Method and apparatus for detecting superparamagnetic material

An apparatus for detecting superparamagnetic material, comprising:

• an excitation coil arranged to apply a magnetic field comprising a first and a second component to an object;
• a sensing device (3) comprising a first detection sub-coil (4a) and a second detection sub-coil (4b);
• and
• a controller arranged to:
• a) apply, by the excitation coil, a magnetic field during a first period to an object to modulate a magnetization of the superparamagnetic material, the magnetic field comprising a first component with a first frequency;
• a-2) position the sensing (3) at a first position from the excitation coil,
• b) receive, a first signal by the first detection sub-coil (4a) in the sensing device (3) and a second signal by the second detection sub-coil (4b) in the sensing device (3);
• c) determine a sensor signal from the first signal and the second signal;
• d) determine a detection signal based on the sensor signal;
• and
• e) determine a parameter indicating an amount of superparamagnetic material by dividing the detection signal by the first signal, and
• f) repeat steps (a-2) to (e) at at least one different position in order to determine a location where the parameter has a maximal value.