Ultrasound probe
An ultrasound probe , also known as an ultrasound head or transducer , is the part of an ultrasound device that establishes contact between the patient being examined and the signal processor unit. The ultrasonic probe sends and receives ultrasonic waves , converts them into electrical impulses and forwards them for processing.
The ultrasonic waves are generated by special crystals embedded in the transducer using the (inverse) piezoelectric effect . A high-frequency electrical alternating voltage stimulates the crystals to vibrate, which causes pressure fluctuations and, as a result, ultrasound. Conversely, an ultrasonic wave hitting the crystal generates an electrical voltage (direct piezo effect), which is finally displayed as a pixel by the ultrasonic device.
Probe types
A distinction is made between different types of probes according to their structure: sector, convex and linear probes. Their areas of application differ according to their different properties (for example the coupling surface).
Sector transducer
Sector probes are available in electro-mechanical and electronic versions. What they all have in common is a small coupling area. In the mechanical sector scanner that was used in the past, a few piezo crystals rotate or oscillate around an axis and thus build a triangular image of the scanned area with an angle of 60 ° to 120 °. A further development of the mechanical sector sound head is the annular phased array applicator, in which the piezo elements are arranged in a ring and thus enable the emitted sound beam to be focused.
Today's sector transducers no longer generate the image with the help of a mechanical pivoting of the crystals, but the targeted electronic control of the individual elements enables the construction of a sector-shaped image (phased array applicator).
This structure causes the typical properties of the sector transducer: the main advantage is the small coupling area. This enables the examination of regions that are difficult to access sonographically, such as the heart area or the inside of the skull in newborns. A clear disadvantage is poor image resolution in the area close to the transducer, which is associated with distortion. Distortions ( "divergences" ) can also occur at the transducer .
Linear array
As the name suggests, the piezo elements in this type of probe are arranged in series. This requires parallel sound propagation, which creates a rectangular image. A clear advantage of the linear transducer is the good resolution, even of structures close to the transducer, which makes it the ideal instrument for examining superficial structures such as the thyroid , the skin and its appendage organs, as well as the joints ( orthopedics ). The main disadvantage is the large area required for coupling, which makes handling difficult when space is limited. The examination of bone-protected regions is only possible to a limited extent.
Convex transducer (curved array)
A convex probe combines some properties of linear and sector probes. As with the linear scanner, the transmission elements are arranged in series. In this case, however, the coupling surface is convexly curved. This results in the advantage of a reduced contact surface compared to the linear transducer with better near-field resolution compared to the sector scanner. This makes this type of probe suitable for general examinations; it is mainly used in the abdominal area ( abdomen ). However, it represents a compromise in every respect, as its resolving power does not match that of the linear transducer and distortions occur in the near and far range. One way out is the virtual convex transducer . This is understood to be a linear transducer, the piezo elements of which are electronically controlled according to the principle of the phased array scanner (see above), so that an image section that is comparable to the convex transducer is created.
Special probes
- TEE probe = swallowing echo :
The probe has the shape of an endoscope (without optics) and is inserted into the esophagus in order to look at the heart from behind and thereby bypass the ribs and the lungs, which are impenetrable to the sound, so that a good representation of the To reach atria. - Vaginal
probe Rod transducer that is inserted vaginally (lateral and frontal sectional view) - Rectal probe Rod
transducer that is inserted rectally to examine the prostate or rectum (radial and lateral-frontal sectional view) - IVUS = Intravascular Ultrasound
Thin probes that are inserted directly into vessels in order to examine them from the inside. (360 ° radial sectional view; 2D image only). The high resolution allows an exact analysis of the vessel wall. - ICE = Intracardiac Echocardiography
Ultrasound catheter that is placed directly in a heart cavity and from there allows images with the highest resolution. (Lateral sector cross-section, 2D image; color Doppler, PW Doppler and CW Doppler) - Endoscopically placeable probes
