Ultrasound Physics Practice Flashcards

Smooth structures, such as the diaphragm, are highly angle-dependent regarding specular reflection. If the transducer angle is slightly altered, echoes can scatter in many different directions, leading to a loss of the image.
The brightness of echoes is determined by impedance mismatch. When impedance is equal at a boundary, there is 100% transmission of the sound energy. A slight mismatch results in partial reflection and partial transmission, while a large mismatch causes nearly total reflection.
Organ capsules appear as bright linear echoes due to equal impedance. In contrast, the internal tissue structures of organs appear gray and textured because of backscatter caused by small reflectors.
A perpendicular beam orientation produces the maximum reflected signal from smooth surfaces because most of the sound is reflected directly back toward the transducer.
When an ultrasound beam strikes a smooth surface at an oblique angle, the sound energy is reflected away from the transducer, which can result in refraction.

The type of reflection that occurs is influenced by the size of the reflector relative to the ultrasound wavelength:
- If reflectors are larger than an ultrasound wavelength, clear specular reflection occurs.
- If reflectors are smaller than the ultrasound wavelength, non-specular or backscatter reflection may occur.
Blood cells produce Rayleigh scattering rather than specular reflection because the cells are much smaller than the wavelength of the ultrasound beam.
Non-specular reflections occur at rough or irregular surfaces, such as the tissue texture inside organs. These reflections allow for the visualization of organ parenchyma, though they do not allow for the visualization of sharp organ boundaries.

Acoustic speckle is formed by the presence of many small scatterers within tissue that produce lower-amplitude echoes in all directions.
There is a distinct difference between the types of echoes used for imaging:
- Strong echoes are used for anatomical imaging and originate from large, smooth tissue boundaries.
- Weak echoes produce acoustic speckle and scatter from tiny structures located inside organs.
Ultrasound systems distinguish between these signals based on strength and consistency; anatomical echoes are stronger and more consistent, while speckle signals are weaker and random.
Acoustic speckle is considered acoustic noise rather than true anatomical information because low-amplitude signals cannot be processed by the system as real structures and instead appear only as bright dots on the screen.
The speckle pattern is a result of wave interference:
- Constructive interference produces stronger echoes.
- Destructive interference produces weaker echoes.

Acoustic impedance mismatch determines the proportion of sound reflected versus transmitted at a boundary. A larger mismatch causes more reflection, while a smaller mismatch allows for more transmission.
Interfaces between tissues with very different impedances produce strong reflections due to the large mismatch.
The air-tissue interface reflects almost all incident ultrasound energy because there is a large impedance mismatch between those two media.
Coupling gel is used to improve the transmission of sound from the transducer into the body by removing air trapped between the transducer and the skin.
Structures located behind bone are often poorly visualized because bone both reflects and absorbs ultrasound energy, meaning very little sound reaches the structures behind it.
A slight impedance mismatch can simultaneously produce reflection and transmission, where some sound returns to the transducer and some continues through the boundary.
If two tissues have identical acoustic impedance, no reflection occurs at the interface; the sound is 100% transmitted, and the boundary is considered acoustically invisible.

Refraction is the change in the direction of the transmitted ultrasound beam when it crosses media with different propagation speeds.
Refraction can only take place if two specific conditions occur simultaneously:
1. There is an oblique angle of incidence.
2. The two tissues have different propagation speeds.
Refraction cannot occur when the ultrasound beam strikes an interface at exactly $90^{\circ}$ (perpendicular angle).
Differences in propagation speed between two media cause refraction and change the direction of the transmitted beam.
Refraction can cause structures to appear in the wrong lateral position on an image. Because the system assumes sound travels in a straight line, the bending of the beam causes the system to misregister the location of the echo and place a copy of the structure in an incorrect lateral position.

The ultrasound system determines the distance to a reflector using the range equation, which utilizes measured travel time and the known propagation speed to calculate distance.
The system must assume a constant propagation speed to allow for simple mathematical calculations, as it cannot measure the exact distance directly.
Incorrect assumptions about propagation speed can produce image distortion by causing the system to calculate the wrong depth or position of a structure.
In soft tissue, the significance of travel time is captured by the $13\,\mu s$ per centimeter rule, which states that it takes $13\,\mu s$ to travel $1\,cm$ one way.
When calculating reflector depth, the system divides the measured travel time by $2$ to account for the fact that the pulse must travel to the reflector and then back to the transducer.