Susceptometers for foreign body detection
View Patent ↗Methods and apparatus for minimizing the effects of temperature drift in a magnetic susceptibility measurement instrument, such as an instrument used in pre-MRI screening for the presence of ferromagnetic foreign bodies. The magnetic field source and magnetic sensors can be combined into a single, rigid unit. The stability and sensitivity required in high quality magnetic susceptibility measurements can be achieved through symmetrical design of the source-sensor unit, minimization of thermal stresses, minimization of temperature variations, use of materials with low thermal expansion coefficients, or through appropriate combinations thereof. Use of patient eye movement where an eye is being screened, use of a water bag between the patient and the instrument, or use of telemedicine to facilitate performance of the necessary computations can also be incorporated.
1. A method for performing pre-MRI screening for ferromagnetic foreign bodies, comprising:
providing a source-sensor unit including at least one applied-field generating element and at least one magnetic sensor;
canceling the signal due to the applied magnetic field at the location of said at least one magnetic sensor by the relative arrangement of said at least one applied-field generating element and said at least one magnetic sensor in said source-sensor unit;
generating an applied magnetic field in the eye of a patient, with said source-sensor unit;
rotating said eye, thereby moving any ferromagnetic foreign body which may be present to modulate the magnetic susceptibility signal from said ferromagnetic foreign body, without substantially changing the magnetic susceptibility response of the patient's body tissues; and
generating a magnetic susceptibility measurement relevant to the presence or absence of a ferromagnetic foreign body in said eye, with said source-sensor unit.
2. The method recited in claim 1 , further comprising rotating said eye in a controlled manner.
3. The method recited in claim 1 , further comprising preserving said cancellation of the applied-field signal as said entire source-sensor unit expands and contracts.