Spectral resolution enhancement by chemical shift imaging

Three-dimensional chemical shift imaging (3D CSI) with appropriate data postprocessing can be used as a tool to improve spectral resolution in samples where large susceptibility differences and limited shim capabilities prevent good sample shimming. Data postprocessing is reduced to the realignment of individual 3D voxel spectra. As a result, the line broadening due to the field inhomogeneity over the sample's volume is reduced to the broadening by inhomogeneity within individual voxels. We compared this method with the resolution enhancement by window multiplication. We demonstrated, theoretically and experimentally, that in the presence of large, lower-order gradients, 3D CSI achieves better resolution enhancement with smaller sensitivity losses. An application of the method to a simple biological system is presented as well.     

Artifacts in chemical shift selective imaging.

The role of susceptibility effects in the production of artifacts in chemical shift images generated by the selective excitation technique is discussed. The effects are demonstrated in images of phantom samples of agarose gels containing small air bubbles. The artifacts can lead to erroneous interpretations, in which the resonances to be resolved exhibit a relatively small chemical shift separation, such as that between water and soluble carbohydrates (sugars).     

Robustness of fat quantification using chemical shift imaging

The purpose of this study was to investigate the effect of parameter changes that can potentially lead to unreliable measurements in fat quantification. Chemical shift imaging was performed using spoiled gradient echo sequences with systematic variations in the following: two-dimensional/three-dimensional sequence, number of echoes, delta echo time, fractional echo factor, slice thickness, repetition time, flip angle, bandwidth, matrix size, flow compensation and field strength. Results indicated no significant (or significant but small) changes in fat fraction with parameter. The significant changes can be attributed to the known effects of T1 bias and two forms of noise bias.     

Quantitative multivoxel proton chemical shift imaging of the breast

The study of focal pathology by single-voxel magnetic resonance spectroscopy (MRS) is hampered by the impossibility to study tissue heterogeneity or compare the metabolite signals in breast lesion directly to those in unaffected tissue. Multivoxel MRS studies, while potentially allowing for truly quantitative tissue characterization, have up to now also been far from quantitative with, for example, the signal-to-noise ratio of the choline (Cho) signal serving as measure of tumor activity. Shown in this study is that in a standard clinical setting with a regular 1.5-T magnetic resonance scanner, it is possible to perform quantitative multivoxel MRS. With the use of literature values for the T1 and T2 relaxation times of Cho and water in fibroglandular breast tissue and tumors, one can determine the concentrations of Cho in different tumor compartments and surrounding tissues in two brief multivoxel MRS measurements. This opens excellent perspectives to quantitative diagnostic and follow-up studies of focal pathology such as lesions suspected of breast cancer.     

Feasibility of proton chemical shift imaging with a stereotactic headframe

To prove feasibility of proton chemical shift imaging ((1)H CSI) during stereotactic procedure, authors performed (1)H CSI in combination with a stereotactic headframe and selected targets according to local metabolic information, evaluated the pathologic results. The (1)H CSI directed stereotactic biopsy was performed in four patients. (1)H CSI and conventional Gd-enhancement stereotactic MRI were performed simultaneously after the fitting of a stereotactic headframe. Focal areas of increased phosphocholine(Cho)/phosphocreatine(Cr) and Lactate/Cr ratios were selected as target sites in the stereotactic MR images. (1)H CSI is possible with the stereotactic headframe in place. Pathologic samples taken from areas of increased Cho/Cr ratios and decreased NAA/Cr ratios provided information upon increased cellularity, mitoses and cellular atypism, and facilitated diagnosis. Pathologic samples taken from areas of increased Lac/Cr ratio showed predominant feature of necrosis. (1)H CSI was feasible with the stereotactic headframe in place. The final pathologic results obtained were concordant with the local metabolic information from (1)H CSI. We believe that (1)H CSI-directed stereotactic biopsy has the potential to significantly improve the accuracy of stereotactic biopsy targeting.     

A method for imaging of chemical shift or magnetic field distributions

A phase encoding method for imaging of chemical shift or magnetic field distributions is described. The method utilizes the spin-echo principle and the time period between signal collection and excitation is constant but the time period between excitation and the 180 degrees pulse is varied by constant steps. The method is relatively easy to apply with the Fourier or projection reconstruction methods.     

A comparison of selective saturation and selective echo chemical shift imaging techniques

We report on a comparative study of two methods of chemical shift imaging which can be used to selectively image fat and water in vivo. Both methods require a B0 field sufficiently homogenous to resolve the methylene and water spectral lines. One method, called CHESS, uses a chemically selective pulse to saturate the unwanted spectral line. The other method, called SECSI, achieves chemical selectivity by using a soft 180 degree RF pulse in forming a spin echo image. Both methods require that the strength of the B1 RF field be accurately calibrated and homogenous. We show by theoretical analysis that the suppression of unwanted spectral lines is sensitive to the first power of B1 errors in the CHESS method but to the second power of B1 errors in the SECSI method. Experiments with phantoms confirmed the expected non linearity of the SECSI method, and showed superior water suppression factors in phantoms with it. Experiments with a large phantom and a living rabbit showed superior results using the SECSI method, and the best results were obtained using a combination of the two techniques.     

Spectrally resolved flow imaging of fluids inside a microfluidic chip with ultrahigh time resolution

Microfluidics has advanced to become a complete lab-on-a-chip platform with applications across many disciplines of scientific research. While optical techniques are primarily used as modes of detection, magnetic resonance (MR) is emerging as a potentially powerful and complementary tool because of its non-invasive operation and analytical fidelity. Two prevailing limitations currently inhibit MR techniques on microfluidic devices: poor sensitivity and the relatively slow time scale of dynamics that can be probed. It is commonly assumed that the time scale of observation of one variable limits the certainty with which one can measure the complementary variable. For example, short observation times imply poor spectral resolution. In this article, we demonstrate a new methodology that overcomes this fundamental limit, allowing in principle for arbitrarily high temporal resolution with a sensitivity across the entire microfluidic device several orders of magnitude greater than is possible by direct MR measurement. The enhancement is evidenced by recording chemically resolved fluid mixing through a complex 3D microfluidic device at 500 frames per second, the highest recorded in a magnetic resonance imaging experiment. The key to this development is combining remote detection with a time ‘slicing’ of its spatially encoded counterpart. Remote detection circumvents the problem of insensitive direct MR detection on a microfluidic device where the direct sensitivity is less than 10^-5 relative to traditional NMR, while the time slicing eliminates the constraints of the limited observation time by converting the time variable into a spatial variable through the use of magnetic field gradients. This method has implications for observing fast processes, such as fluid mixing, rapid binding, and certain classes of chemical reactions with sub millisecond time resolution and as a new modality for on-chip chromatography.     

Cardiac phosphorus-31 two-dimensional chemical shift imaging in patients with hereditary hemochromatosis

Hemochromatosis is a hereditary iron overload syndrome characterized by increased iron storage, followed by liver cirrhosis and is often associated with restrictive cardiomyopathy. The purpose of this study was to detect alterations of cardiac high-energy phosphate metabolism in patients with hereditary hemochromatosis (HHC) prior to the development of structural heart diseases. Therefore cardiac phosphorus-31 two-dimensional chemical shift imaging ((31)P 2D CSI) was employed. Twenty-four male patients (mean age 47.2 +/- 12 years) homozygous for the C282Y mutation in the hemochromatosis associated HFE gene and twenty-four male healthy volunteers (mean age 47 +/- 11 years) as age-matched controls were included in this study. Using a 1.5-Tesla whole-body magnetic resonance scanner, electrocardiograph-triggered transversal 31P 2D CSI was performed. Left ventricle mean phosphocreatine (PCr) to beta-adenosine triphosphate (beta-ATP) ratios of patients with HHC (1.60 +/- 0.41) were significantly decreased in comparison to healthy volunteers (1.93 +/- 0.36; p = 0.004). Furthermore, we detected moderate, negative correlations between left ventricular PCr to beta-ATP ratios and transferrin saturation, cholesterol, low-density lipoprotein as well as triglyceride. This study shows that 31P 2D CSI permits the detection of alterations of cardiac high-energy phosphate metabolism in patients with HHC, but without any evidence for heart disease. The decreased PCr to beta-ATP ratios in HHC might be caused by mitochondrial impairment due to cardiac iron overload.     

Choice of soft pulse shapes for signal excitation in chemical shift selective imaging

The choice of soft pulse shapes for chemical shift selective excitation in chemical shift imaging is discussed. In the presence of inhomogeneities in the static magnetic field resulting from susceptibility anomalies, it is important to optimise pulse bandshape and frequency offset as well as bandwidth, in order to minimize artefacts arising from excitation of unwanted resonances. A comparison of the use of Gaussian and sinc shaped excitation pulses in the chemical shift micro-imaging of grapes serves to illustrate some of the effects that may be observed.     

Mapping phosphorylation rate of fluoro-deoxy-glucose in rat brain by F chemical shift imaging

¹⁹F magnetic resonance spectroscopy (MRS) studies of 2-fluoro-2-deoxy-d-glucose (FDG) and 2-fluoro-2-deoxy-d-glucose-6-phosphate (FDG-6P) can be used for directly assessing total glucose metabolism in vivo. To date, ¹⁹F MRS measurements of FDG phosphorylation in the brain have either been achieved ex vivo from extracted tissue or in vivo by unusually long acquisition times. Electrophysiological and functional magnetic resonance imaging (fMRI) measurements indicate that FDG doses up to 500 mg/kg can be tolerated with minimal side effects on cerebral physiology and evoked fMRI-BOLD responses to forepaw stimulation. In halothane-anesthetized rats, we report localized in vivo detection and separation of FDG and FDG-6P MRS signals with ¹⁹F 2D chemical shift imaging (CSI) at 11.7 T. A metabolic model based on reversible transport between plasma and brain tissue, which included a non-saturable plasma to tissue component, was used to calculate spatial distribution of FDG and FDG-6P concentrations in rat brain. In addition, spatial distribution of rate constants and metabolic fluxes of FDG to FDG-6P conversion were estimated. Mapping the rate of FDG to FDG-6P conversion by ¹⁹F CSI provides an MR methodology that could impact other in vivo applications such as characterization of tumor pathophysiology.     

An investigation of the fluidity of concentration polarisation layers in crossflow membrane filtration of an oil-water emulsion using chemical shift selective flow imaging

A chemical shift selective NMR flow imaging sequence using stimulated echoes for data acquisition is presented. The sequence was tested using a 20% (vol/vol) oil-water emulsion formed from a soluble cutting oil, which was passed through a simple flow phantom to yield two-dimensional velocity distribution maps of the oil droplets and of the water separately. It was then used to investigate the fluidity of concentration polarisation layers formed from the oil droplets during crossflow membrane filtration of a 5% (vol/vol) emulsion of the same cutting oil. A simple membrane filtration module was used for this purpose, with the feedstock emulsion fed into the lumen of a single tubular membrane at a trans-membrane pressure difference P ∼ 70kPa and crossflow Reynolds number, Re, in the range 100–1000. The results confirm a net axial flow rate < 7.5 μm/s (half digital resolution in the velocity dimension) for the oil polarisation layer. Under these conditions, the upper limit for oil flow tangential to the membrane in the polarised layer is less than 15% of the convective flow of oil towards the membrane.     

Chemical shift resolved photoionization cross sections of amorphous carbon nitride

We present a method to derive an upper bound for the entropy density of coupled map lattices with local interactions from local observations. To do this, we use an embedding technique that is a combination of time delay and spatial embedding. This embedding allows us to identify the local character of the equations of motion. Based on this method we present an approximate estimate of the entropy density by the correlation integral.     

A chemical shift imaging study on regional metabolite distribution in a CADASIL family

A chemical shift imaging (CSI) study was performed to directly assess relative concentrations of N-acetylaspartate (NAA), Cho and Cr metabolites in normal- and abnormal-appearing brain tissue of asymptomatic and symptomatic members of a single family with a neuropathologic, genetic and electrophysiological confirmed diagnosis of cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. The aim of the investigation was to evaluate clinical findings and metabolite abnormalities as early appearance of axonal injury in this syndrome. The main findings related statistically significant decreases in the mean metabolite ratios for NAA/Cr, NAA/Cho and Cho/Cr in the anterior parts in comparison with the posterior parts of the centrum semiovale in symptomatic and asymptomatic patients. The effect was considerably greater in the symptomatic patients, indicating a strong correlation between CSI and pathology results. No differences were found between the two areas in the control group. Although lactate signals were hardly detectable in individual spectra, there was a trend toward increased Lac/Cr values in the anterior parts with respect to the posterior parts in the patient group, with the effect particularly evident in the asymptomatic subjects with the gene mutation.