Ultrasound Flashcards
What is are advantages of Power Doppler over Conventional Doppler Imaging?
1) More sensitive to low flow
2) Less dependent on Doppler angle
What is a disadvantage of Power Doppler?
No flow direction information
The display on clinical ultrasound units shows values of both thermal index (TI) and mechanical index (MI). What is the lowest value of TI at which adjustments to the examination should be considered when that value is exceeded?
** Check answer Raphex 2020
The speed of sound in ultrasound imaging is affected by the tissue property of ______.
Impedance
Improved axial resolution in ultrasound image can be best accomplished with _______.
Shorter pulse length (reducing wavelength or number of cycles)
What are common ultrasound frequencies for imaging thick body parts and thin body parts?
Thin: 7-15 MHz
Thick: 2-5 MHz
Higher frequencies attenuate faster, meaning less capable of imaging greater depths
As a rule of thumb, what is the approximate decibal attenuation of an ultrasound beam?
0.5 dB MHz^-1 cm^-1
What causes shadowing artifacts in ultrasound?
Ultrasound assumes a uniform attenuation through material. When the sound wave passes through a highly attenuating object, the reduced signal intensity on the echo shows as a shadow.
How do you calculate the length of the near field?
= d^2 / (4*lambda)
lambda = wavelength
d = transducer length
What causes enhancement artifacts in ultrasound?
Ultrasound assumes a uniform attenuation through material. When the sound wave passes through a weakly attenuating object, the high signal intensity at greater depths on the echo shows as an enhancement.
How do you calculate the divergence angle of the far field of an ultrasound beam>
sin(theta) = 1.22 * (lambda / d)
lambda = wavelength
d = transducer length
How to determine the axial spatial resolution?
= 1/ (2* SPL)
How do you calculate the doppler frequency?
f_d = 2 * f_0 * (v/c) * cos(theta)
f_d = doppler frequency
f_0 = incident frequency
v = speed of sound
c = speed of light
theta = doppler angler
What is the Q factor of an ultrasound beam and how to calculate it?
Q-factor describes the ratio of the center frequency to the bandwidth of the beam
Q = f_0 / bandwidth
What is an application for high Q ultrasound beams?
The long spatial pulse length and narrow frequency bandwidth is good for Doppler imaging which is sensitive to frequency changes
What is an application for low Q ultrasound beams?
The heavy damping to generate the low Q makes for short pulse lengths, allowing for high axial spatial resolution
What is the matching layer? What is the thickness of the matching layer?
The matching layer is used to minimize reflections between the transducer and skin, and is constructed of a material with intermediate impedance between them.
The thickness is 1/4 of the center operating frequency of the transducer.
How is the resonant frequency of a piezoelectric material determined?
The thickness of the crystal is equal to 1/2 the resonant wavelength
What is the thermal index (TI)?
The ratio of the acoustical power prodcued by the transducer required to raise the tissue temperature in the beam area by 1 degree C.
What is the mechanical index (MI)?
A value that estimates the likelihood of cavitation. It increases linearly with output power and decreases by the frequency squared (prop. to f^-2)
What is the threshold intensity for ultrasound biological effects?
100 mW/cm^2
