Development of a Low-Cost Phantom to Assess Absolute Quantification in Multi-Voxel MR Spectroscopy

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Background: Magnetic resonance spectroscopy (MRS) is a technique to measure chemical shift and can be used to analyze biochemical parameters in a region-of-interest (ROI) of the tissue through a procedure called Quantification. Evaluation of accuracy and robustness of a quantification method is extremely important to help validate it in practice. This is particularly important when the method deals with absolute quantification as there is no consensus regarding absolute values found in different ROIs. Usually, such evaluation procedures are initially tested on simulated signals and then on phantom signals in the next step. Most of MRS phantoms designed so far are not multi-purpose, in that they evaluate either quality assurance and reliability, or absolute quantification results. Objectives: Here, we designed a low-cost MRS phantom with flexibility to contain one single solution for quality assurance and reliability tests, as well as different combinations of metabolites with known concentrations for evaluation of (absolute) quantification algorithms. Patients and Methods: Phantom production: Phantom was designed in cylindrical-shape. Phantom can be filled with water or any other liquid. A detachable circular holder plate was designed for the interior of the cylinder of phantom. The holder contained 55 holes to hold 25 standard 10 mL vials with 15 mm diameters for holes. Another circular plate at the bottom with holes of 5mm diameter was used in order to fix the vials in their place. The cone-shape vials could be filled with pure metabolites or any combination of metabolites with desired concentrations. The resulting cylindrical phantom body had an outer diameter of 21 cm, an inner diameter of 14 cm and a height 15 cm. The body of the phantom was made of acrylic glass with removable top that allowed easy access to the interior chamber. An O-ring was used to watertight the lid of the phantom. All materials, laser-cut and production procedure cost less than 100$.Data acquisition: Proton MRS imaging experiments were performed on a 1.5T Siemens Avanto MRI/MRS system in the room temperature using Point REsolved Spectroscopy (PRESS) pulse sequence with manufactures built-in auto-shimming on the volume-of-interest, CHESS water suppression and 3D imaging parameters as follows: TE/TR = 30/1500 ms, voxel size = 888 mm3, NEX = 2, frequency bandwidth = 2000Hz and number of data points = 1024. Each vial was filled with a pure metabolite with known concentration as exists in human normal brain. All solutions of metabolites were calibrated in chemical laboratory with highly sophisticated instruments. : Processing: Signal quantification, residual water peak removal and phase corrections were carried out with the Java-based MRUI quantification package. Receiver coil inhomogeneity was compensated using, 3D T2-weighted images acquired from the pure water phantom. Results: Position of the voxels on the T1-weighted MR scout image was shown. One sample spectrum of Choline in built-in Siemens Syngo MRS Package was shown. Quantification for all metabolites in different slices of each vial, based on QUEST method in jMRUI software, and after being post-processed with the 3D receive coil inhomogeneity profile was depicted. The result was in linear relationship between volume and concentration. Conclusions: We developed a versatile and low-cost phantom for evaluation purposes in different MR imaging and spectroscopy studies, with special capabilities to be utilized for multi-voxel spectroscopy experiments with the goal of absolute quantification.