An ultrasound transducer is an essential component of ultrasound scanners. How do you choose the right ultrasound transducer for the tasks at hand in order to obtain the highest-quality image of the area of interest? Let’s try to figure it out.
We will not go into detail about how ultrasound waves are generated and how an image is formed, since ultrasound physics is the foundation of knowledge for every specialist engaged in ultrasound diagnostics.
Let’s start with a brief classification. First of all, there is classification by design parameters:
Convex transducer. Operates at 2.5–7.5 MHz. It has a shorter length, which makes it easier to achieve even contact with the patient’s skin. However, when using convex transducers, the image width is several centimeters larger than the transducer itself. To accurately identify anatomical landmarks, the physician must take this discrepancy into account. Due to the lower frequency, the scanning depth reaches 20–25 cm. It is typically used to examine deep-lying organs—abdominal and retroperitoneal organs, the genitourinary system, and the hip joints.
Linear transducer. Operates at 5–15 MHz. A key advantage of the linear transducer is the full correspondence between the examined organ and the position of the transducer on the body surface. A drawback of linear transducers is the difficulty of ensuring uniform contact of the transducer surface with the patient’s skin in all cases, which can cause distortion at the image edges. Due to the higher frequency, linear transducers provide high-resolution imaging of the area of interest, but the scanning depth is quite limited (no more than 10 cm, although manufacturers often claim much greater depth—this does not reflect reality). They are mainly used to examine superficial structures such as the thyroid gland, mammary glands, small joints and muscles, as well as for vascular imaging.
Phased array / sector transducer. Operates at 1.5–5 MHz. It has an even greater mismatch between the transducer size and the resulting image, so it is mainly used when a large field of view at depth must be obtained from a small surface area. Sector scanning is especially useful when examining through intercostal spaces. A typical application of a sector transducer is echocardiography (cardiac imaging), transcranial scanning of cerebral vessels, and examination of pleural cavities and lung tissue.
Biplane transducer. This is a combination of two types of emitters (convex + convex or convex + linear) that allows imaging in both longitudinal and transverse planes. There are even triplane transducers, but they have not gained wide popularity in ultrasound diagnostics. Biplane transducers are mainly used in urology to assess the prostate gland.
Pencil transducer (blind CW). Pencil Doppler transducers are used to examine major vessels of the limbs and neck at 2–8 MHz using continuous-wave Doppler (CW Doppler). Transducers that contain a separate emitter and receiver can operate in B-mode and Color Doppler. However, since CW Doppler is available on all transducers in modern ultrasound scanners, the use of pencil transducers has largely lost its relevance. Today, given their low cost, they are still manufactured and used in clinical practice.
Intracavitary transducer. These, in turn, are divided into vaginal, rectal, vaginal-rectal, and transurethral. The scanning surface may be convex, phased, or linear. The viewing angle and frequency of such transducers vary widely. There is also a broad range of intracavitary transducers designed specifically for biopsy, where the biopsy needle passes directly through the body of the transducer. The most popular intracavitary transducer among clinicians is a transducer with a convex scanning head, a scanning frequency of 5–9 MHz, and a field of view from 90° to 180°.
Modifications of traditional transducers: Microconvex transducers. This is a convex transducer used in pediatrics and neurosonography.
Transesophageal transducers. Used for echocardiography. There are pediatric and adult versions with different lengths and diameters. Operating frequency is 4–9 MHz, and the viewing angle is about 90°. Modern variations may have a biplane design, but most often these are sector or microconvex transducers.
Laparoscopic / intraoperative transducers. Used during laparoscopic procedures. Their special feature is the ability to be controlled via a joystick. Intraoperative ultrasound transducers are designed to visualize the surgeon’s actions within the operative field.
Volumetric (3D/4D) convex or linear transducer. Scanning is performed using a scanning head that is driven by a motor located inside the transducer itself. Designed to obtain three-dimensional images. Most widely used in obstetrics.
Another important parameter of ultrasound transducers is the manufacturing technology. Based on the technologies used in ultrasound transducers, the following types are distinguished:
Piezoelectric crystal (standard) transducers. The most commonly used in clinical practice due to their relatively low cost and widespread availability. The more densely the elements are arranged, the higher the resolution that can be achieved. For example, standard-density convex and linear transducers contain 128 elements, while similar high-density emitters have 192 elements. However, the number of elements itself is not the most important factor. The decisive factor is the line density, which depends on the geometric parameters of the transducer (aperture, field of view angle, radius).
Single-crystal transducers. The technological difference is that standard piezoelectric crystal transducers are assembled from separate piezo elements. Even though the differences between them are small, they can negatively affect image quality. In single-crystal transducers, the piezo elements are “cut” from a single crystal. The image obtained with such transducers is less “noisy” (cleaner), even before any computer signal processing.
Matrix transducers. In classic ultrasound emitters, piezo elements (128–256) are arranged in a single row. Matrix transducers have from 3 to 10 such rows—forming a matrix. Multiple rows of piezo elements create a beam that is thinner and more uniform in thickness. The narrower the beam, the higher the image resolution. A similar effect is achieved with high-density transducers, but only at a certain depth (in the focal zone). Matrix transducers visualize the near, mid, and far scanning zones equally well. Such emitters are about twice as expensive as classic ones. Depending on the number of rows, 1.5D and 2D types are distinguished (when the number of elements is nearly the same on both sides and a volumetric image can be obtained).
As a final step when choosing an ultrasound transducer, attention should be paid to the geometric and acoustic parameters—aperture (working surface), radius of curvature, and field-of-view angle, respectively.
If we условно divide ultrasound scanners from all available manufacturers into three groups—basic (entry-level), mid-range, and expert-class systems—it should be noted that the number of supported transducers grows proportionally with the class of the device. Entry-level systems are limited to 1–2 standard transducers for the main types of examinations. Mid-range systems offer a fairly wide selection of transducers with different frequencies and support single-crystal technology. Expert systems, as a rule, have dozens of transducers in their arsenal, satisfying even the most demanding clinician; matrix transducers are manufactured exclusively for “expert” systems. Given these features, when selecting a transducer, try to get the maximum out of the ultrasound machine and obtain the highest-quality, sharpest image—leaving no chance for a diagnostic error.
