2. 2
Ultrasound is cyclic sound pressure with a frequency greater than the upper
limit of human hearing. Although this limit varies from person to person, it
is approximately 25 kilohertz (25,000 hertz) in healthy, young adults and
thus, 25kHz serves as a useful lower limit in describing ultrasound.
SONAR (SOund NAvigation and Ranging)—or sonar—is a technique that
uses sound propagation under water to navigate, communicate or to detect
other vessels. There are two kinds of sonar—active and passive. Acoustic
location in air was used before the introduction of radar. Sonar may also be used
in air for robot navigation
SODAR (an upward looking in-air sonar) is used for atmospheric
investigations.
The term sonar is also used for the equipment used to generate and receive
the sound. The frequencies used in sonar systems vary from infrasonic to
ultrasonic.
4. 4
Ultrasound Physics
Diagnostic Ultrasound X-rays
wave type
longitudinal mechanical
waves
electromagnetic waves
transmission
requirements
elastic medium No medium
generation stressing the medium accelerating electric charges
velocity
depends on the medium
through which it
propagates
It is relatively constant:
299,792.456.2 m/s
5. 5
Velocity of sound in some Biological Materials
Material Velocity of Sound
(m/s)
Impedance (Rayl x 10 -6)
Air 330 0.0004
Fat 1450 1.38
Water 1480 1.48
Average Human Soft Tissue 1540 1.63
Brain 1540 NA
Liver 1550 1.65
Kidney 1560 1.62
Blood 1570 1.61
Muscle 1580 1.7
Lens of eye 1620 NA
Skull Bone 4080 7.8
6. 6
Piezoelectricity
Piezoelectricity is the ability of some materials(notably crystals and
certain ceramics) to generate an electric potential[1] in response to applied
mechanical stress.
This may take the form of a separation of electric charge across the crystal
lattice.
The piezoelectric effect is reversible in that materials exhibiting the direct
piezoelectric effect
The effect finds useful applications such as the production and detection of
sound, generation of high voltages, and electronic frequency generation.
8. 8
From sound to image
Producing a sound
wave1-
Receiving the echoes 2-
Forming the image3-
Sound in the body4-
9. 9
Producing a sound
wave1-
A sound wave is typically produced by a piezoelectric transducer encased in a probe.
Strong, short electrical pulses from the ultrasound machine make the transducer ring at the
desired frequency. The frequencies can be anywhere between 2 and 15 MHz.
The sound is focused either by the shape of the transducer, a lens in front of the transducer, or
a complex set of control pulses from the ultrasound scanner machine. This focusing produces
an arc-shaped sound wave from the face of the transducer.
The wave travels into the body and comes into focus at a desired depth.
Linear Array Transducer
12. Modes of sonography
•
• A-mode: A-mode is the simplest type of ultrasound.
1- A single transducer scans a line through the body.
2- Echoes plotted on screen as a function of depth.
3- Therapeutic ultrasound aimed at a specific tumor or calculus is
also A-mode, to allow for pinpoint accurate focus of the
destructive wave energy.
• B-mode: In B-mode ultrasound:-
1- A linear array of transducers are used.
2- Simultaneously, that array scans a plane through the body.
3- That plane can be viewed as a two-dimensional image.
• M-mode: M stands for motion.
1- A rapid sequence of B-mode scans specific area.
2- images follow each other in sequence on screen, which enables
doctors to see and measure range of motion, as the organ
boundaries that produce reflections move relative to the probe.
• Doppler mode: This mode makes use of the Doppler effect.
•
12
13. Doppler sonography
• The total Doppler effect may result from:
1- motion of the source (probe ) 2- motion of the observer probe
3-motion of the medium ( i.e blood).
• Each of these effects is analyzed separately using spicial computer
program.
13
15. Strengths of sonography
• It images muscle and soft tissue very well and is particularly useful for delineating the
interfaces between solid and fluid-filled spaces.
• It renders "live" images, where the operator can dynamically select the most useful
section for diagnosing and documenting changes, often enabling rapid diagnoses.
• It shows the structure of organs.
• It has no known long-term side effects and rarely causes any discomfort to the
patient.
• Equipment is widely available and comparatively flexible.
• Small, easily carried scanners are available; examinations can be performed at the
bedside.
• Relatively inexpensive compared to other modes of investigation
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16. Weaknessesofultrasonicimaging
• Sonographic devices have trouble penetrating bone. For example, sonography
of the adult brain is very limited.
• Sonography performs very poorly when there is a gas between the transducer
and the organ of interest, due to the extreme differences in acoustic
impedance.
• The depth penetration of ultrasound is limited, even in the absence of bone or
air, making it difficult to image structures deep in the body.
• The method is operator-dependent. A high level of skill and experience is
needed to acquire good-quality images and make accurate diagnoses.
• There is no scout image as there is with CT and MRI. Once an image has been
acquired there is no exact way to tell which part of the body was imaged.
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