Geometric AccuracyHow accurate are distance measurements on an MR scan?
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Linear measurements on MR phantom
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One of the most straightforward assessments of MR quality is to determine the accuracy of distance measurements. MR phantoms for this purpose contain a variety of tubes, grids, and other objects of known size that should be measured in each cardinal direction (slice, frequency, and phase) as well as obliquely. Measurements taken from the scan are compared to the known distances on the phantom, and a percent error can be calculated. In a phantom with a uniformly spaced grid, linearity over the entire field-of-view can be determined using the coefficient of variation of the inter-grid distances measured.
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Various detailed procedures (see references below) for calculating linearity and geometric accuracy are available from the American College of Radiology (ACR), National Electrical Manufacturers Association (NEMA), and the American Association of Physicists in Medicine (AAPM). Linearity errors should typically be less than 1%-2%.
The most common cause of geometric errors is miscalibration of one or more imaging gradients. Gradients tend to drift over time and require periodic re-calibration by service engineers. Occasionally the problem is caused by Bo inhomogeneity due to improper shim adjustments or an occult ferromagnetic object lodged in the scanner bore.
References
American Association of Physicists in Medicine (AAPM). Report No. 100. Acceptance testing and quality assurance procedures for magnetic resonance imaging facilities. 2010.
American College of Radiology. Phantom test guidance for use of the large MRI phantom for the ACR accreditation program, 2018.
National Electrical Manufacturers Association. NEMA Standards Publication MS 2-2008 (R2014). Determination of two-dimensional geometric distortion in diagnostic magnetic resonance images. NEMA, Washington, DC. Can be downloaded for free or purchased in book form here.
National Electrical Manufacturers Association. NEMA Standards Publication MS 12-2016. Quantification and mapping of geometric distortion for special applications. NEMA, Washington, DC. Can be downloaded for free or purchased in book form here.
American Association of Physicists in Medicine (AAPM). Report No. 100. Acceptance testing and quality assurance procedures for magnetic resonance imaging facilities. 2010.
American College of Radiology. Phantom test guidance for use of the large MRI phantom for the ACR accreditation program, 2018.
National Electrical Manufacturers Association. NEMA Standards Publication MS 2-2008 (R2014). Determination of two-dimensional geometric distortion in diagnostic magnetic resonance images. NEMA, Washington, DC. Can be downloaded for free or purchased in book form here.
National Electrical Manufacturers Association. NEMA Standards Publication MS 12-2016. Quantification and mapping of geometric distortion for special applications. NEMA, Washington, DC. Can be downloaded for free or purchased in book form here.
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