(HVL) The attenuation of ultrasound waves in human tissue is characterized as the half value layer, or the half power distance. Half value layer means the distance the sound beam will travel in a tissue before its amplitude or energy is attenuated to half its original value. Air and lung tissue have extremely short half-power distances and represent severe obstacles to the transmission of acoustic energy.

A handle or probe contains the transducer elements and is attached to the system via a relatively long coaxial cable. Handles are selected via high voltage (HV) relays. In most ultrasound machines, several different handles are available to be connected to the system, allowing the operator to select the appropriate transducer for optimal imaging.

See also Probe Cleaning, and Transducer Assembly.

Harmonic is an oscillation of a system at a frequency that is a simple multiple of its fundamental frequency. The fundamental frequency of a sinusoidal oscillation is called the first harmonic. The second harmonic has a frequency doubled that of the fundamental.

See also Fundamental Imaging, Harmonic Imaging, Subharmonic Imaging and Superharmonic Imaging.

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.

Harmonic imaging relies on detection of harmonics of the transmitted frequency produced by bubble oscillation. This method is widely available on ultrasound scanners and uses the same array transducers as conventional imaging. A major limitation of the use of ultrasound contrast agents is the problem that signals from the microbubbles are mixed with those from tissue. Echoes from solid tissue and red blood cells are suppressed by harmonic imaging.
In harmonic mode, the system transmits at one frequency, but is tuned to receive echoes preferentially at double that frequency, and the second harmonic echoes from the place of the bubble. Typically, the transmit frequency lies between 1.5 and 3 MHz and the receive frequency is selected by means of a bandpass filter whose center frequency lies between 3 and 6 MHz.
Color Doppler and real-time harmonic spectral Doppler modes have also been implemented and show a level of tissue motion suppression not available in conventional modes.

See also Harmonic B-Mode Imaging, and Harmonic Power Doppler.