Page 284 - Electrical Engineering Dictionary

t ∈[t 1 ,t 2 ], filtered backprojection an algorithm for
image reconstruction from projections. In
˙ x(t) ∈ F(t, x),
the filtering part of the algorithm, the pro-
jections are measured, their Fourier trans-
where F(t, x) is an appropriately constructed
forms computed, and the transforms are
convex set. See also differential inclusion.
multiplied (filtered) by a weighting func-
fill-in when solving a set of sparse lin- tion. In the backprojection part, the in-
ear equations using Gaussian elimination, it verse Fourier transforms of the weighted
is possible for a zero location to become projections are computed and summed to
nonzero. This new nonzero istermedafill-in. yield the reconstructed image. Filtered back-
projection is the reconstruction algorithm
FILO See first-in-last-out. currently used by almost all commercial
computed tomography scanners. See also
filter (1) a network, usually composed Fourier transform, image reconstruction,
of inductors and capacitors (for lumped cir- projection, Radon transform, reconstruction,
cuit), or transmission lines of varying length tomography.
and characteristic impedance (for distributed
circuit), that passes AC signals over a cer- filtering (1) an estimation procedure in
tain frequency range while blocking signals which the present value of the state vector
at other frequencies. A bandpass filter passes (see the definition) is estimated based on the
signals over a specified range (flow to fhi), data available up to the present time.
and rejects frequencies outside this range. (2) the process of eliminating object, sig-
For example, for a DBS receiver that is to re- nal or image components which do not match
ceive satellite transmitted microwave signals up to some pre-specified criterion, as in the
in a frequency range of 11 GHz to 12 GHz, case of removing specific types of noise from
a band-pass filter (BPF) would allow sig- signals. More generally, the application of an
nals in this frequency range to pass through operator (typically a linear convolution) to a
with minimum signal loss, while blocking all signal.
other frequencies. A low-pass filter (LPF)
would allow signals to pass with minimum finefficiency athermalcharacteristicofan
signal loss as long as their frequency was extended surface that relates the heat transfer
less than a certain “cutoff frequency” above ability of the additional area to that of the
which significant signal blocking occurs. base area.
(2) an operator that transforms image in-
tensity Ix of pixel x into a different intensity final test electrical test performed after
ˆ
Ix, depending on the values of a set of (usu- assembly to separate “good” devices from
ally neighboring) pixels (which may or may “bad.”
not include x). Filtering is performed to en-
hance significant features of an image or to finesse measure of the quality of a Fabry-
remove nonsignificant ones or noise. Perot interferometer; free spectral range di-
vided by linewidth (full width at half maxi-
filter bank a set of filters consisting of a mum).
bank of analysis filters and a bank of syn-
thesis filters. The analysis filters decom- finger stick an insulated stick like a hot-
pose input signal spectra into a number of stick used to actuate a disconnect-switch atop
directly adjacent frequency bands for further a pole.
processing, and the synthesis filters recom-
bine the signal spectra from different fre- finite difference method a numerical
quency bands. technique for solving a differential equation

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2000 by CRC Press LLC