In recent decades, many advanced techniques have been used in research of neuroscience. The application of neuroimaging enables us to ascertain deeper knowledge on brain function. However, there is still a lack of an efficient method for recording and displaying cerebral cortical temperature. In this study, we attempted to use an infrared thermography technique to observe cerebral cortical temperature and create a cortical infrared thermograph (CIT) on the basis of our previous studies. [1,2]

METHODS

Animal model
The experiments were performed on 52 healthy cats of either sex, weighing 2. 8±0. 7 kg. The cats were supplied by the Experimental Animal Center of China Academy of TCM. After the animals' parietal hair was shaved off, a cross- shaped incision was made along the sagittal midline of the skull and the vertical line of the auricular anterior edge under Nembutal anesthesia (i. p. , 45 mg/kg) and Novocain (i. h. ). An incision into the scalp was made first. The subcutaneous partial muscles on the temporal side were cut off, and the ipsilateral skull was removed with an electrical saw. After removing the skull covering cerebral hemisphere, the cerebral cortex was exposed, and the dura mater was retained, for performance of an unilateral operation only on each animal. Retained skin was re- sewed on temporarily to cover the surface of the cerebral hemisphere and protect the cerebral tissues (Fig. 1) after the operation. The animals were maintained continually under application of antibiotics on the local incision and injections of penicillin. In addition, local injections of analgesic were performed daily to relieve the animals of pain.

Experimental instruments
An AGA- 782 model infrared thermovision system (AGA Inc. , Sweden) with a thermosensitivity to 0. 1 degree Celsius (0. 025 degree after computerized processing) and a geometrical resolution angle of 1. 9×10 - 5 Gy was used in this study. The real- time images of animal cerebral cortex were taken by an IR camera at a speed of 25 frames/s, and then shown on both black and white display and a PVW- 1271Q model color monitor. Simultaneously, the digitized thermo- images were transferred to the thermovision's TC- 800 computer and saved on DISCO3. 1 software for processing relevant temperature values. In addition, the images observed for the duration of the experiment were also recorded continuously with a VR- 30 Infrared Video Recorder. A DH- 1 Thermal Calibration Source was used to calibrate the temperature's absolute values in the cortex.

Recording method of infrared thermo- images
Each cat was observed three times at the 24th, 48th and 72nd hours after its operation. Before observation, the cat was fixed to an operation stand in prone position with its head steadily maintained. Local novocaine was applied to the wound. The temporarily sewed skin was opened to expose the cerebral cortex. The IR camera facing the surface of the cortex located about 70 cm away was used to take the images (Fig. 2) . At the 0, 10th, 20th and 30th min after opening the skin flap, the cortical temperatures were recorded and saved by the thermovision system and its computer. During the experiment, the operator petted the animal gently to alleviate its fear and pain. After each observation, the skin was temporarily sewed over again and the cat was maintained until the next observation. The experiments were carried out in a glass- shielded room without a strong infrared radiation source or noticeable air flow. The room temperature fluctuation was kept at less than 0. 5 degree/h.

Computerized processing and analyzing method of cortical thermo- images
On the basis of classical stereotaxic atlas of cat's cerebral cortex, the cortexwas divided into 28 parts (Fig. 3) . A TC- 800 computer installed with the IR imaging software DISCO3. 1 (including MAGNIFY and PROF/SP) was used to analyze the cortical thermo- images and determine the temperature in different parts of the cerebral cortex. After analyzing and comparing the temperature fluctuation (4 times: 0, 10, 20 and 30 min) within 30 min of each observation over 3 days (24 th, 48th and 72nd h after the initial operation), the temperature data were analyzed statistically and the temperature distribution atlas of the cat's cerebral cortex was drawn up.

RESULTS

Animal conditions
After craniotomy, all animals were in good condition and developed no infections around the wound and cortical surface. Due to the application of analgesic, the animals showed little pain reaction, and their motion and other reactions were approximately normal. They drank and ate freely during behavioral observation and survived more than 2 weeks.

Display of cerebral cortical infrared thermo- images in cat
The cerebral cortical infrared thermo- images were clear. The temperature on the cortical surface presented in an uneven distribution, with higher temperature mainly in the following regions: Ant. Ectosylvian, Ant. Sylvian, lower section of Mid. Ectosylvian, Ant. Suprasylvian. Lower temperatures were located in: Post. Suprasylvian, Post. Lateral, Post. Ectosylvian, Ant. Lateral and the posterior section of Mid. Ectosylvian. Generally, the temperature of the anterior- lower region was higher, while that of the posterior- superior region was lower. There were about 5-6 temperature grades observable in each cortical thermo- image, with the temperature differences ranging from 2-4 degrees (Fig. 4 ).

Analysis of cortical thermo- images and diagram of cortical temperature atlas
The observation and analysis of the thermo- images showed that the change in temperature in the same part of the cortex within 3 days was small, with the mean difference value between the areas of highest and the lowest temperature about 2. 3℃ throughout the entire hemisphere. The highest temperature was found in the middle section of the Ant. Ectosylvian (36. 15±0. 86 degrees), while the lowest was in the supra section of Post. Suprasylvian (33. 81±1. 51 degrees). Figure 5 shows the cortical temperature atlas drawn according to the results of computerized processing in 52 cats (26 for each side).

Fluctuations of cat's cortical temperature over short and long- term periods
After opening the temporarily sewed skin, the cortical infrared thermo- imaging did not observably change within 30 min after the operation (Fig. 6), and the temperature in various cortical parts had no abnormal fluctuations. At the 24th, 48th and 72nd h after the operation, the cortical temperature in the all parts was generally consistent, and the position of the higher and lower temperature parts on the thermo- imaging showed no remarkable changes (Fig. 6) . Variance analysis indicated no significant changes in cortical temperature except at the 4th, 16th and 18th areas.

DISCUSSION

Since the electroencephalogram (EEG) recording method was invented, some other neuroimaging techniques have been developed subsequently, such as, mapping of the regional cerebral blood flow (rCBF), the 14C- deoxyglucose method (［14C］- 2DG), X- ray computerized tomography (X- CT), positron emission tomography (PET), single photon emission computerized tomography (SPECT), nuclear magnetic resonance imaging (NMRI) and functional nuclear magnetic resonance imaging (FMRI). These new techniques have greatly enriched the ability of neuroscience to develop further research. Temperature has been used as an index in physiological research in the past, and still plays an important role in modern medical science, as it not only reflects the condition of the organism's local regions as well as the entire body, but also indirectly indicates the status of energy metabolism and local blood circulation. Furthermore, the temperature of external and internal environments, one of the most important factors of life, has played a key role in life processes and maintenance of certain organs. The cerebral cortex, acting as the highest nerve centre, participates in various reflexes and thought activities. In order to maintain its normal function, the cerebral cortex needs to keep normal and steady intracranial temperature for various neural reflexes. Based on the importance of temperature, a method for cortical temperature imaging should be established in order to clarify temperature distribution of brain hemispheres visually, analyze cortical temperature features, and understand the relationship between changes in temperature and cerebral neuronal activity.

There are various ways to measure biological temperature and infrared thermography is one of the more advanced approaches. In the process of thermo- imaging, the IR detector receives the infrared- ray radiation from the measured objects. On the basis of the scanning principle, the radiation signal is transferred to the infrared thermovision system and the imaging of the transformed signal is displayed on a screen. The computer connected to the thermovision can conduct data processing and analysis. Infrared thermography has the following functions and advantages: display of the measured object as an image; image generation without touching the measurement object, therefore, avoiding interference and errors; and continuous dynamic observation carried out over a long- term period in real- time. The method described in this paper is the first to show temperature images of cerebral cortex. In the past, contact thermometers (needle thermal couples and semiconductor temperature measurement thermistors) were used to measure temperatures in brain research. These thermometers required contact with the measurement object, did not demonstrate high rates of accuracy, and had limited areas of measurement accessibility. Furthermore, using needle couples had damaging effects on brain tissue. The difference between cortical infrared thermography and previous methodologies is significant. In the 1960s, researchers used this method to observe the cerebral cortex, and tried to use the temperature to indirectly evaluate the cortical blood circulation. [3- 5] After 1984, a new instrument called the thermoenphaloscope (TES) was developed to monitor rapid changes in IR radiation on the skull, and researchers believed that these fluctuations were caused by cerebral activities. [6,7] Some subsequent studies applied this technique in a series of studies regarding nerve reflex activities in relation to observation of dynamic changes in IR radiation occurring in the cranial top after application of various stimuli. For instance, when visual, auditory, or sensatory stimulation was given, a rapid change in IR radiation from the cranial top was observed. [8- 14] Some researchers have monitored changes in blood circulation during brain operations using infrared thermography. [15- 19] Both these applications of infrared thermography on nerve- ima ging are bold new experiments demonstrating that people have been trying to seek an effective way to record and display cerebral temperature in order to ascertain the relationship between cerebral temperature changes and neuronal activities.

This study is aim to display cerebral cortical thermo- images and obtain relevant temperature data in cats. It achieves the following results beyond the previously referenced research: full exposure of the cerebral hemisphere and generation of many thermo- images of almost the entire cortex; analysis of cortical temperature distribution, a temperature atlas diagram, and attainment of a stylebook's mean values; and observation of the stability of the cortical temperature and short- term fluctuations. This study may lay a good foundation for the application of IR thermography in future cerebral research. After opening the temporarily sewed skin flap for cortical exposure, there were no observed abnormal changes in temperature in all the cortical regions. This may be due to the existence of dura mater, which plays a role in neural shielding. Within 3 days after the craniotomy, the temperature in all cortical parts underwent little change, and the higher and lower temperature sections remained more or less in the same positions. Our research shows that the cortical temperature has a definite regularity in distribution, and the higher and lower temperature areas in different cats are consistent. This phenomenon suggests that the activity of cortical cells in the same parts of different test subjects is identical, and the metabolic activities of the nerve cells and local blood flow are also similar. Furthermore, the temperature reflected in the thermo- images is fairly consistent. These results also indicate that cortical temperature is stable, and the animal model used in this study and the achieved cortical temperature data are reliable. Our results also indicate that the temperature is higher in the anterior hemisphere and lower in the posterior hemisphere of the cerebral cortex, which may be related to the activity of the nerve cells, specifically that the region of higher temperature is located where nerve cells are fairly active in function. We previously applied this method to observe the role of electro- acupuncture (EA) on neural activity, and found that the temperature increased significantly in some cortical parts after EA. The areas with increased temperature were probably the locations of the cortex responding to EA stimulation, which may have resulted from the excitement of the cortical neurons. [20,21] The temperature atlas diagram in this paper shows that the temperature mean values of the same cortical regions on both sides are not completely identical, which is possibly due to different activity and functional status of each side. This study shows that use of cortical infrared thermography (CIT) as a new neuro- thermo- imaging technique can play a practical role in the research of cerebral temperature. For example, it can be used to study the temperature activity from animals to humans, to tele- measure cortical temperature in the brain operation, and to diagnose brain diseases by temperature reaction. We also hope that, with joint efforts of other researchers in this area, this technique will be continuously improved and widely applied in future research.

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