RTEP LEC CT
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A kind of x-ray that can produce 3D pictures
Creation of a cross-section tomographic
section (slices, cuts) of the body using arotating fan beam (x-ray tube), detector array
and computed (complex computer)
reconstruction.
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A CT (computerised tomography) scanner is a
special kind of X-ray machine. Instead ofsending out a single X-ray through your body
as with ordinary X-rays, several beams aresent simultaneously from different angles.
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CAT Computer-Assisted Tomography
or Computed Axial Tomography
CTAT Computerized Transaxialtomography
CRT Computerized Reconstruction
Tomography DAT Digital Axial Tomography
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WILHELM CONRAD
ROENTGEN
1845-1923
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Alessandro Vallebona
Early 1900s
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A method whereby body-section roentgenography isaccomplished by relative
motion of the tube and filmwith respect to the bodyroentgenographed, the tubeand film meanwhile having no
motion relative to oneanother. This can beaccomplished by rotating thebody between a stationary
tube and film
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1950
developedmathematicalsolutions
involved in CT
scan
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1967 1st successful demonstration of CT was conducted by Hounsfieldfrom Central Research Laboratory of EMI in England
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The first patient brain-scan was
done on 1 October 1971. Themachine takes 5 minute per scan
and can be processed by over 2.5
hours.
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Dr. Robert Ledley atGeorgetown
University MedicalCenter developed the
1st scanner capable of
visualizing anysection.
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1979- Cormack and Hounsfield received a
nobel prize for medicine.
1982 - 1979- Cormack and Hounsfieldreceived a nobel prize for physics.
1989 spiral CT introduced
1991 Multiple CT introduced
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1ST GENERATION
2ND GENERATION
3RD GENERATION 4TH GENERATION
5TH GENERATION
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THE GANTRY
COMPUTER
OPERATING CONSOLE
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XRAY TUBE
DETECTOR ARRAY
HIGH VOLTAGE GENERATOR PATIENT COUCH
COLLIMATOR ASSEMBLY
MECHANICAL SUPPORT
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HARDWARE
SOFTWARE
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FLOPPY DISK
MAGNETIC TAPE
COMPACT DISK
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Film
Paper copy
PACS
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Refers to the systematic collection of
information from the patient to produce the
CT imager.
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IMAGE RECONSTRUCTION IS THE PHASE IN
WHICH THE SCAN DATA SET IS PROCESSED
TO PRODUCE AN IMAGE.
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THE TIME FROM THE END OF IMAGING
(END OF DATA COLLECTION) TO IMAGE
APPEARANCE
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1. scanning phase produces data but not an
image
2. the reconstruction phase processes the
acquired data and forms digital image.
3. digital to analog conversion phaseproduces visible and displayed analog image
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PIXEL
MATRIX
FOV (FIELD OF VIEW) VOXEL
CT NUMBERS
WINDOW LEVEL
WINDOW WIDTH
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TWO DIMENSION REPRESENTATION OF
CORRESPONDING TISSUE VOLUME
PIXEL SIZE = FOV DIVIDED BY MATRIX SIZE
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ARRAY OF NUMBERS IN ROWS AND
COLUMNS OF PIXELS DISPLAYED ON A
DIGITAL IMAGE
256X256
512X512
1024X1024
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DIAMETER OF IMAGE RECONSTRUCTION
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TISSUE VOLUME
VOXEL SIZE (CUBIC mm) = PIXEL SIZE(mmSQUARED) X SLICE THICKNESS (mm)
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SMALL PIXEL = IMPROVED SPATIAL
RESOULTION AND CONTAIN HIGH
FREQUENCY INFO LARGE PIXEL IMAGE HAVE REDUCED
SPATIAL RESOLUTION AND CONTAIN LOWFREQUENCY INFO
SPATIAL RESOLUTION IS DETERMINED BYMATRIX SIZE AND FOV
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Number that represents the attenuation
value for each pixel.
Optical density & Brightness level as
displayed on the monitor.
Numbers level ranges from -1000 to +1000for each pixel.
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TISSUE CT NUMBERS
DENSE BONE 1000+
MUSCLE 50
WHITE MATTER 45GRAY MATTER 40
BLOOD 20
CSF 15
WATER 0
FAT -100
LUNGS
AIR
-200
-1000
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REFERS TO THE MANIPULATION OF WL
AND WW TO OPTIMIZE IMAGE CONTRAST
WINDOW WIDTH = RANGE OF CT NUMBERS
DISPLAYED
WINDOW LEVEL = CENTER OF RANGE ANDCENTRAL VALUE OF WINDOW WIDTH
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PART WW WL
BRAIN 80 40
CHEST SOFT TISSUE 400 40
CHEST LUNG 1500 -400
ABDOMEN400 50
SPINE 1600 300
BONE 3000 500
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Spatial resolution
contrast resolution
linearity
noise
artifacts
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Spatial resolution is the CT
system's ability todifferentiate small objects
that are adjacent to oneanother.
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SLICE THICKNESS DECREASE SLICE
THICKNESS TO IMPROVE SPATIAL
RESOLUTION
PIXEL SIZE SMALL PIXEL SIZE improvesspatial resolution
INFLUENCED BY FOV,MATRIX, WIDTH OF THE DETECTORS,
INTERSPACING BETWEEN DETECTOR
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Contrast Resolution is the
ability of a CT scanner todifferentiate small
attenuation differences on the
CT image.
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AS IMAGE NOISE INCREASES CONTRAST
RESOLUTION DECREASES
COLLIMATION DETERMINESSECTION
THICKNESS
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" Property of a detector
characterized by an outputelectrical current that is exactly
linearly proportional to the input
radiation incident on thedetector." (Morgan 1983)
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contains no information. Noise
is characterized by a grainyappearance of the image. Many
authors describe noise as a salt
and pepper pattern on the CTimage.
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RESULT OF LOW
RADIATION;CHARACTERIZED
BY GRAINY IMAGE
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INTERPOLATION = EXTRACTION OF VALUESBETWEEN KNOWN VALUES
EXTRAPOLATION = EXTRACTION OF VALUESOUTSIDE THE KNOWN VALUES
Spiral/helical CT data uses a mathematical
technique called interpolation along with areconstruction algorithm to produce axial imagedata from a volume of data.
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1. QUANTUM NOISE - is a result
of too few photons reaching adetector after being attenuated
by the body.
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2. COMPUTATIONAL NOISE - is primarily
caused by all the statistical fluctuations
that occur from the reconstructionmathematics that are essential to
produce a CT image.
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3. ELECTRONIC NOISE caused by electrical
fluctuations
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"An Artifact is any distortion or error in the
image that is unrelated to the subject being
studied (Morgan 1983).
Wolbarst (1993) describes artifacts asaberrations that arise at the interfaces of
materials significantly different from theradiologic properties of the structures being
scanned.
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SYSTEMATIC DISCREPANCIES BETWEEN
THE CT NUMBERS IN THE
RECONSTRUCTED IMAGE AND THE TRUEATTENUATION COEFFICIENTS OF THE
OBJECT.
NON RANDOM, OR STRUCTURED IMAGENOISE
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Motion artifacts occur primarily because
during reconstruction the mathematical
algorithm is unable to solve for theinconsistencies in attenuation.
Appears as streaks or step-like patterns at
high contrast edges.
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Beam hardening artifacts occurwhen the low energy photons are
absorbed leaving the high-energyphotons to strike the detectors
Appears as a dark ring inside cranialbone and cupping at the center ofthe image
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Ring artifacts appear on a CT image as a
ring or a number of rings superimposed
on the structures being scanned.
The artifact is commonly associated
with third generation CT systems.
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The partial volume effect orvolume averaging is when two or
more different tissue typesoccupy the same pixel and areaveraged together.
Blurring over sharp edges
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GENERALLY, PATIENT RADIATION DOSE IS
HIGHER DURING CT THAN DURINGRADIOGRAPHY OR FLOUROSCOPY.
PATIENT RADIATION DOSE DURING CT ISAPPROXIMATELY 5000 mrad per
EXAMINATION
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REPEAT SCAN
LOW PITCH
THIN SLICE
OVERLAPPING SLICE
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PATIENT DOSE IS DESCRIBED BY CTDI AND
DLP.
CTDI CT DOSE INDEX
DLP DOSE LENGTH PRODUCT
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Lowest area radiation exposure plane of the
gantry and outside patient aperture.
Highest area radiation exposure - near thepatient and is due to scatter radiation
produced in the patient.
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