Vladimir Mlynarik

The role of relaxation times in monitoring proteoglycan depletion in articular cartilage.

Various proton relaxation times (T2, T1ρ, and gadolinium‐diethylene triamine pentaacetic acid [Gd‐DTPA]‐enhanced T1) were measured in articular cartilage in vitro at 3 T to assess their role in visualizing proteoglycan depletion. Cartilage‐bone specimens were obtained from patients who underwent total joint replacement and got a double dose of Gd‐DTPA 2 hours prior surgery. In these specimens, regions of mechanically undamaged cartilage having a decreased content of proteoglycans showed about 15% lower T1 values compared with apparently normal cartilaginous tissue. The expected increase of the T2 relaxation time was not observed in these regions. On the other hand, the T2 and, to a lower degree, T1 relaxation times were found to be increased in regions of cartilage fibrillation. The T1ρ relaxation times obtained were slightly longer than the corresponding T2 values, but both parameters showed almost identical spatial distributions. J. Magn. Reson. Imaging 1999;10:497–502.

MRI visualization of proteoglycan depletion in articular cartilage via intravenous administration of Gd-DTPA.

The effect of intravenous administration of gadolinium diethylenetriamine-pentaacetic acid (Gd-DTPA) on MR images was studied in vitro, using pathologic osteochondral specimens removed during surgery for total endoprosthesis, and in vivo, on a group of volunteers. In ex vivo specimens, lesions of different shape having lower T1 were detected which corresponded to areas with depleted proteoglycans found histologically. In vivo experiments on young volunteers showed that the time course of cartilage enhancement was different for different anatomies. The time for maximum enhancement ranged from 45 min for the ventral femoral condyle to 270 min for patellar cartilage.

Absolute metabolite quantification by in vivo NMR spectroscopy: II. A multicentre trial of protocols for in vivo localised proton studies of human brain

We have performed a multicentre trial to assess the performance of three techniques for absolute quantification of cerebral metabolites using in vivo proton nuclear magnetic resonance (NMR). The techniques included were 1) an internal water standard method, 2) an external standard method based on phantom replacement, and 3) a more sophisticated method incorporating elements of both the internal and external standard approaches, together with compartmental analysis of brain water. Only the internal water standard technique could be readily implemented at all participating sites and gave acceptable precision and interlaboratory reproducibility. This method was insensitive to many of the experimental factors affecting the performance of the alternative techniques, including effects related to loading, standing waves and B1 inhomogeneities; and practical issues of phantom positioning, user expertise and examination duration. However, the internal water standard method assumes a value for the concentration of NMR-visible water within the spectroscopic volume of interest. In general, it is necessary to modify this assumed concentration on the basis of the grey matter, white matter and cerebrospinal fluid (CSF) content of the volume, and the NMR-visible water content of the grey and white matter fractions. Combining data from 11 sites, the concentrations of the principal NMR-visible metabolites in the brains of healthy subjects (age range 20-35 years) determined using the internal water standard method were (mean+/-SD): [NAA]=10.0+/-3.4 mM (n=53), [tCho]=1.9+/-1.0 mM (n=51), [Cr + PCr]=6.5+/-3.7 mM (n=51). Evidence of system instability and other sources of error at some participating sites reinforces the need for rigorous quality assurance in quantitative spectroscopy.

Investigation of apparent diffusion constant as an indicator of early degenerative disease in articular cartilage.

To investigate the apparent diffusion constant (ADC) as a prospective magnetic resonance imaging (MRI) marker of early degeneration in articular cartilage.

High‐resolution 3D proton spectroscopic imaging of the human brain at 3 T: SNR issues and application for anatomy‐matched voxel sizes

In a systematic study on the interdependence of linewidth, signal‐to‐noise ratio (SNR), and spatial resolution in 3D proton spectroscopic imaging (1H‐SI) at 3 T, we demonstrate reduced linewidths with increased spatial resolution due to reduced magnetic inhomogeneity within the brain. High‐precision quantitative data (0.75–0.094 cm3) were obtained for all resolutions, enabling the creation of metabolic maps that display details such as the ventricles, sulci, and gyri. High‐resolution 1H‐SI allows differences in metabolic ratios to be estimated for anatomically defined regions in gray (GM) and white matter (WM). Seven distinct regions in a healthy brain were anatomically segmented and their metabolic ratios were compared quantitatively. Data from a tumor patient are also presented to demonstrate potential clinical applications. Because of the high resolution, the metabolite ratios could be determined for distinct pathologic regions within the tumor and its surroundings. The method was additionally applied to a patient with patchy Pelizaeus Merzbacher disease (PMD), and compared to single‐voxel spectroscopy performed in the same session. High‐resolution SI data were demonstrated in our study to allow the direct matching of anatomic and metabolic images. This may enhance the clinical value of 1H‐SI. Magn Reson Med 49:299–306, 2003.

1H NMR relaxation times of skeletal muscle metabolites at 3 T

This study reports proton relaxation times of water and metabolites in soleus and tibialis anterior muscles of young healthy volunteers at 3 T. The results are in agreement with data reported for 1.5 and 4 T, showing a steady increase of spin-lattice relaxation times of water, creatine and lipids with B0 and no effect of B0 on spin–spin relaxation. Comparison between muscles revealed a longer spin–spin relaxation time of water in soleus than in tibialis anterior muscle (31±1 ms vs. 28±1 ms, p<0.05). These data can be applied to relaxation correction for the absolute quantification of skeletal muscle metabolite concentrations and further sequence optimization.

Direct noninvasive quantification of lactate and high energy phosphates simultaneously in exercising human skeletal muscle by localized magnetic resonance spectroscopy

A novel method based on interleaved localized 31P‐ and 1H MRS is presented, by which lactate accumulation and the accompanying changes in high energy phosphates in human skeletal muscle can be monitored simultaneously during exercise and recovery. Lactate is quantified using a localized double quantum filter suppressing the abundant lipid signals while taking into account orientation dependent signal modulations. Lactate concentration after ischemic exercise directly quantified by DQF 1H spectroscopy was 24 ± 3 mmol/L cell water, while 22 ± 3 mmol/L was expected on the basis of 31P MRS acquired simultaneously. Lactate concentration in a sample of porcine meat was estimated to be 40 ± 7 mmol/L by means of DQF quantitation, versus 39 ± 5 mmol/L by biochemical methods. Excellent agreement is shown between lactate concentrations measured noninvasively by 1H MRS, measured biochemically ex vivo, and inferred indirectly in vivo from changes in pH, Pi, and PCr as obtained from 31P MRS data. Magn Reson Med 57:654–660, 2007.

Evaluation of cartilage repair and osteoarthritis with sodium MRI

The growing need for early diagnosis and higher specificity than that which can be achieved with morphological MRI is a driving force in the application of methods capable of probing the biochemical composition of cartilage tissue, such as sodium imaging. Unlike morphological imaging, sodium MRI is sensitive to even small changes in cartilage glycosaminoglycan content, which plays a key role in cartilage homeostasis. Recent advances in high‐ and ultrahigh‐field MR systems, gradient technology, phase‐array radiofrequency coils, parallel imaging approaches, MRI acquisition strategies and post‐processing developments have resulted in many clinical in vivo sodium MRI studies of cartilage, even at 3 T. Sodium MRI has great promise as a non‐invasive tool for cartilage evaluation. However, further hardware and software improvements are necessary to complete the translation of sodium MRI into a clinically feasible method for 3‐T systems. This review is divided into three parts: (i) cartilage composition, pathology and treatment; (ii) sodium MRI; and (iii) clinical sodium MRI studies of cartilage with a focus on the evaluation of cartilage repair tissue and osteoarthritis. Copyright

Very short echo time proton MR spectroscopy of human brain with a standard transmit/receive surface coil

A method for localized proton spectroscopy of the human brain is proposed which can be used with a standard transmit/receive planar surface coil producing an inhomogeneous RF field. Water suppression is accomplished by a train of full passage adiabatic pulses with optimized frequencies and delays, which account for variation in the water resonance frequency and the spin‐lattice relaxation time. The robust method requires minimal pulse calibration and provides high‐quality spectra even at very short echo times and in the absence of outer volume saturation and, therefore, is well suited for clinical in vivo spectroscopy. Performance of the method is demonstrated on a test object and on MR spectra from the human brain at 3 T. Magn Reson Med 44:964–967, 2000.

Morphological and compositional monitoring of a new cell-free cartilage repair hydrogel technology – GelrinC by MR using semi-quantitative MOCART scoring and quantitative T2 index and new zonal T2 index calculation

OBJECTIVE To evaluate cartilage repair tissue (RT) using MOCART scoring for morphological and T2 mapping for biochemical assessment following implantation of GelrinC, a biosynthetic, biodegradable hydrogel implant. DESIGN MR imaging (1.5/3T) was performed on 21 patients at six sites. Standard protocols were used for MOCART evaluation at 1 week (baseline) 1, 3, 6, 12, 18 and 24 months. Multi-echo SE was used for T2 mapping. Global (T2 in RT divided by T2 in normal cartilage) and zonal T2 index (deep T2 divided by superficial T2) of RT were calculated. RESULTS Average MOCART score was 71.8 (95% CI 62.2 to 81.3) at six, 75.2 (95% CI 62.8 to 87.5) at twelve, 71.8 (95% CI 55.4 to 88.2) at eighteen and 84.4 (95% CI 77.7 to 91.0) at twenty-four months. The global T2 index ranged between 0.8 and 1.2 (normal healthy cartilage) in 1/11 (9%) patients at baseline, 8/12 (67%) at 12 months, 11/13 (85%) at 18 months and 13/16 (81%) at 24 months. The zonal T2 index for RT was <20% difference to the zonal T2 index for normal cartilage in: 6/12 patients (50%) at 12 months, 7/13 (53.8%) at 18 months and 10/16 (63.5%) at 24 months. The standard deviation for T2 showed a significant decrease over the study. CONCLUSIONS The increase of MOCART scores over follow-up indicates improving cartilage repair tissue. Global and zonal T2 repair values at 24 months reached normal cartilage in 81% and 63.5% of the patients respectively, reflecting collagen organization similar to hyaline cartilage.