Comparison of T2 relaxation in blood, brain, and ferritin

Dependence of brain-tissue R2 on MRI field strength

PURPOSE

To quantify T2 relaxation in the brain at 3 T and 7 T to study its field dependence and correlation with iron content, and to investigate whether iron can be separated from other sources of T2 relaxation based on this field dependence.

METHODS

Nine subjects were scanned at both field strengths with the same acquisition technique, which used multiple gradient-echo sampling of a spin echo. This allowed for separation of T2 relaxation from static dephasing by B0 field inhomogeneities and the effects of radiofrequency refocusing imperfections. The average relaxation rates (R2 = 1/T2) in multiple regions of interest in the brain were fitted with a model linear in B0 and correlated with literature iron values.

RESULTS

The relationship between the R2 values at the two field strengths appeared to be linear over all regions of interest. The R2 values (in s-1) in the regions of interest for which both an iron and a lipid mass fraction have been documented in the literature were fitted asR 2 = 9 + 0.9 + 2 · 10 4 [ Fe ] + 5.7 [ lipid ] · B 0 $$ {\mathrm{R}}_2=9+\left(0.9+2\cdotp {10}^4\left[\mathrm{Fe}\right]+5.7\left[\mathrm{lipid}\right]\right)\cdotp {\mathrm{B}}_0 $$ , where[ Fe ] $$ \left[\mathrm{Fe}\right] $$ and[ lipid ] $$ \left[\mathrm{lipid}\right] $$ indicate the putative mass fractions of iron and lipid.

CONCLUSION

The R2 relaxation rate is well described by a constant plus a term linear in B0, with both iron and lipid content contributing to the slope. This indicates that the contributions of lipid and iron to R2 cannot be separated based solely on the field dependence of R2 in the field range of 3-7 T.

MRI contrast agents and retention in the brain: review of contemporary knowledge and recommendations to the future

Gadolinium-based contrast agents (GBCA) were introduced with high expectations for favorable efficacy, low nephrotoxicity, and minimal allergic-like reactions. Nephrogenic systemic fibrosis and proven gadolinium retention in the body including the brain has led to the restriction of linear GBCAs and a more prudent approach regarding GBCA indication and dosing. In this review, we present the chemical, physical, and clinical aspects of this topic and aim to provide an equanimous and comprehensive summary of contemporary knowledge with a perspective of the future. In the first part of the review, we present various elements and compounds that may serve as MRI contrast agents. Several GBCAs are further discussed with consideration of their relaxivity, chelate structure, and stability. Gadolinium retention in the brain is explored including correlation with the presence of metalloprotein ferritin in the same regions where visible hyperintensity on unenhanced T1-weighted imaging occurs. Proven interaction between ferritin and gadolinium released from GBCAs is introduced and discussed, as well as the interaction of other elements with ferritin; and manganese in patients with impaired liver function or calcium in Fahr disease. We further present the concept that only high-molecular-weight forms of gadolinium can likely visibly change signal intensity on unenhanced T1-weighted imaging. Clinical data are also presented with respect to potential neurological manifestations originating from the deep-brain nuclei. Finally, new contrast agents with relatively high relaxivity and stability are introduced. CRITICAL RELEVANCE STATEMENT: GBCA may accumulate in the brain, especially in ferritin-rich areas; however, no adverse neurological manifestations have been detected in relation to gadolinium retention. KEY POINTS: Gadolinium currently serves as the basis for MRI contrast agents used clinically. No adverse neurological manifestations have been detected in relation to gadolinium retention. Future contrast agents must advance chelate stability and relativity, facilitating lower doses.

Oxygen saturation-dependent effects on blood transverse relaxation at low fields

Objective

Blood oxygenation can be measured using magnetic resonance using the paramagnetic effect of deoxy-haemoglobin, which decreases the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2 relaxation time of blood. This \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2 contrast has been well characterised at the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{B}_{{0}}$$\end{document}B0 fields used in MRI (1.5 T and above). However, few studies have characterised this effect at lower magnetic fields. Here, the feasibility of blood oximetry at low field based on \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2 changes that are within a physiological relevant range is explored. This study could be used for specifying requirements for construction of a monitoring device based on low field permanent magnet systems.

Methods

A continuous flow circuit was used to control parameters such as oxygen saturation and temperature in a sample of blood. It flowed through a variable field magnet, where CPMG experiments were performed to measure its \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2. In addition, the oxygen saturation was monitored by an optical sensor for comparison with the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2 changes.

Results

These results show that at low \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{B}_{{0}}$$\end{document}B0 fields, the change in blood \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2 due to oxygenation is small, but still detectable. The data measured at low fields are also in agreement with theoretical models for the oxy-deoxy \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2 effect.

Conclusion

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textit{T}_{2}$$\end{document}T2 changes in blood due to oxygenation were observed at fields as low as 0.1 T. These results suggest that low field NMR relaxometry devices around 0.3 T could be designed to detect changes in blood oxygenation.

Relaxivity-iron calibration in hepatic iron overload: Reproducibility and extension of a Monte Carlo model

The aim of this study was to reproduce relaxivity-iron calibration in hepatic iron overload using a Monte Carlo model, and further extend the model with multiple spin echo (MSE) imaging. As previously reported, relationships between relaxation rates ( R2* and single spin echo R₂ ) and liver iron concentration (LIC) can be characterized by a Monte Carlo model incorporating realistic liver structure, iron distribution, and proton mobility. In this study, relaxivity-iron calibration curves at 1.5 and 3.0 T were simulated using the Monte Carlo model. Furthermore, the model was extended with MSE imaging, and iron calibrations were evaluated using two different fitting models: mononexponential with a constant offset and nonmonoexponential. Results consistent with previous empirical calibrations and Monte Carlo predictions were accurately reproduced for relaxivity-iron calibration. The predicted R2* and single spin echo R₂ increased by a factor of 2.00 and 1.51, respectively, at 1.5 versus 3.0 T. MSE signals and their corresponding R₂ depended strongly on LIC, interecho time, and field strength. Preliminary results showed that a nonmonoexponential model accurately characterizes the simulated MSE signals, and that strong correlations were found between predicted relaxation parameters and LIC. In conclusion, relaxivity-iron calibration is reproducible using the proposed Monte Carlo model. Furthermore, this model can be readily extended to other important applications, including predicting signal behavior for MSE imaging.

Off-resonance saturation as an MRI method to quantify mineral- iron in the post-mortem brain

Purpose

To employ an off‐resonance saturation method to measure the mineral‐iron pool in the postmortem brain, which is an endogenous contrast agent that can give information on cellular iron status.

Methods

An off‐resonance saturation acquisition protocol was implemented on a 7 Tesla preclinical scanner, and the contrast maps were fitted to an established analytical model. The method was validated by correlation and Bland‐Altman analysis on a ferritin‐containing phantom. Mineral‐iron maps were obtained from postmortem tissue of patients with neurological diseases characterized by brain iron accumulation, that is, Alzheimer disease, Huntington disease, and aceruloplasminemia, and validated with histology. Transverse relaxation rate and magnetic susceptibility values were used for comparison.

Results

In postmortem tissue, the mineral‐iron contrast colocalizes with histological iron staining in all the cases. Iron concentrations obtained via the off‐resonance saturation method are in agreement with literature.

Conclusions

Off‐resonance saturation is an effective way to detect iron in gray matter structures and partially mitigate for the presence of myelin. If a reference region with little iron is available in the tissue, the method can produce quantitative iron maps. This method is applicable in the study of diseases characterized by brain iron accumulation and can complement existing iron‐sensitive parametric methods.

Safe guidewire visualization using the modes of a PTx transmit array MR system

PURPOSE

MRI-guided cardiovascular intervention using standard metal guidewires can produce focal tissue heating caused by induced radiofrequency guidewire currents. It has been shown that safe operation is made possible by using parallel transmit radiofrequency coils driven in the null current mode, which does not induce radiofrequency currents and hence allows safe tissue visualization. We propose that the maximum current modes, usually considered unsafe, be used at very low power levels to visualize conductive wires, and we investigate pulse sequences best suited for this application.

METHODS

Spoiled gradient echo, balanced steady-state free precession, and turbo spin echo sequences were evaluated for their ability to visualize a conductive guidewire embedded in a gel phantom when run in maximum current modes at very low power level. Temperature at the guidewire tip was monitored for safety assessment.

RESULTS

Excellent guidewire visualization could be achieved using maximum current modes excitation, with the turbo spin echo sequence giving the best image quality. Although turbo spin echo is usually considered to be a high-power sequence, our method reduced all pulses to 1% amplitude (0.01% power), and heating was not detected. In addition, visualization of background tissue can be achieved using null current mode, also with no recorded heating at the guidewire tip even when running at 100% (reported) specific absorption rate.

CONCLUSION

Parallel transmit is a promising approach for both guidewire and tissue visualization using maximum and null current modes, respectively, for interventional cardiac MRI. Such systems can switch excitation mode instantaneously, allowing for flexible integration into interactive sequences.

Quantitative theory for the transverse relaxation time of blood water

An integrative model is proposed to describe the dependence of the transverse relaxation rate of blood water protons (R_2blood = 1/T_2blood ) on hematocrit fraction and oxygenation fraction (Y). This unified model takes into account (a) the diamagnetic effects of albumin, hemoglobin and the cell membrane; (b) the paramagnetic effect of hemoglobin; (c) the effect of compartmental exchange between plasma and erythrocytes under both fast and slow exchange conditions that vary depending on field strength and compartmental relaxation rates and (d) the effect of diffusion through field gradients near the erythrocyte membrane. To validate the model, whole-blood and lysed-blood R₂ data acquired previously using Carr-Purcell-Meiboom-Gill measurements as a function of inter-echo spacing τ_cp at magnetic fields of 3.0, 7.0, 9.4 and 11.7 T were fitted to determine the lifetimes (field-independent physiological constants) for water diffusion and exchange, as well as several physical constants, some of which are field-independent (magnetic susceptibilities) and some are field-dependent (relaxation rates for water protons in solutions of albumin and oxygenated and deoxygenated hemoglobin, ie, blood plasma and erythrocytes, respectively). This combined exchange-diffusion model allowed excellent fitting of the curve of the τ_cp -dependent relaxation rate dispersion at all four fields using a single average erythrocyte water lifetime, τ_ery = 9.1 ± 1.4 ms, and an averaged diffusional correlation time, τ_D = 3.15 ± 0.43 ms. Using this model and the determined physiological time constants and relaxation parameters, blood T₂ values published by multiple groups based on measurements at magnetic field strengths of 1.5 T and higher could be predicted correctly within error. Establishment of this theory is a fundamental step for quantitative modeling of the BOLD effect underlying functional MRI.

Imaging endogenous macrophage iron deposits reveals a metabolic biomarker of polarized tumor macrophage infiltration and response to CSF1R breast cancer immunotherapy

Iron deposits are a phenotypic trait of tumor-associated macrophages (TAMs). Histological iron imaging and contrast-agent free magnetic resonance imaging (MRI) can detect these deposits, but their presence  in human cancer, and correlation with immunotherapeutic response is largely untested. Here, primarily using these iron imaging approaches, we evaluated the spatial distribution of polarized macrophage populations containing high endogenous levels of iron in preclinical murine models and human breast cancer, and used them as metabolic biomarkers to correlate TAM infiltration with response to immunotherapy in preclinical trials. Macrophage-targeted inhibition of the colony stimulating factor 1 receptor (CSF1R) by immunotherapy was confirmed to inhibit macrophage accumulation and slow mammary tumor growth in mouse models while also reducing hemosiderin iron-laden TAM accumulation as measured by both iron histology and in vivo iron MRI (FeMRI). Spatial profiling of TAM iron deposit infiltration defined regions of maximal accumulation and response to the CSF1R inhibitor, and revealed differences between microenvironments of human cancer according to levels of polarized macrophage iron accumulation in stromal margins. We therefore demonstrate that iron deposition serves as an endogenous metabolic imaging biomarker of TAM infiltration in breast cancer that has high translational potential for evaluation of immunotherapeutic response.

Transverse NMR relaxation in biological tissues

Transverse NMR relaxation is a fundamental physical phenomenon underpinning a wide range of MRI-based techniques, essential for non-invasive studies in biology, physiology and neuroscience, as well as in diagnostic imaging. Biophysically, transverse relaxation originates from a number of distinct scales - molecular (nanometers), cellular (micrometers), and macroscopic (millimeter-level MRI resolution). Here we review the contributions to the observed relaxation from each of these scales, with the main focus on the cellular level of tissue organization, commensurate with the diffusion length of spin-carrying molecules. We discuss how the interplay between diffusion and spin dephasing in a spatially heterogeneous tissue environment leads to a non-monoexponential time-dependent transverse relaxation signal that contains important biophysical information about tissue microstructure.

Quantification of Nanoparticle Enhancement in Polarized Breast Tumor Macrophage Deposits by Spatial Analysis of MRI and Histological Iron Contrast Using Computer Vision

Magnetic resonance imaging applications utilizing nanoparticle agents for polarized macrophage detection are conventionally analyzed according to iron-dependent parameters averaged over large regions of interest (ROI). However, contributions from macrophage iron deposits are usually obscured in these analyses due to their lower spatial frequency and smaller population size compared with the bulk of the tumor tissue. We hypothesized that, by addressing MRI and histological pixel contrast heterogeneity using computer vision image analysis approaches rather than statistical ROI distribution averages, we could enhance our ability to characterize deposits of polarized tumor-associated macrophages (TAMs). We tested this approach using in vivo iron MRI (FeMRI) and histological detection of macrophage iron in control and ultrasmall superparamagnetic iron oxide (USPIO) enhanced mouse models of breast cancer. Automated spatial profiling of the number and size of iron-containing macrophage deposits according to localized high-iron FeMRI or Prussian blue pixel clustering performed better than using distribution averages to evaluate the effects of contrast agent injections. This analysis was extended to characterize subpixel contributions to the localized FeMRI measurements with histology that confirmed the association of endogenous and nanoparticle-enhanced iron deposits with macrophages in vascular regions and further allowed us to define the polarization status of the macrophage iron deposits detected by MRI. These imaging studies demonstrate that characterization of TAMs in breast cancer models can be improved by focusing on spatial distributions of iron deposits rather than ROI averages and indicate that nanoparticle uptake is dependent on the polarization status of the macrophage populations. These findings have broad implications for nanoparticle-enhanced biomedical imaging especially in cancer.

Iron deposition is associated with differential macrophage infiltration and therapeutic response to iron chelation in prostate cancer

Immune cells such as macrophages are drivers and biomarkers of most cancers. Scoring macrophage infiltration in tumor tissue provides a prognostic assessment that is correlated with disease outcome and therapeutic response, but generally requires invasive biopsy. Routine detection of hemosiderin iron aggregates in macrophages in other settings histologically and in vivo by MRI suggests that similar assessments in cancer can bridge a gap in our ability to assess tumor macrophage infiltration. Quantitative histological and in vivo MRI assessments of non-heme cellular iron revealed that preclinical prostate tumor models could be differentiated according to hemosiderin iron accumulation-both in tumors and systemically. Monitoring cellular iron levels during "off-label" administration of the FDA-approved iron chelator deferiprone evidenced significant reductions in tumor size without extensive perturbation to these iron deposits. Spatial profiling of the iron-laden infiltrates further demonstrated that higher numbers of infiltrating macrophage iron deposits was associated with lower anti-tumor chelation therapy response. Imaging macrophages according to their innate iron status provides a new phenotypic window into the immune tumor landscape and reveals a prognostic biomarker associated with macrophage infiltration and therapeutic outcome.

Iron imaging reveals tumor and metastasis macrophage hemosiderin deposits in breast cancer

Iron-deposition is a metabolic biomarker of macrophages in both normal and pathological situations, but the presence of iron in tumor and metastasis-associated macrophages is not known. Here we mapped and quantified hemosiderin-laden macrophage (HLM) deposits in murine models of metastatic breast cancer using iron and macrophage histology, and in vivo MRI. Iron MRI detected high-iron pixel clusters in mammary tumors, lung metastasis, and brain metastasis as well as liver and spleen tissue known to contain the HLMs. Iron histology showed these regions to contain clustered macrophages identified by their common iron status and tissue-intrinsic association with other phenotypic macrophage markers. The in vivo MRI and ex vivo histological images were further processed to determine the frequencies and sizes of the iron deposits, and measure the number of HLMs in each deposit to estimate the in vivo MRI sensitivity for these cells. Hemosiderin accumulation is a macrophage biomarker and intrinsic contrast source for cellular MRI associated with the innate function of macrophages in iron metabolism systemically, and in metastatic cancer.

In vivo measurement of T1 and T2 relaxation times in awake pigeon and rat brains at 7T

PURPOSE

Establishment of regional longitudinal (T₁ ) and transverse (T₂ ) relaxation times in awake pigeons and rats at 7T field strength. Regional differences in relaxation times between species and between two different pigeon breeds (homing pigeons and Figurita pigeons) were investigated.

METHODS

T₁ and T₂ relaxation times were determined for nine functionally equivalent brain regions in awake pigeons and rats using a multiple spin-echo saturation recovery method with variable repetition time and a multi-slice/multi-echo sequence, respectively. Optimized head fixation and habituation protocols were applied to accustom animals to the scanning conditions and to minimize movement.

RESULTS

The habituation protocol successfully limited movement of the awake animals to a negligible minimum, allowing reliable measurement of T₁ and T₂ values within all regions of interest. Significant differences in relaxation times were found between rats and pigeons but not between different pigeon breeds.

CONCLUSION

The obtained T₁ and T₂ values for awake pigeons and rats and the optimized habituation protocol will augment future MRI studies with awake animals. The differences in relaxation times observed between species underline the importance of the acquisition of T₁ /T₂ values as reference points for specific experiments. Magn Reson Med 79:1090-1100, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Investigating the Relationship between Transverse Relaxation Rate (R2) and Interecho Time in MagA-Expressing, Iron-Labeled Cells

Reporter gene-based labeling of cells with iron is an emerging method of providing magnetic resonance imaging contrast for long-term cell tracking and monitoring cellular activities. This report investigates 9.4 T nuclear magnetic resonance properties of mammalian cells overexpressing MagA, a putative iron transport protein from magnetotactic bacteria. MagA-expressing MDA-MB-435 cells were cultured in the presence and absence of iron supplementation and compared to the untransfected control. The relationship between the transverse relaxation rate (R2) and interecho time was investigated using the Carr-Purcell-Meiboom-Gill sequence. This relationship was analyzed using a model based on water diffusion in weak magnetic field inhomogeneities (Jensen-Chandra model) as well as a fast-exchange model (Luz-Meiboom model). Increases in R2 with increasing interecho time were larger in the iron-supplemented, MagA-expressing cells compared to other cells. The dependence of R2 on interecho time in these iron-supplemented, MagA-expressing cells was better represented by the Jensen-Chandra model compared to the Luz-Meiboom model, whereas the Luz-Meiboom model performed better for the remaining cell types. Our findings provide an estimate of the distance scale of microscopic magnetic field variations in MagA-expressing cells, which is thought to be related to the size of iron-containing vesicles.

Longitudinal Changes in Placental Magnetic Resonance Imaging Relaxation Parameter in Murine Pregnancy: Compartmental Analysis

OBJECTIVE

To quantify gestation-dependent longitudinal changes in the magnetic resonance transverse relaxation time (T2) parameter of the major constituent regions of the mouse placenta and to evaluate their relative contributions to changes in overall placental T2.

METHODS

Timed-pregnant CD-1 mice underwent magnetic resonance imaging at 7.0 T field strength, on gestational day 13 (GD13), GD15 and GD17. T2 of the placenta and its constituent high and low blood perfusion regions were quantified. A linear mixed-effects model was used to fit the T2 across gestation, and the significance of coefficients was tested.

RESULTS

A decrease in the T2 values of the placenta and its constituent regions was observed across gestation. The temporal change in T2 was estimated to be -1.85 ms/GD (p < 0.0001) for the placenta, -1.00 ms/GD (p < 0.001) for the high-perfusion zones (HPZs) and -1.66 ms/GD (p < 0.0001) for the low-perfusion zones (LPZs).

CONCLUSION

T2 of the constituent zones of the murine placenta decreases with advancing gestation. While the T2 of the LPZ is smaller than that of the HPZ, there is no difference in their decrease rate relative to that of the whole placenta (p = 0.24). The results suggest an increased role of constituent volume fractions in affecting overall gestation-dependent placental T2 decrease in mice.

Magnetic nanoparticles: surface effects and properties related to biomedicine applications

Due to finite size effects, such as the high surface-to-volume ratio and different crystal structures, magnetic nanoparticles are found to exhibit interesting and considerably different magnetic properties than those found in their corresponding bulk materials. These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles’ size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of nanoparticles of this type in diverse fields. Biomedicine is one of these fields where intensive research is currently being conducted. In this review, we will discuss the magnetic properties of nanoparticles which are directly related to their applications in biomedicine. We will focus mainly on surface effects and ferrite nanoparticles, and on one diagnostic application of magnetic nanoparticles as magnetic resonance imaging contrast agents.

Intravascular contrast agent T2* relaxivity in brain tissue

Dynamic susceptibility-weighted contrast-enhanced (DSC) MRI perfusion measurements depend on estimating intravascular contrast agent (CA) concentrations (C) from signal intensity changes in T2*-weighted images after bolus injection. Generally, linearity is assumed between relaxation and C, but previous studies have shown that compartmentalization of CA and secondary magnetic field perturbations generate deviations from linearity. Physical phantoms using bulk blood have been used to empirically determine the relationship between relaxation rate and C in large vessels. However, the relaxivity of CA in the microvasculature is not easily measured since constructing appropriate phantoms is difficult. Instead, theoretical relaxivity models have been developed. In this study, we empirically tested a non-linear expression based on static dephasing regime (SDR) and linear approximation. Signal-time curves in white (WM) and grey matter (GM) were converted to concentration time curves (CTCs) using both expressions. Parameters for both linear and non-linear formulations were adjusted to give a best agreement between cerebral blood volumes (CBV) calculated from WM and arterial CTCs in a group of normal subjects scanned at 3T. Optimized parameters were used to calculate blood volume in WM and GM in healthy subjects scanned at 3T and in meningioma patients scanned at 1.5T. Results from this study showed that a non-linear SDR formulation gave an acceptable functional form for tissue relaxivity, giving reliable CBV estimates at different field strengths and echo times.

Imaging of iron

Magnetic resonance imaging (MRI) enables a noninvasive in vivo quantification of iron in various organs. Several techniques have been developed that detect signal alterations derived mainly from the magnetic properties of ferritin and hemosiderin, the major iron storage compounds. High magnetic susceptibility of ferritin shortens the transversal relaxation time of nearby water protons and thus induces a focal signal extinction of iron-rich areas in T2-weighted (T2w) MRI. T2w tissue contrast is additionally influenced by other factors such as water content, myelin density, and the presence of other metals. Therefore, more specific methods are needed with higher specificity to iron. These in vivo techniques can be divided into three groups: relaxometry, magnetic field correlation imaging and phase-based contrast covering susceptibility-weighted imaging, and quantitative susceptibility mapping. The differential diagnosis of various neurological disorders is aided by characteristic patterns of iron depositions. Reliable estimates of cerebral tissue iron concentration are equally important in studying physiological age-related as well as pathological conditions in neurodegenerative, neuroinflammatory, and vascular diseases. In the future, monitoring changes in iron storage and content may serve as sensitive biomarker for diagnosis as well as treatment monitoring.

In vivo measurement of transverse relaxation time in the mouse brain at 17.6 T

PURPOSE

To establish regional T1 and T2 values of the healthy mouse brain at ultra-high magnetic field strength of 17.6 T and to follow regional brain T1 and T2 changes with age.

METHODS

In vivo T1 and T2 values in the C57BL/6J mouse brain were followed with age using multislice-multiecho sequence and multiple spin echo saturation recovery with variable repetition time sequence, respectively, at 9.4 and 17.6 T. Gadolinium-tetra-azacyclo-dodecane-tetra-acetic acid phantoms were used to validate in vivo T2 measurements. Student's t-test was used to compare mean relaxation values.

RESULTS

A field-dependent decrease in T2 is shown and validated with phantom measurements. T2 values at 17.6 T typically increased with age in multiple brain regions except in the hypothalamus and the caudate-putamen, where a slight decrease was observed. Furthermore, T1 values in various brain regions of young and old mice are presented at 17.6 T. A large gain in signal-to-noise ratio was observed at 17.6 T.

CONCLUSIONS

This study establishes for the first time the normative T1 and T2 values at 17.6 T over different mouse brain regions with age. The estimates of in vivo T1 and T2 will be useful to optimize pulse sequences for optimal image contrast at 17.6 T and will serve as baseline values against which disease-related relaxation changes can be assessed in mice.

MR T1 and T2 relaxations in cysts and abscesses measured by 1.5 T MRI

OBJECTIVES

The main objective of this study was to make a comparison between the relaxation rates in jaw cysts and abscesses. Such a comparison should provide quantitative information for MR image analysis.

METHODS

A phantom containing 20 odontogenic jaw cysts and 11 jaw abscesses was imaged with 1.5 T MR. T(1) measurements were performed by using a mixed sequence of inversion recovery and spin echo, while T(2) measurements were carried out by the Carr-Purcell Meiboom-Gill (CPMG) sequence. Cystic fluids and abscesses were compared statistically.

RESULTS

In cysts and abscesses, respectively, the mean 1/T(1) was 0.9355 s(-1) and 0.8245 s(-1) and the mean 1/T(2) was 2.4575 s(-1) and 4.7073 s(-1). The 1/T(2) in cysts was very highly significantly different from that in abscesses (p = 0.0001). Both T(1) and T(2) were linearly proportional to material contents. T(2) relaxivities [26.458 ml (g s)(-1) for abscesses and 21.455 ml (g s)(-1) for cysts] were higher than T(1) relaxivities [5.4766 ml (g s)(-1) for abscesses and 10.075 ml (g s)(-1) for cysts].

DISCUSSION

Present T(2) measurements differentiate cysts from abscesses with a confidence interval of 95%. Because in vivo and in vitro image contrasts are changed by the same parameters, the T(2) findings should present valuable information for in vivo MRI. Hence the significant difference and the relaxivities may provide quantitative information for clinicians and researchers making image analyses.

CONCLUSION

T(2) may differentiate cysts from abscesses. The difference in T(2) is related to the material content of samples.

In vivo assessment of age-related brain iron differences by magnetic field correlation imaging

PURPOSE

To assess a recently developed magnetic resonance imaging (MRI) technique called magnetic field correlation (MFC) imaging along with a conventional imaging method, the transverse relaxation rate (R2), for estimating age-related brain iron concentration in adolescents and adults. Brain region measures were compared with nonheme iron concentrations (C(PM) ) based on a prior postmortem study.

MATERIALS AND METHODS

Asymmetric spin echo (ASE) images were acquired at 3T from 26 healthy individuals (16 adolescents, 10 adults). Regions of interest (ROIs) were placed in areas in which age-related iron content was estimated postmortem: globus pallidus (GP), putamen (PUT), caudate nucleus (CN), thalamus (THL), and frontal white matter (FWM). Regression and group analyses were conducted on ROI means.

RESULTS

MFC and R2 displayed significant linear relationships to C(PM) when all regions were combined. Whereas MFC was significantly correlated with C(PM) for every individual region except FWM and detected significantly lower means in adolescents than adults for each region, R2 detected significant correlation and lower means for only PUT and CN.

CONCLUSION

Our results support the hypothesis that MFC is sensitive to brain iron in GM regions and detects age-related iron increases known to occur from adolescence to adulthood. MFC may be more sensitive than R2 to iron-related changes occurring within specific brain regions.

Robust quantification of contrast agent (CA) concentration with magnetic field correlation (MFC) imaging

Contrast-enhanced perfusion studies of the brain by means magnetic resonance imaging (MRI) are used to estimate a number of important brain tissue parameters, including cerebral blood flow and volume. In order to calculate these parameters, the contrast agent (CA) concentration must first be estimated. This is usually accomplished by measurement of a nuclear magnetic resonance (NMR) relaxation rate with the assumption of a linear relationship between the rate and the CA concentration. However, such a linear relationship does not necessarily hold in biological tissues due to compartmentalization of the CA in either the intravascular or extracellular spaces. Here we propose an alternative MRI method of CA quantification based on measurement of the magnetic field correlation (MFC), which is theoretically predicted to have a robust quadratic dependence on the CA concentration even when the CA is compartmentalized. In this study, CA concentration estimation by means of MFC is shown to be more accurate than established methods based on relaxation rates in yeast cell suspensions.

Diffusion effects on the CPMG relaxation rate in a dipolar field

The diffusion in the magnetic dipolar field around a sphere is considered. The diffusion is restricted to the space between two concentric spheres, where the inner sphere is the source of the magnetic dipolar field. Analytical expressions for the CPMG transverse relaxation rate as well as the free induction decay and the spin echo time evolution are given in the Gaussian approximation. The influence of the inter-echo time is analyzed. The limiting cases of small and large inter-echo times as well as the short and long time behavior are evaluated.

Basal ganglia MR relaxometry in obsessive-compulsive disorder: T2 depends upon age of symptom onset

Dysfunction in circuits linking frontal cortex and basal ganglia (BG) is strongly implicated in obsessive-compulsive disorder (OCD). On MRI studies, neuropsychiatric disorders with known BG pathology have abnormally short T2 relaxation values (a putative biomarker of elevated iron) in this region. We asked if BG T2 values are abnormal in OCD. We measured volume and T2 and T1 relaxation rates in BG of 32 adults with OCD and 33 matched controls. There were no group differences in volume or T1 values in caudate, putamen, or globus pallidus (GP). The OCD group had lower T2 values (suggesting higher iron content) in the right GP, with a trend in the same direction for the left GP. This effect was driven by patients whose OCD symptoms began from around adolescence to early adulthood. The results suggest a possible relationship between age of OCD onset and iron deposition in the basal ganglia.

The relationship between hematoma iron content and perihematoma edema: an MRI study

BACKGROUND

Iron neurotoxicity has been linked to delayed neuronal injury and edema formation after intracerebral hemorrhage (ICH). We have previously shown that serum ferritin, an indicator of body iron load, correlates with the relative perihematoma edema volume (RPHEV) on days 3-4 after ICH. We undertook this study to directly examine the relationship between in vivo brain and hematoma iron content, measured by MRI, and RPHEV.

METHODS

We retrospectively reviewed prospectively collected clinical and laboratory data from 36 consecutive patients with acute spontaneous lobar ICH who had MRI performed within 2-4 days of ICH onset. We measured hematoma and edema volumes, and the signal intensity on T(2)-weighted images (T(2)SI), as an estimate of iron content, in the hematoma and contralateral globus pallidus (GP). We calculated the RPHEV and T(2)SI in the hematoma and GP, relative to T(2)SI in the frontal deep white matter which contains negligible iron, to estimate the hematoma and brain iron load. We used Spearman correlation coefficient to determine the association of relative T(2)SI of the hematoma and GP with RPHEV.

RESULTS

We found a significant inverse correlation between the relative T(2)SI in the hematoma (r = -0.75, p < 0.001) and to a lesser extent in the GP (r = -0.34, p = 0.04) and the RPHEV.

CONCLUSIONS

Our findings suggest that in vivo brain and hematoma iron content, as measured by MRI, is linked to perihematoma edema after ICH, and provide further support to existing preclinical evidence linking iron-mediated toxicity to delayed neuronal injury after ICH.

Relaxation by clustered ferritin: a model for ferritin-induced relaxation in vivo

Ferritin, the iron-storing protein of mammals, is known to darken T(2)-weighted MR images. This darkening could be used for the non-invasive measurement of an organ's iron content. Unexplained discrepancies exist between T(2) data obtained in ferritin-containing tissues and aqueous solutions of ferritin. The clustering of the protein induced by trypsin is used to evaluate the effect of ferritin agglomeration on the relaxation rates. Although the longitudinal relaxation is not significantly influenced by clustering, T(2) depends greatly on the stage of agglomeration: the transverse relaxation rate is higher for a clustered sample than for an unclustered sample. Moreover, the field and inter-echo time dependences of the relaxation rate indicate that the relaxation mechanism may be different between small clusters -- where a linear dependence of 1/T(2) on B(0) is observed -- and large clusters -- where a quadratic dependence is observed. These results help to explain the relaxation induced by ferritin in tissues.

Air microbubbles as MR susceptibility contrast agent at 1.5 Tesla

Air microbubbles have been investigated recently at high magnetic field strength (2 Tesla or greater) as potential MR susceptibility contrast agents. We used a phantom to measure their susceptibility at 1.5 T to clarify their usefulness for this purpose. The phantom, filled with fresh Levovist suspension at 4 different doses (67 to 125 mg/mL), was continuously scanned with a gradient-echo technique at a temporal resolution of 10 s. The transverse relaxation increase (R2*) by microbubbles demonstrated a time course of exponential decay at each dose (time-constant, 39 to 57 s). The dependency of R2* on microbubble volume fraction was linear, with a slope of 89 s-1 per percentage microbubble volume fraction. Our study represents the first step towards applying microbubbles as susceptibility contrast agents at 1.5 T.

Correlation of proton transverse relaxation rates (R2) with iron concentrations in postmortem brain tissue from alzheimer's disease patients

Iron accumulates in the Alzheimer's disease (AD) brain and is directly associated with beta-amyloid pathology. The proton transverse relaxation rate (R(2)) has a strong linear relationship with iron concentrations in healthy brain tissue; however, an independent test of this relationship has not been extended to AD brain tissue. In this study in vitro single spin-echo (SE) measurements were made on tissue samples from four human AD brains using a 4.7T MRI research scanner. R(2) values were calculated for 14 cortical and subcortical gray matter (GM) and white matter (WM) regions. Atomic absorption spectroscopy was used to measure iron concentrations in the corresponding excised brain regions. Significant positive linear correlations were observed between R(2) values and iron concentrations in GM regions assessed across individual tissue samples and data averaged by brain region. With the use of a predictive model for R(2), a threshold iron concentration of 55 microg Fe/g wet tissue was determined above which R(2) appears to be dominated by the affects of iron in AD brain tissue. High-field MRI may therefore be a useful research tool for assessing brain iron changes associated with AD.

Experimental validation of a T2rho transverse relaxation model using LASER and CPMG acquisitions

The transverse relaxation rate (R2=1/T2) of many biological tissues are altered by endogenous magnetized particles (i.e., ferritin, deoxyhemoglobin), and may be sensitive to the pathological progression of neurodegenerative disorders associated with altered brain-iron stores. R2 measurements using Carr-Purcell-Meiboom-Gill (CPMG) acquisitions are sensitive to the refocusing pulse interval (2taucp), and have been modeled as a chemical exchange (CE) process, while R2 measurements using a localization by adiabatic selective refocusing (LASER) sequence have an additional relaxation rate contribution that has been modeled as a R2rho process. However, no direct comparison of the R2 measured using these two sequences has been described for a controlled phantom model of magnetized particles. The three main objectives of this study were: (1) to compare the accuracy of R2 relaxation rate predictions from the CE model with experimental data acquired using a conventional CPMG sequence, (2) to compare R2 estimates obtained using LASER and CPMG acquisitions, and (3) to determine whether the CE model, modified to account for R2rho relaxation, adequately describes the R2 measured by LASER for a full range of taucp values. In all cases, our analysis was confined to spherical magnetic particles that satisfied the weak field regime. Three phantoms were produced that contained spherical magnetic particles (10 microm diameter polyamide powders) suspended in Gd-DTPA (1.0, 1.5, and 2.0 mmol/L) doped gel. Mono-exponential R2 measurements were made at 4T as a function of refocusing pulse interval. CPMG measurements of R2 agreed with CE model predictions while significant differences in R2 estimates were observed between LASER and CPMG measurements for short taucp acquisitions. The discrepancy between R2 estimates is shown to be attributable to contrast enhancement in LASER due to T2rho relaxation.

Relaxivities of human liver and spleen ferritin

Ferritin, the iron-storing protein of mammals, is known to darken T2-weighted magnetic resonance images. This darkening can be used to noninvasively measure an organ's iron content. Significant discrepancies exist between T2 data obtained with ferritin-containing tissues and with aqueous solutions of horse spleen ferritin (HSF). The NMR properties of stable human ferritin have never been studied in aqueous solutions. Relaxometry results on human liver and spleen ferritin are reported here, showing that the relaxation induced in aqueous solutions by human ferritins is comparable to that induced by HSF. As a consequence, the differences between ferritin-containing human tissues and ferritin solutions cannot be attributed to different NMR properties of human and horse ferritins, but probably to a clustering of the protein in vivo.

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 method's feasibility. The preliminary results suggest that MFC imaging may be useful for the quantitative assessment of iron within the brain.

Imaging iron stores in the brain using magnetic resonance imaging

For the last century, there has been great physiological interest in brain iron and its role in brain function and disease. It is well known that iron accumulates in the brain for people with Huntington's disease, Parkinson's disease, Alzheimer's disease, multiple sclerosis, chronic hemorrhage, cerebral infarction, anemia, thalassemia, hemochromatosis, Hallervorden-Spatz, Down syndrome, AIDS and in the eye for people with macular degeneration. Measuring the amount of nonheme iron in the body may well lead to not only a better understanding of the disease progression but an ability to predict outcome. As there are many forms of iron in the brain, separating them and quantifying each type have been a major challenge. In this review, we present our understanding of attempts to measure brain iron and the potential of doing so with magnetic resonance imaging. Specifically, we examine the response of the magnetic resonance visible iron in tissue that produces signal changes in both magnitude and phase images. These images seem to correlate with brain iron content, perhaps ferritin specifically, but still have not been successfully exploited to accurately and precisely quantify brain iron. For future quantitative studies of iron content we propose four methods: correlating R2' and phase to iron content; applying a special filter to the phase to obtain a susceptibility map; using complex analysis to extract the product of susceptibility and volume content of the susceptibility source; and using early and late echo information to separately predict susceptibility and volume content.

Ferritin-induced relaxation in tissues: an in vitro study

PURPOSE

To study in vitro the proton relaxation induced in tissues by ferritin, the iron-storing protein of mammals.

MATERIALS AND METHODS

Nuclear magnetic relaxation dispersion (NMRD) profiles of liver and spleen from control and iron-overloaded mice are compared with NMRD profiles of ferritin and Fercayl-a ferritin-like akaganeite particle-in aqueous solutions or in 1% agarose gel.

RESULTS

The relaxation of water protons induced by ferritin and Fercayl in 1% agarose gel is comparable with the relaxation of aqueous solutions of the same compounds, but slower than the relaxation of liver and spleen. The gel is not a good model of tissues containing ferritin. The longitudinal NMRD profiles of control and iron-overloaded liver and spleen are almost identical: ferritin accumulation has only a slight effect on longitudinal relaxation. The transverse NMRD profiles of liver and spleen tissues are linear, but the slope of the linear regression is larger for iron-loaded organs than for control ones, which is a consequence of a higher ferritin concentration in the former. However, the correlation between the slope of the transverse NMRD profiles and the iron concentration is not very good, probably because transverse relaxation is modified by the clustering of ferritin in cells.

CONCLUSION

It could be difficult to develop a general technique for the accurate quantification of ferritin-bound iron by nuclear magnetic resonance or magnetic resonance imaging.

Visualization of beta-amyloid plaques in a transgenic mouse model of Alzheimer's disease using MR microscopy without contrast reagents

The visualization of beta-amyloid plaque deposition in brain, a key feature of Alzheimer's disease (AD), is important for the evaluation of disease progression and the efficacy of therapeutic interventions. In this study, beta-amyloid plaques in the PS/APP transgenic mouse brain, a model of human AD pathology, were detected using MR microscopy without contrast reagents. beta-Amyloid plaques were clearly visible in the cortex, thalamus, and hippocampus of fixed brains of PS/APP mice. The distribution of plaques identified by MRI was in excellent agreement with those found in the immunohistological analysis of the same brain sections. It was also demonstrated that image contrast for beta-amyloid plaques was present in freshly excised nonfixed brains. Furthermore, the detection of beta-amyloid plaques was achieved with a scan time as short as 2 hr, approaching the scan time considered reasonable for in vivo imaging.

MR relaxometry and 1H MR spectroscopy for the determination of iron and metabolite concentrations in PKAN patients

The influence of iron deposits on T2 values and the content of metabolites in the brain of three patients with DNA proved pantothenate kinase-associated neurodegeneration (PKAN, formerly Hallervorden-Spatz syndrome) was studied. An eye-of-the-tiger sign, a typical MR finding for PKAN, was observed in two patients with the same mutation. A hypointensive lesion in a whole globus pallidus was observed in the third patient with the additional mutation. T2 values in the globus pallidus of the patients were about 40% shorter than in controls (71/48 ms in controls vs. patients), which corresponds to the increase of Fe concentration based on the ferritin basis from 17 mg for controls to 48 mg (100 g wet brain weight) in PKAN patients. 1H MR spectroscopy (MRS) has mainly been used to describe neuronal damage represented by decreased NAA (6.4 mmol vs. 9 mmol) and Cr/PCr (7.0 mmol vs. 9.8 mmol) concentrations in the basal ganglia region of the patient group to controls; MRS is much more case-sensitive and describes individual development of the disease as demonstrated in the difference between the spectra of typical PKAN patients (1, 2), and the patient (3) with atypical PKAN development. Any significant changes of metabolite concentration with the exception glutamine, glutamate and GABA were found in the white matter.

Relaxation induced by ferritin: a better understanding for an improved MRI iron quantification

Ferritin, the iron storing protein, is known to darken T2-weighted MRI. This darkening can be used to non-invasively measure iron content. However, ferritin's behavior is not the same in tissue as in solution, a discrepancy that remains unexplained by the recently developed theory matching the NMR properties of ferritin solutions. A better understanding of the relaxation induced by ferritin in tissue could help for the development of new MRI protocols of iron quantification. In this short review, the main relaxation properties of ferritin in solution and in tissue are presented together with a discussion of the possible reasons for the faster transverse relaxation observed in tissues.

Human whole-blood relaxometry at 1.5T: Assessment of diffusion and exchange models

Human whole-blood relaxometry experiments were performed to allow the prediction of blood signal changes with blood oxygen saturation (Y) and refocusing interval (tau180). Such predictions are particularly relevant for spin-echo (SE) blood oxygenation level-dependent (BOLD) experiments and a recently proposed noninvasive fMRI method for measuring cerebral blood volume (CBV). Ensemble fitting of the entire set of T2 estimates, obtained over an extensive range of Y and tau180 values, was performed with the use of both a fast chemical exchange model and a model of diffusion in weak magnetic field inhomogeneities. The diffusion modeling resulted in a large reduction in the residual sum-of-squares compared to the fast exchange modeling. The longitudinal relaxation rate decreased linearly with Y, and increased with hematocrit. The results support the application of the recently reported diffusion model to describe deoxyhemoglobin (dHb)-induced blood transverse relaxation rate enhancement at 1.5 T.

MR contrast of ferritin and hemosiderin in the brain: comparison among gradient-echo, conventional spin-echo and fast spin-echo sequences

OBJECTIVE

To compare the magnetic resonance image contrasts due to ferritin and hemosiderin in the brain tissue among different pulse sequences.

MATERIALS AND METHODS

Fourteen patients with cavernous hemangioma in the brain prospectively underwent MR imaging with T2*-weighted gradient-echo (GRE), T2-weighted conventional spin-echo (SE) and fast spin-echo (FSE) sequences. The relative contrast ratios (CRs) of the hypointense part of cavernous hemangioma, globus pallidus and putamen to the deep frontal white matter were measured on each pulse sequence and statistically analyzed using analysis of variance followed by paired t-test.

RESULTS

In the hypointense part of cavernous hemangioma, relative CRs were significantly lower on T2*-weighted GRE than on T2-weighted SE images (P=0.0001), and on T2-weighted SE than on T2-weighted FSE images (P=0.0001). In the globus pallidus, relative CRs were significantly lower on T2-weighted SE than on T2*-weighted GRE images (P=0.002), and on T2*-weighted GRE than on T2-weighted FSE images (P=0.0002). In the putamen, relative CRs were significantly lower on T2-weighted SE than on T2*-weighted GRE images (P=0.001), and there was no significant difference between CRs on T2-weighted FSE and T2*-weighted GRE images (P=0.90).

CONCLUSION

Hemosiderin showed best image contrast on T2*-weighted GRE images but ferritin showed more prominent image contrast on T2-weighted SE than on T2*-weighted GRE images, which may help to determine an appropriate pulse sequence in neurological diseases associated with excessive ferritin accumulation.

Transverse NMR relaxation as a probe of mesoscopic structure

Transverse NMR relaxation in a macroscopic sample is shown to be extremely sensitive to the structure of mesoscopic magnetic susceptibility variations. Such a sensitivity is proposed as a novel kind of contrast in the NMR measurements. For suspensions of arbitrary-shaped paramagnetic objects, the transverse relaxation is found in the case of a small dephasing effect of an individual object. Strong relaxation rate dependence on the objects' shape agrees with experiments on whole blood. Demonstrated structure sensitivity is a generic effect that arises in NMR relaxation in porous media, biological systems, as well as in kinetics of diffusion limited reactions.

On the theoretical basis of perfusion measurements by dynamic susceptibility contrast MRI

A quantitative analysis was undertaken to calibrate the perfusion quantification technique based on tracking the first pass of a bolus of a blood pool contrast agent. A complete simulation of the bolus passage, of the associated changes in the T2 and T2* signals, and of the data processing was performed using the tracer dilution theory, an analytical theory of the MR signal from living tissues and numerical simulations. The noise was excluded in the simulation in order to analyze the ultimate accuracy of the method. It is demonstrated that the relationship between the contrast agent concentration and the associated changes in the transverse relaxation rate shows essentially different forms in studied tissue and in the reference artery. This effect results in systematic deviations of the measured blood flow, blood volume, and the residue function obtained with conventional processing from their true values. The error depends on the microvascular composition, the properties of the contrast agent, and the weights of the various compartments in the total signal. The results show that dynamic susceptibility contrast MRI can reach the goal of absolute perfusion quantification only with additional input from measurements of the microvascular architecture. Alternatively, the method can be used to provide such information if the perfusion is quantified by another modality.

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 Parkinson's 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.

On T2-shortening by weakly magnetized particles: the chemical exchange model

Chemical exchange (CE) theory is compared with two theories of T2-shortening caused by microscopic magnetic centers: inner- and outer-sphere relaxation theory (long-echo limit) and mean gradient diffusion theory (short-echo limit). The CE equation is shown to be identical to these theories in the respective limits and appropriate parameter relationships are derived for spherical particles. The theories are then compared with computer simulations of spherical particles and with a recent general theory, with good agreement in the asymptotic regions. The CE model also reproduces the essential relaxation characteristics in the intermediate range. Finally, good agreement of a CE model with simulations for magnetized cylinders is also demonstrated. The discussion is limited to weakly magnetized particles such that the maximum phase shift during an echo interval is less than one radian, permitting the use of the Luz-Meiboom CE equation. Published 2001 Wiley-Liss, Inc.

BOLD Contrast on a 3 T Magnet: Detectability of the Motor Areas

To predict the potential and the limitations of functional MRI (fMRI) with a very high field magnet, the detectability and reproducibility of activation were evaluated by comparing the activation induced by a sequential finger movement task at 1.5 T with that at 3 T. The detectability of the premotor area, supplementary motor area (SMA), and ipsilateral sensorimotor area (SM1) showed significant improvement at 3 T. On the other hand, the detectability of contralateral SM1 was not significantly different between 1.5 and 3 T. The degree of activation was proportional to task demand in the ipsilateral SM1 and SMA, whereas that in the contralateral SM1 and SMA was not. FMRI with a 3 T magnet has greater potential for detection of neuronal activation as a functional network. These observations indicated that task demand and static magnetic field strength should be considered in interpretation of fMRI data for clinical usage.

Relaxation induced by ferritin and ferritin-like magnetic particles: the role of proton exchange

Proton T1 and T2 in solutions of ferritin and fercayl (a ferritin-like iron-dextran particle) solutions were measured, over a wide range of various parameters (Bo, temperature, interecho-time and pH). The window of the previously referred linear dependence of 1/T2 on the static field was increased, up to 500 MHz, and the independence of T2 on the echo time was confirmed. Correlation times were extracted from T1 nuclear magnetic relaxation dispersion profiles. In the pH range studied, no strong variation of the relaxivities of ferritin solutions was noticed. Fercayl, which, unlike ferritin, remains stable under large pH variations, is characterized by strongly pH-dependent relaxation rates. This feature is interpreted as due to the effect of proton exchange in the water relaxation process. Outer sphere theory, which ignores proton binding, is shown to be unable to describe the relaxation of ferritin and ferritin-like particles solutions, first because it predicts a quadratic rate dependence on Bo, but also because it severely underestimates the relaxation rate. Explaining relaxation induced by ferritin and ferritin-like particle solutions will likely require a model that accounts for proton binding.