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.

Harmonic B-mode imaging takes advantage of the non-linear oscillation of microbubbles. During harmonic imaging, the sound signal is transmitted at a frequency of around 1.5 to 2.0 MHz and received at twice this frequency. The microbubbles also reflect waves with wavelengths different from the transmitted one, the detectors can be set to receive only the latter ones and create only images of the contrast agent.
Using bandpass filters the transmitted frequency is separated from the received signal to get improved visualization of vessels containing ultrasound contrast agents (USCAs). The signal to noise ratio during the presence of microbubbles in tissue is four- to fivefold higher at the harmonic compared with the basic frequency.
Using harmonic B-mode imaging, harmonic frequencies produced by the ultrasound propagation through tissue have to be taken into account. The tissue reflection produces only a small amount harmonic energy compared to USCAs, but has to be removed by background subtraction for quantitative evaluation of myocardial perfusion.

See also Non-linear Propagation.

A logarithmic measure of sound loudness closely related to the decibel. The unit decibel is used for objective measurements, that means, they measure the actual pressure of the sound waves as recorded using a microphone. The unit phon is used for subjective measurements, which means, measurements made using the ears of a human listener.
A sound has the loudness 'p' phon if it seems to the listener to be equal in loudness to the sound of a pure tone of the frequency 1 kilohertz and strength 'p' decibel. A measurement in phons will be similar to a measurement in decibel, but not identical, since the perceived loudness of a sound depends on the distribution of frequencies in the sound as well as the pressure of the sound waves. In the U.S., sound loudness is frequently measured in sones rather than phons: a sound of loudness x sones has loudness 10 log2 x + 40 phons.

See also Acoustic Noise.

(TCD) Transcranial color Doppler sonography allows to evaluate the presence and flow direction of vessels as well as their relationships to surrounding structures.
A disadvantage of cerebrovascular ultrasonography is the attenuation of the ultrasound signal by the skull. The loss of power through the skull is considerable, the signal to noise ratio is poor and so contrast enhanced Doppler imaging is advantageous. The use of ultrasound contrast agents provides a diagnostic window of sufficient duration and imaging quality to improve an evaluation of the cerebral vessels. Contrast TCD also results in visualization of small arteries and veins and greater length of these vessels.

See also A-Mode, Cranial Bone Thermal Index, Transcranial Color Coded Sonography and Transcranial Window.

(TVS) The transvaginal sonography (transvaginal echography or endovaginal ultrasound), uses a small vaginal transducer (5-7.5 MHz) that is inserted into the vagina to depict the inner female organs (uterus, ovaries, vessels). As a general rule, ultrasound works better when the probe is close to the area being examined. Compared with a sonogram through the abdominal wall, the transvaginal technique produces a sharper image, not only because of the close proximity to the uterus, but also because the better signal to noise ratio of the used transducer.

Indications:

The advantage of transvaginal sonography is being able to get very close to the structures of the pelvis, and thus get better images and a more reliable diagnosis.

See also Pelvic Ultrasound, Hysterosalpingo Contrast Sonography.