Electron paramagnetic resonance imaging using magnetic-field-gradient spinning

A novel X-band CW EPR imaging has been developed using magnetic-field-gradient (MFG) spinning to obtain spatial distributions of electron paramagnetic species. Spinning MFG EPR imaging for 65 projection spectra required just 55 s while conventional imaging took 11 min 40 s, that is, the acquisition time for the new system is one order of magnitude shorter than that for conventional EPR imaging. Spinning MFG EPR imaging allows one to measure reconstructed images in an interactive manner where resolution and condition can be changed quickly.     

Object dependent sweep width reduction with spectral-spatial EPR imaging

For spectral-spatial EPR imaging, prior knowledge about the spatial support of an imaged object can be exploited in two ways. We can shrink the spatial field of view (FOV) to closely wrap the object in a sphere or reduce the sweep width in a projection dependent fashion. Use of a smaller spatial FOV with the same number of samples enhances spatial resolution by reducing voxel volume at the expense of signal-to-noise and a consequent degraded line-width resolution. We have developed another approach to define sweep width that prunes away the portions of the projection sweep with no signal. This reduces data acquisition time for the continuous wave (CW) EPR image proportional to the sweep width reduction. This method also avoids voxel volume reduction. Using the reduced-sweep method, we decreased the data acquisition time by 20% maintaining spatial and linewidth resolution.     

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.     

Three-dimensional diffuse optical imaging of hand joints: System description and phantom studies

In this paper we describe a three-dimensional (3D) continuous wave (CW) diffuse optical tomography (DOT) system and present 3D volumetric reconstruction studies using this DOT system with simple phantom models that simulate hand joints. The CCD-based DOT system consists of 64×64 source/detector fiber optic channels, which are arranged in four layers, forming a cylindrical fiber optic/tissue interface. Phantom experiments are used to evaluate system performance with respective to axial spatial resolution, optical contrast and target position for detection of osteoarthritis where cartilage is the primary target region of interest. These phantom studies suggest that we are able to quantitatively resolve a 2 mm thick “cartilage” and qualitatively resolve a 1 mm thick “cartilage” using our 3D reconstruction approach. Our results also show that optical contrast of 3:1–7:1 between the “disease cartilage” and normal cartilage can be quantitatively recovered. Finally, the target position along axial direction on image reconstruction is studied. All the images are obtained using our 3D finite-element-based reconstruction algorithm.     

Stochastic excitation and Hadamard correlation spectroscopy with bandwidth extension in RF FT-EPR

The application of correlation spectroscopy employing stochastic excitation and the Hadamard transform to time-domain Fourier transform electron paramagnetic resonance (FT-EPR) spectroscopy in the radiofrequency (RF) band is described. An existing, time-domain FT-EPR spectrometer system with a Larmor frequency (L(f)) of 300 MHz was used to develop this technique by incorporating a pseudo-random pulse sequence generator to output the maximum length binary sequence (MLBS, 10- and 11-bit). Software developed to control the EPR system setup, acquire the signals, and post process the data, is outlined. The software incorporates the Hadamard transform algorithm to perform the required cross-correlation of the acquired signal and the MLBS after stochastic excitation. To accommodate the EPR signals, bandwidth extension was accomplished by sampling at a rate many times faster than the RF pulse repetition rate, and subsequent digital signal processing of the data. The results of these experiments showed that there was a decrease in the total acquisition time, and an improved free induction decay (FID) signal-to-noise (S/N) ratio compared to the conventional coherent averaging approach. These techniques have the potential to reduce the RF pulse power to the levels used in continuous wave (CW) EPR while retaining the advantage of time-domain EPR methods. These methods have the potential to facilitate the progression to in vivo FT-EPR imaging of larger volumes.     

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.     

Development and testing of a CW-EPR apparatus for imaging of short-lifetime nitroxyl radicals in mouse head

This article describes a method for reducing the acquisition time in three-dimensional (3D) continuous-wave electron paramagnetic resonance (CW-EPR) imaging. To visualize nitroxyl spin probes, which have a short lifetime in living organisms, the acquisition time for a data set of spectral projections should be shorter than the lifetime of the spin probes. To decrease the total time required for data acquisition, the duration of magnetic field scanning was reduced to 0.5 s. Moreover, the number of projections was decreased by using the concept of a uniform distribution. To demonstrate this faster data acquisition, two kinds of nitroxyl radicals with different decay rates were measured in mice. 3D EPR imaging of 4-hydroxy-2,2,6,6-tetramethylpiperidine-d₁₇-1-¹⁵N-1-oxyl in mouse head was successfully carried out. 3D EPR imaging of nitroxyl spin probes with a half-life of a few minutes was achieved for the first time in live animals.     

A new strategy for fast radiofrequency CW EPR imaging: direct detection with rapid scan and rotating gradients

Rapid field scan on the order of T/s using high frequency sinusoidal or triangular sweep fields superimposed on the main Zeeman field, was used for direct detection of signals without low-frequency field modulation. Simultaneous application of space-encoding rotating field gradients have been employed to perform fast CW EPR imaging using direct detection that could, in principle, approach the speed of pulsed FT EPR imaging. The method takes advantage of the well-known rapid-scan strategy in CW NMR and EPR that allows arbitrarily fast field sweep and the simultaneous application of spinning gradients that allows fast spatial encoding. This leads to fast functional EPR imaging and, depending on the spin concentration, spectrometer sensitivity and detection band width, can provide improved temporal resolution that is important to interrogate dynamics of spin perfusion, pharmacokinetics, spectral spatial imaging, dynamic oxymetry, etc.     

在体电子自旋共振(EPR)波谱和成像技术在生物医学中的应用

活性氧和氮自由基(ROS/RNS)在一系列的人类疾病中扮演着双重角色. 它们可以是氧化剂, 诱导氧化状态, 导致组织损伤. 它们又可以是信号传导因子, 诱发保护性反应, 使得被调节的组织器官经受得起更强的损伤. 鉴于它们在生物医学中的重要作用, 检测它们产生和分布的技术的研究因而变得必要和紧迫. 在体电子自旋共振(EPR)波谱和成像技术渐已被应用于活体生物体系中用以表针和显像ROS/RNS. EPR 波谱特性(包括线宽、强度和寿命)以及空间分布信息已为动物甚至人体病理模型中氧化还原状态和氧分布的检测提供不可缺少的依据. 该文将简单描述和讨论一系列在体EPR 波谱和成像技术在器官和组织中的应用, 其中包括活体组织氧化还原状态, 活体组织氧分布和时间演化, 自由基空间以及谱-空间成像等.     

3D sensitivity encoded ellipsoidal MR spectroscopic imaging of gliomas at 3T

Purpose

The goal of this study was to implement time efficient data acquisition and reconstruction methods for 3D magnetic resonance spectroscopic imaging (MRSI) of gliomas at a field strength of 3T using parallel imaging techniques.

Methods

The point spread functions, signal to noise ratio (SNR), spatial resolution, metabolite intensity distributions and Cho:NAA ratio of 3D ellipsoidal, 3D sensitivity encoding (SENSE) and 3D combined ellipsoidal and SENSE (e-SENSE) k-space sampling schemes were compared with conventional k-space data acquisition methods.

Results

The 3D SENSE and e-SENSE methods resulted in similar spectral patterns as the conventional MRSI methods. The Cho:NAA ratios were highly correlated (P<.05 for SENSE and P<.001 for e-SENSE) with the ellipsoidal method and all methods exhibited significantly different spectral patterns in tumor regions compared to normal appearing white matter. The geometry factors ranged between 1.2 and 1.3 for both the SENSE and e-SENSE spectra. When corrected for these factors and for differences in data acquisition times, the empirical SNRs were similar to values expected based upon theoretical grounds. The effective spatial resolution of the SENSE spectra was estimated to be same as the corresponding fully sampled k-space data, while the spectra acquired with ellipsoidal and e-SENSE k-space samplings were estimated to have a 2.36–2.47-fold loss in spatial resolution due to the differences in their point spread functions.

Conclusion

The 3D SENSE method retained the same spatial resolution as full k-space sampling but with a 4-fold reduction in scan time and an acquisition time of 9.28 min. The 3D e-SENSE method had a similar spatial resolution as the corresponding ellipsoidal sampling with a scan time of 4:36 min. Both parallel imaging methods provided clinically interpretable spectra with volumetric coverage and adequate SNR for evaluating Cho, Cr and NAA.     

Projection-Reconstruction Spectroscopic Imaging forB0Field Plotting and Shimming without Pulsed Gradients

Shimming is important. Noniterative methods are desirable. Such methods exist for shimming a spectrometer with pulsed field gradients, generally based on field maps made by spin-warp Fourier imaging. For spectrometers with no pulsed gradients (or for cases whereT₂is too short to permit echo imaging), an alternative method is presented: projection-reconstruction spectroscopic imaging, which can be accomplished using only the shim coils of a conventional spectrometer. Images so acquired can be used to map the field, even in the presence of multiple spectral components. Noniterative optimization of the axial shims of a GN-300 spectrometer is demonstrated using 1D + 1D spectroscopic images. Prospects for extending the technique to include the radial shims using 3D + 1D spectroscopic images are discussed.     

谱-空间2D ESR成像

在Bruker ER 200D ESR 谱仪上安装一套自制的谱-空间2D ESR成像系统,这套系统由一对梯度场线圈、电源、微机及图像重建程序组成. 用滤波反投影图像重建方法,实现了两种自由基样品的谱-空间2D ESR成像,由2D 图像得到样品中自由基的自旋密度空间分布及相应的波谱参数. 讨论了成像参数与图像精度的关系.     

Fast 3D spatial EPR imaging using spiral magnetic field gradient

Electron paramagnetic resonance imaging (EPRI) provides direct detection and mapping of free radicals. The continuous wave (CW) EPRI technique, in particular, has been widely used in a variety of applications in the fields of biology and medicine due to its high sensitivity and applicability to a wide range of free radicals and paramagnetic species. However, the technique requires long image acquisition periods, and this limits its use for many in vivo applications where relatively rapid changes occur in the magnitude and distribution of spins. Therefore, there has been a great need to develop fast EPRI techniques. We report the development of a fast 3D CW EPRI technique using spiral magnetic field gradient. By spiraling the magnetic field gradient and stepping the main magnetic field, this approach acquires a 3D image in one sweep of the main magnetic field, enabling significant reduction of the imaging time. A direct one-stage 3D image reconstruction algorithm, modified for reconstruction of the EPR images from the projections acquired with the spiral magnetic field gradient, was used. We demonstrated using a home-built L-band EPR system that the spiral magnetic field gradient technique enabled a 4-7-fold accelerated acquisition of projections. This technique has great potential for in vivo studies of free radicals and their metabolism.     

Targeted-ROI imaging in electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) is a technique that has been used for in vivo oxygen imaging of small animals. In continuous wave (CW) EPRI, the measurement can be interpreted as a sampled 4D Radon transform of the image function. The conventional filtered-backprojection (FBP) algorithm has been used widely for reconstructing images from full knowledge of the Radon transform acquired in CW EPRI. In practical applications of CW EPRI, one often is interested in information only in a region of interest (ROI) within the imaged subject. It is desirable to accurately reconstruct an ROI image only from partial knowledge of the Radon transform because acquisition of the partial data set can lead to considerable reduction of imaging time. The conventional FBP algorithm cannot, however, reconstruct accurate ROI images from partial knowledge of the Radon transform of even dimension. In this work, we describe two new algorithms, which are referred to as the backprojection filtration (BPF) and minimum-data filtered-backprojection (MDFBP) algorithms, for accurate ROI-image reconstruction from a partial Radon transform (or, truncated Radon transform) in CW EPRI. We have also performed numerical studies in the context of ROI-image reconstruction of a synthetic 2D image with density similar to that found in a small animal EPRI. This demonstrates both the inadequacy of the conventional FBP algorithm and the success of BPF and MDFBP algorithms in ROI reconstruction. The proposed ROI imaging approach promises a means to substantially reduce image acquisition time in CW EPRI.     

Projection-reconstruction reduced [correction of reduces] FOV imaging

Reduced field-of-view (rFOV) imaging was introduced recently as a rapid imaging technique that improves temporal resolution while maintaining spatial resolution. It is based on undersampling in k-space and utilizes the fact that the dynamics of an evolving process are often confined to a local area within the full FOV. In the work presented here the reduced FOV approach is applied to projection-reconstruction MRI and compared to the original spin-warp implementation. Results are presented that clearly demonstrate the increased robustness of the projection-reconstruction version of rFOV imaging. The technique is successfully applied to an MR-guided biopsy scenario (ex-vivo) and to cine cardiac imaging. Finally an algorithm is proposed that uses the intrinsic advantages of radial k-space sampling to evaluate the projection data to control the adjustment of position and size of the reduced FOV window.     

Fast EPR imaging at 300 MHz using spinning magnetic field gradients

Electron paramagnetic resonance imaging (EPRI) technology has rapidly progressed in the last decade enabling many important applications in the fields of biology and medicine. At frequencies of 300-1200 MHz a range of in vivo applications have been performed. However, the requisite imaging time duration to acquire a given number of projections, limits the use of this technique in many in vivo applications where relatively rapid kinetics occur. Therefore, there has been a great need to develop approaches to accelerate EPRI data acquisition. We report the development of a fast low-frequency EPRI technique using spinning magnetic field gradients (SMFG). Utilizing a 300 MHz CW (continuous wave) EPRI system, SMFG enabled over 10-fold accelerated acquisition of image projections. 2D images with over 200 projections could be acquired in less than 3s and with 20s acquisitions good image quality was obtained on large aqueous free radical samples. This technique should be particularly useful for in vivo studies of free radicals and their metabolism.     

Progressive EPR imaging with adaptive projection acquisition

Continuous wave electron paramagnetic resonance imaging (EPRI) of living biological systems requires rapid acquisition and visualization of free radical images. In the commonly used multiple-stage back-projection image reconstruction algorithm, the EPR image cannot be reconstructed until a complete set of projections is collected. If the data acquisition is incomplete, the previously acquired incomplete data set is no longer useful. In this work, a 3-dimensional progressive EPRI technique was implemented based on inverse Radon transform in which a 3-dimensional EPR image is acquired and reconstructed gradually from low resolution to high resolution. An adaptive data acquisition strategy is proposed to determine the significance of projections and acquire them in an order from the most significant to the least significant. The image acquisition can be terminated at any time if further collection of projections does not improve the image resolution distinctly, providing flexibility to trade image quality with imaging time. The progressive imaging technique was validated using computer simulations as well as imaging experiments. The adaptive acquisition uses 50-70% less projections as compared to the regular acquisition. In conclusion, adaptive data acquisition with progressive image reconstruction should be very useful for the accelerated acquisition and visualization of free radical distribution.     

Water-selective spectral-spatial contrast-enhanced breast MRI for cancer detection in patients with extracapsular and injected free silicone

OBJECTIVE: This study investigates the use of contrast-enhanced, T1-weighted, water-selective spectral-spatial 3D gradient echo magnetic resonance imaging (MRI) with magnetization transfer (3DSSMT) for detecting breast cancer in patients with intraparenchymal silicone. CONCLUSION: Water-selective 3DSSMT provides superior fat and silicone suppression in patients with free silicone as compared with conventional fat saturation. It enables direct, high-quality, high-spatial-resolution, T1-weighted breast MRI of contrast enhancement without the need for subtraction processing and aids diagnosis of cancer in the breast with free silicone.     

Simulation of 4D spectral-spatial EPR images

Electron paramagnetic resonance imaging (EPRI) can be modeled by the forward projection of a 4D synthetic spectral-spatial phantom. We developed a simulation tool for EPRI and carried out a quantitative comparison between simulation and experiment, focusing on the signal and noise characteristics. The signal height in the simulation was compared to that in the experimental projections at gradients of different magnitudes and directions. We investigated the noise power spectrum of an EPR imager and incorporated it into the simulation. The signal and noise modeling of the simulation achieved the same performance as the EPR imager. Using this simulation, various sampling schemes were tried to find an optimized parameter set under the customized noise model of this EPR imager.