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Physics

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In everyday language, sound refers to the experience of the listener. Scientifically, it is a phenomenon, not an experience. Sound is a wave created by a vibrating mechanical disturbance traveling within a compressed and rarified molecular medium. As sound encounters the boundary of another medium, it can bounce off that boundary and return in the opposite direction from which it came from. This is called reflection and the returning sound wave is called an Echo; can be depicted by tying a rope to the end of a chair and giving it a firm flick. As the wave encounters the chair "medium", you will feel the reflected wave return - it will jolt your hand slightly. The transducer spends a majority of its time awaiting for Echo waves to return (long listen) compared to short pulses. 

Sound wave characteristics

 

Continuous-wave Ultrasound

Sound wave is a traveling variation in pressure. 

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  • amplitude (gain)

    • maximal deviation of a wave from its baseline (the resting position, zero)​

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  • cycle

    • going up and down and back to baseline​​

 

  • period (T)

    • time (sec) it takes for 1 cycle​

    • time it takes an air molecule to move back and forth 1 time

    • represents what that particular molecule of air is doing as a function of time

    • eg. it takes you 2 seconds to go up and down back to baseline versus 1 second

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  • frequency

    • number of wave cycles that occur in 1 second​

    • 1 Hz = 1 cycle / second

    • 1 / period (T)

    • megaHz (MHz) = millions of cycles per second

    • human hearing = 20Hz - 20,000 Hz

    • higher frequency = less penetration because the cycle length is shorter and thus does not penetrate as well. Better resolution

    • lower frequency = more penetration because the cycle length is longer and thus can penetrate deeper structures. Less resolution

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  • wavelength

    • depends on the thickness of the piezoelectric crystals​ (eg. the vascular probe is thinner than the phased and curvilinear probe)

    • length of space over which one cycle occurs

    • how far the wave has travelled after one cycle

    • the length between two compressed air regions (not the same as period)

    • represents "snapshot" displacement of ALL the air molecules along that wave at a particular instance in time

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  • power 

    • the rate at which work is performed; proportional to amplitude squared​

    • total energy incident on a tissue in a specified time

 

  • intensity

    • concentration of energy in a sound beam; beam power divided by cross sectional area​

    • proportional to power​​

 

  • propogation speed

    • the speed a sound wave moves through a medium​

    • highest through solids (eg. bone) lowest through gases​

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Pulse-Wave Signal Transmission

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Ultrasound Wave Transmission

  • influenced by the medium the sound wave moves through and "things" it comes in contact with

  • reflection and propogation of sound wave through tissues depends on two important parameters:​

    • acoustic impedence

    • attenuation

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Generation of Ultrasound Images on the Screen

  • reflection

    • occurs at the interface between 2 medium​

  • scatter

    • sound waves redirected chaotically in different directions because of large difference in acoustic impedence between two mediums (eg. air-tissue)

    • reason why gel is used between transducer and skin; to facilitate propagation of ultrasound waves into the body

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Loss of Ultrasound Waves

Acoustic Impedance

  • the resistance of sound wave to move through the tissue

  • progressive weakening of sound waves 

  • proportional to tissue density; fixed property of tissue (see table)

  • the greater the difference in acoustic impedence between two mediums leads to greater reflection of sound waves (eg. air in bowel)

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Attenuation (Transmitted)

  • the progressive loss of sound wave energy as it travels through tissue

  • decrease intensity and amplitude of sound wave as they travel 

  • depends on the distance of travel and wave frequency

  • due to:

    • absorption​

      • the most important cause of attenuation; energy from the US beam is absorbed by the tissue and converted to heat ​

      • most important determinant of depth of US wave

        • low frequency waves not absorbed as much​ (penetrate deeper)

        • high frequency waves are absorbed more (penetrate shallower)

    • deflection

    • divergence

PUTTING IT ALL TOGETHER: US images on the screen come about when sound waves reflect at the interface of two media with different acoustic impedance. The wave that returns back to the transducer is called an ECHO. This ECHO is interpreted by the machine by measuring the magnitude of the returning signal and the time it took to return and converts it into an image on the screen. 

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US transducer generates pulses of sound waves; Doppler ultrasound generate continuous sound waves.

Artifacts

- false images (or parts of images) that do not represent true anatomic structures

- provide insight into tissue makeup and can be diagnositic (eg. gallstone, pulmonary edema)

- due to erroneous ultrasound signaling that occurs in tissue due to violation of one or more of the following assumptions:

  • US echoes originate from a single uniform US beam

  • US waves always travel in a straight path and return after only one reflection

  • speed of sound is constant in all tissues

 

Imaging artifiacts are errors in imaging and occur because of certain assumptions:

  • sound travels in a straight line

  • echos originate only from objects in line with sound wave

  • speed of sound assumed to be constant (1540m/s in soft tissue)

  • reflections arise only from structures positioned in the beam's main axis

  • imaging plane is very thin

  • the strength of the reflection is related to the characteristic of the tissue 

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Attenuation Artifact

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  • acoustic shadowing

    • when the sound wave reaches an object with very high acoustic impedance (eg. gallstone), it will not be able to penetrate it
    • distal to the highly attenuating object is a decrease in the amplitude of the sound wave 
    • as a result, no information can be obtained from behind this object (within the shadowed area) unless multiple angled views used
    • occurs posteriorly to high attenuation objects (reflect, scatter, or absorb ultrasound waves)
    • used to diagnose high attenuation objects (eg. gallstones, simple cysts)

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  • acoustic enhancement

    • when sound wave passes through an area of lower attenuation (eg. fluid-filled) structures located beneath appear hyperechoic 

    • sound waves travel with little attenuation (eg. urinary bladder, gallbladder, large vessels)​

    • sound travels through the fluid-filled structure (has low attenuation) unimpeded and thus the refelcted echos have high-amplitude back to the transducer creating a bright image

    • reason why the bladder can be used to visualize structures behind it

 

  • dirty shadowing (gas scatter)

    • gas is the enemy of ultrasound and causes sound waves to scatter

    • common with aortic imaging; may improve by changing patient position or continuous pressure

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Propagation Artifact

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  • ​reverberation artifact

    • occurs as the sound wave is caught between two media and causes it to bounce back and forth (the two media are strong reflectors)

    • if the lines merge, this is the "comet-tail" sign that is used to rule-out pneumothorax (reverberation between the visceral and parietal pleura)

    • multiple reflections that occurs between the transducer and two or more highly reflective structures

    • occurs when there is a strong reflector causing ECHO to return to the transducer, which then causes a rebound of the sound wave back to the reflector (ongoing cascade occurs)

    • appear as equidistant arcs

    • seen with curved or phased array transducer

      • comet-tail artifact

        • reverberation that occurs between two closely spaced reflector​s

      • ring-down artifact

        • gas bubbles become stimulated and vibrate by the US wave​

        • creates an echogenic line(s) parallel and distal to the bubbles

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  • mirror-image artifact

    • when the sound wave encounters a curved object with very high acoustic impedance (eg. diaphragm) the sound waves get deflected. Because the US system assumes sound travels in a straight line, it will not be able to recognize the redirected sound beams and thus will place them deeper (because it will take longer time to reach the transducer)

    • located on both sides of a strong reflector​ (eg. diaphragm's pleural-air interface)

    • pleural-air & liver interface causing bouncing back and forth of ultrasound waves

    • occurs by the incorrect assumption that the echo returns back to the transducer after a single reflection

    • can be used to rule-out any fluid in the chest (mirror-image only appears if there is NO fluid in the chest)

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  • refraction artifact (redirected) (structure duplication)

    • when an oblique sound beam encounters the interface of 2 different media with different propagation speeds, it gets redirected or refracted

    • occurs when the sound wave travels into a medium of a different propagation speed. The assumption is that sound travels in a straight line. When the sound wave encounters another medium (eg. cyst) the sound wave changes direction thus giving you the false impression of where an object truely is. The artifact will be placed side by side with the true object. 

    • change in direction of sound waves at oblique angles as sound waves travel from one tissue to the next

    • when sound travels across one medium into another and changes speed​

    • a change in the direction of the wave due to a change in propagation speed

    • sound waves are redirected at the boundary

      • edge artifact

        • shadow created along the edge of the second medium (eg. bending​ pencil in a glass of water (Snell's law)

        • usually seen distal to smooth round cavities (eg. bladder, aorta, gallbladder)

        • narrow, hypoechoic shadow lines extending a significant distance distal to the lateral edges of the fluid filled cavity and parrallel to the US beam

      • misregistration of structure position

      • structure duplication​

    • edge artifact (type of refraction) is seen along the curved edges 

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  • side-lobe artifact

    •  strong reflector located outside of the main ultrasound beam may generate echoes that are detectable by the transducer. These echoes will be falsely displayed as having originated from within the main beam. This form of artifact is more likely to be recognized when the misplaced echoes overlap an expected anechoic structure

    • arise from a single piezoelectric element​

    • displays duplicate structures lateral to the actual structures (eg. mistaken for aortic dissection or biliary sludge within gallbladder)

    • mitigated by examining from different angles, adjusting the focal zone, or centering the transducer over the site of interest

 

  • grating-lobe artifact

    • arise from an array of piezoelectric elements​

    • displays duplicate structures lateral to the actual structures (eg. mistaken for aortic dissection or biliary sludge within gallbladder)

    • mitigated by examining from different angles, adjusting the focal zone, or centering the transducer over the site of interest

    • usually seen with phased array

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  • beam-width artifact

    • occurs distal to the focal zone where the ultrasound beam splays to a width greater that the proximal beam

    • as the beam sweeps from side to side, structures in the far field appear linear/stretched horizontally

    • arises from a progressive widening of the ultrasound beam distal to its focal zone​

    • results in a hyperechoic artifact being displayed along the peripheral margins of a normally anechoic structure (eg. bladder)

    • mitigated by adjusting the focal zone and centering the transducer over the structure of interest

    • fix it: adjusting the focal zone closer to the structure of interest in the far field

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  • slice-thickness artifact

    • again, distal to focal zone​

    • occurs when the beam dimension is greater than the reflector size

    • thinner planes have less chance of artifact occurring

    • eg. bladder with pseudo-sludge produced from signal from adjacent bowel gas (out-of plane) causing artifact within the bladder (in-plane); can lead to false diagnosis

    • fiz it: tissue harmonic imaging: the sound beam in this mode is narrower than gray-scale mode or adjusting the focal zone closer to the structure of interest in the far field

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  • speckle artifact

    • "grainy appearance" occuring from scattered sound arising from sound scatterers within the tissue (eg. common carotid artery)

    • reduced by decreasing the gain or using tissue harmonic imaging (THI) ​

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    • speed displacement (speed error)

      • causes echos to be displayed deeper or shallower depending on the tissue it encounters​

      • fat slows sound transmission and the returning signal will take longer to return to the transducer which causes the image to be displayed deeper than it actually is)

      • bone speeds up sound transmission causing the echo to appear shallower than it actually is

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Doppler Artifact

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  • Spectral Doppler Pulsed Wave Artifacts
    • ​signal aliasing artifact
      • ​caused by high velocity laminar flow (eg. Nyquist limit)
      • seen in stenotic or regurgitant lesions
      • flow will be depicted as simultaneously moving towards and away from the transducer at the same time
      • the peak of the curve will be missing (this is the point where the Nyquist limit is exceeded) and will reappear on the other side of the time axis
      • the operator can shift to a "cut and paste" baseline, increase the velocity scale or switch from pulse-wave to continous wave doppler mode
  • Color Doppler Artifact
    • signal aliasing
      • when high flow across a valve moves toward the transducer, the flow may switch from bright red-yellow to blue because the Nyquist limit has been exceeded in that region
    • shadowing
      • the absence of flow distal to a strong signal reflector​
    • ghosting
      • momentary color flashes that do not correspond to flow patterns
      • moving structures with strong reflectors (eg. prothetic heart valves) ​
    • background noise
      • speckled color pattern caused by excessive gain settings ​
    • suboptimal intercept angles
      • underextimation/absence of color signal​
    • electronic interference
      • linear/complex color patterns from surrounding electromagnetic medical devices​
    • variance (green color)
      • when flow rate and direction at a particular site vary significatly from the mean flow rate
      • results from turbulent flow or when the machine misinterprets aliasing as variance 

 

 

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    • edge shadowing ???

    • double image

    • slice thickness 

    • equipment generated 

  • ​occurs in regions where the frequency shift (casued by the high velocity) exceeds the Nyquist limit and a reversal flow pattern will be observed

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  • Correctly setting up the machine to record patient information and save images

  • Choosing appropriate transducer for each type of exam

  • Adjusting depth, frequency, gain, and focal zones to maximize image quality

  • Identifying the orientation of the image on the machine: anterior, posterior, medial, lateral, caudal, cranial sections

beam-width artifact

Tissue Harmonic Imagin (THI)

  • use in cardiac

Pulse wave vs Continuous wave Ultrasound signals

  • Pulse wave Spectral Doppler

    • several cycles of ultrasound signal separated by gaps of no signal

    • usually 2-3 cycles long

    • the following are present:

      • wavelength​

      • period 

      • cycle

      • frequency

      • propogation speed

      • pulse-repetition period

        • the time of onset from one pulse to the onset of the next​​

      • pulse-repetition frequency (PRF)

        • the number of pulses occuring in 1 second​

        • as PRF increases, the pulse repetition period decreases

      • pulse duration

        • the total time it takes for a pulse to occur​

        • the number of wave cycles multipled by the wave period

      • duty factor

        • the percentage of time ultrasound signals are being transmitted (only sent a fraction of the time)​

        • increases if pulse duration and/or PRF increases

  • Continuous wave Doppler

    • usualy 5 to 30 cycles long

    • the following are present:

      • wavelength​

      • period 

      • cycle

      • frequency

      • propogation speed

      • duty factor

        • the percentage of time ultrasound signals are being transmitted (sent 100% of the time)​

  • Frequency: the number of cycles over a period of time; measured in Hz (cycles/sec); 2-15MHz is the operating frequency of ultrasound in diagnostic

  • Period: from beginning to end of cycle or wave measured in time

  • Wavelength: esentially the period measured in distance

  • Frequency and wavelength are inversely proportional 

  • Long wavelength travel deeply into the body but will have low resolution because unable to discriminate tissues adjacent to each other

  • Short wavelength travel shallow into the body but gives higher resolution

Speed

  • US waves travel at different speeds through different tissues (1540 m/s in soft tissue)

    • 500m/s ​in air/lungs (much slower)

    • 3000m/s in bone (solids fastes)

    • this difference in speed affects the way the sound wave bend, reflect and behave (important in artifacts). 

Time = Distance

  • the amount of time it takes the sound / Echo wave to make it back determines where the image gets displayed on the screen

  • near field (top part of screen)

  • far field (bottom part of screen)

Echogenicity (brightness)

  • dtermined by the amplitude of the returning sound waves

  • high impedance structures (eg. gallstone) strongly refelcts high amplitude sound waves and makes they image appear bright

  • low impedance structures (eg. fluid filled) poorly reflects sound waves back to the transducer as a dark anechoic image 

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Hyperechoic (bright)

  • more echogenic than the surrounding tissue

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Hypoehoic

  • less echogenic than surrounding tissue

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Anechoic (dark)

  • completely devoid of echos

Imaging

  • B (2D) mode (grey-scale)

  • M mode

  • Doppler

    • ​illustrates the presence, direction, speed and character of blood flow

      • Color-flow doppler

        • uses multiple beams with multiple samle volumes along each path to obtain flow measurements​

      • Spectral doppler pulsed wave

      • Power-flow doppler

      • continuous wave??

Transducers

  • General mode

    • uses the middle of the transducers range​

  • Penetration mode

    • the low frequency setting within the transducers range​

    • the dots are spread more apart

  • Resolution mode

    • the high frequency setting within the transducers range​

    • the dots are closer together (more resolute)

High frequency = better resolution (poor penetration into the body)

  • great for superficial structures

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Low frequency = resolution not as good (penetration better)

  • FAST exam (eg. heart, aorta)

Footprints

  • convex-array (curvilinear) (curved-abdominal)

    • edges splay out (loss of lateral resolution)​

  • linear-array​

    • high frequency tranducers​

    • same resolution across the image because the scan lines are separated equally and no splaying of the edges

  • phased-array

Indicator

  • toward the patients anatomic right side

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Sagittal (longitudinal) View

  • parallel to the long axis of the body

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Transverse View

  • indicator toward the patients right side

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Coronal View

  • also longitudinal

  • indicator toward the patients head

Holding the probe

Doppler-Mode Imaging

  • doppler effect

  • doppler shift

  • doppler equation

  • intercept angle

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Color Doppler

  • autocorrelation

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Power-flow doppler

  • spectral doppler

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Gain

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Depth

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Resolution

  • spatial resolution

    • lateral resolution​

    • axial resolution

  • temporal resolution

  • focal zone

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