L6.1. CT: Image Formation and Reconstruction Flashcards
What is CT?
Computed Tomography
A volumetric imaging modality based on X-ray absorption
CT allows for what?
Reconstruction of a 2D or 3D absorber map
Who invented CT?
Sir Godfrey Hounsfield
At the laboratories of EMI Limited
What does ‘Data Acquisition’ refer to?
The collection of X-ray transmission measurements through the patient
Requires an X-ray source that produces an X-ray beam, which is collimated into the shape of a fan or cone
Aim of CT
Obtain a specially resolved map of absorption coefficients in one slice of the patient’s body
The map, if sampled at a finite resolution, gives an image
The reconstruction of images from the X-ray measurements involve..
Various steps
The result is a 2D matrix of preselected size and detail
CT vs Conventional plain film radiography
CT has better contrast
CT has worse spatial resolution
When was the first CT scan of a patient?
1st October, 1971
Atkinson Morley’s Hospital, London.
CT is the first imaging modality where the what is essential to the image reconstruction?
The computer
How fast can a 2D cross-sectional image be produced?
Less than a second
How much do Clinical CT scanners cost?
Millions
-> relatively high cost per CT scan
In a projection image, like a standard X-ray projection image, what cannot be determined?
What is done to solve this?
The exact location of interest
Radiologist often take 2 perpendicular projections
Eg lateral and AP
AP
Anterior-posterior
One possible geometry for CT scanner source and detectors
Arc shaped-detector
Source and detector rotate in tandem
Recording projections through a single plane within the body
-> for many different angles
When an x-ray beam passes through an object, what happens?
Some photons are absorbed or scattered
X-ray attenuation/transmission
Reduction of x-ray attenuation depends on?
Atomic composition of the crossed tissues
Density of the crossed tissues
Energy of the photons
What happens to the partially attenuated x-rays after passing through an object?
They are collected by x-ray detectors on the opposite side
Then converted from x-rays photons -> electrical signals
Signals then converted into digital data, then attenuation value is calculated
S1
µ1 + µ2
S2
µ3 + µ4
S4
µ1 + µ3
S3
µ2 + µ4
What law does each projection follow?
Lambert Beer’s Law
We obtain a linear equation system due to this that we can solve for µ1, µ2, µ3, µ4
Attenuated beams symbols
I1, I2, I4, I5
When can the simple equation system be solved?
When one of the absorption coefficients is known
Image reconstruction steps
- Measurements
- Pre-processing
- Raw Data
- Filtering
- Filtered Data
6.1. Interpolation
6.2. Back-projection
- Axial Source images
What is the selected ‘field of view’ divided into?
Small image elements aka pixels
What do pixels that make up each cross-sectional represent?
Small volume of tissue
Called a voxel (volume element)
Density value of each pixel depends on
The composition of the tissue it represents
Expressed in Hounsfield Units (HU)
How are HU calculated?
From the attenuation measurements relative to the attenuation of water
HU range
-1024 to +3071 HU
Lung HU
-500
Fat HU
-100 to -50
CSF HU
+15
Blood HU
+30 to +45
Muscle HU
+10 to +40
Adrenal tumor HU
Less than +10
White matter HU
+20 to +30
Grey matter HU
+37 to + 45
Liver HU
+40 to +60
Bone HU
+700 to +3000
Air HU
-1000
