HOW DOES IT WORKS ?

For  tissue visualiastion two methods are avalable:

1. A-Mode - the curve displayed on the monitor, amplitude of curve straight proportional to the signal intensity. The A-scan  is the apper part ofthe picture (below)

2. B-Mode - in this mode ultrasound crystall mowing along the skin. At the same time transducer send and receive reflected ultasound signals, and get a many A-scans. From A-scans computer can builtd two dimensional tissue cut - B-scan. Amplitude signals replaced by color pixels. The color of pixel depends from signal amplitude.

The x-axis of the image complies to the spreading into the depth. In fact the time the ultrasound travels from the transducer to the tissue and back is proportional to the depth.
The important numbers to define tissue properties are the ultrasound velocity, the ultrasound impedance and the sound resistance. The ultrasound image with a good contrast results in different sound resistance's. It is important, that the ultrasound-system has a fine and high resolution. That means that it is possible to distinguish thin structures in axial direction (direction of the sound) and lateral(side by side). For a high axial resolution it is needed to have short ultrasound waves. The supposition therefore is the highest possible frequency of the sound wave.
Nowdays the systems which are used in the Dermatology contain a frequency area from 7,5 MHz - 75 MHz.
On the one side the limitation depends on the technical possibilities. On the other side the absorption in the tissue sets the upper limits. As the frequency is increasing the absorption grows linear. In compact tissues it grows quadratic, which will lower the penetration depth. So, that it is impossible to present the deeper structures.

The resolution and penetration are depend from freequency of te ultrasound

Basic-principle:
The ultrasound impulse is passing through the tissue-structure with the sound velocity which is characteristic to this structure.
At that time a reciprocal-action with the tissue of the structure arises. The utilization of the received signals (Amplitude and Running Time) is admitting conclusion inferences of the inner quality of the tissue-structure, without destroying the same.

 Wave types and wave spreading:
In the longitudinal wave the molecules (atoms) are swinging parallel to the spreading direction.
Compression- and pulling powers are working in it and therefore it's also called compression wave. It is the real sound wave, which is transmitting the sound through solid and liquid substances. In solid substances the transversal wave will be found, too. But the molecules are swinging vertical, transversal and not into the spreading direction.

The sound velocity is a constant, which depends on the tissue. In combination with the used frequency, the result is the wave length.

sound velocity [m/s]
--------------------------- = wave length [mm]
frequency [Hz]

Technical realization
The sender of the system generates high voltage impulses, which will be send to the transducer inside the applicator. At the same time the transducer will move inside the applicator a specific distance. In this case we are working in the pulse/echo mode which means that the same transducer sends and receives the ultrasound signals.

With a high quality and high speed A/D-converter the electrical impulses which comes back from the transducer will be digitized. With a sampling rate of 100 MHz or more and a dynamic of 8bit the system will give a clear reproduction image of the signals. These information is used for viewing a A-, B-, C- or 3D-scan.

Ultrasound impulses
A piezoelectrical converter/transducer (ceramic,folio) is used to create the ultrasound impulses. This transducer converts electrical energy from the sender into ultrasound waves. Mechanical damping inside the transducer creates a smooth swinging ultrasound impulse. The frequency of the impulse is defined by the thickness of the transducer element, the impulse length and the frequency spectrum.

Sound field
The form of the ultrasound field depends on the reflection of the tissue, the thickness of the element, the frequency and the sound velocity of the tissue.
Completing the sound area with sensitivity of circular reflectors it creates the sonogram.
In the far field a bell-shaped lateral distribution of the sound pressure will be produced. The descent of the sound pressure to 50% defines the beam diameter.

 Reflection and Refraction
A sound wave which hits a border line splits in two parts, one who will be reflected and one that will go through . This will change amplitude, reflection and permeability fact.