Wingchi Edmund Kwok1,2, Clement Ren3, Gloria Pryhuber4, Mitchell Chess1, and Jason C. Woods5
1Department of Imaging Sciences, University of Rochester, Rochester, NY, United States, 2Rochester Center for Brain Imaging, University of Rochester, Rochester, NY, United States, 3Department of Pediatrics, University of Rochester, Rochester, NY, United States, 4Departments of Pediatrics and Environmental Medicine, University of Rochester, Rochester, NY, United States, 5Departments of Pediatrics and Radiology, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH, United States
Synopsis
Our purpose was to study the
feasibility of high-resolution lung ultra-short TE imaging of young children at
3T without sedation and tackle potential challenges. Two subjects aged 7
and 8 with mild cystic fibrosis were recruited.
They were supported by a child life specialist and the use of a mock
magnet. Siemens work-in-progress UTE and
PETRA_D sequences were used for lung imaging.
The images depicted the lung parenchyma, airways and vessels, and revealed
abnormalities such as bronchial wall thickening. The techniques should be useful for the
monitoring of lung development and evaluation of lung diseases in children.Purpose
Imaging
of the lung is useful for the evaluation of lung development and diseases such
as cystic fibrosis in children. While
computed tomography has been the standard imaging modality for the lung, it
carries potentially harmful ionizing radiation that is especially undesirable
for children. Though MRI does not have
ionizing radiation, the short T2* of lung tissue makes it difficult to image
using conventional pulse sequences. Ultra-short
echo-time MRI has been explored for lung imaging, but most studies have been conducted
on adults
1-3. Challenges to
ultra-short TE lung imaging of young children include the need for higher image resolution,
motion artifact prevention and the preference for no sedation. Our objective is to study the feasibility of
high-resolution lung ultra-short TE imaging of young children at 3T without sedation.
Methods
The study was conducted on a Siemens TRIO TIM 3T system. Two young female subjects, aged 7 and 8, were
recruited. Both have mild cystic fibrosis. Prior to MR
imaging, they were brought into a mock magnet with recorded scanning sound to
get accustomed to the MRI environment. A
child life specialist was present to explain the MRI procedures and to provide
support to the subjects. A body matrix
coil was used together with a spine coil to image the lung. Work-in-progress ultra-short TE
sequences obtained from Siemens were used.
They included UTE and PETRA_D; both utilized 3D radial
acquisition with isotropic image resolution.
The two sequences differ in that the readout gradient in PETRA_D is switched on before RF excitation, resulting in missing central k-space points that are acquired at the end of the sequence. Axial images were obtained with FOV 26cm, TR 3ms, TE 0.07 to 0.1ms, matrix size 192 to 256, 3D isotropic resolution 1.01 to 1.32 mm and number of radial views 100,000. To optimize SNR, a flip angle of 4
0 was used, which was close to the Ernst angle of 3.78
0 for TR of 3ms, assuming T1 of lung tissues to be 1374ms at 3T
4. Respiratory gating was
applied to PETRA_D with the use of respiratory bellows and a 30%
threshold for end-expiration acquisition. UTE was not compatible with respiratory gating, and the scan was obtained during free breathing. The scan time was 5:00 mins for UTE and about
9 mins for PETRA_D. The images were evaluated by a radiologist for diagnostic quality and for the hallmarks of cystic fibrosis-specific lung disease.
Results
Both subjects were able to stay still during MR scanning. The ultra-short TE images depicted the lung parenchyma, airways and vessels, and revealed diagnostic information such as peribronchial wall thickening, bronchiectasis and atelectasis (Figs 1-3). For the PETRA_D sequence, the subject's arms were raised above the head to prevent wrap-around
artifacts in the left-right direction. Wrap-around in the head-foot direction was minimized by limiting signal coverage through coil element selection. The UTE sequence did not have wrap-around artifacts due to data oversampling.
Discussion
The use of the mock magnet
and the support from the child life specialist seemed to contribute positively to this study. Both subjects appeared very calm and relaxed
in the MR scanner, which would have helped them stay still. Sedation-free scanning allows the studies to be conducted more easily without an anesthesiologist, and is particularly advantageous to research studies as parents will be more willing to let their children participate.
Both the
PETRA_D and UTE sequences appear to be suitable for imaging young
children. Besides being compatible with respiratory gating that may be needed in some children, PETRA_D
also has the advantage of scanning quietly since the readout gradient changes
in small steps between radial projections. UTE has the
advantage of no wrap-around artifacts since it uses radial oversampling.
This study was conducted at 3T to
provide better SNR to support higher image resolution.
According to the Nyquist criterion, about
200,000 radial views are needed for 2563 image matrix 5. However, it would be sufficient to use 100,000 views that provide an alias free
FOV of about 70% of the diameter 6. With the acquired 3D isotropic resolution,
the axial images could be reformatted to other planes to facilitate evaluation.
In the future, we plan to investigate ultra-short TE lung imaging of even
younger children without sedation 7.
Conclusion
We have
successfully conducted high-resolution 3D ultra-short TE lung imaging of young children at 3T
without sedation. It should be
useful for the monitoring of lung development and for the evaluation of lung
diseases such as of cystic fibrosis.
Acknowledgements
The authors would like to thank Dr. Neils Oesingmann, Dr. David Grodski, and Dr. Taka Natsuaki of Siemens Healthcare for their assistance in obtaining the UTE and PETRA_D sequences for this study. We would also like to thank Jacqueline Wameling for her important involvement as a child life specialist. References
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