Showing papers by "Ravinder Reddy published in 2002"

23Na MRI Accurately Measures Fixed Charge Density in Articular Cartilage

Abstract: One of the initiating steps of osteoarthritis is the loss of proteoglycan (PG) molecules from the cartilage matrix. One method for assessing cartilage integrity, therefore, is to measure the PG content or fixed charge density (FCD) of cartilage. This report shows the feasibility of calculating FCD by (23)Na MRI and introduces MRI protocols for human studies, in vivo. (23)Na MRI was used to measure the sodium concentration inside bovine patellar cartilage. The sodium concentration was then converted to FCD (mM) by considering ideal Donnan equilibrium. These FCD measurements were compared to FCD measurements obtained through standard dimethylmethylene blue PG assays. There was a high correlation (slope = 0.89, r(2) = 0.81) between the FCD measurements obtained by (23)Na MRI and those obtained by the PG assays. These methods were then employed in quantifying the FCD of articular cartilage of human volunteers in vivo. Two imaging protocols were compared: one using a birdcage coil, the other using a transmit/receive surface coil. Both methodologies gave similar results, with the average sodium concentration of normal human patellar cartilage ranging from approximately 240 to 260 mM. This corresponds to FCDs of -158 mM to -182 mM.

Quantifying Sodium in the Human Wrist in Vivo by Using MR Imaging

Abstract: The authors quantified sodium content in the wrist joints of six healthy volunteers with no known history of arthritis or pain. Average sodium concentrations ranged from 115 to 150 mmol/L in noncartilaginous regions and from 200 to 210 mmol/L in cartilaginous regions. The feasibility of quantifying sodium in vivo was demonstrated. This method has potential applications in monitoring the integrity of cartilaginous tissue in vivo.

Fast MRI of RF heating via phase difference mapping.

Abstract: The growth of high-field MRI has brought attention to the fact that many MRI procedures that are routinely practiced at clinical field strengths may not be allowable at higher field strengths, due to an excessive specific absorption rate (SAR) for radiofrequency (RF) power (1). These SAR regulations are in place to prevent excessive heating of tissues during the MRI examination, caused by the interaction of the sample with electric fields (2). This has challenged the MR community to develop methods to analyze and measure power deposition in subjects and tissue-mimicking phantoms. Numerous investigations into the effects on the tissue MR signal from heating have been conducted, either measuring T1 differences (3–5) and water proton chemical shift differences (5–11), or using physical temperature measurement devices (12–14). Computer simulations of Maxwell’s equations, using various computational methods, show that both B1 inhomogeneity and power absorption by the sample become much more severe at higher field strengths (15–18). Chen and Hoult (19) showed experimentally, for the first time, the heating patterns that can be generated when sending current through an MRI coil. Recently, we described (20) a modification of Chen and Hoult’s experiment, wherein a 3D temperature map was obtained by phase difference mapping of the MR signal. This temperature map measured the heating of a phantom caused by the absorption of RF energy from a transmit/receive surface coil during an MRI experiment. This was accomplished using an agarose phantom doped with 40 mM Na4HTm(DOTP). The 23Na signal in this complex exhibited a −0.5 PPM/K temperature-dependent chemical shift coefficient (TDCSC) (21). This frequency shift generated a phase shift governed by Δωτ, where Δω is the frequency shift and τ is the echo time (TE) in a gradient-echo pulse sequence. Appropriate image processing yielding phase difference maps allowed direct quantitation of the temperature increase. The major drawback of this method was the lengthy acquisition times, on the order of 2 min. Here the use of similar 1H-based methods to investigate the heating effects of the birdcage coil is described. The overall goal was to create a procedure that could acquire images very rapidly and would be useful for accessing clinical MRI coils, most of which are 1H coils. Agarose phantoms doped with paramagnetic cobalt and dysprosium compounds were constructed to measure the heating produced by MRI procedures. These paramagnetic compounds enhance the TDCSC of water, making it more sensitive to temperature changes. This is conceptually different from the method of MR thermometry, in which resonances from the shift reagents themselves are measured (see, for example, Refs. 22–24). Additionally, these compounds also enhance the T1 relaxation in water, making rapid image acquisition possible. Furthermore, the addition of these compounds provided the necessary electrolytes to yield electrical conductivity.

Effect of IL-1β-Induced Macromolecular Depletion on Residual Quadrupolar Interaction in Articular Cartilage

Abstract: Purpose Sodium multiple-quantum filtered (MQF) NMR spectroscopy may potentially be used to measure proteoglycan (PG) depletion in cartilage caused by osteoarthritis (OA). The purpose of this work was to quantify the effect of interleukin-1 (IL-1β)-induced macromolecule depletion on the residual quadrupolar interaction (RQI) of sodium in bovine cartilage plugs.