CT angiography data offer new approach to perfusion stroke imaging

Acute stroke is a medical emergency that is potentially treatable. Because "time is brain," however, the imaging workup must be fast and therapy initiated rapidly. Images should first be checked to exclude signs of hemorrhage. Practitioners then need to visualize any early ischemic changes, demonstrate the presence of hypoperfusion, locate any underlying vascular pathology, and elucidate the presence of a potential penumbra.¹ The presence— or absence—of vessel occlusion is particularly important if the patient is to be treated with recombinant tissue plasminogen activator (rtPA).

A variety of imaging techniques can be used to assess irreversible damage to the infarct core and to evaluate tissue at risk of infarction. These include diffusion- and perfusion- weighted MRI, MR angiography, unenhanced CT, CT angiography, and perfusion-weighted CT (Figures 1 and 2).²

MRI is, theoretically, the method of choice for stroke imaging. It can provide precise information about cerebral perfusion, the size and territory of the acute infarction, microangiopathic changes, and the status of cerebral vasculature throughout the entire brain without exposing patients to ionizing radiation. It is not, however, used as the standard modality for acute imaging examinations in the majority of stroke centers. This is due to the limited availability of MR systems, the relatively long time that the scanner would be occupied, noncompliant patients, and various contraindications, such as cardiac pacemakers.

CT typically plays the predominant role in acute stroke imaging. The time needed to complete a CT examination is very short, scanners are more widely available, fewer contraindications must be considered, and patient monitoring is easier. Economic considerations also favor CT.

Concerns over the link between nephrogenic systemic fibrosis and gadolinium-based MR imaging contrast agents³ and the appearance of studies demonstrating no or low incidence of contrast-induced nephropathy following CT stroke imaging with iodinated contrast^4,5 suggest that CT will play an even more important role in stroke imaging in the future.

PERFUSION PROTOCOLS

Information about at-risk brain tissue requires that data on cerebral blood perfusion be obtained. The most common approach is to acquire a dynamic scan series during contrast injection. This can be achieved with either CT or MRI. The following parameters can then be calculated and mapped:

• cerebral blood flow (CBF): volume of blood flow per unit of brain tissue per minute; normal range in gray matter = 50 to 60 mL/100 g/min.
• cerebral blood volume (CBV): volume of blood per unit of brain tissue; normal range = 4 to 5 mL/100 g.
• mean transit time (MTT): time difference between arterial inflow and venous outflow.
• time to peak (TTP): time between contrast injection and attenuation peak.

It is usually sufficient to calculate either MTT or TTP, not both. The choice will depend on the algorithm used; deconvolution algorithms provide both parameters but need more user interaction and calculation time, while maximum-slope algorithms are less time-consuming but provide only TTP values.

Regional changes observed in these maps can be used to predict infarction. The penumbra would be expected to show one of the following:

• increased MTT with moderately decreased CBF ( > 60%), combined with normal or increased CBV (80% to 100% or higher), secondary to autoregulatory mechanisms; or
• increased MTT with markedly reduced CBF ( > 30%) and moderately reduced CBV ( > 60%).