Abstract ID: 373 Quality assurance (QA) in unconventional magnetic resonance (MR) techniques: Diffusion and spectroscopy

Abstract

Purpose Diffusion-weighted imaging (DWI) is an MR quantitative technique, which is widely employed in research applications as well as in clinical practice. Proton MR Spectroscopy (MRS) in vivo provides unique biochemical information that complements MR imaging. Specific test objects and evaluation methods should be used to obtain meaningful information on the performance level of the MR scanner, and the need for QA protocols also in DWI and MRS acquisitions is well recognized by international guidelines. The aim of this work is to present the methods for measuring the main parameters for the evaluation of the quantitative accuracy of diffusion and spectroscopy techniques. Methods Methods to assess the reliability of the spectroscopic chain should include at least the measurement of the signal-to-noise ratio and the characterization of a singlet resonance peak such as that of water or a metabolite (intensity, spectral resolution), the accuracy of spatial selectivity (linearity test) in single or multi-volume mode and finally the efficiency of water-suppression pulses. It is also advisable to check the degree of accuracy of the absolute quantification by verifying the concentration of a mixture of metabolites by means of calculation algorithms available on the console or in offline mode. In DWI, the apparent diffusion coefficient (ADC) is a physical quantity calculated directly from a series of images obtained with different diffusion weighing (b-value) due to the application of high intensity field gradients. ADC is affected by different factors that may increase its inaccuracy and variability (gradient miscalibration and spatial nonlinearity, coupled imaging-gradient, rectified noise, temperature). It is advisable to plan a series of diffusion-weighted acquisition sequences of a phantom containing an aqueous/gelatinous solution and equipped with a temperature verification system. DWI acquisition series should be performed with both short and long maximum b-value, changing the spatial direction of the gradients applied at least along the three main orthogonal axes. Finally, more advanced tests can be devised for the evaluation of parameters derived from the diffusion tensor such as fractional anisotropy or kurtosis index, using isotropic/anisotropic phantoms and suitably heterogeneous materials (non-gaussian diffusion), respectively. Diffusion-weighted imaging (DWI) is an MR quantitative technique, which is widely employed in research applications as well as in clinical practice. Proton MR Spectroscopy (MRS) in vivo provides unique biochemical information that complements MR imaging. Specific test objects and evaluation methods should be used to obtain meaningful information on the performance level of the MR scanner, and the need for QA protocols also in DWI and MRS acquisitions is well recognized by international guidelines. The aim of this work is to present the methods for measuring the main parameters for the evaluation of the quantitative accuracy of diffusion and spectroscopy techniques. Methods to assess the reliability of the spectroscopic chain should include at least the measurement of the signal-to-noise ratio and the characterization of a singlet resonance peak such as that of water or a metabolite (intensity, spectral resolution), the accuracy of spatial selectivity (linearity test) in single or multi-volume mode and finally the efficiency of water-suppression pulses. It is also advisable to check the degree of accuracy of the absolute quantification by verifying the concentration of a mixture of metabolites by means of calculation algorithms available on the console or in offline mode. In DWI, the apparent diffusion coefficient (ADC) is a physical quantity calculated directly from a series of images obtained with different diffusion weighing (b-value) due to the application of high intensity field gradients. ADC is affected by different factors that may increase its inaccuracy and variability (gradient miscalibration and spatial nonlinearity, coupled imaging-gradient, rectified noise, temperature). It is advisable to plan a series of diffusion-weighted acquisition sequences of a phantom containing an aqueous/gelatinous solution and equipped with a temperature verification system. DWI acquisition series should be performed with both short and long maximum b-value, changing the spatial direction of the gradients applied at least along the three main orthogonal axes. Finally, more advanced tests can be devised for the evaluation of parameters derived from the diffusion tensor such as fractional anisotropy or kurtosis index, using isotropic/anisotropic phantoms and suitably heterogeneous materials (non-gaussian diffusion), respectively.