Evaluation of optimized breath-hold and free-breathing 3D ultrashort echo time contrast agent-free MRI of the human lung

Abstract

Purpose: To evaluate an optimized stack of radials ultrashort echo time (UTE) 3D magnetic resonance imaging (MRI) sequence for breath-hold and free-breathing imaging of the human lung.

Materials and methods: A 3D stack of ultrashort echo time radials trajectory was optimized for coronal and axial lower-resolution breath-hold and higher-resolution free-breathing scans using Bloch simulations. The sequence was evaluated in 10 volunteers, without the use of contrast agents. Signal-to-noise ratio (SNR) mean and 95% confidence interval (CI) were determined from separate signal and noise images in a semiautomated fashion. The four scanning schemes were evaluated for significant differences in image quality using Student's t-test. Ten clinical patients were scanned with the sequence and findings were compared with concomitant computed tomography (CT) in nine patients. Breath-hold 3D spokes images were compared with 3D stack of radials in five volunteers. A Mann-Whitney U-test was performed to test significance in both cases.

Results: Breath-hold imaging of the entire lung in volunteers was performed with SNR (mean = 42.5 [CI]: 35.5-49.5; mean = 34.3 [CI]: 28.6-40) in lung parenchyma for coronal and axial scans, respectively, which can be used as a quick scout scan. Longer respiratory triggered free-breathing scan enabled high-resolution UTE scanning with mean SNR of 14.2 ([CI]: 12.9-15.5) and 9.2 ([CI]: 8.2-10.2) for coronal and axial scans, respectively. Axial free-breathing scans showed significantly higher image quality (P = 0.008) than the three other scanning schemes. The mean score for comparison with CT was 1.67 (score 0: n = 0; 1: n = 3; 2: n = 6). There was no significant difference between CT and MRI (P = 0.25). 3D stack of radials images were significantly better than 3D spokes images (P < 0.001).

Conclusion: The optimized 3D stack of radials trajectory was shown to provide high-quality MR images of the lung parenchyma without the use of MRI contrast agents. The sequence may offer the possibility of breath-hold imaging and provides greater flexibility in trading off slice thickness and parallel imaging for scan time.

Keywords: 3D stack of radials; breath-hold; free-breathing; lung parenchyma.

Figure 1

(a): The simulated PSF in the x–y plane with the breath-hold scan parameters and typical lung relaxation parameters at 3T. M_xy = 1 corresponds to magnetization devoid of both T1 and T2 relaxation and for θ = 90°. The radius at FWHM was 0.85 mm (b) a 1-D profile of the PSF depicted at the center plane in Figure 1(a). FWHM was measured to be ~2 mm.

Figure 2

Single slice from coronal and axial lower resolution breath-hold (a) and higher resolution free breathing (b) scans of a normal volunteer.

Figure 3

MIP (25 mm slab thickness) of breath-hold lung images obtained at three different levels for (a) coronal and (b) axial scans. Native resolution was 2 × 2 × 8 mm³ for coronal scan and 2.5 × 2.5 × 8 mm³ for axial scan. Scan time was 18.5 s and 21 s, respectively, for complete coverage along the two directions.

Figure 4

MIP (25 mm slab thickness) of respiratory triggered free breathing lung images obtained at three different levels for (a) coronal and (b) axial scans. Native resolution was 1.5 × 1.5 × 5 mm³ for both scans. Scan time was 5:06 and 6:54 for complete coverage along the coronal and axial directions, respectively.

Figure 5

Semi-quantitative analysis of images for visualization of vessels and airways and the presence of artifacts. CL and AL refer to low resolution breath-hold coronal and axial images while CH and AH refer to high resolution FB coronal and axial scans, respectively. Note that maximum score for visualization was 4 (all the way to the periphery of lungs) while the best score for artifacts was 3 (no artifacts).

Figure 6

Scarring or atelectasis exhibited by a patient (CT and MRI done sequentially). (a) CT image compared with lower resolution breath-hold axial MRI image (red arrows) (b) CT image compared with free breathing axial MRI image (blue arrows). Although CT offers higher resolution, MR images provide comparable visualization.

Figure 7

Sample high resolution non-contrast MR image obtained with an isotropic resolution of 1 mm using a free breathing 3D STAR trajectory. Nominal scan time was 11 min 40 s. Sub-segmental airways are clearly visible on this image.

Figure 8

Comparison images obtained with the (a) 3D STAR and (b) the 3D spokes trajectory. Scan time was 20 s for each case. Resolution was fixed to the same voxel size (32 mm³) for each (2 × 2 × 8 mm³ for 3D STAR and 3.2 × 3.2 × 3.2 mm³ for 3D spokes.