Fast triple-spin-echo Dixon (FTSED) sequence for water and fat imaging.

A number of 'Dixon' techniques based on fast spin echo (FSE) sequence have been proposed and successfully used in many branches of medicine. Some require only one scan, but most of them need multiple scans and long scan times. This article describes a new fast triple-spin-echo Dixon (FTSED) technique suitable for ultra-high field MRI, in which three specific time shifts are introduced in the echo train; thus, three images with defined water-fat phase-differences (0, π, 2π) are encoded in the phase of the acquired images without extreme restrictions upon the echo duration. The water and fat images are then calculated by iterative least-squares estimation method. The sequence was successfully implemented at a 9.4T ultra-high field MRI system and tested on a phantom and a rat.

Are Early Somatic Embryos of the Norway Spruce (Picea abies (L.) Karst.) Organised?

BACKGROUND: Somatic embryogenesis in conifer species has great potential for the forestry industry. Hence, a number of methods have been developed for their efficient and rapid propagation through somatic embryogenesis. Although information is available regarding the previous process-mediated generation of embryogenic cells to form somatic embryos, there is a dearth of information in the literature on the detailed structure of these clusters. METHODOLOGY/PRINCIPAL FINDINGS: The main aim of this study was to provide a more detailed structure of the embryogenic tissue clusters obtained through the in vitro propagation of the Norway spruce (Picea abies (L.) Karst.). We primarily focused on the growth of early somatic embryos (ESEs). The data on ESE growth suggested that there may be clear distinctions between their inner and outer regions. Therefore, we selected ESEs collected on the 56th day after sub-cultivation to dissect the homogeneity of the ESE clusters. Two colourimetric assays (acetocarmine and fluorescein diacetate/propidium iodide staining) and one metabolic assay based on the use of 2,3,5-triphenyltetrazolium chloride uncovered large differences in the metabolic activity inside the cluster. Next, we performed nuclear magnetic resonance measurements. The ESE cluster seemed to be compactly aggregated during the first four weeks of cultivation; thereafter, the difference between the 1H nuclei concentration in the inner and outer clusters was more evident. There were clear differences in the visual appearance of embryos from the outer and inner regions. Finally, a cluster was divided into six parts (three each from the inner and the outer regions of the embryo) to determine their growth and viability. The innermost embryos (centripetally towards the cluster centre) could grow after sub-cultivation but exhibited the slowest rate and required the longest time to reach the common growth rate. To confirm our hypothesis on the organisation of the ESE cluster, we investigated the effect of cluster orientation on the cultivation medium and the influence of the change of the cluster's three-dimensional orientation on its development. Maintaining the same position when transferring ESEs into new cultivation medium seemed to be necessary because changes in the orientation significantly affected ESE growth. CONCLUSIONS AND SIGNIFICANCE: This work illustrated the possible inner organisation of ESEs. The outer layer of ESEs is formed by individual somatic embryos with high metabolic activity (and with high demands for nutrients, oxygen and water), while an embryonal group is directed outside of the ESE cluster. Somatic embryos with depressed metabolic activity were localised in the inner regions, where these embryonic tissues probably have a very important transport function.

Wavelet-based de-noising techniques in MRI.

The paper deals with techniques for the enhancement of magnetic resonance (MR) images using the wavelet analysis, which is assessed from the viewpoint of choosing the mother wavelet and the thresholding technique. Three parameters are used as objective criteria of the quality of image enhancement: the signal-to-noise ratio (SNR), image contrast, and linear approximation of edge steepness. Unlike most of the standard methods, which work exclusively with image magnitude, we also examined the influence of image phase, i.e. the image is processed as a complex signal. In addition to the interpretation of results, a short summary is given that deals with the choice of the optimal mother wavelet and thresholding technique for different types of MR images.

Measurements of time characteristics of the gradient magnetic field.

Imaging techniques based on the principles of nuclear magnetic resonance (NMR) are modern techniques for the study of chemical, biological and physical properties of substances. The most important are their applications in medical sciences. MR imaging of a specimen weighted with diffusion coefficients requires very accurate data on the time course of the gradient pulse. Diffusion coefficients are determined from the drop of the MR signal measured with and without the application of magnetic field gradients. From the accuracy point of view, the defined course of gradients plays an important role in the computation of coefficients. A minimum rise and fall times, a defined magnitude of the excited gradient of the magnetic field and a symmetry of positive and negative pulses (zero integral of pulses of the same magnitude and opposite polarities) are required. To characterize the time course of gradient pulses or either polarity, simple methods of their measurement has been developed and experimentally tested on a 4.7 T tomograph. The method is based on the principle of instantaneous MR frequency measurements at the presence of a gradient pulse following the excitation of a thin layer situated outside the centre of the gradient field.