Characterization of tissue magnetic susceptibility-induced distortions for MRIgRT
The main steps in the data analysis workflow for the numerical simulations using patient data: (a) the volume contours of anatomical structures are delineated on the patient CT data and used as data input; (b) the contours are converted into 3D masks by assigning them bulk susceptibility values (e.g., soft-tissue, bone, air); (c) magnetic field simulations are performed (the inset shows a typical simulation output cross section for a lung patient); and (d) postsimulation data analysis-–distortion values are quantified for the contours representing the interface of anatomical structures defined at step (a).
Experimental validation of the numerical simulation algorithm. A cylindrical phantom, mimicking the water-air interface, was scanned on two different MR scanners (a) 1.5 and (b) 3 T with a wide range of frequency encoding gradient strengths. The simulation results are displayed at the inner water-air interface. The maximum ppm difference was 4.5. The orientation of B 0 was along the x-axis and G E was set along the y-axis (image plane). The Dice similarity index for the simulation results versus experimental data was higher than 0.98 for all cases (slight variations due to changes in SNR). Please note the difference in the gradient strengths between the two image series for 1.5 and 3 T.
Experimental validation of the numerical simulation algorithm. The phantom was prepared to resemble a water-oil interface, with the oil occupying the inner cavity. The phantom was scanned on a 1.5 and 3 T magnet with several frequency encoding gradient strengths. The simulation results are displayed at the inner water-oil interface. The maximum ppm difference was 0.3, which translated into negligible distortion values (within 0.2 mm) for all B 0 and G E scenarios. The direction of B 0 and G E was along the x and y-axis (image plane), respectively. The Dice similarity index for the comparison of simulation versus experimental data was better than 0.98 for all cases. Please note the difference in the gradient strengths between the two image series for 1.5 and 3 T.
Simulation results: susceptibility-induced field distortions dependence on the inhomogeneity size and composition. The outer compartment (diameter of 240 mm) of the annular phantom was assigned the χ water value. The material of the inner compartment was varied to mimic interfaces relevant to patient anatomy. The inset shows (a) the phantom geometry and simulation parameters, i.e., cavity size (diameter varied from 1 to 80 mm) and composition; (b) and (c) show the maximum ppm and the maximum range (difference between max and min values) of ppm values, respectively; (d) ppm offset relative to water.
Simulation results: the maximum distortion values, derived from ppm values using Eq. (1), as a function of B 0 and G E for different air cavity sizes, i.e., the water-air interface of the cylindrical phantom presented in Fig. 4.
Susceptibility effects dependence on the inhomogeneity shape and its relative orientation relative to B 0. The outer and inner compartments of the annular phantom were assigned the χ of water and air values, respectively. The inset shows (a) the phantom geometry and simulation parameters, namely, cavity shape (from ellipse to circle) and B 0 orientation (parallel and perpendicular); (b) the maximum ppm values for two cavity sizes, i.e., 5 and 10 mm.
Susceptibility-induced B 0 perturbations maps for the two MR-linac system configurations in the case of a typical brain patient anatomy. Column (a) displays the simulation results for B 0 along z-axis for the bore-type magnet, and column (b) shows the results for the biplanar magnet configuration with B 0 along y-axis [in the (x,y)-plane].
Susceptibility-induced B 0 perturbations maps for the two MR-linac system configurations in the case of a lung patient. The simulation results are in column (a) for B 0 along z-axis as in a bore magnet configuration, and in column (b) for the biplanar magnet with B 0 along y-axis [in the (x,y)-plane].
The field distortion simulations results for anatomical structures with significantly different susceptibility values in the case of several anatomical sites. The distortions were quantified in terms of maximum, mean, and range of field fluctuations. The corresponding geometric distortions for 0.5 and 3 T are given, assuming an encoding gradient strength of 5 mT/m [see Eq. (1)]. B 0 is along z-axis as for a typical bore-type MR scanner.
Simulation results (same metrics as in Table I) for the case of B 0 along y-axis [in the (x,y)-plane]. This is the B 0 configuration for a biplanar MR scanner.
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