Outracing Lung Signal Decay – Potential of Ultrashort Echo Time MRI

Abstract

Background Magnetic resonance imaging (MRI) of the pulmonary parenchyma is generally hampered by multiple challenges related to patient respiratory- and circulation-related motion, low proton density and extremely fast signal decay due to the structure of the lungs evolved for gas exchange. Methods Systematic literature database research as well as annual participation in conferences dedicated to pulmonary MRI for more than the past 20 years by at least one member of the author team. Results and Conclusion The problem of motion has been addressed in the past by developments such as triggering, gating and parallel imaging. The second problem has, in part, turned out to be an advantage in those diseases that lead to an increase in lung substance and thus an increase in signal relative to the background. To reduce signal decay, ultrashort echo time (UTE) methods were developed to minimize the time between excitation and readout. Having been postulated a while ago, improved hardware and software now open up the possibility of achieving echo times shorter than 200 µs, increasing lung signal significantly by forestalling signal decay and more effectively using the few protons available. Such UTE techniques may not only improve structural imaging of the lung but also enhance functional imaging, including ventilation and perfusion imaging as well as quantitative parameter mapping. Because of accelerating progress in this field of lung MRI, the review at hand seeks to introduce some technical properties as well as to summarize the growing data from applications in humans and disease, which promise that UTE MRI will play an important role in the morphological and functional assessment of the lung in the near future. Key Points: Citation Format

Balanced steady‐state free precession thoracic imaging with half‐radial dual‐echo readout on smoothly interleaved archimedean spirals

“…Experiments were performed on a 1.5T whole‐body MR‐scanner (MAGNETOM Avanto‐Fit, Siemens Healthineers, Erlangen, Germany) using 12‐channel thorax and 24‐channel spine coil for signal reception. Two phantoms and four healthy volunteers (mean age: 40.2 y, range: 30–52 y, 3 male, 1 female) were scanned with a hr‐de‐bSSFP and a hr‐de‐SPGR variant with essentially identical bipolar readout gradient for signal comparison, as well as with a contemporary single‐echo 3D UTE imaging sequence . The study was approved by the Institutional Review Board, and written informed consent was obtained from each volunteer before the examinations.…”

“…The first SPGR setup was used for the comparison between the signal intensity measured in a phantom and in vivo using an identical bipolar half‐radial dual‐echo readout gradient and minimal TR. The second SPGR setup was only used for in vivo scans with imaging parameters similar to the commonly used ones found in the literature on pulmonary UTE imaging . The overall scan time was fixed for both bSSFP and SPGR to the same value, resulting in different number of projections because of the different TR.…”

“…Two phantoms and four healthy volunteers (mean age: 40.2 y, range: 30-52 y, 3 male, 1 female) were scanned with a hr-de-bSSFP and a hr-de-SPGR variant with essentially identical bipolar readout gradient for signal comparison, as well as with a contemporary single-echo 3D UTE imaging sequence. 25,26 The study was approved by the Institutional Review Board, and written informed consent was obtained from each volunteer before the examinations. In vivo thorax scans were obtained in expiratory or inspiratory breath-hold in supine position.…”

“…The second SPGR setup was only used for in vivo scans with imaging parameters similar to the commonly used ones found in the literature on pulmonary UTE imaging. 25,26 The overall scan time was fixed for both bSSFP and SPGR to the same value, resulting in different number of projections because of the different TR.…”

The current status and further prospects for lung magnetic resonance imaging in pediatric radiology

“…The T1-W UTE sequences are available as 3-D sequences in reconstructible isovoxel geometry or as 2-D sequences, which can be executed in 13 s (slice width 2.5 mm) and also as so-called Zero Echo Time sequences. Technical background of these three lung sequences can be found in an excellent review by Wielpütz et al 2019 [33].…”