Physical Principles of CT

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Goal of CT
Minimal superimpostion
Better image contrast
Visualize small differences in tissue contrast
Data Aquisition
Collections of information from the patient to produce the CT image
Axial Aquisition
Slice by Slice
Tube rotates around patient
Data is collected from a single slice
Tube stops
Patient is moved to next location
Volume Data Aquisition
Helical or spiral scanning
Tube rotates around patient and traces a spiral path
Volume of tissue at a time
Attenuation measurements are sent to the computer as raw data
Large number of transmission measurements are needed to reconstruct the CT image
Attenuation
The reduction of intensity of the beam as it passes through an object, either through absorbtion or scatter
Homogenous beam
All photons have the same energy
Known as monochromatic or monoenergeric
Used in Houndsfield fist CT scanner
Quality of the beam does not change
Takes longer to produce
Heterogenous Beam
Photons have different engery values
Called a polychromatic beam
Used in CT tubes now
As the beam passes though equal thickness the attenuation quality and quantity changes
Mean kV will increase because of teh low engery photons being absorbed
Attenuation of x-rays
Compton Scatter
Photoeletric Effect
Compton Scatter
Photon is deflected to a new direction
Retains part of its original energy
Occurs in soft tissue
Photoelectric Effect
Photon energy is completely absorbed by an atomic electron that is ejected from the atom
Occurs in bone
Data Aquisition Geometries
Way the x-ray tube and detectors are arranged
Data Processing
1. Raw data undergoes corrections and reformatting
2. Reconstruction - changes the scan data to a digital image
Reconstruction algorithm is a mathematical procedure used to change raw data into different images
Each pixel in a reconstructed image is given a ____?
CT number or Hounsfield units
Related to the linear attenuation coefficients
Linear attenuation coefficients
The probability per cm thickness of matter and x-ray photon will be attenuated
Water
0
Bone
1000
Air
-1000
Lungs
-550 to -650
Fat
-80 to -100
Kidneys
20 to 40
Pancreas
30 to 50
Blood
50 to 60
Liver
50 to 70
Linear attenuation coefficients is affected by _____?
Energy of the radiation
kV changes the coefficients, coefficients change the CT number
Why is high kV used in CT?
Reduce the dependence on penetrating photons
Reduce the contrast of bone relative to soft tissue
Produce high radiation flux at detector
Ensure optimum detector response
Resolution for a HR monitor?
HR monitor is 2048 x 2048
Windowing
Process of changing the CT image gray scale
Total of 2000 numbers
Window Width
Range of numbers
Contrast
Window Level
Center of the range
Brightness
Format of the CT Image
Matrix size and scan FOV is chosen by the tech before the CT
Display FOV can equal or be less than the scan FOV
Pixel size is 1 to 10mm
Voxel size depends on matrix size and FOV size
Isotropic Imaging
Voxel dimensions of length,width, and height are equal
A perfect cube
Each pixel in the CT image can have a range or ___?
Gray shades
Numerical value of the pixel represents the brightness
Data Flow in a CT Scanner
1. Tube rotates around the patient with a highly collimated beam directed through an area of interest
2.Radiation is attenuated by the patient and the photons are measured by 2 detectors. Reference detector which measures the radiation from the tube and another detector which measures the thransmission through the patient.
3. The transmitted and reference beam are converted to electrical current signals and then amplified
4.The data is converted to digital by the ADC
5.Data processing begins
6. Convolution is performed on the data by an array processor
7.Reconstruction algorithms reconstruct the image
8.Image is displayed or stored
9.Image processing allows post processing operations on displayed images
10. Control terminal is usually an operator's control console, which completely controls the CT system
Advantages of CT
Excellent low contrast resolution
-highly collimated beam
-more sensitive detectors
-contrast resolution is 4
More advantages of CT
Window and leveling
Volume data aquisition in a single breath
Limiatations of CT
Spatial resolution (lpmm) poorer than radiography
Dose is higher
Difficult to image areas of tissue surrounded by dense areas of bone
Metal objects produce streak artifacts
Other artifacts