Artifacts arising from undersampling are not always treatable as incoherent noise for the pattern matching process in Magnetic Resonance Fingerprinting (MRF). To estimate the effect of undersampling ...artifacts on MRF quantitative results, spiral sampling trajectories and their temporal variation is examined.
The effect of sampling trajectories and their variation during the MRF experiment was assessed by characterizing aliasing artifacts. Temporal rearrangements of sampling trajectories were tested and evaluated in simulations and scans of phantoms and in a volunteer brain.
Results show that some temporal variations of sampling patterns can lead to spatial biases in MRF parameter maps. Observed effects are consistent with derived performance indicators for different interleaving schemes, leading to substantially improved MRF sampling patterns.
With the help of the presented simulation framework, MRF implementations can be investigated and improved. This was demonstrated for a spiral FISP (Fast imaging with steady-state free precession) MRF implementation, where a significantly improved interleaving scheme was identified, and confirmed by experiment.
Magnetic resonance field fingerprinting Körzdörfer, Gregor; Jiang, Yun; Speier, Peter ...
Magnetic resonance in medicine,
April 2019, Letnik:
81, Številka:
4
Journal Article
Recenzirano
Purpose
To develop and evaluate the magnetic resonance field fingerprinting method that simultaneously generates T1, T2, B0, and B1+ maps from a single continuous measurement.
Methods
An encoding ...pattern was designed to integrate true fast imaging with steady‐state precession (TrueFISP), fast imaging with steady‐state precession (FISP), and fast low‐angle shot (FLASH) sequence segments with varying flip angles, radio frequency (RF) phases, TEs, and gradient moments in a continuous acquisition. A multistep matching process was introduced that includes steps for integrated spiral deblurring and the correction of intravoxel phase dispersion. The method was evaluated in phantoms as well as in vivo studies in brain and lower abdomen.
Results
Simultaneous measurement of T1, T2, B0, and B1+ is achieved with T1 and T2 subsequently being less afflicted by B0 and B1+ variations. Phantom results demonstrate the stability of generated parameter maps. Higher undersampling factors and spatial resolution can be achieved with the proposed method as compared with solely FISP–based magnetic resonance fingerprinting. High‐resolution B0 maps can potentially be further used as diagnostic information.
Conclusion
The proposed magnetic resonance field fingerprinting method can estimate T1, T2, B0, and B1+ maps accurately in phantoms, in the brain, and in the lower abdomen.
The purpose of this study was to investigate whether the cardiac motion artifact that regularly appears in diffusion-weighted imaging of the left liver lobe might be reduced by acquiring images in ...inspiration, when the coupling between heart and liver might be minimal. 43 patients with known or suspected focal liver lesions were examined at 1.5 T with breath hold acquisition, once in inspiration and once in expiration. Data were acquired with a diffusion-weighted echo planar imaging sequence and two b-values (b50 = 50 s/mm² and b800 = 800 s/mm²). The severity of the cardiac motion artifact in the left liver lobe was rated by two experienced radiologists for both b-values with a 5 point Likert scale. Additionally, the normalized signal S(b800)/S(b50) in the left liver lobe was computed. The Wilcoxon signed-rank test was used comparing the scores of the two readers obtained in inspiration and expiration, and to compare the normalized signal in inspiration and expiration. The normalized signal in inspiration was slightly higher than in expiration (0.349±0.077 vs 0.336±0.058), which would indicate a slight reduction of the cardiac motion artifact, but this difference was not significant (p = 0.24). In the qualitative evaluation, the readers did not observe a significant difference for b50 (reader 1: p = 0.61; reader 2: p = 0.18). For b800, reader 1 observed a significant difference of small effect size favouring expiration (p = 0.03 with a difference of mean Likert scores of 0.27), while reader 2 observed no significant difference (p = 0.62). Acquiring the data in inspiration does not lead to a markedly reduced cardiac motion artifact in diffusion-weighted imaging of the left liver lobe and is in this regard not to be preferred over acquiring the data in expiration.
Bacterial nanocellulose (BNC) is a promising material for the use in medical implants. BNC does not induce unwanted reactions in vivo, is long term stable and possesses unique mechanical properties. ...However, to make the most of these features, BNC must be carefully processed. Details of the cultivation and post-synthetic methods offer various ways to control the properties of BNC. The focus of this work is put on drying of the BNC. Different unconstrained drying methods (climate chamber at 23°C, oven at 100°C, freeze-drying) and constrained drying under excertion of uniaxial pressure at various temperatures have been investigated. The reduction of the high water content of native BNC ( 98%) causes a thickness reduction of the samples. For oven or climate chamber drying a thickness reduction of 98% is observed, while freeze-drying widely preserves the nano- or micro-structure of the fibrous material and leads to a thickness reduction of only 13%. During drying or pressing at high temperature (100°C), i.e. by evaporation of the water, intermolecular hydrogen bonds are formed and interconnect the individual fibres and strands. Consequently mechanical stiffening is observed in tensile tests at small strains. After drying, a densified cellulose nano-fibre network is observed by scanning electron microscopy. Due to the irreversibility of drying by evaporation, the water content and water retention capacity of BNC are not recovered by rehydration. Applying uniaxial pressure before drying further enhances the irreversible reinforcement of the fibre network, while this is not the case when pressing the samples after drying. The presented results show that the properties of BNC can be widely controlled by post-processing steps. Thus, taylor-made BNC can be produced for biomedical applications.
Heart rate variability (HRV) analysis is increasingly used in anaesthesia and intensive care monitoring of spontaneous breathing and mechanical ventilated patients. In the frequency domain, different ...estimation methods of the power spectral density (PSD) of RR-intervals lead to different results. Therefore, we investigated the PSD estimates of fast Fourier transform (FFT), autoregressive modeling (AR) and Lomb–Scargle periodogram (LSP) for 25 young healthy subjects subjected to metronomic breathing. The optimum method for determination of HRV spectral parameters under paced respiration was identified by evaluating the relative error (RE) and the root mean square relative error (RMSRE) for each breathing frequency (BF) and spectral estimation method. Additionally, the sympathovagal balance was investigated by calculating the low frequency/high frequency (LF/HF) ratio. Above 7 breaths per minute, all methods showed a significant increase in LF/HF ratio with increasing BF. On average, the RMSRE of FFT was lower than for LSP and AR. Therefore, under paced respiration conditions, estimating RR-interval PSD using FFT is recommend.
Purpose
To investigate and to provide guidance for sample size selection based on the current practice in MR technical development studies in which healthy volunteers are examined.
Methods
All ...original articles published in Magnetic Resonance in Medicine between 2017 and 2019 were investigated and categorized according to technique, anatomical region, and magnetic field strength. The number of examined healthy volunteers (ie, the sample size) was collected and evaluated, whereas the number of patients was not considered. Papers solely measuring patients, animals, phantoms, specimens, or studies using existing data, for example, from an open databank, or consisting only of theoretical work or simulations were excluded.
Results
The median sample size of the 882 included studies was 6. There were some peaks in the sample size distribution (eg, 1, 5, and 10). In 49.9%, 82.1%, and 95.6% of the studies, the sample size was smaller or equal to 5, 10, and 20, respectively.
Conclusion
We observed a large variance in sample sizes reflecting the variety of studies published in Magnetic Resonance in Medicine. Therefore, it can be concluded that it is current practice to balance the need for statistical power with the demand to minimize experiments involving healthy humans, often by choosing small sample sizes between 1 and 10. Naturally, this observation does not release an investigator from ensuring that sufficient data are acquired to reach statistical conclusions.
Purpose
To find an optimized b‐value distribution for reproducible triexponential intravoxel incoherent motion (IVIM) exams in the liver.
Methods
A numeric optimization of b‐value distributions was ...performed using the triexponential IVIM equation and 27 different IVIM parameter sets. Starting with an initially optimized distribution of 6 b‐values, the number of b‐values was increased stepwise. Each new b‐value was chosen from a set of 64 predefined b‐values based on the computed summed relative mean error of the fitted triexponential IVIM parameters. This process was repeated for up to 100 b‐values. In simulations and in vivo measurements, optimized b‐value distributions were compared to 4 representative distributions found in literature.
Results
The first 16 optimized b‐values were 0, 0.3, 0.3, 70, 200, 800, 70, 1, 3.5, 5, 70, 1.2, 6, 45, 1.5, and 60 in units of s/mm2. Low b‐values were much more frequent than high b‐values. The optimized b‐value distribution resulted in a higher fit stability compared to distributions used in literature in both, simulation and in vivo measurements. Using more than 6 b‐values, ideally 16 or more, increased the fit stability considerably.
Conclusion
Using optimized b‐values, the fit uncertainty in triexponential IVIM can be largely reduced. Ideally, 16 or more b‐values should be acquired.
Purpose
High resolution diffusion‐weighted imaging is limited by susceptibility‐induced distortions and relaxation‐induced blurring. Segmented acquisition techniques can address these limitations at ...the expense of a prolonged scan time. If segmentation is performed along the readout direction, e.g., in RESOLVE (readout segmentation of long and variable echo‐trains), scan time can be reduced by readout (RO) partial Fourier methods, or simultaneous multi‐slice (SMS) methods. In this paper, we present a new approach to additionally accelerate the image acquisition called variable segment (VASE) RESOLVE.
Methods
To avoid discontinuities at the boundaries of the segments, the phase evolution and therefore the effective echo‐spacing needs to be adjusted. To achieve this, we use higher undersampling factors in the outer parts of k‐space. Simultaneously we increase the width of the outer segments resulting in an increase of the echo‐spacing. Because of this variation, we introduce a kind of randomization to the sampling scheme. This enables the use of compressed sensing reconstruction techniques, which results in improved image quality compared to standard parallel imaging methods.
Results
The RMS errors for the VASE RESOLVE acquisitions were lower compared to the standard reconstructions. The VASE RESOLVE in vivo images show a higher apparent signal to noise ratio.
Conclusion
VASE RESOLVE is a new approach to further decrease the acquisition time of RO segmented acquisitions. Compared to RESOLVE with SMS, VASE RESOLVE additionally reduces the acquisition time by a factor of 2.
Purpose
Intravoxel incoherent motion (IVIM) studies are performed with different acquisition protocols. Comparing them requires knowledge of echo time (TE) dependencies. The TE‐dependence of the ...biexponential perfusion fraction f is well‐documented, unlike that of its triexponential counterparts f1 and f2 and the biexponential and triexponential pseudodiffusion coefficients D*, D1∗, and D2∗. The purpose was to investigate the TE‐dependence of these parameters and to check whether the triexponential pseudodiffusion compartments are associated with arterial and venous blood.
Methods
Fifteen healthy volunteers (19‐58 y; mean: 24.7 y) underwent diffusion‐weighted imaging of the abdomen with 24 b‐values (0.2‐800 s/mm2) at TEs of 45, 60, 75, and 90 ms. Regions of interest (ROIs) were manually drawn in the liver. One set of bi‐ and triexponential IVIM parameters per volunteer and TE was determined. The TE‐dependence was assessed with the Kruskal‐Wallis test.
Results
TE‐dependence was observed for f (P < .001), f1 (P = .001), and f2 (P < .001). Their median values at the four measured TEs were: f: 0.198/0.240/0.274/0.359, f1: 0.113/0.139/0.146/0.205, f2: 0.115/0.155/0.182/0.194. D, D*, D1∗, and D2∗ showed no significant TE‐dependence. Their values were: diffusion coefficient D (10−4 mm2/s): 9.45/9.63/9.75/9.41, biexponential D* (10−2 mm2/s): 5.26/5.52/6.13/5.82, triexponential D1∗ (10−2 mm2/s): 1.73/2.91/2.25/2.51, triexponential D2∗ (mm2/s): 0.478/1.385/0.616/0.846.
Conclusion
f1 and f2 show similar TE‐dependence as f, ie, increase with rising TE; an effect that must be accounted for when comparing different studies. The diffusion and pseudodiffusion coefficients might be compared without TE correction. Because of the similar TE‐dependence of f1 and f2, the triexponential pseudodiffusion compartments are most probably not associated to venous and arterial blood.
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