Primary traumatic brain injury describes the lesion sustained to the brain parenchyma at the moment of trauma. Contusions are the most common intra-axial injuries. Other primary lesions are: focal brain injuries (lacerations), hemorrhage, diffuse axonal injury (DAI), or penetrating injuries/blast injuries.
Secondary TBI results from processes initiated by the trauma, as for example brain swelling, cerebral hypoxia, raised intracranial pressure, or hypothalamic–pituitary dysfunction among others.
We will focus on DAI, one of the most disabling conditions of primary traumatic brain. DAI occurs as a result of acceleration–deceleration/rotational force, and is typically located in the long white matter fibre tracts.
Cortex and white-matter have different densities and therefore rotate at different speeds during closed head injury, leading to misaligned axons or stretched axons (rarely sheared).
The stretching of axons causes depolarization, metabolic alterations, cellular swelling, cytotoxic edema, and apoptosis.
2. CLINICAL PERSPECTIVE
We should consider DAI when traumatic patients present with a loss of consciousness without or with minimal lesions on CT
3. IMAGING PERSPECTIVE 
DAI typically presents with hemorrhagic and non-hemorrhagic lesions.
-Often without relevant findings
-Tiny hyperdense foci (microhemorrhage) in:
1.Gray matter–white matter, especially frontotemporal lobes.
2.Corpus callosum, especially splenium.
3.Brainstem, especially dorsolateral midbrain and upper pons.
4.Less common: Deep gray matter, basal ganglia and internal/external capsule, tegmentum, fornix, corona radiata and cerebellar peduncles
-Intraventricular hemorrhage correlates with DAI
-MRI is superior to CT in detecting hemorrhagic and non-hemorrhagic DAI lesions.
-Hemorrhagic lesions: SWI is the best tool to detect haemorrhagic DAIs.
-Non-hemorrhagic lesion: FLAIR is the best current tool to detect small and non-haemorrhagic parenchymal lesions, displayed as hyperintense lesions.
-Prognosis: The region demonstrating restricted diffusion (DWI) is assumed to have suffered irreversible injury. Schaefer et al. demonstrated that the volume of altered DWI shows a stronger correlation with clinical outcome and Galsgow coma scale than FLAIR. DTI (Diffusion tensor imaging) analyses water motion in order to evaluate the integrity of white matter tracts. Neuronal disruption can be implied by reductions in fractional anisotropy (FA).
Nuclear medicine findings
-PET may show hypometabolism in cingulate gyrus, lingual gyrus, and cuneus.
4. OUTCOME 
Different degrees of DAI have been shown to be present in 80–90% of patients with traumatic brain injury. Prognosis greatly depends on its extension (Grade I to III).
Grading: Adams and Gennarelli staging
Stage 1 (mild): Frontal and temporal lobe gray-white matter interface lesions
Stage 2: Lesions in lobar white-matter and corpus callosum
Stage 3 (severe): Lesions of dorsolateral midbrain and upper pons
5.TAKE HOME MESSAGE
Most of the lesions are microscopic and nonhemorrhagic.