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

Automatic spatial and temporal temperature control for MR-guided focused ultrasound using fast 3D MR thermometry and multispiral trajectory of the focal point

Of the different modalities to induce local hyperthermia, focused ultrasound is the only noninvasive technology available at the moment. In addition to the 3D localization of the target region, it has been shown that MRI can provide real-time thermometry and allows online, automatic control of temperature evolution of the focal point. Treatment of a large tissue volume (as compared to the focal spot size, i.e., the ultrasound wavelength) can be achieved rapidly by moving the focal point along an inside-out spiral trajectory. It has been shown previously that under linear conditions of energy deposition versus temperature, the spatial profile of the temperature within a large area can be controlled. In this study, a proportional, integral, and derivative (PID) spatial-and-temporal controller is described for the control of the temperature evolution within the target region under more variable conditions. The aim was to reach a predefined temperature profile after a few successive trajectories. Heat conduction in tissue is exploited to obtain a uniform temperature increase in a volume using discrete sonications without any waiting time. Input data sets consisted of 3D temperature maps provided online by a MR scanner. For each new trajectory, the controller recalculates the number of sonications per surface unit (spatial density of points describing the trajectory) and the applied power. Its performance was tested ex vivo and in vivo. Diameters of the target region ranged from 9 mm to 19 mm. Targeted temperature increase ranged from +8 degrees C to +18 degrees C. Spatiotemporal temperature control showed good stability and fast convergence, for both circular and elliptic ROIs.

Delivery of systemic chemotherapeutic agent to tumors by using focused ultrasound: study in a murine model

PURPOSE

To quantitatively determine the delivery of systemic liposomal doxorubicin to tumors treated with pulsed high-intensity focused ultrasound and to study the mechanism underlying this delivery in a murine model.

MATERIALS AND METHODS

All animal work was performed in compliance with guidelines and approval of institutional animal care committee. C3H mice received subcutaneous injections in the flank of a cell suspension of SCC7, a murine squamous cell carcinoma cell line; mice (n = 32) in drug delivery study received unilateral injections, whereas mice (n = 10) in mechanistic study received bilateral injections. Tumors were treated when they reached 1 cm(3) in volume. In the drug delivery study, doxorubicin hydrochloride liposomes were injected into the tail vein: Mice received therapy with doxorubicin injections and high-intensity focused ultrasound, doxorubicin injections alone, or neither form of therapy (controls). Tumors were removed, and the doxorubicin content was assayed with fluorescent spectrophotometry. In the mechanistic study, all mice received an injection of 500-kDa dextran-fluorescein isothyocyanate into the tail vein, and half of them were exposed to high-intensity focused ultrasound prior to injection. Contralateral tumors served as controls for each group. Extravasation of dextran-fluorescein isothyocyanate was observed by using in vivo confocal microscopy.

RESULTS

Mean doxorubicin concentration in tumors treated with pulsed high-intensity focused ultrasound was 9.4 microg . g(-1) +/- 2.1 (standard deviation), and it was significantly higher (124% [9.4 microg . g(-1)/4.2 microg . g(-1)]) than in those that were not treated with high-intensity focused ultrasound (4.2 microg . g(-1) +/- 0.95) (P < .001, unpaired two-tailed Student t test). Extravasation of dextran-fluorescein isothyocyanate was observed in the vasculature of tumors treated with high-intensity focused ultrasound but not in that of untreated tumors.

CONCLUSION

Pulsed high-intensity focused ultrasound is an effective method of targeting systemic drug delivery to tumor tissue. Potential mechanisms for producing the observed enhancement are discussed.

MRI-guided interstitial ultrasound thermal therapy of the prostate: A feasibility study in the canine model

The feasibility of MRI-guided interstitial ultrasound thermal therapy of the prostate was evaluated in an in vivo canine prostate model. MRI compatible, multielement interstitial ultrasound applicators were developed using 1.5 mm diameter cylindrical piezoceramic transducers (7 to 8 MHz) sectored to provide 180 degrees of angular directional heating. Two in vivo experiments were performed in canine prostate. The first using two interstitial ultrasound applicators, the second using three ultrasound applicators in conjunction with rectal and urethral cooling. In both experiments, the applicators were inserted transperineally into the prostate with the energy directed ventrally, away from the rectum. Electrical power levels of 5-17 W per element (approximately 1.6-5.4 W acoustic output power) were applied for heating periods of 18 and 48 min. Phase-sensitive gradient-echo MR imaging was used to monitor the thermal treatment in real-time on a 0.5 T interventional MRI system. Contrast-enhanced T1-weighted images and vital-stained serial tissue sections were obtained to assess thermal damage and correlate to real-time thermal contour plots and calculated thermal doses. Results from these studies indicated a large volume of ablated (nonstained) tissue within the prostate, extending 1.2 to 2.0 cm from the applicators to the periphery of the gland, with the dorsal margin of coagulation well-defined by the applicator placement and directionality. The shape of the lesions correlated well to the hypointense regions visible in the contrast-enhanced T1-weighted images, and were also in good agreement with the contours of the 52 degrees C threshold temperature and t43 > 240 min. This study demonstrates the feasibility of using directional interstitial ultrasound in conjunction with MRI thermal imaging to monitor and possibly control thermal coagulation within a targeted tissue volume while potentially protecting surrounding tissue, such as rectum, from thermal damage.

Future perspectives for intraoperative MRI

MRI-guided neurosurgery not only represents a technical challenge but a transformation from conventional hand-eye coordination to interactive navigational operations. In the future, multimodality-based images will be merged into a single model, in which anatomy and pathologic changes are at once distinguished and integrated into the same intuitive framework. The long-term goals of improving surgical procedures and attendant outcomes, reducing costs, and achieving broad use can be achieved with a three-pronged approach: 1. Improving the presentation of preoperative and real-time intraoperative image information 2. Integrating imaging and treatment-related technology into therapy delivery systems 3. Testing the clinical utility of image guidance in surgery The recent focus in technology development is on improving our ability to understand and apply medical images and imaging systems. Areas of active research include image processing, model-based image analysis, model deformation, real-time registration, real-time 3D (so-called "four-dimensional") imaging, and the integration and presentation of image and sensing information in the operating room. Key elements of the technical matrix also include visualization and display platforms and related software for information and display, model-based image understanding, the use of computing clusters to speed computation (ie, algorithms with partitioned computation to optimize performance), and advanced devices and systems for 3D device tracking (navigation). Current clinical applications are successfully incorporating real-time and/or continuously up-dated image-based information for direct intra-operative visualization. In addition to using traditional imaging systems during surgery, we foresee optimized use of molecular marker technology, direct measures of tissue characterization (ie, optical measurements and/or imaging), and integration of the next generation of surgical and therapy devices (including image-guided robotic systems). Although we expect the primary clinical thrusts of MRI-guided therapy to remain in neurosurgery, with the possible addition of other areas like orthopedic, head, neck, and spine surgery, we also anticipate increased use of image-guided focal thermal ablative methods (eg, laser, RF, cryoablation, high-intensity focused ultrasound). By validating the effectiveness of MRI-guided therapy in specific clinical procedures while refining the technology that serves as its underpinning at the same time, we expect many neurosurgeons will eventually embrace MRI as their intraoperative imaging choice. Clearly, intraoperative MRI offers several palpable advantages. Most important among these are improved medical outcomes, shorter hospitalization, and better and faster procedures with fewer complications. Certain economic and practical barriers also impede the large-scale use of intraoperative MRI. Although there has been a concerted technical effort to increase the benefit/cost ratio by gathering more accurate information, designing more localized and less invasive treatment devices, and developing better methods to orient and position therapy end-effectors, further research is needed. Indeed, the drive to improve and upgrade technology is ongoing. Specifically, in the context of the real-time representation of the patient's anatomy, we have improved the quality and utility of the information presented to the surgeon, which, in turn, contributes to more successful surgical outcomes. We can also expect improvements in intraoperative imaging systems as well as increased use of nonimaging sensors and robotics to facilitate more widespread use of intraoperative MRI.

Capturing intraoperative deformations: research experience at Brigham and Women's Hospital

During neurosurgical procedures the objective of the neurosurgeon is to achieve the resection of as much diseased tissue as possible while achieving the preservation of healthy brain tissue. The restricted capacity of the conventional operating room to enable the surgeon to visualize critical healthy brain structures and tumor margin has lead, over the past decade, to the development of sophisticated intraoperative imaging techniques to enhance visualization. However, both rigid motion due to patient placement and nonrigid deformations occurring as a consequence of the surgical intervention disrupt the correspondence between preoperative data used to plan surgery and the intraoperative configuration of the patient's brain. Similar challenges are faced in other interventional therapies, such as in cryoablation of the liver, or biopsy of the prostate. We have developed algorithms to model the motion of key anatomical structures and system implementations that enable us to estimate the deformation of the critical anatomy from sequences of volumetric images and to prepare updated fused visualizations of preoperative and intraoperative images at a rate compatible with surgical decision making. This paper reviews the experience at Brigham and Women's Hospital through the process of developing and applying novel algorithms for capturing intraoperative deformations in support of image guided therapy.

MRI Guidance of Focused Ultrasound Therapy of Uterine Fibroids:Early Results

OBJECTIVE

The purpose of this study was to explore our hypothesis that MRI-guided focused ultrasound therapy for the treatment of uterine fibroids will lead to a significant reduction in symptoms and improvement in quality of life. We describe focused ultrasound therapy applications and the method for monitoring the thermal energy deposited in the fibroids, including the MRI parameters required, in a prospective review of 108 treatments.

MATERIALS AND METHODS

Patients presenting with symptomatic uterine fibroids who attained a minimal symptom severity score and who would otherwise have been offered a hysterectomy were recruited. Thermal lesions were created within target fibroids using an MRI-guided focused ultrasound therapy system. The developing lesion was monitored using real-time MR thermometry, which was used to assess treatment outcome in real time to change treatment parameters and achieve the desired outcome. Fibroid volume, fibroid symptoms, and quality-of-life scores were measured before treatment and 6 months after treatment. Adverse events were actively monitored and recorded.

RESULTS

In this study, 79.3% of women who had been treated reported a significant improvement in their uterine fibroid symptoms on follow-up health-related quality-of-life questionnaires, which supports our hypothesis. The mean reduction in fibroid volume at 6 months was 13.5%, but nonenhancing volume (mean, 51 cm(3)) remained within the treated fibroid at 6 months.

CONCLUSION

This early description of MRI-guided focused ultrasound therapy treatment of fibroids includes follow-up data and shows that, although the volume reduction is moderate, it correlates with treatment volume and the symptomatic response to this treatment is encouraging.

Noninvasive thermal ablation of hepatocellular carcinoma by using magnetic resonance imaging-guided focused ultrasound

A number of minimally invasive methods have been tested for the thermal ablation of liver tumors as an alternative to surgical resection. The use of focused ultrasound transducers to ablate deep tumors offers the first completely noninvasive alternative to these techniques. By increasing the flexibility of this technology with modern phased-array transducer design and by combining it with magnetic resonance imaging for targeting and online guidance, a powerful tool results with the potential to offer treatment to a larger population of patients, to reduce trauma to the patient, and to reduce the cost of treatment. In this article, we review previous work with focused ultrasound in the liver and recent experimental results with magnetic resonance imaging guidance.

Patterns of Thermal Deposition in the Skull During Transcranial Focused Ultrasound Surgery

The induction of temperature elevation by focused ultrasound is a noninvasive surgical technique for destroying tissue. This technique has been used clinically in soft tissues such as liver, prostate and breast. It has long been desired to extend this technique to noninvasive treatment of brain tumors. Although the skull was once thought to be an unsurpassable barrier to focused ultrasound treatment, it has been shown that the distortion caused by the skull can be corrected to produce a useful intracranial focus. However, the attenuation experienced by the ultrasound in passing through cranial bone is large, and consequently the skull is subject to the deposition of acoustic energy as heat. The nature and extent of this heating process has been difficult to characterize empirically. It is practically difficult to implant a sufficient number of thermocouples to obtain detailed temperature data directly, and bone is an unsuitable medium in which to perform noninvasive thermometry using proton chemical shift magnetic resonance imaging. Furthermore, skull specimens used experimentally lack active blood perfusion of the skull and the overlying scalp. This paper describes the use of large-scale acoustic and thermal simulations to calculate the distribution of temperature within the skull and brain that can be expected to occur during therapeutically useful focused ultrasound sonications of the brain. The results demonstrate that standing waves may be formed within the skull during transcranial sonication leading to nonuniform skull heating. However, the results also show that these effects can be sufficiently controlled to allow therapeutic ultrasound to be focused in the cranial base region of the brain without causing thermal damage to the scalp, skull or outer surface of the brain.

In vivo feasibility of image-guided transvaginal focused ultrasound therapy for the treatment of intracavitary fibroids

OBJECTIVE

To determine the feasibility of uterine tissue ablation in vivo using a transvaginal focused ultrasound applicator guided by ultrasound imaging.

DESIGN

Randomized in vivo animal study.

SETTING

Academic research environment.

ANIMAL(S)

Healthy anesthetized sheep.

INTERVENTION(S)

Uterine treatment location was determined using a computerized targeting system. Five sonications 10 seconds in duration and averaging 2,000 W/cm(2) of focal ultrasound intensity were applied in each animal's uterus. Animals were euthanized either immediately or 2, 7, or 30 days post-treatment.

MAIN OUTCOME MEASURE(S)

Gross and microscopic analysis of the dissected uterus was used to quantitatively and qualitatively determine the ablated region and treatment side effects.

RESULT(S)

Treatments resulted in coagulative necrosis. Histopathological analysis showed that over 7 days, inflammatory cells appeared and smooth muscle bundles regenerated. By day 30, treated tissues healed and scar tissue formed. None of the animals showed abnormal behavior or medical problems. Complications in three animals were damage to the vaginal wall and colon, possibly due to inadequate applicator cooling and an empty bladder during treatment.

CONCLUSION(S)

Transvaginal image-guided high-intensity focused ultrasound has potential for treating uterine fibroids. Further safety testing of this treatment will prepare it for human use.

MRI monitoring of the effect of tissue interfaces in the penetration of high intensity focused ultrasound in kidney in vivo

In this paper, we studied the effect of interfaces during the application of high intensity focused ultrasound (HIFU) ablation in rabbit kidney in vivo. In kidney ablation, mainly two types of interfaces are encountered: these are muscle-kidney and fat-kidney. It was observed that the intensity for which the probability of cavitation (POC) is one was decreased when HIFU penetrated through interfaces, meaning that an interface is a potential site of cavitation. We utilized the concept of scanning the area to be treated in two dimensions (rectangular grid) by applying low intensity ultrasound (diagnostic scan). When all the points of the grid show decrease of signal in T1-weighted fast spoiled gradient (FSPGR) which indicated heating, complete necrosis was observed in the targeted area during the application of HIFU (therapeutic scan). If ultrasound goes through an interface that includes air spaces, the diagnostic scan indicates spaces with poor ultrasound penetration and as a result, during the application of the therapeutic scan, some sites remain untreated. The muscle-kidney and fat-kidney interfaces cause reflection of ultrasound, which prevents the penetration of ultrasound. Microscopic bubbles in the interface may initiate cavitation, especially at high intensities. However, sometimes these types of interfaces do not include any bubbles and therefore the propagation of ultrasound is not inhibited.

Image-guided HIFU neurolysis of peripheral nerves to treat spasticity and pain

Spasticity, a major complication of central nervous system disorders, signified by uncontrollable muscle contractions, is very difficult to treat effectively. We report on the use of ultrasound (US) image-guided high-intensity focused US (HIFU) to target and suppress the function of the sciatic nerve complex of rabbits in vivo, as a possible treatment of spasticity. The image-guided HIFU device included a 3.2-MHz spherically curved transducer and an intraoperative imaging probe. A focal acoustic intensity of 1480 to 1850 W/cm(2), applied using a scanning method, was effective in achieving complete conduction block in 100% of 22 nerve complexes with HIFU treatment times of 36 +/- 14 s (mean +/- SD). Gross examination showed blanching of the nerve at the HIFU treatment site and lesion volumes of 2.8 +/- 1.4 cm(3) encompassing the nerve complex. Histologic examination indicated axonal demyelination and necrosis of Schwann cells as probable mechanisms of nerve block. With accurate localization and targeting of peripheral nerves using US imaging, HIFU could become a promising tool for the suppression of spasticity.

Perspectives in clinical uses of high-intensity focused ultrasound

Focused ultrasound holds promise in a large number of therapeutic applications. It has long been known that high intensity focused ultrasound can kill tissue through coagulative necrosis. However, it is only in recent years that practical clinical applications are becoming possible, with the development of high power ultrasound phased arrays and noninvasive monitoring methods. These technologies, combined with more sophisticated treatment planning methods allow noninvasive focusing in areas such as the brain, that were once thought to be unreachable. Meanwhile, exciting investigations are underway in microbubble-enhanced heating which could significantly reduce treatment times. These developments have promoted an increase in the number of potential applications by providing valuable new tools for medical research. This paper provides an overview of the scientific and engineering advances that are allowing the growth in clinical focused ultrasound applications. It also discusses some of these prospective applications, including the treatment of brain disorders and targeted drug delivery.

[Focused ultrasound surgery. Basics, current status, and new trends]

There is an increasing interest in high intensity focused ultrasound (HIFU) for thermo ablative tumor therapy. The attractiveness of this method is based on its ability to destroy tumor tissue non invasively while sparing surrounding tissue from outside the body. HIFU induced tissue necroses are sharply circumscribed. Therefore this method was termed focused ultrasound surgery (FUS). The therapeutic potential of FUS is under investigation in several clinical studies. Main objects of these studies are prostate carcinomas, breast kidney and liver tumors. The next innovative step will be the non invasive FUS treatment of brain through the intact skull. Combining FUS with magnetic resonance imaging (MRI) or diagnostic ultrasound allows accurate and online therapy guiding and monitoring. This article gives an overview of the basics, the latest developments and actual clinical studies in the field of focused ultrasound surgery.

Prediction of subtle thermal histopathological change using a novel analysis of Gd-DTPA kinetics

PURPOSE

To investigate Gd-DTPA kinetics as predictors of histopathological changes following focused ultrasound (FUS) thermal ablation for improved planning and assessment.

MATERIALS AND METHODS

Twenty-nine FUS lesions were created in the thigh muscle of eight rabbits under MR-guidance at 1.5 Tesla. Three rabbits were killed at four hours; and 11 lesions were analyzed with histopathology. Temperature-sensitive MRI using proton-resonant frequency-shift was used for time-dependent temperature measurements. Analysis of the uptake kinetics of Gd-DTPA was performed after Gd-DTPA injection, within 20 minutes after heating and again at two hours after heating. The resulting kinetic maps, permeability (K(trans)) and leakage space (v(e)), were correlated to peak temperatures, T(2)-weighted MR, and histopathology.

RESULTS

Images of K(trans) and v(e) reveal regions of histopathological change not visible on conventional post-therapy MR. At early times after heating, v(e) predicts the area of injury more accurately than T(2) (7 +/- 2% vs. 25 +/- 6% underestimation). A circular region of extensive structural/vascular disruption is indicated only on K(trans) maps. The sharp decrease in K(trans) at the boundary of this region occurs at 47.5 +/- 0.5 degrees C, and may be a better estimate of cell death than the conventional method of temperature threshold (55 degrees C for coagulation) used in therapy planning.

CONCLUSION

Our results suggest Gd-DTPA kinetics can predict different histopathological changes following FUS ablation and may be valuable for early prediction.

High-intensity focused ultrasound in the treatment of postpartum hemorrhage: an animal model

OBJECTIVE

To investigate the use of high-intensity focused ultrasound (HIFUS) to reduce uterine artery blood flow in ewes in the postpartum period.

METHODS

HIFUS was applied to the uterine arteries of seven ewes in the postpartum period. Arterial flow velocities were measured before and after the procedure at the site of HIFUS application (target), as well as 3 cm upstream and 3 cm downstream from the target. The uterine arteries were then removed for macroscopic and histological examination.

RESULTS

Maximum flow velocities in the target area increased after the procedure by 350% and those upstream from the target decreased by 65%. Macroscopically, the vessel diameter was shown to have reduced at the site of HIFUS application. Microscopically, both the endothelium and media showed thermal lesions. Tissues surrounding the arteries were macroscopically and microscopically normal.

CONCLUSION

Exposure of uterine arteries to HIFUS reduces the vessel diameter and thus induces a dramatic increase in the maximum flow velocities within the target area. HIFUS may have a role in the treatment of postpartum hemorrhage.

Non-invasive transcranial high intensity focused ultrasound (HIFUS) under MRI thermometry and guidance in the treatment of brain lesions

Non-invasive transcranial high intensity focused ultrasound (HIFUS) therapy given under MRI thermometry and image guidance to awake patients lying within the bore of a 1.5 T MRI scanner (a) to thermally ablate brain lesions such as metastases, (b) to cause precise ablative brain lesions in functional disorders, or (c) to locally open the blood-brain-barrier for targeted therapeutic construct delivery--without the radiation risks of stereotactic radiotherapy--may sound science fiction. Kullervo Hynynen, a Finnish-born ultrasound and MRI physicist, and Ferenc Jolesz, a Hungarian-born neurosurgeon and visionary of image guided surgery, have joined forces at Radiology, Brigham & Women's Hospital, Boston, and they have taken every step to realize the vision above, in highly successful collaboration with the industry (GE, InSightec, TxSonics). The sophisticated transcranial HIFUS instrumentation, supported by profound research data from experimental animals and by the clinical experience from extracranial HIFUS targets (breast fibroadenoma, uterine fibroid), is now coming to a phase I clinical trial in cerebral metastases. It remains to be seen whether transcranial HIFUS will find applications in diffuse gliomas such as (a) thermal ablation of selected areas of glioma tissue, (b) opening the blood-brain-barrier for therapeutic constructs to enter selected areas, or (c) activating such constructs in desired areas. The prophecy of Dr. Jolesz, "this technology will put neurosurgeons out of business", may not fulfill during our lifetime.

Focused ultrasound treatment of uterine fibroid tumors: safety and feasibility of a noninvasive thermoablative technique

OBJECTIVE

The purpose of this study was to determine the safety and efficacy of focused ultrasound surgery with magnetic resonance imaging guidance for the noninvasive treatment of uterine leiomyomas.

STUDY DESIGN

Fifty-five women with clinically significant uterine leiomyomas were treated. Pain and complications were assessed prospectively, and posttreatment magnetic resonance imaging was used to measure the treatment effects. Patients in three of the five centers underwent planned hysterectomy after treatment, which provided pathologic correlation of treatment.

RESULTS

Seventy-six percent of the enrolled patients completed the full treatment session. All treatments were conducted in an outpatient setting with minimal discomfort for subjects and no major complications. Pathologic examination of the uterus confirmed that magnetic resonance imaging guidance provides the safe and accurate delivery of effective levels of thermal energy with a 3-fold increase in volume of histologically documented necrosis, compared with treatment volume (6.6 +/- 0.8 vs 18.4 +/- 3.9 mL, P <.005).

CONCLUSION

Magnetic resonance imaging-guided focused ultrasound surgery appears to be a well-tolerated treatment for uterine leiomyomas.