Magnetic Field HomogeneityAn MR salesman claims his scanner has better "homogeneity" than that of his competitor. What is he talking about?
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Homogeneity refers to the uniformity of a magnetic field in the center of a scanner when no patient is present. Magnetic field homogeneity is measured in parts per million (ppm) over a certain diameter of spherical volume (DSV). For example, a 3.0T magnet may guaranteed to have a homogeneity of <1 ppm over a 40 cm DSV. This means that no two points within ± 20 cm of the magnet isocenter differ in magnetic field strength by more than one part in a million, or by no more than 3.0T x (1/1,000,000) or 0.000003T.
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.
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.
References
Keller P. Technologies for precision field mapping. MetroLab Instruments (www.metrolab.com), Geneva, 2006.
Keller P. Technologies for precision field mapping. MetroLab Instruments (www.metrolab.com), Geneva, 2006.