A device performs MR imaging of a body (7) placed in a stationary and substantially homogeneous main magnetic field, with an RF transmit antenna (6) for radiating RF pulses towards the body (7), which RF transmit antenna (6) has different resonance modes. In order to improve image uniformity in high field MR imaging, the device (1) is arranged to determine the size and/or the aspect ratio of the body (7), and to acquire an MR image of the body (7) by an imaging sequence including RF pulses. The phases and amplitudes of the different resonance modes of the RF transmit antenna (6) excited during irradiation of the RF pulses are controlled on the basis of the size and/or aspect ratio of the body (7).