Abstract
Spatially Targeted Mass Spectrometry (MS) analysis using survey scans with an imaging modality often requires consecutive tissue slices, because of the tissue damage during survey scan or due to incompatible sample preparation requirements between the survey modality and MS. We report two spatially targeted MS analysis workflows based on polarized light imaging guidance that use the same tissue sample for survey and targeted analysis. The first workflow is applicable for thin-slice analysis, and uses transmission-polarimetry-guided Desorption ElectroSpray Ionization Mass Spectrometry (DESI-MS), and confirmatory H&E histopathology analysis on the same slice; this is validated using quantitative digital pathology methods. The second workflow explores a polarimetry-guided MS platform for thick tissue assessment by developing reflection-mode polarimetric imaging coupled with a hand-held Picosecond InfraRed Laser (PIRL) MS ablation probe that requires minimal tissue removal to produce detectable signal. Tissue differentiation within 5–10 s of sampling with the hand-held probe is shown using multivariate statistical methods of the MS profiles. Both workflows were tasked with differentiating necrotic cancer sites from viable cancers using a breast tumour model, and their performance was evaluated. The use of the same tissue surface addresses mismatches in guidance due to intrinsic changes in tissue morphology over consecutive sections.
Highlights
Ambient MS techniques such as Rapid Evaporative Ionization Mass Spectrometry (REIMS), Laser Ablation Electrospray Ionization (LAESI) and Desorption Electrospray Ionization (DESI) Mass Spectrometry have reached widespread utility in characterization of biological samples
For efficient guidance of Desorption ElectroSpray Ionization Mass Spectrometry (DESI-MS) imaging using thin tissue slices, it is desirable to use the same slice for polarimetry guidance, MS analysis and subsequent histopathology staining; the last step is necessary for ground-truth confirmatory imaging[3, 5, 6, 19,20,21,22]
It is important to note that other wide-field imaging techniques such as Raman spectroscopy[23], fluorescence imaging and optical coherence tomography (OCT) can be used as the survey imaging modality to guide the acquisition of targeted MS images
Summary
Received: 15 December 2016 Accepted: 14 February 2017 Published: xx xx xxxx and in situ Sampling of Biological Tissues. The first addresses the thin-slice-scenario limitations of the previous study[18], providing an optimized platform for ‘transmission mode’ polarimetry-guided MS imaging with pathology confirmation in a single tissue slice The utility of this optimized methodology is demonstrated by differentiating necrotic from viable tumour tissue in breast cancer xenograft models subjected to DESI-MS21. The second workflow moves beyond thin tissue sections and explores the use of ‘reflection mode’ polarimetry combined with a novel, hand held MS point sampling probe utilizing Picosecond InfraRed Laser (PIRL)[29] ablation for tissue profiling with MS through guided point sampling This workflow is used to examine the surface of thick tissue ex vivo, and extends the pioneering works of the Takats group in rapid tissue profiling with surgical aerosols produced with diathermy, lasers and ultrasonic aspiration methods[2, 12,13,14]. Total areas of each ROI and their spatial distributions were calculated as percent total area of the segmented image
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