Stratified spherical model for microwave imaging of the brain: Analysis and experimental validation of transmitted power
Summary (2 min read)
Introduction
- For this purpose, a spherical layered model composed of dispersive biological tissues is investigated in the range of [0.5-4].
- Several main factors influence the quality of the MWI: the scattered power, the frequency and the medium in which the test object is immersed, called the matching or the background medium.
- To validate numerical models, interesting works have been done in the microwave imaging community with a 3D printed breast phantoms [17] [18] with Triton X-100 based mixtures to mimic biological tissues.
- For this, a spherically stratified head phantom has been built to experimentally estimate the power transmitted into the brain and to analyze the influence of these layers on the propagation of the EM wave.
II. MODEL AND ELECTROMAGNETIC ANALYSIS
- The spherically stratified model, sketched in Fig. 1 , is a more realistic model of the human head than the planar model in [14] while still allowing an analytic solution for the electric field distribution.
- Since the authors deal with higher frequencies and, thus, shorter wavelengths, they model the CSF separately and the dielectric characteristics of the brain are set with a grey over white matter ratio (GM/WM ratio) of 1.5 as shown in [21].
- A similar ratio was used in [20], as the dielectric properties of the latter two are similar in the frequency range of interest.
- Due to the spherical geometry of the boundary conditions, the electric field can be expanded as an infinite sum of vector spherical harmonics and be expressed analytically.
- These results are valid for a plane wave impinging on the head phantom.
C. Simulation Results and Discussion
- The normalized transmitted power represents the incoming power that can excite any anomaly in the center of the head (worst case scenario) and therefore produce a scattered field.
- Fig. 2 represents a map of the normalized transmitted power plotted in dB for the spherical (left) model.
- The color change corresponds to a drop in the normalized transmitted power in steps of 3dB and up to -36dB (all values below -36dB are depicted as the same dark blue color).
- It indicates that the power transmission is mainly affected by the tissue attenuation which is exponentially increasing with frequency.
- According to these observations, one can freely choose the permittivity of the matching medium with respect to power transmission.
A. Head Prototype
- A 3D printed concentric multilayered spherical structure (see Fig. 3) has been manufactured.
- The authors used the Fused Deposition Modeling technology and white ABS (Acrylonitrile butadiene styrene) for the plastic.
- The filling system consists of 5 entries, allowing to use a different liquid for each shell.
- Several recipes are available to make liquids mimicking the main human head tissues.
- The results for permittivity and conductivity measurements are depicted in Fig. 4, where the dispersive characteristics of their theoretical values can be observed over the frequency band of interest.
B. Measurement Results
- The transmission parameter |𝑆12| between a monopole antenna (port 2) vertically placed in the center of the head phantom and a vertically polarized horn antenna (port 1) placed at 1m distance is measured between 0.5 and 4 GHz with a HP 8720D to ensure far field conditions of a linearly polarized plane wave, along the z-direction.
- This figure shows reasonable agreements between the measured and the simulated data of the same configurations.
- In the lower part of Fig.8, the product of all these influences gives the total influence 𝑅(𝜔), of all the layers on the wave propagation.
- It appears clearly that up to around 1.2GHz the “barrier” formed by the CSF, bone, fat and skin, is beneficial to the power transmission, and the opposite above this limit.
IV. CONCLUSION
- The analysis of the normalized transmitted power of an impinging electromagnetic field onto a simplified model of the human head for MWI applications has been presented using a spherical multilayered model.
- Additionally, the authors presented a methodology to perform measurements outside and inside liquid phantoms using the 3D printing technology.
- The strong attenuation of at least 15dB between 1.5GHz and 3GHz in the measurements matches the predictions made with simple transmission line models [14].
- The experimental results have been compared to theoretical results based on a spherical wave expansion and showed reasonable agreement.
- This information provides insight on the limit of the frequency, where this parameter starts to have a negative impact on the signal transmission between the brain and the matching medium, and therefore on the imaging quality.
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..., elliptical and spherical layered models [59]...
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Cites background from "Stratified spherical model for micr..."
...Significant efforts, including experimental lab test on phantoms, were published in [11]....
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"Stratified spherical model for micr..." refers background in this paper
...Conversely, a multilayered spherical model better approximates the head geometry while still allowing an analytical solution to the electromagnetic scattering problem.(19,20) This article focuses on the analysis of the power transmission through the four main layers between the brain and the background medium, namely the cerebrospinal fluid 1042 | BJELOGRLIC ET AL....
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Frequently Asked Questions (14)
Q2. Why is the electric field expanded as an infinite sum of vector spherical harmonics?
Due to the spherical geometry of the boundary conditions, the electric field can be expanded as an infinite sum of vector spherical harmonics and be expressed analytically.
Q3. How much power does the optimum spherical structure have?
At 1GHz for example, the optimum is at 𝜀𝑚𝑚 = 10, however the normalized transmitted power drops by approximately 3dB if 𝜀𝑚𝑚 = 80 (approximately water at 1GHz), but the imaging resolution would increase by almost a factor of 3.
Q4. What can be done to improve the accuracy of the head MWI?
Depending on the sensitivity of the data acquisition of the imaging system, the frequency and matching medium ranges can be chosen using simplified analytical models and then be fine-tuned using more complex EM solvers and more realistic models of the head.
Q5. What is the normalized transmitted power of the simulated data?
GHz the normalized transmitted power 𝑃𝑁𝑡 drops very rapidly by 15dB due to the strong attenuation in the tissues, which was predicted by both the planar and the spherical model.
Q6. What is the Larmor frequency for 7T MRI?
In [19] for 7T MRI the Larmor frequency is around 300MHz and the brain region is modeled as a combination of CSF, grey matter, and white matter.
Q7. What is the transmission parameter between a monopole antenna and a horn antenna?
The transmission parameter |𝑆12| between a monopole antenna (port 2) vertically placed in the center of the head phantom and a vertically polarized horn antenna (port 1) placed at 1m distance is measured between 0.5 and 4 GHz with a HP 8720D to ensure far field conditions of a linearly polarized plane wave, along the z-direction.
Q8. What is the dielectric properties of the ABS plastic structure of the 3D printed prototype?
The dielectric properties of the ABS plastic structure of the 3D printed prototype were measured in the range of [0.5 − 4] GHz using the Agilent 85070E dielectric probe kit.
Q9. What is the color change of the PNt?
The color change corresponds to a drop in the normalized transmitted power in steps of 3dB and up to -36dB (all values below -36dB are depicted as the same dark blue color).
Q10. What is the optimum frequency for the spherical structure?
The almost free choice of the permittivity (1dB drop of 𝑃𝑁𝑡 for increasing 𝜀𝑚𝑚 from 56 to 80) means also, that it can be used to improve the imaging resolution according to the discussion in the introduction.
Q11. What is the effect of the background medium on the transmission of the electromagnetic field?
The strong attenuation of at least 15dB between 1.5GHz and 3GHz in the measurements matches the predictions made with simple transmission line models [14].
Q12. What is the effect of the filling process on the transmission of the wave?
As the filling process allowed to fill each layer on-site without moving the prototype (see Fig. 5), it was possible to estimate the influence of each layer on the power transmission.
Q13. What is the definition of a plane wave?
because the authors are only interested in the transmission inside the head, this is not a real restriction and the results using a plane wave should be also valid for an antenna directly placed on the head since the propagation of an electromagnetic wave depends only on the properties of the medium and not on the characteristics of the wave, that is plane wave, spherical wave, etc.
Q14. What is the spherically stratified model of the human head?
The spherically stratified model, sketched in Fig. 1 (right), is a more realistic model of the human head than the planar model in [14] while still allowing an analytic solution for the electric field distribution.