Biomedical subjects

M A Bernstein

Publications and source records attributed to M A Bernstein.

At least 19 recordsLinked to original sources

PubMed2001

Effect of windowing and zero-filled reconstruction of MRI data on spatial resolution and acquisition strategy.

Standard, MR spin-warp sampling strategies acquire data on a rectangular k-space grid. That method samples data from the "corners" of k-space, i.e., data that lie in a region of k-space outside of an ellipse just inscribed in the rectangular boundary. Illustrative calculations demonstrate that the data in the corners of k-space contribute to the useful resolution only if an interpolation method such as a zero-filled reconstruction is used. The consequences of this finding on data acquisition and data windowing strategies are discussed. A further implication of this result is that the spatial resolution of images reconstructed with zero-filling (but without radial windowing) is expected to display angular dependence, even when the phase- and frequency-encoded resolutions are identical. This hypothesis is experimentally verified with a slit geometry phantom. It is also observed that images reconstructed without zero-filling do not display the angular dependence of spatial resolution predicted solely by the maximal k-space extent of the raw data. The implications of these results for 3D contrast-enhanced angiographic acquisitions with elliptical centric view ordering are explored with simulations.

Filtration↗

PubMed2001

High-resolution intracranial and cervical MRA at 3.0T: technical considerations and initial experience.

Initial experience with intracranial and cervical MRA at 3.0T is reported. Phantom measurements (corrected for relaxation effects) show S/N (3.0T) = 2.14 +/- 0.08 x S/N (1.5T) in identical-geometry head coils. A 3.0T 3DTOF intracranial imaging protocol with higher-order autoshimming was developed and compared to 1.5T 3DTOF in 12 patients with aneurysms. A comparison by two radiologists showed the 3.0T to be significantly better (P < 0.001) for visualization of the aneurysms. The feasibility of cervical and intracranial contrast enhanced MR angiography (CEMRA) at 3.0T is also examined. The relaxivity of the gadolinium contrast agent decreases by only about 4-7% when the field strength is increased from 1.5 to 3.0T. Cervical 3.0T CEMRA was obtained in eight patients, two of whom had 1.5T studies available for direct comparison. Image comparison suggests 3.0T to be a favorable field strength for cervical CEMRA. Voxel volumes of 0.62-0.73 mm(3) (not including zero-filling) were readily achieved at 3.0T with the use of a single-channel transmit-receive head or cervical coil, a 25 mL bolus of gadoteridol, and a 3D pulse sequence with a 66% sampling efficiency. This spatial resolution allowed visualization of intracranial aneurysms, carotid dissections, and atherosclerotic disease including ulcerations. Potential drawbacks of 3.0T MRA are increased SAR and T(*)(2) dephasing compared to 1.5T. Image comparison suggests signal loss due to T(*)(2) dephasing will not be substantially more problematic than at 1.5T. The dependence of RF power deposition on TR for CEMRA is calculated and discussed.

Brain↗

PubMed2001

Magnetic resonance angiography at 3.0 Tesla: initial clinical experience.

Magnetic resonance (MR) angiography has undergone significant development over the past decade. It has gone from being a novelty application of MR with limited clinical use to replacing catheter angiography in some clinical applications. One of the principal limitations inherent to all MR angiographic techniques is that they remain signal limited when pushed to the limits of higher resolution and short acquisition time. Developments in magnetic gradient hardware, coil design, and pulse sequences now are well optimized for MR angiography obtained at 1.5-T main magnetic field (B-field) strength, with acquisition times and imaging matrix size near their optimal limits, respectively. Recently, the United States Food and Drug Administration (FDA) approved use of clinical magnetic resonance imaging with main magnetic field strengths of up to 4 T. Before FDA approval, use of MR with magnetic field strengths much greater than 1.5 T was essentially reserved for investigational or research applications. The main advantage of high B-field imaging is a significant improvement in the signal-to-noise ratio (SNR), which increases in an approximately linear fashion with field strength in the range of 1.5 to 3.0 T. This increased SNR is directly available when performing MR angiographic acquisitions at higher magnetic field strengths, allowing for better resolution and conspicuity of vessels with similar acquisition times. Little has been reported on the benefits of performing MR angiography at magnetic field strengths >1.5 T. The purpose of this article is to summarize our current experience with intracranial and cervical MR angiographic techniques at 3.0 T.

Equipment Design↗

PubMed2001

Carotid artery: elliptic centric contrast-enhanced MR angiography compared with conventional angiography.

PURPOSE: To determine the accuracy of elliptic centric contrast material-enhanced magnetic resonance (MR) angiography by using conventional angiography as the reference standard. MATERIALS AND METHODS: Fifty patients were examined prospectively with contrast-enhanced MR angiography and conventional angiography. The two examinations were performed within 1 week of each other. Two patients underwent conventional angiography of only one carotid artery, which yielded 98 arteries for comparison. RESULTS: With conventional angiography as the reference standard and by using a 70% threshold for internal carotid arterial diameter stenosis, maximum intensity projection (MIP) images had a sensitivity of 93.3%, specificity of 85.1%, and accuracy of 87.6%, whereas reformatted transverse source images had a sensitivity of 83.3%, specificity of 97.0%, and accuracy of 92.8%. Interobserver variability for conventional angiograms was 0.97, for MIP images was 0.91, and for source images was 0.90. The contrast-enhanced MR angiographic technique had a sensitivity of 88.9% and specificity of 58.1% for the presence of irregularity and/or ulceration. All 50 examinations were triggered appropriately so that minimal or no venous signal intensity was depicted. CONCLUSION: Contrast-enhanced elliptic centric three-dimensional MR angiography offers high-spatial-resolution, venous-suppressed images of the carotid arteries that appear to be adequate to replace conventional angiography in most patients examined prior to carotid endarterectomy.

Aged↗

PubMed2001

Contrast-enhanced magnetic resonance angiography of the cervical vessels: experience with 422 patients.

BACKGROUND AND PURPOSE: Contrast-enhanced magnetic resonance angiography (CEMRA) permits acquisition of high-spatial-resolution, venous-suppressed, 3D MR angiograms of the cervical carotid and vertebral arteries. In this study, an elliptic centric-view ordering with either MR fluoroscopic triggering or test bolus timing was used. The use of CEMRA of the cervical vessels has changed our clinical practice and is replacing conventional angiography for the evaluation of most carotid and vertebral artery diseases. METHODS: We retrospectively reviewed our experience with the use of CEMRA performed in 422 patients from January through December 1999. RESULTS: CEMRA was performed to evaluate transient ischemic attack and ischemic stroke in 239 patients, asymptomatic carotid bruit in 88 patients, and other neurological symptoms in 95 patients. Carotid endarterectomies were performed in 97 patients (103 procedures), and conventional angiography was performed in 12 of these patients. CEMRA was used to evaluate for the presence of an arterial dissection in 85 of the 239 patients with transient ischemic attack and ischemic stroke. Of this group, 32 patients had cervical arterial dissection, and pseudoaneurysm was detected in 11 of these patients. Compared with ultrasonography of the cervical vessels, CEMRA provided additional information in 43 of 422 patients and led to changes in the decision as to whether to perform carotid endarterectomy in 5 patients. CONCLUSIONS: Use of CEMRA permits noninvasive evaluation of patients suspected of having carotid or vertebral disease and avoids the potential complications of conventional angiography.

Adolescent↗

PubMed2001

Contrast-enhanced carotid MR angiography with commercially available triggering mechanisms and elliptic centric phase encoding.

OBJECTIVE: The technical feasibility of contrast-enhanced MR angiography of the carotid arteries was evaluated with routinely available timing sequences and elliptic centric acquisition. The image quality of the contrast-enhanced MR angiography was compared with that of multiple overlapping thin-section acquisition MR angiography (MOTSA MR angiography). SUBJECTS AND METHODS: Sixty-three patients were enrolled. A 2-mL test bolus and commercially available software were used to time the gadolinium bolus. High-resolution contrast-enhanced MR angiography was performed with elliptic centric acquisition. RESULTS: The average time of bolus arrival was 17.3 sec (range, 12-25 sec). In 60 of the 63 patients, we had excellent or good visualization of the carotid bifurcation using contrast-enhanced MR angiography with little or no venous contamination. Two observers ranked delineation of stenosis and morphology of proximal internal carotid artery and overall diagnostic confidence statistically significantly higher for contrast-enhanced MR angiography compared with MOTSA: Ulceration, length of stenosis, and slow flow distal to a critical stenosis were better depicted with contrast-enhanced MR angiography than with MOTSA: Venetian blind artifact, saturation of slow or in-plane flow, and artifactual narrowing in carotid artery kinks plagued MOTSA but were not seen on contrast-enhanced MR angiography. MOTSA was graded superior to contrast-enhanced MR angiography in nine of 120 carotid bifurcations analyzed. CONCLUSION: High-resolution carotid contrast-enhanced MR angiography is technically feasible. We found a 95% success rate using commercially available hardware and software. The image quality with carotid contrast-enhanced MR angiography has improved so that it is equal or superior to the longer MOTSA in most patients.

Adult↗

PubMed2000

Association of muscle power with functional status in community-dwelling elderly women.

BACKGROUND: Identification of the physiologic factors most relevant to functional independence in the elderly population is critical for the design of effective interventions. It has been suggested that muscle power may be more directly related to impaired physical performance than muscle strength in elderly persons. We tested the hypothesis that peak muscle power is closely associated with self-reported functional status in sedentary elderly community-dwelling women. METHODS: We used baseline data that were collected as part of a 1-year randomized controlled clinical trial of a combined program of strength, power, and endurance training in 80 elderly women (mean age 74.8 +/- 5.0 years) with 3.2 +/- 1.9 chronic diseases, selected for baseline functional impairment and/or falls. RESULTS: Functional status at baseline was related in univariate analyses to physiologic capacity, habitual physical activity level, neuropsychological status, and medical diagnoses. Leg power had the strongest univariate correlation to self-reported functional status (r = -.47, p < .0001) of any of the physiologic factors we tested. In a forward stepwise regression model, leg press power and habitual physical activity level were the only two factors that contributed independently to functional status (r = .64, p < .0001), accounting for 40% of the variance in functional status. CONCLUSIONS: Leg power is a strong predictor of self-reported functional status in elderly women.

Activities of Daily Living↗

PubMed2000

Three-dimensional contrast-enhanced MR angiography with real-time fluoroscopic triggering: design specifications and technical reliability in 330 patient studies.

Technical reliability was determined for triggering three-dimensional (3D) contrast material-enhanced magnetic resonance (MR) angiography with MR fluoroscopy. Technical requirements for high reliability were also identified. Reliability was evaluated in 330 consecutive patient studies of the neck, thorax, abdomen, and pelvis. Contrast material arrival was detected fluoroscopically in 325 of the 330 studies (98.5%), and the 3D sequence was successfully triggered in 321 of 330 studies (97.3%). Fluoroscopic triggering of centrically encoded 3D MR angiographic acquisitions is a highly reliable means of obtaining 3D MR angiograms with high spatial resolution.

Abdomen↗

PubMed2000

The effect of oral nutritional supplements on habitual dietary quality and quantity in frail elders.

BACKGROUND: Frail institutionalized elders have a high prevalence of nutritional risk factors, undernutrition, weight loss, and nutrition-related morbidity and excess mortality. Little information is available on effective means to intervene in this setting. HYPOTHESES: We tested the hypothesis that addition of multinutrient oral supplements to the diet of frail elders would improve their overall nutritional status and functional level. METHODS: Fifty nursing home residents aged 88+/-1 yr. were followed for 10 weeks in the course of a randomized controlled trial of supplementation with a multinutrient liquid supplement vs. a non-nutritive placebo drink. Three-day food weighing was used to analyze their habitual dietary intake before and during the final week of the intervention. Nutritional status was further assessed with nutritional biochemistries, anthropometric measurements, and body composition analysis as well as physical and functional performance tests. RESULTS: The nutritional supplement was consumed with high compliance, but did not significantly augment total caloric intake. Supplementation was associated with significant reductions in total energy, protein, fat, water, fiber, and many vitamins and minerals in the habitual diet of these nursing home residents. Nutritional status improved in terms of folate levels in serum, but no other measured vitamin or mineral indices. Body composition analysis revealed a small gain in weight, increases in fat stores, but no improvement in lean tissue mass associated with supplemention. No physical performance or functional gains were associated with supplementation. CONCLUSION: Short-term nutritional supplementation in elders at nutritional risk is offset by simultaneous reduction in voluntary food intake. It seems likely that changing other components of energy expenditure such as physical activity levels or basal metabolism may be required to produce overall improvements in nutritional intake in this setting.

Aged↗

PubMed1999

Concomitant gradient field effects in spiral scans.

Maxwell's equations imply that imaging gradients are accompanied by higher order spatially varying fields (concomitant fields) that can cause artifacts in MR imaging. The lowest order concomitant fields depend quadratically on the imaging gradient amplitude and inversely on the static field strength. Time-varying concomitant fields that accompany the readout gradients of spiral scans cause unwanted phase accumulation during the readout, resulting in spatially dependent blurring. Concomitant field phase errors are independent of echo time and, therefore, cannot be detected using Dixon-type field map measurements that are normally used to deblur spiral scan images. Data acquisition methods that reduce concomitant field blurring increase off-resonant spin blurring, and vice versa. Blurring caused by concomitant fields can be removed by variations of image reconstruction methods developed to correct for spatially dependent resonance offsets with nonrectangular k-space trajectories.

Artifacts↗

PubMed1999

Theoretical limits of spatial resolution in elliptical-centric contrast-enhanced 3D-MRA.

The point spread function (PSF) for contrast-enhanced three-dimensional (3D) MR angiography using the elliptical centric view order is derived. This view order has been shown previously to provide high venous suppression thereby enabling long acquisition times capable of high spatial resolution. The dependence of the PSF on TR, field of view (FOV), scan time, and trigger time are shown explicitly. Theoretical predictions are corroborated with experimental results in phantoms and in vivo. The PSF width decreases as the square root of the product of TR and the two phase encoding FOV's for fixed nominal voxel size. The PSF peak amplitude increases as the reciprocal of this product. Theory and experiment demonstrate that acquisition times over 40 sec provide superior resolution compared to shorter acquisitions, despite falling levels of contrast agent concentration. The analysis predicts that an isotropic spatial resolution of 1 mm before zero filling is possible in a FOV large enough to encompass the carotid and vertebral arteries bilaterally. Magn Reson Med 42:1106-1116, 1999.

Carotid Arteries↗

PubMed1999

A method for the determination of activated receptor phosphorylation state following in vivo drug treatment.

Reversible phosphorylation events can alter many critical cellular functions and has been hypothesized to play a role in the development of tolerance to chronically administered agonists. The technique described here was developed to assess the phosphorylation state of mu-opioid receptor protein isolated from rodent brain tissue following 4 days of continual, subcutaneously released morphine. We have adapted and combined two techniques: back-phosphorylation reactions, which make use of tissue from animal models, and phosphorylation followed by immunoprecipitation, an approach used in cell culture models. mu-receptor protein is precipitated using a receptor-specific antibody, and the protein phosphorylation state is preserved by the use of phosphatase inhibitors. Phosphorylation sites that are dephosphorylated on the isolated protein are then radiolabelled by the addition of purified cAMP-dependent protein kinase (PKA) catalytic subunit and [32P]ATP. The samples are separated by gel electrophoresis, and the resulting bands are quantified using a phosphoimager. The amount of 32P incorporated into the protein is assumed to be inversely related to the degree of phosphorylation prior to the reaction (i.e., the in vivo state of the protein). These estimates of micro-opioid receptor phosphorylation can then be correlated with a behavioral endpoint such as antinociception, to better understand the role of phosphorylation events in tolerance development.

Adenosine Triphosphate↗

PubMed1999

Carotid arteries: maximizing arterial to venous contrast in fluoroscopically triggered contrast-enhanced MR angiography with elliptic centric view ordering.

PURPOSE: To obtain high-spatial-resolution, venous-suppressed, contrast material-enhanced, three-dimensional (3D) magnetic resonance (MR) angiograms of the carotid arteries and aortic arch by using an elliptic centric view ordering with MR fluoroscopic triggering. MATERIALS AND METHODS: Forty consecutive patients with cerebrovascular disease in the differential diagnosis were evaluated with fluoroscopically triggered 3D MR angiography (gadoteridol dose range, 0.1-0.3 mmol per kilogram of body weight; mean acquisition time, 40 second +/- 8 [SD]). The contrast-enhanced 3D MR angiograms were evaluated for overall quality, vascular signal intensity, venous suppression, and motion artifact. Twenty patients also underwent two-dimensional (2D) time-of-flight (TOF) MR angiography. The overall quality of the 2D TOF MR angiograms and comparative quality between the 2D TOF and contrast-enhanced 3D MR angiograms were determined. RESULTS: The contrast-enhanced 3D MR angiograms were of excellent or more than adequate quality for diagnosis in 36 of the 40 studies (90%). In 35 of the 38 contrast-enhanced 3D studies in which the contrast material bolus was detected fluoroscopically, the internal jugular vein signal intensity was either not detectable or barely visible. In 18 of the 20 patients who also underwent 2D TOF MR angiography, the quality of the contrast-enhanced 3D MR angiograms was graded as markedly superior or superior. CONCLUSION: Contrast-enhanced, elliptic centric 3D MR angiography with real-time MR fluoroscopic triggering offers high-spatial-resolution images of the carotid arteries and aortic arch with reliable venous suppression.

Aorta, Thoracic↗

PubMed1998

Inhibition of protein phosphatases alters the expression of morphine tolerance in mice.

Recently our laboratory found that tolerance to morphine-induced antinociception could be completely reversed with intracerebroventricular (i.c.v.) administration of a protein kinase A inhibitor, whereas intrathecal (i.t.) administration of the inhibitor produced hyperalgesia in morphine-tolerant mice. In the experiments described here, we sought to characterize further the role of phosphorylation events in supraspinal versus spinal opioid-mediated pain pathways and how such events might be involved in the development of antinociceptive tolerance. Two phosphatase inhibitors were administered centrally to determine whether they affected morphine-induced antinociception in naive or chronically morphine-treated mice. By the i.c.v. route, okadaic acid enhanced morphine-induced antinociception in tolerant mice and produced toxicity by the i.t. route. The calcineurin inhibitor ascomycin had no effect on antinociception following acute or chronic morphine treatment. These results suggest that increased activity of protein phosphatase types 1 and/or 2A in the brain may contribute to the development of morphine tolerance.

Analgesics, Opioid↗

PubMed1998

Concomitant gradient terms in phase contrast MR: analysis and correction.

Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.

Artifacts↗

PubMed1998

Concomitant magnetic-field-induced artifacts in axial echo planar imaging.

When a linear magnetic field gradient is used, spatially higher-order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher-order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase-encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unreported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image artifacts.

Artifacts↗

PubMed1998

Artifacts induced by concomitant magnetic field in fast spin-echo imaging.

It has been observed that fast spin-echo (FSE) images with a large field of view (>40 cm) in certain directions exhibit unusual ghosting artifacts that cannot be eliminated with existing ghost removal methods. These artifacts have been related to a higher-order magnetic field perturbation (known as the concomitant field, or Maxwell field) concomitant to the linear imaging gradient, in accordance with the Maxwell equations V z B = 0 and V x B approximately 0. Several methods have been developed to eliminate or minimize the effects of the concomitant magnetic field by redesigning the FSE pulse sequences. In the slice-selection direction, the gradient waveforms are made symmetrical about the refocusing RF pulses wherever possible. Surrounding the first refocusing pulse, such symmetry cannot be achieved due to the slice-refocusing gradient, which is often combined with the left crusher. In this case, it is shown how crusher gradients can be reshaped to nullify the phase due to the concomitant field. In the phase-encoding direction, the gradient amplitude is reduced and its duration is prolonged. Artifacts due to the readout gradient are eliminated by reshaping the prephasing lobe, while keeping its area fixed. In all the three directions, the gradient waveforms are adjusted so that they have minimal overlap. Selected methods have been implemented on a clinical scanner, and typically reduce the ghost intensities in phantom and human images by a factor of 3.

Artifacts↗

PubMed1998

mu-Opioid receptor down-regulation and cAMP-dependent protein kinase phosphorylation in a mouse model of chronic morphine tolerance.

Results of radioligand binding and transfected receptor studies indicate that mu-receptor down-regulation and phosphorylation may be critical to the expression of morphine tolerance. In this study, an animal model of morphine tolerance was used to correlate antinociception with changes in receptor number and phosphorylation state. mu-Opioid receptor protein was quantitated by Western immunoassay of brainstem tissue from morphine-treated mice. Degree of receptor phosphorylation was assessed using immunoprecipitation (IP) of the receptor followed by back-phosphorylation. Acutely administered morphine produced no changes in mu-receptor quantity. Chronic morphine administration resulted in a 50% reduction in receptor protein quantity over placebo-treated samples. Back-phosphorylation experiments showed a drop in cAMP-dependent protein kinase A (PKA)-induced receptor phosphorylation shortly after acute morphine administration, followed by a naloxone-reversible increase in phosphorylation of the receptor that correlated with the onset of antinociception. Chronic morphine administration resulted in a decrease in PKA-induced phosphorylation of the mu-receptor. Since it has been shown that PKA activity is enhanced in the brains of morphine-tolerant mice, this decrease in mu-receptor phosphorylation suggests that the mu-receptor may be structurally or conformationally altered in the morphine-tolerant state.

Animals↗