3D TrueFISP imaging of mouse brain at 4.7T and 9.4T

Magnetic resonance spectroscopy in the hippocampus of adult APP/PS1 mice following chronic vitamin D deficiency

Vitamin D (VitD) deficiency can exacerbate AD progression and may cause changes in brain metabolite levels that can be detected by magnetic resonance spectroscopy (MRS). The purpose of this study was to determine whether chronic VitD deficiency in an AD mouse model caused persistent metabolite levels changes in the hippocampus associated with memory performance. Six-month-old APPSwe/PS1ΔE9 (APP/PS1) mice (N = 14 mice/group) were fed either a VitD deficient (VitD-) diet or a control diet. Metabolite level changes in the hippocampus were evaluated by 1H MRS using a 9.4 T MRI. Ventricle volume was assessed by imaging and spatial memory was evaluated using the Barnes maze. All measurements were made at 6, 9, 12, and 15 months of age. At 15 months of age, amyloid plaque load and astrocyte number were evaluated histologically (N = 4 mice/group). Levels of N-acetyl aspartate and creatine were lower in VitD- mice compared to control diet mice at 12 months of age. VitD deficiency did not change ventricle volume. Lactate levels increased over time in VitD- mice and increases from 12 to 15 months were negatively correlated with changes in primary latency to the target hole in the Barns Maze. VitD- mice showed improved spatial memory performance compared to control diet mice. VitD- mice also had more astrocytes in the cortex and hippocampus at 15 months than control diet mice. This study suggests that severe VitD deficiency in APP/PS1 mice may lead to compensatory changes in metabolite and astrocyte levels that contribute to improved performance on spatial memory tasks.

A Genetically Encoded Magnetic Resonance Imaging Reporter Enables Sensitive Detection and Tracking of Spontaneous Metastases in Deep Tissues

Metastasis is the leading cause of cancer-related death. However, it remains a poorly understood aspect of cancer biology, and most preclinical cancer studies do not examine metastasis, focusing solely on the primary tumor. One major factor contributing to this paradox is a gap in available tools for accurate spatiotemporal measurements of metastatic spread in vivo. Here, our objective was to develop an imaging reporter system that offers sensitive three-dimensional (3D) detection of cancer cells at high resolutions in live mice. An organic anion-transporting polypeptide 1b3 (oatp1b3) was used as an MRI reporter gene, and its sensitivity was systematically optimized for in vivo tracking of viable cancer cells in a spontaneous metastasis model. Metastases with oatp1b3-MRI could be observed at the single lymph node level and tracked over time as cancer cells spread to multiple lymph nodes and different organ systems in individual animals. While initial single lesions were successfully imaged in parallel via bioluminescence, later metastases were largely obscured by light scatter from the initial node. Importantly, MRI could detect micrometastases in lung tissue comprised on the order of 1,000 cancer cells. In summary, oatp1b3-MRI enables longitudinal tracking of cancer cells with combined high resolution and high sensitivity that provides 3D spatial information and the surrounding anatomical context.

SIGNIFICANCE

An MRI reporter gene system optimized for tracking metastasis in deep tissues at high resolutions and able to detect spontaneous micrometastases in lungs of mice provides a useful tool for metastasis research.

Deep learning model for automatic segmentation of lungs and pulmonary metastasis in small animal MR images

Lungs are the most frequent site of metastases growth. The amount and size of pulmonary metastases acquired from MRI imaging data are the important criteria to assess the efficacy of new drugs in preclinical models. While efficient solutions both for MR imaging and the downstream automatic segmentation have been proposed for human patients, both MRI lung imaging and segmentation in preclinical animal models remains challenging due to the physiological motion (respiratory and cardiac movements), to the low amount of protons in this organ and to the particular challenge of precise segmentation of metastases. As a consequence post-mortem analysis is currently required to obtain information on metastatic volume. In this work, we have developed a complete methodological pipeline for automated analysis of lungs and metastases in mice, consisting of an MR sequence for image acquisition and a deep learning method for automatic segmentation of both lungs and metastases. On one hand, we optimized an MR sequence for mouse lung imaging with high contrast for high detection sensitivity. On the other hand we developed DeepMeta, a multiclass U-Net 3+ deep learning model to automatically segment the images. To assess if the proposed deep learning pipeline is able to provide an accurate segmentation of both lungs and pulmonary metastases, we have longitudinally imaged mice with fast- and slow-growing metastasis. Fifty-five balb/c mice were injected with two different derivatives of renal carcinoma cells. Mice were imaged with a SG-bSSFP (self-gated balanced steady state free precession) sequence at different time points after the injection of cancer cells. Both lung and metastases segmentations were manually performed by experts. DeepMeta was trained to perform lung and metastases segmentation based on the resulting ground truth annotations. Volumes of lungs and of pulmonary metastases as well as the number of metastases per mouse were measured on a separate test dataset of MR images. Thanks to the SG method, the 3D bSSFP images of lungs were artifact-free, enabling the downstream detection and serial follow-up of metastases. Moreover, both lungs and metastases segmentation was accurately performed by DeepMeta as soon as they reached the volume of ∼ 0.02 m m 3 . Thus we were able to distinguish two groups of mice in terms of number and volume of pulmonary metastases as well as in terms of the slow versus fast patterns of growth of metastases. We have shown that our methodology combining SG-bSSFP with deep learning, enables processing of the whole animal lungs and is thus a viable alternative to histology alone.

Scribble Controls Social Motivation Behavior through the Regulation of the ERK/Mnk1 Pathway

Social behavior is a basic domain affected by several neurodevelopmental disorders, including ASD and a heterogeneous set of neuropsychiatric disorders. The SCRIB gene that codes for the polarity protein SCRIBBLE has been identified as a risk gene for spina bifida, the most common type of neural tube defect, found at high frequencies in autistic patients, as well as other congenital anomalies. The deletions and mutations of the 8q24.3 region encompassing SCRIB are also associated with multisyndromic and rare disorders. Nonetheless, the potential link between SCRIB and relevant social phenotypes has not been fully investigated. Hence, we show that Scribcrc/+ mice, carrying a mutated version of Scrib, displayed reduced social motivation behavior and social habituation, while other behavioral domains were unaltered. Social deficits were associated with the upregulation of ERK phosphorylation, together with increased c-Fos activity. Importantly, the social alterations were rescued by both direct and indirect pERK inhibition. These results support a link between polarity genes, social behaviors and hippocampal functionality and suggest a role for SCRIB in the etiopathology of neurodevelopmental disorders. Furthermore, our data demonstrate the crucial role of the MAPK/ERK signaling pathway in underlying social motivation behavior, thus supporting its relevance as a therapeutic target.

Current Applications and Future Development of Magnetic Resonance Fingerprinting in Diagnosis, Characterization, and Response Monitoring in Cancer

Magnetic resonance imaging (MRI) has enabled non-invasive cancer diagnosis, monitoring, and management in common clinical settings. However, inadequate quantitative analyses in MRI continue to limit its full potential and these often have an impact on clinicians' judgments. Magnetic resonance fingerprinting (MRF) has recently been introduced to acquire multiple quantitative parameters simultaneously in a reasonable timeframe. Initial retrospective studies have demonstrated the feasibility of using MRF for various cancer characterizations. Further trials with larger cohorts are still needed to explore the repeatability and reproducibility of the data acquired by MRF. At the moment, technical difficulties such as undesirable processing time or lack of motion robustness are limiting further implementations of MRF in clinical oncology. This review summarises the latest findings and technology developments for the use of MRF in cancer management and suggests possible future implications of MRF in characterizing tumour heterogeneity and response assessment.

Three-dimensional ultrashort echo time (3D UTE) magnetic resonance imaging (MRI) of the normal and degenerative disco-vertebral complex at 4.7 T: a feasibility study with longitudinal evaluation

OBJECTIVES

To assess feasibility of a three-dimensional ultrashort echo time (3D-UTE)-sequence to evaluate normal and pathological disco-vertebral complex (DVC), with assessment of its different portions in a rat model of degenerative disk disease (DDD) with histological correlation. To assess whether this sequence, in comparison with long echo time T2-weighted sequence, is able to monitor DDD with differentiation of early from chronic DVC changes in pathological mechanical conditions.

METHODS

Five rats were induced with DDD model by percutaneous disk trituration of the tail with an 18-G needle under US-guidance and imaged at 4.7 T. MRI protocol included fat-saturated-T2 (RARE) and 3D-UTE-sequences performed at baseline (day 0. n = 5 animals /10 DVC) and each week (W) from W1 to W10 postoperatively. Visual analysis and signal intensity measurements of SNR and CNR of all DVC portions were performed on RARE and UTE images. Following killing (baseline, n = 1/2 DVC; W2, n = 2/4 DVC; W10, n = 2/4 DVC), histological analysis was performed and compared with MRI.

RESULTS

In normal DVC, unlike conventional RARE-sequences, 3D-UTE allowed complete identification of DVC zonal anatomy including on visual analysis and CNR measurements. In pathological conditions, SNR and CNR measurements of the annulus fibrosus and nucleus pulposus on 3D-UTE distinguished early discitis at W1 from chronic discopathy (P < 0.001 for SNR and P < 0.001 for CNR). Neither the normal complete anatomy of the DVC nor its pathological patterns could be assessed on conventional sequences.

CONCLUSIONS

Unlike conventional sequences, 3D-UTE enables visualization of the complete normal DVC anatomy and enables monitoring of DDD differentiating between early DVC changes from chronic ones.

LEVEL OF EVIDENCE I

Diagnostic: individual cross-sectional studies with the consistently applied reference standard and blinding.

Approximated analytical characterization of the steady-state chemical exchange saturation transfer (CEST) signals

PURPOSE

CEST MRI can indirectly detect low-concentrated molecules via their proton exchange with the bulk water and is widely measured by a sensitivity index, the asymmetry of magnetization transfer ratio (MTRasym ). Because CEST applications are often limited by their low sensitivity or specificity, it is important to characterize MTRasym analytically to optimize its sensitivity or specifity.

METHODS

Approximated analytical solutions of the MTRasym spectrum were derived based on a 2-pool chemical exchange model for slow-to-intermediate exchanges. The optimal saturation pulse power for maximizing the MTRasym or tuning MTRasym to a specific exchange rate and the peak position and linewidth of a MTRasym spectrum were also derived. These approximated analytical solutions were compared with the solutions from the Bloch-McConnell equations using computer simulations.

RESULTS

The approximated analytical solutions of the MTRasym spectra, the optimizing parameters, and the peak and linewidth of MTRasym matched well with the solutions of Bloch-McConnell equations in the slow or slow-to-intermediate exchange regimes.

CONCLUSION

These approximate analytical solutions can provide insights to the understanding of CEST signal property and help the optimization of saturation parameters and the interpretation of CEST data.

Three-dimensional ultrashort echo time (3D UTE) MRI of Achilles tendon at 4.7T MRI with comparison to conventional sequences in an experimental murine model of spondyloarthropathy

BACKGROUND

Due to the very short T₂ of its components, the normal anatomy of Achilles enthesis is impossible to define with "conventional" long echo time (TE) T₂ sequences. However, this is a common site affected by rheumatologic disease. Early abnormalities related to inflammatory processes are impossible to detect in this location.

PURPOSE

To assess the feasibility of a 3D-UTE (ultrashort echo time) sequence to evaluate normal and pathological Achilles entheses, determining both anterior fibrocartilaginous and posterior collagenic portions at 4.7T, in a rat model of spondyloarthropathy (SpA) with histological correlation. To assess whether this sequence detects SpA enthesopathy prior to long TE T₂ sequences, enabling disease monitoring.

STUDY TYPE

Prospective case-control study.

ANIMAL MODEL

Twelve immunocompetent Wistar male rats imaged before (controls); the model was induced in eight rats (16 tendons) imaged at day 6, day 13, and day 21 with regular sacrifice for ex vivo imaging and histological correlation.

FIELD STRENGTH

4.7T Bruker Biospec Systems. 3D balanced steady-state free precession (bSSFP) and 3D-UTE sequences, performed at baseline (day 0, n = 12 animals / 24 tendons), day 6 (n = 8/16), 13 (n = 4/8), and day 21 (n = 2/4).

ASSESSMENT

Visual analysis and signal intensity measurements (signal to noise ratio, SNR) of both bSSFP and UTE images were performed by two independent musculoskeletal radiologists at different locations of the Achilles enthesis and preinsertional area.

STATISTICAL TESTS

Normal and pathological rat values were compared by Wilcoxon signed-rank tests, as well as interobserver differences. MRI findings were compared against histological data.

RESULTS

The 3D-UTE sequence identified the anterior fibrocartilage and posterior collagenic areas of Achilles entheses in all cases. Visual analysis and signal intensity measurements distinguished SpA-affected entheses from healthy ones at days 6 and 13 (P = 0.002 and P = 0.006, respectively). Neither the normal anatomy of the enthesis nor its pathological pattern could be identified on T₂ bSSFP sequences.

DATA CONCLUSION

Unlike bSSFP T₂ sequences, 3D-UTE sequences enable visualization of normal enthesis anatomy and early detection of abnormalities in pathological conditions.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:127-135.

Compressed-Sensing MP2RAGE sequence: Application to the detection of brain metastases in mice at 7T

PURPOSE

To develop a Compressed Sensing (CS)-MP2RAGE sequence to drastically shorten acquisition duration and then detect and measure the T₁ of brain metastases in mice at 7 T.

METHODS

The encoding trajectory of the standard Cartesian MP2RAGE sequence has been modified (1) to obtain a variable density Poisson disk under-sampling distribution along the k_y -k_z plane, and (2) to sample the central part of the k-space exactly at TI₁ and TI₂ inversion times. In a prospective study, the accuracy of the T₁ measurements was evaluated on phantoms containing increasing concentrations of gadolinium. The CS acceleration factors were increased to evaluate their influence on the contrast and T₁ measurements of brain metastases in vivo. Finally, the 3D T₁ maps were acquired with at 4-fold increased spatial resolution. The volumes and T₁ values of the metastases were measured while using CS to reduce scan time.

RESULTS

The implementation of the CS-encoding trajectory did not affect the T₁ measurements in vitro. Accelerating the acquisition by a factor of 2 did not alter the contrast or the T₁ values of the brain metastases. 3D T₁ maps could be obtained in < 1 min using a CS factor of 6. Increasing the spatial resolution enabled more accurately measurement of the metastasis volumes while maintaining an acquisition duration below 5 min.

CONCLUSION

The CS-MP2RAGE sequence could be of great interest in oncology to either rapidly obtain mouse brain 3D T₁ maps or to increase the spatial resolution with no penalty on the scan duration.

3D anatomical and perfusion MRI for longitudinal evaluation of biomaterials for bone regeneration of femoral bone defect in rats

Magnetic Resonance Imaging (MRI) appears as a good surrogate to Computed Tomography (CT) scan as it does not involve radiation. In this context, a 3D anatomical and perfusion MR imaging protocol was developed to follow the evolution of bone regeneration and the neo-vascularization in femoral bone defects in rats. For this, three different biomaterials based on Pullulan-Dextran and containing either Fucoidan or HydroxyApatite or both were implanted. In vivo MRI, ex vivo micro-CT and histology were performed 1, 3 and 5 weeks after implantation. The high spatially resolved (156 × 182 × 195 µm) anatomical images showed a high contrast from the defects filled with biomaterials that decreased over time due to bone formation. The 3D Dynamic Contrast Enhanced (DCE) imaging with high temporal resolution (1 image/19 s) enabled to detect a modification in the Area-Under-The-Gadolinium-Curve over the weeks post implantation. The high sensitivity of MRI enabled to distinguish which biomaterial was the least efficient for bone regeneration, which was confirmed by micro-CT images and by a lower vessel density observed by histology. In conclusion, the methodology developed here highlights the efficiency of longitudinal MRI for tissue engineering as a routine small animal exam.

Computational Modelling of Metastasis Development in Renal Cell Carcinoma

The biology of the metastatic colonization process remains a poorly understood phenomenon. To improve our knowledge of its dynamics, we conducted a modelling study based on multi-modal data from an orthotopic murine experimental system of metastatic renal cell carcinoma. The standard theory of metastatic colonization usually assumes that secondary tumours, once established at a distant site, grow independently from each other and from the primary tumour. Using a mathematical model that translates this assumption into equations, we challenged this theory against our data that included: 1) dynamics of primary tumour cells in the kidney and metastatic cells in the lungs, retrieved by green fluorescent protein tracking, and 2) magnetic resonance images (MRI) informing on the number and size of macroscopic lesions. Critically, when calibrated on the growth of the primary tumour and total metastatic burden, the predicted theoretical size distributions were not in agreement with the MRI observations. Moreover, tumour expansion only based on proliferation was not able to explain the volume increase of the metastatic lesions. These findings strongly suggested rejection of the standard theory, demonstrating that the time development of the size distribution of metastases could not be explained by independent growth of metastatic foci. This led us to investigate the effect of spatial interactions between merging metastatic tumours on the dynamics of the global metastatic burden. We derived a mathematical model of spatial tumour growth, confronted it with experimental data of single metastatic tumour growth, and used it to provide insights on the dynamics of multiple tumours growing in close vicinity. Together, our results have implications for theories of the metastatic process and suggest that global dynamics of metastasis development is dependent on spatial interactions between metastatic lesions.

We used mathematical modelling to formalize the standard theory of metastatic initiation, under which secondary tumours, after establishment in a distant organ, grow independently from each other and from the primary tumour. When calibrated on the experimental data of primary tumour and total metastatic burden in the lungs in an animal model of renal cell carcinoma, the initial model predicted a size distribution of metastatic foci that did not fit with observations obtained experimentally using magnetic resonance imaging (which provided size and number of macro-metastases). The model predicted an increase in the number of lesions, but of smaller size when compared to the data. This led us to revise the standard theory and to propose two hypotheses in order to explain the observations: 1) small metastatic foci merge into larger ones and/or 2) circulating tumour cells may join already established tumours. We then derived a spatial model of tumour growth in order to explore the quantitative implications of tumours merging on global tumour growth and estimated the numbers of required metastatic foci to obtain the observed metastatic volumes.

Water Selective Imaging and bSSFP Banding Artifact Correction in Humans and Small Animals at 3T and 7T, Respectively

INTRODUCTION

The purpose of this paper is to develop an easy method to generate both fat signal and banding artifact free 3D balanced Steady State Free Precession (bSSFP) images at high magnetic field.

METHODS

In order to suppress fat signal and bSSFP banding artifacts, two or four images were acquired with the excitation frequency of the water-selective binomial radiofrequency pulse set On Resonance or shifted by a maximum of 3/4TR. Mice and human volunteers were imaged at 7 T and 3 T, respectively to perform whole-body and musculoskeletal imaging. "Sum-Of-Square" reconstruction was performed and combined or not with parallel imaging.

RESULTS

The frequency selectivity of 1-2-3-2-1 or 1-3-3-1 binomial pulses was preserved after (3/4TR) frequency shifting. Consequently, whole body small animal 3D imaging was performed at 7 T and enabled visualization of small structures within adipose tissue like lymph nodes. In parallel, this method allowed 3D musculoskeletal imaging in humans with high spatial resolution at 3 T. The combination with parallel imaging allowed the acquisition of knee images with ~500 μm resolution images in less than 2 min. In addition, ankles, full head coverage and legs of volunteers were imaged, demonstrating the possible application of the method also for large FOV.

CONCLUSION

In conclusion, this robust method can be applied in small animals and humans at high magnetic fields. The high SNR and tissue contrast obtained in short acquisition times allows to prescribe bSSFP sequence for several preclinical and clinical applications.

Preclinical MR fingerprinting (MRF) at 7 T: effective quantitative imaging for rodent disease models

High-field preclinical MRI scanners are now commonly used to quantitatively assess disease status and the efficacy of novel therapies in a wide variety of rodent models. Unfortunately, conventional MRI methods are highly susceptible to respiratory and cardiac motion artifacts resulting in potentially inaccurate and misleading data. We have developed an initial preclinical 7.0-T MRI implementation of the highly novel MR fingerprinting (MRF) methodology which has been described previously for clinical imaging applications. The MRF technology combines a priori variation in the MRI acquisition parameters with dictionary-based matching of acquired signal evolution profiles to simultaneously generate quantitative maps of T1 and T2 relaxation times and proton density. This preclinical MRF acquisition was constructed from a fast imaging with steady-state free precession (FISP) MRI pulse sequence to acquire 600 MRF images with both evolving T1 and T2 weighting in approximately 30 min. This initial high-field preclinical MRF investigation demonstrated reproducible and differentiated estimates of in vitro phantoms with different relaxation times. In vivo preclinical MRF results in mouse kidneys and brain tumor models demonstrated an inherent resistance to respiratory motion artifacts as well as sensitivity to known pathology. These results suggest that MRF methodology may offer the opportunity for the quantification of numerous MRI parameters for a wide variety of preclinical imaging applications.

Pullulan/dextran/nHA macroporous composite beads for bone repair in a femoral condyle defect in rats

The repair of bone defects is of particular interest for orthopedic, oral, maxillofacial, and dental surgery. Bone loss requiring reconstruction is conventionally addressed through bone grafting. Depending on the size and the location of the defect, this method has limits and risks. Biomaterials can offer an alternative and have features supporting bone repair. Here, we propose to evaluate the cellular penetration and bone formation of new macroporous beads based on pullulan/dextran that has been supplemented with nanocrystalline hydroxyapatite in a rat model. Cross-linked beads of 300–500 µm diameters were used in a lateral femoral condyle defect and analyzed by magnetic resonance imaging, micro-computed tomography, and histology in comparison to the empty defects 15, 30, and 70 days after implantation. Inflammation was absent for both conditions. For empty defects, cellularisation and mineralization started from the periphery of the defect. For the defects containing beads, cellular structures filling out the spaces between the scaffolds with increasing interconnectivity and trabecular-like organization were observed over time. The analysis of calcified sections showed increased mineralization over time for both conditions, but was more pronounced for the samples containing beads. Bone Mineral Density and Bone Mineral Content were both significantly higher at day 70 for the beads in comparison to empty defects as well as compared with earlier time points. Analysis of newly formed tissue around the beads showed an increase of osteoid tissue, measured as percentage of the defect surface. This study suggests that the use of beads for the repair of small size defects in bone may be expanded on to meet the clinical need for a ready-to-use fill-up material that can favor bone formation and mineralization, as well as promote vessel ingrowth into the defect site.

Quantitative rapid steady state T1 magnetic resonance imaging for cerebral blood volume mapping in mice: Lengthened measurement time window with intraperitoneal Gd-DOTA injection

This work demonstrates how the rapid steady state T1 MRI technique for cerebral blood volume fraction (BVf) quantification can be used with intraperitoneal Gd-DOTA injections in mice at 4.7 T. The peak signal amplitude after intravenous administration (0.7 mmol/kg) and the steady state signal amplitude reached 15 min after intraperitoneal administration (6 mmol/kg) in the same mice lead to equivalent BVf measures in the order of 0.02 in the brain. The resulting time window for BVf quantification is ≈30 min and allows for cerebral BVf mapping with increased spatial resolution or signal-to-noise ratio, or for monitoring functional BVf changes. A cerebral BVf increase of up to 25% induced by the vasodilator acetazolamide was observed, validating the vascular origin of the signal. The noninvasive and quantitative rapid steady state T1 technique can be used in serial studies to evaluate new drugs or disease models, such as antiangiogenic therapies in tumors.

In vivo high-resolution 3D overhauser-enhanced MRI in mice at 0.2 T

Overhauser-enhanced MRI (OMRI) offers the potentiality of detecting low-concentrated species generated by specific biological processes. However molecular imaging applications of OMRI need significant improvement in spatial localization. Here it is shown that 3D-OMRI of a free radical injected in tumor-bearing mice can be performed at high anatomical resolution at a constant field. A 30 mm cavity operating at 5.43 GHz was inserted in a C-shaped magnet for proton MRI at 0.194 T. Nude mice with or without brain-implanted C6 rat glioma were positioned in the cavity and injected with TOPCA (1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole-3-carboxylic acid). OMRI was performed in 3D within several minutes in the brain region without high overheating of the animals. Voxel size was 0.5 × 0.5 × 1 mm³ , providing good delineation of brain regions. Signal amplifications ranged from 2 in tumors to 10 in vessels several minutes after TOPCA injection. Time-course of signal enhancement could be measured by 2D OMRI at 15 s time intervals in a localized thin slice. The method opens the way for molecular imaging of biological activities able to generate OMRI-visible free radicals.

In vivo characterization of changing blood-tumor barrier permeability in a mouse model of breast cancer metastasis: a complementary magnetic resonance imaging approach

OBJECTIVES

The current lack of efficacy for any chemo- or molecular therapeutic in the treatment of brain metastases is thought to be due, in part, to the heterogeneous permeability of the blood-brain-barrier (BBB). Little is known about how heterogeneous permeability develops, or how it varies among individual metastases. Understanding the BBB's role in metastasis will be crucial to the development of new, more effective therapies. In this article, we developed the first magnetic resonance imaging-based strategy to detect and measure the volumes of BBB permeable and nonpermeable metastases and studied the development of altered BBB permeability in metastases in vivo, over time in a mouse model of breast cancer metastasis to the brain.

MATERIALS AND METHODS

Animals bearing human experimental brain metastases of breast cancer (231-BR cells) were imaged, using 3-dimensional balanced steady-state free precession to visualize total metastases, and contrast-enhanced T1-weighted spin echo with gadopentetic acid (Gd-DTPA) to visualize which of these displayed contrast enhancement, as Gd-DTPA leakage is indicative of altered BBB permeability.

RESULTS

Metastases detected 20 days after injection showed no Gd-DTPA enhancement. At day 25, 6.1% ± 6.3% (mean ± standard deviation) of metastases enhanced, and by day 30, 28.1% ± 14.2% enhanced (P < 0.05). Enhancing metastases (mid: 0.14 ± 0.18 mm, late: 0.24 ± 0.32 mm) had larger volumes than nonenhancing (mid: 0.04 ± 0.04 mm, late: 0.09 ± 0.09 mm, P < 0.05); however, there was no significant difference between the growth rates of the 2.

CONCLUSIONS

A significant number of brain metastases were uniformly nonpermeable, which highlights the need for developing treatment strategies that can overcome the permeability of the BBB. The model developed herein can provide the basis for in vivo evaluation of both BBB permeable and nonpermeable metastases response to therapy.

In vivo MR tracking of therapeutic microglia to a human glioma model

A knowledge of the spatial localization of cell vehicles used in gene therapy against glioma is necessary before launching therapy. For this purpose, MRI cell tracking is performed by labeling the cell vehicles with contrast agents. In this context, the goal of this study was to follow noninvasively the chemoattraction of therapeutic microglial cells to a human glioma model before triggering therapy. Silica nanoparticles grafted with gadolinium were used to label microglia. These vehicles, expressing constitutively the thymidine kinase suicide gene fused to the green fluorescent protein gene, were injected intravenously into human glioma-bearing nude mice. MRI was performed at 4.7 T to track noninvasively microglial accumulation in the tumor. This was followed by microscopy on brain slices to assess the presence in the glioma of the contrast agents, microglia and fusion gene through the detection of silica nanoparticles grafted with tetramethyl rhodamine iso-thiocyanate, 3,3'-dioctadecyloxacarbocyanine perchlorate and green fluorescent protein fluorescence, respectively. Finally, gancyclovir was administered systemically to mice. Human microglia were detectable in living mice, with strong negative contrast on T(2) *-weighted MR images, at the periphery of the glioma only 24 h after systemic injection. The location of the dark dots was identical in MR microscopy images of the extracted brains at 9.4 T. Fluorescence microscopy confirmed the presence of the contrast agents, exogenous microglia and suicide gene in the intracranial tumor. In addition, gancyclovir treatment allowed an increase in mice survival time. This study validates the MR tracking of microglia to a glioma after systemic injection and their use in a therapeutic strategy against glioma.

In vivo single scan detection of both iron-labeled cells and breast cancer metastases in the mouse brain using balanced steady-state free precession imaging at 1.5 T

PURPOSE

To simultaneously detect iron-labeled cancer cells and brain tumors in vivo in one scan, the balanced steady-state free precession (b-SSFP) imaging sequence was optimized at 1.5 T on mice developing brain metastases subsequent to the injection of micron-sized iron oxide particle-labeled human breast cancer cells.

MATERIALS AND METHODS

b-SSFP sequence parameters (repetition time, flip angle, and receiver bandwidth) were varied and the signal-to-noise ratio, contrast between the brain and tumors, and the number of detected iron-labeled cells were evaluated.

RESULTS

Optimal b-SSFP images were acquired with a 26 msec repetition time, 35° flip angle, and bandwidth of ±21 kHz. b-SSFP images were compared with T(2) -weighted 2D fast spin echo (FSE) and 3D spoiled gradient recalled echo (SPGR) images. The mean tumor-brain contrast-to-noise ratio and the ability to detect iron-labeled cells were the highest in the b-SSFP images.

CONCLUSION

A single b-SSFP scan can be used to visualize both iron-labeled cells and brain metastases.

Optimization of the balanced steady state free precession (bSSFP) pulse sequence for magnetic resonance imaging of the mouse prostate at 3T

INTRODUCTION

MRI can be used to non-invasively monitor tumour growth and response to treatment in mouse models of prostate cancer, particularly for longitudinal studies of orthotopically-implanted models. We have optimized the balanced steady-state free precession (bSSFP) pulse sequence for mouse prostate imaging.

METHODS

Phase cycling, excitations, flip angle and receiver bandwidth parameters were optimized for signal to noise ratio and contrast to noise ratio of the prostate. The optimized bSSFP sequence was compared to T1- and T2-weighted spin echo sequences.

RESULTS

SNR and CNR increased with flip angle. As bandwidth increased, SNR, CNR and artifacts such as chemical shift decreased. The final optimized sequence was 4 PC, 2 NEX, FA 50°, BW ±62.5 kHz and took 14-26 minutes with 200 µm isotropic resolution. The SNR efficiency of the bSSFP images was higher than for T1WSE and T2WSE. CNR was highest for T1WSE, followed closely by bSSFP, with the T2WSE having the lowest CNR. With the bSSFP images the whole body and organs of interest including renal, iliac, inguinal and popliteal lymph nodes were visible.

CONCLUSION

We were able to obtain fast, high-resolution, high CNR images of the healthy mouse prostate with an optimized bSSFP sequence.

Real-time 3D MRI of contrast agents in whole living mice

A specific mouse whole body coil and a dedicated gradient system at 4.7 T were coupled with an ultra-fast 3D gradient echo MRI and keyhole reconstruction technique to obtain 3D whole-body dynamic T(1)-weighted or T(2)*-weighted imaging. The technique was used to visualize the real-time distribution of non-targeting T(1) and T(2)* contrast agent (CA) in a glioma-bearing mouse model. T(1) dynamic contrast-enhancement imaging was performed with a fast imaging with steady-state precession sequence [echo time/repetition time (TE/TR), 1.32/3.7 ms] before and after CA injection (Gd-DOTA and BSA-Gd-DOTA) for 21 min. The temporal resolution was 1 image/6.5 s. T(2)* imaging (TE/TR, 4/8 ms) was performed before and after iron-based (small and ultra-small particles of iron oxide) CA injection for 45 min. The temporal resolution was 1 image/14 s. Signal-to-noise ratio curves were determined in various mouse organs. The whole-body coil and gradient systems made it possible to acquire data with sufficient and homogeneous signal-to-noise ratio on the whole animal. The spatial resolution allowed adequate depiction of the major organs, blood vessels and brain glioma. The distribution and the time-course of T(1) and T(2)* contrasts upon contrast agent injection were also assessed. 3D whole-body mouse MRI is feasible at high spatial resolution in movie mode and can be applied successfully to visualize real-time contrast agent distribution. This method should be effective in future preclinical molecular imaging studies.

Integration and evaluation of a needle-positioning robot with volumetric microcomputed tomography image guidance for small animal stereotactic interventions

PURPOSE

Preclinical research protocols often require insertion of needles to specific targets within small animal brains. To target biologically relevant locations in rodent brains more effectively, a robotic device has been developed that is capable of positioning a needle along oblique trajectories through a single burr hole in the skull under volumetric microcomputed tomography (micro-CT) guidance.

METHODS

An x-ray compatible stereotactic frame secures the head throughout the procedure using a bite bar, nose clamp, and ear bars. CT-to-robot registration enables structures identified in the image to be mapped to physical coordinates in the brain. Registration is accomplished by injecting a barium sulfate contrast agent as the robot withdraws the needle from predefined points in a phantom. Registration accuracy is affected by the robot-positioning error and is assessed by measuring the surface registration error for the fiducial and target needle tracks (FRE and TRE). This system was demonstrated in situ by injecting 200 microm tungsten beads into rat brains along oblique trajectories through a single burr hole on the top of the skull under micro-CT image guidance. Postintervention micro-CT images of each skull were registered with preintervention high-field magnetic resonance images of the brain to infer the anatomical locations of the beads.

RESULTS

Registration using four fiducial needle tracks and one target track produced a FRE and a TRE of 96 and 210 microm, respectively. Evaluation with tissue-mimicking gelatin phantoms showed that locations could be targeted with a mean error of 154 +/- 113 microm.

CONCLUSIONS

The integration of a robotic needle-positioning device with volumetric micro-CT image guidance should increase the accuracy and reduce the invasiveness of stereotactic needle interventions in small animals.

Change of the cerebrospinal fluid volume during brain activation investigated by T(1rho)-weighted fMRI

A voxel in MRI often contains tissue as well as cerebrospinal fluid (CSF). During functional stimulation, volume fractions of these different water compartments may change. To directly image the CSF volume fraction and measure its functional change, we utilized a rotating-frame longitudinal relaxation time (T(1rho))-weighted MRI technique. At 9.4T with a spin-locking frequency of approximately 500Hz, T(1rho) of tissue water and CSF are about 48 and 450ms, respectively. Therefore, the parenchyma signal becomes negligible when a long spin-locking time (e.g., 200ms) is applied, leaving only the CSF signal. Baseline CSF volume fraction (V(csf)) and its change induced by visual stimulation were mapped in isoflurane-anesthetized cats (n=6). In both T(1rho)-weighted fMRI with spin locking times of 200 and 300ms, negative changes with similar magnitudes were observed, indicating that a decrease in V(csf) is a dominant contributor. In the region with voxels containing the visual cortex and CSF compartments, an average baseline V(csf) was 24.6+/-2%, an average CSF volume fraction change (DeltaV(csf)/V(csf)) was -2.45+/-0.6%, and an absolute change in CSF volume fraction (DeltaV(csf)) was -0.6+/-0.15%. A negative correlation was observed between pixel-wise baseline V(csf) and DeltaV(csf)/V(csf), which can be explained by similar DeltaV(csf) among voxels. Our results suggest that the functional reduction of CSF volume fraction could contribute to fMRI signals, especially when the tissue signal is significantly reduced as compared to the CSF with certain experimental techniques or parameters.

In vivo bioprinting for computer- and robotic-assisted medical intervention: preliminary study in mice

We present the first attempt to apply bioprinting technologies in the perspective of computer-assisted medical interventions. A workstation dedicated to high-throughput biological laser printing has been designed. Nano-hydroxyapatite (n-HA) was printed in the mouse calvaria defect model in vivo. Critical size bone defects were performed in OF-1 male mice calvaria with a 4 mm diameter trephine. Prior to laser printing experiments, the absence of inflammation due to laser irradiation onto mice dura mater was shown by means of magnetic resonance imaging. Procedures for in vivo bioprinting and results obtained using decalcified sections and x-ray microtomography are discussed. Although heterogeneous, these preliminary results demonstrate that in vivo bioprinting is possible. Bioprinting may prove to be helpful in the future for medical robotics and computer-assisted medical interventions.

4D retrospective black blood trueFISP imaging of mouse heart

The purpose of this study was to demonstrate the feasibility of steady-state True fast imaging with steady precession (TrueFISP) four-dimensional imaging of mouse heart at high resolution and its efficiency for cardiac volumetry. Three-dimensional cine-imaging of control and hypoxic mice was carried out at 4.7 T without magnetization preparation or ECG-triggering. The k-space lines were acquired with the TrueFISP sequence (pulse repetition time/echo time = 4/2 ms) in a repeated sequential manner. Retrospective reordering of raw data allowed the reconstruction of 10 three-dimensional images per cardiac cycle. The acquisition scheme used an alternating radiofrequency phase and sum-of-square reconstruction method. Black-blood three-dimensional images at around 200 mum resolution were produced without banding artifact throughout the cardiac cycle. High contrast to noise made it possible to estimate cavity volumes during diastole and systole. Right and left ventricular stroke volume was significantly higher in hypoxic mice vs controls (20.2 +/- 2 vs 15.1 +/- 2; P < 0.05, 24.9 +/- 2 vs 20.4 +/- 2; P < 0.05, respectively). In conclusion, four-dimensional black-blood TrueFISP imaging in living mice is a method of choice to investigate cardiac abnormalities in mouse models.

In vivo magnetic resonance imaging of spinal cord injury in the mouse

The feasibility of performing high-resolution in vivo magnetic resonance imaging (MRI) to visualize the injured mouse spinal cord using a three-dimensional (3D)-FIESTA (fast imaging employing steady state acquisition) pulse sequence, in a clip compression injury model, is presented. Images were acquired using a 3-Tesla clinical whole-body MR system equipped with a high-performance gradient coil insert. High-resolution mouse cord images were used to detect and monitor the cord lesions for 6 weeks after spinal cord injury (SCI). The epicenter of the injury appeared as a region of mixed signal intensities on day 2 post-SCI. Regions of signal hypointensity appeared at the lesion site by 2 weeks post-SCI and became more apparent with time. In some mice, large cyst-like lesions were detected rostral to the lesion epicenter, as early as 2 weeks post-SCI, and increased in volume with time. In addition, MRI was used to detect and monitor iron-labeled mesenchymal stem cells (MSCs) after their transplantation into the injured cord. MSCs appeared as large, obvious regions of signal loss in the cord, which decreased in size over time.