Scatter measurements for optical cone-beam computed tomography

摘要

Both x-ray and optical cone-beam CT (CBCT) scanners are limited by scattered photons that contaminate the primary images. Transparent radiochromic gels and plastics are anticipated to produce less scattering than polymerization gels and hence result in reconstructed dose distributions with greater contrast and dynamic range. Four methods of scatter measurement were investigated to quantify scatter fractions using an in-house optical CBCT scanner. The methods consisted of generating either "coin" shadows or primary beamlets in transmission images. The four methods generated similar results of 10percent scatter fractions in gel transmission images for this particular scanner configuration with both leuco crystal violet and carbon black micelle gels. The scatter contributions were distributed as 5percent from micelle gel, 2percent from Teflon wall, 2percent from refractive index matching liquid and 1percent other sources. Applying a uniform background subtraction of 2percent of open field intensity resulted in 0.2percent difference between laser CT and cone-beam CT reconstructed attenuation coefficients for a black micelle gel finger phantom.