###### Abstract  
In this article, we propose a new approach utilizing diffuse optical tomography (DOT) to monitoring the changes in tissues' optical properties and temperature in high-intensity focused ultrasound (HIFU) therapy. By correlating the tissue reduced scattering coefficient (\(\mu^{\prime}_{s}\)) reconstructed by DOT and the temperature measured by a thermocouple, the quantitative relationship between \(\mu^{\prime}_{s}\) and temperature in HIFU treatment was explored. The experiments were conducted using porcine and chicken breast muscle tissues during HIFU; the temperature of each tissue sample was recorded using a thermocouple. To incorporate the temperature dependency of tissue optical properties, both polynomial and exponential models were utilized to fit the experimental data. The results show that the change of \(\mu^{\prime}_{s}\) during HIFU treatment could be detected in real-time using DOT and that this change of \(\mu^{\prime}_{s}\) is quantitatively correlated with tissue temperature. Furthermore, while the tissue-type-dependent relationship between \(\mu^{\prime}_{s}\) and temperature is non-linear in nature, it is stable and repeatable. Therefore, our approach has the potential to be used to predict temperature of tissue during HIFU treatment. diffuse optical tomography (DOT); photon diffusion equation; absorption coefficient; scattering coefficient; high-intensity focused ultrasound (HIFU); temperature +
Footnote †: journal: Medical Engineering, University of South Florida, Tampa, FL 33612, USA; [EMAIL_ADDRESS] (H.  
utilizing a thermocouple. An alternative tool with which to measure temperature is a thermal imaging camera. While the thermal imaging camera provides two-dimensional (2D) measurement of temperature with high precision, the superficial measurement on the surface highly limits its application in HIFU therapy. Recently, magnetic resonance (MR)-guided high-intensity focused ultrasound (MR-HIFU) treatment, an image guided technique which combines high-intensity focused ultrasound ablation with real-time monitoring of temperature change during the treatment [14; 15; 16], was approved by the Food and Drug Administration (FDA) for the treatment of osteoid osteoma in the extremities [17; 18]. Although this treatment modality is encouraging, MRI is too expensive in most situations and still has limitations in spatial and temporal resolutions in temperature measurement. Moreover, spatial and temporal averaging of the measured temperature can lead to an underestimation of the temperature and errors in the estimation of the thermal dose. Another limitation of this method is that it measures only temperature change rather than absolute temperature. Although there have been some developments in this field [19; 20; 21], measuring the absolute temperature is very challenging. It is for these reasons that an alternative approach for non-invasive volumetric temperature measurement is desired. Optical properties of tissue, including the absorption coefficient (\(\mu_{a}\)) and the reduced scattering coefficient (\(\mu_{s}^{{}^{\prime}}\)), provide sufficient information to monitor tissue metabolic status or diagnose disease. Characteristic changes in tissue optical properties can be indicative of disease, which is due to the thermal dissociation of phospholipid cellular membranes and thermal denaturation of intracellular and extracellular proteins when heat is applied to tissue [22; 23; 24; 25]. It was reported that the first change in optical properties was detected at 45 \({}^{\circ}\)C and that there were a 2-fold increase and 7-fold increase in absorption and scattering, respectively, at temperatures above 65 \({}^{\circ}\)C [24]. Therefore, it is conceivable that an alternative approach could be realized by measuring the changes in optical properties, especially \(\mu_{s}^{{}^{\prime}}\), as a function of temperature. Integrating sphere measurement is a common method to measure the optical properties of tissue. However, this method is invasive and measures temperature only at one point.