The goal of this study was to evaluate the noise redu
ction a
chievable from dual energy
computed tomography (CT) imaging (DECT) using filtered ba
ckproje
ction (FBP) and iterative image re
constru
ction algorithms
combined with in
creased imaging exposure. We evaluated the data in the
context of imaging for bra
chytherapy dose
cal
culation, where a
ccurate quantifi
cation of ele
ctron density
ρe and effe
ctive atomi
c number
Zeff is benefi
cial.
A dual source CT scanner was used to scan a phantom containing tissue mimicking inserts. DECT scans were acquired at 80 kVp/140Sn kVp (where Sn stands for tin filtration) and 100 kVp/140Sn kVp, using the same values of the CT dose index CTDIvol for both settings as a measure for the radiation imaging exposure. Four CTDIvol levels were investigated. Images were reconstructed using FBP and sinogram affirmed iterative reconstruction (SAFIRE) with strength 1,3 and 5. From DECT scans two material quantities were derived, Zeff and ρe. DECT images were used to assign material types and the amount of improperly assigned voxels was quantified for each protocol. The dosimetric impact of improperly assigned voxels was evaluated with Geant4 Monte Carlo (MC) dose calculations for an 125I source in numerical phantoms.
Standard deviations for Zeff and ρe were reduced up to a factor ∼2 when using SAFIRE with strength 5 compared to FBP. Standard deviations on Zeff and ρe as low as 0.15 and 0.006 were achieved for the muscle insert representing typical soft tissue using a CTDIvol of 40 mGy and 3 mm slice thickness. Dose calculation accuracy was generally improved when using SAFIRE. Mean (maximum absolute) dose errors of up to 1.3% (21%) with FBP were reduced to less than 1% (6%) with SAFIRE at a CTDIvol of 10 mGy. Using a CTDIvol of 40mGy and SAFIRE yielded mean dose calculation errors of the order of 0.6% which was the MC dose calculation precision in this study and no error was larger than ±2.5% as opposed to errors of up to -4% with FPB.
This phantom study showed that the SAFIRE image reconstruction algorithm provided reduced standard deviations of Zeff and ρe in uniform regions of interest while preserving mean Zeff and ρe values. This resulted in improved material type assignment. The use of SAFIRE improved brachytherapy dose calculations for the materials from the phantom investigated in this study using 125I.