Center Frequency
Please explain how and why the center frequency must be adjusted.
|
|
The next step of routine prescan involves setting the center frequency. This is a relatively fine adjustment that tells the scanner at exactly what frequency the protons of interest are resonating in the magnet's isocenter. (Even after careful coil tuning, patient-to-patient variations in susceptibility may change this value by a few dozen hertz, potentially resulting in a few millimeters of error in spatial localization).
|
Setting the center frequency also allows identification and tuning on a proton species of interest. By virtue of their chemical shifts, water and fat protons resonate at slightly different frequencies (about 220 Hz difference at 1.5T). These separate peaks can be recognized on the crude calibration scan image shown right where signal intensity of the entire imaged volume is displayed as a function of frequency.
For example, the center frequency may be set on water protons, fat protons, or some average of the two. Accurate center frequency setting is particularly important when fat saturation pulses are employed.
|
|
Although automated methods work pretty well, in difficult anatomy where significant quantities of fat or magnetic field inhomogeneities are present, MR technologists frequently check this crude spectrum to insure that the proper peak is being identified. By moving a cursor under the tip of the chosen peak, both coarse and fine adjustments to center frequency can be made. The efficiency of fat suppression can also be observed by watching the lipid peak fall during this manual procedure.
References
Maier JK. Automatic RF frequency adjustment for magnetic resonance scanner. US Patent # 4,806,866 dated 21 Feb 1989.
Maier JK. Automatic RF frequency adjustment for magnetic resonance scanner. US Patent # 4,806,866 dated 21 Feb 1989.
Related Questions
What is the scanner doing during the automatic pre-scan period?
What is the scanner doing during the automatic pre-scan period?