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M Garwood

Publications and source records attributed to M Garwood.

At least 55 records · Page 3Linked to original sources
PubMed1991

An analysis of the intrinsic resonance offset dependence of magnetization generated by stimulated echo pulse sequences for noncoupled spins.

It is demonstrated that the basic radiofrequency pulse train used to generate stimulated echoes (90x-tau TE-90x-tau TM-90x-tau TE-Acq.) is in general characterized by strong amplitude and phase modulations of the transverse magnetization as a function of the resonance offset. Two dephasing techniques which eliminate the modulations are investigated both theoretically and experimentally, and a simple formula is derived for calculating the relative modulation across a spectrum as a function of gradient strength and duration, echo delay, and spectral linewidth.

Magnetic Resonance Spectroscopy↗
PubMed1990

Transmural high energy phosphate distribution and response to alterations in workload in the normal canine myocardium as studied with spatially localized 31P NMR spectroscopy.

Spatially localized phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy has been applied to the study of the normal canine myocardium to measure the relative content of high energy phosphates across the left ventricular wall. Transmural NMR data were acquired in five voxels spanning the wall of the left ventricle using the FLAX-ISIS technique. The validity of the FLAX-ISIS approach in acquiring localized spectra for transmural studies and in providing quantitative information from the localized spectra was examined rigorously by studies involving phantoms, intact rats, and the canine myocardium in vivo. The results indicated that (1) this technique yields spatially resolved spectra with partial overlap between adjacent voxels and virtually no overlap between every other voxel; (2) in the canine heart, signals from subepicardium, midwall, and subendocardium can be detected separately without cross contamination; and (3) relative metabolite contents within a voxel and among voxels can be quantitated. Transmural 31P NMR spectra were acquired with cardiac gating on 29 separate animals either at early systole or late diastole, and at three different workloads with the heart rate peak systolic pressure product (RPP) increasing from 6000 mmHg/min to 35,000 mmHg/min. The data revealed that in the normal canine myocardium, the creatine phosphate (CP) content and the CP/ATP ratio was significantly lower in the subendocardium than in the subepicardium. ATP levels were transmurally constant. Both the CP content and the CP/ATP ratio measured for each voxel remained unaltered in relation to either the phase of the cardiac cycle or approximately fourfold increase in workload. Free ADP levels calculated for each voxel showed that ADP was relatively higher in the subendocardium than the subepicardium, and in all transmural layers was higher than its apparent Km for oxidative phosphorylation. In this domain changes in ADP content with workload and MVO2 are not expected and were not observed.

Adenosine Diphosphate↗
PubMed1990

3-D FLASH imaging using a single surface coil and a new adiabatic pulse, BIR-4.

A new adiabatic pulse, which can induce uniform and arbitrary flip angles despite the presence of transmitter coil magnetic field (B1) inhomogeneities, is employed for 3-D fast imaging using a single surface coil for pulse transmission and signal detection. Computer calculations and phantom, rat, and human surface coil imaging experiments demonstrate the utility of this adiabatic pulse for T1-weighted imaging with a transmitter coil which generates a highly inhomogeneous B1 field profile.

Animals↗
PubMed1990

Correlation between transmural high energy phosphate levels and myocardial blood flow in the presence of graded coronary stenosis.

Spatially localized nuclear magnetic resonance spectroscopy was used to investigate with transmural differentiation the response of myocardial high energy phosphate compounds and inorganic orthophosphate (Pi) to graded reductions in coronary blood flow caused by sustained coronary stenosis. In an open-chest model, localized 31P nuclear magnetic resonance spectra from five layers across the left ventricular wall were obtained simultaneously with transmural blood flow measurements during control conditions and during sustained graded reductions in intracoronary pressure. Both the blood flow, and high energy phosphate and Pi contents displayed transmural heterogeneity in response to decreases in intracoronary pressure. The subendocardial creatine phosphate (CP) level remained unchanged as blood flow was reduced to approximately 0.7 ml/min/g wet wt and decreased precipitously beyond this critical flow level. The relation between CP and flow in the midmyocardium and especially in the subepicardium was more complex. Subepicardial CP content did not correlate well with blood flow; however, in cases in which a coronary stenosis resulted in subendocardial hypoperfusion but subepicardial flow was near or above normal, a close correlation was present between subepicardial and subendocardial CP levels. ATP levels in all layers remained unaltered until blood flow was severely reduced. These results demonstrate that 1) the myocardial high energy phosphate and Pi levels at any transmural layer are not generally determined by O2 and blood flow limitation under basal conditions; 2) during subtotal coronary occlusion, increased oxygen extraction is able to meet myocardial needs until a critical level of stenosis is reached; 3) below a critical flow level, subendocardial CP and Pi contents are closely correlated with absolute subendocardial blood flow; and 4) in the presence of a coronary stenosis, subepicardial CP and Pi contents may change even in the absence of perfusion deficit secondary to loss of subendocardial function.

Adenosine Triphosphate↗
PubMed1989

Gas chromatographic-mass spectrometric analysis of hexose monophosphate shunt activity in cultured cells.

A GC/MS method is described for monitoring the relative amount of glucose degraded to lactate via the hexose monophosphate shunt (HMPS) in neoplastic cells. C6 glioma cells were incubated in medium supplemented with [1-13 C]glucose and medium containing [1-13C]glucose with 0.001 mM phenazine methosulfate (PMS). The ratio of the [13C]lactate to [12C]lactate determined from the measurement of the m/z 219/220 and 117/118 ions of the trimethyl silyl derivative, was used to calculate HMPS activity. PMS increased HMPS activity in C6 glioma cells by 3.2 and 4.8 fold at 2 and 12 hours of incubation respectively. GC/MS results were compared with 1H NMR measurements of the [3-13C]lactate/[3-12C]lactate ratio. The GC/MS method was found to require less sample size and yielded better sensitivity than the NMR method.

Animals↗
PubMed1989

Magnetic resonance imaging with adiabatic pulses using a single surface coil for RF transmission and signal detection.

In order to overcome the problems that arise from nonuniform B1 fields, there has been interest in developing pulses that are insensitive to large variations in RF power. Pulses derived from adiabatic passage principles that can execute spin inversion, excitation, and 90 degrees and 180 degrees plane rotations in the presence of B1 inhomogeneities have recently been described. When driven with optimized modulation functions, these pulses can execute uniform excitation, refocusing, and slice-selective inversion over a 10-fold or greater variation in B1 magnitude. This insensitivity to B1 strength enables the execution of T1- and/or T2-weighted spin-echo imaging experiments using coils, such as the surface coil, with extremely inhomogeneous B1 profiles. We have successfully acquired images with these pulses at 200 MHz using a single surface coil as the transmitter and receiver. Images of the slice definition, the region over which the excitation and refocusing pulses operate with a surface coil, and brain images obtained with slice planes perpendicular to the plane of the surface coil are presented. Results demonstrate that these pulses can be transmitted with a surface coil to yield high-quality T1- and/or T2-weighted images without B1 artifacts.

Animals↗
PubMed1989

Spectroscopic imaging and spatial localization using adiabatic pulses and applications to detect transmural metabolite distribution in the canine heart.

Adiabatic pulses have been employed in spectroscopic imaging and relaxation rate measurements at 4.7 T to demonstrate the feasibility of obtaining spectroscopic data from the complete sensitive volume of a surface coil using the surface coil as a transmitter and receiver. With conventional B1 sensitive pulses, spectroscopic localization or imaging techniques, such as chemical-shift imaging, yield resonance intensities that are distorted severely as a function of space, and maximal signal is detected from a small region within the complete sensitive volume of the coil. With adiabatic pulses, however, this problem is eliminated completely. In addition, a new method of spatial localization is introduced. This method, referred to as FLAX-ISIS, is a derivative of longitudinally modulated Fourier series window and ISIS approaches and utilizes adiabatic inversion and excitation pulses. The method allows construction of localized spectra for multiple regions along the surface coil axis by postacquisition data manipulation of a single set of free induction decays. These techniques were applied to the study of the myocardium using an implanted surface coil in an instrumented closed-chest canine model and in an open-chest preparation. The results demonstrate that one-dimensional techniques are adequate for transmural detection of metabolites provided signal origin is restricted to a column perpendicular to the left ventricle wall.

Animals↗
PubMed1989

Transmural metabolite distribution in regional myocardial ischemia as studied with 31P NMR.

Phosphorus-31 nuclear magnetic resonance (31P NMR) has been applied to study the canine heart prior to and during regional myocardial ischemia induced by partial flow reduction in the left anterior descending coronary artery (LAD). NMR data were acquired in a transmural fashion by restricting the signal to a column perpendicular to the heart wall using B0 gradients and obtaining spectroscopic spatial resolution along the third dimension using the B1 gradient and adiabatic excitation. With this approach, transmural spectra were accumulated in five separate voxels spanning the wall of the left ventricle from the epicardium to the endocardium. In the normal canine myocardium the levels of high-energy phosphates CP and ATP were relatively constant throughout the left ventricular wall, with only minor evidence of free inorganic phosphate in any of the transmural voxels. However, during sustained partial occlusion of the LAD, significant regional differences between the epi- and the endocardium were noted. The data demonstrate the importance of studying cardiac bioenergetics with transmural differentiation.

Animals↗
PubMed1989

In vivo 31P and 2H NMR [corrected] studies of rat brain tumor pH and blood flow during acute hyperglycemia: differential effects between subcutaneous and intracerebral locations.

Surface coil NMR spectroscopy was used to monitor the hyperglycemia-induced alterations in pH and blood flow in vivo in C6 gliomas implanted both subcutaneously and intracerebrally in rats. Tumor pH was calculated from the chemical shift difference between PCr and Pi in the 31P NMR spectra. Subcutaneous glioma pH decreased 0.8 units by 1 h after intraperitoneal administration of an aqueous 50% glucose solution (6 g glucose per kg body weight). In contrast, hyperglycemia failed to significantly alter the pH of intracerebral gliomas which were monitored for 90 min following administration of glucose. Tumor blood flow (TBF) was determined both pre- and post-glucose administration using deuterium NMR by monitoring the time course of D2O washout following intratumoral injection of saline D2O. Subcutaneous and intracerebral TBF were found to have an average change of -78.1% (range -47.4 to -93.3%, n = 5) and -21.1% (range +6.0 to -37.8%, n = 9), respectively. In addition, laser Doppler blood flow measurements of rat skin and subcutaneous glioma revealed a dramatic reduction in blood flow in both tissues following glucose administration. These results indicate that the effects of acute hyperglycemia are site dependent and that hyperglycemia alone is not beneficial for inducing intracellular acidosis in intracerebral tumors.

Animals↗
PubMed1988

31P NMR spectroscopy of the in vivo metabolism of an intracerebral glioma in the rat.

The in vivo high-energy phosphorus metabolic profile and pH of an experimental intracerebral C6 glioma in rats was examined using surface coil 31P NMR spectroscopy. Initially, phosphorus-containing metabolites of the glioma were characterized by in vivo 31P surface coil spectroscopy of subcutaneously implanted tumors and by high-resolution NMR studies of perchloric acid (PCA) extracts of both freeze-clamped subcutaneous tumor tissue and cultured cells. These studies demonstrated that the C6 glioma has reduced levels of phosphocreatine (PCr) compared to the levels found in normal rat brain. Thus, reduced spectral PCr levels were useful as a metabolic indicator for monitoring the spatial selectivity of tumor metabolism distinct from that of adjacent normal brain tissue. To maximize 31P NMR signals from intracerebral tumors, tumor cells were stereotaxically placed superficially in the brain. Proton magnetic resonance imaging (1H MRI) was used to determine the size and location of the resultant brain tumors in order to preselect rats with large superficial tumors for spectroscopic study. 31P NMR spectra of the glioma tumors revealed a consistent reduction in the PCr/ATP ratio, an increase in the Pi/ATP ratio, and a slightly increased tissue pH. No correlation was found between levels of Pi/ATP and tumor pH in subcutaneous or intracerebral gliomas and the amount of necrosis as determined histologically. This study demonstrates that phosphorus metabolites of an experimental brain tumor in the rat can be monitored in vivo with minimal contributions from adjacent normal brain tissue metabolites using surface coil 31P NMR spectroscopy.

Adenosine Triphosphate↗
PubMed1986

Localization of phosphorus metabolites and sodium ions in the rat kidney.

Relative amounts of phosphorus-containing metabolites and sodium ions present in different regions of the in vivo rat kidney were obtained using a surface-coil probe and recently developed NMR rotating-frame methods. During altered physiologic states, changes in distribution of metabolites and sodium ions within the kidney were identified in one-dimensional metabolite maps. This technique may have important applications to disorders commonly found in clinical medicine.

Acidosis↗
PubMed1986

Treatment of fecal incontinence in children with spina bifida: comparison of biofeedback and behavior modification.

Two experiments compared the effects of biofeedback training to behavior modification in the treatment of 33 children aged 5 to 16 who had fecal incontinence secondary to myelomeningocele. Biofeedback involved providing visual feedback and rewards for successively stronger sphincter contractions during training sessions and requiring 50 sphincter contraction exercises daily. Behavior modification involved attempting to defecate immediately after the evening meal each day, receiving a reward for defecating in the toilet without an enema or suppository, and receiving an enema if unsuccessful for two consecutive days. In experiment I, eight children were offered biofeedback alone in an attempt to replicate previous studies. Biofeedback alone was found insufficient; addition of behavior modification was necessary. Experiment II systematically investigated the relative contribution of these two treatments. Overall, patients who received only behavior modification for three months showed as much clinical improvement as patients who received behavior modification plus biofeedback. This suggests that previous reports, because they have not controlled for nonspecific treatment effects, have overestimated the value of biofeedback in this population. However, a subgroup of patients (27%) were identified for whom biofeedback provided additional, specific benefit. These were children who had spinal cord lesions below L-2 and who initially had two or more bowel movements daily. The combination of behavior modification and biofeedback resulted in a greater than 50% reduction in the frequency of incontinence for 64% of patients, and results were well maintained at follow-up one year later.

Adolescent↗
PubMed1985

Surface coil phosphorus-31 nuclear magnetic resonance studies of the intact eye.

The feasibility of employing the surface coil probe technique for the non-invasive study of ocular tissue metabolism by phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR) in enucleated bovine, rabbit, human and rat globes is demonstrated. An assessment of individual phosphorus-metabolite contributions from ocular tissues, including the cornea, lens and iris, to the overall 31P NMR spectrum (NMR spectral acquisition parameters optimized for the lens region of the globe) was accomplished through the combination of surgical ablation and difference spectroscopy. The NMR measurements also provided tissue pH values for the lens and cornea. The strengths and limitations of the surface coil NMR method, which is particularly appropriate for in vivo metabolic studies of ocular tissues such as the lens, are discussed.

Adenosine Triphosphate↗
PubMed1981

Skin potential level: age and recording site interactions.

Two experiments comparing the skin potential level (SPL) of young (30 to 47 years), middle-aged (48 to 65 years), and old (66 to 82 years) men during rest and during stimulus presentations using a nonhydrating electrolyte medium demonstrated that there was an ordered relationship between SPL negativity and age with the older men having less negative levels. Age differences in SPL were dependent upon recording site with larger age differences occurring when SPL was recorded from the medial phalanx than when SPL was recorded from the thenar eminence.

Adult↗
PubMed1981

Skin potential level: age and epidermal hydration effects.

Age differences in skin potential level (SPL) of young adult and aged men were investigated using basal skin potential level (BSPL) procedures at three levels of epidermal hydration. A glycol medium was used at a site of least hydration. A distilled water and agar solution was the medium at two other more hydrated sites which also received either 0 min or 15 min pretreatment in distilled water. A 0.5% KCl electrolyte concentration was used at all sites. Significant effects for age and hydration were dependent on the point in the recording session comparisons were made. The major findings were: (1) the old men had significantly lower SPLs than did the young men at the start of recording with the glycol electrolyte medium; there were no significant age differences at the start of recording with the agar mediums, or at BSPL with any hydration condition. (2) Hydration significantly affected SPL magnitude at the start of recording, but not at BSPL. SPL recorded with the glycol medium at certain points in the recording was significantly related to outdoor activity among the aged men. This effect was independent of differences in cardiovascular status, and cardiovascular status was not significantly correlated with glycol SPL. It is concluded that there is a reduced sweat gland potential in old age which can only be observed when recording conditions produce high epidermal resistance. This reduced sweat gland potential is related to low levels of outdoor activity.

Adolescent↗