11
brain from two-dimensional data. In addition, there are currently many three-dimensional techniques available for certain applications, although not practical in the acute setting. Imaging of the traumatically injured brain should be fast, reliable, diagnostic, and widely available.
The workhorse of any facility continues to be CT scan-ning. Helical CT is extremely fast. It allows the technician to obtain whole-body images in a single breath hold. This is of paramount importance in patients who have sus-tained polytrauma of other organs in addition to brain injuries.
Many investigators have attempted to develop cri-teria for obtaining a CT scan in the acute setting for the evaluation of traumatic brain injury. The American College of Radiology (ACR) conducted a study in 1986 that ultimately divided neurologically traumatized patients into three groups of risk: low, moderate, and high. The low-risk group consisted of patients with minor symptoms without neurological deficits such as headache, dizziness, or scalp injuries. It was proposed that this group of patients could be observed without the need for imag-ing. The moderate- and high-risk groups included those patients with neurological deficits, altered consciousness, seizures, depressed skull fractures, or penetrating injuries.
It was proposed that these patients should receive a non-contrast CT scan of the brain as part of the initial work-up (3). Although the ACR and others have attempted to define the indications for obtaining an imaging study in minor trauma, currently this is a multifactorial decision made in emergency centers across the country. Some researchers have also found that brain injuries detectable by CT imaging may occur in people with relatively minor complaints, normal neurological exam and/or a normal Glasgow Coma Scale (GCS) of 15 (4). Issues such as cost, risk-to-benefit ratio, and medical/legal concerns play roles in the decision to image patients with mild brain injuries or post-concussive disorders.
Computed Tomography
Sir Godfrey N. Hounsfield of Great Britain and Allan M.
Cormack of the United States won the 1979 Nobel Prize in Medicine for their groundbreaking work that made CT a reality in the 1970s. A conventional CT scanner con-sists of a gantry, table, and computer workstation. Early CT scanners were slow and cumbersome. Only one slice could be acquired at a time. The incorporation of slip-ring technology in the 1990s allowed helical CT units with a continually rotating gantry to become available.
With helical CT, the table and the patient are continu-ally moving through the gantry. Many of these units also employ multiple detector arrays that increase the volume of tissue imaged and improve the speed with which the study is performed. Current multiple detector helical CT scanners produce remarkable images and allow versatile
reconstruction of volume data in multiple planes—axial, sagittal, coronal, and oblique projections. These units are four to six times faster than older helical units. An entire CT of the abdomen and pelvis may be performed in a trauma patient with a single breath hold in less than 30 sec. CT scans of the brain may be performed even faster.
The raw data obtained from CT detectors is mathe-matically manipulated using several algorithms and filters to obtain visual data that offers both spatial and contrast information. The visual contrast between different tissues when viewing a CT image is due to differences in the absorption of x-rays. The properties that govern x-ray absorption on plain films are identical for CT. As x-ray pho-tons pass through air and tissue, the useful beam is subject to attenuation—or removal of photons (5). The x-rays may be attenuated (absorbed), scattered in a new direction, or pass through the tissue of interest.
The two extremes of air and metal have character-istic appearances on CT because no x-rays are absorbed by air while nearly all x-rays are absorbed by metal.
Everything in between these two extremes has a spectrum of CT appearances. Visually, air is black, fat is dark, and soft tissue and fluid are shades of gray. Bone is white and metal is extremely bright—often causing streak artifact on the CT images. The Hounsfield Scale, named for the father of computed tomography, allows one to quantitate the actual tomographic densities of these different tissues.
In fact, CT’s ability to distinguish between subtle differ-ences of similar tissues—or contrast resolution, is the true advantage of CT as an imaging modality. Hounsfield Units (HU) are numerical values that are assigned to each pixel (picture element) in a CT image. The scale is roughly –1000 to +3000. Water or simple fluid like cerebrospinal fluid (CSF) is set as the baseline of zero. Air is black, or –1000, while bright metal or iodinated contrast material is close to +3000. Soft tissue ranges from about –200 to +200. Gray matter of the brain is typically around 30–40 Hounsfield units. When interpreting a CT image of the brain, the density of surrounding structures or abnor-malities is often compared to gray matter. A typical radi-ology report may read, “there is an intra-axial mass that is isodense (the same density) to gray matter.”
Another advantage of CT is the ability to manipu-late the raw data to augment certain visual characteris-tics for the person interpreting the image. Bone windows, brain windows, soft tissue windows, lung windows, and so on, are simply different ways of viewing the same raw data to take advantage of contrast differences. These dif-ferent window settings are made by employing unique computer algorithms, or filters, to manipulate the raw data. These settings have both a window level and a win-dow width. A winwin-dow level is the center value of a cer-tain setting, while the window width is the range around the center value. It is the window width that determines
STATIC NEUROIMAGING IN THE EVALUATION OF TBI 131
the visual contrast or “shades of gray” displayed for a cer-tain setting (5). The window level accounts for the tissue brightness that is displayed (6).
The only absolute contraindication to obtaining a non-contrast CT is an unstable patient. Because CT imaging uses ionizing radiation, special care should be used with children and pregnant patients. Organs that are not included in the region of interest may be shielded with lead to protect them from unnecessary exposure to radiation. Although there is no danger with obtaining CT in patients with aneurysm clips or intracranial metal, these objects do create artifacts that render the interpretation of a CT difficult.
In order to perform a CT, the patient is placed in the supine position on the table and moved into the gantry. A CT scout image, or topogram, is obtained in order to determine the area that should be scanned. The scout image looks like a lateral plain film of the skull. The technologist can then orient the plane of the scan and set the slice thickness. Typically, the axial images are obtained parallel to the orbital roof. A trauma CT scan of the brain is performed without contrast in order to identify acute hemorrhage, which appears hyperdense (bright) on CT.
CT may also be obtained after the administration of intravenous, iodinated contrast material. Contrast is usu-ally not needed in the evaluation of acute traumatic brain injury, unless screening of the vasculature is warranted. For suspected vascular injury, dedicated conventional angiog-raphy is often performed rather than CT angiogangiog-raphy because of increased sensitivity and the ability for thera-peutic intervention to be performed quickly if needed.
Thus, routinely giving contrast for a CT in the acute set-ting for a patient with suspected brain injuries is a wasted step and may obscure underlying acute hemorrhage.
Contrast is administered in order to evaluate med-ical pathology for which the blood-brain barrier has been violated. These include inflammatory, infectious, vascu-lar, and neoplastic abnormalities. Contrast is given to identify the enhancement patterns of intracranial mass, cerebral abscess, subdural empyema, encephalitis, and vasculitis. Contrast material is also required for per-forming CT angiography in order to visualize the enhanc-ing arteries and veins.
With the advent of multidetector CT (MDCT), fast imaging of large volumes of tissue is possible, without sac-rificing spatial resolution. MDCT with thin slices can be combined with software packages that optimize vascu-lar enhancement. This allows one to control the intra-venous bolus of contrast, specifying arterial phase and delayed venous phase scanning. This becomes important for CT angiography and evaluation of intracranial vas-culature noninvasively. Post-processing techniques such as maximum intensity projection (MIP), volume render-ing (VR), shaded surface display (SSD), and curved pla-nar reformatted images allow one to manipulate the MDCT data to optimally display intracranial vascular
structures and detect pathology such as arterial occlusion, dissection, pseudoaneurysm formation, and vasospasm.
Goldsher et al. evaluated 36 patients presenting with sub-arachnoid hemorrhage using MDCT angiography of the vertebrobasilar system. Their results suggest that cerebral MDCT angiography is a reliable, rapid, and minimally invasive diagnostic method, for assessing vasospasm of the posterior circulation in patients with subarachnoid hemorrhage (7). More studies are needed at large volume trauma centers to further define the role of MDCT angiography in the evaluation of traumatic intracranial vascular injury.
Sequential axial nonenhanced images of the brain are performed from the base of the skull through the ver-tex with slices that vary in thickness from 5 mm to 7 mm.
Current helical scanners obtain overlapping voxels (vol-ume elements) of tissue continuously from start to finish.
The raw CT data is manipulated and sent to a work-station for processing. Images may then be viewed in different window settings on the workstation, film or Pic-ture Archiving and Communication System (PACS). CT scans of the brain are often viewed in bone windows (win-dow level: 700, win(win-dow width: 3500), brain win(win-dows (window level: 40, window width: 100) and intermediate windows (window level: 80, window width 250)—for purposes of identifying subtle hemorrhages along the sur-face of the brain (1). Many hospitals are now converting to digital PACS systems in order to take advantage of the ability to manipulate data, eliminate film costs and allow clinicians to access a computer version of images and reports.
Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI) is an imaging tech-nique that plays an adjunctive role in brain injury. The ability for tissue/fluid characterization and multiplanar imaging are advantages of MRI. While the imaging times of MRI have shortened with improved equipment and stronger magnets, performing an MRI is still much longer than performing a CT scan. Thus, the use of MRI in patients with acute traumatic brain injury is limited to certain patient populations. When compared to CT, MRI provides exquisite soft tissue detail, evaluation of the brainstem, skull base, and cranial nerves. It displays par-ticularly well the posterior fossa, which is often obscured by a beam-hardening artifact that occurs with CT.
MRI is used primarily in the work-up of chronic conditions or for following the sequelae of traumatic brain injury. It is more accurate in assessing the general age of intracranial blood products. The appearance of evolving intracranial hemorrhage is described in detail later in the text. MR angiography is a technique that is useful to screen for concurrent vascular injuries in the head and neck. However, conventional angiography
remains the gold standard in the evaluation of traumatic vascular injury, both for diagnosis and the opportunity for endovascular therapy such as embolization, angio-plasty or stenting.
The human body is made up of water, natural ele-ments, minerals and macromolecules such as proteins, fats, and carbohydrates. One of the main components is the hydrogen proton. Magnetic resonance imaging relies on the signal emitted by relaxing protons as they are dis-turbed by external magnetic fields and radiofrequency pulses. An MRI unit consists of a powerful external mag-netic field, transmitting coils, receiving coils, patient table and a digital workstation (Figure 11-1). With the appli-cation of a strong external magnetic field, hydrogen protons in the body align themselves either parallel or anti-parallel with this field. Most of the protons align themselves with the lower energy parallel state, creating a net magnetic moment in the direction of the external magnetic field (8). This net magnetic moment in the direc-tion of the external magnetic field is called the longitudinal vector.
Each proton has a spin and a motion called preces-sion. The frequency with which the protons precess depend on the strength of the external magnetic field. The relationship between the precession frequency and the external magnetic field is described by the Larmor equa-tion (5, 8):
0 B0
0is the precession frequency (Hz)
is the gyro magnetic ratio
B0is the strength of the external magnetic field (Tesla) It can be derived from the equation that the preces-sion of protons varies directly with the strength of the mag-netic field. Stronger magnets produce faster precession.
This property allows for a higher signal and improved image quality. Most diagnostic quality magnets available in hospitals or outpatient facilities are in the 1.0–1.5 Tesla range. Magnets in the 3.0–7.0 Tesla range are now being used in academic centers for human and animal research.
Stronger magnets will improve the quality, signal-to-noise ratio, and speed of acquisition of MR images in the future.
A radiofrequency pulse (RF) at the same frequency as the precessing protons is then turned on. This pulse dis-rupts the parallel/antiparallel equilibrium of the protons within the external magnetic field. Some protons are flipped into the higher-energy state, or antiparallel orien-tation. This eliminates the net magnetic longitudinal vec-tor that had been in the direction of the external magnetic field. This disruption of the longitudinal vector, or trans-fer of energy from the RF pulse to the precessing protons is called resonance (8). The RF pulse results in a decrease or elimination of the longitudinal net magnetization and also allows the protons to precess in phase with one another. Because the protons are precessing in phase dur-ing the RF pulse, a net magnetization in the transverse direction is then created. Thus, after the RF pulse is turned on, the longitudinal magnetization in the direction of exter-nal magnetic field decreases, while a new transverse mag-netization is created due to the protons now precessing in phase.
The RF pulse is then turned off. Protons gradually flip back to the lower-energy parallel orientation in the direction of the external magnetic field. The longitudi-nal magnetization vector gradually reappears. The reap-pearance of the longitudinal vector is called longitudinal relaxation and is described by the time constant T1, or longitudinal relaxation time. The transverse magnetiza-tion that had been created by the protons precessing in phase disappears when the RF pulse is turned off. This is described by the transverse relaxation time, T2. T1 relax-ation depends on the strength of the external magnetic field, tissue composition and structure. T2relaxation is affected by magnetic field inhomogeneities.
Each type of tissue has unique T1and T2properties that affect how they will appear on an MR image. When the RF pulse is turned off, special coils around the patient detect the signal of the increasing longitudinal vector and decreasing transverse vector (8). The decay and growth of these relaxation properties may be plotted on a graph.
By changing different parameters and imaging times of the MRI sequence, one can take advantage of the differ-ent points along these relaxation curves, thereby affect-ing the signal of different tissues. These parameters can render an image T1weighted, T2weighted, or a combi-nation of the two. In addition, each tissue has a certain proportion of protons per unit volume. Parameters can also be changed to take advantage of this proton density.
For instance, water or fluid has a long T1and is typically dark on a T1weighted image and bright on a T2weighted FIGURE 11-1
Standard 1.5 Tesla MRI gantry
STATIC NEUROIMAGING IN THE EVALUATION OF TBI 133
image. Fat has a short T1and is typically bright on both T1and T2weighted images.
In order to localize the protons or the signal created within tissue, gradient magnetic fields are applied in a multiplanar fashion. The applied external magnetic field created by the MR gantry is fairly uniform or homoge-nous. Gradients are applied to the main magnetic field to create local differences across the area of interest. In other words, the magnetic field will change across the patient in several directions, determined by the gradients applied. These magnetic field gradients are imposed on the external field in order to change the precession frequency of each proton in the body slightly, allowing spatial localization to take place. Each proton can be identified in space and location because of its unique spin created by applying these known gradient fields across the regions of interest. The magnetic field gradients and radiofrequency excitations help to localize the signal from each voxel (volume element) within the patient being imaged (5, 8). It is beyond the scope of this chapter to describe the specific appearance of each tissue within a myriad of MR pulse sequences that seem to change daily.
CT is the dominant imaging modality for the detec-tion of traumatic brain injury in the acute setting, while MRI is very helpful for the detection of occult lesions, follow-up and the long-term management of patients with traumatic brain injuries. Most initial decisions concern-ing management, such as surgery or no surgery in the emergency setting, are made based on the findings of a non-contrast CT of the brain. MRI may be obtained in the acute setting if there are clinical findings out of pro-portion to the CT findings, suggesting diffuse axonal injury or stroke. MRI also shows the chronic sequelae of the initial injury with excellent detail. Some common indi-cations for obtaining an MRI in a post-traumatic work-up include chronic headaches, stroke, pseudoaneurysm, dementia, hydrocephalus, cranial nerve palsy, cavernous sinus injuries, persistent neurological deficits, or patients with diffuse axonal injury in light of a normal CT of the brain (1, 9, 10). The findings on MRI have implications on predicting patient outcome as well, and this topic will be discussed at the end of the chapter.
Gadolinium is a rare earth element with an atomic number of 64 that can be chelated to form MRI contrast material. Intravenous gadolinium chelate is non-iodinated and contrast reactions or renal failure are much less com-mon than with iodinated contrast material. Gadolinium is paramagnetic, meaning that it affects the T1and T2 characteristics in such a way that enhancement on post-contrast T1images can be seen. Gadolinium provides the same information that iodinated contrast provides on CT—it allows visualization of vasculature and areas of breakdown of the blood-brain barrier. MRI is often used in the subacute or chronic setting of relatively stable patients. Gadolinium is extremely safe and it is often used
in many protocols after noncontrast MR images have been obtained.
BRAIN INJURY
Imaging technology changes quickly; thus it is advanta-geous to learn pathology in an organ-based fashion. No matter how the technology changes, the pathology remains the same. With the basic knowledge of patho-physiology, one can learn and adapt to how a particular condition appears within a myriad of changing imaging modalities. Imaging of patients with traumatic brain injury in the acute setting often begins with a noncontrast CT of the brain (6). All of the available data including his-tory, physical exam, mechanism of injury, and vital signs are used in conjunction with the imaging evaluation to triage these patients into surgical or medical management.
If surgery will be needed, decisions as to the timing of surgery are also made based on the clinical presentation and imaging results. These acute injuries may require early neurosurgical intervention and a timely interpreta-tion of the imaging findings is advantageous in the emer-gency room setting (11).
When interpreting an emergent CT scan of the brain, particular attention should be paid to the midline struc-tures, presence of shift, ventricles, basal cisterns, the pres-ence of herniation, cerebral convexities, and mass effect (12). The skull of infants and young children has open sutures and fontanelles; thus there is more plasticity of the cranial vault, should pathology develop. The adult brain exists in a closed box, the densely ossified skull that does not yield to intracranial pathology. Skull fractures, particularly depressed fractures often herald underlying brain injuries. Another advantage of CT is the exsquisite bony detail it offers. Subtle, nondisplaced fractures in the plane of the scan may go undetected; however, these are felt by many researchers to be clinically insignificant (3,13). CT allows one to characterize fractures, the extent of bony injury and the sequelae of these fractures to the underlying brain parenchyma.
Air present within the skull or brain is termed pneu-mocephalus. The presence of pneumocephalus often her-alds the presence of a fracture, usually communicating with air in the sinuses or outside of the skull. It is imper-ative to recognize fractures involving neurologically exquisite areas such as the orbit or the temporal bone.
Temporal bone fractures, depending on type may lead to cerebrospinal fluid otorrhea, conductive hearing loss, sen-sorineural hearing loss or facial nerve paralysis. Other fractures such as nondisplaced calvareal fractures may be managed conservatively.
There is a finite amount of space within the adult skull. If something occurs within the skull to occupy the space, it occurs at the expense of the brain that is soft and
very pliable. In response to a space-occupying intracra-nial lesion, the brain may become deformed, swollen or edematous. If the mass effect within the skull is severe enough, a herniation syndrome may result. The brain is made up of gray and white matter. The gray matter con-sists of the cerebral cortex, cerebellar cortex, and deep nuclei. The white matter consists of a network of axons—
connections and tracts between these nuclei. Injuries involving the brain itself—the gray or white matter of the brain parenchyma, are called intra-axial injuries. The extra-axial space, on the other hand, is outside of the brain parenchyma. The brain is separated from the inner table of the calvarium by several layers of tissue called meninges.
The meninges are composed of different layers of connective tissue. The dura is the outer layer that is firmly attached to the inner table and periosteum of the skull.
The arachnoid exists between the dura and the pia. The pia is a thin lining that covers the brain and spinal cord.
It follows the cortex of the brain and lines each gyrus, while dipping into the intervening sulci. The subarach-noid space is the space bounded by the arachsubarach-noid layer and pia. It contains cerebrospinal fluid and blood vessels.
There are several areas within the brain where the sub-arachnoid space expands to create cisterns. Abnormali-ties that affect the subarachnoid space may extend into these cisterns (14). The pia and arachnoid comprise the leptomeninges. Injuries outside of the brain parenchyma are called extra-axial injuries.
The brain and spinal cord are bathed in cerebrospinal fluid (CSF). This fluid is also present in the ventricles and cisterns of the brain. There are two lateral ventricles, with frontal, temporal, and occipital horns. The third and fourth ventricles are unpaired, midline structures. The lat-eral ventricles communicate with the third ventricle through the paired foramina of Monro. The third ventri-cle communicates with the fourth ventriventri-cle—which is in the posterior fossa of the brain through the cerebral aqueduct. In the normal state, the CSF volume remains rel-atively constant, although it is constantly in flux. A home-ostasis is reached where the amount of CSF produced in the choroids plexus is the same as the amount of CSF being absorbed into the venous sinuses. Cerebral capillaries form an impermeable blood-brain barrier and the capillaries and ependymal cells form a blood-CSF barrier (14). These bar-riers may break down in pathological conditions such as trauma, infection or tumor. Injuries within the ventricles or CSF spaces, while inside the confines of the brain, are actually outside the brain parenchyma and thus are extra-axial injuries. Sometimes, it is difficult to determine whether a lesion is intra-axial or extra-axial, however there are signs on imaging that help to define the location of lesions.
In the sections that follow, several types of traumatic brain injury will be discussed. The injuries are organized
according to location. Injuries involving the brain parenchyma, or intra-axial space will be discussed first, followed by a discussion of extra-axial manifestations of traumatic brain injury.
Intra-axial: Brain Contusions
Some authors have described brain injuries based on the mechanism of injury—such as impact injuries versus iner-tial injuries, or primary versus secondary injuries (15).
The traumatic neuropathology described in this chapter will be divided into intra-axial injuries and extra-axial injuries. Injury of the brain may occur from blunt or pen-etrating forces. Lesions such as brain contusions or epidural hematomas occur due to direct force, while other injuries, described later, occur due to acceleration/decel-eration or rotational forces.
Direct forces may impact on the skull and underly-ing brain to cause injury to the brain parenchyma. These manifest acutely as brain contusions (Figure 11-2). A con-tusion may be hemorrhagic (Figure 11-3) or nonhemor-rhagic, and all involve the gray matter of the brain. The two main reasons for not giving contrast prior to imaging a patient with acute intracranial injuries are to perform the exam quickly and to look for intracranial blood products.
FIGURE 11-2
Intra-axial hematomas: Axial noncontrast CT demonstrates intra-axial contusions in the bilateral frontal lobes and left temporal lobe (arrows). There is associated vasogenic edema