Physics Flashcards
relationship between Pulse Duration and accuracy
SHORTER pulse duration INCREASES accuracy. This is because pulse duration = #cycles / frequency. A corollary of this is that high frequency gives better accuracy because HIGH FREQUENCY = SHORT PULSE DURATION.
Relationship between DEPTH and Pulse Repetition Frequency?
Depth DECREASES PRF.
Relationship between DEPTH and Pulse Repetition Period?
Depth INCREASES PRP, but only the receive time portion. PRP (usec) = depth (cm) / 13 (usec/cm)
Relationship between DEPTH and Duty Factor?
Depth INCREASES pulse repetition period, therefore DECREASES the Duty factor.
Duty Factor formula?
pulse duration / pulse repetition period x 100.
typical duty factor value?
0.001 or 0.1%
What pulse wave characteristics are adjustable?
Pulse repetition frequency, pulse repetition period, duty factor.
Temporal intensities from smallest to largest?
Ita, Ipa, Imax, Itp
Rank of spatial intensities from largest to smallest?
SPTP (largest)> Im > SPPA > SPTA > SATA (smallest)
What is attenuation measured in?
Decibels. Measure “relative” changes in intensity.
What generally governs attenuation?
path length and sound frequency.
High to low: Gas > solid > liquid
“Attenuation” of +3 dB results in doing what to the intensity signal?
Doubling the intensity signal.
+10dB = 10x intensity signal.
What is the relationship between frequency and attenuation?
Higher frequency attenuates more; higher frequency sound loses more energy.
What is the relationship between distance traveled by sound and attenuation?
Longer distance traveled by a sound wave results in greater attenuation.
What is the relationship between speed and medium?
DENSITY INCREASES speed; STIFFNESS DECREASES speed.
Specular reflection
Regular surface; back to transducer; organized; unidirectional
diffuse reflection
Irregular surface; back to transducer; disorganized
scattering
Small tissue interface (smaller than wavelength)
multiple directions
disorganized
Rayleigh scattering
Multiple directions
organized
Happens when acoustic wave is redirected in many different directions as a result of striking a small particle. Blood is a Rayleigh scatterer.
Relationship between scattering and frequency?
Directly related.
Increased frequency = increased scatter. Includes Rayleigh scatter.
Relationship between Rayleigh scattering and frequency?
RS is directly related to f^4.
IRC formula?
IRC = Intensity Reflection Coefficient.
IRC (%) = 100 x ((Δ impedance) / (Σ impedance)) ^2.
% of the initial intensity that is REFLECTED.
ITC (Intensity transmission coefficient) formula?
ITC + IRC = 1.Therefore, ITC = 1 - IRC.
Requirements for IRC formula?
NORMAL INCIDENCE
Snell’s Law
V1 x sin (transmission angle ) = V2 x sin (incident angle)
Other names for axial resolution
Longitudinal, radial, depth resolution
Attenuation coefficient formula?
1/2 x f (in MHz)
Half value layer thickness
Like half life. Distance for which sound has to travel to lose HALF of original INTENSITY.
units of Impedance
RAYLS. Kg / (m2 x s) . Impedance of Air = 0.0004 Rayls.
Thirteen microsecond rule
1 cm depth corresponds to 13 microsec of transit time (time of flight)
Axial resolution formula
Ax Resolution = Spatial Pulse Length / 2
= (#cycles x wavelength) / 2
= 0.77 x #cycles / f
relationship between speed of PZT and frequency?
Slower PZT speed = LOWER frequency
relationship between thickness of PZT and frequency?
Thick PZT = LOWER frequency
Mathematical relationship between PZT, frequency, and speed in PZT?
Frequency (MHz) = 1/2 x (speed in PZT / thickness (mm))
Doppler equation?
Frequency shift = 2 x ft x vblood cos Θ / csound
What does “scale” on the ultrasound screen correspond to?
Pulse repetition frequency. Also can be thought of as “sampling rate”,
Focal depth formula?
(diam^2)X (f ) / (61.7)
= 1/4 x (diam^2 / wavelength)
what does aliasing look like?
wraparound of the velocity signal onto the other side of the baseline
relationship between gate depth and aliasing
Deeper depth = lower PRF = lower Nyquist limit = INCREASED aliasing
relationship between transducer frequency and aliasing
higher frequency transducer INCREASES aliasing; higher frequencies cause higher Doppler shifts that are more likely to exceed the Nyquist limit.
relationship between blood velocity and aliasing
Higher blood velocity INCREASES aliasing.
Does focusing contribute to attenuation?
No.
absorption is defined as?
transfer of acoustic energy into heat (in tissues)
What is the Nyquist limit?
the highest Doppler frequency that can be measured w/o aliasing. Nyquist limit = PRF/2.
Is axial resolution dependent on the depth of probe?
No. Axial resolution is dependent on the SPL, which is then dependent on the wavelength and cycles in the pulse.
Characteristics of imaging transducers?
Short duration pulses; backing material; reduced sensitivity; lower Q factor; better axial resolution;
Quality factor equation?
QF = main frequency / bandwidth
relationship between duration and bandwidth?
High duration = narrow bandwidth, low duration = wide bandwidth
relationship between Q factor and pulse length?
Short pulse = LOWER QF. Long pulse = HIGH QF.
Temporal resolution is proportional to?
FRAME RATE (Hz)
How does depth influence frame rate?
Low depth = INCREASES frame rate. High depth = DECREASES frame rate
How does sector size influence frame rate?
Increased sector size = increased Tframe = DECREASED frame rate. Therefore, DECREASED temporal resolution
How does # pulses per line (foci) influence frame rate?
Increased #pulse per line = increased T frame = DECREASED frame rate = dec. temp resolution.
Incidentally this INCREASES lateral resolution.
Relationship between transducer DIAMETER and Far field DIVERGENCE?
Large DIAMETER = LOW Divergence.
Small DIAMETER = HIGH Divergence
Relationship between transducer FREQUENCY and Far field DIVERGENCE?
High frequency = LOW Divergence.
Relationship between LATERAL RESOLUTION and Far field DIVERGENCE?
Less Divergence = Better Lateral Resolution.
Best way to get good LATERAL RESOLUTION in the FAR FIELD?
Basically, whichever way translates to a LOW divergence. I.e. high diameter transducer and high frequency transducer
other words for lateral resolution
LATA = lateral, anguilar, transverse, azimuthal
where is the lateral resolution the best?
at the focus. in general, lateral resolution thought to be “good” in end of near zone.
main determinant of lateral resolution is ?
beam diameter. Narrow beam = better resolution.
Effect of focus on beam diameter?
beam diameter DECREASES (smaller) in the near field + focal zone
Effect of focus on focal depth?
focal depth is DECREASES.
Effect of focus on divergence?
Divergence INCREASES.
Effect of focus on focal zone?
Focal zone DECREASES.
Effect of #pulse per line (pulse density) on spatial resolution?
Improves spatial resolution
Amplitude units?
Pascals, g/ cubic cm, particle motion in cm
Intensity formula?
Power/area
Relationship between power and amplitude?
Power proportional to amplitude ^ 2
1 MHz corresponds to roughly what wavelength?
1.5 mm
Impedance formula?
density of medium x speed of sound in medium.
What determines speed of sound in a medium?
Density (decreases speed)
Stiffness (increases speed)
(High to low): solid > liquid > gas.
Tradeoff between frequency, attenuation, and accuracy?
Higher frequency = shorter pulse duration and narrowest beam. Therefore, they have the highest accuracy. But higher frequency also = higher attenuation. .
Lateral resolution formula?
Lateral resolution = (minimum) beam diameter.
higher frequency transducers produce _______ beams and _____________ lateral resolution?
Higher frequency = NARROW pulse = IMPROVED Lateral resolution.
Spatial pulse length formula?
SPL = #cycles x wavelength –> inversely proportional to frequency.
Relationship between frequency and axial resolution?
Higher frequency = shorter spatial pulse length (SPL) = better axial resolution.
what is the diameter of the beam at the focus?
Half the diameter at the outset of the transducer.
How does line density influence temporal resolution?
Increased line density means higher Tframe, which translates to DECREASED temporal resolution. This also translates to better spatial resolution.
What is the effect of PRP or PRF on temporal resolution?
High PRP = LOWER temporal resolution. Low PRF = LOWER temporal resolution.
what is the unit of compensation?
dB.
what does the alteration of “grayscale mapping” correspond to?
Receiver compression.
ALARA principle
As Low As Reasonably Achievable. When modifications of output power or receiver gain are necessary for improving Dx quality, the first + best choice will minimize U/S exposure.
