Automatic Pre-scan

What is the scanner doing during the automatic pre-scan period?  
The final step before scanning officially begins is to perform a prescan. This is generally run as an automated process, but can be performed manually for improved performance. During prescan, several calibrations are performed to insure optimal performance of the specified protocol. The typical steps include: 
  1. Quick shimming. Small adjustments are made to currents passing through gradients and higher order shim coils to optimize Bo field homogeneity in the presence of the patient. A more advanced manual process may be necessary for spectroscopy and chemical shift selective suppression applications.
  2. Coil tuning and matching. Human bodies differ in size and shape and thus create different electromagnetic "loads" on the RF-coils. As such, the resonance frequency of the patient-coil system must be adjusted as well as the coil impedance (complex resistance) for effective energy transfer. 
  3. Center frequency adjustment. Human tissues contain both water and fat in various proportions whose resonance peaks differ by a few hundred hertz (Hz). The scanner must lock on to the correct spectral peak for proper localization. 
  4. Transmitter attenuation/gain adjustment. The attenuator or amplifier gain of the RF pulse must be calibrated for each patient so that proper flip angle can be obtained.
  5. Receive attenuation/gain adjustment. The MR signal intensity must appropriately scaled so it is neither too big nor too small for the amplifier chain.
  6. Dummy cycles. These are multiple runs of a pulse sequence prior to acquiring data to allow a steady state magnetization to develop.
These steps will be described in more detail in subsequent Q&A's.

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References
     Elster AD, Burdette JH. Questions and Answers in MRI, 2nd ed. St. Louis: Mosby, 2001, pp 1-3.
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