1. BMME 560 & BME 590I Medical Imaging: X-ray, CT, and Nuclear Methods
CT Imaging Part 1

2. Today CT Systems CT Image interpretation CT Degradations
Seven generations
Instrumentation
CT Image interpretation
CT number
CT Degradations
Beam hardening
Metal artifacts
Other issues

3. CT Systems CT system designs are classified into “generations”
These are more or less in chronological order, but some earlier generations have lasted longer than later ones.
These are explained well in your book.

4. CT Generations Scanning pencil beam Scanning fan beam
Full fan-beam with rotating detector*
Full fan-beam with stationary detector ring*
Electron-beam CT (EBCT)
Spiral CT*
Multislice CT*
* Most common today

5. Spiral and Multislice CT
Reference 1
The patient moves through the scanner, so that a complete rotation is NOT taken for any given plane. This requires interpolation for the helical data.
The key parameter is the pitch, the axial distance traversed in one complete rotation of the gantry.

6. Electron-beam CT (EBCT)
Detector ring
Electron source
Magnetic steering
Target ring
No mechanical motion required to obtain tomographic data

7. CT Instrumentation Sources differ from radiography sources
Fan-beam sources have slit collimators to keep beam in plane
Slit may be adjustable
This needs to be opened up in multislice CT
More filtration than in projection radiography
Use a “harder” (higher average energy) beam to reduce beam hardening artifacts

8. CT Instrumentation Tube-current modulation
Use higher tube current in this position to get the same number of photons on the detector
Use lower tube current here

9. CT Instrumentation Tube voltage selection
Higher tube voltage = higher effective energy
Less contrast between soft tissues
Lower dose
80 kVp and up to 140 kVp are common
It depends on the reason for the scan and the tissue properties of the region being imaged.

10. CT Instrumentation Detectors
Solid-state scintillator + photodiode array
NOT flat-panel, but strip detectors because they have faster readout
Multislice systems simply stack strips to form slices
Note that in 3G systems, the slice thickness may be controlled by beam collimation, but in 7G systems it is determined by the detector spacing.

11. CT Instrumentation Slip-ring
To perform spiral/helical CT, we must achieve continuous rotation of the source and detector
BUT we must also deliver continuous power to the rotating tube and continuously read data from the rotating detector
Power is delivered through continuous electrical contact between a stationary ring and a “brush” on the rotating gantry (wearing problem?)
Data is communicated through optical links

12. CT Image Reconstruction
Remember that CT is a transmission imaging modality
To obtain Radon-type (line integral) data, we have to perform a log-transform
Measured data
Blank scan data

13. CT Image Interpretation
What does the intensity in the raw reconstructed CT image mean? What are its units?
Problem: Each X-ray tube has its own spectral characteristics, and the characteristics change over the service life of the tube.
Constant calibration is necessary

14. Contrast
Properties of materials

15. CT Image Interpretation
Measured values are normalized via the CT number, which is expressed in Hounsfield units (HU)
But we must measure mwater under the same conditions (tube voltage, current, time of service, etc) as when the CT is taken

16. CT Numbers of Common Materials
Min
Max
Bone
400
1000
Soft tissue 40 80
Water
Fat
-100
-60
Lung
-600
-400
Air
-1000
Most CT scanners range up to 2000, but some can go up to 4000 to accommodate metal implants. What determines the peak CT number resolvable in a scanner?
Reference 1

17. CT Numbers Example problem If the CT number of bone is 1000,
What is the relationship between the linear attenuation coefficients of bone and water for this device?
What is the effective energy of this device for imaging bone?

18. CT Numbers Energy(MeV) Water mu (cm^-1) Bone mu (cm^-1) Bone - 2*water
4.41E+01
1.50E-02
1.67E+00
1.40E+01
2.00E-02
8.10E-01
6.06E+00
3.00E-02
3.76E-01
1.80E+00
4.00E-02
2.68E-01
7.41E-01
5.00E-02
2.27E-01
3.61E-01
6.00E-02
2.06E-01
1.93E-01
8.00E-02
1.84E-01
6.06E-02
1.00E-01
1.71E-01
1.48E-02
1.50E-01
1.51E-01
-1.68E-02
2.00E-01
1.37E-01
-2.27E-02
3.00E-01
1.19E-01
-2.35E-02
4.00E-01
1.06E-01
-2.20E-02
5.00E-01
9.69E-02
-2.05E-02
6.00E-01
8.96E-02
-1.91E-02
8.00E-01
7.87E-02
-1.70E-02
1.00E+00
7.07E-02
-1.54E-02
1.25E+00
6.32E-02
-1.37E-02
1.50E+00
5.75E-02
-1.24E-02
2.00E+00
4.94E-02
-1.04E-02
3.00E+00
3.97E-02
-7.48E-03
4.00E+00
3.40E-02
-5.53E-03
5.00E+00
3.03E-02
-4.06E-03
6.00E+00
2.77E-02
-2.91E-03
8.00E+00
2.43E-02
-1.21E-03
1.00E+01
2.22E-02
4.88E-05
1.50E+01
1.94E-02
2.11E-03
2.00E+01
1.81E-02
3.45E-03

19. CT Numbers Calibration Frequent calibration of the system is necessary
Image a phantom with known material properties, including air and water
In radiation dosimetry applications, may need to develop a conversion curve to electron density

20. CT Limitations Deviations from Radon projections Beam hardening
Partial volume
Photon starvation
Metal artifacts
Motion artifacts
Nonuniformity
Helical scanning
Cone-beam

21. CT Artifacts
Beam hardening occurs when low energies are preferentially absorbed by the subject
The spectrum changes as the beam travels through the subject.
Reference 2

22. CT Artifacts Beam hardening
The beam spectrum reaching the object differs depending on the angle of projection
Also, the response of the detector will vary with energy spectrum

23. CT Artifacts Beam hardening – “cupping” artifact 1 2
Projection line 2 experiences more beam hardening than projection line 1, so line 2 deviates more from the ideal Radon projection. How?

24. CT Artifacts Beam hardening – cupping artifact
In a cylinder, it is possible to calibrate and correct for this. Patients, only partly so.
Reference 2

25. CT Artifacts
Beam hardening – streaks occur near highly-absorbing regions (bone, iodine)
Reference 2

26. CT Artifacts Beam hardening – partial correction
Iterative correction algorithm determines bony locations and estimates beam hardening effects.
Reference 2

27. CT Artifacts Beam-hardening reduction Filtration
Pre-hardening
Bowtie filter
Phantom calibration with different shapes
Iterative post-correction methods
Iterative reconstruction
Hardens the beam more at edges to reduce cupping

28. CT Artifacts Partial volume effect
Consider a fan-beam with a certain slice thickness
If an object is only partially in the slice, its apparent attenuation will be reduced.

29. CT Artifacts
Photon starvation – streak artifacts caused by too few photons at some angles
Reference 2

30. CT Artifacts Addressing photon starvation Get more photons
Tube current modulation
“Adaptive filtration”
Pre-reconstruction method for identifying low-count regions and (linear, not radiation) filtering in axial direction selectively

31. CT Artifacts
Adaptive filtration – partial correction
Reference 2

32. CT Artifacts
Metal artifacts – both photon starvation and beam hardening
Reference 2

33. CT Artifacts
Even though a slice can be completed in 1-2 seconds (or less on newer systems), patient motion is significant
Voluntary motion (wiggly patients)
Involuntary motion (heartbeat, respiration)
If the motion is rigid-body motion, and we know the transformation, we can correct it.

34. CT Artifacts
Motion artifact
Reference 2

35. CT Artifacts
Rigid-body voluntary motion
Translation
Rotation

36. CT Artifacts
Most software-correction methods for voluntary motion are based on rigid-body transforms
Estimate the motion by using the beginning and end projections
Apply the rigid-body transform to the backprojection geometry in reconstruction
Note that people are not necessarily rigid

37. CT Artifacts
Motion correction
Reference 2

38. CT Artifacts
Detector nonuniformity – ring artifacts
Reference 2

39. CT Artifacts
Helical scanning – artifacts occur around regions that vary quickly in axial direction – depending on helical pitch and slice thickness – due to angle-varying partial volume effects
Reference 2

40. CT Artifacts
Cone-beam artifacts: Multislice scanners suffer more from cone-beam oblique angle artifacts
Reference 2

41. CT Artifacts Key points
Many factors can contribute to inconsistencies in the data.
Be able to explain the physical reasons why the artifacts occur and why they are inconsistent.
Frequent calibration can help.
Experienced operators are essential.

42. References
S. Jackson, R. M. Thomas, Cross-sectional Imaging Made Easy, Churchill Livingstone: London, 2004.
J. F. Barrett, N. Keat, “Artifacts in CT: Recognition and Avoidance,” Radiographics, vol. 24, pp , 2004.