25. CT: IMAGE FORMATION Flashcards
1
Q
- What is CT (Computed Tomography)?
A
- it is a volumetric imaging modality
- it is based on X-Ray Absorption
2
Q
- When was CT introduced into medicine?
A
- in the early 1970s
3
Q
- What does CT allow for?
A
- it allows for the reconstruction of a 2D or 3D absorber
map
4
Q
- In which sense it CT superior to X-Ray imaging?
A
- the contrast in the soft tissue of the images is superior
5
Q
- In which sense is X-Ray imaging superior to CT?
A
- the spatial resolution of the images is superior
6
Q
- Who invented the CT?
A
- Sir Godfrey Hounsfield
- he received the Nobel Prize in Medicine in 1979
7
Q
- Where did the first clinical CT scan take place?
A
- it took place in Atkinson Morley’s Hospital
- in London, England
- it took place on the 1st October 1971
8
Q
- What essential role does the computer play in CT scans?
A
- the computer is essential for Image Reconstruction
- this is the first ever imaging modality to be this way
9
Q
- What adjective can describe the rate at which Modern Clinical CT scanners work?
A
- these scanners are very fast
- they can produce a 2D cross-sectional image in less
than one second
10
Q
- How costly are CT scanners?
A
- they are very expensive
- this ranges into the millions
- there is a relatively high cost per CT scan
11
Q
- Why do Radiologists often take two perpendicular projections?
A
IN STANDARD X-RAY PROJECTION IMAGES:
- the exact location of an area of interest cannot be
determined
12
Q
- Which are the names of the 2 perpendicular projections?
A
- Lateral
- Anterior-Posterior (AP)
13
Q
- What does Data Acquisition refer to?
A
- it refers to the collection of X-Ray transmission
measurements - this is done through the patient
14
Q
- What kind of source does Data Acquisition require?
A
- it requires an X-Ray source
- this X-Rays move in a parallel manner
- they combine into the shape of a fan or a cone
15
Q
- What does this image show?
A
- a possible geometry for the CT scanner source and
detectors
BOTH THE SOURCE AND THE ARC-SHAPED DETECTOR:
- rotate in a tandem
- they record the projections through a single plane
within the body
- this brings about many different angles
16
Q
- What happens when an X-Ray beam passes through an object?
A
- some of the photons are absorbed or scattered
17
Q
- What does the reduction of X-ray transmission (attenuation) depend on?
ATTENUATION: reduction of the effect or force
A
- THE ATOMIC COMPOSITION OF THE CROSSED
TISSUES - THE DENSITY OF THE CROSSED TISSUES
- THE ENERGY OF THE PHOTONS
18
Q
- What happens after an X-Ray beam passes through an object?
A
- the partially attenuated X-Rays are collected
- they are collected by the X-Ray detectors on the
opposite side
19
Q
- What happens when these X-Rays are collected by the X-Ray detectors?
A
- they are converted from X-Ray photons
TO electrical signals
20
Q
- What happens to these electrical signals?
A
- they are converted into digital data
- the attenuation value is then calculated
- these attenuations values are converted into different
shades of grey - this results in contrast in the final image
21
Q
- What happens while the X-Ray tube and detectors are rotating around the patient?
A
- many projections are being collected
- they are being collected from consecutive angular
orientations
22
Q
- What do these images show?
A
- they show the Filtered Back
Projection Method - this is the most common method to construct the
image from the Data Acquisition - the forward projection
- the detectors record these projections during the scan
- there are 5 absorbing regions in the horizontal plane
- grey colour indicated absorbtion
- white indicated no absorption
23
Q
- What happens at the Forward Projection?
A
- the detectors record these projections during the scan
- there are 5 absorbing regions in the horizontal plane
- the grey colour indicates that there is absorption
- the white colour indicates that there is no absorption
24
Q
- What happens at the Back Projection?
A
- we will measure the absorption projections
- we measure each region that has overlaps
- these numbers are translated into shades of grey
25
Q
- Explain the meaning of the different colour shades and number in this image.
A
HIGH NUMBERS OF ATTENUATION:
- translate as white
- they mean that the object is very dense
- it absorbs many of the photons
(high absorption)
- very few photons reach the detector
LOW NUMBERS OF ATTENUATION:
- translate as greys
- they mean that the object is not as dense
- it does not absorb many of the photons
(low absorption)
- many more photons reach the detector
26
Q
- What is the aim of a CT?
A
- the aim is to obtain a spatially resolved map of
absorption coefficients in one slice - this slice comes is of the patient’s body
27
Q
- What happens when a resolved map of absorption coefficients in samples at a finite resolution?
A
- it provides an image
28
Q
- What is happening in this image?
A
- we have obtained four different projections
- we then determine the overall attenuation
29
Q
- What is happening in this image?
A
- WE HAVE 4 ATTENUATED BEAMS
- I₁, I₂, I₃, I₄
- EACH PROJECTION FOLLOWS LAMBER BEER’S LAW
- this allows us to obtain a linear equation system
- this helps us to solve for µ₁, µ₂, µ₃, µ₄ - THE SIMPLE EQUATION SYSTEM CAN ONLY BE
SOLVED:
- when one of the Absorption Coefficient is known
30
Q
- What does this image show?
A
AN ARBITRARY OBJECT IS FORMED:
- it is composed of n-by-n
- there are also different materials that require n²
independent equations
- this is solved by taking more projections at different
angles
