Observation of bi-exponential T 1ρ relaxation of in-vivo rat muscles at 3T

摘要

Background: Spin-lattice relaxation in the rotating frame, or T 1ρ relaxation, is normally described by a monoexponential decay model. However, compartmentation of tissues and microscopic molecular exchange may lead to bi-exponential or multi-exponential T 1ρ relaxation behavior in certain tissues under the application of spin lock pulse field strength. Purpose: To investigate the presence of bi-exponential T 1ρ relaxations in in-vivo rat head tissues of brain and muscle. Material and Methods: Five Sprague-Dawley rats underwent T 1ρ imaging at 3T. A B 1-insensitive rotary echo spin lock pulse was used for T 1ρ preparation with a spin lock frequency of 500Hz. Twenty-five T 1ρ-weighted images with spin lock times ranging from 1 to 60 ms were acquired using a 3D spoiled gradient echo sequence. Image intensities over different spin lock times were fitted using mono-exponential as well as biexponential models both on region-of-interest basis and pixel-by-pixel basis. F-test with a significance level P value of 0.01 was used to evaluate whether bi-exponential model gave a better fitting than monoexponential model. Results: In rat brains, only mono-exponential but no apparent bi-exponential T 1ρ relaxation (～70-71 ms) was found. In contrast, bi-exponential T 1ρ relaxation was observed in muscles of all five rats (P 10 -4). A longer and a shorter T 1ρ relaxation component were extracted to be ～37- ～ 41 ms (a fraction of ～80- ～88%) and ～9- ～11 ms (～12-20%), compared to the normal single T 1ρ relaxation of ～30- ～ 33 ms. Conclusion: Bi-exponential relaxation components were detected in rat muscles. The long and the short T 1ρ relaxation component are thought to correspond to the restricted intracellular water population and the hydrogen exchange between amine and hydroxyl groups, respectively.