Faculty Bibliography

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 x 0.5 x 1 mm(3) , 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.

Osteoarthritis is a degenerative disease of articular cartilage that may be associated with a loss of glycosaminoglycans. Quantitative sodium magnetic resonance imaging is highly specific to glycosaminoglycan content and could be used to assess the biochemical degradation of cartilage in early osteoarthritis. However, the reproducibility and repeatability of this technique are not well documented. The aim of this study is to test the reproducibility and repeatability of sodium quantification in cartilage in vivo using intraday and interday acquisitions at 3 T and 7 T, with a radial 3D sequence, with and without fluid suppression. Fluid suppression was obtained by adiabatic inversion recovery (IR WURST) and is expected to improve the sensitivity of the method to glycosaminoglycan content. The root mean square of coefficients of variation are all in the range of 7.5-13.6%. No significant intermagnet, intersequence, intraday, and interday differences in the coefficients of variation were observed. Sodium quantification using IR WURST gave values closer to those reported in the literature for healthy cartilage (220-310 mM) than radial 3D. In conclusion, IR WURST was more accurate in context of sodium measurement, with a reproducibility and repeatability comparable to other compositional magnetic resonance imaging techniques of cartilage. Magn Reson Med, 2011. (c) 2011 Wiley Periodicals, Inc.

The sodium concentration correlates directly with the concentration of proteoglycans (PG) in cartilage, the loss of which is an early signature of osteoarthritis (OA). As a result, quantitative sodium MRI is a promising technique for assessing the degradation of articular cartilage in patients with OA. Sodium relaxation times can also provide information on the degradation of cartilage: it has already been shown on bovine cartilage that T(1) and T(2long) are longer and T(2short) shorter when the PG concentration decreases. In this study, sodium T(1) , T2short * and T2long * relaxation maps were measured in vivo at 7 T on 8 healthy volunteers and in 4 different regions of the cartilage in the knee joint. The patellar, femoro-tibial medial, lateral, and femoral condyle cartilage have an average T(1) approximately 20 ms, but different T2short * (from 0.5 ms to 1.4 ms) and T2long * (from 11.4 ms to 14.8 ms). Statistically significant differences in T(1) , T2short * and T2long * were observed between the different regions in cartilage (p << 10(- 5) ). Statistical differences in T(1) were also observed between male and female data (p << 10(- 5) ). These relaxation times measurements can further be applied as correction factors for sodium concentration maps in vivo and can also be useful as complementary information to quantitative sodium MRI in the quest for detecting early OA. These measurements were done on low resolution sodium images in order to acquire sufficient quality data for fitting (5 images for T(1) and 9 images for T2 *) while keeping the total time of acquisition of the data reasonable for the volunteer's comfort (1 h 15 min)

Sodium MRI has been shown to be highly specific for glycosaminoglycan (GAG) content in articular cartilage, the loss of which is an early sign of osteoarthritis (OA). Quantitative sodium MRI techniques are therefore under development in order to detect and assess early biochemical degradation of cartilage, but due to low sodium NMR sensitivity and its low concentration, sodium images need long acquisition times (15-25min) even at high magnetic fields and are typically of low resolution. In this preliminary study, we show that compressed sensing can be applied to reduce the acquisition time by a factor of 2 at 7T without losing sodium quantification accuracy. Alternatively, the nonlinear reconstruction technique can be used to denoise fully-sampled images. We expect to even further reduce this acquisition time by using parallel imaging techniques combined with SNR-improved 3D sequences at 3T and 7T

PURPOSE: To compare a new birdcage-transmit, 28-channel receive array (28-Ch) coil and a quadrature volume coil for 7T morphologic MRI and T2 mapping of knee cartilage. MATERIALS AND METHODS: The right knees of 10 healthy subjects were imaged on a 7T whole body magnetic resonance (MR) scanner using both coils. 3D fast low-angle shot (3D-FLASH) and multiecho spin-echo (MESE) sequences were implemented. Cartilage signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), thickness, and T2 values were assessed. RESULTS: SNR/CNR was 17%-400% greater for the 28-Ch compared to the quadrature coil (P </= 0.005). Bland-Altman plots show mean differences between measurements of tibial/femoral cartilage thickness and T2 values obtained with each coil to be small (-0.002 +/- 0.009 cm / 0.003 +/- 0.011 cm) and large (-6.8 +/- 6.7 msec/-8.2 +/- 9.7 msec), respectively. For the 28-Ch coil, when parallel imaging with acceleration factors (AF) 2, 3, and 4 was performed SNR retained was: 62%-69%, 51%-55%, and 39%-45%. CONCLUSION: A 28-Ch knee coil provides increased SNR/CNR for 7T cartilage morphologic imaging and T2 mapping. Coils should be switched with caution during clinical studies because T2 values may differ. The greater SNR of the 28-Ch coil could be used to perform parallel imaging with AF2 and obtain similar SNR as the quadrature coil. J. Magn. Reson. Imaging 2012;441-448. (c) 2011 Wiley Periodicals, Inc

Because articular cartilage is avascular and has no intrinsic capacity to heal itself, physical damage to cartilage poses a serious clinical problem for orthopedic surgeons and rheumatologists. No medication exists to treat or reconstitute physical defects in articular cartilage, and pharmacotherapy is limited to pain control. Developments in the field of articular cartilage repair include microfracture, osteochondral autografting, osteochondral allografting, repair with synthetic resorbable plugs, and autologous chondrocyte implantation. MR imaging techniques have the potential to allow in vivo monitoring of the collagen and proteoglycan content of cartilage repair tissue and may provide useful additional metrics of cartilage repair tissue quality

We present in this paper a method to monitor multiphase fluid core saturation through measurement of the sodium NMR signal. In a rock core saturated with water and oil, sodium will be present only in the water phase, and therefore can be used to separate the two fluids. Two dimensional sodium images were taken to monitor the movement of brine into oil saturated rock cores. The measured fluid exchange agrees well with expected behavior from traditional core analysis methods. Indications of damage to the rock structure can be seen from the patterns of fluid imbibition.

BACKGROUND AND PURPOSE: Double Inversion Recovery Magnetic Resonance Imaging (DIR) consists of two adiabatic non-selective inversion pulses applied before a Turbo Spin Echo (TSE) sequence, in order to suppress the signal from two tissues with different longitudinal relaxation times T(1) simultaneously. In the brain, DIR is used to selectively image the gray matter (GM) by nulling the signal from white matter (WM) and cerebrospinal fluid (CSF). The main limitation of the technique remains the intrinsic low SNR due to the specific preparation of the longitudinal magnetization. The recent availability of high field magnets operating at 7 T for human imaging offers the advantage of higher SNR. This study shows the feasibility of brain Double Inversion Recovery Magnetic Resonance Imaging (DIR-MRI) at 7 T in vivo in healthy volunteers. METHODS: The MRI experiments were performed on phantoms at 7 T and on four healthy volunteers at 7 and 3 T. For fat suppression, a chemical shift selective Fat Inversion Recovery (csFatIR) technique was used and compared to the standard fat saturation (FatSat). RESULTS: The csFatIR method resulted to be significantly more efficient than the Fatsat at 7 T and slightly more efficient at 3 T, enabling a clear delineation of GM. CONCLUSIONS: DIR is feasible at 7 T despite the problems associated with B(1) in-homogeneity

The loss of proteoglycans (PG) in the articular cartilage is an early signature of osteoarthritis (OA). The ensuing changes in the fixed charge density in the cartilage can be directly linked to sodium concentration via charge balance. Sodium ions in the knee joint appear in two pools: in the synovial fluids or joint effusion where the ions are in free motion and bound within the cartilage tissue where the Na(+) ions have a restricted motion. The ions in these two compartments have therefore different T and T relaxation times. The purpose of this study is to demonstrate the feasibility of a fluid-suppressed 3D ultrashort TE radial sodium sequence by implementing an inversion recovery (IR) preparation of the magnetization at 7T. This method could allow a more accurate and more sensitive quantification of loss of PG in patients with OA. It is shown that adiabatic pulses offer significantly improved performance in terms of robustness to B and B inhomogeneities when compared to the hard pulse sequence. Power deposition considerations further pose a limit to the RF inversion power, and we demonstrate in simulations and experiments how a practical compromise can be struck between clean suppression of fluid signals and power deposition levels. Two IR sequences with different types of inversion pulses (a rectangular pulse and an adiabatic pulse) were tested on a liquid phantom, ex vivo on a human knee cadaver and then in vivo on five healthy volunteers, with a (Nyquist) resolution of approximately 3.6 mm and a signal-to-noise ratio of approximately 30 in cartilage without IR and approximately 20 with IR. Due to specific absorption rate limitations, the total acquisition time was approximately 17 min for the 3D radial sequence without inversion or with the rectangular IR, and 24:30 min for the adiabatic IR sequence. It is shown that the adiabatic IR sequence generates a more uniform fluid suppression over the whole sample than the rectangular IR sequence