AJR: Top tips for optimizing CT stroke workup

Perfusion CT for stroke diagnosis. Image Source: Radiology

With the imaging workup for acute ischemic stroke is progressing expansively beyond unenhanced head CT, the imaging protocol has become more complex and requires updated attention from radiologists, according to an article published in the January issue of the American Journal of Roentgenology (AJR).

In light of the changes to acute ischemic stroke imaging, the authors released responses to the 10 most frequent questions they receive from colleagues interested in building a stroke program involving perfusion CT imaging.

The authors acknowledged that, in making the initial judgment as to whether to perform CT/perfusion CT or MRI/MRI perfusion, the results are often similar, though their technical advantages and limitations wield subtle variations. For example, MRI may be preferable in younger patients and in those with renal failure or allergies, as MRI yields no radiation and can be performed without intravenous contrast. On the other hand, the relatively large number of MR contraindications (mechanical implants and most pacemakers, for example) and the availability of absolute perfusion thresholds resulting from greater quantitative accuracy bode strongly for CT.

Moreover, "the key element supporting the use of CT in the initial evaluation of patients suspected of acute ischemic stroke is its wide availability, which is especially important in the emergency setting. CT scanners are available throughout the day and night in the wide majority of medical institutions," wrote Carlos Leiva-Salinas, MD, of the department of radiology at the University of Virginia in Charlottesville, and co-authors.

From the clinical standpoint, the authors also emphasized CT as the preferred method for the evaluation of patients with acute cerebral infarction who are in the therapeutic window for thrombolysis, because CT provides more rapid detection of hemorrhages.

Leiva-Salinas and colleagues considered the optimal ordering of perfusion CT and CT angiography (CTA). They observed that many institutions prefer CTA first because the test will provide important CTA data before patient motion increases and degrades the quality of images. Nevertheless, Leiva-Salinas and colleagues recommended performing perfusion CT first, saying that in their "experience, the advantages provided by optimal timing of CTA imaging (allowed by the prior perfusion CT contrast administration) outweigh the disadvantages of perfusion CT-related venous contamination of the CTA images."

The authors also provided a detailed description of what information can be expected from perfusion CT as well as an explanation of the processing that underlies the calculation and pixel-mapping of perfusion parameters. "The primary goal of acute stroke imaging is to provide an assessment of ischemic tissue viability on the basis of the relative extent of infarcted tissue (region of tissue that cannot be salvaged) and penumbra (region that is at risk for infarction in the absence of adequate therapy)," wrote Leiva-Salinas and colleagues.

Leiva-Salinas and co-authors sought to dispel the original model that recommended selecting the arterial input function as close as possible to the arterial territory being evaluated. According to the authors, although more specific arterial input function considerations are important in patients with chronic vascular conditions, "several studies have shown that, in acute stroke patients, the selection of the arterial input function does not have a significant impact on the perfusion CT parametric maps if the ipsilateral carotid artery is not stenotic."

Meanwhile, the authors cited the exclusive dependence of cerebral blood volume values on the selection of a vein as a reference for venous output function. "The site for a reference venous output function should be chosen using a voxel having the maximal area under the time-density curve or, stated differently, the least amount of partial volume averaging. The voxel at the center of the superior sagittal sinus, which has the advantage of being large and orthogonal to axial perfusion CT source images, is adequate for this purpose in a large majority of cases."

The authors rounded out the list of common perfusion CT questions by discussing the interpretation of irregular findings, closing with emphasis on "the importance of CT quality assurance programs" in regard to radiation exposure. "Radiologists and physicists should ensure that all CT protocols respect the as low as reasonably achievable dose principle," the authors stated. And although a comprehensive CT protocol involving head CT, perfusion CT and CTA "can deliver a large mean effective dose (on the order of 7–8 mSv)," Leiva-Salinas pointed out that not every patient requires this workup, concluding that "[p]erfusion CT imaging, when appropriately and correctly performed, is justified and provides safe valuable information that can substantially contribute to the management of acutely ill patients with acute cerebrovascular disease."