Quantitative MRI Measurement of Lung Density Must Account for the Change in T(2)* With Lung Inflation

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 stabstically 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)*.