How are images generated and displayed? In simple terms, ultrasound signals reflected from tissue reflectors are first detected by quartz transducers. The reverse piezoelectric effect then leads to generation of an electrical signal. Processing of this electrical signal leads to display of an ultrasound image on the monitor.
The intensity (amplitude) of the reflected ultrasound signal and the time taken for the ultrasound signal to return from the reflector are two vital pieces of information for signal processing and image display.
The intensity of the returning signal determines the brightness of the displayed pixel. The time taken for the signal to return to the probe determines the depth of a reflector.
The following 2 principles are important when considering image generation:
- Speed of ultrasound in tissues.
- Interaction between ultrasound & tissues.
1. Speed of Sound in Tissues
What does this tell us ?
- Knowledge – The distance travelled by the ultrasound wave is dependent on speed and time. This allows us to determine the depth of a tissue reflector.
- Assumption – The speed of ultrasound in tissues is constant at 1540m/s.
In actual fact, 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 however, the processor assumes a constant speed of sound in all tissues of 1540m/s.
We will learn later on how some of these assumptions impact image display, and lead to problems such as artifacts.
| Tissue Type | Speed of Sound (m/s) |
|---|---|
| Air | 330 |
| Fat | 1450 |
| Water | 1480 |
| Liver | 1550 |
| Kidney | 1560 |
| Blood | 1570 |
| Muscle | 1580 |
| Bone | 4080 |
| Assumed Speed | 1540 |
Table 1.0 shows the speed of sound in tissues of the body.
The equation below is used to determine reflector depth. We divide time by 2 as the time measured starts from when the ultrasound wave is emitted from the transducer until it returns back.
distance(d) = speed(c) x time(t)/2
2. 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 degree of sound wave attenuation in a specific tissue is known as that tissue’s attenuation coefficient.
Attenuation (reduction in intensity / amplitude) of the ultrasound wave 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. Frequency also affects image resolution, with better resolution obtained with higher frequency 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. A low intensity returning signal is displayed as as a dark pixel. A high intensity returning signal is displayed as a bright pixel on the monitor. See image below.
Reflection
This is a key component of ultrasound image generation. Reflected waves arriving at the transducer are converted to electrical energy, and processed into a displayed image. The brightness of the displayed image will depend on the intensity of the reflected beam.
Reflection of ultrasound by tissues depends on:
- The difference in acoustic impedence at the interface between 2 tissues. The larger the difference, the greater the reflection.
- Smooth vs Specular reflectors. Ultrasound waves are uniformly reflected by smooth reflectors such as bone. Specular (uneven) reflectors cause a more uneven pattern of reflection.
- Angle of incidence of the beam. Sound waves return to the transducer with higher intensity when the angle of incidence is at right angles to the reflector. An incident wave hitting the interface at an angle greater than 0 degrees (less than perpendicular) will result in the wave being deflected away from the transducer. In this circumstance, the signal of the returning echo is weakened, creating a darker image (anisotropic artifact).
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 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 line.
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 1.06c.