Computed tomography (CT) and magnetic resonance imaging (MRI) were initially introduced to the medical imaging field in the early 1970s and 1980s, respectively. With their initial application in neuroimaging, their contribution to medical imaging has been tremendous. Subsequent to these beginnings in CT and MRI, numerous imaging applications have been developed to study anatomic structures better, diagnose diseases and disorders of the body systems, and follow treatment patterns. In this chapter, the primary focus will be to provide information on the use of these imaging modalities in neuroimaging as it applies specifically to CT and MRI. An overview, however, of other imaging modalities used in evaluating the brain requires a brief discussion.
Although CT and MRI have taken the spotlight in neuroimaging, basic skull radiography, ultrasound (US), positron emission tomography (PET), and single-emission computed tomography (SPECT) also have their respective roles in assisting the physician in caring for the patient. These imaging modalities can be grouped into two categories according to the type of image information demonstrated. For conventional CT, MRI, and US, images present the morphology of the anatomy within the imaging field-of-view. With PET, SPECT, and the advanced technologies associated with CT, MRI, and US, functional images demonstrate the flow and distribution of blood throughout the brain. Most imaging facilities have the conventional imaging modalities mentioned above available for their physicians' utilization. The advanced imaging technologies that provide functional (physiologic) information are not routinely available in all imaging facilities. For the majority of applications, conventional imaging modalities are sufficient. However, in difficult cases, the functional modalities may be used to assist in detecting diseases and disorders in an earlier stage of development compared to conventional modalities.
Since the development of radiology in 1895, skull radiographs have been used to evaluate patients with head trauma. Skull radiographs taken from several different views may demonstrate fractures and assist with identifying displacement of internal midline structures, which could indicate an intracranial space-occupying mass such as a hematoma. Historically, other radiologic procedures such as pneumoencephalography and cerebral angiography were developed to further the diagnostic ability of radiology in evaluating head trauma. Since the development of CT, pneumoencephalography is no longer used. For trauma patients with a history of mild head injury, a basic x-ray skull series might be ordered as a screening for possible fractures. In some cases, however, where a skull fracture is suspected, a CT of the head is mandatory. Cerebral angiography, a radiologic (x-ray) procedure that is performed to evaluate the blood vessels of the brain is used to examine intracranial vascular lesions such as aneurysms (Figure 2–1), arteriovenous malformations (AVMs), tumors, and atherosclerotic or stenotic lesions. Comparatively, cerebral angiography, still considered to be the "gold standard" when evaluating the cerebral vasculature structures, provides higher spatial resolution than either magnetic resonance angiography (MRA) or computed tomography angiography (CTA). For initial screening examinations, however, imaging procedures such as CTA, MRA, and duplex US have replaced cerebral angiography.