Original contributionImaging of stroke: a comparison between X-ray fluorescence and magnetic resonance imaging methods
Introduction
While iron is known to play an important role in hemorrhagic stroke, the role of other elements has been largely neglected. The elemental changes associated with chronic ischemic lesions are largely unknown, but when they form magnetic minerals (paramagnetic ferritin or hemosiderin or diamagnetic calcifications), they may contribute to signal changes in MR images. Synchrotron rapid scanning X-ray fluorescence (SRS-XRF) mapping simultaneously localizes and accurately quantifies multiple metals in tissue slices or small whole organisms [1], [2], [3], [4]. This well-established, element-specific analytical technique has been applied to map slices of human brain, using newer high-resolution SRS-XRF [5], [6], [7], [8]. Recently, some of these results have been compared with magnetic resonance imaging (MRI) scanning [9], [10], [11].
SRS-XRF imaging has not previously been applied to study human stroke or animal models of stroke [12]. Much of our fundamental understanding of the pathophysiology of ischemic and hemorrhagic stroke has arisen from work on a variety of animal models using behavioral, cell biology and molecular biology techniques [13]. None of the animal models fully replicates human stroke and studies in animals tend to be short-term. In the cases shown here, the strokes were not immediately fatal and occurred years or weeks before death. One of the aims of this work is to determine the feasibility of comparing the measurement of iron content obtained from SRS-XRF with that obtained from MRI using quantitative susceptibility mapping and T2* mapping.
Iron has clinical importance [14], [15], not only because it is required for normal brain function [7] but also because the chemical reactivity of free iron likely contributes to brain damage by leading to the formation of free radicals [14]. It also serves as an important endogenous marker of blood products in T2* weighted gradient echo (GRE) MR that is commonly used to image stroke in vivo [16], [17]. Susceptibility weighted imaging (SWI) based on GRE is the most sensitive MR method both for non-heme iron [14], [18], [19], [20] and heme iron [21], [22], [23], [24], [25]. Hemorrhage, whether primary or secondary to ischemia undergoes a transformation from deoxy-hemoglobin to methemoglobin and later hemosiderin [26]. Each of these sources of iron has different T1 and T2* properties [27]. SWI is sensitive to iron in all three forms [21]. Thus, in suspected acute stroke, SWI serves as a key sequence in detecting hemorrhage within the region of infarction. However, SWI data are known to have a blooming effect that magnifies the hemorrhagic lesions [in GRE with long echo time (TE)], and the phase image is dependent on field strength, echo time, the object's relative orientation to the main field and its geometric shape [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. Since susceptibility maps reconstructed from SWI phase images and T2* maps reconstructed from multi-echo SWI magnitude images are free from the above concerns, they are the most promising way to depict and quantify iron in hemorrhagic lesions [32], [34], [35], [36], [37], [39], [40].
In this study, we aim to determine whether SWI, susceptibility maps or T2* correlate best with elemental mapping of iron (Fe) and calcium (Ca) using SRS-XRF. Based on the known effects outlined above, we hypothesize that susceptibility maps will provide the best spatial correlation of both Fe and Ca found in hemorrhagic and chronic stroke lesions.
Section snippets
Study samples and analysis procedures
Frozen coronal sections of human cadaveric brains were obtained from the Human Brain and Spinal Fluid Resource Center (HSB), Los Angeles, CA, under the University of Saskatchewan ethics approval BioREB 06–250. The known clinical features of the cases are summarized in Table 1. To reduce storage artifacts such as leaching of metals, the frozen slices were fixed in buffered formalin for 6 hours, drained and sealed in plastic immediately prior to initial synchrotron imaging of the surface of the
XRF findings
Visual inspection of the thin (1–3 mm thick) brain slices showed regions of discoloration and histology revealed extensive tissue damage in these areas (Fig. 1). In the XRF maps, the apparent increase in elements around the edge is likely due to the summation of counts arising from both the surface and the edge of these thick slices (i.e., an edge effect) rather than from the deposition of elements from the formalin solution [41]. ROIs of hemorrhage, white matter (WM), gray matter (GM) and
Discussion
In conventional stroke imaging, signal loss in T2* weighted gradient echo images is interpreted as caused by the dephasing effect from spatially varying local magnetic fields due to iron deposits. However, tissue damage or changes in tissue susceptibility can also cause signal loss. Phase images are sensitive to the geometric shape of a hemorrhagic lesion and its orientation to the main field. Phase images were not included in the analysis because they did not have as good a correlation to the
Conclusion
In this study of cadaver brains, SRS-XRF mapping of iron, zinc and calcium revealed their elemental distributions in stroke. These distributions were then compared to the MR data in order to reveal how well either T2* mapping or susceptibility mapping could identify said regions. We found that hemorrhagic regions had higher Fe, Zn and Ca but that these elements did not always co-localize. We also found that regions of ischemic damage in white matter had lower levels of Fe, Zn and Ca than
Acknowledgments
This work is supported by the Canadian Institutes of Health Research (CIHR)/Heart and Stroke Foundation of Canada (HSFC) Synchrotron Medical Imaging Team Grant #CIF 99472.
Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the US Department of Energy Office of Science by Stanford University. The SSRL Structural Molecular Biology Program is supported
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