Ultrashort echo time imaging of the lungs under high-frequency noninvasive ventilation: A new approach to lung imaging

Abstract

Background: Although ultrashort echo time (UTE) sequences allow excellent assessment of lung parenchyma, image quality remains lower than that of computed tomography (CT).

Purpose: To investigate a high-frequency noninvasive ventilation (HF-NIV) technique allowing a stabilized inspiration and to compare image quality with current dedicated MR sequences.

Study type: Prospective.

Population: Ten healthy volunteers.

Field strength/sequence: 3D radial UTE sequence at 1.5T.

Assessment: UTE-HF-NIV sequence was compared with UTE-free-breathing (UTE-FB), reconstructed at end expiration (UTE-Exp) and average (UTE-Avg), and breath-hold VIBE sequences. The distance from lung apex to the dome of the right hemidiaphragm was measured. Visual assessment of the visibility and sharpness of normal anatomical structures was carried out. Dedicated software also quantitatively evaluated vessel-lung and right lung-liver interface sharpness. Apparent signal ratio (Sr) and contrast ratios (Cr) were quantitatively evaluated.

Statistical tests: Wilcoxon signed rank test for visual scores, paired t-test for continuous variables, significance at P < 0.05.

Results: The distance between apex and the right hemidiaphragmatic dome was significantly larger (P < 0.001) with UTE-HF-NIV compared with UTE-FB and VIBE acquisitions. Vessel and airway visibility had identical median visual scores with all UTE methods. Median visual scores for sharpness of vessels and airways were significantly higher (P < 0.001) with HF-NIV (vessels = 3; airways = 2) than in UTE-FB (vessels = 2; airways = 1) and VIBE (vessels = 1; airways = 1). Software-based vessel sharpness evaluation resulted in larger values in 8/10 volunteers with UTE-HF-NIV (67.3 ± 9.8) compared with UTE-Avg (62.3 ± 12.6) but the average difference was not significant (P = 0.28). The sharpness of the lung-liver interface was significantly higher (P < 0.001) with HF-NIV (17.3 ± 5.3) compared with UTE-Avg (14.1 ± 3.9). Significantly higher values (P < 0.01) of Sr and Cr were observed with UTE-HF-NIV compared with UTE-FB and VIBE.

Data conclusion: HF-NIV allowing acquisition at full inspiration significantly improves image quality for lung imaging. This could offer the option to alternate some follow-up CT studies by using this technique.

Level of evidence: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:1789-1797.

Keywords: UTE; lung; respiratory stabilization; ventilation.

© 2019 The Authors. Journal of Magnetic Resonance Imaging published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

Figures

Figure 1

(a) The average distance from lung apex to dome of right hemidiaphragm was significantly increased with the use of HF‐NIV (error bars indicate standard deviation). (b) The median value (thick line) of the visibility score was identical between all UTE acquisition and reconstruction methods for airways (b), and between all methods for central vasculature (c) and peripheral vasculature (d). Upper and lower boundaries of boxes indicate the 25% and 75% quantiles, respectively. In particular, the interquartile range of airway visibility scores included higher values when HF‐NIV was used.

Figure 2

Visual evaluation of sharpness indicated significant increases for central vasculature (a), peripheral vasculature (b), and airways (c). Thick line indicates median, while upper and lower boundaries of boxes indicate the 25% and 75% quantiles, respectively. Software‐based quantification of vessel sharpness indicated a trend to an increase but was not significant (d). However, the average lung–liver interface sharpness was significantly increased with the use of HF‐NIV (e). Error bars indicate standard deviation.

Figure 3

Significantly increased average signal intensity with use of HF‐NIV in the lung parenchyma (a). Significantly decreased signal intensity compared with VIBE sequences in the vessels (b). No significant differences in airway (c) signal intensities. Resulting significantly higher average apparent signal (d) and contrast (e) ratios with the use of HF‐NIV. Error bars indicate standard deviation.

Figure 4

Comparison of the two equally sampled UTE acquisitions in a healthy volunteer. (a) Coronal slice of a free‐breathing acquisition (UTE‐Avg). (b) Coronal slice of an HF‐NIV acquisition. (c) Axial slice of a free‐breathing acquisition (UTE‐Avg). (d) Axial slice of an HF‐NIV acquisition. The difference in lung volume can be appreciated visually. Sharper features are observed in images acquired with HF‐NIV. Comparison of the two equally sampled UTE acquisitions in another healthy volunteer. (e) Coronal slice of a free‐breathing acquisition (UTE‐Avg). (f) Coronal slice of an HF‐NIV acquisition. (g) Axial slice of a free‐breathing acquisition (UTE‐Avg). (h) Axial slice of an HF‐NIV acquisition. The difference in lung volume can be appreciated visually. For this volunteer, contrary to most other cases, the free‐breathing acquisition contained few respiratory motion artifacts. In this case, blurring introduced by the ventilation technique can be observed at the lung bases (f).

Figure 5

Axial 1.30‐mm thick slices at the level of the middle lobe and the lingula bronchovascular bundles of a VIBE (a), UTE‐Avg (b), UTE‐Exp (c), UTE‐HF‐NIV (d) sequences. The sharpness of vessels such as the pulmonary artery branch of the lingula (blue arrows) is increased, as well as the sharpness of bronchial walls with the UTE‐HF‐NIV sequence. A 15‐mm thick maximum intensity projection reformat in a coronal orientation of a VIBE (e), UTE‐Avg (f), UTE‐Exp (g), UTE‐HF‐NIV (h) sequences. Although vessel visibility is similar between various sequences, the sharpness of vessels is increased with the UTE HF‐NIV sequence. Note the increased volume of lungs with the same sequence allowing to display the elongated vessels on a large section.