S J Doran, K K Koerkamp*,

1. S Department of Physics, University of Surrey, Guildford, GU2 7XH, UK Dept of Applied Physics, University of Twente, Enschede, NL S J Doran, K K Koerkamp Technical development of a high resolution CCD-based scanner for 3-D gel dosimetry: (II) Problems encountered S J Doran, K K Koerkamp*, * Department of Physics University of Surrey Department of Applied Physics University of Twente, NL

2. Structure of talk • Factors determining signal detected • Detector and projection screen characteristics • The “ring artifact” and how to remove it • The “correction scan” procedure • Sample containers • The dynamic range problem • Conclusions

3. Signal measured in CCD tomography • Light field L(x,y) • Projection screen PS(x,y) • Detector response D(x,y,S) • Gel absorption G(x,y,q) • Reflection and refraction • None of these quantities is known a priori. • We can estimate L(x,y) relatively easily. • PS(x,y) and D(x,y,S) can be a problem.

4. 0.7 for pixels in detector column s 0.6 for pixels in detector row s 0.5 +5 1 / 0.4 s -5 µ SNR 0.3 0.2 +5 0.1 -5 0.0 0 5 10 15 20 1/2 (n ) averages CCD detector characteristics: (1) Dark response • We started out using a cheap CCD detector (~£120). • The “noise” from the detector has a clear structure. • This has serious consequences for improvement in signal by averaging frames. Image from detector with lens cap on!

5. 1200 1000 800 Measured data points 6th order polynomial fit Pixel value 600 400 200 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Relative intensity CCD detector characteristics: (2) Light response • Response measured by exposing CCD to different light levels, obtained using two polaroids rotated w.r.t. each other. • No need for parallel beam here. Collimating optics and projection screen not used. • Does the response vary with pixel position?

6. Open light field and projection screen • Relatively easy to separate slowly varying L(x,y) from PS(x,y) and D(x,y). • Oscillating the projection screen up and down “smears out” some of the granularity. • How much of the residual noise comes from PS(x,y) and how much from D(x,y)? • Replacing the projection screen shows the extent of the problem. Oscillating projection screen Without oscillating projection screen

7. Pros and cons of two different projection screens • Screen 1 (engineering tracing film) is granular, but produces sharp images. • Screen 2 (opal white perspex) has much less granularity, but the projection images are blurred.

8. Consequences of PS(x,y) and D(x,y) • The “noise” generated is coherent between projections. • This gives rise to a characteristic ring artifact. Ideal object Simulated artifact Experimental artifact Artifact removed by “wobble”

9. CCD detector Hg lamp PC with frame- grabber card “Correction” of the ring artifact via “wobble” • At each projection step, the detector moves randomly relative to the tank by a few pixels. • Can be achieved either by moving tank or camera. • This allows us to sample the response functions of different pixels over the course of the acquisition. • Coherent noise turns into random noise!

10. Containers and the correction scan • Sample container has refractive index different to that of the sample and the matching medium. • This causes partial reflection and refraction. • Containers are imperfect, leading to artifacts in the projections. • Problems can be partly overcome by taking the ratio of images before (“correction scan”) and after irradiation. Scratch mark Before irradiation After irradiation Processed sinogram

11. 6 Gy - 6 Gy Artifacts due to imperfect containers • Minute scratches on the container wall cause spurious reflection and refraction. • These are easily seen as parallel tracks in the sinograms. • They lead to characteristic artifacts at the edge of the field-of-view

12. Dynamic range problems • Video capture card has limited dynamic range (10-bit). • Light travelling through low-dose region saturates ADC. • Light travelling through high-dose region registers a very low signal that is strongly affected by noise. • Extremely important to make absorption of matching medium same as that of unirradiated gel.

13. Conclusions • We now understand many of the causes of artifacts in the OCT images. • Most of the artifacts can be removed by investment in higher quality components (particularly the CCD, projection screen and sample container).