0298

Real-Time Imaging of Lower Airway Collapse at 0.55T

Sarina Kapai¹, Prakash Kumar¹, Ecrin Yagiz¹, Ye Tian¹, Roberta Kato², Marcus Chen³, Marcela Ferrada³, Adrienne Campbell-Washburn³, and Krishna S Nayak¹
¹Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, United States, ²Children's Hospital Los Angeles, Los Angeles, CA, United States, ³National Institutes of Health, Bethesda, MD, United States

Synopsis

Keywords: Rare Disease, Low-Field MRI, Acquisition Methods, Data Acquisition, Pulse Sequence Design, Lower Airway, Tracheomalacia, Lung

Motivation: Tracheomalacia involves intermittent collapse of the lower airway and is difficult to diagnose. Currently, this condition is evaluated with invasive procedures like bronchoscopy or dynamic computer tomography (CT) which utilizes ionizing radiation.

Goal(s): To develop and optimize a high spatial and temporal resolution pulse sequence for evaluating tracheomalacia at 0.55T.

Approach: We optimize contrast (sequence, flip angle) for dynamic imaging in the lower airway for both 2D and SMS real-time imaging.

Results: Spiral acquisition achieves sufficient temporal resolution and the TrueFISP sequence (flip angle=32°) offers optimal contrast for imaging lower airway collapse.

Impact: We demonstrate the feasibility of capturing lower airway collapse with 2D and SMS real-time imaging (at 0.55T) in a way that is informative to the diagnosis and longitudinal monitoring of tracheomalacia.

Introduction

Tracheomalacia (TM) is characterized by damage to cartilaginous structures within the trachea that causes intermittent airway collapse; ultimately, this can lead to significant morbidity and mortality¹. Diagnosis of TM is challenging, often necessitating invasive procedures such as bronchoscopy, which can underestimate the severity of collapse. Dynamic CT has been explored as a potential diagnostic tool for TM; however, it deposits high levels of radiation².

There is an opportunity for MRI to provide value in the evaluation of lower airway collapse patients. Additionally, the decreased susceptibility artifacts experienced at lower field strengths such as 0.55T allow for better imaging of airway dynamics^3,4. We propose a protocol consisting of 2D and SMS real-time sequences that can resolve intermittent collapse with sufficient spatial and temporal resolution; this is similar to prior work at higher field strengths, but with substantially better spatiotemporal resolution⁵. We utilize spiral sampling and explore various sequences and flip angles (FA) to optimize the acquisition. Our primary objective is to outline the technical development required to prove that 0.55T MRI is capable of evaluating TM.

Methods

Experimental Methods
Experiments were performed using a whole body 0.55T system (prototype Magnetom Aera, Siemens Healthineers, Erlangen, Germany) equipped with high-performance shielded gradients (45 mT/m amplitude, 200 T/m/s slew rate). Seven healthy adult volunteers (age 22-30, 3F), and three patients with tracheomalacia/suspected bronchomalacia (age 6-13, M) were enrolled in this study, under a protocol approved by our Institutional Review Board and after providing written informed consent. The imaging protocol includes a real-time TrueFISP localizer, 2D real-time spiral FLASH/FISP/TrueFISP, and SMS real-time spiral TrueFISP. During the scan, patients were instructed to perform breathing maneuvers such as sustained expiration, bearing down, and repeated coughing. Total scan times were ≤1 hour.

Pulse Sequence
2D and SMS real-time spiral scans were implemented in the RTHawk (HeartVista, Inc., Menlo Park, CA) real-time interactive scanning platform⁶. Three different sequences were tested with golden-angle spiral trajectories: FLASH, FISP, and TrueFISP.

Evaluation
Reconstruction for spiral sequences was performed offline with GIRF corrected spiral trajectory and with spatially and temporally constrained reconstruction^7-9. Online low-latency gridding reconstruction was provided to the scan operator without GIRF correction.

Results

Figure 1 illustrates the airway anatomy and its important structures, such as the cartilaginous C-rings that scaffold the trachea and the smooth muscle tissue on the posterior wall¹⁰. Additionally, axial views of the airway are compared for a healthy adult and a patient with TM during inhalation and exhalation.
Figure 2 shows signal intensity normalized by the proton density as a function of FA for each of the FLASH, FISP, and TrueFISP sequences. Experiment and simulation results are compared in order to identify the optimal FAs (FA_FLASH=11°, FA_FISP=24°, FA_TrueFISP=32°).
Figure 3 compares FLASH, FISP, and TrueFISP 2D real-time spiral sequences in the lower airway at 1.8x1.8x6mm³resolution. Each was acquired at the optimal FAs identified in Figure 2. Noise dominates in the FLASH acquisition, while FISP is affected by flow artifacts. TrueFISP was identified as the optimal sequence due to its robustness to both noise and flow artifacts. SNR was calculated by performing an SNR-unit recon and estimating the noise covariance matrix from a FA = 0° acquisition¹¹.
Figure 4 displays 2D real-time spiral scans of a patient with TM at 1.8x1.8x6mm³ spatial resolution and 146ms temporal resolution. Intermittent collapse is easily visualized and a 58% reduction in cross-sectional area was observed.
Figure 5 demonstrates that SMS real-time spiral imaging can resolve lower airway collapse along the inferior-superior axis. Depending on the placement of the slice group, airway collapse is captured in (a) a subset of the slices or (b) all three slices. This confirms that collapse severity varies along the trachea and motivates the need for monitoring collapse along the length of the airway.

Discussion and Conclusion

In this study, we demonstrate the feasibility of resolving intermittent airway collapse in the lower trachea with 0.55T MRI. 2D real-time spiral acquisition was optimized over sequence type and FA to achieve maximum SNR and contrast in the region of interest. Results indicate that TrueFISP is most robust to motion artifacts and flow, while maintaining high CNR/SNR in the lower airway. This permitted both 2D real-time and SMS real-time imaging of intermittent airway collapse in patients with TM. Future work will focus on improving the spatial and temporal resolution of the spiral trajectory scans in order to apply this work to younger patients with smaller anatomical features. Reducing flow artifacts in the SMS real-time acquisitions is also of interest in order to provide monitoring capability of the collapse along the length of the airway.

Acknowledgements

We acknowledge grant support from the National Institutes of Health (U01-HL167613) and research support from Siemens Healthineers.

References

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[4] Lim Y, Kumar P, Nayak KS. Speech production real-time MRI at 0.55 T. Magn Reson Med. 2023;10.1002/mrm.29843. doi:10.1002/mrm.29843

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[7] Campbell-Washburn AE, Xue H, Lederman RJ, Faranesh AZ, Hansen MS. Real-time distortion correction of spiral and echo planar images using the gradient system impulse response function. Magnetic Resonance in Medicine. 2016;75(6):2278-2285. doi:10.1002/mrm.25788

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Figures

Relevant airway anatomy and comparison between a normal airway and an airway with tracheomalacia during inhalation and exhalation. Notably, a diseased airway will collapse from the anterior side during exhalation due to increased intrathoracic pressure.⁹

A study of signal intensity normalized by the proton density as a function of FA for FLASH, FISP and TrueFISP to identify the optimal FA for each sequence. Results for simulation and experiment are compared. Note that the discrepancy between simulated and experimental FLASH results is likely caused by inadequate gradient spoiling.

2D real-time spiral acquisition in oblique axial view of the airway for a healthy volunteer (M, age 24) during free breathing. The airway remains open in a healthy volunteer (see Figure 1). Spatial resolution is 1.8x1.8x6 mm³ and temporal resolution is 146-148 ms. Results are shown for (a) FLASH with a FA of 11° and SNR = 4.75 (b) FISP with a FA of 24° and SNR = 5.94 (c) TrueFISP with a FA of 32° and SNR = 8.25. Overall, TrueFISP offers increased robustness to motion artifacts and flow, and greater SNR in the region of interest.

2D real-time spiral TrueFISP (FA=33°) acquisition in a patient with tracheomalacia (M, 13 years) in (a) sagittal view and (b) oblique axial view. The red arrow indicates the site of significant airway collapse; note the presence of a fluid collection in the open esophagus. A 58% reduction in cross-sectional area is observed. Spatial resolution is 1.8x1.8x6 mm³ and temporal resolution is 146 ms.

SMS real-time spiral TrueFISP (FA=33°) acquisition showing airway collapse across three slices in a patient with tracheomalacia (M, 13 years). Red arrows indicate the presence of a clear collapse. Spatial resolution is 1.8x1.8x6 mm³and temporal resolution is 65 ms.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)

0298

DOI: https://doi.org/10.58530/2024/0298