Ramesh Chandra

NMR relaxation in tissues with weak magnetic inhomogeneities

A theory is presented for describing the effect on the transverse NMR relaxation rate of microscopic spatial inhomogeneities in the static magnetic field. The theory applies when the inhomogeneities are weak in magnitude and the nuclear spins diffuse a significant distance in comparison with a length scale characterizing the inhomogeneities. It is shown that the relaxation rate is determined by a temporal correlation function and depends quadratically on the magnitude of the inhomogeneities. For the case of unrestricted diffusion, a simple algebraic approximation for the temporal correlation function is derived. The theory is illustrated by applying it to a model of randomly distributed magnetized spheres. The theory is also used to fit experimental data for the dependence of the relaxation rate on the interecho time for a Carr‐Purcell‐Meiboom‐Gill pulse sequence. The experimental systems considered are in vitro red blood cell suspensions and samples of human gray matter and rat liver. Magn Reson Med 44:144–156, 2000.

MR imaging of microvasculature.

An imaging technique is proposed in which the contrast is correlated to the morphology of capillaries and other small blood vessels. The technique is based on measurements of the relaxation rates 1/T2 and 1/T*2 before and after the injection of a contrast agent. An image is then formed by mapping the quantity Q ≡ ΔR2/(ΔR*2)2/3, where ΔR2 and ΔR*2 are the changes in the relaxation rates due to the contrast agent. If the contrast agent concentration is sufficiently high, it is shown that Q is given approximately by a simple analytic formula that involves only intrinsic properties of the vascular network and the rate of diffusion. In particular, Q is sensitive to the histologic vessel density. Theoretical predictions for Q are shown to be consistent with experimental data obtained with a rat glioma model and normal cerebral cortex. The imaging technique may be useful in characterizing tumor angiogenesis. Magn Reson Med 44:224–230, 2000.

Magnetic field correlation imaging

A magnetic resonance imaging (MRI) method is presented for estimating the magnetic field correlation (MFC) associated with magnetic field inhomogeneities (MFIs) within biological tissues. The method utilizes asymmetric spin echoes and is based on a detailed theory for the effect of MFIs on nuclear magnetic resonance (NMR) signal decay. The validity of the method is supported with results from phantom experiments at 1.5 and 3 T, and human brain images obtained at 3 T are shown to demonstrate the methods feasibility. The preliminary results suggest that MFC imaging may be useful for the quantitative assessment of iron within the brain. Magn Reson Med, 2006.

Strong field behavior of the NMR signal from magnetically heterogeneous tissues

A theory for the behavior of the nuclear magnetic resonance (NMR) signal obtained from magnetically heterogeneous tissues is developed for the limit of a strong external magnetic field. If B0 is the magnitude of the external magnetic field, it is found that a free‐induction signal decays in a time scaling as 1/B0, a single‐spin echo signal decays in a time scaling as 1/B u200902/3 , and a multiple‐spin echo signal decays in a time scaling as 1/B u200902 . Moreover, it is shown that the form of the signal decay for a multiple‐spin echo sequence may deviate significantly from an exponential. Numerical results for a model consisting of randomly distributed magnetic spheres are used to confirm the theory. In addition, good agreement is demonstrated between the theory and experimental measurements obtained with particle suspensions. The validity and application of the theory to biological tissues are discussed. Magn Reson Med 43:226–236, 2000.

Theory of nonexponential NMR signal decay in liver with iron overload or superparamagnetic iron oxide particles

A quantitative theory is proposed for the nonexponential NMR proton signal decay observed in liver with iron overload or superparamagnetic iron oxide particles. This effect occurs for Carr‐Purcell‐Meiboom‐Gill (CPMG) sequences and is argued to be a direct consequence of the strong magnetic field inhomogeneities generated by the iron, rather than being due to tissue compartments. An approximate mathematical form is given for the signal decay, which is fit to experimental data for samples of rat liver with iron oxide particles, for samples of marmoset liver with hemosiderosis, and for in vivo human liver with hereditary hemochromatosis. The fitting parameters obtained are consistent with the pattern of iron deposition determined from histology. For the case of hereditary hemochromatosis, a good correlation is found between a parameter characterizing the nonexponential decay and the iron concentration. Implications for practical MR quantification of hepatic iron are discussed. Magn Reson Med 47:1131–1138, 2002.

Magnetic field correlation as a measure of iron-generated magnetic field inhomogeneities in the brain.

The magnetic field correlation (MFC) at an applied field level of 3 Tesla was estimated by means of MRI in several brain regions for 21 healthy human adults and 1 subject with aceruloplasminemia. For healthy subjects, highly elevated MFC values compared with surrounding tissues were found within the basal ganglia. These are argued as being primarily the result of microscopic magnetic field inhomogeneities generated by nonheme brain iron. The MFC in the aceruloplasminemia subject was significantly higher than for healthy adults in the globus pallidus, thalamus and frontal white matter, consistent with the known increased brain iron concentration associated with this disease. Magn Reson Med 61:481–485, 2009.

Brain tissue volume measurement from magnetic resonance imaging. A phantom study.

RATIONALE AND OBJECTIVESThe authors appraised the accuracy of a method for brain volume measurement from magnetic resonance images and evaluated the effects of the acquisition matrix, slice thickness, and tissue sampling on the measurement error. METHODS.The method uses two magnetic resonance imaging sequences to account explicitly for partial volume effects. The accuracy was measured with one-, two-, and three- compartmental phantoms that mimic the relaxation properties of brain tissues. The sensitivity of the method to section thickness was measured by repeated scans of human brain. RESULTS.Using a strongly T2-weighted sequence and two-compartmental phantoms, the average error was 5%, with 3% error for phantoms larger than 90 mL. In the three-compartmental phantoms the error varied from 2% to 7%. Varying the section thickness from 5 to 10 mm on three-compartmental phantoms and from 2.5 to 10 mm in the human brain did not significantly affect compartmental volumes. CONCLUSIONS.The experimental study validates the feasibility of monitoring localized volume changes in a three-compartmental model.

Quantitative model for the interecho time dependence of the CPMG relaxation rate in iron-rich gray matter.

A quantitative model is proposed for computing the dependence on the interecho time of the NMR relaxation rate in iron‐rich gray matter obtained with a Carr‐Purcell‐Meiboom‐Gill sequence. The model consists of representing oligodendrocytes as identical magnetic spheres arranged in a spatially random pattern, and in approximating water diffusion as isotropic and unrestricted. Predictions of the model are calculated numerically using a Monte Carlo technique and, for the weak field limit, using an analytic formula. The model is shown to provide a good fit to experimental measurements of in vitro samples of monkey brain at field levels of 1.0 T and 1.5 T. These field levels are not sufficient to fully determine the model parameters, but it is argued that this may be possible at 3.0 T. The model is potentially of value for multiple‐spin‐echo MRI studies of iron‐related neurodegenerative disorders, such as Parkinsons disease. In particular, the model can be applied to correlate MRI data with the cellular distribution of iron in gray matter. Magn Reson Med 46:159–165, 2001.

A micromethod for evaluating the phagocytic activity of human macrophages by ingestion of radio-labelled polystyrene particles.

Studies in vitro of human macrophage function in health and disease have been impeded by the difficulty of obtaining such cells in sufficient number. Unlike animal species, the only readily available source of human macrophages are circulating monocytes. Herein, a method is described whereby the phagocytic rate of small numbers of glass-adherent mononuclear cells can be accurately measured. The method utilizes the ingestion by macrophages of technetium labelled polystyrene particles; both the radiolabel and ingestible substrate are readily available and the labelling process simple and efficient. The phagocytic rate can be expressed as radioactive counts per microgram of cell protein; data is also presented showing that the number of particles ingested per cell can be accurately derived.

Production of 167Tm for medical use

A method is described for producing sufficient amounts of 167Tm for medical use in a carrier free form on a small medical cyclotron. Using enriched 167Er2O3 up to 75 μCi/μAhr can be produced. The biological distribution of 167Tm HEDTA in rabbits is also presented. Most of the activity (up to 70 per cent) is localized in the skeleton and the rest is excreted.