Topic 6. CT Types Flashcards
What are the 5 types of CT
Sequential CT
Spiral CT
Electron beam CT
Multi-Slice CT
Dual Source CT
Name the CT: the patient is moved forward along a longitudinal axis of the CT scanner, PAUSING AT INTERVALS to allow trans-axial images to be captured at each position along the axis.
- LONG SCAN TIME
- Rely on the physical connection of wires between the rotating elements
Sequencial CT
In a Sequencial CT during the acquisition of a slice the table remains stationairy. After completion of the acquisition, the wires _____. Then the table moves to a ____ _____ to perform the next scan
unwind, new position
Name the CT: This CT allows for continuous tube-detector rotation and fast acquisition of data
- Has Slip-ring technology
- continuous acquisition of data allows coverage of larger sections in the same amount of time, no splicing.
Spiral CT
What is slip ring technology found in spiral CTs?
Transmission of energy and data via electrically conductive brushes and rotating brings, enabling spiral CT scanners to rotate continuously.
In spiral CTs because the x-ray tube generates energy for an extended period, more ______ storage capacity is required
heat.
what CT requires requires expanded storage capacity and processing capacity?
Spiral, because a large amount of data is produced in a very short period
Name the CT: This CT was developed to take images of the heart, it generates/directs electrons along a STATIONARY TUNGSTEN RING. Emitted x-rays front the tungsten target ring are collimated, and after passing through
the patient, are collected by the stationary detectors on the opposite side.
- Mostly used to detect the amount of coronary calcium
- temporal resolution is 100ms
Electron beam CT (EBCT)
Name the CT: An increase in the number of detector rows, multiple channels of data can be acquired simultaneously.
Has the advantage of INCREASED LONGITUDINAL COVERAGE THAT CAN BE ACHIEVED PER ROTATION/ slice.
- useful for cardiac acquisition, which deal with an inherently longer scan time compared with imaging of non-moving organs
- simultaneously acquires 300 slices
- 300 slice scanners are equipped with 320 detector rows
MULTI-SLICE CT
Besides 320, CT manufactureres also offer other detector systems for multi slice with the number of
16, 32, 64, 128, 256
Some Multi slice CT systems allow for twice their slice acquisition by ______ sampling in longitudinal direction
doubling (two cone shape x-rays)
By rapid, longitudinal alternation of the focal sport during acquisition, 2 partially ______ sets of projections from slightly differing position are acquired
overlapping.
Compared with systems with z-sampling, double z-sampling results in an IMPROVED longitudinal resolution at the penalty of
a longer total scan time
increasing the number of detectors in a multi slice CT
decreases individual detector width
An Increase in available detector rows=
increase in coverage of Multi-slice CT scanners
Name the CT: Equipped with 2 x-ray tubes rotating at 90° angle to each other
- ADVANTAGE : IMPROVEMENT OF THE TEMPORAL RESOLUTION- - Dual-Energy CT: During scanning both tube detector systems operate using a different tube voltage (kV) → improves tissue differentiation
Dual source CT
Image quality of a CT is determined by ____ size and number of _____ projections
detector size
angular projections
Typical resolution of clinical CT scanners
0.2-1mm with a slice thickness of 0.4-5mm
Higher slice thickness is chosen to reduce radiation dose & improve SNR at expense of axial resolution, why?
captures more in one go, less time spent in machine.
what is the key determinant of the image quality
focal spot of x-ray tube, large focal spot blurs the image
high resolution CT have a focal spot size of
3-5 micrometer
Collimator function
Interaction of x-rays with tissue creates randomly scattered photons which create image noise and cloud the image
artefacts?
cause any discrepancy between the CT number represented in the image and the expected CT numbers (HU units)
list some Common artifacts
- Beam hardening
- Partial volume effect
- Bad detector
- Metal
- Patient motion
Artifacts cause any discrepancy between the CT number represented in the image and the expected CT numbers (HU units)
In extreme cases, a detector element may fail giving a constant output signal. _____ _______ occurs when the x-rays pass through strongly absorbing materials. Lower-energy x-rays are absorbed and the energy peak shifts toward higher energies. Absorption values are underestimated.
Beam hardening
How is beam hardening reduced?
by pre-hardening the beam, by placing thin
metal plate (molybdenum or tungsten) in front of the tube
beam hardening
Artifacts cause any discrepancy between the CT number represented in the image and the expected CT numbers (HU units)
What artifact occurs when A PIXEL REPRESENTS MORE THAN ONE KIND OF TISSUE. Particularly relevant when a tissue boundary lies within a CT slice.
- Blurs the intensity distinction between adjacent tissue
- Can be reduced by higher resolution or sometimes repositioning the patient
Partial volume effect, u can see the blur in the image
Artifacts cause any discrepancy between the CT number represented in the image and the expected CT numbers (HU units)
Detectors must be calibrated and positioned correctly: Detectors need to be centered with respect to the source → otherwise, the image will be blurred
- Detectors need to be calibrated so that the output signal intensity is identical for all detectors with the same incident x-ray intensity.
Uncalibrated detectors create which artifact?
ring artifacts
what artifact is shown here
ring artifact
Metal materials can cause the streaking artifacts due to block parts of projection data
- E.g.: dental fillings, prosthetic devices, surgical clips
- Remove the metal material as possible to reduce artifact
this artifact is?
Metal artifact
CT image of patient w/ metal spine implants:
This artifact occurs when patient MOVES (e.g. breathes) during the scan of one slice
- Cannot be corrected but risk of motion blur can be reduced with shorter acquisition times and with the use of the helical scanning principle
- Detailed imaging of the heart continues to pose problems
Patient Motion artifact
what is this
the effects of reducing projections, individual cross sections visible because there are less, more projections means theirs more so their are so many they blend perfectly, you cant tell
what is this
the effects of reducing rays,
photon starvation
In 1st Gen CTs, state the number of detectors, type of beam. tube detector movement, and duration of the scan
Detectors: 1
Type of beam: pencil like
Tube detector movement: Translate-rotate
Duration of scan: 20-35 min
In 2nd Gen CTs, state the number of detectors, type of beam. tube detector movement, and duration of the scan
Detectors: up to 30
Type of beam: fan shaped
Tube detector movement: Translate-rotate
Duration of scan: less than 90s
In 3rd Gen CTs, state the number of detectors, type of beam. tube detector movement, and duration of the scan
Detectors: 288, 700+
Type of beam: fan shaped
Tube detector movement: Rotate-rotate
Duration of scan: about 5 secs
In 4th Gen CTs, state the number of detectors, type of beam. tube detector movement, and duration of the scan
Detectors: more than 2000
Type of beam: fan shaped
Tube detector movement: Rotate-rotate
Duration of scan: few seconds
MEMORIZE
