BMME 560 & BME 590IMedical Imaging: X-ray, CT, and

Nuclear Methods

CT Imaging Part 1

Today

• CT Systems– Seven generations– Instrumentation

• CT Image interpretation– CT number

• CT Degradations– Beam hardening– Metal artifacts– Other issues

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.

CT Generations

1. Scanning pencil beam

2. Scanning fan beam

3. Full fan-beam with rotating detector*

4. Full fan-beam with stationary detector ring*

5. Electron-beam CT (EBCT)

6. Spiral CT*

7. Multislice CT*

* Most common today

Spiral and Multislice CT

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.

Reference 1

Electron-beam CT (EBCT)

Electron source

Magnetic steering

Target ring

Detector ring

No mechanical motion required to obtain tomographic data

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

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

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.

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.

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

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

0

0

exp ( )

( ) log

j j

jj

j

I I s ds

Ig s ds

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Measured data

Blank scan data

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

Contrast

• Properties of materials

CT Image Interpretation

• Measured values are normalized via the CT number, which is expressed in Hounsfield units (HU)

• But we must measure water under the same conditions (tube voltage, current, time of service, etc) as when the CT is taken

1000 pixel water

water

h

CT Numbers of Common Materials

material Min Max

Bone 400 1000

Soft tissue 40 80

Water 0 0

Fat -100 -60

Lung -600 -400

Air -1000 -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

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?

CT NumbersEnergy(MeV) Water mu (cm^-1) Bone mu (cm^-1) Bone - 2*water

1.00E-02 5.33E+00 54.7392 4.41E+011.50E-02 1.67E+00 17.34144 1.40E+012.00E-02 8.10E-01 7.68192 6.06E+003.00E-02 3.76E-01 2.55552 1.80E+004.00E-02 2.68E-01 1.27776 7.41E-015.00E-02 2.27E-01 0.814464 3.61E-016.00E-02 2.06E-01 0.604416 1.93E-018.00E-02 1.84E-01 0.427968 6.06E-021.00E-01 1.71E-01 0.35616 1.48E-021.50E-01 1.51E-01 0.28416 -1.68E-022.00E-01 1.37E-01 0.251328 -2.27E-023.00E-01 1.19E-01 0.213696 -2.35E-024.00E-01 1.06E-01 0.1902336 -2.20E-025.00E-01 9.69E-02 0.1732224 -2.05E-026.00E-01 8.96E-02 0.1599744 -1.91E-028.00E-01 7.87E-02 0.1403136 -1.70E-021.00E+00 7.07E-02 0.1260672 -1.54E-021.25E+00 6.32E-02 0.1127232 -1.37E-021.50E+00 5.75E-02 0.1026432 -1.24E-022.00E+00 4.94E-02 0.0884544 -1.04E-023.00E+00 3.97E-02 0.071904 -7.48E-034.00E+00 3.40E-02 0.0625344 -5.53E-035.00E+00 3.03E-02 0.0565632 -4.06E-036.00E+00 2.77E-02 0.0524928 -2.91E-038.00E+00 2.43E-02 0.0473664 -1.21E-031.00E+01 2.22E-02 0.0444288 4.88E-051.50E+01 1.94E-02 0.0409344 2.11E-032.00E+01 1.81E-02 0.0397056 3.45E-03

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

www.scanditronix-wellhofer.com

CT Limitations

• Deviations from Radon projections– Beam hardening– Partial volume– Photon starvation– Metal artifacts– Motion artifacts– Nonuniformity– Helical scanning– Cone-beam

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

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

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?

CT Artifacts

• Beam hardening – cupping artifact

Reference 2

In a cylinder, it is possible to calibrate and correct for this. Patients, only partly so.

CT Artifacts

• Beam hardening – streaks occur near highly-absorbing regions (bone, iodine)

Reference 2

CT Artifacts

• Beam hardening – partial correction

Reference 2Iterative correction algorithm determines bony locations and estimates beam hardening effects.

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

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.

CT Artifacts

• Photon starvation – streak artifacts caused by too few photons at some angles

Reference 2

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

CT Artifacts

• Adaptive filtration – partial correction

Reference 2

CT Artifacts

• Metal artifacts – both photon starvation and beam hardening

Reference 2

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.

CT Artifacts

• Motion artifact

Reference 2

CT Artifacts

• Rigid-body voluntary motion

Translation

Rotation

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

CT Artifacts

• Motion correction

Reference 2

CT Artifacts

• Detector nonuniformity – ring artifacts

Reference 2

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

CT Artifacts

• Cone-beam artifacts: Multislice scanners suffer more from cone-beam oblique angle artifacts

Reference 2

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.

References

1. S. Jackson, R. M. Thomas, Cross-sectional Imaging Made Easy, Churchill Livingstone: London, 2004.

2. J. F. Barrett, N. Keat, “Artifacts in CT: Recognition and Avoidance,” Radiographics, vol. 24, pp. 1679-1691, 2004.