CT made easy

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Introduction

The computed tomograpic (CT) scanner is revolutionary.

It does not use an ordinary image reseptor, but instead a well-collimated x-ray beam directed on the patient, and the attenuated image response is transmitted to a computer.

The computer reconstructs the image and displays it on a monitor.

The reconstructions are accomplished with algorithms adapted for computer processing.

History of CT

The first demonstration of the technique was done in 1970, 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 earlier developed the mathematics used to reconstruct CT images.

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

Principles of use

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

It is extremly complicated to understand the precise method on how the CT makes these images. You need to have good knowlegde of physics, engineering and computer science.

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

Principles of use

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

When the machine makes one sweep over the patient the internal structures of the body attenuate the x-ray beam according to their mass density and effective atomic number.

The machine takes several sweeps of the body and collects this in a computer, this computer then reconstructs the images of the anatomic structures in that slice.

Principles of use

The detector signal during each sweep is registered in increments with values as high as 1000. The value og each increment is related to the x-ray attenuation coefficient of the total path trough the tissue.

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

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, single detector, 5-min scan time

Second generation: translate- rotate configuration, fan-shaped beam, detector array, 30 s- scan time

The different generations of CT

Third generation: rotate- rotate configuration, fan-shaped beam, detector array, 1-s scan time, disadvantage: ring artefacts

Fourth generation: rotate-stationary configuration, fan-shaped beam, detector array, 1-s scan time

Third generation scanner

In these scanners the x-ray tube and detector array are 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 an advantage for good image reconstruction.

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

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 aquired signal will result in a ring on the reconstructed image.

Software-corrected image reconstruction algorithms minimize such artefacts.

Fourth generation scanners

Radiation detection is accomplished through a fixed circular array, which contains as many as 8000 individual elements.

The fixed detector array does not result in a constant beam path from the source to all the detectors, but it allowes 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

Fifth generation scanners

Development of CT is always going on. The producers wish to make a CT scanner with improved image quality at a lesser patient dose.

Rotate-nutate scanners: Toshiba has produced a novel extension of the fourth generation. To maintain the x-ray source at the same distance from the patient as the detectors, the detector array nutates, as the x-ray source rotates.

Fifth generation scanners

Electron-beam CT (EBCT) is a fundamentally different way to produce CT images. Imatron came 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.

System components

The gantry The computer The operating console

The gantry

Includes the x-ray tube, the detector array, the high-voltage generator, the patient support couch and the mechanical support for each.

X-ray tube: it has special requirements. The power capacity must be high. The anode heating capacity must be atleast several million heat units (MHU).

The gantry

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

Focal-spot size is important. CT scanners designed for imaging using high spatial resolution incorporate x-ray tubes with small focus-spot.

Detector assembly: Early scanners had one detector. Modern scanners have up to 8000, devided in to groups; scintillation detectors and gas-filled detectors.

The gantry

Scintillation detectors: Containes scintillation crystal-photodiode

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

But the space between each detector is big, so the overall detection efficiency may only be 50 %. They give dose to patient but do not contribute to the image.

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-krypton mixture)

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

The gantry

Collimators: Required for the same reason as conventional x-ray. Correct collimation reduces patient dose and improves image quality due to less scattered 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 the patient dose.

The gantry

Improper adjustment of this collimator is the cause of most 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.

The gantry

High-voltage generator; All CT scanners operate on three-phase or

high-frequenzy power. Most manufactors built them into the gantry

or by mounting on the rotating wheel of the gantry. It reduces the amount of space needed, and winding and unwinding a power cable is unnecessary.

The gantry

Patient positioning and support couch; It has to be made of a material with a low

atomic number (carbon fiber) so that is does not interfere with x-ray beam transmission and patient imaging.

It should move smoothly for accurate patient positioning, and is especially important for spiral CT

Computer

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

Depending on the format the computer has to do up to 250 000 equations at the same time!

In the computer there is a microprocessor and a primary memory. These determine the reconstruction time= the time from end of scanning to image appearance.

Array processors are becoming more common. They are faster than the microprocessor and can reconstruct an image in less than 1 s.

Operating console

Many CT scanners have 2 or 3 consoles. One for the CT radiologic technologist to operate the

scanner. One for an other technologist to postprocess images. One for the physician to view the image, manipulate

contrast, size and general visual appearance. A typical operating console contains controls and

monitors for the various technique factors.

Image characteristics

With CT, the x-rays form a stored electronic image that is displayed as a matrix of intensities.

The CT scan format consists of many cells with its own number which is shown as a brightness level.

A matrix of 512 x 512 = 262 144 cells of information.

Image characteristics

Each cell is a pixel (picture element) The numerical information in each pixel is a

CT number/ Houndsfield Unit (HU) It is a two dimensional representation of a

corresponding tissue volume. The diameter of image reconstruction is

called the field of view (FOV)

Image characteristics

When the FOV is increased for a fixed matrix ( for example: 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 size grows smaller.

Pixel size = FOV/matrix size The tissue volume is known as a voxel (volume

element) Voxel size= Pixel size x slice thickness

CT numbers

Each pixel is displayed on the video monitor as a level of brightness and on the photographic image as a level of optical density.

The levels correspond to a range of CT numbers from -1000 up to +1000 for each pixel.

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

CT numbers

The CT number is related to the x-ray attenuation coefficient of the tissue contained in the voxel.

Remember: the degree of x-ray attenuation is determined by the avarage energy of the x-ray beam and the effective atomic number of the absorber and is expressed by the attenuation coefficient.

CT numbers

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

The screen only shows 32 grayscales.

Image reconstruction

Filtered back projection = all the projections during on CT examination is stored in the computers memory, and the reconstructions are made by these.

With filter we do not mean a metal filter as in the tube of the x-ray, but it is a mathematical function. A difficult one!

Image reconstruction

In CT there over 250 000 pixels to reconstruct from, that means that the machine has to solve 250 000 equations to find the solutions for the images.

Image quality

Spatial resolution Contrast resolution Noise Linearity Uniformity

Spatial resolution

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

The degree of blurring is a measure of spatial resolution of the system and is controlled by several factors.

If you take a scan over an area that has a high contrast interface, for example the brain and the skull, the image will be blurred.

The system will fix some of the blurring, and smoothen the picture.

Spatial resolution

This, however, reduses the spatial resolution because of some features of the scanner.

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

The detector size and design of prepatient and postpatient collimation affect the level of scatter radiation and influence the spatial resolution by affecting the contrast of the system.

Also the x-rays’ focal spot has influence on spatial resolution.

Contrast resolution

Contrast resolution = the ability to distinguish one soft tissue from another without regard for size or shape.

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

Contrast resolution

Imagine a scan over abdomen, where you have spine, liver and fat. The atomic numbers are different, but in conventional x- ray it is difficult to seperate them. With CT and the CT numbers it makes it easy! With HU the CT can amplify these contrast differences, and make the contrast high. Then we can cleary see differences between tissue.

Noise

Noise= the precentage of standard deviation of a large number of pixels obtained with a water-bath scan.

Noise depends on the following factors:1: kilovolt peak filtration2: Pixel size3: Slice thickness4: Detector efficiency5: Patient dose

Noise

Example:If you scan a homogeneous medium like water

the pixel value should be zero. But because the system is not perfect some pixel values will be both higher and lower than zero. These variations in HU will show in the image as graininess, and is what we call noise. The larger the variations in pixel value, the more noise you get in the image.

Linearity

The CT must be calibrated frequently so that the HU are correct.

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 it’s a sign of

malfunction of the CT. It may not show on the visual image, but could greatly affect

quantitative analysis of tissue, the determination of tissue composition based on CT number.

Uniformity

When you scan a uniform object (water) the pixel value should be zero (for water!).But since the machine is very complicated mechanically this does not happen. The value may drift from day to day/hour by hour.

If it is scanned, and the pixel value is constant in all regions of the reconstructed image, this is called Spatial uniformity

Uniformity

There is also a test for this, where you scan a bucket of water and plot the numbers along an axis of the image. If this axis is within 2 standard deviations of the mean value, the system has acceptable spatial uniformity.

Because of the x-ray beam hardening, there may be a decrease of CT numbers, so the middle of the image is darker than it should.

This is called ”cupping” artefact.

Summary

The collimated x-ray beam is directed to the patient.

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

The signal from the detector array is measured by a computer.

The image is reconstructed in the computer. The image is displayed on a TV monitor.

Summary

CT makes transverse images (axial images) The internal structures of the body attenuate the x-

ray beam according to their mass density and atomic number.

All data are processed in digital form. The resulting computer image is an electronic matrix

of intensities. Matrix size is generally 512x512 individual cells or

pixels.

Summary

In each pixel is numerical information called a CT number or HU.

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

The voxel (volume element) is determined by multiplying the square of the pixel size by the thickness of the CT scan slice.

HU -1000=air, 0= water, 1000= dense bone

Summary

The CT scanner has exellent contrast resolution 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 the system.

System noise is determined by the numbers of x-rays used by the detector array to produce the image.

Questions...

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