Image Reconstruction Bushong Chapter 4 Bushong Chapter 4.

Image ReconstructionImage Reconstruction

Bushong Chapter 4Bushong Chapter 4

IntroIntro

• Image reconstruction involves filtered Image reconstruction involves filtered back projection, resulting in a digital back projection, resulting in a digital matrix, which can be post processed matrix, which can be post processed for additional image analysisfor additional image analysis

• The object of image reconstruction The object of image reconstruction from projections is to compute and from projections is to compute and assign a computed tomography (CT) assign a computed tomography (CT) number to each pixelnumber to each pixel

IntroIntro

• Computed tomography imaging involves Computed tomography imaging involves data acquisition, image reconstruction, data acquisition, image reconstruction, and image displayand image display

• Between data acquisition and image Between data acquisition and image reconstruction is reconstruction is preprocessingpreprocessing – – reformatting and convolutionreformatting and convolution

• Following image display is Following image display is postprocessing, recording, and archivingpostprocessing, recording, and archiving

The CT ComputerThe CT Computer

• The CT computer must have exceptional The CT computer must have exceptional capacity to manipulate extensive data capacity to manipulate extensive data obtainedobtained

• A CT computer consists of four principal A CT computer consists of four principal components – an input device, a CPU, an components – an input device, a CPU, an output device, and memoryoutput device, and memory

• Input/output devices are ancillary pieces of Input/output devices are ancillary pieces of computer hardware designed to place raw computer hardware designed to place raw data into a computer and receive processed data into a computer and receive processed data from the computerdata from the computer


• Input devices include keyboard, tape, Input devices include keyboard, tape, disk, CD-ROM, video display terminal, CT disk, CD-ROM, video display terminal, CT detector, laser scanner, and plasma detector, laser scanner, and plasma screenscreen

• Output devices include video display Output devices include video display terminal, laser camera, dry image terminal, laser camera, dry image processor, printer, and image transmitterprocessor, printer, and image transmitter

• Hard copy refers to an image on film or Hard copy refers to an image on film or printedprinted


• Soft copy mean the image is Soft copy mean the image is displayed on a cathode ray tube displayed on a cathode ray tube (CRT), flat panel display, or stored on (CRT), flat panel display, or stored on magnetic or optical disksmagnetic or optical disks

• The brains of a computer are in the The brains of a computer are in the central processing unit (CPU), which central processing unit (CPU), which contains the microprocessor, the contains the microprocessor, the control unit, and primary memorycontrol unit, and primary memory


• The microprocessor is the “computer The microprocessor is the “computer on a chip” or wafer, of silicon on a chip” or wafer, of silicon fabricated into many diodes and fabricated into many diodes and transistorstransistors

• The control unit interprets The control unit interprets instruction, sequences tasks, and instruction, sequences tasks, and generally runs computer functionsgenerally runs computer functions


• RAM, ROM, or WORM are the three RAM, ROM, or WORM are the three principal types of solid state memoryprincipal types of solid state memory– Random Access MemoryRandom Access Memory– Read Only MemoryRead Only Memory– Write Once Read Many times memoryWrite Once Read Many times memory

• Look up tables (LUT) are software Look up tables (LUT) are software components components


• Primary memory exists as read only Primary memory exists as read only memory or random access memory to memory or random access memory to store data as it is used in computationsstore data as it is used in computations

• Primary memory may be on the CPU or Primary memory may be on the CPU or on an additional circuit boardon an additional circuit board

• Primary memory is solid state, made of Primary memory is solid state, made of silicon (semiconductor) technology, silicon (semiconductor) technology, and very fast but limited in sizeand very fast but limited in size


• Secondary memory is required when primary Secondary memory is required when primary memory is insufficient and when data needs memory is insufficient and when data needs to be transferred to another locationto be transferred to another location

• Secondary memory is useful for bulk storage Secondary memory is useful for bulk storage of information, such as imagesof information, such as images

• Secondary memory can be on-line as with Secondary memory can be on-line as with magnetic hard drive disks or off-line as with magnetic hard drive disks or off-line as with magnetic tape and magnetic or optical disksmagnetic tape and magnetic or optical disks


• The analog-to-digital converter (ADC) is The analog-to-digital converter (ADC) is a special type of computer that converts a special type of computer that converts the analog signal from each CT detector the analog signal from each CT detector to a digital form for computer to a digital form for computer manipulationmanipulation

• An array processor is a special type of An array processor is a special type of computer that is designed to do only a computer that is designed to do only a special task, such as image special task, such as image reconstruction, and it does that task reconstruction, and it does that task very fastvery fast


• The software of a computer is the The software of a computer is the collection of programs written in collection of programs written in computer language to implement the computer language to implement the many tasks of a computermany tasks of a computer

• There are two general types of There are two general types of software software – Operating systemsOperating systems– Application programsApplication programs


• Operating system such as Microsoft Operating system such as Microsoft DOS, Microsoft Windows, and UNIX DOS, Microsoft Windows, and UNIX manages the computer hardwaremanages the computer hardware

• Application programs are written in a Application programs are written in a higher level languagehigher level language

• Application programs for CT include the Application programs for CT include the algorithms for image reconstruction and algorithms for image reconstruction and programs for postprocessing analysisprograms for postprocessing analysis


• The CT computer must have the capacity to The CT computer must have the capacity to solve a large number of equations solve a large number of equations simultaneouslysimultaneously

• To produce a 512 x 512 matrix, 512To produce a 512 x 512 matrix, 5122 2 or or 262,144 equations must be solved 262,144 equations must be solved simultaneouslysimultaneously

• The laboratory environment should be The laboratory environment should be controlled to less than 30% relative humidity controlled to less than 30% relative humidity and below 20 degrees C (70 degrees F) to and below 20 degrees C (70 degrees F) to ensure best computer operationensure best computer operation


• The time from the end of imaging The time from the end of imaging (end of data collection) to image (end of data collection) to image appearance is the reconstruction timeappearance is the reconstruction time

• Reconstruction times of 1s and less Reconstruction times of 1s and less are commonare common

• Most of the action of a CT computer is Most of the action of a CT computer is accomplished with multiple accomplished with multiple microprocessorsmicroprocessors


• Most image reconstruction is done Most image reconstruction is done with an Array Processorwith an Array Processor

• The array processor is designed to The array processor is designed to perform many specific calculations perform many specific calculations very quickly, but can do nothing elsevery quickly, but can do nothing else

Back ProjectionBack Projection

• The analog image projection recorded The analog image projection recorded by each detector element is received by each detector element is received and transferred by the data acquisition and transferred by the data acquisition system (DAS) to the ADC so ti becomes system (DAS) to the ADC so ti becomes a digital image projectiona digital image projection

• Each digital image projection acquired Each digital image projection acquired by each detector during a CT by each detector during a CT examination is stored in the computer examination is stored in the computer memorymemory


• Computed tomography images are Computed tomography images are reconstructed from these image reconstructed from these image projections by convoluted back projectionprojections by convoluted back projection

• The image is reconstructed with The image is reconstructed with simultaneous filtered back projection of all simultaneous filtered back projection of all the image projectionsthe image projections

• Reconstruction algorithms are a set of well Reconstruction algorithms are a set of well defined computer software steps designed defined computer software steps designed to produce a specific output (image) from to produce a specific output (image) from a given input (signal profiles)a given input (signal profiles)


• Four reconstruction algorithms have been Four reconstruction algorithms have been applied to CT – Fourier transformation, applied to CT – Fourier transformation, analytic, iterative, and back projectionanalytic, iterative, and back projection

• Back projection with a convolution filter – Back projection with a convolution filter – filtered back projection – is most widely filtered back projection – is most widely applied in CTapplied in CT

• Volume and surface rendered images are Volume and surface rendered images are produces with different convolution filtersproduces with different convolution filters


• The word filter as used here is not a The word filter as used here is not a metallic filter of Al or Cu placed in an x-ray metallic filter of Al or Cu placed in an x-ray beam to reduce low energy x-raysbeam to reduce low energy x-rays

• Filter, or more correctly, convolution filter, Filter, or more correctly, convolution filter, refers to a mathematical manipulation of refers to a mathematical manipulation of the data designed to change the the data designed to change the appearance of the imageappearance of the image

• A convolution filter – sometimes called a A convolution filter – sometimes called a kernelkernel – is a mathematic process applied to – is a mathematic process applied to an image projection before back projectionan image projection before back projection


• Convolution filters are also called Convolution filters are also called reconstruction algorithmsreconstruction algorithms

• A high-frequency convolution filter A high-frequency convolution filter suppresses high-frequency signals, causing suppresses high-frequency signals, causing the image to have a smooth appearance and the image to have a smooth appearance and possible improvement in contrast resolutionpossible improvement in contrast resolution

• Back projection results in a blurred imaged Back projection results in a blurred imaged because x-ray attenuation is not uniform because x-ray attenuation is not uniform over the entire path lengthover the entire path length


• The convolution filter is applied to the The convolution filter is applied to the image projections prior to reconstruction image projections prior to reconstruction and the result is a sharper imageand the result is a sharper image

• A low-frequency convolution filter A low-frequency convolution filter suppresses low-frequency signals, suppresses low-frequency signals, resulting in edge enhancement and resulting in edge enhancement and possible improvement in spatial resolutionpossible improvement in spatial resolution

• Most CT imagers have in excess of 20 Most CT imagers have in excess of 20 convolution filters available that are convolution filters available that are operator selectableoperator selectable


• Image projections from all angles are Image projections from all angles are overlappedoverlapped

• Projection angulation results in image Projection angulation results in image blur, which can be accommodated by blur, which can be accommodated by convolution filtersconvolution filters

• There are many types of convolution There are many types of convolution filters – some enhance contrast filters – some enhance contrast resolution, some enhance spatial resolution, some enhance spatial resolutionresolution


• Spatial frequency related how rapidly Spatial frequency related how rapidly subject contrast changessubject contrast changes

• A bone-soft tissue interface A bone-soft tissue interface represents high spatial frequency represents high spatial frequency (small object, high contrast)(small object, high contrast)

• Gray/white matter of the brain Gray/white matter of the brain represents low spatial frequency represents low spatial frequency (large object, low contrast)(large object, low contrast)


• The spatial frequencies of various The spatial frequencies of various tissues are enhanced or suppressed by tissues are enhanced or suppressed by using appropriate convolution filtersusing appropriate convolution filters

• High pass convolution filters are used High pass convolution filters are used for imaging of bone, inner ear, etc,for imaging of bone, inner ear, etc,

• High pass convolution filters (bone High pass convolution filters (bone algorithms) result in images with algorithms) result in images with enhanced edges, short scale of enhanced edges, short scale of contrast, and more noisecontrast, and more noise


• Low pas convolution filters are used for Low pas convolution filters are used for imaging soft tissue such as brain, liver, imaging soft tissue such as brain, liver, etcetc

• Low pass convolution filters (smoothing Low pass convolution filters (smoothing algorithms) appear less noisy with long algorithms) appear less noisy with long scale of contrastscale of contrast

• Image reconstruction time is 1s or less Image reconstruction time is 1s or less and is determined by ADC rate, CPU clock and is determined by ADC rate, CPU clock speed, amount of data collected, and speed, amount of data collected, and convolution filter chosenconvolution filter chosen

Image DisplayImage Display

• All CT images are digital and All CT images are digital and formatted as a matrixformatted as a matrix

• A matrix is an orderly array of cells A matrix is an orderly array of cells fashioned in rows and columnsfashioned in rows and columns

• Current CT images produce 512x512 Current CT images produce 512x512 and 1024x1024 matricesand 1024x1024 matrices

• A 1024x1024 image is reconstructed A 1024x1024 image is reconstructed from 1,048,576 simultaneous from 1,048,576 simultaneous equations into 1,048,576 matrix cellsequations into 1,048,576 matrix cells


• A larger matrix size results in improved A larger matrix size results in improved spatial resolutionspatial resolution

• A larger matrix size requires longer A larger matrix size requires longer reconstruction timesreconstruction times

• Each matrix cell is a picture element Each matrix cell is a picture element (pixel)(pixel)

• Each pixel is a two dimensional Each pixel is a two dimensional representation of a volume element representation of a volume element (voxel)(voxel)


• Voxel size is the product of pixel size Voxel size is the product of pixel size and section thicknessand section thickness

• The diameter of the reconstructed The diameter of the reconstructed image is the field of view (FOV)image is the field of view (FOV)

• When FOV is increased and matrix When FOV is increased and matrix size is constant, pixel size increases size is constant, pixel size increases and spatial resolution is reducedand spatial resolution is reduced

• Either smaller FOV or larger matrix Either smaller FOV or larger matrix size results in smaller pixel sizesize results in smaller pixel size


• When FOV is constant and matrix When FOV is constant and matrix size increased, pixel size is reduced size increased, pixel size is reduced and spatial resolution improvedand spatial resolution improved

• In general, pixel size is the limiting In general, pixel size is the limiting spatial resolution of a CT scannerspatial resolution of a CT scanner

• Small pixel images have improved Small pixel images have improved spatial resolution and contain high spatial resolution and contain high frequency informationfrequency information


• Large pixel images have reduced Large pixel images have reduced spatial resolution and contain more spatial resolution and contain more low frequency informationlow frequency information

• Matrix size describes the number of Matrix size describes the number of pixels in an imagepixels in an image

• Spatial resolution is determined by Spatial resolution is determined by matrix size and FOVmatrix size and FOV


• Larger matrix size, for example Larger matrix size, for example 1024x1024 instead of 512x512, results in 1024x1024 instead of 512x512, results in smaller pixels and better spatial resolutionsmaller pixels and better spatial resolution

• Smaller matrix size is useful for pediatric Smaller matrix size is useful for pediatric imagingimaging

• Smaller matrix size is useful for biopsy Smaller matrix size is useful for biopsy localizationlocalization

• The normal scanned FOV is approximately The normal scanned FOV is approximately 20cm for head or pediatric body, 35cm for 20cm for head or pediatric body, 35cm for body, and 48cm for large bodybody, and 48cm for large body


• Localizer images are used to plan extent Localizer images are used to plan extent of anatomy to be imagedof anatomy to be imaged

• Localizer images are an example of digital Localizer images are an example of digital radiographic imagesradiographic images

• Computed tomography vendors identify Computed tomography vendors identify localizer images by various nameslocalizer images by various names– ScoutScout– PilotscanPilotscan– TopogramTopogram– SurviewSurview


• Localizer images are made with the x-Localizer images are made with the x-ray beam on continuously while the ray beam on continuously while the patient couch moves through the beampatient couch moves through the beam

• Localizer images may be AP, PA, or Localizer images may be AP, PA, or laterallateral

• The display FOV (DFOV) can be equal to The display FOV (DFOV) can be equal to or less than the scanned FOV (SFOV)or less than the scanned FOV (SFOV)

• Pixel size is the quotient of FOV by Pixel size is the quotient of FOV by matrix sizematrix size


• There is a subtle difference between There is a subtle difference between SFOV and DFOVSFOV and DFOV

• Scanned field of view is usually set to Scanned field of view is usually set to cover the anatomic part – head, body, cover the anatomic part – head, body, large bodylarge body

• Displayed field of view is usually Displayed field of view is usually employed as a postprocessing tool to employed as a postprocessing tool to provide a magnified image of a part of provide a magnified image of a part of the SFOVthe SFOV


• Magnification using original image Magnification using original image projections – target zoom – results in projections – target zoom – results in improvement in spatial resolutionimprovement in spatial resolution

• Magnification resulting from pixel Magnification resulting from pixel enlargement – photo zoom – is easier enlargement – photo zoom – is easier and faster but the image has less and faster but the image has less spatial resolution than the originalspatial resolution than the original


• Postprocessing includesPostprocessing includes– Pan and zoomPan and zoom– Histogram analysisHistogram analysis– Measurement and Rio'sMeasurement and Rio's– AnnotationAnnotation– Windowing and LevelingWindowing and Leveling– ReconstructionsReconstructions– And general image manipulation featuresAnd general image manipulation features


• Postprocessing does not result in Postprocessing does not result in additional information, just the same additional information, just the same or less information presented or less information presented differentlydifferently

CT NumbersCT Numbers

• The solution of the simultaneous equations The solution of the simultaneous equations by filtered back projection results in a by filtered back projection results in a numerical value (CT number) for each pixelnumerical value (CT number) for each pixel

• The CT number is directly related to the x-The CT number is directly related to the x-ray linear attenuation coefficient for the ray linear attenuation coefficient for the tissue in that voxeltissue in that voxel

• The standard scale of CT numbers is the The standard scale of CT numbers is the Hounsfield scale of Hounsfield unites (HU) Hounsfield scale of Hounsfield unites (HU) – Not to be confused with heat unitsNot to be confused with heat units


• HUHU– Bone = 1000HUBone = 1000HU– Water = 0HUWater = 0HU– Air = -1000HUAir = -1000HU

• One HU = .1% difference between One HU = .1% difference between the linear attenuation coefficient of the linear attenuation coefficient of tissue compared to the linear tissue compared to the linear attenuation coefficient of waterattenuation coefficient of water


• Pixel brightness if proportional to HUPixel brightness if proportional to HU– High HU is brightHigh HU is bright– Low HU is darkLow HU is dark

• The Hounsfield scale ranges from -The Hounsfield scale ranges from -1000 to 1000 and some imager have 1000 to 1000 and some imager have CT number scales from -2000 to 6000CT number scales from -2000 to 6000

• The video monitor can display perhaps The video monitor can display perhaps 256 shades of gray but the eye can 256 shades of gray but the eye can detect only approximately 20detect only approximately 20


• The range of CT numbers displayed is The range of CT numbers displayed is the Window Width (WW)the Window Width (WW)

• The central value of the WW is the The central value of the WW is the Window Level (WL)Window Level (WL)

• Reducing the WW increases contrastReducing the WW increases contrast• The WL selects the CT number at the The WL selects the CT number at the

center of the displayed gray scalecenter of the displayed gray scale• WW and WL allow the entire CT or HU WW and WL allow the entire CT or HU

number scale to be visualizednumber scale to be visualized


• Wide WW is used for bone imagine, Wide WW is used for bone imagine, narrow WW is used for soft tissuenarrow WW is used for soft tissue

• Windowing refers to the manipulation of Windowing refers to the manipulation of WL and WW to optimize image contrastWL and WW to optimize image contrast

• A wide WW results in a gray image – A wide WW results in a gray image – long gray scale, low contrastlong gray scale, low contrast

• A narrow WW results in a black/white A narrow WW results in a black/white image – short gray scale, high contrastimage – short gray scale, high contrast


• Window level is the center of WWWindow level is the center of WW

• A CT image is optimized for that A CT image is optimized for that tissue having the same CT number tissue having the same CT number as the WLas the WL

Post ProcessingPost Processing

• Most widely applied multiplanar reconstruction Most widely applied multiplanar reconstruction algorithms result in maximum intensity algorithms result in maximum intensity projection and shaded surface displayprojection and shaded surface display

• Multiplanar reconstruction produces coronal Multiplanar reconstruction produces coronal and sagittal images from axial imagesand sagittal images from axial images

• Quantitative CT compares vertebral bone CT Quantitative CT compares vertebral bone CT numbers with a standard phantom imaged numbers with a standard phantom imaged simultaneously to assay bone mineralizationsimultaneously to assay bone mineralization

Maximum Intensity Projection Maximum Intensity Projection (MIP)(MIP)

• Multiple MIP images reconstructed at Multiple MIP images reconstructed at different angles and viewed in rotation different angles and viewed in rotation may be required to separate may be required to separate superimposed vesselssuperimposed vessels

• MIP was first employed in MRIMIP was first employed in MRI• MIP is the basis for CT angiographyMIP is the basis for CT angiography• To create the proper image plane, the To create the proper image plane, the

technologist must have a good foundation technologist must have a good foundation in anatomy, especially vascular anatomyin anatomy, especially vascular anatomy

MIPMIP

• MIP selects voxels along a row or column in a MIP selects voxels along a row or column in a volume of interest with the highest CT volume of interest with the highest CT number – or a specified range of CT numbers number – or a specified range of CT numbers – for display– for display

• Bones usually have higher CT numbers than Bones usually have higher CT numbers than contrast filled vessels and must be software contrast filled vessels and must be software excludedexcluded

• Multiple overlapping reconstruction reduces Multiple overlapping reconstruction reduces the “beading” artifact sometimes seen in MIPthe “beading” artifact sometimes seen in MIP

MIPMIP

• Along any row or column, the voxel with Along any row or column, the voxel with the highest CT number is displayedthe highest CT number is displayed

• Contrast enhanced voxels are displayed Contrast enhanced voxels are displayed in preference to soft tissue voxelsin preference to soft tissue voxels

• MIP images do not provide depth MIP images do not provide depth informationinformation

• MIP images are volume rendered imagesMIP images are volume rendered images• Shaded surface images are surface Shaded surface images are surface

rendered imagesrendered images

MIPMIP

• The width of the region of interest The width of the region of interest should be a small as possible to should be a small as possible to reduce background noise in the reduce background noise in the presence of contrast filled vesselspresence of contrast filled vessels

Image Reconstruction Bushong Chapter 4 Bushong Chapter 4.

Documents

following image display

intro image reconstruction

object of image reconstruction

ct computer input devices

dry image processor

ct computer soft copy

computer inputoutput

image transmitter hard