Odile Jolivet

Famis: A software package for functional feature extraction from biomedical multidimensional images

An increased number of image sequences is acquired in all modalities of the biomedical imaging field in order to study displacement or metabolism of a tracer or a contrast agent. It requires effective processing methods to estimate the underlying physiological components. We have developed a software package based on factor analysis algorithms which can adapt to various imaging modalities and its extension to double-indexed image sequences. We describe general characteristics of the software and present the main points of the user-friendly interface. The performances of the package are discussed and the possibilities of the methodology are illustrated using an example in magnetic resonance imaging.

Precision of magnetic resonance velocity and acceleration measurements: Theoretical issues and phantom experiments

Magnetic resonance (MR) sequences have been developed for acquiring multiple components of velocity and/or acceleration in a reasonable time and with a single acquisition. They have many parameters that influence the precision of measurements: NS, the number of flow‐encoding steps; NEX, the number of signal accumulations; and ND, the number of dimensions. Our aims were to establish a general relationship revealing the precision of these measurements as a function of NS, ND, and NEX and to validate it by experiments using phantoms. Previous work on precision has been restricted to two‐step (NS = 2) or 1D (ND = 1) MR velocity measurements. We describe a comprehensive approach that encompasses both multistep and multidimensional strategies. Our theoretical formula gives the precision of velocity and acceleration measurements. It was validated experimentally with measurements on a rotating disk phantom. This phantom was much easier to handle than fluid‐based phantoms. It could be used to assess both velocity and acceleration sequences and provided accurate and precise assessments over a wide, adjustable range of values within a single experiment. Increasing each of the three parameters, NS, ND, and NEX, improves the precision but makes the acquisition time longer. However, if only one parameter is to be assessed, maximizing the number of steps (NS) is the most efficient way of improving the precision of measurements; if several parameters are of interest, they should be measured simultaneously. By contrast, increasing the number of signals accumulated (NEX) is the least efficient strategy. J. Magn. Reson. Imaging 2001;13:445–451.

Interobserver Variability in Assessing Segmental Function can be Reduced by Combining Visual Analysis of CMR Cine Sequences with Corresponding Parametric Images of Myocardial Contraction

OBJECTIVEnTo evaluate if color-coded parametric images could help subjective visual analysis and improve interobserver agreement in the evaluation of segmental contraction (SC) in CMR.nnnBACKGROUNDnRoutine evaluation of SC in CMR remains mostly based on visual analysis of cine loops and subsequent interobserver variability remains a potential drawback.nnnMATERIALS AND METHODSnThree short axis cine loops were obtained in 33 subjects (18 myocardial infarction, 15 control), and 528 segments were analyzed. From each cine loop a single static parametric image resuming wall motion information was generated using Factor Analysis of Medical Image Sequences. Three readers (R1, R2, R3) scored left ventricular SC in 4 classes in 2 steps: visual assessment of cine loops alone and by combining cine loops with the corresponding parametric image. Reference segmental scores were obtained by consensus. Global contraction indexes were calculated in each step of the analysis.nnnRESULTSnWhen parametric images were combined with cine loop assessment, interobserver agreement was enhanced for paired readers: R1-R2: kappa = 0.66 (combined analysis) vs. kappa = 0.60 (cine alone); R2-R3: kappa = 0.67 vs. kappa = 0.65; R3-R1: kappa = 0.71 vs. kappa = 0.67 and absolute agreement with consensus was higher for the 3 readers: R1: 91% vs. 85%; R2: 87% vs. 83% and R3: 94% vs. 89%. When considering global wall motion indexes, interobserver agreement was also enhanced: R1 vs. R2 : r = 0.91 vs. 0.85; R2 vs. R3: 0.95 vs. 0.91; R3 vs R1: 0.98 vs. 0.91.nnnCONCLUSIONnAdding a color-coded static parametric image to routine subjective visual assessment of SC reduces interobserver variability.

Functional MR imaging of the human sensorimotor cortex during haptic discrimination.

THIS study attempted to determine whether haptic discriminations of shape (haptic task) activate the same tissue in the central cortical region of normal human subjects as do finger movements (opposition task). Opposition and haptic tasks both activated the central sulcus, as expected from previous imaging studies. The haptic task activated about 50% of the cortical territory activated by the opposition task. The results suggest that exploratory digital movements performed to collect precise somatosensory information and automatic movements performed during finger positioning activate partially overlapping parts of the sensorimotor cortex.

Implementation and optimization by the simplex method of a 3D double echo sequence in steady-state free precession.

Under steady-state conditions, the resulting echoes have very complex T1 and T2 relationships. Many authors exploited these echoes in different sequences to produce either T1- or T2-weighted images. The simultaneous acquisition of two echoes in a single sequence provides two images of clearly different contrasts. We implemented such a sequence, in a 3D-acquisition mode, combining the advantages of thin and contiguous slices with those of a multi-echo sequence. The contrast of the images was correlated with theoretical results, derived from Bloch equations. In order to estimate the acquisition parameters (alpha, TE, TR) to obtain an optimal T1- or T2-contrast between two tissues, a computer simulation of these equations was used in conjunction with the simplex method. The results show that this sequence improves the clinical efficiency of MRI, particularly in neurological and articular disease.

Optimization Of Acquisition Parameters In Fast Magnetic Resonance Imaging

This paper presents a numerical analysis of signal and contrast intensities from small tip angle, gradient reversal (FLASH) imaging. Because of the steady-state effects the characteristics of the images change considerably. A numerical technique is used to determine the acquisition parameters (flip angle, TE, TR) that maximize signal intensity from a single tissue, and maximize both T1-induced and T2-induced contrast for a pair of tissues.