Soft Tissue Visualization Using a Highly Efficient Megavoltage Cone ...

(a): 0.025cGy receptor 1 (b): 0.035cGy receptor 2
3.2 Reconstructed Images
3.2.1 Phantom Images
Figure 3: QC-3V phantom images
Figure 4 shows the QUASAR and contrast/spatial resolution (ball) phantom images that were acquired
with receptor 1. The total delivered dose was 18cGy and no preprocessing filtering used. From Figure
4(a) 2% electron density variation between acrylic and trabecular bone is resolvable without applying
the adaptive filtering. Applying the resolution/dose adaptive filter will reduce the standard deviation in
pixel intensities by a factor of ~0.5. Using the ratio of pixel intensity difference (due to density
difference) to root mean square uncertainty in pixel intensities of receptor 1, it can be estimated that
the receptor 2 due to its higher CNR compared to receptor 1 and in combination with preprocessing
adaptive filtering is capable of resolving 1% contrast with radiation doses as low as 10cGy.
The left image on figure 5 illustrates that muscle insert with contrast of 0.5% is resolvable with
receptor 1 using 163cGy dose and without applying dose/resolution adaptive filtering. By using only
the odd projections used for the previous reconstruction (163cGy) that corresponds to the dose value of
81.5cGy (center image of figure 5), the 0.5% contrast is still detectable. The right hand image of figure
5 shows that the minimum required dose in which the 0.5 % contrast becomes discernible with
receptor 1 and by applying the adaptive filter is ~41MU (threshold of detection of 0.5% contrast). Note
that all figure 5 images have the cupping artefacts that are due to the scattered, and no scatter/beam
profile corrections have been applied to these images.
Figures 4(b) and 6(a) illustrate that the 2mm air holes and gold seeds implanted (1mm x 3mm) within
a head phantom are resolvable.
Figure 6 (a) shows axial, coronal and sagittal images of a head phantom that is acquired with receptor
2 by delivering only 1cGy. Figure 6 (b) displays the raw image (no adaptive filtering) of the 2D
projection of this phantom acquired with 0.005cGy. The mean/standard deviation (SD) ratios of this
2D projection (on open field regions) before and after applying dose/resolution adaptive filter are 26
and 68, respectively. This indicates the possibility of the use of receptor 2 for 4D-CT.