Recent technological advances suggest, however, that perfusion imaging of the contralateral hemisphere through the temporal bone window will be possible. If a constant concentration of contrast agent is delivered to brain tissue using a constant UCA infusion rate, then after destruction

with high MI ultrasound, new microbubbles BAY 73-4506 nmr will enter this field with a certain velocity, will travel a determined distance and fill a certain tissue volume depending on blood velocity. The intensity of the echo response signal is directly related to the contrast agent concentration in the tissue; therefore, the blood flow assessment is based on monitoring the intensity of the echo response signal of the insonated volume after bubbles destruction. Low-MI ultrasound imaging can be used to monitor microbubble replenishment in real time (Fig. 2) following the application of destruction pulses at high MI. The behavior of the refill kinetics can be assessed with an exponential curve fit [2]. The parameters of this exponential curve are related to cerebral blood flow: blood flow velocity is directly related to the rate constant β, the fractional vascular volume is related to the plateau echo CP-690550 manufacturer enhancement

(A), and the product of both (A · β) is associated with blood flow [3]. More sophisticated algorithms for characterization of microbubble refill have been recently introduced [4] and [5], which should increase reproducibility and improve the quantification of cerebral blood flow with ultrasound perfusion imaging. Since individual

microbubbles can be depicted flowing through small vessels in the brain with low MI imaging, it is possible to track these bubbles and map perfusion over time. Dynamic microvascular microbubble maps provide excellent demarcation of MCA infarctions (Fig. 3) and provide impressive displays of low velocity tissue microbubble refill following destruction with high mechanical index imaging. In brain regions showing delayed contrast bolus arrival on perfusion-weighted MRI, Metformin mouse ultrasound shows decreased or absent microbubble refill kinetics. This new technique has been applied for diagnosis of acute ischemic stroke. Recent results demonstrate that real-time ultrasound perfusion imaging with analysis of microbubble replenishment correctly identifies ischemic brain tissue in acute MCA stroke [6]. Pulse compression methods are being combined with the nonlinear bubble imaging techniques discussed above for highly sensitive contrast imaging with very low noise. These advances will lead to further improvements of microbubble imaging of the brain microcirculation, making ultrasound perfusion imaging a viable application for bedside assessment of acute stroke patients. Ultrasound applied as an adjunct to thrombolytic therapy improves recanalization of occluded intracerebral vessels and microbubbles can amplify this effect.