IMAGE RECONSTRUCTION. ALGORITHM-A SET OF RULES OR DIRECTIONS FOR GETTING SPECIFIC OUTPUT FROM...
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Transcript of IMAGE RECONSTRUCTION. ALGORITHM-A SET OF RULES OR DIRECTIONS FOR GETTING SPECIFIC OUTPUT FROM...
![Page 1: IMAGE RECONSTRUCTION. ALGORITHM-A SET OF RULES OR DIRECTIONS FOR GETTING SPECIFIC OUTPUT FROM SPECIFIC INPUT.](https://reader033.fdocuments.in/reader033/viewer/2022061305/551424d8550346d8488b59bd/html5/thumbnails/1.jpg)
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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PRE-PROCESING
REFORMATTED RAW DATA
CONVOLUTION WITH FILTER
IMAGE RECONSTRUCTION ALGORITHM
RECONSTRUCTED IMAGE
IMAGE:IMAGE:
STORAGEDISPLAY
RECORDINGARCHIVING
DETECTORS
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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
2MM
2MM
SLICE
INCREMENT
2MM
1MM
SLICE
INCREMENT
CONTIGUOUS 50% OVERLAP
2MM
4MM
SLICE
INCREMENT
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 AVAILABLERAW
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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
• IT’S 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
SCAN DATA PREPROCESSING RAW DATA
CONVOLUTION
CONVOLVED DATA DATA
BACKPROJECTION
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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 TODAY’S
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
NOISE
STANDARD
SMOOTHING
EDGE-ENHANCEMENT
SHARP
HIGH PASS FILTER
LOW PASS FILTER
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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 DATAIMAGE
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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 THICKNESS1 MM SLICE INCREMENT
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STAIRCASE ARTIFACT
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3-D
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EXTRUSION
IS 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
A
B
A= WIDTH B= HEIGTH
AREA OF THE PIXEL = A x B
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VOXEL VOLUME
A
B
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