Decay of the spin echo in a pulsed magnetic field gradient NMR

The additional attenuation of the spin echo of a pulsed magnetic field gradient NMR caused by the diffusion of the spins carrying molecules is calculated for a time dependent diffusion coefficient. The cases of a Constant D₀, of an exponential decay with time, and an abrupt drop from D₀ to zero at time t₀ are considered. In all cases the time dependence of the experimentally observed attenuation is less steep than that of D. The time scale, however, Changes very little. The calculation was based on the consideration that the diffusion transports the precessing spins to a location with a different Larmor frequency so that a complete reversal after the 180° pulse is not possible any more. Hence, any deviation from a straight line in a plot of the excess attenuation versus δ²(1 ? δ/3Δ) may by interpreted in terms of a time dependence of the diffusion coefficient.     

In vitro detection of apoptosis using oscillating and pulsed gradient diffusion magnetic resonance imaging

Cellular apoptosis, a common pathway towards tumor regression, is induced by many radiotherapy and chemotherapy regimens. Imaging methods that can detect apoptosis may be able to assess treatment response earlier than typical tumor volume measurements. In this paper, a wide range of diffusion experiments and a simple model of diffusion in tissues were used to probe the microstructural effects of apoptosis. Experiments were conducted on acute myeloid leukemia cell pellets, where apoptosis was induced by treatment with the chemotherapeutic agent cisplatin. Seventy‐two hours following cisplatin treatment, pulsed and oscillating gradient diffusion measurements were utilized to assess effects across a broad range of structural scales. The presence of apoptosis, which was histologically confirmed by TUNEL (terminal deoxynucleotidyl transferase UTP nick end labelling) staining, significantly changed diffusion properties. To describe these changes, the data were fit to the parallel plane model, which characterizes the effects of restricted diffusion in terms of three parameters: d, the restricted size, D_free, the intrinsic, free diffusion coefficient of the solvent, and D_rest, the long time or “restricted” diffusion coefficient. Apoptotic samples exhibited significant decreases in parameters d and D_free and a significant increase in D_rest. These changes appear consistent with the established morphological effects of apoptosis. In particular, the decrease in d may be a result of the combined effects of cell shrinkage, nuclear fragmentation and membrane blebbing, the decrease in D_free may relate to cytosolic condensation, while the increase in D_rest can be attributed to increased membrane permeability and extracellular volume fraction. By non‐invasively detecting apoptosis, the methods reported in this study have the potential to improve upon current MRI methods for monitoring therapeutic response. Furthermore, these methods may offer sufficient specificity to differentiate between apoptosis and other modes of cell death, such as oncosis or necrosis. Copyright © 2014 John Wiley & Sons, Ltd.     

In vivo magnetic resonance imaging and spectroscopy of humans with a 4 T whole-body magnet

A research-type 4 T whole-body magnet, built by Siemens AG, Erlangen, FRG, was used to investigate magnetic resonance at high field strengths. Designs for head and body coils operating at 170 MHz are described. Proton images of the human head and body are degraded by dielectric resonances and penetration effects. The nature of the dielectric resonances was demonstrated in phantoms containing distilled and saline doped water. Radiation damping at 170 MHz generates secondary echoes after a spin echo sequence. This effect was observed in phantoms and with reduced amplitude in the human head. Hydrogen spectra of the human head were selected utilizing stimulated and spin echoes. The latter technique allows the volume size to be reduced to 1 cm3. Examples of brain tumors that have been routinely investigated with volumes of 8 cm3 are given. Natural abundance carbon and phosphorus spectra of muscle and liver demonstrate the expected increase in spectral resolution and signal to noise ratio. Carbon spectra from the liver show the glycogen signal. Fluorine spectroscopy was used to study the time course of the absorption and emptying of a fluorinated antibiotic from the human stomach.     

The effects of simultaneous pulsing in different gradient coils on the nuclear magnetic resonance imaging of oblique slices

The definition and mapping of oblique planes by magnetic resonance imaging (MRI) requires the simultaneous application of two or three orthogonal gradients to define the desired intermediate direction of the frequency encoding or "readout" gradient. Each of the three main gradient coils produces different patterns of eddy currents. Consequently, the application of dephasing and rephasing lobes of these gradients will produce echoes at slightly different times for each gradient. If two or three gradients are applied simultaneously to create an arbitrary view direction, the resulting echo will therefore be shifted in time and considerably reduced in intensity. In this article, we present an analysis of the behavior of the magnetization in a typical two-dimensional Fourier transform pulse sequence for the imaging of oblique slices. The theoretical displacements in time and reduction in intensity of the echo amplitudes are calculated and compared to the experimental behavior. We show that, in spite of this phenomenon, the final image suffers only marginally in signal-to-noise ratio, provided the slice width is small compared to the field of view. This is due to the fact that there always exists a cycle in the sequence in which the phase-encoding gradient almost completely compensates for the above described effect.     

An open-source software tool for the generation of relaxation time maps in magnetic resonance imaging

Background  

In magnetic resonance (MR) imaging, T1, T2 and T2* relaxation times represent characteristic tissue properties that can be quantified with the help of specific imaging strategies. While there are basic software tools for specific pulse sequences, until now there is no universal software program available to automate pixel-wise mapping of relaxation times from various types of images or MR systems. Such a software program would allow researchers to test and compare new imaging strategies and thus would significantly facilitate research in the area of quantitative tissue characterization.     

Utility of gradient recalled echo magnetic resonance imaging for the study of myelination in cuprizone mice treated with fingolimod

The availability of high‐field‐strength magnetic resonance imaging (MRI) systems has brought about the development of techniques that aim to map myelination via the exploitation of various contrast mechanisms. Myelin mapping techniques have the potential to provide tools for the diagnosis and treatment of diseases, such as multiple sclerosis. In this study, we evaluated the sensitivity of T₂*, frequency shift and susceptibility measures to myelin levels in a cuprizone mouse model of demyelination. The model was supplemented with two different dosages of fingolimod, a drug known to positively affect demyelination. A decrease in grey–white matter contrast with the cuprizone diet was observed for T₂*, frequency shift and susceptibility measures, together with myelin basic protein antibody findings. These results indicate that T₂*, frequency shift and susceptibility measures have the potential to act as biomarkers for myelination. Susceptibility was found to be the most sensitive measure to changes in grey–white matter contrast. In addition, fingolimod treatment was found to reduce the level of demyelination, with a larger dosage exhibiting a greater reduction in demyelination for the in vivo MRI results. Overall, susceptibility mapping appears to be a more promising tool than T₂* or frequency shift mapping for the early diagnosis and treatment of diseases in which myelination is implicated.     

Negative socio-emotional resonance in schizophrenia: a functional magnetic resonance imaging hypothesis

The aim of the present study is to use neuroscience theories about brain function (mirror-neurons MN) to draw inferences about the mechanisms supporting emotional resonance in two different groups of schizophrenia patients (with flat affect FA+ n = 13 and without flat affect FA- n = 11). We hypothesize that FA+ will not activate key brain areas involved in emotional processing. Conversely, FA- will have a functional mirror system for emotional resonance confirmed by activation of the prefrontal cortex and behavioral results. To test this hypothesis, we compared the two groups using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) displaying a passive visual task (44 negative IAPS pictures and 44 neutral pictures). A random-effects analysis, for schizophrenia patients FA-, revealed significant loci of activation in the left mesial prefrontal (MPFC), right orbitofrontal (OFC) and left anterior cingulate cortices (ACC). Correlational analyses carried out between self-report ratings of negative feelings and BOLD signal changes revealed the existence of positive correlation in the LACC, LMPFC and ROFC. Conversely, FA+ did not show significant activation in the prefrontal cortex. We propose that negative emotional resonance induced by passively viewing negative pictures may be a form of "mirroring" that grounds negative feelings via an experiential mechanism. Hence, it could be argued that FA- were able to 'feel' emotions through this resonance behavior. Conversely, we suggest that the dysfunction seen in the FA+ group is a failure or distortion in the development of the MN system. This could be due to genetic or other endogenous causes, which affected prefrontal cortex MN involved in emotional resonance.     

Optimal classification of long echo time in vivo magnetic resonance spectra in the detection of recurrent brain tumors

We describe the optimal high-level postprocessing of single-voxel (1)H magnetic resonance spectra and assess the benefits and limitations of automated methods as diagnostic aids in the detection of recurrent brain tumor. In a previous clinical study, 90 long-echo-time single-voxel spectra were obtained from 52 patients and classified during follow-up (30/28/32 normal/non-progressive tumor/tumor). Based on these data, a large number of evaluation strategies, including both standard resonance line quantification and algorithms from pattern recognition and machine learning, were compared in a quantitative evaluation. Results from linear and non-linear feature extraction, including ICA, PCA and wavelet transformations, and also the data from resonance line quantification were combined systematically with different classifiers such as LDA, chemometric methods (PLS, PCR), support vector machines and ensemble methods. Classification accuracy was assessed using a leave-one-out cross-validation scheme and the area under the curve (AUC) of the receiver operator characteristic (ROC). A regularized linear regression on spectra with binned channels reached 91% classification accuracy compared with 83% from quantification. Interpreting the loadings of these regressions, we find that lipid and lactate signals are too unreliable to be used in a simple machine rule. Choline and NAA are the main source of relevant information. Overall, we find that fully automated pattern recognition algorithms perform as well as, or slightly better than, a manually controlled and optimized resonance line quantification.     

Effect of strength training on the relationship between magnetic resonance relaxation time and muscle fibre composition

Summary The effect of muscle hypertrophy on the relationship between magnetic resonance (MR) relaxation time and muscle fibre composition was investigated. Relaxation time and muscle fibre composition were measured in five subjects before and after a 20-week period of strength training. Muscle fibre composition in all subjects exhibited a significant shift to a predominance of fast-twitch (FT) fibres as a result of 20-week strength training (% area FT fibres: mean values from 49.8%, SD 17.9% to 57%, SD 5.6%; P<0.05). Longitudinal relaxation time (T₁) and transverse relaxation time (T₂) were prolonged significantly after strength training (T₁ mean values from 334.9 ms, SD 13.6 to 359.0 ms, SD 9.0, P<0.001; T₂ from 27.5 ms, SD 0.9 to 30.8 ms, SD 2.3, P<0.05). A constant relationship was observed in changes caused by strength training in muscle fibre composition (% area FT) and relaxation time, with a high correlation obtained between both parameters. These results indicate that MR relaxation time can be used for non-invasive estimation of muscle fibre composition.     

The analysis of waiting time and utilization of computed tomography and magnetic resonance imaging in Croatia: a nationwide survey

AimTo assess the variation in the waiting time for diagnostic imaging (DI) services among Croatian public hospitals and the utilization of computed tomography (CT) and magnetic resonance imaging (MRI) scanners.MethodsWe analyzed aggregated data from public hospitals. Counties were classified according to economic strength, and utilization was expressed as the average number of exams per machine. We compared the waiting times for 2018 and utilization for 2015 according to hospital category (high and low level) and economic strength by county.ResultsThe waiting time was longer for MRI compared with CT, 268 vs 77.61 days. Overall CT waiting time was in the unfavorable European Health Consumer Index category. High-level hospitals had longer waiting time for MRI and CT. The waiting time positively correlated with economic strength for MRI (P = 0.019), but not for CT. In low-level hospitals, MRI utilization ranged from 104 to 6032, whereas CT utilization ranged from 48 to 17 852. In high-level hospitals, MRI utilization ranged from 3846 to 11 026, while CT utilization ranged from 503 to 17 234. CT (P = 0.041) and MRI (P = 0.031) utilization in high-level hospitals was significantly higher than in low-level hospitals.ConclusionThe waiting times for CT and MRI were exceptionally long regardless of the hospital category, with highly varying utilization. Croatia performed more exams per scanner compared with other EU countries, but not significantly so. High-level hospitals'' utilization was significantly higher than that of low-level hospitals, and CT utilization was significantly higher than EU average, while the difference for MRI utilization was not significant.

Radiology has a pivotal position within the health care system owing to the indispensable role of diagnostic imaging (DI) procedures, in particular computed tomography (CT) and magnetic resonance imaging (MRI) in current diagnostic and follow-up algorithms. The demand for DI is continuously growing, expectedly prolonging the waiting time. Waiting time is a topic of high interest when evaluating health system performance, therefore health services need to utilize CT and MRI scanners to the fullest extent. The assessment and minimization of waiting time is of utmost importance not only from a medical but also from a social and political perspective. Long waiting times for medical procedures represent a major obstacle toward achieving optimal (or even standard) levels of care in public health care systems (1-3). Waiting for diagnostic tests accounts for the majority of lengthy waits in the public health care system, leading to lower patients’ satisfaction. Besides affecting patients’ physical and mental health, waiting time also has economic implications for the individual patient and society (4). Achieving a more efficient and high-performing health care system represents a primary concern for most patients and a major aim for health authorities. Implementation of good radiological practice can be an efficient strategy to reduce inefficient and unnecessary spending in health care. Although Croatian counties differ in available radiological and economic resources, the association of these factors with waiting time for DI and characteristics of waiting time have not been studied thus far.This article aims to assess the variation in waiting time for CT and MRI services across different hospital categories, and the level of utilization and its association with counties'' economic strength in Croatia.     

A simple procedure for determination of the dead time of a stopped-flow instrument

The reducing reaction of 2,6-dichlorophenolindophenol by L-ascorbic acid was used to determine the dead time of a stopped-flow instrument. Because this reaction is irreversible, the dead time could be determined by a simple graphical analysis. The dead time values determined by the present method were comparable to those by other methods previously reported.     

Light sedation with short habituation time for large-scale functional magnetic resonance imaging studies in rats

Traditionally, preclinical resting state functional magnetic resonance imaging (fMRI) studies have been performed in anesthetized animals. Nevertheless, as anesthesia affects the functional connectivity (FC) in the brain, there has been a growing interest in imaging in the awake state. Obviously, awake imaging requires resource- and time-consuming habituation prior to data acquisition to reduce the stress and motion of the animals. Light sedation has been a less widely exploited alternative for awake imaging, requiring shorter habituation times, while still reducing the effect of anesthesia. Here, we imaged 102 rats under light sedation and 10 awake animals to conduct an FC analysis. We established an automated data-processing pipeline suitable for both groups. Additionally, the same pipeline was used on data obtained from an openly available awake rat database (289 measurements in 90 rats). The FC pattern in the light sedation measurements closely resembled the corresponding patterns in both onsite and offsite awake datasets. However, fewer datasets had to be excluded due to movement in rats with light sedation. The temporal analysis of FC in the lightly sedated group indicated a lingering effect of anesthesia that stabilized after the first 5 min. In summary, our results indicate that the light sedation protocol is a valid alternative for large-scale studies where awake protocols may become prohibitively resource-demanding, as it provides similar results to awake imaging, preserves more scans, and requires shorter habituation times. The large amount of fMRI data obtained in this work are openly available for further analyses.     

31P-Nuclear magnetic resonance spectroscopy study of the time course of energy metabolism during exercise and recovery

Summary This study evaluated the time courses of intracellular pH and the metabolism of phosphocreatine (PCr) and inorganic phosphate (P) at the onset of four exercise intensities and recoveries. Non-invasive evaluation of continuous changes in phosphorus metabolites has become possible using³¹P-nuclear magnetic resonance spectroscopy (³¹P-MRS). After measurements at rest, six healthy male subjects performed 4 min of femoral flexion exercise at intensities of 0 (loadless), 10, 20 and 30 kg · m · min^–1 in a 2.1 T superconducting magnet with a 67-cm bore. Measurements were continuously made during 5 min of recovery. During a series of rest-exercise-recovery procedures,³¹P-MRS were accumulated using 32 scans · spectrum^–1 requiring 12.8 s each. At the onset of exercise, PCr decreased exponentially with a time constant of 27–32 s regardless of the exercise intensity. The time constant PCr resynthesis during recovery was about 27–40 s. The PCr kinetics were independent of exercise intensity. There were similar Pi kinetics at the onset of all types of exercise, while those of Pi recovery became significantly longer at the higher exercise intensities (P < 0.05). Furthermore, the intracellular pH indicated temporary alkalosis just at the onset of exercise, probably due to absorption of hydrogen ions by PCr hydrolysis, and then decrease at a point about 40%–50% of the preexercise PCr. The pH recovery time was longer than that for the P_i or PCr kinetics. By using a more efficient resolution system it was possible to obtain the phosphorus kinetics during exercise and to follow PCr resynthesis within the first few minutes of recovery. From our results it was concluded that in general the time course of PCr and Pi metabolism were unaffected by the exercise intensity, both at the onset of exercise and during recovery, with the exception of P_i recovery.     

磁生物效应研究中圆片永磁磁源空间磁感强度的计算方法

本文根据电磁学中的比奥－－沙伐－－拉普拉斯定律，推倒出了圆片永磁磁源空间磁感强度的计算公式；并使用数值积分和计算机程序，建立了该公式的计算方法；最后通过实例计算，得出了圆片永磁磁源空间磁感强度的量值分布。为磁生物效应研究中磁源的使用提供了理论依据。