Cone beam X-ray scatter removal via image frequency modulation and filtering

1. Cone beam X-ray scatter removal via image frequency modulation and filtering Ali Bani-Hashemi, Ekkehard Blanz, Jonathan Maltz, Dimitre Hristov and Michelle Svatos

2. Overview • Problem: patient scatter in cone beam imaging • Approaches to scatter reduction • Theory: scatter removal via modulation • Practical implementation • Conclusion

3. Scatter in general cone beam imaging Megavoltage (MV) cone beam imaging Kilovoltage (kV) cone beam imaging using C-arm Lack of detector collimation (as used in regular CT) leads to serious scatter problem

4. Example: Scatter in MV cone beam imaging Acquired projection image Pelvis phantom imaged by linac equipped with flat panel detector Measured scatter distribution

5. Scatter in cone beam imaging Effects of scatter on image quality: • “Cupping“ artifact • Decreased contrast • Increased noise

6. Approaches to scatter removal / artifact reduction • Measurement of scatter using beam stop methods • Hardware: bow-tie filter, antiscatter grid • Monte Carlo modeling of photon transport through the patient • Convolution-superposition methods • Analytical scatter models • Heuristic methods based on approximate geometry • Image enhancement methods

7. We explore a novel approach to scatter removal – spatial amplitude modulation: • Primary photons always originate at a distant well-defined focal spot • Scattered photons have diffuse origins at points closer to the detector KEY IDEA: • If a spatial frequency modulator is placed between the object and detector, the effective magnification of the modulator will be different for primary and scattered photons • Thus, the image due to primary photons will be modulated to a higher spatial frequency than the scatter • Additionally, owing to the diffuse nature of the scatter, the image of the modulator due to the scatter will be blurred and consist of frequency components well below the modulator frequency

8. Cone beam imaging with modulatorAnalysis with “worst case“ planar object Realistic imaging geometry: Plane of patient closest to source is subject to highest magnification and so has closest modulation frequency

9. Desired modulation performance Baseband spectrum (primary+scatter) Measured scatter distribution

10. Desired modulation performance Spectra of modulated primary photons Measured scatter distribution

11. Desired modulation performance Spectra of modulated scatter originating from patient plane closest to source Measured scatter distribution

12. Recovering the primary image Quadrants “undamaged” by scatter spectrum Quadrants contaminated by scatter spectrum must be discarded Measured scatter distribution

13. Recovering the primary image 1. Copy good quadrants to clean Fourier space 2. Reconstruct missing quadrants by enforcing conjugate symmetry of Fourier transform of real-valued functions

14. Cone beam imaging with modulatorAnalysis and simulation for “worst case“ planar object Modulator at maximum (optimal) frequency (appears aliased on this display) Modulator at 1/10th maximum frequency (shown for illustrative purposes only)

15. Cone beam imaging with modulatorAnalysis and simulation for “worst case“ planar object These values are optimal in that they allow for modulation of the highest bandwidth images by exactly fitting the baseband and modulated spectra to the full detector bandwidth

16. Analysis and simulation for “worst case“ planar object Magnified source Magnified modulator Planar object

17. Analysis and simulation for “worst case“ planar object Baseband spectrum Modulated primary spectra Magnified modulator

18. Analysis and simulation for “worst case“ planar object Exaggerated amount of isotropic scatter Contains baseband and modulated scatter

19. Analysis and simulation for “worst case“ planar object

20. Comparison of primary and scatter Fourier spectra • Scatter components always appear at lower frequencies than the modulated primaries • Reconstruction of primary distribution from Fourier quadrants I and II is always possible when scatter distribution has sufficiently narrow bandwidth

21. Simulated performance

22. Simulated performance • Simulated performance, even in the presence of exaggerated amounts of scatter, is good • Can the ideal modulator be realized physically? • How close can we get to an ideal modulator?

23. Physical realization • In simulation, we have modulated transmittance. In reality, we must modulate fluence. • Appears we must use an attenuating modulator: • This modulator must multiply fluence by 2D sinusoid: • What is the shape of the attenuating modulator? • We have set for B=1 and A=1/2 for maximum modulation.

24. Physical realization • Contains singularities where argument of log is zero. • Difficult to fabricate, delicate, painful to touch • Attenuation profile is dependent on photon energy

25. Physical realization: projection of photons through modulator • Modulator is designed to provide maximum attenuation of approximately 10 attenuation lengths at design frequency • High spectral purity is achieved at design frequency

26. Physical realization: projection of photons through modulator: 50keV photons with 100keV modulator • Modulator performance at 50keV is okay, but this is because of lead K-edge • Some clipping is evident • Extra harmonic components in spectrum will lead to aliasing

27. Physical realization: projection of photons through modulator: 85keV photons with 100keV modulator • Modulator performance at 85keV is may be unacceptable due to larger amount of power in unwanted harmonics

28. Future work • Performance will only be acceptable over ranges of energies where attenuation coefficient is a relatively flat function of photon energy • May be useful for imaging with 60Co and synchrotron sources • Possibility of using alloy with low energy dependent attenuation over range of interest • Monte Carlo simulation of imaging performance is not feasible owing to need for very fine discretization of modulator profile

29. Conclusion • We have shown theoretical validity of novel approach to scatter removal in cone beam CT • Derived modulator profile for fabrication in Pb • Energy dependence of modulator is major drawback • Less severe drawbacks are attenuation of primary photons and reduction in usable detector resolution