Noninvasive cerebrospinal fluid shunt flow measurement by Doppler ultrasound using ultrasonically excited bubbles: A feasibility study

Because normal cerebrospinal fluid (CSF) has almost no natural Doppler scat terers, patency testing of ventriculoperitoneal cerebrospinal fluid shunts (small silastic tubing with lumen diameter of similar to 1 mm draining exce ssive CSF from the brain) cannot be performed by Doppler ultrasound, We hav e developed a low-frequency bubble excitation system that generates microbu bble scatterers in both distilled water and CSF, Doppler ultrasound can the n be used for flow measurement in a ventricluloperitoneal shunt. By using l ow duty-cycle (similar to 10%), low-frequency (similar to 30 kHz), and low- amplitude (similar to 30 kPa) ultrasound, a population of microbubbles can be maintained for sufficiently long times (> 10) min) for Doppler ultrasoun d measurement, although bubble initiation is inconsistent, The minimum pres sure needed for bubble maintenance was found to decrease with increasing bu rst length and duty cycle. It has been possible to detect the presence of C SF shunt flow down to a mean flow rate of 3 mL/h (mean velocity similar to 0.6 mm/s). The bubble maintenance scheme developed satisfies the safety par ameters specified by the American Institute of Ultrasound in Medicine (AIUM ) and the US Food and Drug Administration (FDA). Results from both in vitro and in vivo ( externalized shunts) experiments indicate the feasibility of this scheme for determining realistic CSF shunt flows, though some practic al problems remain before the technique will be ready for clinical use. (C) 1999 World Federation for Ultrasound in Medicine & Biology.