The molecular makeup of a biological sample describes its structure and often unveils functional aspects, such as normal versus diseased. By scanning a sample with mass spectrometry (MS) and combining that information, scientists can explore the distribution of biomarkers, proteins, and other molecules, along with their masses. Thus imaging MS is essentially molecular microscopy. Some of the biggest advances in this technology come from new applications, which range from basic science to healthcare.
With about 15 years of experience in the imaging field, both academic and industrial, Shannon Cornett, imaging business manager at Bruker, has a broad perspective on imaging MS. When asked about the most interesting recent advances, he points out two. One is coupling matrix-assisted laser desorption/ionization (MALDI) imaging MS with other imaging modalities, including PET and MRI. This started in academia and has carried over into industry. “Moving from an academic endeavor to industry bodes well for the potential that imaging MS brings to all other modalities,” Cornett says.
The other recent advance that Cornett cites is doing chemistry on sample sections to affect what compounds can be imaged. As an example, he points out the derivatization of neurotransmitters, which are not readily detected by imaging MS. “The tissue derivatization, though, makes these molecules more detectable,” he says.
Image: Molecular histology of rat brain. Image courtesy of Bruker.
In even the recent past, scientists critical of MALDI imaging often said “you get what you get with imaging MS”. “Now, we’re moving into the next phase where we’re able to control what compounds we can image,” Cornett explains. To do this, scientists need better ways to prepare samples, and vendors are providing them. “The field has grown to the point of supporting sample preparation as a separate subindustry,” Cornett states.
Turning up translation
To explore more applications of imaging MS, there is no one better to ask than Richard M. Caprioli, the Stanford Moore Chair in Biochemistry and director of the mass spectrometry research center at Vanderbilt University School of Medicine. He was the first to use MS as an imaging technology.
To Caprioli, the most exciting aspect of imaging MS is watching the translation of the technology into the clinic as a form of anatomical pathology. “Imaging MS allows pathologists to examine a biopsy and make a molecular decision—not a subjective one—on whether it is cancer or not, or whether it is grade 2 or 3,” he says. “It’s not based on the appearance of the sample after staining, but rather on the molecular makeup.”
For now, this approach is not approved for clinical use, but Caprioli says, “it’s more or less ready to go.” It’s taken years to develop imaging MS to the point where it can be used in a pathology lab, and now it’s close to being ready. “What needs to be done a little better is making the instrumentation attuned to the specific application,” Caprioli says.
“A research-grade instrument’s bells and whistles might not be necessary in a pathology lab.” In short, the technology needs to be a little easier to use, which can come from simply turning an imaging MS platform into a specialized tool for specific pathology needs. “Maybe it’s designed to where a pathologist just pushes a button here or there and doesn’t even need to look at the spectrum,” Caprioli envisions. “Instead, you have a computer search for a signature of, say, eight to twelve proteins, and if they are in the sample, then the disease is easily and accurately identified.”
The technology’s curve
The research use of imaging MS has improved with technological advances, as well. Robert Cody, product manager for mass spectrometry at JEOL USA, talks about how imaging MS started with just secondary ion mass spectrometry (SIMS) and the Laser Microprobe Mass Analyzer. As he says, “SIMS is still an extremely important MS imaging method, but it doesn’t always give easily interpretable information about the composition of organic molecules.” He says that MALDI is the “most well-known and most mature mass spec imaging technique.”
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To analyze a sample in a more natural state, scientists often choose some form of ambient ionization, such as desorption electrospray ionization (DESI). Cody calls this “the most well-established and commercially available ambient ionization imaging technique.”
Xin Yan—a postdoctoral student in the lab of Richard Zare, Marguerite Blake Wilbur Professor of Natural Science at Stanford University—and other Stanford scientists used DESI imaging MS, Zare says, “to examine mouse brain to understand the mechanism of aging, and they have found several molecules including lipids and amino acids from different structures in the brain that show differences as a function of age.” Zare adds, “Why this excites me is the wild dream that by administering some drug it might be possible to reverse these changes and thus reverse brain aging!”
Other combinations of technology also promise to push imaging MS ahead, and examples are easy to find. “Our progress in mass spectrometry imaging has resulted from collaborations with our customers and their innovations in combination with our technology,” Cody says.
“JEOL and Osaka University developed the SpiralTOF MALDI Imaging system with unique ion optics that provide a 17-meter ion flight path in a compact one-meter space.” He adds, “The long flight path makes it possible to obtain ultrahigh-resolution mass spectra, but it also has an important consequence for MALDI/time-of-flight mass spectrometry imaging.” The bumps in samples change the time of flight, which reduces the accuracy of the data, but using such a long flight path reduces the change in the flight time caused by small variations in the flatness of the sample.
Cody sees other exciting advances in the technology behind imaging MS. Another that he mentions is the development of laser ablation DART (direct analysis in real time) imaging (LADI) by Rabi Musah and her student Kristen Fowble at the University of Albany. “Fowble visited our lab and set up the LADI apparatus to obtain high-resolution images of hardwood from endangered species that were provided by Ed Espinoza from the U.S. Fish and Wildlife Forensic Lab,” Cody says. “Images of hardwoods, such as Brazilian rosewood and mahogany, are providing insights into wood anatomy that support the use of DART for identifying illegally traded timber and aid in understanding the chemical functions of wood structures.”
Image: Laser Ablation DART Imaging (LADI) shows the spatial distribution for a molecule (m/z 303.1232) in a Dalbergia normandii wood sample. (LADI image courtesy of Kristen Fowble and Rabi Musah (University of Albany), and D. normandii sample and optical image courtesy of Edgar Espinoza (US Fish and Wildlife Forensic Lab).)
So, from translational research to illegal timber, imaging MS provides basic and applied uses that can be performed even by non-experts. Moreover, the ease of using this technology is just beginning.