One of the most attractive potential uses of magnetic resonance spectroscopy (MRS) is as a noninvasive tool to quantitatively assess metabolite content. Assessment of metabolite content, commonly referred to as absolute quantification, requires accurate determination of the calibration factor between the quantity of excited nuclei within the measurement volume and the amplitude of the corresponding spectral peak in the processed spectrum. Controlling for all of the transformations and parameters that affect this calibration factor incurs a heavy burden. As a result, nearly all MRS results are presented in terms of arbitrary units or as ratios, which are difficult to interpret and of limited utility. We have developed a method that eases the burden of metabolite quantification with respect to a subset of these transformations;those that occur during and following data acquisition. The conceptual approach is to inject a pre-calibrated, stable artificial signal into the data. The artificial signal is transmitted through a second RF coil and acquired by the receive coil in parallel with the real signal arising from the sample. An artificial peak appears in the processed spectrum and can be used as a calibration factor to convert the real peaks into units of metabolite content. The distinguishing feature of this approach is that the pseudo-signal is introduced via inductive coupling. Inductive coupling is also the mechanism by which the magnetic field created by excited nuclei in the sample couples to the main RF coil so any subsequent transformations of the data have an equal effect on the real and pseudo-signals. This makes the calibration factor immune to changes in coil loading conditions, amplifier gain and data processing algorithms. The biggest advantage of this approach is that it removes the cumbersome requirement for coil loading conditions to be replicated in the calibration and experimental samples. Our prototype probe used a surface coil to excite and receive the MR signals. This simple coil was appropriate for demonstrating feasibility in vitro but, like all surface coils, it created a nonuniform B1 field, which made it impractical for in vivo use. Our primary goal in this proposal is to demonstrate in vivo utility of this method. We propose to build two additional probes that create uniform B1 fields to excite and receive the signals. These probes will allow us to validate the method by comparison against biochemical assays of metabolite content in rat hind limb, and to demonstrate the ability to accurately quantify changes in metabolite content in human skeletal muscle during physiological perturbations. This project focuses on MRS measurements in skeletal muscle but the methods will be directly applicable to other organs and could easily be adapted to MR imaging protocols.
We have developed a new method of converting magnetic resonance spectroscopy (MRS) data into units of metabolite concentration. We propose to validate the method in animal studies and demonstrate its value in humans. This is a methods development project and we are not proposing to test any biological hypotheses. However, successful completion of the project could have widespread impact on a variety of diseases by providing researchers and clinicians with a more practical noninvasive tool for measuring metabolite concentration.
