With recent advances in magnet technology, ultrahigh field magnets (7T and higher) with bore sizes capable of accommodating the adult human head have become available from a variety of research and clinical vendors. The increased field strength confers the intrinsic advantages of increased SNR and for spectroscopic studies, increased spectral resolution and spectral simplification of J coupled resonances such as glutamate and glutamine. Thus, spectroscopic imaging at 7T should provide an ideal platform for evaluating these compounds. Unfortunately, despite the presence of 7T systems dating from late 1990s, there have been few reports of their use in spectroscopic imaging studies. This limitation is largely due to the intrinsic inefficiencies of generating sufficient B1 strength at high field and the resulting increases more than a factor of 10 in power deposition. Thus, many conventional spectroscopic imaging sequences result in long echo times and can exceed FDA guidelines for tissue heating when applied at 7T. To overcome these limitations we will develop novel methods for spectroscopic imaging at 7T which combine both pulse sequence design, B1 shimming and the development of multi-geometry transceiver arrays. Although contrast enhanced and FLAIR imaging are routinely used for monitoring the response to radiochemotherapy in patients with malignant gliomas, false positives (pseudoprogression) arising from necrosis and inflammation in the absence of tumor progression during the acute and sub-acute periods (first 60 days of treatment) occur in 20-40% of the patients being treated. This severely limits the interpretation of early imaging studies, delaying optimal therapeutic response and decreasing survival times. In addition to the common findings of increased choline, lactate and mobile lipids (which are also found in inflammatory cells), cerebral tumors appear to show elevated concentrations glutamine. This is consistent with the major role which glutamine plays in biosynthetic and anapleurotic activities in proliferating tumor cells. Thus, short TE MRSI measurements of glutamine have the potential to significantly aid the serial evaluation of tumors in response to therapy. Therefore, to evaluate the utility of the methods, we will determine if MRSI measurements of glutamine can resolve progression from pseudoprogression in patients with malignant gliomas being treated with radiochemotherapy.
Although spectroscopic imaging at 7T should provide an ideal platform for measurements of amino acids such as glutamate and glutamine, decreased transmission efficiency, large signal intensity losses and power deposition make use of conventional methods difficult. To overcome these limitations we will develop novel methods for spectroscopic imaging at 7T, which combine both pulse sequence design and new ways of shaping RF fields spatially with multi-element coil arrays, which dramatically decrease power deposition and maintain optimal signal detection. We will use these methods to determine if spectroscopic imaging of glutamine can aid in the monitoring the response of malignant gliomas to radiochemotherapy.
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|Hetherington, Hoby P; Avdievich, Nikolai I; Kuznetsov, Andrey M et al. (2010) RF shimming for spectroscopic localization in the human brain at 7 T. Magn Reson Med 63:9-19|