Answer #6

The correction answer is C: 75 Hz

To generate a 2D cine video image, the ultrasound machine strings together a number of still images or frames. For each image it collects one line of data at a time as scanning occurs across a plane so it can generate the frame as shown in the figure .

To generate a 2D image you need a large number of scanned lines. The more lines you have the longer it will take to scan that plane and create an the frame before the machine can start all over on the next frame. The machine then puts these frames together and we perceive a moving 2D video image. For a decent video, the human eye needs about 30 frames/sec or 30 Hz as minimum the frame rate.

To calculate the frame rate we first calculate the time needed to get one line in a frame. This is from the pulse repetition period (PRP), or the time between ultrasound pulses. Recall that the PRP is just the viewing depth in cm x 13--giving us the answer in microseconds. So here 4cm x 13 = 52 microseconds for each line.

Now we have 256 lines for one frame in this question. So if each line takes 52 microseconds, then 256 lines will take 52 x 256 or a total of 13,312 microseconds. This is the same as saying 13.3 milliseconds per frame.

Thus the frame rate (number of frames per second) would be 1/13.3 milliseconds or 0.075 Kilohertz which is 75 Hz.

Note that for answers D and E, the units are megahertz (MHz) which are very high for a 2D frame rate and nonsensical. They can be eliminated easily.