Noninvasive measurement of cerebral blood flow and glucose metabolic rate in the rat with high-resolution animal positron emission tomography (PET): a novel in vivo approach for assessing drug action in the brains of small animals

Conscious rat PET imaging with soft immobilization for quantitation of brain functions: comprehensive assessment of anesthesia effects on cerebral blood flow and metabolism

BACKGROUND

Animal brain functions evaluated by in vivo imaging under anesthesia can be affected by anesthetic agents, resulting in incorrect assessment of physiological brain function. We therefore performed dynamic positron emission tomography (PET) imaging of conscious rats using recently reported soft immobilization to validate the efficacy of the immobilization for brain function assessments. We also determined the effects of six anesthetic agents-a mixed anesthetic agent (MMB), ketamine + xylazine (KX), chloral hydrate (Chloral), pentobarbital (PTB), propofol (PF), and isoflurane (IFL)-on brain function by comparison with conscious rats.

RESULTS

The immobilization enabled 45-min dynamic [¹⁸F]FDG-PET acquisition with arterial blood sampling using conscious rats without the use of special techniques or invasive surgery. The spatial resolution and quantitativity of [¹⁸F]FDG-PET were not significantly lower for conscious rats than for anesthetized rats. While MMB, Chloral, PTB, and PF showed ubiquitous reduction in the cerebral metabolic rates of glucose (CMR_glu) in brain regions, KX and IFL showed higher reductions in cerebellum and interbrain, and cerebellum, respectively. Cerebral blood flow (CBF) was reduced by MMB, KX, PTB, and PF; increased by IFL; and unaltered by Chloral. The magnitude of decrease in CMR_glu and CBF for MMB were not larger than for other five anesthetic agents, although blood glucose levels and body temperature can be easily affected by MMB.

CONCLUSION

The six anesthetic agents induced various effects on CMR_glu and CBF. The immobilization technique presented here is a promising tool for noninvasive brain functional imaging using conscious rats to avoid the effects of anesthetic agents.

Optimization of phase-contrast MRI for the estimation of global cerebral blood flow of mice at 11.7T

PURPOSE

To optimize phase-contrast (PC) MRI for the measurement of global cerebral blood flow (CBF) in the mouse at 11.7T.

METHODS

We determined proper velocity encoding (VENC) for internal carotid arteries (ICAs) and vertebral arteries (VAs). Next, we optimized spatial resolution of the sequence. To shorten scan time without compromising data quality, we further optimized repetition time and developed a reduced field-of-view (FOV) scheme for ICA and VA PC MRI. Whole-brain volume was determined with T₂ -weighted image to obtain unit-volume CBF.

RESULTS

Peak flow velocities were 13.8 ± 1.7, 14.4 ± 0.6, 6.5 ± 1.7, and 6.7 ± 1.3 cm/s for left ICA, right ICA, left VA, and right VA, respectively. Thus, VENC values of 20 and 10 cm/s were chosen for ICA and VA PC MRI, respectively. An in-plane spatial resolution of 50 × 50 μm² was found to provide a reasonable trade-off between reducing partial-volume effects and maintaining signal-to-noise ratio. Because of the fact that saturated spins in the imaging slice are rapidly replaced by fresh spins, TR of the sequence can be decreased to as short as 15 ms without reducing signal intensity, thereby substantially lowering scan time. Moreover, reduced FOV along the phase-encoding direction was able to shorten scan time by 33.3% while maintaining measurement accuracy. With these optimizations, it took 96 seconds to evaluate CBF with a test-retest variability of approximately 5% and an inter-rater correlation of >0.95. Global unit-volume CBF was found to be 279.5 ± 11.1 mL of blood/100 ml of tissue/min.

CONCLUSION

We have optimized PC MRI for noninvasive quantification of blood flow in mice at 11.7T.

Quantitation of rat cerebral blood flow using 99mTc-HMPAO

INTRODUCTION

Technetium-99m-hexamethylpropyleneamine oxime (^99mTc-HMPAO) is potentially useful for the assessment of cerebral blood flow (CBF) in small animals. In this paper, a procedure for quantitation of rat CBF using ^99mTc-HMPAO was determined.

METHODS

Biodistribution of ^99mTc-radioactivity in normal rats was determined after intravenous administration of ^99mTc-HMPAO. Acetazolamide treated rats were intravenously administered with the mixture of ^99mTc-HMPAO and N-isopropyl-[¹²⁵I]iodoamphetamine ([¹²⁵I]IMP), and arterial blood was then collected for 5min. After blood sampling, the brain radioactivity concentration was measured with the auto-well γ counter.

RESULTS

The brain radioactivity concentration after intravenous administration of ^99mTc-HMPAO was steady from 14s to 60min post-injection. A double tracer experiment using ^99mTc-HMPAO and [¹²⁵I]IMP showed that 19s was the average of the optimal integration interval of arterial blood ^99mTc-radioactivity concentration to obtain CBF values measured by ^99mTc-HMPAO identical to those determined by [¹²⁵I]IMP. The CBF value determined by ^99mTc-HMPAO, calculated by dividing the brain radioactivity concentration at 5min post-injection by the integrated arterial blood radioactivity concentration until 19s post-injection, was well correlated with CBF as determined by [¹²⁵I]IMP.

CONCLUSION

These results suggest that the CBF quantitation procedure described in this paper could be useful for rat CBF assessment.

Rat brain slices oxidize glucose at high rates: a (13)C NMR study

Since glucose is the main cerebral substrate, we have characterized the metabolism of various (13)C glucose isotopomers in rat brain slices. For this, we have used our cellular metabolomic approach that combines enzymatic and carbon 13 NMR techniques with mathematical models of metabolic pathways. We identified the fate and the pathways of the conversion of glucose carbons into various products (pyruvate, lactate, alanine, aspartate, glutamate, GABA, glutamine and CO(2)) and determined absolute fluxes through pathways of glucose metabolism. After 60 min of incubation, lactate and CO(2) were the main end-products of the metabolism of glucose which was avidly metabolized by the slices. Lactate was also used at high rates by the slices and mainly converted into CO(2). High values of flux through pyruvate carboxylase, which were similar with glucose and lactate as substrate, were observed. The addition of glutamine, but not of acetate, stimulated pyruvate carboxylation, the conversion of glutamate into succinate and fluxes through succinate dehydrogenase, malic enzyme, glutamine synthetase and aspartate aminotransferase. It is concluded that, unlike brain cells in culture, and consistent with high fluxes through PDH and enzymes of the tricarboxylic acid cycle, rat brain slices oxidized both glucose and lactate at high rates.

Development of an H(2)(15)O steady-state method combining a bolus and slow increasing injection with a multiprogramming syringe pump

An (15)O-labeled water (H(2)(15)O) steady-state method for quantitative measurement of cerebral blood flow (CBF), which is less stressful to small animals with a few point blood sampling, was developed. After a simulation using a dose meter to achieve stable H(2)(15)O radioactivity in the blood with a multiprogramming syringe pump programmed for slowly increasing injection volume, 10 rats were studied with the injection method. Arterial blood was sampled every minute during 6-minute positron emission tomography (PET) scans. After the PET scan, N-isopropyl-p-[(125)I]-iodoamphetamine ((125)I-IMP) was injected into the same rat to measure CBF using the autoradiography method based on a microsphere model. Regions of interest were placed on the whole brain in H(2)(15)O-PET and (125)I-IMP-autoradiography images, and CBF values calculated from both methods were compared. Radioactivity in the dose meter achieved equilibrium ∼1 minute after starting the H(2)(15)O injection. In rat studies, radioactivity in the blood and brain rapidly achieved equilibrium at 2 minutes after administration. The correlation of CBF values of H(2)(15)O PET (49.2±5.4 mL per 100 g per minute) and those of (125)I-IMP autoradiography (49.1±5.2 mL per 100 g per minute) was excellent (y=1.01x-0.37, r(2)=0.97). The H(2)(15)O steady-state method with a continuously increasing injection is useful for CBF measurement in small animal studies, especially when multiple scans are required in the same animal.

Non-invasive quantification of cerebral blood flow for rats by microPET imaging of 15O labelled water: the application of a cardiac time-activity curve for the tracer arterial input function

OBJECTIVE

In-vivo quantitative cerebral blood flow (CBF) measurement using positron emission tomography (PET) has typically employed invasive arterial blood sampling procedure to determine the arterial input function (AIF). The present study was performed to provide a non-invasive quantitative CBF measurement technique for rats using a dedicated animal PET.

METHODS

CBF was measured in 10 male rats (Fischer 344, 247-290 g) under alpha-chloralose anesthesia (30 mg x kg . h, intravenous infusion) by dynamic PET imaging employing the intravenous bolus injection of H2(15)O. Unlike other conventional PET methods, no arterial blood sampling was employed. Instead, a cardiac time-activity curve (TAC) obtained from the dynamic PET imaging was used to determine the AIF. For the validation of this technique, CBF was also measured by calculating the washout rate of the tracer (H2(15)O) following an intracarotid bolus injection. CBF measurements by two independent methods were done while modulating and maintaining the body temperature at two different levels (32+/-1 and 37+/-1 degrees C by the rectal temperature). Two methods were compared by the linear regression analysis.

RESULTS

CBF (ml x 100 g x min) values (mean+/-SD) were 45.2+/-6.05 (intravenous) and 47.4+/-8.64 (intracarotid) at the hypothermic condition (32 degrees C), and 55.1+/-4.88 (intravenous) and 54.4+/-4.60 (intracarotid) at the normothermic condition (37 degrees C). There was a good agreement between the two methods (r=0.70).

CONCLUSIONS

Our cardiac TAC analysis technique for small animals can be used for the non-invasive quantification of CBF using the PET-based in-vivo imaging technique.

Availability of N-isopropyl-p-[125I]iodoamphetamine (IMP) as a practical cerebral blood flow (CBF) indicator in rats

Since a very complicated technique is necessary to measure cerebral blood flow (CBF) with [14C]iodoantipyrine in small animals, a practical and easy method is needed. In this paper, the differential uptake ratio (DUR) at 5 min after injection of N-isopropyl-p-[125I]iodoamphetamine (IMP) was estimated as an index of CBF in normal rats and compared with the quantitative CBF value measured using [15O]H2O and dynamic PET scan. A good correlation between the two values was obtained. The results indicate that DUR of [125I]IMP at 5 min after injection in rats is a useful and practical indicator of CBF since neither arterial blood sampling nor metabolite correction is necessary.

Nonlinear coupling of neural activity and CBF in rodent barrel cortex

The relationship between neural activity and accompanying changes in cerebral blood flow (CBF) and oxygenation must be fully understood before data from brain imaging techniques can be correctly interpreted. Whether signals in fMRI reflect the neural input or output of an activated region is still unclear. Similarly, quantitative relationships between neural activity and changes in CBF are not well understood. The present study addresses these issues by using simultaneous laser Doppler flowmetry (LDF) to measure CBF and multichannel electrophysiology to record neural activity in the form of field potentials and multiunit spiking. We demonstrate that CBF-activation coupling is a nonlinear inverse sigmoid function. Comparing the data with previous work suggests that within a cortical model, CBF shows greatest spatial correlation with a current sink 500 microm below the surface corresponding to sensory input. These results show that care must be exercised when interpreting imaging data elicited by particularly strong or weak stimuli and that hemodynamic changes may better reflect the input to a region rather than its spiking output.

Electron paramagnetic resonance assessment of brain tissue oxygen tension in anesthetized rats

The adequacy of cerebral tissue oxygenation (PtO(2)) is a central therapeutic end point in critically ill and anesthetized patients. Clinically, PtO(2) is currently measured indirectly, based on measurements of cerebrovascular oxygenation using near infrared spectroscopy and experimentally, using positron emission tomographic scanning. Recent developments in electron paramagnetic resonance (EPR) oximetry facilitate accurate, sensitive, and repeated measurements of PtO(2). EPR is similar to nuclear magnetic resonance but detects paramagnetic species. Because these species are not abundant in brain (or other tissues) in vivo, oxygen-responsive paramagnetic lithium phthalocyanine crystals implanted into the cerebral cortex are used for the measurement of oxygen. The line widths of the EPR spectra of these materials are linear functions of PtO(2). We used EPR oximetry in anesthetized rats to study the patterns of PtO(2) during exposure to various inhaled and injected general anesthetics and to varying levels of inspired oxygen. Rats anesthetized with 2.0 minimum alveolar anesthetic concentration isoflurane maintained the largest PtO(2) (38.0 +/- 4.5 mm Hg) and rats anesthetized with ketamine/xylazine had the smallest PtO(2) (3.5 +/- 0.3 mm Hg) at a fraction of inspired oxygen (FIO(2)) of 0.21, P < 0.05. The maximal PtO(2) achieved under ketamine/xylazine anesthesia with FIO(2) of 1.0 was 8.8 +/- 0.3 mm Hg, whereas PtO(2) measured during isoflurane anesthesia with FIO(2) of 1.0 was 56.3 +/- 1.7 mm Hg (P < 0.05). These data highlight the experimental utility of EPR in measuring PtO(2) during anesthesia and serve as a foundation for further study of PtO(2) in response to physiologic perturbations and therapeutic interventions directed at preventing cerebral ischemia.

IMPLICATIONS

Using in vivo electron paramagnetic resonance oximetry, we studied the patterns of cerebral tissue oxygenation (PtO(2)) during exposure to various inhaled and injected general anesthetics, and to varying levels of inspired oxygen. These data show that inhaled anesthetics result in larger levels of PtO(2) in the brain than do several injectable anesthetics. The results highlight the experimental utility of electron paramagnetic resonance in measuring PtO(2) during anesthesia and serve as a foundation for further study of PtO(2) in response to physiologic perturbations and therapeutic interventions directed at preventing cerebral ischemia.

[Biomedical imaging in pharmacology with nuclear medical imaging methodologies: positron emission tomography (PET) and single photon emission computed tomography (SPECT)]

The nuclear imaging technologies, positron emission tomography (PET) and single photon emission computed tomography (SPECT), have the power to non-invasively obtain dynamic and real-time information on the in vivo behaviors of radiolabeled molecules not only in humans but also in experimental animals. Thus, PET and SPECT can image molecular interactions of biological processes in vivo directly and reveal biological phenomena that are hidden from view. Furthermore, these imaging procedures also can be repeatedly performed before and after interventions, thereby allowing each subject to be used as its own control. In these studies, the radiolabeled compounds used as imaging probes for non-invasive assays of biochemical processes should have defined in vivo behaviors that can provide valuable information on the physiological and pharmacological processes. This paper describes the principle of the nuclear medical imaging systems, rational design of radiolabeled imaging probes, and the application to in vivo investigation of the change of various neurotransmission systems under disease and drug treatment. The efficient utilization of these nuclear medical imaging technologies will accelerate biomedical studies and drug development.

Challenges in small animal noninvasive imaging

The current status and challenges of small animal non-invasive imaging is briefly reviewed. The advantages of non-invasive studies on living animals versus post-mortem studies are evaluated. An argument is advanced that even in post-mortem situations, non-invasive imaging may play an important role in efficiently characterizing small animal phenotypes as well as pathology. Issues of data interpretation under anesthetized conditions in live animal studies are also reviewed. The five imaging technologies covered include CT, PET, ultrasound, MRI and optical imaging. The structural and physiological information content of these different modalities is reviewed along with the ability of these techniques to scale down for use in small mammals such as mice and rats. In general, it was found that most of these technologies scale favorably to the study of small mammals, generally providing more physiological information than when used on the larger human scale. This suggests that these types of small mammal imaging capabilities will play a very significant role in the full utilization of these important animal models in biomedical research.

The biological application of small animal PET imaging

The short history of small animal PET is reviewed in the context of its application in the laboratory. Early work has demonstrated a role for the technique in both drug development and in the in vivo monitoring of neuroreceptor function with time. As spatial resolution approaches 1 mm, challenges in quantification remain. However, the ability to carry out animal PET studies that are analogous to human PET will form an important bridge between laboratory and clinical sciences.

In vivo imaging of neuronal activation and plasticity in the rat brain by high resolution positron emission tomography (microPET)

The study of neural repair and neuroplasticity in rodents would be enhanced by the ability to assess neuronal function in vivo. Positron emission tomography (PET) is used to study brain plasticity in humans, but the limited resolution and sensitivity of conventional scanners have generally precluded the use of PET to study neuroplasticity in rodents. We now demonstrate that microPET, a PET scanner developed for use with small animals, can be used to assess metabolic activity in different regions of the conscious rodent brain using [18F]fluorodeoxyglucose (FDG) as the tracer, and to monitor changes in neuronal activity. Limbic seizures result in dramatically elevated metabolic activity in the hippocampus, whereas vibrissal stimulation results in more modest increases in FDG uptake in the contralateral neocortex. We also show that microPET can be used to study lesion-induced plasticity of the brain. Cerebral hemidecortication resulted in diminished relative glucose metabolism in the neostriatum and thalamus ipsilateral to the lesion, with subsequent, significant recovery of metabolic function. These studies demonstrate that microPET can be used for serial assessment of metabolic function of individual, awake rats with a minimal degree of invasiveness, and therefore, has the potential for use in the study of brain disorders and repair.

Issues in measuring glucose metabolism of rat brain using PET: the effect of harderian glands on the frontal lobe

We estimated the effect of the Harderian gland (an orbital gland of land vertebrates) on the measurement of cerebral metabolic rate of glucose (CMRGIc) of the rat brain using positron emission tomography (PET) for animal use. The Harderian gland had the high accumulation of 18-F labeled deoxyglucose (FDG) after intravenous injection. By placing the large regions of interest (ROI) (twice the full width at half maximum in diameter), the CMRGIc in the frontal region was slightly higher compared with the CMRGIc after Harderian gland resection, but the parietal and occipital regions and the cerebellum had the similar level of CMRGIc before and after Harderian gland resection. Therefore the Harderian gland has a slight effect on the frontal lobe CMRGIc, but such overestimation can be within the permissible range for PET study of rat brains.

Use of scanner characteristics in iterative image reconstruction for high-resolution positron emission tomography studies of small animals

The purpose of this work was to improve of the spatial resolution of a whole-body positron emission tomography (PET) system for experimental studies of small animals by incorporation of scanner characteristics into the process of iterative image reconstruction. The image-forming characteristics of the PET camera were characterized by a spatially variant line-spread function (LSF), which was determined from 49 activated copper-64 line sources positioned over a field of view (FOV) of 21.0 cm. This information was used to model the image degradation process. During the course of iterative image reconstruction, the forward projection of the estimated image was blurred with the LSF at each iteration step before the estimated projections were compared with the measured projections. The imaging characteristics of the high-resolution algorithm were investigated in phantom experiments. Moreover, imaging studies of a rat and two nude mice were performed to evaluate the imaging properties of our approach in vivo. The spatial resolution of the scanner perpendicular to the direction of projection could be approximated by a one-dimensional Gaussian-shaped LSF with a full-width at half-maximum increasing from 6.5 mm at the centre to 6.7 mm at a radial distance of 10.5 cm. The incorporation of this blurring kernel into the iteration formula resulted in a significantly improved spatial resolution of about 3.9 mm over the examined FOV. As demonstrated by the phantom and the animal experiments, the high-resolution algorithm not only led to a better contrast resolution in the reconstructed emission scans but also improved the accuracy for quantitating activity concentrations in small tissue structures without leading to an amplification of image noise or image mottle. The presented data-handling strategy incorporates the image restoration step directly into the process of algebraic image reconstruction and obviates the need for ill-conditioned "deconvolution" procedures to be performed on the projections or on the reconstructed image. In our experience, the proposed algorithm is of special interest in experimental studies of small animals.

Effects of binge pattern cocaine administration on dopamine D1 and D2 receptors in the rat brain: an in vivo study using positron emission tomography

The aim of the present study was to determine the effect of "binge" pattern cocaine administration on dopamine D1 and D2 receptors in the rat brain. Male Sprague Dawley rats were injected three times at 1 hr intervals with saline or cocaine (15 mg/kg) each day for 2, 7, or 14 d. The in vivo binding of [11C]SCH23390 (dopamine D1 receptor antagonist) and [11C]N-methylspiperone (NMSP; dopamine D2 receptor antagonist) in the striatal region was measured by a high-resolution positron emission tomography at 1 and 3.5 hr, respectively, after the last cocaine or saline injection. Acute (2 d) binge cocaine administration did not change the in vivo binding potential of [11C]SCH23390 or the binding of [11C]NMSP in the striatum. After 7 d of binge cocaine administration, a significant decrease in the binding potential of [11C]SCH23390 was observed, whereas no change in the binding of [11C]NMSP was found. After 14 d of binge cocaine administration, the in vivo binding was significantly reduced for both [11C]SCH23390 and [11C]NMSP. Separate saturation experiments indicated that the observed alterations of in vivo binding were attributable mainly to apparent alterations in the affinity and not the number of binding sites. These results suggest that both dopamine D1 and D2 receptors may have altered physiologically available binding sites after binge pattern cocaine administration.

Positron emission tomography for quantitative determination of glucose metabolism in normal and ischemic brains in rats: an insoluble problem by the Harderian glands

To examine the reliability of quantitative positron emission tomography studies in the rat (Rat-PET), we assessed the influence of radioactivity accumulated in the Harderian glands on PET CMRglc determination. We measured CMRglc by PET and ex vivo dissection methods by using 2-[18F]fluoro-2-deoxy-D-glucose in rats with and without focal brain ischemia. The CMRglc values obtained by PET, after correcting with recovery coefficients, were higher than those measured by the ex vivo method at rostral slices, and reduction of the CMRglc in the ischemic brain was not demonstrated by PET in the frontal cortex. The radioactivity accumulated in the Harderian glands prevents the quantitative determination of CMRglc using Rat-PET.