CT Made ECasy

7/29/2019 CT Made ECasy

1/47

CT made easy


2/47

Introduction

The computed tomograpic (CT) scanner isrevolutionary.

It does not use an ordinary image reseptor, butinstead a well-collimated x-ray beam directed on thepatient, and the attenuated image response istransmitted to a computer.

The computer reconstructs the image and displays iton a monitor.

The reconstructions are accomplished withalgorithms adapted for computer processing.


3/47

History of CT

The first demonstration of the technique was done in1970, but the components to construct a CT scanner

was avalible 20 years before this. In 1982, Godfrey Houndsfield shared the Nobel prize

in physics with Alan Cormack. Cormack had earlierdeveloped the mathematics used to reconstruct CTimages.

No other x-ray equipment development are asimportant as the CT. Some say that MRI and UL areas equally important, but they are, however, not x-ray equipments.


4/47

Principles of use

CT is a tranaxional/transverese image. Thatmeans that you get axial pictures of the body.

It is extremly complicated to understand theprecise method on how the CT makes theseimages. You need to have good knowlegdeof physics, engineering and computerscience.

The basic principles can be demonstrated ifyou consider the simplest CT systems.


5/47

Principles of use

The x-ray source and and detector are connected sothat they move at the same time.

When the machine makes one sweep over thepatient the internal structures of the body attenuatethe x-ray beam according to their mass density andeffective atomic number.

The machine takes several sweeps of the body andcollects this in a computer, this computer thenreconstructs the images of the anatomic structures inthat slice.


6/47

Principles of use

The detector signal during each sweep is

registered in increments with values as highas 1000. The value og each increment isrelated to the x-ray attenuation coefficient ofthe total path trough the tissue.

Through the use of simultaneous equations,a matrix of values is obtained that representsa cross section of anatomy.


7/47

The different generations of CT

There are four/five generations of CT

scanners. The fifth is still under development. The first generation: translate-rotate

configuration, pencil- shaped beam, singledetector, 5-min scan time

Second generation: translate- rotateconfiguration, fan-shaped beam, detectorarray, 30 s- scan time


8/47

The different generations of CT

Third generation: rotate- rotate configuration,

fan-shaped beam, detector array, 1-s scantime, disadvantage: ring artefacts

Fourth generation: rotate-stationaryconfiguration, fan-shaped beam, detector

array, 1-s scan time


9/47

Third generation scanner

In these scanners the x-ray tube and detector arrayare rotated concentrically about the patient.

They can produce an image in one sec. It uses a curvilinear array containing many detectors

and a fan beam.

The curvilinear detector array results in a constant

source-to-detector path length, which was anadvantage for good image reconstruction.

This also allows for better x-ray beam collimation toreduce the effect of scatter radiation.


10/47

Third generation scanner

One disadvantage is the ring artefacts. They

occur for several reasons. Each detector views a ring of anatomy , so if

any single detector malfunctions, the aquiredsignal will result in a ring on the

reconstructed image. Software-corrected image reconstruction

algorithms minimize such artefacts.


11/47

Fourth generation scanners

Radiation detection is accomplished through a fixedcircular array, which contains as many as 8000

individual elements.

The fixed detector array does not result in a constantbeam path from the source to all the detectors, but itallowes each detector to be calibrated and its signal

normalized during a scan. They are generally without ring artefacts

Disadvantage: patient dose and cost of buying


12/47

Fifth generation scanners

Development of CT is always going on. The

producers wish to make a CT scanner withimproved image quality at a lesser patientdose.

Rotate-nutate scanners: Toshiba has

produced a novel extension of the fourthgeneration. To maintain the x-ray source atthe same distance from the patient as thedetectors, the detector array nutates, as the

x-ray source rotates.


13/47

Fifth generation scanners

Electron-beam CT (EBCT) is a fundamentally

different way to produce CT images. Imatroncame up with the idea for scanning the heart.

Currently , EBCT is used to scan all tissues,but especially when ultrafast imaging is

helpful. EBCT images are produced in 50 ms.


14/47

System components

The gantry

The computer The operating console


15/47

The gantry

Includes the x-ray tube, the detector array,

the high-voltage generator, the patientsupport couch and the mechanical supportfor each.

X-ray tube: it has special requirements. The

power capacity must be high. The anodeheating capacity must be atleast severalmillion heat units (MHU).


16/47

The gantry

High speed rotors are used in most tubes for thebest heat dissipation.

Focal-spot size is important. CT scanners designedfor imaging using high spatial resolution incorporatex-ray tubes with small focus-spot.

Detector assembly: Early scanners had one

detector. Modern scanners have up to 8000, devidedin to groups; scintillation detectors and gas-filleddetectors.


17/47

The gantry

Scintillation detectors:

Containes scintillation crystal-photodiode

assemblies. They convert light into electronicsignals. They are highly efficient at detecting x-rays,almost 90 % of the x-rays are absorbed andcontribute to the output signal.

But the space between each detector is big, so theoverall detection efficiency may only be 50 %. Theygive dose to patient but do not contribute to theimage.


18/47

The gantry

Gas-filled detectors: Contructed of a large metallic chamber with baffles

spaced with 1 mm intervals. The baffles are like grid stripes and devide the large

chambers into small ones. Each small chamber is one detector. It is sealed and filled under preassure with an inert

gas with high atomic number (xenon/xenon-kryptonmixture)

The overall total detetction efficiency is 45 %, almostthe same as scintillation detectors.


19/47

The gantry

Collimators: Required for the same reason asconventional x-ray. Correct collimation reduces

patient dose and improves image quality due to lessscattered radiation.

In CT there are normally two collimators.

One is the prepatient collimator; on the x-ray tube

housing/adjacent to it. It limits the area of the patient that intercepts the

useful beam and thereby the slice thickness and thepatient dose.


20/47

The gantry

Improper adjustment of this collimator is the cause ofmost of the un-necessary dose to patient.

The predetector collimator; located under the patient,over the detector array.

Reduces scatter radiation improves image quality

When coupled correctly with the prepatient detector,it defines the slice thickness.

Has nothing to do with patient dose.


21/47

The gantry

High-voltage generator;

All CT scanners operate on three-phase orhigh-frequenzy power.

Most manufactors built them into the gantryor by mounting on the rotating wheel of the

gantry. It reduces the amount of spaceneeded, and winding and unwinding a powercable is unnecessary.


22/47

The gantry

Patient positioning and support couch;

It has to be made of a material with a lowatomic number (carbon fiber) so that is doesnot interfere with x-ray beam transmissionand patient imaging.

It should move smoothly for accurate patientpositioning, and is especially important forspiral CT


23/47

Computer

It is unique for the CT and a must! A ultra-high speeddigital computer is needed for making CT images.

Depending on the format the computer has to do upto 250 000 equations at the same time! In the computer there is a microprocessor and a

primary memory. These determine thereconstruction time= the time from end of scanning

to image appearance. Array processors are becoming more common. They

are faster than the microprocessor and canreconstruct an image in less than 1 s.


24/47

Operating console

Many CT scanners have 2 or 3 consoles.

One for the CT radiologic technologist to operate thescanner.

One for an other technologist to postprocess images.

One for the physician to view the image, manipulatecontrast, size and general visual appearance.

A typical operating console contains controls andmonitors for the various technique factors.


25/47

Image characteristics

With CT, the x-rays form a stored electronic

image that is displayed as a matrix ofintensities.

The CT scan format consists of many cellswith its own number which is shown as a

brightness level. A matrix of 512 x 512 = 262 144 cells of

information.


26/47


Each cell is a pixel (picture element)

The numerical information in each pixel is aCT number/ Houndsfield Unit (HU)

It is a two dimensional representation of acorresponding tissue volume.

The diameter of image reconstruction iscalled the field of view (FOV)


27/47


When the FOV is increased for a fixed matrix ( forexample: from 12 to 20 cm) the size of each pixel is

increased proportionately. When the matrix size is increased for a fixed FOV

(for example 512 x 512 to 1024 x1024)the pixel sizegrows smaller.

Pixel size = FOV/matrix size

The tissue volume is known as a voxel (volumeelement)

Voxel size= Pixel size x slice thickness


28/47

CT numbers

Each pixel is displayed on the video monitoras a level of brightness and on the

photographic image as a level of opticaldensity.

The levels correspond to a range of CTnumbers from -1000 up to +1000 for eachpixel.

-1000 is air, +1000 is dense bone and 0 iswater.


29/47

CT numbers

The CT number is related to the x-rayattenuation coefficient of the tissue containedin the voxel.

Remember: the degree of x-ray attenuationis determined by the avarage energy of the

x-ray beam and the effective atomic numberof the absorber and is expressed by theattenuation coefficient.


30/47

CT numbers

By the scale of HU there is a range of 2000different gray scales with imformation, butmost of it goes lost.

The screen only shows 32 grayscales.


31/47

Image reconstruction

Filtered back projection = all the projectionsduring on CT examination is stored in thecomputers memory, and the reconstructionsare made by these.

With filter we do not mean a metal filter as in

the tube of the x-ray, but it is a mathematicalfunction. A difficultone!


32/47

Image reconstruction

In CT there over 250 000 pixels toreconstruct from, that means that themachine has to solve 250 000 equations tofind the solutions for the images.


33/47

Image quality

Spatial resolution

Contrast resolution Noise

Linearity

Uniformity


34/47

Spatial resolution

If you scan a regular geometric structure that has asharp interface,the image at the interface will be

blurred. The degree of blurring is a measure of spatialresolution of the system and is controlled by severalfactors.

If you take a scan over an area that has a high

contrast interface, for example the brain and theskull, the image will be blurred. The system will fix some of the blurring, and

smoothen the picture.


35/47

Spatial resolution

This, however, reduses the spatial resolutionbecause of some features of the scanner.

The larger the pixel size and the lower the subjectcontrast, the poorer the spatial resolution will be.

The detector size and design of prepatient andpostpatient collimation affect the level of scatterradiation and influence the spatial resolution byaffecting the contrast of the system.

Also the x-rays focal spot has influence on spatialresolution.


36/47

Contrast resolution

Contrast resolution = the ability to distinguishone soft tissue from another without regardfor size or shape.

Contrast resolution is superior in CT,principally because of x-ray beam

collimation.


37/47

Contrast resolution

Imagine a scan over abdomen, where youhave spine, liver and fat. The atomic

numbers are different, but in conventional x-ray it is difficult to seperate them. With CTand the CT numbers it makes it easy! WithHU the CT can amplify these contrast

differences, and make the contrast high.Then we can cleary see differences betweentissue.


38/47

Noise

Noise= the precentage of standard deviation of alarge number of pixels obtained with a water-bath

scan. Noise depends on the following factors:

1: kilovolt peak filtration

2: Pixel size

3: Slice thickness4: Detector efficiency

5: Patient dose


39/47

Noise

Example:

If you scan a homogeneous medium like waterthe pixel value should be zero. But becausethe system is not perfect some pixel valueswill be both higher and lower than zero.These variations in HU will show in the

image as graininess, and is what we callnoise. The larger the variations in pixel value,the more noise you get in the image.


40/47

Linearity

The CT must be calibrated frequently so that the HU arecorrect.

There is a test you can do with a phantom and a water bucket. The result from this test should show a linear line passing

through the CT number of water (0)

If the test shows deviation from linearity its a sign ofmalfunction of the CT.

It may not show on the visual image, but could greatly affectquantitative analysis of tissue, the determination of tissuecomposition based on CT number.


41/47

Uniformity

When you scan a uniform object (water) thepixel value should be zero (for water!).Butsince the machine is very complicatedmechanically this does not happen. Thevalue may drift from day to day/hour by hour.

If it is scanned, and the pixel value isconstant in all regions of the reconstructedimage, this is called Spatial uniformity


42/47

Uniformity

There is also a test for this, where you scan abucket of water and plot the numbers along

an axis of the image. If this axis is within 2standard deviations of the mean value, thesystem has acceptable spatial uniformity.

Because of the x-ray beam hardening, there

may be a decrease of CT numbers, so themiddle of the image is darker than it should.

This is called cupping artefact.


43/47

Summary

The collimated x-ray beam is directed to thepatient.

The attenuated image-forming x-ray beam ismeasured by a detector array.

The signal from the detector array ismeasured by a computer.

The image is reconstructed in the computer.

The image is displayed on a TV monitor.


44/47

Summary

CT makes transverse images (axial images)

The internal structures of the body attenuate the x-

ray beam according to their mass density and atomicnumber.

All data are processed in digital form.

The resulting computer image is an electronic matrix

of intensities.

Matrix size is generally 512x512 individual cells orpixels.


45/47

Summary

In each pixel is numerical information called a CTnumber or HU.

The pixel is a two-dimensional representation of acorrensponding tissue volume.

The voxel (volume element) is determined bymultiplying the square of the pixel size by the

thickness of the CT scan slice. HU -1000=air, 0= water, 1000= dense bone


46/47

Summary

The CT scanner has exellent contrastresolution because of the reduction of scatter

radiation by the x-ray beam collimators. The ability to scan low-contrast anatomic

structures is limited by the noise of thesystem.

System noise is determined by the numbersof x-rays used by the detector array toproduce the image.


47/47

Questions...

?

CT Made ECasy

Documents

Transcript of CT Made ECasy