Difference between revisions of "Team:Slovenia/Model"

 
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                     <b>Controlling device</b>
 
                     <b>Controlling device</b>
 
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                 </a>
                 <a class="item" href="#int" style="margin-left: 10%">
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                 <a class="item" href="//2016.igem.org/Team:Slovenia/Model" style="color:#DB2828;">
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                    <i class="selected radio icon"></i>
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                    <b>Modeling of</b>
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<br />
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<b style = "margin-left: 12%">ultrasound propagation</b>
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                     <i class="selected radio icon"></i>
 
                     <b>Introduction</b>
 
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                     <b>Conclusion</b>
 
                     <b>Conclusion</b>
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                    <b>References</b>
 
 
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                 <a class="item" href="//2016.igem.org/Team:Slovenia/Software">
 
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<div class="article" id="context">
 
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<div class="main ui citing justified container"><h1 class = "ui left dividing header"><span class="section">&nbsp;</span>Modeling of ultrasound propagation</h1>
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<div class="main ui citing justified container"><h1><span class = "section colorize"> &nbsp; </span>Modeling of ultrasound propagation</h1>
<div>
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<div class = "ui segment" style = "background-color: #ebc7c7; ">
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<p><b><ul><li>We generated a model for the propagation of the ultrasound waves through tissue.
 +
<li>The source is available under GNU general public licence on <a href = "https://github.com/zigapusnik/UltrasoundPropagationModel">GitHub.</a>
 +
</ul></b></p>
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</div>
 
<div class = "ui segment">
 
<div class = "ui segment">
<h3><span id = "intro" class = "section"> &nbsp; </span>Ultrasound propagation modeling</h3>
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<h3><span id = "intro" class = "section colorize"> &nbsp; </span></h3>
 
<p>Ultrasound are sound waves with frequencies higher than upper audible limit of human hearing, i.e. higher than 20 kHz.  
 
<p>Ultrasound are sound waves with frequencies higher than upper audible limit of human hearing, i.e. higher than 20 kHz.  
 
In the frequency range between 0.8 and 5 Mhz, the sound wave length is between 2 and 0.3 mm. Ultrasound is mainly used for diagnostic  
 
In the frequency range between 0.8 and 5 Mhz, the sound wave length is between 2 and 0.3 mm. Ultrasound is mainly used for diagnostic  
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</div>
 
</div>
 
<!--<div>
 
<!--<div>
<h2><span id ="usesInMedicine" class = "section"> &nbsp; </span>Use of high intensity focused ultrasound in medicine</h2>
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<h2><span id ="usesInMedicine" class = "section colorize"> &nbsp; </span>Use of high intensity focused ultrasound in medicine</h2>
 
<p>
 
<p>
 
Parkinson&#39;s disease (PD) is considered as the most frequent movement disorder. At the same time, it also represents second most common
 
Parkinson&#39;s disease (PD) is considered as the most frequent movement disorder. At the same time, it also represents second most common
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</p>
 
</p>
 
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<h1 class = "ui left dividing header"><span class="section colorize">&nbsp;</span>Modeling</h1>
 
<div class = "ui segment">
 
<div class = "ui segment">
<h3><span id = "modeling" class = "section"> &nbsp; </span>Sound modeling with wave equation</h3>
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<h3><span id = "mod" class = "section colorize"> &nbsp; </span>Sound modeling with wave equation</h3>
 
<p>The code for the ultrasound propagation model is written in C++ programming language with OpenGL library. The model was parallelized using OpenMP to take advantage of multi-core processor architecture and to achieve better performance. Source code is available on <a href = "https://github.com/zigapusnik/UltrasoundPropagationModel">GitHub.</a></p>
 
<p>The code for the ultrasound propagation model is written in C++ programming language with OpenGL library. The model was parallelized using OpenMP to take advantage of multi-core processor architecture and to achieve better performance. Source code is available on <a href = "https://github.com/zigapusnik/UltrasoundPropagationModel">GitHub.</a></p>
 
<br />
 
<br />
 
<p>
 
<p>
 
Sound is a vibration that propagates as a mechanical wave of pressure and displacement through the medium. The speed of its propagation also depends on the properties of the medium.
 
Sound is a vibration that propagates as a mechanical wave of pressure and displacement through the medium. The speed of its propagation also depends on the properties of the medium.
This means that the speed of sound varies greatly between different types of tissues <x-ref>azhari2010basics</x-ref>. The speed of sound propagation in different tissue in comparison to its propagation through air is represented in (<ref>2</ref>).
+
This means that the speed of sound varies greatly between different types of tissues <x-ref>azhari2010basics</x-ref>. The speed of sound propagation in different tissue in comparison to its propagation through air is represented in <ref>2</ref>.
 
</p>
 
</p>
 
 
<table class="ui fixed single line celled table" data-ref="2">
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In order to achieve a realistic simulation of the sound propagation through brain an MR image represented in <ref>3</ref> was used <x-ref>mollerpocket</x-ref>. Furthermore, we performed the tissue segmentation with the aid of medical experts, i.e. our team members, students of medicine (Kosta, Samo and Nina) (<ref>4</ref>).
 
In order to achieve a realistic simulation of the sound propagation through brain an MR image represented in <ref>3</ref> was used <x-ref>mollerpocket</x-ref>. Furthermore, we performed the tissue segmentation with the aid of medical experts, i.e. our team members, students of medicine (Kosta, Samo and Nina) (<ref>4</ref>).
 
</p>
 
</p>
<div style = "float:left; width:50%">
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<div style = "float:left; width:50%;">
 
<figure data-ref="3">
 
<figure data-ref="3">
<img src="//2016.igem.org/wiki/images/0/0b/T--Slovenia--Head.png"> </img>
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<img src="//2016.igem.org/wiki/images/0/0b/T--Slovenia--Head.png">
 
<figcaption>
 
<figcaption>
<p style="text-align:justify">An MR of transversal cut at the middle head area.
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<b>An MR of transversal cut at the middle head area.</b>
</p></figcaption>
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</figcaption>
 
</figure>
 
</figure>
 
</div>
 
</div>
<div style = "float:right; width:50%">
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<div style = "float: left; width:50%;">
 
<figure data-ref="4">
 
<figure data-ref="4">
 
<img src="//2016.igem.org/wiki/images/f/f8/T--Slovenia--Segm_head.jpg"> </img>
 
<img src="//2016.igem.org/wiki/images/f/f8/T--Slovenia--Segm_head.jpg"> </img>
<figcaption>
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<figcaption><b>The segmentation of head section.</b>
<p style="text-align:justify">The segmentation of head section, where the green color represents skin, blue color represents bone, cyan color represents the cerebrospinal fluid, dark grey color represents grey brain matter and light grey color represents white brain matter. While the speed of sound does not vary much between dark and white grey matter it is sufficient to model only brain matter in general.</figcaption>
+
<p style="text-align:justify">The green color represents skin, blue color represents bone, cyan color represents the  
 +
cerebrospinal fluid, dark grey color represents grey brain matter and light grey color represents white brain matter. Since the speed of sound does not vary
 +
much between dark and white grey matter it is sufficient to model only brain matter in general.</figcaption>
 
</p>
 
</p>
 
</figure>
 
</figure>
 
</div>
 
</div>
<p style = "clear:both">
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<p style = "clear:left; clear:right;">
 
 
 
</p>
 
</p>
<div style = "clear:left; clear:right; float:left; ; width:50%">
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<div style = "clear:left; clear:right; float:left; width:50%;">
 
<figure data-ref = "6">
 
<figure data-ref = "6">
 
<img src = "//2016.igem.org/wiki/images/a/a8/T--Slovenia--GaussianBeam.jpg"> </img>
 
<img src = "//2016.igem.org/wiki/images/a/a8/T--Slovenia--GaussianBeam.jpg"> </img>
<figcaption>
+
<figcaption><b>The representation of a Gaussian beam function in two dimensions using a probe with a focus of 7 cm.</b>
<p style="text-align:justify">The representation of a Gaussian beam function in two dimensions using a probe with a focus of 7 cm. Although focusing is not good, one can notice the elongation of a focused area. This is the case even in simulation with higher frequencies. This suggests that efficient focusing in the tissue requires at least two probes.</figcaption>
+
<p style="text-align:justify">Although focusing is not good, one can notice the elongation of a focused area. This is the case even in simulation with higher frequencies. This suggests that efficient focusing in the tissue requires at least two probes.</figcaption>
 
</p>
 
</p>
 
</figure>
 
</figure>
 
</div>
 
</div>
 
<p>
 
<p>
The probes in our model are capable of emitting sound waves at frequencies ranging from 0.5 Mhz to 5 Mhz at acoustic pressure from 10 kPa to 100 kPa. The focused probes that we used have focal length of 7 cm. Besides focused probes we also modeled unfocused probes. Both types of probes have an aperture of 4.4 cm.
+
The probes in our model are capable of emitting sound waves at frequencies ranging from 0.5 Mhz to 5 Mhz at acoustic pressure from 10 kPa to 100 kPa. The focused probes that we used have focal length of 7 cm. Besides focused probes we also modelled unfocused probes. Both types of probes have an aperture of 4.4 cm.
 
In general propagation of sound from the focused probe can be approximated with an analytic function of a Gaussian beam (<ref>6</ref>), but this representation fails when we want to model wave interferences from multiple probes. For such cases wave equation model needs to be established.  
 
In general propagation of sound from the focused probe can be approximated with an analytic function of a Gaussian beam (<ref>6</ref>), but this representation fails when we want to model wave interferences from multiple probes. For such cases wave equation model needs to be established.  
 
</p>
 
</p>
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\end{equation}
 
\end{equation}
  
While wave equation cannot be solved analytically, one can approximate it with finite difference method. Let's presume we want to approximate the solution on a squared area. First we need to discretize the area to obtain the grid of evenly distributed points $(x_{i}, y_{j})$, $i,j = 1,...,n$. The Laplacian operator can now be approximated as:
+
While wave equation cannot be solved analytically, one can approximate it with finite difference method. Let's presume we want to approximate the solution on a squared area. First, we need to discretize the area to obtain the grid of evenly distributed points $(x_{i}, y_{j})$, $i,j = 1,...,n$. The Laplacian operator can now be approximated as:
  
 
\begin{equation}
 
\begin{equation}
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\end{equation}
 
\end{equation}
 
</p>
 
</p>
<div style = "float:left; width:45%">
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<div style = "float:left; width:47%">
 
<figure data-ref="5">
 
<figure data-ref="5">
 
<img src="//2016.igem.org/wiki/images/a/ab/T--Slovenia--Scheme.png" >
 
<img src="//2016.igem.org/wiki/images/a/ab/T--Slovenia--Scheme.png" >
 
<figcaption>
 
<figcaption>
<p style="text-align:justify">A flowchart representing an iterative computation of wave equation.
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<b>A flowchart representing an iterative computation of wave equation.</b>
</p></figcaption>
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</figcaption>
 
</figure>
 
</figure>
 
</div>
 
</div>
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We model a propagation of the ultrasound waves on a 500 x 500 grid, where distances between points are exactly 0.3 cm. This translates to a stimulated area of 15 by 15 cm.  
 
We model a propagation of the ultrasound waves on a 500 x 500 grid, where distances between points are exactly 0.3 cm. This translates to a stimulated area of 15 by 15 cm.  
 
The model computes propagation of waves in 2 dimensions. While the computational complexity of calculating the solutions of wave equation in 3 dimensions is asymptotically much higher than in 2 dimensional space, it is unnecessary to demonstrate the ability to focus the ultrasound by modulation of the intensity, frequency and geometry of the ultrasound probes.  
 
The model computes propagation of waves in 2 dimensions. While the computational complexity of calculating the solutions of wave equation in 3 dimensions is asymptotically much higher than in 2 dimensional space, it is unnecessary to demonstrate the ability to focus the ultrasound by modulation of the intensity, frequency and geometry of the ultrasound probes.  
However, in 3 dimensional space the intensity in the focal point should increase, since addition of sound waves is coming from all directions not just from left and right. Three or more probes could be combined to define the focal point in 3D. In our model it is also possible to observe the main sound characteristics such as the attenuation, reflection and occlusion. Probes are modeled as the set of finite elements. The pressure of every finite element on the probe is set according to sine function with the selected amplitude and frequency, thus every finite element acts like independent source of the acoustic pressure.  
+
However, in 3 dimensional space the intensity in the focal point should increase, since addition of sound waves is coming from all directions not just from left and right. Three or more probes could be combined to define the focal point in 3D. In our model it is also possible to observe the main sound characteristics such as the attenuation, reflection and occlusion. Probes are modelled as the set of finite elements. The pressure of every finite element on the probe is set according to sine function with the selected amplitude and frequency, thus every finite element acts like independent source of the acoustic pressure.  
 
</p>
 
</p>
</div>
 
 
 
</div>
 
</div>
 
<div>
 
<div>
<h1><span class="section">&nbsp;</span>Results</h1>
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<h1><span id = "res" class = "section colorize"> &nbsp; </span>Results</h1>
<div class = "ui segment">Modeling of ultrasound propagation</h1>
+
<div class = "ui segment">
 +
 
<p>
 
<p>
 
Simulations computed with our model (<ref>7</ref>) clearly show that it is possible to target small regions of brain tissue. We can also observe the one dimensional cut of acoustic pressure, where focusing is even more evident (<ref>8</ref>). The probe on the right is slightly displaced due to the asymmetry of the tissue. This clearly shows that models need to be applied to calculate the optimal position of ultrasound probes.  
 
Simulations computed with our model (<ref>7</ref>) clearly show that it is possible to target small regions of brain tissue. We can also observe the one dimensional cut of acoustic pressure, where focusing is even more evident (<ref>8</ref>). The probe on the right is slightly displaced due to the asymmetry of the tissue. This clearly shows that models need to be applied to calculate the optimal position of ultrasound probes.  
Such software will include automatic tissue segmentation, modeling of propagation of ultrasound waves and will assist experts in probe positioning. Our model is a step in that direction, since it can be used as a tool for positioning the probes. In the model that we generated focused and unfocused probes can be added or removed and positioned. Additionally probe's acoustic pressure and frequency can be set.   
+
Such software will include automatic tissue segmentation, modeling of propagation of ultrasound waves and will assist experts in probe positioning. Our model is a step in that direction, since it can be used as a tool for positioning the probes. In the model that we generated focused and unfocused probes can be added or removed and positioned. Additionally, probe's acoustic pressure and frequency can be set.   
 
</p>
 
</p>
 
<div align = "center">
 
<div align = "center">
 
<figure data-ref="7">
 
<figure data-ref="7">
 
<img class="playGif ui big centered image" src="//2016.igem.org/wiki/images/4/4a/T--Slovenia--USModelThumbnail.png" data-alt="//2016.igem.org/wiki/images/c/c0/T--Slovenia--Usmodel.gif">
 
<img class="playGif ui big centered image" src="//2016.igem.org/wiki/images/4/4a/T--Slovenia--USModelThumbnail.png" data-alt="//2016.igem.org/wiki/images/c/c0/T--Slovenia--Usmodel.gif">
<figcaption>
+
<figcaption><b>The propagation of ultrasound waves through brain tissue as computed with our model.</b>
<p style="text-align:justify">The propagation of ultrasound waves through brain tissue as computed with our model. One can observe the small focused area in the middle of the brain (Fornix). We can also detect green specks in the bone region, which are caused by the reflection of the sound waves. Since the speed of sound in the bone is much higher than the speed in soft tissue, the acoustic wavelengths in the bone are increased.
+
<p style="text-align:justify">One can observe the small focused area in the middle of the brain (Fornix). We can also detect green specks in the bone region, which are caused by the reflection of the sound waves. Since the speed of sound in the bone is much higher than the speed in soft tissue, the acoustic wavelengths in the bone are increased.
 
</p>
 
</p>
 
</figcaption>
 
</figcaption>
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<figure data-ref="8" align = "center">
 
<figure data-ref="8" align = "center">
 
<img class="playGif ui big centered image" src="//2016.igem.org/wiki/images/c/cc/T--Slovenia--1dThumbnail.png" data-alt="//2016.igem.org/wiki/images/7/7b/T--Slovenia--Us1d.gif">
 
<img class="playGif ui big centered image" src="//2016.igem.org/wiki/images/c/cc/T--Slovenia--1dThumbnail.png" data-alt="//2016.igem.org/wiki/images/7/7b/T--Slovenia--Us1d.gif">
<figcaption><p style="text-align:justify">The vertical cut of the acoustic pressure through the focal point. The waves that travel from the right to the left, are slightly deformed on the right side due to the wave interferences from two separate probes.
+
<figcaption><b>The vertical cut of the acoustic pressure through the focal point.</b>
 +
<p style="text-align:justify">The waves that travel from the right to the left, are slightly deformed on the right side due to the wave interferences from two separate probes.
 
</p></figcaption>
 
</p></figcaption>
 
</figure>
 
</figure>
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<div>
 
<div>
 
<div class = "ui segment">
 
<div class = "ui segment">
<h3><span id="conclusion" class = "section"> &nbsp; </span>Conclusion and outlook</h3>
+
<h3><span id="con" class = "section colorize"> &nbsp; </span>Conclusion and outlook</h3>
 
<p>
 
<p>
 
We generated a model for the propagation of the ultrasound waves through tissue. In our case a transversal cut of the middle head area was taken. Two probes were adjusted in such a manner that the focusing on small area was obtained.  
 
We generated a model for the propagation of the ultrasound waves through tissue. In our case a transversal cut of the middle head area was taken. Two probes were adjusted in such a manner that the focusing on small area was obtained.  
 
With this model we can confirm the potential to use the ultrasound as a platform for therapeutic applications, but for more advanced use the software tools needs to be further developed. Further development will include automatic tissue segmentation from patient medical images, computing propagation of ultrasound waves in 3 dimensions aimed to guide the experts with the probe positioning.  
 
With this model we can confirm the potential to use the ultrasound as a platform for therapeutic applications, but for more advanced use the software tools needs to be further developed. Further development will include automatic tissue segmentation from patient medical images, computing propagation of ultrasound waves in 3 dimensions aimed to guide the experts with the probe positioning.  
The low intensity ultrasound image could be used to receive the feedback of the sonogram of the tissue for each probe that could be used to drive ultrasound stimulation for cell activation in the desired area. It is likely that the specific shape of the stimulated tissue could be selected rather than just a single focal point.
+
The low intensity ultrasound image could be used to receive the feedback of the sonogram of the tissue for each probe that could be used to drive ultrasound stimulation for cell activation in the desired area. It is likely that the specific shape of the stimulated tissue could be selected rather than just a single focal point.
 
</p>
 
</p>
 
</div>
 
</div>
 
<div>
 
<div>
<h1 class = "ui left dividing header"><span id="ref-title" class = "section">&nbsp; </span>References</h1>
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<h1 class = "ui left dividing header"><span id="ref-title" class = "section colorize">&nbsp; </span>References</h1>
 
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<div class="citing" id="references"></div>
 
<div class="citing" id="references"></div>
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Latest revision as of 14:47, 19 October 2016

Ultrasound propagation modeling

  Modeling of ultrasound propagation

  • We generated a model for the propagation of the ultrasound waves through tissue.
  • The source is available under GNU general public licence on GitHub.

 

Ultrasound are sound waves with frequencies higher than upper audible limit of human hearing, i.e. higher than 20 kHz. In the frequency range between 0.8 and 5 Mhz, the sound wave length is between 2 and 0.3 mm. Ultrasound is mainly used for diagnostic purposes. Unlike light ultrasound can easily penetrate the tissue and is at those intensities completely harmless. It is therefore used also in prenatal diagnostics where non-invasivness is of vital importance. Short wavelengths additionally enable us to create small regions of high intensity focused ultrasound waves at the focal point, which can be used for therapeutic purposes escoffre2016therapeutic for example for the tissue ablation. High intensity focused ultrasound has been already used in thermal ablation therapies escoffre2016therapeutic. In vital organs like brain it is particularly important to focus high intensity ultrasound beams at the small targeted structure in order to avoid unwanted tissue damage.

Based on our improvement of the ultrasound responsiveness of the cells it would be feasible to introduce a genetic device into the target tissue and then selectively activate only specific cells by using focused ultrasound, causing a sufficient intensity of the acoustic pressure. This would for example enable the stimulation of neurons in the selected brain section to achieve non-invasive deep brain stimulation or to stimulate cells to produce different hormones.

For this purpose we designed a model of the ultrasound propagation. The model can guide us in the process of the ultrasound device calibration with the potential to use multiple ultrasound probes.

 Modeling

  Sound modeling with wave equation

The code for the ultrasound propagation model is written in C++ programming language with OpenGL library. The model was parallelized using OpenMP to take advantage of multi-core processor architecture and to achieve better performance. Source code is available on GitHub.


Sound is a vibration that propagates as a mechanical wave of pressure and displacement through the medium. The speed of its propagation also depends on the properties of the medium. This means that the speed of sound varies greatly between different types of tissues azhari2010basics. The speed of sound propagation in different tissue in comparison to its propagation through air is represented in 2.

Name Speed (m/s)
Skin 1730
White matter 1570
Grey matter 1570
Skull 4080
Cerebrospinal Fluid 1500
Air 343


In order to achieve a realistic simulation of the sound propagation through brain an MR image represented in 3 was used mollerpocket. Furthermore, we performed the tissue segmentation with the aid of medical experts, i.e. our team members, students of medicine (Kosta, Samo and Nina) (4).

An MR of transversal cut at the middle head area.
The segmentation of head section.

The green color represents skin, blue color represents bone, cyan color represents the cerebrospinal fluid, dark grey color represents grey brain matter and light grey color represents white brain matter. Since the speed of sound does not vary much between dark and white grey matter it is sufficient to model only brain matter in general.

The representation of a Gaussian beam function in two dimensions using a probe with a focus of 7 cm.

Although focusing is not good, one can notice the elongation of a focused area. This is the case even in simulation with higher frequencies. This suggests that efficient focusing in the tissue requires at least two probes.

The probes in our model are capable of emitting sound waves at frequencies ranging from 0.5 Mhz to 5 Mhz at acoustic pressure from 10 kPa to 100 kPa. The focused probes that we used have focal length of 7 cm. Besides focused probes we also modelled unfocused probes. Both types of probes have an aperture of 4.4 cm. In general propagation of sound from the focused probe can be approximated with an analytic function of a Gaussian beam (6), but this representation fails when we want to model wave interferences from multiple probes. For such cases wave equation model needs to be established.

Acoustic wave equation is a second-order linear partial differential equation that governs the propagation of acoustic waves through the medium. The general form of the equation is: \begin{equation} \frac{\partial^2 p}{\partial t^2} = c^2(\nabla^2 p) \end{equation} where $c$ is speed of sound in the medium, $p$ is acoustic pressure (the deviation from ambient pressure) and $\nabla^2$ is Laplacian operator (sum of second order partial derivatives with respect to each independent variable). In two dimensional Cartesian coordinate system we get the following equation: \begin{equation} \frac{\partial^2 p}{\partial t^2} = c^2(\frac{\partial^2 p}{\partial x^2} + \frac{\partial^2 p}{\partial y^2}) \end{equation} While wave equation cannot be solved analytically, one can approximate it with finite difference method. Let's presume we want to approximate the solution on a squared area. First, we need to discretize the area to obtain the grid of evenly distributed points $(x_{i}, y_{j})$, $i,j = 1,...,n$. The Laplacian operator can now be approximated as: \begin{equation} \nabla^2 p_{i, j} \approx \frac{1}{h^2}(p_{i + 1, j} + p_{i-1, j} + p_{i, j + 1} + p_{i, j - 1} - 4p_{i,j}) \end{equation} where $h$ is Euclidean distance between two adjacent points. Since the frequencies between 0.8 and 5 Mhz correspond to the wavelength between 2 and 0.3 mm, $h$ does not need to be smaller than 0.3 mm. We now obtain a $n*n$ system of the second order ordinary differential equations: \begin{equation} \ddot{p}_{i, j} = \frac{c^2}{h^2}(p_{i + 1, j} + p_{i-1, j} + p_{i, j + 1} + p_{i, j - 1} - 4p_{i,j}) \end{equation} These equation can be simplified to the first order differential equations with the introduction of a new variable that represents the speed: $(v_{i, j} = \dot{p}_{i, j})$. The new system now has the following form: \begin{equation} \begin{aligned} \dot{p}_{i, j} = v_{i, j} \\ \dot{v}_{i, j} = \frac{c^2}{h^2}(p_{i + 1, j} + p_{i-1, j} + p_{i, j + 1} + p_{i, j - 1} - 4p_{i,j}) \end{aligned} \end{equation} The dispersion of the waves is already included in the wave equation. However, the absorption of sound waves in tissue is not included. The absorption can be approximated with the introduction of the attenuation coefficient $k$. We fitted attenuation coefficient to experimental data sprawls1989ultrasound to obtain, $k = 16000$ for soft tissue and $k = 160000$ for bone tissue. The final equation now has the form: \begin{equation} \dot{v}_{i, j} = \frac{c^2}{h^2}(p_{i + 1, j} + p_{i-1, j} + p_{i, j + 1} + p_{i, j - 1} - 4p_{i,j}) - k*v_{i,j} \end{equation}

A flowchart representing an iterative computation of wave equation.

Finally we can solve the given system with iterative numeric methods such as Euler's method or Runge-Kutta method. The most widely known method of the Runge-Kutta family is RK4, which was also used in our model. While RK4 introduces some computational overhead it also significantly increases the numerical stability of the system in comparison to the Euler's method. The most basic steps used for the calculation of ultrasound propagation are displayed in (5).

We model a propagation of the ultrasound waves on a 500 x 500 grid, where distances between points are exactly 0.3 cm. This translates to a stimulated area of 15 by 15 cm. The model computes propagation of waves in 2 dimensions. While the computational complexity of calculating the solutions of wave equation in 3 dimensions is asymptotically much higher than in 2 dimensional space, it is unnecessary to demonstrate the ability to focus the ultrasound by modulation of the intensity, frequency and geometry of the ultrasound probes. However, in 3 dimensional space the intensity in the focal point should increase, since addition of sound waves is coming from all directions not just from left and right. Three or more probes could be combined to define the focal point in 3D. In our model it is also possible to observe the main sound characteristics such as the attenuation, reflection and occlusion. Probes are modelled as the set of finite elements. The pressure of every finite element on the probe is set according to sine function with the selected amplitude and frequency, thus every finite element acts like independent source of the acoustic pressure.

  Results

Simulations computed with our model (7) clearly show that it is possible to target small regions of brain tissue. We can also observe the one dimensional cut of acoustic pressure, where focusing is even more evident (8). The probe on the right is slightly displaced due to the asymmetry of the tissue. This clearly shows that models need to be applied to calculate the optimal position of ultrasound probes. Such software will include automatic tissue segmentation, modeling of propagation of ultrasound waves and will assist experts in probe positioning. Our model is a step in that direction, since it can be used as a tool for positioning the probes. In the model that we generated focused and unfocused probes can be added or removed and positioned. Additionally, probe's acoustic pressure and frequency can be set.

The propagation of ultrasound waves through brain tissue as computed with our model.

One can observe the small focused area in the middle of the brain (Fornix). We can also detect green specks in the bone region, which are caused by the reflection of the sound waves. Since the speed of sound in the bone is much higher than the speed in soft tissue, the acoustic wavelengths in the bone are increased.

The vertical cut of the acoustic pressure through the focal point.

The waves that travel from the right to the left, are slightly deformed on the right side due to the wave interferences from two separate probes.

  Conclusion and outlook

We generated a model for the propagation of the ultrasound waves through tissue. In our case a transversal cut of the middle head area was taken. Two probes were adjusted in such a manner that the focusing on small area was obtained. With this model we can confirm the potential to use the ultrasound as a platform for therapeutic applications, but for more advanced use the software tools needs to be further developed. Further development will include automatic tissue segmentation from patient medical images, computing propagation of ultrasound waves in 3 dimensions aimed to guide the experts with the probe positioning. The low intensity ultrasound image could be used to receive the feedback of the sonogram of the tissue for each probe that could be used to drive ultrasound stimulation for cell activation in the desired area. It is likely that the specific shape of the stimulated tissue could be selected rather than just a single focal point.

  References