Evaluation of coils for imaging histological slides: Signal-to-noise ratio and filling factor

Autor: Youssef Zaim Wadghiri, Chao Zhang, Latifa Fakri-Bouchet, Evelyn B. Voura, Dung Minh Hoang
Rok vydání: 2013
Předmět:
Zdroj: Magnetic Resonance in Medicine. 71:1932-1943
ISSN: 0740-3194
Popis: In biomedical research, the use of animal models of human disease offers the opportunity to test and optimize magnetic resonance imaging (MRI) pulse sequences, and to compare image features with corresponding histopathology, the current gold standard for disease assessment. Despite these great advantages, the coregistration steps between MRI and histological sections obtained through light microscopy have presented important practical challenges (1–5). The main difficulties encountered during the validation steps stem from both the slice section misalignments and the significant difference in slice thicknesses between sub-millimetric in vivo MRI (ranging from 100 μm to 1 mm thickness) and histology sections (thicknesses commonly ranging from 5 to 100 μm). These limitations have been partially addressed by acquiring three-dimensional (3D) ex vivo images of excised and perfused organs of interest. The scanning is usually assessed over long imaging times using a dedicated coil closely fitting the sample to improve the sensitivity (6–20). The accumulation of repeated scans results in motion-free and highly resolved 3D MRI datasets enabling a precise virtual realignment of the image slice of interest to closely match the physical histology section (21–26). However, important discrepancies remain between these two analyses (22). These notable differences can be attributed to sample changes associated with postmortem tissue processing due to fixation and dehydration resulting in tissue deformation, as well as artifacts caused by sectioning and the chemicals used during the staining process (27–33). These discrepancies have been addressed using time-consuming postprocessing techniques when establishing 3D atlases (21–24). But this approach may be more difficult to systematically implement for every existing transgenic mouse line or murine disease model due to limited resources or lack of image postprocessing expertise. Alternatively, histological slides can conceivably be scanned directly by both MRI and light microscopy (34–36). Coregistration would then be easily achievable provided that MRI sensitivity and the experimental conditions allow direct imaging with adequate spatial resolution of histological sections ranging from 5 to 100 μm. Along these lines, efforts have been devoted to the design of dedicated radiofrequency (RF) coil geometries that closely matches the shape of the tissue of interest. Nabuurs and coworkers (36) proposed the use of an inductively coupled loop combined with a commercial linear birdcage mouse head coil capable of housing the slide setup. The insertion of the coupled loop was aimed at driving the effective RF coverage toward the tissue of interest thereby reciprocally increasing the resulting sensitivity. This solution led to a less than fourfold gain in sensitivity under the best geometrical inductive coupling conditions compared with results obtained using the linear birdcage mouse head coil alone. Alternatively, Meadowcroft et al. (35) proposed a unique RF coil design based on a flat U-shaped structure that closely matches the planar nature of a histological tissue sample providing the most optimal filling factor. The authors demonstrated excellent RF homogeneity throughout the region of interest (24 × 24 mm) within the coil cavity designed to house a sample encased on dual glass coverslips. The length (L) of the coil was doubled (L = 48 mm) by inserting an equal-sized Teflon spacer between the two flat copper strips next to the U-shaped cavity leading to the driving port. The presence of this spacer both provided some distributed capacitance and allowed for the current to be spread evenly throughout the strip before reaching the sample cavity from the connections to the tuning capacitor. Although the sensitivity of this new probe design was not compared with other commonly available RF coil structures, the authors demonstrated their ability to acquire highly detailed MRI from 60 μm sections of freshly mounted tissue sandwiched between two coverslips. This setup enabled the accurate correlation between MRI and light microscopy by direct imaging of postmortem tissue samples from either human Alzheimer’s Disease (AD) subjects or mice of a strain exhibiting some characteristics of AD (34). Here, we build on the work of Meadowcroft et al. to accommodate both wider and thicker tissue slides by devising an in-house set of five histology MRI probes. All newly designed RF histology resonators were systemically characterized by mapping their RF homogeneity and examining how each performed against commercially available surface and whole mouse head coils. In addition, we examined the effect of the spacer on the resulting coil sensitivity by replacing the Teflon with either air or glass as alternative insulators. We intended to image both freshly mounted tissue and premounted specimens originating from either paraffin embedding or cryosections having thicknesses ranging from 5 to 1000 μm. To this effect, our study investigated the best conditions for sample preparation for either case in order to minimize MRI artifacts. We demonstrated that our new five-coil set can accommodate off-the-shelf standardized histology preparations ranging from dual coverslips to glass slides. MRI examples of tissue derived from whole mouse organs and human specimens illustrate both the performance of each coil, as well as the near perfect coregistration with histology.
Databáze: OpenAIRE
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