Ultrasound (Sonography) Explained

Sonography (synonyms: ultrasound, echography) is a diagnostic procedure used in radiology to produce cross-sectional images of almost any organ in any slice. The generation of a sonogram works by emitting high-frequency sound waves at the surface of the body, which are reflected by the tissue to be examined. Although sonographic examination is a radiological procedure, the vast majority of it is performed by physicians in other disciplines. The use of sonography is often the first diagnostic procedure in the examination of a patient, but it can also be used, for example, to monitor the course of various diseases or in prenatal care. The reason for the widespread use of sonography is the relatively low risk of damage compared to conventional X-ray examinations. The first medical application of sonography was performed by the American neurologist Karl Dussik in 1942. The basic idea of sonography came from World War I, when ultrasound waves were used to locate submarines.

The procedure

The principle of sonography is based on the use of a sound in the range of 1 MHz to about 20 MHz, which is generated by a large number of crystal elements in the ultrasound probe through the piezoelectric effect (occurrence of an electric voltage on a solid when it is elastically deformed). These crystals are located directly next to the transducer (contact surface in the transducer). Sound lines are generated by the crystals in the transducer. The density of the sound lines determines the resolving power of the generated sonogram. Due to this, the sound waves are bundled and focused so that the image generated is more faithful to the image. After the generated sound waves are emitted from the transducer, they encounter various tissue structures in the body, from which they are reflected. This causes an energy attenuation in the tissue, which is stronger the higher the frequency range of the waves. As a result of the increased energy loss in the high frequency range, the penetration depth of the ultrasound waves in the tissue decreases. However, the generated frequency of the transducers cannot be reduced arbitrarily, since higher frequencies are associated with a shorter wavelength and thus have a better resolving power. When the generated sound wave impinges on a tissue structure, the degree of reflection of the sound wave is directly dependent on the tissue properties. Each type of tissue has a different number of reflective structures that vary in density and number. Although reflections occur at every tissue on which ultrasound waves impinge, it is still possible that not every reflected sound wave results in a sufficiently strong backscatter signal to be detected in the sonogram. If the reflection occurs at the tissue, the sound waves are partially transmitted back to the transducer where they are received by the crystal elements. The received information is now processed by means of a beamformer (method for locating sound sources) and sent on as electrical pulses for digitization. Digitization is performed by a receiver and following this process the sonograms become visible on the monitor. Of crucial importance for the propagation of ultrasonic waves is impedance. Impedance represents a phenomenon that is of concern in the propagation of all sound waves and describes the resistance that opposes the propagation of the waves. To reduce the impedance phenomenon, a specific gel is used during a sonographic examination, which prevents the sound from being reflected by air spaces between the transducer and the body surface. The following systems are used to display the received ultrasound waves and for image reconstruction:

  • A-mode method (synonym: amplitude-modulated method): in this method, which is a technically simple method for imaging the echo signals, the imaging function is based on the amplitude displacement of the individual ultrasound waves. After the sound waves have been reflected and scattered by the tissue, the returning echo signals impinge on the transducer and are displayed as amplitudes connected in series.As an indication for the use of an A-mode process counts, for example, the quality control in the welding seam technology.
  • B-mode method (synonym: brightness-mode method): In contrast to the amplitude-modulated method, this method produces a two-dimensional sectional image in which the delineation of the various tissue structures is achieved by different brightness levels. In this method, the intensity of the returning ultrasound waves encodes the image in gray levels. Depending on the echo intensity, the individual pixels are electronically processed with different densities. With the aid of the B-mode method, it is possible to run the individual sonograms as an animated sequence of images, so that the method can also be referred to as a real-time method. This two-dimensional real-time procedure can be coupled with other procedures such as the M-mode or the Doppler sonographic examination. The shape of the transducer for scanning is done by a convex shaped scanner.
  • M-mode method (synonym: motion mode): this method is predestined for recording motion sequences, such as when recording the function of the entire heart or a single valve. Scanning is performed by using a circular vector scanner from which the beams can propagate in diverse directions.
  • Doppler sonographic procedures (see below Doppler sonography/Introduction).
  • Multidimensional applications: Three-dimensional and four-dimensional sonographic examination have been introduced as additional procedures in recent years. With the help of the 3D procedure, it is possible to create spatial images. The 4D procedure offers the option to perform a dynamic functional examination by imaging another plane in combination with the 3D procedure, for example.

In addition to the further developments in the field of multidimensional sonography, especially further developments have been made in digital signal processing. Especially through the increased computing power of the processors of ultrasound equipment, it has now become possible to precisely separate the ambient noise from the previously generated sound waves, so that the image resolution could be improved. Furthermore, the use of contrast agents for ultrasound examination has been optimized, which has resulted in sonographic vascular examination becoming more precise. Contrast-enhanced ultrasound (CEUS) has become an indispensable standard in the management of malignant diseases. The procedure detects with greater certainty than other imaging techniques whether a tumor is benign or malignant. This is particularly true for solid organs such as the liver, kidney and pancreas. During chemotherapy, immunotherapy or radiotherapy, CEUS can be used to detect whether the therapy has reduced or completely eliminated tumor perfusion. Thus, the procedure can also be used for therapy control and initial therapy monitoring.Contrast sonography is the procedure of first choice for tumor patients in whom kidney function is limited, a pacemaker prevents the use of magnetic resonance imaging (MRI), radiation exposure should be avoided, or an iodine allergy is present. Advantages of sonographic examination include the following:

  • It is a low-risk and commonly used procedure with a very high quality standard, which does not require exposure to radiation that is hazardous to health.

The disadvantages of sonographic examination are the following:

  • Since it is a very complex procedure, learning it is considered difficult for the doctor. Due to this, the objectivity of the procedure is considered low.
  • Moreover, the resolution of the procedure is lower than, for example, computed tomography.

The following ultrasound applications, among others, are presented below: