Final answer:
The CNR in MRI is controlled by the magnetic field strength, with a practical example demonstrating the calculation of the field strength leading to an induced voltage in a pacemaker wire moving within an MRI unit's magnetic field.
Step-by-step explanation:
The CNR (contrast to noise ratio) in MRI is primarily controlled by the magnetic field strength of the MRI unit. This can be illustrated by an example where a patient with a pacemaker (which should be noted as a contraindication for MRI) experiences an induced Hall voltage due to the movement of the pacemaker wire within the strong magnetic field.
To calculate the magnetic field strength when a 20.0-mV Hall voltage is induced in a 10.0-cm-long section of wire moving at 10.0 cm/s perpendicular to the magnetic field, the following formula from physics is used: V = B×l×v, where V is the voltage, B is the magnetic field strength, l is the length of the wire, and v is the velocity. By rearranging the formula, we can calculate B as B = V / (l×v). Therefore, the magnetic field strength can be found to be 2.0 T (Tesla) given that V = 20.0 mV, l = 10.0 cm, and v = 10.0 cm/s.