May 16, 2012Qs 1.Diffusion tensor imaging (DTI) in muscle fiber trackingBackground of Diffusion tensor imaging (DTI)The arrangement of fibers in a muscle is in such a way that if affects the force produced and shortens the values of velocity. This is more evident in pinnate muscles. In the mentioned muscles fibers are inserted at an oblique angle to the aponeurosis. This arrangement is what causes an increase in sarcomeres that lie parallel and consequently the force production. This increase in production of force causes the velocity to shorten as a shortening component is produced orthogonally to the muscle axis (Heemskerk et.
al, 2006, p. 21). For quite a good number of years there was only pennation measurement technique which was used for measuring cadaver specimens. However, this method had so many errors due to fixation artifacts. As a result ultrasound was introduced for measuring pennation angles. Again, this method was challenged by some of the pennation angles of some muscles are heterogeneous. Ultrasound was also found to use 2D imaging. It was due to these inefficiencies that a more accurate method that would provide 3D fiber reconstruction was found. Diffusion tensor imaging technique (DTI)Diffusion tensor imaging (DTI) is based on the association between water diffusion direction, cellular geometry of tissues, white matter of the CNS and the cardiac muscles.
Trajectories of local fiber can be reconstructed by measuring diffusion in six non collinear directions. A tensor model can be used to describe this type of diffusion. Tensor has three Eigen values. These values describe the diffusion coefficient magnitude in three directions that are all orthogonal. The coefficient gradient is also defined in three eigenvectors and these specify the directions (Heemskerk et al, 2005, p.
1336). Fiber trajectories are reconstructed by following the greatest diffusion route and summing up points at regular intervals. In addition the eigenvector that is correspondent to the largest eigenvalue is considered to be a coincident to the longitudinal axis. Measurements of angle pennation that are obtained using DTI fiber tracking are in agreement to those done using direct anatomical inspection. Any change in these measurements is in order as they change with the change in foot angle. DTI is also feasible in human muscle.
It can also be used in studies of muscle micro architecture and muscle injury. The use of DTI helps us understand the structure of muscles and their function in relation to age and health. It is also useful in studying the heterogeneous patterns of muscle architecture and its impact on muscle mechanics. Nevertheless, a number of practical complications are encountered in these studies. Some of the complications include a short transverse relaxation time (T2) in the muscles. This causes a rapid delay in signal that consequently lowers signal to noise ratio.
This short transverse relaxation time limits the amount of diffusion weighing that would be obtained in echo time (Heemskerk et. al, 2006 p. 279). It also limits the radio frequency coil length and the size of static magnetic field. In a simpler way, DTI has the capability of collecting information concerning diffusion weighed images. It then puts together all the information concerning where water has the possibility of diffusing. Diffusion tensor imaging uses an ellipsoid to show where water can diffuse. A long thin ellipsoid is a representation of good diffusion in the axis of the ellipsoid.
A sphere indicates that water has diffused uniformly in all directions. A tensor is a description of an ellipsoid for every voxel (Heemskerk et al, 2005, p. 1336).