Unilateral MRI using a rastered projection Christina L. Bray, Joseph P. Hornak * Magnetic Resonance Laboratory, Center for Imaging Science, Rochester Institute of Technology, Rochester, NY 14623-5604, USA Received 23 January 2007; revised 19 June 2007 Available online 4 July 2007 Abstract Unilateral magnetic resonance techniques, where magnet and radio frequency (RF) coil are placed on one side of the sample, can provide valuable information about a sample which otherwise cannot be accommodated in conventional high spectral resolution mag- netic resonance systems. A unilateral magnetic resonance imaging approach utilizing the stray field from a disc magnet and a butterfly geometry RF coil is described. The coil excites spins in a volume centered around an arc through the sample. Translating the RF coil relative to the magnet and recording the signal at each translational location creates a projection of the signal in a tomographic slice through the sample. Rotating the RF coil relative to the sample and repeating the translation creates projections through the sample at different angles. Backprojecting this information yields an image. A proof of concept device operating on this principle at 12.4 MHz was constructed and characterized. Projections through three phantoms are presented with a 1.2–4 cm field of view, thickness of 102 lm, and at a distance of 3 mm from the RF coil and 14 mm from the magnet. The edge spread function (ESF) was measured resulting in a 4 mm full width at half maximum (FWHM) line spread function (LSF) estimation using a Gaussian model. An example of one reconstructed image is presented. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Unilateral MRI; Single sided MRI; Surface MRI; Instrumentation; Imaging; MRI; Backprojection reconstruction 1. Introduction The majority of nuclear magnetic resonance experiments are performed in a mode where the source of the applied static magnetic field (B o ) and oscillating magnetic field (B 1 ) at the resonance frequency m are created with magnets and coils that surround, or are at least located on opposite sides of, the sample. Unilateral or single-sided magnetic resonance is performed by placing both the B o and B 1 sources on the same side of the sample. There are many potential applications of unilateral MRI, ranging from the macroscopic to the microscopic. On the macro scale, geophysical applications dominate. These include utility mapping, prospecting for aquifers, contaminant abatement, archeology, and earth dam assess- ment. An MRI based technique would compliment ground penetrating radar (GPR) which fails under conditions of high moisture and moderate conductivity where MRI should perform well. Moving towards microscopic dimen- sions, medical, food, and materials applications prevail. In food science, a unilateral system could be used for assessing moisture and oil content in foods and grains. In medicine, applications include dermatology, lymphology, and vascu- larity. In materials science, applications might range from studies of water in absorbing products, such as paper tow- els and diapers, to moisture ingress in organic light emit- ting diodes (OLED) and liquid crystal displays (LCD), to polymer rheology. The terms unilateral and single-sided have been used in the literature to describe a variety of magnetic resonance systems [1–23]. We restrict our summary to include those systems where both B o and B 1 magnetic fields are applied unilaterally. Therefore, stray-field (STRAFI) [16] and Gra- dient-At-Right-angles-to-Field (GARField) [17] NMR and MRI; and magnetic resonance sounding, NMR geotomog- raphy, and geophysical surface NMR [18–23] where either 1090-7807/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2007.06.010 * Corresponding author. Fax: +1 585 475 5988. E-mail address: jphsch@rit.edu (J.P. Hornak). www.elsevier.com/locate/jmr Journal of Magnetic Resonance 188 (2007) 151–159