Tracer-specific PET and SPECT templates for automatic co-registration of functional rat brain images

Objectives: Template based spatial co-registration of PET and SPECT data is an important first step in its semi- automatic processing, facilitating VOI- and voxel-based analysis. Although this procedure is standard in human, using corresponding MRI images, these systems are often not accessible for preclinical research. Alternatively, manual co-registration of images to a MRI template is often performed. However, this is operator dependent and can introduce bias. Therefore, we constructed several tracer-specific PET and SPECT rat brain templates for automatic co-registration, spatially aligned with a widely used MRI-based template in Paxinos stereotactic space [1]. Methods: PET (18F-FDG, 11C-PK11195, and 11C-MeDAS) and SPECT (99mTc-HMPAO) brain scans were acquired from healthy male Sprague-Dawley and Wistar rats. Symmetrical left-right templates were constructed by averaging the scans. Within-modality registration was performed by minimizing the sum of squared difference and template to MRI registration by normalized mutual information maximization algorithm. For validation purposes, PET scans were acquired from a rat model of multiple sclerosis (MS) where focal demyelination was induced by injection of lysolecithin (or control saline) in right corpus callosum and striatum. Parametric SUV images were created for automatic co-registration. The validity of the templates was assessed by estimation of registration accuracy errors, inter-subject variability, right-to-left asymmetry indices, and voxel-based analysis of the MS model [2]. Results: The obtained mean registration errors were 0.097-1.277mm for PET, and 0.059-0.477mm for SPECT. These values are below spatial resolution of the cameras (1.4mm and 0.8mm, respectively) and in agreement with human literature [3]. Results from voxel-based analyses (Figure 1) correspond with those previously reported using VOI-based analysis [4], and correlate with the regions where lesion was induced. Conclusion: The constructed tracer-specific templates allow accurate registration of functional rat brain data, using automatic normalization algorithms available in standard packages (e.g., SPM, FSL), supporting either VOI- or voxel-based analysis. The templates will be made freely available for the research community

A Standardized Method for the Construction of Tracer Specific PET and SPECT Rat Brain Templates:Validation and Implementation of a Toolbox

High-resolution anatomical image data in preclinical brain PET and SPECT studies is often not available, and inter-modality spatial normalization to an MRI brain template is frequently performed. However, this procedure can be challenging for tracers where substantial anatomical structures present limited tracer uptake. Therefore, we constructed and validated strain- and tracer-specific rat brain templates in Paxinos space to allow intra-modal registration. PET [18F]FDG, [11C]flumazenil, [11C]MeDAS, [11C]PK11195 and [11C]raclopride, and SPECT [99mTc]HMPAO brain scans were acquired from healthy male rats. Tracer-specific templates were constructed by averaging the scans, and by spatial normalization to a widely used MRI-based template. The added value of tracer-specific templates was evaluated by quantification of the residual error between original and realigned voxels after random misalignments of the data set. Additionally, the impact of strain differences, disease uptake patterns (focal and diffuse lesion), and the effect of image and template size on the registration errors were explored. Mean registration errors were 0.70 ± 0.32 mm for [18F]FDG (n = 25), 0.23 ± 0.10mm for [11C]flumazenil (n = 13), 0.88 ± 0.20 mm for [11C]MeDAS (n = 15), 0.64 ± 0.28 mm for [11C]PK11195 (n = 19), 0.34 ± 0.15 mm for [11C]raclopride (n = 6), and 0.40 ± 0.13 mm for [99mTc]HMPAO (n = 15). These values were smallest with tracer-specific templates, when compared to the use of [18F]FDG as reference template (p<0.001). Additionally, registration errors were smallest with strain-specific templates (p<0.05), and when images and templates had the same size (p ≤ 0.001). Moreover, highest registration errors were found for the focal lesion group (p<0.005) and the diffuse lesion group (p = n.s.). In the voxel-based analysis, the reported coordinates of the focal lesion model are consistent with the stereotaxic injection procedure. The use of PET/SPECT strain- and tracer-specific templates allows accurate registration of functional rat brain data, independent of disease specific uptake patterns and with registration error below spatial resolution of the cameras. The templates and the SAMIT package will be freely available for the research community [corrected]

Test-retest repeatability of [18F]MC225-PET in rodents:A tracer for imaging of P-gp function

In longitudinal PET studies, animals are repeatedly anesthetized which may affect the repeatability of PET measurements. The aim of this study was to assess the effect of anesthesia on the P-gp function as well as the reproducibility of [18F]MC225 PET scans. Thus, dynamic PET scans with blood sampling were conducted in 13 Wistar rats. Seven animals were exposed to isoflurane anesthesia 1 week before the PET scan ("Anesthesia-exposed" PET). A second group of six animals was used to evaluate the reproducibility of measurements of P-gp function at the blood-brain barrier (BBB) with [18F]MC225. In this group, two PET scans were made with a 1 week interval ("Test" and "Retest" PET). Pharmacokinetic parameters were calculated using compartmental models and metabolite-corrected plasma as an input function. "Anesthesia-exposed" animals showed a 28% decrease in whole-brain volume of distribution (VT) (p < 0.001) compared to "Test", where the animals were not previously anesthetized. The VT at "Retest" also decreased (19%) compared to "Test" (p < 0.001). The k2 values in whole-brain were significantly increased by 18% in "Anesthesia-exposed" (p = 0.005) and by 15% in "Retest" (p = 0.008) compared to "Test". However, no significant differences were found in the influx rate constant K1, which is considered as the best parameter to measure the P-gp function. Moreover, Western Blot analysis did not find significant differences in the P-gp expression of animals not pre-exposed to anesthesia ("Test") or pre-exposed animals ("Retest"). To conclude, anesthesia may affect the brain distribution of [18F]MC225 but it does not affect the P-gp expression or function

Pharmacokinetic Modeling of (R)-[11C]verapamil to Measure the P-Glycoprotein Function in Nonhuman Primates

(R)-[(11)C]verapamil is a radiotracer widely used for the evaluation of the P-glycoprotein (P-gp) function at the blood-brain barrier (BBB). Several studies have evaluated the pharmacokinetics of (R)-[(11)C]verapamil in rats and humans under different conditions. However, to the best of our knowledge, the pharmacokinetics of (R)-[(11)C]verapamil have not yet been evaluated in nonhuman primates. Our study aims to establish (R)-[(11)C]verapamil as a reference P-gp tracer for comparison of a newly developed P-gp positron emission tomography (PET) tracer in a species close to humans. Therefore, the study assesses the kinetics of (R)-[(11)C]verapamil and evaluates the effect of scan duration and P-gp inhibition on estimated pharmacokinetic parameters. Three nonhuman primates underwent two dynamic 91 min PET scans with arterial blood sampling, one at baseline and another after inhibition of the P-gp function. The (R)-[(11)C]verapamil data were analyzed using 1-tissue compartment model (1-TCM) and 2-tissue compartment model fits using plasma-corrected for polar radio-metabolites or non-corrected for radio-metabolites as an input function and with various scan durations (10, 20, 30, 60, and 91 min). The preferred model was chosen according to the Akaike information criterion and the standard errors (SE %) of the estimated parameters. 1-TCM was selected as the model of choice to analyze the (R)-[(11)C]verapamil data at baseline and after inhibition and for all scan durations tested. The volume of distribution (V(T)) and the efflux constant k(2) estimations were affected by the evaluated scan durations, whereas the influx constant K(1) estimations remained relatively constant. After P-gp inhibition (tariquidar, 8 mg/kg), in a 91 min scan duration, the whole-brain V(T) increased significantly up to 208% (p < 0.001) and K(1) up to 159% (p < 0.001) compared with baseline scans. The k(2) values decreased significantly after P-gp inhibition in all the scan durations except for the 91 min scans. This study suggests the use of K(1), calculated with 1-TCM and using short PET scans (10 to 30 min), as a suitable parameter to measure the P-gp function at the BBB of nonhuman primate

Evaluation of [C-11]CB184 for imaging and quantification of TSPO overexpression in a rat model of herpes encephalitis

PURPOSE: Evaluation of translocator protein (TSPO) overexpression is considered an attractive research tool for monitoring neuroinflammation in several neurological and psychiatric disorders. [11C]PK11195 PET imaging has been widely used for this purpose. However, it has a low sensitivity and a poor signal-to-noise ratio. For these reasons, [11C]CB184 was evaluated as a potentially more sensitive PET tracer. METHODS: A model of herpes simplex encephalitis (HSE) was induced in male Wistar rats. On day 6 or 7 after virus inoculation, [11C]CB184 PET scans were acquired followed by ex vivo evaluation of biodistribution. In addition, [11C]CB184 and [11C]PK11195 PET scans with arterial blood sampling were acquired to generate input for pharmacokinetic modelling. Differences between the saline-treated control group and the virus-treated HSE group were explored using volumes of interest and voxel-based analysis. RESULTS: The biodistribution study showed significantly higher [11C]CB184 uptake in the amygdala, olfactory bulb, medulla, pons and striatum (p < 0.05) in HSE rats than in control rats, and the voxel-based analysis showed higher bilateral uptake in the pons and medulla (p < 0.05, corrected at the cluster level). A high correlation was found between tracer uptake in the biodistribution study and on the PET scans (p < 0.001, r2 = 0.71). Pretreatment with 5 mg/kg of unlabelled PK11195 effectively reduced (p < 0.001) [11C]CB184 uptake in the whole brain. Both, [11C]CB184 and [11C]PK11195, showed similar amounts of metabolites in plasma, and the binding potential (BPND) was not significantly different between the HSE rats and the control rats. In HSE rats BPND for [11C]CB184 was significantly higher (p < 0.05) in the amygdala, hypothalamus, medulla, pons and septum than in control rats, whereas higher uptake of [11C]PK11195 was only detected in the medulla. CONCLUSION: [11C]CB184 showed nonspecific binding to healthy tissue comparable to that observed for [11C]PK11195, but it displayed significantly higher specific binding in those brain regions affected by the HSE. Our results suggest that [11C]CB184 PET is a good alternative for imaging of neuroinflammatory processes

SAMIT

<p>SAMIT (Small Animal Molecular Imaging Toolbox)</p> <p>The aim of this toolbox is to facilitate the construction of new tracer specific templates and the subsequent voxel-based analysis of small animal PET and SPECT brain images. In human studies, the analysis of functional neuroimaging data is frequently performed with the SPM software (Wellcome Department of Cognitive Neurology, University College London, UK). We decided to develop a toolbox producing minimal changes to the original SPM code, compatible with the most recent versions of SPM (SPM8 and SPM12). Moreover, this toolbox is focused in the analysis of small animal PET and SPECT functional brain images.</p> <p>Further documentation is under development.</p

Nuclear medicine imaging in concussive head injuries in sports

Concussions in sports and during recreational activities are a major source of traumatic brain injury in our society. This is mainly relevant in adolescence and young adulthood, where the annual rate of diagnosed concussions is increasing from year to year. Contact sports (e.g., ice hockey, American football, or boxing) are especially exposed to repeated concussions. While most of the athletes recover fully from the trauma, some experience a variety of symptoms including headache, fatigue, dizziness, anxiety, abnormal balance and postural instability, impaired memory, or other cognitive deficits. Moreover, there is growing evidence regarding clinical and neuropathological consequences of repetitive concussions, which are also linked to an increased risk for depression and Alzheimer’s disease or the development of chronic traumatic encephalopathy. With little contribution of conventional structural imaging (computed tomography (CT) or magnetic resonance imaging (MRI)) to the evaluation of concussion, nuclear imaging techniques (i.e., positron emission tomography (PET) and single-photon emission computed tomography (SPECT)) are in a favorable position to provide reliable tools for a better understanding of the pathophysiology and the clinical evaluation of athletes suffering a concussion

Comparison of calculated attenuation corrected FDG brain scans with CT

Without attenuation correction (AC) PET images exhibit strong artifacts and do not allow quantitative imaging. Therefore, current PET-CT systems perform a low-dose CT-AC scan. Although of low dose, this still increases the total radiation dose to the patient which is a special concern when scanning children. In addition, patient motion between or during the CT-AC scan and the PET scan can introduce motion AC artifacts. For brain scans a calculated AC is possible which alleviates both problems. Aim: To compare the calculated AC corrected brain scans, using the Siemens Neuro-AC tool, with the gold-standard i.e. CT-AC. Methods: For Alzheimer suspected patients, 10 FDG brain scans were included. (Age: 55-77, M: 6, F: 4) Subjects where positioned using a CT scout scan after which a CT-AC was performed (80 kV, 40 mAs). Scans of 5 min duration where made, 30 min after the injection of 200 MBq FDG. During the uptake period, patients remained in a darkened and quiet room. The scans were reconstructed using the CT-AC scan and the Neuro-AC tool. The reconstructions resulted in different dimensions of the data. Therefore, the scans where first co-registered and then both absolute and relative error images where calculated (Vinci). To prevent extra-cranial voxels to influence the analysis, the different images where masked using the CT-AC corrected FDG scan. The current analysis is based on a visual interpretation of the data, displayed in transaxial, coronal and sagittal sections. Results: Relative errors were largest in areas with low uptake which is a direct result of the division by low values. This is particularly evident for the skull. The smallest relative errors were observed for the cortical regions particularly the more cranial slices. More caudal structures, particularly at the level of the nasal cavities were more susceptible for errors. Conclusion: Calculated AC using the Neuro-AC tool may give large relative errors, although these are usually in low uptake areas. For a visual interpretation of FDG brain scans, particularly of cortical areas, its use seems appropriate. The same holds for the quantitative assessment (i.e. using SUV) of follow-up scans of patients. Generally, the use of SUV ratios is advised since this will reduce potential systematic errors. However, more research is required if this approach is to be used in a research settin