Altered Nigral Amide Proton Transfer Imaging Signal Concordant With Motor Asymmetry in Parkinson's Disease: A Multipool CEST MRI Study

Parkinson's disease (PD) is a neurodegenerative disorder primarily affecting motor functions, characterized by asymmetrical motor symptoms. This asymmetry is a hallmark of PD, with patients often presenting unilateral motor symptoms that progress over time [1, 2]. The underlying mechanisms driving this asymmetry, particularly at the neuronal level, remain a key area of research. A study published in this issue explored the use of chemical exchange saturation transfer (CEST) MRI imaging to investigate asymmetrical changes in brain regions associated with PD, providing new insights into the molecular changes accompanying this disease. The study specifically focused on the substantia nigra (SN) and red nucleus (RN), key regions involved in motor function, and assessed the potential of CEST parameters as noninvasive biomarkers for PD. It received high scores from the reviewers, and it has been selected as an Editor's Pick. This editorial will review its significance to the field. CEST is an advanced molecular MRI technique that enhances the detection of solute molecules with exchangeable protons [3-5]. This method involves applying a saturation pulse to the solute protons for a duration ranging from a few milliseconds to several seconds, followed by measuring the resultant decrease in the water signal due to a cumulative saturation transfer effect. A notable variation of CEST imaging is amide proton transfer (APT), which targets amide protons of mobile proteins and peptides, resonating at 3.5 ppm from water. Previously, APT-weighted imaging, utilizing an asymmetry analysis method, has been applied to PD [6-8]. However, this technique is affected by various confounding factors [9]. This study demonstrated the significant potential of CEST imaging to highlight asymmetries in the SN and RN in PD patients with varying levels of motor asymmetry. Instead of using the conventional asymmetry analysis method, the study employed a multiple-pool fit technique to resolve each CEST effect. This approach not only provides a more specific quantification of the APT effect but also captures several other effects, including the relayed nuclear Overhauser enhancement (rNOE), direct water saturation, and magnetization transfer effects [10]. A significant decrease in the APT effect, but not other effects, was observed on the more affected side compared to the less affected side. This reduction likely indicates the neurodegenerative processes occurring in the more affected hemisphere of the brain. Furthermore, the study identified a positive correlation between the APT signal asymmetry index and the motor asymmetry index in the SN for the group of PD patients with severe motor asymmetry. This relationship suggests that APT signals can effectively reflect PD progression, offering a noninvasive method to assess the lateralization of motor symptoms. One potential type of future research inspired by this study could be the refining of imaging techniques to more fully characterize PD-related changes. One opportunity is exploration of other proton pools (with different exchange rates and frequencies) in the water saturation spectrum, which can be done by using different saturation strengths for resolving each effect more accurately. Larger and multicenter studies will be needed to confirm and extend these results, so as to validate CEST imaging as a clinical tool for monitoring PD. While the exact mechanisms driving APT signal changes are still not fully understood, future research of patient groups with specific biochemical changes, such as PD patients, can add to further elucidating underlying contributions. This Editor's Pick highlights the potential of CEST imaging, particularly APT, as a valuable tool for investigating the asymmetrical molecular changes in PD. By providing a noninvasive means to assess neurodegeneration and symptom lateralization, CEST imaging may significantly enhance our understanding of PD and contribute to the development of targeted treatment strategies. Data sharing is not applicable to this article as no new data were created or analyzed in this study.