This section provides some useful definitions or information in
regard to important terms not easily found on the Internet:
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Absorption is the transfer of energy from the ultrasound
beam to the tissue. It is proportional to frequency
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Apodization is a method for reducing side lobes in some
arrays. It gradually decreases the vibration of the transducer surface with
distance from its center. It is usually accomplished by using more power to
excite the innermost elements.
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Axial resolution is the minimum separation between two
interfaces located in a direction parallel to the beam so that they can be
imaged as two different interfaces
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Decibel is a way to express the ratio of two sound
intensities: dB=10log10I1/I2 being I1 the reference. For instance: +3
dB = I multiplied by 2 and -3 db = I divided by 2
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Diffraction is the change in the directions and intensities
of a group of waves after passing by an obstacle or through an aperture
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Duty factor is the lapse of time the transducer is actively
transmitting sound
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Echo ranging is the relationship between transit time and
reflector depth expressed as t = 2d/c
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Grating lobes as side lobes are secondary ultrasound beams
projecting off-axis at predictable angles to the main beam. Side lobes are
too small to produce important artifacts.
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Half Value Layer (HVL) is the distance the sound beam
penetrates into a tissue when its intensity has been reduced to one half of
its initial value
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Huygens' principle states that an expanding sphere of waves
behaves as if each point on the wave front were a new source of radiation of
the same frequency and phase
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Impedance is the product of the density of a material and
the speed of sound in that material
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Pulse average intensity I(PA) is the average intensity
during the pulse
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Lateral resolution is the minimum separation of two
interfaces aligned along a direction perpendicular to the ultrasound beam.
It depends on the beam width
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Partial Volume Artifact (slice thickness or volume averaging
artifact), that occurs when the slice thickness is wider than the scanned
structure
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Q-value means the degree that a transducer is finely tuned
to specific narrow frequency range. For instance: Low Q means wide bandwitdh
and High Q means narrow bandwidth
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Range resolution is the ability to determine the depth of
reflectors
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Rayleigh scatterers are objects whose dimensions are much
less than the ultrasound wavelength. Scattering increases with frequency
raised to the 4th power and provides much of the diagnostic information from
ultrasound
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Refraction is the bending of a wave beam when it crosses at
an oblique angle the interface of two materials, through which the waves
propagate at different velocities
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Snell's law governs the direction of the transmitted beam
when refraction occurs:
sin qt = (c2/c1) x sin qi (qt and qi are transmit and incident angles
respectively)
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Spatial Average Intensity (SA) is the acoustic power within
the beam, divided by the beam area
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Spatial Peak Intensity (SP) is the point in the sound field
with maximum intensity
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Side lobes are energy in the sound beam falling outside the
main beam
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Spatial resolution means how closely two reflectors -or
scattering regions, can be to one another while they can be identified as
different reflectors
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Subdicing is a technique used to overcome grating lobes:
each major transducer element is devided into smaller parts, each one being
a half wave lenght
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Temporal (instantaneous) Peak Intensity I(TP) or I(IP) is
the maximum intensity during the pulse
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Time Average Intensity I(TA): average intensity calculated
over the time between pulses:
ITA= I(PA) x Duty factor
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Wavelength is l=c/f (c = propagation speed; f =
frequency)
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Aliasing is an artifact that lowers the frequency components
when the PRF is less than 2 times the highest frequency of a Doppler signal
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Beat frequency, for CW Doppler, is the Doppler shift
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Doppler shift is the change in the perceived frequency
relative to the transmitted frequency. Doppler shift frequency: fD =
fr - f0 = 2f0v/c
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Doppler shift frequency with incident angle: fD =
2f0v/c cos q
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Ensemble length -packet size, shots per line- is the number
of pulses per scan line. In color Doppler, each line of sight most be pulsed
several times
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FFT. Fast Fourier Transform analyzer is a common device that
performs spectral analysis in ultrasound instruments. In this case, it
displays different quadrature Doppler frequencies, or reflector velocities
when a sample volume cursor is used (Doppler frequency is proportional to
reflector velocity) along time
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High pass filter is the wall filter
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Nyquist Frequency is the maximum frequency that can be sampled without
aliasing. NF = PRF/2 (PRF stands for Pulse Repetition Frequency)
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Quadrature detection is a signal processing method for
directional Doppler in which the signal reference frequency for two channels
differ in phase by 1/4 period. The output Doppler signal phase for both
channels also depends on the Doppler shift, whether positive or negative
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Spectral analysis is the quantitative analysis to display
the distribution of frequencies
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Variance is the variation of Doppler frequencies within each
pixel during a pulse packet, efective to detect turbulence with color
Doppler