Computed Tomography :Basic Principle and its generations

Computed Tomography :Basic Principle and its generations

Presented by :Rituraj Mishra.B.Sc.MIT, 2013Moderator: Mr. Ranjit Kumar Jha

INTRODUCTIONComputed Tomography is a well accepted imaging modality for evaluation of the entire body.Computed Tomography(CT) Scan Machines Uses X- rays, a powerful form of Electromagnetic Radiation.The images are obtained directly in the axial plane of varying tissue thickness with the help of a computer.Some pathology can be seen in saggital or coronal plane by reconstruction of the images by computer.CT has undergone several evolutions and nowadays multi- detectors CT scanners have been evolved which have better application in clinical field.

COMPARISION OF CT WITH CONVIENTIONAL RADIOGRAPHY

Conventional radiography suffers from the collapsing of 3D structures onto a 2D image.

CT gives accurate diagnostic information about the distribution of structures inside the body.

COMPARISION OF CT WITH CONVIENTIONAL RADIOGRAPHY. A conventional X-ray image is basically a shadow.Shadows give you an incomplete picture of an object's shape.

This is the basic idea of computer aided tomography. In a CT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles

Imagine you are standing in front of a wall, holding a pineapple against your chest with your right hand and a banana out to your side with your left hand. Your friend is looking only at the wall, not at you. If there's a lamp in front of you, your friend will see the outline of you holding the banana, but not the pineapple -- the shadow of your torso blocks the pineapple. If the lamp is to your left, your friend will see the outline of the pineapple, but not the banana. In order to know that you are holding a pineapple and a banana, your friend would have to see your shadow in both positions and form a complete mental image. This is the basic idea of computer aided tomography4

Comparison of CT with Conventional Radiography Radiographic procedure is qualitative and not quantitative

Differential absorption of X ray is recorded in film so qualitative measurement and differential absorption of x rays are recorded by special detectors so quantitative.(minute differences.)5

ADVANTAGE OF COMPUTED TOMOGRAPHY OVER CONVIENTIONAL RADIOGRAPHY.

To overcome superimposition of structures.To improve contrast of the image.To measure small differences in tissue contrast.

TOMOGRAPHYImaging of Layer/Slice.Principle Images of structures lying above and below the plane are blurred out due to motion unsharpness while the structures lying in plane of interest appear sharp in in the image.

Tomography

PRINCIPLE OF COMPUTED TOMOGRAPHY

The internal structure of the object can be reconstructed from multiple projections of the object.Mathematically principle of CT was first developed in 1917 by Radon.Proved that image of unknown object could be produced if one had several number of projections throughout the object.

VARIOUS PARAMETERS OF CTSLICEMATRIXPIXELVOXELCT NUMBERWINDOWINGWINDOW WIDTHWINDOW LEVELPITCH

SLICE/CUTThe cross section portion of body which is scanned for production of CT image is called Slice.The slice has width and therefore volume.The width is determined by width of the x rays beam.

Cross Sectional SlicesThink like looking into the loaf of bread by cutting into the thin slices and then viewing the slice individually.

MATRIXThe CT image is represented as the Matrix of the number.A two dimensional array of numbers arranged in rows and columns is called Matrix.Each number represent the value of the image at that location.

PIXELEach square in a matrix is called a pixel.Also known as picture element.

VOXELEach individual element or number in the image matrix represents a three dimensional volume element in object called VOXEL.

CT NUMBERThe numbers in the image matrix is called CT NUMBER.Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object.

HOUNSFIELD UNITS(HU)

Related to different composition and nature of Tissue.The CT NUMBER is also known as Hounsfield units(HU).Represent the density of tissue.Different Tissue have different CT number Range in HU.

Air- 1000Fat-100Pure water 0CSF15White matter45Gray matter40Blood20Bone/calcification+1000

TISSUE AND CT NUMBER APPROXIMATE

WINDOWING is a system where the CT no. range of interest is spread cover the full grey scale available on the display system WINDOW WIDTH Means total range of CT no.values selected for gray scale interpretation. It corresponds to contrast of the image.WINDOW LEVEL represents the CT no. selected for the centre of the range of the no. displayed on the image. It corresponds to brightness of image .

PitchThe relationship between patient and tube motion is called Pitch.It is defined as table movement during each revolution of x-ray tube divided by collimation width.For example: For a 5mm section, if patient moves 10mm during the time it takes for the x-ray tube to rotate through 360, the pitch is 2.Increasing pitch reduces the scan time and patient dose.

STEPS OF CT IMAGE FORMATION

21

Phase of CT scanning1.Scanning the patient or data Acquisitiona)X-ray Generatorb)X-ray Tubec)X-ray Filtration Systemd)Detector System

2.Reconstructiona)Simple back projectionb)Iterative methodc)Analytical method

3.Display

DATA ACQUISTION

The scanning process begins with data acquisitionData Acquisition refers to a method by which the patient is systematically scanned by the X ray tube and detectors to collect enough information for image reconstruction

Major components of Data Acquisition System(DAS)a)X-ray GeneratorsGenerators are located on rotating scan frames within the CT gantry to accommodate slip Ring.Power: 50 to 80kwFrequency: 5 to 50kHzKVp: 80-120mA:80-500

b) X-ray TubeRotating anode x-ray tube with unique cooling.Small focal spot size (0.6mm) to improve spatial resolution.Anode heating capacity:1MHU to 7MHU Cooling rate:1MHU per minute.

c)X-ray Beam Filtration SystemCT employs monochromatic beam but radiation fromCT X-ray tube is polychromatic. so, X-ray beam is shaped by compensation filter.a)Pre patient Collimators: Reduces the patient dose.b)Post patient Collimators: Reduces the scattered radiation detectors.

Overall Functions ofCollimators.

To decrease scatter radiationTo reduce patient dose To improve image qualityCollimator width determines the slice thickness

d)DetectorsThe detectors gather information by measuring the x-ray transmission through the patient.Two types:Scintillation crystal detector(Cadmium tungstate+ Si Photodiode) Can be used in third and fourth generation scannersXenon gas ionisation chamber Can be used in third generation scanners only

2)ReconstructionReproduction of an image from raw data is called Reconstruction.A)Simple back projectionThe image is created by reflecting the attenuation profiles back in same direction they were obtained.

B)Iterative methodIt start with assumption that all point in matrix have same value and it was compared with measured value and make correction until Values come with in acceptable range.It contain three correction factor1. SIMULTANEOUS RECONSTRUCTION2. RAY BY RAY CORRECTION3. POINT BY POINT CORRECTION

C)Analytical Method

Today commonly used .

Two popular method used in that method are:-

1. 2-D FOURIER ANALYSIS

2.FILTERED BACK PROJECTION

2-D FOURIER ANALYSISIn it any function of time or space can be represented by the sum of various frequencies and amplitude of sine andcosine waves.For example the actual projected image of original object is more rounded than those shown which would be slowly simplify and corrected by Fourier transformation.

2-D FOURIER ANALYSIS

FILTERED BACK PROJECTIONSame as back projection except that the image is filtered, or Modified to exactly counterbalance the effect of sudden density Changes , which cause blurring(star like pattern) in simple back projection.

3)DisplayThe reconstructed image is displayed on the monitor. It is a digital image.It consists of 2D representation of 3D object in the form of pixels.CT pixel size is determined by dividing the FOV by matrix Size which is generally 512*512.

PIXEL SIZE= FOV (mm)/ MATRIX SIZE

Generations of CT ScanFirst Generation Narrow pencil beamSingle detector Detector used is made up of NaI.Translate Rotate movements of Tube- detector combinationScan time-5mins.Designed only for evaluation of brain.

First generation CT Scanner

Head kept enclosed in a water bathPaired detectorsA reference detector

Second Generation CT Scanner

Second GenerationNarrow fan beamLinear detector array(5 to30)Translate-Rotate movements of Tube-Detector combinationFewer linear movements are needed as there are more detectors to gather the data.Between linear movements, the gantry rotated 30oScan time~30secs(advantage over first generation)

Third Generation

Rotate(tube)Rotate(detectors) Motion.Pulsed wide fan beam.Arc of detectors(600-900)Detectors are perfectly aligned with the X-Ray tubeBoth Xenon and scintillation crystal detectors can be usedScan time< 5secsDisadvantage: Ring Artifacts due to electronic drift between many detectors.

Fourth Generation

Fourth GenerationComplete circular array of about 1200 to 4800 stationary detectorsSingle x-ray tube rotates with in the circular array of detectorsWide fan beam to cover the entire patientScan time of newer scanners is about s or,