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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 3  |  Issue : 3  |  Page : 138-144

A multipurpose positioning device for magnetic resonance imaging-guided focused ultrasound surgery


1 Department of Electrical Engineering, Cyprus University of Technology; Research and Development , Medsonic Ltd., Limassol, Cyprus
2 Department of Electrical Engineering, Cyprus University of Technology, Limassol, Cyprus
3 Department of Bioengineering, City University, London, England

Date of Web Publication7-Dec-2017

Correspondence Address:
Christakis Damianou
Department of Electrical Engineering, Cyprus University of Technology, 30 Archbishop Kyprianou Str, 3036 Limassol
Cyprus
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/digm.digm_33_17

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  Abstract 

Background and Objectives: An magnetic resonance imaging (MRI)-guided focused ultrasound (MRgFUS) positioning device was developed with 3 identical Cartesian stages. The robotic system can be utilized to move a focused ultrasound transducer for performing various MR-guided applications. Materials and Methods: A single element spherically focused transducer of 4 cm diameter, focusing at 10 cm, and operating at 1.14 MHz was used during the evaluation of the robotic system. The propagation of ultrasound was either lateral or superior to inferior. MRI thermometry algorithms were developed to assess the thermal effects of MRgFUS. The proposed robotic system was developed using a three-dimensional printer. Results: The system was tested successfully in a gel phantom for various tasks (robot motion, functionality, and MR compatibility). Controlled thermal lesions were created in the gel phantom. The lesion creation was monitored successfully using MRI thermometry. Conclusions: The system was tested successfully for its functionality and its MR compatibility. This system has the potential to be used for focused ultrasound applications in the brain, breast, abdominal, and thyroid.

Keywords: Abdominal, brain, breast, magnetic resonance imaging, robot, thyroid, ultrasound


How to cite this article:
Yiallouras C, Yiannakou M, Menikou G, Damianou C. A multipurpose positioning device for magnetic resonance imaging-guided focused ultrasound surgery. Digit Med 2017;3:138-44

How to cite this URL:
Yiallouras C, Yiannakou M, Menikou G, Damianou C. A multipurpose positioning device for magnetic resonance imaging-guided focused ultrasound surgery. Digit Med [serial online] 2017 [cited 2021 Dec 8];3:138-44. Available from: http://www.digitmedicine.com/text.asp?2017/3/3/138/220130


  Introduction Top


Magnetic resonance-guided focused ultrasound (MRgFUS) is a technology that uses intense sound to ablate soft tissue using magnetic resonance imaging (MRI) for monitoring and guidance.[1],[2] A number of organs that ultrasound can penetrate were investigated for clinical success. Most important applications reported are for brain,[3] prostate,[4],[5] breast,[6] fibroids,[7] and liver.[8] With the advancements in MRI compatible robots, it would be possible in the future that applications in various organs will be explored since MRgFUS is a minimally invasive technology.

The initial robotic systems for MRgFUS were functioning using hydraulic principles.[9],[10],[11] However, the hydraulic positioning systems had serious repeatability and reliability problems and were eventually abandoned. Insightec (Tirat Carmel, Israel) produced the first MRgFUS system that used piezoelectric motors to move the transducer inside an MRI scanner. This system was integrated on the patient's table. This system was eventually commercialized and was dedicated initially for the treatment of uterine fibroids,[7] leiomyoma,[12] and adenomyosis.[13] This system received the Food and Drug Administration approval in 2004 for fibroid treatment. The InSightec system was deployed for other applications such as the treatment of prostate cancer,[4] breast cancer,[14],[15] liver,[8],[16] and for pain palliation of bone metastases.[17],[18] Recently, Insightec developed a transcranial MRgFUS system specialized for the treatment of essential tremor.[19]

Later, Philips Healthcare (Best, the Netherlands) produced and commercialize the Sonalleve MRgFUS system [20] with 5 degree of freedom. This system was also integrated on the patient's table. Sonalleve MRgFUS received CE mark for clinical use for fibroid treatment and noninvasive palliative pain treatment of bone metastases.

The objective of this study was to develop a 3 Cartesian axes MRI compatible robotic system that can be either placed on the table of the MRI or attached in the bore of the static magnet. The robotic system may carry a single element transducer. The advantage of using the proposed system over the phased array system is that the focus is steered mechanically, instead of electronically. This makes the system less complex and affordable. Moreover, because the robotic system can be placed on the magnet, it can be used to access many sites that are accessible to ultrasound (brain, thyroid, breast, liver, bones, and kidney). Because of its simple design, it can be used also for preclinical studies. For this study, the robotic system can be placed on the table of the MRI scanner.


  Materials and Methods Top


This proposed technology is a continuation of other MRgFUS technologies designed for other applications by our group. For example, the article by Damianou et al.[21] and Mylonas and Damianou [22] described a robot dedicated for brain ablation. The article by Yiallouras et al. 2014[23] describes a robotic system for endorectal ablation using MRgFUS for prostate cancer (included a linear and an angular axis). Recently, a robotic system was developed with a linear and an angular axis for treating endovaginally gynecological tumors (Epaminonda et al. 2016).[24] With this experience gained, we were able to design this modular robotic system with 3 linear axes that can access targets for many organs (brain, abdominal, fibroids, breast, and thyroid).

Each Cartesian stage is identical, thus making the system versatile. For example, the X-stage can be placed in the bottom and then the Y-stage can be placed perpendicularly to the X-stage. Another possible geometry is to place the Z-stage perpendicularly to the X-stage, and then place the Y-stage on the moving plate of the Z-stage.

Mechanical design of the robotic system

[Figure 1]a shows the design drawing of one of the stages of the robotic system. Each stage included a motor holder that supported the ultrasonic motor. A screw type shaft was coupled to the motor shaft in one end, and in the other end, it was supported by a plastic structure. A plate that is coupled to the next stage was coupled to the screw shaft. Smooth motion of this plate was achieved through a guide structure that accommodated the moving plate. In the bottom of this guide structure, a mechanism was added so that attachment to the moving plate of another stage was possible. Furthermore, the moving plate had a structure that allows stages to be connected in the plate. With this configuration, each stage could serve as a stage of any Cartesian axis (X or Y or Z). One linear optical encoder for each individual stage was used (EM1-2-500-I EM1, US Digital Corporation, Vancouver, WA 98684, USA). The optical encoders work in conjunction with a polymer plastic strip material, and it uses a light-emitting diode source and a monolithic detector. The encoder output was connected to the counter input of a data acquisition (DAQ) board USB 6251 (NI, Austin, USA). [Figure 1]b shows the photo of a single stage.
Figure 1: (a) Drawing of a single-stage axis (all other axes were made using the same approach).(b) Photo of the single-stage axis

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Each stage was driven by a piezoelectric motor (USR60-S3N, Shinsei Kogyo Corp., Tokyo, Japan). The robotic system was designed so that it can be placed on the table of any high-field MRI system. It has a maximum a length of 25 cm, a width of 25 cm, and a height of 36 cm. The motion range of the robotic system was 9 cm for the X stage, 9 cm for the Y stage, and 8 cm for the Z stage. The robotic system weighted around 2 kg.

The robotic system was developed using the software Microstation (V8, Bentley Systems, Inc.). Drawings of the individual parts were sent to a three-dimensional (3D) digital manufacturing machine (FDM400, Stratasys, 7665 Commerce Way, Eden Prairie, Minnesota, 55344, USA) for production using acrylonitrile butadiene styrene plastic. [Figure 2]a shows the complete drawing of the system. [Figure 2]b shows a photo of the robotic system. [Figure 2]c shows a photo of the robotic system as it was placed inside an MRI scanner.
Figure 2: (a) Complete drawing of the robotic system.(b) Photo of the developed robotic system.(c) Photo of the robotic system as it was placed on the table of the magnetic resonance imaging scanner

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Because of the use of piezoelectric motors and encoders that require electricity during operation, the proposed robotic system is actually classified as MRI conditional, according to the ASTM standards (F2503, F2052, F2213, F2182, and F2119) which are excellently described by Stoianovici et al.[25]

Accuracy

The motion accuracy of the motor was evaluated using a digital caliper. One side of the caliper was fixed on a stationary part of the positioning device, and the other edge was fixed on a movable part. The movement of a specific step was measured using the incremental distance of the caliper. The resolution of the caliper was 10 μm.

The relationship between the measured distance (D) and number of encoder pulses (P) is linear.

D = 0.2+αP (1)

When the number of pulses is zero, the distance moved is not zero because during the execution of the programming code, some overhead time is always needed. Therefore, from this equation, as the user selects the distance to be moved (D), the program extracts the number of pulses that the encoder has to measure. The constant α depends on the encoder type (for the linear encoder is 500 pulses per inch). During the evaluation test at any given P, twenty (50) values of Y were measured. The distance move by each stage was measured and compared to the planned distance. The difference in distance for each axis was measured for each stage.

Weight loading

Different weight (increments of 0.2 kg) was placed on the moving plate of each axis. The maximum load was recorded that disables the motor to rotate.

Software

A user-friendly program was developed written in C # (Visual Studio 2010 Express, Microsoft Corporation, USA) having the following functionalities: (a) transferring of MRI images from the MRI scanner to the computer which controlled the therapeutic system, (b) motion control of the 3 Cartesian axes, (c) experimental details (phantom type, animal type, etc.), (d) archiving of functions activated, (e) transducer coordinates, (f) images of an MR compatible camera (MRC Systems GmbH, Heidelberg, Germany), (g) control of the signal generator (power, time ON, time OFF, frequency, duty factor, and pulse repetition period), (h) temperature acquired from thermocouple, and (k) temperature extracted from MR thermometry.

Robot drivers

A DC supply (24 V, 2 A) was used to drive the Shinsei motors. The control of the robot movement was achieved through a USB 6251 DAQ interface card (National Instruments, Austin, Texas, USA).

Focused ultrasound system

The functionality of the robot was evaluated by creating lesions in a commercially available polyacrylamide gel (ONDA Corporation, Sunnyvale CA, USA). The FUS system consisted of a signal generator (HP 33120A, Agilent Technologies, Englewood, CO, USA), an RF amplifier (250 W, AR, Souderton, PA, USA), and a spherical transducer made from piezoelectric ceramic (Sonic Concepts, USA). The transducer operated at 1.14 MHz had a focal length of 10 cm and diameter of 4 cm. The gel under evaluation was immersed in a container that included degassed water. The transducer was placed on the holder (plastic case) of the robotic system which was immersed in the water tank, thus providing excellent acoustical coupling between the gel and transducer. [Figure 3] shows the experimental setup used to evaluate the functionality and efficacy of the robotic system. An extension arm was mounted on the Z-stage, and the transducer holder was then mounted at the other end of the arm. This setup was used to test the functionality of the robotic system in the laboratory setting. In the clinical setting, a gel pad will be used to couple to the target.
Figure 3: Experimental setup that evaluated the functionality and efficacy of the robotic system

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Magnetic resonance imaging

The robotic FUS system was tested in a 1.5 T MR system (Signa, General Electric, Fairfield, CT, USA) using a GP FLEX coil (USA instruments, Cleveland, OH, USA). To test the MRI compatibility of the robotic system, the signal in the gel phantom was acquired. The signal to noise ratio (SNR) was measured under different states (activation of encoder, motor, or FUS transducer). The MRI pulse sequence of T1-weighted spoiled gradient (spoiled gradient echo sequence [SPGR]) was used with the following parameters: repetition time = 40 ms, echo time (TE) =20 ms, field of view = 21 cm, matrix = 128 × 128, flip angle = 20°, Number of excitations = 1.

Magnetic resonance thermometry

The temperature during FUS exposure was estimated using the proton resonance frequency shift phenomenon.[26] This method relates the associated phase shift derived from the frequency shift of the MR signal due to the local temperature elevation (ΔΤ). This relationship is described by equation 1:



Where ϕ(Τ) and ϕ(Τ0) are the absolute phases of the MR signal at a starting and final temperature T and Τ0, respectively, γ is the gyromagnetic ratio, α is the PRF change coefficient, B0 is the magnetic field strength, and TE is the echo time.

The SPGR was used to extract the MR thermometry maps. Phase maps were reconstructed by calculating the phase on a pixel-by-pixel basis after combining pixel data from real and imaginary channels. Although the scanner is capable of producing directly phase image reconstructions, the applied intrascan gradient nonlinearity corrections induced phase interpolation problems. Transient phase shifts developed during the acquisition of nontreated (mask) and treated images were compensated by subtracting internal references in regions away from the treated site. Temperature elevation was returned by the software as the maximum value in a prescribed region of interest (ROI) that was manually positioned. Temperature color-coded maps were produced by adjusting the color map (blue to red) for a range of minimum to maximum ROI temperature.


  Results Top


[Table 1] shows the range, distance difference, and maximum load that can be carried for each stage. Note that, because all stages are identical, the results are almost the same. Any changes that were recorded (distance error) are attributed to the measurement error.
Table 1: Range, distance difference, and maximum load that can be carried for each stage

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[Figure 4] shows the SNR of the phantom using SPGR measured for different component states (activation or not) for the encoder, motor, and transducer. When the robotic system was placed out of the MRI (control), the SNR was high (35). When the robotic system was introduced, the SNR remained high (around 33). The activation of the encoder appeared to cause minimal effect on the SNR (31). When the transducer was activated, the SNR dropped further (28) despite the use of a 10 MHz low-pass filter. Further drop in the SNR (26) was caused due to the activation of the motor.
Figure 4: Signal-to-noise ratio using spoiled gradient echo sequence measured for different component activations (encoder, motor, and transducer)

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[Figure 5] shows an MRI thermometry map of the acoustic beam in a plane perpendicular to the transducer beam using SPGR (coronal plane). The acoustical power used was 45 W for 60 s duration. Ultrasound was activated in the first 5 images and deactivated in the remaining images. Each image was acquired roughly every 10 s.
Figure 5: Magnetic resonance imaging thermometry map of the beam in a plane perpendicular to the transducer beam using spoiled gradient echo sequence (coronal plane). The acoustical power used was 45 W for 60 s duration. Ultrasound was activated in the first 5 images and deactivated in the remaining images. Each image was acquired roughly every 10 s

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[Figure 6] shows the photo of lesions produced in the gel phantom when the Y-axis was moved 5 times with a step of 5 mm. The lesions were created using the single element spherically focused transducer operating at 1.14 MHz. The exposure times were 25 s with an acoustical power of 45 W. This figure demonstrates the functionality of the robot placing lesions on the gel phantom. The lesion width varied from 2.1 to 2.6 mm. The length of the lesion varied from 46 to 52 mm. The variability in the lesion size is most likely attributed to the variability of absorption within the gel.
Figure 6: Photo of lesions in the gel phantom by moving the Y-axis with a step of 5 mm using a single element spherically focused transducer operating at 1.14 MHz (radius of curvature = 10 cm, diameter = 4 cm). The exposure times was 25 with an acoustical power of 45 W

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  Discussion Top


An MRI-conditional robotic system was developed that carries a focused ultrasound transducer. Nevertheless, the signal drop from 35 to 28 since the transducer activation caused minimal effect on the MR thermometry, especially if the transducer was at a distance from the imaging plane. Therefore, it can access organs that currently receive a lot of research and clinical attention (brain, fibroids, abdominal, breast, and thyroid). The robotic system was capable of moving the ultrasonic transducer in 3Ds. The range of motion of the system 9 cm (x-axis), 9 cm (y-axis), and 8 cm (Z-axis) is sufficient for all the potential applications.

The presence of the transducer, encoder, and motor caused signal drop in the SPGR MRI sequence. This was caused because all three components required electricity. The signal dropped from 35 (control) to 26 (motor activation). The activation of the encoder caused some droop in the signal (31). The activation of the transducer caused some further drop in the signal (28) due to radio frequency artifacts, despite the use of a 10 MHz low-pass filter. Finally, the maximum drop was caused when the motor was activated (26). The drop in the signal due to the motor and encoders was not a major issue since during the acquisition of data for MR thermometry, these two components were not activated (stage is stationary). The effect of the FUS transducer was the only one that matters since during MR thermometry, the transducer was activated. Nevertheless, the signal drop from 35 to 28 since the transducer activation caused minimal effect on the MR thermometry, especially if the transducer was at a distance from the imaging plane.

The heating experiment of [Figure 6] demonstrated the accurate movement of the transducer. We envision this system as being suitable for investigating novel therapeutic strategies with the intent to be used in conjunction with a phased array transducer.

In this article, we presented a simple robotic system that can carry a single element spherically focused transducer. The major challenge of this technology was the coupling of the transducer to the organ. This challenge has already been implemented by the team of Theraclion that established acoustic coupling to the thyroid or to the breast [27] with a transducer placed on the top of the target. The coupling media used was in the form of a gel pad.[27] Although this is a simple way to couple to targets using the top to bottom approach, it was not implemented in this paper. In our opinion, the coupling for such targets is not a major issue anymore and cannot harm the deployment of such robotic systems. This same concept was shown in another study performed by our group.[21]

The capabilities of this system are summarized in [Figure 7]. [Figure 7]a shows that we if could place the robotic system upside down and then fix it in the MRI scanner, we could increase the capabilities of this robot. Fixing this robot into the MRI scanner will require the collaboration with the manufacturer of the MRI scanner. [Figure 7]b shows the modification that needs to be done in the arm of the system so that access to brain is achieved (lateral). [Figure 7]c shows the modification that needs to be done in the arm of the system so that access to the abdominal area is achieved (superior to inferior). [Figure 7]d shows the access method for the thyroid or breast (same superior to inferior as in the case of the abdominal). These configurations of [Figure 7]b and [Figure 7]c (or 7d) requires the proper coupling to the target. It might seem challenging, but just it was achieved by the group of Theraclion [27] for non-MRI applications, it could be achieved also for the proposed MR applications. The coupling mechanisms do not require any magnetic materials, and therefore, the MR compatibility should be easy to implement. Finally, this robotic system can be used also for conducting MRgFUS experiments in animals of any size.
Figure 7: (a) System placed upside down in the magnetic resonance imaging scanner, (b) modification that needs to be done in the arm of the system so that access to brain (lateral), (c) modification that needs to be done in the arm of the system so that access to the abdominal area (superior to inferior), and (d) the access method for the thyroid or breast (same superior to inferior as in the case of the abdominal)

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An angular axis (theta axis) can be attached to the Y-axis in the future so that the maneuverability of the system is improved. In addition, a phi axes can be attached to the theta axis to provide an additional stage in a different angular axis. One major limitation of this robotic system is that with obese patients, the room allowed for the robotic system is limited and possibly these patients will be excluded for treatment.


  Conclusions Top


An MRI-conditional robotic system was developed that can access targets either laterally, or superior to inferior. The robotic system was capable of moving the ultrasonic transducer in 3Ds. This robotic system can be used also for conducting MRgFUS experiments in animals of reasonable size.

Financial support and sponsorship

This work was supported by the Project PROFUS E! 6620. PROFUS is implemented within the framework of the EUROSTARS Program and is co-funded by the European Community and the Research Promotion Foundation, under the EUROSTARS Cyprus Action of the EUREKA Cyprus Program (Project Code: EUREKA/EUSTAR/0311/01).

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1]


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