Validation of CEST MR Imaging of Creatine and Phosphocreatine in Muscle

项目总结 肌酸(Cr)和磷酸肌酸(Pcr)是肌酸激酶反应的两种主要代谢物 在肌肉能量学中的重要作用。然而，现有的检测铬或聚合酶链式反应的方法，如质子和磷 磁共振波谱(1H和31P MRS)，具有低灵敏度、低空间分辨率和 部分体积效应。此外，1HMRS只能测量总铬(TCR=铬+聚合酶链式反应)，不能区分聚合酶链式反应 和铬，因此与测量能量代谢的相关性不大。化学交换饱和转移 (CEST)是一种提高对溶液中代谢物敏感性的新方法，它采用频率选择性 射频脉冲使交换的质子饱和，并检测水信号的后续变化。此外，CEST 来自铬和聚合酶链式反应的信号以与水不同的频率偏移(分别为2ppm和2.7ppm)产生， 它提供了分别成像这两种代谢物的能力。然而，尽管CEST已经表明 承诺并引起了很多关注，但几乎没有人验证CEST数据是否反映了实际情况 代谢物水平，以及没有全面评估定位肌酸和聚合酶链式反应的特异性和敏感性。 活着。首先，对于2ppm的铬的CEST成像(CEST@2ppm)，尽管铬的胍胺质子在2ppm Ppm，蛋白质精氨酸残基有类似的化学位移(≈2ppm)，因此可能会混淆测量 铬的含量。在以前对铬的CEST成像的验证中，只有主要组织代谢物的贡献是 考虑过，但忽略了来自蛋白质的贡献。这可能是因为很难模仿 使用简单模型模体研究蛋白质的精氨酸残基是因为有许多类型的蛋白质 在生物组织中含有不同比例的精氨酸残留物。第二，尝试对CEST进行评估 此前也曾在运动后对肌肉进行体内铬成像。然而，由于 运动后肌肉铬含量快速恢复(2分钟)，难以获得高信噪比和可靠性 用于评价其特异性和敏感性的信号。这两个挑战阻碍了它的验证，并限制了它 申请。拟议的研究将验证CEST磁共振成像铬和聚合酶链式反应在肌肉中的应用 应对这两个挑战。在目标1中，我们将使用透析来去除铬和其他小分子 肌肉组织匀浆样品，以研究蛋白质对肌肉中CEST@2ppm的影响。 以及对含有主要代谢物的模体的测量，组织匀浆的透析 可以对CEST信号的来源进行全面调查。在目标2中，我们将使用 硝酸胍N-甲基转移酶缺乏症(GAMT-/-)小鼠 提供了一种理想的实验模型来改变肌酸和聚合酶链式反应的浓度，以研究其在 活着。通过这两个目标，我们将验证该方法，这将对未来的临床非常重要。 翻译并最终将改进许多肌肉疾病的非侵入性MRI诊断。

PROJECT SUMMARY Creatine (Cr) and phosphocreatine (PCr) are two major metabolites of the creatine kinase reaction that play vital roles in muscle energetics. However, existing methods for detecting Cr or PCr e.g. proton and phosphorus magnetic resonance spectroscopy (1H and 31P MRS), suffer from low sensitivity, low spatial resolution, and partial volume effect. Moreover, 1H MRS can measure only total Cr (tCr=Cr+PCr) and cannot differentiate PCr and Cr, and thus has little relevance for measuring energy metabolism. Chemical exchange saturation transfer (CEST) is a novel approach to increase sensitivity to metabolites in solution, which applies frequency selective RF pulses to saturate exchanging protons and detect subsequent changes in water signals. In addition, CEST signals from Cr and PCr arise at different frequency offsets from water (2 ppm and 2.7 ppm, respectively), which provides the ability to image these two metabolites separately. However, although CEST has shown promise and has attracted a lot of attention, there has been almost no validation that CEST data reflect actual metabolite levels, and no comprehensive evaluation of the specificity and sensitivity for mapping Cr and PCr in vivo. First, for CEST imaging of Cr at 2 ppm (CEST@2ppm), although Cr has guanidine amine protons at 2 ppm, protein arginine residues have similar chemical shifts (≈2 ppm), and thus may confound measurements of Cr. In previous validation of CEST imaging of Cr, only contributions from the major tissue metabolites were considered, but contributions from proteins were ignored. This may be due to that it is difficult to mimic the arginine residues of proteins using simple model phantoms because there are many types of proteins which contain different proportions of arginine residues in biological tissues. Second, attempts to evaluate CEST imaging of Cr in vivo have been also previously performed in muscle following exercise. However, due to the quick recovery of Cr content in muscle (2 mins) following exercise, it is hard to acquire high SNR and reliable signals for evaluating its specificity and sensitivity. These two challenges hinder its validation and limit its applications. The proposed study will validate the use of CEST MR imaging Cr and PCr in muscle through addressing these two challenges. In Aim 1, we will use dialysis to remove Cr and other small molecules from samples of muscle tissue homogenates to investigate the influence of proteins on CEST@2ppm in muscle. Together with measurements on phantoms containing major metabolites, the dialysis of tissue homogenates can provide a comprehensive investigation of the origins of CEST signals. In Aim 2, we will use Guanidinoacetate N-Methyltransferase deficiency (GAMT-/-) mice with a controlled Cr supplementation which provide an ideal experimental model to vary Cr and PCr concentrations to study its specificity and sensitivity in vivo. Through these two Aims, we will validate the method, which will be very important for future clinical translation and will ultimately improve non-invasive MRI diagnoses of many muscular disorders.