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Ultrasound Physics and Procedure of Ultrasound

Written By onci on Wednesday, December 21, 2011 | 11:32 PM

Ultrasound Physics

Question Answer
Define acoustic impedence density of tissue times the speed of sound in that tissue. Speed of sound in body tissue is taken to be 1540 meters/second.
Amplitude is strength or height of a signal
Attenuation is progressive weaking of the sound beam as it travels through body tissue caused by scatter absorption and reflection
Focal zone where resolution is the highest.
Pulse repetition rate is number of times per second a transmit- receive cycle occurs
Resolution is the ability to distinguish between 2 adjacent structures (interfaces)
Axial Resolution is the ability to distinguish 2 structures lying close together front to back.Known as depth, range and longitudinal resolutions. AR(mm)=SPL(mm)/2spacial pulse length is the wavelength X cycles per pulse.Wavelength is the speed of sound in soft tissue
Lateral resolution is the ability to: resolve two structures lieing closely together laterally or side by side. Also known as transverse, aximuthal, horizontal, perpendicular and trans-axial resolution.As the beam width varies along the beam axis, so does lateral resolution.
The freshnel zone is the portion of the sound beam from the transducer to the focal zone. Also referred to as the near field
The focal zone is the area of the beam where___ the resolution is highest
The Fraunhofer zone is the area of the beam that_ The Fraunhofer zone is the area of the beam that_
hertz is ______________ the standard unit of frequency, equal to one cycle per second. 1,000,000 is 1 MHz
A mode is_____ also known as amplitude mode. One dimensional image displaying the amplitude strength of the returning echo signals along the vertical axis and the time (therefore the distance from the transducer) along the horizontal axis
Axial is Depth axis. Resolution along the ultrasound beam. Represents the smallest spacing required between two reflectors to be resolved as independent reflectors
B-Mode is also known as brightness mode.A method of displaying the intensity of an echo by varying the brightness of a dot to correspond to echo strength
Damping material is material attached to the back of the transducer crystal to decrease ring time (continued vibration of crystal internal responses). Secondary ring secondary reverberations stopped/decreased by backing material. Decreasing ring time increases axial res.
The most common material for crystals in transducers is lead zirconate titanate
A phased array consists of multiple transducer elements mounted compactly in a line. All elements pulsed as group with small time delays providing beam steering,focusing. Result is vector/ sector . Cardiac, intercostal, endovacavity imaging. Phased array is electronically steered.
An annular array is a system that employs crystals of the same frequency arranged in a circle. The circular transducers are electronically focused at several depths.The beam may be reflected off a mirror into a water bath. An annular array is electronically steered system.
A Linear sequenced array is___ multiple transducer elements mounted in straight or curved bar. Groups of elements electronically pulsed at once to act as single larger element. Pulsing occurs sequentialy down length transducer face, moving sound beam from end-end.
A Linear sequenced array is___ A linear sequenced array is a electronically steered system
What factors need to be known to calculate the distance to a reflector? Travel time and speed. Range equation T=2d/ctime = distance/speed
What factor is the temporal resolution of a real-time scanner dependent on? Frame rate
Five factors which describe waves wavelengthamplitudeperiod frequency velocity
How is the propagation of sound in a medium related to the medium's stiffness and density? propagation of sound is directly related to stiffness and inversely related to density. As the density increases speed decreases, as the stiffness increases speed increases
What reflector information can be obtained from a M-Mode display? Distance Motion
What is the purpose of TGC controls? Depth gain compensation controls compensate for attenuation of sound as the distance from the transducer to target increases
What is the purpose of grayscale? Gray scale displays a range of reflection amplitudes, from white to black. Grayscale is an important factor in contrast resolution.
What does the ratio of the maximum amplitude to the minimum amplitude represent? Dynamic range. Typically 110-120 dB
What are the 3 mechanisms of action for US bioeffects? HeatCavitation Mechanical
If the frequency decreases what happens to the spatial pulse length? The spatial pulse length increases. The spatial pulse length is the distance from the beginning of a pulse to the end of that pulse.Lower frequencies reach a greater depth so have a greater spatial pulse length
What two factors increase the intensity of a sound beam? power increase and focus of the beam
Does attentuation for soft tissue increase or decrease with increased tissue thickness? Increase.
What is the Q factor? Q factor=length of time sound persists. Optimal low Q factor=short pulse length. Q factor improves with better mfg technique of transducers. Transducers with high bandwidth low Q factor are desirable. Normal range Q factor is 2-4
What factor is the operating frequency dependent upon? Crystal thickness
What is the primary factor which determines the velocity or propagation speed of the sound wave? The medium through which the sound travels
What is refraction? The change in direction of an ultrasound beam when it passes from one medium to another in which eleasticity and density differ from those of the first medium
What are the units for attenuation? dB (decibels)
What are the unit for intesity? w/cm2
What term best describes the magnitude of a signal? Amplitude
Which value indicates the average intesity over one on-off cycle?Spatial Average (SA)Temporal Average (TA)Spatial Peak, Temporal Average (SPTA)Spatial Peak, Temporal Peak (SPTP) answer is Temporal Average
List Five factors which affect the acoustic impedance of a medium densitycompressibiltyvelocitytemperaturestiffness
List 3 factors which affect lateral resolution: 1.distance from the transducer2.number of cycles in a pulse3.frequencyLateral resolution is the ability to resolve two structures lieing side by side or adjacent to one another perpendicular to the beam as two distinct echoes.
What is Lateral resolution equal to? Lateral resolution is equal to the beam diameter.
Name 3 factors the Q factor is dependent upon 1.Crystal damping2.Crystal thickness3.Operating frequencyQ factor is unitless. It refers to the ability of the tranducer to emit a pure or clean ultrasound frequency.Q factor = Main Frequency /bandwidth
In order to eliminate flicker what frame rate is necessary in a real time imaging system? > 15-20 frames per second
What is the reciprocal of the pulse repetion frequency? Pulse repetiton periodPulse repetition frequency is # of pulses occur in one second. Pulse repetition period is time it takes from begining of a pulse to beginning of next pulse. As imaging depth increases, PRF decreases.
The temporal resolution of a real time scanner is dependent on whicht factor? Frame rate. Temporal resolution is the ability to determine the position of a moving structure at any particular moment in time. The higher the frame rate, the better the temporal resolution.
What type of display mode operation causes a brightening of the spot on the CRT, rather than a deflection, each time a reflection is received? B-Mode
What is the purpose of the scan converter? purpose of the scan converter is to accept output video signal from receiver & store the image before it is sent to the TV monitor for final display. 2 types of scan converters, analog and digital. Digital scan converters are used in current US systems
Describe enhancement. Strong echos received from structures beyond a weakly attenuating region
List at least 5 parameters of a US system which require periodic testing SensitivityAxial resolutionLongitudinal resolutionDepth calibrationRange resolitionDead zone
What is the formula for the sound - on time? Total time X duty factor
The more bits per pixel, the more shades of gray can be assigned. True or false True
What is the purpose of changing the excitation time from one frame to the next (phase) in phased array transducers Allows for steering and focusing of the US pulse
What is refraction? An artifact that occurs when the sound wave changes direction when traveling from one media to another. This results in the reflector being improperly position on the display
Lateral resolution will vary depending upon (more than one answer may apply).depthbeam diametertransducer frequencyfocusing depth, beam diameter and focusing
An US transducer that has multiple concentric piezoelectric elements that are energized sequentially from the inside out allowing for circular symmetry of beam is called:1. linear sequenced array2. curvilinear array3.phased array4. annular array annular array
What function does the pulser of an Ultrasound system have? Produces an electrical voltage which drives the transducer and tells the display where the US pulses are produced
What is the PRF? Pulse repetition frequency is the number of electrical pulses produced per second. It is determined by the sound source. In diagnostic imaging PRF ranges from 1-10 kHz. The PRF changes with imaging depth.The PRF decreases with increased imaging depth.
Operating frequency = propagation speed of crystal (mm/s) / 2X thickness (mm) The thicker the crystal the lower the frequencyThe thinner the crystal the higher the frequency
Characteristics of the matching layer: The matching layer is also known as the quarter wave layer as its thickness is one quarter the wavelength.Its acoustic impedance is intermediate to the crystal and soft tissue.
The matching layer is important to: Improve sensitivity of the transducer - ability to detect weak echosImprove sound propagation or transmission into bodyProtect the crystalDecrease the amount of relection at the transducer/skin interface
The components of a pulse-echo system are: PulserTransducerReceiverDisplay
List the factors which determine the US pulse amplitude and intensity: Both amplitude and intensity are determined by the sound source. Both can be changed by the operator.Amplitude is the strength of the sound wave and equals the maximum variation in an acoustic variable.Intensity varies as sound wave travels thru body
If PRF increases the duty factor: increases. DF is the time the sound is being transmitted. Any factor that increases the time an US machine transmits increases the DF
The ratio of output power to input power is defined as : Gain. units are dB. Too much overall gain results in artifactual echos in fluid filled structures, Too little gain negates overall echo information
TGC curve - near gain is at skin level. After the flat line of delay there is an increase of the curve realtive to cm deep to the skin called -slope rate to the knee and then the flat line of the Far gain. The TGC curve compensates for the loss (attenuation) of the sound beam as it passes through the body.
Backing material characteristics: its accoustic impedance must be identical to the crystal to prevent or reduce reflection at the transducer and backing material interface
Backing material is made of: epoxy resin with tungsten powder Araldite
Pulses with shorter pulse length have ______bandwidth wide
Pulses with longer pulse length have _____ bandwidth narrow
The distribution and characteristics of a US field or Sound Beam depends on: diameter of the piezioelectric element (sound source), frequency bandwidth, focasing characteristics, attenuation, velocity of the medium and diffratction.
The near field equation is : [trx diameter (mm)]²/ 4X wavelength(mm)
The length of the near flield depends upon: transducer diameter and frequency
Calculate wavelength of a 2.5 mHz transducer with a diameter of 13 mm.Then calculate the NFL of the same transducer. 1540 m/s / 2.5 mHz = 0.00062m or .62mmNFL = 13 mm² / 4 X 0.62 = 169 /2.48 = 68 mm or 6.8 cm
calculate the NFL of a trx with 5 mHz. wavelength = velocity X frequency1540 m/s / 5.0 MHz = 0.31 mmNFL = 13 mm² / 4 X 0.31 mm = 169 / 1.24 = 136 mm
When the transducer frequency goes up the NFL __ ___ goes up. When transducer frequency doubles the NFL doubles. When transducer frequency is halved the NFL is halved
Whats the difference between external focusing and internal focusing? External focusing is achieved by placing an acoustic lense infront of the crystal.Internal focusing shaping or curving of the crystal (used on frequencies 5 MHz or lower.
An example of a nonspecular reflector is:a liver surfaceb diaphramc RBCd any structure not producing a strong echo C RBC
The spatial pulse length:a determines penetrationb usually decreases with frequencyc is improved with rectificationd determines lateral resolution SPL = # of cycles / wavelength As wavelength is shortened, SPL decreases with higher freqiuency
Axial resolution is:a improved in the focal zoneb depends on the TGC slopec is improved by digital scan convertersd depends on the wavelength d depends on the wavelengthAlso called longitudinal, range or depth resolution is determined by the wavelength. Axial resolution improves with increasing frequency
Lateral Resolution :a. and ring-down are the sameb.depends on the beam diameterc. improves with frequencyd. cannot be measured in the far zone B. Depends on the beam diameter
The beam of an unfocused transducer diverges :a. becasuse of inadequate dampingb. in the Freshnel zonec. in the Fraunhofer zoned. when the pulse length is long enough c. in the Fraunhofer zone
Reverberation artifacts are a result of a electronic noiseb. improper TGCc. presence of two or more strong reflecting surfacesd. angle of incidence is too small c. presence of two or more strong reflecting surfaces
Grating lobes are necessary for the proper operation of a linear phased array:A. TrueB. False B. False
A linear array cannnot be dynamically focused:A TrueB False B FalseThe focusing of a linear array is dependent on timing and is thus capable of dynamic focusing
Electronically sterred transducers always produce higher resolution images than do mechanically steered transducersA. trueB. false A. true
An annular arary uses mechanical beam steeringA. TrueB. False B. False
Decreasing spatial pulse length :a. reduces the field of viewb. reduces lateral resolutionc improves axial resolutiond. improves lateral resolution c improves axial resolution
How much will a 3.5 MHz transducer attenuate after passing through 2 cm of tissue?a. 7 dBb. 3.5 dBc. 17dBd. 1.75 dB b. 3.5 dB.5db*3.5MHz*2cm = 3.5dB
Propagation speed errors result in:a.reverberationb. improper axial positionc. shadowingd. a Doppler shift b. improper axial position
Enhancement is caused by:a. strongly reflected structuresb. propogation speed errorsc. Snell's lawd. weakly attenuating structures d. weakly attenuating structures
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