Image Formation & US Interactions

Ultrasound Interactions

Knowledge of the following 2 principles are necessary to understand how an image of tissues being scanned comes to be displayed on the screen:

Speed of sound in tissues – This allows depth of tissue being scanned to be determined.
Interaction of ultrasound with tissues – reflection, attenuation, refraction & scattering.

Speed of Sound in Tissues

The speed of sound in different tissues is not the same. It is dependent on tissue elasticity and density. Stiffer and less dense materials propagate sound waves at faster speeds. For the purposes of medical imaging, the ultrasound machine assumes a constant speed of sound in tissues of 1540m/s.

Tissue Type	Speed of Sound m/s
Air	330
Fat	1450
Water	1480
Liver	1550
Kidney	1560
Blood	1570
Muscle	1580
Bone	4080

Table 1.0. The speed of sound in various tissues of the body

Table 1.0 shows the speed of sound in tissues of the body. For imaging purposes, the ultrasound machine assumes a constant velocity of sound in tissues of 1540m/s.

Knowledge of the speed of sound in tissues allows for determination of the depth of the tissue reflector (echo-ranging). This is given by the following equation:

distance(d) = speed(c) x time(t)/2

Fig 1.04. Image shows calculation of distance of reflector from probe. This allows for depth determination of the reflector. The pixel is then displayed at a depth of 4cm on the display monitor.

Interaction Between Ultrasound and Tissues.

When ultrasound encounters tissues or interfaces between tissues of differing physical properties, the ultrasound beam can be absorbed, reflected, scattered or refracted. These are all causes for ultrasound beam attenuation. The property of the degree of sound wave attenuation in specific tissue is known as that tissue’s attenuation coefficient.

Fig 1.05. Image shows interaction of ultrasound waves with tissues. This determines the intensity of the returning ultrasound waves to the transducer.

Attenuation (reduction in intensity / amplitude) of the ultrasound wave also occurs with increasing depth of penetration. Ultrasound waves with higher frequency undergo more attenuation and consequently less penetration. In contrast, ultrasound waves with lower frequency are better for imaging deeper structures as penetration is better. Frequency also affects image resolution, with better resolution obtained with with higher frequncy ultrasound.

Higher Frequency = Better Image Resolution but Poor Penetration.

Lower Frequency = Better Penetration but Decreased Image Resolution.

The interaction of sound waves with tissues determines the intensity of the reflected beam that is sensed by the piezoelectric crystals. A lower intensity returning signal will be displayed as as a darker pixel, while a higher intensity returning wave will be displayed as a brighter pixel on the monitor. See image below.

Fig 1.06. Image of internal jugular vein cannulation. Blood in the jugular vein is a poor reflector of ultrasound waves, hence it appears black (anechoeic). The needle is a strong reflector and appears bright (hyperechoeic).

Reflection

This is a key component of ultrasound image generation, with reflected waves arriving at the transducer being converted to electrical energy, and subsequently processed into a displayed image. The brightness of the displayed image will depend on the intensity of the reflected beam. Reflection depends on the following:

The difference in acoustic impedence at the interface between 2 tissues. The larger the difference, the greater the reflection.
Smooth vs Specular reflectors. Smooth reflectors such as bone reflect ultrasound waves uniformly when compared to specular (uneven) reflectors which cause a more uneven pattern of reflection.
Angle of incidence of the beam. An angle of incidence that is perpendicular (0 degrees) to a smooth interface results in the largest amount of sound waves returning to the transducer. An incident wave hitting the interface at an angle of incidence greater than 0 degrees (less than perpendicular) will result in the wave being deflected away from the transducer at an angle equal to the angle of incidence in the opposite direction. In this circumstance, the signal of the returning echo is weakened creating a darker image (anisotropic artifact).

Fig 1.07. a) Reflection of US wave by a smooth reflector with incident beam perpendicular to reflector. b) Incident beam at an angle to reflector, causing reflection and refraction. c) scattered reflection caused by irregular surface. Image from: Eggers, Jürgen. Frontiers of neurology and neuroscience. 2006.

Refraction

Refraction is the alteration of direction of the sound wave (bending) after it strikes the interface of different tissue with different impedances.

Fig 1.08 Image licensed under Creative Commons Attribution 4.0 International

Fig 1.08. Image shows ultrasound beam refraction, which is governed by Snell’s Law. This causes the position of the reflector being imaged to appear altered. Refraction can also cause improper brightness of image to be displayed. These artefacts occur because the ultrasound machine assumes that the ultrasound beam only travels in a straight direction.

Fig 1.09. Duplication artefact caused by refraction. Refraction of US through the rectus sheath causes an artefactual double aorta to be seen.

Absorbtion

Absorption is another cause of ultrasound intensity attenuation. This occurs when the sound wave energy is transformed to heat. As a result, none of the ultrasound wave energy returns to the transducer to contribute to image generation.

Scattering

Scatter occurs when the ultrasound waves hits reflectors that have an irregular surface. It can also occur when the beam hits a small reflector, causing scattering of the ultrasound waves. This results in reduced intensity of the reflected beam. See Fig 1.07c.