The value of MRI in defining target volumes and organs at risk is established. Numerous difficulties appear to stand in the way of using MRI alone in dose planning, with the result that this imaging modality is used in most cases in conjunction with computerized X-ray tomography (CT). The aim of this paper is to appreciate these difficulties: geometrical distortion, chemical shifts, dosimetric accuracy. Geometrical distortion measurements were carried out on two 1.5 T MR scanners and the effect of chemical shift and magnetic susceptibility were evaluated in volunteers. The effect on dosimetric calculations of uncertainty in determining electron densities was evaluated too. Geometrical distortion remained at small values: less than 2 mm and 3 mm for field of view of 20 cm and 45 cm. The chemical shift and magnetic susceptibility values obtained, ranging from 0.3 to 3 mm, were well below the theoretical values. The assignment of relative electron densities to only two structures in MR images seems to permit dose planning that is identical with that obtained with CT. None of the technical obstacles mentioned represents a stumbling block. The access to MRI facility could represent a persisting problem.

Context: Cognitive impairments are common in patients with Parkinson’s disease (PD) and are heterogeneous in their presentation. The “dual syndrome hypothesis” suggests the existence of two distinct subtypes of mild cognitive impairment (MCI) in PD: a frontostriatal subtype with predominant attentional and/or executive deficits and a posterior cortical subtype with predominant visuospatial, memory, and/or language deficits. The latter subtype has been associated with a higher risk of developing dementia.Objective: The objective of this study was to identify structural modifications in cortical and subcortical regions associated with each PD-MCI subtype.Methods: One-hundred and fourteen non-demented PD patients underwent a comprehensive neuropsychological assessment as well as a 3T magnetic resonance imaging scan. Patients were categorized as having no cognitive impairment (n = 41) or as having a frontostriatal (n = 16), posterior cortical (n = 25), or a mixed (n = 32) MCI subtype. Cortical regions were analyzed using a surface-based Cortical thickness (CTh) method. In addition, the volumes, shapes, and textures of the caudate nuclei, hippocampi, and thalami were studied. Tractometric analyses were performed on associative and commissural white matter (WM) tracts.Results: There were no between-group differences in volumetric measurements and cortical thickness. Shape analyses revealed more abundant and more extensive deformations fields in the caudate nuclei, hippocampi, and thalami in patients with posterior cortical deficits compared to patients with no cognitive impairment. Decreased fractional anisotropy (FA) and increased mean diffusivity (MD) were also observed in the superior longitudinal fascicle, the inferior fronto-occipital fascicle, the striato-parietal tract, and the anterior and posterior commissural tracts. Texture analyses showed a significant difference in the right hippocampus of patients with a mixed MCI subtype.Conclusion: PD-MCI patients with posterior cortical deficits have more abundant and more extensive structural alterations independently of age, disease duration, and severity, which may explain why they have an increased risk of dementia.

Written informed consent was obtained from all participants after full information of the procedure. The study was approved by the institutional ethics committees (CPP Nord-Ouest IV, 2012-A 01317-36). Additional information on this study group is detailed in the original paper.

This paper deals with a new approach of registration in multimodal imaging. Modalities involved are Digital Subtracted Angiography (DSA, 2D) and Magnetic Resonance Angiography (MRA, 3D). Our approach is an hybrid one, mixing feature and intensity based approaches. This approach is based on the extraction of a anatomical referential common to both MRA and DSA. This step appears to be the "geometric-like" aspect. Then, a high level optimization scheme gives the best registration, using an iconic similarity measure. Several ways of matching planar and tomographic imaging are proposed through superimposition, point to point matching or 3D data fusion. The results obtained prove the methods efficiency in a clinical context.