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An ultrasound department seeks your advice regarding scans they perform to visualise and confirm foetal heart beating. They wish to know what imaging frame rate should be set in order to visualise the heart movement reliably but achieve the maximum possible reduction in ultrasound exposure of the foetus. They explain that the typical scan conditions are a foetal heartrate of 150 beats per minute, imaged using a maximum scan display depth of 10 cm. You research the scanner and discover that: (a) the line density for imaging is fixed at 100 lines per frame (b) the pulse repetition frequency is affected by a minimum delay of 200 microseconds at the end of each pulse-echo cycle for the scanner to process the returning signals before the next pulse-echo cycle is commenced, but will otherwise adjust with the selected frame rate and (c) the scanner is designed to run initially at the maximum frame rate achievable for the depth of scan that is set, but that the frame rate may then be adjusted down by the operator. Note: The frame rate is expressed in frames per second or fps. Which of the following is the best advice to give? Please select one answer only Select one: A. Adjust the frame rate down to 5 fps and achieve an exposure reduction of approximately 83 % B. Adjust the frame rate down to 2.5 fps and achieve an exposure reduction of approximately 92 % C. Adjust the frame rate down to 5 fps and achieve an exposure reduction of approximately 94% D. Adjust the frame rate down to 2.5 fps and achieve an exposure reduction of approximately 97 % E. Adjust the frame rate down to 10 fps and achieve an exposure reduction of approximately 67 % F. Adjust the frame rate down to 10 fps and achieve an exposure reduction of approximately 87 %
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