Computerized Tomography Process Flashcards
Phases of CT imaging
- Scanning the patient
- Data Acquisition
i. Tube or tube and detector move
ii. Multiple attenuation measurements are taken around the object - Image reconstruction
- Image Display
- Image archival (recording)
stopping of x-rays with transfer of energy
Absorption
deflection of x-rays
Scatter
No. of x-ray photons falling on an object
Incident Intensity
No. of photons passing through
Transmitted Intensity
obtained by measuring and comparing the incident and transmitted intensities
degree of attenuation
cross sectional portion of the body which is scanned for the production of CT image
slice
The slice has width and therefore _____
volume
determined by the width of the x-ray beam
width
The image is represented as a _____ of numbers.
MATRIX
A two dimensional array of numbers arranged in rows and columns.
Matrix
Each individual element or number in the image matrix represents a three dimensional volume element in the object,
VOXEL
The VOXEL is represented in the image as a two-dimensional element called
PIXEL - (picture element
The numbers in the image matrix
CT numbers
Each ____ has a number which represents the x-ray attenuation in the corresponding voxel of the object
pixel
To obtain a visual image, the CT numbers are assigned
different shades of gray on a gray scale.
Basic components (Data Acquisition)
X-ray tube
Collimators
Detector/s
____ traverses the object and enters the detecto
Collimated x-ray beam
determined from the difference between incident intensity and transmitted intensity
attenuation
In the basic CT system the x-ray tube and detector are translated ____ so that the beam scans the object
linearly
Imaginary line between Tube & Detector
Ray
Attenuation along a Ray
Ray Sum
The set of ray sums in one direction
View
The attenuation for each ray sum when plotted as function of its position is called
attenuation profile
Attenuation of objects with different densities will change the attenuation profile
Object with low attenuation
Object with high attenuation
after a view is recorded, the tube and detector rotate a small angle and the entire process is repeated until many views have been recorded for the same slice
translate –rotate CT
The image is created by reflecting the attenuation profiles back in the same direction they were obtained
BACK PROJECTION
The resultant image closely resembles the original object but it shows star shaped patterns around objects and streaks
‘Star’ and ‘streak’ artifacts
applied to each point along the attenuation profile to eliminate these artifacts
filter function
used to create sharper (higher resolution) or smoother (lower noise) images
Different filter functions
process of applying the filter function to the attenuation profile
CONVOLUTION
(Super imposition of the filter function and the attenuation profile)
CONVOLUTION
minimizes the artifacts by changing the back projected information
convolution
stored as numerical values as a function of position in the profile.
This data is used in the image reconstruction process
difference profile
set of difference profiles for a complete scan
SCAN DATA FILE or raw data
The actual size of a pixel is equal to the size of the area scanned
Field of View
How many levels of gray can the human eye distinguish
20 levels of gray
How is the low distinguishing power of the human eye solved for viewing of CT images
Window width and Window level
Range of CT numbers that is displayed using the useful level of gray scale
Window width
separates one CT number from another
Small window
condenses more than one CT number in each of gray level
Large window
Center of the range of CT numbers displayed by the window
Window level
The level control moves the visible gray scale up and down the CT number scale
Window level
As an honour to Hounsfield who was the pioneer of invention of CT the CT numbers are called
Hounsfield units
range of Hounsfield units Is from
-1000 to +3000
Standard reference points are -1000 for
Air
Standard reference points are 0 for
Water
