General Overview Of Common Techniques

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Although a more thorough discussion of techniques is provided in other chapters of this textbook, a brief introduction to the more common techniques will be beneficial to understanding the optimal approach to cranial neuroimaging. Plain films are an essential tool. X-rays generate two-dimensional images based on attenuation of a collimated beam. The attenuation is proportional to the electron density of the structures through which it passes with the final image reflective of tissue density. This results in high spatial resolution and thus sensitivity for the detection of fractures. However, detection of abnormalities requires that there be no obscuration of the region of interest by overlying bone or other dense material. Thus, whereas a skull fracture of the calvarium is often readily detected, a fracture involving the skull base is typically obscured due to overlying bone. Plain films also provide a coarse view of the extracranial soft tissues, but intracranial soft tissues are obscured. Because the primary goal of ED neuroimaging is the detection of intracranial soft tissue abnormalities, the role of plain films is largely limited to the primary assessment of spinal trauma, the assessment of some facial fractures, assessment of some subtle, linear skull fractures oriented in the axial plane of acquisition, or as part of the skeletal survey for child abuse.

Computed tomography (CT) has largely supplanted plain films for neuroimaging. CT images are generated using a rotating x-ray tube that projects a collimated beam through the brain with resultant attenuation of the beam proportional to the electron density (tissue density) of the tissue through which it passes. Thus, rather than interrogating the tissue of interest from one direction (as in conventional x-rays), computerized processing of the circumferentially projected attenuated rays enables reconstruction of tissue maps largely unobscured by overlying tissues. Differentiation of adjacent tissues such as bone, gray and white matter, and cerebrospinal fluid (CSF) is possible due to perceivable differences in tissue density. Vasogenic and cytotoxic edema and hemorrhage alter the density of the tissue effected, making CT highly sensitive in the detection of disease. Further, the fine anatomic detail enables detection of morphologic alterations such as fractures and parenchymal swelling.

Magnetic resonance imaging (MRI) involves placing a patient inside an externally applied magnetic field. Due to its odd number of nuclides, hydrogen will align with this magnetic field. These "magnetized" hydrogen atoms are then "energized" by the input of an applied radiofrequency pulse and the pattern of energy release provides information about the biophysical properties of the tissue being evaluated. The shifts in water (hydrogen) distribution that characterize all pathologic processes alter these biophysical properties. Detectable changes occur prior to a detectable change in tissue density. Thus, MRI is more sensitive than CT in detecting parenchymal pathologic processes such as cerebral ischemia, infection, or metastases. Equally important is that the presence of certain blood products (deoxy- and methemoglobin, hemosiderin) develops over time postbleeding. These cause a detectable alteration in local magnetic field. MRI is thus nearly equivalent to CT in the detection of fresh bleeding, but is much more sensitive in the detection of staid blood. Blood becomes CT "normal" in density 7 to 10 days postbleed, but can generate an abnormal magnetic field for months to years. Lastly, MRI enables multiplanar acquisition while the patient remains supine. In CT, imaging requires patient positioning specific for the plane of interest. This can be difficult for the very young, the elderly, and for trauma patients.

Notably, cortical bone is relatively depleted of hydrogen. Thus, the cortex of bone is nearly invisible on MRI. Although this is of benefit in that parenchyma can be assessed without compromise from adjacent dense bone (e.g., infratemporal region and posterior fossa assessment is slightly limited in CT due to artifact from the adjacent skull base) (Fig 2.29.-1.A and Fig 2.2.9.-1B), MRI is inadequate for the assessment of bony integrity. Compared to CT, MRI is frequently less (or even not)

available in many Emergency Departments; is contraindicated in the presence of devices such as pacemakers, cochlear implants, and metallic foreign matter near vital structures; requires greater patient cooperation; and impedes monitoring of critically ill patients. As a result, despite its superior sensitivity to most pathological processes, MRI plays an important but more limited role in the ED.

FIG. 229-1. A. Axial CT obtained in a patient presenting with a left cerebellar infarct. Initial CT demonstrates a focal area of decreased attenuation in the left cerebellar hemisphere difficult to detect due to artifact generated by adjacent bone ( arrow). Incidental note is made of similar obscuration of the inferior temporal lobe by artifact (arrowheads). B. Axial MRI clearly demonstrates an area of infarction in the left cerebellum. Visualization is clear with no artifact generated despite proximity to bone.

Ultrasound generates images based upon transmittance of sound waves. In general, sonographic imaging enables evaluation of soft tissue structures provided that air or bone does not obscure the area of interest. In neuroimaging, most of the structures of interest are obscured by the calvarium. However, in infants, patency of the fontanel provides an adequate sonographic window enabling high sensitivity for the detection of intracranial masses, hemorrhages, or hydrocephalus. In infants and very young children, ultrasound is the study of choice for intracranial assessment.

Ultrasound can also be used to assess for traumatic globe injury or masses. However, this technique is highly operator-dependent, is limited in the assessment of the orbital apex, and is typically performed by an ophthalmologist.

Ultrasound images can be generated in which flowing blood produces color images scaled to flow velocity. These images give information on lumen caliber and hemodynamic alterations (such as occur with a distal occlusion or proximal stenosis). Ultrasound has become the screening technique of choice in assessing for common carotid bifurcation atherosclerotic disease. Transcranial Doppler and power Doppler ultrasound enables assessment of the distal most aspects of the internal carotid arteries, the proximal cerebral arteries, the circle of Willis, and the superficial parenchyma. Assessment of the posterior circulation and deeper parenchyma remains limited.

The vasculature can also be assessed noninvasively with CT angiography (CTA) or MR angiography (MRA). CTA and MRA are appealing techniques because they are noninvasive and can be incorporated into a comprehensive Ct or MR examination with minimal additional time or difficulty ( Fig 229-2A and Fig 229-2.B). MRA

utilizes a gradient echo pulse sequence that results in increased intravascular signal intensity with normal flow. CTA is performed following the peripheral venous administration of iodinated contrast. An image similar to a conventional arteriogram can be reconstructed from the axial images. Both are sensitive to large vessel occlusion or narrowing in the internal carotid, vertebral, basilar, and first and second segments of the anterior, middle, and posterior cerebral arteries. Furthermore, both techniques can be modified to look at venous structures (i.e., CTV or MRV). CTA requires less patient cooperation with an assessment of the vasculature complete in approximately 30 s. However, vascular detail is limited when vessels are in direct apposition to bone. Vascular assessment with MRA is not limited in the presence of bone. It is useful for assessing the vertebral arteries as they course through the bony transverse foramina and skull base, and in assessing the cephalad course of the internal carotid artery.

FIG. 229-2. A. MR angiogram demonstrates an area of focal stenosis involving the basilar artery (curved arrow). However, the right posterior cerebral artery is clearly demonstrated excluding large vessel occlusion (straight arrow). B. MR axial FLAIR image demonstrates and area of hyperintensity corresponding to an acute right parietal infarct (arrowhead).

Both CTA and MRA techniques have resolution inferior to catheter angiography. For example, neither CTA nor MRA can reliably detect intracranial aneurysms less than 3 mm. The primary use of both techniques is as a screening tool for vascular occlusion and generally should be supplemented with catheter angiography when clinical suspicion remains. Catheter angiography utilizes plain x-rays in conjunction with administration of intra-arterial contrast through a catheter placed from a femoral artery approach and positioned proximal to the vasculature to be assessed. The intra-arterial contrast delineates vascular anatomy and detects pathologic changes such as narrowing and occlusion. It is the definitive procedure for assessment of the intra- and extracranial vasculature. However, there is a 1 to 2 percent risk of stroke associated with catheter angiography.

Near infrared spectroscopy (NIRS) measures changes in absorbance of an infrared light beam projected transcranially. Specifically, near infrared light (700 to 1000 nm) has relatively good transcranial penetration and hemoglobin (Hb) has characteristic absorption spectra. The absorbance of light is dependent on the presence of Hb and the oxygen content of the Hb. Thus, this technology can be used to assess for the presence of hemorrhage and to assess the oxygen state of the superficial microcirculation. For example, in one investigation, 24 of 27 patients with delayed traumatic intracranial hemorrhages demonstrated increases in the NIRS absorbance.1 NIRS units are portable and relatively inexpensive, but the role for NIRS remains to be determined.

Radioisotope scans are performed following the administration of radioisotopes that emit photons. Photon imaging studies can be used to assess ventriculoperitoneal shunt patency, CSF leaks, hydrocephalus, Ommaya reservoirs, and brain death. Anatomic scans can also be performed that enable detection of space-occupying lesions. In general, nuclear medicine neuroimaging studies are infrequently utilized in the ED setting due to the superior resolution, increased availability, and expedience of CT and MR.

There are numerous available neuroimaging techniques for cranial imaging. Fortunately, the majority of imaging can be performed with plain films, CT, or MR. The choice of modality varies depending on the clinical question to be answered, the availability of the modality, and the ability of the patient to tolerate the examination. Although a consensus is often difficult to attain, some guiding principles exist. The remainder of this review focuses on guiding principles for the assessment of the most common emergency neurologic complaints.

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