Regions Of Low Signal Research Articles

Filter design for phased-array mr image reconstruction using super algorithm

Receiver coils in magnetic resonance (MR) systems are used to acquire the signals emitted by the nuclei. Surface coils provide a high signal-to-noise ratio due to their small sensitive region but the usable field of view is also limited to the size of the sensitive region. The use of array coils permits to increase the sensitive region. The outputs from the receiver channels are combined to construct a single composite image from the data of many coils. The image reconstruction is usually performed by using root sum-of-squares (RSS) method, which combines data without the knowledge of the coils sensitivity but it is known to introduce bias in low-signal regions. SUPER reconstruction permits to yield a composite image with higher contrast between high- and low-signal regions than RSS algorithm, by preliminary estimating coil sensitivities using low-pass filtering of original images. The purpose of this work is to introduce a theory for obtaining estimated coil sensitivity maps from the individual coil images and successively combining data from array elements using SUPER algorithm. Performance evaluation and comparison with RSS reconstruction were carried out with MR acquisitions performed using a 32-elements cardiac phased-array coil in a 3 T scanner. © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 41B: 85–93, 2012

Turbid Scene Enhancement Using Multi-Directional Illumination Fusion

Ambient light is strongly attenuated in turbid media. Moreover, natural light is often more highly attenuated in some spectral bands, relative to others. Hence, imaging in turbid media often relies heavily on artificial sources for illumination. Scenes irradiated by an off-axis single point source have enhanced local object shadow edges, which may increase object visibility. However, the images may suffer from severe nonuniformity, regions of low signal (being distant from the source), and regions of strong backscatter. On the other hand, simultaneously illuminating the scene from multiple directions increases the backscatter and fills-in shadows, both of which degrade local contrast. Some previous methods tackle backscatter by scanning the scene, either temporally or spatially, requiring a large number of frames. We suggest using a few frames, in each of which wide field scene irradiance originates from a different direction. This way, shadow contrast can be maintained and backscatter can be minimized in each frame, while the sequence at large has a wider, more spatially uniform illumination. The frames are then fused by post processing to a single, clearer image. We demonstrate significant visibility enhancement underwater using as little as two frames.

SU‐E‐T‐283: Deforming Planning CT Images to CBCT Images Using MIMVista

Purpose: The overarching goal of this project is to be able to use daily CBCT images to calculate the dose delivery for a single treatment. To calculate the dose, planning CT images can be deformed to match the patient anatomy at the time of delivery as shown in the daily CBCT. Our current research is looking into methods to deform the planning CT to best match the CBCT using the commercial software MIMVista. Methods: The OBI (on‐board imager) of a Varian Linac at Karmanos Cancer Institute was used to get a set of daily CBCT images for a prostate patient. The images were processed in MatLab on a voxel‐by‐voxel basis to enhance areas of low signal and to convert HU for the CBCT to HU for the planning CT. Efforts were also made to decrease signal saturation in the bone through high pixel value masking. The planning CT images were deformed to the new CBCT images using MIMVista software. Results: Converting the HUs from CBCT to planning CT values followed by applying different scaling factors gave some success in improving the deformation of the images using MIMVista. However, the resulting deformation is clearly not yet accurate enough to be applied clinically. Deformation was performed by aligning bony anatomy, therefore bony anatomy deformed significantly better than tissue. Regions of low signal, variable bladder filling, and bowel gas pockets present difficulties that still need to be resolved. Conclusions: While the CBCT image adjustments we have made have improved MIMVistaˈs ability to deform planning CT images to CBCT images, work still needs to be done to reduce the noise in the CBCT images so that the planning CT images will be able to be deformed accurately.

Utility of delayed gadolinium-enhanced MRI (dGEMRIC) for qualitative evaluation of articular cartilage of patellofemoral joint

Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) was used for the measurement of relative proteoglycan depletion of articular cartilage in the patellofemoral (PF) joint following a proprietary protocol, which was compared with the X-ray images, proton density weighted MR images (PDWI) and arthroscopic findings. The study examined 30 knees. The ages ranged from 16 to 74 (average 40.3) years. The Gd-DTPA(2-)containing contrast medium was used in a single dose. The subjects were made to exercise the knee joint for 10 min; and MR images were taken 2 h after intravenous injection of contrast medium. T1-calculated images were produced and the region of interest (ROI) was set as follows. (1) ROI1: entire articular cartilage in a slice through the center of the patella. (2) ROI2: low signal region in T1-calculated images, which were set in a blind fashion by two observers. (3) ROI3: articular cartilage on one side that includes ROI2 where low signal region were detected (medial or lateral). ROI3 was set to examine the contrast of ROI2 with surrounding articular cartilage. The average T1 values of ROI1 was 393.5+/-33.6 ms for radiographic grade 0 and 361.3+/-11.1 ms for grade I, which showed a significant difference (P=0.036). The T1 value of ROI2 was 351.6+/-28.2 ms for grade I, 361.9+/-38.3 ms for grade II, 362.1+/-67.7 ms for grade III, and 297.8+/-54.1 ms for grade IV according to arthroscopic Outerbridge classification. All cases, that demonstrated decrease of T1 values on dGEMRIC (ROI2), showed abnormal arthroscopic or direct viewing findings. The ratio (ROI3/ROI2) in cases of only slight damage classified as Outerbridge grade I (6 cases) was an average of 1.04+/-0.02 and was 1.0 or greater in all cases, thereby indicating well-defined contrast with the surrounding cartilage. The diagnosis of damage in articular cartilage was possible in all 16 cases with radiographic K-L grade I on dGEMRIC, while the intensity changes were not found in 10 of 16 cases on PDWI. The dGEMRIC with a single-dose would be useful on a diagnosis of the area demonstrating early relative proteoglycan depletion in the articular cartilage of the PF joint prior to any discernible changes in the subchondral bone on X-ray images and exceeds to plain MR images for examining deterioration of articular cartilage.

Application of partial differential equation‐based inpainting on sensitivity maps

Inpainting is an image interpolation method. Partial differential equation (PDE)-based digital inpainting techniques are finding broad applications. In this paper, PDE-based inpainting techniques are applied to the field of MR parallel imaging. A novel model and its corresponding numerical method are introduced. This model is then applied to sensitivity maps. Coil sensitivity maps are important for parallel imaging, and they often require extrapolation and hole filling (holes being dark regions of low signal in MR images). These problems can be solved simultaneously by the application of inpainting techniques. Experiments for determining coil sensitivity maps for phantoms and cardiac MR images demonstrate the accuracy of the proposed model. Images generated using sensitivity encoding (SENSE) that utilizes inpainted sensitivity maps, thin-plate spline (TPS) estimated sensitivity maps, and Gaussian kernel smoothed (GKS) sensitivity maps are compared. From the experimental results, it can be seen that inpainted sensitivity maps produce better results than GKS sensitivity maps. The TPS method generates results similar to those of the inpainting technique but is much more time-consuming.

Effect of dislocations on local transconductance in AlGaN/GaN heterostructures as imaged by scanning gate microscopy

The spatial variations of transconductance in AlGaN/GaN heterostructures were mapped using a conducting tip atomic force microscope. The conducting tip locally modulates the two-dimensional electron gas (2DEG) while the change in the drain current was monitored as a function of tip position. A spatial resolution of 250 nm was obtained. This technique enables us to investigate the role of defects in transistor performance. In particular, when biased near the depletion of the 2DEG, the transconductance map displays a cell structure, with low signal regions correlating with the positions of negatively charged threading dislocations.

Effect of Internal Flows on Sunyaev‐Zeldovich Measurements of Cluster Peculiar Velocities

Galaxy clusters are potentially powerful probes of the large-scale velocity field in the Universe because their peculiar velocity can be estimated directly via the kinematic Sunyaev-Zeldovich effect (kSZ). Using high-resolution cosmological simulations of an evolving cluster of galaxies, we evaluate how well the average velocity obtained via a kSZ measurement reflects the actual cluster peculiar velocity. We find that the internal velocities in the intracluster gas are comparable to the overall cluster peculiar velocity, 20 to 30% of the sound speed even when a cluster is relatively relaxed. Nevertheless, the velocity averaged over the kSZ map inside a circular aperture matched to the cluster virial region provides an unbiased estimate of a cluster's radial peculiar velocity with a dispersion of 50 to 100 km/s, depending on the line of sight and dynamical state of the cluster. This dispersion puts a lower limit on the accuracy with which cluster peculiar velocity can be measured. Although the dispersion of the average is relatively small, the velocity distribution is broad; regions of low signal must be treated with care to avoid bias. We discuss the extent to which systematic errors might be modelled, and the resulting limitations on using galaxy clusters as cosmological velocity tracers.

Magnetic resonance imaging of lung water content and distribution in term and preterm infants.

An increase in lung liquid may contribute to respiratory disease in preterm infants. Uneven distribution of lung liquid may cause heterogeneity in the lung disease seen in these infants. We used magnetic resonance imaging to investigate lung water content and distribution in 16 preterm (24-31 weeks) and 9 term infants in the first week of life. Images of lung parenchyma were examined and relative proton density quantified to give an index of lung water. Lung water content and distribution were compared between preterm and term infants, and in preterm infants regional signal distribution between dependent and nondependent lung on T1 weighted images was also compared after turning between prone and supine positions. Relative proton density was higher in preterm than in term lung (p < 0.008) and greater in dependent than in nondependent regions, particularly in the preterm (p < 0.001). Repositioning preterm infants rapidly redistributed signal intensities, with more even distribution lying prone than supine (p < 0.001). Small, low-signal regions were seen in the lung parenchyma in preterm but not in term infants, which may indicate peribronchial fluid or overdistension of compliant lung units. We conclude that lung water content is higher in preterm than in term infants and is associated with gravity-related changes consistent with dependent atelectasis.

Mediastinal and diffuse pulmonary haemangiolymphangioma

A 17-year-old male patient presented to the Department of Pulmonary Medicine with a history of dyspnoea and haemoptysis. At the age of 14 an unenhanced CT scan of the thorax demonstrated a large soft tissue mass involving the mediastinum surrounding the great vessels, trachea, oesophagus and the hila. A large pericardial effusion, small bilateral pleural effusions and linear interstitial opacification of the lungs was noted. A sternotomy revealed inflamed pericardial and fat tissue, which oozed blood tinged lymph like fluid, 700 cc of which was drained from the pericardium. Mediastinal tissue and a right middle lobe lung biopsy was performed, a pericardial window was created into the right pleural space. On gross inspection; the mediastinal tissue was purplish in colour, it contained grey coloured nodules, and some cysts up to 6 mm in diameter. The histology of the mediastinal lesion revealed an angiomatous disease process with lymphatic and blood vessel components. In many areas the walls of the vessels were sclerotic and there was evidence of recent bleeding, with numerous haemosiderin-laden macrophages. The biopsy specimen of the right middle lobe revealed haemangiolymphangioma within the lung interstitium. A contrast enhanced CT scan (Fig. 1) and MR scan (Fig. 2) were performed at the age of 15. The MR scan demonstrated no extension of the mediastinal tumour into the neck. Axial T1 weighted spin echo images demonstrated a heterogeneous intermediate signal mediastinal mass, with integral low signal regions, likely due to cysts. The patient was admitted 2 weeks later for a sternotomy, radical debridement of the mediastinal mass, ligation of the thoracic duct, bilateral parietal pleurectomy and wedge biopsy of the lingula. The pathologist confirmed the diagnosis of haemangiolymphangioma of the mediastinum and interstitium of the lingula (Fig. 3). The thymus was normal. On this admission, the patient’s oxygen-haemoglobin saturation was greater than 91% on room air. Spirometry demonstrated a total lung capacity of 2.96 l (56% of predicted), a forced vital capacity of 1.97 l (44% of predicted), a forced expiratory volume in 1 s of 1.69 l * Corresponding author. Present address: The Department of Medical Imaging, The Royal Hobart Hospital, G.P.O. Box 1061L, Hobart, Tasmania 7001, Australia. Tel.: +61 362 247351; fax: +61 362 314457.

Angiosarcoma of the liver: MR imaging pre- and post-chemotherapy

Angiosarcoma of the liver is a rare tumor of mesenchymal origin. We report the MR appearance of angiosarcoma of the liver on T1-weighted, T2-weighted, and serial gadolinium enhanced spoiled gradient echo images pre- and post-intravenous chemotherapy. On pre-treatment images, angiosarcoma lesions were well defined, high signal with central regions of low signal on T2-weighted images, and showed peripheral nodular enhancement on serial post gadolinium images. Lesions resembled hemangiomas, however low signal central regions on T2-weighted images was a distinguishing feature. Post-treatment images acquired 11, 15, and 17 months after initiation of therapy, demonstrated decrease in diameter or resolution of angiosarcoma masses. Lesions increased in signal on T1 weighted images and decreased in signal on T2 weighted images approaching the signal intensity of liver. Treated liver lesions enhanced negligibly on gadolinium enhanced images.

Stability of a class of directionally sensitive asymmetric nonlinear neural networks

Asymmetrically connected inhibitory shunting networks are believed to occur in many areas of the brain. One such area is the visual system; these networks are found useful in explaining many peripheral visual phenomena. This paper examines a class of asymmetrically connected networks for stability and uniqueness of outputs; both time-invariant and moving inputs are considered. Under different sets of constaints upon the network characteristics and operating region, it is possible to obtain: • • bounded-input-bounded-output stability of the solutions; • • stability and uniqueness of the output to any input for a network of finite extent, with time-invariant inputs giving time-invariant outputs; • • localization of edge effects on a finite network, and stability and uniqueness of the output to any input for a network of infinite extent, with time-invariant or traveling-wave inputs giving time-invariant or traveling-wave outputs respectively. The constraints required for these three sets of characteristics are successively more stringent. The most interesting constraints are those required for localization of edge effects on a finite network; they are not satisfied by a network in which the output from each node is obtained by sharply thresholding the node potential. However, they can be satisfied if the network operates in a relatively low signal region, below saturation of the outputs. An example of a network which does satisfy the constraints is given by certain field effect transistor (FET) implementations.

Inhomogeneity correction using an estimated linear field map.

A fast and robust method for correcting magnetic resonance image distortion due to field inhomogeneity is proposed and applied to spiral k-space scanning. The method consists of acquiring a local field map, finding the best fit to a linear map, and using it to deblur the image distortions due to local frequency variations. The linear field map is determined using a maximum likelihood estimator with weights proportional to the pixel intensity. The method requires little additional computation and is robust in low signal regions and near abrupt field changes. Additionally, it can be used in combination with other deblurring methods. The application of this method is illustrated in conjunction with a multislice, T2-weighted, breath-held spiral scan of the liver.

MR high-resolution imaging of arterial intimal hyperplasia at 1.5 T

Intimal hyperplasia is the earliest microscopic change detectable in the arterial wall in the development of arteriosclerosis and atherosclerosis. To enable high-resolution imaging of this, a standard 1.5-T machine was modified using inserted gradient coils of 30 cm internal diameter and surface radio-frequency coils. Six rabbit aorta specimens with injury-induced intimal hyperplasia and two normal rabbit aorta specimens were imaged using a spin-echo sequence of TE 40. Pixel sizes as small as 20 µ×20 µ could be obtained and the area of intimal hyperplasia could be measured at 80 µ×40 µ. The results were compared with those made by a computer-linked microscope. In the injured aortas, there was a low-signal region on magnetic resonance imaging which corresponded to the media of the vessel wall when compared with the histology.T1 andT2 constants for the media were determined and compared with those of a normal specimen. TheT1 constant was shorter for the media of the injured aorta than in the normal. This could be related to the disruption of the internal elastic lamina and migration of muscle cells to the intima which occurs following injury. The popliteal artery of four normal volunteers was imaged and the area of the wall could be measured at a pixel size of 312 µ×160 µ. This suggests thatin vivo studies of intimal hyperplasia arteriosclerosis and atherosclerosis could be made.

視床梗塞により生じたふくそう眼振の２症例

Convergence nystagmus, which is a main finding in Sylvian aqueduct syndrome is not common. We examined two patients of convergence nystagmus associated with infarction of the thalamus. The patients, aged 21 and 64-years, visited our clinic because of sudden dizziness. There was no cochlear sign in either case. Neurotological examinations, including electronystagmography (ENG), were carried out. Both patients had up-beat nystagmus, convergence nystagmus and abnormal vertical eye movements; however, no upward gaze palsy was observed. One patient showed a normal response to caloric testing, but the other showed rt-canal paresis (41%). On computerized tomography (CT scan), no lesion could be identified in the midbrain or thalamus of either patient. However, in one patient, T2-weighted MRI revealed a high signal region and Ti-weighted MRI demonstrated a low signal region in the lt-medial part of the thalamus. In the second patient, T2-weighted MRI revealed multiple bilateral lacunar high signal lesions in the thalamus, the putamen and in the region of the deep white matter. The etiology of abnormal vertical eye movements and convergence nystagmus is discussed with emphasis on the neurotological examinations and MRI.Convergence nystagmus may be associated with lesions of the posterior commisure and nucleus in the medial part of the thalamus, and abnormal vertical eye movements may be attributed to a lesion in the tract from ri-MLF to the posterior commisure.

In vitro MR microscopy of the hippocampus in Alzheimer's disease

We used MR microscopy at 7 tesla to identify the anatomy of the degenerating hippocampus in Alzheimer's disease (AD), which we then correlated with the histopathologic findings in the same specimens. The specimens studied were resected postmortem from 13 patients with confirmed AD and from nine age-matched controls. We imaged the specimens in the coronal plane using either three-dimensional Fourier encoding or single-slice Carr, Purcell, Meiboom, Gill (CPMG) spin echo sequences. On all specimens imaged with the CPMG pulse sequence, we calculated the T2 relaxation times for subfields within the hippocampus. Histologic sections were taken from each specimen and compared with the corresponding MR image. Using histologic boundaries, we quantified the number of neuritic plaques and neurofibrillary tangles in each hippocampal subfield. We measured the area, morphometric characteristics, and width of identifiable signal variant regions on each image and compared these measurements with the histopathologic findings. The mean cross-sectional area of the hippocampus in AD was decreased by 31% compared with the control group. This atrophy was highly correlated with tangle counts within the hippocampus, but not with plaque counts. The width of the gray matter in hippocampal area CA1, as identified by MR, correlated with the total area of the hippocampus. An age-related decrease in the size of a low-signal region that corresponds histologically to input projections comprising part of the perforant pathway was identified. Measurements of the T2 relaxation times of hippocampal subfields showed little regional variability and were not accurate indicators of disease presence or severity (p > 0.05).

Motion Associated Susceptibility Artifacts

A bowel labeling agent is important for improving abdominal MR. Besides providing contrast between the bowel and other organs, the contrast agent itself is a potential source of artifacts. The artifacts created by superparamagnetic particles (SPP) subjected to motion have been studied in vitro at 0.5 T, and compared to artifacts created by a paramagnetic compound. Apart from the expected static effects of the SPP, movement induced additional artifacts were seen as signal displacements in the phase-encoding direction. The artifacts were obvious at an iron concentration of 1 mg Fe/ml, barely visible at 0.2 mg Fe/ml, and completely absent at 0.1 mg Fe/ml. Artifacts were also evident with the SPP outside the imaging slice. This further emphasizes the importance of choosing the lowest effective dose when using SPP contrast agents. For the paramagnetic agent, motion propagated artifacts consisted of high and low signal regions in a mosaic pattern.

Striatonigral degeneration

We report a patient with striatonigral degeneration in whom T2-weighted MRI imaging revealed low signal and atrophy in the putamen. Neuropathologic studies confirmed putaminal atrophy and iron deposition in the MRI low signal regions.

An enhanced electron-stimulated-desorption ion angular distribution method for imaging molecular orientations of adsorbed species

A striking improvement has been made in the quality of positive ion angular distribution data obtained in electron-stimulated-desorption ion angular distribution (ESDIAD). The improved method has been used to study configurational changes in chemisorbed CO on Ni(110) which are induced by intermolecular interactions at high coverage. CO molecules adsorbed with normal C–O bond orientations on ridge Ni sites tilt away from the normal by ±19° in directions perpendicular to the ridges for CO coverages above 0.75 CO/Ni. Evidence for the C–O tilting has also been studied as a function of electron energy. This has demonstrated that an O(1s) excitation process leads to intense and sharp O+ ESDIAD beams. The improved ESDIAD method involves the removal of a soft x-ray background signal which originates from electron impact on the metal surface, and is therefore present in all photographic ESDIAD measurements made to date. The angular distribution of the background effect is independent of electron energy and current density. The resultant ‘‘pure ESDIAD’’ patterns show more quantitative details in high and low signal regions than has been seen before.