Single Point Imaging Research Articles

Interval velocity analysis using multiparameter common image gathers

We developed a nonconventional approach to interval velocity analysis. The motivation for this approach is based on the argument that when the subsurface structure is complex, velocity error cannot be related to a single parameter. The suggested analysis uses multiparameter common image gathers (MPCIGs), generated by standard prestack depth migration. The parameterization of these multiparameter gathers is directly related to the structural characteristics of the subsurface image points. The undesirable summation, which is usually involved in the generation of conventional common image gathers, is avoided. During the velocity analysis procedure, depth slices taken out of the calculated MPCIGs are examined. Each depth slice contains all seismic data that were migrated into a single image point associated with the specific depth slice. When the MPCIGs are generated with the correct velocity function, each depth slice holds all structural information associated with the corresponding image point. Through detailed analysis of 2D synthetic and real data examples, the influence of migration velocity errors on the accuracy of the migrated multiparameter gathers is demonstrated. A Kirchhoff-based algorithm is used for the migration along with a layer-stripping method, relying on velocity scans, for the analysis. A velocity correctness criterion was also verified, along with some suggestions on the practical usage of the method.

Two-dimensional T2 distribution mapping in rock core plugs with optimal k-space sampling

Spin-echo single point imaging has been employed for 1D T2 distribution mapping, but a simple extension to 2D is challenging since the time increase is n fold, where n is the number of pixels in the second dimension. Nevertheless 2D T2 mapping in fluid saturated rock core plugs is highly desirable because the bedding plane structure in rocks often results in different pore properties within the sample. The acquisition time can be improved by undersampling k-space. The cylindrical shape of rock core plugs yields well defined intensity distributions in k-space that may be efficiently determined by new k-space sampling patterns that are developed in this work. These patterns acquire 22.2% and 11.7% of the k-space data points. Companion density images may be employed, in a keyhole imaging sense, to improve image quality. T2 weighted images are fit to extract T2 distributions, pixel by pixel, employing an inverse Laplace transform. Images reconstructed with compressed sensing, with similar acceleration factors, are also presented. The results show that restricted k-space sampling, in this application, provides high quality results.

Echo-based Single Point Imaging (ESPI): A novel pulsed EPR imaging modality for high spatial resolution and quantitative oximetry

A novel time-domain spectroscopic EPR imaging approach, that is a unique combination of already known techniques, is described. The first one is multi-gradient Single Point Imaging involving pure phase-encoding where the oximetry is based on T2∗. Line width derived from T2∗ is subject to susceptibility effects and therefore needs system-dependent line width calibrations. The second approach utilizes the conventional 90°–τ–180° Spin-Echo pulse sequence where the images are obtained by the filtered back-projection after FT of the echoes collected under frequency-encoding gradients. The spatially resolved oximetry information is derived from a set of T2-weighted images. The back-projection images suffer susceptibility artifacts with resolution determined by T2∗, but the oximetry based on T2 is quite reliable. The current approach combines Single Point Imaging and the Spin-Echo procedure to take advantage the enhanced spatial resolution associated with the former and the T2 dependent contrast of the latter. Pairs of images are derived choosing two time points located at identical time intervals on either side of the 180° pulse. The refocusing pulse being exactly in the middle of the two points ensures that artifacts associated with susceptibility and field inhomogeneities are eliminated. In addition, the net phase accumulated by the two time points being identical results in identical field of views, thus avoiding the zoom-in effect as a function delay in regular SPI and the associated interpolation requirements employed in T2∗-weighted oximetry. The end result is superior image resolution and reliable oximetry. In spite of the fact that projection–reconstruction methods require less number of measurements compared to SPI, the enormous advantage in SNR of the SPI procedure makes the echo-based SPI equally efficient in terms of measurement time. The Fourier reconstruction, line width independent resolution and the true T2-weighting make this novel procedure very attractive for in vivo EPR imaging of tissue oxygen quantitatively.

Quantification of superparamagnetic iron oxide with large dynamic range using TurboSPI

This work proposes the use of TurboSPI, a multi-echo single point imaging sequence, for the quantification of labeled cells containing moderate to high concentrations of iron oxide contrast agent. At each k-space location, TurboSPI acquires several hundred time points during a spin echo, permitting reliable relaxation rate mapping of large-R2∗ materials. An automatic calibration routine optimizes image quality by promoting coherent alignment of spin and stimulated echoes throughout the multi-echo train, and this calibration is sufficiently robust for in vivo applications. In vitro relaxation rate measurements of SPIO-loaded cervical cancer cells exhibit behavior consistent with theoretical predictions of the static dephasing regime in the spin echo case; the relaxivity measured with TurboSPI was 10.47±2.3s−1/mG, comparable to the theoretical value of 10.78s−1/mG. Similar measurements of micron-sized iron oxide particles (0.96μm and 1.63μm diameter) show a reduced relaxivity of 8.06±0.68s−1/mG and 7.13±0.31s−1/mG respectively, indicating that the static dephasing criterion was not met. Nonetheless, accurate quantification of such particles is demonstrated up to R2∗=900s−1, with a potentially higher upper limit for loaded cells having a more favorable R2′:R2 ratio. Based on the cells used in this study, reliable quantification of cells loaded with 10pg of iron per cell should be possible up to a density of 27millioncells/mL. Such quantification will be of crucial importance to the development of longitudinal monitoring for cellular therapy and other procedures using iron-labeled cells.

Depth Calculation Method of Integral Imaging Based on Gaussian Beam Distribution Model

In this paper, we propose a concept of the optimal viewing distance range and a depth calculation method of integral imaging. By considering the light emanating from a single elemental image point as a Gaussian beam, we analyze the integral imaging depth range and deduce the depth calculation formulas in different display modes. Experiments in focused mode were carried out, and good results finally confirmed the feasibility of the proposed method.

MR imaging of ore for heap bioleaching studies using pure phase encode acquisition methods

Various MRI techniques were considered with respect to imaging of aqueous flow fields in low grade copper ore. Spin echo frequency encoded techniques were shown to produce unacceptable image distortions which led to pure phase encoded techniques being considered. Single point imaging multiple point acquisition (SPI–MPA) and spin echo single point imaging (SESPI) techniques were applied. By direct comparison with X-ray tomographic images, both techniques were found to be able to produce distortion-free images of the ore packings at 2T. The signal to noise ratios (SNRs) of the SESPI images were found to be superior to SPI–MPA for equal total acquisition times; this was explained based on NMR relaxation measurements. SESPI was also found to produce suitable images for a range of particles sizes, whereas SPI–MPA SNR deteriorated markedly as particles size was reduced. Comparisons on a 4.7T magnet showed significant signal loss from the SPI–MPA images, the effect of which was accentuated in the case of unsaturated flowing systems. Hence it was concluded that SESPI was the most robust imaging method for the study of copper ore heap leaching hydrology.

Reporting of quantitative oxygen mapping in EPR imaging

Oxygen maps derived from electron paramagnetic resonance spectral–spatial imaging (EPRI) are based upon the relaxivity of molecular oxygen with paramagnetic spin probes. This technique can be combined with MRI to facilitate mapping of pO 2 values in specific anatomic locations with high precision. The co-registration procedure, which matches the physical and digital dimensions of EPR and MR images, may present the pO 2 map at the higher MRI resolution, exaggerating the spatial resolution of oxygen, making it difficult to precisely distinguish hypoxic regions from normoxic regions. The latter distinction is critical in monitoring the treatment of cancer by radiation and chemotherapy, since it is well-established that hypoxic regions are three or four times more resistant to treatment compared to normoxic regions. The aim of this article is to describe pO 2 maps based on the intrinsic resolution of EPRI. A spectral parameter that affects the intrinsic spatial resolution of EPRI is the full width at half maximum (FWHM) height of the gradient-free EPR absorption line in frequency-encoded imaging. In single point imaging too, the transverse relaxation times ( T 2 ∗ ) limit the resolution since the signal decays by exp ( - t p / T 2 ∗ ) where the delay time after excitation pulse, t p , is related to the resolution. Although the spin densities of two point objects may be resolved at this separation, it is inadequate to evaluate quantitative changes of pO 2 levels since the linewidths are proportionately affected by pO 2. A spatial separation of at least twice this resolution is necessary to correctly identify a change in pO 2 level. In addition, the pO 2 values are blurred by uncertainties arising from spectral dimensions. Blurring due to noise and low resolution modulates the pO 2 levels at the boundaries of hypoxic and normoxic regions resulting in higher apparent pO 2 levels in hypoxic regions. Therefore, specification of intrinsic resolution and pO 2 uncertainties are necessary to interpret digitally processed pO 2 illustrations.

Oscillating gradient measurements of fast oscillatory and rotational motion in the fluids

We demonstrate the combination of oscillating gradient waveforms with single-point imaging techniques to perform measurements of rapidly oscillating and/or rotating fluid motion. Measurements of Fourier components of motion can be performed over a wide range of frequencies, while the immunity of single-point imaging to time-evolution artefacts allows applications to systems with great susceptibility variations. The processing approaches, displacement resolution, and the diffusive attenuation are analyzed. Measurements of high-speed flow rotation in a spiral phantom, periodic displacements of oscillating gas in a thermoacoustic device and of cavitating liquid reveal a variety of motion spectra. The potential systems for study with the technique include turbulent motion, cavitation, and multiphase flows in general.

Effect of morphology on water sorption in cellular solid foods. Part II: Sorption in cereal crackers

Experimental dynamical moisture profiles of crackers with a fine and coarse morphology are successfully predicted using a pore scale network model. Experimental profiles are obtained using a single point imaging (SPI) NMR technique that enables 3D mapping of the moisture content of relatively immobile water at low water activity.The relative vapor conductivity trough the structure is 33% and 64% for the fine and coarse structured crackers, respectively. It can be argued that this is due to their difference in cell connectivity and not directly related to their difference in average cell diameter (0.33 and 0.75mm, respectively). It was found that the retarded local sorption dynamics of the solid matrix has a noticeable influence on the moisture profiles that arise in the first hours. This is crucial for the moisture sorption dynamics of sub centimeter size samples, for which there is a distinct non-equilibrium between the vapor and the sorbed water phase. The local sorption at low water activity is a factor 3 faster for the fine structure cracker compared to the coarse one. This is due to their differences in average lamellae thickness (54 and 93μm, respectively).However, for the description of the overall moisture sorption dynamics of the few cm thick samples, on a time scale of days, it valid to assume local equilibrium and to use an effective diffusivity model. The relative vapor conductivity together with the porosity and the derivative of the sorption isotherm determines the effective moisture diffusivity for these open structures, which is a factor 3 lower for the fine structured cracker compared to the coarse one. The single sided moisture sorption in the 2.5 thick cracker samples is not even completed after 5days, mainly because at higher water content (near 20%) there is very little gradient in relative humidity to drive the vapor transport. This is reflected in the predicted effective moisture diffusivities which for the coarse cracker decrease from 16×10−9m/s2 (at 1% MC, 16% aw) to 7.6×10−10m/s2 (at 20% MC, 86% aw).

The hydration of reactive cement-in-polymer dispersions studied by nuclear magnetic resonance

The behaviour of two novel cement-in-polymer (c/p) dispersions, namely cement-in-poly(vinyl acetate) and cement-in-poly(vinyl alcohol) upon exposure to water at room temperature was investigated by a combination of various NMR methods. The swelling, cracking, and the water ingress were monitored non-destructively using 1H single point imaging. The hydration of the cement matrix was investigated using 29Si NMR whilst 13C CPMAS NMR spectra allowed the quantification of the kinetics of the hydrolysis reaction of poly(vinyl acetate) into poly(vinyl alcohol). The polymer controls the rate of water ingress and swelling which in turn determines the behaviour of the c/p dispersions upon exposure to water. For the cement-in-poly(vinyl alcohol), the rates of water ingress and swelling are much faster than the hydration of the clinker whilst for the cement-in-poly(vinyl acetate) the slow rates of the two processes allow the formation of a cementious matrix which assures the stability of the sample.

Improvements in MR imaging of solids through gradient waveform optimization

Magnetic resonance imaging (MRI) is known to provide a useful approach for the exploration of the chemistry and dynamics of a wide range of soft condensed materials. However, its application to solids has been limited to those materials with relatively narrow resonances. The time needed to obtain an image of a solid with a given resolution and signal-to-noise ratio (SNR) is directly proportional to the line width of the resonance. For MRI to become practical for the imaging of solids it will have to rely on the development and use of MR sequences that avoid the issues raised by line broadening of the resonance. In this paper we review the latest contributions towards MR imaging of solids from our laboratory, in particular, applications using optimized gradient waveforms. Acoustic noise reduction and SNR improvement obtained with modifications of the standard single-point imaging sequence are presented and discussed using examples.

Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA)

Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k-space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k-space is filled with radial half-projections, whereas the centre is measured single pointwise on a Cartesian trajectory. This hybrid sequence combines the features of single point imaging with radial projection imaging. No hardware changes are required. Using this method, 3D images with an isotropic resolution of 1 mm can be obtained in less than 3 minutes. The differences between PETRA and the ultrashort echo time (UTE) sequence are evaluated by simulation and phantom measurements. Advantages of pointwise encoding time reduction with radial acquisition are shown for tissue with a T(2) below 1 ms. The signal to noise ratio and Contrast-to-noise ratio (CNR) performance, as well as possible limitations of the approach, are investigated. In-vivo head, knee, ankle, and wrist examples are presented to prove the feasibility of the sequence. In summary, fast imaging with ultrashort echo time is enabled by PETRA and may help to establish new routine clinical applications of ultrashort echo time sequences.

A method for single image restoration based on the principal ergodic

We present a method to extract from a single image both object and point spread function using low contrast features of an extended field of view. Invoking the principal ergodic on stochastic turbulent phenomena, we show that the aberration parameters, characteristics of the earth's turbulence, can be recovered from multiple features within an isoplanatic patch. The ensemble statistics is replacing the spatial statistics of a single realization to derive an equivalent modulation transfer function and to apply usual deconvolution techniques such as Richardson-Lucy algorithms. The reliability of this postprocessing treatment has been tested on synthetic data, on solar granulation observations performed at La Lunette Jean Rosch du Pic du Midi, and during the event of the Venus transit at La Tour Solaire de Meudon.

Magnetic resonance imaging near metal implants

The desire to apply magnetic resonance imaging (MRI) techniques in the vicinity of embedded metallic hardware is increasing. The soft-tissue contrast available with MR techniques is advantageous in diagnosing complications near an increasing variety of MR-safe metallic hardware. Near such hardware, the spatial encoding mechanisms utilized in conventional MRI methods are often severely compromised. Mitigating these encoding difficulties has been the focus of numerous research investigations over the past two decades. Such approaches include view-angle tilting, short echo-time projection reconstruction acquisitions, single-point imaging, prepolarized MRI, and postprocessing image correction. Various technical advances have also enabled the recent development of two alternative approaches that have shown promising clinical potential. Here, the physical principals and proposed solutions to the problem of MRI near embedded metal are discussed.

TU-C-204C-02: Introducton to Cancer Biology

This presentation will focus on methods to validate non‐invasive PET radiotracers for the quantification of loco‐regional hypoxia. MicroPET images will be shown of different hypoxia tracers and their uptake kinetics discussed. The use of compartmental analysis to define parametric images will be compared against single late time point imaging. Digital autoradiography will be used to determine the intra‐tumoral distributions of different hypoxia tracers, which will be compared against endogenous hypoxia‐related proteins, exogenous hypoxia markers and hypoxia reporter‐gene expression. Experiments to validate PET imaging of tumor hypoxia using image guided partial oxygen probes will be discussed. The lecture will conclude with efforts to clinical quantify the hypoxia distribution in head and neck patients. Learning Objectives: 1. Understand the different methods of detecting hypoxia (direct pO2 probe measurement, immunohistochemistry and surrogate imaging approaches). 2. Learn how potential hypoxia radiotracers are validated. 3. Understand the difference between static images of hypoxia radiotracer uptake versus parametric images of tumor hypoxia. 4. Understand the challenges of dose painting loco‐regional hypoxia

TH-C-202-03: The Use of Novel Radiotracers in PET - Potential Applications for Dose Painting

The aim of this presentation will be discuss the potential of PET imaging methods to define the biological characteristics of tumors. The talk will cover radiotracers whose selectively are based on tumor metabolism (FDG) hypoxia (FMISO) cellular proliferation (FLT) and finally specific tumor receptor (radiolabeled hormones and antibodies). The clinical acquisition protocols will be discussed with regards to single time point static imaging versus dynamic PET imaging strategies. Such images can provide useful supplemental data which when combined with the anatomical definition of the tumor may provide the basis for further treatment optimization. Learning Objectives: 1. Understand the mechanism by different radiotracers are entrapped by cancer cells. 2. Understand how PET images can be acquired and provide information useful to determine the radiobiology of the cancer. 3. Understand how to obtain and process dynamic image data sets and how they relate to single static time point imaging.

Robotics and Dynamic Image Analysis for Studies of Gene Expression in Plant Tissues

Gene expression in plant tissues is typically studied by destructive extraction of compounds from plant tissues for in vitro analyses. The methods presented here utilize the green fluorescent protein (gfp) gene for continual monitoring of gene expression in the same pieces of tissues, over time. The gfp gene was placed under regulatory control of different promoters and introduced into lima bean cotyledonary tissues via particle bombardment. Cotyledons were then placed on a robotic image collection system, which consisted of a fluorescence dissecting microscope with a digital camera and a 2-dimensional robotics platform custom-designed to allow secure attachment of culture dishes. Images were collected from cotyledonary tissues every hour for 100 hours to generate expression profiles for each promoter. Each collected series of 100 images was first subjected to manual image alignment using ImageReady to make certain that GFP-expressing foci were consistently retained within selected fields of analysis. Specific regions of the series measuring 300 x 400 pixels, were then selected for further analysis to provide GFP Intensity measurements using ImageJ software. Batch images were separated into the red, green and blue channels and GFP-expressing areas were identified using the threshold feature of ImageJ. After subtracting the background fluorescence (subtraction of gray values of non-expressing pixels from every pixel) in the respective red and green channels, GFP intensity was calculated by multiplying the mean grayscale value per pixel by the total number of GFP-expressing pixels in each channel, and then adding those values for both the red and green channels. GFP Intensity values were collected for all 100 time points to yield expression profiles. Variations in GFP expression profiles resulted from differences in factors such as promoter strength, presence of a silencing suppressor, or nature of the promoter. In addition to quantification of GFP intensity, the image series were also used to generate time-lapse animations using ImageReady. Time-lapse animations revealed that the clear majority of cells displayed a relatively rapid increase in GFP expression, followed by a slow decline. Some cells occasionally displayed a sudden loss of fluorescence, which may be associated with rapid cell death. Apparent transport of GFP across the membrane and cell wall to adjacent cells was also observed. Time lapse animations provided additional information that could not otherwise be obtained using GFP Intensity profiles or single time point image collections.

Robotics and Dynamic Image Analysis for Studies of Gene Expression in Plant Tissues

Gene expression in plant tissues is typically studied by destructive extraction of compounds from plant tissues for in vitro analyses. The methods presented here utilize the green fluorescent protein (gfp) gene for continual monitoring of gene expression in the same pieces of tissues, over time. The gfp gene was placed under regulatory control of different promoters and introduced into lima bean cotyledonary tissues via particle bombardment. Cotyledons were then placed on a robotic image collection system, which consisted of a fluorescence dissecting microscope with a digital camera and a 2-dimensional robotics platform custom-designed to allow secure attachment of culture dishes. Images were collected from cotyledonary tissues every hour for 100 hours to generate expression profiles for each promoter. Each collected series of 100 images was first subjected to manual image alignment using ImageReady to make certain that GFP-expressing foci were consistently retained within selected fields of analysis. Specific regions of the series measuring 300 x 400 pixels, were then selected for further analysis to provide GFP Intensity measurements using ImageJ software. Batch images were separated into the red, green and blue channels and GFP-expressing areas were identified using the threshold feature of ImageJ. After subtracting the background fluorescence (subtraction of gray values of non-expressing pixels from every pixel) in the respective red and green channels, GFP intensity was calculated by multiplying the mean grayscale value per pixel by the total number of GFP-expressing pixels in each channel, and then adding those values for both the red and green channels. GFP Intensity values were collected for all 100 time points to yield expression profiles. Variations in GFP expression profiles resulted from differences in factors such as promoter strength, presence of a silencing suppressor, or nature of the promoter. In addition to quantification of GFP intensity, the image series were also used to generate time-lapse animations using ImageReady. Time-lapse animations revealed that the clear majority of cells displayed a relatively rapid increase in GFP expression, followed by a slow decline. Some cells occasionally displayed a sudden loss of fluorescence, which may be associated with rapid cell death. Apparent transport of GFP across the membrane and cell wall to adjacent cells was also observed. Time lapse animations provided additional information that could not otherwise be obtained using GFP Intensity profiles or single time point image collections.

A new approach combining different MRI methods to provide detailed view on swelling dynamics of xanthan tablets influencing drug release at different pH and ionic strength

The key element in drug release from hydrophilic matrix tablets is the gel layer that regulates the penetration of water and controls drug dissolution and diffusion. We have selected magnetic resonance imaging (MRI) as the method of choice for visualizing the dynamic processes occurring during the swelling of xanthan tablets in a variety of media. The aims were (i) to develop a new method using MRI for accurate determination of penetration, swelling and erosion fronts, (ii) to investigate the effects of pH and ionic strength on swelling, and (iii) to study the influence of structural changes in xanthan gel on drug release. Two dimensional (2D) MRI and one dimensional single point imaging (SPI) of swollen xanthan tablets were recorded, together with T 2 mapping. The border between dry and hydrated glassy xanthan—the penetration front—was determined from 1D SPI signal intensity profiles. The erosion front was obtained from signal intensity profiles of 2D MR images. The swelling front, where xanthan is transformed from a glassy to a rubbery state (gel formation), was determined from T 2 profiles. Further, the new combination of MRI methods for swelling front determination enables to explain the appearance of the unusual “bright front” observed on 2D MR images in tablets swollen in HCl pH 1.2 media, which represents the position of swelling front. All six media studied, differing in pH and ionic strength, penetrate through the whole tablet in 4 h ± 0.3 h, but formation of the gel layer is significantly delayed. Unexpectedly, the position of the swelling front was the same, independently of the different xanthan gel structures formed under different conditions of pH and ionic strength. The position of the erosion front, on the other hand, is strongly dependent on pH and ionic strength, as reflected in different thicknesses of the gel layers. The latter are seen to be the consequence of the different hydrodynamic radii of the xanthan molecules, which affect the drug release kinetics. The slowest release of pentoxifylline was observed in water where the thickest gel was formed, whereas the fastest release was observed in HCl pH 1.2, in which the gel layer was thinnest. Moreover, experiments simulating physiological conditions showed that changes of pH and ionic strength influence the xanthan gel structure relatively quickly, and consequently the drug release kinetics. It is therefore concluded that drug release is greatly influenced by changes in the xanthan molecular conformation, as reflected in changed thickness of the gel layer. A new method utilizing combination of SPI, multi-echo MRI and T 2 mapping eliminates the limitations of standard methods used in previous studies for determining moving fronts and improves current understanding of the dynamic processes involved in polymer swelling.

Optimal k-space sampling for single point imaging of transient systems

Optimal k-space sampling for single point imaging of transient systems