WE-G-217A-06: Spatial Accuracy Quantification of an MR System

To develop a phantom and measurement protocol for quantifying spatial accuracy of an MR imaging system over its entire imaging volume.

The measurement protocol is comprised of a phantom, a set of MR sequence parameters for imaging the phantom, and analysis software for calculating spatial errors in the acquired phantom images. The phantom covers the entire imaging volume of the scanner above the patient table. It consists of layers of tooling foam which does not produce any detectable signal on conventional MR images, embedded with a matrix of oil capsules to serve as markers. To account for possible spatial errors in the construction of the phantom, the phantom was imaged with CT to create a gold standard data set. On MR scanners, the phantom is acquired with a 3D FGRE sequence that covers an extended FOV of 61.44 mm and with bandwidth = ±62.5 kHz. Error measurements are performed by detecting markers in the image sets and identifying them based on their known locations on the phantom. The spatial error of a marker is defined as the difference between its locations on the MR and CT image sets.

The phantom was constructed and the measurement protocol was executed on two different MR scanners. Some markers were located in areas of severe field inhomogeneity or gradient nonlinearity, and could not be adequately detected for analysis. Maximum errors over concentric spherical regions were observed by plotting the error of each marker as a function of their distance from isocenter.

The proposed phantom and protocol can be an effective tool for verifying the spatial accuracy of an MR system, which in turn can improve the accuracy and confidence of MR guided therapies. Data from this protocol may also be used in the development of advanced distortion correction algorithms. Employed by General Electric Healthcare.