Mass spectrometry (MS) is a sensitive analytic technique that can provide a chemical fingerprint of a biological tissue on the basis of mass-to-charge (m/z) ratios of its molecular constituents. Mass Spectrometry Imaging (MSI) combines the multiplexed (m/z) measurement capability of MS with a surface sampling process to deliver a spatially resolved chemical content (or molecular) map of the target tissue. MSI provides a molecular image of the distribution of m/z values characteristic to the disease state from which an image of the disease site that includes the boundary and infiltrating regions can be created. A mass spectrometry imaging approach is particularly suited to the investigation of molecular signatures of intratumoural heterogeneity such as necrotic and hypoxic areas as well as stroma. Housed and fully integrated within the multi-modality pre-clinical imaging facility STTARR (http://www.sttarr.com/splash.html) we are uniquely positioned to perform MSI methods development as well as preclinical validation in house.
Contrast Agent Mass Spectrometry Imaging Reveals Tumour Heterogeneity
The clinical translation of MSI into a universal cancer imaging technique requires access to MS profiles characteristic to tumours and healthy tissues. This requirement has motivated many ex vivo studies to identify and validate disease specific MS profiles with utility in intraoperative MSI. We have extended the utility of intraoperative MSI imaging to tumour site mapping without the need for disease specific MS profiles through developing Contrast Agent Mass Spectrometry Imaging (CA-MSI) which uses tumour subregion-specific formation of chemical adducts of magnetic resonance imaging agent Gadoteridol to reveal tumour margins, the location of tumour blood vessels and necrotic areas without requiring validated cancer MS profiles. Human breast cancer tumours grown in mice were subjected to CA-MSI using Gadoteridol, revealing tumour margins and vasculature from the localization of [Gadoteridol+K]+ and [Gadoteridol+Na]+ adducts, respectively. As such, the technique is applicable to all tumours passively targeted by imaging agents. This broad method is applicable to tumours for which currently no established MS profiles exist to guide conventional imaging with mass spectrometry. Therefore, our discovery extends the utility of MSI to a wider variety of tumour cases and disease type compared to what was previously possible and overcomes the requirement for validated cancer markers – the bottleneck that prevented MSI from becoming applicable to a wide cancer base.
Picosecond InfraRed Laser Ablation Mass Spectrometry for scar-free surgery with molecular level guidance
We are interested in the application of cold laser ablation technology as an analytic tool and a new desorption source for MSI with the added benefit of minimal damage to the tissue being sampled, which is particularly attractive for in vivo MSI to preserve healthy tissues. Picosecond InfraRed Laser (PIRL) is capable of cutting through biological tissues in the absence of significant thermal damage. As such, PIRL is a standalone surgical scalpel with the added bonus of minimal post-operative scar tissue formation. Unlike competing cauterizing lasers, a cold ablation source is able to ablate biological material without significant thermal or mechanical damage to the tissues that surround the ablation site. This laser scalpel releases the molecule constituents of the tissue to gas phase in the form of a plume of laser-ablated material. This plume contains molecular fingerprints that can report on the type of tissue being sampled, and could be further captured and analyzed by mass spectrometry (ultimately in real time) to provide surgical guidance. Further engineering work will lead to full integration of cold ablation laser with mass spectrometry for real-time intraoperative surgical guidance on the basis of Simultaneous Mapping of Ablated Residues from Tissues (SMART Laser Scalpel). We have validated PIRL as an ion source for mass spectrometry. A tandem of PIRL Ablation with ElectroSpray Ionization (PIR-LAESI) mass spectrometry is demonstrated and characterized for tissue molecular imaging. PIR-LAESI offers a 20-30 μm vertical resolution (~3 μm removal per pulse) and a lateral resolution of ~100 μm. PIR-LAESI was used to map the distribution of endogenous and exogenous molecules in biological tissues, producing localization maps that could be corroborated by the literature, and other MSI techniques. We further showed that PIR-LAESI was capable of desorption ionization of proteins as well as phospholipids. This comparative study illustrates that PIR-LAESI is an ion source for ambient mass spectrometry applications. As such, a future PIRL scalpel combined with secondary ionization and mass spectrometry has the potential to provide molecular feedback to guide PIRL surgery.