Final answer:
Axial resolution in ultrasound imaging is determined by the echo times used to measure distances to tissues and requires higher frequency sound waves for finer detail resolution. The ability to distinguish between closely spaced structures (1.00 mm) depends on the sound's wavelength, speed, and time resolution.
Step-by-step explanation:
The axial resolution of an ultrasound is determined by the ability to distinguish between echoes from structures that are close together in the direction of the ultrasound beam. It is proportional to the wavelength of the sound waves used; shorter wavelengths result in better resolution. Echo times are used by ultrasound scanners to determine distances to reflecting surfaces within a patient. To calculate the difference in echo times for tissues at 3.50 cm and 3.60 cm beneath the surface, one must consider the speed of sound in the tissue and apply the formula for distance based on the time taken by the sound waves to return. Moreover, the period T of the ultrasound must be smaller than the minimum time resolution to see smaller details, which implies the need for a higher frequency. The minimum frequency can be calculated using the speed of sound within the tissue and the minimum time resolution required.
Using the Rectangular Criterion from optics, similar concepts apply to ultrasound imaging where the ability to distinguish between two points (or tissues) is related to the wavelength and the numerical aperture of the system. Modern ultrasound devices construct detailed images by measuring echoes in a phased array pattern, navigating a directional beam across the area of interest, enhancing resolution. Therefore, to have good axial resolution and distinguish details as small as 1.00 mm, the frequency of the ultrasound must be high enough to allow for sufficient discrimination of these small time differences.