Comparison of Intra- and Inter-Patient Intensity Standardization Methods for Multi-Parametric Whole-Body MRI.

To test and compare different intensity standardization approaches for whole-body multi-parametric MRI, allowing for the compensation of univocal representation of image intensities between scans, which pose problems in image quantification, assessment of changes between a baseline and follow-up scan and hinder performance of image processing and machine learning algorithms. 

Approach: In this work, we present a comparison on the accuracy of intensity standardization approaches with increasing complexity, for intra- and inter-patient multi-parametric whole-body MRI. Several approaches were used: z-scoring of the intensities, piecewise linear mapping and deformable mapping of intensity distributions into established reference intensity space. Additionally, the use reference image and average population distribution reference; as well as, whole image and region of interest standardization approaches were investigated. All methods were validated on a data set of 18 whole-body anatomical and diffusion-weighted MR scans consisting of baseline and follow-up examinations acquired from advanced prostate cancer patients and healthy volunteers. 

Main results: The piecewise linear intensity standardisation approach provided the best compromise between standardization accuracy and method stability, with average deviations in intensity profile of 0.011-0.027 and mean absolute difference of 0.29-0.37 standard score (intra-patient) and 0.014-0.056 (inter-patient), depending on the type of used MR modality. 

Significance: Linear piecewise approaches showed the overall best performance across multiple validation metrics, mostly because of its robustness. The inter-patient standardization proved to perform better when using population average reference image; in contrary to intra-patient approach, where the best results were achieved by standardizing towards a reference image taken as the baseline scan.