MR-temperature maps. This 2.51 mm by 2.32 mm, unfocused CMUT heated a HIFU phantom by 14 degrees C in 2.5 min. This temperature rise was successfully monitored by MR thermometry in a 3.0 T General Electric scanner.

Temperature mapping considerations in the breast with line scan echo planar spectroscopic imaging

A line-scan echo planar spectroscopic imaging (LSEPSI) sequence was used to serially acquire spectra from 4,096 voxels every 6.4 s throughout the breasts of nine female subjects in vivo. Data from the serial acquisitions were analyzed to determine the potential of the technique to characterize temperature changes using either the water frequency alone or the water-methylene frequency difference. Fluctuations of the apparent temperature change under these conditions of no heating were smallest using the water-methylene frequency difference, most probably due to a substantial reduction of motion effects both within and without the imaged plane. The approach offers considerable advantages over other methods for temperature change monitoring in the breast with magnetic resonance but suffers from some limitations, including the unavailability of lipid and water resonances in some voxels as well as a surprisingly large distribution of water-methylene frequency differences, which may preclude absolute temperature measurement.

MR-guided prostate interventions

In this article the current issues of diagnosis and detection of prostate cancer are reviewed. The limitations for current techniques are highlighted and some possible solutions with MR imaging and MR-guided biopsy approaches are reviewed. There are several different biopsy approaches under investigation. These include transperineal open magnet approaches to closed-bore 1.5T transrectal biopsies. The imaging, image processing, and tracking methods are also discussed. In the arena of therapy, MR guidance has been used in conjunction with radiation methods, either brachytherapy or external delivery. The principles of the radiation treatment, the toxicities, and use of images are outlined. The future role of imaging and image-guided interventions lie with providing a noninvasive surrogate for cancer surveillance or monitoring treatment response. The shift to minimally invasive focal therapies has already begun and will be very exciting when MR-guided focused ultrasound surgery reaches its full potential.

MR imaging-guided interventions in the genitourinary tract: an evolving concept

MR imaging-guided interventions are well established in routine patient care in many parts of the world. There are many approaches, depending on magnet design and clinical need, based on MR imaging providing excellent inherent tissue contrast without ionizing radiation risk for patients. MR imaging-guided minimally invasive therapeutic procedures have advantages over conventional surgical procedures. In the genitourinary tract, MR imaging guidance has a role in tumor detection, localization, and staging and can provide accurate image guidance for minimally invasive procedures. The advent of molecular and metabolic imaging and use of higher strength magnets likely will improve diagnostic accuracy and allow targeted therapy to maximize disease control and minimize side effects.

A rapid three-dimensional high intensity focused ultrasound beam plotting

The goal of this study was to develop a rapid three-dimensional High Intensity Focused Ultrasound (HIFU) beam plotting system for visualizing the acoustic energy distribution, achieved using a three-dimensional positioning system and data acquisition hardware. Beam plots of a 1.1MHz HIFU transducer were performed to evaluate the capability of the proposed system. A three-dimensional beam plotting was completed in only about 30 minutes and covered a predetermined volume of 5mm x 20mm x 5mm with a spatial resolution of 0.2mm x 0.001mm x 0.2mm. The FWHM (Full Width at Half Maximum) dimensions of the ultrasound field on axial and lateral directions were measured at 12.51mm and 1.4mm respectively. The proposed system can not only perform 2-dimensional beam plots but also can aid in the visualization of a three-dimensional ultrasound beam pattern volume within a relatively short scanning time.

Mid-term outcome of magnetic resonance-guided focused ultrasound surgery for uterine myomas: from six to twelve months after volume reduction

STUDY OBJECTIVE

To clarify the volume change ratio of uterine myomas treated with magnetic resonance-guided focused ultrasound surgery (MRgFUS), in relation to the signal intensity of T(2)-weighted magnetic resonance (MR) images.

DESIGN

Prospective study (Canadian Task Force classification II-3).

SETTING

Department of Gynecology, Shinsuma General Hospital, Kobe, Japan.

PATIENTS

Forty-eight myomas in 35 patients were followed up with MR images 6 months after MRgFUS, and 23 myomas in 17 patients were followed up 12 months after MRgFUS. Before treatment, the myomas were classified into 3 types on the basis of the signal intensity of T(2)-weighted MR images as follows: Type 1, low intensity; type 2, intermediate intensity; type 3, high intensity.

INTERVENTIONS

Thermal ablation therapy was performed with an MRgFUS system (ExAblate 2000).

MEASUREMENTS AND MAIN RESULTS

MRgFUS produced a greater volume reduction in type 1 and type 2 myomas than in type 3 myomas. Nonperfused areas always diminished in the period after MRgFUS; however, the volume change was affected by the volume change ratio of perfused areas inside the treated myomas.

CONCLUSION

At present, type 3 myomas should be exempted from the application of MRgFUS, because the nonperfused ratio immediately after the procedure was small compared with that in type 1 and type 2 myomas, and the subsequent volume change was unfavorable.

MRgFUS for pain relief as palliative treatment in recurrent cervical carcinoma: a case report

INTRODUCTION

Focused ultrasound under real-time MR guidance and control (MRgFUS) can be used for the thermal ablation of tissue. Currently this technique is used clinically for the noninvasive treatment of uterine leiomyomas and is in clinical evaluation for breast cancer, adenomyosis and other indications. MRgFUS is being tested for pain relief in patients suffering from bone metastases. This is the first case to report on MRgFUS for pain relief in patients suffering from recurrent cervical carcinoma.

CASE REPORT

A 29-year-old patient with recurrent squamous cell carcinoma of cervix following radical hysterectomy, chemotherapy and radiation was treated by MRgFUS due to pelvic mass unresponsive to conventional treatment that caused intractable pain. Following two treatments the patient experienced a marked reduction in pain and increase in Karnovsky Performance Status (KPS) from 50% to 80%.

DISCUSSION

Palliative treatment of pain with noninvasive MRgFUS in cases of recurrent cervical carcinoma may be a safe and efficient alternative to other invasive techniques.

Spatio-temporal control of gene expression and cancer treatment using magnetic resonance imaging-guided focused ultrasound

Local temperature elevation may be used for tumor ablation, gene expression, drug activation, and gene and/or drug delivery. High-intensity focused ultrasound (HIFU) is the only clinically viable technology that can be used to achieve a local temperature increase deep inside the human body in a noninvasive way. Magnetic resonance imaging (MRI) guidance of the procedure allows in situ target definition and identification of nearby healthy tissue to be spared. In addition, MRI can be used to provide continuous temperature mapping during HIFU for spatial and temporal control of the heating procedure and prediction of the final lesion based on the received thermal dose. The primary purpose of the development of MRI-guided HIFU was to achieve safe noninvasive tissue ablation. The technique has been tested extensively in preclinical studies and is now accepted in the clinic for ablation of uterine fibroids. MRI-guided HIFU for ablation shows conceptual similarities with radiation therapy. However, thermal damage generally shows threshold-like behavior, with necrosis above the critical thermal dose and full recovery below. MRI-guided HIFU is being clinically evaluated in the cancer field. The technology also shows great promise for a variety of advanced therapeutic methods, such as gene therapy. MR-guided HIFU, together with the use of a temperature-sensitive promoter, provides local, physical, and spatio-temporal control of transgene expression. Specially designed contrast agents, together with the combined use of MRI and ultrasound, may be used for local gene and drug delivery.

High-intensity focused ultrasound for the targeted destruction of uterine tissues: experiences from a pilot study using a mobile HIFU unit

BACKGROUND

High intensity focused ultrasound (HIFU) is a novel method which offers the non-invasive ablation of tissues without harming overlying organs or skin. It has been introduced successfully in urology for the ablation of prostatic hyperplasia and seems to be promising in the treatment of uterine fibroids. In this study we aimed to examine the feasibility and possible side effects of HIFU treatment of uterine tissues using an experimental mobile HIFU unit with ultrasound guidance.

METHODS

For these experiments, a 1.07 MHz ultrasound source was used which allows treatment depths between 0 and 10 cm. In 12 patients scheduled to have abdominal hysterectomy, 5-60 impulses of HIFU were applied through the intact skin upon uterine tissues directly prior to the surgical procedure. Tissue intensities lay between 3,200 and 6,300 W/cm(2) and a fixed pulse length of 4 s was used.

RESULTS

No side effects were encountered other than one first-degree skin burn and the treatment was well tolerated. Histology showed clearly demarcated coagulative necrosis in the targeted tissues. Treatment was concluded in less than 45 min for each patient.

CONCLUSION

Focused ultrasound is an effective method to selectively destroy tissue within the uterus and the transabdominal access route is very feasible. This study shows that a mobile ultrasound source can be used safely and effectively to destroy uterine tissues, such as fibroids, without major side effects.

Sustained Relief of Leiomyoma Symptoms by Using Focused Ultrasound Surgery

OBJECTIVE

To assess several measures of the long-term outcome of magnetic resonance-guided focused ultrasound surgery for symptomatic uterine leiomyomata.

METHODS

Data on 359 women completing 24-month follow-up in all clinical trials of magnetic resonance-guided focused ultrasound surgery for uterine leiomyomata were analyzed. Quality of life outcomes, measured by the symptom severity score of the Uterine Fibroid Symptoms Quality Of Life Questionnaire were assessed for 24 months after treatment. Clinical endpoints, including uterine shrinkage, the need for additional leiomyoma treatment, and the time to additional leiomyoma treatment, were all assessed. The nonperfused volume ratio after treatment, calculated from the gadolinium-enhanced magnetic resonance imaging after treatment and the best measure of tissue necrosis after treatment, was used to assess outcome based on completeness of leiomyoma ablation.

RESULTS

Women undergoing magnetic resonance-guided focused ultrasound surgery for symptomatic uterine leiomyomata have durable symptom relief, as measured by the symptom severity score at 24 months, with significantly greater improvement with more complete ablation (P<.001). Survival analysis demonstrates a significant reduction in the percentage of women undergoing additional leiomyoma treatment (P=.001) in women in the high nonperfused volume group. The mean shrinkage and mean residual nonperfused volume ratio are both significantly above zero at 6 months in the high nonperfused volume group (P<.001). The incidence of adverse events is low. However, for women with minimal treatment, the risk of additional procedures is high.

CONCLUSION

Magnetic resonance-guided focused ultrasound surgery is an effective treatment for uterine leiomyomata and results in sustained symptomatic relief.

LEVEL OF EVIDENCE

III.

A fast and conformal heating scheme for producing large thermal lesions using a 2D ultrasound phased array

The treatment conformability and the total treatment time of large tumors are both important issues in ultrasound thermal therapy. Previous heating strategies all show their restrictions in achieving these two issues to satisfactory levels simultaneously. This work theoretically presents a new heating strategy which is capable of both increasing the treatment conformability and shortening the treatment time, when using a 2D ultrasound phased array transducer. To perform this, a set of the multiple-foci patterns (considered the basic heating units) were temporally switched to steer the beam at different focal planes with the lesion length being well-controlled. Then, to conformally cover an irregular target volume, the 2D phased array was laterally shifted by a positioning system to deposit a suitable heating unit to cover a subvolume part. Results demonstrated that the totally treatment time can be largely reduced. The heating rate can be increased up to 0.96 cm3/min compared to the previously reported 0.26 cm3/min. Also, the proposed scheme showed that the tumor regions can be completely treated with the normal tissue damage at satisfactory level. The feasibility of the proposed strategy for irregular tumor treatment was also demonstrated. This study offers useful information in large tumor treatment in ultrasound thermal therapy.

Thermal therapy for breast tumors by using a cylindrical ultrasound phased array with multifocus pattern scanning: a preliminary numerical study

The purpose of this study is to investigate the feasibility of using a 1 MHz cylindrical ultrasound phased array with multifocus pattern scanning to produce uniform heating for breast tumor thermal therapy. The breast was submerged in water and surrounded by the cylindrical ultrasound phased array. A multifocus pattern was generated and electrically scanned by the phased array to enlarge the treatment lesion in single heating. To prevent overheating normal tissues, a large planning target volume (PTV) would be divided into several planes with several subunits on each plane and sequentially treated with a cooling phase between two successive heatings of the subunit. Heating results for different target temperatures (T(tgt)), blood perfusion rates and sizes of the PTV have been studied. Furthermore, a superficial breast tumor with different water temperatures was also studied. Results indicated that a higher target temperature would produce a slightly larger thermal lesion, and a higher blood perfusion rate would not affect the heating lesion size but increase the heating time significantly. The acoustic power deposition and temperature elevations in ribs can be minimized by orienting the acoustic beam from the ultrasound phased array approximately parallel to the ribs. In addition, a large acoustic window on the convex-shaped breast surface for the proposed ultrasound phased array and the cooling effect of water would prevent the skin overheating for the production of a lesion at any desired location. This study demonstrated that the proposed cylindrical ultrasound phased array can provide effective heating for breast tumor thermal therapy without overheating the skin and ribs within a reasonable treatment time.

Interstitial devices for minimally invasive thermal ablation by high-intensity ultrasound

Interstitial ultrasound applicators have been proposed for treating deep-seated tumours that cannot be reached with extra-corporeal high-intensity focused ultrasound. In addition, interstitial ultrasound offers several advantages compared with conventional ablation technology (radiofrequency, microwaves, cryotherapy) in terms of penetration, speed of coagulation, ability to direct and control the thermal lesion and compatibility with image monitoring. The ultrasound source is brought as close as possible to the target in order to minimize the effects of attenuation and phase aberration along the ultrasound pathway. The present paper is a review of the interstitial applicators that were described during the last decade in the literature. It is presented in three sections. The technical aspects common to all applicators are first described. For example, most-described applicators are sideview applicators whose active element is water-cooled and operates at rather high frequency (above 3 MHz) in order to promote heating. Then the different potential techniques for monitoring treatment administered by the interstitial route are presented and illustrated through a review of image-guided interstitial thermal ablation. Three major techniques of imaging are used for guiding interstitial treatment: MRI, ultrasound and fluoroscopy. The third section goes in to further detail on diverse described medical applications.

Treatment monitoring and thermometry for therapeutic focused ultrasound

Therapeutic ultrasound is currently enjoying increasingly widespread clinical use especially for the treatment of cancer of the prostate, liver, kidney, breast, pancreas and bone, as well as for the treatment of uterine fibroids. The optimum method of treatment delivery varies between anatomical sites, but in all cases monitoring of the treatment is crucial if extensive clinical acceptance is to be achieved. Monitoring not only provides the operating clinician with information relating to the effectiveness of treatment, but can also provide an early alert to the onset of adverse effects in normal tissue. This paper reviews invasive and non-invasive monitoring methods that have been applied to assess the extent of treatment during the delivery of therapeutic ultrasound in the laboratory and clinic (follow-up after treatment is not reviewed in detail). The monitoring of temperature and, importantly, the way in which this measurement can be used to estimate the delivered thermal dose, is dealt with as a separate special case. Already therapeutic ultrasound has reached a stage of development where it is possible to attempt real-time feedback during exposure in order to optimize each and every delivery of ultrasound energy. To date, data from MR imaging have shown better agreement with the size of regions of damage than those from diagnostic ultrasound, but novel ultrasonic techniques may redress this balance. Whilst MR currently offers the best method for non-invasive temperature measurement, the ultrasound techniques under development, which could potentially offer more rapid visualisation of results, are discussed.

A novel strategy to increase heating efficiency in a split-focus ultrasound phased array

Focus splitting using sector-based phased arrays increases the necrosed volume in a single sonication and reduces the total treatment time in the treatment of large tumors. However, split-focus sonication results in a lower energy density and worse focal-beam distortion, which limits its usefulness in practical treatments. Here, we propose a new heating strategy involving consecutive strongly focused and split-focus sonications to improve the heating efficiency. Theoretical predictions including linear and thermal-dose-dependent attenuation change were employed to investigate potential factors of this strategy, and ex vivo tissue experiments were conducted to confirm its effectiveness. Results showed that the thermal lesions produced by the proposed strategy could be increased when comparing with the previous reported strategies. The proposed heating strategy also induces a thermal lesion more rapidly, and exhibits higher robustness to various blood perfusion conditions, higher robustness to various power/heating time combinations, and superiority to generate deep-seated lesions through tissues with complex interfaces. Possible mechanisms include the optimization of the thermal conduction created by the strongly focused sonication and the temperature buildup gained from thermally induced tissue attenuation change based on the theoretical analysis. This may represent a useful technique for increasing the applicability of split-focus and multiple-focus sonication techniques, and solve the obstacles encountered when attempting to use these methods to shorten the total clinical treatment time.

Targeted drug delivery to the brain using focused ultrasound

Drug delivery to the brain remains a challenging field. The presence of a physiological barrier, the blood-brain barrier (BBB), complicates the delivery of drugs to the brain. Although several methods have been developed for drug delivery to the brain, they have problems such as being invasive or lacking in target specificity. On the other hand, ultrasound has emerged as a treatment method and a diagnostic technology. Several studies have shown the feasibility of using ultrasound for the localized and reversible disruption of the BBB. In this review, I would like to review the recent advancement of ultrasound-induced MRI-guided BBB disruption technique and other methods for delivering drugs to the brain.

Magnetic resonance-guided high-intensity ultrasound ablation of the prostate

OBJECTIVES

This paper describes our work in developing techniques and devices for magnetic resonance (MR)-guided high-intensity ultrasound ablation of the prostate and includes review of relevant literature.

METHODS

Catheter-based high-intensity ultrasound applicators, in interstitial and transurethral configurations, were developed to be used under MR guidance. Magnetic resonance thermometry and the relevant characteristics and artifacts were evaluated during in vivo thermal ablation of the prostate in 10 animals. Contrast-enhanced MR imaging (MRI) and diffusion-weighted MRI were used to assess tissue damage and compared with histology.

RESULTS

During evaluation of these applicators, MR thermometry was used to monitor the temperature distributions in the prostate in real time. Magnetic resonance-derived maximum temperature thresholds of 52 degrees C and thermal dose thresholds of 240 minutes were used to control the extent of treatment and qualitatively correlated well with posttreatment imaging studies and histology. The directional transurethral devices are selective in their ability to target well-defined regions of the prostate gland and can be rotated in discrete steps to conform treatment to prescribed boundaries. The curvilinear applicator is the most precise of these directional techniques. Multisectored transurethral applicators, with dynamic angular control of heating and no rotation requirements, offer a fast and less complex means of treatment with less selective contouring.

CONCLUSIONS

The catheter-based ultrasound devices can produce spatially selective regions of thermal destruction in prostate. The MR thermal imaging and thermal dose maps, obtained in multiple slices through the target volume, are useful for controlling therapy delivery (rotation, power levels, duration). Contrast-enhanced T1-weighted MRI and diffusion-weighted imaging are useful tools for assessing treatment.

A review of magnetic resonance imaging-guided focused ultrasound surgery of uterine fibroids

Uterine fibroids are a significant cause of morbidity for women of reproductive age. Over the past decade, minimally invasive treatment options are becoming increasingly popular. A new, Food and Drug Administration-approved noninvasive treatment option is magnetic resonance-guided focused ultrasound surgery, which has the potential to become a treatment of choice for selected patients. We review the technical aspects of the procedure of magnetic resonance-guided focused ultrasound surgery for treatment of uterine fibroids, potential difficulties with treatment planning, and clinical trial results to date. We also describe current developments in treatment imaging and treatment optimization.

Uterine Leiomyomas: MR Imaging–guided Focused Ultrasound Surgery—Results of Different Treatment Protocols1

PURPOSE

To prospectively assess patient response (after 12 months) to magnetic resonance (MR) imaging-guided focused ultrasound surgery in treatment of uterine leiomyomas by using two treatment protocols.

MATERIALS AND METHODS

This prospective clinical trial was approved by institutional review boards and was HIPAA compliant. After giving informed consent, patients with symptomatic leiomyomas were consecutively enrolled and treated at one of five U.S. centers by using an original or a modified protocol. Outcomes were assessed with the symptom severity score (SSS) obtained at baseline and 3, 6, and 12 months after treatment. Adverse events (AEs) were recorded. Statistical analysis included Student t test, Fisher exact test, analysis of covariance, Spearman correlation, and logistic regression.

RESULTS

One hundred sixty patients had a mean SSS of 62.1 +/- 16.3 (standard deviation) at baseline, which decreased to 35.5 +/- 19.5 at 3 months (P<.001) and to 32.3 +/- 19.8 at 6 months (P<.001) and was 32.7 +/- 21.0 at 12 months (P<.001). Ninety-six patients (mean age, 46.0 years +/- 4.6) were treated with an original protocol, and 64 (mean age, 45.9 years +/- 3.9) were treated with a modified protocol. Patients in the modified group had a significantly greater SSS decrease at 3 months (P=.037) than those in the original group, and 73% of those in the original group and 91% of those in the modified group reported a significant decrease in SSS (of 10 points or greater) at 12 months. No serious AEs were recorded. Fewer AEs were reported in the modified group than in the original group (25% vs 13% reporting no event). Of evaluable patients, fewer in the modified group chose alternative treatment (28%) than in the original group (37%).

CONCLUSION

MR imaging-guided focused ultrasound surgery results in symptomatic improvement, sustained to 12 months after treatment. Treatment with a modified protocol results in greater clinical effectiveness and fewer AEs.

Key factors that affect sonoporation efficiency in in vitro settings: the importance of standing wave in sonoporation

Ultrasound-induced intracellular drug delivery, sonoporation, is an appealing and promising technique for next generation drug delivery system. Many types of molecules, such as plasmid DNAs, siRNAs and peptides, have been demonstrated to be delivered into the cell by ultrasound with the aid of microbubbles both in vitro and in vivo. Although there are many reports on in vitro sonoporation, the efficiency of successful sonoporation and the viabilities of cells after the procedure documented in each report vary in a wide range, and the reasons for these differences are not fully understood. In this study, we have investigated how different experimental settings would affect sonoporation efficiency and cell viabilities after the procedure. Our results show that the fashion of cell culture (e.g. in suspension or in monolayer culture) and the presence of standing wave have a great impact on the overall results. These results indicate that in vitro sonoporation settings should be carefully evaluated in each experiment. The fact that standing wave is necessary to achieve high sonoporation efficiency may be a problematic issue for clinical application of sonoporation, as it may be difficult (although not impossible) to create standing wave in a human body.

MR thermometry for monitoring tumor ablation

Local thermal therapies are increasingly used in the clinic for tissue ablation. During energy deposition, the actual tissue temperature is difficult to estimate since physiological processes may modify local heat conduction and energy absorption. Blood flow may increase during temperature increase and thus change heat conduction. In order to improve the therapeutic efficiency and the safety of the intervention, mapping of temperature and thermal dose appear to offer the best strategy to optimize such interventions and to provide therapy endpoints. MRI can be used to monitor local temperature changes during thermal therapies. On-line availability of dynamic temperature mapping allows prediction of tissue death during the intervention based on semi-empirical thermal dose calculations. Much progress has been made recently in MR thermometry research, and some applications are appearing in the clinic. In this paper, the principles of MRI temperature mapping are described with special emphasis on methods employing the temperature dependency of the water proton resonance frequency. Then, the prospects and requirements for widespread applications of MR thermometry in the clinic are evaluated.

Conformal thermal therapy using planar ultrasound transducers and adaptive closed-loop MR temperature control: demonstration in gel phantoms andex vivotissues

MRI-guided transurethral ultrasound therapy offers a minimally invasive approach for the treatment of localized prostate cancer. Integrating a multi-element planar transducer with active MR temperature feedback can enable three-dimensional conformal thermal therapy of a target region within the prostate gland while sparing surrounding normal tissues. Continuous measurement of the temperature distribution in tissue enables dynamic compensation for unknown changes in blood flow and tissue properties during treatment. The main goal of this study was to evaluate the feasibility of using active temperature feedback on a clinical 1.5 T MR imager for conformal thermal therapy. MR thermometry was performed during heating in both gel phantoms and excised tissue with a transurethral heating applicator, and the rotation rate and power were varied based on the thermal measurements. The capability to produce a region of thermal damage that matched a target boundary was evaluated. The influence of a cooling gradient (to simulate cooling of the rectum or urethra) on the desired pattern of thermal damage was also investigated in gel phantoms. Results showed high correlation between the desired target boundary and the 55 degrees C isotherm generated during heating with an average distance error of 0.9 mm +/- 0.4 mm (n = 6) in turkey breasts, 1.4 mm +/- 0.6 mm (n = 4) in gel phantoms without rectal cooling and 1.4 mm +/- 0.6 mm (n = 3) in gel phantoms with rectal cooling. The results were obtained using a temporal update rate of 5 s, a spatial resolution of 3 x 3 x 10 mm for the control point, and a temperature uncertainty of approximately 1 degrees C. The performance of the control algorithm under these conditions was comparable to that of simulations conducted previously by our group. Overall, the feasibility of generating targeted regions of thermal damage with a transurethral heating applicator and active MR temperature feedback has been demonstrated experimentally. This method of treatment appears capable of accounting for unpredictable and varying tissue properties during the treatment.

Thermal ablation of uterine fibroids using MR-guided focused ultrasound-a truly non-invasive treatment modality

Uterine fibroids are a significant source of morbidity for women of reproductive age. Definitive treatment has traditionally been a hysterectomy, but increasingly women are not prepared to undergo such an invasive procedure for a benign and usually self-limiting condition. Although a number of minimally invasive techniques are now available, focused ultrasound has a considerable advantage over them as it is completely non-invasive and does not require an anaesthetic. Improvements in imaging techniques, particularly magnetic resonance imaging (MRI), have enabled the accurate planning, targeting and monitoring of treatments. We review the early experience of focused ultrasound surgery for the treatment of fibroids, and, in particular, the results of the recent phase I, II and III multi-centre clinical trials. These trials and other studies which demonstrate that MR-guided focused ultrasound ablation is feasible, safe and appears to have an efficacy that is comparable with other treatment modalities are described. This technique has the advantages of being non-invasive and being deliverable as an out-patient procedure.

Future directions in MR imaging of the female pelvis

New technology continues to change the field of MR imaging. This article describes select areas of technical development that are likely to have an increasing clinical impact on MR imaging of the female pelvis, including high-field imaging, parallel imaging, contrast agents, diffusion-weighted imaging and spectroscopy, and MR-guided intervention.

Real-time adaptive methods for treatment of mobile organs by MRI-controlled high-intensity focused ultrasound

Focused ultrasound (US) is a unique and noninvasive technique for local deposition of thermal energy deep inside the body. MRI guidance offers the additional benefits of excellent target visualization and continuous temperature mapping. However, treating a moving target poses severe problems because 1) motion-related thermometry artifacts must be corrected, 2) the US focal point must be relocated according to the target displacement. In this paper a complete MRI-compatible, high-intensity focused US (HIFU) system is described together with adaptive methods that allow continuous MR thermometry and therapeutic US with real-time tracking of a moving target, online motion correction of the thermometry maps, and regional temperature control based on the proportional, integral, and derivative method. The hardware is based on a 256-element phased-array transducer with rapid electronic displacement of the focal point. The exact location of the target during US firing is anticipated using automatic analysis of periodic motions. The methods were tested with moving phantoms undergoing either rigid body or elastic periodical motions. The results show accurate tracking of the focal point. Focal and regional temperature control is demonstrated with a performance similar to that obtained with stationary phantoms.

MAGNETIC RESONANCE IMAGING-GUIDED FOCUSED ULTRASOUND FOR THERMAL ABLATION IN THE BRAIN

INTRODUCTION

Magnetic resonance imaging (MRI)-guided focused ultrasound is a novel technique that was developed to enable precise, image-guided targeting and destruction of tumors by thermocoagulation. The system, ExAblate2000, is a focused ultrasound delivery system embedded within the MRI bed of a conventional diagnostic MRI scanner. The device delivers small volumetric sonications from an ultrasound phased array transmitter that converge energy to selectively destroy the target. Temperature maps generated by the MRI scanner verify the location and thermal rise as feedback, as well as thermal destruction. To assess the safety, feasibility, and precision of this technology in the brain, we have used the ExAblate system to create predefined thermal lesions in the brains of pigs.

METHODS

Ten pigs underwent bilateral craniectomy to provide a bone window for the ultrasound beams. Seven to 10 days later, the animals were anesthetized and positioned in the ExAblate system. A predefined, 1-cm frontal para ventricular region was delineated as the target and treated with multiple sonications. MRI was performed immediately and 1 week after treatment. The animals were then sacrificed and the brains removed for pathological study. The size of individual sonication points and the location of the lesion were compared between the planned dose maps, posttreatment MRI scans, and pathological specimen.

RESULTS

High-energy sonications led to precise coagulation necrosis of the specified targets as shown by subsequent MRI, macroscopic, and histological analysis. The thermal lesions were sharply demarcated from the surrounding brain with no anatomic or histological abnormalities outside the target.

CONCLUSION

MRI-guided focused ultrasound proved a precise and an effective means to destroy anatomically predefined brain targets by thermocoagulation with minimal associated edema or damage to adjacent structures. Contrast-enhanced T1-, T2-, and diffusion-weighted MRI scans may be used for real-time assessment of tissue destruction.

A case of hepatocellular carcinoma treated by MR-guided focused ultrasound ablation with respiratory gating

Focused ultrasound surgery (FUS) is a method of noninvasive focal thermal ablation. Temperature-sensitive phase-difference magnetic resonance (MR) imaging allows monitoring of the focal point and measurement of tissue temperature elevation in real time, ensuring delivery of a therapeutic dose. A newly developed respiratory monitoring system enables us to track liver tumors, which move with respiration. We report our initial experience using MR-guided FUS with respiratory gating in successfully treating a hepatocellular carcinoma 15 mm in diameter.

Blood flow occlusion via ultrasound image-guided high-intensity focused ultrasound and its effect on tissue perfusion

This study investigated the induction of tissue necrosis by arterial blood flow occlusion using ultrasound image-guided high-intensity focused ultrasound (HIFU). We constructed a prototype HIFU transducer in combination with an imaging probe that provided color Doppler imaging and ultrasound contrast imaging. The HIFU beam was aimed into a branch of the renal artery in vivo. The renal artery branches of eight rabbits were occluded by HIFU at an intensity of 4 kW/cm(2) (from 2 to 10 times of each sonication for 5 s). When the HIFU exposure was successful, complete cessation of blood flow was observed by color Doppler imaging with success rate of 100% (8/8). Furthermore, lack of perfusion was observed in the renal cortex with a contrast-enhanced image. Postmortem histologic evaluation showed a wedge-shaped area of infarction in six of seven cases, corresponding to the lack of the contrast medium in the ultrasound image. These results demonstrated that ultrasound image-guided HIFU can be used to induce arterial occlusion, thus producing infarction and necrosis of the perfused tissue.

High intensity focused ultrasound: physical principles and devices

High intensity focused ultrasound (HIFU) is gaining rapid clinical acceptance as a treatment modality enabling non-invasive tissue heating and ablation for numerous applications. HIFU treatments are usually carried out in a single session, often as a day case procedure, with the patient either fully conscious, lightly sedated or under light general anaesthesia. A major advantage of HIFU over other thermal ablation techniques is that there is no necessity for the transcutaneous insertion of probes into the target tissue. The high powered focused beams employed are generated from sources placed either outside the body (for treatment of tumours of the liver, kidney, breast, uterus, pancreas and bone) or in the rectum (for treatment of the prostate), and are designed to enable rapid heating of a target tissue volume, while leaving tissue in the ultrasound propagation path relatively unaffected. Given the wide-ranging applicability of HIFU, numerous extra-corporeal, transrectal and interstitial devices have been designed to optimise application-specific treatment delivery. Their principle of operation is described here, alongside an overview of the physical mechanisms governing HIFU propagation and HIFU-induced heating. Present methods of characterising HIFU fields and of quantifying HIFU exposure and its associated effects are also addressed.

Magnetic Resonance–Guided Focused Ultrasound Surgery for the Noninvasive Curative Ablation of Tumors and Palliative Treatments: A Review

This article reviews and discusses the up-to-date data on and feasibility of focused ultrasound surgery. This technique uses high-energy ultrasound beams that can be directed to penetrate through the skin and various soft tissues, focus on the target, and destroy tumors by increasing the temperature at the targeted tissue volume. The boundaries of the treatment area are sharply demarcated (focused) without causing damage to the surrounding organs. Although the idea of using sound waves to ablate tumors was first demonstrated in the 1940 s, only recent developments have enabled this technology to become more controlled and, hence, more feasible. The major breakthrough toward its clinical use came with coupling the thermal ablative process to advanced imaging. The development of magnetic resonance as the foundation to guide and evaluate the end results of focused ultrasound surgery treatment, the image guidance of the ultrasound beam, and the development of a reliable method for tissue temperature measurement and real-time feedback of the extent of tissue destruction have pushed this novel technology forward in oncological practice.

Ultrasound interstitial thermal therapy (USITT) for the treatment of uterine myomas

Uterine myomas (fibroids) are the most common pelvic tumors occurring in women, and are the leading cause of hysterectomy. Symptoms can be severe, and traditional treatments involve either surgical removal of the uterus (hysterectomy), or the fibroids (myomectomy). Interstitial ultrasound technologies have demonstrated potential for hyperthermia and high temperature thermal therapy in the treatment of benign and malignant tumors. These ultrasound devices offer favorable energy penetration allowing large volumes of tissue to be treated in short periods of time, as well as axial and angular control of heating to conform thermal treatment to a targeted tissue, while protecting surrounding tissues from thermal damage. The goal of this project is to evaluate interstitial ultrasound for controlled thermal coagulation of fibroids. Multi-element applicators were fabricated using tubular transducers, some of which were sectored to produce 180° directional heating patterns, and integrated with water cooling. Human uterine fibroids were obtained after routine myomectomies, and instrumented with thermocouples spaced at 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 cm from the applicator. Power levels ranging from 8-15 W per element were applied for up to 15 minute heating periods. Results demonstrated that therapeutic temperatures >50° C and cytotoxic thermal doses (t43) extended beyond 2 cm radially from the applicator (>4 cm diameter). It is anticipated that this system will make a significant contribution toward the treatment of uterine fibroids.

High-intensity focused ultrasound principles, current uses, and potential for the future

High-intensity focused ultrasound (HIFU) continues to be a very attractive option for minimally invasive procedures. Using well-established principles, this ablative therapy can be used to treat a number of benign and malignant diseases with few side effects. During the last 15 years, there has been an enormous amount of work, both laboratory based and in the form of clinical trials, aimed at developing devices that can deliver treatments with safe and effective outcomes. In this article, we aim to outline the principles of HIFU, describe the current commercially available machines and their applications, and discuss the role of HIFU in the future.

Magnetic resonance-guided focused ultrasound surgery for uterine fibroids: relationship between the therapeutic effects and signal intensity of preexisting T2-weighted magnetic resonance images

OBJECTIVE

This study was undertaken to clarify the relationship between the signal intensity of T2-weighted magnetic resonance images and the therapeutic effect of magnetic resonance-guided focused ultrasound surgery (MRgFUS) on uterine fibroids.

STUDY DESIGN

Ninety-five fibroids in 63 patients were classified into 3 types based on the signal intensity of T2-weighted magnetic resonance images as follows: type 1, low intensity; type 2, intermediate intensity; type 3, high intensity. The treated area ratio of MRgFUS and the volume reduction ratio 6 months after treatment were used as the indices of therapeutic effect.

RESULTS

The treated area ratio of type 3 fibroids was the lowest among the 3 types (P < .01). The volume reduction ratio correlated with the treated area ratio (r = 0.64; P < .01).

CONCLUSION

The efficacy of MRgFUS correlates with the signal intensity of T2-weighted magnetic resonance images. Type 1 and type 2 fibroids are suitable candidates for MRgFUS, whereas type 3 fibroids are not.