week 2 lec 1: history Flashcards
why was CT introduced?
limitations of general x-ray: - superimposition of structures
- need a large difference in attenuation to visualise changes in density
what is a tomographic image
a “slice” of the patient’s anatomy; generally in the axial plane
what is a pixel and what is a CT pixel
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
what is a voxel and how do we calculate it
voxel is the 3D analogue of a pixel
FOV and matrix size/ by pixel size (XY) ≈ voxel size (slice thickness) of the reconstructed image
what is anisotropic voxel and isotropic voxel
anisotropic; not a perfect cube
if slice thickness not equal to 0.5mm
isotropic; perfect cube
slice thickness 0.5mm
how do voxels impact the image
The smaller the voxel size, the greater (or higher) spatial resolution
what is required for a CT scan to be performed
- 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
what is data acquisition
-Sampling ‘real life’ signals and converting these into digital numeric values for processing; the collection of x-ray transmission measurements from the patient / object
what happens in the data acquisition phase
- 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”
what is image reconstruction
-Converts raw data (slice profiles) to image data and utilises image reconstruction algorithms
what is image display
- 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)
3 types of post-processing and what happens after this
- edge enhancement, multi-planar reformations, 3D reconstructions
- Sent to PACS for storage, reporting, etc.
what are some characteristics of the 1st experimental CT scanner?
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”
what factors affect spatial resolution (8)
- Field of view
- Pixel size
- Focal spot size
- Motion of the patient
- Pitch
- Kernel (algorithm)
- Slice thickness
- Detector size
how many generations of CT scanners are there and what are the most commonly used ones now?
5 generations
- 3rd generation
What are 5 characteristics of the first generation: rotate/ translate, translation/rotation CT scanner
- 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
what are 4 characteristics of the 2nd generations: translate/ rotate (fan beam) CT scanners
- 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
list 2 characteristics of all original CT scanners (1974-1987)
- 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
list three characteristics of the 3rd generation (rotate/rotate) CT scanner
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
list three benefits and three issues with the third generation CT scanner
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
what is ring artefact?
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
list 5 characteristics of the 4th generation (rotate/ stationary) CT scanner
- 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
list 5 characteristics of the 5th generation CT scanner
- 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
what were the issues with the 5th generation CT scanner
- 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
what is helical CT
- Helical CT scanners acquire data while the table is moving
- All helical scanners are based on 3rd generation configuration
what are the benefits of helical CT
- 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
what is helical/ spiral CT
-Required a rethink of data and high voltage cable connection
what information do slip rings transfer
-Slip rings are used to transfer:
. Data: low voltage
. Mains AC: medium voltage
. High voltage power
what are slip rings
-A metal ring mounted on a rotating part of a machine to provide a continuous electrical connection through brushes on stationary contacts
why were slip rings created
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
what is multiple detector array multi-slice CT
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
benefits of multiple detector array multi-slice CT
Opening up the collimator in a single array scanner increases the slice thickness, reducing spatial resolution in the slice thickness dimension
