03-09-2012, 01:36 PM
Reflection-mode ultrasound imaging
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Overview
• Ultrasound: acoustic waves with frequency > 20kHz. Medical ultrasound typically 1-10MHz.
• Ultrasound imaging is fundamentally a non-reconstructive, or direct, form of imaging. (Minimal post-processing required.)
• Two-dimensions of spatial localization are performed by diffraction, as in optics.
• One-dimension of spatial localization is performed by pulsing, as in RADAR.
• The ultrasonic wave is created and launched into the body by electrical excitation of a piezoelectric transducer.
• Reflected ultrasonic waves are detected by the same transducer and converted into an electrical signal.
• Basic ultrasound imaging system is shown below.
What does ultrasound image?
Reflection-mode ultrasound images display the reflectivity of the object, denoted R(x, y, z).
The reflectivity depends on both the object shape and the material in a complex way.
Two important types of reflections are surface reflections and volumetric scattering.
Summary
In reflection-mode ultrasound imaging, the images are “representative reproductions” of the reflectivity of the object. A cyst,
which is a nearly homogeneous fluid-filled region, has reflectivity nearly 0, so appears black on ultrasound image. Liver tissue,
which has complicated cellular structure with many small mechanical inhomogeneities that scatter the sound waves, appears as a
fuzzy gray blob (my opinion). Boundaries between organs or tissues with different impedances appear as brighter white curves in
the image.