Physics, Techniques and ProceduresHigh pass filter
a filter that removes frequencies below a certain threshold. In
Doppler ultrasound, a high-pass filter ("wall filter") is used to remove the strong signals from slowly moving tissues such as vessel walls and
cardiac leaflets.
These structures may cause specular reflection, with echo amplitudes about 1 000 times higher than those of the backscattered echoes from blood. Without high-pass filtering, these high-amplitude, low-frequency Doppler signals may severely disturb flow information by creating "wall thump" (noise in the speakers) and clutter signals (high amplitude spikes in the time - velocity spectrum). The threshold of the high-pass filter is usually selectable. Too high a threshold may remove signals also from slowly moving blood. When measuring blood flow velocity in low velocity vessels such as veins, the high pass filter should be adjusted to its minimum value.
In spatial filtering of digital images (see filtering of images), enhancement of the higher spatial frequencies will produce better defined edges and more noise in the images ("high-pass spatial filtering"). See also edge enhancement.
HJS