Abstract Archives of the RSNA, 2003
Jiang Hsieh PhD, PRESENTER: Nothing to Disclose
Abstract:
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Purpose: Use of CT scanners for oncology treatment planning has received
significant interest. Since the patient needs to be positioned in a similar
fashion as in a therapy machine, a part of the patient is often placed outside
the field-of-view (FOV) defined by the CT scanner. The patient anatomy outside
of the FOV may lie in the treatment beam path and therefore must be considered
when calculating X-ray attenuation and dose delivery. In addition, truncated
projections often produce image artifacts and make attenuation estimation
difficult. We propose a reconstruction algorithm that allows adequate
estimation of the object outside the FOV.
Methods and Materials: Since the total attenuation of each ideal projection in
a parallel sampling geometry remains constant over views, we can accurately
estimate the sum of missing projection data. To overcome the small fluctuation
due to non-perfect calibrations and patient motion, we use the projections of
two neighboring non-truncated views as the basis. The missing attenuation for
each view is the difference between the linearly interpolated total attenuation
and the actual measurement. Next, we use the magnitude and slope of the
projection samples at the truncation to estimate the size and the location of a
water cylinder that can best fit to the projection data. Additional expansion
and contraction of the fitted projection takes place based on the difference
between the total attenuation of the fitted missing projection and the
previously calculated projection. Continuity constraints are placed on the
fitting parameters.
Results: Extensive phantom and clinical experiments were conducted to validate
our approach. For quantitative analysis, we positioned the phantoms from 0cm to
8cm outside the 50cm FOV at 1cm increments. Eight different phantoms were used:
48cm poly, 35cm poly, 20cm GE QA resolution phantom, 20cm QA low-contrast
phantom, 20 cm QA water phantom, 20 cm linearity phantom, 5 in. water phantom,
and a thorax phantom. Results show that CT number accuracy is fully recovered
inside the 50cm FOV in all cases. The maximum amount of CT number deviation
outside FOV is about 40HU. The shape of the truncated object has been recovered
to an accuracy of a few millimeters.
Conclusion: We present an algorithmic approach to extend the FOV of a CT
scanner beyond the hardware limit. Extensive phantom and clinical experiments
have shown that the algorithm can successfully restore images inside the FOV
without residual truncation artifacts. Regions outside the FOV is adequate for
oncology applications. (J.H., E.C., B.G., A.H., S.M., T.J.M. are employees of
GE Medical Systems.)
Hsieh PhD, J,
A Reconstruction Algorithm to Extend the Field-of-view Beyond the Scanner Limit. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3107061.html