Ultrasound imaging works by transmitting high frequency sound waves into the body and receiving echoes from tissue to form an image. Ultrasound waves are generated by compressing and releasing tissue with a transducer. As the waves propagate through different tissues, they may be absorbed, refracted, reflected, scattered, or transmitted. The echo signals are used to generate an image on the screen by modulating brightness or motion. Image quality can be affected by artifacts from assumptions made during image formation not matching reality or by speckle from interference of signals from small scatterers within tissues.
2. SOUND
• Human : 20 Hz – 20 kHz
• Ultrasound : 20 kHz – above
• Medical US : 1-10 MHz
3. ULTRASOUND WAVES
• Propagation: wave-like fashion (acoustics), by
expansion and compression of material
• Can be absorbed, refracted, focused, reflected
and scattered
• Image: acquired by echoes produced by
scattering or reflection of complex tissues
4. WAVE GENERATION
• Transducers, as both generators and receivers
• Compressing a small volume of tissue, then
releasing it
• Due to elastic properties of the
material, compressed volume expand to its past
equilibrium, making neighboring volumes compress
• As this process continues successively through
tissues, wave is generated
• Longitudinal waves (mostly in US) and shear waves
34. A-MODE
Medical Imaging- Systems and Signals
Prince, J.L. – Links, J.M.
• Envelope detected, gain compensated signal, Amplitude
Mode
35. M-MODE
• Time evolution of repetitive A-Mode signals, which are
set to images as columns: Motion Mode
36. B-MODE
Medical Imaging- Systems and Signals
Prince, J.L. – Links, J.M.
• As transducer moves along the x-plane, A-mode
signals are keyed to the x-position of the transducer
• Then, the keyed signals produce B-Mode image by
brightness modulating -- Brightness Mode
37. TEXTURE (SPECKLE) IN B-MODE IMAGE
Image from
gehealthcare.com
• Granular, with a mottled dot pattern
38. SPECKLE
• Tissues have lots of scatterers, which are:
very numerous
randomly positioned throughout the tissue
• Generally scatterers are too close to be resolved
on images.
• In fact, an echo and the corresponding «dot» on
the US image is a combined signal from a group of
scatterers.
• So interference, constructiveness and
destructiveness plays a mojer role in imaging them
• Interestingly many characteristics of the
pattern, depend also on the imaging system itself.
(freq.)
39. ARTIFACTS
• Artifacts: structures and features on the image that
we construct, which is not 1-1 with the real object
• Assumptions that giving rise to incorrect results in
artifacts in imaging- and the reality:
Assumptions Reality
Reflectors on the beam axis Beam has a certain beamwidth, so
sometimes echoes are picked up
from structures off the axis
c is constant along the tissue Slight changes in c is present,
which leads to erronous location
Echo strength indicates organ
echogenicity
Slight deflections and refractions of
an US beam might occur, causing
to wrong location
43. BIBLIOGRAPHY
• Medical Imaging Signals and Systems, J.L.
Prince, J.M. Links
• Medical Imaging Systems, A. Macovski
• Foundations of Biomedical Ultrasound, R.S.C.
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