Search PubMed⌕ Search

Biomedical subjects

J A Helpern

Publications and source records attributed to J A Helpern.

At least 19 recordsLinked to original sources
PubMed2003

Motion correction in MRI using an apparatus for dynamic angular position tracking (ADAPT).

Motion during MRI examinations is a serious problem that degrades the quality of the data (images) acquired. Motion can be corrected during the postprocessing of the data; however, this approach is suboptimal and is typically limited to in-plane or translational motion. An apparatus for dynamic angular position tracking (ADAPT) for prospective angular motion correction has been developed. This application is capable of "tracking" the scanned region of interest by performing dynamic adjustments of orientation of the scanning plane. The operation of the apparatus is based on deuterium MR spectroscopy and does not rely on the use of magnetic field gradients. Orientation-sensitive deuterium quadrupolar interaction in a single crystal attached to a subject is used to monitor the angular position in magnetic fields. Measurements are performed with an independent spectrometer channel in the background of the MRI scans. This apparatus is very cost- and time-efficient because it utilizes the hardware already available on many spectrometers and can be used in parallel with MRI scans. Potentially, rotations by a fraction of one degree can be easily corrected and the angular position information can be rapidly updated.

Artifacts↗
PubMed2001

RF excitation profiles with FAIR: impact of truncation of the arterial input function on quantitative perfusion.

This study investigates the impact of imaging coil length and consequent truncation of the arterial input function on the perfusion signal contrast obtained in the flow-sensitive alternating inversion recovery (FAIR) perfusion imaging measurement. We examined the difference in perfusion contrast achieved with head, head and neck, and body imaging coils based on the hypothesis that the standard head coil provides a truncated input function compared with that provided by the body coil and that this effect will be accentuated at long inversion times. The TI-dependent cerebral response of the FAIR sequence was examined at 1.5 T by varying the TI from 200 to 3500 msec with both the head and whole body coils (n = 5) as well as using a head and neck coil (n = 3). Difference signal intensity DeltaM and quantitative cerebral blood flow (CBF) were plotted against TI for each coil configuration. Despite a lower signal-to-noise ratio, relative CBF was significantly greater when measured with the body or head and neck coil compared with the standard head coil for longer inversion times (two-way ANOVA, P < or = 0.002). This effect is attributed to truncation of the arterial input function of labeled water by the standard head coil and the resultant inflow of unlabeled spins to the image slice during control image acquisition, resulting in overestimation of CBF. The results support the conclusion that the arterial input function depends on the anatomic extent of the inversion pulse in FAIR, particularly at longer mixing times (TI > 1200 msec at 1.5 T). Use of a head and neck coil ensures adequate inversion while preserving SNR that is lost in the body coil.

Adult↗
PubMed2001

A quantitative comparison of motion detection algorithms in fMRI.

An important step in the analysis of fMRI time-series data is to detect, and as much as possible, correct for subject motion during the course of the scanning session. Several public domain algorithms are currently available for motion detection in fMRI. This paper compares the performance of four commonly used programs: AIR 3.08, SPM99, AFNI98, and the pyramid method of Thévenaz, Ruttimann, and Unser (TRU). The comparison is based on the performance of the algorithms in correcting a range of simulated known motions in the presence of various degrees of noise. SPM99 provided the most accurate motion detection amongst the algorithms studied. AFNI98 provided only slightly less accurate results than SPM99, however, it was several times faster than the other programs. This algorithm represents a good compromise between speed and accuracy. AFNI98 was also the most robust program in presence of noise. It yielded reasonable results for very low signal to noise levels. For small initial misalignments, TRU's performance was similar to SPM99 and AFNI98. However, its accuracy diminished rapidly for larger misalignments. AIR was found to be the least accurate program studied.

Algorithms↗
PubMed1999

Rapid and continuous monitoring of cerebral perfusion by magnetic resonance line scan assessment with arterial spin tagging.

A new approach is presented for rapid and continuous monitoring of cerebral perfusion which is based upon line-scan MR column imaging with arterial spin tagging (AST) of endogenous water. Spin tagging of arterial water protons is accomplished using adiabatic fast passage inversion, followed by acquisition of the perfusion sensitive MR signal from a column placed at the desired level through the brain using line scan localization techniques. A perfusion sensitive line scan is followed by a non-perfusion sensitive line scan, and perfusion is calculated pixel-by-pixel from the intensity difference of the two lines. Continuous perfusion measurements are reported with temporal resolution of 10 s in pixels of volume 0.027 cm3 or less. Examples of the methodology are given during hypercapnic challenge induced with carbon dioxide, and during an ischemic event induced by reversible middle cerebral artery occlusion. The method is also used to characterize the signal response as a function of arterial inversion time and post inversion acquisition delay. These methods permit rapid and continuous monitoring of cerebral perfusion with high spatial resolution, and can be interleaved with MR measurements of diffusion and T1 to follow the progression of cerebral events during physiological or pharmacological intervention.

Animals↗
PubMed1999

MR perfusion imaging in human brain using the UNFAIR technique. Un-inverted flow-sensitive alternating inversion recovery.

Pulsed arterial spin labeling magnetic resonance techniques have been developed recently to estimate cerebral blood flow (CBF). Flow-sensitive alternating inversion recovery (FAIR) is one such technique that has been implemented successfully in humans. Un-inverted FAIR (UNFAIR) is an alternative technique in which the flow-sensitive image is acquired following inversion of all spins outside the slice of interest, and the control image is acquired without any spin labeling. This approach is potentially more efficient than FAIR since the UNFAIR control image is entirely flow independent and need only be acquired once. Here, we describe implementation of the sequence on a clinical 1.5 T magnetic resonance system. Both FAIR and UNFAIR perfusion-weighted images were obtained from six normal volunteers. Wash-in/wash-out curves measured in cortical gray and white matter were practically identical for the two techniques, as predicted by our model.

Adult↗
PubMed1998

The influence of preischemic hyperglycemia on acute changes in brain water ADCw following focal ischemia in rats.

The effect of preischemic hyperglycemia on the acute decline of brain apparent diffusion coefficient of water (ADCw) following cerebral ischemia was studied in a rat model of middle cerebral artery occlusion (MCAO). ADCw was measured by NMR with a newly developed spin-echo line-scan protocol that provides for an ADCw calculation every 15 s at a spatial resolution of 3.4 microl/pixel. A remote controlled occluding device was used to initiate ischemia from outside the magnet, allowing for continuous monitoring of ADCw before, during and after MCAO. Preischemic hyperglycemia (25-30 mM) was achieved via i.v. infusion of 50% glucose. The decline in ADCw following ischemia was analyzed to obtain three-time constants: the time from onset of ischemia to initial significant ADCw decline below baseline level (i.e., 20% of maximal decline, T0.20), the time to decline by 50% (T0.50), and the time to decline by 95% (T0.95). Mean (+/-S.D.) values for T0.20, T0.50, T0.95 were: 39.6+/-7.2, 54. 0+/-7.8, 105.0+/-15.0 s for the normoglycemic group (n=7), and 49. 2+/-33.0, 116.4+/-2.4, 351.0+/-189.0 s for the hyperglycemic group (n=6), respectively. Hyperglycemia significantly prolongs T0.50 and T0.95 but does not affect T0.20. The temporal profiles of ADCw decline following ischemia under normo- and hyperglycemia are distinctively different from the known time course of membrane depolarization under similar experimental conditions, suggesting that mechanisms other than membrane depolarization and cell swelling may contribute to changes in ADCw in cerebral ischemia.

Animals↗
PubMed1998

Perfusion imaging using FOCI RF pulses.

Pulsed arterial spin-tagging techniques for perfusion measurements (e.g., echo planar MR imaging and signal targeting with alternating radiofrequency (EPISTAR), flow-sensitive alternating inversion recovery (FAIR), quantitative imaging of perfusion using a single subtraction (QUIPPS), uninverted FAIR (UNFAIR)) generally use hyperbolic secant (HS) pulses for spin inversion. The performance of these techniques depends on the inversion efficiency, as well as the sharpness of the slice profiles. Frequency offset corrected inversion (FOCI) pulses, a recently proposed HS variant, can provide slice profiles with edges that can be up to 10 times sharper than those obtained with conventional HS pulses. In this communication, the implementation and application of the C-shape FOCI pulse for perfusion imaging in rat brain with the FAIR technique is summarized. Despite providing a more rectangular slice profile than a conventional HS pulse, it is demonstrated both theoretically and experimentally that the FAIR perfusion signal is not increased by using a FOCI tagging pulse. However, the use of a FOCI inversion pulse is shown to significantly minimize static signal subtraction errors that are common with conventional HS pulses. Finally, the suitability of the pulse for perfusion studies is demonstrated, in vivo, on rat brain.

Animals↗
PubMed1997

The influence of preischemic hyperglycemia on acute changes in the apparent diffusion coefficient of brain water following global ischemia in rats.

We report the effect of increased plasma glucose levels on changes in the apparent diffusion coefficient of brain water (ADCw) during the first few minutes of global ischemia in rats. Brain ADCw values were acquired every 15 s using a diffusion-weighted line-scan MR pulse sequence. Preischemic hyperglycemia was achieved by infusion of 50% dextrose (i.v.) prior to KCl-induced cardiac arrest global ischemia. Analysis based on single voxels (3.4 microl) in brain demonstrated significant differences in the time course of ADCw decline between normoglycemic (n = 8) and hyperglycemic (n = 6) groups. Mean data from the hyperglycemic group indicated a biphasic decline of ADCw that was characterized by an initial rapid drop followed by a plateau of approximately 1 min before gradually declining and leveling off to its minimum value. In the normoglycemic group, ADCw declined to the same value as in the hyperglycemic group, but without a notable plateau. In the cerebral cortex, the times to maximal and half maximal ADCw drop following global ischemia in the hyperglycemic group were 3.96 and 2.26 min respectively. Corresponding time intervals for the normoglycemic group were 1.86 and 1.14 min, respectively. The time course for changes in ADCw demonstrated here is significantly different than that for anoxic depolarization reported under similar experimental conditions and suggests that events other than the complete loss of membrane ionic homeostasis and subsequent cell swelling may be involved in the initial decline of ADCw in global cerebral ischemia.

Animals↗
PubMed1997

The application of diffusion-weighted line-scanning for the rapid assessment of water ADC changes in stroke at high magnetic fields.

Rapid changes in the apparent diffusion coefficient of water following brain ischemia have been extensively studied using echo planar diffusion imaging at low fields (2.0 T). There is a desire to perform these studies at higher fields (> 3.0 T) where the benefits of improved signal-to-noise can be exploited. Unfortunately, EPI diffusion is technically difficult to implement at high fields because of large magnetic susceptibility effects. This article demonstrates the feasibility of employing a line-scan diffusion protocol for ADCw measurements in stroke. The technique was applied on a 4.0 T system to monitor the decline in ADCw following the induction of focal cerebral ischemia in rat. ADCw data were acquired every 15 s with 10 b-values or every 22.5 s with 15 b-values, with a cubic spatial resolution of 1.5 mm. The results demonstrate that estimates of ADCw can be acquired with coefficients of variation under 3.0%, and with a combination of spatial and temporal resolution comparable to that previously reported for EPI.

Animals↗
PubMed1997

Perfusion imaging by un-inverted flow-sensitive alternating inversion recovery (UNFAIR).

A new pulse sequence for estimating cerebral blood flow called UNFAIR, which uses a combination of sequential hyperbolic secant preparatory pulses, is introduced. This sequence is based on the same generalized conditions as previously introduced inversion recovery techniques except that the spins in the image slice of interest always have +z magnetization and the in-flowing spins are alternately inverted and uninverted. CBF-weighted images of rat brain under conditions of normocpnia and hypercapnia are presented and demonstrate the expected CBF response. A model describing the signal response to this pulse sequence is also presented and compared with in-vivo data acquired from gray and white matter.

Animals↗
PubMed1995

Abnormalities of skeletal muscle metabolism during nerve stimulation determined by 31P nuclear magnetic resonance spectroscopy in severe congestive heart failure.

These data confirm the presence of abnormal skeletal muscle metabolic function in patients with chronic heart failure. These changes are independent of patient compliance and central hemodynamic response to conventional exercise, and may explain, in part, the marked decrease in exercise tolerance often observed in patients with chronic severe heart failure. The etiology of these abnormalities of metabolic function and nutritive flow remains uncertain. TNS may prove useful in further elucidation of the peripheral metabolic consequences of chronic heart failure.

Acute Disease↗
PubMed1995

Preliminary observations of transverse relaxation rates obtained at 3 tesla from the substantia nigra of adult normal human brain.

The transverse relaxation rates R2 and R2* were measured at 3 T in the substantia nigra of the midbrain of adult normal human controls. The relaxation rate arising from magnetic inhomogeneities, R2', was calculated from the relationship R2* = R2 + R2'. No significant differences were found for any parameters between left and right side substantia nigra regions, however, a significant inverse correlation was found between R2 and R2' (r = 0.70, p = 0.035). Water diffusion, in the presence of paramagnetic metal ions, may play a role in determining the relative contributions to reversible and irreversible transverse relaxation and may explain the inverse correlation demonstrated here.

Aged↗
PubMed1995

Diffusion-weighted imaging in epilepsy.

Diffusion-weighted imaging (DWI) is a relatively new magnetic resonance imaging (MRI) technique that can be used to probe the microenvironment of water. Contrast in DWI depends on properties different from traditional T1 and T2 contrast, and is derived form the translational motion of water molecules. Since it is reasonable to think that a change in the microenvironment of water might be reflected in a change in water diffusion characteristics, the quantitative assessment of the (apparent) diffusion coefficient ADCw may represent a unique means of assessing tissue status. DWI has already shown great utility in the study of cerebral ischemia in animal models and has proved useful in the early identification of cerebral ischemia in patients. More recent reports have indicated a potential for DWI in studying epilepsy. Here, we briefly review some of what is known about the measurement of ADCw in ischemia and compare these results with what has recently been reported for epilepsy. In this manner we hope to better understand the underlying mechanisms behind changes in water diffusion associated with specific pathologies.

Animals↗
PubMed1995

Increased iron-related MRI contrast in the substantia nigra in Parkinson's disease.

Elevated iron levels in the substantia nigra (SN) of the brain in Parkinson's disease (PD) may mediate lipid peroxidative reactions, promoting SN neuronal death. To assess SN iron accumulation in living PD patients and its relation to motor performance, we measured, in 13 nondemented PD patients and 10 normal control subjects, simple reaction time (SRT) and simple movement time (SMT), followed by head MRI in a 3-tesla system. We measured T2 and T2* in the right and left SN of all subjects and calculated R2', the relaxation rate due to local magnetic field in-homogeneities, from these values. Asymmetries of 1/T2 (R2), 1/T2* (R2*), or R2' versus asymmetries of SRT and SMT were assessed in eight PD subjects who had not taken anti-PD medication(s) for 12 hours. The average of right and left SN values for R2 was lower, and R2* and R2' were higher, in PD patients than in controls (R2, p = 0.046; R2*, p = 0.001; R2', p < 0.001). R2' best predicted group differences. The asymmetry of SRT performance was highly correlated with asymmetries of SN R2* (0.91; p = 0.001) and R2' (0.72; p = 0.03). These results strongly suggest that the increases in iron levels seen postmortem in the SN in PD are reflected in increased iron-related MRI contrast at 3 tesla in living PD patients. Correlations with motor performance in PD suggest that the clinical severity of PD may be related to SN iron accumulation.

Adult↗
PubMed1994

Assessment of relative brain iron concentrations using T2-weighted and T2*-weighted MRI at 3 Tesla.

In this paper a new method is presented for the relative assessment of brain iron concentrations based on the evaluation of T2 and T2*-weighted images. A multiecho sequence is employed for rapid measurement of T2 and T2*, enabling calculation of the line broadening effect (T2'). Several groups have failed to show a correlation between T2 and brain iron content. However, quantification of T2', and the associated relaxation rate R2', may provide a more specific relative measure of brain iron concentration. This may find application in the study of brain diseases, which cause associated changes in brain iron levels. A new method of field inhomogeneity correction is presented that allows the separation of global and local field inhomogeneities, leading to more accurate T2* measurements and hence, T2' values. The combination of T2*, and T2-weighted MRI methods enables the differentiation of Parkinson's disease patients from normal age-matched controls based on differences in iron content within the substantia nigra.

Aged↗
PubMed1994

A low flip angle spin-echo technique for producing rapid diffusion weighted MR images.

A method is described for producing rapid diffusion-weighted images using a modified low flip angle imaging technique. Utility of the method is demonstrated by the quantification of diffusion coefficients in a rat model of focal ischemia. The method may be readily applied to animal research studies using NMR research systems with modest gradient capabilities.

Animals↗
PubMed1994

Correction of motional artifacts in diffusion-weighted MR images using navigator echoes.

Patient motion can seriously degrade the quality of diffusion-weighted MR images obtained using standard 2DFT imaging procedures. The main source of error arises from an MR signal phase-shift error which is proportional to the magnitude of the motion. A modified pulse sequence is proposed which uses the phase information from an additional spin echo to correct for patient motion. Application of this technique is demonstrated for a human brain study, which greatly improves the quantification of diffusion values from regions of brain tissue.

Artifacts↗