These hematologic factors should be sought when unexplained strokes occur in children or young adults, in families whose members have had frequent strokes, in pregnant or parturient women, and in women who are migraineurs or taking birth control pills. According to Markus and Hambley, whose review of this subject is recommended, screening for lupus anticoagulant, anticardiolipin antibodies, deficiency of proteins C and S, and antithrombin III is probably justified, but mainly in these special circumstances. This special category of vascular thrombosis is taken up in later sections, including the distinctions between arterial or venous clotting.
Technologic advances continue to enhance the clinical study of stroke patients; they allow the demonstration of both the cerebral lesion and the affected blood vessel. CT demonstrates and accurately localizes even small hemorrhages, hemorrhagic infarcts, subarachnoid blood, clots in and around aneurysms, arteriovenous malformations, and established regions of infarction as well as adjacent regions of ischemic (penumbral) tissue.
Magnetic resonance imaging demonstrates flow voids in vessels, hemosiderin and iron pigment, and the alterations resulting from ischemic necrosis and gliosis. MRI is particularly advantageous in demonstrating small lacunar lesions deep in the hemispheres and abnormalities in the brainstem (a region obscured by adjacent bone in CT). Diffusion-weighted magnetic resonance techniques are particularly useful to detect infarction within minutes of the stroke, that is, considerably earlier than CT and other MRI sequences (Fig. 33-3). The various MRI imaging sequences used in the diagnosis and dating of stroke are discussed below and in Chap. 2 and in Table 33-4.
Arteriography, enhanced by digital processing of images, accurately demonstrates stenoses and occlusions of the intracranial and extracranial vessels as well as aneurysms, vascular malformations, and other blood vessel diseases such as arteritis and vasospasm. Conventional contrast angiography has largely been supplanted by magnetic resonance angiography (MRA), venography (MRV), and CT angiography for the visualization of large intracranial arteries and veins (see Fig. 2-2J and K). These techniques have the advantage of relative safety (injection of contrast media is required for CT angiography) but do not give refined images of the smaller blood vessels. MRA depicts the “time of flight” of blood through vessels and is not as accurate as CT angiography in measuring the degree and morphology of changes within a cerebral or intracranial vessel.
Partly as a result of the increased use and availability of endovascular thrombectomy, it has become useful to determine if there is ischemic, but not yet infarcted, region of the brain. The main techniques are CT perfusion and MR perfusion. Both involve the rapid acquisition of images during passage of a contrast agent through tissue. A time-intensity curve is derived and from this curve, images of blood flow, blood volume, and transit time can be displayed (Fig. 33-4). A penumbral-ischemic region is identified by increased transit time (or reduced flow) with relatively maintained blood volume. In comparison, infarcted tissue is identified by reduced blood volume that is below the threshold for irreversible tissue damage; this region should correspond to the findings on diffusion weighted imaging.
Other procedures for the investigation of cerebrovascular disease include Doppler ultrasound flow studies, which demonstrate atheromatous plaques and stenoses of large vessels, particularly of the carotid but also of the vertebrobasilar arteries. The transcranial Doppler technique has reached a degree of precision whereby occlusion or spasm of the main vessels of the circle of Willis can be detected and roughly quantitated. Various methods of measuring regional blood flow with positron emission tomography (PET) and radionuclide imaging find use in special instances discussed in appropriate sections of the chapter.