Procedure of the examination
The area to be examined with the ultrasound is first covered with a gel. The gel is needed because air between the tissue and the transducer must be avoided. The examination is performed with light pressure on the tissue.
The structures to be examined are scanned in a fan-shaped manner in different directions, changing the position of the joint. Finally, all structures are assessed by moving joints. An ultrasound examination always proceeds in the same way, regardless of the organ tissue being scanned: depending on the structure to be examined, the patient lies or sits on an examination couch.
Depending on the structure to be examined, the patient lies or sits on an examination couch. If an abdominal sonography is planned, the patient must appear to be fasting for this examination, as the air in the gastrointestinal tract from previous food intake would disturb the ultrasound image. First, the doctor applies a gel to the skin located above the structure to be examined.
This gel has a high water content, which prevents the sound from being reflected by air inclusions between the skin surface and the air. This is the only way to create a usable image, which is why the examiner must always make sure that there is no air between the gel and the transducer. As soon as the gel layer becomes too thin, the image becomes worse, so that it is sometimes necessary to re-apply gel several times during an examination.
The decisive device in the ultrasound examination is the so-called transducer, which is sometimes also called a probe. It is connected by a cable to the actual ultrasound device, which has a monitor on which the image is displayed. In addition, this device is operated by means of several buttons that allow, for example, to change the brightness, to create a still image or to place a color doppler (see below) over the image.
The probe is both responsible for emitting the ultrasound and for receiving it again after reflection.There are different types of probes. A distinction is made between sector, linear and convex probes, which are used in different areas due to their different characteristics. The sector probe has only a small coupling surface, which is practical if you want to examine structures that are difficult to access, such as the heart.
The use of sector probes produces the typical fan-shaped ultrasound image on the screen. A disadvantage of these probes, however, is the poor image resolution near the transducer. The linear probes have a large contact surface and parallel sound propagation, which is why the resulting image is rectangular.
They therefore have a good resolution and are particularly suitable for examining superficial tissue such as the thyroid gland. The convex probe is practically a combination of sector and linear probe. In addition, there are some special probes, for example the TEE probe, which is swallowed, the vaginal probe, the rectal probe and the intravascular ultrasound (IVUS), where thin probes can be inserted directly into the vessels.
As a rule, the probe is placed on top of the gel previously applied to the body. The desired structure can then be targeted by moving the probe back and forth or by bending it. The probe now emits short, directional sound wave impulses.
These waves are reflected or scattered to a greater or lesser extent by the successive different tissue layers. This phenomenon is called echogenicity. The transducer is not only a sound transmitter but also a sound receiver.
It therefore picks up the reflected rays again. From the transit time of the reflected signals a reconstruction of the reflecting object can be made. The reflected sound waves are converted into electrical impulses, amplified and displayed on the screen of the ultrasonic device.
Liquids (e.g. blood or urine) show a low echogenicity, these are displayed on the monitor as black pixels. Structures with a high echogenicity, on the other hand, are shown as white pixels, including those structures that reflect sound to a high degree, such as bones or gases. The doctor looks at the two-dimensional image produced on the monitor during the examination and provides information about the size, shape and structure of the organs being examined. If desired, the physician can either print the image, which produces a so-called sonogram (this is often done to give pregnant women a picture of their unborn child), or make a video recording.
