Presentation is loading. Please wait.

Presentation is loading. Please wait.

CT Scanning Dr. Craig Moore

Similar presentations

Presentation on theme: "CT Scanning Dr. Craig Moore"— Presentation transcript:

1 CT Scanning Dr. Craig Moore
Medical Physicist & Radiation Protection Adviser Radiation Physics Service CHH Oncology

2 Brief History of CT Scanning

3 First CT Scanner - 1972 Originally called CAT A = axial
80 x 80 resolution 4 min. per rotation 8 grey levels overnight reconstruction

4 Here and Now 512 x 512 or 1024 x 1024 resolution Sub second rotation
4096 grey levels 100’s slices per rotation

5 Components of a CT Scanner




9 Principal components of CT scanner
X-ray tube, collimator, and detector array on a rotating gantry Rotation axis is referred to as Z axis Fan beam wide enough to cover patient cross-section Narrower width in the z-axis Behind the patient is a bank of detectors Patient lies on a couch that is moved longitudinally through the gantry

10 X-ray Tube Tube parallel to patient movement – minimise anode heel effect X-rays are produced by firing electrons at a metal target – typically tungsten Capable of producing long exposure times at high mA – get very hot (require heat capacities up to 4MJ and active cooling mechanisms) Continuous scanning limited to around 90s Focal spot size typically mm Beam heavily filtered (6-10 mm Al filters) to optimise spectrum Stops attenuation coefficients varying with depth via beam hardening

11 Collimation and Filtration
Want a monoenergetic beam to avoid beam hardening artefacts As beam passes through patient low energies are filtered This results in the apparent reduction of attenuation and CT number of tissues Computer reconstruction assumes monoenergetic beam Not possible with X-ray tubes so they are heavily filtered At least 6 mm aluminium or copper Some manufacturers use shaped filters such as bow-tie filters to even out the dose distribution (conform to the shape of an elliptical patient) Pre-patient collimator is mounted on the X-ray tube Beam is approx 50cm wide to cover cross section of patient The size is variable in the z-axis Multi-slice scanners between 0.5 and 40 mm thick beams Post patient collimation is not used with multi-slice scanner

12 The Tube & Collimators

13 Detectors Requirements: Small enough to allow good spatial resolution
Up to 1000 detectors per scanner Typically 1.5 mm width but can be a small as 0.5mm High detection efficiency Fast response Wide dynamic range – massive variation in X-ray intensity Stable and noise free No afterglow There needs to be separation between detectors to prevent light crossover This reduces efficiency from 98% to 80%

14 Detectors




18 Xenon v Solid State Xenon Solid state
Single detector chamber sub-divided by electrodes No longer used in multi-slice scanners Solid state Detector array made up of individual elements Scintillant such as cadmium tungstate and a silicone photodiode Rare earth ceramics or bismuth germanate

19 Generations of CT Scanner





24 CT Imaging Conventional radiography suffers from the collapsing of 3D structures onto a 2D image However, CT scanning has extremely good low contrast resolution, enabling the detection of very small changes in tissue type Almost true depiction of subject contrast CT gives accurate diagnostic information about the distribution of structures inside the body Generation of images in transaxial section Perpendicular to the axis of rotation of the X-ray tube about the body Perpendicular to the craniocaudal axis of patient

25 Number of detectors and projections
Typically, for a 3rd generation scanner: 650 – 900 detectors 1000 to 2000 projections per rotation

26 Collapse of 3D Data into 2D Plane
Image contrast 2:1 Planar imaging 2D representation of 3D Distribution of Tissue No depth information Structures at different depths are superimposed Loss of contrast Subject Contrast 4:1 X rays

27 Typical CT Image


29 CT Images Commonly calculated on 512x512 matrix, but 256x256 and 1024x1024 are also used Each pixel is more accurately described as a voxel, because it has depth information The value stored in each voxel is referred to as the CT number which is related to the attenuation of a particular tissue: CTn = 1000 x (µt - µw)/ µw Sometimes referred to as Hounsfield Units Each CT number is assigned a certain shade of grey in the resulting image CT number represents x-ray attenuation coefficient of the corresponding voxel within the patient

30 CT Numbers Tissue Range of CT Numbers Bone 500-3000 Muscle 40-60
Brain (grey matter) 35-45 Brain (white matter) 20-30 Fat -60 to -150 Lung -300 to -800

31 Image Display CT image represented by a range of CT numbers from to (ie 4000 levels of grey) Human eye dose not have the capacity to distinguish so many grey levels If 4000 shades of grey displayed altogether there would be very little difference between different tissues

32 Window Width and Level The appearance of the image on the screen can be changed by altering the window width and level Window width refers to the range of CT numbers selected for display This range of CT numbers is centred at a particular level called the window level e.g. if imaging bone window level should be ~1000 Can spread a small range of CT numbers over a large range of grayscale values

33 Only see CT numbers +/- 250 around -593 Window Level –12
Window Width 500 Good contrast in lungs Only see CT numbers +/- 250 around -593 Window Level –12 Window Width 400 Good soft tissue contrast Only see CT numbers +/- 200 around -12

34 How do we get the images? Tube and detector rotate smoothly around the patient X-rays are produced continuously and the detectors sample the X-ray beam approx 1000 times during one rotation Typically 2 to 4 revolutions per second



37 In reality not always parallel to detectors
Each voxel is traversed by one or more x-ray beams for every measurement (1000 per rotation) Number of measurements taken in single axial section depends on number of detectors Number of samples per rotation Assume 800 detectors measured at 0.5° intervals per 360 ° rotation This is 576,000 measurements More than needed as we only need 260,000 measurements (512 x 512)

38 How do we get the picture?
Back Projection Reverse the process of measurement of projection data to reconstruct image Each projection if smeared back across the reconstructed image

39 Back Projection – the basics
Consider cylindrical uniform body with a hole down the centre A beam passing through this body from one direction will have a transmitted profile in its central region This single measurement cannot determine the position of the hole other than identifying that it is in the line of the pencil beam passing through the centre of the body Pixel values along this line are decreased by the amount of attenuation measured These values are projected back along the field of view A second projection at 90° provides a second band of grey This is then projected back across field of view Progressive projections are shown in the final figure – a star like pattern We now have an image that looks similar to what we are scanning

40 Back Projection Back Project each planar image onto three dimensional image matrix 3 6 3 3 3 6 6 3 3 3 6 3

41 Back Projection Back Project each planar image onto three dimensional image matrix 3 6 3 1 2 1 1 2 1 1 2 1

42 Back Projection Back Project each planar image onto three dimensional image matrix 3 6 3 3 2 2 1 3 2 1 2 1 2 1 6 3 4 3 3 2 1 2 3 2 1

43 Back Projection Back Project each planar image onto three dimensional image matrix 3 6 3 3 4 6 4 3 6 6 8 6 6 3 4 6 4 3 3 6 3

44 Back Projection Back Project each planar image onto three dimensional image matrix 3 6 3 3 4 6 4 3 6 6 8 6 6 3 4 6 4 3 3 6 3

45 Back Projection More views – better reconstruction
1/r blurring, even with infinite number of views

46 Filtered Back Projection
Back projection produces blurred transaxial images Projection data needs to be filtered before reconstruction Different filters can be applied for different diagnostic procedures Smoother filters for viewing soft tissue Sharp filters for high resolution imaging Back projection same as before Data from neighbouring beams are used Some data is subtracted Some data is added Filters are convolved with the blurred image data in Fourier Space

47 Filtered Back Projection




51 Filtered Back Projection
Filter planar views prior to back projection Correction of 1/r blurring requires ‘Ramp’ Filter Gives increasing weight to higher spatial frequencies Amplifies Noise signal

52 Filtered Back Projection
In Practice Use modifications of Ramp Filter Compromise between Noise and Spatial Resolution

53 Problems with Filtered Back Projection
Back projection is mathematically correct, but real life images require Filtered Back Projection Back Projection can introduce noise and streaking artefacts Not good with attenuation correction Filtered Back Projection can reduce noise and artefacts, but may degrade resolution

54 Iterative Reconstruction
Been around for years but only recently has computing power meant IR can be used in practice A raw image is taken, this is compared to a ‘perfect image’ and adjusted a little This process is repeated until the desired level of image quality is achieved

55 Helical and Multi-Slice Scanning

56 Helical Scanning Have discussed simplest form of CT scanning
Can produce transaxial slices with the patient being moved along the z-axis between each rotations ‘Step and shoot’ This is now very rare Helical scanning is now the standard Slip rings Continuous table feed through gantry

57 Helical Scanning Slip ring technology
X-ray tube has to be supplied with constant power Detectors have to pass signals to computer Not possible if gantry was wired – cables would become entangled and overstretched Slip ring is a metal ring mounted on the gantry Good connection while the gantry is free to rotate



60 First we need the ‘scout view’
Scout views are needed prior to the scan Performed to allow the planning of the CT sequence Scout views are produced lines by line at a fixed projection angle Typically AP

61 Helical Scanning Patient moves continuously through the gantry as the X-ray tube and detectors rotate Continuous acquisition of data in a single exposure Can be visualised as a ribbon wrapped around the body This technology minimises slice misregistration

62 Contiguous scan



65 Helical Scanning The position at which sections can be reconstructed can be anywhere within scanned volume other than at the ends For example: 300 mm long volume scanned 10 mm slice width Pitch = 2 Only 15 rotations required From the measured data, 30 contiguous slices, each 10mm thick can be reconstructed (for a single slice scanner) For thinner slices we need multi-slice scanners

66 Advantages of Helical Scanning
Speed No need to pause between scans for table movement Pitches greater than 1 allowed (reduction in dose) Longer scan lengths within breath hold Reduced patient movement artefacts Increased throughput Reduced use of contrast medium

67 Disadvantages of Helical Scanning
Broadening of Slice profile Effective slice thickness increases – poorer z axis resolution Higher noise Helical artefacts not seen in axial scanning Possibility of very high dose if pitch < 1 Lot of tube heating and loading

68 Multi Slice CT

69 Multiple detectors in single row
Remember!! Multiple detectors in single row

70 Multislice CT In its original form, CT scanning was two-dimensional – 2D slices through the body True 3D imaging requires isotropy Voxel size must be equal in all directions Under these circumstances, data generated in a 3D matrix can be reconstructed in any plane

71 Multislice CT Voxel size in axial plane is dependent on matrix size and field of view Typically 1mm Single slice helical scanners have the capability of collimating the beam width (in the patient direction) to 1mm, but this is restricted by scan time Not possible in practice so we need to have multi-slice technology


73 Multislice CT Key to 3D scanning is the multislice scanner
These scanners use solid state detectors with multiple rows of detectors Typical configuration for an 8 slice scanner 12 curved detector rows Each row has approx 800 detectors Each row has minimum possible gap between them Central rows have approx half the length of 2 outer rows Length of central rows 0.5 – 1mm Rows can be used separately or in combination

74 Multislice CT Four possible combinations here are possible:
(a) 8 x 1mm slices (b) 8 x 2mm slices (c) 4 x 4mm slices (d) 2 x 8mm slices Eight slice scanner here is capable of producing scans at four slice widths, 1,2,4, or 8mm In each case, slice width is determined by the detector size and by collimation Scanners up to 256 slices now available

75 Beam width is varied

76 Computer reconstruction
Physical acquisition Computer reconstruction


78 Single slice Multi slice Single slice Multi slice 3 rotations One rotation


80 Dose and Multi-Slice Scanners
Considerations similar to those of single slice scanners Dose utilisation on z axis usually poorer than with single slice scanners X ray beam width is generally broader than the total imaged width Geometric efficiency down to 50% for very small slice thicknesses (sub mm)

81 Broader beam to negate the effect of penumbra
Geometric Efficiency Broader beam to negate the effect of penumbra Extra beam/x = geo eff

82 Dose and Helical CT All helical scanning requires extra irradiation at the end of each run to obtain sufficient interpolation data to reconstruct the required volume On multi-slice scanners this extra length can be quite long

83 Helical & Multi-Slice in the UK
Helical & Multi-Slice scanning represent significant steps forward in CT Better scanning of previous scans Expansion of workload Nearly all scanners sold in UK are multi-slice Technology is still advancing 32/40/64/256 slice scanners now available More slices in the future?

84 Next Lecture: Dosimetry
Operator controls and affects on dose and image quality artefacts

Download ppt "CT Scanning Dr. Craig Moore"

Similar presentations

Ads by Google