Fig. 6.1 a rows) to the transducer. The returning echoes are, in reverse, converted by the crystals into electrical pulses that are then used to compute the sonographic image.
The sound waves are reflected at the interfaces (A, B, C) between media of different acoustic density (i.e., different sound propagation). The reflection of the sound waves is proportionate to the difference in acoustic density: a moderate difference (interface A in Fig. 6.1 a) will reflect and return a portion of the sound beam to the transducer, with the remaining sound waves to be transmitted and propagated further into deeper tissue layers.
If the difference in acoustic density increases (interface B in Fig. 6.1 h), the intensity of the reflected sound also increases, and that of the transmitted sound decreases proportionately. If the acoustic densities are vastly different (interface B in Fig. 6.1 b), the sound beam is completely reflected and total acoustic shadowing (45) results (total reflection). Acoustic shadowing is observed behind bone (ribs), stones (in kidneys or the gallbladder), and air (intestinal gas).
Figure 6.3 illustrates acoustic shadowing (45) behind an air-containing bowel loop (46). Echoes are not elicited if no differences in acoustic density are encountered: homogeneous fluids (blood, bile, urine, and cyst content, but also ascites and pleural effusion) are seen as echo-free (black) structures, e.g., the gallbladder (14) and hepatic vessels (10, 11) in Figure 6.3.
The processor computes the depth from which the echo originated from the registered temporal difference between emission of the sound beam pulse and reception of the echo. Echoes from tissues close to the transducer (A) arrive earlier (tA) than echoes from deeper tissues (tB, tc) (Fig. 6.1).
An echo reflected repeatedly back and forth (Fig. 6.2) before it returns to the transducer has a travel time that is no longer proportionate to the distance of its origin. The processor incorrectly assigns these reverberation echoes (51) to a deeper level (Fig. 10.1).
Additional distortion occurs through propagation speed errors introduced by programming the processor based on the assumption that the propagation speed of sound in tissue is constant, whereas in actual fact it is different for each type of tissue. While sound travels through the liver with a speed of about 1570 m/sec, it travels through fat with a lower speed of 1476 m/sec. The assumed medium speed stored in the processor leads to small differences but no major distortion.
If the propagation speed of adjacent tissue is vastly different (bone: 3360 m/sec vs. air: 331 m/sec), total reflection takes place (Fig. 6.1b along interface B) and acoustic shadowing ensues (45). For this reason a coupling gel is needed to assure direct contact between transducer and skin, with no air trapped in between.
^ Interface B
Operating Sonographic Equipment
The steps relevant for operating a sonographic unit are introduced here by means of a medium-sized unit (Toshiba). First, the patient's name has to be entered correctly (A, B) for proper identification. The keys for changing the program (C) or transducer (D) are found on the upper half of the control panel.
On most panels the freeze button (E) is in the right lower corner. When activated, this will prevent the real-time images from changing. We recommend having one finger of the left hand always resting on this button, thus minimizing any delay in freezing the desired image for measuring, annotating, or printing. The overall amplification of the received echoes is controlled by the gain knob (F).
Depth gain compensation: For selective enhancement of echoes received from different depths, the amplication can also be selectively adjusted with slide-pots (G) to compensate for depth-related losses in signal. Moving the image depth up or down, usually in small increments, increases or deceases the field of view (I). A "trackball" (I) places the dot or range markers (calipers) anywhere on the display. In general, this must be preceded by activating the measurement mode or annotation mode. To facilitate the review by others, the appropriate body marker (L) should be selected and the position of the transducer marked by the trackball (I) before the image is printed (M). The remaining functions are less relevant and can be learned by working with the unit.
A Begin with a new patient B Enter name (ID)
C Menu selection, e.g., abdomen, thyroid gland D Change of transducer E Freeze F Gain
G Depth gain compensation (DGC) H Image depth/field of view
I Trackball for positioning the dot or range markers J Measurements K Annotation L Body marker M Image recording
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