How much is .000001?
10-6 or micro
What are acoustic propagation properties?
the effects of the medium upon the sound wave
What are biologic effects?
the effects of the sound wave upon the biologic tissue
What are the characteristics of period?
Def: time required to complete one cycleunits: microseconds or any unit of timeTV: .o6 to .5 microsecondDetermined by: sound sourceCBS: No
What intensity has the highest value?
SPTP- Spatial peak Temporal peak
What intensity is most relevant for thermal bioeffects?
SPTA- Spatial Peak Temporal Average
Which intensity has the lowest value?
SATA-Spatial Average Temporal Average
What are the three components of attenuation?
absorption (primary, sound converted into heat) , reflection, scattering
What is the oder of attenuation in different media?
Air>>Bone&Lung>>Soft Tissue>>Water**Attenuation in blood is less than that in soft tissue
What is specular reflection?
reflections from a smooth reflector (mirror) are specular and return in one direction
What is diffuse reflection or backscatter?
When a boundary is rough, reflected sound is disorganized and random
What is scattering?
The distribution of sound randomly in all directions*higher frequency sound scatters to a greater extent
What is Rayleigh Scatteirng?
If a reflector is much smaller than the wavelength of sound, sound is uniformly distributed in all directions*omnidirectional, red blood cell, frequency to the fourth power
What is the attenuation coefficient?
the amount of attenuation per centimeter. a way to report attenuation without dealing with how far sound travels, Units-dB/cm*in soft tissue it is 1/2 of the transducers frequency .5 dB/cm/MHz
What is impedance?
a number associated with a medium. Impedance is calculated not measured. represented by the letter Z*reflection of an ultrasound wave depends on different acoustic impedances of the media on either side of the boundary. Impedance= density x propagation speed
What is incident intensity?
~The intensity of the sound wave at the instant prior to striking a boundary (w/cm squared)
What is reflected intensity?
is the intensity that, AFTER striking a boundary, changes direction RETURNS BACK from where it came. (w/cm squared)
What is transmitted intensity?
intensity that, after striking a boundary, continues on in the same general direction that it was traveling
What is the reflection percentage in different biologic media?
soft tissue- air-99%soft tissue - bone -50%soft tissue - soft tissue - < 1%
What is reflection with normal incidence?
reflection occurs only if the two media at the boundary have different acoustic impedances
What is transmission with normal incidence?
these are simply reflection questions. whatever is not reflected must be transmitted.
What is refraction?
transmission with a bend*refraction is a change in direction as sound transmits from one medium to anotherRequires:1. oblique incidence2. different speeds *cannot occur with normal incidence or with identical prropagation speeds
How thick is the active element?
1/2 the wavelength thickalso called the ceramic, PZT or crystal
What is the purpose of the case?
~to protect the internal components from damage, and insulates the pt from electrical shock.**don't use a transducer with a cracked case due to the potential for electrical shock to the patient
on-quarter wavelength thick, impedances: PZT > matching layer > gel > skin
Backing material/ damping element?
reduce ringing, short pulses create more accurate images short pulse length and durationlow sensitivitywide bandwidth (broadband)low Qdecreased output power
What is bandwidth?
the range of frequencies between the highest and lowest frequency emitted from the transducerbandwidth=highest frequency-lowest frequencysynonyms: the main frequency emitted by the transducer is called the center, resonant, primary or natural frequency
What is quality factor?
a unitless number related to extent of dampinglow q = damping and wide bandwidth, imaging (pulsed) transducershigh-q = no damping and narrow bandwidth, CW & therapeutic transducers
What determines the resonant frequency of a continuous wave transducer?
sound frequency = electrical frequency
What determines the resonant frequency of a pulsed transducer?
the main or center frequency of sound from a pulsed transducer is determined by 2 characteristics of the crystal:the thickness and propagation speed of the PZT material
For pulsed transducers, describe the crystal and PZT for higher and lower frequency.
Higher frequency:-thin crystal-fast PZTLower Frequency:-thick crystal-slow PZT**when a PZT crystal Is half as thick, the sound's frequency is twice as high
How is focal depth determined?
transducer diameter or aperturefrequency of the ultrasound
What characteristics create a shallow focus?
small diameterlow frequency
What elements create a deep focus?
large diameterhigh frequency
What are the characteristics of less divergance?
larger aperture or larger diameter active elementhigh frequency narrower beam in far fieldimproved lateral resolution in the far field
What are the characteristics of more divergance?
smaller aperture or smaller diameter active elementlow frequency wider beam in the far fielddegraded lateral resolution in the far field
What is frequency of a CW determined by?
How is frequency of Pulsed Wave determeind?
thickness of ceramic and speed of sound in ceramic
How is focal length determeind?
aperture of active element and frequency of sound
How is divergence determined?
aperture of active element and frequency of sound
What is diffraction pattern?
v-shaped wave also called Huygen's wavelet.
explains the hourglass shape of an imaging transducer's sound beameach tiny part of the surface of the large transducer face may be considered an individual tiny sound source creating a Huygen's wavelet
What is resolution?
the ability to image accurately
Describe axial resolution
the ability to distinguish two structures that are close together front to back, parallel to or along the beams main axis distance units-shorter pulses create better axial resolution---> short pulse means a short spatial pulse length or a short pulse durationcannot be changed by the sonographer-a new transducer is needed to change numerical value of AR is smaller, transducers are designed with backing material to have few cycles per pulse, so that the numerical axial resolution is low and the image accuracy is superior .05-.5 mm
Axial resolution equation
AR = spatial pulse length/ 2
What improves axial resolution?
less ringing-fewer cycles in pulse (fewer carrs in train)higher frequency, shorter wavelength,
Lateral resolution definition
minimum distance that two structures are separated by side-to-side or perpendicular to the sound beam that produces two distinct echoesUnits-distance, mmLR = beam diameter, since beam diameter varies with depth, the LR also varies with depth. also called beam width variation or point spread artifactLR degrades at deeper depths (in the far zone)usually not as good as AR, because sound pulses are wider than they are short
What are special notes for high frequency pulsed ultrasound?
high frequency sound -improved AR in entire image-improved LR in the far far fieldBeam shape and divergence-in the far field, higher f pulsed sound has narrower beams compared to lower f in the far field
How does focusing alter beams?
narrower waist in the us beamshallower focussmaller focal zone
Where is focusing mainly effective?
in the near field and the focal zone
What are the two general types of focusing?
fixed-also called conventional or mechanicaladjustable by electronics also called phased array
What are the three methods of focusing?
lens-externalcurved PZT internal focusingElectronic focusing-adjustable, phased array technology provides dynamic, variable focusing orr multi-focusing
Single crystal transducers are always ______ focus.
Phased array transducers are said to have better what?
overall lateral resolution because the sonographer can move the focus to the anatomic region of clinical importance
What are the units of the acoustic variables?
pressure-pascalsdensity-kg/cm cubeddistance-distance, mm or cm
What are acoustic parameters?
describe the features of a particular sound wave
What waves are audible?
between 20 Hz and. 20,000 Hz, heard by man
What waves are ultrasound?
greater than 20,000. Hz, no frequency too high
What waves are infrasound?
less than 20 Hz, no. frequency, too low
What terms are associated with shallow imaging?
high PRFshort PRPhigh duty factor
What terms are associated with deep imaging?
low PFlong PRPlow duty factor