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121 Cards in this Set
- Front
- Back
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What is the utlrasound wavelength in soft tissue at 2 MHz?
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0.77 mm
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What material property do ultrasound wavelengths depend upon?
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Compressibility
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What is the relationship between velocity and material compressibility?
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Velocity is proportional to the square root of material compressibility
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Are sound velocities in bone high or low?
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High
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What is the average veolicty of sound in soft tissue?
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1540 m/s
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Is sound velocity in fat higher or lower than in soft tissue?
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Slightly lower
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What type of image artifact do reduced sound velocities create?
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Displacement artifact
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What units are ultrasound intensities normally expressed in?
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mW/cm2
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How to calculate beam power?
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Intensity x area
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effeuvia
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outflow in stem of particles
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What is the formula for decibels?
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10 x log (I/Io)
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What is the formual for acoustic impedance?
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Z = v x p
v - sound velocity of the material p - density fo the material |
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What is the unit of acoustic impedance?
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Rayl
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Is acoustic impedance dependent on frequency in the diagnostic range?
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No
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What is the approximate value of acoustic impedance for soft tissues?
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1.6 x 10^6 Rayl
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Piezoelectric crystals have ____ acoustic impedances. Air and lung have _____ acoustic impedance. Bone has _____ acoustic impedance.
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very high; low; high
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What is the formula for intensity reflected at an interface between materials?
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(Z2 - Z1)^2 / (Z2 + Z1)^2
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What is the formula for intensity transmitted?
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(4 * Z1 * Z2) / (Z2 + Z1)^2
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What percent of the beam do tissue/air interfaces reflect?
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~100%
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What are reflections from rough surfaces bigger than the ultrasound wavelength?
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Nonspecular reflections
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What structures contain many scattering sites?
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Kidney, pancreas, spleen, liver
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What two things cause attenuation?
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Scatter and absoprtion
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What is the unit for attenuation?
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dB/cm per Mhz
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What is the approximate attenuation coefficient for soft tissue?
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0.5 dB/cm per Mhz
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What are other names for depth gain compensation?
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time gain compensation, time varied gain, and swept gain
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What materials are ultrasound transducers made from?
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lead-zirconate-titanate (PZT), plastic polyvinylidene difluoride (PVDF), monocrystalline transducers
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Do medical ultrasound machines use pulsed or continuous-wave mode transducers?
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Pulsed
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What determines the resonant frequency of the transducer?
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Thickness
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What is the relationship between transducer thickness and wavelength?
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thickness is usually equal to 1/2 the wavelength
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High frequency transducers are [thin/thick] and low frequency transducers are [thin/thick]
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thin; thick
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Most transducers are designed to be ____ and therefore emit ____ pulses.
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broadband; short
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What is placed behind transducers to reduce vibration (ring down time)?
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Damping material
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What does damping of a transducer do to bandwidth and pulse length?
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Broadens bandwidth and shortens pulses
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What is placed in front of the transducer and what is it use for?
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matching material to improve energy transmission
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How thick is the matching layer?
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1/4 wavelength of the speed of sound in that material
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What is the design criteria for matching material?
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Impedance between transducer and tissue
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What is another name for near field?
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Fresnel zone
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What is the length of the near field?
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r^2 / lamda
r = radius of transducer lamda = wavelength |
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What is the near field for a 10mm diamteter transducer operating at 3.5 Mhz?
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~6 cm
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Doubling transducer size does what to the near field?
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Increases length fourfold
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Doubling transducer frequency does what to the near field?
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Halves the wavelength, so doubles the near field length
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What is another name for far field?
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Fraunhofer zone
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Does ultrasound imaging occur in the far field?
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No
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How to achieve ultrasound beam focusing?
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Curved crystal or acoustic lens. Alternatively, used phased array.
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How many elements do typical US linear arrays have?
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128 to 256
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What type of array geometries are there?
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Linear and curvilinear
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What type of FOV do curvilinear arrays have?
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Diverging
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How many elements do typical phased arrays have?
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96
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What is another name for pulse repetition frequency?
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Pulse rate
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Approximately how many wavelengths does each pulse contain?
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Two
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What is the approximate duration of an US pulse?
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~1 us
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What is a common PRF value?
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~4 kHz
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The choice of what controls the penetration depth?
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pulse repetition frequency
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How does a high PRF affect echo listening time and subsequently penetration depth?
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Shortens listening time; decreases penetration depth
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What is the depth for a return time of 13 us in soft tissue?
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1 cm (total trip 2 cm)
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What creates two-dimensional images from echo data from distinct beam directions?
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Scan converter
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What is the typical matrix and bit depth for US?
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512 x 512, 8 bit
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How many bytes are used in color displays?
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3 bytes; 24 bits
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What mode dispalys depth on the horizontal axis and echo intensity on the vertical axis?
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A-mode
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What mode displays time on the horizontal axis and depth on the vertical axis?
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T-M mode (time-motion)
also known as M-mode |
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What is T-M mode also known as?
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M-mode
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What mode displays an image of a section of tissue?
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B-mode
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Abdominal imaging transducers are in what frequency range?
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1 to 6 Mhz
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Peripheral imaging transducers are in what frequency range?
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5 to 13 MHz
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What are phased array transducers used for clinically?
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Imaging the heart because they have a small footprint
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What is the advantage of harmonic imaging?
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Elimination of fundamental frequency clutter (noise)
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What kind of transducer is used in harmonic imaging?
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Very broadband
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What kind of contrast agent produces harmonic frequencies?
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Microbubbles
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Why are second and higher harmonics not used imaging?
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Too much attenuation
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Two standard pulses which are phase reversed to each other is used for what?
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Phase inversion harmonic imaging
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What is the advantage of phase inversion harmonic imaging?
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Cancels out soft tissue and improves visualization of contrast agents
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What are the size of microbubbles?
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3-6 um
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What do the microbubbles contain?
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air, nitrogen, insoluble gases
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What is the formula for DOppler frequency shift?
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f x (v/c)
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What type of doppler provides depth information as well as Doppler frequency shift?
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Pulsed wave doppler
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What is used to eliminate low frequencies in Doppler?
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wall filter
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What is duplex scanning?
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real-time imaging with Doppler detection
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_____ pulse lengths are used in pulsed Doppler to improve accuracy of frequency shift.
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Longer
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Reverse flow in the center of a vessel is what kind of artifact?
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Aliasing
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How to avoid aliasing artifacts, what can be done?
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PRF must be twice the highest Doppler frequency shift
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What is the the minimum PRF for a 1 kHz doppler shift?
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2 kHz
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How to calculate resistive index?
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(Vmax - Vmin)/Vmax
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Red signifies what direction in Doppler?
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towards transducer
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Blue signifies what direction in Doppler?
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away from transducer
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What is the advantage of power doppler over color doppler?
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More sensitive and less no aliasing artifacts
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Power doppler uses ____ frame rates than color doppler
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slower
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What are motion artifacts in power doppler called?
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flash artifacts
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What is the axial resolution compared to pulse length?
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axial resolution is approximately half the pulse length
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What is the approximate axial resolution at 2 Mhz?
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~ 1 mm
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What is teh approximate axial resolution at 4 Mhz?
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~ 0.5 mm
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How does transducer frequency affect axial resolution?
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it doesn't
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How does axial resolution change with depth?
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it doesn't
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What determines ultrasound lateral resolution?
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Ultrasound beam width
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How does lateral resolution compare to axial resolution?
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4 times worse
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What can you increase to improve lateral resolution?
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Lines per frame
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Lateral resolution can be controlled by adjusting what?
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Focal position
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Using multiple focal lengths improves what?
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Lateral resolution
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Using multiple focal lengths is at the expense of what?
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Reduced frame rate
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What is elevational resolution?
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Resolution in the plane perpendicular to the image plane
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Transducer height is directly related to what resolution?
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Elevational resolution
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Is elevational resolution depth dependent?
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Yes
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What can be used to perform elevational focusing?
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Acoustic lens
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How can slice thickness be improved?
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By use of 1.5D arrays, which focus in elevational direction
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What is an 1.5D array?
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Has 6 rows of transducers in the slice thickness direction
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How many transducer elements are usually found in 1.5D arrays?
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~6 x 192
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What kind of artifact comes from textured appearance of small closely spaced structures?
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Speckle
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What artifact is the result of random signals in the electronic preamplifier?
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Noise
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What causes artifacts in the form of spatial distortions?
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Refraction
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What can arise because of the division of a smooth transducer into a large number of small elements?
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Ghost images
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What is the spatial peak intensity?
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Maximum beam intensity
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Ultrasound intensity varies over the ______ extent of the beam.
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lateral
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What is the maximum beam intensity?
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spatial peak intensity
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What are measures of ultrasound beam intensity?
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Spatial peak intensity
Spatial average intensity Temporal peak intensity Temporal average intensity |
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Which intensity measurements are most predictive of thermal effects?
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Spatial peak
Temporal average |
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Intensities in B-mode are approximately what?
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10 mW/cm2
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Doppler ultrasound intensities can exceed what?
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1000 mW/cm2
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Which index predicts cavitation effects?
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Mechanical index
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Tissue heating is a concern in which mode?
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spectral Doppler
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What is the definition of thermal index?
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Ratio of acoustic power produced by transducer to power required to raise tissue temp by 1 C
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What tissues is thermal index specified for?
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soft tissue (TIS)
bone (TIB) cranial bone (TIC) |
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Which organization reviews ultarsound safety?
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American Institute of Ultrasound in Medicine (AIUM)
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