* Acoustic Shadowing: This artifact occurs when a structure absorbs or reflects most of the ultrasound waves, causing a shadow behind the structure. It does not cause improper positioning of a reflector on the display.
* Speckle: This is a form of noise in ultrasound imaging that appears as granular texture. It can affect image quality but does not lead to improper positioning of reflectors.
* Enhancement: This artifact occurs when the area behind a weakly attenuating structure appears brighter.
It affects the brightness of the image but does not affect the position of reflectors.
* Range Ambiguity: This occurs when an echo is received after the next pulse has been sent out, causing the reflector to be placed at an incorrect depth on the display. This is because the system assumes the echo came from the most recent pulse.
References:
"Ultrasound Physics and Instrumentation" by Frank Miele
ARDMS Sonography Principles and Instrumentation study materials

Comprehensive and Detailed Explanation From Exact Extract:
Using multiple focal zones improves lateral resolution but requires additional time for each focal zone acquisition, which reduces frame rate and therefore degrades temporal resolution.
According to sonography instrumentation reference:
"Multiple focal zones improve lateral resolution but at the expense of temporal resolution due to longer acquisition time for each frame." Therefore, the correct answer is D: Temporal.

The artifact demonstrated in the image is mirroring. This occurs when the ultrasound beam encounters a strong reflector, such as a diaphragm or pleura, and is reflected back and forth between the object and the transducer. This results in a duplicate image appearing on the other side of the strong reflector, creating a mirror image artifact. It is crucial for sonographers to recognize and differentiate this artifact from actual anatomical structures to avoid misinterpretation.
Reference:
American Registry for Diagnostic Medical Sonography (ARDMS) Sonography Principles and Instrumentation study materials.
Diagnostic Ultrasound: Principles and Instruments by Kremkau, F. W. (latest edition).

* Gray-scale evaluation in a tissue-mimicking phantom involves assessing the uniformity and accuracy of the gray-scale representation of the tissues.
* Option C typically contains structures designed to test the machine's ability to accurately depict varying levels of echogenicity, which is essential for proper gray-scale evaluation.
* This area will have a range of echo intensities that help in determining the contrast resolution and the ability of the system to distinguish between different tissue types based on their gray-scale values.
References:
* ARDMS Sonography Principles and Instrumentation guidelines on tissue-mimicking phantoms and image quality evaluation.

Ensemble length, also known as packet size or Doppler packet, refers to the number of pulses used to calculate each Doppler measurement. Increasing the number of pulses in an ensemble length improves the accuracy of velocity measurements by providing more data points for the Doppler shift analysis. This leads to better estimation of mean velocities and reduces the variability of the measurements, although it may slightly decrease temporal resolution due to the longer time required to acquire the data.
ARDMS Sonography Principles and Instrumentation guidelines
Edelman, S. K. (2017). Understanding Ultrasound Physics.