Spectral analysis is the quantitative analysis method to display the distribution of frequencies. A difficult Doppler signal is separated into the frequency components so that the range of frequencies in a Doppler shifted signal can be analyzed. This allows measurement of blood flow velocity by positioning of a probing cursor in the artery (on the screen), and the signal representing blood flow velocity is generated. The peaks and ebbs create the spectrum, corresponding to systolic and diastolic blood flow. The signal is both visual and auditory.

A tri-frequency probe emits three different frequencies. Probes with tri-frequency capabilities allow a wide range of scanning applications from a single probe.

See also Multi-Frequency Probe and Dual Frequency Phased Array Transducer.

An ultrasound (US) scanning gel has the same conductivity as the human body and is applied between the transducer and the skin surface. Air is a bad conductor of US, so this acoustic gel is used to conducts the sound beam and allows the ultrasound probe to pass smoothly over the skin.
The gel will be removed after the examination, and it will not stain skin or clothing. The basic dermatological requirement of a scanning gel is that it be free of skin irritants or sensitizers. In addition, effective preservatives with low incidence of skin reaction are required to prevent microbiological degradation of the gel. The broad range of patients imaged with ultrasound, from pregnant women and infants to the infirm or elderly dictates that the risk of skin reaction must be minimized.
The effect of small bubbles in the ultrasound couplant under the transducer is to disperse the ultrasound which results in clouding of the image. This effect is most clearly seen on anechoic regions of the image which becomes cloudy. Air bubbles, regardless of their size, degrade the performance of ultrasound in all medical applications including imaging, Lithotripsy and physical therapy.
There are some chemicals, including mineral oil, silicone oil, alcohol, surfactants, and fragrances that can degrade the acoustic lens, destroy bonding, or change the acoustic properties of the lens. The use of scanning gels or lotions in diagnostic ultrasound containing these chemicals should be avoided. In therapeutic ultrasound, ultrasound transmission gels and lotions commonly contain oils and other chemicals not intended for use with diagnostic imaging transducers.

See also Ultrasound Therapy and Ultrasound Physics.

Ultrasound machines, widely used in medical imaging, are essential tools in the field of diagnostic ultrasound. These devices utilize high-frequency sound waves to create real-time images of internal body structures. Ultrasound machines consist of several key components that work together to generate diagnostic images. These include:
B-mode machines represent the vast majority of machines used in echocardiology, obstetrical scans, abdominal scans, gynecological scans, etc. B-mode ultrasound machines usually produce the sector (or pie segment-shaped) scans. These ultrasound scans require either a mechanical scanner transducer (the transducer moves to produce the sector scan), or a linear array transducer operated as a phased array.


Ultrasound machines come in different types, each catering to specific clinical needs. The two primary types are stationary and portable ultrasound machines:


See also Handheld Ultrasound, Ultrasound System Performance, Equipment Preparation, Coaxial Cable, and Microbubble Scanner Modification, Environmental Protection and Ultrasound Accessories and Supplies.

The traveling ultrasonic wave causes a low-level ultrasound radiation force when this energy is absorbed in tissues (absorbed dose). This force produces a pressure in the direction of the beam and away from the transducer. It should not be confused with the oscillatory pressure of the ultrasound wave itself. The pressure that results and the pressure gradient across the beam are very low, even for intensities at the higher end of the range of diagnostic ultrasound. Mechanical effects like radiation forces lead to stress at tissue interfaces. The effect of the force is manifest in volumes of fluid where streaming can occur with motion within the fluid. The fluid velocities which result are low and are unlikely to cause damage.
The effects of ultrasound radiation force (also called Bjerknes Forces) were first reported in 1906 by C. A. and V. F. K. Bjerknes, when they observed the attraction and repulsion of air bubbles in a sound field.
While incompressible objects do experience radiation forces, compressible objects driven at their resonant frequency experience far larger forces and can be observably displaced by low-amplitude ultrasound waves. A microbubble driven near its resonance frequency experiences a large net radiation force in the direction of ultrasound wave propagation. Ultrasound pulses of many cycles can deflect resonant microbubbles over distances on the order of millimeters.
In addition to primary radiation force, which acts in the direction of acoustic wave propagation, a secondary radiation force for which each individual bubble is a source and receptor causes the microspheres to attract or repel each other. The result of this secondary force is that a much larger concentration of microbubbles collects along a vessel wall than might otherwise occur.

See also Acoustically Active Lipospheres.