Ultrasound Physics and Artifacts Part 2
Review of ultrasound physics and artifacts for the ABR core exam. Prepare to succeed.
Show Notes/Study Guide:
What is the cause of a side lobe artifact?
We assume that ultrasound beams leave the transducer almost in a straight uniform line but, you have the primary beam and secondary beams that surround the primary beam that come off the transducer in a radial pattern. If this “side lobe” beams get reflected to the transducer you can see a side lobe artifact which typically appear echogenic and curvilinear. Look this up to see examples of how this can manifest on images.
What is anisotropy and why is this a potential problem for musculoskeletal ultrasound?
Anisotropy can occur if uniform rope-like structures like a tendon reflect the incident ultrasound wave away from the transducer. The transducer then doesn’t receive the echo and will make that area look hypoechoic which can mimic a tendon tear or other tendon pathology. To check for this, change the ultrasound angle slightly and see if the artifact goes away and you now see an echogenic tendon. If you change the angle and see the tendon in the region that previously looked like a tear, you have confirmed that what you were seeing was artifact from anisotropy and not a true tendon tear.
How do you prevent color flash artifact?
Color flash artifact results from motion of tissue in the ultrasound beam or motion of the transducer over tissue that causes a flash of color when using color Doppler imaging. A good example is flashes of color on a breast ultrasound when a patient is talking, due to movement of the chest tissues from respiration/vibration from talking. To prevent this artifact, take measures to prevent motion of the ultrasound probe or motion of the patient while imaging with color Doppler imaging. Another strategy is to reduce the color gain to make this flash artifact less likely to occur.
True or false? Ultrasound velocity is independent of the frequency of the emitted soundwaves.
True. Velocity of the sound wave is determined only by the type of material the sound wave is passing through. This is calculated as the inverse of the square root of the compressibility of the material. Less compressible materials have higher sound wave velocities. Ultrasound waves travel faster through bone and slower through air.
What is acoustic impedance?
Acoustic impedance is an intrinsic property of a material that is the product of the density of the material and the velocity of sound within the material. Differences in acoustic impedance between materials determine how much energy is reflected at the interface of the materials. Interfaces between low acoustic impedance and high acoustic impedance materials will reflect the most energy. Examples of highly reflective interfaces are bone/soft tissue and soft tissue/air interfaces. An ultrasound image is essentially an acoustic impedance map.
What is the difference between non-specular and specular reflections?
Specular reflections are from large smooth surfaces and reflect a lot of soundwaves back to the transducer. Nonspecular reflections are from rough, irregular materials that scatter rather than reflect a lot of the soundwaves. The echoes from specular reflections are the primary component of ultrasound images.
What does ultrasound gel do to improve the image quality of an ultrasound system?
The gel displaces air between the ultrasound transducer and the patient’s soft tissues, thereby getting rid of the large difference in acoustic impedance that would otherwise occur if air existed between the transducer and patient’s skin—this promotes transmission of ultrasound into the patient’s body and reduces reflection of ultrasound beams by air outside of the patient’s body.
If a surface being imaged is larger than the wavelength of an ultrasound beam does this promote reflection or scatter of the ultrasound beam?
Large surfaces promote reflection of the ultrasound beam. In general, if the surface is larger than the wavelength of ultrasound, reflection will occur. If a surface is smaller than the wavelength of ultrasound, scatter will occur.
Does increased scatter increase or decrease the echogenicity of a structure?
Increases in scatter increase the echogenicity on an image. ECHOgenicity literally has “echo” in the name, and this is because the echogenicity is related to the degree of echoes from a tissue. Therefore, cysts are anechoic—because they transmit the ultrasound wave without scatter, and they are therefore “anechoic” or another way of saying this is “without scatter”.
What are two major factors that increase attenuation of ultrasound in the body?
Absorption and scatter of the ultrasound beam.
What is the primary determinant of the frequency of an ultrasound probe?
The thickness of the piezoelectric crystal determines the frequency. Higher frequency probes are thinner/have thinner crystals and lower frequency probes are thicker/have wider crystals. In general, smaller frequency probes give more detailed images. Lower frequencies have better depth penetration.
What is the pulse repetition frequency and how does changing the pulse repetition frequency affect an ultrasound image?
The pulse repetition frequency (PRF) is a measure of how many soundwaves are transmitted per unit time. In general, an ultrasound system listens about 100x longer than it transmits and the longer you listen (the lower the pulse repetition frequency) the more time sound beam must penetrate tissue and reflect to the probe (thereby obtaining deeper echoes). So lower frequencies and lower pulse repetition frequencies result in increased depth penetration with tradeoffs of lower spatial and temporal resolution.
