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
