Ultrasound physics Flashcards
Frequency of Ultrasound Waves
> 20 Hz
Period (US)
T = 1/f
What aspect of a sound wave stays constant
frequency
Speed of sound (in different materials)
Sq rt (elastic property/inertial property) or Sq rt (bulk modulus/density) ~ Sq rt (tissue stiffness/density).
Think of running on hard ground (stiffer-faster) compared to soft ground (slower). Think of air runner is running through, air -fast, water - slow.
Power (US)
~ Pressure (amplitude) ^2
Intensity (US)
I = Power/Area
Attenuation (US)
A = f x u (dB/cm)/ MHz, u = attenuation coefficient, ~0.5 in soft tissue.
u increases with tissue density
Half value layer. Thickness of tissue in which US beam is attenuated by 50%.
Speed of sound in soft tissue
1540 m/s
Range (US)
R = t * c/2, distance ultrasound waves are transmitted back to receiver.
Set the range to image specific depth of tissue. Probe will “listen” for transmitted waves at specific depth.
Pulse echo duration (US)
PD = # of cycles / f
Spatial pulse length
SPL = # of cycles x wavelength
Pulse Repetition Period
PRP = 1 / PRF, time of pulse echo duration + listening time
Duty Factor (US)
PD / PRP, percentage of time spent producing ultrasound waves (as opposed to listening)
Acoustic impedance
Z = p (density) * c, c = sq rt (B/p). Increases with denser tissues.
Think of spring analogy at tissue interface. Stiffer spring reflects sound: /\/\/\/\IIIII
Reflection (US)
R = [(Z2 - Z1)/(Z2 + Z1)] ^ 2
Types of reflection (US)
Specular (at an angle on smooth surface), nonspecular (at angles, not on smooth surface, think of reflection from broken glass chips)
Refraction
sin (at) / sin (ai) = c2 / c1, change in speed changes angle of sound wave
Relationship between piezoelectric material and frequency generated
Thicker PZT -> longer wavelengths (shorter frequency).
Think of guitar string. Longer string -> longer wavelength produced
f = c / (2 * PZT)
Matching layer
Acoustic impedance value between PZT and soft tissue to decrease reflection of sound waves.
ML = 1/4 * lambda (L)
Gel gets rid of air between probe and skin and smooths out skin
Function of dampening block (US)
Stops PZT from making sound waves in order to be able to receive reflected sound waves. Also makes the sound waves propagate away from probe.
Think of putty or wet rag on top of a cymbal.
Quality factor (US)
Q = fo (resonant frequency) / fr (range)
Measure of purity of sound waves.
A mode (US)
Amplitude mode. Used to determine where the tissue interfaces are. Used rarely.
B mode (US)
Brightness mode. Scans line by line to form an image.
M mode (US)
Motion mode. Takes a line (A mode) and plots the motion over time.
Time Game Compensation
As the sound waves increase in depth they are more attenuated, so the reflected signal is a both a measure of the acoustic impedance and the attenuation. To correct for attenuation, deeper reflected sound waves are given more amplitude.
Transducer types (US)
Linear array - fires PZT crystals sequentially down the probe.
Curvilinear - good for deeper structures. Used in abdominal imaging.
Phase array - fires PZT crystals all at the same time. Beam can be steered. Good for small windows - transvaginal, between ribs.
Near field (US) in soft tissue
N = (d^2 * f)/(4*1.54), where d = diameter of PZT array.
This is the focal distance in tissue.
Longer transducer -> longer focal zone length.
Far field (US)
~ 1/f
Higher frequencies, more information at increased depths. Measure of divergence.
Axial resolution (US)
= 1/2 SPL
Improved with higher frequencies. Objects half the spatial pulse length cannot be accurately resolved. Same at ALL depths.
Lateral resolution (US)
LR = beam width
Best lateral resolution is at the focal point where beam width is most narrow.
Higher frequency beam (narrower beam) = higher lateral resolution.
Improved with higher scan density
Elevational resolution (US)
ER = beam height
Best at focal point. Improved with phase array in the Z plane.
Temporal resolution (US)
TR = frame rate = Nline * Tline. Tline = 2D / c
> 24/s our eyes see a continuous video.
Can be improved by decreasing depth, decreasing field of view, decreasing line density.
Increasing the number of focal points on each line decreases temporal resolution linearly.
Doppler shift
~ f * v * cos (o), where v is the velocity of the object and o is the Doppler angle
optimal Doppler angle is 30-60 degrees*
Continuous wave doppler versus Pulsed Wave doppler
Continuous wave doppler cannot provide depth or angle information, hence only high pitched noise to indicate flow.
Pulsed wave doppler can measure velocity.
Different types of Pulsed wave doppler
Color - measures a region’s velocity
Spectral - measures a focal area’s velocity
Power - no direction. good for picking up low flow. No aliasing, no dependence on Doppler angle.
Resistive Index
RI = (PSV - EDV)/PSV
< 1 (EDV positive, always forward flow) organs that need constant perfusion - brain, kidney
= 1
> 1 high resistance. Organs that don’t need constant flow - skeletal muscles
Advantage of harmonic frequencies
Pure reflection at tissue boundary. No scatter. Better S/N and contrast. Better lateral resolution. Lower side lobe and grating lobe artifacts.
Lower axial resolution*
Low Damping (US)
Narrow bandwidth, long spatial pulse length. Used for doppler
High Damping (US)
Broad bandwidth. High axial spatial resolution.
Stand off pad (US)
For superficial findings, using a spacer between the US probe and the finding will place finding in the focal zone.
Aliasing
Super high velocities displayed as low (below baseline).
Reduced by increasing Doppler angle, increasing the PRF, increasing the scale, or using a lower frequency transducer.
Cavitation (US)
Most likely to happen with CE US, low frequency and high pressure.
Guidelines for US during pregnancy
Pulsed doppler should NOT be used.
M-mode is recommended instead of spectral Doppler for HR.
Keep TI under 1.0
Compound Imaging (US)
Using electronics to steer wave. Sharpens edges. Loss of posterior shadowing.
