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Increasing which of the following is MOST likely to improve the axial resolution of an ultrasound image?
A. Zoom X
B. Frequency ✔
C. Wavelength X
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Transesophageal echocardiography relies on ultrasound transduction for image formation. Piezoelectric crystals in the ultrasound transducer convert electrical energy to ultrasound waves. Ultrasound waves are characterized by wavelength (the distance or length of a single wave) and frequency (the number of sound waves per second). The relationship among the frequency (f), wavelength (λ), and velocity (v) of an ultrasound wave is defined by the formula:
v = f × λ
The spatial resolution of an ultrasound image is the measure of its ability to distinguish between different structures that are close to one another in space. Axial, lateral, and elevational resolution (the 3 components of spatial resolution) are measured relative to the direction of the ultrasound wave (Figure 1).
Figure 1. Components of spatial resolution. ROI, region of interest. © 2022 American Society of Anesthesiologists.
Axial resolution is the minimum distance between 2 structures along the ultrasound wave that can be imaged as 2 separate structures. Axial resolution is limited to 1 wavelength. Thus, the shorter the wavelength, the better the axial resolution. Since frequency is inversely related to wavelength, higher frequencies will provide better axial resolution.
The zoom function on transesophageal echocardiography machine enlarges an image but does not improve the axial resolution.
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