Ultrasound technology offers a non-invasive way to visualize structures inside the body without using radiation, using a device technically called a transducer, though commonly referred to as the ultrasound “wand” or probe. This handheld instrument sends and receives sound waves beyond the range of human hearing to create an image on a screen. Because different medical procedures require examining organs at various depths and locations, transducers come in several sizes and shapes. This ensures the best possible image quality for each specific type of scan.
The Standard External Transducer
The device most people imagine when they think of an ultrasound is the standard external transducer, often used for abdominal, pelvic, and fetal imaging. This probe is designed to be easily manipulated by a technician, fitting comfortably in the palm of the hand. Its shape is curvilinear or convex, meaning the surface that touches the skin has a gentle curve. This curved surface allows the sound waves to spread out, providing a wide, fan-shaped view of deeper structures within the body.
The actual area that transmits the sound waves, known as the footprint, is generally wide to cover a larger surface area on the skin. A typical array of crystals on the convex probe might be around two to three inches long. This design is optimal for deep penetration, making it the preferred tool for examining organs like the liver, kidneys, and the developing fetus.
Specialized Probes for Targeted Scans
While the external transducer works well for general imaging, specialized procedures require endocavity probes, such as the transvaginal and transrectal transducers. These are necessary for obtaining high-resolution images of organs close to a body cavity. Their design allows them to be placed closer to the target organ, enabling the use of higher-frequency sound waves for greater detail. This results in sharper images of structures like the prostate, uterus, and ovaries.
These probes are long and slender, much narrower than the external devices. A transvaginal probe is typically less than 1.5 inches in diameter, comparable to a finger. Transrectal probes are similarly slim, with a circumference often falling between 58 to 74 millimeters. To ensure patient comfort and hygiene, these probes are always covered with a sterile sheath and coated with a lubricating gel before insertion.
How the Transducer Creates an Image
Every ultrasound transducer operates on the core principle of converting electrical energy into sound energy and back again. The device contains numerous piezoelectric crystals that vibrate rapidly when an electrical current is applied. These vibrations generate high-frequency sound waves, which are emitted in short pulses into the patient’s body. The transducer acts as both a speaker, sending out the sound, and a microphone, listening for the returning echoes.
As the sound waves travel through tissues, they encounter boundaries between different structures. At these boundaries, a portion of the sound wave is reflected back to the transducer. The machine measures the time and strength of each returning echo. This information is processed to calculate the depth and density of the reflecting structure, which is then translated into a visual image on the screen. The familiar ultrasound coupling gel is necessary to create a seamless connection between the probe and the skin, as air pockets prevent imaging.