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IMAGE RECONSTRUCTION
ALGORITHM-A SET OF RULES OR DIRECTIONS FOR GETTING SPECIFIC OUTPUT
FROM SPECIFIC INPUT
ALGORITHM MUST TERMINATE AFTER FINATE
NUMBER OF STEPS
PRE-PROCESING
REFORMATTED RAW DATA
CONVOLUTION WITH FILTER
IMAGE RECONSTRUCTION ALGORITHM
RECONSTRUCTED IMAGE
IMAGE:IMAGE:
STORAGEDISPLAY
RECORDINGARCHIVING
DETECTORS
IMAGE RECONSTRUCTION PERFORMED BY:
• ARRAY PROCESSOR OR PROCESSORS
RECONSTRUCTION WHEN ONE OR MORE TECHNICAL FACTORS ARE MODIFIED IS
CALLED:
• RETROSPECTIVE RECONSTRUCTION
FACTORS THAT CAN BE CHANGED DURING RECONSTRUCTION
• DFOV
• MATRIX
• SLICE THICKNESS
• SLICE INCREMENTATION
• ANGLE
SFOV LARGE DFOV
SFOV SMALL DFOV
TARGETED (ZOOMED) RECON
SFOV VERY SMALL DFOVTARGETED (ZOOMED) RECON
DIFFERENT MATRICES
80 X 80 512 X 512
USUALLY MATRIX IS PERMANENTLY SET AT 512 X 512
SLICE THICKNESS
• RECONSTRUCTION IN DIFFERENT THICKNESS ONLY POSSIBLE IN THE MULTISLICE UNIT
SLICE INCREMENTATION
• RECONSTRUCTION IN DIFFERENT SLICE INCREMENTATION POSSIBLE IN SINGLE AND MULTISLICE UNITS
INCREMENTATION
2MM
2MM
SLICE
INCREMENT
2MM
1MM
SLICE
INCREMENT
CONTIGUOUS 50% OVERLAP
2MM
4MM
SLICE
INCREMENT
100% GAP
SLICE vs
RECONSTRUCTIONINCREMENTATION
• SLICE THICKNESS
• RECON INCREMENT
• IMAGE QUALITY
• IMAGE QUALITY
TOMO ANGLE 360º
TUBE
TOMO ANGLE 180º
TUBE
TOMO ANGLE
• ANGLE IMAGE QUALITY
TYPES OF DATA
• MEASUREMENT DATA
• RAW DATA
• CONVOLVED DATA
• IMAGE DATA
PREREQUISITE FOR DATA RECONSTRUCTION
• DATA AVAILABLERAW
MEASUREMENT DATA(SCAN DATA)
• DATA THAT ARISES FROM DETECTORS. IT NEEDS TO BE PREPROCESSED TO ELIMINATE ARTIFACTS.
RAW DATA
• IT’S THE RESULT OF SCAN DATA BEING PRE-PROCESSED
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.
MATHEMATICAL FILTER CAN ALSO BE CALLED:
• KERNEL
• ALGORITHM
• PASS FILTER
TYPES OF FILTERS
• SMOOTH
• STANDARD
• SHARP
• EXTRA SHARP
TYPES OF FILTERS (SIEMENS)
• B20 (SMOOTH)
• B40 (STANDARD)
• A80 (SHARP)• A91 (VERY SHARP)
SMOOTH SHARP
IMAGE DATA(RECONSTRUCTED DATA)
• CONVOLVED DATA THAT HAVE BEEN BACKPROJECTED INTO THE IMAGE MATRIX TO CREATE CT IMAGES DISPLAYED ON THE MONITOR.
DETECTORS
SCAN DATA PREPROCESSING RAW DATA
CONVOLUTION
CONVOLVED DATA DATA
BACKPROJECTION
BEAM GEOMETRIES AND DATA ACQUSITION
• PARALLEL –SLOW
• FAN -FASTER
Algorithms applicable to CT
Back projection Iterative methods Analytic methods
BACK PROJECTION
• ALSO CALLED SUMMATION METHOD OR LINEAR SUPERPOSITION METHOD
Example image of how a cube generates different projections depending on the angle of projection
ITERATIVE ALGORITHMS
• SIMULTANEOUS ITERATIVE RECONSTRUCTION TECHNIQUE
• ITERATIVE LEAST SQUARES TECHNIQUE
• ALGEBRAIC RECONSTRUCTION TECHNIQUE
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.
ART USED BY HOUNSFIELD IN HIS FIRST EMI BRAIN
SCANNER
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
ANALYTIC RECONSTRUCTION ALGORITHMS
• FOURIER RECONSTRUCTION
• FILTERED BACK PROJECTION
USED IN MODERN CT SCANNERS
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.
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
FOURIER RECONSTRUCTION
• USED IN MRI
• NOT USED IN CT BECAUSE OF COMPLICATED MATHEMATICS
ALGORITHM (FILTER) vs NOISE
DETAIL
NOISE
STANDARD
SMOOTHING
EDGE-ENHANCEMENT
SHARP
HIGH PASS FILTER
LOW PASS FILTER
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)
RECONSTRUCTION IN MULTISLICE SPIRAL CT
• INTERLACED SAMPLING
• LONGITUDINAL INTERPOLATION
• FAN BEAM RECONSTRUCTION
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
MPRMULTIPLANAR
RECONSTRUCTION(REFORMATTING)
IT USES DATAIMAGE
STACKS OF IMAGES ONLY IN ONE PLANE CAN BE USED
FOR ONE TYPE OF MPRDIFFERENT PLANE IMAGES
CAN NOT BE COMBINED
ONLY IMAGES IN ONE PLANE CAN BE COMBINED!
CURVED MPR
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
STAIRCASE ARTIFACT
3-D
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
EXTRUSION
PIXEL AREA
A
B
A= WIDTH B= HEIGTH
AREA OF THE PIXEL = A x B
VOXEL VOLUME
A
B
C
A= WIDTH B= HEIGTH
C-DEPTH (SLICE THICKNESS)
VOLUME OF THE VOXEL = A x B x C
DATA ACQUSITION FOR 3-D
• CONVENTIONAL SLICE BY SLICE
• VOLUME DATA ACQUSITION
PROBLEMS WITH CONVENTIONAL SLICE BY SLICE ACQUISITION IN 3-D
GENERATION• MOTION - STAIR-STEP ARTIFACT
• MIREGISTRATION
STAIR-STEP ARTIFACT
SEVERE STAIR-STEP ARTIFACT
PROCESSING FOR 3-D
• SEGMENTATION
• TRESHOLDING
• OBJECT DELINEATION
• RENDERING
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.
SEGMENTATION
THRESHOLDING
• METHOD OF CLASSIFYING THE TYPES OF TISSUES REPRESENTED BY EACH OF THE VOXELS. CT NUMBER IS USED TO DETERMINE THIS.
TRESHOLDING (IN SEGMENTATION)
DELINEATION
• BOUNDARY EXTRACTION
• VOLUME EXTRACTION
DELINEATION
RENDERING TECHNIQUES
• SURFACE RENDERING – SHADED SURFACE DISPLAY (SSD)
• VOLUME RENDERING
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
ADVANTAGE OF SSD
• NOT MUCH COMPUTING POWER REQUIRED
• ONLY CONTOUR IS USED
DISADVANTAGES OF SSD
• INFO OF STRUCTURES INSIDE OR BEHIND THE SURFACE IS NOT DISPLAYED!!
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.
ADVANTAGES OF VOLUME RENDERING (VR)
• UNLIKE SSD, VOLUME RENDERING ALLOWS SEEING THROUGH SURFACES. IT ALLOWS THE VIEWER TO SEE BOTH INTERNAL AND EXTERNAL STRUCTURES.
DISADVANTAGE/S
• IT REQUIRES GREAT COMPUTING POWER – SOPHISTICATED COMPUTER EQUIPMENT
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
MIP ALLOWS ONLY THE VOXEL WITH THE
BRIGHTEST VALUE TO BE SELECTED
VR vs MIP
ADVANTAGES OF MIP
• NO NEED FOR SOPHISTICATED COMPUTER HARDWARE- IT USES LESS THAN 10% OF DATA
DISADVANTAGE/S OF MIP
• ARTIFACT- STRING OF BEADS
• NO SUPERIMPOSED STRUCTURES DEMONSTRATION
ARTIFACTS
• SSD – MANY FALSE SURFACES
• MIP – MIP ARTIFACT
• VR - FEW
