Relaxometry and magnetometry of ferritin

Non-invasive and high-sensitivity scanning detection of magnetic nanoparticles in animals using high-Tc scanning superconducting-quantum-interference-device biosusceptometry

Although magnetic nanoparticles (MNPs) have been widely applied to animals in biomedicine, MNPs within animals should be examined in real time, in vivo, and without bio-damaged possibility to evaluate whether the bio-function of MNPs is valid or to further controls the biomedicinal process because of accompanying complex problems such as MNPs distribution and MNPs biodegradation. The non-invasive and high-sensitivity scanning detection of MNPs in animals using ac susceptometry based on a high-T(c) superconducting quantum interference device (SQUID) is presented. The non-invasive results and biopsy results show good agreement, and two gold-standard biomedicine methods, Prussian blue stain and inductively coupled plasma, prove the magnetic results. This confirms that the future clinical diagnosis of bio-functional MNPs could be operated by using scanning SQUID biosusceptometry as conveniently as an ultrasonic probe.

Evaluation of the specificity and sensitivity of ferritin as an MRI reporter gene in the mouse brain using lentiviral and adeno-associated viral vectors

The development of in vivo imaging protocols to reliably track transplanted cells or to report on gene expression is critical for treatment monitoring in (pre)clinical cell and gene therapy protocols. Therefore, we evaluated the potential of lentiviral vectors (LVs) and adeno-associated viral vectors (AAVs) to express the magnetic resonance imaging (MRI) reporter gene ferritin in the rodent brain. First, we compared the induction of background MRI contrast for both vector systems in immune-deficient and immune-competent mice. LV injection resulted in hypointense (that is, dark) changes of T(2)/T(2)(*) (spin-spin relaxation time)-weighted MRI contrast at the injection site, which can be partially explained by an inflammatory response against the vector injection. In contrast to LVs, AAV injection resulted in reduced background contrast. Moreover, AAV-mediated ferritin overexpression resulted in significantly enhanced contrast to background on T(2)(*)-weighted MRI. Although sensitivity associated with the ferritin reporter remains modest, AAVs seem to be the most promising vector system for in vivo MRI reporter gene imaging.

Preferential patterns of myocardial iron overload by multislice multiecho T*2 CMR in thalassemia major patients

T*(2) multislice multiecho cardiac MR allows quantification of the segmental distribution of myocardial iron overload. This study aimed to determine if there were preferential patterns of myocardial iron overload in thalassemia major. Five hundred twenty-three thalassemia major patients underwent cardiac MR. Three short-axis views of the left ventricle were acquired and analyzed using a 16-segment standardized model. The T*(2) value on each segment was calculated, as well as the global value. Four main circumferential regions (anterior, septal, inferior, and lateral) were defined. Significant segmental variability was found in the 229 patients with significant myocardial iron overload (global T*(2) <26 ms), subsequently divided into two groups: severe (global T*(2) <10 ms) and mild to moderate (global T*(2) between 10 and 26 ms) myocardial iron overload. A preferential pattern of iron store in anterior and inferior regions was detected in both groups. This pattern was preserved among the slices. The pattern could not be explained by additive susceptibility artifacts, negligible in heavily iron-loaded patients. A significantly higher T*(2) value in the basal slice was found in patients with severe iron overload. In conclusion, a segmental T*(2) cardiac MR approach could identify early iron deposit, useful for tailoring chelation therapy and preventing myocardial dysfunction in the clinical setting.

Magnetic resonance assessment of iron overload by separate measurement of tissue ferritin and hemosiderin iron

With transfusional iron overload, almost all the excess iron is sequestered intracellularly as rapidly mobilizable, dispersed, soluble ferritin iron, and as aggregated, insoluble hemosiderin iron for long-term storage. Established magnetic resonance imaging (MRI) indicators of tissue iron (R(2), R(2)*) are principally influenced by hemosiderin iron and change slowly, even with intensive iron chelation. Intracellular ferritin iron is evidently in equilibrium with the low-molecular-weight cytosolic iron pool that can change rapidly with iron chelation. We have developed a new MRI method to separately measure ferritin and hemosiderin iron, based on the non-monoexponential signal decay induced by aggregated iron in multiple-spin-echo sequences. We have initially validated the method in agarose phantoms and in human liver explants and shown the feasibility of its application in patients with thalassemia major. Measurement of tissue ferritin iron is a promising new means to rapidly evaluate the effectiveness of iron-chelating regimens.

Separate MRI quantification of dispersed (ferritin-like) and aggregated (hemosiderin-like) storage iron

A new MRI method is proposed for separately quantifying the two principal forms of tissue storage (nonheme) iron: ferritin iron, a dispersed, soluble fraction that can be rapidly mobilized, and hemosiderin iron, an aggregated, insoluble fraction that serves as a long-term reserve. The method utilizes multiple spin echo sequences, exploiting the fact that aggregated iron can induce nonmonoexponential signal decay for multiple spin echo sequences. The method is validated in vitro for agarose phantoms, simulating dispersed iron with manganese chloride, and aggregated iron with iron oxide microspheres. To demonstrate feasibility for human studies, preliminary in vivo data from two healthy controls and six patients with transfusional iron overload are presented. For both phantoms and human subjects, conventional R(2) and R(2)* relaxation rates are also measured in order to contrast the proposed method with established MRI iron quantification techniques. Quantification of dispersed (ferritin-like) iron may provide a new means of monitoring the risk of iron-induced toxicity in patients with iron overload and, together with quantification of aggregated (hemosiderin-like) iron, improve the accuracy of estimates for total storage iron.

Measurement of brain iron distribution in Hallevorden-Spatz syndrome

PURPOSE

To investigate spatial distribution of iron accumulation in the globus pallidus (GP) in patients with Hallevorden-Spatz syndrome (HSS) using phase imaging. We compared sensitivity of a phase imaging technique to relaxation rate measurement methods (R1,R2,R2*) for iron quantification.

MATERIALS AND METHODS

R1, R2, and R2* were measured in GP structure of the brain of eight pantothenate kinase-associated neurodegeneration (PKAN) patients and a healthy volunteer using a 3T magnetic resonance imaging (MRI) scanner. The phase of gradient-echo images was preprocessed to eliminate phase 2pi wrapping and filtered to remove phase background variations. Phase gap across GP structure was used as a metric for iron effects quantification.

RESULTS

Among the relaxation rates the most sensitive to iron accumulation was the R2* rate. The R1 and R2 rates demonstrated only small variations in this group of subjects. Up to an order of magnitude phase gap changes were measured between one PKAN patient and an age-matched healthy volunteer. Assuming that phase gap differences scale linearly with iron concentration we estimate that up to 2 mg Fe/g ww accumulates in GP of these patients.

CONCLUSION

Our results demonstrate significantly higher sensitivity of the phase measurements for quantitative assessment of iron concentration compared to the relaxation rate measurements. Phase measurements could potentially be used for monitoring a progression and a response to therapy in PKAN.

The Mössbauer and magnetic properties of ferritin cores

BACKGROUND

Mössbauer and magnetization measurements, singly or in combination, extract detailed information on the microscopic or internal magnetism of iron-based materials and their macroscopic or bulk magnetization. The combination of the two techniques affords a powerful investigatory probe into spin relaxation processes of nanosize magnetic systems. The ferritin core constitutes a paradigm of such nano-magnetic system where Mössbauer and magnetization studies have been broadly combined in order to elucidate its composition, the initial steps of iron nucleation and biomineralization, particle growth and core-size distribution. In vivo produced and in vitro reconstituted wild-type and variant ferritins have been extensively studied in order to elucidate structure/function correlations and ferritin's role in iron overloading or neurodegenerative disorders.

SCOPE OF REVIEW

Studies on the initial stages of iron biomineralization, biomimetic synthetic analogues and ferrous ion retention within the ferritin core are presented. The dynamical magnetic properties of ferritin by Mössbauer and magnetization measurements are critically reviewed. The focus is on experiments that reveal the internal magnetic structure of the ferritin core. Novel magnetic measurements on individual ferritin molecules via AFM and nanoSQUID investigations are also mentioned.

MAJOR CONCLUSIONS

A complex two-phase spin system is revealed due to finite-size effects and non-compensated spins at the surface of the anti-ferromagnetic ferritin core. Below the blocking temperature surface spins participate in relaxation processes much faster than those associated with collective magnetic excitations of interior spins.

GENERAL SIGNIFICANCE

The studies reviewed contribute uniquely to the elucidation of the spin-structure and spin-dynamics of anti-ferromagnetic nanolattices and their possible applications to nano/bio-technology.

Modified natural nanoparticles as contrast agents for medical imaging

The development of novel and effective contrast agents is one of the drivers of the ongoing improvement in medical imaging. Many of the new agents reported are nanoparticle-based. There are a variety of natural nanoparticles known, e.g. lipoproteins, viruses or ferritin. Natural nanoparticles have advantages as delivery platforms such as biodegradability. In addition, our understanding of natural nanoparticles is quite advanced, allowing their adaptation as contrast agents. They can be labeled with small molecules or ions such as Gd(3+) to act as contrast agents for magnetic resonance imaging, (18)F to act as positron emission tomography contrast agents or fluorophores to act as contrast agents for fluorescence techniques. Additionally, inorganic nanoparticles such as iron oxide, gold nanoparticles or quantum dots can be incorporated to add further contrast functionality. Furthermore, these natural nanoparticle contrast agents can be re-routed from their natural targets via the attachment of targeting molecules. In this review, we discuss the various modified natural nanoparticles that have been exploited as contrast agents.

Multislice multiecho T2* cardiac magnetic resonance for the detection of heterogeneous myocardial iron distribution in thalassaemia patients

The present study investigated myocardial T2* heterogeneity in thalassaemia major (TM) patients by cardiac magnetic resonance (CMR), to determine whether is related to inhomogeneous iron overload distribution. A total of 230 TM patients consecutively referred to our laboratory were studied retrospectively. Three short-axis views (basal, medium and apical) of the left ventricle (LV) were obtained by multislice multiecho T2* CMR. T2* segmental distribution was mapped on a 16-segment LV model. The level of heterogeneity of the T2* segmental distribution, evaluated by the coefficient of variation (CoV), was compared with that of a surrogate data set, to determine whether the inhomogeneous segmental distribution of T2* could be generated by susceptibility artefacts. Susceptibility artefacts offer an explanation for the T2* heterogeneity observed in patients without iron overload. In subjects with global T2* below the lower limit of the normal, T2* heterogeneity increased abruptly which could not be explained by artefactual effects. Some segmental T2* values were below and others above the limit of normal threshold (20 ms) in 104 (45%) TM patients. Among these patients, 74% showed a normal T2* global value. In conclusion, a true heterogeneity in the iron overload distribution may be present in TM patients. Heterogeneity seemingly appears in the borderline myocardial iron and stabilizes at moderate to severe iron burden.

In vivo atherosclerotic plaque characterization using magnetic susceptibility distinguishes symptom-producing plaques

OBJECTIVES

We investigated the role of iron deposition in atherosclerotic plaque instability using a novel approach of in vivo plaque characterization by a noninvasive, noncontrast magnetic resonance-based T2* measurement. This approach was validated using ex vivo plaque analyses to establish that T2* accurately reflects intraplaque iron composition.

BACKGROUND

Iron catalyzes free radical production, a key step for lipid peroxidation and atherosclerosis development. The parameter T2* measures tissue magnetic susceptibility, which historically has been used to quantify hepatic and myocardial iron. The T2* measurement has not been used for in vivo plaque characterization in patients with atherosclerosis.

METHODS

Thirty-nine patients referred for carotid endarterectomy were prospectively enrolled to undergo preoperative carotid magnetic resonance imaging (MRI) and postoperative analysis of the explanted plaque. Clinical history of any symptoms attributable to each carotid lesion was recorded. We could not complete MRI in 4 subjects because of their claustrophobia, and 3 patients scanned before the institution of a neck stabilizer had motion artifact, precluding quantification.

RESULTS

Symptomatic patients had significantly lower plaque T2* values (20.0 +/- 1.8 ms) compared with asymptomatic patients (34.4 +/- 2.7 ms, p < 0.001). Analytical methods demonstrated similar total iron (138.6 +/- 36.5 microg/g vs. 165.8 +/- 48.3 microg/g, p = NS) but less low molecular weight Fe(III) (7.3 +/- 3.8 microg/g vs. 17.7 +/- 4.0 microg/g, p < 0.05) in the explanted plaques of symptomatic versus asymptomatic patients, respectively, which is consistent with a shift in iron from Fe(III) to greater amounts of T2*-shortening forms of iron. Mass spectroscopy also showed significantly lower calcium (37.5 +/- 10.8 mg/g vs. 123.6 +/- 19.3 mg/g, p < 0.01) and greater copper (3.2 +/- 0.5 microg/g vs. 1.7 +/- 0.1 microg/g, p < 0.01) in plaques from symptomatic patients.

CONCLUSIONS

In vivo measurement of intraplaque T2* using MRI is feasible and distinguishes symptom-producing from non-symptom-producing plaques in patients with carotid artery atherosclerosis. Symptom-producing plaques demonstrated characteristic changes in iron forms by ex vivo analysis, supporting the dynamic presence of iron in the microenvironment of atherosclerotic plaque.

MRI of the basement membrane using charged nanoparticles as contrast agents

The integrity of the basement membrane is essential for tissue cellular growth and is often altered in disease. In this work a method for noninvasively detecting the structural integrity of the basement membrane, based on the delivery of cationic iron-oxide nanoparticles, was developed. Cationic particles accumulate due to the highly negative charge of proteoglycans in the basement membrane. The kidney was used to test this technique because of its highly fenestrated endothelia and well-established disease models to manipulate the basement membrane charge barrier. After systemic injection of cationic or native ferritin (CF or NF) in rats, ex vivo and in vivo MRI showed selective accumulation of CF, but not NF, causing a 60% reduction in signal intensity in cortex at the location of individual glomeruli. Immunofluorescence and electron microscopy demonstrated that this CF accumulation was localized to the glomerular basement membrane (GBM). In a model of GBM breakdown during focal and segmental glomerulosclerosis, MRI showed reduced single glomerular accumulation of CF, but a diffuse accumulation of CF in the kidney tubules caused by leakage of CF through the glomerulus. Cationic contrast agents can be used to target the basement membrane and detect the breakdown of the basement membrane in disease.

3D morphology of the human hepatic ferritin mineral core: new evidence for a subunit structure revealed by single particle analysis of HAADF-STEM images

Ferritin, the major iron storage protein, has dual functions; it sequesters redox activity of intracellular iron and facilitates iron turn-over. Here we present high angle annular dark field (HAADF) images from individual hepatic ferritin cores within tissue sections, these images were obtained using spherical aberration corrected scanning transmission electron microscopy (STEM) under controlled electron fluence. HAADF images of the cores suggest a cubic morphology and a polycrystalline (ferrihydrite) subunit structure that is not evident in equivalent bright field images. By calibrating contrast levels in the HAADF images using quantitative electron energy loss spectroscopy, we have estimated the absolute iron content in any one core, and produced a three dimensional reconstruction of the average core morphology. The core is composed of up to eight subunits, consistent with the eight channels in the protein shell that deliver iron to the central cavity. We find no evidence of a crystallographic orientation relationship between core subunits. Our results confirm that the ferritin protein shell acts as a template for core morphology and within the core, small (approximately 2 nm), surface-disordered ferrihydrite subunits connect to leave a low density centre and a high surface area that would allow rapid turn-over of iron in biological systems.

Controlled aggregation of ferritin to modulate MRI relaxivity

Ferritin is an iron storage protein expressed in varying concentrations in mammalian cells. The deposition of ferric iron in the core of ferritin makes it a magnetic resonance imaging contrast agent, and ferritin has recently been proposed as a gene expression reporter protein for magnetic resonance imaging. To date, ferritin has been overexpressed in vivo and has been coexpressed with transferrin receptor to increase iron loading in cells. However, ferritin has a relatively low T(2) relaxivity (R(2) approximately 1 mM(-1)s(-1)) at typical magnetic field strengths and so requires high levels of expression to be detected. One way to modulate the transverse relaxivity of a superparamagnetic agent is to cause it to aggregate, thereby manipulating the magnetic field gradients through which water diffuses. In this work, it is demonstrated by computer simulation and in vitro that aggregation of ferritin can alter relaxivity. The effects of aggregate size and intraaggregate perturber spacing on R(2) are studied. Computer modeling indicates that the optimal spacing of the ferritin molecules in aggregate for increasing R(2) is 100-200 nm for a typical range of water diffusion rates. Chemical cross-linking of ferritin at 12 A spacing led to a 70% increase in R(2) compared to uncross-linked ferritin controls. To modulate ferritin aggregation in a potentially biologically relevant manner, ferritin was attached to actin and polymerized in vitro. The polymerization of ferritin-F-actin caused a 20% increase in R(2) compared to unpolymerized ferritin-G-actin. The R(2)-value was increased by another 10% by spacing the ferritin farther apart on the actin filaments. The modulation of ferritin aggregation by binding to cytoskeletal elements may be a useful strategy to make a functional reporter gene for magnetic resonance imaging.

A comparative Mössbauer study of the mineral cores of human H-chain ferritin employing dioxygen and hydrogen peroxide as iron oxidants

Ferritins are ubiquitous iron storage and detoxification proteins distributed throughout the plant and animal kingdoms. Mammalian ferritins oxidize and accumulate iron as a ferrihydrite mineral within a shell-like protein cavity. Iron deposition utilizes both O(2) and H(2)O(2) as oxidants for Fe(2+) where oxidation can occur either at protein ferroxidase centers or directly on the surface of the growing mineral core. The present study was undertaken to determine whether the nature of the mineral core formed depends on the protein ferroxidase center versus mineral surface mechanism and on H(2)O(2) versus O(2) as the oxidant. The data reveal that similar cores are produced in all instances, suggesting that the structure of the core is thermodynamically, not kinetically controlled. Cores averaging 500 Fe/protein shell and diameter approximately 2.6 nm were prepared and exhibited superparamagnetic blocking temperatures of 19 and 22 K for the H(2)O(2) and O(2) oxidized samples, respectively. The observed blocking temperatures are consistent with the unexpectedly large effective anisotropy constant K(eff)=312 kJ/m(3) recently reported for ferrihydrite nanoparticles formed in reverse micelles [E.L. Duarte, R. Itri, E. Lima Jr., M.S. Batista, T.S. Berquó and G.F. Goya, Large Magnetic Anisotropy in ferrihydrite nanoparticles synthesized from reverse micelles, Nanotechnology 17 (2006) 5549-5555.]. All ferritin samples exhibited two magnetic phases present in nearly equal amounts and ascribed to iron spins at the surface and in the interior of the nanoparticle. At 4.2 K, the surface spins exhibit hyperfine fields, H(hf), of 436 and 445 kOe for the H(2)O(2) and O(2) samples, respectively. As expected, the spins in the interior of the core exhibit larger H(hf) values, i.e. 478 and 486 kOe for the H(2)O(2) and O(2) samples, respectively. The slightly smaller hyperfine field distribution DH(hf) for both surface (78 kOe vs. 92 kOe) and interior spins (45 kOe vs. 54 kOe) of the O(2) sample compared to the H(2)O(2) samples implies that the former is somewhat more crystalline.

Developing MR reporter genes: promises and pitfalls

MR reporter genes have the potential to monitor transgene expression non-invasively in real time at high resolution. These genes can be applied to interrogate the efficacy of gene therapy, to assess cellular differentiation, cell trafficking, and specific metabolic activity, and also assess changes in the microenvironment. Efforts toward the development of MR reporter genes have been made for at least a decade, but, despite these efforts, the field is still in its early developmental stage. This reflects the fact that there are potential pitfalls, caused by the low sensitivity of detection, the need for substrates with their associated undesirable pharmacokinetics, and/or the difficult and, in some cases, delayed interpretation of signal changes. Nevertheless, significant progress has been made during the last few years. Whereas enzyme-based reporters were initially applied to NMR spectroscopic monitoring of changes in phosphor and fluorine metabolism, MRI-based approaches are now emerging that rely on: (1) enzyme-based cleavage of functional groups that block water (proton) exchange or protein binding of MR contrast agents; (2) expression of surface receptors that enable binding of specific MR contrast agents; (3) expression of para- and anti-ferromagnetic (metallo)proteins involved with iron metabolism, such as tyrosinase, transferrin receptor, and ferritin. After an introduction to the basic principles of designing promoters, expression vectors, and cloning of transgenes, a fresh look is provided on the use of reporter genes for optical (including bioluminescent) and nuclear imaging, with which MR reporter genes compete. Although progress in the use of MR reporter genes has been slow, newer strategies that use metalloproteins or alternative contrast mechanisms, with no need for substrates, promise rapid growth potential for this field.

Theoretical MRI contrast model for exogenous T2 agents

The rational development of new generations of MRI contrast agents (CAs) requires a scheme for predicting contrast enhancement. Previous contrast predictions have been based largely on empirical results in specific systems. Here we present a general theoretical model for evaluating the minimum concentration of T2 CA required for satisfactory image contrast. This analytic contrast model is applicable to a wide range of T2-type agents and delivery scenarios, and requires only a few readily evaluated parameters. We demonstrated the model by predicting contrast produced by superparamagnetic ferumoxide and the iron storage protein, ferritin. We then experimentally verified the predictions using suspensions of Feridex(R) and ferritin in phantoms. The model was also used to compare the contrast efficacy of the metal ions in two clinically approved T1- and T2-type CAs. In the Appendix we present a numerical formalism that is useful for relating image contrast and agent concentration when gradient-echo (GRE) T2*-weighted (T2*W) pulse sequences are used.

The magnetic susceptibilities of iron deposits in thalassaemic spleen tissue

The iron-specific magnetic susceptibility of tissue iron deposits is used in the field of non-invasive measurement of tissue iron concentrations. It has generally been assumed to be a constant for all tissue and disease types. The iron-specific magnetic susceptibilities chi(Fe) for spleen tissue samples from 7 transfusion dependent beta-thalassaemia (beta-thal) patients and 11 non-transfusion dependent beta-thalassaemia/Haemoglobin E (beta/E) patients were measured at 37 degrees C. Both groups of patients were iron loaded with no significant difference in the distribution of spleen iron concentrations between the two groups. There was a significant difference between the mean chi(Fe) of the spleen tissue from each group. The non-transfusion dependent beta/E patients had a higher mean (+/-standard deviation) spleen chi(Fe) (1.55+/-0.23 x 10(-6) m(3)/kg Fe) than the transfusion dependent beta-thal patients (1.16+/-0.25 x 10(-6) m(3)/kg Fe). Correlations were observed between chi(Fe) of the spleen tissue and the fraction of magnetic hyperfine split sextet in the (57)Fe Mössbauer spectra of the tissues at 78 K (Spearman rank order correlation r=-0.54, p=0.03) and between chi(Fe) of the spleen tissue and the fraction of doublet in the spectra at 5 K (r=0.58, p=0.02) indicating that chi(Fe) of the spleen tissue is related to the chemical speciation of the iron deposits in the tissue.

Cellular MRI contrast via coexpression of transferrin receptor and ferritin

Recently there has been growing interest in the development and use of iron-based contrast agents for cellular imaging with MRI. In this study we investigated coexpression of the transferrin receptor and ferritin genes to induce cellular contrast in a biological system. Expression of transgenic human transferrin receptor and human ferritin H-subunit was induced in a stably transfected mouse neural stem cell line. When grown in iron-rich medium, the transgenic cells accumulated significantly more iron than control cells, with a trend toward an increase in reactive oxygen species, but no detrimental effects on cell viability. This cellular iron significantly increased the transverse relaxivities, R2 and R2*, at 1.5 T and 7 T. By comparing measurements in the same cell samples at 1.5 T and 7 T, we confirmed the expected increase in relaxivity with increasing field strength. Finally, supplemented transgenic cells transplanted into mouse brain demonstrated increased contrast with surrounding neural tissue on T2*-weighted MR brain images compared to controls. These results indicate that dual expression of proteins at different critical points in the iron metabolism pathway may improve cellular contrast without compromising cell viability.

Methods for noninvasive measurement of tissue iron in Cooley's anemia

To examine the relationship between myocardial storage iron and body iron burden, as assessed by hepatic storage iron measurements, we studied 22 patients with transfusion-dependent thalassemia syndromes, all being treated with subcutaneous deferoxamine, and 6 healthy subjects. Study participants were examined with a Philips 1.5-T Intera scanner using three multiecho spin echo sequences with electrocardiographic triggering and respiratory navigator gating. Myocardial and hepatic storage iron concentrations were determined using a new magnetic resonance method that estimates total tissue iron stores by separately measuring the two principal forms of storage iron, ferritin and hemosiderin. In a subset of 10 patients with beta-thalassemia major, the hepatic storage iron concentration had been monitored repeatedly for 12-14 years by chemical analysis of tissue obtained by liver biopsy and by magnetic susceptometry. In this subset, we examine the relationship between hepatic iron concentration over time and our current magnetic resonance estimates of myocardial iron stores. No significant relationship was found between simultaneous estimates of myocardial and hepatic storage iron concentrations. By contrast, in the subset of 10 patients with beta-thalassemia major, the correlation between the 5-year average of hepatic iron concentration and the current myocardial storage iron was significant (R = .67, P = .03). In these patients, myocardial storage iron concentrations seem to reflect the control of body iron over a period of years. Magnetic resonance methods promise to provide more effective monitoring of iron deposition in vulnerable tissues, including the liver, heart, and endocrine organs, and could contribute to the development of iron-chelating regimens that more effectively prevent iron toxicity.

Multispectral quantitative magnetic resonance imaging of brain iron stores: a theoretical perspective

OBJECTIVES

To review published magnetic resonance imaging (MRI) iron quantification techniques in the context of quantitative MRI and MR relaxation theories. To analyze comparatively and as a function of age the simultaneous measurements of the proton density (PD), the relaxation times (T1 and T2), and the longitudinal to transverse relaxation times ratio (T1/T2) of brain regions known to accumulate iron preferentially.

METHODS

Twenty-seven human subjects were scanned with the mixed turbo spin echo pulse sequence, which is multispectral in PD, T1, and T2. Quantitative MRI (Q-MRI) maps of PD, T1, T2, and T1/T2 were generated, and region of interest measurements were performed in 5 brain regions, namely, frontal white matter (WM), genu of corpus callosum, caudate nucleus, putamen, and globus pallidus.

RESULTS

Relaxation time measurements are consistent with results of others and provide further confirmation to our basic understanding of the relaxation effects of iron stores in the brain. Specifically, we found that the iron-rich globus pallidus exhibits enhanced T1 and T2 relaxation relative the iron poorer gray matter tissues (caudate nucleus and putamen) and also relative to the WM matter tissues (frontal WM and genu of the corpus callosum). We also observe that under riding this hypothesis-because we do not have independent confirmation-that iron caused relaxation enhancement, are the normal brain aging patterns, which suggest that the brain tissues become wetter with increasing age. Also noted is the virtual removal of age dependence observed for the T1/T2 ratio of WM tissues, further suggesting that this ratio may become of clinical significance in the diagnosis of neoplastic processes as well as for quantifying iron in tissue.

CONCLUSIONS

The theoretical underpinnings of published brain iron Q-MRI techniques have been reviewed. We also examined MR relaxation theory essentials in relation to H-proton relaxation phenomena in diamagnetic tissues as well as theoretical extensions to describe relaxation effects in tissues containing iron deposits with a focus on ferritin. Also reported are in vivo Q-MRI results of 27 human brains obtained with a multispectral technique that uses the mixed turbo spin echo pulse sequence and a model conforming Q-MRI algorithms.

Magnetic resonance imaging of iron deposition in neurological disorders

Deposition of iron in the brain is proposed to play a role in the pathophysiology of the normal aging process and neurodegenerative diseases. Whereas iron is required for normal neuronal metabolism, excessive levels can contribute to the formation of free radicals, leading to lipid peroxidation and neurotoxicity. Magnetic resonance imaging (MRI) is a powerful tool to detect excessive iron in the brain and longitudinally monitor changes in iron levels. Iron deposition is associated with a reduction in the T2 relaxation time, leading to hypointensity on spin-echo and gradient-echo T2-weighted images. The MRI changes associated with iron deposition have been observed both in normal aging and in various chronic neurological diseases, including multiple sclerosis, Alzheimer disease, and Parkinson disease. Magnetic resonance imaging metrics providing information about iron concentrations include R2, R2', and R2*. The purpose of this review is to discuss the role of iron and its detection by MRI in various neurological disorders. We will review the basic biochemical properties of iron and its influence on MRI signal. We will also summarize the sensitivity and specificity of MRI techniques in detecting iron. The MRI and pathological findings pertaining to brain iron will be reviewed with respect to normal aging and a variety of neurological disorders. Finally, the biochemistry and pathophysiology surrounding iron, oxidative stress, free radicals, and lipid peroxidation in the brain will be discussed, including therapeutic implications. The potential role of iron deposition and its assessment by MRI provides exciting potential applications to the diagnosis, longitudinal monitoring, and therapeutic development for disorders of the brain.

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.

Physical interactions of static magnetic fields with living tissues

Clinical magnetic resonance imaging (MRI) was introduced in the early 1980s and has become a widely accepted and heavily utilized medical technology. This technique requires that the patients being studied be exposed to an intense magnetic field of a strength not previously encountered on a wide scale by humans. Nonetheless, the technique has proved to be very safe and the vast majority of the scans have been performed without any evidence of injury to the patient. In this article the history of proposed interactions of magnetic fields with human tissues is briefly reviewed and the predictions of electromagnetic theory on the nature and strength of these interactions are described. The physical basis of the relative weakness of these interactions is attributed to the very low magnetic susceptibility of human tissues and the lack of any substantial amount of ferromagnetic material normally occurring in these tissues. The presence of ferromagnetic foreign bodies within patients, or in the vicinity of the scanner, represents a very great hazard that must be scrupulously avoided. As technology and experience advance, ever stronger magnetic field strengths are being brought into service to improve the capabilities of this imaging technology and the benefits to patients. It is imperative that vigilance be maintained as these higher field strengths are introduced into clinical practice to assure that the high degree of patient safety that has been associated with MRI is maintained.

Formation of protein-coated iron minerals

The ability of iron to cycle between Fe(2+) and Fe(3+) forms has led to the evolution, in different forms, of several iron-containing protein cofactors that are essential for a wide variety of cellular processes, to the extent that virtually all cells require iron for survival and prosperity. The redox properties of iron, however, also mean that this metal is potentially highly toxic and this, coupled with the extreme insolubility of Fe(3+), presents the cell with the significant problem of how to maintain this essential metal in a safe and bioavailable form. This has been overcome through the evolution of proteins capable of reversibly storing iron in the form of a Fe(3+) mineral. For several decades the ferritins have been synonymous with the function of iron storage. Within this family are subfamilies of mammalian, plant and bacterial ferritins which are all composed of 24 subunits assembled to form an essentially spherical protein with a central cavity in which the mineral is laid down. In the past few years it has become clear that other proteins, belonging to the family of DNA-binding proteins from starved cells (the Dps family), which are oligomers of 12 subunits, and to the frataxin family, which may contain up to 48 subunits, are also able to lay down a Fe(3+) mineral core. Here we present an overview of the formation of protein-coated iron minerals, with particular emphasis on the structures of the protein coats and the mechanisms by which they promote core formation. We show on the one hand that significant mechanistic similarities exist between structurally dissimilar proteins, while on the other that relatively small structural differences between otherwise similar proteins result in quite dramatic mechanistic differences.

High-field magnetic resonance imaging of brain iron: birth of a biomarker?

The brain has an unusually high concentration of iron, which is distributed in an unusual pattern unlike that in any other organ. The physiological role of this iron and the reasons for this pattern of distribution are not yet understood. There is increasing evidence that several neurodegenerative diseases are associated with altered brain iron metabolism. Understanding these dysmetabolic conditions may provide important information for their diagnosis and treatment. For many years the iron distribution in the human brain could be studied effectively only under postmortem conditions. This situation was changed dramatically by the finding that T2-weighted MR imaging at high field strength (initially 1.5 T) appears to demonstrate the pattern of iron distribution in normal brains and that this imaging technique can detect changes in brain iron concentrations associated with disease states. Up to the present time this imaging capability has been utilized in many research applications but it has not yet been widely applied in the routine diagnosis and management of neurodegenerative disorders. However, recent advances in the basic science of brain iron metabolism, the clinical understanding of neurodegenerative diseases and in MRI technology, particularly in the availability of clinical scanners operating at the higher field strength of 3 T, suggest that iron-dependent MR imaging may soon provide biomarkers capable of characterizing the presence and progression of important neurological disorders. Such biomarkers may be of crucial assistance in the development and utilization of effective new therapies for Alzheimer's and Parkinson's diseases, multiple sclerosis and other iron-related CNS disorders which are difficult to diagnose and treat.

Electron nanodiffraction and high-resolution electron microscopy studies of the structure and composition of physiological and pathological ferritin

Structures of core nanocrystals of physiological (horse spleen, human liver, and brain) and pathological human brain of patients with progressive supranuclear palsy (PSP) and Alzheimer's disease (AD) ferritin molecules were determined using electron nanodiffraction and high-resolution transmission electron microscopy. The poly-phasic structure of the ferritin cores is confirmed. There are significant differences in the mineral composition between the physiological and pathological ferritins. The physiological ferritin cores mainly consist of single nanocrystals containing hexagonal ferrihydrite (Fh) and hematite (Hm) and some cubic magnetite/maghemite phase. In the pathological cores, Fh is present but only as a minor phase and Hm is absent. The major phases are a face-centered-cubic (fcc) structure with a = 0.43 nm and a high degree of disorder, related to wustite, and a cubic magnetite-like structure. These two cubic phases are also present in human aged normal brain. Evidence for the presence of hemosiderin together with ferritin in the pathological brains is deduced from the similarities of the diffraction patterns with those from patients with primary hemochromatosis, and differences in the shapes and protein composition of the protein shell. These findings suggest a disfunction of the ferritin associated with PSP and AD, associated with an increase in the concentration of brain ferrous toxic iron.

Quantitative T2 in the occipital lobe: the role of the CPMG refocusing rate

PURPOSE

To investigate the dependence of occipital gray and white matter T(2) on the Carr-Purcell-Meiboom-Gill (CPMG) refocusing interval, thereby testing the basis of a novel functional magnetic resonance imaging (fMRI) method for blood volume quantification, and addressing recent questions surrounding T(2) contrast in the occipital lobe.

MATERIALS AND METHODS

A CPMG sequence with 1 x 1 x 5 mm(3) resolution was used to quantify T(2) in a single axial slice at the midlevel of the occipital lobe in 23 healthy adult volunteers. Refocusing intervals of 8, 11, and 22 msec were compared. A Bayesian classifier was used to classify a 1 x 1 x 1 mm(3) T(1)-weighted three-dimensional data set into gray matter, white matter, and cerebrospinal fluid, with an average 95% a posteriori probability used as the threshold for inclusion into a tissue-specific region of interest (ROI).

RESULTS

The usual T(2) contrast between the gray and white matter (i.e., T(2GM) > T(2WM)) was observed, with a highly significant effect of tissue type on the estimated T(2) (P < 10(-5)). The observed T(2) gradually decreased with increasing refocusing interval, for a decrease of 3.3 +/- 1.5 msec in gray matter and 3.0 +/- 1.5 msec in white matter between the 8 and 22 msec refocusing interval acquisitions.

CONCLUSION

The observed T(2) shortening is consistent with the effect of the dramatic decrease in T(2) of partly deoxygenated blood on this range of refocusing rates.

Noninvasive measurement of iron: report of an NIDDK workshop

An international workshop on the noninvasive measurement of iron was conducted by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) on April 17, 2001, to assess the current state of the science and to identify areas needing further investigation. The workshop concluded that a clear clinical need is evident for quantitative, noninvasive, safe, accurate, and readily available means of measuring body storage iron to improve the diagnosis and management of patients with iron overload from such disorders as hereditary hemochromatosis, thalassemia major, sickle cell disease, aplastic anemia, and myelodysplasia, among others. Magnetic resonance imaging (MRI) potentially provides the best available technique for examining the 3-dimensional distribution of excess iron in the body, but further research is needed to develop means of making measurements quantitative. Biomagnetic susceptometry provides the only noninvasive method to measure tissue iron stores that has been calibrated, validated, and used in clinical studies, but the complexity, cost, and technical demands of the liquid-helium-cooled superconducting instruments required at present have restricted clinical access to the method. The workshop identified basic and clinical research opportunities for deepening our understanding of the physical properties of iron and iron toxicity, for further investigation of MRI as a method for quantitative determinations of tissue iron, especially in liver, heart and brain, and for development of improved methods and more widely available instrumentation for biomagnetic susceptometry.

Anomalous nuclear magnetic relaxation of aqueous solutions of ferritin: an unprecedented first-order mechanism

Ferritin, the iron-storing protein, speeds up proton transverse magnetic relaxation in aqueous solutions. This T(2) shortening is used in MRI to quantify iron in the brain and liver. Current theoretical models underestimate the relaxation enhancement by ferritin at imaging fields, and they do not predict the measured dependence of the rate enhancement on the magnetization of the particles. Here it is shown that a proton exchange dephasing model (PEDM) overcomes these limitations by allowing a first-order relaxation mechanism. The PEDM considers proton exchange between bulk water and exchangeable protons located at the surface of the hydrated iron oxide nanometric core of the protein. Relaxation is shown to depend on the distribution of the frequency shifts of the adsorption sites; the observed properties agree with a Lorentzian distribution. Computer simulations utilizing recent Mössbauer spectroscopy data show that the distribution of these shifts is effectively Lorentzian.

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.

Magnetic investigations of human mesencephalic neuromelanin

Pigmentation of neurons in substantia nigra is due to neuromelanin, a pigment that stores large amounts of iron. Human mesencephalic neuromelanin has been investigated by means of magnetic susceptibility measurements as a function of temperature. Magnetic measurements provide a physico-chemical characterization of the iron cluster buried in the organic melanin matrix and support the view that iron is not simply chelated, but rather is organized in a three-dimensional network. The paramagnetism of isolated iron ions is observed, in agreement with electron paramagnetic resonance spectroscopy. Furthermore, antiferromagnetic grains with a large size distribution function are present. These grains contain N spins coupled antiferromagnetically; however, N(1/2) spins are decoupled from the grain bulk and parallelly aligned. The latter subgrains are superparamagnetic with a blocking temperature ranging between 5 K and room temperature. This behavior has not been observed in synthetic melanin, where the paramagnetic contribution is strongly enhanced. Preliminary results on pigment isolated from patients affected by Parkinson's disease, a neurodegenerative pathology involving primarily pigmented neurons in substantia nigra pars compacta, show a lower total magnetization compared to control neuromelanin. The temperature behavior of zero field cooling and field cooling magnetizations is similar for both. The significant depletion of iron content in Parkinson's disease neuromelanin could indicate a progressive Fe migration from its storage environment to the cytosol.

Ferromagnetic resonance of horse spleen ferritin: core blocking and surface ordering temperatures

In nature, ferritin, an iron-storage molecule, is found in species ranging from bacteria to man. In the past 50 years its chemical, physical, and magnetic properties have been studied, searching to relate function and structure. Horse spleen ferritin has been investigated by EPR at temperatures between 7 and 290 K. These spectra change from an isotropic line at 290 K to an anisotropic one at 19 K, with a behavior consistent with a system of particles that undergoes superparamagnetic relaxation. A blocking temperature of (116+/-9) K is obtained. A new temperature-dependent signal is observed in the low field region at temperatures higher than 80 K. At 7 K no EPR signal appears, suggesting (14+/-5) K as the Néel temperature of surface spins. Analysis of the temperature dependence of the distance between EPR lines extrema, under the view of two theoretical models, allowed the evaluation of magnetic parameters. These parameters are 2K/M=2.7 x 10(3) Oe and MV=1.9 x 10(-17) emu or K/M=1.3 x 10(3) Oe and MV=2.0 x 10(-17) emu, where K is the anisotropy energy per unit volume, M is the sample magnetization, and V is the superparamagnetic core volume. The results are also discussed, and some structural models in the literature are considered.

Nuclear magnetic relaxation dispersion of ferritin and ferritin-like magnetic particle solutions: a pH-effect study

The relaxation mechanism of water protons in the presence of ferritin is still being debated. In this work, the pH dependence of the relaxation induced by ferritin and Fercayl, a ferritin-like akaganeite particle, is studied through T1 and T2 nuclear magnetic relaxation dispersion (NMRD) profiles. To differing extents, the relaxation brought about by both systems is significantly affected by pH. A proton exchange time of 33 ns (at pH 6 and 37 degrees C) is deduced from the fittings of Fercayl T1 NMRD profiles. The linearity of the relationship between 1/T2 and the magnetic field B0 for ferritin and Fercayl solutions is not altered by changes in pH. The parameters of this linearity strongly depend on pH for the latter, while remaining unchanged for the former. These results are interpreted in terms of an exchange between protons belonging to hydroxyl groups at the surface of the particle and bulk water protons.

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.

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

In a study of interregional variation of the longitudinal relaxation rate (R(1)) in human brain at 3 T, R(1) maps were acquired from 12 healthy adults using a multi-slice implementation of the T one by multiple readout pulses (TOMROP) sequence. Mean R(1) values were obtained from the prefrontal cortex (0.567 +/- 0.020 sec(-1)), caudate head (0.675 +/- 0.019 sec(-1)), putamen (0.749 +/- 0.023 sec(-1)), substantia nigra (0.873 +/- 0.037 sec(-1)), globus pallidus (0.960 +/- 0.034 sec(-1)), thalamus (0.822 +/- 0.027 sec(-1)), and frontal white matter (1.184 +/- 0.057 sec(-1)). For gray matter regions other than the thalamus, R(1) showed a strong correlation (r = 0.984, P < 0.0001) with estimated regional nonheme iron concentrations ([Fe]). These R(1) values also showed a strong correlation (r = 0.976, P < 0.0001) with estimates of 1/f(w) obtained from MRI relative proton density measurements, where f(w) represents tissue water content. When white matter is included in the consideration, 1/f(w) is a better predictor of R(1) than is [Fe]. An analysis based on the fast-exchange two-state model of longitudinal relaxation suggests that interregional differences in f(w) account for the majority of the variation of R(1) across gray matter regions. Magn Reson Med 45:71-79, 2001.

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.

The correlation between phase shifts in gradient-echo MR images and regional brain iron concentration

The purpose of this study was to investigate the relationship between the magnetic susceptibility of brain tissue and iron concentration. Phase shifts in gradient-echo images (TE = 60 ms) were measured in 21 human subjects, (age 0.7-45 years) and compared with published values of regional brain iron concentration. Phase was correlated with brain iron concentration in putamen (R2 = 0.76), caudate (0.72), motor cortex (0.68), globus pallidus (0.59) (all p < 0.001), and frontal cortex (R2 = 0.19, p = 0.05), but not in white matter (R2 = 0.05,p = 0.34). The slope of the regression (degrees/mg iron/g tissue wet weight) varied over a narrow range from -1.2 in the globus pallidus and frontal cortex to -2.1 in the caudate. These results suggest that magnetic resonance phase reflects iron-induced differences in brain tissue susceptibility in gray matter. The lack of correlation in white matter may reflect important differences between gray and white matter in the cellular distribution and the metabolic functions of iron. Magnetic resonance phase images provide insight into the magnetic state of brain tissue and may prove to be useful in elucidating the relationship between brain iron and tissue relaxation properties.

Mineralization in ferritin: an efficient means of iron storage

Ferritins are a class of iron storage and mineralization proteins found throughout the animal, plant, and microbial kingdoms. Iron is stored within the protein shell of ferritin as a hydrous ferric oxide nanoparticle with a structure similar to that of the mineral "ferrihydrite." The eight hydrophilic channels that traverse the protein shell are thought to be the primary avenues by which iron gains entry to the interior of eukaryotic ferritins. Twenty-four subunits constitute the protein shell and, in mammalian ferritins, are of two types, H and L, which have complementary functions in iron uptake. The H chain contains a dinuclear ferroxidase site that is located within the four-helix bundle of the subunit; it catalyzes the oxidation of ferrous iron by O(2), producing H(2)O(2). The L subunit lacks this site but contains additional glutamate residues on the interior surface of the protein shell which produce a microenvironment that facilitates mineralization and the turnover of iron(III) at the H subunit ferroxidase site. Recent spectroscopic studies have shown that a di-Fe(III) peroxo intermediate is produced at the ferroxidase site followed by formation of a mu-oxobridged dimer, which then fragments and migrates to the nucleation sites to form incipient mineral core species. Once sufficient core has developed, iron oxidation and mineralization occur primarily on the surface of the growing crystallite, thus minimizing the production of potentially harmful H(2)O(2).

T1 and T2 in the brain of healthy subjects, patients with Parkinson disease, and patients with multiple system atrophy: relation to iron content

PURPOSE

To investigate the potential of magnetic resonance imaging for identification and quantification of brain iron in healthy subjects, patients with Parkinson disease, and patients with multiple system atrophy.

MATERIALS AND METHODS

Forty-nine subjects were studied at 1.5 T. Regional T1 and T2 values were compared among groups and also with histopathologic estimates of iron concentration.

RESULTS

In healthy subjects, interregional T1 and T2 differences in the cortex and basal ganglia showed a good correlation with reported values for iron concentration, and intraregional variations were generally consistent with reported variability of iron concentration. Patients with multiple system atrophy had T1 and T2 shortening in the globus pallidus consistent with reported increases in ferritin-bound iron and changes in the putamen consistent with accumulation of hemosiderin (posterior portion) and neuromelanin (remainder). Both groups of patients had changes in the cortex that are consistent with decreased ferritin concentration and T2 changes in white matter consistent with demyelination. Patients with Parkinson disease also had a (nonsignificant) T2 shortening in the substantia nigra that was suggestive of iron accumulation.

CONCLUSION

Most of the T1 and T2 findings appear to be related to changes in iron content and form and may possibly be used as indicators of such changes.

Iron uptake by ferritin: NMR relaxometry studies at low iron loads

Twenty ferritin samples were prepared at pH 6.5 with average loadings of 0-89 Fe atoms per molecule. Nuclear magnetic relaxation times T1 and T2 were measured at 3 degrees C, 23 degrees C, and at 37 degrees C and at field strength from 0.025 to 1.5 T. The field dependence, temperature dependence, and approximate equality of T1 and T2 at low fields all suggest that nuclear magnetic relaxation in this range is caused primarily by solitary Fe3+ ions. The relaxivity (relaxation rate per mM ferritin) increases quickly with initial iron loading, reaches a peak at 13-14 Fe atoms per molecule, and then declines. This provides supportive evidence for the formation of antiferromagnetically-coupled clusters during early stages in iron loading; the failure to see a similar peak in an earlier study may be related to the nonphysiological pH that was used. Above 50 atoms per molecule, the relaxivity remains approximately constant, except that 1/T2 at high fields increases slightly, consistent with early core growth. The residual ionic relaxivity in this region is consistent with about three solitary Fe3+ ions remaining on the protein shell, indicating that spin cancellation is not complete. A similar value is obtained by extrapolating relaxation data at high loadings (up to 3000 Fe atoms per molecule), suggesting that these uncoupled spins persist on the protein shell even after an appreciable core has been built.