Technical notes A volume resolution phantom for MRI Sang Yun Moon, Joseph P. Hornak ⁎ Magnetic Resonance Laboratory, Rochester Institute of Technology, Rochester, NY 14623, USA Received 16 February 2009; revised 26 May 2009; accepted 4 July 2009 Abstract Multisite quantitative magnetic resonance imaging (qMRI) of volume requires a small isotropic point spread-function (PSF) that is spatially, temporarily, and platform invariant. A phantom which will allow rapid assessment of this metric throughout the imaged volume without repositioning will assist certification of imaging sites for use in qMRI studies based on volume. This paper presents a phantom design for this purpose with a three-dimensional repeating pattern throughout its 800-cm 3 volume. The image of the pattern from the phantom contains a series of positive signal points and lines which can be used to measure the PSF, gradient linearity, gradient orthogonality, and B 0 homogeneity at multiple locations throughout its volume. The phantom is readily constructed, can be filled with any nuclear magnetic resonance signal-bearing liquid, and the design is scalable to cover larger volumes. © 2010 Elsevier Inc. All rights reserved. Keywords: Magnetic resonance imaging; Resolution phantom; MRI; Qualitative MRI; qMRI; Linearity; Resolution; Point spread function Quantitative magnetic resonance imaging is often used to stage disease and determine the efficacy of pharmaceuticals [1]. Accurate qMRI typically requires a magnetic resonance imaging (MRI) system to operate at a consistently higher level of performance than is needed for conventional qualitative MRI [2,3]. Existing phantoms were not designed to evaluate information critical to qMRI on current scanners. Therefore, special phantoms are needed to assess system performance for qMRI. Quantitative MRI of volume is an example of where current phantoms are imperfect. Quantitative MRI based on volume changes requires that the imaging system have a small isotropic point spread- function (PSF) [4] and that it be spatially invariant. A longitudinal study adds the constraint that the PSF be temporally invariant, and a broad cross-sectional study can add the requirement of platform invariance. These concepts are illustrated in Fig. 1 for two point samples in an imaged space. In the first example, the imaging system produces an image (Fig. 1C) of the samples (Fig. 1A) that is the convolution of the points with an anisotropic spatially invariant PSF (Fig. 1B). The image shows blurred points, both offset by the same amounts δ 1 and δ 2 from their original positions. In MRI, a constant offset across the image is generally acceptable. In the second example, the two point samples are imaged with a spatially variant PSF (Fig. 1B and D). The points are still blurred, but differently, and now distances in the image are distorted (Fig. 1E). Spatial distortions are caused by the variation in the offset part of the PSF across the image. In MRI, anisotropic PSFs are frequently the norm as rectangular parallelepiped voxels are often used. When small cubic voxels are prescribed, a small isotropic PSF is achievable on many systems in some portion of the imageable volume. Nonlinearities in the gradients, which are either inherent in the gradient coil design or a consequence of B 0 inhomogeneities, make the PSF aniso- tropic, spatially variant and too large at other locations in the magnet. A phantom and procedure for assessing the PSF and its spatial, temporal, and platform variances would greatly benefit qMRI studies of volume. Constant time imaging [5] and phantom-based [6–8] techniques have been proposed for measuring the PSF. The former approach, applied to a uniform signal phantom, yields an image weighted by the integral of the PSF and, thus, can monitor the spatial, temporal and platform variance in the Available online at www.sciencedirect.com Magnetic Resonance Imaging 28 (2010) 286 – 289 ⁎ Corresponding author. Magnetic Resonance Laboratory, Center for Imaging Science, Rochester, NY 14623-5604, USA. Tel.: +1 585 475 2904; fax: +1 585 475 5988. E-mail address: jphsch@rit.edu (J.P. Hornak). 0730–725X/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.mri.2009.07.002