Physics Chapter 4
Terms
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- Pulse Waves in soft tissue
- Smaller the pulse the better the resolution
- Pulse Duration
-
.3 ms to 2.o ms
Controlled by sound source only
Cannot be adjusted by tech
Pulse duration = #cycles/frequency MHz - Spatial Pulse Length
-
1.0 max
# of cycles x wave length mm. - PRP Pulse Repetition Period
-
Time from beginning of one pulse to beginning of next pulse
Changing Depth changes PRP
Longer to get to object the greater the PRP - PRF Pulse Repetition Frequency
-
How many pulses in a second
depth increases prp increases
depth decreases prf increases - Duty Factor
-
How long in time- pulse duration/PRP x 100
% of time crystals vibrate
99.8% of time the transducer is listening
.2 to .5% transducer is transmitting
Pulse duration decreases
duty factor increases - Shallow Imaging
-
less listening time
shorter prp
higher prf
higher duty factor - Deeper Imaging
-
More listening time
longer prp
lower prf
lower duty factor - Logarithms Scales
- compare signals
- Attenuate - When reporting attenuation in dB, authors will often omit the negative sign. Because attenuation means a weakeneing or a decrease the dB's must be negative
-
weakening of beam through the body recorded in decibels
attenuation is determined by two factors
Path length and frequency of sound.
Distance and attenuation are directly related - More attenuaton
-
Longer distances
Higher frequencies - Less Attenuation
-
Shorter distances
lower frequencies -
Soft Tissue
-3dB=2x
-6dB=4x
-9dB=8x
-10db=10x -
=1/2
=1/4
=1/8
1/10 - Sound wave weakening is determined by
-
Path Length
Frequency -
Incidence Beam
Transmission Beam
Reflected Beam -
from the transducer
through the medium
reflected back to the sound source - Three processes that contribute to attenuation
-
1. Reflection-bouces off
2. Scattering-Rayleigh
3. Absorbtion- 80% - Two forms of reflection are
-
Specular-smooth
diffuse - rough - Diffuse Reflection
- most interfaces in the body are not smooth, but have some irregularities. When wave reflects off an irregular surface, it radiates in more that one direction also called backscatter
- Specular Reflection
- When boudary is smooth, the sound is reflected in only one direction. Specular reflection occurs when a light wave strikes a mirror. Wave is slightly off axis.
- Scattering
- the random redirection of sound in many directions. Sound scatters when the tissue interface is small. Higher frequency sound beams scatter much more than lower frequency beams. scattering is directly related to frequency
- Rayleigh Scattering
-
A special form of scattering that occurs when the structure;s dimensions are much smaller that the beams wavelength ie red blood cells result in rayleigh scattering
Rayleigh scattering is organized
when frequecy doubles, rayleigh scatterng is 16 x greater
Rayliegh scattering is related to frequency to the forth power - What is the significance of attenuation in diagnostic sonography?
-
Higher the frequency sound waves produce shorter pulses, which usually create more accurate images. However, higher frequency sound attenuates more and is less capable of traveling to substantial depths.
Goal: use the highest frequency that still provides images to the depth of the structures od clinical interest - Absorbtion
- The third and most sizeable component of attenuation is absorbtion. It occurs when ultrasonic energy is converted into another energy form such as heat. Like scattering, absorbtion is directly related to frequency. higher frequency waves attenuate more than lower frequency waves. Sound traveling in bone undergoes extensive absorption
- Atteunuation Coefficient is the number of decibels of attenuation that occurs when sound travels one centimeter
- the units of attenuation coefficient are dB/cm decibels per centimeter.
- Attenuation coefficient in soft tissue
-
atten. codf.(dB/cm)=frequency (MHz)/ 2
The attenuation coefficient is one half of the frequency - Attenuation of ultrasound in medium
-
Water-extremely low
Blood urine biologic fluids -low
fat-low
soft tissue - intermediate
muscle-higher
bone and lung - even higher
air - extremely high - Attenuation in muscle
- Sound attenuates 2x as much across the fibers and when traveling along the length of fibers.
-
Half Value Layer Thickness
in clinical imaging, half value layer ranges from .25 to 1.0cm - Half value layer thickness is the distance that sound travels in a tissue that reduces the intensity of sound to one-half of its original value.
- Reflection and transmission
- The bases for ultrasonic imaging are reflection and transmission
- Impedance
- Impedance is the acoustic resistance to sound traveling in a medium. Acoustic impedance is calculated by multiplying the density of a medium by the speed that sound travels in the medium.
- Name three components of attenuation
- Absorption, reflection and scattering
- As the path length increases, the attenuation of ultrasound in soft tissue _______________
- Increases
- Attenuation in bone is _____________ attenuation in soft tissue
- greater than
- What are the units of attenuation?
- Decibels (dB)
- Attenuation in lung tissue is _______________ attenuation in soft tissue
- greater than
- Attenuation in air is ____________ attenuation in soft tissue
- greater than
- T or F In a given medium, attenuation is unrelated to speed of sound
- True. Attenuation and propagation speed are unrelated
- What is the relationship between ultrasound frequency and the attenuation codefficient in soft tissue
- In soft tissue, the attenuation coefficient in dB per centimeter is approx. one half of the ultrasonic frequency in MHz.
- What are the units of the half-value layer thickness
- distance:centimeters
- As frequency decreases, depth of penetration
- Increases
- As path length increases, the half boundary layer
- remains the same
- Impedance is a characteristic of
- only the medium
- As the path length increases, the attenuation coefficient of ultrasound in soft tissue
- remains the same
- Acoustic impedance=________X _____
- Impedance + density(kg/m3)x propagation speed (m/s)
-
Normal Incidence also called
perpendicular
orthogonal
right angle
90 degrees - means that the incident sound beam strikes the boundary at exactly 90 degrees
-
Oblique Incidence
means not at right angles or non perpendicular - Occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees
- Refraction Occurs
-
1. different prop. speeds
2. oblique incidence
Density x propagation speed -
Reflection Occurs
same as incident angle - Different impedances