0018-9294 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2019.2901530, IEEE Transactions on Biomedical Engineering TBME- 1 Abstract— Objective: The purpose of this study is to propose a palm-sized cryoprobe system with a built-in thermocouple (TC) for highly accurate and sensitive temperature measurements and to verify the effectiveness of the system. Methods: Conventional cryoprobe systems based on the boiling effect of a refrigerant have triple coaxial tubes. In the proposed system, the outer and middle coaxial tubes are made of two different metals that are welded only at the probe tip, thereby forming a TC. The thermoelectric properties of the built-in TC and measurement accuracy were investigated in agar in a constant-temperature chamber. The system was also applied in a penicillin G-induced rat brain epilepsy model. Results: We verified that the built-in TC has appropriate thermoelectric properties and that the system more accurately and sensitively measured transient temperature changes at the probe tip wall compared to conventional systems, showing the cooling performance of the system. In the rat model, epileptiform activities disappeared on freezing and reliable cell necrosis was achieved at an end temperature of −45.2 ± 1.6 ºC. Conclusions: The proposed system is suitable for reliable cryosurgery. Significance: The system is probably to be valuable for clarifying the relationship between freezing temperature and cell necrosis in vivo. Index Terms—cryoprobe, cryosurgery, palm-sized cryoprobe system, built-in thermocouple, triple coaxial tube, refrigerant expansion, boiling effect. I. INTRODUCTION RYOSURGERY is an alternative surgical technique for resection and is mainly applied to tumors [2]. Compared with conventional resection, cryosurgery has advantages of being a less-invasive and simple procedure [2, 3]. Since Arnott's initial study [4], numerous cryoprobe systems and techniques have been described [5]. In terms of the extreme freezing power, most cryosurgeons prefer the boiling effect to the powerful freezing technique [6]. Several cryoprobe systems are based on the boiling effect and have been proposed in several academic journals [6-14]. The properties of these cryoprobes are initially investigated using phantom experiments and numerical simulations [15-17]. However, these studies are performed under certain assumptions of thermophysical properties of soft biological Manuscript received. This work was supported by a Grant-in-Aid for Specially Promoted Research (Project No. 20001008) and a Grant-in-Aid for Young Scientists (B) (Project Number 17K13019) from MEXT of Japan, and a Hiroshima City University Grant for Special Academic Research (General Study). *T. Tokiwa is with Hiroshima City University, Hiroshima 731-3194, Japan. (e-mail: tokiwa@hiroshima-cu.ac.jp). L. Zimin is with Waseda University. H. Ishiguro is with the Kyushu Institute of Technology. T. Inoue, S. Nomura, and M. Suzuki are with Yamaguchi University Graduate School of Medicine. H. Kajigaya was with Nippon Veterinary and Life Science University. T. Yamakawa is with the Fuzzy Logic Systems Institute (FLSI). tissue and with transient temperature changes that are actually affected by thermal conductivity and diffusivity in a surgical area, in which values are spatially and temporally changed during surgery. Moreover, unexpected situations on experimental devices, such as clogging of a refrigerant, may occur during surgery. We have examined possible use of cryotherapy as treatment for epilepsy [9, 18]. In a series of experiments, we showed that penicillin G (PG)-induced epileptiform discharges (EDs) could be eliminated at the onset of the freezing procedure. However, the EDs were occasionally provoked again after the freezing period due to inadequate freezing protocols (e.g., freezing speed, area, and temperature). These EDs were unexpected and were not visually detectable during freezing or immediately after several hours of freezing [18]. Therefore, safe and reliable cryosurgery with a cryoprobe system requires accurate measurement of transient temperature changes at the probe-tip wall. In particular, to evaluate the lethal temperature of cells, there is a need to measure the temperature at the external probe wall where cells are in direct contact. Of nine cryoprobe systems based on the boiling effect [6-14], seven can measure transient temperature changes [6, 8, 10-14]. The changes were monitored by the thermocouple (TC) whose wiring traveled around the probe external wall. However, these methods have the following three problems: 1) the wire has an adverse effect on tissues in the operative field; 2) wiring disconnection may occur during surgery because the TC wire is in contact with the surgical area; and 3) measuring precision is low because adhesive located between the TC and probe wall causes thermal resistance. Rabin’s cryoprobe system [6] does not depend on the size of the probe tip because the TC is attached in the inner tube of the probe system. Since the TC wiring runs through the inner tube, the wiring also has no impact on the surgical area, but does affect the refrigerant passage. Moreover, the measured temperature is not that around the freezing area, but rather the boiling temperature of the refrigerant, because the measurement point is not on the external probe wall. To address these problems, we propose a cryoprobe system with a built-in TC that was designed based on our previous palm-sized cryoprobe system [9]. Since the TC function is structurally part of the external probe wall, TC wiring at the probe tip is not necessary and the system can measure the temperature of the surgical area with greater accuracy and sensitivity compared with conventional cryoprobe systems. To evaluate the effectiveness of the system, we investigated its freezing characteristics in agar. We then used the system in a Wistar rat epilepsy model, in which epileptiform activities are induced by injection of an epileptogenic substance. A preliminary version of this work has been reported in [19, 20]. A Palm-Sized Cryoprobe System with a Built-in Thermocouple and Its Application in an Animal Model of Epilepsy Tatsuji Tokiwa*, Member, IEEE, Lev Zimin, Hiroshi Ishiguro, Takao Inoue, Hiroshi Kajigaya, Sadahiro Nomura, Michiyasu Suzuki, and Takeshi Yamakawa, Fellow, IEEE C