When comparing homogeneity specifications from different vendors, it is important to be sure the quoted DSVs are the same. Magnetic homogeneities will always look better if smaller DSVs are quoted, and inhomogeneity increases dramatically as the DSV is increased. For example, a scanner that has a homogeneity of 1 ppm over a 40 cm DSV may only have homogeneity of 3 ppm over a 45 cm DSV.
You should also be aware that when your new MR magnet arrives fresh from the factory, its homogeneity will likely be 100 ppm or more -- a far cry from the 1-3 ppm promised to you by your sales representative. But do not despair. Once the magnet is installed and ramped up to full field, it will undergo a set of checks and calibrations that will bring it to within homogeneity specifications. How this is done will be described in the next several questions.
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An additional warning concerning comparison of homogeneities among various scanners , , ,
When comparing the quoted homogeneities between two magnets you must not only make sure the DSV's are the same, but also that the homogeneities are measured in the same units. There are two different ways to measure the field distortion: peak-to-peak (P-P) and root-mean-square (RMS). The P-P value is approximately 1.4 x larger than the RMS value. Most vendors will report the RMS value (because it is smaller and so their ppm homogeneity will appear better), but not always. Just make sure you are comparing "apples to apples"!
Keller P. Technologies for precision field mapping. MetroLab Instruments (www.metrolab.com), Geneva, 2006.