Final answer:
The frame rate in ultrasound imaging of deep tissue is lower due to the longer travel time of the sound waves. Higher-frequency ultrasound provides greater detail but is limited to shallower tissue, while lower frequencies are used for deeper penetration with less detail resolution.
Step-by-step explanation:
When imaging deep tissue with ultrasound, the frame rate must be adjusted to accommodate the longer time it takes for the ultrasound waves to travel to the deep tissue and back. Since the speed of sound in tissue is approximately 1540 m/s, the echo time for ultrasound to penetrate deep tissue and return will be longer, leading to a reduction in frame rate. The use of higher-frequency ultrasound -- which provides greater detail but does not penetrate tissue as deeply as lower frequencies -- is limited to shallower applications due to this principle. When scanning deeper tissue with lower frequency, such as 0.8 to 1 MHz for ultrasound diathermy, we are limited to a frequency that provides sufficient penetration to reach the target depth.
Accurately calculating penetration depth and understanding the trade-off between detail resolution and depth penetration is important in medical imaging and therapy. For example, abdominal scans may use a 7-MHz frequency which limits the penetration depth to about 0.11 m but allows for 1-mm detail resolution, which is attainable and sufficient for many diagnostic purposes. Conversely, in ultrasound diathermy, the typical intensities of 10³ to 10⁴ W/m² and frequencies between 0.8 to 1 MHz ensure deep penetration to treat muscles, with the skill of the therapist being essential to avoid tissue damage.