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

Today CT Systems CT Image interpretation CT Degradations 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 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

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.

Electron-beam CT (EBCT) Detector ring Electron source Magnetic steering Target 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 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 mwater under the same conditions (tube voltage, current, time of service, etc) as when the CT is taken

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

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 Numbers Energy(MeV) Water mu (cm^-1) Bone mu (cm^-1) Bone - 2*water 54.7392 4.41E+01 1.50E-02 1.67E+00 17.34144 1.40E+01 2.00E-02 8.10E-01 7.68192 6.06E+00 3.00E-02 3.76E-01 2.55552 1.80E+00 4.00E-02 2.68E-01 1.27776 7.41E-01 5.00E-02 2.27E-01 0.814464 3.61E-01 6.00E-02 2.06E-01 0.604416 1.93E-01 8.00E-02 1.84E-01 0.427968 6.06E-02 1.00E-01 1.71E-01 0.35616 1.48E-02 1.50E-01 1.51E-01 0.28416 -1.68E-02 2.00E-01 1.37E-01 0.251328 -2.27E-02 3.00E-01 1.19E-01 0.213696 -2.35E-02 4.00E-01 1.06E-01 0.1902336 -2.20E-02 5.00E-01 9.69E-02 0.1732224 -2.05E-02 6.00E-01 8.96E-02 0.1599744 -1.91E-02 8.00E-01 7.87E-02 0.1403136 -1.70E-02 1.00E+00 7.07E-02 0.1260672 -1.54E-02 1.25E+00 6.32E-02 0.1127232 -1.37E-02 1.50E+00 5.75E-02 0.1026432 -1.24E-02 2.00E+00 4.94E-02 0.0884544 -1.04E-02 3.00E+00 3.97E-02 0.071904 -7.48E-03 4.00E+00 3.40E-02 0.0625344 -5.53E-03 5.00E+00 3.03E-02 0.0565632 -4.06E-03 6.00E+00 2.77E-02 0.0524928 -2.91E-03 8.00E+00 2.43E-02 0.0473664 -1.21E-03 1.00E+01 2.22E-02 0.0444288 4.88E-05 1.50E+01 1.94E-02 0.0409344 2.11E-03 2.00E+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 In a cylinder, it is possible to calibrate and correct for this. Patients, only partly so. Reference 2

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

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

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 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. 1679-1691, 2004.