Quantification and reproducibility assessment of the regional brain T2 relaxation in naïve rats at 7T

Quantitative T2 mapping-based longitudinal assessment of brain injury and therapeutic rescue in the rat following acute organophosphate intoxication

Acute intoxication with organophosphate (OP) cholinesterase inhibitors poses a significant public health risk. While currently approved medical countermeasures can improve survival rates, they often fail to prevent chronic neurological damage. Therefore, there is need to develop effective therapies and quantitative metrics for assessing OP-induced brain injury and its rescue by these therapies. In this study we used a rat model of acute intoxication with the OP, diisopropylfluorophosphate (DFP), to test the hypothesis that T2 measures obtained from brain magnetic resonance imaging (MRI) scans provide quantitative metrics of brain injury and therapeutic efficacy. Adult male Sprague Dawley rats were imaged on a 7T MRI scanner at 3, 7 and 28 days post-exposure to DFP or vehicle (VEH) with or without treatment with the standard of care antiseizure drug, midazolam (MDZ); a novel antiseizure medication, allopregnanolone (ALLO); or combination therapy with MDZ and ALLO (DUO). Our results show that mean T2 values in DFP-exposed animals were: (1) higher than VEH in all volumes of interest (VOIs) at day 3; (2) decreased with time; and (3) decreased in the thalamus at day 28. Treatment with ALLO or DUO, but not MDZ alone, significantly decreased mean T2 values relative to untreated DFP animals in the piriform cortex at day 3. On day 28, the DUO group showed the most favorable T2 characteristics. This study supports the utility of T2 mapping for longitudinally monitoring brain injury and highlights the therapeutic potential of ALLO as an adjunct therapy to mitigate chronic morbidity associated with acute OP intoxication.

In vivo mapping of rat brain partial white matter content using hexachlorophene-induced neurotoxicity model

Segmentation of the white matter in MRI scans of the rat brain presents a significant challenge due to the low contrast. Existing anatomical reference maps of rat brain are usually constructed from fixed tissue, which may suffer from geometrical distortions due to fixation/processing. To significantly increase the in vivo contrast between white and gray matter in the rat brain we used a known neurotoxicant hexachlorophene, which produces selective white matter damage. This model was used to map white matter in the rat brain and estimate the partial white matter content in any given imaging voxel. Hexachlorophene was administered to rats at a dose of 30 mg/kg orally once a day over five consecutive days. A significant white matter changes were observed using quantitative T2 maps, from which the partial white matter content throughout the whole rat brain was derived. Several assumptions were made: hexachlorophene affects T2 relaxation only in the white matter; T2 of gray matter is relatively uniform in the brain; apparent T2 value in a given voxel is a combination of T2s from white and gray matter portions of that voxel, hexachlorophene affects nearly 100 % of white matter. The partial white matter map of the rat brain was constructed with the resolution of 0.2 × 0.2 × 1.0 mm per voxel. This map could be adjusted for segmentation of the brain tissue with preset threshold of the white matter content, or to establish the tissue composition in any region of interest among other applications.

Electrical impedance myography detects dystrophin-related muscle changes in mdx mice

BACKGROUND

The lack of functional dystrophin protein in Duchenne muscular dystrophy (DMD) causes chronic skeletal muscle inflammation and degeneration. Therefore, the restoration of functional dystrophin levels is a fundamental approach for DMD therapy. Electrical impedance myography (EIM) is an emerging tool that provides noninvasive monitoring of muscle conditions and has been suggested as a treatment response biomarker in diverse indications. Although magnetic resonance imaging (MRI) of skeletal muscles has become a standard measurement in clinical trials for DMD, EIM offers distinct advantages, such as portability, user-friendliness, and reduced cost, allowing for remote monitoring of disease progression or response to therapy. To investigate the potential of EIM as a biomarker for DMD, we compared longitudinal EIM data with MRI/histopathological data from an X-linked muscular dystrophy (mdx) mouse model of DMD. In addition, we investigated whether EIM could detect dystrophin-related changes in muscles using antisense-mediated exon skipping in mdx mice.

METHODS

The MRI data for muscle T2, the magnetic resonance spectroscopy (MRS) data for fat fraction, and three EIM parameters with histopathology were longitudinally obtained from the hindlimb muscles of wild-type (WT) and mdx mice. In the EIM study, a cell-penetrating peptide (Pip9b2) conjugated antisense phosphorodiamidate morpholino oligomer (PPMO), designed to induce exon-skipping and restore functional dystrophin production, was administered intravenously to mdx mice.

RESULTS

MRI imaging in mdx mice showed higher T2 intensity at 6 weeks of age in hindlimb muscles compared to WT mice, which decreased at ≥ 9 weeks of age. In contrast, EIM reactance began to decline at 12 weeks of age, with peak reduction at 18 weeks of age in mdx mice. This decline was associated with myofiber atrophy and connective tissue infiltration in the skeletal muscles. Repeated dosing of PPMO (10 mg/kg, 4 times every 2 weeks) in mdx mice led to an increase in muscular dystrophin protein and reversed the decrease in EIM reactance.

CONCLUSIONS

These findings suggest that muscle T2 MRI is sensitive to the early inflammatory response associated with dystrophin deficiency, whereas EIM provides a valuable biomarker for the noninvasive monitoring of subsequent changes in skeletal muscle composition. Furthermore, EIM reactance has the potential to monitor dystrophin-deficient muscle abnormalities and their recovery in response to antisense-mediated exon skipping.

Performance of the prospective T2 MRI biomarker of neurotoxicity in a trimethyltin model in rats at 7 T

The assessment of the sensitivity and specificity of any potential biomarker against the gold standard is an important step in the process of its qualification by regulatory authorities. Such qualification is an important step towards incorporating the biomarker into the panel of tools available for drug development. In the current study we analyzed the sensitivity and specificity of T2 MRI relaxometry to detect trimethyltin-induced neurotoxicity in rats. Seventy-five male Sprague-Dawley rats were injected with a single intraperitoneal dose of either TMT (8, 10, 11, or 12 mg/kg) or saline (2 ml/kg) and imaged with 7 T MRI before and 3, 7, 14, and 21 days after injection using a quantitative T2 mapping. Neurohistopathology (the gold standard in the case of neurotoxicity) was performed at the end of the observation and used as an outcome qualifier in receiver-operator characteristic (ROC) curve analysis of T2 changes as a predictor of neurotoxicity. TMT treatment led to a significant increase in T2 values in many brain areas. The biggest changes in T2 values were seen around the lateral ventricles, which was interpreted as ventricular dilation. The area under the ROC curve for the volume of the lateral ventricles was 0.878 with the optimal sensitivity/specificity of 0.805/0.933, respectively. T2 MRI is a promising method for generating a non-invasive biomarkers of neurotoxicity, which shows the dose-response behavior with substantial sensitivity and specificity. While its performance was strong in the TMT model, further characterization of the sensitivity and specificity of T2 MRI with other neurotoxicants is warranted.

Optimization of Detection of Gadodiamide Brain Retention in Rats Using Quantitative T2 Mapping and Intraperitoneal Administration

BACKGROUND

Currently, the gadolinium retention in the brain after the use of contrast agents is studied by T₁ -weighted magnetic resonance imaging (MRI) (T₁ w) and T₁ mapping. The former does not provide easily quantifiable data and the latter requires prolonged scanning and is sensitive to motion. T₂ mapping may provide an alternative approach. Animal studies of gadolinium retention are complicated by repeated intravenous (IV) dosing, whereas intraperitoneal (IP) injections might be sufficient.

HYPOTHESIS

T₂ mapping will detect the changes in the rat brain due to gadolinium retention, and IP administration is equivalent to IV for long-term studies.

STUDY TYPE

Prospective longitudinal.

ANIMAL MODEL

A total of 31 Sprague-Dawley rats administered gadodiamide IV (N = 8) or IP (N = 8), or saline IV (N = 6) or IP (N = 9) 4 days per week for 5 weeks.

FIELD STRENGTH/SEQUENCES

A 7 T, T₁ w, and T₂ mapping.

ASSESSMENT

T₂ relaxation and image intensities in the deep cerebellar nuclei were measured pre-treatment and weekly for 5 weeks. Then brains were assessed for neuropathology (N = 4) or gadolinium content using inductively coupled plasma mass spectrometry (ICP-MS, N = 12).

STATISTICAL TESTS

Repeated measures analysis of variance with post hoc Student-Newman-Keuls tests and Hedges' effect size.

RESULTS

Gadolinium was detected by both approaches; however, T₂ mapping was more sensitive (effect size 2.32 for T₂ vs. 0.95 for T₁ w), and earlier detection (week 3 for T₂ vs. week 4 for T₁ w). ICP-MS confirmed the presence of gadolinium (3.076 ± 0.909 nmol/g in the IV group and 3.948 ± 0.806 nmol/g in the IP group). There was no significant difference between IP and IV groups (ICP-MS, P = 0.109; MRI, P = 0.696). No histopathological abnormalities were detected in any studied animal.

CONCLUSION

T₂ relaxometry detects gadolinium retention in the rat brain after multiple doses of gadodiamide irrespective of the route of administration.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 1.

The effects of long-term methylphenidate administration and withdrawal on progressive ratio responding and T2 MRI in the male rhesus monkey

Methylphenidate is a frequently prescribed drug treatment for Attention-Deficit/Hyperactivity Disorder. However, methylphenidate has a mode of action similar to amphetamine and cocaine, both powerful drugs of abuse. There is lingering concern over the long-term safety of methylphenidate, especially in a pediatric population, where the drug may be used for years. We performed a long-term evaluation of the effects of chronic methylphenidate use on a behavioral measure of motivation in male rhesus monkeys. Animals were orally administered a sweetened methylphenidate solution (2.5 or 12.5 mg/kg, twice a day, Mon-Fri) or vehicle during adolescence and into adulthood. These animals were assessed on a test of motivation (progressive ratio responding), during methylphenidate treatment, and after cessation of use. Moreover, animals were evaluated with quantitative T₂ MRI about one year after cessation of use. During the administration phase of the study animals treated with a clinically relevant dose of methylphenidate generally had a higher rate of responding than the control group, while the high dose group generally had a lower rate of responding. These differences were not statistically significant. In the month after cessation of methylphenidate, responding in both experimental groups dropped compared to their previous level of performance (p = 0.19 2.5 mg/kg, p = 0.06 12.5 mg/kg), and responding in the control animals was unchanged (p = 0.81). While cessation of methylphenidate was associated with an acute reduction in responding, group differences were not observed in the following months. These data suggest that methylphenidate did not have a significant impact on responding, but withdrawal from methylphenidate did cause a temporary change in motivation. No changes in T₂ MRI values were detected when measured about one year after cessation of treatment. These data suggest that long-term methylphenidate use does not have a negative effect on a measure of motivation or brain function / microstructure as measured by quantitative T₂ MRI. However, cessation of use might be associated with temporary cognitive changes, specifically alteration in motivation. Importantly, this study modeled use in healthy individuals, and results may differ if the same work was repeated in a model of ADHD.

W. Emerging technologies and their impact on regulatory science

There is an evolution and increasing need for the utilization of emerging cellular, molecular and in silico technologies and novel approaches for safety assessment of food, drugs, and personal care products. Convergence of these emerging technologies is also enabling rapid advances and approaches that may impact regulatory decisions and approvals. Although the development of emerging technologies may allow rapid advances in regulatory decision making, there is concern that these new technologies have not been thoroughly evaluated to determine if they are ready for regulatory application, singularly or in combinations. The magnitude of these combined technical advances may outpace the ability to assess fit for purpose and to allow routine application of these new methods for regulatory purposes. There is a need to develop strategies to evaluate the new technologies to determine which ones are ready for regulatory use. The opportunity to apply these potentially faster, more accurate, and cost-effective approaches remains an important goal to facilitate their incorporation into regulatory use. However, without a clear strategy to evaluate emerging technologies rapidly and appropriately, the value of these efforts may go unrecognized or may take longer. It is important for the regulatory science field to keep up with the research in these technically advanced areas and to understand the science behind these new approaches. The regulatory field must understand the critical quality attributes of these novel approaches and learn from each other's experience so that workforces can be trained to prepare for emerging global regulatory challenges. Moreover, it is essential that the regulatory community must work with the technology developers to harness collective capabilities towards developing a strategy for evaluation of these new and novel assessment tools.

A Multicenter Preclinical MRI Study: Definition of Rat Brain Relaxometry Reference Maps

Similarly to human population imaging, there are several well-founded motivations for animal population imaging, the most notable being the improvement of the validity of statistical results by pooling a sufficient number of animal data provided by different imaging centers. In this paper, we demonstrate the feasibility of such a multicenter animal study, sharing raw data from forty rats and processing pipelines between four imaging centers. As specific use case, we focused on T1 and T2 mapping of the healthy rat brain at 7T. We quantitatively report about the variability observed across two MR data providers and evaluate the influence of image processing steps on the final maps, using three fitting algorithms from three centers. Finally, to derive relaxation times from different brain areas, two multi-atlas segmentation pipelines from different centers were performed on two different platforms. Differences between the two data providers were 2.21% for T1 and 9.52% for T2. Differences between processing pipelines were 1.04% for T1 and 3.33% for T2. These maps, obtained in healthy conditions, may be used in the future as reference when exploring alterations in animal models of pathology.

Evaluation of Myelin Radiotracers in the Lysolecithin Rat Model of Focal Demyelination: Beware of Pitfalls!

The observation that amyloid radiotracers developed for Alzheimer's disease bind to cerebral white matter paved the road to nuclear imaging of myelin in multiple sclerosis. The lysolecithin (lysophosphatidylcholine (LPC)) rat model of demyelination proved useful in evaluating and comparing candidate radiotracers to target myelin. Focal demyelination following stereotaxic LPC injection is larger than lesions observed in experimental autoimmune encephalitis models and is followed by spontaneous progressive remyelination. Moreover, the contralateral hemisphere may serve as an internal control in a given animal. However, demyelination can be accompanied by concurrent focal necrosis and/or adjacent ventricle dilation. The influence of these side effects on imaging findings has never been carefully assessed. The present study describes an optimization of the LPC model and highlights the use of MRI for controlling the variability and pitfalls of the model. The prototypical amyloid radiotracer [¹¹C]PIB was used to show that in vivo PET does not provide sufficient sensitivity to reliably track myelin changes and may be sensitive to LPC side effects instead of demyelination as such. Ex vivo autoradiography with a fluorine radiotracer should be preferred, to adequately evaluate and compare radiotracers for the assessment of myelin content.

Comparison of quantitative T2 and ADC mapping in the assessment of 3-nitropropionic acid-induced neurotoxicity in rats

To assess the relative performance of MRI T₂ relaxation and ADC mapping as potential biomarkers of neurotoxicity, a model of 3-nitropropionic acid (NP)-induced neurodegeneration in rats was employed. Male Sprague-Dawley rats received NP (N = 20, 16-20 mg/kg, ip or sc) or saline (N = 6, 2 ml/kg, ip) daily for 3 days. MRI was performed using a 7 T system employing quantitative T₂ and ADC mapping based on spin echo pulse sequence. All maps were skull stripped and co-registered and the changes were quantified using baseline subtraction and anatomical segmentation. Following the in vivo portion of the study, rat brains were histologically examined. Four NP-treated rats were considered responders based on their MRI and histology data. T₂ values always increased in the presence of toxicity, while ADC changes were bidirectional, decreasing in some lesion areas and increasing in others. In contrast to T₂ in some cases, ADC did not change. The effect sizes of T₂ and ADC signals suggestive of neurotoxicity were 2.64 and 1.66, respectively, and the variability of averaged T₂ values among anatomical regions was consistently lower than that for ADC. The histopathology data confirmed the presence of neurotoxicity, however, a more detailed assessment of the correlation of MRI with histology is needed. T₂ mapping provides more sensitive and specific information than ADC about changes in the rat brain thought to be associated with neurotoxicity due to a higher signal-to-noise ratio, better resolution, and unidirectional changes, and presents a better opportunity for biomarker development.

3D scaffolds for brain tissue regeneration: architectural challenges

Critical analysis of experimental studies on 3D scaffolds for brain tissue engineering.

A two-pool model to describe the IVIM cerebral perfusion

IntraVoxel Incoherent Motion (IVIM) is a magnetic resonance imaging (MRI) technique capable of measuring perfusion-related parameters. In this manuscript, we show that the mono-exponential model commonly used to process IVIM data might be challenged, especially at short diffusion times. Eleven rat datasets were acquired at 7T using a diffusion-weighted pulsed gradient spin echo sequence with b-values ranging from 7 to 2500 s/mm² at three diffusion times. The IVIM signals, obtained by removing the diffusion component from the raw MR signal, were fitted to the standard mono-exponential model, a bi-exponential model and the Kennan model. The Akaike information criterion used to find the best model to fit the data demonstrates that, at short diffusion times, the bi-exponential IVIM model is most appropriate. The results obtained by comparing the experimental data to a dictionary of numerical simulations of the IVIM signal in microvascular networks support the hypothesis that such a bi-exponential behavior can be explained by considering the contribution of two vascular pools: capillaries and somewhat larger vessels.

In vivo measurement of T1 and T2 relaxation times in awake pigeon and rat brains at 7T

PURPOSE

Establishment of regional longitudinal (T₁ ) and transverse (T₂ ) relaxation times in awake pigeons and rats at 7T field strength. Regional differences in relaxation times between species and between two different pigeon breeds (homing pigeons and Figurita pigeons) were investigated.

METHODS

T₁ and T₂ relaxation times were determined for nine functionally equivalent brain regions in awake pigeons and rats using a multiple spin-echo saturation recovery method with variable repetition time and a multi-slice/multi-echo sequence, respectively. Optimized head fixation and habituation protocols were applied to accustom animals to the scanning conditions and to minimize movement.

RESULTS

The habituation protocol successfully limited movement of the awake animals to a negligible minimum, allowing reliable measurement of T₁ and T₂ values within all regions of interest. Significant differences in relaxation times were found between rats and pigeons but not between different pigeon breeds.

CONCLUSION

The obtained T₁ and T₂ values for awake pigeons and rats and the optimized habituation protocol will augment future MRI studies with awake animals. The differences in relaxation times observed between species underline the importance of the acquisition of T₁ /T₂ values as reference points for specific experiments. Magn Reson Med 79:1090-1100, 2018. © 2017 International Society for Magnetic Resonance in Medicine.