Chemical Imaging Method Research Articles

High-Resolution Chemical Imaging through Tissue with an X-ray Scintillator Sensor

We describe a novel method for high-resolution chemical imaging on a surface embedded in tissue. The sensor surface consists of an X-ray scintillator film coated in a thin film loaded with chemical indicator dye. A narrow scanning X-ray beam is used to excite luminescence from X-ray scintillators located within the beam. This luminescence passes through the indicator film, and the spectrum is analyzed to measure chemical concentrations at that location. A pH sensor is demonstrated with a dynamic range between pH 6-9 and noise level of 0.05 pH units using methyl-red dyed pH paper. The location of the interface between two types of scintillator films is obtained with 0.30 mm spatial resolution even though the images are highly blurred by 10 mm of chicken breast. This work has important applications for detecting pH changes on surfaces of implanted medical devices.

Multiphoton Ionization and Fragmentation of Hydrogen Chloride: A Diatomic Still Good for a Surprise

The competition between multiphoton ionization and fragmentation in the diatomic molecule hydrogen chloride is reviewed. Emphasis is laid on recent experimental results employing chemical imaging methods in order to obtain kinetic energy distributions and angular distributions of photoproducts. The energy range considered is 15 to 20 eV, equivalent to the absorption of three or four photons in the ultraviolet wavelength range. The role of Rydberg states as resonantly excited intermediate states in the ionization/fragmentation processes is assessed. Mixing among Σ1+ states gives rise to peculiarly shaped double minimum potential energy curves which allow for the production of hydrogen and chlorine atomic and ionic fragments via several competing pathways, in addition to the production of molecular HCl+ ions. States with different electronic properties show a qualitatively different behaviour from Σ+ states. Accidental resonances between states of differing orbital angular momentum or multiplicity serve to override these differences and cause subtle as well as significant deviations from the unperturbed behaviour.

Advances in CNS tumour imaging

Brain tumours are the second most common group of paediatric malignancies after leukaemia. Unlike those in adults, most paediatric brain tumours are primitive lesions. The most common location is the posterior cranial fossa, where tumours prevail between ages 1 and 11 years. Conversely, tumours occurring in children younger than 1 year and older than 12 years are located more commonly in the supratentorial compartment. Magnetic resonance imaging (MRI) is an invaluable tool for identifying and characterising brain tumours, presurgical planning and evaluating the effectiveness of surgery, chemotherapy and radiotherapy. Advanced MRI modalities, including diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI) and spectroscopy, provide significant additional information regarding quantities, such as tumour cellularity, haemodynamics and metabolism. DWI provides information regarding diffusion of water molecules from which quantitative values, the apparent diffusion coefficient (ADC), can be calculated. The most important application of DWI in the field of brain tumour imaging is in estimation of cellularity, which correlates grossly with tumour grade. Diffusion tensor imaging (DTI) and fibre tractography are important tools for preoperative mapping of brain tumours, including the identification of the anatomical relationship between tumour and functional areas, such as major fibre tracts. PWI measures cerebral haemodynamics at the microcirculation level. The cerebral blood volume (CBV), defined as the volume of blood per unit of brain tissue and expressed in ml/100 g, is the parameter most commonly evaluated in the field of brain tumour imaging. The average relative CBV of high-grade lesions is significantly higher than that of low-grade neoplasms—a direct function of neovascularity, regardless of the integrity of the blood-brain barrier. There are several techniques for performing PWI. The most widely available is T2*-weighted dynamic susceptibility contrast (DSC) imaging, which requires intravenous administration of a bolus of paramagnetic contrast material and rapid acquisition of images over time during the first pass of contrast material through the capillary bed. DSC-PWI poses a number of technical challenges in the paediatric age group, including the need for constant, high-flow contrast material injection by power injectors, and stable, sufficiently sized intravenous access catheters, which may be problematic in the newborn and infant age groups. Arterial spin labelling (ASL), a perfusion technique that does not use an intravenous tracer, while promising, is less widely available in the clinical arena. Its use is currently limited to a few centres where high fieldstrength (i.e. 3 T and higher) magnets are available. Magnetic resonance spectroscopy (MRS) allows noninvasive detection and measurement of normal and abnormal metabolites and plays an important role in the diagnostic work-up of brain tumours in children. Available 1H-MRS techniques include single-voxel and multi-voxel (chemical shift imaging) methods. Acquisition is possible at short (20– 30 ms), intermediate (135–144 ms), and long (270–280 ms) echo times, yielding different possibilities for metabolite identification and peak analysis. In most instances of brain tumours, a pathological spectrum (non-specific for tumour type) consisting of increased choline (Cho)/creatine (Cr) and Disclaimer Dr. Rossi has no financial interests, investigational or offlabel uses to disclose.

Comparison of chemometric methods in the analysis of pharmaceuticals with hyperspectral Raman imaging

AbstractChemical imaging method of vibrational spectroscopy, which provides both spectral and spatial information, creates a three‐dimensional (3D) dataset with a huge amount of data. When the components of the sample are unknown or their reference spectra are not available, the classical least squares (CLS) method cannot be applied to create visualized distribution maps. Raman image datasets can be evaluated even in such cases using multivariate (chemometric) methods for extracting the needed hidden information. The capability of chemometrics‐assisted Raman mapping is evaluated through the analysis of pharmaceutical tablets (considered as unknown) with the aim of estimating the pure component spectra based on the collected Raman image. Six chemometric methods, namely, principal component analysis (PCA), maximum autocorrelation factors (MAF), sample–sample 2D correlation spectroscopy (SS2D), self‐modeling mixture analysis (SMMA), multivariate curve resolution–alternating least squares (MCR‐ALS), and positive matrix factorization (PMF), were compared. SMMA was found to be the best choice to determine the number of components. MCR‐ALS and PMF provided the pure component spectra with the highest quality. MCR‐ALS was found to be superior to PMF in the estimation of Raman scores (which correspond to the concentrations) and yielded almost the same results as CLS (using the real reference spectra). Thus, the combination of Raman mapping and chemometrics could be successfully used to characterize unknown pharmaceuticals, identify their ingredients, and obtain information about their structures. This may be useful in the struggles against illegal and counterfeit products and also in the field of pharmaceutical industry when contaminants are to be identified. Copyright © 2011 John Wiley & Sons, Ltd.

H 1-MR-spectroscopy of cerebellum in adult attention deficit/hyperactivity disorder

Introduction Neurobiological research has implicated the cerebellum as one possible site of neurophysiological dysfunction in ADHD. Latest theoretical conceptualizations of the cerebellum as core site of the brain to model motor as well as cognitive behavior puts further weight to the assumption that it might play a key role in ADHD pathophysiology. Methods 30 medication free adult ADHD patients and 30 group matched (gender, age and education) healthy controls were investigated using the method of chemical shift imaging (CSI) of the cerebellum. The vermis, left and right cerebellar hemispheres were processed separately. Results We found significantly increased glutamate-glutamine (Glx) to creatine (Cre) ratios in the left cerebellar hemisphere. No other differences in measured metabolite concentrations were observed. Discussion To our knowledge this is the first evidence for neurochemical alterations in cerebellar neurochemistry in adult ADHD. They relate well to recent hypotheses that the cerebellum might control mental activities by internal models.

Chemical shift imaging without water suppression at 3 T

Proton magnetic resonance spectroscopy without water suppression is possible but is hampered by the presence of sideband artifacts. The aim of this study was to develop a chemical shift imaging method without water suppression for clinical routine with reduced sideband artifacts. Spectra from ten healthy volunteers were acquired using a 3T (TimTrio, Siemens, Erlagen, Germany) scanner with a Point RESolved Spectroscopy sequence for volume selection. Postprocessing was performed in three steps: correcting the water peak position in all spectra (chemical shift correction), subtracting the Gaussian convolution of all free induction decay signals (FIDs) (water signal reduction), and subtracting the FID of a water phantom from the volunteer's FID signal (reduction of sidebands). For the postprocessing customized software was developed with Matlab 2007b. The described technique provides spectra with reduced water signal and sidebands. Quantitative analysis showed that there is a good agreement between spectra obtained with water suppressing radiofrequency pulses and the new method. Moreover, spectra obtained with the new method do not need phase correction. The new method offers sufficient reduction of the water peak and sidebands. Its simplicity allows its use in clinical applications.

Rosette spectroscopic imaging: Optimal parameters for alias‐free, high sensitivity spectroscopic imaging

To optimize the Rosette trajectories for fast, high sensitivity spectroscopic imaging experiments and to compare this acquisition technique with other chemical shift imaging (CSI) methods. A framework for comparing the sensitivity of the Rosette Spectroscopic Imaging (RSI) acquisition to other spectroscopic imaging experiments is outlined. Accounting for hardware constraints, trajectory parameters that provide for optimal sampling and minimal artifact production are found. Along with an analytical expression for the number of excitations to be used in an RSI experiment that is provided, the theoretical precompensation weights used for optimal image reconstruction are derived. The spectral response function for RSI is shown to be approximately the same as the point spread function of standard Fourier reconstructions. While the signal-to-noise ratio (SNR) for an RSI experiment is reduced by the inherent nonuniform sampling of these trajectories, their circular k-space support and speed of spatial encoding leads to greater SNR efficiency and improvements in the total data acquisition time relative to the gold standard CSI approach with square k-space support and to similar efficiency to spiral CSI acquisitions. Numerical simulations and in vivo experimental data are presented to demonstrate the properties of this data acquisition technique. This work demonstrates the use of Rosette trajectories and how to achieve improved efficiency for these trajectories in a two-dimensional spectroscopic imaging experiment.

Potential of Raman Spectroscopy and Imaging Methods for Rapid and Routine Screening of the Presence of Melamine in Animal Feed and Foods

The impact of melamine-contaminated animal feed ingredients on food safety has become a major public concern ever since melamine was identified as the organic compound responsible for the deaths of a significant number of cats and dogs in 2007 by way of adulterated pet food. Melamine, a common industrial chemical often added to resins to improve flame resistance and proposed as an alternative form of fertilizer-N for plant growth, was found to be intentionally added to animal feed in amounts ranging from 0.2% to 8% of total mass as a way to boost the products’ apparent protein content. It was also used as a binder in the production of pelleted feed for animals. In addition to melamine, a small amount of cyanuric acid, ammeline, and ammelide were also detected in pet feed and in the tissues and urine of dead pets that had consumed the contaminated food. Even though it is possible that cyanuric acid, ammeline, and ammelide were added, their presence more likely resulted from the degraded derivatives of melamine. There is a great concern that melamine will again enter the food chain and be consumed by humans and animals. As part of the Food Protection Plan, US federal agencies such as the FDA and FSIS and other organizations have established GCMS and LC-MS/MS procedures for the analysis of melamine in food/feed commodities. Although they can detect melamine contaminants in trace amount, these time-consuming laboratory procedures require chemical solvents for the extraction steps and depend on expensive mass spectrometry. Rapid, nondestructive, and routine methods for the specific detection of melamine in raw feed materials are increasingly important, not only for public health concerns but also for melamine screening to prevent protein fraud. Undoubtedly, the well-defined mass spectroscopic technique is preferred due to its low detection limit and capability for structural elucidation; however, since adulteration of raw materials by melamine usually occurs in higher concentrations in order to affect protein content, the high sensitivity of the mass spectroscopic technique may not be necessary. In addition, mass spectrometry might not be sufficiently rapid to screen for the presence of melamine in a large number of food/feed materials from very different sources, because the identification process includes sample-specific extraction procedures, which are labor-intensive and time-consuming. Fast melamine screening requires minimal sample preparation (e.g., no extraction or centrifugation), routine analysis of a number of samples without reagents, minimal procedural steps, and easy operation and interpretation of results. The Raman technique, which has been used to obtain structural information on melamine, is an alternative approach that can be applied to solid materials with no sample pretreatment. In addition, the use of the Fourier transform (FT) methodology and a 1064 nm excitation laser in the near-infrared (NIR) region provides precise wavenumber measurement and good-quality Raman spectra by reducing the interference from fluorescence and photodecomposition of colorants present in food and feed. Raman studies of melamine and melamine-modified resins have been reported in the literature, and the results have revealed the feasibility of the Raman technique for the structural characterization of melamine state in resins. However, there have been few reports on Raman investigation and identification of melamine in complex food and feed systems. The objectives of this study were (1) to identify the characteristic Raman bands in melamine-contaminated wheat flour, corn gluten, and soybean meal mixtures; and (2) to develop simple and universal ratio algorithms for qualitative and quantitative analysis of melamine in mixtures. The ultimate goal is to develop both Raman spectroscopy and Raman chemical imaging methods for rapid, accurate, nondestructive, specific, and routine screening of the presence of melamine in food and feed for public/animal safety and security.

Development of a scanning surface probe for nanoscale tip-enhanced desorption/ablation

We report on the development of a versatile scanning apparatus for nanoscale surface sampling that utilizes the interaction of laser radiation at a sharp probe tip to effect desorption/ablation on opaque substrates. The process, which currently yields surface craters as small as approximately 50 nm diameterx5 nm deep, has been demonstrated with both metal-coated and bare silicon tips. Desorption/ablation under the tip occurs at illumination intensities below the corresponding optical far-field threshold, suggesting that the latter process should not degrade the spatial resolution attainable for proposed chemical imaging methods based on the scanning surface probe.

Towards chemical analysis of nanostructures in biofilms I: imaging of biological nanostructures

Due to their direct influence on the stability of bacterial biofilms, a better insight into the nanoscopic spatial arrangement of the different extracellular polymeric substances (EPS), e.g., polysaccharides and proteins, is important for the improvement of biocides and for process optimization in wastewater treatment and biofiltration. Here, the first application of a combination of confocal laser-scanning microscopy (CLSM) and atomic force microscopy (AFM) to the investigation of river-water biofilms and related biopolymers is presented. AFM images collected at selected areas of CLS micrographs dramatically demonstrate the heterogeneity of biofilms at the nanometer scale and the need for a chemical imaging method with nanoscale resolution. The nanostructures (e.g., pili, flagella, hydrocolloids, and EPS) found in the extracellular matrix are classified according to shape and size, which is typically 50-150 nm in width and 1-10 nm in thickness, and sets the demands regarding spatial resolution of a potential chemical imaging method. Additionally, thin layers of the polysaccharide alginate were investigated. We demonstrate that calcium alginate is a good model for the EPS architecture at the nanometer scale, because of its similar network-like structure.

Energy-filtered X-ray photoemission electron microscopy and its applications to surface and organic materials

Energy-filtered X-ray photoemission electron microscopy (EXPEEM) is a new surface chemical imaging method that combines X-ray photoelectron spectroscopy (XPS) and photoemission electron microscopy (PEEM). We have developed a collinear type EXPEEM system using a Wien-filter-type electron energy analyzer. The collinear arrangement has the advantage of carrying out an easy alignment of the electron optical axis. We have measured EXPEEM images, μ-X-ray absorption near edge structure (μ-XANES) and μ-XPS of Au on Ta and Ag(DM) 2. We discuss the advantage of EXPEEM and future applications to organic devices.

Raman Chemical Imaging for Ingredient-specific Particle Size Characterization of Aqueous Suspension Nasal Spray Formulations: A Progress Report

This study was conducted to evaluate the feasibility of using Raman chemical imaging (i.e., Raman imaging microspectroscopy) to establish chemical identity, particle size and particle size distribution (PSD) for a representative corticosteroid in aqueous nasal spray suspension formulations. The Raman imaging PSD protocol was validated using polystyrene (PS) microsphere size standards (NIST-traceable). A Raman spectral library was developed for the active and inactive compounds in the formulation. Four nasal sprays formulated with beclomethasone dipropionate (BDP) ranging in size from 1.4 to 8.3 microm were imaged by both Raman and brightfield techniques. The Raman images were then processed to calculate the PSD for each formulation. Within each region examined, active pharmaceutical ingredient (API) particles are unambiguously identified and the total number of those particles, particle size and PSD of API free of excipients and PSD of API particles adhered to other excipients are reported. Good statistical agreement is obtained between the reported and measured sizes of the PS microspheres. BDP particles were clearly distinguishable from those of excipients. Raman chemical imaging (RCI) is able to differentiate between and identify the chemical makeup of multiple components in complex BDP sample and placebo mixtures. The Raman chemical imaging method (coupled Raman and optical imaging) shows promise as a method for characterizing particle size and shape of corticosteroid in aqueous nasal spray suspension formulations. However, rigorous validation of RCI for PSD analysis is incomplete and requires additional research effort. Some specific areas of concern are discussed.

Raman imaging for quantification of the volume fraction of biodegradable polymers in histological preparations

Postretrieval analysis of biodegradable polymeric constructs for degradation rates requires correct identification of the degradable polymer, de novo tissue and the confounding presence of a secondary polymer used for embedding. Similarities between the structures of many tissue engineering polymers may make them difficult to distinguish from the polymer used to embed explants prior to histological sectioning. In this study, we assessed the feasibility of a chemical imaging method, Raman microscopy, to discriminate between more than one polymer species. From the perspective of spectroscopy, this is not a straightforward process because of the emergence of multiple peaks, ubiquity of embedding medium, and presence of observations sourcing from points sampled at the interface of two phases. A multivariate K-means data clustering method was used to discriminate between different polymeric components. The method was able to classify 95% of the observations to the correct category. The remaining data displayed multiple memberships because of (1) the laser spot coinciding with the interfaces of more than one phase or (2) infiltration of histological embedding polymer. Combined with multivariate analysis methods, this technique may prove useful in the future for tissue engineering and biomaterials analysis of degradation rates of, and tissue ingrowth into, polymer scaffolds.

In vivo detection of gray and white matter differences in GABA concentration in the human brain

A novel selective multiple quantum filtering-based chemical shift imaging method was developed for acquiring GABA images in the human brain at 3 T. This method allows a concomitant acquisition of an interleaved total creatine image with the same spatial resolution. Using T 1-based image segmentation and a nonlinear least square regression analysis of GABA-to-total creatine concentration ratios in frontal and parietal lobes of healthy adult volunteers as a function of the tissue gray matter fraction, the mean GABA concentration in gray and white matter was determined to be 1.30 ± 0.36 μmol/g and 0.16 ± 0.16 μmol/g (mean ± SD, n = 13), respectively. It is expected that this method will become a useful tool for studying GABAergic function in the human brain in vivo.

Visible and near-infrared chemical imaging methods for the analysis of selected forensic samples

This study investigated various chemical imaging methods for the forensic analysis of paints, tapes and adhesives, inks and firearm propellants (absorption and photoluminescence in the UV–vis–NIR regions). Results obtained using chemical imaging technology were compared with those obtained using traditional techniques. The results show that chemical imaging offers significant advantages in the forensic context, for example the ability to display visual and spectral results side by side and to reduce sample preparation, hence minimizing the risk of contamination. Chemical imaging produced a greater discriminating power than traditional techniques for most evidence types. Chemical imaging also eliminated different brands of ammunition based on the fluorescence characteristics of the propellant grains preserving the evidence for further analysis. It is expected that this technology will find broader forensic applications in the future.

Molecular mapping of periodontal tissues using infrared microspectroscopy

BackgroundChronic periodontitis is an inflammatory disease of the supporting structures of the teeth. Infrared microspectroscopy has the potential to simultaneously monitor multiple disease markers, including cellular infiltration and collagen catabolism, and hence differentiate diseased and healthy tissues. Therefore, our aim was to establish an infrared microspectroscopy methodology with which to analyze and interpret molecular maps defining pathogenic processes in periodontal tissues.MethodsSpecific key cellular and connective tissue components were identified by infrared microspectroscopy and using a chemical imaging method.ResultsHigher densities of DNA, total protein and lipid were revealed in epithelial tissue, compared to the lower percentage of these components in connective tissue. Collagen-specific tissue mapping by infrared microspectroscopy revealed much higher levels of collagen deposition in the connective tissues compared to that in the epithelium, as would be expected. Thus inflammatory events such as cellular infiltration and collagen deposition and catabolism can be identified by infrared microspectroscopy.ConclusionThese results suggest that infrared microspectroscopy may represent a simple, reagent-free, multi-dimensional tool with which to examine periodontal disease etiology using entirely unprocessed tissue sections.

Ultrashort TE chemical shift imaging (UTE‐CSI)

A fundamental modification to the conventional chemical shift imaging (CSI) method is described that improves the imaging of species with short T2's (i.e., less than approximately 2 ms). This approach minimizes the delay before each k-space point is collected. This results in different time delays, T(d), for different free induction decay (FID) acquisitions in k-space. On a clinical 1.5 T system this yields an effective delay due to transmit/receive switching of 70 micros and an echo time (TE) from the center of the excitation pulse to the center of k-space of 170 micros, as compared with 1-2 ms for conventional CSI techniques. Using this method, the signal decay before acquisition is greatly reduced, thus enabling imaging of species with very short T2)(e.g., 200 micros) and increasing the signal-to-noise ratio (SNR) of species with intermediate T2. Increases in the SNR of the short T2 components of 23Na in the heart, and 31P acquisitions of bone are about 27% and 400%, respectively, compared to an optimized conventional CSI approach. The imperfections of this approach are also described, and the magnitude of the resultant image artifacts is quantified for different practical imaging situations. These artifacts were not found to be significant in the described applications. This new method allows us to obtain information on short T2 components without degrading the image quality from long T2 components.

Image analysis of the spatial distribution of paramagnetic Pr3+ ions in a polymer gel by a 1H‐chemical shift NMR imaging method

Abstract1H‐chemical shift NMR imaging patterns of a poly(methacrylic acid) gel containing water with paramagnetic praseodymium ions(Pr3+) were successfully observed, in order to elucidate spatial distribution of Pr3+ ions in the gel. The 1H chemical shift of water associated with Pr3+ ions in the gel moves largely downfield. By analyzing these experimental results, the immersion process of Pr3+ ions into the network of the polymer gel was spatially clarified. Further, it is shown that the chemical shift NMR imaging method is a useful means for determining the spatial distribution of paramagnetic metal ions in polymer gels.

Resonance Raman imaging of the NADPH oxidase subunit cytochrome b558 in single neutrophilic granulocytes.

We have employed confocal resonance Raman (RR) imaging to visualize the subcellular distribution of the NADPH oxidase subunit cytochrome b558 in both resting and phagocytosing neutrophils. Our Raman microscopic technique is a label-free, chemical (vibrational) imaging method that can be applied to individual, intact cells, thus probing cytochrome b558 in its native environment. The Raman signal from cytochrome b558 is resonantly and selectively enhanced in neutrophils by using 413 nm excitation. Experiments on resting neutrophils show a cytoplasmic distribution of cytochrome b558, with several areas of high content. Upon phagocytosis of polystyrene particles, we found that part of the cytochrome b558 is translocated toward the ingested beads. This is in accordance with immunocytochemistry studies combined with electron and fluorescence microscopy. As compared to these methods, RR microscopy requires minimal sample preparation and disturbance. Moreover, it allows the determination of the redox state of cytochrome b558 inside the cell, which reflects its NADPH oxidase activity.

Mixing of concentrated oil-in-water emulsions measured by nuclear magnetic resonance imaging

Mixing of concentrated oil-in-water emulsions in a horizontal, concentric-cylinder geometry was studied using nuclear magnetic resonance imaging. Time-of-flight and chemical shift imaging methods were used to measure velocity profiles and concentration maps in an emulsion that was mixed after being allowed to cream for several hours. The results revealed detailed information about mixing in concentrated emulsions. In the initial state, before mixing, the emulsion system formed three layers: an upper, oil-rich, “creamed” layer; a lower, water-rich layer; and a bulk concentration layer in between. It was found that the thickness of the creamed layer remained constant during mixing, while the oil concentration in that layer decayed exponentially as a function of time. It was also observed that most of the emulsion is quiescent while mixing occurs; the only motion detected occurs in a thin layer close to the rotating, outer cylinder. The velocity profile only begins to transition to its steady-state configuration after the emulsion becomes well mixed. A simple model is introduced that gives a plausible explanation of these experimental observations. These results indicate that the mixing mechanism in concentrated emulsions is significantly different from that in single-phase liquids.