Ultrasound

Ultrasound is a diagnostic technique that uses ultrasounds. The latter can be used in the "execution of a" simple ultrasound, or combined with a CT to obtain images of body sections (CT-Echotomography), or to acquire information and blood flow images (Echocolordoppler).

In-depth articles

Principle of operation

In physics, ultrasounds are longitudinal elastic mechanical waves characterized by short wavelengths and high frequencies. Waves have typical properties:

• They carry no matter
• They bypass obstacles
• They combine their effects without modifying each other.

Sound and light are made up of waves.
The waves are characterized by an oscillatory motion in which the stress of an element is transmitted to the neighboring elements and from these to the others, until it propagates to the whole system. This motion, resulting from the "coupling of individual motions, is a type of collective motion, due to the presence of elastic bonds between the components of the system. It gives rise to the propagation of a perturbation, without any transport of matter, in any direction within the system itself. This collective motion is called a wave. The propagation of ultrasound takes place in matter in the form of a wave motion which generates alternating bands of compression and rarefaction of the molecules that make up the medium.
Just think of when a stone is thrown into a pond and you will understand the concept of a wave.

The wavelength is understood as the distance between two consecutive points in phase, ie having, at the same instant, identical amplitude and direction of motion. Its unit of measurement is the meter, including its submultiples. The range of lengths d " wave used in ultrasound is between 1.5 and 0.1 nanometers (nm, i.e. one billionth of a meter).
The frequency is defined as the number of complete oscillations, or cycles, that the particles make in the unit of time and is measured in Hertz (Hz). The range of frequencies used in ultrasound is between 1 and 10-20 Mega Hertz ( MHz, ie one million Hertz) and is sometimes even greater than 20MHz. These frequencies are not audible to the human ear.
Waves propagate with a certain speed, which depends on the elasticity and density of the medium they pass through. The propagation speed of a wave is given by the product of its frequency by its wavelength (vel = freq x length d "wave).

To propagate, ultrasounds need a substrate (the human body for example), of which they transiently alter the elastic forces of cohesion of the particles. Depending on the substrate, therefore depending on its density and the cohesion forces of its molecules, there will be a different propagation speed of the wave inside it.
Acoustic Impedance is defined as the intrinsic resistance of matter to be crossed by ultrasound. It affects their speed of propagation in matter and is directly proportional to the density of the medium multiplied by the speed of propagation of the ultrasounds in the medium itself (IA = vel x density). The different tissues of the human body all have a different impedance, and this is the principle on which the ultrasound technique is based.
For example, air and water have low acoustic impedance, liver fat and muscle have intermediate and bone and steel have very high. In addition, thanks to this property of the tissues, the ultrasound machine is sometimes able to see things that the CT (Computed Tomography) does not see, such as hepatic steatosis, that is the accumulation of fat in the hepatocytes (liver cells), hematomas from contusion (extravasation of blood) and other types of isolated fluid or solid collections.

In ultrasound, ultrasounds are generated for piezoelectric effect high frequency. By piezoelectric effect we mean the property, possessed by some quartz crystals or some types of ceramics, of vibrating at high frequency if connected to an electric voltage, therefore if crossed by an alternating electric current. These crystals are contained inside the ultrasound probe placed in contact with the skin or tissues of the subject, called a transducer, which thus emits beams of ultrasounds that cross the bodies to be examined and undergo an "attenuation that is in direct relationship with the emission frequency of the transducer. Therefore, the higher the frequency of the ultrasounds, the greater their penetration into the tissues, with a higher resolution of the images. For the study of the abdominal organs, working frequencies between 3 and 5 Mega Hertz are usually used, while higher frequencies, greater than 7.5 Mega Hertz, with greater resolving capacity, are used for the evaluation of superficial tissues (thyroid, breast, scrotum, etc.).
The points of passage between fabrics with different acoustic impedance are called Interfaces. Each time the ultrasound meets an interface, the beam comes in part reflex (go back) and in part refracted (i.e. absorbed by the underlying tissues). The reflected beam is also called an echo; it, in the return phase, goes back to the transducer where it excites the crystal of the probe generating an electric current. In other words, the piezoelectric effect transforms ultrasound into electrical signals which are then processed by a computer and transformed into an image on the video in real time.
It is therefore possible, through the analysis of the characteristics of the reflected ultrasound wave, to obtain useful information to differentiate structures with different densities. The reflection energy is directly proportional to the variation in acoustic impedance between two surfaces. For significant variations, such as the passage between the air and the skin, the ultrasound beam can undergo total reflection; for this it is necessary to use gelatinous substances between the probe and the skin. They have the purpose of eliminating the air.

Methods of execution

Ultrasound can be done in three different ways:

A-Mode (Amplitude Mode = amplitude modulations): is currently superseded by B-Mode. With the A-Mode, each echo is presented as a deflection of the baseline (which expresses the time it takes for the reflected wave to return to the receiving system, ie the distance between the interface that caused the reflection and the probe), as a "peak" whose amplitude corresponds to the intensity of the signal that generated it. It is the simplest way to represent the ultrasound signal and is of the one-dimensional type (ie it offers an analysis in only one dimension). It gives information only on the nature of the structure under examination (liquid or solid). A-Mode is still used, but only in ophthalmology and neurology.

TM-Mode (Time Motion Mode): in it, the A-Mode data is enriched by the dynamic data. A two-dimensional image is obtained in which each echo is represented by a luminous point. The points move horizontally in relation to the movements of the structures. If the interfaces are stationary, the bright spots will also remain stationary. it is similar to A-Mode, but with the difference that the movement of the echo is also recorded. This method is still used in cardiology, especially for demonstrations of valve kinetics.

B-Mode (Brightness Mode or brightness modulation): this is a classic echo-graphic image (ie a section of the body) of the representation on a television monitor of the echoes coming from the structures under examination. The image is constructed by converting the reflected waves into signals whose brightness (shades of gray) is proportional to the "intensity of the echo"; the spatial relationships between the various echoes "build" on the screen the image of the section of the organ under examination It also offers two-dimensional images.

The introduction of grayscale (different shades of gray to represent echoes of different amplitude) has improved the quality of the ultrasound image more. Thus all bodily structures are represented with tones ranging from black to white. The white dots signify the presence of a "called image." hyperechoic (for example a calculation), while the black points of an "image hypoechoic (for example liquids).

According to the scanning technique, the B-Mode ultrasound can be static (or manual) or dynamic (real-time). With real-time ultrasounds the image is constantly reconstructed (at least 16 complete scans per second) in phase dynamic, providing a continuous representation in real time.

CONTINUE: Applications of "ultrasound"