Visual demonstration of aliasing in planar nuclear medicine imaging: The importance of correct collimator selection by nuclear medicine practitioners

Aliasing artefact is an imaging distortion phenomenon experienced in a wide variety of medical imaging modalities. This case report illustrates its occurrence during planar gamma camera nuclear medicine imaging under non-clinical conditions using experimental incorrect selection of collimators. In accordance with provision of an optimal service, nuclear medicine practitioners are recommended to have sufficient technical expertise along with knowledge of gamma camera operation. The purpose, construction and interaction of collimators used during planar imaging are presented herein with specific regards to the aliasing phenomenon. Furthermore, this case report recommends the careful planning of worklists to avoid frequent collimator changes to reduce the risk of human error.     

Isolating physiologic noise sources with independently determined spatial measures

To properly account for the presence of physiologic noise in fMRI data, parallel measurement of pulse and respiratory data is necessary. In some cases, this parallel measurement is difficult or impossible due to the experimental paradigm or lack of available monitoring equipment. We present a robust method for determining the direct-sampled pulse and respiratory data for a subject from the fMRI data itself, utilizing an independently determined spatial weighting matrix. It is shown that temporal independent component analysis can reliably separate the spatial and temporal patterns of physiologic noise through correlation if the parallel measurement is made. The spatial patterns thus determined can be applied to a separate scan of the same subject to produce the temporal pattern specific to this independent scan. The robustness of this method leads to the more general method of creating spatial weight matrices in standard brain space averaged over multiple subjects in order to acquire the physiologic signals without the necessity of any (further) parallel measurements. The resulting cardiac and respiratory estimators can effectively be used in a manner similar to that of a direct-sampled physiologic signal, e.g., direct input to retrospective correction methods, evaluation of cardiac and respiratory effects of tasks, etc. Spatial mixing matrices for estimating cardiac and respiratory sources for the acquisition protocols described here (and others as they are developed) are offered to investigators and can be obtained through e-mail from the corresponding author.     

Quantitative analysis of spatial sampling error in the infant and adult electroencephalogram

The purpose of this report was to determine the required number of electrodes to record the infant and adult electroencephalogram (EEG) with a specified amount of spatial sampling error. We first developed mathematical theory that governs the spatial sampling of EEG data distributed on a spherical approximation to the scalp. We then used a concentric sphere model of current flow in the head to simulate realistic EEG data. Quantitative spatial sampling error was calculated for the simulated EEG, with additive measurement noise, for 64, 128, and 256 electrodes equally spaced over the surface of the sphere corresponding to the coverage of the human scalp by commercially available "geodesic" electrode arrays. We found the sampling error for the infant to be larger than that for the adult. For example, a sampling error of less than 10% for the adult was obtained with a 64-electrode array but a 256-electrode array was needed for the infant to achieve the same level of error. With the addition of measurement noise, with power 10 times less than that of the EEG, the sampling error increased to 25% for both the infant and adult, for these numbers of electrodes. These results show that accurate measurement of the spatial properties of the infant EEG requires more electrodes than for the adult.     

Quality of the primate photoreceptor lattice and limits of spatial vision

Quantitative analysis of a primate photoreceptor lattice shows that the foveal lattice is a highly regular hexagonal structure with a positional correlation length of at least 130 photoreceptors. This result indicates that the photoreceptor lattice is not sufficiently disordered to prevent aliasing in the fovea. but rather could provide the metric with which the visual system determines spatial separation even for tasks involving hyperacuity.     

An image space approach to Cartesian based parallel MR imaging with total variation regularization

The Cartesian parallel magnetic imaging problem is formulated variationally using a high-order penalty for coil sensitivities and a total variation like penalty for the reconstructed image. Then the optimality system is derived and numerically discretized. The objective function used is non-convex, but it possesses a bilinear structure that allows the ambiguity among solutions to be resolved technically by regularization and practically by normalizing a pre-estimated norm of the reconstructed image. Since the objective function is convex in each single argument, convex analysis is used to formulate the optimality condition for the image in terms of a primal-dual system. To solve the optimality system, a nonlinear Gauss-Seidel outer iteration is used in which the objective function is minimized with respect to one variable after the other using an inner generalized Newton iteration. Computational results for in vivo MR imaging data show that a significant improvement in reconstruction quality can be obtained by using the proposed regularization methods in relation to alternative approaches.     

A review of electrocardiogram filtering

Analog filtering and digital signal processing algorithms in the preprocessing modules of an electrocardiographic device play a pivotal role in providing high-quality electrocardiogram (ECG) signals for analysis, interpretation, and presentation (display, printout, and storage). In this article, issues relating to inaccuracy of ECG preprocessing filters are investigated in the context of facilitating efficient ECG interpretation and diagnosis. The discussion covers 4 specific ECG preprocessing applications: anti-aliasing and upper-frequency cutoff, baseline wander suppression and lower-frequency cutoff, line frequency rejection, and muscle artifact reduction. Issues discussed include linear phase, aliasing, distortion, ringing, and attenuation of desired ECG signals. Due to the overlapping power spectrum of signal and noise in acquired ECG data, frequency selective filters must seek a delicate balance between noise removal and deformation of the desired signal. Most importantly, the filtering output should not adversely impact subsequent diagnosis and interpretation. Based on these discussions, several suggestions are made to improve and update existing ECG data preprocessing standards and guidelines.     

Elimination of errors caused by first-order aliasing in velocity encoded cine-MR measurements of postoperative jets after aortic coarctation: in vitro and in vivo validation

The aim of this study was to evaluate a velocity-encoded cine-MR (VEC-MR) sequence in measuring flow velocities up to two times the velocity encoding value (VENC) in a flow phantom and to validate the method for assessing poststenotic jet velocities in postoperative patients after aortic coarctation. In vitro, a flow phantom was used (0.5T; TR/TE: 51/8 ms, flip angle=30 degrees, FOV=280 mm, 128x256 matrix VENC 40 or 80 cm/s). On binary images, maximum flow velocities (V(max)) were calculated with a region of interest (ROI, 8 pixels). With aliasing, V(max) was calculated by VENC+(V(aliasing)). In vivo, 16 postoperative patients after aortic coarctation underwent double-oblique VEC-MR imaging through the aortic arch (ECG triggering, 16 phases/RR, TR=600-800 ms, flow-encoding cranio-caudal, VENC=2 m/s). Peak systolic velocities were measured and transthoracic Doppler echocardiography (TTDE) was performed. In vitro, there were excellent correlations for MR velocity measurements with and without aliasing ( r=0.99) and for true and MR-derived flow velocities ( r=0.99). In vivo, there was good correlation between VEC-MR and TTDE-assessed V(max) values in the aorta at the former coarctation site ( r=0.90, n=16). Aliasing occurred in 13 patients. VEC-MR is a useful modality for assessing jet velocities in the follow-up of patients after aortic coarctation. Despite of aliasing, accurate velocity measurements up to two times VENC are possible using binary images.     

Effect of sampling array irregularity and window size on the discrimination of sampled gratings

The effect of sampling irregularity and window size on orientation discrimination was investigated using discretely sampled gratings as stimuli. For regular sampling arrays, visual performance could be accounted for by a theoretical analysis of aliasing produced by undersampling. For irregular arrays produced by adding noise to the location of individual samples, the incidence of perceived orientation reversal declined and the spatial frequency range of flawless performance expanded well beyond the nominal Nyquist frequency. These results provide a psychophysical method to estimate the spatial density and the degree of irregularity in the neural sampling arrays that limit human visual resolution.