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R T Constable

Publications and source records attributed to R T Constable.

At least 55 records · Page 3Linked to original sources
PubMed1994

Development and evaluation of tracking algorithms for cardiac wall motion analysis using phase velocity MR imaging.

Phase velocity magnetic resonance imaging (MRI) has shown considerable potential for tracking distinct regions of the myocardium throughout the cardiac cycle. Phase contrast MR imaging produces multiple images, each phase encoded for velocity in a different direction, in which individual pixels depict the local motion of the tissue. In this work we present in detail three algorithms for tracking motion based on these images. Both simulated and phantom data are used to examine some of the problems encountered in practice in tracking points based on velocity maps. Solutions to these problems are offered when possible. The impact of noise and low order phase errors in the data on each of the three tracking approaches is examined. In addition, problems due to tissue expansion and contraction, to 2D versus 3D tracking, and to round off errors from motion which is small relative to pixel size or slice thickness, are considered. An example using data obtained in vivo is included to demonstrate the efficacy of the best of the three tracking algorithms in measuring left ventricular circumferential shortening preinfarct and postinfarct in a canine model.

Algorithms↗
PubMed1994

Gradient moment nulling in fast spin echo.

The fast spin echo sequence combines data from many echo signals in a Carr-Purcell-Meiboom-Gill echo train to form a single image. Much of the signal in the second and later echoes results from the coherent addition of stimulated echo signal components back to the spin echo signal. Because stimulated echoes experience no dephasing effects during the time that they are stored as Mz magnetization, they experience a different gradient first moment than does the spin echo. This leads to flow-related phase differences between different echo components and results in flow voids and ghosting, even when the first moment is nulled for the spin echo signal. A method of gradient moment nulling that correctly compensates both spin echo and stimulated echo components has been developed. The simplest solution involves nulling the first gradient moment at least at the RF pulses and preferably at both the RF pulses and the echoes. Phantom and volunteer studies demonstrate good suppression of flow-related artifacts.

Artifacts↗
PubMed1994

MR of the head and neck: comparison of fast spin-echo and conventional spin-echo sequences.

PURPOSE: To compare conspicuousness of head and neck lesions on fast spin-echo sequences and conventional spin-echo sequences. METHODS: Forty consecutive patients with 61 head and neck lesions were evaluated. Lesion conspicuousness was qualitatively compared on conventional spin-echo and fast spin-echo sequences, using both spin-density and T2-weighted images. Thirty-six lesions had surgical or pathologic confirmation, and 25 were assigned a presumptive diagnosis based on clinical evaluation and imaging findings seen on conventional spin-echo T1- and T2-weighted sequences. Forty lesions were related to neoplasms; 21 lesions consisted of infectious, vascular, or inflammatory abnormalities. RESULTS: Fast spin-echo sequences provided improved lesion conspicuousness in 91% of spin-density images, in 77% of T2-weighted images, and in 84% of the combined spin-density and T2-weighted images. CONCLUSION: By providing shorter imaging times and equal or superior lesion conspicuousness, long-repetition-time fast spin-echo sequences can replace long-repetition-time conventional spin-echo sequences in evaluation of the head and neck.

Adipose Tissue↗
PubMed1993

Functional brain imaging at 1.5 T using conventional gradient echo MR imaging techniques.

There is considerable interest in the use of MR imaging to study brain function. Recently, it has been demonstrated that small changes in signal intensity occur in the visual cortex in T2*-weighted imaging in response to appropriate visual stimuli. Similar responses to activation have also been recorded in motor cortex as well as frontal lobes. To date most of these studies have been carried out at very high field strength or they have used echo planar imaging. We report our preliminary results showing that the effects of activation of visual and motor areas of the brains of normal volunteers can be recorded using conventional MR imaging methods on a standard 1.5 T clinical scanner. Using gradient-echo imaging sequences, we have been able to map activated visual and motor cortex with high spatial resolution in multiple planes, and are using this technique to examine the relationship between physiological response and stimulus parameters. Signal changes of the order of 2-12% in images with TE = 45 msec, TR = 120 msec, and alpha = 40 degrees, permit excellent depiction of the regions affected.

Brain↗
PubMed1993

Inversion-recovery fast spin-echo MR imaging: efficacy in the evaluation of head and neck lesions.

To compare the efficacies of fast spin-echo (FSE) and inversion-recovery FSE (IRFSE) magnetic resonance (MR) imaging in evaluating head and neck disorders, the authors evaluated 46 lesions in 23 consecutive patients. Twenty-seven lesions were related to neoplasms; 19 lesions resulted from infectious, allergic, or radiation-induced inflammation. Conventional T1-weighted, FSE, and IRFSE images were obtained in all patients. The FSE and IRFSE images were qualitatively compared in an unblinded manner for conspicuity of lesion margins and extent. IRFSE imaging improved conspicuity of 22 lesions (48%) and showed equal conspicuity of 18 (39%). IRFSE imaging proved most useful for small lesions with long T2 relaxation times that were surrounded by fat. IRFSE imaging improved visibility of small optic nerve gliomas, salivary gland inflammation, peripheral nerve tumors, and small lymph nodes. Early changes secondary to spread of tumor across fascial planes were also well visualized with IRFSE sequences. In six lesions (13%) that did not have long T2 relaxation times, the FSE images provided better conspicuity. The authors conclude that by improving conspicuity of small lesions adjacent to or surrounded by fat, IRFSE sequences can supplement FSE sequences in imaging the head and neck.

Adolescent↗
PubMed1992

The loss of small objects in variable TE imaging: implications for FSE, RARE, and EPI.

The importance to MR image quality of the order of acquisition of different phase-encoded views with sequences that have variable TR and TE has been recently reported. It has been shown that the effective point spread function (PSF) may be manipulated by varying TE or TR, or both, with each phase-encoding step. This paper explores the behavior of the PSF in a variable TE sequence and its dependence on both imaging and tissue parameters. It is shown that the PSF is different for each tissue type and that its effect on tissue contrast is a function of both the shape and size of the structure. The important problem of signal loss from small objects that arises when the effective PSF is broad and the difficulty in detecting this phenomenon in practical MR images is illustrated. It is shown that the PSF can produce significant blurring and loss of object contrast in fast spin-echo images but that this blurring may be not be obvious in practice because the noise is unaffected by the PSF. It is also shown that the signal from small lesions with short T2 can easily be lost through this blurring mechanism. The importance of signal loss from small objects and its implication for the clinical use of such sequences as fast spin-echo or rapid acquisition relaxation-enhanced and echo planar imaging is stressed.

Humans↗
PubMed1992

Factors influencing contrast in fast spin-echo MR imaging.

Multi-echo pulse sequences for producing T2-weighted images in much reduced imaging times have recently been developed for routine clinical use. A number of recent articles have described the contrast obtained with fast spin-echo (FSE) sequences and have generally indicated that they depict tissues very similarly to conventional spin-echo (SE) imaging. There are, however, some important differences in contrast between some tissues in FSE images. This work presents a detailed study of the contrast obtained with FSE imaging sequences and examines the image sequence and tissue parameters which influence contrast. The use of multiple refocusing pulses produces several subtle effects not seen in conventional SE imaging sequences, and in this study the precise nature and extent of such effects are described. The relative contributions to image contrast of magnetization transfer, the decoupling of J-modulation effects, the production of stimulated echoes and direct saturation effects, of diffusion and of the effects of the differential attenuation of different spatial frequencies, are each quantified. The mechanisms responsible for the brighter fat signal seen in FSE images, as well as the loss of signal from some other tissues, are explained. Computer simulations, phantom experiments, and clinical images are all used to support the conclusions.

Brain↗
PubMed1992

Multicoil high-resolution fast spin-echo MR imaging of the female pelvis.

A fast spin-echo pulse sequence was combined with multiple surface coils used simultaneously in the form of a "multicoil" in magnetic resonance imaging studies of the female pelvis. This combination allowed maximal resolution with maintenance of the signal-to-noise ratio (S/N) at an acceptable level, and the S/N with the multicoil system was substantially better than that achieved with a body coil. Excellent image quality and demonstration of anatomic detail were afforded by use of this technique.

Adolescent↗
PubMed1991

Data extrapolation for truncation artifact removal.

Clinicians typically obtain high-resolution clinical MR images in an effort to avoid the truncation artifacts that often arise in Fourier transform reconstruction of limited data. A method for reducing these artifacts in MR images, at the reconstruction stage, would allow for reduced imaging times, through the collection of fewer phase encode steps and increased signal-to-noise ratios, through increased pixel size. The approach to reducing truncation artifacts in MR images is developed and a simple algorithm is presented which significantly reduces truncation artifacts in images with as few as 96 phase encode steps. The algorithm is compared with a more sophisticated method of reconstructing truncation-free images and is shown to be equivalently effective. Three clinical examples are shown illustrating the success of the method.

Algorithms↗
PubMed1991

A quantitative comparison of the TERA modeling and DFT magnetic resonance image reconstruction techniques.

The resolution of magnetic resonance images reconstructed using the discrete Fourier transform (DFT) algorithm is limited by the effective window generated by the finite data length. The transient error reconstruction approach (TERA) is an alternative reconstruction method based on autoregressive moving average (ARMA) modeling techniques. Quantitative measurements comparing the truncation artifacts present during DFT and TERA image reconstruction show that the modeling method substantially reduces these artifacts on "full" (256 X 256), "truncated" (256 X 192), and "severely truncated" (256 X 128) data sets without introducing the global amplitude distortion found in other modeling techniques. Two global measures for determining the success of modeling are suggested. Problem areas for one-dimensional modeling are examined and reasons for considering two-dimensional modeling discussed. Analysis of both medical and phantom data reconstructions are presented.

Algorithms↗
PubMed1990

Why MEM does not work in MR image reconstruction.

This paper discusses the theory and application of the Maximum Entropy Method (MEM) to the reconstruction of Magnetic Resonance (MR) images. It is shown that the MEM is inappropriate for MR image reconstruction and that the usual heuristic justification is invalid in this case. The application of the MEM in MR image reconstruction is characterized as merely one of many constrained regularization approaches.

Fourier Analysis↗
PubMedDate not supplied

Coupled-spin fast spin-echo MR imaging.

Distinguishing between lipid and water-containing tissues is clinically important. Current techniques rely on the chemical shift difference between fat and water resonances or differences in relaxation times of the tissues, or a combination of both. A method is presented for separating the signals of lipid protons from those of water protons by using fast spin-echo magnetic resonance imaging based on the principle that lipid protons behave differently from water protons in multi-echo sequences. Two images are acquired with different echo train lengths and echo spacing but with identical TEs, and then subtracted to exploit differences in the behavior of lipid and water protons in multi-echo sequences. The method is insensitive to B0 inhomogeneities or susceptibility effects and provides separate lipid and water images with a high signal-to-noise ratio. The advantages of the method are demonstrated with phantom studies and clinical examples.

Female↗
PubMedDate not supplied

Functional MR imaging using gradient-echo echo-planar imaging in the presence of large static field inhomogeneities.

At 1.5 T, the field strength of most clinical MR imagers, gradient-echo imaging is the primary imaging method for measuring brain activation, as such sequences are highly sensitive to changes in blood oxygenation or T2* effects. Unfortunately, gradient-echo sequences are also extremely sensitive to magnetic field inhomogeneities, and this sensitivity has precluded examination of regions of cortex near field inhomogeneities with functional MR imaging. This article presents a gradient-echo echo-planar imaging method that uses variable amplitude scaling on the slice-select refocusing lobe to generate images compensated for static field inhomogeneities. A technique for constructing composite images to be used in statistical tests for activation is also presented. The method is shown to produce clean activation maps in the presence of large static field inhomogeneities. The technique retains the sensitivity of gradient-echo imaging to changes in blood oxygenation while removing the sensitivity to large static field inhomogeneities.

Algorithms↗
PubMedDate not supplied

Motion artifact in T2-weighted fast spin-echo images of the liver: effect on image contrast and reduction of artifact using respiratory triggering in normal volunteers.

The purpose of our study was to evaluate the effect of respiratory motion on the image contrast of T2-weighted fast spin-echo (FSE) images of the liver as well as the reduction of motion artifact using respiratory triggering of the data acquisition. We imaged the livers of 10 healthy volunteers using a fast spin-echo T2-weighted sequence. Images were obtained both without and with patient triggering. Triggered images were acquired in a segmented fashion during multiple sequential breath-holds using an echo train of 8 or 16, both with and without flow compensation (gradient moment nulling). Ratios of signal difference to noise (SD/N) of the liver and gallbladder as well as the liver and spleen were compared for all sequences. All of the triggered images showed statistically significant improvement of SD/N for the liver and gallbladder as well as for the liver and spleen when compared with the nontriggered images. Triggered images obtained with an echo train length of 8 and, with flow compensation, showed the highest SD/N ratios. In one volunteer whose liver contained multiple small cysts, the triggered images showed improved visualization of individual cysts and identified a larger number of cysts. Respiratory motion causes a significant loss of contrast on T2-weighted fast spin-echo images of the liver. This can be reduced by using a segmented data acquisition triggered by the respiratory cycle obtained during sequential breath-holds.

Adult↗
PubMedDate not supplied

Activation of human prefrontal cortex during spatial and nonspatial working memory tasks measured by functional MRI.

Separate working memory domains for spatial location, and for objects, faces, and patterns, have been identified in the prefrontal cortex (PFC) of nonhuman primates. We have used functional magnetic resonance imaging to examine whether spatial and nonspatial visual working memory processes are similarly dissociable in human PFC. Subjects performed tasks which required them to remember either the location or shape of successive visual stimuli. We found that the mnemonic component of the working memory tasks affected the hemispheric pattern of PFC activation. The spatial (LOCATION) working memory task preferentially activated the middle frontal gyrus (MFG) in the right hemisphere, while the nonspatial (SHAPE) working memory task activated the MFG in both hemispheres. Furthermore, the area of activation in the left hemisphere extended into the inferior frontal gyrus for nonspatial SHAPE task. A perceptual target (DOT) detection task also activated the MFG bilaterally, but at a level approximately half that of the working memory tasks. The activation in the MFG occurred within 3-6 s of task onset and declined following task offset. Time-course analysis revealed a different pattern for cingulate gyrus, in which activation occurred upon task completion. Cingulate gyrus activation was greatest following the SHAPE task and was greater in the left hemisphere. The present results support the prominent role of the PFC and, specifically, the MFG in working memory, and indicate that the mnemonic content of the task affects the relative weighting of hemispheric activation.

Adult↗
PubMedDate not supplied

High quality zoomed MR images.

A zooming technique based on zero filling of the Fourier space is presented for high quality magnification of magnetic resonance magnitude images. Comparison with conventional linear interpolation methods on two clinical examples indicates that Fourier magnification is preferable because it avoids image artifacts and provides superior image quality. It is recommended that this technique become the standard method of magnification on all imagers.

Fourier Analysis↗
PubMedDate not supplied

Contrast, resolution, and detectability in MR imaging.

With the introduction of fast scan techniques and high field imagers, the ability to achieve very high resolution MR images in reasonable imaging times is now possible. Increased resolution allows for better detection of small, high contrast pathological features, but at some cost. Increasing resolution leads to a nonrecoverable decrease in signal-to-noise ratio per pixel and a loss of low contrast detectability for constant imaging time. This article examines the tradeoffs between image resolution, signal-to-noise ratio, and low contrast detectability in MR imaging. Contrast detail curves are presented for images collected in a constant imaging time, with constant field of view and bandwidth but at different resolutions, and these are compared with theoretical curves. The problem of measuring contrast levels in magnitude images, with different resolutions and receiver attenuation values, is discussed and a definition that accommodates these parameters developed. In addition, a clinical example is shown demonstrating a decrease in soft tissue differentiation with increasing resolution, again for fixed imaging time. The results indicate that moving to high resolution imaging matrices requires consideration be given to the sacrifice in low contrast detectability that occurs. Most importantly, it is shown that filtering a high resolution image to a lower resolution image, through nearest neighbor averaging, does not regain the detectability lost in initially collecting the high resolution image.

Brain↗