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
-
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)
-
Spatial Average Intensity (SA) is the
acoustic power within the beam, divided by the beam area
-
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
-
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)
-
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
-
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
