# 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
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

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