Quantitative MRI measurement of lung density must account for the change in <i>T</i> with lung inflation

<jats:title>Abstract</jats:title><jats:sec><jats:title>Purpose</jats:title><jats:p>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.</jats:p></jats:sec><jats:sec><jats:title>Materials and Methods</jats:title><jats:p>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<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-2.gif" xlink:title="urn:x-wiley:10531807:media:JMRI21866:tex2gif-stack-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<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-3.gif" xlink:title="urn:x-wiley:10531807:media:JMRI21866:tex2gif-stack-3" /> and lung water density across lung volumes, imaging plane, and spatial locations in the lung.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>In normal subjects (n = 10), T<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-4.gif" xlink:title="urn:x-wiley:10531807:media:JMRI21866:tex2gif-stack-4" /> (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<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-5.gif" xlink:title="urn:x-wiley:10531807:media:JMRI21866:tex2gif-stack-5" />.</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>Data show that T<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-6.gif" xlink:title="urn:x-wiley:10531807:media:JMRI21866:tex2gif-stack-6" /> 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<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-7.gif" xlink:title="urn:x-wiley:10531807:media:JMRI21866:tex2gif-stack-7" />. J. Magn. Reson. Imaging 2009. © 2009 Wiley‐Liss, Inc.</jats:p></jats:sec>