Core Pixels Research Articles

Cluster Correction for Cluster Analysis-Based Multibaseline InSAR Phase Unwrapping

In recent years, multibaseline phase unwrapping (MBPU) has been widely studied, but it is still suffering from the problem of low noise robustness. In view of this, two cluster correction methods for the cluster-analysis (CA)-based MBPU algorithms are presented to improve the accuracy and effectiveness of the algorithm. In the first method, a suitable box centered on each pixel is selected and the cluster with the highest frequency in the box is taken as the corrected cluster to which the central pixel belongs; In the second method, all pixels are divided into core pixels and noncore pixels according to the predefined density, and then the cluster of the noncore pixels is corrected to the one that appears most frequently in the selected box centered on the noncore pixels. Experiments on both simulated and real datasets confirm the effectiveness of the proposed cluster correction methods.

Intensity contrast of solar plage as a function of magnetic flux at high spatial resolution

Magnetic elements have an intensity contrast that depends on the type of region they are located in (for example quiet Sun, or active region plage). Observed values also depend on the spatial resolution of the data. Here we investigate the contrast-magnetic field dependence in active region plage observed near disk center with SUNRISE during its second flight in 2013. The wavelengths under study range from the visible at 525 nm to the near ultraviolet (NUV) at 300 nm and 397 nm. We use quasi-simultaneous spectropolarimetric and photometric data from the Imaging Magnetograph eXperiment (IMaX) and the Sunrise Filter Imager (SuFI), respectively. We find that in all wavelength bands, the contrast exhibits a qualitatively similar dependence on the line-of-sight magnetic field, BLOS, as found in the quiet Sun, with the exception of the continuum at 525 nm. There, the contrast of plage magnetic elements peaks for intermediate values of BLOS and decreases at higher field strengths. By comparison, the contrast of magnetic elements in the quiet Sun saturates at its maximum value at large BLOS. We find that the explanation of the turnover in contrast in terms of the effect of finite spatial resolution of the data is incorrect with the evidence provided by the high-spatial resolution SUNRISE data, as the plage magnetic elements are larger than the quiet Sun magnetic elements and are well-resolved. The turnover comes from the fact that the core pixels of these larger magnetic elements are darker than the quiet Sun. We find that plages reach lower contrast than the quiet Sun at disk center at wavelength bands formed deep in the photosphere, such as the visible continuum and the 300 nm band. This difference decreases with formation height and disappears in the Ca II H core, in agreement with empirical models of magnetic element atmospheres.

In patients with suspected acute stroke, CT perfusion-based cerebral blood flow maps cannot substitute for DWI in measuring the ischemic core.

BackgroundNeuroimaging may guide acute stroke treatment by measuring the volume of brain tissue in the irreversibly injured “ischemic core.” The most widely accepted core volume measurement technique is diffusion-weighted MRI (DWI). However, some claim that measuring regional cerebral blood flow (CBF) with CT perfusion imaging (CTP), and labeling tissue below some threshold as the core, provides equivalent estimates. We tested whether any threshold allows reliable substitution of CBF for DWI.Methods58 patients with suspected stroke underwent DWI and CTP within six hours of symptom onset. A neuroradiologist outlined DWI lesions. In CBF maps, core pixels were defined by thresholds ranging from 0%-100% of normal, in 1% increments. Replicating prior studies, we used receiver operating characteristic (ROC) curves to select thresholds that optimized sensitivity and specificity in predicting DWI-positive pixels, first using only pixels on the side of the brain where infarction was clinically suspected (“unilateral” method), then including both sides (“bilateral”). We quantified each method and threshold’s accuracy in estimating DWI volumes, using sums of squared errors (SSE). For the 23 patients with follow-up studies, we assessed whether CBF-derived volumes inaccurately exceeded follow-up infarct volumes.ResultsThe areas under the ROC curves were 0.89 (unilateral) and 0.90 (bilateral). Various metrics selected optimum CBF thresholds ranging from 29%-32%, with sensitivities of 0.79–0.81, and specificities of 0.83–0.85. However, for the unilateral and bilateral methods respectively, volume estimates derived from all CBF thresholds above 28% and 22% were less accurate than disregarding imaging and presuming every patient’s core volume to be zero. The unilateral method with a 30% threshold, which recent clinical trials have employed, produced a mean core overestimation of 65 mL (range: –82–191), and exceeded follow-up volumes for 83% of patients, by up to 191 mL.ConclusionCTP-derived CBF maps cannot substitute for DWI in measuring the ischemic core.

Neuroprotective Efficacy of Methylene Blue in Ischemic Stroke: An MRI Study

Methylene blue (MB) has unique energy-enhancing and antioxidant properties and is FDA-approved drug to treat methemoglobinemia and cyanide poisoning. This study evaluated the efficacy of MB to treat ischemic stroke in rats using longitudinal MRI and behavioral measures. Rats were subjected to 60-minute middle-cerebral-artery occlusion. In a randomized double-blinded design, vehicle or MB was administered after reperfusion. The initial lesion volumes at 30 minutes post-ischemia were not significantly different between the two groups (P = 0.92). The final infarct volumes two days after stroke increased in the vehicle group but decreased in the MB group, yielding a 30% difference in infarct volume (P = 0.03). Tracking tissue fate on a pixel-by-pixel basis showed that MB salvaged more initial core pixels compared to controls (22±3% versus 11±3%, P = 0.03), and more mismatch pixels compared to controls (83±3% versus 61±8%, P = 0.02). This study demonstrates MB treatment minimizes ischemic brain injury and improves functional outcomes.

Effects of Cerebral Ischemic and Reperfusion on T2∗-Weighted MRI Responses to Brief Oxygen Challenge

This study characterized the effects of cerebral ischemia and reperfusion on T2*-weighted magnetic resonance image (MRI) responses to brief oxygen challenge (OC) in transient (60 minutes) cerebral ischemia in rats. During occlusion, the ischemic core tissue showed no significant OC response, whereas the perfusion-diffusion mismatch tissue showed markedly higher percent changes relative to normal tissue. After reperfusion, much of the pixels with initial exaggerated OC responses showed normal OC responses, and the majority of these tissues were salvaged as defined by endpoint T2 MRI. The initial core pixels showed exaggerated OC responses after reperfusion, but the majority of the core pixels eventually became infarct, suggesting exaggerated OC responses do not necessarily reflect salvageable tissue. Twenty-four hours after stroke, basal T1 increased in the ischemic core. Oxygen challenge decreased T1 significantly in the core, indicative of the substantial increases in dissolved oxygen in the core as the result of hyperperfusion. We concluded that exaggerated T2*-weighted MRI responses to OC offer useful insight in ischemic tissue fates. However, exaggerated OC pixels are not all salvageable, and they exhibited complex dynamics depending on reperfusion status, hyperperfusion, and edema effects.

Abstract 3417: Pixel by Pixel Comparison of CTP-Defined Infarct Core on Concurrent Non-Contrast CT

Introduction Interobserver agreement for the presence of early ischemic changes on non-contrast CT (NCCT) in potential thrombolysis candidates can be poor even among experienced raters. CT perfusion (CTP) may demonstrate infarct core and penumbra according to proposed thresholds but the extent of correlation between CTP defined core and hypodensity on hyperacute NCCT is unclear Methods A pixel by pixel comparison between NCCT and concurrent CTP obtained <6hrs from symptom onset was performed for patients with symptomatic arterial occlusions. Segmentation was performed using time-averaged CTP raw images to compare core and normal tissue pixels for grey and white matter separately. Core was defined as relative Cerebral Blood Flow (CBF) of <45% of normal tissue. NCCT was co-registered to the baseline frames of the concurrent CTP using a rigid body transformation. Core pixels were extracted from CTP and transposed to NCCT. Hounsfield unit (HU) values in CTP-defined core and normal tissue were quantified for grey and white matter. Receiver operating characteristic (ROC) curve analysis was performed to assess the predictive value of NCCT hypodensity and CTP-defined core. Results CT and CTP were compared for 33 patients. Mean age was 72 years (SD 12), median NIHSS was 16 (IQR 11-20) and mean time from symptom onset to CTP was 191 minutes (SD 63). Area under the curve (AUC) for prediction of grey and white matter core by NCCT was 0.641 and 0.601 respectively. The most frequently occurring optimum HU cut-points were 29 and 28 for grey and white matter respectively .These cut-points were associated with low overall specificity for detecting core (Median ± IQR= 0.48, 0.32-0.59 and 0.36, 0.26-0.44) but higher sensitivity (Median ±IQR 0.79, 0.62-0.87 and 0.83, 0.73-0.89) for grey and white matter Conclusion Objectively determined hypodensity on NCCT has low sensitivity and specificity for prediction of ischemic core tissue defined by CT perfusion. Thresholds were similar for both grey and white matter. NCCT scans without established hypodensity may still have irreversibly infarcted pixels which can be detected with CTP. Prospective evaluation of the added value of CTP-defined core on decision making in acute stroke is needed.

Probing ischemic tissue fate with BOLD fMRI of brief oxygen challenge

It has been recently shown that at-risk tissue exhibits exaggerated T2⁎-weighted MRI signal increases during transient oxygen challenge (OC), suggesting that the tissue is still metabolically active. This study further characterized the effects of transient OC on T2⁎-weighted MRI in permanent focal stroke rats (N=8) using additional quantitative measures. The major findings were: i) the ischemic core cluster showed no significant response, whereas the mismatch cluster showed markedly higher percent changes relative to normal tissue in the acute phase. ii) Many of the mismatch pixels showed exaggerated OC responses which became hyperintense on T2-weighted MRI at 24h. The area with exaggerated OC responses was larger than the mismatch, suggesting that some tissue with reduced diffusion were potentially at risk. iii) Basal T2⁎-weighted intensities on the perfusion–diffusion contourplot were high in normal tissue and low in the core, with a sharp transition in the mismatch. iv) OC-induced changes on the perfusion–diffusion contourplot dropped as perfusion and diffusion values fell below their respective viability thresholds. v) Basal T1 increased slightly in the ischemic core (P<0.05). OC decreased T1 in normal (P<0.05) but not in mismatch and core pixels. vi) OC decreased CBF in normal (P<0.05) but not in mismatch and core pixels. T2⁎-weighted MRI of OC has the potential to offer unique clinically relevant data.

Characterizing Tissue Fate After Transient Cerebral Ischemia of Varying Duration Using Quantitative Diffusion and Perfusion Imaging

The purpose of this study was to investigate the effects of reperfusion on ischemic lesion evolution and pixel-by-pixel apparent diffusion coefficient-cerebral blood flow (ADC-CBF) dynamics of core and mismatch tissues after 35, 60, and 95 minutes of transient focal ischemia in rats (n=28). Serial diffusion-, perfusion-, and T2-weighted imaging were performed up to 24 hours. The evolution of the magnetic resonance image-derived lesion volume was investigated and ADC-CBF scatterplots were performed to prospectively characterize the ADC and CBF dynamics of core and mismatch tissues with different fates. For comparison, similar analysis was performed on a historical 60-minute transient ischemia and permanent ischemia group. ADC-derived lesions markedly decreased on reperfusion at 35 minutes to an average of 15+/-5% of prereperfusion lesion size (P<0.00001). At 24 hours, lesion volume as determined by T2 imaging increased again to 51+/-10% of prereperfusion lesion size. In the 95-minute group, ADC lesions only briefly decreased on reperfusion and then secondarily enlarged at 180 minutes, almost reaching prereperfusion lesion volume. Pixel-based analysis demonstrated that >85% of mismatch pixels were salvaged by reperfusion independent of ischemia duration. Recanalization at 35, 60, and 95 minutes resulted in recovery of 46%, 28%, and 9% of core pixels, respectively. Core and mismatch pixels that were ultimately salvaged had persistently higher (P<0.001) CBF values during ischemia in all reperfusion groups, associated with higher (P<0.05) ADC values. This study demonstrated substantial salvage of mismatch tissue after reperfusion independent of ischemia duration and substantial permanent recovery of initial core pixels with early reperfusion. Severity of CBF reduction during ischemia seems to be the main factor determining tissue fate.

Estimation of forest stand volumes by Landsat TM imagery and stand-level field-inventory data

Strategic decisions concerning the timing and location of forest operations require accurate information about the state of forests. In regional forest management, it is essential that the inventory data cover all the forests of the area. A field inventory is expensive, and due to cost-effectiveness the forests not under active management are omitted, which tends to create gaps. Some inventory data for these gaps can potentially be obtained by remote sensing. In this study, forest stand parameters were estimated by means of Landsat TM images and existing stand-level forest data of neighbouring area. The non-parametric k-nearest-neighbour ( kNN) estimator was employed in the estimation of stand volumes by tree species. Different spectral features were tested in the estimation, and the average values of the pixels within a stand core proved to give the best volume estimates. When the kNN estimator and the average of the stand core pixels were employed in the estimation, the estimation error (RMSE) of the total volume was 48%. The errors for other volume estimates were higher; for spruce the RMSE was 81%, and in the case of the volumes of pine and broad-leaved trees, the RMSEs were over 100%. The estimation results obtained are not accurate enough for forest management purposes at the forest stand level. However, the approximate volume estimates derived by the method can be useful in areas where no other forest information is available.

An evaluation of per-parcel land cover mapping using maximum likelihood class probabilities

The Centre for Ecology and Hydrology (CEH) has recently developed a per-parcel classification procedure which integrates remotely sensed imagery with digital cartography. The aim of the work reported here was to compare the per-parcel approach with a conventional per-pixel classification through examination of maximum likelihood class probabilities. An Airborne Thematic Mapper (ATM) image with 1.25 m spatial resolution was used as a source of training data. The ATM image was spatially degraded to 10 m resolution to form the remotely sensed input and the study area was subdivided into land parcels by manual digitizing. The per-parcel approach extracted raster data from a core area described by a shrunken version of a land parcel, thus eliminating mixed boundary pixels. Boundary pixels were shown to be more variable than the core pixels and their removal improved classification confidence. The per-parcel approach calculated the mean spectral response of the extracted core pixels. This was shown to remove within-parcel variation and improve classification confidence in comparison to per-pixel class allocations. A parcel-based representation was shown to be most appropriate for mapping agricultural land cover in comparison to semi-natural areas, because agricultural landscapes have an inherent parcel structure. When land cover is heterogeneous, as in many semi-natural areas, a per-pixel classification would appear to be more appropriate. A hybrid classifier, which could switch between per-parcel and per-pixel mapping, was suggested as a powerful land cover mapping tool.

細径内視鏡の泌尿器科領域における有用性

To clearly observe the upper urinary tract with a thin flexible endoscope, we developed a novel real-time image processing system. The dark cladding portion in an image guide was electrically painted out by the light which was emitted from each core pixels to obtain bright image. Thirty patients were examined using a thin flexible endoscope with this system at retrograde pyelography or puncture of a renal or prostatic cyst. The endoscope which was 0.75mm in diameter had 3,000 pixels of image fiber. The objectives were clearly seen without the dilation of the ureteral orifice. Finer and more colorful image could be obtained with this system. However, visualization was not clear in 4 patients because of poor manipulation capability at the endoscope tip. There were no complications, such as perforation of the ureter. We concluded that a thin endoscope with the real-time processing system would be useful for the diagnosis or treatment of the upper urinary tract or a cystic disease.

A width-independent algorithm for character skeleton estimation

The paper presents an algorithm for the estimation of skeletons of thick characters. We directly identify the core pixels of the skeleton forming the core skeletal segments based on labeling of the character boundary with some local properties. The core skeletal pixel is defined as the midpoint of a line segment normal to the boundary pixels. These core skeletal segments are extended and joined systematically based on certain global properties resulting in the final skeleton. The algorithm is independent of the width of the character and is capable of yielding a skeleton close to our intuitive notion of character shape. The topological description of the character is constructed more or less as a by-product of the skeletomization process. The description forms the basis for character recognition using syntactic methods. The algorithm is well suited for parallel implementation.