Jeremy Tan1,2,3, Adam C. Waspe1,2, Charles Mougenot4, Kullervo Hynynen1,5, James M. Drake1,2, and Samuel Pichardo3,6
1University of Toronto, Toronto, ON, Canada, 2Hospital for Sick Children, Toronto, ON, Canada, 3Thunder Bay Regional Research Institute, Thunder Bay, ON, Canada, 4Philips Healthcare, Toronto, ON, Canada, 5Sunnybrook Research Institute, Toronto, ON, Canada, 6Electrical Engineering, Lakehead University, Thunder Bay, ON, Canada
Synopsis
Accurate thermometry during abdominal high-intensity focused
ultrasound is severely compromised by motion and susceptibility artifacts. A
hybrid artifact correction method has been developed using principal component
analysis as a multi-baseline method and projection onto dipole fields as a
near-referenceless approach. The hybrid algorithm was tested using
free-breathing porcine and human subjects and achieved an average temperature
stability and precision of 0.31 (±0.22) °C and 1.18 (±0.94) °C, respectively in
the kidney.Purpose
Motion and susceptibility changes, caused by respiration
(periodic) and peristalsis (aperiodic), create intense MR thermometry artifacts
in the abdomen. This novel hybrid method aims to remove both periodic and
aperiodic artifacts by combining principal component analysis (PCA) and
projection onto dipole fields (PDF). PCA has previously been used for thermometry
motion correction, based on the assumption that organ motion produces an
equivalent shift in both magnitude and phase images.
1 However, this
premise is violated when artifacts appear in stationary tissue, due to
susceptibility changes outside of the field of view. In this hybrid method, PCA
serves as a multi-baseline method and corrects both motion- and
susceptibility-based periodic artifacts. The implementation is similar to
previous work in facial recognition
2 and does not require any motion
tracking tools or navigator echoes. Aperiodic motion is handled by the PDF
algorithm, originally created for background phase removal in quantitative
susceptibility mapping
3. It functions as a near-referenceless
method, correcting aperiodic artifacts induced by susceptibility changes at
air-tissue interfaces.
Methods
The hybrid method incorporates PCA and PDF.
PCA: During a learning step, an atlas of eigenimages is computed based on
images acquired in a pre-heating period. Newly acquired images are then
projected onto the subspace spanned by the eigenimages and a reference image is
generated as a sum of these vectors. This PCA reference image is then employed
by the PDF method to correct for aperiodic artifacts.
PDF: All phase patterns can be generated as a dipole response to
either i) local susceptibility sources in tissue or ii) far-reaching
susceptibility differences at air-tissue interfaces. By projecting the complete
dipole response (i and ii) onto a subspace which spans only the dipole response
from the surroundings (ii), the local susceptibility sources can be isolated
from the fluctuating external sources. This allows for accurate thermometry
inside the tissue regardless of external influences.
Evaluation: Non-heating
experiments were performed to acquire thermometry data of
in vivo kidneys
in free-breathing pigs and human volunteers. Data was collected on a Philips 3T
Achieva scanner (FOV: 300 x 300 mm
2, voxel size = 1.34 mm, slice
thickness = 11 mm, TE/TR = 16/26 ms, flip angle = 20°, acquisition matrix = 200
x 198, reconstruction matrix = 224, ETL = 9, NEX = 1, dynamic time = 0.58 s).
Data from an
in vivo porcine head and neck hyperthermia experiment
4
was also processed (Philips 3T Achieva scanner, FOV: 400 x 400 mm
2,
voxel size = 2.08 mm, slice thickness = 7 mm, TE/TR = 16/45 ms, flip angle =
18°, acquisition matrix = 192 x 191, reconstruction matrix = 192, ETL = 11, NEX
= 1, dynamic time = 0.88 s).
Results
Thermometry data of
in vivo kidneys (Figure 1) was
processed with the PCA-PDF algorithm and a standard subtraction method for
comparison. Figure 2 displays progressive correction of both periodic and
aperiodic artifacts. Temperature stability (temporal standard deviation of
spatial average) and temperature precision (temporal average of spatial
standard deviation), measured in the upper portion of the kidney can be seen in
Figure 3 for both methods. The PCA-PDF method improves average temperature
stability and precision by roughly an order of magnitude as compared with subtraction. Temperature stability is improved from 3.33 (±1.45) ºC to 0.31 (±0.22) ºC and temperature precision is improved from 11.53 (±13.68) ºC to 1.18 (±0.94) ºC. Figure
4 shows preservation of heat-induced phase change in the hyperthermia data. Also shown is a comparison with a multi-baseline method
4 which uses echo-navigator data for classification. PCA-PDF corrects fluctuating artifacts and improves
stability beyond the performance of the navigator echo assisted multi-baseline method
4, without collecting any supplementary data.
Conclusion
The PCA-PDF hybrid method demonstrates effective motion correction across a variety of artifacts in free-breathing specimens. The low standard deviation of the PCA-PDF temperature stability and precision illustrates the algorithm's robust correction of complex artifacts in each subject. The algorithm also maintains high-fidelity temperature reporting and improved stability compared to methods that rely on navigator echoes. Ongoing efforts are directed at improving the performance of PCA-PDF at tissue borders and improving discrimination between overlapping heat and artifacts.
Acknowledgements
Authors acknowledge financial support from the Canadian Institutes of Health Research, the Discovery Program of the Natural Sciences and Engineering Research Council of Canada, and the Federal Economic Development Agency for Southern Ontario. CM is an employee of Philips.References
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