Harmonic imaging of the vertebrobasilar system.

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Abstract
Background and Purpose Gas bubbles of ultrasound contrast agents resonate at frequencies used for diagnostic ultrasound and produce harmonics or multiples of the transmitted frequency. Processing of the second harmonic frequency results in a reduction of the signal-to-noise ratio and the signal-to-tissue artifacts. This study is the first to evaluate second harmonic imaging in the cerebral circulation. Methods We used a duplex system (HP SONOS 2500) in connection with a 1.8/3.6-MHz (second harmonic) and a 2.5-MHz (conventional) sector transducer. Levovist (6.5 mL; 400 mg/mL) was injected intravenously for second harmonic and conventional color duplex imaging in 13 healthy volunteers (age range, 23 to 34 [median, 29] years). Results When second harmonic imaging was compared with conventional color duplex imaging, more cerebellar arteries were detected (35 versus 31), the duration of blooming artifact was significantly reduced (7.9 versus 29.9 seconds; P =.03), and the duration of diagnostically useful signal enhancement was increased (248.5 versus 117.4 seconds; P =.0003), but the maximal investigation depth was reduced (8.4 versus 9.3 cm; P =.001). When conventional and second harmonic duplex were compared, there was a significant ( P <.04) difference in the systolic blood flow velocity in the vertebral and basilar arteries. Conclusions Second harmonic color duplex imaging in the vertebrobasilar system increases the time of diagnostic useful signal enhancement and produces a better spatial resolution compared with conventional color duplex imaging.