| dc.description.abstract |
<p>Imaging respiratory induced tumor motion in the radiation therapy treatment room
could eliminate the necessity for large motion encompassing margins that result in
excessive irradiation of healthy tissues. Currently available image guidance technologies
are ill-suited for this task. Two-dimensional fluoroscopic images are acquired with
sufficient speed to image respiratory motion. However, volume information is not
present, and soft tissue structures are often not visible because a large volume is
projected onto a single plane. Currently available volumetric imaging modalities
are not acquired with sufficient speed to capture full motion trajectory information.
Four-dimensional cone-beam computed tomography (4D CBCT) using a gantry mounted 2D
flat panel imaging device has been proposed but has been limited by high doses, long
scan times and severe under-sampling artifacts. The focus of the work completed in
this thesis was to find ways to improve 4D imaging using a gantry mounted 2D kV imaging
system. Specifically, the goals were to investigate methods for minimizing imaging
dose and scan time while achieving consistent, controllable, high quality 4D images.</p><p>First,
we introduced four-dimensional digital tomosynthesis (4D DTS) and characterized its
potential for 3D motion analysis using a motion phantom. The motion phantom was programmed
to exhibit motion profiles with various known amplitudes in all three dimensions and
scanned using a 2D kV imaging system mounted on a linear accelerator. Two arcs of
projection data centered about the anterior-posterior and lateral axes were used to
reconstruct phase resolved DTS coronal and sagittal images. Respiratory signals were
obtained by analyzing projection data, and these signals were used to derive phases
for each of the projection images. Projection images were sorted according to phase,
and DTS phase images were reconstructed for each phase bin. 4D DTS target location
accuracies for peak inhalation and peak exhalation in all three dimensions were limited
only by the 0.5 mm pixel resolution for all DTS scan angles. The average localization
errors for all phases of an assymetric motion profile with a 2 cm peak-to-peak amplitude
were 0.68, 0.67 and 1.85 mm for 60 <super> o <super/> 4D DTS, 360<super> o <super/>
CBCT and 4DCT, respectively. Motion artifacts for 4D DTS were found to be substantially
less than those seen in 4DCT, which is the current clinical standard in 4D imaging.
</p><p>We then developed a comprehensive framework for relating patient respiratory
parameters with acquisition and reconstruction parameters for slow gantry rotation
4D DTS and 4D CBCT imaging. This framework was validated and optimized with phantom
and lung patient studies. The framework facilitates calculation of optimal frame
rates and gantry rotation speeds based on patient specific respiratory parameters
and required temporal resolution (task dependent). We also conducted lung patient
studies to investigate required scan angles for 4D DTS and achievable dose and scan
times for 4D DTS and 4D CBCT using the optimized framework. This explicit and comprehensive
framework of relationships allowed us to demonstrate that under-sampling artifacts
can be controlled, and 4D CBCT images can be acquired using lower doses than previously
reported. We reconstructed 4D CBCT images of three patients with accumulated doses
of 4.8 to 5.7 cGy. These doses are three times less than the doses used for the only
previously reported 4D CBCT investigation that did not report images characterized
by severe under-sampling artifacts. </p><p>We found that scan times for 200<super>
o <super/> 4D CBCT imaging using acquisition sequences optimized for reduction of
imaging dose and under-sampling artifacts were necessarily between 4 and 7 minutes
(depending on patient respiration). The results from lung patient studies concluded
that scan times could be reduced using 4D DTS. Patient 4D DTS studies demonstrated
that tumor visibility for the lung patients we studied could be achieved using 30<super>
o <super/> scan angles for coronal views. Scan times for those cases were between
41 and 50 seconds. Additional dose reductions were also demonstrated. Image doses
were between 1.56 and 2.13 cGy. These doses are well below doses for standard CBCT
scans. The techniques developed and reported in this thesis demonstrate how respiratory
motion can be imaged in the radiotherapy treatment room using clinically feasible
imaging doses and scan times.</p>
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