The effect of position on magnetic resonance evaluation of cerebrospinal fluid shunt function

Since cerebrospinal fluid shunt malfunction is common and its diagnosis often requires invasive testing, we have evaluated magnetic resonance imaging (MRI) as a noninvasive test for shunt function. In prior experiments, MRI has been shown to be capable of detecting flows as low as 0.4 cc h-1 in shunt systems. Because patient movement and position in an MRI scanner are variable, we have studied the effects of shunt position on the ability to reliably detect the rate of CSF flow. Flow related enhancement of fluid flowing from 0 to 2 cc h-1 was measured in a specialized section of tubing using multiple spin echo scans (TR2000,TE20). The angle of the specialized tubing was varied relative to the axis of the MRI scanner and then the intensity of the MR signal at specific points in the tubing was measured at different flow rates. No flow standards were used as controls. The results show that the linear relationship between signal intensity and flow that had previously been demonstrated was significantly altered by changes of position. Positions angled superior to the axis of the scanner enhanced the signal while lateral changes in the plane of the scan degraded the signal. These results suggest that in order for MRI to become an accurate test for shunt function, techniques must be developed to overcome the effects of shunt position.