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.

A phantom is used to control the imaging performance of ultrasound transducers. The spatial resolution, dead zone, linear fidelity, depth of penetration and image uniformity is important for the image quality. For the axial and lateral resolution, the standard definition is the resolution of objects parallel and perpendicular to the path of the sound beam. Ultrasound pictures created by scans of specially designed ultrasound phantoms can quantify the imaging quality and transducer performance.
Phantoms contain one or more materials that simulate a tissue in its interaction with ultrasound. Several phantoms are available specifically for quality control. Transducer characterization consists of a standard pulse echo analysis and insertion loss measurement for each probe. The quality variation from the baseline should be tracked over a period.

A vaginal probe is a small ultrasound transducer (e.g. a biplane probe) that is inserted directly into the vagina. A sterile cover is slipped over the handheld transducer (also handle or probe), which is then covered with lubricating gel and placed in the vagina.
This technique produces a sharp image, not only because of the close proximity to the uterus, but also because this transducer is more sensitive.

A zone is a focal region of the ultrasound beam. An ultrasound beam can be directed and focused at a transmit focal zone position. The axial length of the transmit focal zone is a function of the width of the transmit aperture.
The field to be imaged is deepened by focusing the transmit energy at progressively deeper points in the body, caused by the beam properties. Typically, multiple zones are used. The main reason for multiple zones is that the transmit energy needs to be greater for points that are deeper in the body, because of the signal's attenuation as it travels into the body.

Beam zones:
The tightest focus and the narrowest beam widths for most conventional transducers are in the mid-field within the zone where the acoustic lens is focused. The ultrasound beam is less well focused and, therefore, wider in the near and far fields which are superficial and deep to the elevation plane focal zone. The beam width is greater in the near and far fields, making lesions in these locations more subject to a partial volume artifact.

See also Derated Quantity.

From ALOKA Co., Ltd.;
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The system also use Super High Density transducers (found our high-end systems) to enhance imaging resolution. These multi-frequency transducers provide a broad range of imaging frequencies to optimize scan frequency for depth of view.'