P MRS of human brain tumors at 3 T using 1 H – 31 P polarization transfer

Introduction: It is well known that tumors have an abnormal phospolipid metabolism[1]. This is typically reflected in the higher phosphomonoester and total choline compound levels observed by in vivo P and H MRS respectively, which may be used as biomarkers for diagnosis and treatment evaluation [1,2,3,4]. For practical and sensitivity reasons H MRS has been used most extensively in the clinic to assess higher choline compound levels in brain tumors. However, the choline resonance in H MR spectra of these tumors may be quite variable as the intensity of this signal also depends on other factors such as cell density. From extract and cell studies it appears that in particular the phosphocholine (PC) over glycerophophocholine (GPC) and phosphoethanolamine (PE) over glycerophosphoethanolamine (GPE) ratios are potential biomarkers for tumor presence and malignancy [1,5]. Unfortunately, the resonances for these compounds are not resolved in H MR spectra obtained in vivo. With P MR spectroscopy the options to observe these compounds separately are much better, however this comes with a lower sensitivity. Moreover, the spectral region with these resonances is contaminated with signals of other phosphomonoester and diester substances. To increase sensitivity we have developed a H P polarization method that achieves a high efficiency of polarization transfer for the signals of PE, GPE, PC and GPC by using chemical shift selective refocusing pulses [6]. This P MR method of using refocused insensitive nuclei enhanced by polarization transfer (RINEPT) has the additional advantage of a distinct selection of the resonances of these compounds. Aim: to explore the potential of this new approach in the assessment of human brain tumors at 3T.