A filter in electronics is a circuit that only passes certain signals. In ultrasound is a filter a device to suppress acoustic or electromagnetic waves of certain frequencies, letting other frequencies pass, e.g. high pass filter, low pass filter. The high pass filter is the wall filter (also called thump filter) used in Doppler devices to eliminate low frequency Doppler shifts caused by clutter. For blood flow measurement, a low pass filter is often used to strip out high frequency noise, leaving only the biological components of interest. Frame averaging is a form of a low pass filter.

The wall filter is designed to exclude Doppler signals with low frequency, and high amplitude (highpass filter) from moving tissue, such as a vessel wall. Wall filter performance is critical for effectiveness of a color Doppler system.

(CD) Color Doppler is an ultrasound imaging mode, which visualizes the presence, direction and velocity of flowing blood in a wide range of flow conditions. It provides an estimate of the mean velocity of flow within a vessel by color coding the flow and displaying it superimposed on the 2D gray scale image. The flow direction is arbitrarily assigned the color red or blue, indicating flow toward or away from the transducer.
Color (colour, Brit.) Doppler ultrasound is capable of evaluating a wider area than other Doppler modes than for example Duplex or power Doppler, and therefore makes it less likely to miss flow abnormalities. It is also easier to interpret. Color flow is not as precise as conventional Doppler and is best used to scan a larger area and then use conventional Doppler for detailed analysis at a site of potential flow abnormality.

Adjustments for color Doppler in case of too much color:
Adjustments for color Doppler in case of not enough color:
See also Color Power Doppler, Autocorrelation, Color Priority, Triplex Exam and Color Saturation.

B-scan comined with D-scan (D=Depth) is used to avoid image inhomogeneity. Different transmitter signals for each depth are applied and prefiltered pseudoinversely according to the transfer properties of the covering tissue. Pulse compression techniques with nonlinearly frequency modulated signals are used to gain the required energy for inverse filtering.
D-scan is a modified C-scan used in nondestructive testing with the display of amplitudes. In the 2D graphical presentation, time of flight values are displayed in the top view on a test surface.

See also A-Scan, B-Scan and C-Scan.

From Medison Co.,Ltd.;
'The SONOACE 9900 offers exceptional B/W image clarity by newly developed FINE™ Filter technology, which contributes to edge enhancement and average filtering in real-time. OSIO™(Organ Specified Image Optimization)sets the best diagnosis environment with minimum manual operation and increases patient throughput. Also, color images are improved with better color Doppler performance, high color sensitivity, fine pixel image, and reduced flash artifact across all probes.'

'The SONOACE 9900 PRIME from Medison offers full range of applications specially designed for your practice. With the addition of the new C-Square Processing and PSAD Beamformer, SONOACE 9900 PRIME provides higher contrast resolution and clearer image quality in all application ranges. Based on super clear image resolution and versatile 3D functions, SONOACE 9900 PRIME offers you the true value of future ultrasound technology.'