Probes send out and receive information via the piezoelectric or the pressure-electricity effect. The probe relies on

a complex, delicate, and expensive arrangement of crystals.

These crystals convert electrical energy to mechanical energy

in the form of sound waves. Returning sound waves are

translated back into electricity by the probe. Probe mainte ­

nance is of utmost importance; a probe must never be used

if cracked or otherwise significantly damaged.

Frequency. The higher the frequency of sound waves

emitted by the probe, the greater the tissue resolution, but

the lower the depth of penetration. Different types of probes

exist for different clinical questions. Low-frequency probes

(2-5 MHz) are used in thoracic and abdominal imaging to

visualize deeper structures. High-frequency probes (8--10 MHz)

are used in procedural applications, such as central line

placement and nerve blocks, to visualize more superficial

structures with more detailed resolution.

Echogenicity. Images are described in terms of echogenicity. Dense bone is highly reflective, appearing bright

or hyperechogenic. Less dense organ parenchyma appears

grainy or echogenic. Fluid-filled structures or acute bleeding do not reflect, appearing black or anechogenic. Air has

an irregular reflective surface and appears as bright scatter

with dirty posterior shadows.

Orientation. A marker on the US probe corresponds to

an indicator on the screen. By accepted standard in emergency medicine and radiology, the indicator is always on the

physician's left side of the screen. In the sagittal (longitudinal) anatomic plane, the probe marker is pointed at the

patient's head, resulting in the head being displayed toward

the left side of the screen and the feet toward the right

(Figure 8- lA). In the coronal (transverse) anatomic plane,

the probe marker is pointed at the patient's right, resulting

in the patient's right side being displayed on the left side of

the screen, similar to viewing a CT scan image (Figure 8-1B).

Modes. The most commonly used mode is the brightness (B) mode on the US machine. Other modes include

the motion (M) mode, often used to measure the fetal

heartbeat, as well as the Doppler and color flow modes to

measure blood flow.

A

8