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 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) ().