IMAGE RECONSTRUCTION

ALGORITHM-A SET OF RULES OR DIRECTIONS FOR GETTING SPECIFIC OUTPUT FROM SPECIFIC INPUT

ALGORITHM MUST TERMINATE AFTER FINATE NUMBER OF STEPS

CONVOLUTION WITH FILTER DETECTORS PRE-PROCESING REFORMATTED RAW DATA CONVOLUTION WITH FILTER IMAGE RECONSTRUCTION ALGORITHM IMAGE: STORAGE DISPLAY RECORDING ARCHIVING RECONSTRUCTED IMAGE

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 DFOV TARGETED (ZOOMED) RECON

USUALLY MATRIX IS PERMANENTLY SET AT 512 X 512 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 1MM SLICE INCREMENT 2MM 4MM SLICE INCREMENT 2MM CONTIGUOUS 50% OVERLAP 100% GAP

SLICE vs RECONSTRUCTION INCREMENTATION SLICE THICKNESS RECON INCREMENT 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 AVAILABLE RAW

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 RAW DATA CONVOLVED DATA DATA PREPROCESSING CONVOLUTION CONVOLVED DATA DATA BACK PROJECTION

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

BETTER THAN FILTERED BACK PROJECTION REDUCTION AND NOISE REDUCTION 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-PROJECTION CONVOLUTION 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 HIGH PASS FILTER SHARP EDGE-ENHANCEMENT STANDARD LOW PASS FILTER SMOOTHING NOISE

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

MPR MULTIPLANAR RECONSTRUCTION (REFORMATTING)

IT USES DATA IMAGE

STACKS OF IMAGES ONLY IN ONE PLANE CAN BE USED FOR ONE TYPE OF MPR DIFFERENT 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 THICKNESS 1 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 B A A= WIDTH B= HEIGTH AREA OF THE PIXEL = A x B

VOXEL VOLUME B A 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