Quantitative magnetic susceptibility mapping without phase unwrapping using WASSR.

The magnetic susceptibility of tissue within and around an image voxel affects the magnetic field and thus the local frequency in that voxel. Recently, it has been shown that spatial maps of frequency can be used to quantify local susceptibility if the contributions of surrounding tissue can be deconvolved. Currently, such quantitative susceptibility mapping (QSM) methods employ gradient recalled echo (GRE) imaging to measure spatial differences in the signal phase evolution as a function of echo time, from which frequencies can be deduced. Analysis of these phase images, however, is complicated by phase wraps, despite the availability and usage of various phase unwrapping algorithms. In addition, lengthy high-resolution GRE scanning often heats the magnet bore, causing the magnetic field to drift over several Hertz, which is on the order of the frequency differences between tissues. Here, we explore the feasibility of applying the WAter Saturation Shift Referencing (WASSR) method for 3D whole brain susceptibility imaging. WASSR uses direct saturation of water protons as a function of frequency irradiation offset to generate frequency maps without phase wraps, which can be combined with any image or spectroscopy acquisition. By utilizing a series of fast short-echo-time direct saturation images with multiple radiofrequency offsets, a frequency correction for field drift can be applied based on the individual image phases. Regions of interest were delineated with an automated atlas-based method, and the average magnetic susceptibilities calculated from frequency maps obtained from WASSR correlated well with those from the phase-based multi-echo GRE approach at 3T.

Mapping magnetic susceptibility anisotropies of white matter in vivo in the human brain at 7 T.

High-resolution magnetic resonance phase- or frequency-shift images acquired at high field show contrast related to magnetic susceptibility differences between tissues. Such contrast varies with the orientation of the organ in the field, but the development of quantitative susceptibility mapping (QSM) has made it possible to reproducibly image the intrinsic tissue susceptibility contrast. However, recent studies indicate that magnetic susceptibility is anisotropic in brain white matter and, as such, needs to be described by a symmetric second-rank tensor( ̅χ). To fully determine the elements of this tensor, it would be necessary to acquire frequency data at six or more orientations. Assuming cylindrical symmetry of the susceptibility tensor in myelinated white matter fibers, we propose a simplified method to reconstruct the susceptibility tensor in terms of a mean magnetic susceptibility, MMS=(χ(//)+2 χ(⊥))/3 and a magnetic susceptibility anisotropy, MSA=χ(//)-χ(⊥), where χ(//) and χ(⊥) are susceptibility parallel and perpendicular to the white matter fiber direction, respectively. Computer simulations show that with a practical head rotation angle of around 20°-30°, four head orientations suffice to reproducibly reconstruct the tensor with good accuracy. We tested this approach on whole brain 1 × 1 × 1 mm(3) frequency data acquired from five healthy subjects at 7 T. The frequency information from phase images collected at four head orientations was combined with the fiber direction information extracted from diffusion tensor imaging (DTI) to map the white matter susceptibility tensor. The MMS and MSA were quantified for regions in several large white matter fiber structures, including the corona radiata, posterior thalamic radiation and corpus callosum. MMS ranged from -0.037 to -0.053 ppm (referenced to CSF being about zero). MSA values could be quantified without the need for a reference and ranged between 0.004 and 0.029 ppm, in line with the expectation that the susceptibility perpendicular to the fiber is more diamagnetic than the one parallel to it.

Multi-parametric neuroimaging reproducibility: a 3-T resource study.

Modern MRI image processing methods have yielded quantitative, morphometric, functional, and structural assessments of the human brain. These analyses typically exploit carefully optimized protocols for specific imaging targets. Algorithm investigators have several excellent public data resources to use to test, develop, and optimize their methods. Recently, there has been an increasing focus on combining MRI protocols in multi-parametric studies. Notably, these have included innovative approaches for fusing connectivity inferences with functional and/or anatomical characterizations. Yet, validation of the reproducibility of these interesting and novel methods has been severely hampered by the limited availability of appropriate multi-parametric data. We present an imaging protocol optimized to include state-of-the-art assessment of brain function, structure, micro-architecture, and quantitative parameters within a clinically feasible 60-min protocol on a 3-T MRI scanner. We present scan-rescan reproducibility of these imaging contrasts based on 21 healthy volunteers (11 M/10 F, 22-61 years old). The cortical gray matter, cortical white matter, ventricular cerebrospinal fluid, thalamus, putamen, caudate, cerebellar gray matter, cerebellar white matter, and brainstem were identified with mean volume-wise reproducibility of 3.5%. We tabulate the mean intensity, variability, and reproducibility of each contrast in a region of interest approach, which is essential for prospective study planning and retrospective power analysis considerations. Anatomy was highly consistent on structural acquisition (~1-5% variability), while variation on diffusion and several other quantitative scans was higher (~<10%). Some sequences are particularly variable in specific structures (ASL exhibited variation of 28% in the cerebral white matter) or in thin structures (quantitative T2 varied by up to 73% in the caudate) due, in large part, to variability in automated ROI placement. The richness of the joint distribution of intensities across imaging methods can be best assessed within the context of a particular analysis approach as opposed to a summary table. As such, all imaging data and analysis routines have been made publicly and freely available. This effort provides the neuroimaging community with a resource for optimization of algorithms that exploit the diversity of modern MRI modalities. Additionally, it establishes a baseline for continuing development and optimization of multi-parametric imaging protocols.

Estimation of mean and median pO2 values for a composite EPR spectrum.

Electron paramagnetic resonance (EPR)-based oximetry is capable of quantifying oxygen content in samples. However, for a heterogeneous environment with multiple pO2 values, peak-to-peak linewidth of the composite EPR lineshape does not provide a reliable estimate of the overall pO2 in the sample. The estimate, depending on the heterogeneity, can be severely biased towards narrow components. To address this issue, we suggest a postprocessing method to recover the linewidth histogram which can be used in estimating meaningful parameters, such as the mean and median pO2 values. This information, although not as comprehensive as obtained by EPR spectral-spatial imaging, goes beyond what can be generally achieved with conventional EPR spectroscopy. Substantially shorter acquisition times, in comparison to EPR imaging, may prompt its use in clinically relevant models. For validation, simulation and EPR experiment data are presented.

Evaluation of oxygen-response times of phthalocyanine-based crystalline paramagnetic spin probes for EPR oximetry.

The goal of the present study was to evaluate the temporal response of particulate-based EPR oximetry probes to changes in partial pressure of oxygen (pO(2)). In order to accurately evaluate the oxygen-response time, we developed a method for rapid modulation of pO(2) in a chamber containing the probe using an oscillator-driven speaker-diaphragm setup. The apparatus was capable of producing sinusoidal changes in pO(2) at frequencies up to 300 Hz or more. The pressure-modulation setup was used to evaluate the temporal response of some of the most commonly used phthalocyanine-based particulate probes. For validation, the time-response of the probes was compared to that of a high sensitivity pressure sensor. The results revealed that some particulate probes could respond to changes in pO(2) with a temporal response of 3.3 ms (300 Hz). The observations were interpreted in the light of their crystalline packing in favor of oxygen diffusion. The results of the present study should enable the selection of probes for oximetry applications requiring high temporal resolution.

Methods for noninvasive imaging of tissue hypoxia.

The purpose of this review is to provide an overview of the methods available for imaging tissue oxygenation. The following imaging methods are reviewed: phosphorescence, near-infrared (NIR), positron emission tomography (PET), magnetic resonance imaging ((19)F MRI and BOLD MRI), and electron paramagnetic resonance (EPR). The methods are based on different principles and differ in their ability to accurately quantify tissue oxygenation, either the absolute value of a particular measure of oxygenation (partial pressure of oxygen, concentration), or a parameter related to it (oxygen saturation). Methods that can provide images of relative changes in oxygenation or visualization of hypoxia in a specific tissue of interest are also considered valuable tools for biomedical research and clinical applications.

A comparative evaluation of EPR and OxyLite oximetry using a random sampling of pO(2) in a murine tumor.

Methods currently available for the measurement of oxygen concentrations (oximetry) in viable tissues differ widely from each other in their methodological basis and applicability. The goal of this study was to compare two novel methods, particulate-based electron paramagnetic resonance (EPR) and OxyLite oximetry, in an experimental tumor model. EPR oximetry uses implantable paramagnetic particulates, whereas OxyLite uses fluorescent probes affixed on a fiber-optic cable. C3H mice were transplanted with radiation-induced fibrosarcoma (RIF-1) tumors in their hind limbs. Lithium phthalocyanine (LiPc) microcrystals were used as EPR probes. The pO(2) measurements were taken from random locations at a depth of approximately 3 mm within the tumor either immediately or 48 h after implantation of LiPc. Both methods revealed significant hypoxia in the tumor. However, there were striking differences between the EPR and OxyLite readings. The differences were attributed to the volume of tissue under examination and the effect of needle invasion at the site of measurement. This study recognizes the unique benefits of EPR oximetry in terms of robustness, repeatability and minimal invasiveness.

Quasi Monte Carlo-based isotropic distribution of gradient directions for improved reconstruction quality of 3D EPR imaging.

In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstructed image along with fast and reliable data acquisition is highly desirable for many biological applications. An accurate representation of uniform distribution of projection data is necessary to ensure high reconstruction quality. The current techniques for data acquisition suffer from nonuniformities or local anisotropies in the distribution of projection data and present a poor approximation of a true uniform and isotropic distribution. In this work, we have implemented a technique based on Quasi-Monte Carlo method to acquire projections with more uniform and isotropic distribution of data over a 3D acquisition space. The proposed technique exhibits improvements in the reconstruction quality in terms of both mean-square-error and visual judgment. The effectiveness of the suggested technique is demonstrated using computer simulations and 3D EPRI experiments. The technique is robust and exhibits consistent performance for different object configurations and orientations.

Enhanced resolution for EPR imaging by two-step deblurring.

The broad spectrum of spin probes used for electron paramagnetic resonance imaging (EPRI) result in poor spatial resolution of the reconstructed images. Conventional deconvolution procedures can enhance the resolution to some extent but obtaining high resolution EPR images is still a challenge. In this work, we have implemented and analyzed the performance of a postacquisition deblurring technique to enhance the spatial resolution of the EPR images. The technique consists of two steps; noniterative deconvolution followed by iterative deconvolution of the acquired projections which are then projected back using filtered backprojection (FBP) to reconstruct a high resolution image. Further, we have proposed an analogous technique for iterative reconstruction algorithms such as multiplicative simultaneous iterative reconstruction technique (MSIRT) which can be a method of choice for many applications. The performance of the suggested deblurring approach is evaluated using computer simulations and EPRI experiments. Results suggest that the proposed procedure is superior to the standard FBP and standard iterative reconstruction algorithms in terms of mean-square-error (MSE), spatial resolution, and visual judgment. Although the procedure is described for 2D imaging, it can be readily extended to 3D imaging.

Uniform distribution of projection data for improved reconstruction quality of 4D EPR imaging.

In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstruction in a limited acquisition time is a high priority. It has been shown for the case of 3D EPRI, that a uniform distribution of the projection data generally enhances reconstruction quality. In this work, we have suggested two data acquisition techniques for which the gradient orientations are more evenly distributed over the 4D acquisition space as compared to the existing methods. The first sampling technique is based on equal solid angle partitioning of 4D space, while the second technique is based on Fekete points estimation in 4D to generate a more uniform distribution of data. After acquisition, filtered backprojection (FBP) is applied to carry out the reconstruction in a single stage. The single-stage reconstruction improves the spatial resolution by eliminating the necessity of data interpolation in multi-stage reconstructions. For the proposed data distributions, the simulations and experimental results indicate a higher fidelity to the true object configuration. Using the uniform distribution, we expect about 50% reduction in the acquisition time over the traditional method of equal linear angle acquisition.

In vivo imaging of free radicals and oxygen.

Free radicals are highly reactive compounds that play an essential role in many biological processes, both beneficial and deleterious. Detection and quantification of these species is critical to develop a better understanding of normal and pathophysiological functions at the cellular and tissue levels. Electron paramagnetic resonance (EPR) spectroscopy is the technique most commonly used for this purpose through the detection of exogenous probes or spin traps that interact with the free radical species of interest. Over the past several years, the spatial and temporal distribution of free radicals within cells and tissues has been of particular interest. This chapter briefly explains the principles and challenges in the use of EPR for biological samples and introduces the concept of EPR for free radical imaging purposes. In addition, specific examples are given for the use of EPR imaging in four principal areas: free radical probes, nitric oxide (NO), redox state, and oxygen (O(2)) concentration.

EPR oximetry in three spatial dimensions using sparse spin distribution.

A method is presented to use continuous wave electron paramagnetic resonance imaging for rapid measurement of oxygen partial pressure in three spatial dimensions. A particulate paramagnetic probe is employed to create a sparse distribution of spins in a volume of interest. Information encoding location and spectral linewidth is collected by varying the spatial orientation and strength of an applied magnetic gradient field. Data processing exploits the spatial sparseness of spins to detect voxels with nonzero spin and to estimate the spectral linewidth for those voxels. The parsimonious representation of spin locations and linewidths permits an order of magnitude reduction in data acquisition time, compared to four-dimensional tomographic reconstruction using traditional spectral-spatial imaging. The proposed oximetry method is experimentally demonstrated for a lithium octa-n-butoxy naphthalocyanine (LiNc-BuO) probe using an L-band EPR spectrometer.

Skeletal myoblasts transplanted in the ischemic myocardium enhance in situ oxygenation and recovery of contractile function.

It is unclear whether oxygen plays a role in stem cell therapy. Hence, the determination of local oxygenation (Po(2)) in the infarct heart and at the site of transplantation may be critical to study the efficacy of cell therapy. To demonstrate this, we have developed an oxygen-sensing paramagnetic spin probes (OxySpin) to monitor oxygenation in the region of cell transplantation using electron paramagnetic resonance (EPR) spectroscopy. Skeletal myoblast (SM) cells isolated from thigh muscle biopsies of mice were labeled with OxySpin by coculturing the cells with submicron-sized (270 +/- 120 nm) particulates of the probe. Myocardial infarction was created by left coronary artery ligation in mice. Immediately after ligation, labeled SM cells were transplanted in the ischemic region of the heart. The engraftment of the transplanted cells and in situ Po(2) in the heart were monitored weekly for 4 wk. EPR measurements revealed the retention of cells in the infarcted tissue. The myocardial Po(2) at the site of SM cell therapy was significantly higher compared with the untreated group throughout the 4-wk period. Histological studies revealed differentiation and engraftment of SM cells into myotubes and increased incidence of neovascularization in the infarct region. The infarct size in the treated group was significantly decreased, whereas echocardiography showed an overall improvement in cardiac function when compared with untreated hearts. To our knowledge, this the first report detailing changes in in situ oxygenation in cell therapy. The increased myocardial Po(2) positively correlated with neoangiogenesis and cardiac function.

Automated on-the-fly detection and correction procedure for EPR imaging data acquisition.

Fast and reliable data acquisition is a major requirement for successful and useful biological electron paramagnetic resonance imaging (EPRI) experiments. Even a technologically advanced and professionally supervised EPRI system can exhibit instabilities initiated by perturbations such as animal motion, microphonics, and temperature changes. As a result, part of an acquired data set may become corrupted with excessive noise and distortions, which in turn may degrade the quality of the reconstructed image. In this work an automated scheme to monitor the system performance and stability over the course of an experiment is demonstrated. This method ensures that the quality of the acquired data is maintained during the experiment. For this purpose, four parameters including noise content and integration of each acquired projection are quantified and measured against those of the zero-gradient (ZG) projection, which is set as a quality benchmark. Projections with parameter values that differ substantially from the expected values are identified as damaged and consequently are reacquired. Therefore, the proposed technique not only effectively monitors the quality of acquisition, it also saves a substantial amount of acquisition time because it eliminates the necessity of repeating the entire experiment in cases in which only a small fraction of the data are corrupted.