1. Aetna considers cerebral computed tomography (CT) perfusion imaging experimental and investigational for assessing persons suspected of having an acute stroke or in triaging persons with stroke for thrombolytic therapy because there is inadequate published evidence that CT cerebral perfusion imaging improves outcomes over standard non-contrast CT scanning.

2. Aetna considers CT cerebral perfusion imaging experimental and investigational for evaluating persons with chronic cerebral ischemia, cerebral vasospasm, head trauma, cerebral gliomas, herpes simplex virus encephalitis, or for use in the balloon occlusion test, or for other indications because there is inadequate scientific evidence to support its use for these indications.

Computed tomography (CT) perfusion imaging provides a quantitative measurement of regional cerebral blood flow.  A perfusion CT study involves sequential acquisition of CT sections during intravenous administration of an iodinated contrast agent.  Analysis of the results allows the physician to calculate the regional cerebral blood volume, the blood mean transit time through the cerebral capillaries, and the regional cerebral blood flow.

Currently, non-contrast computed tomography is used to detect intracerebral hemorrhage in stroke patients who are being considered for thrombolytic therapy.

CT perfusion imaging has been proposed to be used primarily as a method of evaluating patients suspected of having an acute stroke whenever thrombolysis is considered.  CT perfusion imaging may provide information about the presence and site of vascular occlusion, the presence and extent of ischemia, and about tissue viability.  This information may help the clinician determine whether thrombolysis is appropriate.

Potential advantages of CT perfusion imaging are that it can be performed using standard CT scanners, which are more widely available and less expensive than MRI, and it is less invasive than CT angiography.  CT perfusion imaging can be performed rapidly, and involves injection of a relatively small amount of contrast agent.

Current literature on CT perfusion imaging has focused on its feasibility and technical capabilities.  Prospective clinical studies are needed to determine the clinical value of CT perfusion imaging over standard non-contrast computed tomography in the assessment of patients with symptoms suggestive of acute stroke, and in the triage of patients in whom thrombolytic therapy is contemplated.

The Council on Cardiovascular Radiology of the American Heart Association provided guidelines and recommendations for perfusion imaging in cerebral ischemia (Latchaw et al, 2003).  It stated that quantitative CT perfusion may possibly be useful to differentiate between reversibly and irreversibly ischemic tissues in patients with acute stroke.  However, large prospective and appropriately blinded studies are needed to ascertain the value of this technique.  There are no data regarding the ability of this technique to predict the potential for hemorrhage following thrombolysis, as there is for the diffusible tracer techniques.  Furthermore, no recommendation can be made for the use of CT perfusion in patients with chronic ischemia, vasospasm, head trauma, or as part of the balloon occlusion test, the traditional method for identifying patients at risk for stroke.

In a review on imaging viable brain tissue with CT scan during acute stroke, Meuli (2004) stated that perfusion CT is now ready to be used in clinical trials as a decision-making tool to tailor more precisely the thrombolytic therapy to the individual patient.

Ding et al (2006) simultaneously examined regional cerebral blood volume (rCBV) and permeability surfaces (rPS) in glioma patients to determine their correlation with histological grade using CT perfusion imaging.  A total of 22 patients with gliomas underwent multi-slice CT perfusion imaging pre-operatively.  Low-grade and high-grade groups were categorized corresponding to WHO grade II gliomas and WHO grade III or IV gliomas, respectively, as determined by histopathological examination.  Regional cerebral blood volume and rPSs were obtained from regions of maximal abnormality in tumor parenchyma on CBV and PS color perfusion maps.  Perfusion parameters were compared using the Kruskal-Wallis test in order to evaluate the differences in relation to tumor grade.  The Pearson coefficients of rCBV and rPS for each tumor grade were assessed using SPSS 13.0 software.  Regional cerebral blood volume and rPS provided significant P-value in differentiating glioma grade (low-grade gliomas 3.28 +/- 2.01 versus 2.12 +/- 3.19 ml/100 g/min, high-grade gliomas 8.87 +/- 4.63 versus 12.11 +/- 3.18 ml/100 g/min, p < 0.05).  Receiver operating characteristic (ROC) curves revealed better specificity and sensitivity in PS than in CBV for glioma grade.  A significant correlation between rCBV and rPS was observed in high-grade gliomas (r = 0.684). Regional cerebral blood volume in oligodendrogliomas were higher than in other low-grade gliomas, whereas their rPS values did not show a parallel difference.  The authors concluded that perfusion CT provides useful information for glioma grading and might have the potential to significantly impact clinical management and follow-up of cerebral gliomas.

Marco de Lucas et al (2006) noted that an early diagnosis is crucial in herpes simplex virus encephalitis patients in order to institute acyclovir therapy and reduce mortality rates.  Magnetic resonance imaging is considered the gold standard for evaluation of these patients, but is frequently not available in the emergency setting.  These investigators reported the first case of a CT perfusion study that helped to establish a prompt diagnosis revealing abnormal increase of blood flow in the affected temporo-parietal cortex at an early stage.