1. Advanced CT systems and Their Performance

2. Scanner without covers

3. Scanner without covers

4. Scanner with covers

5. High cost and Low efficiency
Disadvantages
Generations
source
Source collimation
detector
Detector collimation
Source- Detector movement
Advantages
single
multiple
Pencil beam
Fan- beamlet
Fan- beam
Narrow
cone- beam
many
Stationary ring
Multiple arrays
1st Gen.
2nd Gen.
3rd Gen.
4th Gen.
5th Gen.
6th Gen.
7th Gen. no
yes
Trans.+Rotates
Rotates together
Source Rotates only
No movement
3rdGen.+
bed trans.
No scatter
Faster than 1G
Faster than 2G
Higher efficiency than 3G
Ultrafast for cardiac
faster 3D imaging
slow
Low efficiency
High cost and Low efficiency
high scatter
high cost
higher cost
8th Gen.
wide
FPD
Large 3D
Relatively slow

6. 4th Generation CT Scanners Rotate/Stationary
Part No...., Module No....Lesson No
4th Generation CT Scanners Rotate/Stationary
Module title
Fan beam geometry
More than 4800 detectors
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

7. 5th generation: Electron Beam CT (EBCT)
Part No...., Module No....Lesson No
5th generation: Electron Beam CT (EBCT)
Module title
- x-ray source is not x-ray tube but a focused, steered, microwave-accelerated EB incident on a tungsten target.
- It has no moving parts .
- Target covers one-half of the imaging circle; detector array covers the other half.
Images in
less than
50ms.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

8. Electron Beam CT (EBCT)
Part No...., Module No....Lesson No
Module title
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

9. Part No...., Module No....Lesson No
EBCT(CONT’D)
Module title
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

10. Part No...., Module No....Lesson No
Module title
Spiral (Helical) CT: Reciprocating rotation (A) versus fast continuous rotation using slip-ring technology (B)
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

11. Part No...., Module No....Lesson No
Module title
Helical Scanning:
Continuous Gantry Rotation
Continuous Table Movement
Continuous Data Acquisition
One set of detectors
Beam collimation = slice thickness
Helical scanning goes by several different names, depending on which company that you happen to be talking to. <describe>
Essentially they all describe the same concept, which a continuous movement of the table, coupled with continuous acquisition.
In single slice spiral scanners, the X-ray tube emits a beam as it rotates around the patient. On the other side of the patient, opposite the tube, a set of detectors rotates in synchrony with the tube movement, registering the radiation that has passed thru the patient.
In a single detector unit, the tube may emit a beam that is approx 5mm thick,and the detector unit is collimated to register that 5mm beam, leading to the creation of a single 5 mm thick image.
Challenges:
Tube cooling, and patient comfort – breathhold difficult.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

12. Part No...., Module No....Lesson No
Module title
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

13. Part No...., Module No....Lesson No
Module title
(A) Pitch = (B) Pitch = 2
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

14. Part No...., Module No....Lesson No
MULTISLICE SPIRAL CT
Part No...., Module No....Lesson No
Module title
Introduced at the 1998.
They are based multiple detector. rows ranging between 8, 16, 24, 32 and 64 depending on the manufacturer.
The overall goal is to improve the volume coverage speed performance.
Complete x-ray tube/detector array rotation in less than 1s.
Partial scan images can be obtained in approximately 100ms.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

15. 256-slice cone-beam CT detector

16. REAL-TIME CT FLUOLOROSCOPY
Part No...., Module No....Lesson No
REAL-TIME CT FLUOLOROSCOPY
Module title
CT fluoroscopy acquire dynamic images in real time.
Fast continuous imaging, fast image reconstruction & continuous image display.
Patient movement is low during Tube rotation.
Fast image Reconstruction algorithm is required.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

17. Part No...., Module No....Lesson No
CT ANGIOGRAPHY (CTA)
Module title
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

18. CT VIRTUAL REALITY IAMAGING
Part No...., Module No....Lesson No
CT VIRTUAL REALITY IAMAGING
Module title
The use of virtual reality is the creation the inner views of tubular structures.
Offers both endoluminal and extra luminal information.
It reduces complication (eg. infection and perforation).
Four requirements:
data acquisition
image processing
3D rendering
image display and analysis.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

19. What is displayed in CT images?
Water: 0HU Air: -1000HU
Typical medical scanner display:
[-1024HU,+3071HU],
Range:
12 bit per pixel is required in display.

20. Hounsfield scales for typical tissues

21. W L
-1024
+3071
For most of the display device, we can only display 8 bit gray scale.
This can only cover a range of 2^8=256 CT number range. Therefore,
for a target organ, we need to map the CT numbers into [0,255] gray
scale range for observation purpose. A window level and window
width are utilized to specify a display.
255

22. Windowing in CT image display

23. Scintillator Properties of CT
Transparency
X-ray stopping power
Light output and efficiency for lower dose
Primary speed (Fast decay time or quicker response)
Luminescent afterglow for quicker speed response
Radiation damage

24. PRIMARY SPEED
Primary speed is the rise time of the output signal in response to a constant x-ray input (~ seconds supporting 0.5 second scanning time).
It is also the time constant for the first component of exponential decay of that output after the input is turned off.
Clinical Significance:
Primary speed is critical to maintaining high resolution during sub-second scanning. It must be fast enough to prevent blurring especially at the perimeter of the scan FOV.
A slower primary speed can be seen as shading across high contrast edges, such as the skin-air interface.

26. AFTERGLOW
Afterglow is the second time component of the exponential decay of the output after the x-ray source is turned off.
Clinical Significance:
Afterglow will result in arcing artifacts extending from low attenuation anatomy into areas of higher attenuation. It also decreases in plane spatial resolution.

27. RADIATION DAMAGE
Radiation damage is the darkening of the material with radiation exposure over time.
It results in a gain or a shift in output for a given x-ray exposure.
It can also cause changes in Z- Axis uniformity. This is especially true for translucent materials.
Clinical Significance:
Radiation damage causes changes in gain that require frequent re-calibration.
It can result in changes in Z-axis uniformity which are more severe. These can cause rings or spots especially when scanning anatomy that changes rapidly along Z.

28. TRANSPARENCY
A transparent material allows light to be transmitted with very little scatter. Most light rays can pass through with a shorter more direct path for light.
In a translucent material light rays scatter many times as they travel from the creation site to the photodiode. This longer path length can result in more self absorption, lower net light output, and greater susceptibility to radiation damage.
Clinical Significance:
Transparency results in higher light output, better signal to noise, better Z-axis uniformity and reduced radiation damage.

29. X-RAY STOPPING POWER
Stopping power is defined as the thickness of material needed to stop 98% of incident x-rays in the typical 140 kVp CT beam (~2-3 mm).
Clinical Significance:
The thinner the material needed to stop 98% of the x-rays, the greater the light output at the diode.

30. LIGHT OUTPUT
Relative light signal at the diode for a given x-ray input (~70% at 610nm).
Clinical Significance:
Low light output can result in performance due to less electronic noise vs. quantum noise for thin slice, large patient, low dose application.
It can also result in low signal artifacts such as streaking at the shoulders and hips.

31. EMMISSION SPECTRUM
The emission spectrum of a scintillator is the relative intensity of light output at a given wavelength.
Clinical Significance:
In addition to diode matching for optimal electronic signal output, the emission spectrum of a scintillator can impact the design flexibility of detector systems and its long term stability.
In addition to x-ray radiation, scintillator emission in the photo active range can impact detector aging.

32. DIODE MATCHING and RELATIVE OUTPUT
Total signal output is a function of how well the emission spectrum of the detector material and sensitivity spectrum of photo diode match.
The closer the output spectrum of the detector matches the sensitivity profile of the photo diode, the higher the resultant electrical signal.
Clinical Significance:
Can reduce effective light output with the expected low signal impacts when scanning large patients with thin slices.

33. Relative Diode Sensitivity
HiLight: 610nm
Gadolinium Oxysulfide:
Cadmium Tungstate:
Relative Output
HiLight: 60% x 70% = 42%
Gadolinium Oxysulfide: 40% x 80% = 32%
Cadmium Tungstate: 42% x 30% = 13%

34. Part No...., Module No....Lesson No Quality criteria for CT images
Module title
Quality criteria for CT images
Part …: (Add part number and title)
Module…: (Add module number and title)
Lesson …: (Add session number and title)
Learning objectives: Upon completion of this lesson, the students will be able to: …
. (Add a list of what the students are expected to learn or be able to do upon completion of the session)
Activity: (Add the method used for presenting or conducting the lesson – lecture, demonstration, exercise, laboratory exercise, case study, simulation, etc.)
Duration: (Add presentation time or duration of the session – hrs)
Materials and equipment needed: (List materials and equipment needed to conduct the session, if appropriate)
References: (List the references for the session)
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

35. Part No...., Module No....Lesson No
Module title
The slice sensitivity profile (SSP) 1) For conventional CT and spiral/helical CT. 2) SSP is wider for 360-degree linear interpolation algorithms.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

36. Scanner performance: technical parameters (I)
Part No...., Module No....Lesson No
Module title
Scanner performance: technical parameters (I)
CT Number Accuracy
CT number depends on tube voltage, filtration, object thickness
CT number of water is by definition equal to 0
Measured CT number should be < ± 4 HU in the central ROI
CT Number Linearity
It concerns the linear relationship between the calculated CT number and the linear attenuation coefficient of each element of the object
Deviations from linearity should be < ± 5 HU
CT Number Uniformity
It relates to the fact that a CT number of each pixel in the image of an homogeneous object should be the same over various regions
The difference in the CT number between a peripheral and a central region of an homogeneous test object should be < 8HU
Differences are largely due to beam hardening phenomenon
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

37. Scanner performance: technical parameters (II)
Part No...., Module No....Lesson No
Module title
Scanner performance: technical parameters (II)
Spatial Resolution
The high contrast resolution determines the minimum size of detail visualized in the plane of the slice with a contrast >10%.
It is affected by:
the reconstruction algorithm
the detector width
the effective slice thickness
the object to detector distance
the X-ray tube focal spot size
the matrix size.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

38. Scanner performance: technical parameters (III)
Part No...., Module No....Lesson No
Module title
Scanner performance: technical parameters (III)
Spatial Resolution
The low contrast resolution determines the size of detail that can be visibly reproduced when there is only a small difference in density relative to the surrounding area
Low contrast resolution is considerably limited by noise.
The perception threshold in relation to contrast and detail size can be determined, for example, by means of a contrast-detail curve.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

39. Scanner performance: technical parameters (IV)
Part No...., Module No....Lesson No
Module title
Scanner performance: technical parameters (IV)
Slice Thickness
The slice thickness is determined in the center of the field of view.
The use of post-patient collimation to reduce the width of the image slice leads to very significant increases in the patient dose
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

40. Part No...., Module No....Lesson No
Module title
CT number uniformity
Axial image of an homogenous phantom
can be assessed at the same time as measuring noise, by placing four additional ROI (N, E, S and W) at positions near the edge of the image of a uniform phantom
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

41. Part No...., Module No....Lesson No
CT number linearity
Module title
CT number linearity is assessed using a phantom containing inserts of a number of different materials (materials should cover a wide range of CT numbers
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

42. Part No...., Module No....Lesson No
Module title
Low contrast resolution
Typical image of the Catphan LCR insert
Low contrast resolution (or low contrast detectability) is often quoted as the smallest visible object at a given contrast for a given dose
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

43. Part No...., Module No....Lesson No
Module title
Spatial resolution (high contrast)
The number of line pairs per cm just visible in the image is approximately equivalent to the 2% value of the MTF
This result can then be compared with the 2% MTF
The resolution is quoted as the spatial frequency (in line pairs / cm) at which the modulation falls to 50%, 10% or 2% MTF.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

44. Part No...., Module No....Lesson No
Module title
Z-Sensitivity (Imaged slice width)
Plan view of a test object used to measure imaged slice widths for axial scans, to assess the accuracy of post patient collimation, and to calculate the geometric efficiency for the scanner
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

45. Part No...., Module No....Lesson No
Module title
Dosimetry - CTDI in air (helical)
Axial slice positions
Helical scan (pitch 1)
The Computed Tomography Dose Index (CTDI) in air can be measured using a 10cm pencil ionization chamber, bisected by the scan plane at the isocentre.
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

46. Part No...., Module No....Lesson No
Module title
Dosimetry - CTDI in Perspex Phantoms
Insert to plug holes
Body phantom (or annulus
to fit over haed phantom)
Head phantom
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

47. Part No...., Module No....Lesson No
Module title
Dosimetry - CTDI in Perspex Phantoms
Central and peripheral CTDI’s are used to calculate weighted CTDI, CTDIw:
CTDIws can be compared against diagnostic reference levels for standard scan examinations ) (
CTDI 3 2 + 1 C = p
100, c w n
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

48. CT Noise Characteristics
For low mAs values
standard deviation decreases with increasing mAs
For higher mAs values
standard deviation stays fairly constant
Transition point mAs should not increase throughout scanner life
Standard
Deviation
mAs

49. CT Noise Characteristics
Excessive noise can be caused by
detector sensitivity
electronic noise in detector amplifier circuits
reduced output per mAs

50. Imaging performance (Noise)
Part No...., Module No....Lesson No
Module title
Imaging performance (Noise)
Noise is generally assessed using cylindrical phantoms, which are either filled with water or made of a tissue equivalent material
Once an axial image of the phantom has been acquired, noise is obtained from the standard deviation in CT number in a region of interest (ROI) placed centrally within the image 50
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

51. Imaging performance (Noise)
Part No...., Module No....Lesson No
Module title
Imaging performance (Noise)
Region of
interest (ROI) 51
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

52. SNR is dependent on dose, as in X-ray.
Notice how images become grainier and our ability to see small objects decreases as dose decreases.
There are some similarities with X-ray. But we also see some important differences.

53. Actions that can influence image quality
Part No...., Module No....Lesson No
Module title
Actions that can influence image quality
Avoid bad viewing conditions (e.g. lack of monitor brightness or contrast, poor spatial resolution, etc)
Improve insufficient skill to use the workstation capabilities to visualize images (window level, inversion, magnification, etc).
Reduce artifacts due to incorrect digital post-processing (creation of false lesions or pathologies)
Compromise between image quality and compression level in the images
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources