Axial resolution refers to the ability of an ultrasound system to distinguish between two structures that are close to each other along the path of the ultrasound beam. It is primarily determined by the spatial pulse length, which is the product of the wavelength and the number of cycles in a pulse. Shorter wavelengths result in shorter spatial pulse lengths, thereby improving axial resolution. This is because shorter wavelengths (which correspond to higher frequencies) allow for better differentiation between closely spaced structures along the beam's axis.
American Registry for Diagnostic Medical Sonography (ARDMS). Sonography Principles and Instrumentation (SPI) Examination Review Guide.
Question 49
What is an advantage of pulsed-wave Doppler over continuous-wave Doppler?
Comprehensive and Detailed Explanation From Exact Extract:
The primary advantage of pulsed-wave Doppler is its ability to provide range resolution. This means the operator can select a specific depth (sample volume) from which Doppler signals are acquired, allowing precise localization of blood flow velocities.
According to sonography instrumentation reference:
“Pulsed-wave Doppler allows for range resolution, enabling velocity measurements at a specific depth along the ultrasound beam path.”
Continuous-wave Doppler does not have range resolution, as it measures velocities along the entire beam path.
Therefore, the correct answer is C: Range resolution.
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Question 50
What improves the temporal resolution of color flow imaging?
Temporal resolution refers to the ability of the ultrasound system to distinguish events occurring closely in time. In color flow imaging, temporal resolution is affected by the frame rate, which can be increased by decreasing the width of the color field of view. This is because a narrower color field requires fewer scan lines to be processed, allowing for more frames to be captured per second.
ARDMS Sonography Principles and Instrumentation guidelines
Edelman, S. K. (2017). Understanding Ultrasound Physics.
Comprehensive and Detailed Explanation From Exact Extract:
Ensemble length, also known as packet size, refers to the number of ultrasound pulses sent along each scan line when performing color Doppler or power Doppler imaging. Increasing the ensemble length allows for more accurate velocity estimation and improved sensitivity to low flow but comes at the expense of decreased frame rate.
Official sonography instrumentation reference states:
“Ensemble length refers to the number of pulses per scan line used in color Doppler imaging. A larger ensemble improves sensitivity to slow flow and provides more accurate frequency (velocity) estimates but reduces temporal resolution.”
Thus, the correct answer is B: Number of pulses along a scan line.
