Determination of the Defining Boundary in Nuclear Magnetic Resonance Diffusion Experiments

While nuclear magnetic resonance diffusion experiments are widely used to resolve structures confining the diffusion process, it has been elusive whether they can exactly reveal these structures. This question is closely related to X-ray scattering and to Kac's "hear the drum" problem. Although the shape of the drum is not "hearable", we show that the confining boundary of closed pores can indeed be detected using modified Stejskal-Tanner magnetic field gradients that preserve the phase information and enable imaging of the average pore in a porous medium with a largely increased signal-to-noise ratio.Comment: 13 pages, 2 figure

Impact of velocity- and acceleration-compensated encodings on signal dropout and black-blood state in diffusion-weighted magnetic resonance liver imaging at clinical TEs.

PurposeThe study aims to develop easy-to-implement concomitant field-compensated gradient waveforms with varying velocity-weighting (M1) and acceleration-weighting (M2) levels and to evaluate their efficacy in correcting signal dropouts and preserving the black-blood state in liver diffusion-weighted imaging. Additionally, we seek to determine an optimal degree of compensation that minimizes signal dropouts while maintaining blood signal suppression.MethodsNumerically optimized gradient waveforms were adapted using a novel method that allows for the simultaneous tuning of M1- and M2-weighting by changing only one timing variable. Seven healthy volunteers underwent diffusion-weighted magnetic resonance imaging (DWI) with five diffusion encoding schemes (monopolar, velocity-compensated (M1 = 0), acceleration-compensated (M1 = M2 = 0), 84%-M1-M2-compensated, 67%-M1-M2-compensated) at b-values of 50 and 800 s/mm2 at a constant echo time of 70 ms. Signal dropout correction and apparent diffusion coefficients (ADCs) were quantified using regions of interest in the left and right liver lobe. The blood appearance was evaluated using two five-point Likert scales.ResultsSignal dropout was more pronounced in the left lobe (19%-42% less signal than in the right lobe with monopolar scheme) and best corrected by acceleration-compensation (8%-10% less signal than in the right lobe). The black-blood state was best with monopolar encodings and decreased significantly (p ConclusionAll of the diffusion encodings used in this study demonstrated suitability for routine DWI application. The results indicate that a perfect value for the level of M1-M2-compensation does not exist. However, among the examined encodings, the 84%-M1-M2-compensated encodings provided a suitable tradeoff

Revealing Hidden Potentials of the q-Space Signal in Breast Cancer

Mammography screening for early detection of breast lesions currently suffers from high amounts of false positive findings, which result in unnecessary invasive biopsies. Diffusion-weighted MR images (DWI) can help to reduce many of these false-positive findings prior to biopsy. Current approaches estimate tissue properties by means of quantitative parameters taken from generative, biophysical models fit to the q-space encoded signal under certain assumptions regarding noise and spatial homogeneity. This process is prone to fitting instability and partial information loss due to model simplicity. We reveal unexplored potentials of the signal by integrating all data processing components into a convolutional neural network (CNN) architecture that is designed to propagate clinical target information down to the raw input images. This approach enables simultaneous and target-specific optimization of image normalization, signal exploitation, global representation learning and classification. Using a multicentric data set of 222 patients, we demonstrate that our approach significantly improves clinical decision making with respect to the current state of the art.Comment: Accepted conference paper at MICCAI 201

Diffusionstensor - Magnetresonanztomographie : Phantomentwicklung und Optimierung der Messtechnik für Anwendungen am Rückenmark

In dieser Arbeit werden quantitative DTI-Messungen mit maschinell hergestellten, einfach reproduzierbaren, kostengünstigen DTI-Phantomen aus Polyamidfäden und Plexiglasspindeln oder Schrumpfschläuchen vorgestellt. Die Phantome haben ähnliche MR- und Diffusionseigenschaften (FA > 0.7, ADC etwa 1 µm²/ms, T2 > 300 ms) wie neuronales Gewebe in vivo. Mit Hilfe der Phantome werden Messfehler untersucht die durch den Bias diffusionsgewichteter Magnitudenbilder, aber nicht durch Streuung der Daten entstehen. Es wird gezeigt dass es eine systematische Eigenvektorverschiebung in Richtung 'attraktiver' Orientierungen gibt und dass Eigenwerte des Diffusionstensors überschätzt werden können wenn die Diffusion unterschätzt wird. Die Orientierungsabhängigkeit der FA und des ADC wird mit den Phantomen und in vivo demonstriert, die verwendeten Bildgebungsparameter sind dabei im Bereich klinisch eingesetzter Diffusionssequenzen. Die gemessene FA des Corpus Callosum variiert zwischen 0.74 und 0.81 wenn das Gradientenschema rotiert wird. Desweiteren wird die DTI-Bildgebung für das Rückenmark optimiert. Es wird gezeigt, dass echoplanare Bildgebungssequenzen signaleffzienter sind als Turbo-Spin-Echo und stimulierte Echo Sequenzen. Eine verbesserte Methode der Magnetisierungsrückgewinnung bei Inner-Volume-Sequenzen mit doppelt refokussierender Spin-Echo Diffusionswichtung wird vorgestellt. Trotzdem bleibt die Verkürzung des Echozugs mit paralleler Bildgebung zeiteffizienter als Inner-Volume-Techniken. Quantitative Diffusionwerte werden mit einer probabilistischen Voxelklassiffikation bestimmt um benutzerbedingte Variationen zu minimieren. Im Bereich der Halswirbelsäule liegen die ermittelten quantitative Werte des ADC zwischen 0,993 und 1,071 µm²/ms und der FA zwischen 0,624 und 0,646. Die Standardabweichung in fünf Messungen war für beide Werte kleiner als 3,5% des Messwertes

Symmetry of the gradient profile as second experimental dimension in the short-time expansion of the apparent diffusion coefficient as measured with NMR diffusometry

The time-dependent apparent diffusion coefficient as measured by pulsed gradient NMR can be used to estimate parameters of porous structures including the surface-to-volume ratio and the mean curvature of pores. In this work, the short-time diffusion limit and in particular the influence of the temporal profile of diffusion gradients on the expansion as proposed by Mitra et al. (1993) is investigated. It is shown that flow-compensated waveforms, i.e. those whose first moment is zero, are blind to the term linear in observation time, which is the term that is proportional to mean curvature and surface permeability. A gradient waveform that smoothly interpolates between flow-compensated and bipolar waveform is proposed and the degree of flow-compensation is used as a second experimental dimension. This two-dimensional ansatz is shown to yield an improved precision when characterizing the confining domain. This technique is demonstrated with simulations and in experiments performed with cylindrical capillaries of 100 μm radius