Biomedical subjects

C H Meyer

Publications and source records attributed to C H Meyer.

At least 55 records · Page 3Linked to original sources

PubMed2000

[Correlation of morphologic changes between optical coherence tomography and topographic angiography in a case of gyrate atrophy].

PURPOSE: To characterize ultrastrructual changes in atrophic disease of the retina, RPE and choroid as seen with gyrate atrophy using two new diagnostic modalities, optical coherence tomography (OCT) and topographic angiography. PATIENT AND METHOD: OCT images were taken in a patient with pericentral choroidal atrophy using a slit-lamp-adapted OCT system. Ophthalmoscopy, conventional and topographic angiographic findings were correlated to the reflectivity changes as seen on OCT. RESULTS: Areas of chorioretinal atrophy correlated with a loss of reflectivity in the RPE-choriocapillaris complex on OCT. Additionally OCT identified a thinning of the nerve fiber layer. Topographic angiography demonstrated an extensive defect, seen as an area of depression, consistent with a loss of choriocapillaris and larger-sized choroidal vessels. In contrast to conventional angiography, central islands were not found to demonstrate structural intensity, while the midperipheral surrounding area was clearly elevated to physiological levels. CONCLUSION: OCT and topographic angiography provide in vivo insight into morphologic changes within neurosensory retina and choroid caused by pericentral choroidal atrophy.

Choroid↗

PubMed2000

The real-time interactive 3-D-DVA for robust coronary MRA.

A graphical user interface (GUI) has been developed which enables interactive feedback and control to the real-time diminishing variance algorithm (DVA). This interactivity allows the user to set scan parameters, view scan statistics, and view image updates during the course of the scan. In addition, the DVA has been extended to simultaneously reduce motion artifacts in three dimensions using three orthogonal navigators. Preliminary in vivo studies indicate that these improvements to the standard DVA allow for significantly improved consistency and robustness in eliminating respiratory motion artifacts from MR images, particularly when imaging the coronary arteries.

Algorithms↗

PubMed1999

Fast magnetic resonance coronary angiography with a three-dimensional stack of spirals trajectory.

In this work, three-dimensional (3D) spiral imaging has been utilized for magnetic resonance coronary angiography. Spiral-based 3D techniques can dramatically reduce imaging time requirements compared with 3D Fourier Transform imaging. The method developed here utilized a "stack of spirals" trajectory, to traverse 3D k-space rapidly. Both thick-slab volumes encompassing the entire coronary tree with isotropic resolution and thin-slab volumes targeted to a particular vessel of interest were acquired. Respiratory compensation was achieved using the diminishing variance algorithm. T2-prepared contrast was also applied in some cases to improve contrast between vessel and myocardium, while off-resonance blurring was minimized by applying a linear correction to the acquired data. Images from healthy volunteers were displayed using a curved reformatting technique to view long segments of vessel in a single projection. The results demonstrate that this 3D spiral technique is capable of producing high-quality coronary magnetic resonance angiograms.

Algorithms↗

PubMed1999

Prospective MR signal-based cardiac triggering.

A cardiac motion compensation method using magnetic resonance signal-based triggering is presented. The method interlaces a triggering pulse sequence with an imaging sequence. The triggering sequence is designed to measure aortic blood velocity, from which cardiac phase can be inferred. The triggering sequence is executed repeatedly and the acquired data processed after each sequence iteration. When the desired phase of the cardiac cycle is detected, data are acquired using the imaging sequence. A signal-processing unit of a conventional scanner is used to process the triggering data in real time and issue triggering commands. Alternatively, a workstation, with a bus adaptor, can access data as they are acquired, process and display the data, and issue triggering commands. With a graphical user interface, the triggering pulse sequence and data-processing techniques can be modified instantaneously to optimize triggering. The technique is demonstrated with coronary artery imaging using both conventional two-dimensional Fourier transform scans and spiral trajectories.

Aorta, Thoracic↗

PubMed1999

Reduced spatial side lobes in chemical-shift imaging.

Density-weighted k-space sampling with spiral trajectories is used to reduce spatial side lobes in chemical-shift imaging (CSI). In this method, more time is spent collecting data at the center of k space and less time at the edges of k space in order to make the sampling density proportional to a given apodization function, subject to constraints imposed by gradient performance and Nyquist sampling. The efficient k-space coverage of spiral-based trajectories enables good control over the sampling density within practical in vivo scan times. The density-weighted acquisition is compared to a conventional, nonweighted spiral sampling without the application of a window function. For a fixed voxel size and imaging time, the noise variance is observed to be the same for both cases, while spatial side lobes are greatly reduced with the variable-density sampling. This method is demonstrated on a normal volunteer by imaging of brain metabolites at 1.5 T with both single slice CSI and volumetric CSI. Magn Reson Med 42:314-323, 1999.

Brain↗

PubMed1998

3D MR coronary artery segmentation.

Coronary arteries are segmented from the blood pool using mathematical morphology operations from a 3D magnetic resonance spiral acquisition on a continuously breathing healthy volunteer. The segmented volume is maximal intensity projected at different views to yield coronary angiograms showing the left anterior descending artery (LAD), right coronary artery (RCA), and left circumflex artery (LCX). Magnetic resonance coronary angiography provides a retrospective rotating view of the coronary artery tree that complements oblique reformatted sections.

Algorithms↗

PubMed1998

New real-time interactive cardiac magnetic resonance imaging system complements echocardiography.

OBJECTIVES: We conducted an initial clinical trial of a newly developed cardiac magnetic resonance imaging (CMRI) system. We evaluated left ventricular (LV) function in 85 patients to compare the clinical utility of the CMRI system with echocardiography, the current noninvasive gold standard. BACKGROUND: Conventional CMRI systems require cardiac-gating and respiratory compensation to synthesize a single image from data acquired over multiple cardiac cycles. In contrast, the new CMRI system allows continuous real-time dynamic acquisition and display of any scan plane at 16 images/s without the need for cardiac gating or breath-holding. METHODS: A conventional 1.5T Signa MRI Scanner (GE, Milwaukee, Wisconsin) was modified by the addition of an interactive workstation and a bus adapter. The new CMRI system underwent clinical trial by testing its ability to evaluate global and regional LV function. The first group (A) consisted of 31 patients with acceptable echocardiography image quality. The second group (B) consisted of 31 patients with suboptimal echocardiography image quality. The third group (C) consisted of 29 patients with severe lung disease or congenital cardiac malformation who frequently have suboptimal echo study. Two independent observers scored wall motion and image quality using the standard 16-segment model and rank-order analysis. RESULTS: CMRI evaluation was complete in less than 15 min. In group A, no significant difference was found between ECHO and CMRI studies (p = NS). In group B, adequate visualization of wall segments was obtained 38% of the time using ECHO and 97% of the time using CMRI (p < 0.0001). When grouped into coronary segments, adequate visualization of at least one segment occurred in 18 of 30 patients (60%) with ECHO and in all 30 patients (100%) with CMRI (p < 0.0001). In group C, adequate visualization of the wall segments was obtained in 58% (CI 0.53-0.62) of the time using echocardiography and 99.7% (CI 0.99-1.0) of the time using CMRI (p < 0.0001). CONCLUSIONS: The new CMRI system provides clinically reliable evaluation of LV function and complements suboptimal echocardiography. In comparison with the conventional CMRI, the new CMRI system significantly reduces scan time, patient discomfort and associated cost.

Adolescent↗

PubMed1997

Reducing flow artifacts in echo-planar imaging.

Echo-planar imaging (EPI) is very susceptible to flow artifacts. Two ways to improve its flow properties are presented. First, "partial flyback" is proposed to reduce artifacts arising from flow in the readout direction. Near the center of k-space, only the even echoes of the EPI echo-train are used. Partial flyback is shown to improve the readout-flow properties at the expense of a slight worsening of the phase-encode flow and off-resonance properties. We recommend that the flyback region acquire 95% of the energy in k-space. Second, "inside-out" EPI is used to reduce artifacts arising from flow in the phase-encode direction. Data collection begins at the center of k-space, with separate interleaves to acquire the top and bottom, halves of k-space. Partial flyback is combined with partial-Fourier EPI and inside-out EPI. Partial-flyback inside-out EPI has worse off-resonance properties than partial-flyback partial-Fourier EPI but demonstrates better flow properties and does not require partial k-space reconstruction.

Heart↗

PubMed1997

Real-time interactive MRI on a conventional scanner.

A real-time interactive MRI system capable of localizing coronary arteries and imaging arrhythmic hearts in real-time is described. Non-2DFT acquisition strategies such as spiral-interleaf, spiral-ring, and circular echo-planar imaging provide short scan times on a conventional scanner. Real-time gridding reconstruction at 8-20 images/s is achieved by distributing the reconstruction on general-purpose UNIX workstations. An X-windows application provides interactive control. A six-interleaf spiral sequence is used for cardiac imaging and can acquire six images/s. A sliding window reconstruction achieves display rates of 16-20 images/s. This allows cardiac images to be acquired in real-time, with minimal motion and flow artifacts, and without breath holding or cardiac gating. Abdominal images are acquired at over 2.5 images/s with spiral-ring or circular echo-planar sequences. Reconstruction rates are 8-10 images/s. Rapid localization in the abdomen is demonstrated with the spiral-ring acquisition, whereas peristaltic motion in the small bowel is well visualized using the circular echo-planar sequence.

Abdomen↗

PubMed1997

Unilateral pallidotomy for Parkinson's disease promptly improves a wide range of voluntary activities--especially gait and trunk movements.

26 patients with Parkinson's disease were assessed in the OFF state 2-3 days before and one week after pallidotomy for the time to complete each of 25 standardized motor tasks testing a wide range of voluntary activities important for daily living. After pallidotomy there were substantial improvements across this wide range of activities. Patients completed tasks that they could not perform preoperatively. In general for individual tasks (a) preoperative and postoperative scores were directly related, and (b) the absolute improvement (preop time-postop time) was directly related to preoperative performance: numerical improvements were greater in patients who were worse preoperatively. When the 25 tasks were ranked according to improvement relative to preoperative performance postoperative improvements were successively greater for (a) limbs ipsilateral to pallidotomy, (b) contralateral limbs, and (c) standing and walking. The best results were for activities in which the trunk plays a large part.

Activities of Daily Living↗

PubMed1997

Spiral spin-echo magnetic resonance imaging of the pelvis with spectrally and spatially selective radiofrequency excitation: comparison with fat-saturated fast spin-echo imaging.

OBJECTIVE: The authors describe their initial clinical experience in comparing a spiral spin-echo technique with a fat-saturated fast spin-echo technique for imaging the pelvis. METHODS: A total of 18 patients were imaged with both spiral spin-echo and fat-saturated fast spin-echo magnetic resonance imaging. The spiral spin-echo technique combines a spectrally and spatially selective radiofrequency excitation with a spiral k-space trajectory. This technique permits rapid acquisition of T2-weighted water-only images. RESULTS: The spiral spin-echo images were judged superior to the fat-saturated fast spin-echo images in terms of uniformity of fat suppression and absence of flow-related artifacts. However, the overall image quality of the spiral spin-echo images was inferior to that of the fat-saturated fast spin-echo images, as a result of blurring caused by off resonance effects. The two techniques were judged equivalent in terms of conspicuity of the abnormality and diagnostic information. CONCLUSION: The authors conclude that the spiral spin-echo technique holds promise, particularly if combined with a deblurring algorithm.

Adolescent↗

PubMed1996

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.

Abdomen↗

PubMed1995

A velocity k-space analysis of flow effects in echo-planar and spiral imaging.

A velocity k-space formalism facilitates the analysis of flow effects for imaging sequences involving time-varying gradients such as echo-planar and spiral. For each sequence, the velocity k-space trajectory can be represented by kv(kr); that is, its velocity-frequency (kv) position as a function of spatial-frequency (kr) position. In an echo-planar sequence, kv is discontinuous and asymmetric. However, in a spiral sequence, kv is smoothly varying, circularly symmetric, and small near the kr origin. To compare the effects of these trajectory differences, simulated images were generated by computing the k-space values for an in-plane vessel with parabolic flow. Whereas the resulting echo-planar images demonstrate distortions and ghosting that depend on the vessel orientation, the spiral images exhibit minimal artifacts.

Artifacts↗

PubMed1995

Coronary angiography with magnetization-prepared T2 contrast.

A magnetization-prepared, T2-weighted sequence (T2 Prep) is used to suppress muscle and venous structures. When combined with lipid suppression, this technique improves the visualization of the coronary arteries. T2 Prep was designed to be rebust in the presence of flow as well as B0 and B1 inhomogeneities and may be combined with virtually any imaging technique. Here, it is implemented with both a single-slice spiral acquisition and a multi-slice spiral method that acquires up to 15 slices in a single breath-holding interval.

Arteries↗

PubMed1995

Discrimination of large venous vessels in time-course spiral blood-oxygen-level-dependent magnetic-resonance functional neuroimaging.

A technique is described for discriminating blood-oxygen-level-dependent (BOLD) signal changes originating from large venous vessels and those that arise from the cortical parenchyma based on examining the temporal delay of each pixel's response. Photic stimulation experiments were performed with a conventional 1.5 T scanner and correlated each pixel's time-course with sine and cosine functions at the frequency of the stimulus. It was found that the signal in pixels anatomically associated with gray matter was delayed between 4 and 8 s compared with the stimulus, whereas the signal in pixels correlated with visible vessels and sulci was generally delayed from 8 to 14 s. This larger delay is consistent with the longer time required for blood to reach the larger vessels. Finally, stimulus-induced NMR phase changes were observed for the largest vessels, which are attributed to bulk susceptibility shifts.

Brain↗

PubMed1995

Magnetic resonance fluoroscopy using spirals with variable sampling densities.

The imaging of dynamic processes in the body is of considerable interest in interventional examinations as well as kinematic studies, and spiral imaging is a fast magnetic resonance imaging technique ideally suited for such fluoroscopic applications. In this manuscript, magnetic resonance fluoroscopy pulse sequences in which interleaved spirals are used to continuously acquire data and reconstruct one movie frame for each repetition time interval are implemented. For many applications, not all of k-space needs to be updated each frame, and nonuniform k-space sampling can be used to exploit this rapid imaging strategy by allowing variable update rates for different spatial frequencies. Using the appropriate reconstruction algorithm, the temporal updating rate for each spatial frequency is effectively proportional to the corresponding k-space sampling density. Results from a motion phantom as well as in in vivo gadolinium diethylenetriaminopentaacetic acid (Gd-DTPA) bolus tracking studies in a rat model demonstrate the high temporal resolution achievable using these techniques as well as the tradeoffs available with nonuniform sampling densities. This paper focuses on the acquisition of real-time dynamic information, and all images presented are reconstructed retrospectively. The issues of real-time data reconstruction and display are not addressed.

Animals↗

PubMed1995

The diminishing variance algorithm for real-time reduction of motion artifacts in MRI.

A technique has been developed whereby motion can be detected in real time during the acquisition of data. This enables the implementation of several algorithms to reduce or eliminate motion effects from an image as it is being acquired. One such algorithm previously described is the acceptance/rejection method. This paper deals with another real-time algorithm called the diminishing variance algorithm (DVA). With this method, a complete set of preliminary data is acquired along with information about the relative motion position of each frame of data. After all the preliminary data are acquired, the position information is used to determine which data frames are most corrupted by motion. Frames of data are then reacquired, starting with the most corrupted one. The position information is continually updated in an iterative process; therefore, each subsequent reacquisition is always done on the worst frame of data. The algorithm has been implemented on several different types of sequences. Preliminary in vivo studies indicate that motion artifacts are dramatically reduced.

Algorithms↗

PubMed1995

Hitchcock's experimental series of foetal implants for Parkinson's disease: co-grafting ventral mesencephalon and striatum.

Following 4 previous experimental series of foetal implants (mesencephalon) to treat patients with Parkinson's disease subjects (N7) in the fifth series were treated with co-grafts of foetal mesencephalon and striatum implanted stereotactically into the caudate nucleus bilaterally. The clinical outcome, better than in the previous series, included improvements lasting through 18 months follow-up in activities of daily living, clinical neurological motor examination, timed motor tasks, and dyskinesia-with reduction in the patients' need for dopaminergic medication.

Brain Tissue Transplantation↗