RESOLVING ANATOMICAL DETAILS IN LUNG PARENCHYMA - THEORY AND EXPERIMENT FOR A STRUCTURALLY AND MAGNETICALLY INHOMOGENEOUS LUNG IMAGING MODEL

Lung parenchyma is, structurally and magnetically, a very inhomogeneou s system, The strong local field gradients due to magnetic susceptibil ity variation across the air-tissue interfaces impose a spatially depe ndent phase on spins that are inhomogeneously distributed. As a result , the slice selection process can cause destructive interference and c orresponding partial cancellation of the NMR signals from different pa rts of the slice, The mechanism of this effect was studied by expandin g the internal magnetic field gradient as a Fourier series and determi ning the Fourier components with a foam model, which consists of air b ubbles preferentially at the alveolar diameter. The effect of signal c ancellation as a function of slice thickness is characterized by a sin e-like function with destructive interference of higher orders shown a s lobes and the zeroth order as the main peak. Computer simulation of the slice selection process was conducted to illustrate the combined e ffects of signal cancellation and spatial average, and their dependenc e on slice thickness, Finally, images of rat lungs are presented to de monstrate the significant improvement in image quality by avoiding the high order destructive interference of NMR signals in the slice selec tion process.