Validation of CEST MR Imaging of Creatine and Phosphocreatine in Muscle

项目摘要 肌酸（Cr）和磷酸肌酸（PCr）是肌酸激酶反应的两种主要代谢产物， 在肌肉能量学中的重要作用。然而，用于检测Cr或PCR（例如质子和磷）的现有方法 磁共振波谱（1H和31 P MRS）具有低灵敏度、低空间分辨率， 部分体积效应此外，1HMRS只能测量总Cr（tCr=Cr+PCr），不能区分PCr 和Cr，因此对于测量能量代谢几乎没有相关性。化学交换饱和转移 （CEST）是一种增加对溶液中代谢物的灵敏度的新方法，其应用频率选择性 RF脉冲使交换质子饱和并检测水信号的后续变化。此外，CEST 来自Cr和PCr的信号出现在与水不同的频率偏移处（分别为2 ppm和2.7ppm）， 其提供了分别对这两种代谢物成像的能力。然而，尽管CEST显示， 承诺，并吸引了很多关注，几乎没有验证CEST数据反映了实际的 代谢物水平，并且没有全面评价Cr和PCr映射的特异性和敏感性， vivo.首先，对于2 ppm处的Cr的CEST成像（CEST@2ppm），尽管Cr在2 ppm处具有胍胺质子， ppm，蛋白质精氨酸残基具有相似的化学位移（<102 ppm），因此可能混淆测量 的Cr。在之前的Cr CEST成像验证中，仅对主要组织代谢物的贡献进行了分析。 考虑，但蛋白质的贡献被忽略。这可能是由于很难模仿 蛋白质的精氨酸残基使用简单的模型模型，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基。第二，尝试评价CEST 先前也在运动后肌肉中进行了体内Cr成像。但由于 运动后肌肉中Cr含量恢复快（<12 min），难以获得高信噪比， 用于评估其特异性和灵敏度的信号。这两个挑战阻碍了它的有效性，并限制了它的 应用.拟议的研究将通过以下方式验证CEST MR成像Cr和PCr在肌肉中的使用： 应对这两个挑战。在目标1中，我们将使用透析来去除Cr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。 连同对含有主要代谢物的体素的测量，组织匀浆的透析 可以提供CEST信号起源的全面调查。在目标2中，我们将使用 胍基乙酸N-甲基转移酶缺乏症（GAMT-/-）小鼠， 提供了一个理想的实验模型，以改变Cr和PCr浓度，以研究其特异性和敏感性， vivo.通过这两个方面的研究，对该方法进行了验证，对今后的临床应用具有重要意义 翻译，并将最终改善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.