Mass spectrometry imaging (MSI) is a powerful tool for detecting lipids in tissue sections, with matrix-assisted laser desorption/ionization (MALDI) and desorption electrospray ionization (DESI) as its key ionization techniques. In this study, we examine how MALDI compares with state-of-the-art DESI ionization in identifying lipids in heterogeneous samples, specifically atherosclerotic plaques. Carotid plaques (n = 4) from patients undergoing endarterectomy were snap-frozen, stored at −80°C, and then sectioned for MSI analysis and H&E staining. Measurements were conducted using a SYNAPT XS mass spectrometer in positive ion mode, employing MALDI with a 2,5-dihydroxybenzoic acid (DHB) matrix and DESI with a methanol: water (98:2) (v/v) solvent. Our comparison covered spectral profiles, sensitivity, and image quality generated by these two techniques. We found that both MALDI and DESI are highly suitable techniques for detecting a wide range of lipids in atherosclerotic plaque sections. DESI-MSI exhibited higher ion counts for most lipid classes than MALDI-MSI and provided sharper images. MALDI detected larger amounts of ceramide and hexosylceramide species, possibly due to its efficient generation of dehydrated ions. In contrast, DESI showed greater peak intensities of cholesteryl ester and triacylglyceride species than MALDI, consistent with reduced fragmentation. These findings establish the relative merits of DESI and MALDI and demonstrate their complementarity as techniques for lipid research in MSI.

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Multimodal intravascular ultrasound and photoacoustic (IVUS/PA) imaging is a promising diagnostic tool for cardiovascular diseases like atherosclerosis. IVUS/PA catheters typically require two independent transducers due to different frequency requirements, potentially increasing the catheter size. To facilitate multimodal imaging within conventional catheter dimensions, we designed, fabricated, and characterized a dual-transducer acoustic stack where a low-frequency (LF) PA receiver sits as a matching layer for the high-frequency (HF) US transducer. While the HF transducer operates around 50 MHz, the LF receiver targets frequencies below 15 MHz to capture most of the PA energy from atherosclerotic plaque lipids. Simulation results reveal that this configuration could increase the sensitivity of the HF transducer by 3.54 dB while maintaining bandwidth. Phantom experiments with fabricated stacks showed improved performance for the US transducer, validating the enhanced sensitivity and bandwidth. Following improvements in stack fabrication, the proposed acoustic stack is a viable design that can significantly enhance diagnostic accuracy for atherosclerosis, providing high-resolution, multifrequency imaging within a compact catheter form factor.@en

Photoacoustic imaging offers optical contrast images of human tissue at acoustic resolution, making it valuable for diverse clinical applications. However, quantifying tissue composition via optical contrast remains challenging due to the unknown light fluence within the tissue. Here, we propose a method that leverages known chromophores (e.g., arterial blood) to improve the accuracy of quantitative photoacoustic imaging. By using the optical properties of a known chromophore as a fluence marker and integrating it into the optical inversion process, we can estimate the unknown fluence within the tissue. Experimentally, we demonstrate that this approach successfully recovers both the spectral shape and magnitude of the optical absorption coefficient of an unknown chromophore. Additionally, we show that the fluence marker method enhances conventional optical inversion techniques, specifically (i) a straightforward iterative approach and (ii) a gradient-based method. Our results indicate an improvement in accuracy of up to 24.4% when comparing optical absorption recovery with and without the fluence marker. Finally, we present the method's performance and illustrate its applications in carotid plaque quantification.

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Spectral photoacoustic imaging in combination with unmixing techniques may be applied to retrieve information about high-risk features present in atherosclerotic plaques, possibly providing prognostic insights into future stroke events. We present the photoacoustic spectral contrast found in 12 systematically scanned advanced atherosclerotic plaques in the near-infrared wavelength range (850–1250 nm). The main absorbers are lipid, water, and hemoglobin, with the highest photoacoustic intensities at the lipid's second overtone at 1190 and 1210 nm. Linear unmixing resulted in visualizing regions with high lipid and hemoglobin absorption, corresponding to the histological presence of lipid and intraplaque hemorrhage. A non-negative matrix factorization approach reveals differences in lipid spectral contrast, providing potential insights into the vulnerability of atherosclerotic plaque. These results provide a reference for future, more complex, in vivo photoacoustic imaging of carotid artery atherosclerosis, potentially contributing to assessing the risk of future events and treatment decision.

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