Ultrasound Physics

Ultrasound Basics

  • Need a wave + a medium for the wave to travel through

  • The wave will have areas of compression & rarefaction

    • Compression - sound particles are closer together and dense = represented by peaks in wave

    • Rarefaction - sound particles are looser/less particles and not so close together = represented by troughs in wave

  • Speed of the wave is calculated by:

    • c = f(lambda)

      • c = speed of wave

      • f = frequency

      • lambda = wavelength

    • The speed of the sound wave will significantly be affected by the medium through which it is traveling

Note some key differences between electromagnetic radiation and sound waves

  • Electromagnetic radiation (i.e. XR) - the speed of the wave is constant regardless of the type of radiation because XR, UV, gamma rays etc all travel at the speed of light, their frequenct however will vary

    • Therefore we can classify EMR based on wavelength

  • Sound waves - here we will set the frequency (and basically keep it constant) and its speed will then vary based on the medium it has to travel through

    • Therefore we will classify sound waves based on frequency (frequency is what we control basically)

Misc Facts

  • High frequency waves

    • Higher resolution

    • Lower penetration

US Physics Lecture

  • US imaging uses compression (aka longitudinal) waves

    • Note shear waves are used in US elastography, that is really the only exception

  • 1.5 mHz frequency has a 1 mm wavelength in soft tissue

  • Higher frequency = higher resolution

  • Lower frequency = high penetration = can see deeper shit

  • Wavelength

    • Distance between peak to peak or low to low

    • Shorter wavelengths

    • Longer wavelengths

    • 1.5 mHz wave

  • Sound velocity

    • Determined by the material only, wavelength and frequency do not determine velocity

    • Need to know this equation = velocity = frequency X wavelength

      • Velocity is fixed based on the tissue it is traveling through and the frequency and wavelength then have to make it work so that when they are multiplied it equals the speed of sound for that medium

    • Stiffer materials = faster

    • Fat = 1400 m/s

    • Soft tissie = 1500 m/s

    • Bone = 4000 m/s

  • Intensity (aka power)

    • Measured in decibels

    • The intensity received by probe relative to the intensity sent out by the probe

    • This is a relative measurement, so you need to have something you are comparing it too

    • This is the amplitude of the wave, if you increase power you increase the amplitude (how tall is the peak basically)

    • Higher amplitude is more uniform and less shit in between amplitudes

    • If you increase your power by 3 dB you are doubling the power

    • if you decrease by 3 dB you half the power

    • If you increase by 10 dB you 10X the power (and same if you decrease you -10X)

    • If you increase by 20 dB you 100x the power (and same if you decrease you -100X)

    • Decreasing is called attenuation

    • Increasing is called amplification

    • If you increase the transducer power you will increase the pressure

    • If you de3crease the power you decrease the pressure

  • (specular) Reflection

    • Need a smooth surface and the surface has to be bigger than the wavelength

    • Edge of liver is smooth and looks good and linear

  • Diffuse reflection = scattering = little different from specular except when the wave hits the target it goes in all different directions

    • Will happen with rough surface and if not bigger than wavelength

    • Inside of liver looks grainy because there’s a bunch of small structures in there

  • Acoustic Impedance

    • property of the material, basically how hard is it for the wave to get through the material, bone is highest

    • Measured in Rayl

    • Impedance = density X speed of sound in that material

    • When wave travels between two different materials it will have a change in appearance on imaging and the change is determined by how different the impedance is between the two different materials

    • This the reason why if you hold the probe above the body you don’t get an image because 99% of the waves are reflected when transitioning from air to soft tissue

    • Note that it is not only losing power going through the tissue transition but it also has to come back to the transducer so basically twice it has to do this

  • Refraction

    • Also occurs when wave goes through two different tissues and is caused by the different velocity in the two tissues

    • Only the wavelength will change (frequency and velocity stays the same) - why - because that’s just how it works

    • When the wave hits the need substance it will cause bending of the wave

    • This causes two artifacts

      • Refraction artifact

        • Beam goes straight hist target and comes back

        • Another beam from same transducer hits muscle first, the wave bends, then hits target and comes back

        • Double of a single structure (double aorta)

      • Vessel wall artifact/ Edge artifact - just another type of refraction artifact

        • Darks edges than go down from the edges of the vessel wall

        • Looks like dark straight lines

  • Attenuation

    • Basically loss of strength due to heat

    • Lose 0.5 dB for every cm and MHz = 0.5 dB/cm/MHz (each way)

      • Multiply by transducer strangth —> use 2 transducer then you lose 1 per cm or some shit

    • For abdomen use a 2-6 MHz probe )20 cm depth)

    • For breast use 10-15 MHz probe (5 cm depth)

  • Echogenicity

US lecture

  • B mode

    • The US mode you typically think of when using US

  • M Mode

    • Basically looking only at one line in the picture and how it changes over time

    • Y axis is depth, X axis is time

  • Speckle

    • Property of the image

    • Similar but different from noise because noise is random and speckle is not random

    • When you send out the US beam and they come back to the transducer they will all interact with each other, idk something like this

    • Caused by interference pattern of reflections from tissue

    • This is essentially non-random noise

  • Linear Array (array = probe)

    • Good for superficial structures, therefore typically uses higher frequencies

    • Has a rectangular FOV

    • Used in carotid doppler (superficial structure)

  • Curved (convex) (curvilinear) array

    • Used for deep structures, therefore uses lower frequencies

    • Probe is curved so the more superficial shit will be distorted by the curve

  • Phased arrays

    • All the crystals fire but are timed differently

    • This lets us steer or focus the beam to see our target better

    • Small footprint, small origin where the beams come out

    • Uses in cardiac and neonatal brain imaging

  • Spatial compounding

    • This is steering the beam at different angles and then generates a single image from all this

    • Reduces speckle and shadowing (need to know if you were looking for shadowing to diagnose something)

    • Reduces frame rate, and therefore worse temporal resolution

    • Good for superficial and small structures

  • Panoramic/Extended FOV imaging

    • Large FOV with software stitching

  • Harmonic Imaging

    • When beam is sent out the wave will not maintain the same shape throughout its course, it will gradually change shape

    • When the wave shape occurs the frequency also changes

    • Will get more power in harmonics

    • 2nd harmonic is 2x original Hz, 3rd is 3x and so on

    • So harmonic imaging is sending out a 1 mgHz beam but the probe will only record the beams that come back at the multiplier (2x)

    • Better lateral resolution

    • Less reverberation and lobes

    • Less noise

    • Worse axial and temporal resolution

  • Contrast agents

    • Larger than iodine and gadolidium so that they do not extravasate

    • Why are they bright —> create their own pressure so when we hit them with the beam they compress and they make their own pressure waves that get sent back to the probe

    • Need to use low mechanical index (MI < 0.5), if too high will pop the bubbles,

    • Flashing is when you pop the bubbles on purpose

    • Too much contrast - too much reflection , cannot see anything deep to the contrast agent

    • Higher frequencies = better axial resolution

    • Lower concentrations of the contrast agent give a more uniform signal

    • Limited evaluation of structures deeper than 10 cm

Ultrasound (lecture 12)

  • Doppler

    • Have a wave sent out and wave that comes back when it is reflected

    • If the thing it hits when it is reflected is moving it will change the frequency

    • If moving toward the source = higher frequency, and therefore decreased wavelength

    • If moving away = lower frequency

    • Perpendicular motion = no frequency change

    • You can measure the change in frequency = doppler shift

      • If coming head on = bigger shift

      • If moving at angle = less shift, until perpendicular and then no shift

    • Doppler angle of insonation

      • Typically 30-60 degrees, needs to be kept under 60 degrees though

        • Why under 60, idk some math formula and if over 60 the error gets much higher quickly

      • This is the angle you make with the moving object

  • Bernoulli principle

    • Narrow the lumen = faster speed and lower pressure

    • Wider lumen = lower speed and higher pressure

  • Power doppler

    • Does not depend on angle

    • Sensitive for slow flow

    • Uses absolute values so basically says if there is flow or not but not what direction

  • d

  • d

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