Optimized 3D ultrashort echo time pulmonary MRI

Abstract

Purpose: To optimize 3D radial ultrashort echo time MRI for high resolution whole-lung imaging.

Methods: 3D radial ultrashort echo time was implemented on a 3T scanner to investigate the effects of: (1) limited field-of-view excitation, (2) variable density readouts, and (3) radial oversampling. Improvements in noise performance and spatial resolution were assessed through simulation and phantom studies. Their effects on lung and airway visualization in five healthy male human subjects (mean age 32 years) were compared qualitatively through blinded ordinal scoring by two cardiothoracic radiologists using a nonparametric Friedman test (P < 0.05). Relative signal difference between endobronchial air and adjacent lung tissue, normalized to nearby vessel, was used as a surrogate for lung tissue signal. Quantitative measures were compared using the paired Student's t-test (P < 0.05). Finally, clinical feasibility was investigated in a patient with interstitial fibrosis.

Results: Simulation and phantom studies showed up to 67% improvement in SNR and reduced blurring for short T2* species using all three optimizations. In vivo images showed decreased artifacts and improved lung tissue and airway visualization both qualitatively and quantitatively.

Conclusion: The use of limited field-of-view excitation, variable readout gradients, and radial oversampling significantly improve the technical quality of 3D radial ultrashort echo time lung images.

Keywords: MRI; lung; radial imaging; ultrashort echo time.

Copyright © 2012 Wiley Periodicals, Inc.

Figures

FIG. 1

Pulse sequence diagram for dual-echo variable density 3D UTE consisting of short (high bandwidth) slab excitation, an arc-length optimized out-and-back radial readout utilizing variable gradients, followed by a gradient spoiler.

FIG. 2

Respiratory motion can be minimized with real-time gating to end-expiration through adaptive feedback from the respiratory bellows signal with a 50% acceptance window (shaded in gray).

FIG. 3

Phantom images acquired with trapezoidal gradients (a) and variable density gradients (b) with vials arranged left to right from short to long T2* (0.53–3.1 ms). Note the longest T2* phantom (11.6 ms) is not shown for brevity. Note the increase in noise and decrease in edge sharpness in the short T2* vials. Similar trends are observed in experimental and theoretical measures for edge width (c), and SNR (d). For the shortest T2* species (T2* = 0.53 ms) there is a 67% gain in SNR with variable density sampling compared with ramp sampled trapezoidal gradients. Disagreement in edge width likely arises from off-resonance effects that were not included in the theoretical model.

FIG. 4

Effective flip angle (a) and signal at end of RF (b) for hard, SLR, and symmetric Sinc pulses as a function of pulse width. T2 was fixed at 0.5 ms. The effective flip angle is dominated by RF bandwidth effects and is largely independent of the pulse shape. However, signal is highly sensitive to RF shape due to signal decay during the RF pulse and is substantially higher for SLR pulses. The flip angle excitation profiles for SLR pulses (c) shows accurate inner volume excitation with outer volume excitation apparent with long RF pulses. Outer volume signal rapidly decays and is not detectable in the signal (d). Signal broadening occurs with long RF pulses.

FIG. 5

Scoring results from the consensus reader study of the five normal volunteers with statistical differences noted (P < 0.05). Consistent improvement in image quality was found with variable density readout gradients.

FIG. 6

Sagittal reformats of a 3D UTE from the same volunteer acquired with a hard RF pulse with trapezoid readout (a), selective RF pulse with trapezoid readout (b), and selective RF pulse with variable density readout (c). Severe artifacts are visible superiorly due to wrap from outside the FOV in (a) overlying the right upper lobe. Arrows show the right major fissure with progressive improvement in its visibility with this optimized UTE technique.

FIG. 7

Axial reformats of a 3D UTE from a volunteer acquired with a hard RF pulse with trapezoid readout (a), selective RF pulse with trapezoid readout (b), and selective RF pulse with variable density readout (c). Arrows point to a small subsegmental airway. This airway is obscured by artifacts in the acquisition using a hard pulse RF excitation. The bronchial wall is visible on both acquisitions with selective excitation, with slightly higher quality in the variable density acquisition. Note the generalized improvement in overall image quality from left to right.

FIG. 8

Axial and sagittal reformats of HRCT (a), 3D UTE (b), and the second echo of the 3D UTE sequence (TE = 2.1 ms). HRCT was performed 11 weeks after the 3D UTE scan. Both HRCT and 3D UTE scans show similar fibrosis patterns and extent, which is not well visualized in late echo MRI images. HRCT images show substantially higher spatial resolution, likely due to higher nominal resolution and reduced respiratory motion.