Microwave Induced Thermoacoustic Tomography

1. Microwave Induced Thermoacoustic Tomography By: Zaid Al-Husseini

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

3. 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 impedances…etc)

4. 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.

5. Sample Experimental Set Up

6. 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 ultrasound’s 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.

7. Images from Thermoacoustic Tomography

8. Advantages Takes advantage of Microwave imaging’s 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.

9. 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