Quantitative MRI measurement of lung density must account for the change in T(2) (*) with lung inflation

Abstract

Purpose: To evaluate lung water density at three different levels of lung inflation in normal lungs using a fast gradient echo sequence developed for rapid imaging.

Materials and methods: Ten healthy volunteers were imaged with a fast gradient echo sequence that collects 12 images alternating between two closely spaced echoes in a single 9-s breathhold. Data were fit to a single exponential to determine lung water density and T(2) (*). Data were evaluated in a single imaging slice at total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). Analysis of variance for repeated measures was used to statistically evaluate changes in T(2) (*) and lung water density across lung volumes, imaging plane, and spatial locations in the lung.

Results: In normal subjects (n = 10), T(2) (*) (and [lung density/water density]) was 1.2 +/- 0.1 msec (0.10 +/- 0.02), 1.8 +/- 0.2 ms (0.25 +/- 0.04), and 2.0 +/- 0.2 msec (0.27 +/- 0.03) at TLC, FRC, and RV, respectively. Results also show that there is a considerable intersubject variability in the values of T(2) (*).

Conclusion: Data show that T(2) (*) in the lung is very short, and varies considerably with lung volume. Thus, if quantitative assessment of lung density within a breathhold is to be measured accurately, then it is necessary to also determine T(2) (*).

Figure 1

Relative location of lung ROIs within lung VOI for coronal (left image) and sagittal (right image) imaging plane. ROIs start at the most medial margin in the coronal imaging plane and the most posterior position of the lung in the sagittal plane.

Figure 2

Average signal of the lung from 12 successive images divided by the average of the last 4 images, showing the approach to steady-state for a single subject. Error bars show the SD over the 5 repetitions at each image number divided by the average of the last 4 images. Results are shown for the (a) sagittal and (b) coronal imaging plane at RV, FRC, and TLC. The signal of the lung does not reach steady state until image 4 in both imaging planes and at all 3 lung volumes.

Figure 3

Effects of using different TE values on estimates of T₂*. Spatial results are from data acquired at RV (a,b: scans 1 and 4 in Table 1), and FRC (c,d: scans 2 and 5 in Table 1) in the coronal (a,c) and sagittal (b,d) imaging planes at either a fixed TE2 (square–solid line) or TE[max] (circle–dotted line). There was no significant main effect for TE2 (TE[fixed] vs TE[max]) for data acquired at sagittal RV (P = 0.36), coronal RV (P = 0.90) and coronal FRC (P = 0.11) for T₂* averaged over all lung ROIs. There was a significant effect main effect for TE2 for sagittal FRC (P = 0.0002). There was also a significant lung region (ROI) by echo time interaction for RV and FRC in both planes (P< 0.04).

Figure 4

Effects of lung volume and scan plane on (a,c) fractional lung water density and (b,d) T₂*. The results are from data acquired at RV, FRC, and TLC for a maximum TE2 (3.4 msec at RV, 2.4 msec at FRC, and 1.8 msec at TLC). There was a significant main effect for lung volume at P <0.0001.

Figure 5

Percent difference in fractional lung water density across subjects between two methods of calculating fractional lung water density. The first method calculates lung water density by back-extrapolating the signal to and echo time of zero based on images acquired at two echo times and calculates T₂* on a subject by subject basis (2 measurement). The second method also back-extrapolates the signal to an echo time of zero, but the fit is based on a fixed T₂* and an image acquired at a single short echo time (Fixed T₂*). There is up to a 21.5% ± 13.3% (a: coronal) and a 19.8% ± 26.5% (b: sagittal) error in lung density calculated assuming mean values of T₂* across subjects due to a considerable inter-subject variability in T₂*.