Imaging in Moderate to Severe Traumatic Brain Injury

David N. Alexander

BACKGROUND: IMAGING TECHNIQUES

The two most commonly used imaging procedures in traumatic brain injury (TBI) are computerized tomography (CT) of the brain and magnetic resonance imaging (MRI or MR) of the brain. Less commonly used imaging procedures that are helpful in selected patients with brain injury for specific purposes include lateral and AP (anteroposterior) skull x-rays (SXRs), cerebral angiography, CT angiography (CTA), magnetic resonance angiography (MRA), magnetic resonance venography (MRV), single photon emission computerized tomography (SPECT) and positron emission tomography (PET), diffusion tensor imaging (DTI), and functional MR imaging (fMRI). Each of these techniques has its own principles, benefits, and limitations. They have been employed in some settings and will be briefly described; however, the primary focus of this chapter is CT and MR.

Computerized Tomography

CT measures the density of structures in the brain displayed in two-dimensional slices that vary in thickness from 2 mm to 1 cm. CT is the initial imaging procedure of choice in acute moderate to severe TBI. CT is readily available, with rapid scanning time, and has excellent imaging of acute blood, fractures, foreign bodies, and hemorrhagic contusion.

High-density structures, such as bone and acute blood, appear white on CT, whereas low-density structures, such as air or cerebrospinal fluid (CSF), appear dark. The white matter of the brain is slightly less dense than gray matter; so white matter is darker than gray matter on CT imaging. Edema reduces the density of the brain, making it appear darker. So density, and therefore whiteness on the scan, is as follows: bone > acute blood > gray matter > white matter > CSF > air. One can choose to view a specific range or a subset of the data obtained; referred to as “windowing.” Generally, the CT scan is windowed to visualize brain matter best; a second set of images are windowed to highlight bone densities (“bone windows”) to allow better visualization of fractures.

Magnetic Resonance Imaging

MRI is the best single test for assessing injury to the brain in moderate and severe TBI in the subacute or chronic phase after TBI. Its advantages over CT in the subacute phase include excellent imaging of the posterior fossa, assessment of axonal injury, and visualization of cortical and subcortical nonhemorrhagic contusions and edema. The signal characteristics of hematomas on MRI are highly variable. The appearance is greatly influenced by the state of hemoglobin (ferrous vs. ferric), the field strength of the magnet, the pulse sequence used, the status of the RBCs (intact vs. lysed), the age of the clot, the hematocrit, state of oxygenation, and size of the clot.

MR requires a longer scan time than CT, and is therefore less useful for agitated patients or patients who cannot remain still during the test. MR is not useful for bone imaging or for fractures. MR cannot be used in patients with pacemakers, or a variety of other metallic implants. When there is concern for venous sinus injury, MRV provides excellent imaging of the sinuses. MR is more sensitive than CT in visualizing reversible signal changes that are the result of seizure activity, which can include focal cortical lesions, focal swelling, and sulcal enhancement, and these findings need to be differentiated from structural injury due to TBI.

MR is an imaging procedure without ionizing radiation. The basic principle of MRI is the imaging of proton magnetism. Protons, a component of water, are ubiquitous in the body and brain. The magnetic field lines up the charged protons like little magnets, then the magnetic field is removed or perturbed. Sequences of perturbation and data acquisition determine imaging characteristics:

• T1-weighted images show the general structure of the brain.

• T2-weighted and FLAIR (fluid attenuation inversion recovery) images show white matter changes.

• DWI (diffusion weighted imaging) and derived apparent diffusion coefficient (ADC) maps are sensitive to early swelling of cells in ischemic infarction and disruption of white matter tracts, and may show injured neurons and glia in TBI.

Gradient recalled echo (GRE) and susceptibility weighted imaging (SWI) sequences are very sensitive to paramagnetic compounds, including deoxyhemoglobin, ferritin, and hemosiderin seen in blood products, and show up as dark areas associated with hemorrhage.

Skull X-Rays

Historically, SXR was the initial neuroradiologic procedure for imaging head trauma. Conventional radiographs were used as early as the Spanish-American War to evaluate the cranial vault for depressed fractures and for localizing radiopaque materials. SXRs have been supplanted by CT scan, although a modified lateral SXR is generally obtained as a “scout film” in preparation for the CT.

Angiography, Including CTA and MRA/V

Angiography defines the intra- and extracranial circulation to the brain. Improvements in CT angiography make this noninvasive study the imaging procedure of choice for visualizing the anatomy of the cerebral vasculature, resulting in less need for conventional angiography. CTA is less invasive and intravenous contrast media is used in doses similar to that of a CT brain with contrast. Catheter cerebral angiogram is used uncommonly in TBI, and then generally for treatment of an identified vascular malformation, such as an incidental aneurysm or arteriovenous malformation (AVM). Both CTA and catheter angiography may be contraindicated for patients with allergy to contrast media, or if kidney injury is present. Intravenous sodium bicarbonate prior to and after the contrast injection, combined with N-acetylcysteine, either oral or intravenous are used to reduce the likelihood of contrast-induced nephropathy or worsening nephropathy in those with chronic kidney injury. Time of flight MR angiography (TOF-MRA) is an alternative imaging procedure to study cerebral vasculature, requiring no contrast agent, but it does have less spatial resolution and less sensitivity than CTA or catheter angiography. If cerebral venous thrombosis is suspected, MRV is a useful and sensitive test.

Single Photon Emission Computerized Tomography

SPECT is a nuclear medicine tomographic imaging technique using γ rays and a γ camera. A common γ-emitting isotope used in SPECT, 99mTc-HMPAO (technetium 99m-hexamethylpropylene amine oxime), is useful in the detection of regional cerebral blood flow. SPECT is uncommonly used in TBI and its clinical value is limited.

Positron Emission Tomography

PET utilizes positron-emitting radiopharmaceuticals to map the physiology, biochemistry, and hemodynamics of the brain. 2-Deoxy-2-[18F]fluoro-D-Glucose (FDG) is the most common radiopharmaceutical used in PET to measure regional glucose metabolism in the brain. A variety of other substrates and radiolabeled compounds can be injected and a cross-sectional map of the brain shows the quantitative distribution, utilization, or binding of these substrates in the brain. The radiation exposure from a PET scan is about the same as a CT. The clinical value of PET in TBI is under investigation.

Diffusion Tensor Imaging

DTI measures the diffusion of water molecules and their vectors/direction along white matter tracts, using MR techniques. The most commonly studied result is the fractional anisotropy (FA) value, which reflects the degree to which molecular diffusion is linear. It is measured on a unit-less scale from 0 to 1. CSF, with no directional diffusion is zero, and extreme unidirectional diffusion (e.g., along a axon) is maximal at 1 representing infinite anisotropic diffusion. The FA maps are typically color coded such that fibers traveling left to right or vice versa are red, fibers traveling in AP direction are green, and fiber tracts traveling vertically (superior ↔ inferior) are blue. Disruption of white matter tracts, as is typically seen in the context of traumatic axonal injury (TAI), is more dramatically visualized with DTI. Further research is needed to establish the clinical utility of this test [1] and the technical complexities of use of this emerging technology are described in a recent review in the Journal of Neurotrama [2].

Functional MR Imaging

fMRI measures changes in blood oxygenation levels in specific volumes (voxels) of the brain. The brain rapidly changes its blood flow/oxygen delivery to parts of the brain as they become metabolically more or less active. Both resting state and connectivity patterns can be examined. Recent reports in a small minority of patients with severe brain injury and in a minimally conscious state or vegetative state have shown metabolic patterns that indicate consciousness using fMRI to measure response to questions/commands [3].

PATHOLOGY: IMAGING APPEARANCE OF TBI

The pathologic changes typically associated with moderate to severe closed head injury include traumatic diffuse axonal injury, intra- and extra-axial hemorrhages, focal cortical contusions (FCCs), and hypoxic-ischemic injury (HII). Open or penetrating head injury leads to direct disruption of brain parenchyma. Other phenomena include skull fractures and diffuse brain swelling. The main mechanical phenomena causing brain damage are that of contact and acceleration (Table 21.1).

TABLE 21.1 Summary of TBI Pathology

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