Computed Tomography

In general, computed tomography (CT) is the initial imaging modality used to evaluate patients with known or suspected head trauma to detect potentially life-threatening conditions that may require immediate surgical intervention. CT is widely available, easy to obtain, and can be performed quickly. It can be used with all manner of life-support and monitoring equipment. The advantages of multidetector CT scanners are a shorter scanning time and the ability to rapidly provide multiplanar and three-dimensional (3D) images. These advances have increased the sensitivity of CT in detecting the sequelae of head trauma, particularly skull base and temporal bone fractures. CT angiography (CTA) is useful in the evaluation of traumatic vascular injury and has the relative advantage of speed and fewer flow-related artifacts compared with magnetic resonance angiography (MRA).

Limitations of CT include low contrast in the immature brain, which decreases its sensitivity in detecting edema in infants, and beam hardening artifact, which may partially obscure small extraaxial collection or subtle cortical contusions adjacent to bone, particularly in the posterior fossa and skull base. Although advances in CT scanners have improved their sensitivity in detecting TBI, CT is known to underestimate the degree and extent of traumatic parenchymal injuries when compared with MR imaging (MRI). Nonetheless, acute imaging findings on CT have been proposed as criteria for grading and predicting outcome in persons with TBI (the Marshall Classification and subsequent modifications by Maas et al.).¹

CT has been used liberally in children with scalp contusions. Recently, the exposure of children’s brains to excessive radiation has become a concern after Pearce et al.² demonstrated that children exposed to radiation to the head from CT scans have an increased risk of the development of brain tumors many years later. For patients with a minor head injury (Glasgow Coma Scale [GCS] score of 13 to 15), clinical guidelines are available regarding indications for obtaining a CT scan of the head, such as the New Orleans Criteria, the Canadian CT Head Rule, and guidelines from the Pediatric Emergency Care Applied Research Network, with high sensitivity for detecting injuries that require neurosurgical intervention.³ The American College of Radiology also publishes Appropriateness Criteria for imaging in the setting of head trauma, including a special section for children younger than 2 years in whom clinical assessment and criteria are less reliable than for older children.⁴

Adaptive statistical iterative reconstruction is a reconstruction technique that reduces image noise and improves low-contrast detectability and image quality. It provides higher diagnostic performance at a lower dose with no loss of image sharpness, noise, and artifacts.⁵

Magnetic Resonance Imaging

Because of its more limited availability and longer scanning times, MRI generally is a secondary modality in the evaluation of acute head trauma. MRI usually is performed in the subacute and chronic phases when the findings on CT do not correlate with the patient’s clinical condition, or when patients have unexpected neurologic deterioration or are not responding as expected. MRI offers the advantages of direct multiplanar imaging and greater overall sensitivity and specificity for the detection of TBI. It is particularly useful in the detection of small extraaxial hematomas, nonhemorrhagic intraaxial contusions, edema, brainstem injury, posterior fossa abnormalities, and DAI.⁶ Contraindications to MRI include incompatible vascular clips, metallic implants, ocular foreign bodies, and most pacemakers.

Specific MRI sequences are invaluable in the evaluation of TBI. Fluid attenuation inversion recovery sequence (FLAIR) uses an inversion recovery pulse with a long inversion time that nulls signal from cerebrospinal fluid (CSF). FLAIR is sensitive for delineation of foci of signal abnormality adjacent to the ventricles and subarachnoid space.

T2* gradient-echo sequence (GRE) uses a pair of bipolar gradient pulses instead of a refocusing 180-degree pulse to enhance magnetic susceptibility caused by magnetic field distortion. Susceptibility-weighted imaging (SWI) is a technique that uses the signal loss from out-of-phase protons with different magnetic susceptibilities. SWI is more sensitive than GRE for detection of blood products such as deoxyhemoglobin, methemoglobin, and hemosiderin.⁷

Diffusion-weighted imaging (DWI) assesses the diffusion of water protons with use of a bipolar gradient pulse. The normal diffusion of water protons along the gradient reduces the MR signal. Areas of restricted diffusion will retain the MR signal and represent acute cerebral injury in persons with ischemia/hypoxia, trauma, metabolic disorders, and infection. Reduced diffusion is present in lesions with a low nucleus-cytoplasm ratio. DWI is susceptible to the intrinsic T2* signal of the tissue because it is a gradient sequence, known as the T2 shine-through effect. To separate the DWI signal from T2 shine through, the apparent water diffusion coefficient (ADC) is calculated by acquiring images with different gradient duration and amplitude (b-values), thus eliminating the T1 and T2* values.⁸ Diffusion tensor imaging (DTI) assesses the anisotropy of the brain tissue by evaluating differences in the direction of diffusion of water molecules in normal and abnormal tissues and provides information about the orientation and integrity of the white matter tracts.

MRA and MR venography, which usually are performed without intravenous contrast material using the time-of-flight technique, are useful if a vascular injury is suspected.

MR spectroscopy (MRS) assesses the distribution and quantification of naturally occurring molecules within the central nervous system (see Chapter 25). Various techniques are available. The most commonly used technique is the point-resolved spectroscopy sequence. Both short time to echo (TE) and long TE acquisitions can be obtained. All acquisitions provide sensitive, noninvasive analysis of brain metabolites and cellular biochemical changes. Decreased N-acetyl aspartate (NAA) has been reported to correlate with abnormal neuropsychological function tests in persons with TBI.⁹

Perfusion MRI of the brain can be assessed either through a dynamic T2/T2* acquisition during a bolus injection of intravenous contrast material or with unenhanced techniques such as arterial spin labeling and blood–oxygen level dependent sequences (see Chapters 27 and 28). Perfusion MRI provides the cerebral blood volume, cerebral blood flow, and mean transit time in targeted areas of the brain.

Magnetization transfer imaging (MTI) relies on the principle that protons bound in macromolecules of tissues exhibit T1 relaxation coupling with protons in the aqueous phase or water. When an off-resonance saturation pulse is applied, it selectively saturates the protons that are bound in macromolecules. These protons subsequently exchange longitudinal magnetization with free water protons, leading to a reduction in the detected signal intensity. The magnetization transfer ratio (MTR) may provide a quantitative index of the structural integrity of tissue. Animal models of rotational acceleration suggest that quantitative MTI offers increased sensitivity to detection of histopathologically proven damage, such as axonal swelling, compared with conventional MRI. Although the precise mechanism for MTR reduction is incompletely understood for mild head trauma, the extent of the abnormality usually increases as the MTR value decreases. Abnormal MTR also has been found in normal-appearing white matter on MRI and is an apparent predictor of poor outcome.¹⁰ Functional MRI has shown changes in regional brain activation in patients with TBI and has been used for assessment of clinical outcome of these patients.

Other Imaging Modalities

Magnetoencephalography (MEG) detects the magnetic waves that are created by the electric current along the axons. The advantage of MEG over electroencephalography is that the magnetic waves are less susceptible to distortion caused by the skull than the electric currents (see Chapter 28). This method of imaging has found that brain dysfunction is present in a significantly greater number of patients with minor head trauma and postconcussive syndrome than is shown by MRI or electroencephalography. This method shows excessive abnormal low-frequency magnetic activity, which provides objective evidence of brain injury in these patients that correlates with the degree of symptomatic recovery.

Single-photon emission CT can detect abnormalities in cerebral blood flow; however, alterations in cerebral blood flow are not always associated with traumatic lesions on imaging. A negative initial single-photon emission CT scan after trauma seems to be a strong predictor of a favorable clinical outcome. A worse prognosis is associated with large lesions, multiple lesions, and lesions in the brainstem, temporal lobes, parietal lobes, or basal ganglia.

Positron emission tomography (PET) measures the cerebral metabolism of various substrates (see Chapter 25). Fluorodeoxyglucose is the primary substrate used in the measurement of glucose metabolism, which should correspond to neuronal viability. PET can be used in patients with DAI to determine the extent of damage and prognosis. PET also has been helpful in delineating the extent of reversibility of lesions. The major limitation of PET is the inability to distinguish functional abnormalities from structural damage.

Scalp Hematoma

A scalp contusion often results in subcutaneous edema. If the contusion is of sufficient severity to cause a hemorrhage, a hematoma is formed under the skin. Scalp hematomas usually resolve without complication, with the exception of subgaleal hematomas in infants, which pose the risk of life-threatening hypovolemia. The size and location of the contusion and the patient’s age are important factors when evaluating a patient with a scalp contusion, because large scalp hematomas (particularly involving the parietal or temporal regions in infants younger than 12 months) are associated with an increased risk of skull fracture and intracranial injury.

Skull Fractures

The likelihood of intracranial injury increases significantly when a skull fracture is present in a child. However, the absence of a skull fracture does not preclude intracranial injury and has little prognostic significance in pediatric head trauma. The skull of a younger child is thinner and more pliable than the adult skull. Therefore children have a higher incidence of both skull fractures and traumatic intracranial injury without the presence of a fracture, particularly between the ages of 6 months to 2 years. The parietal and occipital bones are most commonly involved, followed by the frontal and temporal bones. Falls and motor vehicle accidents account for most skull fractures in children.

Fractures in infants may be linear, depressed, diastatic, compound (stellate or “egg shell”), “ping pong” (buckled), or penetrating. In this age group, fractures are usually the result of a fall from the arms of a caregiver, or they result from an object striking the head. Fractures tend to pass through points of weakness and course toward a suture, synchondrosis, foramen, or canal. As the calvarium becomes more mature, comminuted fractures may occur.

Linear calvarial fractures are more frequent in all age groups, followed by depressed or comminuted calvarial fractures and basilar fractures. Linear fractures usually heal without complication. Complex fractures in which the dura mater is torn may be complicated by herniation of the pia and arachnoid layers, creating a leptomeningeal cyst. The CSF pulsations lead to progressive erosion of skull around the fracture, known as a “growing fracture,” which appears as an angular or linear lytic lesion in the skull with scalloped margins (see Chapter 23). Underlying brain contusion is not uncommon.

Depressed fractures are comminuted skull fractures in which broken bones are displaced inward by at least the thickness of the skull. Depressed skull fractures usually result from blunt force trauma to the head and may result in increased intracranial pressure (ICP), epidural hematoma, and parenchymal contusion or laceration. Depressed fractures usually are treated surgically to prevent complications and also for cosmetic reasons. Fractures displacing the dural sinuses often are treated conservatively because of the potential risk of fatal hemorrhage with intervention (e-Fig. 39-2). Depressed fractures are categorized as compound fractures when a laceration of the overlying scalp is present and as penetrating fractures when an underlying dural tear is present, allowing potential communication between the external environment and the brain. Fractures that communicate with the paranasal sinuses, middle ear, or the mastoid air cells within the intracranial compartment are considered compound fractures. Pneumocephalus indicates the presence of a compound fracture (e-Fig. 39-3). The most serious complications of compound skull fractures are CSF leak and infection. Increased risk factors for infection include visible contamination (hair, skin, fat, or bone), a meningeal tear, and delayed treatment for more than 8 hours after the initial injury. A rare type of compound skull fracture is the elevated skull fracture, which occurs when the fracture is elevated outward above the outer table of the skull.

Diastatic skull fractures involve the cranial sutures. Although diastatic fractures usually involve the lambdoid suture in newborns and infants, they may involve any suture in any age group. The normal suture becomes widened, measuring more than 2 mm. Asymmetric sutures, even if they are less than 2 mm in width, should raise suspicion for a diastatic fracture. An overriding suture is indicative of a diastatic fracture.

Fractures involving the skull base are associated with increased risk of vascular or cranial nerve injury. Basilar fractures most commonly involve the temporal bone, causing bleeding into the middle ear and mastoid air cells. Temporal bone fractures are classified according to their orientation to the long axis of the temporal bone as longitudinal, transverse, or complex. This classification is helpful in determining the risk of complications. Longitudinal fractures represent the majority (70% to 90%) and may be associated with facial nerve injury, incudostapedial dissociation, and pneumocephalus. Transverse temporal bone fractures are less common but have a higher risk of permanent injury to the facial nerve and vestibulocochlear nerve, as well as cochlear disruption, causing facial paralysis, sensorineural hearing loss, and perilymph fistula, respectively. Temporal bone fractures involving the petrous apex or the carotid canal are associated with increased risk of carotid dissection, occlusion, pseudoaneurysm, and/or jugular vein injury.

Studies report a sensitivity of 94% to 99% for the detection of linear or depressed skull fractures with routine radiographis of the calvarium. These radiographs usually are obtained in suspected cases NAT as part of a skeletal survey to document additional areas of injury. Standard CT has a lower overall sensitivity for the detection of linear fractures parallel to the plane of imaging. Images and 3D CT reconstructions often are diagnostic in difficult cases. CT has a relatively high degree of sensitivity for the detection of depressed and basilar skull fractures and fractures of the facial bones, sinuses, and orbits. In children, skull fractures typically heal in 6 to 8 weeks but may remain radiographically apparent for 1 year.

Extraaxial and Intraventricular Hemorrhage