…….CT Physics - Continued V.G.WimalasenaPrincipal School of radiography

Correction for star artifacts - Filtering A filter function is applied to each point along the attenuation profile to eliminate these artifacts A filter function is applied to each point along the attenuation profile to eliminate these artifacts Different filter functions are used to create sharper (higher resolution) or smoother (lower noise) images Different filter functions are used to create sharper (higher resolution) or smoother (lower noise) images Attenuation profile Filter function Filtered profile

Convolution? The process of applying the filter function to the attenuation profile is called CONVOLUTION The process of applying the filter function to the attenuation profile is called CONVOLUTION (Super imposition of the filter function and the attenuation profile)

Result of convolution The convolution minimizes the artifacts by changing the back projected information. The convolution minimizes the artifacts by changing the back projected information.

The dark and white bands in the illustration add up in such a way as to yield an image which accurately represents scanned object The dark and white bands in the illustration add up in such a way as to yield an image which accurately represents scanned object Impulse signals from linear traverses following angular rotations 1 st back projection 2 nd back projection 3 rd back projection Final back projection

Practical method of Generation of Attenuation profiles Attenuation profiles are a recording of X-ray attenuation verses position in the object. Attenuation profiles are a recording of X-ray attenuation verses position in the object. Attenuation is related to the ratio between incident intensity (I 0 ) and transmitted intensity (I t ) Attenuation is related to the ratio between incident intensity (I 0 ) and transmitted intensity (I t ) I0I0 ItIt

X-ray intensity is measured by a DETECTOR which converts x-ray photons to electrical current X-ray intensity is measured by a DETECTOR which converts x-ray photons to electrical current The reference (Input) detector measures the incident intensity (I 0 ) The reference (Input) detector measures the incident intensity (I 0 ) The output detector measures the transmitted intensity (I t ) The output detector measures the transmitted intensity (I t ) I 0 = incident intensity = tube output ItIt

I 0 should remain constant I 0 should remain constant I t will change depending on the attenuation of the object scanned (e.g. centered, round object of uniform density) I t will change depending on the attenuation of the object scanned (e.g. centered, round object of uniform density) I o =1000 I t =1000 ItIt

Calibration profile If the object scanned is a centered, round water bath each attenuation profile is called a CALIBRATION PROFILE. and the set of profiles are called the Calibration File or Cal file If the object scanned is a centered, round water bath each attenuation profile is called a CALIBRATION PROFILE. and the set of profiles are called the Calibration File or Cal file

DATA Calibration Subtraction of calibration file from the attenuation profiles of the object called data calibration Subtraction of calibration file from the attenuation profiles of the object called data calibration (consider an object containing two areas of different densities as shown) (consider an object containing two areas of different densities as shown)

Scan Data File The difference profile are stored as numerical values as a function of position in the profile. The difference profile are stored as numerical values as a function of position in the profile. This data is used in the image reconstruction process This data is used in the image reconstruction process The set of difference profiles for a complete scan is called SCAN DATA FILE or raw data The set of difference profiles for a complete scan is called SCAN DATA FILE or raw data

Back projection of scan data 1. Back projection of the corrected attenuation profiles is accomplished by feeding the numerical values of each point along the profile into a matrix. (Since the scanning motion is circular, the matrix is usually round) --

2. Close rays are averaged to attain a pixel value from the ray that passes closest to its center 3. Every pixel is assigned one value per view. 4. The final value is the sum of these divided by the number of views (as shown)

6. When all the measurements have been projected back into the matrix and all pixel values determined, the image can be displayed

7. The number of pixels in the image corresponds to the MATRIX SIZE. This is given as 256 x 256, 320x 320, 512 x 512. this refers to the number of pixels across a vertical or horizontal diameter of the image. This is given as 256 x 256, 320x 320, 512 x 512. this refers to the number of pixels across a vertical or horizontal diameter of the image.

8. The actual size of a pixel is equal to the size of the area scanned, called the FIELD OF VIEW (FOV) divided by the matrix size. ( Small pixel sizes helps to display small objects that can be resolved by the system) ( Small pixel sizes helps to display small objects that can be resolved by the system)

CT Numbers CT numbers are calculated from the measured attenuation values using the equation given below. CT numbers are calculated from the measured attenuation values using the equation given below.

Range of CT numbers Tremendous range of attenuations that might be displayed in a CT image requires a large number scale. +/ is commonly used and some systems have extended this range further on the high end to differentiate bone densities Tremendous range of attenuations that might be displayed in a CT image requires a large number scale. +/ is commonly used and some systems have extended this range further on the high end to differentiate bone densities

Limitation in display of CT numbers The human eye can distinguish only about 20 levels of gray in the gray scale and cannot appreciate the large range of information contained in CT scan The human eye can distinguish only about 20 levels of gray in the gray scale and cannot appreciate the large range of information contained in CT scan This is solved by using the ‘window width’ & ‘window level’ This is solved by using the ‘window width’ & ‘window level’

Window width This is the range of CT numbers that is displayed using the useful levels of gray scale This is the range of CT numbers that is displayed using the useful levels of gray scale Small window separates one CT number from another Small window separates one CT number from another Large window condenses more than one CT number in each of gray level Large window condenses more than one CT number in each of gray level

Window level This is the centre of the range of CT numbers displayed by the window This is the centre of the range of CT numbers displayed by the window The level control moves the visible gray scale up and down the CT number scale The level control moves the visible gray scale up and down the CT number scale

Hounsfield unit system & CT numbers As an honour to Hounsfield who was the pioneer of invention of CT the CT numbers are called Hounsfield units. As an honour to Hounsfield who was the pioneer of invention of CT the CT numbers are called Hounsfield units. The range of Hounsfield units Is from to The range of Hounsfield units Is from to Standard reference points are for Air and 0 for water. Standard reference points are for Air and 0 for water.

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