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Biomedical subjects

C H Meyer

Publications and source records attributed to C H Meyer.

At least 19 recordsLinked to original sources
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

An increased frequency of IgE-producing B cell precursors contributes to the elevated levels of plasma IgE in atopic subjects.

BACKGROUND: The production of specific IgE, which underlies the allergic response, may be a normal correlate of the immune response to a certain class of antigen (allergens), or could represent a unique response driven by regulatory signals that are absent in non-allergic individuals. If atopic subjects do possess a regulatory environment favoring IgE production, they may display not only allergen-specific IgE, but also higher levels of total IgE and higher frequencies of IgE-producing B lymphocytes. OBJECTIVE: To address the contribution of antibody-producing cell number to the circulating IgE titre in atopic vs non-atopic subjects. METHODS: Frequency determination by limiting dilution of EBV transformants and Poisson distribution analysis. Titres of total and allergen-specific IgM, IgG, and IgE by specific ELISA. RESULTS: In contrast to findings reported by others, the atopic subjects had a significantly higher frequency of IgE-producing B cells than non-atopics (0.79% of total Ig-producing cells, as compared with 0.17% for the control group; P < 0.01), suggesting that one factor contributing to the high plasma IgE titres in atopic subjects is the high frequency of B lymphocytes with the potential to produce IgE. Although only the atopic subjects produced allergen-specific IgE, the frequency of specific IgE-producing B cells was undetectable in both groups. CONCLUSION: Atopic subjects have higher frequencies of IgE-producing B cell precursors than non-atopics. A correlation exists between IgE-producing B cell frequency and levels of circulating IgE.

Animals↗
PubMed1994

Magnetic resonance velocity imaging using a fast spiral phase contrast sequence.

Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.

Blood Flow Velocity↗
PubMed1994

Real-time motion detection in spiral MRI using navigators.

A technique has been developed whereby motion can be detected in real time during the acquisition of data. This enables the implementation of an algorithm to accept or reject and reacquire data during a scan. Frames of data with motion are rejected and reacquired on the fly so that by the end of the scan, a complete motion-free data set has been acquired. The algorithm has been implemented on several different types of sequences. Preliminary in vivo studies indicate that motion artifacts are dramatically reduced.

Abdomen↗
PubMed1994

Comparison of the levels of the major allergens Der p I and Der p II in standardized extracts of the house dust mite, Dermatophagoides pteronyssinus.

Allergy to the house dust mite Dermatophagoides pteronyssinus is mediated by IgE to the major allergens Der p I and Der p II in the majority of mite-allergic patients. In recent years, standardized preparations of D. pteronyssinus, commercially available from several sources, have become widely used for the diagnosis and immunotherapy of mite allergy. As standardization implies uniformity of allergen composition and potency, we directly compared the absolute and relative quantities of Der p I and Der p II in six different commercial standardized extracts of D. pteronyssinus. Our findings reveal variability in levels of both Der p I and Der p II, producing ratios of Der p I/Der p II ranging from 1.1/1 to 6/1. Although the content of minor allergens in the extracts was not evaluated here, their contribution to the overall reactivity of mite-allergic patients to the commercial extracts was judged to be minimal. This was demonstrated by showing that plasma depleted of reactivity to both Der p I and Der p II had virtually no residual IgE directed against extract components. The variation in the proportion of Der p I and Der p II among different D. pteronyssinus extracts is likely to influence their biological effectiveness. Patients with reactivity against only Der p I or Der p II, who were found to comprise approximately one-third of the mite-allergic population, may not respond optimally to extracts containing relatively low levels of the allergen to which they are sensitive.

Allergens↗
PubMed1992

Deblurring for non-2D Fourier transform magnetic resonance imaging.

For several non-2D Fourier transform imaging methods, off-resonant reconstruction does not just cause geometric distortion, but changes the shape of the point spread function and causes blurring. This effect is well known for projection reconstruction and spiral k-space scanning sequences. We introduce here a method that automatically removes blur introduced by magnetic field inhomogeneity and susceptibility without using a resonant frequency map, making these imaging methods more useful. In this method, the raw data are modulated to several different frequencies and reconstructed to create a series of base images. Determination of degree of blur is done by calculating a focusing measure for each point in each base image and a composite image is then constructed using only the unblurred regions from each base image. This method has been successfully applied to phantom and in vivo images using projection-reconstruction and spiral-scan sequences.

Algorithms↗
PubMed1992

Fast spiral coronary artery imaging.

A flow-independent method for imaging the coronary arteries within a breath-hold on a standard whole-body MR imager was developed. The technique is based on interleaved spiral k-space scanning and forms a cardiac-gated image in 20 heartbeats. The spiral readouts have good flow properties and generate minimal flow artifacts. The oblique slices are positioned so that the arteries are in the plane and so that the chamber blood does not obscure the arteries. Fat suppression by a spectral-spatial pulse improves the visualization of the arteries.

Adipose Tissue↗
PubMed1992

Cerebral blood flow in patients with a subarachnoid haemorrhage during treatment with tranexamic acid.

Many clinicians currently use antifibrinolytic therapy (AFT) routinely in the management of subarachnoid haemorrhage (SAH). Many others do not, either because they remain unconvinced that AFT reduces the risk of rebleeding, or that the medication itself causes serious complications and in particular cerebral ischaemia. Nineteen randomly selected patients were studied, 9 receiving tranexamic acid (9 g a day) and the remaining 10 placebo, with SAH confirmed by CT scanning and by lumbar puncture. There was no difference between the active and placebo group regarding the age, sex, clinical grade, CT scan and angiographic appearance. The intravenous Xe133 technique was used for serial determinations of hemispheral cerebral blood flow. The cerebral blood flow remained stable during the first week following subarachnoid haemorrhage, and then fell progressively, reaching its bottom level by the end of the second week. The cerebral blood flow levelled out during the third week at the end of which a sharp elevation, well above the first week's post-subarachnoid haemorrhage level, was noted. This rebound rise of cerebral blood flow was observed for both cerebral hemispheres. Cerebral flow was greatest in the contralateral (to side of ruptured aneurysm) brain hemisphere save for the peak observed during the first week post-subarachnoid haemorrhage. The difference between the ipsi- and contralateral hemispheres was most pronounced in patients receiving active treatment. Analysis of variance showed that cerebral blood flow was reduced by the active treatment and especially more so on the ipsilateral side with the ruptured aneurysm. The usefulness of AFT should therefore be reconsidered.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Supratentorial masses: stereotactic or freehand biopsy?

CT-guided stereotactic biopsy is now an accepted method of tissue sampling in intracranial mass lesions but many surgeons still practise freehand burrhole biopsy. This study compares two groups of patients who had either stereotactically guided (n = 153) or freehand (n = 217) biopsy. Stereotactic biopsy has a lower incidence of both mortality (2.6%) and morbidity (1.3%) than freehand (7.8 and 7.8%) while diagnostic accuracy is 92.1 and 64.9%, respectively. The success rate for stereotactic biopsy is independent of the size and depth of the lesion while freehand biopsy is most successful for large, superficial lesions but its success never exceeds 88%. The stereotactic technique is superior to the freehand for all intracranial biopsies regardless of size or site.

Adult↗
PubMed1990

Simultaneous spatial and spectral selective excitation.

Using a k-space interpretation of small-tip excitation, a single excitation pulse has been designed that is simultaneously selective in space and resonant frequency. An analytic expression for the response of this pulse has been derived. The pulse has been implemented on a 1.5-T imaging system. The pulse has been applied to a rapid gradient-echo imaging sequence that forms both water and fat images within a breath-holding interval. These rapid images are free of the chemical shift artifacts at organ boundaries that typically afflict conventional rapid images. The pulse can be applied to a variety of other sequences, such as multislice water/fat sequences and rapid k-space scanning sequences.

Adipose Tissue↗
PubMed1990

Benefits and risks of antifibrinolytic therapy in the management of ruptured intracranial aneurysms. A double-blind placebo-controlled study.

One hundred patients with a verified subarachnoid haemorrhage were studied in a double blind, placebo-controlled trial at a single centre to determine the value and relative risks of tranexamic acid (TXA) in the management of ruptured intracranial aneurysms. The incidence of recurrent haemorrhage between active and placebo groups was identical (12%) and the mortality from recurrent haemorrhage was 7% and 5%, respectively. The overall incidence of cerebral infarction before surgery, at discharge and at 6 months follow-up was greater in the TXA group (27%) than in the control group (11%). Post-operative cerebral ischaemia was significantly more frequent in the active, 18 of 29 as compared to 6 of 32 patients, in the placebo group. In a fifth of the patients in whom cerebral blood flow was estimated there was a significant reduction of cerebral blood flow (CBF) on the side of the ruptured aneurysm in the TXA treated group. It is suggested that this may be the cause of the increased incidence of cerebral ischaemia in this group. There was no significant difference in the incidence of cerebral vasospasm, hydrocephalus, visual disturbances and gastrointestinal disturbances. More fatalities were encountered from ischaemia and recurrent haemorrhage in the TXA group but these differences did not reach statistical significance at the 5% level. Given that disability was due to either vasospasm or recurrent haemorrhage than a patient under TXA treatment was significantly more likely to have disability due to vasospasm (p less than 0.04); the reverse was true for the placebo patient (p less than 0.05).

Adult↗
PubMed1990

Fibrinolytic activity after subarachnoid haemorrhage and the effect of tranexamic acid.

Seventy-four patients with recent subarachnoid haemorrhage were randomly allocated to placebo or tranexamic acid treatment. Fibrinolytic activity in the blood and cerebrospinal fluid was assessed before treatment, one week later and two weeks later. The natural history of fibrinolysis following subarachnoid haemorrhage was obtained from analysis of the placebo group. Following subarachnoid haemorrhage, fibrin degradation products and plasminogen activity in the cerebrospinal fluid were elevated. Subsequently, fibrin degradation products in the cerebrospinal fluid fell progressively over the following 2 weeks. Changes in cerebrospinal fluid plasminogen activity correlated with those of blood plasminogen activity. Complications such as rebleeding, hydrocephalus or cerebral thrombosis could not be predicted from analysis of fibrinolytic activity. Tranexamic acid treatment resulted in a reduction in cerebrospinal fluid and blood plasminogen activity. The relevance of fibrinolysis in cerebrospinal fluid and blood to the management of subarachnoid haemorrhage is discussed.

Antifibrinolytic Agents↗
PubMed1986

[Side effects following intravenous regional anesthesia (IVRA) on the central nervous system and heart rate].

Forty patients undergoing upper limb surgery received intravenous regional anaesthesia (IVRA). Electroencephalographic (EEG) and electrocardiographic (ECG) assessments were performed the day before surgery without any premedication and postoperatively before, during and after the release of the cuff. For IVRA, vasocontrictorfree 0.5% mepivacaine solution at a dosage of 3 mg/kg body wt. was applied, followed by an additional injection of 20 ml 0.9% NaCl solution. The EEG and ECG did not show any evidence of toxical effects on brain function or cardiac rhythm. In response to the sedative property of mepivacaine stages of sleep were observed in all patients following the release of the cuff. Because of this effect on alertness, ambulant patients should not participate actively in traffic.

Adolescent↗