Generating resolution-enhanced images for correction of partial volume effects in emission tomography: a multiresolution approach

Partial volume effects (PVE) are consequences of the limited spatial resolution in emission tomography. They lead to a loss of signal in tissues of size similar to the point spread function and induce activity spillover between regions. Although PVE can be corrected for by using algorithms that provide the correct radioactivity concentration in a series of regions of interest (ROI), so far little attention has been given to the possibility of creating improved images as a result of PVE correction. The objective of our study was therefore to develop a methodology for PVE correction to enable not only the accurate recuperation of activity concentrations, but also to generate PVE corrected images. In the multiresolution analysis that we define here, details of a high resolution image H (MRI or CT) are extracted, transformed and integrated in a low resolution image L (PET or SPECT). A discrete wavelet transform of both H and L images is performed by using the "a trous" algorithm, which allows the spatial frequencies to be easily obtained at a level of resolution common to H and L. A model is then inferred to build the lacking details of L from the high frequency details in H. The process was successfully tested on synthetic and simulated data, proving the ability to obtain accurately corrected images. Quantitative PVE correction was found to be comparable with a method considered as a reference but limited to ROI analyses. Improvement was obtained also in two examples of clinical images, the first using a combined PET/CT scanner with a lymphoma patient and the second using a FDG brain PET and corresponding T1-weighted MRI in an epileptic patient.