  A key challenge in cardiology is the non-invasive imaging of electric current
propagation occurring in the cardiovascular system at an intra-cardiac scale. A
promising approach for directly mapping the current dynamics is to monitor the
associated stray magnetic field. However, in this magnetic field approach, the
spatial resolution deteriorates significantly as the standoff distance between
the target and the sensor increases. Existing sensors usually remain relatively
far from the target and provide only centimetre-scale resolution because their
operating temperature is not biocompatible. Here we demonstrate
millimetre-scale magnetocardiography of living rats using a solid-state quantum
sensor based on nitrogen-vacancy centres in diamond. The essence of the method
is a millimetre proximity from the sensor to heart surface, which enhances the
cardiac magnetic field to greater than nanoteslas and allows the mapping of
these signals with intra-cardiac resolution. From the acquired magnetic images,
we also estimate the source electric current vector, flowing from the right
atria base via the Purkinje fibre bundle to the left ventricular apex. Our
results establish the solid-state quantum sensor's capability to probe cardiac
magnetic signals from mammalian animals and reveal their intra-cardiac
electrodynamics. This technique will enable the study of the origin and
progression of myriad cardiac arrhythmias including flutter, fibrillation, and
tachycardia.
