Indications for neuroimaging are diverse and include traumatic and non-traumatic conditions. The major brain neuroimaging modalities are CT and MRI, with adjunctive roles for conventional angiography and ultrasound. Plain films of the calvarium have an extremely limited role, as they can detect bony injury but cannot detect underlying brain injury that may be present even in the absence of a skull fracture.
CT has been in general clinical use in the emergency department (ED) in the United States since the early 1980s. The modality was simultaneously and independently described by the British physicist Godfrey N. Hounsfield and the American Allan M. Cormack in 1973, and the two were co-recipients of the Nobel Prize for Medicine in 1979.1,2Advances in computers and the introduction of multi-slice helical technology have dramatically enhanced the resolution, sensitivity, and specificity of CT since its introduction. CT relies on the differential attenuation of x-ray by body tissues of differing density. The image acquisition occurs by rapid movement of the patient through a circular gantry opening equipped with multiple x-ray sources and detectors. A three-dimensional volume of image data is acquired; this volume can be displayed asaxial, sagittal, or coronal slices or as a three-dimensional image. When performed without intravenous(IV) contrast, CT is considered to be excellent for detection of bony abnormalities, acute hemorrhage,cerebral edema, hydrocephalus, or mass effect. It is less sensitive for acute ischemic stroke but becomes more sensitive with the passage of time, as will be discussed later. Adding IV contrast improves the sensitivity of CT for neoplastic, infectious, and vascular abnormalities. IV contrast can be used to generate CT arteriograms, CT venograms, and CT perfusion maps. These will be described in more detail later in this paper. CT does raise some safety concerns with regard to long-term biological effects of the ionizing radiation and carcinogenesis. The radiation exposure to the fetus in a shielded patient undergoing head CT is minimal.3 Most commercially available CT scanners have a weight capacity of approximately 450 pounds, although some manufacturers now offer units with capacities up to 650 pounds, and a portable head CT scanner with a manufacturer-reported unlimited weight capacity is also now available.4
MRI has been in wide clinical use in the U.S. since the late 1980s. The modality was co-invented by the American Paul C. Lauterbur and the British physicist Sir Peter Mansfield, who shared the 2003 Nobel Prize in Medicine for their work.5 MRI allows imaging of the brain by creating variations in the gradient of a magnetic field and analyzing the radio waves emitted in response to objects within the field. Advantages of MRI include its noninvasive nature and its apparent safety in pregnancy.6 It also has no known permanent harmful biological effects.7 Traditionally, contraindications have included the presence of ferromagnetic material within the body, including electronic devices (such as pacemakers) or metallic debris (such as shrapnel), especially when they are located in sensitive structures (such as the eye or brain). However, there are now over 230 published prospective cases of patients with pacemakers having safely undergone low-field MRI, making MRI a possible imaging option in these patients.8 Magnetic effects on tattoos, including first-degree burns and burning sensation, have been reported, although these appear rare and more likely to interfere with completion of MRI than to cause significant harm.9-11
Joshua Broder; Robert Preston
December 1, 2007