Microwave Induced Thermoacoustic Tomography

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Microwave Induced Thermoacoustic Tomography

• By Zaid Al-Husseini

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Sections

• Introduction to MITT (Microwave Induced
  Thermoacoustic Tomography)
• Experimental Set up (Apparatus)
• Discussion of Results
• Advantages
• Disadvantages

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Introduction to MITT

• Method to Image biological tissue
• Electromagnetic microwaves are pulsated towards a
  biological material resulting in the absorption
  of heat and hence a mechanical expansion of the
  material.
• Acoustic waves are created by thermal expansion
  of material which propagate throughout the
  material in all directions in the form of sound.
• Increasing the pulsation of the electromagnetic
  waves induces a higher rate of expansion and
  contraction in the material resulting in higher
  frequencies in the acoustic waves ie. Ultrasound.
• The higher the acoustic frequency produced, the
  lower the wavelength of the acoustic waves which
  results in a higher resolution of images. Since
• ? vsound / fsound
• Unlike conventional acoustic imaging MITT depends
  on the difference in dielectric constants between
  tissues whereas ultrasound imaging depends on
  acoustic impedance between tissues. This gives
  MITT the potential for imaging objects invisible
  to ultrasound (due to equal impedancesetc)

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Apparatus and Experimental Setup

• General Apparatus simulating a biological
  material (usually animal fat tissue w/muscle)
  enclosed in mineral oil and is exposed to
  microwave generator.
• Within the apparatus is an ultrasonic transducer
  which detects Ultrasonic Thermoacoustic waves.
• Data is collected from several angles of the
  object and is processed by a computer.
• The data is collected through linear scanning of
  the ultrasonic transducer which results in
  multiple one-dimensional images.
• The computer uses the obtained one-dimensional
  images to reconstruct the data into
  two-dimensional images.
• Time delay and velocity manipulation of produced
  acoustic waves are used to obtain distance
  variations of the material and reconstruct/image
  the material sample.

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Sample Experimental Set Up
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Discussion of Results

• Microwave induced acoustic pressure is
  proportional to the intensity of the incidental
  microwaves. The different dialectic constants
  between different tissues provides good imaging
  in MITT which is fundamentally different than
  ultrasounds reliance on acoustic impedance
  differences.
• Using a non-focused transducer allows for a
  larger reception angle than focused one allowing
  for a wider ranged data set.
• Higher frequencies of microwave pulses (e.g. 9
  GHz) resulted in higher resolution but less depth
  penetration than low frequencies due to large
  attenuation of signal at higher frequencies.
• Lower frequencies allowed for much more depth
  penetration due to higher wavelengths but a lower
  Signal noise ratio (SNR) than higher frequencies.
• Strong microwave absorption results in decreased
  clarity of produced images (eg. High muscle
  tissue absorption).
• Broadening of the pulse signal proportionally to
  the transducer surface allowed for better
  optimization of image depiction at boundaries.
• Axial resolution (alleviate stretching problems)
  by reducing transducer diameter at the cost of a
  lower SNR.

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Images from Thermoacoustic Tomography
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Advantages

• Takes advantage of Microwave imagings
  non-ionizing radiation and high imaging contrast
  benefits due to the use of dielectric property
  differences of materials
• Benefits from Ultrasounds exceptional spatial
  resolution.
• Low Power emissions make it acceptable for use on
  humans and biological tissue and thus make them
  harmless (based on IEEE standards).
• High Depth penetration, spatial resolution and
  contrast at low/high frequencies allows detection
  of tumors at earlier stages.

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Disadvantages

• Limited Depth Recognition and high frequencies
  due to large attenuation resulting in sacrifice
  between resolution and depth.
• Most Depth is limited to about 40-45 mm which is
  impractical for most breast tumor detection.
• Sacrifice between axial resolution improvement at
  the cost of lower SNR due to widening of
  transducer reception