rad t 265 ct lecture
DESCRIPTION
Rad T 265 CT Lecture. History Equipment Image Production/Manipulation. History of CT. 1895 - Roetgen discovers x-rays 1917 - Radon develops recontruction formulas 1963 - Cormack develops mathematics for x- ray absoprtion in tissue 1972 - Housfield demonstrates CT. Dateline. Dateline 2. - PowerPoint PPT PresentationTRANSCRIPT
▪History▪Equipment▪Image Production/Manipulation
▪1895 - Roetgen discovers x-rays▪1917 - Radon develops recontruction
formulas▪1963 - Cormack develops mathematics for x-
ray absoprtion in tissue▪1972 - Housfield demonstrates CT
Dateline
▪1975 - first whole body CT▪1979 - Housfield and Cormack win Nobel
prize▪1983 - EBCT▪1989 - spiral CT▪1991 - multi-slice CT
▪Original idea was to move the patient not the beam.▸The intent was to produce a homogeneous or monoenergetic
beam.▪Original scanner used a radioisotope instead of a tube.
▪To date there have been four accepted generations with some consideration as EBCT to be the fourth.
▪The first fourth generation scanner was unveiled in 1978 four years after the first scanner.
▪Pencil thin beam - highly collimated
▪Single radiation detector▪180 translations at 1
degree of rotation▪One image projection per
translation▪5 minutes of scan time per
image▪Heads only
Translate/rotate
▪Fan shaped beam▪Multiple detectors - a
detector array▪18 translations with 10
degrees between them.▪Multiple image
projections per translation
▪30 second scan time per image
▪Head and body imager
Translate/rotate
▪Fan beam that covers the entire width of the patient
▪Several hundred detectors in a curvilinear detector array
▪Both the source and the detector array move
▪Hundreds of projections are obtained during each rotation, thereby producing better spatial and contrast resolution.
▪Scan time is reduced to one second or less per image
Rotate/rotate
▪Still a fan beam▪Thousands of detectors
are now used▪Thousands of
projections are acquired producing better image quality
▪Sub-second scan times▪Various arcs of
scanning are possible increasing functionality
Rotate/stationary
▪Intended for rapid imaging
▪Scan time less than 100 msec
▪No tube, instead tungsten rings are used
▪Four rings allow four slices to be acquired simultaneously
▪No moving parts
▪Third or fourth generation scanners with constant patient movement
▪Use slip ring technology▪Can cover a lot of anatomy in a short period
of time
spiralfirst
<1 s300 sscan time
1024x102480x80matrix
1 mm13 mmslice th
15 lp/cm3 lp/cmspatial res
CT image circa 1971
▪X-ray source▪Detector array▪Collimator▪High voltage generator
▪10,000 rpm anodes▪8 MHU▪Tube is parallel the patient to reduce anode
heel effect▪200 - 800 mA
▪Bow tie filters are used to ‘even out’ the beam intensity at the detectors
▪Primary purpose is to harden the beam▸Reduces artifacts
▪CT uses a high kVp to minimize photoelectric effect
▪High kVp allows the maximum number of photons to get to the dectector array
▪All current scanners use high frequency generators
▸High frequency generators are much smaller than three phase units allowing for a smaller footprint and less voltage fluctuation
▪Early scanners used scintillation crystal photomultiplier detectors as a single element
▪Currently two types of detector arrays▸Gas filled▸Solid state
▪Filled with high pressure xenon▪Fast response time with no afterglow or lag▪50% dectection efficiency▪Can be tightly packed▸Less interspacing, fewer lost photons
▪Ion chambers are approximately 1 mm wide▪Geometric efficiency is 90% for the entire
array▪Total detector efficiency = geometric
efficiency x intrinsic efficiency
▪Cadmium tungstate▸Scintillator
▪Material is optically coupled with a photodiode▪Nearly 100 % efficiency▪Due to design they cannot be tightly packed
▪80 % total detector efficiency▪Automatically recalibrate▪Reduced noise▪Reduced patient dose▪More expensive than gas filled
▪Amplifies the signal▪Converts the analog signal to digital(ADC)▪Transmits the signal to the computer
Located between the detector array and the computer
▪Multiple detector arrays allow for multiple slices to be acquired simultaneously
▸Pre-patient▪Controls patient dose▪Determines dose profile
▸Post-patient▸Controls slice thickness
▪Most common process is filtered back projection▪Fourier transformation▪Analytic▪Iterative
▪Data acquisition▪Preprocessing▸Reformatting and convolution
▪Image reconstruction▪Image display▪Post-processing activities
▪Suppress low spatial frequencies resulting in images with high spatial resolution
▸Bone▸Inner ear▸High-res chest
▪Suppress high spatial frequencies▪Most commonly used filters▪Images appear smoother▸Less noisy
▪Images are displayed on a matrix▪Today most are 512 x 512 or 1024 x 1024▸The original matrix was 80 x 80
▪The matrix consists of pixels▪Pixels represent voxels
▪The diameter of the reconstructed image is the FoV
▪Generally, pixel size is the limiting factor in spatial resolution.
▪The smaller the pixel the higher the spatial resolution.
▪Pixel size (spatial resolution) is determined by matrix size and FoV.
▪Post-processing does not increase the amount of information available. It presents the original information in a different format
▪This is numerical value assigned to each pixel.
▪CT numbers are derived from the attenuation coefficient of the tissue in the voxel.
▪CT numbers are also called Hounsfield units
Att CoeffCT numbertissue0.461000bone0.23150muscle0.18745white matter0.18440gray matter0.18220blood0.18115CSF0.180water0.162-100fat0.094-200lungo.0003-1000air
▪Atomic number▪Tissue density▪Beam energy
▪I=Ioe-µx
▪Based on a homogenous beam
Attenuation
▪The higher the CT number the brighter the pixel
▪Calculation▪Positive and Negative▪Numbers for various anatomical structures
▪Water is 0.206
µT - µi µI
X 1000
▪Air = -1000▪Lungs = -200▪Fat = -50 to – 100▪Water = 0▪CSF = 15▪Blood = 42-50▪Gray matter = 40▪White matter = 45▪Muscle = 50▪Bone = >500
▪This is the range of CT numbers displayed.▪The wider the width the lower the contrast.▸Think scale of contrast, a long scale (wide width)
has low contrast.
▪Level is the center number of the width.▪Usually, this represents the anatomy of
interest.▪You can see by the similarities between CT
numbers that the level doesn’t change much.
▪Increase pixels increase resolution▪Decrease voxel size increase resolution▪Typically need to increase technique with higher res
▪The most common is maximum intensity projection (MIP)
▪Also, volume rendering is used to provide an image with depth. Used to be called shaded-surface display (SSD).
▪Quantitative CT uses a phantom to establish a bone mineral density exam.
▪This is the basis for CT angiography.▪Voxels are selected for their intensity along a
proscribed axis of reconstruction.▪MIP images are volume rendered
▪ROI▪Measurement▸Linear▸Volume
▪Magnification
▪Spiral scanners greatly improved sagittal and coronal reconstructions because they limited movement.
▪Multi-slice scanners are even better because they have smaller slice thicknesses and isotropic voxels.
Axial image
Conventional CT
Spiral
▪Source moves, detectors probably not▪Source stops and starts▪Patients moves between exposures
▪Source moves, detectors may move▪Patient moves during exposure
▪Couch movement per rotation divided by slice thickness
▪Contigous spiral: pitch = 1, 10mm of movement with a slice thickness of 10mm
▪Extended spiral: pitch = 2, 20mm of movement with a slice thickness of 10mm.
▪Overlapping spiral: pitch = ½
▪The lower the pitch the better the z-axis resolution.
▪The narrower the collimation the better the z- axis resolution.
▪Increase pitch, decrease dose▪When pitch exceeds 1, interpolation filters
must be applied
▪Spiral scanners don’t acquire true axial images so interpolation becomes necessary at larger pitches.
▪So data is interpolated and then back filtered.
▪Image noise is higher for spiral CT than conventional CT regardless of the scanning parameters.
▪Faster image acquisition▪Contrast can be followed better▪Reduced patient dose at pitches > 1▪Physiologic imaging▪Improved 3d and reconstructions▪Less partial volume
▪Fewer motion artifacts▪No misregistration▪Increased throughput▪Real time biopsy