IMAGE RECONSTRUCTION

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IMAGE RECONSTRUCTION
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ALGORITHM-A SET OF RULES OR DIRECTIONS FOR
GETTING SPECIFIC OUTPUT FROM SPECIFIC INPUT
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ALGORITHM MUST TERMINATE AFTER FINATE NUMBER OF
STEPS
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DETECTORS
PRE-PROCESING
REFORMATTED RAW DATA
CONVOLUTION WITH FILTER
IMAGE RECONSTRUCTION ALGORITHM
IMAGE STORAGE DISPLAY RECORDING ARCHIVING
RECONSTRUCTED IMAGE
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IMAGE RECONSTRUCTION PERFORMED BY

• ARRAY PROCESSOR OR PROCESSORS

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RECONSTRUCTION WHEN ONE OR MORE TECHNICAL FACTORS
ARE MODIFIED IS CALLED

• RETROSPECTIVE RECONSTRUCTION

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FACTORS THAT CAN BE CHANGED DURING RECONSTRUCTION

• DFOV
• MATRIX
• SLICE THICKNESS
• SLICE INCREMENTATION
• ANGLE

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SFOV LARGE DFOV
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SFOV SMALL DFOV
TARGETED (ZOOMED) RECON
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SFOV VERY SMALL DFOVTARGETED (ZOOMED)
RECON
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DIFFERENT MATRICES
80 X 80
512 X 512
USUALLY MATRIX IS PERMANENTLY SET AT 512 X 512
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SLICE THICKNESS

• RECONSTRUCTION IN DIFFERENT THICKNESS ONLY
  POSSIBLE IN THE MULTISLICE UNIT

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SLICE INCREMENTATION

• RECONSTRUCTION IN DIFFERENT SLICE INCREMENTATION
  POSSIBLE IN SINGLE AND MULTISLICE UNITS

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INCREMENTATION
CONTIGUOUS
50 OVERLAP
100 GAP
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SLICE vs RECONSTRUCTIONINCREMENTATION

• SLICE THICKNESS
• RECON INCREMENT

• IMAGE QUALITY
• IMAGE QUALITY

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TOMO ANGLE 360º
TUBE
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TOMO ANGLE 180º
TUBE
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TOMO ANGLE

• ANGLE IMAGE QUALITY

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TYPES OF DATA

• MEASUREMENT DATA
• RAW DATA
• CONVOLVED DATA
• IMAGE DATA

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PREREQUISITE FOR DATA RECONSTRUCTION

• DATA AVAILABLE

RAW
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MEASUREMENT DATA(SCAN DATA)

• DATA THAT ARISES FROM DETECTORS. IT NEEDS TO BE
  PREPROCESSED TO ELIMINATE ARTIFACTS.

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RAW DATA

• ITS THE RESULT OF SCAN DATA BEING PRE-PROCESSED

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CONVOLVED DATA

• FILTERED BACKPROJECTION IS THE ALGORITHM USED BY
  MODERN CT.
• IT REQUIRES FILTERING AND THEN BACKPROJECTION.
  RAW DATA IS FILTERED USING MATHEMATICAL
  FILTER.(CONVOLUTION)
• IT REMOVES BLURR. CONVOLUTION CAN ONLY BE APPLIED
  TO RAW DATA.

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MATHEMATICAL FILTER CAN ALSO BE CALLED

• KERNEL
• ALGORITHM
• PASS FILTER

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TYPES OF FILTERS

• SMOOTH
• STANDARD
• SHARP
• EXTRA SHARP

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TYPES OF FILTERS (SIEMENS)

• B20 (SMOOTH)
• B40 (STANDARD)
• A80 (SHARP)
• A91 (VERY SHARP)

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SMOOTH
SHARP
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IMAGE DATA(RECONSTRUCTED DATA)

• CONVOLVED DATA THAT HAVE BEEN BACKPROJECTED INTO
  THE IMAGE MATRIX TO CREATE CT IMAGES DISPLAYED ON
  THE MONITOR.

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DETECTORS
PREPROCESSING
SCAN DATA
RAW DATA
CONVOLUTION
CONVOLVED DATA DATA
BACK PROJECTION
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BEAM GEOMETRIES AND DATA ACQUSITION

• PARALLEL SLOW
• FAN -FASTER

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Algorithms applicable to CT
Back projection
Iterative methods
Analytic methods
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BACK PROJECTION

• ALSO CALLED SUMMATION METHOD OR LINEAR
  SUPERPOSITION METHOD

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Example image of how a cube generates different
projections depending on the angle of projection
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ITERATIVE ALGORITHMS

• SIMULTANEOUS ITERATIVE RECONSTRUCTION TECHNIQUE
• ITERATIVE LEAST SQUARES TECHNIQUE
• ALGEBRAIC RECONSTRUCTION TECHNIQUE

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ITERATIVE RECON DEFINITION

• IT STARTS WITH ASSUMPTION AND COMPARES THIS
  ASSUMPTION WITH A MEASURED VALUE, MAKES
  CORRECTIONS TO BRING THE TWO VALUES IN AGREEMENT.
  THIS PROCESS REPEATS OVER AND OVER.

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ART USED BY HOUNSFIELD IN HIS FIRST EMI BRAIN
SCANNER
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ITERATIVE TECHNIQUES ARE NOT USED IN TODAYS
COMMERCIAL SCANNERS THEY ARE VERY SLOW
BETTER THAN FILTERED BACK PROJECTION IN METAL
ARTIFACT REDUCTION AND NOISE REDUCTION
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ANALYTIC RECONSTRUCTION ALGORITHMS

• FOURIER RECONSTRUCTION
• FILTERED BACK PROJECTION

USED IN MODERN CT SCANNERS
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FILTERED BACK-PROJECTIONCONVOLUTION METHOD

• PROJECTION PROFILE IS FILTERED OR CONVOLVED TO
  REMOVE THE TYPICAL STAR LIKE BLURRING THAT IS
  CHARACTERISTIC OF THE SIMPLE BACK PROJECTION.

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STEPS IN FILTERED BACK PROJECTION

• ALL PROJECTION PROFILES ARE OBTAINED
• THE LOGARITHM OF DATA IS OBTAINED
• LOGARITHMIC VALUES ARE MULTIPLIED BY DIGITAL
  FILTER
• FILTERED PROFILES ARE BACKPROJECTED
• THE FILTERED PROJECTIONS ARE SUMMED AND THE
  NEGATIVE AND POSITIVE COMPONENTS ARE CANCELLED

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FOURIER RECONSTRUCTION

• USED IN MRI
• NOT USED IN CT BECAUSE OF COMPLICATED MATHEMATICS

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ALGORITHM (FILTER) vs NOISE
DETAIL
HIGH PASS FILTER
SHARP
EDGE-ENHANCEMENT
STANDARD
LOW PASS FILTER
SMOOTHING
NOISE
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RECONSTRUCTION IN SPIRAL CT

• FILTERED BACK PROJECTION USED INTERPOLATION
  BACAUSE OF THE CONTINUOUS MOVEMENT OF THE PATIENT
  IN THE Z-DIRECTION.
• ( TO ELIMINATE MOTION BLURR)

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RECONSTRUCTION IN MULTISLICE SPIRAL CT

• INTERLACED SAMPLING
• LONGITUDINAL INTERPOLATION
• FAN BEAM RECONSTRUCTION

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RECONSTRUCTION ALGORITHM COMPARISON

• ANALYTIC METHODS ARE FASTER THAN ITERATIVE
  ALGORITHMS.
• FILTERED BACK PROJECTION IS USED IN MODERN CT
  SCANNERS
• ITERETIVE METHODS ARE BETTER THAN FILTERED BACK
  PROJECTION IN METAL ARTIFACT REDUCTION AND NOISE
  REDUCTION

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MPRMULTIPLANAR RECONSTRUCTION(REFORMATTING)
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IT USES DATA
IMAGE
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STACKS OF IMAGES ONLY IN ONE PLANE CAN BE USED
FOR ONE TYPE OF MPRDIFFERENT PLANE IMAGES CAN
NOT BE COMBINED
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ONLY IMAGES IN ONE PLANE CAN BE COMBINED!
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CURVED MPR
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FOR BEST QUALITY IMAGES

• USE VOLUMETRIC DATA ACQUSITION
• RECON YOUR ORIGINAL THICKER SLICES INTO VERY THIN
  SLICES (2mm OR 1 mm)
• SELECT RECON INCREMENT THAT WOULD CREATE AT LEAST
  50 OVERLAP BETWEEN SLICES

EXAMPLE 2 MM SLICE THICKNESS 1 MM SLICE
INCREMENT
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STAIRCASE ARTIFACT
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3-D
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EXTRUSIONIS A MODELING TECHNIQUE THAT GENERATES
A 3-D OBJECT FROM A 2 D PROFILE ON THE COMPUTER
SCREEN.IT USES IMAGE DATA TO BUILD 3-D OBJECT
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EXTRUSION
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PIXEL AREA
B
A
A WIDTH B HEIGTH AREA OF THE PIXEL A x B
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VOXEL VOLUME
B
A
C
A WIDTH B HEIGTH C-DEPTH (SLICE
THICKNESS) VOLUME OF THE VOXEL A x B x C
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DATA ACQUSITION FOR 3-D

• CONVENTIONAL SLICE BY SLICE
• VOLUME DATA ACQUSITION

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PROBLEMS WITH CONVENTIONAL SLICE BY SLICE
ACQUISITION IN 3-D GENERATION

• MOTION - STAIR-STEP ARTIFACT
• MIREGISTRATION

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STAIR-STEP ARTIFACT
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SEVERE STAIR-STEP ARTIFACT
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PROCESSING FOR 3-D

• SEGMENTATION
• TRESHOLDING
• OBJECT DELINEATION
• RENDERING

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SEGMENTATION

• PROCESSING TECHNIQUE USED TO IDENTIFY THE
  STRUCTURE OF INTEREST IN A GIVEN IMAGE. IT
  DETERMINES WHICH VOXEL ARE PART OF THE OBJECT AND
  SHOULD BE DISPLAYED AND WHICH ARE NOT AND SHOULD
  BE DISCARDED.

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SEGMENTATION
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THRESHOLDING

• METHOD OF CLASSIFYING THE TYPES OF TISSUES
  REPRESENTED BY EACH OF THE VOXELS. CT NUMBER IS
  USED TO DETERMINE THIS.

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TRESHOLDING (IN SEGMENTATION)
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DELINEATION

• BOUNDARY EXTRACTION
• VOLUME EXTRACTION

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DELINEATION




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RENDERING TECHNIQUES

• SURFACE RENDERING SHADED SURFACE DISPLAY (SSD)
• VOLUME RENDERING

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SURFACE RENDERING-SSD

• SIMPLER OF THE TWO METHODS. DISPLAYS THE IMAGE
  ACCORDING TO ITS CALCULATIONS OF HOW THE LIGHT
  RAYS WOULD BE REFLECTED TO THE VIEWERS EYES.
• COMPUTER CREATES INTERNAL REPRESENTATION OF
  SURFACES

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ADVANTAGE OF SSD

• NOT MUCH COMPUTING POWER REQUIRED
• ONLY CONTOUR IS USED

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DISADVANTAGES OF SSD

• INFO OF STRUCTURES INSIDE OR BEHIND THE SURFACE
  IS NOT DISPLAYED!!

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VOLUME RENDERING

• SOPHISTICATED TECHNIQUE. 3-D IMAGES HAVE BETTER
  QUALITY THAN IN SURFACE RENDERING. USES ENTIRE
  DATA SET FROM 3-D SPACE. IT REQUIRES MORE
  COMPUTING POWER.

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ADVANTAGES OF VOLUME RENDERING (VR)

• UNLIKE SSD, VOLUME RENDERING ALLOWS SEEING
  THROUGH SURFACES. IT ALLOWS THE VIEWER TO SEE
  BOTH INTERNAL AND EXTERNAL STRUCTURES.

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DISADVANTAGE/S

• IT REQUIRES GREAT COMPUTING POWER SOPHISTICATED
  COMPUTER EQUIPMENT

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MAXIMUM INTENSITY PROJECTION

• VOLUME RENDERING 3-D TECHNIQUE THAT IS NOW
  FREQUENTLY USED IN CTA ( CT ANGIO) IT USES LESS
  THAN 10 OF DATA IN 3-D SPACE. IT DOES NOT NEED
  SOPHISTICATED COMPUTING.

IT ORIGINATED IN MRA
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MIP ALLOWS ONLY THE VOXEL WITH THE BRIGHTEST
VALUE TO BE SELECTED
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VR vs MIP
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ADVANTAGES OF MIP

• NO NEED FOR SOPHISTICATED COMPUTER HARDWARE- IT
  USES LESS THAN 10 OF DATA

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DISADVANTAGE/S OF MIP

• ARTIFACT- STRING OF BEADS
• NO SUPERIMPOSED STRUCTURES DEMONSTRATION

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ARTIFACTS

• SSD MANY FALSE SURFACES
• MIP MIP ARTIFACT
• VR - FEW