The present invention provides a phantom (200) for use in a magnetic resonance (MR) imaging system (110) with a set of resonating volumes (206) positioned in a base body (202), whereby the base body (202) has a spherical or ellipsoid shape in accordance with a volume of interest (203) of the MR imaging system (110), and the resonating volumes (206) are located at a circumference of the base body (202). The phantom is used in a method for evaluating the magnetic field of a main magnet (114) of a magnetic resonance (MR) imaging system (110), comprising the steps of positioning the phantom (200) within the main magnet (114), performing a 3D spectroscopic MR measurement of the phantom (200) using the MR imaging system (110), thereby measuring resonances of the resonating volumes (206), assigning the measured resonances to the resonating volumes (206), and evaluating the magnetic field of the main magnet (114) from the MR measurement of the phantom (200) based on the measured resonances of the resonating volumes (206). Accordingly, the MR imaging system itself is directly used for determining the magnetic field of its main magnet. Accordingly, the MR imaging system itself can be used as measurement equipment, instead of a separate NMR magnetometer, which is required for conventional determination of the magnetic field.