Body Size, Cerebral Blood Flow, Ambient Temperature, and Relative Brain Temperatures in Newborn Infants under Incubator Care

Subtle changes in body temperature affect the outcomes of ill newborns. However, the temperature profile of neonatal brains remains largely unknown. In open-cot care, increased cerebral perfusion is correlated with higher superficial brain temperatures. This study investigated the dependence of brain temperature (relative to rectal temperature) on ambient temperature, body size, cerebral perfusion, and metabolism in infants receiving incubator care. Rectal, scalp, and brain temperatures, superior vena cava flow, and brain oxygenation were assessed using echocardiography, thermo-compensatory temperature monitoring, and near-infrared spectroscopy in 60 newborns. These infants had a mean postconceptional age of 36.9 (2.2) weeks and weighed 2348 (609) g at the time of evaluation. The ambient temperature was maintained at 30.0 (1.0) °C. A higher rectal temperature was associated with greater postconceptional age (p = 0.002), body weight (p < 0.001), and head circumference (p < 0.001). Relative scalp, superficial brain, and deep brain temperatures were associated with smaller head circumference (p < 0.001, p = 0.030, and p = 0.015, respectively) and superior vena cava flow (p = 0.002, p = 0.003, and p = 0.003, respectively). In infants receiving incubator care, larger head sizes and increased brain perfusion were associated with lower relative scalp and brain temperatures. When considered alongside previous reports, cerebral perfusion may contribute to maintaining stable cerebral tissue temperature against ambient temperature changes.

The Association of Metabolic Brain MRI, Amyloid PET, and Clinical Factors: A Study of Alzheimer's Disease and Normal Controls From the Open Access Series of Imaging Studies Dataset

BACKGROUND

Although brain activities in Alzheimer's disease (AD) might be evaluated MRI and PET, the relationships between brain temperature (BT), the index of diffusivity along the perivascular space (ALPS index), and amyloid deposition in the cerebral cortex are still unclear.

PURPOSE

To investigate the relationship between metabolic imaging measurements and clinical information in patients with AD and normal controls (NCs).

STUDY TYPE

Retrospective analysis of a prospective dataset.

POPULATION

58 participants (78.3 ± 6.8 years; 30 female): 29 AD patients and 29 age- and sex-matched NCs from the Open Access Series of Imaging Studies dataset.

FIELD STRENGTH/SEQUENCE

3T; T1-weighted magnetization-prepared rapid gradient-echo, diffusion tensor imaging with 64 directions, and dynamic 18 F-florbetapir PET.

ASSESSMENT

Imaging metrics were compared between AD and NCs. These included BT calculated by the diffusivity of the lateral ventricles, ALPS index that reflects the glymphatic system, the mean standardized uptake value ratio (SUVR) of amyloid PET in the cerebral cortex and clinical information, such as age, sex, and MMSE.

STATISTICAL TESTS

Pearson's or Spearman's correlation and multiple linear regression analyses. P values <0.05 were defined as statistically significant.

RESULTS

Significant positive correlations were found between BT and ALPS index (r = 0.44 for NCs), while significant negative correlations were found between age and ALPS index (rs  = -0.43 for AD and - 0.47 for NCs). The SUVR of amyloid PET was not significantly associated with BT (P = 0.81 for AD and 0.21 for NCs) or ALPS index (P = 0.10 for AD and 0.52 for NCs). In the multiple regression analysis, age was significantly associated with BT, while age, sex, and presence of AD were significantly associated with the ALPS index.

DATA CONCLUSION

Impairment of the glymphatic system measured using MRI was associated with lower BT and aging.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY STAGE: 1.

Sleep and Thermoregulation in Birds: Cold Exposure Reduces Brain Temperature but Has Little Influence on Sleep Time and Sleep Architecture in Jackdaws (Coloeus monedula)

Birds have an electrophysiological sleep state that resembles mammalian rapid-eye-movement (REM) sleep. However, whether its regulation and function are similar is unclear. In the current experiment, we studied REM sleep regulation in jackdaws (Coloeus monedula) by exposing the birds to low ambient temperature, a procedure that selectively suppresses REM sleep in mammals. Eight jackdaws were equipped with electrodes to record brain activity and neck muscle activity and a thermistor to record cortical brain temperature. Recordings covered a three-day period starting with a 24 h baseline day at an ambient temperature of 21 °C, followed by a 12 h cold night at 4 °C, after which the ambient temperature was restored to 21 °C for the remaining recovery period. Cold exposure at night caused a significant drop in brain temperature of 1.4 °C compared to the baseline night. However, throughout the cold night, jackdaws expressed NREM sleep and REM sleep levels that were not significantly different from the baseline. Also, EEG spectral power during NREM sleep was unaffected by cold exposure. Thus, while cold exposure had a clear effect on brain temperature in jackdaws, it did not have the same REM sleep suppressing effect reported for mammals. These findings suggest that the REM-sleep-like state in birds, unlike REM sleep in mammals, is protected against the influence of low temperature.

Peripheral and Central Nervous System Biomarkers of Inflammation in Functional Seizures: Assessment with Magnetic Resonance Spectroscopy

Purpose

Inflammation may link trauma to clinical symptoms in functional seizures (FS). We compared brain temperature and metabolites in FS, psychiatric (PCs) and healthy controls (HCs) and quantified their associations with serum biomarkers of inflammation and clinical symptoms.

Patients and Methods

Brain temperature and metabolites were measured with whole-brain magnetic resonance spectroscopic imaging (MRSI) and compared between groups in regions of interest and the whole brain. Relationships with inflammatory biomarkers and symptoms were assessed with Pearson correlations.

Results

Brain temperature was higher in FS than HCs in the orbitofrontal cortex (OFC) and anterior cingulate gyrus (ACG) and lower in the occipital cortex and frontal lobe. PCs showed lower temperatures than HCs in the frontal lobe including precentral gyrus and in the cerebellum. Myo-inositol (MINO) was higher in FS than HCs in the precentral gyrus, posterior temporal gyrus, ACG and OFC, and choline (CHO) was higher in the occipital lobe. CHO was higher in PCs than HCs in the ACG and OFC, and N-acetylaspartate (NAA) was higher in the ACG. There were no significant correlations with the serum inflammatory biomarkers. In FS, brain temperature correlated with depression, quality of life, psychological symptoms, and disability, CHO correlated with disability, and MINO correlated with hostility, disability, and quality of life.

Conclusion

Some of the previously identified neuroimaging abnormalities in FS may be related to comorbid psychiatric symptoms, while others, such as abnormalities in sensorimotor cortex, occipital regions, and the temporo-parietal junction may be specific to FS. Overlapping MINO and temperature increases in the ACG and OFC in FS suggest neuroinflammation.

Predicting brain temperature in humans using bioheat models: Progress and outlook

Brain temperature, regulated by the balance between blood circulation and metabolic heat generation, is an important parameter related to neural activity, cerebral hemodynamics, and neuroinflammation. A key challenge for integrating brain temperature into clinical practice is the lack of reliable and non-invasive brain thermometry. The recognized importance of brain temperature and thermoregulation in both health and disease, combined with limited availability of experimental methods, has motivated the development of computational thermal models using bioheat equations to predict brain temperature. In this mini-review, we describe progress and the current state-of-the-art in brain thermal modeling in humans and discuss potential avenues for clinical applications.

A Study on REM Sleep Homeostasis in the Day-Active Tree Shrew (Tupaia belangeri): Cold-Induced Suppression of REM Sleep Is Not Followed by a Rebound

The function and regulation of rapid-eye-movement (REM) sleep is a topic of ongoing debate. It is often assumed that REM sleep is a homeostatically regulated process and that a need for REM sleep builds up, either during prior wakefulness or during preceding slow wave sleep. In the current study, we tested this hypothesis in six diurnal tree shrews (Tupaia belangeri), small mammals closely related to primates. All animals were individually housed and kept under a 12:12 light-dark cycle with an ambient temperature of 24 °C. We recorded sleep and temperature in the tree shrews for 3 consecutive 24 h days. During the second night, we exposed the animals to a low ambient temperature of 4 °C, a procedure that is known to suppress REM sleep. Cold exposure caused a significant drop in brain temperature and body temperature and also resulted in a strong and selective suppression of REM sleep by 64.9%. However, contrary to our expectation, the loss of REM sleep was not recovered during the subsequent day and night. These findings in a diurnal mammal confirm that the expression of REM sleep is highly sensitive to environmental temperature but do not support the view that REM sleep is homeostatically regulated in this species.

Concordance between focal brain temperature elevations and focal edema in temporal lobe epilepsy

OBJECTIVE

Neuroinflammation (NI) is a pathophysiological factor in many neurological disorders, including epilepsy. Because NI causes microstructural tissue damage that worsens with increasing brain temperature, abnormally elevated brain temperatures may be a surrogate measure of the biochemical consequences of NI. This study investigated whether patients with temporal lobe epilepsy (TLE) have abnormal brain temperature elevations (T_CRE ) in seizure-producing regions that show evidence of edema and/or microstructural damage.

METHODS

Twenty adults with TLE and 20 healthy controls (HCs) were scanned at 3-Tesla. T_CRE in each voxel was calculated (T_CRE  = -102.61(Δ_H20-CRE)  + 206.1°C) by non-invasive volumetric magnetic resonance spectroscopic imaging and thermometry (MRSI-t). Multi-shell diffusion images were processed by neurite orientation and density imaging (NODDI). Voxel wise two-sample t tests computed group differences in imaging data. Multimodal data fusion (joint independent component analysis [ICA]) determined the spatial coupling of T_CRE with NODDI.

RESULTS

T_CRE analyses showed that, compared to HCs, TLEs had higher T_CRE (p < .001). NODDI analyses showed increased extracellular free water (p_FWE  < 0.05) in the medial temporal lobes, with the most pronounced increases ipsilateral to seizure onset. TLEs also had increased angular dispersion of neurites (p < .001) and decreased neurite density (p_FWE  <0.05) in the ictal-onset medial temporal lobe, as well as more widespread, bilateral patterns of abnormalities. Focal increases in T_CRE were spatially concordant with increased free water in the left inferior and middle temporal gyri and the associated cortex. In TLE, ICA loadings extracted from this region of overlap were associated with greater mood disturbance (r = .34, p = .02) and higher depression scores (r = .37, p = .009).

SIGNIFICANCE

The spatial concordance between focal T_CRE elevations and edema in TLE supports the notion that brain thermometry visualizes the correlates of focal NI. MRSI-t-based T_CRE elevations may, therefore, be a useful biomarker for identifying seizure-producing tissue in patients with focal epilepsy caused by brain damage.

Measuring Brain Temperature in Youth Bipolar Disorder Using a Novel Magnetic Resonance Imaging Approach: A Proof-of-concept Study

BACKGROUND

There is evidence of alterations in mitochondrial energy metabolism and cerebral blood flow (CBF) in adults and youth with bipolar disorder (BD). Brain thermoregulation is based on the balance of heat-producing metabolism and heat-dissipating mechanisms, including CBF.

OBJECTIVE

To examine brain temperature, and its relation to CBF, in relation to BD and mood symptom severity in youth.

METHODS

This study included 25 youth participants (age 17.4 ± 1.7 years; 13 BD, 12 control group (CG)). Magnetic resonance spectroscopy data were acquired to obtain brain temperature in the left anterior cingulate cortex (ACC) and the left precuneus. Regional estimates of CBF were provided by arterial spin labeling imaging. Analyses used general linear regression models, covarying for age, sex, and psychiatric medications.

RESULTS

Brain temperature was significantly higher in BD compared to CG in the precuneus. A higher ratio of brain temperature to CBF was significantly associated with greater depression symptom severity in both the ACC and precuneus within BD. Analyses examining the relationship of brain temperature or CBF with depression severity score did not reveal any significant finding in the ACC or the precuneus.

CONCLUSION

The current study provides preliminary evidence of increased brain temperature in youth with BD, in whom reduced thermoregulatory capacity is putatively associated with depression symptom severity. Evaluation of brain temperature and CBF in conjunction may provide valuable insight beyond what can be gleaned by either metric alone. Larger prospective studies are warranted to further evaluate brain temperature and its association with CBF concerning BD.

A daily temperature rhythm in the human brain predicts survival after brain injury

Patients undergo interventions to achieve a 'normal' brain temperature; a parameter that remains undefined for humans. The profound sensitivity of neuronal function to temperature implies the brain should be isothermal, but observations from patients and non-human primates suggest significant spatiotemporal variation. We aimed to determine the clinical relevance of brain temperature in patients by establishing how much it varies in healthy adults. We retrospectively screened data for all patients recruited to the Collaborative European NeuroTrauma Effectiveness Research in Traumatic Brain Injury (CENTER-TBI) High Resolution Intensive Care Unit Sub-Study. Only patients with direct brain temperature measurements and without targeted temperature management were included. To interpret patient analyses, we prospectively recruited 40 healthy adults (20 males, 20 females, 20-40 years) for brain thermometry using magnetic resonance spectroscopy. Participants were scanned in the morning, afternoon, and late evening of a single day. In patients (n = 114), brain temperature ranged from 32.6 to 42.3°C and mean brain temperature (38.5 ± 0.8°C) exceeded body temperature (37.5 ± 0.5°C, P < 0.0001). Of 100 patients eligible for brain temperature rhythm analysis, 25 displayed a daily rhythm, and the brain temperature range decreased in older patients (P = 0.018). In healthy participants, brain temperature ranged from 36.1 to 40.9°C; mean brain temperature (38.5 ± 0.4°C) exceeded oral temperature (36.0 ± 0.5°C) and was 0.36°C higher in luteal females relative to follicular females and males (P = 0.0006 and P < 0.0001, respectively). Temperature increased with age, most notably in deep brain regions (0.6°C over 20 years, P = 0.0002), and varied spatially by 2.41 ± 0.46°C with highest temperatures in the thalamus. Brain temperature varied by time of day, especially in deep regions (0.86°C, P = 0.0001), and was lowest at night. From the healthy data we built HEATWAVE-a 4D map of human brain temperature. Testing the clinical relevance of HEATWAVE in patients, we found that lack of a daily brain temperature rhythm increased the odds of death in intensive care 21-fold (P = 0.016), whilst absolute temperature maxima or minima did not predict outcome. A warmer mean brain temperature was associated with survival (P = 0.035), however, and ageing by 10 years increased the odds of death 11-fold (P = 0.0002). Human brain temperature is higher and varies more than previously assumed-by age, sex, menstrual cycle, brain region, and time of day. This has major implications for temperature monitoring and management, with daily brain temperature rhythmicity emerging as one of the strongest single predictors of survival after brain injury. We conclude that daily rhythmic brain temperature variation-not absolute brain temperature-is one way in which human brain physiology may be distinguished from pathophysiology.

In Vivo Magnetic Resonance Thermometry for Brain and Body Temperature Variations in Canines under General Anesthesia

The core body temperature tends to decrease under general anesthesia. Consequently, monitoring the core body temperature during procedures involving general anesthesia is essential to ensure patient safety. In veterinary medicine, rectal temperature is used as an indicator of the core body temperature, owing to the accuracy and convenience of this approach. Some previous studies involving craniotomy reported differences between the brain and core temperatures under general anesthesia. However, noninvasive imaging techniques are required to ascertain this because invasive brain temperature measurements can cause unintended temperature changes by inserting the temperature sensors into the brain or by performing the surgical operations. In this study, we employed in vivo magnetic resonance thermometry to observe the brain temperatures of patients under general anesthesia using the proton resonance frequency shift method. The rectal temperature was also recorded using a fiber optic thermometer during the MR thermometry to compare with the brain temperature changes. When the rectal temperature decreased by 1.4 ± 0.5 °C (mean ± standard deviation), the brain temperature (white matter) decreased by 4.8 ± 0.5 °C. Furthermore, a difference in the temperature reduction of the different types of brain tissue was observed; the reduction in the temperature of white matter exceeded that of gray matter mainly due to the distribution of blood vessels in the gray matter. We also analyzed and interpreted the core temperature changes with the body conditioning scores of subjects to see how the body weight affected the temperature changes.

Whole-Brain Metabolic Abnormalities Are Associated With Mobility in Older Adults With Multiple Sclerosis

BACKGROUND

Older adults with multiple sclerosis (MS) experience mobility impairments, but conventional brain imaging is a poor predictor of walking abilities in this population.

OBJECTIVE

To test whether brain metabolites measured with Magnetic Resonance Spectroscopy (MRS) are associated with walking performance in older adults with MS.

METHODS

Fifteen older adults with MS (mean age: 60.9, SD: 5.1) and 22 age-matched healthy controls (mean age: 64.2, SD: 5.7) underwent whole-brain MRS and mobility testing. Levels of N-acetylaspartate (NAA), myo-inositol (MI), choline (CHO), and temperature in 47 brain regions were compared between groups and correlated with walking speed (Timed 25 Foot Walk) and walking endurance (Six-Minute Walk).

RESULTS

Older adults with MS had higher MI in 23 areas, including the bilateral frontal (right: t (21.449) = -2.605, P = .016; left: t (35) = -2.434, P = .020), temporal (right: t (35) = -3.063, P = .004; left: t (35) = -3.026, P = .005), and parietal lobes (right: t (21.100) = -2.886, P = .009; left: t (35) = -2.507, P = .017), and right thalamus (t (35) = -2.840, P = .007). MI in eleven regions correlated with walking speed, and MI in twelve regions correlated with walking endurance. NAA was lower in MS in the bilateral thalami (right: t (35) = 3.449, P < .001; left: t (35) = 2.061, P = .047), caudate nuclei (right: t (33) = 2.828, P = .008; left: t (32) = 2.132, P = .041), and posterior cingulum (right: t (35) = 3.077, P = .004; left: t (35) = 2.972, P = .005). NAA in four regions correlated with walking speed and endurance. Brain temperature was higher in MS patients in four regions, but did not correlate with mobility measures. There were no group differences in CHO.

CONCLUSION

MI and NAA may be useful imaging end-points for walking ability as a clinical outcome in older adults with MS.

Brain temperature monitoring in newborn infants: Current methodologies and prospects

Brain tissue temperature is a dynamic balance between heat generation from metabolism, passive loss of energy to the environment, and thermoregulatory processes such as perfusion. Perinatal brain injuries, particularly neonatal encephalopathy, and seizures, have a significant impact on the metabolic and haemodynamic state of the developing brain, and thereby likely induce changes in brain temperature. In healthy newborn brains, brain temperature is higher than the core temperature. Magnetic resonance spectroscopy (MRS) has been used as a viable, non-invasive tool to measure temperature in the newborn brain with a reported accuracy of up to 0.2 degrees Celcius and a precision of 0.3 degrees Celcius. This measurement is based on the separation of chemical shifts between the temperature-sensitive water peaks and temperature-insensitive singlet metabolite peaks. MRS thermometry requires transport to an MRI scanner and a lengthy single-point measurement. Optical monitoring, using near infrared spectroscopy (NIRS), offers an alternative which overcomes this limitation in its ability to monitor newborn brain tissue temperature continuously at the cot side in real-time. Near infrared spectroscopy uses linear temperature-dependent changes in water absorption spectra in the near infrared range to estimate the tissue temperature. This review focuses on the currently available methodologies and their viability for accurate measurement, the potential benefits of monitoring newborn brain temperature in the neonatal intensive care unit, and the important challenges that still need to be addressed.

Preventive Administration of the Heat Shock Protein Hsp70 Relieves Endotoxemia-Induced Febrile Reaction in Pigeons ( ) and Rats

The stress-inducible 70 kDa heat shock protein (Hsp70) can exert a protective effect on endotoxemia and sepsis due to its ability to interact with immune cells and modulate the immune response. However, it remains unknown whether Hsp70 is able to relieve endotoxemia-induced fever. We carried out a comparative study of the effects of preventive administration of the human recombinant Hsp70 (HSPA1A) on lipopolysaccharide (LPS)-induced endotoxemia in pigeons and rats with preimplanted electrodes and thermistors for recording the thermoregulation parameters (brain temperature, peripheral vasomotor reaction, muscular contractile activity). Additionally, we analyzed the dynamics of the white blood cell (WBC) count in rats under the same conditions. It was found that preventive administration of Hsp70 relieves the LPS-induced febrile reaction in pigeons and rats and accelerates the restoration of the WBC count in rats. The data obtained suggest that these warm-blooded animals share a common physiological mechanism that underlies the protective effect of Hsp70.

Temperature Dynamics in Rat Brains Exposed to Near-Field Waveguide Outputs at 2.8 GHz

Biological effects in the microwave band of the radiofrequency (RF) spectrum are thermally mediated. For acute high-power microwave exposures, these effects will depend on transient time-temperature histories within the tissue. In this article, we summarize the transient temperature response of rats exposed to RF energy emanating from an open-ended rectangular waveguide. These exposures produced specific absorption rates of approximately 36 and 203 W/kg in the whole body and brain, respectively. We then use the experimentally measured thermal data to infer the baseline perfusion rate in the brain and modify a custom thermal modeling tool based upon these findings. Finally, we compare multi-physics simulations of rat brain temperature against empirical measurements in both live and euthanized subjects and find close agreement between model and experimentation. This research revealed that baseline brain perfusion rates in rat subjects could be larger than previously assumed in the RF thermal modeling literature, and plays a significant role in the transient thermal response to high-power microwave exposures. © 2021 Bioelectromagnetics Society.

Selective head-neck cooling after concussion shortens return-to-play in ice hockey players

We aimed to investigate whether selective head–neck cooling could shorten recovery after sports-related concussions (SRCs). In a nonrandomized study of 15 Swedish professional ice hockey teams, 29 concussed players received immediate head and neck cooling for ≥30 min (initiated at 12.3 ± 9.2 min post-SRC by a portable cooling system), and 52 SRC controls received standard management. Players receiving head–neck cooling had shorter time to return-to-play than controls (7 vs 12.5 days, p < 0.0001), and 7% in the intervention group versus 25% in the control group were out of play for ≥3 weeks (p = 0.07). Immediate selective head–neck cooling is a promising option in the acute management of SRC that should be addressed in larger cohorts.

Rethinking the Conditions and Mechanism for Glymphatic Clearance

Critical studies that form the foundation of the glymphatic system and the clearance of metabolic by-products of unwanted proteins from the brain are reviewed. Concerns are raised about studying glymphatic flow in anesthetized animals and making assumptions about the whole brain based upon data collected from a cranial window on the cortex. A new model is proposed arguing that the flow of cerebral spinal fluid and parenchymal clearance in the perivascular system of unwanted proteins is regulated by circadian changes in brain temperature and blood flow at the level of the microvasculature.

Sub-minute prediction of brain temperature based on sleep-wake state in the mouse

Although brain temperature has neurobiological and clinical importance, it remains unclear which factors contribute to its daily dynamics and to what extent. Using a statistical approach, we previously demonstrated that hourly brain temperature values co-varied strongly with time spent awake (Hoekstra et al., 2019). Here we develop and make available a mathematical tool to simulate and predict cortical temperature in mice based on a 4-s sleep-wake sequence. Our model estimated cortical temperature with remarkable precision and accounted for 91% of the variance based on three factors: sleep-wake sequence, time-of-day ('circadian'), and a novel 'prior wake prevalence' factor, contributing with 74%, 9%, and 43%, respectively (including shared variance). We applied these optimized parameters to an independent cohort of mice and predicted cortical temperature with similar accuracy. This model confirms the profound influence of sleep-wake state on brain temperature, and can be harnessed to differentiate between thermoregulatory and sleep-wake-driven effects in experiments affecting both.

Brain Temperature Measured by Magnetic Resonance Spectroscopy to Predict Clinical Outcome in Patients with Infarction

Acute ischemic stroke is characterized by dynamic changes in metabolism and hemodynamics, which can affect brain temperature. We used proton magnetic resonance (MR) spectroscopy under everyday clinical settings to measure brain temperature in seven patients with internal carotid artery occlusion to explore the relationship between lesion temperature and clinical course. Regions of interest were selected in the infarct area and the corresponding contralateral region. Single-voxel MR spectroscopy was performed using the following parameters: 2000-ms repetition time, 144-ms echo time, and 128 excitations. Brain temperature was calculated from the chemical shift between water and N-acetyl aspartate, choline-containing compounds, or creatine phosphate. Within 48 h of onset, compared with the contralateral region temperature, brain temperature in the ischemic lesion was lower in five patients and higher in two patients. Severe brain swelling occurred subsequently in three of the five patients with lower lesion temperatures, but in neither of the two patients with higher lesion temperatures. The use of proton MR spectroscopy to measure brain temperature in patients with internal carotid artery occlusion may predict brain swelling and subsequent motor deficits, allowing for more effective early surgical intervention. Moreover, our methodology allows for MR spectroscopy to be used in everyday clinical settings.

Repeatability and Reproducibility of in-vivo Brain Temperature Measurements

Background: Magnetic resonance spectroscopic imaging (MRSI) is a neuroimaging technique that may be useful for non-invasive mapping of brain temperature (i.e., thermometry) over a large brain volume. To date, intra-subject reproducibility of MRSI-based brain temperature (MRSI-t) has not been investigated. The objective of this repeated measures MRSI-t study was to establish intra-subject reproducibility and repeatability of brain temperature, as well as typical brain temperature range. Methods: Healthy participants aged 23-46 years (N = 18; 7 females) were scanned at two time points ~12-weeks apart. Volumetric MRSI data were processed by reconstructing metabolite and water images using parametric spectral analysis. Brain temperature was derived using the frequency difference between water and creatine (TCRE) for 47 regions of interest (ROIs) delineated by the modified Automated Anatomical Labeling (AAL) atlas. Reproducibility was measured using the coefficient of variation for repeated measures (COVrep), and repeatability was determined using the standard error of measurement (SEM). For each region, the upper and lower bounds of Minimal Detectable Change (MDC) were established to characterize the typical range of TCRE values. Results: The mean global brain temperature over all subjects was 37.2°C with spatial variations across ROIs. There was a significant main effect for time [F (1, 1,591) = 37.0, p < 0.0001] and for brain region [F (46, 1,591) = 2.66, p < 0.0001]. The time*brain region interaction was not significant [F (46, 1,591) = 0.80, p = 0.83]. Participants' TCRE was stable for each ROI across both time points, with ROIs' COVrep ranging from 0.81 to 3.08% (mean COVrep = 1.92%); majority of ROIs had a COVrep <2.0%. Conclusions: Brain temperature measurements were highly consistent between both time points, indicating high reproducibility and repeatability of MRSI-t. MRSI-t may be a promising diagnostic, prognostic, and therapeutic tool for non-invasively monitoring brain temperature changes in health and disease. However, further studies of healthy participants with larger sample size(s) and numerous repeated acquisitions are imperative for establishing a reference range of typical brain TCRE, as well as the threshold above which TCRE is likely pathological.

Cold Exposure After Exercise Impedes the Neuroprotective Effects of Exercise on Thermoregulation and UCP4 Expression in an MPTP-Induced Parkinsonian Mouse Model

Moderate exercise and mild hypothermia have protective effects against brain injury and neurodegeneration. Running in a cold environment alters exercise-induced hyperthermia and outcomes; however, evaluations of post-exercise cold exposure related to exercise benefits for the brain are relatively rare. We investigated the effects of 4°C cold exposure after exercise on exercise-induced thermal responses and neuroprotection in an MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced Parkinsonian mouse model. Male C57BL/6J mice were pretreated with MPTP for five consecutive days and follow-up treadmill exercise for 4 weeks. After 1-h running at a 22°C temperature, the mice were exposed to a 4°C environment for 2 h. An MPTP injection induced a transient drop in body and brain temperatures, while mild brain hypothermia was found to last for 4 weeks after MPTP treatment. Preventing brain hypothermia by exercise or 4°C exposure was associated with an improvement in MPTP-induced striatal uncoupling protein 4 (UCP4) downregulation and nigrostriatal dopaminergic neurodegeneration. However, 4°C exposure after exercise abrogated the exercise-induced beneficial effects and thermal responses in MPTP-treated mice, including a low amplitude of exercise-induced brain hyperthermia and body temperature while at rest after exercise. Our findings elucidate that post-exercise thermoregulation and UCP4 expression are important in the neuroprotective effects of exercise against MPTP toxicity.

Reproducibility of whole-brain temperature mapping and metabolite quantification using proton magnetic resonance spectroscopy

Assessing brain temperature can provide important information about disease processes (e.g., stroke, trauma) and therapeutic effects (e.g., cerebral hypothermia treatment). Whole-brain magnetic resonance spectroscopic imaging (WB-MRSI) is increasingly used to quantify brain metabolites across the entire brain. However, its feasibility and reliability for estimating brain temperature needs further validation. Therefore, the present study evaluates the reproducibility of WB-MRSI for temperature mapping as well as metabolite quantification across the whole brain in healthy volunteers. Ten healthy adults were scanned on three occasions 1 week apart. Brain temperature, along with four commonly assessed brain metabolites-total N-acetyl-aspartate (tNAA), total creatine (tCr), total choline (tCho) and myo-inositol (mI)-were measured from WB-MRSI data. Reproducibility was evaluated using the coefficient of variation (CV). The measured mean (range) of the intra-subject CVs was 0.9% (0.6%-1.6%) for brain temperature mapping, and 4.7% (2.5%-15.7%), 6.4% (2.4%-18.9%) and 14.2% (4.4%-52.6%) for tNAA, tCho and mI, respectively, with reference to tCr. Consistently larger variability was found when using H₂ O as the reference for metabolite quantifications: 7.8% (3.3%-17.8%), 7.8% (3.1%-18.0%), 9.8% (3.7%-31.0%) and 17.0% (5.9%-54.0%) for tNAA, tCr, tCho and mI, respectively. Further, the larger the brain region (indicated by a greater number of voxels within that region), the better the reproducibility for both temperature and metabolite estimates. Our results demonstrate good reproducibility of whole-brain temperature and metabolite measurements using the WB-MRSI technique.

Infusion of Cold Saline into the Carotid Artery Does Not Affect Outcome After Intrastriatal Hemorrhage

Localized brain hypothermia (HYPO) can be achieved by infusing cold saline into the carotid artery of animals and patients. Studies suggest that HYPO improves behavioral and histological outcomes in focal ischemia models. Given that ischemic stroke and intracerebral hemorrhage (ICH) share pathophysiological overlap, we tested whether cold saline infusion is safe and neuroprotective when given during collagenase-induced ICH. Eighty-five adult male Sprague-Dawley rats were used. Experiment 1 investigated brain and body temperature changes associated with a cold saline infusion paradigm that was scaled from patients according to brain weight and blood volume (3 mL/20-minute infusion). Experiment 2 determined whether HYPO aggravated bleeding volume. Experiment 3 investigated if cerebral edema or elemental concentrations were altered by HYPO. We also collected core body temperature and activity data through telemetry. Experiment 4 investigated whether behavioral outcomes (e.g., skilled reaching) and tissue loss were influenced by HYPO. Our HYPO protocol decreased the ipsilateral striatal temperature by ∼0.20°C (p < 0.001), with no other effects. HYPO did not affect hematoma volume (p = 0.64), cerebral edema (p = 0.34), or elemental concentrations (p = 0.49) at 24 hours post-ICH. Although ICH caused persistent behavioral impairments, HYPO did not improve behavioral outcomes (measured by a neurological deficit scale, cylinder, and the staircase test; p > 0.05 for all). Brain tissue loss was not different between groups on day 28 post-ICH (p = 0.90). Although cold saline infusion appears to be safe in the acute post-ICH period, there was no evidence that this therapy improved outcome. However, our treatment protocol was relatively mild and additional interventions might help improve efficacy. Finally, our findings may also speak to the safety of this cooling approach in focal ischemia where hemorrhagic transformation is a risk; future studies on this issue are needed.

An In Vivo Assessment of Regional Brain Temperature during Whole-Body Cooling for Neonatal Encephalopathy

OBJECTIVE

To assess differences in regional brain temperatures during whole-body hypothermia and test the hypothesis that brain temperature profile is nonhomogenous in infants with hypoxic-ischemic encephalopathy.

STUDY DESIGN

Infants with hypoxic-ischemic encephalopathy were enrolled prospectively in this observational study. Magnetic resonance (MR) spectra of basal ganglia, thalamus, cortical gray matter, and white matter (WM) were acquired during therapeutic hypothermia. Regional brain tissue temperatures were calculated from the chemical shift difference between water signal and metabolites in the MR spectra after performing calibration measurements. Overall difference in regional temperature was analyzed by mixed-effects model; temperature among different patterns and severity of injury on MR imaging also was analyzed. Correlation between temperature and depth of brain structure was analyzed using repeated-measures correlation.

RESULTS

In total, 53 infants were enrolled (31 girls, mean gestational age: 38.6 ± 2 weeks; mean birth weight: 3243 ± 613 g). MR spectroscopy was acquired at mean age of 2.2 ± 0.6 days. A total of 201 MR spectra were included in the analysis. The thalamus, the deepest structure (36.4 ± 2.3 mm from skull surface), was lowest in temperature (33.2 ± 0.8°C, compared with basal ganglia: 33.5 ± 0.9°C; gray matter: 33.6 ± 0.7°C; WM: 33.8 ± 0.9°C, all P < .001). Temperatures in more superficial gray matter and WM regions (depth: 21.9 ± 2.4 and 21.5 ± 2.2 mm) were greater than the rectal temperatures (33.4 ± 0.4°C, P < .03). There was a negative correlation between temperature and depth of brain structure (r_rm = -0.36, P < .001).

CONCLUSIONS

Whole-body hypothermia was effective in cooling deep brain structures, whereas superficial structures were warmer, with temperatures significantly greater than rectal temperatures.

Imaging the effect of the circadian light-dark cycle on the glymphatic system in awake rats

Homeostasis and the daily rhythms in brain function and temperature are coupled to the circadian light–dark cycle. MRI was used to study the redistribution of intraventricular contrast agent in awake rats during the night when they are active and during the day when at rest. Redistribution is lowest during the day and highest at night and parallels the gradients and regional variations in brain temperatures reported in the literature. The brain areas of low parenchymal redistribution are associated with high temperatures and have a high density of blood vessels that may be an essential part of the organization of the glymphatic system regulating brain temperature, blood gases, nutrients, metabolites, and waste products over the light–dark cycle.

The glymphatic system functions in the removal of potentially harmful metabolites and proteins from the brain. Dynamic, contrast-enhanced MRI was used in fully awake rats to follow the redistribution of intraventricular contrast agent entrained to the light–dark cycle and its hypothetical relationship to the sleep–waking cycle, blood flow, and brain temperature in specific brain areas. Brain areas involved in circadian timing and sleep–wake rhythms showed the lowest redistribution of contrast agent during the light phase or time of inactivity and sleep in rats. Global brain redistribution of contrast agent was heterogeneous. The redistribution was highest along the dorsal cerebrum and lowest in the midbrain/pons and along the ventral surface of the brain. This heterogeneous redistribution of contrast agent paralleled the gradients and regional variations in brain temperatures reported in the literature for awake animals. Three-dimensional quantitative ultrashort time-to-echo contrast-enhanced imaging was used to reconstruct small, medium, and large arteries and veins in the rat brain and revealed areas of lowest redistribution overlapped with this macrovasculature. This study raises new questions and theoretical considerations of the impact of the light–dark cycle, brain temperature, and blood flow on the function of the glymphatic system.

Dynamics of sleep oscillations is coupled to brain temperature on multiple scales

KEY POINTS

Sleep spindle frequency positively, duration negatively correlates with brain temperature. Local heating of the thalamus produces similar effects in the heated area. Thalamic network model corroborates temperature dependence of sleep spindle frequency. Brain temperature shows spontaneous microfluctuations during both anesthesia and natural sleep. Larger fluctuations are associated with epochs of REM sleep. Smaller fluctuations correspond to the alteration of spindling and delta epochs of infra-slow oscillation.

ABSTRACT

Every form of neural activity depends on temperature, yet its relationship to brain rhythms is poorly understood. In this work we examined how sleep spindles are influenced by changing brain temperatures and how brain temperature is influenced by sleep oscillations. We employed a novel thermoelectrode designed for measuring temperature while recording neural activity. We found that spindle frequency is positively correlated and duration negatively correlated with brain temperature. Local heating of the thalamus replicated the temperature dependence of spindle parameters in the heated area only, suggesting biophysical rather than global modulatory mechanisms, a finding also supported by a thalamic network model. Finally, we show that switches between oscillatory states also influence brain temperature on a shorter and smaller scale. Epochs of paradoxical sleep as well as the infra-slow oscillation were associated with brain temperature fluctuations below 0.2°C. Our results highlight that brain temperature is massively intertwined with sleep oscillations on various time scales.

Limitations of Local Brain Cooling on Generalized Motor Seizures from Unknown Foci in Awake Rats

Local brain cooling of an epileptic focus at 15°C reduces the number of spikes on an electrocorticogram (ECoG), terminates seizures, and maintains neurological functions. In this study, we attempted to suppress generalized motor seizures (GMSs) by cooling a unilateral sensorimotor area. GMSs were induced in rats by intraperitoneal injection of bicuculline methiodide, an antagonist of gamma-aminobutyric acid. While monitoring the ECoG and behavior, the right sensorimotor cortex was cooled for 10 min using an implanted device. The number of spikes recorded from the cooled cortex significantly decreased to 71.2% and 62.5% compared with the control group at temperatures of 15 and 5°C (both P <0.01), respectively. The number of spikes recorded from the contralateral mirror cortex reduced to 61.7% and 62.7% (both P <0.05), respectively. The ECoG power also declined to 85% and 50% in the cooled cortex, and to 94% and 49% in the mirror cortex by the cooling at 15 and 5°C, respectively. The spikes regained in the middle of the cooling period at 15°C and in the late period at 5°C. Seizure-free durations during the 10-min periods of cooling at 15 and 5°C lasted for 4.1 ± 2.2 and 5.9 ± 1.1 min, respectively. Although temperature-dependent seizure alleviation was observed, the effect of local cortical cooling on GMSs was limited compared with the effect of local cooling of the epileptic focus on GSMs.

Effects of Brain Temperature on the Outcome of Patients with Traumatic Brain Injury: A Prospective Observational Study

A prospective observational study collected temperature data from 51 patients in 11 neurosurgical centers and follow-up outcome information at 6 months in 49 patients. Brain temperature (T_br) was measured directly by an intraventricular temperature sensor. Axillary temperature (T_ax) and rectal temperature (T_re) were measured by electric thermometers. T_br was 0.4 to 1.5°C higher than body temperature. T_re correlated well with the T_br (coefficient: 0.7378; p < 0.05). Among all patients, Glasgow Coma Scale (GCS) scores on admission were significantly lower in the patients with post-operatively extreme peak temperature (T_peak, < 37°C or >39°C in first 24 h) and major temperature variation (T_vari > 1°C in first 12 h; p < 0.05, p < 0.01, respectively). Among the patients with no temperature intervention, the extreme T_peak group showed a lower Glasgow Outcome Scale-Extended (GOS-E) score at 6 months (p < 0.05) with lower GCS scores on admission (p < 0.01), compared with the moderate T_peak group. Remarkably, the major T_vari group showed significantly lower GOS-E scores (p < 0.05) with the same GCS scores as the minor T_vari group. Thus, T_re is the better candidate to estimate T_br. Spontaneously extreme T_peak in TBI represents both more serious injury on admission and worse prognosis, and T_vari might be used as a novel prognostic parameter in TBI. Brain temperature is therefore one of the critical indicators evaluating injury severity, prognostication, and monitoring in the management of TBI. This prospective observational study has been registered in ClinicalTrials.gov ( https://clinicaltrials.gov ), and the registration number is NCT03068143.

Acupuncture inhibition of methamphetamine-induced behaviors, dopamine release and hyperthermia in the nucleus accumbens: mediation of group II mGluR

Methamphetamine (METH) increases metabolic neuronal activity in the mesolimbic dopamine (DA) system and mediates the reinforcing effect. To explore the underlying mechanism of acupuncture intervention in reducing METH-induced behaviors, we investigated the effect of acupuncture on locomotor activity, ultrasonic vocalizations, extracellular DA release in the nucleus accumbens (NAcs) using fast-scan cyclic voltammetry and alterations of brain temperature (an indicator of local brain metabolic activity) produced by METH administration. When acupuncture was applied to HT7, but not TE4, both locomotor activity and 50-kHz ultrasonic vocalizations were suppressed in METH-treated rats. Acupuncture at HT7 attenuated the enhancement of electrically stimulated DA release in the NAc of METH-treated rats. Systemic injection of METH produced a sustained increase in NAc temperature, which was reversed by the DA D1 receptor antagonist SCH 23390 or acupuncture at HT7. Acupuncture inhibition of METH-induced NAc temperature was prevented by pre-treatment with a group II metabotropic glutamate receptors (mGluR2/3) antagonist EGLU into the NAc or mimicked by injection of an mGluR2/3 agonist DCG-IV into the NAc. These results suggest that acupuncture reduces extracellular DA release and metabolic neuronal activity in the NAc through activation of mGluR2/3 and suppresses METH-induced affective states and locomotor behavior.

Simian immunodeficiency virus transiently increases brain temperature in rhesus monkeys: detection with magnetic resonance spectroscopy thermometry

PURPOSE

To evaluate brain temperature effects of early simian immunodeficiency virus (SIV) infection in rhesus macaques using proton magnetic resonance spectroscopy (MRS) thermometry (MRSt) and to determine whether temperature correlates with brain choline or myo-inositol levels.

METHODS

Brain temperature was retrospectively determined in serial MRS scans that had been acquired at baseline and at 2 and 4 weeks post-SIV infection (wpi) in 16 monkeys by calculating the chemical shift difference between N-acetylaspartate (NAA) and water peaks in sequentially acquired water-suppressed and unsuppressed point-resolved spectroscopy (PRESS) spectra. Frontal and parietal cortex, basal ganglia, and white matter spectra were analyzed.

RESULTS

At 2 wpi, brain and rectal temperatures increased relative to baseline and normalized at 4 wpi. Brain temperatures correlated with choline levels in several brain areas, but not with myo-inositol levels.

CONCLUSION

These data indicate that SIV transiently increases brain temperature soon after infection and that temperature is correlated with transient changes in choline levels. Given that choline levels are associated with brain inflammation in SIV-infected monkeys, our findings suggest that the SIV-induced temperature increase reflects brain inflammation. We conclude that MRSt may be informative in human immunodeficiency virus models and may be useful for assessing effects of treatments that reduce inflammation. This study also illustrates that existing MRS data sets containing unsuppressed water spectra can be used to measure tissue temperature, an important physiological parameter.

Magnetic Resonance Spectroscopy Thermometry at 3 Tesla: Importance of Calibration Measurements

To demonstrate the importance of calibration measurements in 3 Tesla proton magnetic resonance (MR) spectroscopy (¹H-MRS) thermometry for human brain temperature estimation for routine clinical applications. In vitro proton MR spectroscopy to obtain calibration constants of the water-chemical shift was conducted at 3 Tesla with a temperature-controlled phantom, containing a pH-buffered aqueous solution of N-acetyl aspartate (NAA), creatine (Cr), methylene protons of Cr (Cr2), dimethyl silapentane sulfonic acid (DSS), and sodium formate (NaFor). Estimations of absolute human brain temperature were performed utilizing the correlation of temperature to the water-chemical shift for the resonances of NAA, Cr, and Cr2. Data for calibration of the metabolites' chemical shift differences and in vivo temperature estimations were acquired with single-voxel point-resolved spectroscopy (PRESS) sequences (repetition time/echo time = 2000/30 ms; voxel size 2 × 2 × 2 cm³). Spectroscopy data were quantified in the time-domain, and a Pearson correlation analysis was performed to estimate the correlation between the chemical shift of metabolites and measured temperatures. The correlation coefficients (r) of our calibration measurements were NAA 0.9975 (±0.0609), Cr -0.9979 (±0.0621), Cr2 - 0.9973 (±0.0577), DSS -0.9976 (±0.0615), and NaFor -0.8132 (±2.348). The mean calculated brain temperature was 37.78 ± 1.447°C, and the mean tympanic temperature was 36.83 ± 0.2456°C. Calculated temperatures derived from Cr and Cr2 provided significant (p = 0.0241 and p = 0.0210, respectively) correlations with measured temperatures (r = 0.4108 and r = -0.4194, respectively). Calibration measurements are vital for ¹H-MRS thermometry. Small numeric differences in measured signal and data preprocessing without any calibration measurements reduce accuracy of temperature calculations, which indicates that calculated temperatures should be interpreted with caution. Application of this method for clinical purposes warrants further investigation and a more practical approach.

Simultaneous infrared thermal imaging and laser speckle imaging of brain temperature and cerebral blood flow in rats

Infrared thermal imaging of brain temperature changes is useful for evaluating cortical activity and disease states, such as stroke. However, the changes depend on a balance between changes in heat generation from metabolism and in heat convection related to blood flow. To discriminate between these effects and gain a clearer understanding of neurovascular metabolic coupling, brain temperature imaging must be improved to measure temperature and blood flow simultaneously. We develop an imaging technique that shows a two-dimensional (2-D) distribution of absolute brain temperature and relative cerebral blood flow changes in anesthetized rats by combining infrared thermal imaging with laser speckle imaging. The changes in brain metabolism and cerebral blood flow are achieved using two different anesthetics (isoflurane and α-chloralose) to evaluate our system. Isoflurane increased cerebral blood flow but decreased metabolism, whereas α-chloralose decreased both parameters. This technique enables simultaneous visualization of brain surface changes in temperature and cerebral blood flow in the same regions. This imaging system will permit further study of neurovascular metabolic coupling in normal and diseased brains.

Working Dogs Drinking a Nutrient-Enriched Water Maintain Cooler Body Temperature and Improved Pulse Rate Recovery After Exercise

Exercise-related physiological changes were evaluated in hydrated, exercise-conditioned working dogs with free access to tap water (TW) with or without a nutrient-enriched water supplement (NW). Physiological samples and measures were collected before and after work-related field tasks in a warm and moderately humid ambient environment. In a cross-over design study, 12 dogs (age range 8-23 months) were evaluated on 3 separate occasions within each period with exercise bouts up to 30 min, on days -4, 3, and 11. Dogs were offered either ad libitum TW or portion-controlled NW daily plus ad libitum TW. Prior to and serially after exercise, pulse rate (PR), core (BT_core) and ear (BT_ear) temperature were recorded. Urine was collected first thing in the morning, whereas blood samples collected and body weight (BW) recorded pre- and immediately post exercise. Ambient temperature was above 21.7°C (71°F) and relative humidity ranged from 36 to 76%. Activity parameters, AM urine measures, post-exercise percent change of BW, resting PR and resting BT_core did not differ between treatment groups on any exercise day. At the completion of exercise, mean BT_core for all dogs ranged from 104.8 to 105.6°F. Immediate post-exercise BT_ear was always lower compared to BT_core and means ranged from 103.3 to 104.0°F. The effect of time was highly significant (P < 0.001) for both BT measures with both BT_core and BT_ear recovering to resting levels by 60 min post exercise. PR and several blood values showed a significant main effect of time. Over the recovery period, dogs in the NW group had lower mean BT_ear and PR by 0.6°F and 3.4 bpm, respectively. Daily ingestion of a NW in combination with free access to TW can reduce the post-exercise-related BT_core and BT_ear hyperthermia, and improve pulse rate recovery following exercise in this population of working dogs undergoing 30 min bout of exercise.

Brain temperature but not core temperature increases during spreading depolarizations in patients with spontaneous intracerebral hemorrhage

Spreading depolarizations (SDs) are highly active metabolic events, commonly occur in patients with intracerebral hemorrhage (ICH) and may be triggered by fever. We investigated the dynamics of brain-temperature (T_brain) and core-temperature (T_core) relative to the occurrence of SDs. Twenty consecutive comatose ICH patients with multimodal electrocorticograpy (ECoG) and T_brain monitoring of the perihematomal area were prospectively enrolled. Clusters of SDs were defined as ≥2 SDs/h. Generalized estimating equations were used for statistical calculations. Data are presented as median and interquartile range. During 3097 h (173 h [81-223]/patient) of ECoG monitoring, 342 SDs were analyzed of which 51 (15%) occurred in clusters. Baseline T_core and T_brain was 37.3℃ (36.9-37.8) and 37.4℃ (36.7-37.9), respectively. T_brain but not T_core significantly increased 25 min preceding the onset of SDs by 0.2℃ (0.1-0.2; p < 0.001) and returned to baseline 35 min following SDs. During clusters, T_brain increased to a higher level (+0.4℃ [0.1-0.4]; p = 0.006) when compared to single SDs. A higher probability (OR = 36.9; CI = 36.8-37.1; p < 0.001) of developing SDs was observed during episodes of T_brain ≥ 38.0℃ (23% probability), than during T_brain ≤ 36.6℃ (9% probability). Spreading depolarizations - and in particular clusters of SDs - may increase brain temperature following ICH.

Heroin Contaminated with Fentanyl Dramatically Enhances Brain Hypoxia and Induces Brain Hypothermia

While opioid abuse is an established medical and public health issue, the increased availability of highly potent synthetic opioids, such as fentanyl, has given rise to acute health complications, including a comatose state and death during drug overdose. Since respiratory depression that leads to acute hypoxia is the most dangerous complication of opioid drug use, we examined the effects of intravenous heroin and heroin contaminated with 10% fentanyl on oxygen levels in the nucleus accumbens (NAc) monitored using high-speed amperometry in freely moving rats. Additionally, we examined the effects of heroin, fentanyl, and their mixture on locomotion and temperatures in the NAc, temporal muscle, and skin. Both fentanyl and heroin at human-relevant doses (400 and 40 μg/kg, respectively) induced rapid, strong and transient decreases in NAc oxygen, indicative of brain hypoxia. When the heroin-fentanyl mixture was injected, the NAc hypoxic response was greatly potentiated in its duration, suggesting sustained hypoxia. In contrast to modest, monophasic brain temperature increases caused by heroin alone, the heroin-fentanyl mixture induced a biphasic temperature response, with a prominent postinjection decrease resulting from peripheral vasodilation. This hypothermic effect, albeit much smaller and more transient, was typical of fentanyl injected alone. Our findings indicate that accidental use of fentanyl instead of heroin, or even a relatively minor contamination of “street heroin” with fentanyl, poses great danger for acute health complications, including a comatose state and death.

Intravenous Heroin Induces Rapid Brain Hypoxia and Hyperglycemia that Precede Brain Metabolic Response

Heroin use and overdose have increased in recent years as people transition from abusing prescription opiates to using the cheaper street drug. Despite a long history of research, many physiological effects of heroin and their underlying mechanisms remain unknown. Here, we used high-speed amperometry to examine the effects of intravenous heroin on oxygen and glucose levels in the nucleus accumbens (NAc) in freely-moving rats. Heroin within the dose range of human drug use and rat self-administration (100–200 μg/kg) induced a rapid, strong, but transient drop in NAc oxygen that was followed by a slower and more prolonged rise in glucose. Using oxygen recordings in the subcutaneous space, a densely-vascularized site with no metabolic activity, we confirmed that heroin-induced brain hypoxia results from decreased blood oxygen, presumably due to drug-induced respiratory depression. Respiratory depression and the associated rise in CO₂ levels appear to drive tonic increases in NAc glucose via local vasodilation. Heroin-induced changes in oxygen and glucose were rapid and preceded the slow and prolonged increase in brain temperature and were independent of enhanced intra-brain heat production, an index of metabolic activation. A very high heroin dose (3.2 mg/kg), corresponding to doses used by experienced drug users in overdose conditions, caused strong and prolonged brain hypoxia and hyperglycemia coupled with robust initial hypothermia that preceded an extended hyperthermic response. Our data suggest heroin-induced respiratory depression as a trigger for brain hypoxia, which leads to hyperglycemia, both of which appear independent of subsequent changes in brain temperature and metabolic neural activity.

Prediction of brain tissue temperature using near-infrared spectroscopy

Broadband near-infrared spectroscopy (NIRS) can provide an endogenous indicator of tissue temperature based on the temperature dependence of the water absorption spectrum. We describe a first evaluation of the calibration and prediction of brain tissue temperature obtained during hypothermia in newborn piglets (animal dataset) and rewarming in newborn infants (human dataset) based on measured body (rectal) temperature. The calibration using partial least squares regression proved to be a reliable method to predict brain tissue temperature with respect to core body temperature in the wavelength interval of 720 to 880 nm with a strong mean predictive power of [Formula: see text] (animal dataset) and [Formula: see text] (human dataset). In addition, we applied regression receiver operating characteristic curves for the first time to evaluate the temperature prediction, which provided an overall mean error bias between NIRS predicted brain temperature and body temperature of [Formula: see text] (animal dataset) and [Formula: see text] (human dataset). We discuss main methodological aspects, particularly the well-known aspect of over- versus underestimation between brain and body temperature, which is relevant for potential clinical applications.

Brain Hyperglycemia Induced by Heroin: Association with Metabolic Neural Activation

Glucose enters the brain extracellular space from arterial blood, and its proper delivery is essential for metabolic activity of brain cells. By using enzyme-based biosensors coupled with high-speed amperometry in freely moving rats, we previously showed that glucose levels in the nucleus accumbens (NAc) display high variability, increasing rapidly following exposure to various arousing stimuli. In this study, the same technology was used to assess NAc glucose fluctuations induced by intravenous heroin. Heroin passively injected at a low dose optimal for maintaining self-administration behavior (100 μg/kg) induces a rapid but moderate glucose rise (∼150-200 μM or ∼15-25% over resting baseline). When the heroin dose was doubled and tripled, the increase became progressively larger in magnitude and longer in duration. Heroin-induced glucose increases also occurred in other brain structures (medial thalamus, lateral striatum, hippocampus), suggesting that brain hyperglycemia is a whole-brain phenomenon but changes were notably distinct in each structure. While local vasodilation appears to be the possible mechanism underlying the rapid rise in extracellular glucose levels, the driving factor for this vasodilation (central vs peripheral) remains to be clarified. The heroin-induced NAc glucose increases positively correlated with increases in intracerebral heat production determined in separate experiments using multisite temperature recordings (NAc, temporal muscle and skin). However, glucose levels rise very rapidly, preceding much slower increases in brain heat production, a measure of metabolic activation associated with glucose consumption.

Body water conservation through selective brain cooling by the carotid rete: a physiological feature for surviving climate change?

Some mammals have the ability to lower their hypothalamic temperature below that of carotid arterial blood temperature, a process termed selective brain cooling. Although the requisite anatomical structure that facilitates this physiological process, the carotid rete, is present in members of the Cetartiodactyla, Felidae and Canidae, the carotid rete is particularly well developed in the artiodactyls, e.g. antelopes, cattle, sheep and goats. First described in the domestic cat, the seemingly obvious function initially attributed to selective brain cooling was that of protecting the brain from thermal damage. However, hyperthermia is not a prerequisite for selective brain cooling, and selective brain cooling can be exhibited at all times of the day, even when carotid arterial blood temperature is relatively low. More recently, it has been shown that selective brain cooling functions primarily as a water-conservation mechanism, allowing artiodactyls to save more than half of their daily water requirements. Here, we argue that the evolutionary success of the artiodactyls may, in part, be attributed to the evolution of the carotid rete and the resulting ability to conserve body water during past environmental conditions, and we suggest that this group of mammals may therefore have a selective advantage in the hotter and drier conditions associated with current anthropogenic climate change. A better understanding of how selective brain cooling provides physiological plasticity to mammals in changing environments will improve our ability to predict their responses and to implement appropriate conservation measures.

Eye and Ear Temperature Using Infrared Thermography Are Related to Rectal Temperature in Dogs at Rest or With Exercise

Rectal body temperature (BT) has been documented in exercising dogs to monitor thermoregulation, heat stress risk, and performance during physical activity. Eye (BTeye) and ear (BTear) temperature measured with infrared thermography (IRT) were compared to rectal (BTrec) temperature as the reference method and assess alternative sites to track hyperthermia, possibly to establish BTeye IRT as a passive and non-contact method. BT measures were recorded at 09:00, 11:30, 12:30, and 02:30 from Labrador Retrievers (N = 16) and Beagles (N = 16) while sedentary and with 30-min play-exercise (pre- and 0, 15, 30-min post-exercise). Total exercise locomotor activity counts were recorded to compare relative intensity of play-exercise between breeds. BTrec, BTeye, and BTear were measured within 5 min of the target time. Each BT method was analyzed by analysis of variance for main effects of breed and time. Method differences were compared using Bland-Altman plots and linear regression. Sedentary BT differed by breed for BTrec (p < 0.0001), BTear (p < 0.0001), and BTeye (p = 0.06) with Labs having on average 0.3-0.8°C higher BT compared to Beagles. Readings also declined over time for BTeye (p < 0.0001) and BTear (p < 0.0001), but not for BTrec (p = 0.63) for both breeds. Total exercise (30-min) activity counts did not differ (p = 0.53) between breeds. Time and breed interaction was significant in response to exercise for both BTrec and BTear (p = 0.035 and p = 0.005, respectively), with a marginal interaction (p = 0.09) for BTeye. All the three methods detected hyperthermia with Labs having a higher increase compared to Beagles. Both BTear and BTeye were significantly (p < 0.0001) related to BTrec in all dogs with sedentary or exercise activity. The relationship between BTeye and BTrec improved when monitoring exercise hyperthermia (r = 0.674) versus measures at rest (r = 0.381), whereas BTear was significantly related to BTrec regardless of activity (r = 0.615-0.735). Although BT readings were significantly related, method bias (p < 0.02) was observed for BTeye to slightly underestimate BTrec, whereas no bias was observed between BTear and BTrec. This study demonstrates that IRT technology effectively measures both ear and eye temperature and enables effective monitoring of BT changes at rest, with exercise, and between breeds. However, ear, and not eye, temperature is a better reflection of rectal temperature.

The Role of ADC-Based Thermometry in Measuring Brain Intraventricular Temperature in Children

BACKGROUND AND PURPOSE

To determine the feasibility of apparent diffusion coefficient (ADC)-based thermometry to assess intraventricular temperature in children.

METHODS

ADC maps were generated from diffusion tensor imaging data, which were acquired with diffusion gradients along 20 noncollinear directions using a b-value of 1000 s/mm(2) . The intraventricular temperature was calculated based on intraventricular ADC values and the mode method as previously reported. The calculated intraventricular temperature was validated with an estimated brain temperature based on temporal artery temperature measurements. We included 120 children in this study (49 females, 71 males, mean age 6.63 years), 15 consecutive children for each of the following age groups: 0-1, 1-2, 2-4, 4-6, 6-8, 8-10, 10-14, and 14-18 years. Forty-three children had a normal brain MRI and 77 children had an abnormal brain scan. Polynomial fitting to the temperature distribution and subsequent calculation of mode values was performed. A correlation coefficient and a coefficient of determination were calculated between ADC calculated temperatures and estimated brain temperatures. Linear regression analysis was performed to investigate the two temperature measures.

RESULTS

ADC-based intraventricular temperatures ranged between 31.5 and 39.6 °C, although estimated brain temperatures ranged between 36.3 and 38.1 °C. The difference between the temperatures is larger for children with more than 8,000 voxels within the lateral ventricles compared to children with less than 8,000 voxels. The correlation coefficient between ADC-based temperatures and the estimated brain temperatures is .1, the respective R(2) is .01 indicating that 1% of the changes in estimated brain temperatures are attributable to corresponding changes in ADC-based temperature measurements (P = .275).

CONCLUSIONS

ADC-based thermometry has limited application in the pediatric population mainly due to a small ventricular size.

Coupling of cerebral blood flow and oxygen consumption during hypothermia in newborn piglets as measured by time-resolved near-infrared spectroscopy: a pilot study

Hypothermia (HT) is a potent neuroprotective therapy that is now widely used in following neurological emergencies, such as neonatal asphyxia. An important mechanism of HT-induced neuroprotection is attributed to the associated reduction in the cerebral metabolic rate of oxygen ([Formula: see text]). Since cerebral circulation and metabolism are tightly regulated, reduction in [Formula: see text] typically results in decreased cerebral blood flow (CBF); it is only under oxidative stress, e.g., hypoxia-ischemia, that oxygen extraction fraction (OEF) deviates from its basal value, which can lead to cerebral dysfunction. As such, it is critical to measure these key physiological parameters during therapeutic HT. This report investigates a noninvasive method of measuring the coupling of [Formula: see text] and CBF under HT and different anesthetic combinations of propofol/nitrous-oxide ([Formula: see text]) that may be used in clinical practice. Both CBF and [Formula: see text] decreased with decreasing temperature, but the OEF remained unchanged, which indicates a tight coupling of flow and metabolism under different anesthetics and over the mild HT temperature range (38°C to 33°C).

MRS thermometry calibration at 3 T: effects of protein, ionic concentration and magnetic field strength

MRS thermometry has been utilized to measure temperature changes in the brain, which may aid in the diagnosis of brain trauma and tumours. However, the temperature calibration of the technique has been shown to be sensitive to non-temperature-based factors, which may provide unique information on the tissue microenvironment if the mechanisms can be further understood. The focus of this study was to investigate the effects of varied protein content on the calibration of MRS thermometry at 3 T, which has not been thoroughly explored in the literature. The effects of ionic concentration and magnetic field strength were also considered. Temperature reference materials were controlled by water circulation and freezing organic fixed-point compounds (diphenyl ether and ethylene carbonate) stable to within 0.2 °C. The temperature was measured throughout the scan time with a fluoro-optic probe, with an uncertainty of 0.16 °C. The probe was calibrated at the National Physical Laboratory (NPL) with traceability to the International Temperature Scale 1990 (ITS-90). MRS thermometry measures were based on single-voxel spectroscopy chemical shift differences between water and N-acetylaspartate (NAA), Δ(H20-NAA), using a Philips Achieva 3 T scanner. Six different phantom solutions with varying protein or ionic concentration, simulating potential tissue differences, were investigated within a temperature range of 21-42 °C. Results were compared with a similar study performed at 1.5 T to observe the effect of field strengths. Temperature calibration curves were plotted to convert Δ(H20-NAA) to apparent temperature. The apparent temperature changed by -0.2 °C/% of bovine serum albumin (BSA) and a trend of 0.5 °C/50 mM ionic concentration was observed. Differences in the calibration coefficients for the 10% BSA solution were seen in this study at 3 T compared with a study at 1.5 T. MRS thermometry may be utilized to measure temperature and the tissue microenvironment, which could provide unique unexplored information for brain abnormalities and other pathologies.

Non-invasive measurement of brain temperature using radiometric thermometry: experimental validation and clinical observations in asphyxiated newborns

BACKGROUND

Therapeutic hypothermia (HT) has been shown to decrease death and severe disability in infants with hypoxic-ischemic encephalopathy (HIE). Rectal temperature (RT) is used to determine the temperature set-points for treatment with HT, however experimental studies have shown significant differences between RT and brain temperature during HT. Knowledge of actual brain temperature during HT might allow better determination of optimal degree of cooling and improve outcomes.

OBJECTIVES

To compare measurements of brain temperature obtained by non-invasive radiometric thermometry (RadT) to direct tissue measurements in an experimental model of HT, and to use RadT in newborn infants with HIE undergoing HT.

STUDY DESIGN

RadT measurements of brain temperature were compared to fiber optic (Luxtron) thermometry measurements placed at a depth of 1.5 centimeters into the brain of cooled miniswine. Following validation studies, brain RadT and RT measurements were continuously recorded in thirty infants with HIE during HT and rewarming.

RESULTS

RadT and Luxtron probe temperatures were comparable in miniswine throughout a temperature range similar to therapeutic HT. RadT measurements of brain temperature were higher than RT in 60% of infants with HIE undergoing HT. Higher RadT measurements compared to RT were associated with cerebral white matter abnormalities (p = 0.01).

CONCLUSIONS

RadT provides a safe, passive and non-invasive way to measure brain temperature that can be used in the clinical setting. RadT may be helpful in determining the optimal degree of cooling and identifying infants at highest risk of brain injury.

Altered coupling of regional cerebral blood flow and brain temperature in schizophrenia compared with bipolar disorder and healthy subjects

Previous studies have suggested that schizophrenia patients have dysfunctional thermoregulation. The aim of this study was to examine whether brain temperature (BT) in schizophrenia patients differs from that in patients with bipolar disorder and healthy subjects by using magnetic resonance imaging. We also evaluated the possible relationship between BT and cerebral blood flow (CBF). We analyzed the temperature of lateral ventricles as the mean BT using diffusion-weighted imaging (DWI) thermometry, and evaluated the relationships between the BT and the CBF using pseudo-continuous arterial spin labeling (pCASL) among 3 diagnostic groups, 22 male patients with schizophrenia, 19 male patients with bipolar disorder, and 23 healthy male subjects. There were significant positive correlations between BT in the lateral ventricles and CBF in both the patients with bipolar disorder and healthy subjects. By contrast, there were significant negative correlations in patients with schizophrenia. We could not detect the significant difference in the surrogates of BT among three diagnostic groups. We showed that patients with schizophrenia, but not those with bipolar disorder, have dysfunctional thermoregulation in the brain. Brain temperature is highly dependent on cerebral metabolism and CBF, and thus uncoupling of cerebral metabolism and CBF may occur in schizophrenics.

Non-invasive monitoring of brain temperature by near-infrared spectroscopy

Optical techniques are promising methods for measuring tissue temperature noninvasively due to the transparency of tissue to near infrared-light and the temperature dependent light-absorbing properties of endogenous absorbers, particularly water. Besides being noninvasive, the instruments are compact and portable, permitting bedside monitoring.

MRS water resonance frequency in childhood brain tumours: a novel potential biomarker of temperature and tumour environment

(1)H MRS thermometry has been investigated for brain trauma and hypothermia monitoring applications but has not been explored in brain tumours. The proton resonance frequency (PRF) of water is dependent on temperature but is also influenced by microenvironment factors, such as fast proton exchange with macromolecules, ionic concentration and magnetic susceptibility. (1)H MRS has been utilized for brain tumour diagnostic and prognostic purposes in children; however, the water PRF measure may provide complementary information to further improve characterization. Water PRF values were investigated from a repository of MRS data acquired from childhood brain tumours and children with apparently normal brains. The cohort consisted of histologically proven glioma (22), medulloblastoma (19) and control groups (28, MRS in both the basal ganglia and parietal white matter regions). All data were acquired at 1.5 T using a short TE (30 ms) single voxel spectroscopy (PRESS) protocol. Water PRF values were calculated using methyl creatine and total choline. Spectral peak amplitude weighted averaging was used to improve the accuracy of the measurements. Mean PRF values were significantly larger for medulloblastoma compared with glioma, with a difference in the means of 0.0147 ppm (p < 0.05), while the mean PRF for glioma was significantly lower than for the healthy cohort, with a difference in the means of 0.0061 ppm (p < 0.05). This would suggest the apparent temperature of the glioma group was ~1.5 °C higher than the medulloblastomas and ~0.7 °C higher than a healthy brain. However, the PRF shift may not reflect a change in temperature, given that alterations in protein content, microstructure and ionic concentration contribute to PRF shifts. Measurement of these effects could also be used as a supplementary biomarker, and further investigation is required. This study has shown that the water PRF value has the potential to be used for characterizing childhood brain tumours, which has not been reported previously.

Critical role of peripheral vasoconstriction in fatal brain hyperthermia induced by MDMA (Ecstasy) under conditions that mimic human drug use

MDMA (Ecstasy) is an illicit drug used by young adults at hot, crowed "rave" parties, yet the data on potential health hazards of its abuse remain controversial. Here, we examined the effect of MDMA on temperature homeostasis in male rats under standard laboratory conditions and under conditions that simulate drug use in humans. We chronically implanted thermocouple microsensors in the nucleus accumbens (a brain reward area), temporal muscle, and facial skin to measure temperature continuously from freely moving rats. While focusing on brain hyperthermia, temperature monitoring from the two peripheral locations allowed us to evaluate the physiological mechanisms (i.e., intracerebral heat production and heat loss via skin surfaces) that underlie MDMA-induced brain temperature responses. Our data confirm previous reports on high individual variability and relatively weak brain hyperthermic effects of MDMA under standard control conditions (quiet rest, 22-23°C), but demonstrate dramatic enhancements of drug-induced brain hyperthermia during social interaction (exposure to male conspecific) and in warm environments (29°C). Importantly, we identified peripheral vasoconstriction as a critical mechanism underlying the activity- and state-dependent potentiation of MDMA-induced brain hyperthermia. Through this mechanism, which prevents proper heat dissipation to the external environment, MDMA at a moderate nontoxic dose (9 mg/kg or ~1/5 of LD50 in rats) can cause fatal hyperthermia under environmental conditions commonly encountered by humans. Our results demonstrate that doses of MDMA that are nontoxic under cool, quiet conditions can become highly dangerous under conditions that mimic recreational use of MDMA at rave parties or other hot, crowded venues.