Positioning device for MRI-guided high intensity focused ultrasound system

Treatment of canine and feline sarcoma using MR-guided focused ultrasound system

PURPOSE

In recent years, veterinary medicine has enhanced its applications beyond traditional approaches, progressively incorporating the Focused Ultrasound (FUS) technology. This study investigated the ability of FUS to precisely ablate naturally occurring canine and feline soft tissue sarcomas (STS).

METHODS

Six dogs and four cats with superficial tumours were enrolled in the study. The tumours were treated with a Magnetic Resonance guided FUS (MRgFUS) robotic system featuring a single element spherically focused transducer of 2.6 MHz. The tumours were then removed by surgery and sent for hematoxylin and eosin (H&E) staining.

RESULTS

The MRgFUS system was capable of inflicting well-defined overlapping lesions in the tumours. The anatomical sites of the treated tumours were the neck, leg, face, back and belly. Coagulative necrosis was evidenced by histopathology assessment in 80% of cases.

CONCLUSION

Therefore, this technology can be a therapeutic solution for veterinary cancer and a model for advancing the knowledge on human STS.

Magnetic Resonance Imaging-Guided Focused Ultrasound Positioning System for Preclinical Studies in Small Animals

OBJECTIVES

A positioning device compatible with magnetic resonance imaging (MRI) used for preclinical studies in small animals was developed that fits in MRI scanners up to 7 T. The positioning device was designed with two computer-controlled linear stages.

METHODS

The positioning device was evaluated in an agar-based phantom, which mimics soft tissues, and in a rabbit. Experiments with this positioning device were performed in an MRI system using the agar-based phantom. The transducer used had a diameter of 50 mm, operated at 0.5 MHz, and focused energy at 60 mm.

RESULTS

Magnetic resonance thermometry was used to assess the functionality of the device, which showed adequate deposition of thermal energy and sufficient positional accuracy in all axes.

CONCLUSIONS

The proposed system fits in MRI scanners up to 7 T. Because of the size of the positioning device, at the moment, it can be used to perform preclinical studies on small animals such as mice, rats, and rabbits.

Robotic system for magnetic resonance guided focused ultrasound ablation of abdominal cancer

BACKGROUND

A prototype robotic system that uses magnetic resonance guided focused ultrasound (MRgFUS) technology is presented. It features three degrees of freedom (DOF) and is intended for thermal ablation of abdominal cancer.

METHODS

The device is equipped with three identical transducers being offset between them, thus focussing at different depths in tissue. The efficacy and safety of the system in ablating rabbit liver and kidney was assessed, both in laboratory and magnetic resonance imaging (MRI) conditions.

RESULTS

Despite these organs' challenging location, in situ coagulative necrosis of a tissue area was achieved. Heating of abdominal organs in rabbit was successfully monitored with MR thermometry.

CONCLUSIONS

The MRgFUS system was proven successful in creating lesions in the abdominal area of rabbits. The outcomes of the study are promising for future translation of the technology to the clinic.

Simple methods to test the accuracy of MRgFUS robotic systems

Background

Robotic‐assisted diagnostic and therapeutic modalities require a highly accurate performance to be certified for clinical application. In this paper, three simple methods for assessing the accuracy of motion of magnetic resonance‐guided focused ultrasound (MRgFUS) robotic systems are presented.

Methods

The accuracy of motion of a 4 degrees of freedom robotic system intended for preclinical use of MRgFUS was evaluated by calliper‐based and magnetic resonance imaging (MRI) methods, as well as visually by performing multiple ablations on a plastic film.

Results

The benchtop results confirmed a highly accurate motion in all axes of operation. The spatial positioning errors estimated by MRI evaluation were defined by the size of the imaging pixels. Lesions arrangement in discrete and overlapping patterns confirmed satisfactory alignment of motion trajectories.

Conclusions

We believe the methods presented here should serve as a standard for evaluating the accuracy of motion of MRgFUS robotic systems.

Focused ultrasound robotic system for very small bore magnetic resonance imaging

Background

A magnetic resonance imaging (MRI) compatible robotic system for focused ultrasound was developed for small animal like mice or rats that fits into a 9.4 T MRI scanner (Bruker Biospec 9420, Bruker Biospin, Ettlingen, Germany). The robotic system includes two computer‐controlled linear stages.

Materials and Methods

The robotic system was evaluated in a mouse‐shaped, real‐size agar‐based mimicking material, which has similar acoustical properties as soft tissues. The agar content was 6% weight per volume (w/v), 4% w/v silica while the rest was degassed water. The transducer used has a diameter of 4 cm, operates with 2.6 MHz and focuses energy at 5 cm.

Results

The MRI compatibility of the robotic system was evaluated in a 9.4 T small animal scanner. The efficacy of the ultrasonic transducer was evaluated in the mimicking material using temperature measurements.

Conclusions

The proposed robotic system can be utilized in a 9.4 T small animal MRI scanner. The proposed system is functional, compact and simple thus providing a useful tool for preclinical research in mice and rats.

MRI-guided frameless biopsy robotic system with the inclusion of unfocused ultrasound transducer for brain cancer ablation

BACKGROUND

A magnetic resonance image (MRI) guided robotic system dedicated for brain biopsy was developed. The robotic system carries a biopsy needle and a small rectangular unfocused, single element, planar ultrasonic transducer which can be potentially utilized to ablate small and localized brain cancer.

MATERIALS AND METHODS

The robotic device includes six computer-controlled axes. An agar-based phantom was developed which included an olive that mimics brain target. A rectangular ultrasonic transducer operated at 4 MHz was used.

RESULTS

The functionality of the robotic system was assessed by means of ultrasound imaging, MRI imaging, and MR thermometry, demonstrating effective targeting. The heating capabilities of the ultrasonic transducer were also evaluated.

CONCLUSIONS

A functional MRI-guided robotic system was produced which can perform frameless brain biopsy. In the future, if a tumour is proven malignant, the needle can be pulled-out and a small ultrasonic transducer can be inserted to ablate the tumour.

Design and validation of an MR-conditional robot for transcranial focused ultrasound surgery in infants

PURPOSE

Current treatment of intraventricular hemorrhage (IVH) involves cerebral shunt placement or an invasive brain surgery. Magnetic resonance-guided focused ultrasound (MRgFUS) applied to the brains of pediatric patients presents an opportunity to treat IVH in a noninvasive manner, termed "incision-less surgery." Current clinical and research focused ultrasound systems lack the capability to perform neonatal transcranial surgeries due to either range of motion or dexterity requirements. A novel robotic system is proposed to position a focused ultrasound transducer accurately above the head of a neonatal patient inside an MRI machine to deliver the therapy.

METHODS

A clinical Philips Sonalleve MRgFUS system was expanded to perform transcranial treatment. A five degree-of-freedom MR-conditional robot was designed and manufactured using MR compatible materials. The robot electronics and control were integrated into existing Philips electronics and software interfaces. The user commands the position of the robot with a graphical user interface, and is presented with real-time MR imaging of the patient throughout the surgery. The robot is validated through a series of experiments that characterize accuracy, signal-to-noise ratio degeneration of an MR image as a result of the robot, MR imaging artifacts generated by the robot, and the robot's ability to operate in a representative surgical environment inside an MR machine.

RESULTS

Experimental results show the robot responds reliably within an MR environment, has achieved 0.59 ± 0.25 mm accuracy, does not produce severe MR-imaging artifacts, has a workspace providing sufficient coverage of a neonatal brain, and can manipulate a 5 kg payload. A full system demonstration shows these characteristics apply in an application environment.

CONCLUSIONS

This paper presents a comprehensive look at the process of designing and validating a new robot from concept to implementation for use in an MR environment. An MR conditional robot has been designed and manufactured to design specifications. The system has demonstrated its feasibility as a platform for MRgFUS interventions for neonatal patients. The success of the system in experimental trials suggests that it is ready to be used for validation of the transcranial intervention in animal studies.

MRI guided focused ultrasound robotic system for animal experiments

BACKGROUND

In this paper an MRI-guided focused ultrasound (MRgFUS) robotic system was developed that can be used for conducting experiments in small animals.The target for this robotic system regarding motion was to move a therapeutic ultrasound transducer in two Cartesian axes.

METHODS

A single element spherically focused transducer of 3 cm diameter, focusing at 7 cm and operating at 0.4 MHz was used. The positioning device incorporates only MRI compatible materials. The propagation of ultrasound is a bottom to top approach. The 2-D positioning device is controlled by custom-made software and a custom-made electronic system which controls the two piezoelectric motors.

RESULTS

The system was tested successfully in agar/silica/evaporated milk phantom for various tasks (robot motion, MR compatibility, and MR thermometry). The robotic system is capable of moving the focused ultrasound transducer to perform MR-guided focused ultrasound experiments in small animals.

CONCLUSIONS

This system has the potential to be deployed as a cost effective solution for performing experiments in small animals.

MRI-guided coupling for a focused ultrasound system using a top-to-bottom propagation

Background

A novel magnetic resonance imaging (MRI)-conditional coupling system was developed that accommodates a focused ultrasound (FUS) transducer. With this coupling system, the transducer can access targets from top to bottom. The intended clinical application is treatment of fibroids using FUS with the patient placed in supine position.

Methods

The coupling system was manufactured using a rapid prototyping device using acrylonitrile butadiene styrene (ABS) plastic. Coupling to a gel phantom was achieved using a water bag filled with degassed water. The FUS transducer was immersed in the water bag.

Results

The coupling system was successfully tested for MRI compatibility using fast-gradient pulse sequences in a gel phantom. The robotic system with its new coupling system was evaluated for its functionality for creating discrete and multiple (overlapping) lesions in the gel phantom.

Conclusions

An MRI-conditional FUS coupling system integrated with an existing robotic system was developed that has the potential to create thermal lesions in targets using a top-to-bottom approach. This system has the potential to treat fibroid tumors with the patient lying in supine position.

Feasibility of an electromagnetic compatibility method for MRgFUS using a wire mesh screen

This study evaluated an electromagnetic compatibility method for high-intensity focused ultrasound (HIFU) and magnetic resonance (MR) imaging in an MR-guided focused ultrasound surgery using a conductive wire mesh screen. This screen has a good ultrasound transmission and shielding effectiveness. A hybrid acoustic simulation method was developed to analyze the effects of mesh parameters and the HIFU working frequency on the acoustic field. Experiments were performed to measure both acoustic pressure profile and radiated electromagnetic noise. With the proposed mesh screen, the electromagnetic radiation emission was reduced by 14dB at 128MHz while the acoustic focal intensity was reduced by less than 11% using one screen. This shielding method is easy to implement and requires no additional phase correction method. This method also improves the quality of MR images.

Evaluation of focused ultrasound algorithms: Issues for reducing pre-focal heating and treatment time

BACKGROUND

Due to the heating in the pre-focal field the delay between successive movements in high intensity focused ultrasound (HIFU) are sometimes as long as 60s, resulting to treatment time in the order of 2-3h. Because there is generally a requirement to reduce treatment time, we were motivated to explore alternative transducer motion algorithms in order to reduce pre-focal heating and treatment time.

MATERIALS AND METHODS

A 1 MHz single element transducer with 4 cm diameter and 10 cm focal length was used. A simulation model was developed that estimates the temperature, thermal dose and lesion development in the pre-focal field. The simulated temperature history that was combined with the motion algorithms produced thermal maps in the pre-focal region. Polyacrylimde gel phantom was used to evaluate the induced pre-focal heating for each motion algorithm used, and also was used to assess the accuracy of the simulation model.

RESULTS

Three out of the six algorithms having successive steps close to each other, exhibited severe heating in the pre-focal field. Minimal heating was produced with the algorithms having successive steps apart from each other (square, square spiral and random). The last three algorithms were improved further (with small cost in time), thus eliminating completely the pre-focal heating and reducing substantially the treatment time as compared to traditional algorithms.

CONCLUSIONS

Out of the six algorithms, 3 were successful in eliminating the pre-focal heating completely. Because these 3 algorithms required no delay between successive movements (except in the last part of the motion), the treatment time was reduced by 93%. Therefore, it will be possible in the future, to achieve treatment time of focused ultrasound therapies shorter than 30 min. The rate of ablated volume achieved with one of the proposed algorithms was 71 cm(3)/h. The intention of this pilot study was to demonstrate that the navigation algorithms play the most important role in reducing pre-focal heating. By evaluating in the future, all commercially available geometries, it will be possible to reduce the treatment time, for thermal ablation protocols intended for oncological targets.

Review of MRI positioning devices for guiding focused ultrasound systems

BACKGROUND

This article contains a review of positioning devices that are currently used in the area of magnetic resonance imaging (MRI) guided focused ultrasound surgery (MRgFUS).

METHODS

The paper includes an extensive review of literature published since the first prototype system was invented in 1991.

RESULTS

The technology has grown into a fast developing area with application to any organ accessible to ultrasound. The initial design operated using hydraulic principles, while the latest technology incorporates piezoelectric motors. Although, in the beginning there were fears regarding MRI safety, during recent years, the deployment of MR-safe positioning devices in FUS has become routine. Many of these positioning devices are now undergoing testing in clinical trials.

CONCLUSION

Existing MRgFUS systems have been utilized mostly in oncology (fibroids, brain, liver, kidney, bone, pancreas, eye, thyroid, and prostate). It is anticipated that, in the near future, there will be a positioning device for every organ that is accessible by focused ultrasound.

MR compatible positioning device for guiding a focused ultrasound system for the treatment of brain deseases

BACKGROUND

A prototype magnetic resonance imaging (MRI)-compatible positioning device that navigates a high intensity focused ultrasound (HIFU) transducer is presented. The positioning device has three user-controlled degrees of freedom that allow access to brain targets using a lateral coupling approach. The positioning device can be used for the treatment of brain cancer (thermal mode ultrasound) or ischemic stroke (mechanical mode ultrasound).

MATERIALS AND METHODS

The positioning device incorporates only MRI compatible materials such as piezoelectric motors, ABS plastic, brass screws, and brass rack and pinion.

RESULT

The robot has the ability to accurately move the transducer thus creating overlapping lesions in rabbit brain in vivo. The registration and repeatability of the system was evaluated using tissues in vitro and gel phantom and was also tested in vivo in the brain of a rabbit.

CONCLUSION

A simple, cost effective, portable positioning device has been developed which can be used in virtually any clinical MRI scanner since it can be placed on the table of the MRI scanner. This system can be used to treat in the future patients with brain cancer and ischemic stroke.

In vitro and in vivo brain ablation created by high-intensity focused ultrasound and monitored by MRI

In this paper, magnetic resonance imaging (MRI) is investigated for monitoring small and large lesions created by high-intensity focused ultrasound (HIFU) in freshly excised lamb brain and in rabbit brain in vivo. A single-element spherically focused transducer of 5 cm diameter, focusing at 10 cm and operating at 1 MHz was used. A prototype MRI-compatible positioning device that is used to navigate the transducer is described. The effects of HIFU were investigated using T1-W and T2-W fast spin echo (FSE) and fluid-attenuated inversion recovery (FLAIR). T2-W FSE and FLAIR show better anatomical details within the brain than T1-W FSE, but with T1-W FSE, the contrast between lesion and brain is higher for both thermal and bubbly lesions. The best contrast between lesion and brain with T1-W FSE is obtained with TR above 500 ms, whereas with T2-W FSE, the best contrast is observed between 40 and 60 ms. The maximum contrast to noise ratio (CNR) measured with T1-W FSE was approximately 20. With T2-W FSE, the corresponding CNR was approximately 12. With this system, we were able to create large lesions (by producing overlapping lesions), and it was possible to monitor these lesions with MRI with excellent contrast. The length of the lesions in vivo brain was much higher than the length in vitro, indicating that the penetration in the in vitro brain is limited, possibly by reflection due to trapped bubbles in the blood vessels. This paper demonstrates that HIFU has the potential to treat brain tumors in humans. This could be done either using a single-element transducer with a frequency around 1 MHZ or using a multi-element transducer.