Unit 3: Diagnostic Imaging

Non-contrast CT is the modality of choice in identifying stroke as either ischemic or hemorrhagic,³ and should be done as early as possible in order to begin anticoagulation (heparin) therapy and to maintain the option of thrombolytic therapy within the first three hours of onset. Intracranial hemorrhage is immediately evident as a hyperintensity within the hyperacute (< 6 hours) and acute (6-24 hours) periods, with subarachnoid hemorrhage almost as well detected.⁷ In cases of aneurysmal SAH, CT is effective at evaluating the size and location of the bleed, and can indicate the possibility of associated vasospasm, the presence of communicating hydrocephalus, and possibly identify the vessel of origin.⁶ Absence of evidence of hemorrhage correlated with clinical symptoms indicates ischemic stroke instead, and CT is sometimes able to detect subtle signs of diffuse swelling associated with large area of infarction, although not as reliably as MRI.^{4, 5}

CT angiography (CTA) involves helical (spiral) scanning of the brain during an extended injection of intravenous contrast media, and is an important adjunct in the imaging of carotid stenosis and intracranial aneurysms. CTA has shown 92% correlation with angiography in determining the degree of carotid stenosis, with up to 100% in identifying surgical lesions.² Its accuracy may be limited, however, in the presence of calcified plaques.² In the evaluation of cerebral aneurysms, CTA has the advantages of being able to demonstrate the aneurysm neck, identify the extent of intra-aneurysmal clot and/or calcification, and show the relationships of the anuerysm with adjacent vessels and bone.⁶ Reported accuracy of CTA diagnosis of aneurysms is 88%, compared to 89% with that of angiography, although it can be limited by the presence of venous or other surrounding structures that complicate accurate interpretation. Improper scanning parameters can also obscure small aneurysms.⁶ CTA also has the disadvantage of requiring intravenous contrast.²

A representative protocol for CTA of the brain includes a total of 2cc/kg (100-150cc) of 300 concentration nonionic contrast, injected at a rate of 2-3 cc/sec. A delay of 20 seconds from the start of the injection to the initiation of the scan provides optimal visualization of the carotid arteries.^{4, 6}

A recent innovation in MRI technique that shows great promise in the evaluation of ischemic stroke is diffusion weighted imaging (DWI). Ischemia causes a shift in the water balance between the intra- and extracellular environment in cerebral tissue. This results in an accumulation of sodium and water within the cell which results in a slowing of the normal rate of movement of water molecules (Brownian motion). DWI can immediately detect areas of slower motion, identifying the area of ischemia or infarction within 1 hour of onset.¹³

Carotid duplex sonography combines real-time gray-scale ultrasound with Doppler evaluation of blood flow, thereby providing information of both the structural anatomy of a vessel and the physiological movement of the blood within. Anatomic information includes the size of the vessel lumen and the structure of the vessel wall. An increase in the size of the intima & media layers of the arterial wall beyond 0.8mm is an indicator of disease in areas of stenosis less than 50%.¹⁰ As the vessel lumen narrows with increasing stenosis (>50%), the flow velocity increases, becoming more detectable with Doppler. Velocity will continue to increase with increasing stenosis, until reaching the >95% stage, where flow velocity will drop suddenly as the narrowing nears occlusion. In addition to increased flow velocity within the stenosis, high velocity jets of flow are created immediately distal to the stenosis (poststenotic), which are readily demonstrated with color Doppler imaging.¹⁰

As discussed in Unit 1, stroke rates can be significantly reduced if surgical carotid endarterectomy is performed in appropriate patients. Because of certain pitfalls associated with CDS, it has not been accepted as reliable for surgical determination on its own, but is being combined with MRA, another noninvasive technique, for correlation. It is reported that the number of patients requiring conventional angiography can be reduced by 80% with no effect on patient outcomes when correlation between these two modalities exists.^{2, 10} The strengths of each modality in this case can overcome the associated limitations of the other. Common pitfalls and limitations in CDS include the following:

• 5% of ultrasound studies are technically inadequate due to the presence of calcified plaque in the lesion, restricted acoustic window due to patient obesity, or an anatomically high carotid bifurcation.²
• Velocity measurements are not as reliable in patients with cardiovascular disease.²
• The quality of the study is largely technologist dependent, often requiring other tests to confirm the results.¹
• Ultrasound is unreliable in determining the presence of tandem lesions.¹
• There is greater inter- and intraobserver variability than with MRA or angiography.¹
• If the vessels are very tortuous, it is difficult to get an accurate axial orientation of the vessel.¹
• A soft plaque can mimic blood flow, resulting in inaccurate evaluation of the stenosis.¹
• Vertebral arteries are often small and difficult to visualize.¹

The cerebral arteriogram is an invasive procedure that involves the placement by the physician of an arterial vascular catheter, usually into the femoral artery, that is directed through the abdominal and thoracic segments of the aorta until it reaches the aortic arch. It is then directed into the cerebral circulation via the brachiocephalic, left common carotid, or left subclavian arteries for the injection of contrast into selected vessels. The "four vessel" cerebral arteriogram will demonstrate the entire cerebral vascular anatomy via contrast injection into the right and left internal carotid arteries and vertebrobasilar circulation. Frontal and lateral projections are obtained, as well as various oblique projections when indicated. Most angiographic images in the modern special procedures department are high resolution digital, or 14"x14" cut film. This provides the highest spatial resolution of all the modalities. A detailed discussion of catheterization and procedural techniques is beyond the scope of this discussion.

As previously mentioned, angiography is an invasive procedure with risks associated with two main areas, the catheter placement and manipulation, and the intravascular contrast media. Risks to the patient from placement and manipulation of the catheter include hemorrhage, injury to the artery from the needle puncture, injury to the part of the body supplied by the artery, emboli to the distal extremity (from the puncture site), emboli from accidental dislodging of atherosclerotic plaques in the aorta or branch vessels, stroke from dislodged plaque, air embolus introduced during the contrast injection, thrombus formed at the tip of the catheter, or death. Angiography carries a 1.2% incidence of carotid associated infarction (National Institute of Health Clinical Advisory, September 28, 1994).¹

Risks to the patient from iodinated contrast media include all potential complications normally associated with the use of i.v. contrast. In addition, because of the concentrated bolus required for cerebrovascular imaging, neurologic complications, most commonly in the form of seizures, may occur. Keeping the amount of contrast media used to a minimum and using nonionic/low osmolar contrast agents reduces the risk of these complications.⁸

Angiography does have certain limitations. It can only demonstrate the lumen of the vessel, whereas other modalities are able to provide information about the vessel wall and the composition of a plaque or thrombus. Also, because it is essentially a two dimensional medium, at least two projections, often more, must be performed in order to profile an atherosclerotic lesion to accurately evaluate the degree of stenosis. The accurate demonstration of stenotic lesions is dependent on the proper orientation of the lesion with the radiographic projection, therefore a stenosis may be overestimated or underestimated.¹ However, the new technique of rotational angiography can overcome much of this type of problem.⁶

The cerebral arteriovenous malformation (AVM) is not generally considered as a cause of stroke per se, because of the differences in etiology between the two; stroke being mainly an acquired condition associated with aging, and the AVM a congenital condition seen in younger individuals. However, AVM's produce similar neurological symptoms, and are diagnosed and treated in much the same way, therefore a discussion of cerebrovascular disease is incomplete without considering them. An AVM is a tortuously dilated collection of abnormal vessels characterized by multiple enlarged feeder arteries and dilated venous outflow vessels, resulting in rapid flow of blood through the vessels. The accelerated blood flow produces three pathological conditions. Blood is shunted away from normal brain tissue, resulting in inadequate blood supply. Because the flow is rapid and the vessels are enlarged, hemorrhage is a frequent occurance. Neurological deficits may result from the mass effect of the growing lesion on adjacent brain structures. The symptoms produced by these factors include severe headache, seizures, and progressive neurological deficits.¹¹

Cerebral AVM's occur due to congenital abnormal development of the cerebral vasculature. Structurally, the abnormal vessels have characteristics of both arteries and veins. As these structures have a tendency to enlarge, they vary in size. Secondary conditions that may occur include thrombosis, calcification and thrombosis.¹¹

Most AVM's become symptomatic during the second and third decades of life. 80% are manifest before age 40. 10% of intial bleeds are fatal, while 30% produce some degree of disability.¹¹

Screening imaging modalities commonly used in the diagnosis and evaluation of AVM's are CT, MRI, and MRA, with confirmation and surgical evaluation by angiography. In order to help determine the course of treatment, information must be obtained about the location, size, and the number of feeder vessels of the AVM. MRA has shown particular promise as a non-invasive modality because of its ability to demonstrate the nidus of the AVM as well as delineate multiple feeder arteries and draining veins¹⁵. Surgery carries low risks of mortality and morbidity if the AVM is small. Most small AVM's are resected unless they are located in the vital areas of the brain stem or basal ganglia. For those located in non-surgical areas, radiotherapy of at least 2500 rads can effectively reduce the AVM and alleviate symptoms. However, this is not a viable option with AVM's larger than 3cm. due to increasing risk to healthy tissue and less effectiveness in reducing the lesion. The most attractive option for large AVM's is selective transcatheter embolization. This involves placement of a small calibre "microcatheter" as close as possible to the nidus of the AVM, and the injection of an embolic material to induce thrombosis. Materials currently used for embolization include Ivalon, detachable balloons, coils, and methylacrylate glue.^11,13,14

Less frequently occuring types of AVM's are telangectasias, cavernous angiomas, venous angiomas, and vein of Galen aneurysms.¹¹