Hannes Dillinger

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Dillinger

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Hannes

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0000-0002-5597-5273

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Publications 1 - 8 of 8

Fundamentals of turbulent flow spectrum imaging
Dillinger, Hannes; McGrath, Charles; Guenthner, Christian; et al. (2022)
Magnetic Resonance in Medicine
Purpose To introduce a mathematical framework and in-silico validation of turbulent flow spectrum imaging (TFSI) of stenotic flow using phase-contrast MRI, evaluate systematic errors in quantitative turbulence parameter estimation, and propose a novel method for probing the Lagrangian velocity spectra of turbulent flows. Theory and Methods The spectral response of velocity-encoding gradients is derived theoretically and linked to turbulence parameter estimation including the velocity autocorrelation function spectrum. Using a phase-contrast MRI simulation framework, the encoding properties of bipolar gradient waveforms with identical first gradient moments but different duration are investigated on turbulent flow data of defined characteristics as derived from computational fluid dynamics. Based on theoretical insights, an approach using velocity-compensated gradient waveforms is proposed to specifically probe desired ranges of the velocity autocorrelation function spectrum with increased accuracy. Results Practical velocity-encoding gradients exhibit limited encoding power of typical turbulent flow spectra, resulting in up to 50% systematic underestimation of intravoxel SD values. Depending on the turbulence level in fluids, the error due to a single encoding gradient spectral response can vary by 20%. When using tailored velocity-compensated gradients, improved quantification of the Lagrangian velocity spectrum on a voxel-by-voxel basis is achieved and used for quantitative correction of intravoxel SD values estimated with velocity-encoding gradients. Conclusion To address systematic underestimation of turbulence parameters using bipolar velocity-encoding gradients in phase-contrast MRI of stenotic flows with short correlation times, tailored velocity-compensated gradients are proposed to improve quantitative mapping of turbulent blood flow characteristics.
Beat phenomena of oscillating readouts
Dillinger, Hannes; Peereboom, Sophie M.; Kozerke, Sebastian (2024)
Magnetic Resonance in Medicine
PurposeTo demonstrate slowly varying, erroneous magnetic field gradients for oscillating readouts due to the mechanically resonant behavior of gradient systems.MethodsProjections of a static phantom were acquired using a one-dimensional (1D) EPI sequence with varying EPI frequencies ranging from 1121 to 1580 Hz on clinical 3T systems (30 mT/m, 200 T/m/s). Phase due to static B0 inhomogeneities was eliminated by a complex division of two separate scans with different polarities of the EPI readout. The temporal evolution of phase was evaluated and related to the mechanical resonances of the gradient systems derived from the gradient modulation transfer function. Additionally, the impact of temporally varying mechanical resonance effects on EPI was evaluated using an echo-planar spectroscopic imaging sequence.ResultsA beat phenomenon resulting in a slowly varying phase was observed. Its temporal frequency was given by the difference between the EPI frequency and the mechanical resonance frequency of the activated gradient axis. The maximum erroneous, oscillating phase during phase encoding was +/- 0.5 rad for an EPI frequency of 1281 Hz. Echo-planar spectroscopic imaging images showed the resulting time-dependent stretching/compression of the FOV.ConclusionOscillating readouts such as those used in EPI can result in low-frequency, erroneous phase contributions, which are explained by the beat phenomenon. Therefore, EPI phase-correction approaches may need to include beat effects for accurate image reconstruction.
On Blood and Mechanical Motion Sensitization of Encoding and Decoding in Magnetic Resonance Imaging
Dillinger, Hannes (2022)
The assessment of three-dimensional blood flow dynamics by means of Phase-Contrast Magnetic Resonance Imaging (PC-MRI) holds promising potential for the non-invasive diagnosis of diseases of individual vessels and entire structures such as the heart. A variant of PC-MRI, 4D Flow MRI, offers novel biomarkers, which are gaining importance for the classification of e.g. valvular heart disease. The evaluation of mean blood flow and turbulence, by additionally encoding the Reynolds stress tensor, may further enable the quantification of stenosis severity, the assessment of hemolysis and other factors. As these markers can influence clinical decision making, knowledge of the accuracy and precision of the estimated parameters is of utmost importance. Given the intrinsic limitations of PC-MRI in terms of spatial and temporal resolution and the employed hardware, estimated flow parameters are sensitized to the blood flow itself but also to other, undesired contributions. In addition, long acquisition times in 4D Flow MRI hamper its clinical applicability and patient acceptance. In previous works, the echo planar imaging (EPI) readout technique has been suggested to reduce the time needed for acquisition of 4D Flow MRI exams. Here, it is shown that employing EPI for 4D Flow MRI results in misregistration, velocity estimation errors and degrading spatial resolution depending on blood flow patterns. Therefore, it is concluded that for shortening scan time other acceleration methods such as compressed sensing in conjunction with standard gradient echo (GRE) imaging are favorable. Current turbulence encoding models for PC-MRI are based on assumptions regarding the time scales of the underlying flow field. In this work, the turbulence encoding model is derived and related to diffusion MRI and turbulence theory. Results of PC-MRI simulations employing large eddy simulation (LES) data as input show that current turbulence encoding models need to be revisited as they systematically underestimate turbulence parameters. Subsequently, a correction method based on probing the Lagrangian turbulence spectrum is presented and used to gauge the encoding model to reinstate the accurateness of turbulence parameter estimation. The method is demonstrated for PC-MRI of stenotic flows. While encoding and reconstruction techniques can be further optimized, MRI hardware limitations remain. Based on a linear, time-invariant description of the MRI gradient system, the influence of mechanical resonances on PC-MRI data are highlighted. It is shown that residual background phases, which result in biased velocity estimation, can be reduced both in amplitude and spatial order by PC-MRI sequence optimization. The influence of mechanical resonances on spatial encoding is evaluated, demonstrating that EPI readouts are particularly vulnerable to undesired contributions due to mechanical motion of the gradient system. The gradient performance of lower-field systems is shown to benefit from reduced Lorentz forces, mitigating the influence of mechanical resonances. This benefit is contrasted with the reduced signal-to-noise ratio of lower-field systems. Based on a comparison of an MRI system operated at standard and at lower-field strength, increased gradient fidelity as well as reduced sound pressure levels are demonstrated for the lower-field configuration.
Ramping down a clinical 3 T scanner: a journey into MRI and MRS at 0.75 T
Guenthner, Christian; Peereboom, Sophie Marie; Dillinger, Hannes; et al. (2023)
Magnetic Resonance Materials in Physics, Biology, and Medicine
ObjectLower-field MR is reemerging as a viable, potentially cost-effective alternative to high-field MR, thanks to advances in hardware, sequence design, and reconstruction over the past decades. Evaluation of lower field strengths, however, is limited by the availability of lower-field systems on the market and their considerable procurement costs. In this work, we demonstrate a low-cost, temporary alternative to purchasing a dedicated lower-field MR system.Materials and MethodsBy ramping down an existing clinical 3 T MRI system to 0.75 T, proton signals can be acquired using repurposed C-13 transmit/receive hardware and the multi-nuclei spectrometer interface. We describe the ramp-down procedure and necessary software and hardware changes to the system.ResultsApart from presenting system characterization results, we show in vivo examples of cardiac cine imaging, abdominal two- and three-point Dixon-type water/fat separation, water/fat-separated MR Fingerprinting, and point-resolved spectroscopy. In addition, the ramp-down approach allows unique comparisons of, e.g., gradient fidelity of the same MR system operated at different field strengths using the same receive chain, gradient coils, and amplifiers.DiscussionRamping down an existing MR system may be seen as a viable alternative for lower-field MR research in groups that already own multi-nuclei hardware and can also serve as a testing platform for custom-made multi-nuclei transmit/receive coils.
Fourier transform temporal diffusion spectroscopy
Hennel, Franciszek; Dillinger, Hannes; Leupold, Jochen; et al. (2023)
Journal of Magnetic Resonance
Temporal diffusion spectroscopy (TDS) currently uses the oscillating gradient spin echo (OGSE) experiment to measure the spectral density of translational velocity autocorrelation at single frequencies. Due to timing restrictions imposed by the transverse relaxation, the frequency selectivity and the sampling density of OGSE are limited, especially at low frequencies. We propose to overcome this problem by adopting the principles of Fourier transform spectroscopy. The new method of Fourier transform TDS (FTDS) uses two broadband gradient waveforms with different relative delays to make the spin echo attenuation sensitive to a broad range of diffusion frequencies with different harmonic modulations and calculates the spectrum by discrete Fourier transform. The method was validated by a measurement of diffusion spectra in highly restrictive tissues of a celery stalk and provided results consistent with OGSE, however, on a denser frequency grid.
Direct comparison of gradient Fidelity and acoustic noise of the same MRI system at 3 T and 0.75 T
Dillinger, Hannes; Kozerke, Sebastian; Guenthner, Christian (2022)
Magnetic Resonance in Medicine
Purpose To analyze the difference between gradient fidelity and acoustic noise of the same MRI scanner operated at product field strength (3 T) and lower field strength (0.75 T). Methods Gradient modulation transfer functions (GMTFs) were measured using a four-slice 2D phase-encoded chirp-based sequence on the same scanner operated at 3 T and, following ramp-down, at 0.75 T with identical gradient specifications (40 mT/m, 200 T/m/s). Calibrated audio measurements were performed at both field strengths to correlate audio spectra with GMTFs. Results While eddy currents were independent of field strength, mechanical resonances were substantially decreased at lower field, resulting in a reduction of GMTF distortions by up to 95% (88% on average) at the mechanical resonances of the gradient system. Audio spectra amplitudes were reduced by up to 87% when comparing 0.75 T versus 3 T. Conclusion Lower static fields lead to reduced Lorentz forces on the gradient coil and, in turn, to reduced mechanical resonances, thereby improving gradient fidelity. Simultaneously, the reduction of acoustic noise may help to improve patient comfort.
On the limitations of echo planar 4D flow MRI
Dillinger, Hannes; Walheim, Jonas; Kozerke, Sebastian (2020)
Magnetic Resonance in Medicine
Toward an accurate estimation of wall shear stress from 4D flow magnetic resonance downstream of a severe stenosis
Corso, Pascal; Walheim, Jonas; Dillinger, Hannes; et al. (2021)
Magnetic Resonance in Medicine
Purpose First, to investigate the agreement between velocity, velocity gradient, and Reynolds stress obtained from four-dimensional flow magnetic resonance (4D flow MRI) measurements and direct numerical simulation (DNS). Second, to propose and optimize based on DNS, 2 alternative methods for the accurate estimation of wall shear stress (WSS) when the resolution of the flow measurements is limited. Thirdly, to validate the 2 methods based on 4D flow MRI data. Methods In vitro 4D MRI has been conducted in a realistic rigid stenosed aorta model under a constant flow rate of 12 L/min. A DNS of transitional stenotic flow has been performed using the same geometry and boundary conditions. Results Time-averaged velocity and Reynolds stresses are in good agreement between in vitro 4D MRI data and DNS (errors between 2% and 8% of the reference downsampled data). WSS estimation based on the 2 proposed methods applied to MRI data provide good agreement with DNS for slice-averaged values (maximum error is less than 15% of the mean reference WSS for the first method and 25% for the second method). The performance of both models is not strongly sensitive to spatial resolution up to 1.5 mm voxel size. While the performance of model 1 deteriorates appreciably at low signal-to-noise ratios, model 2 remains robust. Conclusions The 2 methods for WSS magnitude give an overall better agreement than the standard approach used in the literature based on direct calculation of the velocity gradient close to the wall (relative error of 84%). © 2021 International Society for Magnetic Resonance in Medicine

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