Mass Spectrometry Imaging Significance Section:
Mass spectrometry imaging (MSI) is powerful that enables untargeted analysis of the spatial distribution of a variety of molecular species. It has the capability to image thousands of molecules in a single experiment without labeling. Unlike other techniques used understandingin-tissue distribution including radio and visualizationfluorescent labeling, MSI is of spatial distributionstechnique that can be used in thinsimultaneous imagingsamplesections multiple analytes makes this a asingle valuablesample. chemicalAdditionally, analysisfluorescent tooland radio labeling of compounds can significantly change penetration measurements, may require extensive synthetic and analytical steps, and labeled molecules useful forindistinguishable from their corresponding biological metabolites and/or degradation products. MSI was first introduced into biological sciences at the end of the 20th century, and has rapidly expanded today integrating itself into many clinical and pharmaceutical applications. Matrix assisted laser desoprtion and ionization (MALDI) is a common mass spectrometry ionization mechanism biological for imaging studies. MALDI uses a laser beam characterization.A summaryirradiate a matrix-coated tissue section mounted on a mobile x-y stage, collecting an array of mass spectra at a specified interval, which are then compiled to form an image of the workflow of the analyte. When the matrix is applied to the tissue, biological molecules co-crystallize into the matrix. which allows them to be ionized is depicteddetected inby Figurethe mass spectrometer. Following spectra collection, software is used to select an individual mass-to-charge (m/z) value, and the intensity 1. After each minimalpixel butis then combined into a heatmap image depicting the relative distribution of that m/z value throughout the sample’s surface. In order careful samplepreparation, thedetermine generalthe setupidentity of a specific m/z value, tandem MS (MS/MS) fragmentation can be performed on ions from each pixel, thefragments can usedtopiecetogetherthe structure of anthe MSIunknown experimentmolecule. involvesOtherwise, definingthe molecule can be identified based on its intact mass by accurate mass matching to databases of known molecules within a certain mass error range. MSI an also expanding into multi-modal imaging systems. Because MSI has high chemical specificity, while other imaging modalities typically have have high spatial resolution, complementary modalities are combined (x, answernewbiologicalquestions.The ability of MSI y)grid overimage themultiple surfacemolecules in a single analysis run makes it indispensable of the sample,pharmacuetical withand theclinical gridapplications. areaHere, chosenwe bydescribe thenovel multi-modal image pairings that allow us user. gainstructural chemicalinsightintopancreatic cancer biological questions that is otherwise unable Themassbe spectrometerinvestigated thenwith ionizesa thesingle moleculesmodality onsystem. theThe surfaceresearch goal of thethese sample studiesis collects combine the chemical specificity a massversatility spectrumof atmass eachspectrometry pixelimaging on with section,pathological withimaging thetools resultingto understandcancer metastasis and chemotherapy response in pancreatic cancer.spatial resolution defined by the pixel size. After collecting the spectra, computational software can be used to select an individual mass-to-charge (m/z) value, and the intensity of the m/z is extracted from each pixel’s spectrum. These intensities are then combined into a heatmap image depicting the relative distribution of that m/z value throughout the sample’s surface. In order to determine the identity of a specific m/z value, tandem MS (MS/MS) fragmentation can be performed on ions from each pixel, and the fragments can be used to piece together the structure of the unknown molecule. Otherwise, the molecule can be identified based on its intact mass by accurate mass matching to databases of known molecules within a certain mass error range.2, 3
With the numerous technological advances in recent years, MSI is becoming a more routine tool in clinical practice and the pharmaceutical industry.4-6 Advances include improvements in reproducible sample preparation to ensure reliable interpretation of data and instrumentation that allows for high acquisition speeds and lower spatial resolution, improving throughput and depth of instrumentation. The credibility of MSI experiments has further been enhanced by the development of methods for absolute quantitation of detected molecules. To help with large computational endeavors, statistical workflows and machine learning algorithms have been implemented to handle the large imaging datasets being produced with modern day instrumentation. MSI can also be combined with other complementary imaging modalities, such as microscopy, Raman spectroscopy, and MRI, to strengthen any biological conclusions. With both hardware and software improvements, 3-dimensional (3D) renderings and even single-cell resolution using MSI are emerging as future frontiers. With all the advances in this field, MSI is rapidly evolving and requires continuous development to match the current demand.
Overall, the aim of this review is to provide an informative resource for those in the MSI community who are interested in improving MSI data quality and analysis or using MSI for novel applications. Particularly, we discuss advances for the last 2 years in sample preparation, instrumentation, quantitation, statistics, and multi-modal imaging that have allowed MSI to emerge as a powerful technique in the clinic. Also, several, novel biological applications are highlighted that demonstrate the potential for the future of the field.