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MRS432- CT > week 2 lec 1: history > Flashcards

week 2 lec 1: history Flashcards

1
Q

why was CT introduced?

A

limitations of general x-ray: - superimposition of structures
- need a large difference in attenuation to visualise changes in density

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2
Q

what is a tomographic image

A

a “slice” of the patient’s anatomy; generally in the axial plane

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3
Q

what is a pixel and what is a CT pixel

A

The smallest element of an image that can be individually processed in a video display system
- CT pixel is the average x-ray attenuation properties of the tissue in the corresponding voxel

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4
Q

what is a voxel and how do we calculate it

A

voxel is the 3D analogue of a pixel

FOV and matrix size/ by pixel size (XY) ≈ voxel size (slice thickness) of the reconstructed image

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5
Q

what is anisotropic voxel and isotropic voxel

A

anisotropic; not a perfect cube
if slice thickness not equal to 0.5mm
isotropic; perfect cube
slice thickness 0.5mm

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6
Q

how do voxels impact the image

A

The smaller the voxel size, the greater (or higher) spatial resolution

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7
Q

what is required for a CT scan to be performed

A
  • X-rays: generator, x-ray tube, collimator
  • Detectors: for conversion of x-rays to electrical signal
  • Analogue to Digital converter (ADC): for converting electrical signal into digital values (data)
  • Computer: data reconstruction, image/ data storage
  • Monitors: image display
  • Gantry, table, patient
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8
Q

what is data acquisition

A

-Sampling ‘real life’ signals and converting these into digital numeric values for processing; the collection of x-ray transmission measurements from the patient / object

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9
Q

what happens in the data acquisition phase

A
  • An x-ray tube produces photons that pass through the patient; attenuation
  • The attenuated beam exits the patient with a series of electronic detectors; this information is recorded as a slice profile
  • X-ray tube and detectors rotate through at least 180 degrees
  • At each angle, data is collected by the detectors (forms the slice profile)
  • Intensity values are converted to digital values(ADC)
  • Slice profiles typically are “filtered”
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10
Q

what is image reconstruction

A

-Converts raw data (slice profiles) to image data and utilises image reconstruction algorithms

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11
Q

what is image display

A
  • CT images are displayed on a monitor

- Require altering of the displayed contrast: done through the use of window width (WW) and window level (WL)

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12
Q

3 types of post-processing and what happens after this

A
  • edge enhancement, multi-planar reformations, 3D reconstructions
  • Sent to PACS for storage, reporting, etc.
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13
Q

what are some characteristics of the 1st experimental CT scanner?

A

ACTA CT system
Used gamma ray source, 9 day image acquisition, 3 hours for image reconstruction
The first CT system that could make images of any part of the body, and did not require the “water tank”

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14
Q

what factors affect spatial resolution (8)

A
  • Field of view
  • Pixel size
  • Focal spot size
  • Motion of the patient
  • Pitch
  • Kernel (algorithm)
  • Slice thickness
  • Detector size
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15
Q

how many generations of CT scanners are there and what are the most commonly used ones now?

A

5 generations

- 3rd generation

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16
Q

What are 5 characteristics of the first generation: rotate/ translate, translation/rotation CT scanner

A
  • Translate / Rotate using a “pencil” beam
  • Only 1-2 x-ray detectors used in the Z direction (two different slices)
  • Translated linearly to acquire 160 rays across a 24cm FOV
  • Rotated slightly between translations to acquire 180 projections at 1-degree intervals
  • About 90sec- 4.5 minutes/scan with 1.5 minutes to reconstruct slice
  • Detector signal decayed slowly, affecting measurements made temporally too close together
  • Pencil beam geometry allowed very efficient scatter reduction; best of all scanner generations
  • Due to time taken; almost exclusively focussed on neurological examination
17
Q

what are 4 characteristics of the 2nd generations: translate/ rotate (fan beam) CT scanners

A
  • Still translate/rotate; but rotation now 3–4° instead of 1°
  • Image quality improved
  • Scan time decreased; about 18sec for single slice
  • Incorporated linear array of 30 detectors
  • More data acquired to improve image quality (600 rays x 540 views)
  • Narrow fan beam allowed more scattered radiation to be detected: scatter in X / Y direction
18
Q

list 2 characteristics of all original CT scanners (1974-1987)

A
  • In all original CT scanners (1974 to 1987), the x-ray power was transferred to the x-ray tube using high voltage cables
  • The rotating frame would spin 360 degrees in one direction and acquire an image, and then spin 360 degrees back in the other direction to acquire a second slice
  • In between each slice, the gantry would come to a complete stop and then reverse directions while the patient table would be moved forward by an increment equal to the slice thickness
19
Q

list three characteristics of the 3rd generation (rotate/rotate) CT scanner

A

Introduction of rotate–rotate

  • Increased number of detectors (over 800)
  • Angle of fan beam increased to cover entire patient: eliminated need for translational motion
  • Curved detector array
  • Mechanically joined x-ray tube and detector array rotate together
20
Q

list three benefits and three issues with the third generation CT scanner

A 
Benefits:
-Scan times decreased to 1-10 seconds
-Decreased motion artefact
-Image reconstruction times decreased
Issues:
-A pause time between rotation
-Slice misregistration (patient breathing is different each breath)
-Ring artefacts
21
Q

what is ring artefact?

A

Drift in the signal levels of the detectors over time affects the values that are back-projected to produce the CT image, causing ring artefacts

22
Q

list 5 characteristics of the 4th generation (rotate/ stationary) CT scanner

A
  • Designed to overcome the problem of ring artefacts
  • Stationary ring of about 4,800 detectors
  • Introduction of rotate only
  • Fixed detectors
  • Higher dose per scan
  • Photomultipliers replaced with photodiodes coated with a rare earth (glows)
  • Tube inside of detector ring
  • Short focus to skin distance; higher skin doses
  • These didn’t stay around long; increased dose
23
Q

list 5 characteristics of the 5th generation CT scanner

A
  • Electron Beam CT (EBCT)
  • Designed for cardiac examinations
  • No moving parts
  • Stationary ring of tungsten
  • Highly energetic focused electron beam steered magnetically, causing x-ray emission where the electron beam strikes the tungsten
  • Electron beam steered around the patient to strike the annular tungsten target
  • Capable of 50-msec scan times; can produce fast-frame-rate; CT ‘movies’ of the beating heart
24
Q

what were the issues with the 5th generation CT scanner

A
  • Space required for unit
  • Very expensive scanner
  • Determination of procedures vs. cost
  • Has multiple rows of detectors, however now up to 320 rows of detectors in 6th / 7th (variation of 3rd) generation scanners
25
Q

what is helical CT

A
  • Helical CT scanners acquire data while the table is moving
  • All helical scanners are based on 3rd generation configuration
26
Q

what are the benefits of helical CT

A
  • By avoiding the time required to translate the patient table; scan time can be much shorter
  • Allows the use of less contrast agent and increases patient throughout
  • In some instances the entire scan be done within a single breath-hold of the patient
27
Q

what is helical/ spiral CT

A

-Required a rethink of data and high voltage cable connection

28
Q

what information do slip rings transfer

A

-Slip rings are used to transfer:
. Data: low voltage
. Mains AC: medium voltage
. High voltage power

29
Q

what are slip rings

A

-A metal ring mounted on a rotating part of a machine to provide a continuous electrical connection through brushes on stationary contacts

30
Q

why were slip rings created

A

In the mid 1980’s, an innovation called the power slip ring was developed so that the elaborate x-ray cable and drum system could be abandoned

  • Allows electric power to be transferred from a stationary power source onto the continuously rotating gantry
  • Now: CT scanners with slip rings can rotate continuously and don’t have to slow down to start/stop
  • The innovation of the power slip ring has created a renaissance in CT: spiral or helical scanning
31
Q

what is multiple detector array multi-slice CT

A

The collimator spacing is wider and more of the x-rays that are produced by the tube are used in producing image data
-With multiple detector array scanners, slice thickness is determined by detector size, not by the collimator

32
Q

benefits of multiple detector array multi-slice CT

A

Opening up the collimator in a single array scanner increases the slice thickness, reducing spatial resolution in the slice thickness dimension

MRS432- CT (15 decks)

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