Faculty Bibliography

PURPOSE: To disentangle the free diffusivity (D0 ) and cellular membrane restrictions, by means of their surface-to-volume ratio (S/V), using the frequency-dependence of the diffusion coefficient D(omega), measured in brain tumors in the short diffusion-time regime using oscillating gradients (OGSE). METHODS: In vivo and ex vivo OGSE experiments were performed on mice bearing the GL261 murine glioma model (n = 10) to identify the relevant time/frequency (t/omega) domain where D(omega) linearly decreases with omega-1/2 . Parametric maps (S/V, D0 ) are compared with conventional DWI metrics. The impact of frequency range and temperature (20 degrees C versus 37 degrees C) on S/V and D0 is investigated ex vivo. RESULTS: The validity of the short diffusion-time regime is demonstrated in vivo and ex vivo. Ex vivo measurements confirm that the purely geometric restrictions embodied in S/V are independent from temperature and frequency range, while the temperature dependence of the free diffusivity D0 is similar to that of pure water. CONCLUSION: Our results suggest that D(omega) in the short diffusion-time regime can be used to uncouple the purely geometric restriction effect, such as S/V, from the intrinsic medium diffusivity properties, and provides a nonempirical and objective way to interpret frequency/time-dependent diffusion changes in tumors in terms of objective biophysical tissue parameters. Magn Reson Med, 2015. (c) 2015 Wiley Periodicals, Inc.

Expression of myeloperoxidase (MPO) activity in the blood vessel wall by cells participating in the innate immune response is a known indicator of downstream acute vascular events (heart attacks, strokes). MPO activity associates with increased risk of rupture of brain aneurysms in humans [1]. Consequently, non-invasive imaging of MPO activity using a clinically acceptable imaging substrate would greatly assist in performing differential diagnosis in patients facing potential risk of aneurysm rupture. We devised an imaging setup in which a novel paramagnetic probe based on highly water-soluble, negatively charged 5-hydroxytryptamide (5HT) of DOTAGAGd could be tested ex vivo in human aneurysmal tissue. In the presence of DOTAGAGd-5HT substrate and serum albumin, we observed an increase in molar longitudinal relaxivity of Gd (r1; 2.3 fold at 0.47 T) as a result of MPO-driven catalysis. In contrast to bis-tryptamides of DTPA [2], DOTAGAGd-based MPO substrate showed better solubility and a higher enzyme-mediated molar relaxivity increase due to the combined effect of a lower relaxivity of DOTAGA-chelated gadolinium (III) and lower binding to plasma proteins of mono-5HT substrates. Furthermore, we tested DOTAGAGd- 5HT for 1) mapping MPO activity in tissue regions using thick frozen histology sections and 2) corroborating micro- MRI (muMRI) images with ultrathin human sections examined using fluorescent MPO substrate (Cy3-5HT). Surgical samples of human saccular brain aneurysm clippings were obtained under the approved UMMS IRB protocol with patient consent (n=23). Frozen aneurysm sections (8 and 50 mum thick) were fixed with acetone. Red fluorescence of Cy3-5HT substrate was localized in several areas in the adventitia of the ruptured aneurysm section (R) as well as within the partially thrombosed lumina of human samples. Parallel thick non-consecutive sections were subjected to muMRI imaging following incubation with 0.5 mM DOTAGAGd-5HT and hydrogen peroxide. Direct muMRI of tissue sections was performed using a set of homebuilt histological coils tuned to operate in a 7T Bruker muMRI system interfaced to a 200 mm horizontal bore magnet [3]. The highly detailed muMR images (57 mum in-plane resolution) of thin tissue sections acquired in less than 8-hours revealed the presence of high focal T1w-enhancement corresponding to the blood vessel wall (R) and the plaque (UR) areas of the same sections. The set of tissue samples examined so far confirm positive correlation between normalized MPO enzymatic activity found in the tissue and the increased risk of developing a rupture within a 5-year period. Therefore, low molecular weight, highly stable DOTAGA-based chelates are promising molecular imaging probes for further clinical translation. The initial assessment of a potential link between brain aneurysm instability and vascular wall inflammation is further evidenced by our imaging setup enabling the direct muMRI analysis of human histology samples and the testing of novel MR enzyme-specific probes in situ. (Figure Presented)

Dynamic contrast enhanced (DCE) MRI has emerged as a reliable and diagnostically useful functional imaging technique. DCE protocol typically lasts 3-15 minutes and results in a time series of N volumes. For automated analysis, it is important that volumes acquired at different times be spatially coregistered. We have recently introduced a novel 4D, or volume time series, coregistration tool based on a user-specified target volume of interest (VOI). However, the relationship between coregistration accuracy and target VOI size has not been investigated. In this study, coregistration accuracy was quantitatively measured using various sized target VOIs. Coregistration of 10 DCE-MRI mouse head image sets were performed with various sized VOIs targeting the mouse brain. Accuracy was quantified by measures based on the union and standard deviation of the coregistered volume time series. Coregistration accuracy was determined to improve rapidly as the size of the VOI increased and approached the approximate volume of the target (mouse brain). Further inflation of the VOI beyond the volume of the target (mouse brain) only marginally improved coregistration accuracy. The CPU time needed to accomplish coregistration is a linear function of N that varied gradually with VOI size. From the results of this study, we recommend the optimal size of the VOI to be slightly overinclusive, approximately by 5 voxels, of the target for computationally efficient and accurate coregistration.

Current clinical Gd(3+)-based T1 magnetic resonance imaging (MRI) contrast agents (CAs) are suboptimal or unsuitable, especially at higher magnetic fields (>1.5 Tesla) for advanced MRI applications such as blood pool, cellular and molecular imaging. Herein, towards the goal of developing a safe and more efficacious high field T1 MRI CA for these applications, we report the sub-acute toxicity and contrast enhancing capabilities of a novel nanoparticle MRI CA comprising of manganese (Mn(2+)) intercalated graphene nanoparticles functionalized with dextran (hereafter, Mangradex) in rodents. Sub-acute toxicology performed on rats intravenously injected with Mangradex at 1, 50 or 100 mg/kg dosages 3 times per week for three weeks indicated that dosages