The effects of ultrasound on different tissue types
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- The effects of ultrasound on different tissue types
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The effects of ultrasound on different tissue types
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[post_date] => 2025-01-09 11:12:55
[post_date_gmt] => 2025-01-09 16:12:55
[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.
In ultrasound imaging, high-frequency vibrational waves are generated by a tool applied to the surface of a patient’s body. The waves produced by this source propagate through the soft tissues in the patient’s body, scattering and reflecting from the different tissues that they encounter as they travel. The waves are recorded again as they exit the patient’s body in order to determine the relative amounts of absorption and scattering that the waves experienced. This information may be used to infer the presence and distribution of tissues with which the waves interacted as they passed through the body.
In the simplest ultrasound experiments, a pulse of fixed-frequency sonic waves is applied to a specific part of the body. The frequency and phase of the sound waves that are reflected back to the emitter after the cessation of the pulse are measured and compared to the known properties of the pulse. The change in signal strength between the transmitted waves and the received waves gives the total loss, or attenuation--- defined as the logarithmic decrease in amplitude (in dB) for each centimeter of thickness of tissue through which the sound waves travelled. By scanning the combined emitter/detector over an area of the patient’s body, a two-dimensional image can be formed based on the attenuation at each point in the patient’s body. The image shows dark areas where waves were not reflected by the body, and light areas where the waves were strongly reflected back towards the sensor.
In Figure 1, data from a series of ultrasound experiments was compiled and analyzed in order to infer the attenuation of various common tissues. This data is plotted against the frequency of sound waves used for the experiment. The slope of each trend line on the plot is indicated beside it.
Figure 1: A plot of the attenuation for various body tissues as a function of the frequency used in the the ultrasound apparatus. The slopes of the trendlines are inset (Jiménez et al. 2014).
[post_title] => The effects of ultrasound on different tissue types
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[questions] => Array
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[question] => Which of the following is NOT a commonality between the sound waves used in ultrasound and the light waves used in conventional microscopes?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
By definition, waves are signals that are periodic in time and space.
Refraction, reflection, and diffraction all occur when any type of wave meets an interface.
Interaction with tissues via scattering always reduces amplitude of the wave, usually by absorption of energy by the tissue.
Sound waves in fluids are purely longitudinal, and so they are unavoidably polarized along the direction of travel.
The orientation of the polarization may NOT be varied for sound waves.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Waves interact with tissues by diffraction, refraction, and reflection
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[1] => Array
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[each_answer] => B. The waves are periodic in time and space
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[2] => Array
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[each_answer] => C. The waves undergo attenuation as they interact with tissues
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[3] => Array
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[each_answer] => D. The orientation of the wave polarization may be varied.
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which of the following properties of the ultrasound waves may change as they pass from one tissue type to the next?
I. The frequency
II. The speed
III. The wavelength
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
Speed of a wave = (frequency wavelength)
Sound waves travel at different speeds in different media
The frequency of a wave does not change when it undergoes scattering
Only the speed and wavelength change as a sound wave passes from one tissue to another.
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[answers] => Array
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[0] => Array
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[each_answer] => A.I and II
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[1] => Array
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[each_answer] => B.I and III
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[2] => Array
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[each_answer] => C.I, II, and III
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[3] => Array
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[each_answer] => D.II and III
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => If the trends on the plot can be extrapolated beyond the given limits, at frequencies higher than 11 MHz, which of the following materials appear darker in images of breast tissue and skin?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
The darker tissue is the one that absorbs more of the sound waves.
Extrapolation is invalid either when the x–axis interval over which the data is extrapolated is large compared the y–scatter in the data divided by the slope of the line.
On the plot, the two materials have trendlines with different apparent slopes.
At 11 MHz, it appears that extrapolations of the two lines will have intersected, causing skin to display a lower attenuation than breast tissue.
Breast tissue would thus appear darker on images of breast and skin.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. The data cannot be extrapolated due to the scatter of the data about the trend lines
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[1] => Array
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[each_answer] => B. Skin
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[each_answer] => C. Breast
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[each_answer] => D. The two materials would absorb equally
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[3] => Array
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[quiz_unique_key] => 2261298308
[question] => Which of the following equations provides the most accurate model for the relationship between attenuation (L) and frequency (f) for waves traveling through the liver?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
The plot has log-log scale
log-log plots transform power laws into straight lines
The slope of a line in a log-log plot indicates the exponent of the power law
The attenuation increases as a power law, L(f) = f65
)
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[0] => Array
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[each_answer] => A.
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[1] => Array
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[each_answer] => B. 
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[2] => Array
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[each_answer] => C. L(f) = (6/5)f
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[3] => Array
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[each_answer] => D. L(f) = 6f+5
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[quiz_unique_key] => 2377279144
[question] => In the log-log coordinates used in the Figure, a linear trend (black line) line has been fit to the blood data (open triangles). Which of the following statements accurately describe this fit?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
Residuals are defined as the difference between a y-value of a datapoint and the y value predicted by the trendline
The blood data shows a clear curvature, leading to nonuniform scatter of the data about the trendline and thus nonuniform residual scatter
Nonuniform residuals are indicative of a need for a more sophisticated fitting model that can account for nonlinear patterns.
Nonuniform residuals thus indicate that the linear fit is incomplete
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[answers] => Array
(
[0] => Array
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[each_answer] => A. Uniform scatter of the residuals and therefore a valid fitting model.
)
[1] => Array
(
[each_answer] => B. Nonuniform scatter of the residuals and therefore an incomplete fitting model.
)
[2] => Array
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[each_answer] => C. Nonuniform scatter of the residuals and therefore a valid fitting model.
)
[3] => Array
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[each_answer] => D. Uniform scatter of the residuals and therefore an incomplete fitting model.
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[559627|1] => D
[559627|2] => D
[559627|3] => C
[559627|4] => A
[559627|5] => B
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