Diffusion Tensor Brain MRI in the Detection of Structural Abnormality of the White Substance in Concussion

Overview

Concussions in sports are a major public health problem because of their frequency, and are often underdiagnosed because of an unspecific clinical picture or sometimes masked by the concussion itself. Support data has been constantly evolving in recent years, including the last publication of the Berlin Consensus in 2016 specifying support in the field. However, to date, there is no tool to predict the severity of a concussion or to predict when it will return to play objectively and reliably. Brain MRI done after the head injury is most often normal. However, previous studies agree that there is a persistent electrophysiologic disturbance several weeks after the injury, and the specific pathophysiology of white matter changes after a head injury remains controversial. Diffusion tensor imaging (DTI), in addition to morphological sequences, is capable of assessing white matter microstructure and fibrous tract integrity or not. Several parameters, such as the seemingly normal white matter fractional anisotropy (FA) coefficient, the mean diffusivity and the radial diffusivity, may be altered in the aftermath of a concussion, indicating axonal damage not visible on conventional MRI sequences. Previous studies have evaluated these parameters with sometimes contradictory results: some have found an increase in AF in specific regions such as the cortico-spinal tract and the corpus callosum, others have found a decrease in AF. So far, assessment of a player's condition on and off the field after a head injury has been based on clinical criteria alone, sometimes far too subjective. The player may choose to mask their symptoms to allow them to return to the game faster, or feign more than they have. Once pathologies such as bone fractures or intracranial hematomas are ruled out by conventional imaging, there is no longer any tool for a more accurate diagnosis of possible microstructural alterations of brain tissue and for monitoring of the patient. The advent of new MRI techniques such as diffusion imaging, and particularly diffusion tensor imaging (DTI), is a promising tool to better understand white matter involvement in diffuse axonal lesions.

Diffusion imaging is based on analyzing the movements of water molecules in a tissue. The parameter measuring the intensity of this diffusion is the apparent coefficient of diffusion (ADC) which varies in particular as a function of the cell density. Diffusion imaging is therefore a modality that reflects tissue density, and in this case is capable of revealing pathological processes in white matter. Diffusion tensor imaging (DTI), a newer and complementary modality, allows estimation of the preferred directions of diffusion of water molecules in a tissue. Compared to "single" scattering imaging that uses scattering gradients applied in 3 directions, DTI uses these same gradients applied in at least 6 directions (20 for our study). In white matter, diffusion is constrained by the axon cell membrane and the myelin sheath, and preferably follows the fiber bundles tangentially. Diffusion tensor modeling allows the determination within a voxel of the principal direction (maximum diffusivity) of proton motion, which corresponds to the orientation of the white matter fibers. Thus, DTI allows the study of tissue microstructural organization. This degree of organization of SB is measurable through diffusion tensor parameters, such as the fraction of anisotropy or fractional anisotropy (FA). When the diffusion is constrained in a main direction, it is said to be anisotropic and the FA tends towards 1. On the other hand, when it is carried out indifferently in all directions of space, it is said to be isotropic and the FA then tends towards a zero value. It would appear, therefore, that DTI may be a more sensitive means than diffusion for revealing apparently normal SB abnormalities related to head trauma For patients who have a concussion, doctors in the sports medicine department will offer them MRI after the visit. At this visit, the investigator will inform (orally and in writing) the subject about the study and, after obtaining the subject's consent, note demographic and clinical information, date of concussion, complications, medical history and SCAT 5 (standardized tool for concussions) score. After the inclusion and exclusion criteria have been verified, the participant will be invited within 7 days to perform an MRI. For control subjects, following presentation of the study by one of the investigators, they will be summoned for the baseline visit followed by the first MRI. The 15-minute baseline visit will be conducted by one of the study investigators. The investigator will inform (orally and in writing) the subject about the study and after obtaining their consent will record demographic information, medical history and check inclusion and exclusion criteria. MRI will be done next. After a delay of 1 month ± 5 days, the second MRI will be scheduled for all subjects. The first MRI consists of several sequences (the total acquisition time is less than one hour) (no contrast injection for these tests). The second is shorter and involves the acquisition of the sequence in diffusion voltage (7 minutes) and that of anatomical locating images (4 minutes).