In this paragraph, we describe the principles of ultrasonography.

The ultrasound scanner is made of one or more probes.

The probes emit a short burst of ultrasound and then receive the echoes back, such sequence is repeated several times per second.

When the ultrasound passes through the body of the patients, it may encounter anatomical structures that partially or completely reflect them, and hence, an echo is generated.

The ultrasound scanner measures the time elapsed from the emission of the ultrasounds and the detection of the echoes, and then it calculates the distance between the probe and the structure that generated the echoes. The intensity of the echo is also measured and a dot is plotted on the screen. The intensity of white is proportional to the intensity of the echo received. A black and white image will be plotted several times per second on the screen representing the echoes received back from the tissues of the patients originated from the probe.

Structures that produce few or no echo are represented as black or dark grey and defined hypoechogenic. Structures that reflect the ultrasound more are represented as white lines and are described as hyperechogenic. If most ultrasound is reflected back, little or no further echoes will be produced underneath the hyperechogenic structure.

In some cases, it is possible to measure the frequency shift of the echoesproduced by moving objects, such frequency shift is named Doppler effect. When this function is activated (usually called Duplex mode), the dots are plotted in red or blue according to the direction and speed of the body who generated the echo. The Doppler effect is useful to visualise the movement of blood in vessels.

The ultrasounds cannot pass through air. A gel should be smeared on the skin of the patient, and more important, when the ultrasound beam encounters a structure full of air (e.g. the trachea), it will be completely reflected back, and no echoes will be received (and plotted) from the area below, on the screen arises a dark “shadow”, which is an artifact effect not corresponding to any actual anatomical structure. In case of subcutaneous emphysema, ultrasonography will be very difficult.

There are many different ultrasound scanners available.

Among the several different probes available on the market, the best one to perform neck ultrasounds is the linear probe. This one emits a thin layer of parallel ultrasound wave. Usually the frequency of the ultrasounds ranges from 7.5 to 12 MHz; the small length of wave will provide a sharp image 1–7 cm away from the probe.

On any ultrasound probes, there is a marker of orientation, in the picture is a green light, and in other devices, it may be a notch or a small knob on the side of the probe. It is very important to keep the marker on the right side of the patient during transverse scan or on the cranial side of the patient during longitudinal scan. If the orientation of the probe is not known by the user, ultrasonography may be very confusing and even harmful for the patient.

On the ultrasound scanner, there are usually many different knobs and buttons, which may puzzle the inexperienced user. For the ultrasound-assisted PDT, only very few functions as on-off switch, probe selector, depth, gain, focus and freeze are really needed. The “depth” control sets the maximum distance from the probe which is plotted on the screen, and it is suggested to set it at 5 cm or less. The “gain” control sets the intensity of the screen, and the user should set it according to his/her capability to recognise anatomical structures. The “focus” control improves the quality of the image at a given distance from the probe, and it is suggested to keep the focus at 1 or 2 cm from the probe, in the area that will be affected by the procedure. The freeze button is used to stop ultrasonography and leave the last image plotted on the screen to observe it or to print it.

Modern ultrasound scanners have many functions to improve the quality of the image that are extremely useful when performing a diagnostic exam. In case of ultrasound-assisted PDT, such features are useless, and they should be disabled if they slow the updating on frameworks on the screen. It is better to have a smooth-flowing real-time movie of the neck rather than a razor-sharp static image.

The ultrasonography provides many information not properly required for an ultrasound-assisted PDT. On the side of the screen, a marker followed by several dots similar to the notches on a ruler represents the depth and is very useful to esteem the size of any structure seen on screen.