Answer #10
The correct answer is B. This question is testing your knowledge of the Huygen's Principle and how phased arrays work.
Modern transducers do not need moving parts to make the ultrasound beam scan back and forth across a plane to make a 2 D image. Instead electrical tricks can be used to make a transducer scan a line across a plane. Phasing is one such technique. There is a timed delay for the excitation pulse of nearby crystal elements so that each one produces a wavelet at different times. The summation of these resultant wavelets creates a scan line that changes direction based on how the time delay of pulses was arranged.
Huygen's Principle actually states that the resulting wave from the sum of many wave sources may be determined by connecting the tangential surfaces of the spherical wavefronts of each wave. This is shown in the figure. An easy way to figure out the direction a beam made by phase delay is to draw a line through the phase delay marks and take the line perpendicular to it. As shown in the figure, this will give us line B.