Ultrasonography, its mechanism, components and diseases which we diagnose on ultrasound

Ultrasonography

Ultrasonography:

Ultrasonography is noninvasive procedure to look the human organs by ultrasonic waves.

Diseases which diagnose on ultrasound:

Cysts.
Gallstones.
Abnormal enlargement of the spleen.
Abnormal growths in the liver or pancreas.
Liver cancer.

Fatty liver disease

Mechanism:

Ultrasound propagates in media in the form of alternating zones of compression and expansion of matter. Sound waves, including ultrasonic ones, are characterized by an oscillation period  - the duration of one complete cycle of elastic oscillation of the medium; frequency  - the number of oscillations per unit time; length  - the distance between the points of one phase and the propagation velocity, which depends mainly on the elasticity and density of the medium. The wavelength is inversely proportional to its period. The higher the frequency of the wave, the higher the resolution of the ultrasonic sensor. In medical ultrasound diagnostic systems, frequencies from 2 to 29 M Hz are commonly used. The resolution of modern ultrasonic devices can reach fractions of an mm.

Any medium, including body tissues, prevents the propagation of ultrasound, that is, it has different acoustic resistance, the value of which depends on their density and the speed of propagation of sound waves. The higher these parameters, the greater the acoustic impedance. Such a general characteristic of any elastic medium is denoted by the term “acoustic impedance ".

Having reached the boundary of two media with different acoustic resistance, the beam of ultrasonic waves undergoes significant changes: one part of it continues to propagate in the new medium, being absorbed by it to one degree or another, the other is reflected. The reflection coefficient depends on the difference in the acoustic impedance values ​​of adjacent tissues: the greater this difference, the greater the reflection and, of course, the greater the intensity of the recorded signal, which means the lighter and brighter it will look on the screen of the device. A complete reflector is the boundary between tissues and air.

Based on the period of passage of the wave reflected from the interface, the method allows measuring the distance to the border between the densities of two bodies in the simplest version of the implementation. The speed of movement of the density interface, as well as the difference in densities that create the interface, can be determined using more complex research methods (for example, based on the Doppler effect).

The rules of geometric optics govern the propagation of ultrasonic vibrations. They propagate in a straight path and at a constant speed in a homogenous medium. Some rays are reflected and some are refracted at the boundary of distinct mediums with differential acoustic density, maintaining their rectilinear propagation. The higher the gradient difference in the acoustic density of the boundary media, the greater part of the ultrasonic vibrations is reflected. Since 99.99% of the vibrations are reflected at the border of the transition of ultrasound from the air to the skin, during ultrasound scanning of a patient, it is necessary to lubricate the skin surface with an aqueous jelly, which acts as a transition medium. Reflection depends on the angle of incidence of the beam (the largest in the perpendicular direction) and the frequency of ultrasonic vibrations (at a higher frequency, most is reflected)

For the study of the abdominal cavity and retroperitoneal space, as well as the pelvic cavity, a frequency of 2.5 - 3.5 MHz is used, for the study of the thyroid gland, a frequency of 7.5 MHz is used.

Of particular interest in diagnostics is the use of the Doppler effect. The essence of the effect is to vary the frequency of the sound as the source and receiver of the sound move closer together. The frequency of the reflected signal changes as sound is reflected from a moving object (frequency shift occurs). When the primary and reflected signals are superimposed, beats occur, which are heard using headphones or a loudspeaker.

Components of ultrasound diagnostic system:

3-Types of sensors:

All ultrasonic sensors are divided into mechanical and electronic. In mechanical scanning is carried out due to the movement of the emitter (it either rotates or swings). In electronic scanning is done electronically. The disadvantages of mechanical sensors are noise, vibration produced by the movement of the emitter, as well as low resolution. Mechanical sensors are obsolete and are not used in modern scanners. Electronic sensors contain emitter arrays for example, from 512 or 1024x4 elements providing three types of ultrasonic scanning due to digital beam forming: linear (parallel), convex and sector. Accordingly, the sensors or transducers of ultrasonic devices are called linear, convex and sector. The choice of sensor for each study is carried out taking into account the depth and nature of the position of the organ.

 1-Ultrasound wave generator:

The generator of ultrasonic waves is a sensor that simultaneously plays the role of a receiver of reflected echo signals. The generator operates in a pulse mode, sending about 1000 pulses per second. In the intervals between the generations of ultrasonic waves, the piezoelectric sensor captures the reflected signals.

2-Ultrasonic sensors:

As a detector or transducer, a complex sensor is used, consisting of several hundreds or thousands of small piezo crystalline transducers operating in the same or different modes, similar to digital antenna arrays. A focusing lens is built into the classic sensor, which makes it possible to create focus at a certain depth. Due to digital beam forming in modern sensors, it is also possible to implement its dynamic depth focusing with multidimensional anodization.

3-Linear sensor:

Linear sensors use a frequency of 5-15 MHz the advantage of the linear transducer is the complete correspondence of the examined organ to the position of the transducer itself on the body surface. The disadvantage of linear sensors is the difficulty of ensuring uniform contact of the transducer surface with the patient's skin in all cases, which leads to distortion of the resulting image at the edges. Also, due to the higher frequency, linear sensors make it possible to obtain an image of the studied area with high resolution, but the scanning depth is rather small (no more than 11 cm). They are used mainly for the study of superficially located structures - the thyroid gland, mammary glands, small joints and muscles, as well as for the study of blood vessels.

4-Convex probe:

The frequency of the convex probe is 1.8-7.5 MHz because it is shorter, it is easier to obtain a consistent fit against the patient's skin. When utilizing convex sensors, however, the generated image is several centimeters broader than the sensor's size. This discrepancy must be taken into account by the doctor in order to define the anatomical landmarks. The scanning depth is 20-25 cm due to the decreased frequency. It's frequently used to look at organs that are deep inside the body, such as the abdominal organs and retroperitoneal region, the genitourinary system, and the hip joints.

5-Sector sensor:

The sector sensor operates at a frequency of 1.5-5 MHz it has an even greater discrepancy between the size of the transducer and the resulting image, therefore it is used mainly in cases where it is necessary to obtain a large view at depth from a small part of the body. The most appropriate use of sector scanning in the study, for example, through the intercostal spaces. A typical application of a sector transducer is echocardiography, a study of the heart.

6-Ultrasonic emission gel:

In contrast to the audible range, ultrasound is noticeably attenuated and distorted by thin (fractions of an mm) obstacles, and high scanning resolution is possible only with minimal distortion of the amplitude and sound transit time. With a simple application of the sensor, an air gap of constantly changing thickness and geometry is formed. Ultrasound is reflected from both interlayer boundaries, weakening and interfering with the useful reflection. To eliminate reflective boundaries at the point of contact, special gels are used to fill the area between the sensor and the skin.