Color flash artifact occurs due to tissue motion. This artifact is a type of color Doppler artifact that happens when there is movement of tissue or transducer, which causes the Doppler system to incorrectly interpret the motion as blood flow. This results in a flash of color appearing on the image where there is actually no flow. Tissue motion affects the Doppler signal, leading to misinterpretation by the system, and hence the artifact appears as a flash of color.
Reference:
ARDMS Sonography Principles and Instrumentation (SPI) Exam Study Guide
"Diagnostic Ultrasound: Principles and Instruments" by Frederick W. Kremkau

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.
Reference:
American Registry for Diagnostic Medical Sonography (ARDMS). Sonography Principles and Instrumentation (SPI) Examination Review Guide.

Comprehensive and Detailed Explanation From Exact Extract:
Axial resolution refers to the ability to distinguish two structures that are located along the path of the ultrasound beam. It is directly related to the spatial pulse length (SPL), which is the product of the number of cycles per pulse and the wavelength. A shorter SPL leads to better axial resolution.
According to sonography instrumentation reference:
"Axial resolution improves as spatial pulse length decreases because shorter pulses can better differentiate structures lying close together along the beam axis." Therefore, the correct answer is B: Spatial pulse length.
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* Increasing the pulse repetition frequency (PRF) helps to optimize the color Doppler imaging by reducing aliasing.
* Aliasing occurs when the PRF is too low to accurately sample the rapid blood flow velocities, leading to incorrect color representation.
* By increasing the PRF, the system can more accurately measure higher velocities without distortion, improving the overall quality of the color Doppler image.References:
* ARDMS Sonography Principles and Instrumentation guidelines on Doppler imaging and techniques to reduce aliasing.

Frame rate in color flow Doppler is influenced by several factors, including the imaging depth. Decreasing the depth reduces the time it takes for sound waves to travel to the imaging area and back to the transducer. This allows for more frames to be captured per second, thereby increasing the frame rate. Higher frame rates improve temporal resolution, making it easier to visualize moving structures.
References:
ARDMS Sonography Principles & Instrumentation Guidelines
Hagen-Ansert SL. Textbook of Diagnostic Ultrasonography. 8th ed. St. Louis, MO: Mosby; 2017.