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Abstract

Purpose/Introduction: For cancer and metabolic disorders the concentration of lactate as a metabolite of the anaerobic glycolysis is of particular relevance. Chemical Exchange Saturation Transfer (CEST) MRI enables the non-invasive detection of intracellular lactate1. We demonstrate dynamic measurements of the lactate level after exercise. Employing 31P spectroscopic imaging, it could additionally be shown that the measured effect cannot be solely explained by a change in pH value. Subjects and Methods: It was shown that the hydroxyl proton of lactate yields a CEST effect at 0.4 ppm relative to water1. Employing the metric2 MTRASYM,Lactate = Z(Dx = -0.4 ppm) - Z(Dx =+0.4 ppm) allows to quantify the lactate effect. Imaging: Z–spectra were obtained by centric–reordered 2D–GRE–CEST MRI implemented on a 7–T whole–body scanner (MAGNETOM; Siemens, Germany) using a 28–ch Rx/1–ch Tx 1H knee coil. For saturation 20 Gaussian–shaped rf pulses (tp = 0.1 s, DC = 50 , B1 = 0.9 lT) were applied. 22 adapted frequency offsets were acquired across the water peak resulting in a total acquisition time of 2 min 3 s. After three baseline measurements an exercise was conducted using an MRcompatible foot pedal until exhaustion (*3 min). Subsequent six additional measurements were performed. Results: The total measurement time of close to 2 min allows a dynamic examination of the lactate effect. Figure 1 shows the asymmetry at 0.4 ppm for all 9 measurements. Highlighted in green are the 3 baseline measurements and in red the 6 measurements after exercise. In the mainly used medial gastrocnemius (MG) a decrease of the effect over time can be observed. Discussion/Conclusion: Measuring the MTRASYM at 0.4 ppm allows the non-invasive investigation of lactate in vivo. We were able to measure an increase of lactate effect after plantar dorsi flexion as well as the decrease with time similar to already published work with creatine3. Since pH and lactate effect showed different behaviors in different muscle areas we could exclude pH as a sole cause of the effect. This could allow to study the anaerobic glycolysis in cancer or metabolic disorders.