Rad T 265 CT Lecture

▪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