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Aperture of CT is usually....
70cm
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3 major systems of CT
imaging system, computer system & image display, recording, storage & communication system
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Imaging system consists of
Gantry-xray tube, generators, collimators, filter, detectors & detector electronics (DAS)--all located in scan room
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Computer system performs what on images?
image manipulation & image processing techniques
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Computer systems include
input/output devices, interface devices, back projection processors, storage devices & communication hardware
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Gantry houses?
xray tube, detectors, slip rings, high voltage generator, collimators, and data acquisition system
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Anode is how big usually?
5 inches in diameter
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Tilting range of gantry is?
=/- 12 to 30 degrees at 0.5 degree increments
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2 most important characteristics of the computer system...
- large storage capacity
- & fast & efficient processing of various kinds of data
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Array Processor is what?
it is capable of the high-speed calculations needed in CT. It may have multiple dedicated processors & image storage
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Operating systems are?
programs that control the hardware components & overall computer operations.
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Ct software (part of computer system) consists of?
- reconstruction
- pre-processing
- image post processing
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Reconstruction builds image from
raw data collected by detectors
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Pre-Processing does what
performs corrections on data before it is reconstructed (example is beam hardening)
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image post processing does what?
operates on reconstructed data/images for viewing (MPR, 3D)
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low ct # (-1000) is white
true or false
false...low ct numbers are black
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High ct numbers are white
true or false
true--bone (+1000)
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Image post processing uses various techniques that modify what
the reconstructed images displayed for viewing through image processing software & algoritms
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Info content in processed images is always equal to the original image
true or false
false alway less than or equal...never more info
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techniques of image post processing consist of
linear & nonlinear
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Linear is what
it is part of image post processing--image smoothing & image enhancement
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Nonlinear is what
- part of image post processing
- concerned w/gray level mapping, contrast enhancement, contrast stretching, histogram modification, histogram stretching & windowing
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Image post processing uses what image data to produce new 2D & 3D images
axial image data set stored in the computer
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What is the most common image processing technique?
windowing
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Windowing affects what?
ct number (brightness & contrast)
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Range of CT numbers in the image is referred to as
window width
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window width is the _______ number of shades of gray that can be displayed.
max
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a large ww indicates ____ contrast
low
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a short gray scale indicates ___ ww
small
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fewer shades of gray indicate a ______ ww
short
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Center or midpoint of range of ct numbers is:
window level
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ct number for air
-1000 (black)
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Bone ct number is
+1000 (white)
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-
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lung ww & wl
- ww=-1500 to -2000
- wl= -700 to -400
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-
-
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a wide window width should be used to
encompass tissues of greatly differing attenuation within the image
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narrow window width should be used to display
soft tissues within structures that contain different tissues of similar densities
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window widths should be centered near what
the average of attenuation of the tissues of interest
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a large ww indicates __________ contrast
less (decrease)
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a narrow ww indicates_______contrast
more (increase)
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as ww increases, contrast _____
decreases (more shades of gray)
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as ww _________, contrast increases and has less shades of gray.
decreases
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MPR
- multiplanar reconstruction/reformation (aka image reformation)
- -computer program that can create coronal, sagittal & paraxial images from the original transverse axial images
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sagittal
passes through the body from anterior to posterior & divides the body into right and left
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coronal
passes through the body from right to left & divides the body into anterior and posterior
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paraxial
cuts through the sagittal and coronal planes in longitudinal direction
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disadvantages of MPR
image detail is decreased ( the image is never better than the original transverse axial slice)
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High contrast spatial resolution is the scanners ability to
resolve closely placed objects that are significantly different from their background
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in-plane spatial resolution is measured how
in line pairs per cm or mm
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MTF
- modulation transfer function-
- -it is used to compare the performance of different ct systems
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term for--compares the performance of different ct systems
modulation transfer function
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factors that affect high contrast resolution
- focal spot size & shape
- detector cell size
- scanner geometry
- sampling frequency
- & reconstruction algorithms
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focal spot affects resolution how
- small spot = higher spatial resolution
- large spot = lower spatial resolution
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a large focal spot size does what to resolution
lowers it
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sampling frequency affects resolution how?
smaller the slice thickness the better the resolution
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convolution algorithms affect resolution how
- specific to each body part and emphasizes the body part being imaged
- ex. bone kernal produces a sharper image, increases spatial resolution (but also increases noise)
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how does field of view affect spatial resolution?
reconstructed fov (sampling intercal has to be sufficiently small to support the reconstruction of small objects
take smaller samples to get better resolution
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bigger the matrix means ______ pixel
smaller
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pixel size is equal to
fov divided by matrix size
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cross plane spatial resolution
uses MPR & MIP to reconstruct 3D images
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resolution of new CT scanners is
isotropic
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low contrast resolution aka
sensitivity of the systems
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low contrast spatial resolution is the ability oc the ct system to
show the different tissue denisities
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ct can show tissue density differences of
0.25-0.5%
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factors that affect low contrast spatial resolution
- low contrast detectability LCD
- size, contrast, noise, kV, slice thickness
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an increase in tube current will _____ noise
reduce
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a thicker slice will produce __________ noise
less noise (however greater dose)
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Temporal resolution is
indication of CT systems ability to freeze motion of the scanned object
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to have better temporal resolution we need to increase?
scan speed
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what type of imaging is used for cardiac imaging to help minimize motion?
- gating--uses ECG to match diastole with scan time
- does not actually improve temporal resolution, but makes it more appealing to the eye
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linearity refers to the relationship of
ct #'s to the linear attenuation coefficient of the object to be imaged
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linearity can be checked how often?
daily calibration
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Uniformity is what?
- dictates that for a uniform phantom, the ct measurements should not change with the location of the selected regions of interest or phantom position
- -should be maintained within a reasonable range (2HU)
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exposure is the concentration of
- -radiation at a particular point on a patient
- -measure of the amount of ionization (amount of radiation to which the pt is exposed)
- -produced in a specific mass of air by xrays or gamma radiation
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exposure is measured by
ionization chamber
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Unit of EXPOSURE is
Roentgen (R)
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Absorbed dose is what
- aka radiation dose
- amount of energy absorbed per unit of mass of material (pt)
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Unit of ABSORBED DOSE is
rad (r) & grays (Gy)
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Any risk associated with radiation is related to the amount of
energy absorbed
-
-
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Effective dose is
- -aka quantity effective dose 'E'
- -used to quantify the risk from the partial-body exposure to that from an equivalent whole body dose
- -basically a weighted average of organ dose
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unit of measurement of EFFECTIVE DOSE is
rem or sieverts
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-
-
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types of radiation bio effects
stochstic & deterministic
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stochastic effects is
the probability (rather than severity) of the effect occurring....it is dependent on dose
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probability of stochastic effect _______ with dose
increases
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stochastic or deterministic have threshold dose?
stochastic
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linear no threshold dose model response states
- (stochastic effect)
- radiation risk increases as the dose increases and that there is no threshold dose
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Which radiation bio effects occur years after exposure
stochastic
-
examples of stochastic effects are
cancer, leukemia & hereditary effects
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stochastic effects are considered to be _____ effects
late
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Deterministic effects aka
nonstochastic effects
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deterministic effects is
where the severity of the effect (rather than the probability) increases with the increasing of the dose
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Threshold dose
relatively high doses that can kill cells and cause degenerative changes in tissue that have been exposed to radiation
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examples of deterministic effects are
skim erythema, epilation, pericarditis & cataracts
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threshold dose for cataracts is
2Gy or (200rad)
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CT beam geometry refers to
size and shape of xray beam emanating from xray tube and passing through the pt to strike a set of detectors that collect radiation attenuation data
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types of dosimetry concepts
film, tld & ionization chamber
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Ionization chamber is what
- type of dosimetry
- instrument used to accurately quantify radiation exposure
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phantoms are used to
standarize the measurement of the dose and provide a clinically realistic geometry
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MSAD
- multiple-scan average dose
- first ct dose descriptor
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CTDI
- CT dose index
- next dose descriptor after MSAD
- developed by the FDA
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what is the CTDI
it represents the mean absorbed dose in the scanned object volume and therefore the unit of CTDI is the Gy
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dose length product (DLP) is
dose descriptor used in CT dose studies & reported in the literature & on CT scanners
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DLP provides what
a measurement of the total amount of exposure for a series of scans
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DLP is ______ proportional to scan length
directly
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2 types of factors that effect dose in CT
indirect & direct
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Indirect factors that affect dose in ct
have direct influence on image quality, but NO direct effect on radiation dose
ex. reconstruction filter
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Direct factors that affect dose in ct
increase or decrease dose to the patient and are under the direct control of the technologist
ex. exposure factors, collimation, pitch, pt centering, # of detectors & overanging
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dose is ______ proportional to mAs
directly
if mAs is doubled, dose will be doubled (not takind AEC into account)
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Effective mAs (used in MSCT)
- to keep the effective dose constant, as pitch increases, the true mAs must also increase.
- ex. increase pitch from 1 to 2, increases teh mAs from 100 to 200 while the CTDI remains unchanged
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effective mAs=
true mAs / pitch
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peak kV determines
power of the photons coming from xray tube
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higher kV indicates the photons have
higher energies & can penetrate thicker objects
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Radiation dose is proportional to the ______ of the kVp
square
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quantity of photons is _____ by the square of the kVp
increased
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Collimation is used to define
- beam width for the exam
- it reflects the efficient use of xray beam at the detector
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Collimation effects dose how
narrow collimation, increases dose
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Pitch definition
ratio of the distance the table travels per rotation to the total collimated beam width
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If pitch increases by 2, the dose will
be reduced by 1/2
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increasing pitch will ___ image quality
decrease
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decreasing pitch will _______ image quality
increase
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The measured radiation dose is _____ proportional to the # of detector rows
inversely
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As the number of detector rows increase, the dose will
decrease
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ATCM
- automatic tube current modulation
- (ct's form for AEC)
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ATMC is used to
optimize the dose to the pt while maintaining constant image quality, regardless of the pt in the z-axis
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2 types of automatic tube current modulation
(ATMC)
angular & longitudinal
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angular modulation
part of ATMC-xy axis
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longitudinal modulation
part of ATMC-z axis
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Use of angular & longitudinal modulation (ATMC) can reduce dose by ______, compared with use of only angular modulation.
52%
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decrease noise will _____ dose
increase
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to reduce Noise in an image by a factor of 2, requires an increase in dose by a factor of
4
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to improve spatial resolution (pixel size) by a factor of 2 it requires an increase in dose by
8
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to decrease slice thickness and pixel size by a factor of 2 requires an increase in dose by a factor of
16
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if double mAs you increase dose by a factor of
2
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if increase square of kVp you will _____ the dose
double
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single slice spiral/helical ct invented by
Dr. Kalender & Peter Vock
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4 steps of scanning sequence in single slice ct
- 1. tube must be accerated to constant speed of motion
- 2. tube produces xrays that are transmitted through pt to fall on the detectors, pt holds breath & data is collected for axial slice
- 3. tube & detectors slow down & come to a stop, pt resumes breathing
- 4. cables unwind, & then pt & table are indexed to next scan position
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Interscan delay is
ISD--steps 1, 3 & 4
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Scan rate is equal to
- number of scans per group divided by
- sum of time to collect slices +ISD
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Limitations of slice by slice sequential ct scanners
- 1 long exam times
- 2 certain portions of anatomy are omitted due to pt breathing
- 3 innactuarte generation of 3D images & MPRs (steplike artifact)
- 4 Only a few slices are scanned during max contrast enhancement
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ISD was eliminated why
by slip/ring technology
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2 interpolation reconstruction algorithms for single slice spiral ct
350 deg & 180 deg Linear Interpolation algorithm
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