The resonant frequency of a pulsed wave transducer is determined by the thickness of the piezoelectric element and the speed of sound within that element. The resonant frequency is inversely proportional to the element thickness and directly proportional to the speed of sound in the material. Thinner elements and higher sound speeds result in higher resonant frequencies, while thicker elements and lower sound speeds result in lower resonant frequencies.
ARDMS Sonography Principles and Instrumentation guidelines
Kremkau, F. W. (2015). Diagnostic Ultrasound: Principles and Instruments. Elsevier.

Comprehensive and Detailed Explanation From Exact Extract:
Reflection is maximized when the ultrasound beam strikes a tissue interface at 90 degrees (perpendicular).
This angle provides optimal return of echoes for imaging.
According to sonography instrumentation reference:
"Maximal reflection occurs when the sound beam strikes a boundary at 90 degrees." Therefore, the correct answer is D: Reflection.
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The tissue-equivalent phantom image with the red oval indicates an area where axial resolution can be evaluated. Axial resolution refers to the ability to distinguish between two structures that are close together along the axis of the ultrasound beam. It is determined by the spatial pulse length (SPL) of the ultrasound wave. In phantoms, this is typically tested by observing the ability to separate closely spaced targets along the beam's path.
Reference:
ARDMS Sonography Principles & Instrumentation Guidelines
Hedrick WR, Hykes DL, Starchman DE. Ultrasound Physics and Instrumentation. 4th ed. Philadelphia, PA: Elsevier Saunders; 2005.

Increasing the packet length in color Doppler imaging means increasing the number of pulses used to interrogate each scan line. This improves the accuracy of velocity measurements and sensitivity to low flow velocities but has the drawback of decreasing the frame rate. A longer packet length requires more time to acquire the necessary data, which reduces the number of frames that can be processed and displayed per second. Consequently, while color Doppler imaging becomes more precise, the temporal resolution (frame rate) decreases. Reference:
ARDMS Sonography Principles and Instrumentation guidelines
"Color Doppler, Power Doppler, and Spectral Doppler" by Michael J. Riccabona

* M-mode (Motion mode) is used in ultrasound to measure and display the movement of structures over time.
* This mode is particularly useful in cardiac imaging to assess the motion of heart walls and valves.
* M-mode provides a one-dimensional view of the motion of tissues and is often used in conjunction with
2D imaging for a comprehensive assessment.
* It is essential in evaluating the dynamic function of organs, especially in cardiology, where precise measurements of cardiac structures' movement are crucial.References:
* ARDMS Sonography Principles and Instrumentation guidelines on modes of ultrasound imaging and their clinical applications.