Doppler effect in living tissue (ultrasound)
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- Doppler effect in living tissue (ultrasound)
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Doppler effect in living tissue (ultrasound)
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[post_date] => 2025-01-09 08:22:30
[post_date_gmt] => 2025-01-09 13:22:30
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Ultrasound is defined as any sound pressure wave which has a frequency above what a human being can hear. Generally speaking, this is any sound above 20kHz. One of the most common uses of ultrasound in medicine is ultrasonic imaging, or sonography. Sonography works by sending ultrasonic waves through the body, and then measuring the reflected waves (or ‘echo’) back from whatever tissues it bounces off. These echoes will reflect differently depending on the attenuation coefficient of the tissue it encounters (table 1), and computers can translate these differences into a visual image. A large attenuation coefficient means that the beam is quickly attenuated (weakened) as it passes through the medium, and a small attenuation coefficient means that the medium is relatively transparent to the beam.
The more consistent the density of medium which the ultrasound waves travel through, the more clear the image will be. Because dry skin contains many pockets of air, a gel is used as a conductive medium that enables a tight bond between the skin and the probe or transducer.
Sonography isn't the only application of ultrasound in medicine however. Like any transmitted wave, ultrasound is also subject to the Doppler effect, meaning that ultrasonic waves can be transmitted through the body in order to determine the speed and direction of moving objects. This is utilized in Doppler echocardiography, which is an imaging technique that measures the speed and direction of blood flow. If blood is flowing in the wrong direction, or at too high or low of a velocity, this can indicate a heart valve defect, as well as a structural abnormalities of the heart which would allow blood to flow improperly to opposite sides of the heart.
Sonography isn't the only application of ultrasound in medicine however. Like any transmitted wave, ultrasound is also subject to the Doppler effect, meaning that ultrasonic waves can be transmitted through the body in order to determine the speed and direction of moving objects. This is utilized in Doppler echocardiography, which is an imaging technique that measures the speed and direction of blood flow. If blood is flowing in the wrong direction, or at too high or low of a velocity, this can indicate a heart valve defect, as well as structural abnormalities of the heart which would allow blood to flow improperly to opposite sides of the heart.
Table 1. Attenuation coefficients of sound in various bodily tissues
Citation: Culjat, Martin O.; Goldenberg, David; Tewari, Priyamvada; Singh, Rahul S. (2010). A Review of Tissue Substitutes for Ultrasound Imaging. Ultrasound in Medicine & Biology 36 (6): 861–873.
[post_title] => Doppler effect in living tissue (ultrasound)
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[question] => Suppose a sonography machine emits a sound wave of 500 kHz into soft tissue. What would be the size of the wavelength, given that the speed of sound in soft tissue is 1540 m/s?
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[answer] => 4
[description] => Reason for the Correct Answer:
The wave equation is all one needs to solve this problem. v =λf , where v is velocity, λ is wavelength, and f is frequency.
Remember that kHz is kilohertz, therefore it is 500,000 Hz. The velocity of sound in soft tissues is given in the question.
To find wavelength, simply divide the velocity by the frequency:
1540 m/s / 500,000s-1 = 0.00309 m = 3.09 mm
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[each_answer] => A. 0.309 mm
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[each_answer] => B. 30.9 mm
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[each_answer] => C. 0.0309 mm
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[each_answer] => D. 3.09 mm
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[question] => A technician using Doppler echocardiography notices an increase of frequency reflected from the blood near the end of a heart beat in the left ventricle. What might this indicate?
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[description] => Reason for the Correct Answer:
Blood flowing at a constant rate would not produce a change in frequency.
The doppler effect indicates that objects moving away from the receiver will lower in pitch, and objects moving toward a receiver will increase in pitch.
If blood flows backwards in the heart (also known as valvular regurgitation) toward the probe, it would be indicated on a doppler echocardiogram by an increase in frequency.
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[each_answer] => A. Blood is flowing backwards toward the probe, possibly indicating valvular regurgitation
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[each_answer] => B. Blood is flowing at a constant rate and therefore it doesn’t necessarily indicate anything
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[each_answer] => C. Blood is flowing faster away from the probe, possibly indicating high blood pressure
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[each_answer] => D. Blood is flowing incorrectly in the heart from left to right, indicating a possible heart defect
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[question] => A larger than normal decrease in frequency during a heartbeat on a Doppler echocardiogram may indicate what kind of medical condition?
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[answer] => 2
[description] => Reason for the Correct Answer:
Improper blood flow between chambers would not be indicated by a change in speed but rather direction of flow.
A larger than normal decrease in frequency would indicate that the blood is moving faster than normal out of the heart, due to the doppler effect
If the blood is moving faster than normal, it must be under higher pressure than normal, meaning the patient would have hypertension.
.
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[each_answer] => A. Slower than normal blood flow out of the heart, indicating possible low blood pressure (hypotension)
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[each_answer] => B. Faster than normal blood flow out of the heart, possible high blood pressure (hypertension)
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[each_answer] => C. Blood is able to back-flow towards the receiver when valves are closed (Regurgitant heart valves)
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[each_answer] => D. Change in the normal direction of flow, indicating possible improper blood flow between chambers of the heart
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[question] => Sonographers apply a cold gel onto a patient’s skin where the probe is placed before imaging with ultrasound. What is the most likely reason for the application of this gel?
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[description] => Reason for the Correct Answer:
Ultrasound waves are pressure waves, and produce no ionizing radiation.
Ultrasound is specifically used to observe blood flow, therefore reducing it via vasoconstriction would not be beneficial.
Because the contact between probe and skin is not perfect, the gel acts to create a consistent ‘seal’ between the two, which would decrease acoustic attenuation and improve the clarity of the image.
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[each_answer] => A. The cold gel makes the surface between the skin and probe more consistent for a clearer image
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[each_answer] => B. The cold gel would filter any ionizing radiation emitted from the probe, making the imaging much safer
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[each_answer] => C. The cold gel would reduce blood flow via vasoconstriction to provide a clearer image
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[each_answer] => D. The cold gel would make for less friction between the skin and probe, making the imaging more comfortable
)
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[quiz_unique_key] => 2377279144
[question] => Based on the data provided in table 1, would the acoustic attenuation of an ultrasound be greater in bone or in the brain?
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[answer] => 3
[description] => Reason for the Correct Answer:
Attenuation is the gradual loss of intensity as sound travels through a medium.
The higher the attenuation coefficient, the greater the attenuation.
Sound attenuation coefficient is larger for bone compared to brain, therefore attenuation would be greater in bone than brain.
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[each_answer] => A. Not enough information is provided
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[each_answer] => B. The acoustic attenuation would be equal
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[each_answer] => C. The acoustic attenuation would be greater in bone
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[each_answer] => D. The acoustic attenuation would be greater in brain
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