New diffusion phantoms dedicated to the study and validation of high‐angular‐resolution diffusion imaging (HARDI) models

Abstract

We present new diffusion phantoms dedicated to the study and validation of high-angular-resolution diffusion imaging (HARDI) models. The phantom design permits the application of imaging parameters that are typically employed in studies of the human brain. The phantoms were made of small-diameter acrylic fibers, chosen for their high hydrophobicity and flexibility that ensured good control of the phantom geometry. The polyurethane medium was filled under vacuum with an aqueous solution that was previously degassed, doped with gadoliniumtetraazacyclododecanetetraacetic acid (Gd-DOTA), and treated by ultrasonic waves. Two versions of such phantoms were manufactured and tested. The phantom's applicability was demonstrated on an analytical Q-ball model. Numerical simulations were performed to assess the accuracy of the phantom. The phantom data will be made accessible to the community with the objective of analyzing various HARDI models. During the last decade, diffusion-weighted (DW) imaging (DWI) has become an established technique for the diagnosis of ischemia (1) and investigations of the anatomical connectivity of the human brain (2). Presently, no manufacturer delivers any phantoms dedicated to diffusion imaging, due to the complexity of their design. However, diffusion phantoms have numerous applications. They include calibration, validation of tractography algorithms, and validation of diffusion models. The phantom design should comply with the concrete application. For example, calibration requires a large region of interest (ROI) with a specific apparent diffusion coefficient (ADC), fractional anisotropy (FA), and principal orientation(s) to reduce the impact of acquisition noise on the measurements. On the other hand, to validate tractography, one would typically use a phantom made up of long bundles, similar to those found in brain white matter. To circumvent the intrinsic limitations of diffusion tensor imaging (DTI; i.e., the inability to resolve multiple fiber populations), a number of high-angular-resolution diffusion imaging (HARDI) models were introduced (3-12). They were conceived with the aim of providing an unbiased estimate of the probability density function (PDF) describing the displacements of the water molecules during a predefined time interval. Some models deliver only the radial projections of the PDF, known as the orientation distribution function (ODF). The phantoms employed in the studies of HARDI models could be adjusted to different fiber configurations (crossing, kissing, merging, and splitting), and angular distribution. Several diffusion phantom designs were proposed, based on fibrous vegetables (13), biological tissues (14), plastic capillaries (15-17), or textile fibers (18 -20). In this work, we present a novel diffusion phantom dedicated to the validation of HARDI models. We developed two versions of this phantom corresponding to 45°and 90°fi ber crossings, and used them to test the analytical Q-ball model. MATERIALS AND METHODSThe design of diffusion phantoms dedicated to st...

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