Ultrasound Teaching Manual

Physical Principles and Technical Fundamentals

Image Generation

Ultrasound images are generated not by X-rays but by sound waves that are sent by a transducer into the human body and reflected there. In abdominal ultrasound, the frequencies used generally are between 2.5 and 5.0 megahertz (MHz; see p. 11). The primary condition required for sound wave reflections is the presence of so-called "impedance mismatches." These occur at the interface between two tissue layers with different sound transmission properties (interfaces in Fig. 8.1). It is interesting to note that different soft tissues show only minor differences in the transmission speed of sound (Table 8.1).

Only air and bone are marked by massively different sound transmission in comparison with other human tissue. For this reason ultrasound units can be operated at a preselected medium frequency of approximately 1540 m/s without producing any major inaccuracies in the calculated origin ("depth") of the echo.

The processor computes the depth of origin of the echo from the time difference detected between emission of the sound impulse and return of the echo.

Echoes from tissue close to the transducer (A) arrive earlier (tA) than echoes from deeper tissues (tB, tC in Fig. 8.1a). The mean frequency is strictly theoretical since the processor cannot know which type of tissue the sound waves traversed.

Sound Transmission in Human Tissue

Air

  331 m/s

Liver

1549 m/s

Spleen

1566 m/s m = 1540 m/s

Muscle

1568 m/s

Bone

3360 m/s

Table 8.1

Fig. 8.1 a b

Which Component of the Sound Wave is Reflected?

Fig. 8.1a shows on the left three tissue blocks traversed by sound waves that differ only minimally in their transmission velocity (indicated by similar gray values). Each interface only reflects a small portion of the original sound waves () as echo (). The right-hand diagram shows a larger impedance mismatch at the interface A between the different tissues (Fig. 8.1b). This increases the proportion of reflected sound waves () in comparison to the tissues shown on the left. However, what happens if the sound waves hit air in the stomach or a rib? This causes a so-called "total reflection," as illustrated at interface B in Fig. 8.2b: The transducer does not detect any residual sound waves deep to this structure from which it can generate an image. Instead, the total reflection creates an acoustic shadow (45).

Conclusion:

Intestinal or pulmonary air and bone are impenetrable by sound waves, precluding any imaging deep to these structures. The goal will later be to work around intestinal air or ribs by maneuvering the transducer. The pressure applied to the transducer against the abdominal wall (see p. 21) and the acoustic gel that displaces air between the surface of the transducer and the patient's skin (see p. 22) play a significant role.

From a "Snowstorm" to an Image .

Do not get discouraged if at first you can only make out a blinding "snowstorm" on ultrasound images. You will be surprised how soon you will learn to recognize the ultrasound morphology of individual organs and vessels. Fig. 8.2 visualizes the gallbladder (14) as a black structure and shows two round polyps (65) within it. The surrounding gray "snowstorm" corresponds to the hepatic parenchyma (9) which is traversed by hepatic vessels (10, 11). How can you quickly work out which structures in the image appear bright and which are dark? The key lies in the concept of echogenicity (see p. 9).

Fig. 8.2 a Gallbladder with polyps b

What Does the Term "Echogenicity" Mean?

Tissues or organs with many intrinsic impedance mismatches produce many echoes and appear "hyperechoic" = bright. In contrast, tissue and organs with few impedance mismatches appear "hypoechoic" = dark. Consequently, homogeneous fluids without impedance mismatches (blood, urine, bile, cerebrospinal fluid, pericardial or pleural effusion, ascites, cyst secretion) appear "anechoic" = black. The number of impedance mismatches does not depend on the physical density (= mass per unit of volume). This is best illustrated with a fatty liver (9). On this noncontrasted CT scan (Fig. 9.1a), the parenchyma of a fatty liver appears darker (i.e., less dense) than hepatic vessels or normal liver (Fig. 9.1b).

Please use the following terms:

These fluids are anechoic (= black):

Hyperechoic (= bright)

pericardial or pleural effusion,

Hypoechoic (= dark)

ascites, cysts, blood, urine, bile,

Anechoic (= black)

cerebrospinal fluid

This is due to the lesser density of fat in comparison with normal liver tissue. On ultrasound the fatty deposits produce more impedance mismatches (Fig. 9.1c) than in normal liver tissue (Fig. 9.1d). Consequently, a fatty liver appears more echogenic (brighter) on ultrasound despite its significantly lower physical density.

A common misunderstanding:

What do ultrasound examiners mean when they refer to a "dense liver"? Either they are not expressing themselves clearly or they have failed to grasp the fundamental principle of ultrasound imaging and how it differs from radiography. Ultrasound does not visualize physical tissue densities but differences in sound transmission (impedance mismatches) which are unrelated to density.

Fig. 9.1 a CT: Fatty liver b CT: Normal liver c Ultrasound: Fatty liver d Ultrasound: Normal liver

Generation and Frequency Ranges of Sound Waves

Sound waves are generated by the reverse "piezoelectric effect." The pressure waves of an echo distort crystals, causing them to emit an electrical impulse. The reverse takes place during transmission. A transducer includes many such crystals. Depending on the impulse applied, they can produce sound waves of various frequencies specified in megahertz (MHz). A "3.75-MHz" transducer does not exclusively emit pressure waves (sound waves) at a frequency of 3.75 MHz. That is merely the specified median frequency (= "center frequency"). In fact, such a transducer may emit sound wave frequencies between, for example, 2 and 6 MHz. So-called "multi frequency transducers" have the additional capability to increase or decrease this center frequency and the surrounding bandwidth of transmitted sound frequencies. In thin patients or children, for instance, the bandwidth can be shifted (say 4-8 MHz with a center frequency of 6 MHz) to achieve better spatial resolution. However, this decreases the depth penetration of the sound waves.

In very obese patients, the use of lower frequencies (1-5 MHz with a center frequency of 2.5 MHz) can be appropriate to achieve the necessary penetration, but at the cost of lower resolution (see p. 11). Newer methods base their image generation on frequency shifts or harmonic frequencies of the echo in relation to the original ultrasound impulse (see p. 13).

Operating an Ultrasound Unit

Many controls on different ultrasound units are quite similar in function and arrangement regardless of the manufacturer. Therefore this section will look at the console of one unit supplied by Samsung (Fig. 10.1), which will then be used to introduce common technical terms.

Selection of Transducer and Preset

After you have switched on the unit (A) and it has booted, select the appropriate preset (PS) and the appropriate transducer for the respective examination and enter the current patient data (PD). You will usually select a linear array transducer (L) for evaluating the thyroid gland and the extremities but a convex array transducer (C) for abdominal examinations. The sector...