Myocardial late gadolinium enhancement: a head-to-head comparison of motion-corrected balanced steady-state free precession with segmented turbo fast low angle shot.

AIM: To evaluate the image quality and diagnostic agreement with a head-to-head comparison of late gadolinium enhancement (LGE) images acquired by the motion-corrected (MOCO) balanced steady-state free precession (bSSFP) phase sensitivity inversion recovery (PSIR) and conventional segmented fast low angle shot (FLASH) PSIR methods15,16 in a patient cohort with a wide spectrum of cardiovascular disease. MATERIALS AND METHODS: In 59 consecutive patients, signal-to-noise ratios (SNRs), contrast-to-noise ratios (CNRs) of the normal myocardium (NM), LGE, and blood pool (BP) were pair-wise compared between the two different sequences. A further semi-qualitative score system (graded 1 -4) was used to compare the overall image quality (OIQ). The diagnostic agreement of the two techniques were evaluated by both transmural severity and absolutely quantitative size of LGE. RESULTS: The SNRs of the NM, LGE, and BP of MOCO bSSFP were 4.8±3.4, 53.6±35.6 and 43.2±29.3, compared with 3.9±3.6 (p=0.126), 27.7±18.5 (p<0.001) and 24.3±13.4 (p<0.001) of FLASH LGE, respectively. The CNRs of LGE to NM, LGE to BP, and BP to NM were 48.3±33.1 versus 23.8±16.7 (p<0.001), 6.5±21.6 versus 3.8±10.8 (p<0.001), and 38.3±27.2 versus 20.3±10.7 (p=0.448), respectively. The OIQ of MOCO bSSFP was higher than that of segmented FLASH (median 4 versus median 3, p<0.001). For quantification of LGE size, there is good agreement and high correlation (r=0.992, p<0.001) between the two methods. CONCLUSIONS: MOCO bSSFP is a feasible, robust sequence for LGE imaging, especially for patients with arrhythmia and those incapable of breath-holding due to severe heart failure.

The impact of a perceptual and adaptive learning module on transoesophageal echocardiography interpretation by anaesthesiology residents.

BACKGROUND: The role of transoesophageal echocardiography (TOE) in anaesthetic practice is expanding. We evaluated the effect of a TOE perceptual and adaptive learning module (PALM) on first-yr anaesthesiology residents' performance, in diagnosing cardiac pathology by TOE. METHODS: First-yr residents were assigned to a group (n = 12) that used a TOE PALM or a control group that did not (n = 12). Both groups received a TOE pretest that measured their accuracy and response times. The PALM group completed the PALM and a posttest within 30 min and a delayed test six months later. The control group received a delayed test six months after their pretest. Accuracy and fluency (accurate responses within 10 s) were measured. RESULTS: The PALM group had statistically significant improvements for both accuracy and fluency (P < 0.0001) in diagnosing cardiac pathology by TOE. After six months, the PALM group's performance remained significantly higher than their pretest values for accuracy (P = 0.0002, d = 2.7) and fluency (P < 0.0001, d = 2.3). CONCLUSIONS: In this pilot study, exposure to a PALM significantly improved accuracy and fluency in diagnosing TOE cardiac pathology, in a group of first-year anaesthesiology residents. PALMs can significantly improve learning and pattern recognition in medical education.

PCATMIP: enhancing signal intensity in diffusion-weighted magnetic resonance imaging.

Diffusion-weighted MRI studies generally lose signal intensity to physiological motion, which can adversely affect quantification/diagnosis. Averaging over multiple repetitions, often used to improve image quality, does not eliminate the signal loss. In this article, PCATMIP, a combined principal component analysis and temporal maximum intensity projection approach, is developed to address this problem. Data are first acquired for a fixed number of repetitions. Assuming that physiological fluctuations of image intensities locally are likely temporally correlated unlike random noise, a local moving boxcar in the spatial domain is used to reconstruct low-noise images by considering the most relevant principal components in the temporal domain. Subsequently, a temporal maximum intensity projection yields a high signal-intensity image. Numerical and experimental studies were performed for validation and to determine optimal parameters for increasing signal intensity and minimizing noise. Subsequently, a combined principal component analysis and temporal maximum intensity projection approach was used to analyze diffusion-weighted porcine liver MRI scans. In these scans, the variability of apparent diffusion coefficient values among repeated measurements was reduced by 59% relative to averaging, and there was an increase in the signal intensity with higher intensity differences observed at higher b-values. In summary, a combined principal component analysis and temporal maximum intensity projection approach is a postprocessing approach that corrects for bulk motion-induced signal loss and improves apparent diffusion coefficient measurement reproducibility.

Robust water/fat separation in the presence of large field inhomogeneities using a graph cut algorithm.

Water/fat separation is a classical problem for in vivo proton MRI. Although many methods have been proposed to address this problem, robust water/fat separation remains a challenge, especially in the presence of large amplitude of static field inhomogeneities. This problem is challenging because of the nonuniqueness of the solution for an isolated voxel. This paper tackles the problem using a statistically motivated formulation that jointly estimates the complete field map and the entire water/fat images. This formulation results in a difficult optimization problem that is solved effectively using a novel graph cut algorithm, based on an iterative process where all voxels are updated simultaneously. The proposed method has good theoretical properties, as well as an efficient implementation. Simulations and in vivo results are shown to highlight the properties of the proposed method and compare it to previous approaches. Twenty-five cardiac datasets acquired on a short, wide-bore scanner with different slice orientations were used to test the proposed method, which produced robust water/fat separation for these challenging datasets. This paper also shows example applications of the proposed method, such as the characterization of intramyocardial fat.

Approximation, torsion, and amodally-completed surfaces.

Consider a stereoscopic display simulating two rectangular patches, the lower frontoparallel and the upper slanted around the vertical axis. When the two patches are amodally completed and appear as the unoccluded parts of a smooth surface partially hidden by a foreground frontoparallel surface, either real or illusory, their relative slant is underestimated with respect to a baseline condition in which they are perceived as separate rectangles. Slant assimilation was studied in three experiments using with- vs. without-occluder displays and two methods, slant matching and speeded classification of twist direction. In Experiments 1 and 2 we found slant assimilation in with-occluder displays and slant contrast in without-occluder displays. In Experiment 3 we isolated a component of slant assimilation attributable to the mere presence of the occluder. Twist classification performance was impaired even when edge geometry hindered amodal completion, but the performance loss was larger when surface patches were amodally completed. To minimize the required amount of torsion, input fragments are misperceived, indicating that in limiting conditions amodal completion is mediated by approximation rather than interpolation. Slant assimilation decreases as twist angle increases, up to a limit above which the visual system does not support the formation of a smooth amodal surface with torsion.

Joint estimation of water/fat images and field inhomogeneity map.

Water/fat separation in the presence of B 0 field inhomogeneity is a problem of considerable practical importance in MRI. This article describes two complementary methods for estimating the water/fat images and the field inhomogeneity map from Dixon-type acquisitions. One is based on variable projection (VARPRO) and the other on linear prediction (LP). The VARPRO method is very robust and can be used in low signal-to-noise ratio conditions because of its ability to achieve the maximum-likelihood solution. The LP method is computationally more efficient, and is shown to perform well under moderate levels of noise and field inhomogeneity. These methods have been extended to handle multicoil acquisitions by jointly solving the estimation problem for all the coils. Both methods are analyzed and compared and results from several experiments are included to demonstrate their performance.

A network flow method for improved MR field map estimation in the presence of water and fat.

Field map estimation is an important problem in MRI, with applications such as water/fat separation and correction of fast acquisitions. However, it constitutes a nonlinear and severely ill-posed problem requiring regularization. In this paper, we introduce an improved method for regularized field map estimation, based on a statistically motivated formulation, as well as a novel algorithm for the solution of the corresponding optimization problem using a network flow approach. The proposed method provides theoretical guarantees (local optimality with respect to a large move), as well as an efficient implementation. It has been applied to the water/fat separation problem and tested on a number of challenging datasets, showing high-quality results.

Extended coverage first-pass perfusion imaging using slice-interleaved TSENSE.

Parallel imaging applied to first-pass, contrast-enhanced cardiac MR can yield greater spatial coverage for a fixed temporal resolution. The method combines rate R=2 acceleration using TSENSE with shot-to-shot interleaving of two slices. The square root R SNR loss is largely compensated for by a longer effective repetition time (TR) and increased flip angle associated with slice interleaving. In this manner, increased spatial coverage is achieved while comparable or better image quality is maintained. Single-heartbeat temporal resolution was accomplished with spatial coverage of eight slices at heart rates up to 71 bpm, six slices up to 95 bpm, and four slices up to 143 bpm. Experiments in normal subjects (N=6) were performed to assess signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) values.

Fluctuation analysis of mitochondrial NADH fluorescence signals in confocal and two-photon microscopy images of living cardiac myocytes.

A fluctuation analysis was performed on the reduced nicotine adenine dinucleotide (NADH) fluorescence signal from resting rabbit myocytes using confocal and two-photon microscopy. The purpose of this study was to establish whether any co-ordinated biochemical processes, such as binding, metabolism and inner mitochondrial membrane potential, were contributing to NADH signal fluctuations above background instrument noise. After a basic characterization of the instrument noise, time series of cellular NADH fluorescence images were collected and compared with an internal standard composed of NADH in the bathing medium. The coefficient of variation as a function of mean signal amplitude of cellular NADH fluorescence and bathing media NADH was identical even as a function of temperature. These data suggest that the fluctuations in cellular NADH fluorescence in resting myocytes are dominated by sampling noise of these instruments and not significantly modified by biological processes. Further analysis revealed no significant spatial correlations within the cell, and Fourier analysis revealed no coherent frequency information. These data suggest that the impact of biochemical processes, which might affect cellular NADH fluorescence emission, are either too small in magnitude, occurring in the wrong temporal scale or too highly spatially localized for detection using these standard optical microscopy approaches.

Interpolation processes in the visual perception of objects.

Visual perception of objects depends on segmentation and grouping processes that act on fragmentary input. This paper gives a brief overview of these processes. A simple geometry accounting for contour interpolation is described, and its applications to 2D, 3D, and spatiotemporal object interpolation processes are considered. A method is described for distinguishing interpolation based on this geometry from more global or top-down influences. Results suggest a separation between interpolation based on relatively local stimulus relations, which give rise to precise boundary representations, and processes involving recognition from partial information, which do not. Aspects of the model-especially the unified treatment of illusory and occluded objects-raise questions about the nature of seeing. Although it is often believed that illusory objects are perceived, while occluded objects are inferred, I suggest that both research and theory converge in supporting a more unified account. Illusory and occluded contours and surfaces do not divide into the real, the perceived, and the inferred, but are all represented, and in key respects, derive from identical perceptual processes.

Ghost artifact cancellation using phased array processing.

In this article, a method for phased array combining is formulated which may be used to cancel ghosts caused by a variety of distortion mechanisms, including space variant distortions such as local flow or off-resonance. This method is based on a constrained optimization, which optimizes SNR subject to the constraint of nulling ghost artifacts at known locations. The resultant technique is similar to the method known as sensitivity encoding (SENSE) used for accelerated imaging; however, in this formulation it is applied to full field-of-view (FOV) images. The method is applied to multishot EPI with noninterleaved phase encode acquisition. A number of benefits, as compared to the conventional interleaved approach, are reduced distortion due to off-resonance, in-plane flow, and EPI delay misalignment, as well as eliminating the need for echo-shifting. Experimental results demonstrate the cancellation for both phantom as well as cardiac imaging examples.

Adaptive sensitivity encoding incorporating temporal filtering (TSENSE).

A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase encodes. The UNFOLD technique is based on time interleaving of k-space lines in sequential images and exploits the property that the outer portion of the field-of-view is relatively static. The differences in spatial sensitivity of multiple receiver coils may be exploited using SENSE or SMASH techniques to eliminate the aliased component that results from undersampling k-space. In this article, an adaptive method of sensitivity encoding is presented which incorporates both spatial and temporal filtering. Temporal filtering and spatial encoding may be combined by acquiring phase encodes in an interleaved manner. In this way the aliased components are alternating phase. The SENSE formulation is not altered by the phase of the alias artifact; however, for imperfect estimates of coil sensitivities the residual artifact will have alternating phase using this approach. This is the essence of combining temporal filtering (UNFOLD) with spatial sensitivity encoding (SENSE). Any residual artifact will be temporally frequency-shifted to the band edge and thus may be further suppressed by temporal low-pass filtering. By combining both temporal and spatial filtering a high degree of alias artifact rejection may be achieved with less stringent requirements on accuracy of coil sensitivity estimates and temporal low-pass filter selectivity than would be required using each method individually. Experimental results that demonstrate the adaptive spatiotemporal filtering method (adaptive TSENSE) with acceleration factor R = 2, for real-time nonbreath-held cardiac MR imaging during exercise induced stress are presented.

Separating processes in object perception.

In this article, I consider research by Needham and colleagues examining the role of object knowledge on infant's segregation of scenes into objects. I suggest that research in this area would benefit from closer connections to computational, psychophysical, and neurophysiological research on adult perceptual segmentation and grouping. I sketch a framework for understanding the components of object perception and apply it to the paradigm and displays used by Needham. This analysis suggests two ideas. First, it would be valuable to demonstrate the role of object knowledge in cases that are less impoverished in terms of perceptual information for segregation and more typical of object arrangements in ordinary scenes. Second, some method is needed to distinguish whether infants' object knowledge affects perceptual organization of new scenes or produces specific beliefs, inferences, or expectations about particular objects and scenes. As a specific example of the benefits of connecting developmental and adult research, some recent research in adult perception is described. The research indicates that in adult object segregation, two types of processes may be distinguished: basic perceptual processes of object segregation and more cognitive processes involving recognition. I suggest that Needham's research may be revealing the developmental origins of the latter processes.

Low-latency temporal filter design for real-time MRI using UNFOLD.

To improve real-time control of interventional procedures such as guidance of catheters, monitoring of ablation therapy, or control of dosage during drug delivery, the image acquisition and reconstruction must be high speed and have low latency (small time delay) in processing. A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase-encodes. A design and implementation of the UNFOLD method which achieves the desired low latency with a recursive temporal filter is presented. The recursive filter design is characterized for this application and compared with more commonly used moving average filters. Experimental results demonstrate low-latency UNFOLD for two applications: 1) high-speed, real-time imaging of the heart to be used in conjunction with cardiac interventional procedures; and 2) the injection of drugs into muscle tissue with contrast enhancement, i.e., monitoring needle insertion and injection of a drug with contrast enhancement properties. Proof-of-concept was demonstrated by injecting a contrast agent. In both applications the UNFOLD technique was used to double the frame rate.

Surface integration influences depth discrimination.

Image fragments arising from partial occlusion may be perceptually unified by a surface integration process on the basis of similar color or texture. In a new objective measure pitting surface feature similarity against binocular disparity, observers discriminated whether a colored circle had either crossed or uncrossed disparity relative to a surrounding gray rectangle. Sensitivity to disparity was impaired only when (1) the configuration of the other surface fragments in the display supported the integration of a surface behind the rectangle and circle, and (2) matched the color of the central circle. Results were consistent with the hypothesis that a surface integration process integrated similarly-colored surface fragments into a smooth surface, even when those fragments were at different depths. Surface integration caused small and reliable effects on depth perception despite unambiguous disparity information. Perceived depth does not depend solely upon disparity, and may be determined after three-dimensional figural unity is established.

The dynamic specification of surfaces and boundaries.

Sequential changes in small separated texture elements can produce perception of a moving form with continuous boundaries. This process of spatiotemporal boundary formation may exist to provide a robust means of detecting moving objects that occlude more distant textured surfaces. Whereas most research on spatiotemporal boundary formation has been focused on boundary and shape perception, two experiments are reported here on the perception of surface qualities in spatiotemporal boundary formation. In experiment 1 a free-report procedure was used to investigate whether surface perception can be determined by dynamic information alone, apart from static spatial differences. Results showed that dynamic information was sufficient to determine the appearance of a surface. This dynamic information may play an important role in other aspects of perception. In experiment 2, it was shown that dynamically specifying an extended, opaque surface facilitated edge perception. Implications for the relation of boundary and surface perception and for theories of perceptual transparency are discussed.

Interactions between spatial and spatiotemporal information in spatiotemporal boundary formation.

The surface and boundaries of an object generally move in unison, so the motion of a surface could provide information about the motion of its boundaries. Here we report the results of three experiments on spatiotemporal boundary formation that indicate that information about the motion of a surface does influence the formation of its boundaries. In Experiment 1, shape identification at low texture densities was poorer for moving forms in which stationary texture was visible inside than for forms in which the stationary texture was visible only outside. In Experiment 2, the disruption found in Experiment 1 was removed by adding a second external boundary. We hypothesized that the disruption was caused by boundary assignment that perceptually grouped the moving boundary with the static texture. Experiment 3 revealed that accurate information about the motion of the surface facilitated boundary formation only when the motion was seen as coming from the surface of the moving form. Potential mechanisms for surface motion effects in dynamic boundary formation are discussed.

A common mechanism for illusory and occluded object completion.

New phenomena and results are reported that implicate a common contour interpolation mechanism in illusory and occluded (modal and amodal) object completion. In 3 experiments, a speeded classification task was used to study novel quasimodal displays in which occluded and illusory contours join. Results showed the same advantages in speed and accuracy over control displays for quasimodal, illusory, and occluded displays. The implications of quasimodal displays, along with another new display type in which contour linkages must precede determination of modal or amodal appearance, are considered. These logical considerations and empirical results suggest that amodal and modal completion depend on a common underlying mechanism that connects edges across gaps.

Spatio-temporal boundary formation: the role of local motion signals in boundary perception.

Spatio-temporal boundary formation (SBF) refers to a perceptual process responsible for perception of moving, bounded surfaces from sequential changes in spatially separated local elements. Previous research has indicated that this process produces perception of global form, continuous boundaries and global motion from spatially and temporally sparse element changes. In the present paper, we sought to distinguish between two classes of models for SBF: form-precedes-motion and motion-precedes-form models. Experiment 1 tested the effects of the addition of spurious motion signals, a manipulation that should affect a motion-precedes-form computation but not a form-precedes-motion computation. Shape identification in a 10-alternative forced-choice procedure was disrupted by this manipulation, supporting the former class of models. A particular computational scheme, edge orientation from motion (EOFM) instantiating a motion-precedes-form model is described and tested in Experiment 2. The EOFM model should be disrupted when initiating element changes occur in a certain type of sequential order, relative to randomly arranged changes. Sequential changes markedly disrupted performance, supporting this EOFM approach. The results favor motion-precedes-form models of SBF and are consistent with the particular computational scheme proposed.

Surface completion complements boundary interpolation in the visual integration of partly occluded objects.

Previous research on perceptual completion has emphasized how the spatial relationships of edges influence the visual integration of the image fragments that result from partial occlusion. We report studies testing the hypothesis that the similarity of surface features also influences visual integration, complementing edge interpolation processes. Using displays that separated edge interpolation processes from surface-feature interpolation processes, we tested the hypotheses that a surface completion process integrates image fragments with similar surface features, and that surface completion is constrained by amodally interpolated and amodally extended boundaries. Both edge relatability and surface-feature similarity were manipulated in a series of paired-comparison and classification tasks. The results of these studies supported the hypotheses and were extended to surface features of colors, textures, and color gradients. Results also suggest that, under certain conditions, surface completion may interact with and influence edge interpolation.