- □ (A) Depth
- □ (B) Wavelength
- □ (C) Frequency
Ultrasound waves are characterized by their frequency (f) and wavelength (λ). These variables have significant impact on the depth of penetration and image resolution. The product of these variables is the velocity (v) of the ultrasound signal. Frequency and wavelength are inversely related:
v = f × λ
Spatial resolution lends detail to an ultrasound image and has three geometric components: axial, lateral, and elevational resolution (Figure). Axial, or linear, resolution is the shortest distance along the ultrasound beam that two objects can separately reflect the ultrasound signal. In other words, axial resolution is parallel to the ultrasound beam. Since axial resolution is limited to one wavelength, shorter wavelengths (and thus higher frequencies) will have better axial resolution. Therefore, increasing the frequency will improve the axial resolution of a transesophageal echocardiogram.
Increasing the depth of an image will decrease the pulse repetition frequency and worsen the axial resolution. Increasing the wavelength will in turn decrease the frequency of the signal and worsen axial resolution.
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1 Perrino AC Jr, Reeves ST, eds A Practical Approach to Transesophageal Echocardiography
3rd ed. Philadelphia, PA
. 2nd ed. Philadelphia, PA Elsevier Saunders 2011 :2-20