1. Wednesday Meeting Mario Djukelic, 22.03.2017, Slide 1
Quantitative Image Quality Evaluation
for kV Cone-beam CT-based IGRT
2. Wednesday Meeting Mario Djukelic, 22.03.2017, Slide 2
Introduction
• Kilovoltage (kV) cone-beam Computed
Tomography (CT) based Image Guided
Radiotherapy (IGRT) system is a quality assurance
measuring tool and a vital equipment for target
localization plus patient setup verification in an
advanced radiotherapy treatment.
• The kV cone-beam CT helps in differentiating the
soft tissues and offering the volumetric images of
the patient using low radiation doses while the
IGRT focuses on guidance and imaging during the
radiation treatment.
• The American Association of Physicist in Medicine
(AAPM) task group (TG) 142 and 179 recommends
a set of quality assurance (QA) programs to
guarantee consistent performance for kV cone-
beam CT system (Lim & Zin, 2017).
3. Wednesday Meeting Mario Djukelic, 22.03.2017, Slide 3
Materials and Methods
• For the kV cone-beam CT system, use a flat-panel
imager with a primary filtration of 6.8 mm and an
active area of 41 by 41 cm2 besides 1024 by 1024
detector elements of 16-bit pixel depth.
• Scan the phantoms using an optimized imaging
protocol that captures 660 projection images in a single
rotation of 5.5 frames per second and reconstruct the
scanned images using a filtered back-projection
reconstruction algorithm known as Feldkamp 3D.
• Evaluate the imaging performance of the kV cone-beam
CT system using Catphan-600 and an image quality
phantom referred to as CIRS-062QA.
• Evaluate the image uniformity in both the vertical and
the horizontal directions using the pixel values across
the center of the uniformity CT slice that covers 80% of
the phantom.
4. Wednesday Meeting Mario Djukelic, 22.03.2017, Slide 4
Results
• Figure 3 demonstrates the uniformity profiles for
Catphan-600 and CIRS-062QA phantom in both
horizontal and vertical directions. In essence, the
Catphans-600 has a greater image uniformity in
comparison with the CIRS-062QA.
• The results also demonstrates that Catphan-600 has
0.0163 integral non-uniformity while CIRS-062QA has
0.0504. The implication of this finding is that lower
integral non-uniformity values offers better image
uniformity and vice versa.
• The CIRS-062QA phantom has a low contrast visibility
value of 1.83 while the Catphan-600 value is 2.42. The
lower value for CIRS-062QA phantom results into less
attenuation of beam and minimizes the image noise since
the low contrast visibility has a direct relationship with
the image noise. For this reason, CIRS-062QA phantom
has a better visualization of soft tissue.
5. Wednesday Meeting Mario Djukelic, 22.03.2017, Slide 5
Discussion
• The image acquired by CIRS-062QA phantom had a
cupping artefact as a result of the hardening and beam
scattering yielded by the phantom holder. Apparently,
the captured images had some parts of the phantom
holder as demonstrated in figure 4(a) hence resulting in
more scattering and beam hardening as shown in figure
4(b).
• The spatial resolution of the imaging system is function
of the size of the focal spot, x-ray scintillator type,
voxel size, and the quantity of the images. Figure 5 (b)
shows the MTF results with the spatial frequencies for
Catphan-600 at f50 and f10 are 5.26 lp/cm and 9.24
lp/cm and those of CIRS-062QA being 4.08 lp/cm and
6.63 lp/cm respectively. Since the imaging parameters
were consistent, the discrepancy in spatial resolution is
a result of the differences in beam attenuation and
image visibility for the background materials.
6. Wednesday Meeting Mario Djukelic, 22.03.2017, Slide 6
Conclusion
• Image quality metrics of a kV cone-beam CT system have been assessed using the
two commercial phantoms, that is, Catphan-600 and CIRS-062QA.
• The developed algorithms for the quantitative image quality assessment offers a set
of reference values that are phantom-specific to be used for a kV cone-beam CT
system.
• The variations in image quality metrics for the phantoms have been caused by the
differences in the geometric design and size of the phantoms.
• For future studies, there is need to look at ways of enhancing the setup of the CIRS-
062QA phantom, to remove the screws and phantom holder from the field-of-view.
7. Wednesday Meeting Mario Djukelic, 22.03.2017, Slide 7
References
• Lim, S. Y., & Zin, H. M. (2017). Quantitative image quality evaluation for kV cone-beam CT-based IGRT.
In Journal of Physics: Conference Series (Vol. 851, No. 1, p. 012029). IOP Publishing.
