Biomedical subjects

Vikas Gulani

Publications and source records attributed to Vikas Gulani.

7 recordsLinked to original sources

PubMed2006

Vascular dynamics and BOLD fMRI: CBF level effects and analysis considerations.

Changes in the cerebral blood flow (CBF) baseline produce significant changes to the hemodynamic response. This work shows that increases in the baseline blood flow level produce blood oxygenation-level dependent (BOLD) and blood flow responses that are slower and lower in amplitude, while decreases in the baseline blood flow level produce faster and higher amplitude hemodynamic responses. This effect was characterized using a vascular model of the hemodynamic response that separated arterial blood flow response from the venous blood volume response and linked the input stimulus to the vascular response. The model predicted the baseline blood flow level effects to be dominated by changes in the arterial vasculature. Specifically, it predicted changes in the arterial blood flow time constant and venous blood volume time constant parameters of +294% and -24%, respectively, for a 27% increase in the baseline blood flow. The vascular model performance was compared to an empirical model of the hemodynamic response. The vascular and empirical hemodynamic models captured most of the baseline blood flow level effects observed and can be used to correct for these effects in fMRI data. While the empirical hemodynamic model is easy to implement, it did not incorporate any explicit physiological information.

Algorithms↗

PubMed2006

Diffusion tensor magnetic resonance imaging.

Molecular diffusion plays an important role in many biologic phenomena. The ability to study diffusion, therefore, is extremely useful in physiology and medicine. MRI offers a non-invasive window to diffusion, particularly water self-diffusion. MRI techniques, which provide diffusion sensitivity or quantitation (diffusion tensor MRI [DTI]), have found widespread application in neuroscience and medicine, including the evaluation of stroke, brain development, tumor imaging, and demyelinating disorders. We discuss the tensor nature of diffusion and provide an overview of how DTI offers unique information on tissue organization, water mobility, and disease states, particularly those of neuro-ophthalmologic interest.

Brain↗

PubMed2005

Hepatic fat fraction: MR imaging for quantitative measurement and display--early experience.

UNLABELLED: The institutional review board approved this HIPAA-compliant study. After all five patients with nonalcoholic fatty liver disease signed a consent, they underwent magnetic resonance (MR) imaging for hepatic fat quantification. The purpose of this study was to develop a fast and accurate method to acquire and display quantitative maps of the percentage of hepatic fat. In-phase and out-of-phase gradient-echo MR imaging was performed with dual flip angles (70 degrees, 20 degrees) to resolve ambiguity of the dominant constituent. T2* corrections were also estimated and applied to generate color-coded maps of the estimated percentage of hepatic fat. MR imaging results were compared with biopsy results in two of five patients, and the technique was validated qualitatively and quantitatively with a water-oil phantom. Results of the phantom study confirmed that the dual-flip angle algorithm can be used to correctly identify the dominant constituent, allowing depiction of 0%-100% of fat content. The estimated liver fat fraction was comparable to quantitative fat measurements at biopsy in both patients (MR imaging, 18.3% +/- 2.8 [standard deviation] and 28.6% +/- 2.4, vs quantitative histopathologic analysis, 11.2% and 28.5%, respectively). SUPPLEMENTAL MATERIAL: radiology.rsnajnls.org/cgi/content/full/2373041639/DC1

Adult↗

PubMed2005

Improved time efficiency and accuracy in diffusion tensor microimaging with multiple-echo acquisition.

In high-field NMR microscopy rapid single-shot imaging methods, for example, echo planar imaging, cannot be used for determination of the apparent diffusion tensor (ADT) due to large magnetic susceptibility effects. We propose a pulse sequence in which a diffusion-weighted spin-echo is followed by multiple gradient-echoes with additional diffusion weighting. These additional echoes can be used to calculate the ADT and T*2 maps. We show here that this results in modest but consistent improvements in the accuracy of ADT determination within a given total data acquisition time. The method is tested on excised, chemically fixed rat spinal cords.

Animals↗

PubMed2004

Inversion recovery TrueFISP: quantification of T(1), T(2), and spin density.

A novel procedure is proposed to extract T(1), T(2), and relative spin density from the signal time course sampled with a series of TrueFISP images after spin inversion. Generally, the recovery of the magnetization during continuous TrueFISP imaging can be described in good approximation by a three parameter monoexponential function S(t) = S(stst)(1-INV exp(-t/T(*) (1)). This apparent relaxation time T(*) (1) <or= T(1) depends on the flip angle as well as on both T(1) and T(2). Here, it is shown that the ratio T(1)/T(2) can be directly extracted from the inversion factor INV, which describes the relation of the signal value extrapolated to t = 0 and the steady-state signal. Analytical expressions are given for the derivation of T(1), T(2), and relative spin density directly from the fit parameters. Phantom results show excellent agreement with single point reference measurements. In human volunteers T(1), T(2), and spin density maps in agreement with literature values were obtained.

Algorithms↗

PubMed2004

Towards a single-sequence neurologic magnetic resonance imaging examination: multiple-contrast images from an IR TrueFISP experiment.

OBJECTIVE: The objective of this study was to reconstruct images bearing multiple contrasts from a single sequence magnetic resonance imaging (MRI) experiment. MATERIALS AND METHODS: Using a segmented IR-TrueFISP imaging sequence, the signal recovery after inversion and alpha/2 preparation was sampled in 6 volunteers. These images were used to generate T1, T2, and spin-density maps, allowing construction of images with multiple contrasts, including T1-, T2-, spin-density-weighted, and also FLAIR contrast. Traditionally acquired images bearing the corresponding contrast were obtained for comparison. Regression analysis was performed to compare the synthetic and traditionally acquired images for the whole brain and a region of interest in the occipital region. RESULTS: The synthetic images closely reproduced the contrast from the "standard" examination. Using regression analysis, the obtained image signal intensities for the calculated images compare favorably (P <0.0001-<<0.000001) with images acquired using multiple sequences. CONCLUSIONS: Perfectly registered images with any desired contrast based on T1, T2, and spin density, along with underlying quantitative maps, can be obtained using a single IR-TrueFISP sequence.

Brain↗

PubMed2002

Intracellular acidification and volume increases explain R(2) decreases in exercising muscle.

Exercise-induced decreases in the (1)H transverse relaxation rate (R(2)) of muscle have been well documented, but the mechanism remains unclear. In this study, the hypothesis was tested that R(2) decreases could be explained by pH decreases and apparent intracellular volume (V(i)') increases. (31)P and (1)H spectroscopy, biexponential R(2) analysis, and imaging were performed prior to and following fatiguing exercise in iodoacetate-treated (IAA, to inhibit glycolysis), NaCN-treated (to inhibit oxidative phosphorylation), and untreated frog gastrocnemii. In all exercised muscles, the apparent intracellular R(2) (R(2i)') and pH decreased, while intracellular osmolytes and V(i)' increased. These effects were larger in NaCN-treated and untreated muscles than in IAA-treated muscles. Multiple regression analysis showed that pH and V(i)' changes explain 70% of the R(2i)' variance. Separate experiments in unexercised muscles demonstrated causal relationships between pH and R(2i)' and between V(i)' and R(2i)'. These data indicate that the R(2) change of exercise is primarily an intracellular phenomenon caused by the accumulation of the end-products of anaerobic metabolism. In the NaCN-treated and untreated muscles, the R(2i)' change increased as field strength increased, suggesting a role for pH-modulated chemical exchange.

Animals↗