ct scanning dr. craig moore medical physicist & radiation protection adviser radiation physics...

CT Scanning

Dr. Craig Moore

Medical Physicist & Radiation Protection Adviser

Radiation Physics Service

CHH Oncology

Brief History of CT Scanning

First CT Scanner - 1972• Originally called CAT

• A = axial

• 80 x 80 resolution

• 4 min. per rotation

• 8 grey levels

• overnight reconstruction

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

Components of a CT Scanner

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

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 0.6 - 1mm• Beam heavily filtered (6-10 mm Al filters) to optimise spectrum• Stops attenuation coefficients varying with depth via beam hardening

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

The Tube & Collimators

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%

Detectors

Xenon v Solid State

• Xenon – 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

Generations of CT Scanner

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

Number of detectors and projections

• Typically, for a 3rd generation scanner:

– 650 – 900 detectors– 1000 to 2000 projections per rotation

Collapse of 3D Data into 2D Plane

• Planar imaging– 2D representation of 3D

Distribution of Tissue– No depth information– Structures at different

depths are superimposed• Loss of contrast

Image contrast 2:1

Subject Contrast 4:1

X rays

Typical CT Image

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

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

Image Display

• CT image represented by a range of CT numbers from -1000 to + 3000 (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

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

Window Level –593

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

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

• 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)

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

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

Back Projection

• Back Project each planar image onto three dimensional image matrix

3 3

3

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Back Projection


3 36

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Back Projection


3 36

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22 3 2

3 4 3

2 3 2

Back Projection


3 3

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Back Projection

• More views – better reconstruction

• 1/r blurring, even with infinite number of views

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


• Filter planar views prior to back projection

• Correction of 1/r blurring requires ‘Ramp’ Filter– Gives increasing weight to

higher spatial frequencies– Amplifies Noise

SPECT FIlters

-0.2

0

0.2

0.4

0.6

0.8

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0 0.2 0.4 0.6 0.8 1

Frequency (fraction of Nyquist)

Ramp

Shepp-Logan

ModifiedShepp_Logan

Hanning

Hamming

Butterworth

signal


• In Practice– Use modifications

of Ramp Filter– Compromise

between Noise and Spatial Resolution

SPECT FIlters

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Frequency (fraction of Nyquist)

Ramp

Shepp-Logan

ModifiedShepp_Logan

Hanning

Hamming

Butterworth

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

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

Helical and Multi-Slice Scanning

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

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

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

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

Contiguous scan

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

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

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

Multi Slice CT

Remember!!

Multiple detectors in single row

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

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

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

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

Beam width is varied

Computer reconstruction

Physical acquisition

Single slice Multi slice

3 rotations One rotation

Single slice

Multi slice

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)

Geometric Efficiency

Broader beam to negate the

effect of penumbra

Extra beam/x = geo eff

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

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?

Next Lecture:

• Dosimetry

• Operator controls and affects on dose and image quality

• artefacts

ct scanning dr. craig moore medical physicist & radiation protection adviser radiation physics...

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