Optimal Point Spread Function Research Articles

Age and gender dependence of human cardiac phosphorus metabolites determined by SLOOP 31P MR spectroscopy

The aim of this study was to apply (31)P magnetic resonance spectroscopy (MRS) using spatial localization with optimal point spread function (SLOOP) to investigate possible age and gender dependencies of the energy metabolite concentrations in the human heart. Thirty healthy volunteers (18 males and 12 females, 21-67 years old, mean = 40.7 years) were examined with the use of (31)P-MRS on a 1.5 T scanner. Intra- and interobserver variability measures (determined in eight of the volunteers) were both 3.8% for phosphocreatine (PCr), and 4.7% and 8.3%, respectively, for adenosine triphosphate (ATP). High-energy phosphate (HEP) concentrations in mmol/kg wet weight were 9.7 +/- 2.4 (age < 40 years, N = 16) and 7.7 +/- 2.5 (age >or= 40 years, N = 14) for PCr, and 5.1 +/- 1.0 (age < 40 years) and 4.1 +/- 0.8 (age >or= 40 years) for ATP, respectively. Separated by gender, PCr concentrations of 9.2 +/- 2.4 (men, N = 18) and 8.0 +/- 2.8 (women, N = 12) and ATP concentrations of 4.9 +/- 1.0 (men) and 4.2 +/- 0.9 (women) were measured. A significant decrease of PCr and ATP was found for volunteers older than 40 years (P < 0.05), but the differences in metabolic concentrations between both sexes were not significant. In conclusion, age has a minor but still significant impact on cardiac energy metabolism, and no significant gender differences were detected.

The AGILE on-board Kalman filter

On-board reduction of particle background is one of the main challenges of space instruments dedicated to gamma-ray astrophysics. We present in this paper a discussion of the method and main simulation results of the on-board background filter of the Gamma-Ray Imaging Detector (GRID) of the AGILE mission. The GRID is capable of detecting and imaging with optimal point spread function gamma-ray photons in the range 30 MeV–30 GeV. The AGILE planned orbit is equatorial, with an altitude of 550 km. This is an optimal orbit from the point of view of the expected particle background. For this orbit, electrons and positrons of kinetic energies between 20 MeV and hundreds of MeV dominate the particle background, with significant contributions from high-energy (primary) and low-energy protons, and gamma-ray albedo-photons. We present here the main results obtained by extensive simulations of the on-board AGILE-GRID particle/photon background rejection algorithms based on a special application of Kalman filter techniques. This filter is applied (Level-2) sequentially after other data processing techniques characterizing the Level-1 processing. We show that, in conjunction with the Level-1 processing, the adopted Kalman filtering is expected to reduce the total particle/albedo-photon background rate to a value ( ⩽ 10 – 30 Hz ) that is compatible with the AGILE telemetry. The AGILE on-board Kalman filter is also effective in reducing the Earth–albedo-photon background rate, and therefore contributes to substantially increase the AGILE exposure for celestial gamma-ray sources.

Coronagraphic Imaging with the Hubble Space Telescope and the Space Telescope Imaging Spectrograph11Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5‐26555. This study is part of the STIS IDT protoplanetary disk key project.

ABSTRACTThe unfiltered Space Telescope Imaging Spectrograph (STIS) CCD in tandem with focal plane wedges and a Lyot stop provides a simple white‐light coronagraph with a bandpass of 0.2–1.0 μm, which has been used since late 1998 to image nebulosity around stars in the ranges 0.34≤V≤14 and -0.03≤B− V≤1.65. The residual starlight seen in STIS coronagraphic images includes diffraction spikes due to the Hubble Space Telescope (HST) secondary support structure, lacks distinct Airy rings, and varies smoothly with radius from the star. The point‐spread function (PSF) shape is a strong function of the source spectral energy distribution: we find that the PSF template needs to differ from the occulted source color by Δ(B− V)≤0.08 mag. Optimal PSF removal is achieved for contemporary template observations matched to HST orbital phase of the science observations. Use of noncontemporary PSF templates can degrade the limiting contrast by up to a factor of 10–12 at r≤2′′. These systematic effects are negligible when the nebular surface brightness is comparable to the residual starlight, and they become progressively more important as surface brightness decreases. STIS has been used to detect circumstellar envelopes and protoplanetary disks with surface brightnesses spanning 10−4 to 10−7 per HST resolution element per Fstar at 2″ from the star, debris disks with FIR/Fstar≥0.001, and emission‐line nebulosity associated with bipolar outflows. The limiting contrast for single‐orbit integrations with the star placed at a location where the coronagraphic wedge is 1 .″0 wide is 10−8 per HST resolution element per Fstar for 6≤V≤8 stars. Deeper imagery can be obtained by placing the star off the active detector area. When the star is 5″ from the detector, a limiting contrast for a single‐orbit integration of 2.5 × 10-10 per HST resolution element per Fstar is reached. At this contrast level, background objects and the stellar color variability prevent further improvement by co‐adding data from multiple orbits.

Cardiac metabolism and function in patients with multiple sclerosis: a combined 31P-MR-spectroscopy and MRI study.

Involvement of the autonomic system in multiple sclerosis (MS) may concur with dysfunction of the cardiovascular system. The introduction of potentially cardiotoxic immunosuppressive drugs like Mitoxantrone into the treatment of MS warrants proper assessment of preexisting heart disease. However, systematic analyses of functional and metabolic derangements in MS are missing. Using quantitative 31P-MR-spectroscopy (MRS) and MR-imaging (MRI) metabolic and functional parameters were analyzed in patients with MS in comparison to healthy volunteers. 14/15 patients with MS could be included in the study, as the MRS examination of one patient had to be excluded from analysis due to movement during the examination. Using chemical shift imaging (CSI) and AMARES, phosphocreatine (PCr) to adenosine triphosphate (ATP) ratios, characterizing myocardial high-energy phosphate metabolism, were determined. Additionally, absolute concentrations of PCr and ATP were calculated by SLOOP (Spatial Localization with Optimal Pointspread Function). Analysis of functional changes was performed by Cine-MRI. 14 healthy volunteers matched for age and gender served as control. A significant decrease of absolute PCr concentration was observed in patients with MS compared to matched volunteers (p < 0.05), whereas ATP concentrations showed no significant changes (p = 0.27). Metabolite ratios calculated by SLOOP or AMARES showed a tendency to be reduced in patients, however, did not reach significance (p = 0.08, SLOOP; p = 0.47, AMARES). Using volunteers' mean values +/- 2 x SD as cut off value revealed PCr changes in 5 of 14 patients, whereas only 2 also had pathologic PCr/ATP ratios. Functional analysis by MRI depicted depressed left ventricular ejection fraction in 4 patients. The reduction in cardiac high-energy phosphates in some patients with MS points to a subclinical involvement of the heart. This may be important for treatment with potentially cardiotoxic drugs. Longitudinal studies are need to understand the clinical relevance of our findings.

Advances in human cardiac 31P-MR spectroscopy: SLOOP and clinical applications.

Phosphorus magnetic resonance spectroscopy (31P-MRS) has revealed a lot about the biochemistry of physiological and pathological processes in the heart. Nevertheless, until today, cardiac 31P-MRS has not had any clinical impact, albeit some pioneering studies demonstrated that 31P-MRS can indeed provide diagnostic information. In this paper, the development of techniques for human cardiac 31P-MRS over the past decade is reviewed, and the requirements for a reliable clinical measurement protocol are discussed. Spatial localization with optimal pointspread function (SLOOP) is a new method to achieve spatial localization and absolute quantitation. Its properties are detailed, and preliminary findings in patients with dilated cardiomyopathy or myocardial infarction are presented.

Beyond k-Space: Spectral Localization Using Higher Order Gradients

Chemical shift imaging (CSI) often suffers from the inconvenient shape of its spatial response function (SRF), which affects both localization and signal-to-noise ratio. Replacing the magnetic field gradients for phase encoding by higher order magnetic fields allows a better adjustment of the SRF to the structures in the sample. We combined this principle with the SLOOP (spectral localization with optimal pointspread function) technique to simultaneously obtain spectra from several arbitrarily shaped compartments within a sample. Linear combinations of the fields of the shim coils are used to generate the pulsed fields for phase encoding. Their shapes are matched to the given sample geometry by numerical optimization. Using this method, spectra from a phantom were obtained that show a higher signal-to-noise ratio and a strongly reduced contamination compared to an equivalent CSI experiment.

Concentrations of human cardiac phosphorus metabolites determined by SLOOP 31P NMR spectroscopy

Human cardiac 31P nuclear magnetic resonance (NMR) spectra are usually quantified in relative terms, i.e., the ratio of metabolite signals is calculated. If 31P NMR spectroscopy of the heart is to emerge as a clinically relevant diagnostic modality, reliable quantification of absolute concentrations of 31P metabolites is required. We applied spectral localization with optimal point spread function (SLOOP) 31P NMR spectroscopy to measure absolute concentrations of phosphocreatine (PCr) and adenosine triphosphate (ATP) in human myocardium. The accuracy of the quantification was first validated in a phantom study. Seven healthy volunteers (aged 19–29 years) were then examined at 1.5 T using a nominal spatial resolution of 25 mL. SLOOP allowed us to obtain localized spectra from compartments anatomically matched to the left ventricular wall. The a priori knowledge of the anatomical structure was obtained from 1H images. The spatially varying effects of saturation, off-resonance, and sensitivity were considered during the reconstruction process. Metabolites were quantified with reference to an external 31P standard. Concentrations of 9.0 ± 1.2 and 5.3 ± 1.2 mmol/kg wet wt (mean ± SD, n = 9) were determined for PCr and ATP in normal heart, respectively. The influence of nuclear Overhauser enhancement on metabolite quantification is discussed. Magn Reson Med 41:657–663, 1999. © 1999 Wiley-Liss, Inc.

Concentrations of human cardiac phosphorus metabolites determined by SLOOP 31P NMR spectroscopy.

Human cardiac 31P nuclear magnetic resonance (NMR) spectra are usually quantified in relative terms, i.e., the ratio of metabolite signals is calculated. If 31P NMR spectroscopy of the heart is to emerge as a clinically relevant diagnostic modality, reliable quantification of absolute concentrations of 31P metabolites is required. We applied spectral localization with optimal point spread function (SLOOP) 31P NMR spectroscopy to measure absolute concentrations of phosphocreatine (PCr) and adenosine triphosphate (ATP) in human myocardium. The accuracy of the quantification was first validated in a phantom study. Seven healthy volunteers (aged 19-29 years) were then examined at 1.5 T using a nominal spatial resolution of 25 mL. SLOOP allowed us to obtain localized spectra from compartments anatomically matched to the left ventricular wall. The a priori knowledge of the anatomical structure was obtained from 1H images. The spatially varying effects of saturation, off-resonance, and sensitivity were considered during the reconstruction process. Metabolites were quantified with reference to an external 31P standard. Concentrations of 9.0 +/- 1.2 and 5.3 +/- 1.2 mmol/kg wet wt (mean +/- SD, n = 9) were determined for PCr and ATP in normal heart, respectively. The influence of nuclear Overhauser enhancement on metabolite quantification is discussed.

Localized Spectroscopy from Anatomically Matched Compartments: Improved Sensitivity and Localization for Cardiac31P MRS in Humans

Several pioneering studies have demonstrated that localized31P NMR spectroscopy of the human heart might become an important diagnostic tool in cardiology. The main limitation is due to the low sensitivity of these experiments, allowing only crude spatial resolution. We have implemented a three-dimensional version of SLOOP (“spectral localization with optimal pointspread function”) on a clinical instrument. SLOOP takes advantage of all availablea prioriinformation to match the size and the shape of the sensitive volumes to the anatomical structures in the examined subject. Thus, SLOOP reduces the contamination from adjacent organs and improves the sensitivity compared to conventional techniques such as ISIS or chemical shift imaging (CSI). Initial studies were performed on six healthy volunteers at 1.5 T. The good localization properties are demonstrated by the absence of resonances from blood in the heart spectra, and by PCr-free spectra from the liver. Compared to conventional CSI, the signal-to-noise ratio of the SLOOP heart spectra was improved by approximately 30%. Taking into account the varying excitation angle in the inhomogeneous B1field of the surface coil, the SLOOP model computes the local spin saturation at every point in space. Therefore, no global saturation correction is required in the quantitative evaluation of local spectra. In this study, we found a PCr/γ-ATP ratio in the left ventricular wall of 1.90 ± 0.33 (mean ± standard deviation).

Spectral localization with optimal pointspread function

The SLIM experiment ( X. Hu, D. N. Levin, P. C. Lauterbur, and T. Spraggins, spectral localization by imaging, Magn. Reson. Med. 8, 314, 1988 ) promises to be an efficient technique for localized spectroscopy in vivo, allowing simultaneous acquisition of regional spectra from several, arbitrarily shaped compartments. In the present contribution, we evaluate how sensitivity is affected in this experiment, and we identify a source of localization errors stemming from inhomogeneities within the compartments. We present SLOOP (“spectral localization with optimal pointspread function”) as an extension of the SLIM technique. SLOOP optimizes sensitivity and minimizes localization errors by choosing an optimal set of phase-encoding gradients that matches the pointspread function to the shape of the volumes of interest. Experimental results obtained in vitro on a rabbit kidney are shown. SLOOP is also compared to Fourier series localization techniques and to spectroscopic imaging and is presented as a generalization of both of these that samples k space in a nonuniform fashion.