Mapping intratumoural heterogeneity such as vasculature and margins are important during intraoperative applications. Desorption Electrospray Ionization Mass Spectrometry (DESI-MS) has demonstrated potential for intraoperative tumour imaging using validated MS profiles. The clinical translation of DESI-MS into a universal label-free imaging technique thus requires access to MS profiles characteristic to tumours and healthy tissues. Here, we developed Contrast Agent Mass Spectrometry Imaging (CA-MSI) that utilizes a Magnetic Resonance Imaging (MRI) contrast agent targeted to disease sites, as a label, to reveal tumour heterogeneity in the absence of known 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. The localization of the [Gadoteridol+K]+ adduct as revealed through DESI-MS complements the in vivo MRI results. DESI-MS imaging is therefore possible for tumours for which no characteristic MS profiles are established. Further DESI-MS imaging of the flux of the contrast agent through mouse kidneys was performed indicating secretion of the intact label.