**1.4.1 Tissue investigation**

The conventional US currently used in practice is called "2D, gray scale" sonography. The technique is based on US property with frequencies > 20 kHz and constant acoustic power to cross tissues with an average speed of 1540 cm/sec. US are returned with different acoustic power at a variable timeframe depending on the acoustic density of the crossed environment and on the position of the reflecting element. There is a proportional relationship between the intensity of the echo and the density of a crossed biological environment; therefore, ultrasound is a noninvasive tissue density assessment procedure. There are two categories of echoes according to their frequency: basic (similar to the incident beam, i.e. 3 MHz) and "harmonic" (multiples of the emission frequency, i.e. 6 MHz). Harmonic echoes arise from the non-linear vibration of the tissues. Due to their high frequency, the result is a high quality image. Harmonic ultrasound technique ("coded harmonics" or "tissue harmonic imaging," THI) combined with the pulse inversion procedure ("pulse inversion harmonic imaging") allows to obtain information regardless of the depth where the region of interest lies and is available on most commercial equipment (Choudhry et al, 2000).

Image resolution (minimum size at which a reflected structure is distinctly shown on the screen) is essential for tumor detection. The number of crystals included into the transducer and the nominal frequency of the ultrasound beam (the higher the frequency the better the resolution) also contribute to the ultrasound image quality. The gain of echoes ("gain"

the method, considered today the most common diagnostic imaging procedure in the world (Derchi & Claudon, 2009). In the near future the ultrasound examination will be unrestrictedly generalized with the introduction of the procedure as basic training for

Ultrasound is useful for the practitioner as a first imaging procedure in direct correlation to the clinical examination. The method has a very good cost/quality ratio, the image is very accurate and precise and the information has a dynamic character ("real time imaging"). It is important to note that ultrasound has its limitations which the examining physician must take into account. Thus, the ultrasound image contains a number of artifacts and is mainly limited by phenomena such as US attenuation related to distance and density. Ultrasound is

The principles of ultrasonography are complex. The US picture is generated by ultrasound penetration into the human body. US are reflected as echoes and converted by the transducer into signals. The ultrasound image is multimodal. There are multiple US procedures systematized into "clinical applications" and the information obtained is tissular ("morphometric" type) and vascular ("hemodynamic "type). Finally, US examination is a "real time" procedure which reflects the movements of the organs. It is mandatory to connect the US information to the clinical and functional-biochemical data in order to obtain

The conventional US currently used in practice is called "2D, gray scale" sonography. The technique is based on US property with frequencies > 20 kHz and constant acoustic power to cross tissues with an average speed of 1540 cm/sec. US are returned with different acoustic power at a variable timeframe depending on the acoustic density of the crossed environment and on the position of the reflecting element. There is a proportional relationship between the intensity of the echo and the density of a crossed biological environment; therefore, ultrasound is a noninvasive tissue density assessment procedure. There are two categories of echoes according to their frequency: basic (similar to the incident beam, i.e. 3 MHz) and "harmonic" (multiples of the emission frequency, i.e. 6 MHz). Harmonic echoes arise from the non-linear vibration of the tissues. Due to their high frequency, the result is a high quality image. Harmonic ultrasound technique ("coded harmonics" or "tissue harmonic imaging," THI) combined with the pulse inversion procedure ("pulse inversion harmonic imaging") allows to obtain information regardless of the depth where the region of interest lies and is available on most commercial equipment

Image resolution (minimum size at which a reflected structure is distinctly shown on the screen) is essential for tumor detection. The number of crystals included into the transducer and the nominal frequency of the ultrasound beam (the higher the frequency the better the resolution) also contribute to the ultrasound image quality. The gain of echoes ("gain"

medical students, as part of clinical examination.

**1.4 Ultrasound techniques and procedures** 

the final diagnosis.

**1.4.1 Tissue investigation** 

(Choudhry et al, 2000).

**1.3 Advantages and disadvantages of the ultrasound method** 

an operator-dependent method, thus its reproducibility is reduced.

function) and the time compensation of echoes gain ("time gain compensation" function) measured in decibels, image depth and the number of focuses, echoes acquisition and their representation rate are other elements that allow information rendering with the same quality at an approximate ultrasound penetration of 20 to 25 cm. All these phenomena contribute to image the tissue echostructure. The sonography has a very good capacity to discriminate lesions depending on their consistency (a parenchymal cyst is detectable at a 2- 3 mm size!; a solid nodule is distinctly shown at sizes of 5 - 6 mm). In addition, the method allows accurate assessment of the lesion size and the evaluation of organ motility (gastrointestinal tract, heart, main vessels).

The ultrasound image is planar, two-dimensional (2D ultrasound). In recent years equipments have been developed, allowing 3D reconstruction of ultrasound images. The resulting images are not planes but volumes that can be static (3D) or dynamic (4D). 3D/4D exploration has recognized obstetrical applications but other areas of application of this method have been identified in recent years, mainly in oncology. 3D/4D ultrasound provides accurate information in connection with the space, shape, size and tumors texture. This procedure may be useful to measure the real tumor volume as well as the healthy surrounding parenchyma which should to be removed in case of surgery. Combining this procedure with intratumoral circulation assessment methods allows more accurate diagnosis of the tumor nature (Badea et al, 2007).
