Brain injury is often defined differently in published reports. Although many authors use the term brain injury to mean acute traumatic damage to the central nervous system (CNS), others use the term head injury, which allows inclusion of skull injuries, fractures, or soft tissue damage to the face or head without any obvious neurologic consequences. (See the diagram below.)
Kraus et al define brain injury as "physician-diagnosed physical damage from acute mechanical energy exchange resulting in concussion, hemorrhage, contusion, or laceration of the brain."  Brain contusions commonly are identified in patients with traumatic brain injury (TBI) and represent regions of primary neuronal and vascular injury.  Contusions are formed in 2 ways: direct trauma and acceleration/deceleration injury.
Images of brain contusion are provided below.
See Can't-Miss Findings on Noncontrast Head CT, a Critical Images slideshow, to identify several different abnormalities depicted in noncontrast CT studies.
Surgical resection of the contused brain tissue is indicated when the patient has brain swelling that increases the intracranial pressure above an acceptable degree. Alahamadi et al performed a study to identify the factors that predict radiologic and clinically significant progression of brain contusions in patients who did not originally require surgery and underwent conservative treatment. Of 98 patients studied, 44 had significant progression on computed tomography (CT) scans and 19 required surgery.  In a study of patients with severe head injury due to blunt trauma, 50.8% of patients survived their injury and 13.2% achieved a good functional outcome at 6 months of follow-up. 
CT scanning is the preferred acute imaging modality, because scans can be performed quickly; newer CT scanners can complete a scan within 5 minutes, with virtually no motion artifacts. CT scan findings help identify abnormalities that may need acute intervention. CT scanning can be performed in the presence of life support equipment. [5, 6, 7, 8, 9, 10] With CT scans, the true volume of neuronal damage in the contused tissue can be underestimated. The detection of superficial contusions using CT scans is hampered by artifacts from adjacent bone. [11, 12, 13]
Magnetic resonance imaging (MRI) is more sensitive and accurate than CT for detecting contusions because of its multiplanar capability and greater sensitivity for edema.  Imaging findings in brain contusions tend to vary because of the stages of evolution common to these lesions. Initially, CT findings can be normal or minimally abnormal because the partial volumes between the dense microhemorrhages and the hypodense edema can render contusions isoattenuating relative to the surrounding brain. [15, 16]
MRI findings typically demonstrate the lesions from the onset of injury, but many facilities cannot perform MRI on an emergent basis. In addition, MRI examination can take up to an hour to perform, and patients may require sedation to minimize motion artifacts. Not all hospitals have MRI-compatible life-support devices, and the patient's body habitus must be physically compatible with the size of the machine.
Skull radiographs are notoriously unhelpful in predicting underlying brain injury. However, scalp hematomas or skull fractures are usually good indicators of a significant direct force to a focal region. As such, the radiographic findings are usually associated with underlying brain contusions, although significant brain injury may occur without these findings.
Degree of confidence
The rate of false-negative findings in skull radiographs is high, but few false-positive findings occur.
Contusions may progress with time. Imaging findings in brain contusions tend to vary because of the stages of evolution common to these lesions.
Acute CT scans initially demonstrate isoattenuating contusions that become more evident on follow-up CT scanning. CT scans, as shown below, often demonstrate progression over time in the size and number of contusions and the amount of hemorrhage in the contusions. Initially, CT scan findings can be normal or minimally abnormal because the partial volumes between the dense microhemorrhages and the hypodense edema can render contusions isoattenuating relative to the surrounding brain tissue. [11, 12, 13]
Gliding contusions (examples of which appear in the image below) are due to sagittal angular acceleration with stretching and tearing of the parasagittal veins. Gliding contusions are often hemorrhagic, not only from the differential motion of subcortical structures (commonly referred to as shear injury), but also from tearing of parasagittal veins. When the brain abruptly shifts at the time of impact, the subcortical tissues glide more than the cortex. The convexities of each hemisphere are anchored to the dura by arachnoid granulations. Gliding contusions also tend to be bilateral.
The image below shows a CT scan compared with a xenon blood-flow image. On CT scan, the contusions are seen in the bifrontal regions as hyperintense areas. The corresponding xenon blood-flow image shows dark regions that indicate decreased perfusion in the contused areas of the brain.
The image below shows CT and MRI scans of acute contusions.
Degree of confidence
CT scanning is an excellent modality for defining contusions. Contusions often are not appreciated on the first CT scan obtained immediately after trauma, but they become obvious on follow-up scans.
Initially, the false-negative rate is high, but false-positive findings are negligible.
Contusions often are not appreciated on the first CT scan obtained immediately after trauma, but they become obvious on follow-up scans.
Magnetic Resonance Imaging
MRI scans typically demonstrate brain contusions from the onset of injury. MRI is sensitive to hyperacute hemorrhagic contusions (< 12 h).
On MRI, contusions are isointense to hyperintense on T1-weighted and hyperintense on T2-weighted images, as shown in the images below. Gradient-echo MRIs may reveal hypointensity, which is critical to the detection and delineation of contusions.
Use of fast fluid-attenuated inversion recovery (FLAIR) sequences has made detection of accompanying subarachnoid hemorrhage possible, with a sensitivity that is equal to or greater than that of CT. Using FLAIR sequences, subarachnoid hemorrhage produces dramatic hyperintensity in the normally hypointense cerebrospinal fluid. Additionally, companion FLAIR images show the extent of contusions better than most traditional MRIs.
Use of diffusion-weighted imaging (DWI) in acute brain trauma has not been described in depth in the literature. DWI allows the rapid detection of an ischemic region after the onset of brain injury. The signal intensity is increased in the affected region on DWIs. DWIs show areas of restricted diffusion in areas associated with cerebral contusions.
Akiyami et al reported that susceptibility-weighted MRI can be very sensitive in visualizing and detecting microhemorrhages. In their study, they found that susceptibility-weighted MRI detected a mean of 76 ± 52 (total, 1132) hypointense spotty lesions and that T2-weighted images detected a mean of 21 ± 19 (total, 316) lesions. 
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see Medscape Reference topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography (MRA) scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with difficulty moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
Degree of confidence
The degree of confidence is excellent with FLAIR imaging. MRI is sensitive but nonspecific for brain injuries.
MRI is the criterion standard for defining contusions. Ischemic lesions often depicted in the normal aging brain may be difficult to distinguish from traumatic injuries in the acute phase.
Currently, functional imaging is not widely used in TBI assessment (Ref 28).
Umile et al note that single-photon emission computed tomography (SPECT) blood-flow imaging with technetium-99m (99m Tc) hexamethylpropyleneamine oxime (HMPAO) uptake is sensitive enough to detect diffuse changes in patients with decreased blood flow due to depression. Many studies of mild, moderate, and severe TBI in the acute, subacute, and chronic stages have shown that SPECT is more sensitive than CT and MRI, and scans can depict changes even when findings are normal.  Comparisons consistently show that in mild cases of TBI, SPECT scans identify more brain abnormalities than conventional CT scanning or MRI. SPECT findings are particularly sensitive in patients with mild postconcussive symptoms. SPECT scans can depict focal changes in 53% of patients with mild head injury who showed few abnormal findings on MRI and CT scans. 
SPECT results are correlated with the severity of injury. A negative finding on SPECT in the first 4 weeks is predictive of a good outcome. SPECT findings also can help in predicting a poor outcome, posttraumatic headaches, and clinical deterioration in patients with intracerebral hematomas. Studies correlating SPECT with neuropsychologic testing have been inconsistent. One study reported that SPECT can reveal significant increases in blood flow following cognitive rehabilitation therapy 2 years after TBI; these findings were correlated with improvements on neuropsychological tests. 
It is unclear whether positron emission tomography (PET) scanning is more useful than MRI or CT scanning. Currently, there are no accepted clinical guidelines for TBI assessment using PET. PET images are more anatomically detailed than SPECT images. Limitations to the use of PET scanning are the time required to complete the images, the cost, and the lack of widespread availability. Scans require from 2 to 40 minutes to complete. PET scans may cost approximately $2,000 per examination. Also, not all facilities have access to PET scanners. [21, 22]
Xenon is an inert noble gas that is diffusible across the blood brain barrier. The inhalation of nonradioactive xenon can be used to calculate physiologic aspects of brain function, including regional cerebral blood flow, by using CT scanning. Xenon imaging has been used for the documentation of brain death. Brain trauma is a potential application for xenon CT scanning.
At the current time, xenon blood-flow imaging has not achieved widespread use because of a number of factors. Xenon gas is expensive; without a rebreathing apparatus, approximately 10 L of xenon is required for each study, adding additional costs to each examination. Also, anesthetic equipment must be used, and special software must be purchased for the CT scanner. The anesthetic effects of xenon are somewhat worrisome as well. Multiple, rapid-sequence images must be obtained at the same level, limiting the total number of section levels that can be obtained. This factor may limit the scan to a few regions, when a given patient may have multiple pathologic areas.
Degree of confidence
SPECT blood flow imaging has excellent sensitivity.
Caution must be taken not to mistakenly attribute the diffuse changes seen in psychiatric disorders with focal TBI changes.
Focal vascular spasm may be seen in the acute/subacute injury phase.
Traumatic aneurysms are rare, but may occur as a result of either blunt or penetrating brain trauma. Intracranial aneurysms may result from fracture, with laceration of the adjacent artery by bone spicules, or by shearing forces secondary to rapid deceleration injury. Penetrating wounds, usually from bullets or shrapnel, are another etiology. Most traumatic aneurysms are pseudoaneurysms, meaning that they are hematoma cavities contained by the soft tissues adjacent to an arterial laceration. As the hematoma organizes and resolves, it becomes surrounded by a fibrous pseudocapsule, which is highly prone to rupture.
Cerebral aneurysms are potentially catastrophic lesions and many persons who do survive aneurysms rupture are permanently disabled. Treatment of unruptured aneurysms has low morbidity and mortality, which makes accurate and timely diagnosis important.
Conventional angiography remains the definitive procedure for the preoperative evaluation of aneurysms. However, the cross-sectional imaging modalities, including computed tomography angiography and MRA, add indispensable information concerning the size of the lesion, location, presence of thrombus, associated hemorrhage, and the condition of surrounding brain tissue.
Spasm is a good indicator of subarachnoid hemorrhage and underlying brain injury. Few false-positive findings occur, but angiography may not reveal the true extent of injury.