Common ultrasound probes by applications

Commonly known as probes, ultrasound transducers come in a wide array of shapes and sizes designed for specific diagnostic applications. Here are some descriptions of the probes commonly found in hospital, clinical, and medical practice settings:

There is the cardiac transducer, whose primary use is echocardiography. In some cases, because the frequency is low, it can used for abdominal studies as well. Next, there is a vascular transducer, which is typically used for carotid arteries and veins, including those in the legs. It can also be used for the thyroid, needle-guided injections, and in some cases, when the frequency is high enough, for breast exams as well. (It is important to note that when using it for breast exams, it should not take the place of conventional mammography, but rather be used in conjunction with it.) Abdominal transducers are used for organs, such as kidney, liver, spleen, and stomach. Typically ob/gyn studies are performed with abdominal transducers. Transvaginal transducers are used to perform studies on women at various stages of pregnancy, during every ultrasound. Then, there is the endorectal transducer, which is used to check for rectal cancer. Urologists primarily use this transducer. The transesophageal echocardiography (TEE) transducer is used to evaluate cardiac studies by use of the echo transducer that produces the sound waves for TEE. The probe is attached to a thin tube that passes through your mouth, down your throat and into your esophagus. Because the esophagus is so close to the upper chambers of the heart, very clear images of those heart structures and valves can be obtained.

What’s in a Wave?

Exactly how does ultrasound work? Pulsed ultrasound, because of its high frequency, can be aimed in a specific direction, and it will obey the laws of geometric optics with regard to reflection, transmission and refraction. In other words, it will bounce off an object in varying degrees. When an ultrasound wave meets an an object, the wave is reflected, refracted or absorbed. These reflected sound waves are processed. The transducer, though it is emitting ultrasound in rapid pulses, actually acts as a receiver most of the time. The ultrasound images can then be displayed on an oscilloscope screen or a video monitor and can be recorded on videotape, thermal paper or radiographic film.

Sound waves in an ultrasound are sent out very quickly--at a speed of approximately 1540 m/sec in soft tissues. The thickness, size and location of various soft tissue structures in relation to the origin of the ultrasound beam are calculated at any point in time. The strength of the reflected sound wave depends on the difference in "acoustic impedance" between adjacent structures. The acoustic impedance of a tissue is related to its density. The greater the difference in acoustic impedance between two adjacent tissues, the more reflective will be their boundary. Higher frequency ultrasound waves have a longer near field and less divergence in the far field; they permit better resolution of small structures. However, more energy is absorbed and scattered by the soft tissues so that higher frequencies have less penetrating ability. Conversely, a transducer producing lower frequencies will provide greater depth of penetration but less well defined images.

The transducer of a real-time scanner typically contains more than 300 crystals arranged in a row; each emits and receives an ultrasound beam in rapid succession to form a sweep. For example, the part of the abdomen under the probe is "swept" about 30 times a second and a moving picture will be formed, not unlike the principle in a movie projector. Beam density and dynamic range control technologies are further being incorporated into each scanner's design to optimize the ultrasound image.

Best Practices for Disinfection

In recent years, properly disinfecting probes has become a point of concern. In investigations, some medical facilities were discovered not to be disinfecting the probes properly, putting patients at risk for infection. Standard cleaning procedures for ultrasound probes used in examinations often do not completely remove bacteria from the devices and can still pose a risk of cross-infection in patients. Millions of ultrasounds are being performed annually, and about a third of those are done trans-vaginally. It’s vitally important to examine and establish the proper protocols for disinfection of probes at any facility, rendering the probe as sterile as possible to eliminate the possibility of passing infections between patients. Several methods of disinfection have been developed that promise a more complete sterilization. 

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