WE3D-4 THERMOACOUSTIC CT RA Kruger’, WL Kiser, Jr., KD. Miller, HE Reynolds Indiana University Medical Center, 541 Clinical Dr., Indianapolis, IN 46202-5 11 1 DR Reinecke, GA Kruger, PJ Hofacker OptoSonics, Inc., 7210 Georgetown Rd., Suite 400, Indianapolis, IN 46268 ABSTRACT We have developed instrumentation for measuring the tissue-absorption properties of radio waves in the human body using thermoacoustic interactions. The imaging principles upon which this instrumentation is based are applicable to other irradiation sources, such as visible and infrared. We present the imaging reconstruction methodology that we have developed for mapping radiation absorption patterns in three dimensions. An example use of the technique to monitor breast tumor response to chemotherapy is presented. INTRODUCTION The thermoacoustic effect was first described in 1880 by Alexander Graham Bell, who observed sound emanating from a sheet of rubber that was being illuminated by an intermittent beam of sun light.’ Nearly a century after this first-reported thermoacoustic phenomenon, several groups reported the generation of thermoacoustic signals in soft tissue and soft-tissue-mimicking phantoms at microwave fr~uencies.2f.4,s,6.~,a.~ turbid media at optical and infrared wavelengths.’”’’ Some groups have attempted to produce medical images based on thermoacoustic signal generation, but have met with only limited success.’2,’3.’4.‘s.’6 Then in 1995, we proposed a general methodology for reconstructing three- dimensional maps of thermoacoustic absorption in soft tissue.” With that work as a starting point, we embarked on a research program whose aim was to develop a new technology for imaging the breast. We call this new imaging technology thennoacoustic computed tomography (Tcv e first reported similar findings in Since 1996, we have taken TCT fiom a new imaging concept to a promising new diagnostic imaging tool. We have produced the world’s first TCT images of a human breast in vivo, which demonstrated good soft tissue differentiation among normal breast tissues, and a spatial resolution of 1 - 3 line millimeters (FWHM) using safe levels of 434 MHz Recently, we successfully imaged breast cancer in viva3 While our imaging research is focused on the breast using 434 MHz radio waves, our work is readily extendable to other organs and alternative irradiation frequencies. MAMMOGRAPHY AT FlF FREQUENCIES We have chosen to implement TCT at 434 MHz, because we believe that radio or microwave fiequencies may be better suited to imaging the breast (and other organs) than are x rays or ultrasound. From 10 MHz - 20 GHz, the radiation absorption increases as the water content of the tissue increases. High water-content tissues, like glandular tissues and blood vessels, should be well differentiated from surrounding fatty tissues, which have significantly lower water content. More importantly, a number of studies have documented that cancerous tissues display consistently higher water fiactions than corresponding “normal” tissues. For example, the water content of epidermis is 60.9%, that of carcinoma of the skin is 81.6%;19 normal liver has a water content of 71.4%, while hepatoma tissue has 8 1.9%:’ Not coincidentally, in vitro experiments have demonstrated enhanced RF absorption by breast cancer relative to “normal” breast tissue over the range of 100 - 1000 MHZ?’.~~’”~”~~ This phenomenon has been attributed to an increase in bound water and sodium within malignant cells. To understand the origin of enhanced absorption by breast cancer over this frequency range, consider the following model. Foster and Shepps have proposed an empirical model to relate conductivity aand permittivity &of high- water-content tissues to the water fractionp @ > 0.7) of these tissues over the fiequency range of 0.01 < f < 18 GHz?~ According to this model we can write: where &,r(p) is a parameter extracted fiom the Debye relaxation (p) is the conductivity at 0.1 GHz, and 933 0-7803-5687-X/00/$10.00 0 2000 IEEE 2000 IEEE MTT-S Digestproperties of free water. These empirical formulae fit a wide range of tissue data for p > 0.7. The absorption of radio- and micro-waves by tissue is characterized by the tissue’s absorption coefficient a, which in turn is a function of the its conductivity C, permittivity E, and the fiequency f of the radiation according to:*’ where free space, respectively. The absorption of tissue can therefore be related to its water content by substituting Eqs. 1 and 2 into Eq. 3. and bare the permittivity and permeability of 0.1 1 -0 m: Breast tumor “signal” vs. kequency. Using this model, we have calculated the differential obsorption for a 1.0 cm spherical “tumor” (p = .8) embedded within “normal” breast tissue (p = 0.7) at a depth of 3.0 cm. The result is plotted in Fig. 1. The differential absorption peaks near 300 MHz, not far from an FCC-approved frequency for medical applications at 434 MHz. While we are not the first group to recognize the potential of imaging soft tissue with radio waves, we are the first to develop a viable imaging technology that allows us to overcome the diffiction limitations of using long- wavelength radio waves which have previously limited the spatial resolution to approximately !h the wavelength of the RF This is because our imaging strategy is a hybrid. Image contrast is determined by the absorption properties of soft tissue at 434 MHz, but the spatial resolution is determined by the sound propagation properties of soft tissue, and details of the detector my. IMAGING THEORY Consider a mass of soft tissue irradiated with a short pulse of radiation. Thermoacoustic waves will be produced throughout the volume of tissue, the strength of which depends on the local absorption properties within the tissue volume. Assume these waves are detected by an array of sonic detectors placed outside the body following some time delay. The excess pressure p, (t) that reaches a detector at position r and time t can be expressed as a volume integral:” where A(r’) is the fractional energy-absorption per-unit- volume of soft tissue at position r’, and ris the pulse width. Equation 4 is valid, provided Tis short enough. In practice r 1 kv meets this criterion. Thus, the pressure recorded at position r and time t = is the sum (integral) of all pressure waves induced over the s&ce of a sphere of radius Ir - rl in the tissue due