The device commonly referred to as an ultrasound wand is technically known as a transducer or a probe. This handheld instrument emits and receives high-frequency sound waves that travel through the body and reflect off internal structures. These reflected sound waves, or echoes, are processed by a computer to construct a real-time image of the underlying anatomy. Because different medical examinations require imaging various depths and organs, transducer size and shape are specifically engineered for distinct applications.
Dimensions of External Probes
The most frequently encountered transducers are external probes, designed to be moved across the skin surface. These probes come in various shapes, determined by the type of internal view they capture. The curvilinear or convex probe, often used for abdominal, obstetric, and gynecological scans, is one of the largest types. Its curved scanning surface is generally sized similarly to a small bar of soap. Its large, rounded face spreads the sound waves in a fan shape, necessary to capture a wide field of view of deep organs like the liver or a fetus.
Another common type is the linear array probe, characterized by a flat scanning surface and a rectangular shape comparable to a small pack of playing cards. This flat design is ideal for imaging structures close to the skin, such as blood vessels, muscles, tendons, or the thyroid gland, providing high-resolution detail for superficial structures. For cardiac imaging, a phased array probe is used, which has a noticeably smaller footprint, often with a square or triangular-shaped scanning surface. This compact design enables the probe to fit between the ribs, providing a clear acoustic window to the heart without obstruction from bone.
Specialized Internal Probes
Specialized internal probes are necessary for certain examinations to achieve a close, high-resolution view of organs within body cavities. These endocavitary probes are designed to be long and slender, prioritizing patient comfort during insertion. The transvaginal probe, used for detailed pelvic and early pregnancy imaging, is typically less than 1.5 inches (about 3.8 cm) in diameter at its widest point. Its shape is comparable to a finger, focusing the imaging array on the tip of the wand.
Transrectal probes, used primarily for prostate imaging, are also slender, with a circumference ranging from approximately 58 mm to 74 mm (a diameter of roughly 1.8 to 2.3 cm). Before insertion, these specialized probes are covered with a protective sheath, which is discarded after the procedure. A lubricating gel is applied to the outside of this cover, which facilitates smooth insertion and promotes acoustic coupling, helping sound waves travel efficiently into the body.
Factors Driving Probe Size Variation
The primary drivers of transducer size and design are the physics of sound waves and the specific clinical application. The relationship between frequency and penetration dictates the necessary size of the array elements. Probes operating at lower frequencies (2 to 5 megahertz, or MHz) are used for deep imaging, such as abdominal scans, because lower-frequency sound waves penetrate farther into the body. These probes often require a larger physical size to house the necessary array elements.
Conversely, probes used for superficial structures, such as vascular or musculoskeletal exams, utilize higher frequencies (7 to 18 MHz). These higher-frequency waves provide finer image detail, or resolution, but they cannot penetrate as deeply. Because the area being scanned is close to the surface, these high-frequency probes can be smaller while still delivering the required image quality. Finally, the size of the array is also determined by the required field of view; a large organ like the liver demands a wider acoustic window from a larger probe, while imaging the heart between the ribs necessitates a much smaller probe.