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

Abstract

Background

Spin-lattice relaxation in the rotating frame, or T_1ρ relaxation, is normally described by a mono-exponential 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 B₁-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 bi-exponential 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 mono-exponential 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.