Resolving anatomical details in lung parenchyma: theory and experiment for a structurally and magnetically...

Lung parenchyma is, structurally and magnetically, a very inhomogeneous system. The strong local field gradients due to magnetic susceptibility variation across the air-tissue interfaces impose a spatially dependent phase on spins that are inhomogeneously distributed. As a result, the slice selection process can cause destructive interference and corresponding partial cancellation of the NMR signals from different parts of the slice. The mechanism of this effect was studied by expanding the internal magnetic field gradient as a Fourier series and determining the Fourier components with a foam model, which consists of air bubbles preferentially at the alveolar diameter. The effect of signal cancellation as a function of slice thickness is characterized by a sinc-like function with destructive interference of higher orders shown as lobes and the zeroth order as the main peak. Computer simulation of the slice selection process was conducted to illustrate the combined effects of signal cancellation and spatial average, and their dependence on slice thickness. Finally, images of rat lungs are presented to demonstrate the significant improvement in image quality by avoiding the high order destructive interference of NMR signals in the slice selection process.