Higher frequencies are attenuated by tissue more than lower frequencies. This means that the higher the frequency the lower the depth of penetration but the greater the resolution.
Harmonic imaging allows the use of a lower frequency pulse to be picked up and sampled at the second harmonic (higher frequency) where the low frequency allows greater penetration and high frequency provides better resolution.

See also Skinline.

(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
The effect of different frequencies on tissue penetration:
The higher the frequency the less the penetration, the lower the frequency the greater the penetration. As frequency increases, resolution improves but the imaging depth or penetration decreases. The lower the axial resolution, the more detail can be seen.
Usual frequencies for pediatric ultrasound: 5.0mHz to 7.5mHz and 10mHz.
Usual frequencies for adult ultrasound: 2.0mHz to 3.0mHz.

See also Doppler Interrogation Frequency, Multi-frequency Probe, and Huygens Principle.

Axial resolution is the minimum separation between two interfaces located in a direction parallel to the beam (objects above and below each other) so that they can be imaged as two different interfaces. The axial space resolution directly relates with the wave frequency, but higher frequencies have lower penetration into tissues.
The axial resolution is inversely proportional to the frequency of the transducer depending on the size of the patient. The higher the frequency the lower the axial resolution in large patients. This state results from the rapid absorption of the ultrasound energy with lower penetration. Lower frequencies are utilized to increase depth of penetration.

See also Damping.

(THI) Tissue harmonic imaging (also called native harmonic imaging) is a signal processing technique which addresses ultrasound limitations like penetration and resolution. Tissue harmonic imaging reduces noise and clutter by improving signal to noise ratio and resolution. The signal penetration in soft tissue increases as the transmit frequency is decreased, by simultaneous decreased image resolution. As an ultrasound wave propagates through the target media a change occurs in the shape and frequency of the transmitted signal. The change is due to the normal resistance of tissue to propagate sound energy. This resistance and the resulting signal change is called a harmonic oscillation.
For harmonic imaging the input frequency doubles the output frequency, for example a transmit frequency of 3.0 MHz. which would provide maximum penetration will return a harmonic frequency of 6.0 MHz. The returning higher frequency signal has to only travel one direction to the probe. The advantages of high frequency imaging and the one-way travel effect are decreased reverberation, beam aberration, and side lobes, as well as increased resolution and cystic clearing.

Conventional, CT and MR imaging technologies are limited in their availability, to depict soft tissue, or to show dynamic activity, like cardiac muscle contractility and blood flow. Easy applicability, real-time sonography and biopsy facilitation are important advantages in veterinarian medicine. Veterinary ultrasound has a very high sensitivity to show the composition of soft tissues, but the low specificity is a disadvantage. High ultrasound system performance includes Doppler techniques, contrast enhanced ultrasound, 3D ultrasound, and tissue harmonic imaging to improve resolution.
Technical and physical requirements of veterinary ultrasound are the same as in human ultrasonography. The higher the sound frequency, the better the possible resolution, but the poorer the tissue penetration. Image quality is depended of the ultrasound equipment. For example, a 10 MHz transducer is excellent for imaging of superficial structures; a 3.5 or 5.0 megahertz transducer allows sufficient penetration to see inner structures like the liver or the heart. In addition, the preparation and performing of the examination is similar to that of humans. The sound beam penetrates soft tissue and fat well, but gas and bone impede the ultrasonic power. Fluid filled organs like the bladder are often used as an acoustic window, and an ultrasound gel is used to conduct the sound beam.