(SNR) Signal to Noise Ratio is the ratio of the amplitude of a signal to undesired noise. The larger the signal noise ratio, the easier is the detection and measurement of a signal. The sensitivity of any device is limited by the SNR. The SNR for sound is often expressed in decibel.

Increasing the frequency of the transmitted power improves the image quality of ultrasound, but the improvement in resolution results in a decreased signal to noise ratio (SNR). Higher acoustic power levels can prevent the loss in SNR, but among other reasons, ultrasound regulations limit this to avoid heating or cavitation.
Coded excitation increase the signal to noise ratio without the loss of resolution by using coded waveforms. Coded excitation allows transmitting a long wide-band pulse with more acoustic power and high penetration of the sound beam.

(CEUS) Contrast agents increase the reflection of ultrasonic energy, improve the signal to noise ratio and caused by that the detection of abnormal microvascular and macrovascular disorders. Contrast enhanced ultrasound is used in abdominal ultrasound (liver sonography) as well as in cerebrovascular examinations e.g., for an accurate grading of carotid stenosis. The used contrast agents are safe and well tolerated.

The quality of the enhancement depends on:
The additional use of ultrasound contrast agents (USCAs) may overcome typical limitations like poor contrast of B-mode imaging or limited sensitivity of Doppler techniques. The development of new ultrasound applications (e.g., blood flow imaging, perfusion quantification) depends also from the development of pulse sequences for bubble specific imaging. In addition, contrast enhanced ultrasound improves the monitoring of ultrasound guided interventions like RF thermal ablation.

See also Contrast Enhanced Doppler Imaging, Contrast Harmonic Imaging, Contrast Imaging Techniques and Contrast Pulse Sequencing.

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.

An undesirable background interference or disturbance that affects image quality.
The noise is commonly characterized by the standard deviation of signal intensity in the image of a uniform object (phantom) in the absence of artifacts. The measured noise may depend on the particular phantom used due to variable effects. Noisy images appear when the signal to noise ratio is too low. There are various noise sources in any electronic system, including Johnson noise, shot noise, thermal noise.

See also Interference Artifact.