The results of the Doppler ultrasound examination are very dependent on the sonographer. Accuracy and finesse are acquired by experience. Here are some techniques to help improve your accuracy.
Choose an alternate Doppler frequency. Higher Doppler frequencies provide small sample volume sizes and greater sensitivity to slow or low flow. When using high Doppler frequencies penetration is an issue. In this situation only superficial vessels may be imaged due to the greater attenuation in tissue. Lower Doppler frequencies offer greater penetration and a lower aliasing velocity.
Angle the Doppler beam. Optimal visualization of a structure is obtained at perpendicular angles. However, to detect flow within a structure an angle is needed. In all examinations the ideal angle is less than 60°.
Change the PRF. The PRF should be set high enough that aliasing does not occur, but not so high that the spectral trace is so small it is difficult to accurately measure velocities, or detect low frequencies. The PRF should be changed throughout the examination especially in pathologic states such as stenosis, occlusion or clot.
Adjust the Doppler Gain. Increasing sensitivity for a Doppler signal can be achieved by increasing the receiver gain to the point where noise is seen on the Doppler display. Once the signal is obtained, decrease the gain to remove the noise so that an adequate range of gray levels is displayed. If the gain level is set too high a mirror image artifact will occur. This will be displayed as positive and negative flow around the baseline. If the gain setting is too low a Doppler signal may not be detected.
