▪ History ▪ Equipment ▪ Image Production/Manipulation

▪ Roetgen discovers x-rays ▪ Radon develops recontruction formulas ▪ Cormack develops mathematics for x- ray absoprtion in tissue ▪ Housfield demonstrates CT Dateline

▪ first whole body CT ▪ Housfield and Cormack win Nobel prize ▪ EBCT ▪ spiral CT ▪ 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 s scan time 1024x102480x80 matrix 1 mm13 mm slice th 15 lp/cm3 lp/cm spatial 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 ▪ 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 numbertissue bone muscle white matter gray matter blood CSF 0.180water fat lung o air

▪ Atomic number ▪ Tissue density ▪ Beam energy

▪ I=I o e -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 µT - µi µI X 1000

▪ Air = ▪ Lungs = -200 ▪ Fat = -50 to – 100 ▪ Water = 0 ▪ CSF = 15 ▪ Blood = ▪ 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