cram cards Flashcards
acoustic variables include
pressure
density
distance (particles in motion)
sound frequency is determined by __________and is ____________ within given medium
the source, constant
the speed of sound in a given medium is determined by
stiffness and density
US machines assume the speed of sound in the human body to be
1540 m/sec
in addition to its Doppler capabilities, continuous wave US may also by used to produce images , T or F?
False
US must be pulsed to create images
Pulse repetition frequency (PRF) is limited by
imaging depth
Duty factor (DF) is the percentage of time that the US system is producing sound T or F?
True
which type of resolution is determined by the SPL (spatial pulse length)
axial resolution
the go return time to a reflector located 1cm from the transducer is
13 microseconds
as sound propagates through soft tissue, the lower the frequency, the greater the attenuation?
False
Which of the following are components of attenuation?
a. absorption
b. scattering
c. frequency
d. a and b
e. a, b, and c
d. attenuation is affected by frequency, but it is not one of its components
components are; absorption, scattering and reflection
T or F
echoes from specular reflectors are weaker, or lower in amplitude than echoes returning from Rayleigh scatteres?
False
Echoes from specular reflectors (irregular interfaces) are about 1000x stronger!
Rayleigh scattering: sound is scattered in all directions (red blood cells -particles smaller than wavelength)
attenuation coefficient for soft tissues is
.5dB/cm/MHz
T or F
adequate penetration of the US beam to the region of interest outweighs the desire for high resolution images
True
T or F
a transducer is a device that converts one form of energy to another
True
which of the following may be found in all transducers?
a. piezoelectric crystal
b. backing layer
c. scan converter
d. a and b
e. a, b, and c
d. a and b
the purpose of the backing material is to reduce
ringing
the optimal frequency at which a transducer operates is its resonance frequency, which is determined by :
a. backing layer
b. thickness of the crystal
c. matching layer
b.thickness of the crystal
for CONTINUOUS wave US the acoustic frequency is determined by the ______________ applied
electrical frequency of the exciting voltage applied by the machine to the crystal
For PULSED US the PRF is determined by _____ the frequency of the US is determined by __________
number of electrical pulses applied by the US machine to the crystal
thickness of the crystal and propagation speed of crystal
T or F
the range of frequencies contained in a pulsed US wave is called the transducer’s resonance frequencies
False
it is called the bandwith
T or F
for continuous wave, transducer frequency is determined primarily by the thickness of the crystal
False (true for pulsed US)
frequency is determined by the electrical frequency of voltage
T or F
generally, poorly dampened crystals produce long pulses with narrow frequency bandwidths, while highly dampened crystals produce short pulses with wide frequency bandwidths
True
A-mode display represents echoes as
spikes rising up from a time-distance baseline
T or F
B-mode provides the operator with a better temporal resolution than A-mode?
False
M-mode. takes a B-mode scan line and moves it across the display screen; it is the only mode that provides information regarding the position of reflectors with respect to time
which of the following transducer types are capable of electronic focusing and beam steering?
a. linear sequential array
b. linear phased array
c. annular array
d. all of the above
b.
-only array transducers can be focused electronically
- only phased array can be electronically steered
(may no longer hold true- Cross beam??/)
the effective beam forming surface of the transducer is known as the
a. beam profiler
b. ultrasonic fear
c. aperture
d. lens
c. aperture
the near zone is called?
a. Fraunhofer zone
b. Fresnel zone
c. transition point
d. none of the above
b. Fresnel zone
T or F
NFL (near field length increases as
1. transducer frequency increases
2. transducer diameter decreases
- T
2. F
lateral resolution is equal to
beam width
**axial resolution Far exceeds lateral resolution
the TGC is a component of which of the following
a. transmitter
b. receiver
c. scan converter
d. none of the above
b. receiver
the scan converter may also by referred to as
image memory or storage
Axial resolution is determined by
spatial pulse length
T or F
Preprocessing functions occur after the image is stored in memory and include all receiver functions
False
Doppler shift is defined as
reflected frequency minus incident frequency
Horizontal accuracy of the US system is dependent on the proper function of the system’s
position encoder
which factors contribute to possible bioeffects?
a. degree of power output
b. degree of sound beam focusing
c. degree of system gain
d. a and b
d, a and b
the highest beam intensity is
SPTP
which of the following imaging modalities exhibits the lowest SPTA acoustic output values?
a. color flow Doppler
b. M-mode
c. B-scan imaging
d. pulsed doppler
c. B- scan imaging
the primary reason the Doppler angle (cos0) is applied to the Doppler equation is :
to estimated flow velocity
wider beam aperture results in ________ (more/less) directional beam and (more/less) divergent beam
more directional
less divergent
more intense
the shape of the US beam is called the
beam profile
frequency is
of cycles per second
- determined by the source
- constant in given medium
period is
time required to complete one ocillation
reciprocal of frequency
Wavelength is
distance over which one cycle occurs
wave equation
Speed of sound
WL= ————————–
frequency
wavelength is __________ proportional to the speed of sound
directly
wavelength is _________ proportional to frequency
indirectly
Propagation speed is determined by medium’s stiffness; increased stiffness will _________ speed,
increase
increased density will
decrease speed
Amplitude is
max variation from the baseline
Amplitude is determined by
determined by the source
(the voltage applied to transducer)
decreases as sound s travel through tissue
affects power and intensity
intensity is
concentration of power in the US beam
- determined by the source
- decreases as sound travels through tissue
intensity= power/ beam area I= watts/cm^2
power is
proportional to amplitude squared
wavelength is affected by source or medium
both
near zone length=
6
larger crystal diameter= _________________ near zone length
longer
higher transmitter frequency = ____________ near zone length
longer
focusing is defined as the process of _________ beam width to improve lateral resolution
reducing
focusing is mainly effective in the near zone
what are the two methods of focusing a US beam?
-mechanical focusing
(external)
for higher frequencies; lenses ans mirrors
(internal)
shaping and curving crystal for 5MHz and lower
-electronic focusing
beam focusing improves ___________ resolution and system __________
lateral
sensitivity
spatial resolution is
axial and lateral
axial resolution is determined by
SPL
axial res. =_______________
2
frequency is important, because wavelength varies with frequency
lateral resolution results in ___________ echoes being detected, it is equal to _______________
separate
beam width
which is better axial or lateral resolution?
axial resolution is better at all frequencies
Doppler effect is a
change in sound frequency, because of motion between sound source and receiver. Positive (moving towards) negative (moving away)
Doppler: Flow towards the transducer has an ___________ frequency, flow away from the transducer has a ______ frequenct
increased
decreased
Doppler shift frequencies fall into what range
-10 KHz to 10 KHz
Doppler shift is
reflected frequency - incident frequency
Doppler equation
Doppler shift = received frequency - source frequency
in order to calculate blood cell velocity the Doppler __________must be known
angle
the magnitude of the Doppler shift depends on its
angle
clinically useful Doppler angles
30- 60 degrees
Doppler angle = 0 (cos 1) = >
maximum Doppler shift occurs
Doppler angle= 90 ( cos 0) =>
no detectable Doppler shift
in order to determine the velocity of RBC, three bits of information need to be known
Speed of sound C
tramsmitted sound frequency
Doppler angle
types of Doppler
continuous wave Doppler
pulsed wave Doppler
color flow Doppler
Power Doppler
relationship of PRF to PRP?
reciprocals
PRF definition
# of US pulses generated by the transducer per second (4-10KHz)
controlled by pulser (sound source)
what limits PRF?
image depth - go-return time
PRP includes both the pulse duration and the
echo reception time
PRF and PRP are controlled and influenced by
operator, sound source (not the medium)
Duty factor is
% of time the US machine is producing a pulse
DF= pulse duration / PRP
SPL determines what kind of resolution?
axial
SPL is determined by what?
sound source and medium
is not adjustable
SPL formula
wavelength x # of cycles in a pulse
Range equation
reflector distance
1.54mm/us x go return time
________________________________
2
to calculated the go return time to a reflector in 1cm distance formula
time (usec)= reflector distance (mm) / 0.77(mm/usec)
time= 13 usec to reflector and back
attenuation is
decrease in amplitude and intensity
units of attenuation
dB, always negative when referring to attenuation
3 components of attenuation
absorption
scattering
reflection
the greater the frequency the greater the attenuation
absorption is
dissipation of sound energy into other forms of energy , mainly heat
makes up 70% of attenuation coefficient
doubling of frequency ________ the absorption
doubles
Reflection occurs ar
interface
part of the energy is reflected, part of it is transmitted into the second medium
Refraction is
bending or change in direction of the transmitted soiund wave at interface.
wavelength may change to accommodate new speed of sound
Snell’s law
when sound strikes at oblique incidence;
if medium2 has a higher speed of sound than medium 1, then the angle of transmission is greater than the angle of incidence. and vice versa
scattering
reflection of US wave at
- rough or irregular interfaces (diffuse scattering, or reflection)
- reflectors smaller than wavelength (Rayleigh scattering)
attenuation coefficient
amount of attenuation per cm
unit; dB/cm
attenuation coefficient in soft tissue is
0.5dB/cm/MHz or half the frequency
attenuation formula
attenuation= path length x attenuation coefficient
attenuation increases with frequency and path length
bandwidth
difference between highest and lowest frequency emitted by transducer
SPL is _________ related to bandwidth
inversely
poorly dampened crystals have long pulse length and narrow bandwidth
A-mode
-first form of echo display
-hardly used anymore (ophthalmic, , )
-series of spikes
y= strength (amplitude)
X= distance from transducer
B-mode (brightness)
- basis of all US today
- strength of echo displayed as brightness on monitor
positioning encoder allows the creation of a 2D image (B scan)
M mode
-the only mode that provides information of position with respect to time
-uses one B-mode scan line
Y= depth
X= time
- used in echocardiography
Hugen’s principle
each particle on the surface of a transducer can be considered a small separate sound source
US system components
- transducer
- transmitter (pulser)
- receiver
- detector
- scan converter
- display
transmitter (pulser, sounder) is responsible for
exciting piezoelectric crystal
receiver is responsible for _______echoes;
4 components
returning
MAIN PROFILER-amplification (system gain)
TGC- compensation
SIGNAL PROCESSOR- compression (compresses ans balances dynamic range)
VIDEO AMPLIFIER- demodulation (converts voltages to video signal)
the imaging effect of changing the overall Gain control is the same as increasing the output gain by the same factor, but it
doesn’t have the increased exposure effect
the typical dynamic range of signals sent to the scan converter is about
40dB (monitor; 20dB)
scan converter is also called
image memory or storage
does not produce a visible image
transducer array types
linear (rectangular images)
Convex (curved, trapezoid)
annular (ring) pie shaped
power
typical power output?
in pulsed wave doppler mode?
20-100mW
up to 2,000mW
US beam can be described in both (time and space)
Spatial -space
Temporal -time
with a focused transducer, SP intensity is always _________ than in an unfocused transducer, why?
Higher because of a small beam area
Temporal average intensity is greatly affected by
PRF
if PRF increases DF increases and TA will ve higher
highest intensity
SPTP
Lowest intensity
SATA
highest acoustic power during
color flow imaging
AIUM stands for
American institute for ultrasound in medicine
