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 Table of Contents  
REVIEW ARTICLE
Year : 2019  |  Volume : 5  |  Issue : 2  |  Page : 52-55

The role of phantoms in magnetic resonance imaging-guided focused ultrasound surgery


Department of Electrical Engineering, Cyprus University of Technology, Limassol, Cyprus

Date of Web Publication23-Sep-2019

Correspondence Address:
Christakis Damianou
30 Arch. Kyprianou, 3036 Limassol
Cyprus
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/digm.digm_13_19

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  Abstract 


This article reviews the role of mimicking materials used in focused ultrasound surgery (FUS) under magnetic resonance imaging. FUS is a noninvasive surgery that has many applications in oncology and neurology. Phantoms (mimicking materials) are mostly based in agar or gelatin phantoms.

Keywords: Magnetic resonance imaging, phantom, ultrasound


How to cite this article:
Damianou C. The role of phantoms in magnetic resonance imaging-guided focused ultrasound surgery. Digit Med 2019;5:52-5

How to cite this URL:
Damianou C. The role of phantoms in magnetic resonance imaging-guided focused ultrasound surgery. Digit Med [serial online] 2019 [cited 2019 Oct 20];5:52-5. Available from: http://www.digitmedicine.com/text.asp?2019/5/2/52/267607




  Introduction Top


Focused ultrasound surgery (FUS) has the potential to heat tumors and therefore is used extensively for oncological applications in the liver, breast, brain, fibroids,[1],[2],[3],[4],[5] kidney,[6] and prostate.[7] This procedure is minimally invasive because FUS is applied externally to the body. FUS produces localized heating, and therefore, the heating is not affecting the surrounding tissue or intervening tissue. The procedure can be monitored either by ultrasound imaging[8],[9] or by magnetic resonance imaging (MRI).[10],[11],[12],[13],[14],[15] Although at the beginning of the deployment of robotics systems in the MRI, the task was considered challenging, recently it has been shown[16],[17],[18],[19],[20],[21] that the fear of MRI compatibility does not exist anymore. Ultrasonic imaging is the simplest and most inexpensive method to guide FUS. However, MRI offers superior contrast than ultrasound and offers magnetic resonance (MR) thermometry, which can monitor thermal heating nearly in real time. In addition, MRI is the gold standard regarding the diagnosis of tumors. Therefore, despite the expensive capital investment needed to install MRI, the numerous benefits of MRI make it as an attractive solution to guide focused ultrasound.

FUS was explored in many organs or tissues that are accessible by ultrasound. For example, it was used in the kidney,[22],[23] eye,[24] prostate,[7],[25],[26],[27] brain,[28],[29],[30],[31],[32],[33] liver[34],[35],[36],[37] breast cancer,[38],[39],[40] bone,[41],[42],[43],[44] and gynecological tumors.[45],[46],[47],[48],[49],[50]


  Materials and Methods Top


Data were acquired from articles search in PubMed.


  Results Top


In the last years, there were a lot of initiatives to develop mimicking materials (phantoms) that have ultrasonic properties close to humans (especially attenuation, absorption, and propagation velocity). With ultrasonic phantoms, experiments in animals can be minimized. Phantoms are less expensive than experimental animals. Moreover, phantoms are more ergonomic compared to experimental animals. With the use of preservatives, phantoms can be used repeatedly for many months. One category of phantoms that are quite functional for ultrasound research is an agar-based phantom.[47] This phantom uses plastic to mimic the brain and agar-based mixture to mimic brain tissue.[51] This phantom is MRI-compatible and is ergonomic. Recently, agar-based phantoms were fully characterized[52] and therefore, all thermal and acoustical properties are known. Agar-based phantoms can mimic various organs. The breast was mimicked using plastic for ribs and agar-based mixture for breast tissue.[53] With this phantom, one issue that was examined, was the effect of the ribs on the delivery of focused ultrasound. In another phantom,[44] plastic was used to mimic bone, and the agar-based mixture was used to mimic tissue that exists within the bone. With this phantom, the objective was to assess the beam distortion due to the presence of the bone. The results revealed MR temperature maps which were similar to maps obtained in humans.


  Conclusions Top


MRI-guided focused ultrasound is a field that it is growing very fast. Although many successful clinical applications exist (for example, treatment of essential tremor[54] or treatment of fibroids[55]), the deployment of other clinical applications is slow. One reason is the extensive animal evaluation needed. Phantoms can play an important role to speed up the deployment of new applications in MRI-guided FUS. The role of three-dimensional printing[56],[57] is important and can assist the design of FUS phantoms. FUS is mostly applied for oncological applications. Recently, it has shown that it may have important applications in neurological applications such as the treatment of essential tremor,[54] or for sonothrombolysis,[58],[59],[60],[61],[62],[63] or MRI-guided sonothrombolysis,[64],[65],[66],[67] or for plaque removal from arteries.[68],[69] Recently, agar-based phantoms have been fully characterized,[52] and thus, not only acoustical properties are known (acoustic propagation, attenuation) but also thermal properties such as thermal conductivity. This creates a mimicking material which is closer to the acoustical and thermal properties of tissue. The melting point of Agar is close to 85°C and therefore is suitable for focused ultrasound experiments. Gelatin-based mimicking materials have melting point close to 45°C and therefore, not suitable for focused ultrasound experiments. Gelatin phantoms though can be a useful mimicking option for experimentation for diagnostic ultrasound which does not produce significant heating.

Mimicking materials (phantoms) can be created to mimic either bone (using Acrylonitrile Butadiene Styrene (ABS)) or to mimic soft tissue (agar-based mixture). With skull phantoms, the objective was to assess the beam distortion due to the presence of the skull.

In future, more advanced phantoms will be needed to assess the focused ultrasound propagation for other types of organs (accessible by focused ultrasound). There is also need to find materials that can adjust the value of thermal conductivity and thermal conductivity (thermal properties). Trying to modify the thermal properties (conductivity or specific heat), alters other properties such as acoustic attenuation or scattering or propagation velocity (acoustical properties). Hence, there will be some trade-offs between thermal and acoustical properties. One cannot achieve both categories in a mixture. Therefore, there are still many challenges regarding the design of mimicking materials for focused ultrasound.

Another property of mimicking materials that was never assessed is acoustic absorption. This is attributed to the fact that measurement of ultrasonic absorption is a very complicated procedure. In order to measure the ultrasonic absorption one needs to measure the rate of temperature change by avoiding conduction. This was measured in the past using thermocouples. This is a complicated procedure and might not result to a very accurate measurement due to the effect of the size of the thermocouple. One was to overcome this problem now is to use data acquired using MR thermometry. To the best of our knowledge, such measurement (ultrasonic absorption) does not exist for mimicking materials (phantoms) either for agar based or gelatin based.

It is also possible that these mimicking materials are used for other technologies such as radio frequency, laser, or microwaves. For these technologies, the acoustical properties of phantoms might not be of interest, but as long as the thermal properties are preserved, the phantoms can be of intense interest.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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