RF Probe and Noise Effects

• A lot of the time in clinical MRI the body is much bigger than the region that you are interested in imaging. Limit the FOV to target the region of imaging interest.
  
  
  
  
  
  
  
• If the rf probe is not sensitive to spins over a large volume, then you shouldn't specify a large FOV, because there is no information obtained where $B_{1}$ is zero.
• Similarly, if you don't excite a large region of the whole body, then you shouldn't specify a large FOV, because there is no information obtained where $M_{\perp}$ is zero
  
  
  
  
  
  
  
• 
• What about noise? Can think of signal as image signal and noise signal.
  

  $$s(k) + \epsilon(k) \stackrel{\raisebox{0.5ex}{\footnotesi... ...le{\cal D}$}}{\Leftrightarrow} \hat{\rho}(x) + \hat{\eta}(x)$$ (12.1)

  
  

• The image signal, $s(k)$, should lead to a band limited signal in $x$-space that is repeated due to sampling.
• However, the noise signal, $\epsilon(k)$, is not band limited in either domain. Assuming white noise gaussian distributed about zero and same at all frequencies.
• Detector is going to have some finite bandwidth, so we don't measure an infinite amount of noise and drown out signal, but will alias all noise at frequencies higher than nyquist frequency back into the image.
• Should carefully band limit detection to prevent aliasing of high frequency noise back into the image and lowering SNR.
  
  
  
  
  
  
  
  
  
  
• Look in book at example 3D imaging examples where excited region can be mistaken, if take slice thickness too literally.