Methods of detecting an aldehyde-containing compound in a subject or in a sample from a subject are described herein, comprising administering an aldehyde-binding compound of Formula I to the subject, or combining such a compound with the sample; and detecting the product of the compound of Formula I and the aldehyde-containing compound. Detection of the product may involve imaging, such as MRI, CEST-MRI or positron emission tomography (PET) imaging; or may involve fluorescence or an electrochemical detection method. Biologically relevant aldehydes detected according to the described method can be used to monitor conditions such as brain injury, neurodegenerative disorders such as Alzheimer's disease, diabetes, heart disease, and cancer.