Brain Imaging: Anatomy, Trauma, and Tumors

1

Identify the normal parts of the brain labeled 1 through 6 in Figure 40-1 .

For the answer, see the figure legend.

2

What imaging modalities can be used to evaluate the brain?

The two major noninvasive cross-sectional imaging modalities used in neuroimaging are computed tomography (CT) and magnetic resonance imaging (MRI). Catheter angiography, also known as digital subtraction angiography (DSA) or conventional angiography, is reserved for the detection of intracranial vascular processes such as aneurysms and intracranial vasculitis. The advantage to catheter angiography is the potential for intervention at the time of diagnosis, such as coiling of an aneurysm. Myelography involves the injection of contrast material into the subarachnoid space by lumbar puncture, allowing for visualization of the spinal cord and nerve roots which are seen as “filling defects” within the contrast-opacified cerebrospinal fluid (CSF). Myelography (usually followed by postmyelography CT) is reserved for patients with contraindications to MRI (e.g., patients with noncompatible pacemakers or severe claustrophobia), patients with surgical hardware that may cause significant artifact obscuring the spinal canal on MRI, patients with equivocal MRI findings, and obese patients who exceed weight limits for MRI scanners.

3

What are the clinical indications for obtaining CT and MRI of the brain?

A CT scan can be a good initial examination for evaluating the brain in many acute settings, including trauma. It can assess for acute intracranial hemorrhage, mass effect, hydrocephalus, or stroke. CT is superior to MRI for evaluation of the bony structures. MRI provides greater soft tissue contrast and is often complementary to the CT examination. MRI can better characterize neoplasms (primary or secondary), neurodegenerative disorders, inflammatory diseases, and infectious processes. Diffusion-weighted imaging (DWI) is the most sensitive MRI technique for identifying acute ischemic infarcts. MRI can also better detect acute and chronic intracranial hemorrhage, nonhemorrhagic brain injury, and brainstem injury, and it is often performed in acute or subacute settings when neurologic findings are unexplained by CT findings.

4

What are contraindications for performing MRI of the brain?

Contraindications include ferromagnetic implanted electronic devices, such as certain pacemakers, neurostimulator devices, non–MRI-compatible vascular clips, ferromagnetic metallic implants, and foreign bodies in the eye. If history is unavailable, prior imaging studies or radiography can evaluate for the presence of foreign bodies. A major change in device safety relates to advancements in cardiac implantable electronic devices. There are now FDA-approved, “MR conditional” pacemakers on the market, and MRI can be performed if the manufacturer's guidelines are followed. In addition, however, a few academic institutions are safely performing MRI studies on patients with non-MR conditional pacemakers after careful evaluation by a physician. MRI can be performed on patients with aneurysm clips if the model and type is documented to be safe. Most aneurysm clips used after 1995 are made of titanium and are MRI-compatible. Severe claustrophobia is a relative contraindication, and, if necessary, most patients with this condition can successfully undergo MRI with sedation.

5

Define the terms “intra-axial” and “extra-axial”, which are commonly used to localize intracranial pathologic conditions.

An intra-axial lesion arises from the brain parenchyma. An extra-axial lesion is one that arises outside of the brain substance, and it may be pial, dural, subdural, epidural, or intraventricular in origin. The most common extra-axial mass is a meningioma ( Figure 40-2 ). In adults, the most common solitary intra-axial masses are primary brain tumors and metastatic disease. An intra-axial mass lesion expands the brain, resulting in gyral swelling and effacement of the cerebral sulci. The gray-white matter interface may be blurred. An extra-axial mass lesion is characterized by obtuse margins and results in crowding of the underlying sulci. Additional features include a cleft of CSF (see Figure 40-2, A ), dura, or small vessel between the mass and brain, as well as bony remodeling of the skull.

6

What is the Glasgow Coma Scale (GCS), and how is it used?

The GCS provides a reliable, objective way of determining a patient's conscious state and is most often used in the setting of head injuries. Responsiveness and awareness is measured by evaluating eye opening response, verbal response, and motor response. A score between 3 and 15 is assigned. Scores of 13 to 15 correspond to mild closed head injury; 9 to 12 correspond to moderate head injury; and 8 or less correspond to severe brain injury. The GCS does not correlate with survival outcome in cases of severe head trauma with coma.

7

What is the imaging modality of choice in the setting of acute head trauma?

The imaging modality of choice in the setting of acute head trauma is an unenhanced brain CT scan. CT is a readily available, fast, and accurate method for detecting acute intracranial hemorrhage. On CT, acute hemorrhage is hyperattenuating relative to brain and may display mass effect. Emergency physicians and neurosurgeons want to know the exact cause of clinical symptoms in a trauma patient. Specifically, the most significant concern is whether there is a treatable lesion. CT plays a primary role in evaluating the extent of trauma and in determining appropriate management. It is sensitive in distinguishing brain contusions from extra-axial hematomas (subdural and epidural). CT is also excellent for detecting facial and calvarial fractures. When diffuse axonal shear injury is suspected, MRI is more sensitive and can be obtained when the patient is clinically able to tolerate this examination. CT is also useful when foreign bodies (such as bullet fragments) are suspected, and MRI is contraindicated.

8

What is the role of imaging after concussion or other minor head trauma?

The need for a head CT in a patient with minor head trauma, classically defined as having a normal or near-normal (13 to 15) GCS and loss of consciousness, disorientation, or memory loss, is controversial. The majority of the time, these CT scans are found to be normal. The Canada CT Head Rule is a set of guidelines developed with the goal of providing evidence-based clinical decision support. In this set of guidelines, any one of the following “high-risk” clinical criteria were found to be 100% sensitive for a positive head CT (demonstrating injury requiring neurosurgical intervention): GCS <15 at 2 hours after injury, suspected open skull or skull base fracture, greater than 2 episodes of vomiting, or age greater than 65 years old. Sensitivity was reported to be 98% with the following “medium-risk” criteria: amnesia less than 30 minutes prior to impact or a dangerous mechanism of injury. More recent criteria were developed based on the New Orleans Criteria (NOC) trial. These criteria differ from the Canada Head CT rule because they only apply to patients with a GCS of 15 and define a positive head CT as having evidence of any acute injury—not necessarily an injury requiring neurosurgical intervention. The NOC recommends a noncontrast head CT in the following settings: headache, vomiting, age older than 60, intoxication, post-traumatic seizure, physical evidence of trauma above the clavicles, and short-term memory deficits. The NOC are considered 98% to 99% sensitive for identification of any intracranial injury.

9

What are the advantages and drawbacks of MRI in assessing a trauma patient?

MRI is more sensitive in distinguishing different ages of hemorrhage (hyperacute, acute, subacute, and chronic), in detecting diffuse axonal shear injury, and in assessing injury in the posterior fossa and undersurfaces of the frontal and temporal lobes. However, MRI scan times are significantly longer, and performing MRI in patients on ventilators and other monitoring devices can be cumbersome. MRI is less sensitive in detecting fractures. In addition, MRI is contraindicated in some patients with foreign bodies (for example, near the orbit). CT, on the other hand, is readily available and fast; it remains the imaging modality of choice in the setting of trauma.

10

How does one differentiate a subdural hematoma from an epidural hematoma?

An epidural hematoma is usually caused by an arterial injury (most commonly, the middle meningeal artery is injured as a result of a fracture of the temporal bone through which it courses) ( Figure 40-3, A ). An epidural hematoma is extra-axial, runs between the periosteum of the inner table of the skull and the dura, and is confined by the lateral sutures (especially the coronal sutures) where the dura inserts. As a result, an epidural hematoma is usually lenticular in shape and can cross the midline. Prompt surgical clot evacuation is typically performed for large epidural hematomas to prevent brain herniation and its associated complications.

In contrast, a subdural hematoma is usually caused by injury to the bridging cortical veins ( Figure 40-3, B ). A subdural hematoma runs between the dura and the pia-arachnoid meninges on the surface of the brain and is not confined by the lateral sutures. As a result, subdural hematomas are usually crescentic in shape. In the midline, the dural reflection is attached to the falx cerebri, however. Subdural hematomas (in contrast to epidural hematomas) do not cross the midline, but rather track in the interhemispheric fissure. Subdural hematomas are commonly seen in patients with closed head trauma; elderly patients with minor head trauma; and patients with rapid decompression of hydrocephalus and extra-axial hematomas, where abrupt changes in intracranial pressure make the bridging cortical veins vulnerable. They may also be seen in pediatric patients in the setting of nonaccidental trauma (child abuse).