Small Animal Brain Research Articles

Impact of attenuation correction strategies on the quantification of High Resolution Research Tomograph PET studies

In this study, the quantitative accuracy of different attenuation correction strategies presently available for the High Resolution Research Tomograph (HRRT) was investigated. These attenuation correction methods differ in reconstruction and processing (segmentation) algorithms used for generating a μ-image from measured 2D transmission scans, an intermediate step in the generation of 3D attenuation correction factors. Available methods are maximum-a-posteriori reconstruction (MAP-TR), unweighted OSEM (UW-OSEM) and NEC-TR, which transforms sinogram values back to their noise equivalent counts (NEC) to restore Poisson distribution. All methods can be applied with or without μ-image segmentation. However, for MAP-TR a μ-histogram is a prior during reconstruction. All possible strategies were evaluated using phantoms of various sizes, simulating preclinical and clinical situations. Furthermore, effects of emission contamination of the transmission scan on the accuracy of various attenuation correction strategies were studied. Finally, the accuracy of various attenuation corrections strategies and its relative impact on the reconstructed activity concentration (AC) were evaluated using small animal and human brain studies. For small structures, MAP-TR with human brain priors showed smaller differences in μ-values for transmission scans with and without emission contamination (<8%) than the other methods (<26%). In addition, it showed best agreement with true AC (deviation <4.5%). A specific prior designed to take into account the presence of small animal fixation devices only very slightly improved AC precision to 4.3%. All methods scaled μ-values of a large homogeneous phantom to within 4% of the water peak, but MAP-TR provided most accurate AC after reconstruction. However, for clinical data MAP-TR using the default prior settings overestimated the thickness of the skull, resulting in overestimations of μ-values in regions near the skull and thus in incorrect AC for cortical regions. Using NEC-TR with segmentation or MAP-TR with an adjusted human brain prior showed less overestimation in both skull thickness and AC for these structures and are therefore the recommended methods for human brain studies.

COMPARISON OF ISODOSE DISTRIBUTIONS IN CANINE BRAIN IN HETEROGENEITY‐CORRECTED VERSUS UNCORRECTED TREATMENT PLANS USING 6 MV PHOTONS

Magnetic resonance (MR) images may be useful for radiation planning due to greater contrast resolution. One disadvantage of MR images for radiation planning is the inability to incorporate electron density information into the dose calculation algorithm. To assess the magnitude of this problem, we evaluated radiation dose distribution in canine brain by comparing computed tomography (CT)-based radiotherapy plans with and without electron density correction. Computerized radiotherapy plans were generated for 13 dogs with brain tumors using 6 MV photons. A tissue-contouring program was used to outline the gross tumor volume (GTV) and the planning target volume (PTV) for each patient. Two treatment plans were generated for each dog. First, the plan was optimized without heterogeneity correction. Then the heterogeneity correction was implemented without changing any other plan parameters. Isodose distributions and dose volume histograms (DVHs) were used to compare the two plans. The D95 (dose delivered to 95% of the volume) within the PTV was calculated for each treatment plan and differences in the D95s were compared. The mean D95s without and with heterogeneity correction were 49.1 +/- 0.7 and 48.9 +/- 1.0Gy, respectively. The absolute mean percent dose difference without and with heterogeneity correction was 1.0 - 0.9% (-1.3-3.2%) and was not considered to be clinically significant. We found no clinically significant difference between CT-based radiotherapy plans without and with heterogeneity correction for brain tumors in small animals, which supports the use of MR-based treatment planning for radiotherapy of small animal brain tumors.

Theoretical model of temperature regulation in the brain during changes in functional activity.

The balance between metabolic heat production, heat removal by blood flow, and heat conductance defines local temperature distribution in a living tissue. Disproportional local increases in blood flow as compared with oxygen consumption during functional brain activity disturb this balance, leading to temperature changes. In this article we have developed a theoretical framework that allows analysis of temperature changes during arbitrary functional brain activity. We established theoretical boundaries on temperature changes and explained how these boundaries depend on physiology (blood flow and metabolism) and external (heat exchange with the environment) experimental conditions. We show that, in regions located deep in the brain, task performance should be accompanied by temperature decreases in regions where blood flow increases (activated regions) and by temperature increases in regions where blood flow decreases (deactivated regions). The sign of temperature effect may be reversed for superficial cortex regions, where the baseline brain temperature is lower than the temperature of incoming arterial blood due to the heat exchange with the environment. Importantly, due to heat conductance, the temperature effect is not localized to the activated region but extends to a surrounding tissue at rest over the distances regulated by the temperature-shielding effect of blood flow. This temperature-shielding effect quantifies the means by which cerebral blood flow prevents "temperature perturbations" from propagating away from the perturbed regions. For small activated regions, this effect also substantially suppresses the magnitude of the temperature response, making it especially important for small animal brains.

Mouse Models of Human Cancers Consortium Workshop on Nervous System Tumors

Nervous system tumors are clinically challenging neoplasms that form within the central and peripheral nervous system. Although there have been many clinical trials using novel agents for the treatment of primary brain tumors, there have been few advances that positively affect overall patient survival. Over the past several years, there has been significant progress in the development of accurate small-animal spontaneous brain tumor models, small-animal neuroimaging, and tools for the bioinformatic analysis of complex molecular data sets, all of which have contributed to an improved understanding of the pathogenesis of human brain tumors. Whereas these models will continue to be of great value in basic science investigations, they can also be used to identify and validate potential therapies for brain tumors and to evaluate these drugs in preclinical trials. The National Cancer Institute recently convened a workshop to review the current state of small-animal brain tumor modeling and to make recommendations about the use of these models to improve the clinical outcome for patients with brain tumors. In this meeting report, we outline the current state of small-animal models for brain tumors, the potential applications of these models, and the recommendations made by the workshop participants for the use of mouse models in the preclinical evaluation of potential brain tumor therapies.

Analysis of mouse brain using a clinical 1.5 T scanner and a standard small loop surface coil

With increasing numbers of in vivo experiments in the field of neuroscience, the interest in methods for in vivo imaging of animal brains as small as those of mice has increased. Because highly specialized small bore scanners with high field strengths are not commonly available, clinical magnetic resonance imaging (cMRI) scanners have been used in the past to image rat and more recently also mouse brains in combination with specifically developed RF coils. These studies have demonstrated that imaging of small animal brains is feasible, and that tumor volumes measured by cMRI correlate well with histological tumor volume analysis. This protocol describes the cMRI settings at 1.5 T for imaging of mouse brain with resolutions up to 120 × 120 μm using an inexpensive, commercially available small loop surface coil. This allows easy establishment of a small animal MRI facility without the need for cost intensive dedicated small animal scanners or special custom made coils. In this study, we demonstrate high-resolution imaging of intracranial xenografts in a mouse glioma model and monitor the treatment effect of external field irradiation by cMRI.

Magnetic-resonance-imaging–coupled broadband near-infrared tomography system for small animal brain studies

A novel magnetic-resonance-coupled broadband near-infrared (NIR) tomography system for small animal brain studies is described. Several features of the image formation approach are new in NIR tomography and represent major advances in the path to recovering high-resolution hemoglobin and oxygen saturation images of tissue. The NIR data were broadband and continuous wave and were used along with a second-derivative-based estimation of the path length from water absorption. The path length estimation from water was then used along with the attenuation spectrum to recover absorption and reduced scattering coefficient images at multiple wavelengths and then to recover images of total hemoglobin and oxygen saturation. Going beyond these basics of NIR tomography, software has been developed to allow inclusion of structures derived from MR imaging (MRI) for the external and internal tissue boundaries, thereby improving the accuracy and spatial resolution of the properties in each tissue type. The system has been validated in both tissue-simulating phantoms, with 10% accuracy observed, and in a rat cranium imaging experiment. The latter experiment used variation in inspired oxygen (FiO2) to vary the observed hemoglobin and oxygen saturation images. Quantitative agreement was observed between the changes in deoxyhemoglobin values derived from NIR and the changes predicted with blood-oxygen-level-dependent (BOLD) MRI. This system represents the initial stage in what will likely be a larger role for NIR tomography, coupled to MRI, and illustrates that the technological challenges of using continuous-wave broadband data and inclusion of a priori structural information can be met with careful phantom studies.

An application of a new planar positron imaging system (PPIS) in a small animal: MPTP-induced parkinsonism in mouse.

Recent animal PET research has led to the development of PET scanners for small animals. A planar positron imaging system (PPIS) was newly developed to study physiological function in small animals and plants in recent years. To examine the usefulness of PPIS for functional study in small animals, we examined dopaminergic images of mouse striata in MPTP-induced parkinsonism. Male C57BL/6NCrj mice were treated with MPTP 7 days before the PPIS study. Scans were performed to measure dopamine D1 receptor binding and dopamine transporter availability with [11C]SCH23390 (about 2 MBq) and [11C]beta-CFT (about 2 MBq), respectively. After the PPIS study, dopamine content in the striatum was measured by HPLC. The MPTP treatment significantly reduced dopamine content in the striatum 7 days after treatment. In the MPTP-treated group, [11C]beta-CFT binding in the striatum was significantly decreased compared with the control group, while striatal [11C]SCH23390 binding was not affected. Dopamine content in the striatum was significantly correlated with the striatal binding of [11C]beta-CFT. The present results suggest that PPIS is able to determine brain function in a small animal. Using PPIS, high throughput imaging of small animal brain functions could be achieved.

In vivo 2D J-resolved magnetic resonance spectroscopy of rat brain with a 3-T clinical human scanner

A clinical 3-T scanner equipped with a custom-made transmit/receive birdcage coil was used to collect 2D J-resolved single-voxel spectroscopy in vivo of rat brain. Four adult Wistar rats were scanned twice each, with a 2-week interval. Voxel size was approximately 5 × 10 × 5 mm 3. Total spectroscopic acquisition time was 14 min for collection of two 4:20 min water-suppressed acquisitions and one 4:20 min acquisition acquired in the absence of water suppression. The unsuppressed water data were used in post-processing to reduce residual water side bands, as well as for metabolite signal normalization to account for variations in coil loading and voxel size. Peak areas were estimated for resonances from N-acetyl aspartate (NAA), creatine, choline, taurine, glutamate, and combined glutamate and glutamine. T 2-relaxation times were estimated for NAA and creatine. The average deviation from the mean of repeated measures for glutamate, combined glutamate and glutamine, and taurine ranged from 7.6% to 18.3%, while for NAA, creatine, and choline, the deviation was less than 3%. The estimated T 2 values for NAA (mean ± SD = 330 ± 57 ms) and creatine (174 ± 27 ms) were similar to those reported previously for rat brain and for human gray and white matter. These results indicate that reliable, small animal brain MR spectroscopy can be performed on a human clinical 3-T scanner.

Use of an in-field-of-view shield to improve count rate performance of the single crystal layer high-resolution research tomograph PET scanner for small animal brain scans

The count rate performance of the single LSO crystal layer high-resolution research tomograph (HRRT-S) PET scanner is limited by the processing speed of its electronics. Therefore, the feasibility of using an in-field-of-view (in-FOV) shield to improve the noise equivalent count rates (NECR) for small animal brain studies was investigated. The in-FOV shield consists of a lead tube of 12 cm length, 6 cm inner diameter and 9 mm wall thickness. It is large enough to shield the activity in the body of a rat or mouse. First, the effect of this shield on NECR was studied. Secondly, a number of experiments were performed to assess the effects of the shield on the accuracy of transmission scan data and, next, on reconstructed activity distribution in the brain. For activities below 150 MBq NECR improved only by 5–10%. For higher activities NECR maxima of 1.2E4 cps at 200 MBq and 2.2E4 cps at 370 MBq were found without and with shield, respectively. Listmode data taken without shield, however, were corrupted for activities above 75 MBq due to data overrun problems (time tag losses) of the electronics. When the shield was used data overrun was avoided up to activities of 150 MBq. For the unshielded part of the phantom, transmission scan data were the same with and without shield. The estimated scatter contribution was approximately 8.5% without and 5.5% with shield. Reconstructed emission data showed a difference up to 5% in the unshielded part of the phantom at 5 mm or more from the edge of the shielding. Of this 5% about 3% results from the difference in the uncorrected scatter contribution. In conclusion, an in-FOV shield can be used successfully in an HRRT PET scanner to improve NECR and accuracy of small animal brain studies. The latter is especially important when high activities are required for tracers with low brain uptake or when multiple animals are scanned simultaneously.

Use of the Leksell Gamma Knife for localized small field lens irradiation in rodents.

For most basic radiobiological research applications involving irradiation of small animals, it is difficult to achieve the same high precision dose distribution realized with human radiotherapy. The precision for irradiations performed with standard radiotherapy equipment is +/-2 mm in each dimension, and is adequate for most human treatment applications. For small animals such as rodents, whose organs and tissue structures may be an order of magnitude smaller than those of humans, the corresponding precision required is closer to +/-0.2 mm, if comparisons or extrapolations are to be made to human data. The Leksell Gamma Knife is a high precision radiosurgery irradiator, with precision in each dimension not exceeding 0.5 mm, and overall precision of 0.7 mm. It has recently been utilized to treat ocular melanoma and induce targeted lesions in the brains of small animals. This paper describes the dosimetry and a technique for performing irradiation of a single rat eye and lens with the Gamma Knife while allowing the contralateral eye and lens of the same rat to serve as the "control". The dosimetry was performed with a phantom in vitro utilizing a pinpoint ion chamber and thermoluminescent dosimeters, and verified by Monte Carlo simulations. We found that the contralateral eye received less than 5% of the administered dose for a 15 Gy exposure to the targeted eye. In addition, after 15 Gy irradiation 15 out of 16 animals developed cataracts in the irradiated target eyes, while 0 out of 16 contralateral eyes developed cataracts over a 6-month period of observation. Experiments at 5 and 10 Gy also confirmed the lack of cataractogenesis in the contralateral eye. Our results validate the use of the Gamma Knife for cataract studies in rodents, and confirmed the precision and utility of the instrument as a small animal irradiator for translational radiobiology experiments.

Near-infrared imaging in the small animal brain: optimization of fiber positions.

We investigate fiber placement issues associated with a hybrid magnetic resonance imaging (MRI) near-infrared (NIR) imaging technique for small animal brain studies. Location of the optical fibers on the cranium is examined, with an emphasis on maximizing the recovered resolution and contrast in the region of interest, which in this case is the murine brain. In a series of simulation studies, singular value decomposition of the Jacobian is used in order to determine the measurement sites that provide the most information about the region of interest. The modeling results indicate that data collected using fibers arranged on one side of the head near the brain contain as much information about optical changes within the brain as those positioned equally spaced around the entire periphery of the head. Practical space limitation considerations favor the one-sided fiber array geometry in the case where the NIR acquisition is expected to occur simultaneously with MRI.

High-T/sub c/ superconducting surface coil for 2 tesla magnetic resonance imaging of small animals

The performance of small-volume Magnetic Resonance Imaging (MRI) depends on the system noise determined by noise of a probe and/or of a preamplifier (not by the body noise). Several demonstrations confirmed that, for selected applications, high-T/sub c/ superconductor MRI receiver coils have superior properties to those of comparable copper coils. We report on the outstanding performance of modified twin horseshoe YBCO rf surface probes in a 2 Tesla scanner operating at 77 K. They were used for MR imaging of spinal cord injuries in rats and for imaging of brain of small animals. The probes were designed with a virtual ground plane, thus reducing the coil-to-ground dielectric losses and making its resonant frequency less sensitive to the body proximity. Each coil was fabricated using large area 500 nm thick double-sided YBa/sub 2/Cu/sub 3/O/sub x/ films deposited on sapphire r-cut substrates. We compare the performance of the 2 Tesla high critical temperature superconductor (HTS) probe with that made of a copper coil. Designing and cryo-packaging of HTS MRI probes is discussed.

Visualization of the topographical structure of the anesthetized mouse brain by MR microimaging.

A nuclear magnetic resonance (NMR) spectrometer equipped with a magnet producing a high and extremely uniform magnetic field (7.05 T) was combined with a strong field gradient coil (3.5 mT/cm) and applied to MR microimaging of the mouse brain to visualize its topographical structure. Since the proton-density-weighted condition (long repetition time (TR) and short echo time (TE); TR/TE = 3,000 ms/10.4 ms) was found to be the most suitable for imaging the mouse brain, mid-sagittal and coronal sections in 1-mm- or 0.3-mm-thick slices were imaged according to the multislice spin echo sequence with 2 or 8 acquisitions, a 2 kHz pulse width and a 256 x 256 data matrix. As expected, the resolution of MR microimaging was comparable to that of the histological sections. The white matter especially, could be distinguished from the gray matter in some regions of the brain. Coronal sections of the brain also showed that the hippocampal CA1-CA3 regions were distinguishable from the other regions. The results suggested that the present MR microimaging technique might be a useful tool for the study of topological anatomy and submicroscopic research using brains of small laboratory animals.

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.

The cerebral metabolic rate of glucose (CMRglc) and cerebral blood flow (CBF) in rats were estimated noninvasively with a high-resolution animal positron emission tomography (PET) system that we recently developed. Qualifications of CMRglc and CBF were attempted with radiotracers, 18F-2-fluoro-2-deoxyglucose and 15O-water, respectively. From the data obtained with PET, the CMRglc and CBF values under pretreatment conditions were respectively calculated as 51 +/- 14 mumol/min/100 g and 61 +/- 7 ml/min/100 g under pentobarbital-anesthesia. These values were in close agreement with published data obtained with the autoradiographic technique. Moreover, an increase of 69% (mean) in CBF was detected after i.v. acetazolamide administration. The results indicate that this high-resolution animal PET scanner holds great potential for the assessment of drug-related metabolic and circulatory effects in intact and experimental animals.

Changes in CSF and brain soluble proteins following vigabatrin treatment in rats.

1. Following the discovery of vacuoles in the white matter of the brain of small animals treated with vigabatrin (GVG) it was decided to investigate possible reasons for the occurrence of these vacuoles and to explore the possibility of finding CSF markers which could be applicable for monitoring toxicity in humans. 2. An animal model was developed to study the changes of protein synthesis and to assay soluble brain proteins by isoelectric focusing and two-dimensional electrophoretic techniques. 3. Five groups of rats were treated either with 300 mg kg-1 day-1 GVG, 50 mg kg-1 GVG every other day, 300 mg kg-1 day-1 sodium valproate, 100 mg kg-1 day-1 sodium valproate or sham treated. 4. All animals were given the drug in a liquid full nutrient diet. The dietary intake of the different groups was adjusted to the group which showed the smallest dietary intake, to compensate for possible differences between groups due to nutritional factors. 5. The rats on 300 mg kg-1 day-1 GVG had a 30% reduction of body weight and a 6% reduction of their brain weight, compared with the lower GVG dose group, the two valproate groups and the sham treated group. 6. The synthesis of soluble proteins in the cerebral cortex, hippocampus and cerebellum was decreased in rats given GVG at 300 mg kg-1 day-1 and was increased in rats given valproate at 300 mg kg-1 day-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependent changes in fluorescent neurons in the brain of Notoplana acticola, a polyclad flatworm.

The formaldehyde-glutaraldehyde-sucrose (FGS) method for in situ localization of catecholamines has been applied to the nervous system of the marine polyclad flatworm Notoplana acticola. This histochemical fluorescence technique revealed the presence of a small population of fluorescent cells within the brain. The number and positions of these neurons were constant in animals of the same size, but varied with the size of the worm. The brains of small animals (8 mm in length) were found to contain 20 fluorescent cells, whereas the largest animals studied (30 mm in length) were found to have 28 such cells. Various intermediate cell numbers were found in animals between these two sizes. The origin of the newly added fluorescent cells is uncertain. Peripheral fluorescence was found in association with the tentacular ocelli (eyespots) and interneurons within the ventral submuscular nerve plexus. The fluorescent spectrum from these cells measured in situ had a lambda max of 526 nm. Treatment with HCl shifts this peak to 530 nm. L-dopamine fluoresces with a similar peak emission before HCl treatment (525.5 nm) and shifts to the appropriate longer wavelength (530 nm) following acidification. This strongly suggests that the fluorescent substance in the neurons is dopaminergic in nature.

A reliable, rapid and inexpensive method for producing and implanting chronic cannulae into brains of small animals.

A new method for constructing and implanting cannulae into brains of small animals is described. This system allows drug microinjections to be performed on multiple occasions with little tissue damage, at a very low cost.

Experimental technique and parameters for obtaining in vivo NMR high energy phosphate spectra from small animal brain

Experimental technique and parameters for obtaining in vivo NMR high energy phosphate spectra from small animal brain

In vivo electrochemistry: behavior of micro electrodes in brain tissue

Micro graphite electrodes, chronically implanted in small animal brains, have been used for some time to monitor continuously changes of electroactive species in situ. This study examines some of the fundamental electrochemical characteristics of these electrodes in solution and in vivo. A restricted mass transport model is proposed which appears to explain the unusual chronoamperometric results obtained during brain measurements.

Simple apparatus for blocking small animal brains

A modification of previously described devices for blocking small animal brains is presented. The apparatus is easily constructed and allows for blocking at varous angles of both fixed and unfixed tissue of most small laboratory animals.