Ultrasound Flashcards

Question

Transducers

Answer 1

Devices that convert one form of energy into another, such as ultrasound waves into electrical signals.

Answer 2

Multiple transducers arranged in a specific pattern to improve image quality.

Answer 3

Different ways ultrasound information is displayed for analysis.

Answer 4

Different display modes for visualizing ultrasound echoes. ## Footnote Example sentence: A-mode displays information versus time.

Answer 5

Equipment used to generate and receive ultrasound waves for imaging.

Answer 6

Technique used to measure blood flow velocity using ultrasound.

Answer 7

Doppler method using continuous transmitted and received waves. ## Footnote Example sentence: Continuous Wave Doppler uses two crystals.

Answer 8

Doppler method using pulsed waves at a constant frequency. ## Footnote Example sentence: Pulsed Wave Doppler collects one sample as a function of time.

Answer 9

Doppler technique that displays blood flow velocity in color on top of a grey scale image. ## Footnote Example sentence: Color Flow Imaging uses multiple pulses along each line.

Answer 10

Complications of ultrasound Local heating Cavitation: The pressure changes cause microbubbles in a liquid to expand then collapse. There is an increased risk of cavitation in: - Gas-containing structures (e.g. bow- el, lung) - Low frequency pulses (i.e. longer wavelengths) - Higher power or intensity of pulses - - Use of ultrasound contrast agent. Mechanical damage to cell membranes Potential risks associated with high-intensity ultrasound systems. ## Footnote Example sentence: Ultrasound can cause tissue heating and bubble formation. The mechanical index (MI) is the measure of the maximum amplitude of the pressure pulse and indicates the risk of cavitation. Should be < 0.7 , <0.5 for neonates The thermal index (TI) measures the ability of the ultrasound to heat up the local tissue. TI = power emitted / that required to in- crease temperature by 1°c Thermal index - Indicates risk of local heating - TI 0 - 1.0 safe - Decreased threshold in: febrile pa- tients, fetal scanning, eye - Should never use TI > 3 in fetal scan- ning Mechanical index - Indicates risk of cavitation - MI < 0.7 for general use - MI < 0.5 for fetal scanning - MI > 0.7 should never be used with ultrasound contrast agents

Answer 11

Linear transducers produce a rectangular field of view with uniform beam density throughout. They are useful for imaging shallow structures and small parts. produce an ultrasound beam that is emitted at 90 degrees to the transducer head. contain 256-512 elements (crystals), making them the largest assembly. Frequency range 4-12 MHz The sensitivity of the image reduces at extremes of steering and lateral resolution is best in the centre of the field of view due to a larger effective aperture. The benefits of a phased array include; a small faced transducer allowing for imaging in small spaces and being able to change the focus of the ultrasound beam. A phased array ultrasound transducer is typically 2-3 cm long, consisting of 64-128 elements with its surface . It is a smaller assembly than a sequential array and can be either linear or curvilinear. Small surface area but large field of view. Frequency range 2Mhz – 7.5Mhz Convex (sequential) arrays, also known as curvilinear or curved linear arrays, are similar to linear arrays but with piezoelectric elements arranged along with a curved transducer head. Ultrasound beams are emitted at 90 degrees to the transducer head. This arrangement results in a trapezoidal field of view due to the divergence of the ultrasound beam with increasing depth. This allows for a wider field of view but with decreased line density at depth and reduced lateral resolution. Frequency range 2.5-7.5 MHz * As a general rule, if the shape at the top of the images matches the shape at the bottom of the image it is a sequential array. If the shapes are different (e.g. rectangular at the top and curved at the bottom) it is a phased array. * endocavitaty probe - very large FOV for small transducer surface - transvaginal, transrectal Endovascular - within vessels, hollow visci * types - single (1PZT crystal that makes ultrasound wave- old ) element vs array

Answer 12

Particles closer together - compression Far apart - rarefaction Frequency - 2-18 MHz (1 Hz = 1 wavelength per second) Velocity - dependent on, and constant for, the material through which the wave is passing. c = √ (ƙ / ρ) Where: c = speed ƙ = rigidity ρ = density Wavelength - distance between points of peak compression Intensity - watts per metre2

Answer 13

Spatial resolution - Axial resolution ability to differentiate between two ob- jects in the axial plane, i.e. along the path of the ultrasound beam, depends on the length of the ultrasound pulse and the wavelength. The resolution is increased by: Low Q value of backing material (shorter pulse length) Shorter wavelength i.e. increased fre- quency -Lateral resolution to the direction of the ultrasound beam and depends on the beam width which, in turn, depends on the diameter of the PZT crystals and the focusing corresponds to ~1/3 of the transducer diameter. Beam width = focal length x λ / D Where: λ = wavelength D = diameter -Slice thickness The higher the frequency the smaller the slice thickness. It is usually larger than the beam width. For standard 2D transducers the slice thickness is fixed.

Answer 14

Spatial resolution - ability to differentiate between 2 objects in separate planes Temporal resolution- is the ability of the system to display events occurring at different times as sepa- rate images. It is measured in frames per sec-ond pulse repetition frequency (PRF) is the number of pulses of ultrasound sent out by the transducer per second. It depends on the velocity of sound and the depth of the tissue being imaged - the deeper the tissue, the longer the transducer has to wait for the ech- oes to come back i.e. lower PRF. Spatial resolution of images is enhanced by short spatial pulse length and focusing. Compared with low-frequency pulses, high-frequency pulses have shallow depth of penetration owing to increased attenuation. Temporal resolution of a two-dimensional image is improved when frame rate is high

Answer 15

Doppler effect refers to a shift in frequency perceived by an object in motion relative to the source of a wave. When applying this to ultrasonography, the transducer is a stationary source of ultrasound waves of a known frequency (depending on the mode) and velocity (1540 meters/second) which reflect off tissues, and specifically for doppler ultrasound, red blood cells (RBCs). Many of these sound waves are scattered in all directions, some are absorbed by tissues, and some of them are even dissipated as heat. The waves that are reflected back to the ultrasound transducer are used to construct the image.

Answer 16

The higher the frequency, the greater the amount of attenuation that will occur in any given tissue. Attenuation will occur not only in the beam of sound produced by the transducer as it propagates through tissue, but also in the returning echoes as they travel back to the transducer. Spatial resolution of images is enhanced by short spatial pulse length and focusing. Compared with low-frequency pulses, high-frequency pulses have shallow depth of penetration owing to increased attenuation. Temporal resolution of a two-dimensional image is improved when frame rate is high Frequency selection: Many scanner / transducer combinations permit changes of frequency. High frequencies give better sensitivity to low flow and have better spatial resolution. Low frequencies have better penetration and are less susceptible to aliasing at high velocities.

Answer 17

Duplex ultrasound involves using high frequency sound waves to look at the speed of blood flow, and structure of blood vessels .

Answer 18

The Nyquist limit states that the sampling fre- quency must be greater than twice the high- est frequency of the input signal in order to be able to accurately represent the image. Nyquist limit = PRF / 2 If the velocity of the flow is greater than the Nyquist limit, the Doppler shift exceeds the scale and “wrap-around” occurs.

Answer 19

An electronic filter is applied to the returning data to eliminate low frequency signals as these are usually produced by low velocity structures such as vessel walls. If the filter is inappropriately applied the real signals from low velocity blood flow are eliminated.

Answer 20

Blood flowing closer the inside of the vessel wall is slower than flow in the middle of the vessel. This large range of frequencies in a particular moment in time produces a widen- ing of the spectral graph and different colours in colour Doppler. This also occurs with turbulent flow (e.g. stenotic vessels) as the turbu- lence creates flow of different velocities and directions.

Answer 21

Enhancement: Fluid filled structures are weakly attenuating and a larger proportion and greater amplitude beam passes through to structures in the re- gion behind. The machine interprets this as an increase in acoustic reflection and these structures show up brighter on the image. The structures located behind fluid and homogeneous tissues generally appear brighter (whiter) than other structures, creating an impression that the echoes have been amplified. In reality, however, the amplitude of the sound has not changed. When sound traverses a structure that absorbs less acoustic energy than surrounding tissues, the area located behind that structure will appear relatively brighter than its surroundings. This acoustic enhancement phenomenon is a typical feature of cysts and homogeneous solid masses, such as lymphomas. Shadowing: Hard calcific substances and soft tissue-air interfaces reflect almost all of the sound waves. No information is received from the area behind the structure. Heterogeneous breast carcinoma with posterior acoustic shadowing. Sound is diffusely scattered within the heterogeneous internal structure of the tumor. The scattering, along with sound absorption in the tissue, creates a posterior acoustic shadow, associated with approximately 60% of primary breast carcinomas.

Answer 22

Sound bounces off a strongly reflecting object which acts as a mirror and reflects the pulse to another tissue interface. The inter- pretation of the image is that the second in- terface is beyond the first surface, much like the reverberation artifact This most often happens at the diaphragm wherein the liver is seen in the chest cavity due to sound waves being reflected off the diaphragm.

Answer 23

Multiple reflections to and fro between the transducer face and a relatively strongly re- flecting interface near the surface produces a series of delayed echoes. These look like stripes within a fluid filled structure. Two types of reverberation artefact exist: 1. Comet tail: from metal or calcified objects 2. Ring down: from a collection of gas bubbles Ring-down" is an ultrasound artifact that appears as a solid streak or a series of parallel bands radiating away from abdominal gas collections.

Answer 24

Resonance is the condition that arises when, at a specific frequency (or frequencies), the reflection of sound waves within the resonating item provide constructive interference or re-inforcement of vibrations within the item. If 2 sound waves of the same wavelength cross in the same phase, they combine and are reinforced (constructive interference). If, how- ever, they are in different phases they cancel each other out (destructive interference).

Answer 25

Diffraction is a phenomenon observed by all waves (electromagnetic, light, sound, vibration, gravity, etc.). Diffraction is what happens as a wave propagates. Beams spread, waves scatter around obstacles, can deflect light, change its frequency and modulate its phase and amplitude.

Answer 26

Flow velocity estimation requires the flow to be as parellel to the direction of the ultra- sound beam as possible. If it is perpendicular, i.e. traveling across the beam, flow is difficult to detect. The angle of insonation should be less than 60° at all times to allow the most accurate estimation of velocity

Answer 27

Panoramic ultrasound is an ultrasound technique which stitches multiple B-mode images together to create a single composite image with an increased field of view (FOV) It is useful in the evaluation of masses/objects of interest which are larger than the typical FOV

Answer 28

Side lobe artifacts occur where side lobes reflect sound from a strong reflector that is outside of the central beam, and where the echoes are displayed as if they originated from within the central beam. Ultrasound transducer crystals expand and contract to produce primary ultrasound beams in the direction of expansion and contraction. Secondary beams occur because the crystals also expand and contract radially. These radial beams are called side lobe beams. Side lobe beams are low-intensity beams that surround the central beam. Side lobe artifacts are echogenic, linear or curvilinear artifacts. Strong reflectors include bowel gas adjacent to the gallbladder or urinary bladder.

Answer 29

Refraction - sound speed error

Ultrasound Flashcards

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