Yu Gao1,2, Yingli Yang3, Novena Rangwala1, and Peng Hu1,2
1Radiological Sciences, University of California, Los Angeles, Los Angeles, CA, United States, 2Physics and Biology in Medicine, University of California, Los Angeles, Los Angeles, CA, United States, 3Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, United States
Synopsis
DWI is a
promising imaging biomarker for tumor response evaluation, and the purpose of
this work is to develop a distortion-free DW sequence that is reliable for
adaptive treatment planning. ADC accuracy of our proposed diffusion-prepared
segmented bSSFP-based sequence against the standard spin-echo single-shot echo-planar-imaging
was quantitatively validated on a diffusion phantom. Geometric reliability was
confirmed on both phantom and in-vivo experiments. Preliminary patient study on
a MRI-Guided radiotherapy system showed high geometric accuracy and promising
tumor detection capability. All these demonstrate the feasibility of using the
DW-SSFP sequence for longitudinal tumor response evaluation and treatment
planning. Purpose
MRI
is increasingly incorporated into the workflow of radiotherapy. Recently, a
real time MRI-guided radiotherapy system (ViewRay™) has been commercially
introduced, which is capable of MRI and radiotherapy simultaneously while the
patient is in the treatment position. Such a system can potentially enable a
scientifically and logistically feasible adaptive radiotherapy strategy,
wherein the treatment plan is adapted throughout the course of therapy based on
tumor response assessment based on diffusion MRI. However, the most commonly
used spin echo single-shot echo-planar-imaging (SS-EPI) diffusion sequence suffers
from low spatial resolution and severe geometric distortion, which is
particularly problematic for radiotherapy because any geometric inaccuracy
directly translates to mis-calculated radiation dose and potentially radiation
target miss. The purpose of the current study was to develop a reliable,
distortion-free DW sequence that is practicable for tumor response evaluation
and radiation treatment re-planning.
Methods
Sequence: The DWI sequence used in this
study combines a diffusion preparation module with a segmented 3D steady-state
free precession (3D-SSFP) readout module, which has been previously proposed
for diagnostic imaging at higher field strength (1.5T or higher) [1, 2]. The
diffusion module, which was inserted at the beginning of each k-space segment, used
a twice-refocused spin-echo (TRSE) with diffusion gradient to generate
diffusion weighting. At the end of diffusion encoding, the magnetization is restored
using a 90°
RF pulse, which is followed by a spoiler. Table 1 lists the imaging protocol
parameters that were used in this study.
ADC Accuracy: A commercially available
diffusion phantom (High Precision Device, Inc.) was used to validate the ADC
accuracy of our DW-SSFP sequence. The
phantom consists of 13 vials filled with aqueous solutions
of polymer polyvinylpyrrolidone (PVP) at 0%, 10%, 20%, 30%, 40% and 50% concentrations
(Fig. 1a), and weighted mean apparent diffusion coefficient (ADC) for each vial
was provided as a reference. Our TRSE DW-SSFP sequence was compared with the standard
SS-EPI sequence on a Siemens Prisma 3T scanner. A total of twelve DW-SSFP
measurements and six SS-EPI measurements with the same phantom positioning was
conducted on the same/multiple days to verify the short-term/long-term
reproducibility.
Geometric
Reliability: The
standard SS-EPI and our DW-SSFP sequences were compared against Turbo Spin Echo
(TSE) for geometrical reliability in both phantom and healthy volunteer (three
brain scans and three liver scans). In the phantom study, the angle between
three fiducial markers was measured and compared to quantify the shape
invariability. In the volunteer study, contours of the brain and liver were
drawn in the three types of images, and the area differences between SS-EPI, DW-SSFP
and TSE were calculated.
Tumor Detection: A pilot study was performed on
the ViewRay system, which includes a tri-Cobalt 60 radiotherapy system and a
0.35T MRI system with Siemens pulse programming interface. The TSE, SS-EPI and
DW-SSFP images of one liver metastasis patient, one glioblastoma (GBM) patient,
and one peritoneal sarcoma patient were acquired.
Results and
Discussion
ADC
accuracy measurements are summarized in Table 2. Mean ADC values calculated from DW-SSFP agreed with both SS-EPI and reference values, indicating
ADC reliability of the DW-SSFP sequence. Extensive sequence development and optimization are underway to further improve ADC accuracy and stability.
Phantom
and in-vivo geometric reliability results are shown in Fig 1. Distortion from
EPI is apparent by visual inspection. Quantitative fiducial marker angle
measurements from the three type of sequences were: TSE: 89.37°±0.21°, SS-EPI:
87.57°±0.48°, DW-SSFP: 89.33°±0.20°. Absolute area difference between SS-EPI
and TSE was 253.43±57.45 (mm2), and absolute area difference between DW-SSFP
and TSE was 38.07±23.96 (mm2). The DW-SSFP-based contour matched
well with TSE whereas the SS-EPI-based contour had large derivation from TSE (Fig. 1(d-f)).
All these suggest that DW-SSFP is substantially more accurate and consistent than SS-EPI in
terms of geometric reliability.
For
all three cancer patients, ADC maps from SS-EPI and DW-SSFP were concordant,
and tumor regions exhibited different ADC patterns compared with normal surrounding,
suggesting the feasibility of detecting and observing tumor response based on
our DW-SSFP. The peritoneal sarcoma case is shown in Fig. 2, where the tumor
region was highlighted on both ADC maps due to surrounding muscle gave low
signal given long TE and low field strength of the ViewRay. Comparing with our DW-SSFP,
distortion of the SS-EPI was severe which made it unacceptable for treatment
planning.
Conclusion
Our DW-SSFP sequence provides desirable ADC accuracy and high geometric
reliability. Pilot study on ViewRay demonstrates the feasibility of
quantitative longitudinal evaluation of tumor response and consequently
treatment re-planning using the DW-SSFP sequence.
Acknowledgements
This research was supported, in part, by ViewRay, Inc. References
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