Interregional variation of longitudinal relaxation rates in human brain at 3.0 T: Relation to estimated iron and water contents

Gelman, Neil; Ewing, James R.; Gorell, Jay M.; Spickler, Eric M.; Solomon, Enez G.

doi:10.1002/1522-2594(200101)45:1<71::aid-mrm1011>3.0.co;2-2

Cited by 249 publications

(271 citation statements)

References 31 publications

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“…2,9,10 However, depending on the technique chosen for data acquisition, a variety of different contrasts may be obtained, which reflect the T 1 or T 2 relaxation properties of mobile water protons, magnetization transfer between water and macromolecular protons, or extra-and intracellular distribution of exogenous contrast agents in brain tissue.…”

Section: In Vivo Brain Mr Imagingmentioning

confidence: 99%

“…Without myelin, the WM/GM contrast in T 1 W, T 2 W, and magnetization transfer MR imaging would be substantially reduced because the water content of the non-myelin portion of brain WM is not different from that of GM (both about 80%). 2,9,10,53,57 In plain T 2 W MR imaging, additional T 2 shortening by myelin may contribute to the WM/GM contrast, although to a limited degree. 2 Whereas immobilized myelin macromolecules accelerate the transverse relaxation of affected water protons (T 2 < 15 ms), the myelin water comprises only a small volume fraction (<15%) in WM, 2,21 and its exchange with other compartments is slow.…”

Section: Myelinmentioning

confidence: 99%

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<i>In Vivo</i> Brain MR Imaging at Subnanoliter Resolution: Contrast and Histology

2016

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This article provides an overview of in vivo magnetic resonance (MR) imaging contrasts obtained for mammalian brain in relation to histological knowledge. Emphasis is paid to the (1) significance of high spatial resolution for the optimization of T 1 , T 2 , and magnetization transfer contrast, (2) use of exogenous extra-and intracellular contrast agents for validating endogenous contrast sources, and (3) histological structures and biochemical compounds underlying these contrasts and (4) their relevance to neuroradiology. Comparisons between MR imaging at subnanoliter resolution and histological data indicate that (a) myelin sheaths, (b) nerve cells, and (c) the neuropil are most responsible for observed MR imaging contrasts, while (a) diamagnetic macromolecules, (b) intracellular paramagnetic ions, and (c) extracellular free water, respectively, emerge as the dominant factors. Enhanced relaxation rates due to paramagnetic ions, such as iron and manganese, have been observed for oligodendrocytes, astrocytes, microglia, and blood cells in the brain as well as for nerve cells. Taken together, a plethora of observations suggests that the delineation of specific structures in high-resolution MR imaging of mammalian brain and the absence of corresponding contrasts in MR imaging of the human brain do not necessarily indicate differences between species but may be explained by partial volume effects. Second, paramagnetic ions are required in active cells in vivo which may reduce the magnetization transfer ratio in the brain through accelerated T 1 recovery. Third, reductions of the magnetization transfer ratio may be more sensitive to a particular pathological condition, such as astrocytosis, microglial activation, inflammation, and demyelination, than changes in relaxation. This is because the simultaneous occurrence of increased paramagnetic ions (i.e., shorter relaxation times) and increased free water (i.e., longer relaxation times) may cancel T 1 or T 2 effects, whereas both processes reduce the magnetization transfer ratio.

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Section: In Vivo Brain Mr Imagingmentioning

confidence: 99%

Section: Myelinmentioning

confidence: 99%

<i>In Vivo</i> Brain MR Imaging at Subnanoliter Resolution: Contrast and Histology

2016

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“…Many experiments have correlated the relaxation rates R 2 ϭ 1/T 2 and R 2 * ϭ 1/T 2 * with the age of the individual at the time of the imaging (13)(14)(15)(16)(17). In the absence of independent measurements of tissue iron content, these studies estimated the iron content from the empirical models of iron accumulation developed by Hallgren and Sourander in 1958 (18).…”

mentioning

confidence: 99%

Correlation of R2 with total iron concentration in the brains of rhesus monkeys

Hardy

Gash

Yokel

et al. 2005

Magnetic Resonance Imaging

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Purpose: To estimate the relationship between R 2 ϭ 1/T 2 as measured with a double echo spin echo sequence and total iron concentration in gray matter structures in the brains of aging rhesus monkeys. Materials and Methods:Using a 1.5-T magnetic resonance (MR) imager, we collected double echo spin echo images of the brains of 12 female rhesus monkeys aged between 9 and 23 years. From the double echo images, the transverse relaxation rate R 2 ϭ 1/T 2 was calculated in selected gray matter regions. After the animals were euthanized, their brains were excised and tissue punches were taken of the substantia nigra, globus pallidus, and gray matter regions of the cerebellum. Some of the tissue punches were assayed for total iron using atomic absorption spectroscopy. Results:The range of tissue iron concentration spanned from 15 to 450 g/g wet weight, with the highest levels in the globus pallidus and the lowest levels in the cerebellum. The results show that R 2 was highly correlated with the total iron concentration and that the relationship between R 2 and tissue iron concentration appeared to depend upon the iron concentration. For concentrations above approximately 150 g/g wet weight, R 2 increased with a sensitivity of 0.0484 Ϯ 0.0023 second -1 ( g/g) -1 . In contrast, where the iron concentration was below 150 g/g, R 2 increased at 0.0013 Ϯ 0.0073 second -1 ( g/g) -1 . The bilinear behavior may reflect changes with age in the relative amounts of iron distributed diffusely and in granular form in the globus pallidus and substantia nigra. Histological sections of the tissues stained for iron and ferritin support this hypothesis and indicate that the distribution of ferritin is similar to the distribution of iron. Conclusion:This study reaffirms the value of measuring the MR relaxation rate R 2 for a noninvasive estimate of iron content in the brain and identified limitations in the relationship at low tissue iron concentrations.

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“…This bias can arise because of components that have not been explicitly included in the model. However, it has previously been shown that macromolecular and iron components, which are rather well captured by MT and R 2 * respectively,31, 32 are the dominant contributors to R 1 11, 15, 16, 33, 34, 35. Any orientation‐dependent or higher‐order relationships that exist between the qMRI maps are not captured by the model either, and may also be a source of bias 36…”

Section: Discussionmentioning

confidence: 99%

“…In the absence of exogenous contrast agents, the measured R 1 is dominated by contributions from free water spins, bound water spins at macromolecular sites and a smaller, spatially varying contribution from iron sites 15, 16. Under conditions of fast exchange, the measured R 1 can be expressed as a weighted sum of the relaxivities of these compartments14:

R_{1} = R_{1 normalf} + f_{M} r_{1 normalM} + f_{FE} r_{1 FE} + true \underset{j}{true\sum} f_{j} r_{1 j} .

…”

Section: Theory: Linear Relaxometry Modelmentioning

confidence: 99%

Synthetic quantitative MRI through relaxometry modelling

2016

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Quantitative MRI (qMRI) provides standardized measures of specific physical parameters that are sensitive to the underlying tissue microstructure and are a first step towards achieving maps of biologically relevant metrics through in vivo histology using MRI. Recently proposed models have described the interdependence of qMRI parameters. Combining such models with the concept of image synthesis points towards a novel approach to synthetic qMRI, in which maps of fundamentally different physical properties are constructed through the use of biophysical models. In this study, the utility of synthetic qMRI is investigated within the context of a recently proposed linear relaxometry model. Two neuroimaging applications are considered. In the first, artefact‐free quantitative maps are synthesized from motion‐corrupted data by exploiting the over‐determined nature of the relaxometry model and the fact that the artefact is inconsistent across the data. In the second application, a map of magnetization transfer (MT) saturation is synthesized without the need to acquire an MT‐weighted volume, which directly leads to a reduction in the specific absorption rate of the acquisition. This feature would be particularly important for ultra‐high field applications. The synthetic MT map is shown to provide improved segmentation of deep grey matter structures, relative to segmentation using T 1‐weighted images or R 1 maps. The proposed approach of synthetic qMRI shows promise for maximizing the extraction of high quality information related to tissue microstructure from qMRI protocols and furthering our understanding of the interrelation of these qMRI parameters.

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Interregional variation of longitudinal relaxation rates in human brain at 3.0 T: Relation to estimated iron and water contents

Cited by 249 publications

References 31 publications

<i>In Vivo</i> Brain MR Imaging at Subnanoliter Resolution: Contrast and Histology

<i>In Vivo</i> Brain MR Imaging at Subnanoliter Resolution: Contrast and Histology

Correlation of R2 with total iron concentration in the brains of rhesus monkeys

Synthetic quantitative MRI through relaxometry modelling

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