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

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

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.

Image from detector with lens cap on! 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. (n averages ) 1/2 5 10 15 20 1 / s µ SNR 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 for pixels in detector column for pixels in detector row -5 +5 Image from detector with lens cap on!

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? 200 400 600 800 1000 1200 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Relative intensity Pixel value Measured data points 6th order polynomial fit

Oscillating projection screen Without oscillating projection screen 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

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.

Artifact removed by “wobble” 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”

“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! Hg lamp CCD detector PC with frame- grabber card

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

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 - 6 Gy 6 Gy

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.

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).