Temperature Effect of HIFU with Thermal Dose Estimation

In the 21st century, cancer is seen as the biggest obstacle to the prolongation of life expectancy and the most important cause of death in every country in the world. According to the GLOBOCAN 2018 report produced by the International Agency for Research on Cancer (IARC), it is predicted that 18.1 million new cancer cases will be diagnosed based on all age groups and genders for 2018 and approximately 9.6 million people will die due to cancer. Considering all these situations, as well as the studies to increase the effectiveness of classical methods for cancer treatment, the search for alternative treatment methods have been accelerated. High Intensity Focused Ultrasound (HIFU) is a promising method for cancer researchers in recent years. Dosimetric studies are required for the widespread use of HIFU in the clinic.In this study, temperature characterization results, which are the basis of our research for the application of HIFU with dosimetric approach to colon cancer cell lines, are given.


INTRODUCTION
Cancer is a malignant tumor that is caused by the irregular division and proliferation of cells in a tissue or organ and is one of the most important health problems of our time and it has long been tried to treat with traditional methods such as radiotherapy, chemotherapy and surgical interventions. During treatment, as well as malignant tissues, healthy tissues are also damaged by traditional methods. The existence of psychological and physiological trauma caused by these methods on the patient has been the sole reason for the researchers to find alternative treatment methods for many years. While increasing the effectiveness of traditional treatment methods, reducing the side effects is the most common point of today's research.
High Intensity Focused Ultrasound (HIFU) has shown promising advances for cancer researchers in recent years and has been used in the treatment of certain types of cancer [1]. Systems developed for the treatment of diseases such as prostate cancer [2][3][4], bone metastases, uterine fibroids [5][6][7] and Parkinson's have been approved by the FDA. However, the effectiveness of HIFU in the treatment of other cancer types is still in the form of pre-clinical and clinical studies.
HIFU are focused ultrasonic waves emitted from a piezoceramic (PZT-Lead Zirconate Titanate) transducer. In this technology, just like collecting light beams through a magnifying glass at a single point, the sound waves generated in the transducer are passed through acoustic lenses to focus on a single point as shown in Figure 1. This full text paper was peer-reviewed at the direction of IEEE Instrumentation and Measurement Society prior to the acceptance and publication.
978-1-7281-4460-3/20/$31.00 ©2020 IEEE The acoustic energy density at the focal point is between 1,000 and 10,000 W / cm 2 depending on the input power applied and the characteristics of the environment in which the ultrasonic wave travels, so that the tissue temperature can reach very high temperatures of more than 80 °C in a very short time, so that the cancerous tissue at the focal point can be destroyed by ablation [8,9].
However, despite all these advantages, HIFU is not a priority method in the treatment of cancer. Conventional methods such as radiotherapy, chemotherapy and surgical intervention are still preferred in the treatment of many cancer types. The most important reason behind not using HIFU as a primary treatment option are the lack of a measurable dosimetric standard. In order to carry out dosimetric studies, firstly, temperature characterization must be performed effectively. Therefore, it is very important that temperature characterizations are performed in an effective manner in order to ensure the safety of treatment.
HIFU is thought to be a substitute for hyperthermia applications for cancer treatment in the clinic due to its temperature effect. The most important parameters in terms of dosimetry in the treatment of hyperthermia applied in the clinic are the temperature and duration of administration. Using these two variables, "thermal dose" has been defined by Sapareto and Dewey for hyperthermia applications in cancer treatment [8].
Thermal applications with equivalent minute units (EM43) at 43 °C are usually defined in terms of thermal dose (TD). This concept seeks to provide a parameter for comparing treatments at various intervals at various temperatures. The therapy procedure can be correlated with exposure time at a defined reference temperature using the temperature information obtained as a function of time and a statistical explanation of this thermal harm association.
Thermal Dose is a concept which can be used to characterize HIFU therapy in a laboratory, describing a treatment time which must be halved to achieve the same thermal damage over the reference temperature value for each 1 °C rise. Thermal dose is determined in the most general sense by the following equation [8,9]: In this equation, T represents the mean value of the temperature applied during the time ∆t, and R denotes a constant number equal to 0.25 for T <43 °C and 0.5 for T> 43 °C.
In this study, temperature characterization results, which are the basis of our research for the application of HIFU with dosimetric approach to colon cancer cell lines, are given.

A. Temperature Measurements
In order to determine the effects of HIFU on colon cancer cell lines, it is necessary to determine the temperature profiles will be formed in the 24-well plates in which the cells are cultured.
The test setup developed for HIFU applications is given in Figure 2 with all hardware components. The H-102 HIFU transducer from Sonic Concepts (SC) was used as the focused ultrasound source. The basic frequency of the H-102 HIFU transducer is 1.1 MHz, the active radius is 70 mm and the focal point is 62.6 mm (from the transducer face). The measuring system also has a signal generator (Fluke) and a power amplifier (EIN 3100L RF Power Amplifier).
The thermocouples used for temperature measurements are the Physitemp Ultra Thin IT Series Flexible Micro probe IT-24P. Such thermocouples are polyester separated and thin polyurethane coated wire designed to measure temperature in animal brains and other tissues in testing applications. The ultra-thin thermocouple is very vulnerable, but it reacts fast (4 ms). Has a maximum outer diameter of 0.23 mm and a wire cross section of 0.13 mm x 0.07 mm with insulated tips.
For multi-channel temperature measurement with LabVIEW software, a PC-based data acquisition and monitoring framework has been created. The hardware components of the system include a PCI data acquisition (Daqboard IOtech 2000), a signal organizer module (DBK83 of IOtech) for thermocouple signals (Analog Input), and a personal computer (PC) as illustrated in Figure 2. In addition, a schematic view of the placement of the 3axis motion system, 24-well plate and HIFU transducers used to determine the temperature distributions generated by the HIFU application process in the 24-well plates is given in Figure 3. Prior to measurements, 5 T-type ultra-fine thermocouples were placed 1 mm apart from the center of one of the wells of the 24-well plate. To fully simulate the temperature that would affect the cells, the corresponding well of the 24-well plate was filled with 1.5 mL of cell medium (Dulbecco's Modified Eagle Medium; DMEM). The HIFU converter is placed on the bottom of the water tank. The well plate filled with cell culture medium was placed on the HIFU transducer with its special holder as shown in Figure 1 and Figure 4. The water used in the study was distilled two times, deionized using ultraviolet light and degassed (4 mg / lt) as specified in IEC 61161 standard. The HIFU focal point was located on the desired well with the help of the 3-axis positioning system, while HIFU operation proceeded. After the focal point has been located in the center of the appropriate well area, temperature changes in all wells have been recorded using the LabVIEW interface. Therefore, according to input power and the duration of operation, the temperature values of the wells could be accurately calculated. The HIFU was applied to the focused well at different input power values for 30 seconds during temperature characterization. Thermal doses are determined using equation (1) for the temperature values obtained. Figure 4 also provides an image showing the temperature distributions obtained by using a thermal camera at the focal point during HIFU application to the tissue-mimicking material (TMM) [10]. In this application, 18 W of the input power of HIFU is applied for 1 minute on TMM and the images are taken every five seconds from the TMM surface.

B. Uncertainty Budget
Uncertainty of measurements was calculated in compliance with Guidelines of uncertainty in measurements [11]. This Guide sets out general rules for assessing and expressing uncertainty in measurements intended to apply to a broad spectrum of measurements.
Two different types of errors affect mainly the thermocouples used in temperature characterizations. These errors must be defined in order to reliably measure the increase in temperature that results from the absorption of ultrasounds.
The first form of error is the artefact of thermocouples [12][13][14]. Thermocouple artefacts may occur either because of the thermocouple's heat conduction or because of differences in heat capacity or acoustic absorption between the thermocouple and the surrounding (water) environment.
The second type of error occurs from the viscous heating on the thermocouple surface. Due to the thermocouple artefacts, an extra temperature increase is added over the tissue material's real temperature increment.
Another disadvantage in the use of thermocouples for temperature measurement in HIFU characterization is the positioning error. The thermocouple should be well matched with the focused beam. Since the size of the lesion at the focal point is in the order of 1 mm to 2 mm in width and 10 mm to 15 mm in length, a slight misalignment between the beam and the junction results in a significant decrease in the temperature measured.
Repeatability in uncertainty can be described as measuring the proximity of a consensus between reciprocal temperature determinations acquired in repeated areas. These are the circumstances under which the same operator(s) acquires reciprocally equivalent temperature measurements using the same equipment over a short time using the same procedure on one tissue in the same measurement area. The quantity to be acquired is the standard deviation of the repeatability uncertainty. The main elements of uncertainty and the cumulative standard uncertainty derived from temperature measurements are shown in Table 1. Combined uncertainty was determined by calculating the square root of the sum of the square root of the sum of the variations using the coverage factor k=2, which corresponds to a coverage possibility of approximately 95% for a normal distribution.

III. RESULTS AND DISCUSSION
Equation 1 is the most generally accepted thermal dose definition and states that the treatment time should be halved despite the desired 1 °C increase in temperature. Considering the studies in the literature indicating that the temperature value of 43 °C is used as reference for hyperthermia applications, when the application time is selected as 30 seconds for thermal dose values of 60, 120 and 240 equivalent minutes (units are indicated as min), the temperature values that should be applied are calculated as in Table 2. Once all these values have been determined, the input voltage values to be applied to the HIFU transducer in order to accurately generate the desired thermal doses are determined by means of a 24-well plate and thermocouple measuring apparatus as shown in Figures 2 and 3. After all the conditions required to create a 60 min, 120 min and 240 min thermal dose were met, the temperature characterizations were performed in the well applied HIFU with the help of Physitemp thermocouples. The resulting temperature distribution graphs are shown in the Figure 5. As can be seen clearly from Figure 5, an extreme temperature increase occurs at the focal point in the wells where HIFU is applied. However, when we move a few mm away from the focal point, a very large temperature difference occurs. In the well where the HIFU was applied, only a significant increase in temperature was observed in the focal point and around 2 mm diameter circular region, while no significant increase in temperature was observed in the other regions of the well. The average of all the temperature values obtained, that are necessary to produce 60 min, 120 min and 240 min thermal dose during 30 s HIFU application, was calculated as 50 °C, 51 °C and 52 °C respectively.

IV. CONCLUSION
This paper is the first phase of a two-stage study. In this study, the temperature effect of HIFU was determined by the ultrasonic thermal dose approach suggested by Sapareto and Dewey in 1984 [8]. This approach allows measuring the length of time it needs to stay at a different temperature to the same result by using the time a certain temperature is applied. Conversely, this process can be equated to an equal treatment time at 43 °C if a process is given at any temperature. Comparisons between processes at various temperatures can therefore be made by conversion to equivalent time at 43 °C in every process.
For the realization of thermal characterization studies thermocouples were used in our study. Thermocouples can only measure the increase in temperature at certain locations in which they are integrated. The biggest contribution to the uncertainty of these thermocouples is from the misalignment. A computer-controlled three axis motion system has been developed and the focus point is determined very precisely to eliminate errors arising from positioning thermocouples. temperature characterization measurements were then carried out.
First of all, dose values should be effectively determined in order to determine the effectiveness of HIFU in colon cancer-cell lines with a dosimetric approach. For this purpose, for 30 seconds of application times, the temperature values which should be applied to reach the desired thermal doses were calculated using Equation 1. Then, the input voltage values that should be applied to the HIFU in order to reach the desired temperature values at certain application times were determined by repeated experiments. As noted in many previous studies [15][16][17][18], a dramatic increase in temperature was observed at the focal point of the HIFU transducer, while the temperature was close to room temperature a few millimeters from the focal point ( Figure 4). Experiments for each determined thermal dose values were repeated at least three times, thus ensuring that the same temperature was always reached at the respective dose values.
Therefore the determination of dose has critical importance in high intensity focused ultrasound applications especially since the uncontrolled application of ultrasound may result the cauterization of healthy tissue. Moreover the dose concept will give a change for quantitative compare of the applications realized by different operators at different times and environments.