Faraday's Law of Induction and MR

• All of the spinning protons lead to a rapidly changing magnetic field. Can detect rapidly changing magnetic fields ...

  
  
  
  
  
  
  
  

• The MR signal is detected by induction. Faraday's Law of Induction says that a changing magnetic field through a closed loop of wire induces an emf in the wire
  $$\begin{eqnarray*} emf = - \frac{d \Phi}{d t} \end{eqnarray*}$$
  
  
  
  
  

  You remember Faraday's Law of Induction from your freshman physics class, ..., please nod and say yes

• the derivative describes how the magnetic field is changing in time, and the flux,
  $$\begin{eqnarray*} \Phi = \int_{\mbox{receive coil area}} \vec{B}_{transmit} \cdot d\vec{S}_{receive} \end{eqnarray*}$$
  
  
  
  
  
  describes the magnitude or amount of magnetic field passing through the loop

• in MR the coherent groups of spins, or isochromats act like small bar magnets, and their precession gives the time dependence which generates emf in coils (in the lab frame) which we can measure

• (IN-LINE QUESTION) How should a single loop detection coil be positioned relative to a body and magnetic field?