A method of operating an MRI apparatus, wherein the MRI apparatus comprises - a superconducting magnet, - a ramp control module configured to initiate a ramping procedure at a suitable time to ramp the magnet to a target frequency fT, and configured to feed, during a ramp-up procedure, current to the magnet until the magnet current or the corresponding magnet centre frequency has reached a predefined target current or frequency, - an ammeter shunt arranged to measure the magnet current during a ramping procedure, so that the magnet frequency can be estimated and - a shunt monitoring module configured to derive an aging characteristic of the ammeter shunt on the basis of patient scan centre frequencies, which method comprises the steps of - identifying and recording centre frequencies (f c1,..., f cn) during successive patient scans, wherein the centre frequencies are determined during the imaging procedure of a patient scan from signals received by a body coil or local coils of the MRI apparatus, and - analysing the centre frequencies (f c1,..., f cn) to identify a number of decay-related characteristics of the main magnetic field of the MRI apparatus, - performing a ramp-up procedure, wherein during ramp-up, current is fed to the magnet until the magnet current or the corresponding frequency has reached a predefined target current or frequency, characterized by the method further comprising the steps of recording a measurement of the ammeter shunt current at the end of the ramp-up procedure, - determining an aging characteristic of the ammeter shunt from the recorded magnet current measurement (I 10) and a centre frequency (f c) identified in a patient scan following the ramp-up procedure, - correcting the ammeter shunt current readings during a subsequent ramp-up procedure using the aging characteristic. - a superconducting magnet, - a ramp control module configured to initiate a ramping procedure at a suitable time to ramp the magnet to a target frequency fT, and configured to feed, during a ramp-up procedure, current to the magnet until the magnet current or the corresponding magnet centre frequency has reached a predefined target current or frequency, - an ammeter shunt arranged to measure the magnet current during a ramping procedure, so that the magnet frequency can be estimated and - a shunt monitoring module configured to derive an aging characteristic of the ammeter shunt on the basis of patient scan centre frequencies, which method comprises the steps of - identifying and recording centre frequencies (f c1,..., f cn) during successive patient scans, wherein the centre frequencies are determined during the imaging procedure of a patient scan from signals received by a body coil or local coils of the MRI apparatus, and - analysing the centre frequencies (f c1,..., f cn) to identify a number of decay-related characteristics of the main magnetic field of the MRI apparatus, - performing a ramp-up procedure, wherein during ramp-up, current is fed to the magnet until the magnet current or the corresponding frequency has reached a predefined target current or frequency, - identifying and recording centre frequencies (f c1,..., f cn) during successive patient scans, wherein the centre frequencies are determined during the imaging procedure of a patient scan from signals received by a body coil or local coils of the MRI apparatus, and - analysing the centre frequencies (f c1,..., f cn) to identify a number of decay-related characteristics of the main magnetic field of the MRI apparatus, - performing a ramp-up procedure, wherein during ramp-up, current is fed to the magnet until the magnet current or the corresponding frequency has reached a predefined target current or frequency, characterized by the method further comprising the steps of recording a measurement of the ammeter shunt current at the end of the ramp-up procedure, - determining an aging characteristic of the ammeter shunt from the recorded magnet current measurement (I 10) and a centre frequency (f c) identified in a patient scan following the ramp-up procedure, - correcting the ammeter shunt current readings during a subsequent ramp-up procedure using the aging characteristic. - determining an aging characteristic of the ammeter shunt from the recorded magnet current measurement (I 10) and a centre frequency (f c) identified in a patient scan following the ramp-up procedure, - correcting the ammeter shunt current readings during a subsequent ramp-up procedure using the aging characteristic.