Optimizing Two-Dimensional Transesophageal Echocardiographic Imaging
Stanton K. Shernan1
Stanton K. Shernan2
1OUTLINE AUTHOR
2ORIGINAL CHAPTER AUTHOR
▪ KEY POINTS
Ultrasound waves become weakened or attenuated as they traverse through biological tissues, especially with the use of high-frequency transducers and greater image depths.
Axial resolution refers to the minimum distance between two structures oriented parallel to the ultrasound beam axis that permits visualization of the structures as separate, distinct reflectors on the monitor screen.
Axial resolution of two-dimensional (2D) echocardiographic images is optimized by shortening the pulse duration through the use of high-frequency transducers with wide-frequency bandwidths and appropriate damping.
Lateral resolution refers to the ability to resolve two adjacent structures that are oriented perpendicular to the beam axis as separate entities. Lateral resolution also refers to the ability of the beam to detect single small objects across the width of the beam.
Lateral resolution of 2D echocardiographic images is optimized by avoiding the use of excessive power or gain, and using a focused transducer with a high frequency and large aperture diameter.
Temporal resolution for 2D echocardiographic imaging can be optimized while maintaining line density by minimizing depth, minimizing the sector angle, and using high-frequency transducers.
Preprocessing refers to modifications of the signal that determine the specific numeric values assigned to the echo intensities prior to storage in the computer memory
Postprocessing (gray-scale mapping) determines the range of pixel values assigned to a particular brightness level after retrieval from computer memory, and only affects the brightness of the displayed pixel rather than its original stored value.
The analog-to-digital converter converts an analog signal to a digitized format by assigning it discrete numeric values, using a binary number system.
I. IMPACT OF ULTRASOUND PHYSICAL PROPERTIES ON IMAGE ACQUISITION
A. Ultrasound Physics
According to the formula for the velocity of sound propagation (v): v = f × λ, the ultrasound wavelength (λ) is dependent on both the frequency (f) which is determined by properties of the selected transducer and the velocity (v) which is determined by the medium through which the beam is directed.
B. Interaction of Ultrasound with Biological Tissues
Sound wave propagation is affected by the density and homogeneity of the interacting medium.
The amount of reflected ultrasound is directly proportional to the difference in the acoustic impedance between two different tissues, the angle of impact, interface surface irregularities, the size of the interface relative to the ultrasound wavelength, and attenuation of the sound wave.
Structures of greater density, such as calcified tissue or prosthetic material, will reflect ultrasound waves to a greater extent and thus appear more strongly echogenic.
As an ultrasound wave traverses through tissues, it becomes weakened or attenuated,
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