First MR images obtained during megavoltage photon irradiation from a prototype integrated linac-MR system
The linac-MR system prototype configuration consisting of the magnet structure and associated linac and gantry. X-ray beam, magnet structure, and gradient coils rotate in unison within the transverse plane about a single rotational axis. The linac target is on the magnet side of the red cylinder by the item called “linac.” All indicated measurements are in mm. Distance from linac target to magnet isocenter is 800 mm. The magnet pole diameter is 632 mm.
Picture taken during the construction of the prototype which shows the biplanar magnet (in blue) with flat gradients. The magnet is physically connected to a cylindrical magnetic shield assembly (in gray on the left) surrounding the 6 MV waveguide (not visible). There is an additional magnetic shielding plate (not shown) between the magnet and the cylinder (see Fig. 3 for design details). The dark instrument inside the magnet is a “jig” to measure the magnetic-field distributions inside the MR magnet used in the shimming process of the magnet.
CT slice through the middle of the test phantom used for MR images. The white diamond is the acrylic block immersed in solution. Holes in the white diamond acrylic are of 2.52, 3.45, and 4.78 mm diameter.
Typical nonuniformed meshed pattern of a particular shielding scenario and the resulting magnetic field from FEM simulation is shown where magnetic fields and Ga are shown as white areas. Item 1 is a cross section of the MR magnet, where the white horizontal rectangular forms correspond to the MR magnet structure. The meshed colorwash depicts the magnetic field inside and just outside the magnet structure. The vertical shield plates are indicated. The light blue area depicts the extension of the 0.5 G. Items 2 and 3 depict the areas to be occupied by the MLC and the linac waveguide, respectively.
Measurement of magnetic field along the central axis of the linac waveguide with and without magnetic shield assembly in place. The shield assembly is placed at 65 cm from the magnet isocenter. The inset has the same axes as the main plot, with the magnetic-field axis magnified 20 times.
Profile through the CR image of the radiation beam emitted by the linac-MR prototype. The CR plate was placed at approximately 50 cm from the linac target. The flattening filter and the primary collimator are in the beam but no secondary jaws.
All images are obtained with a gradient echo sequence, flip , , , , slice , matrix , , and number of . Each image was obtained in 38 s. The line shown in (a) indicates the location of the profiles plotted in Fig. 8. Image (a) is obtained with linac powered up without beam on, (b) with radiation pulse outside the acquisition window, and (c) with radiation pulse inside the acquisition window.
Signal intensity profiles through the MR images of Fig. 7 along the line shown in Fig. 7(a). The plots identified as (a), (b), and (c) correspond to the images (a), (b), and (c) in Fig. 7.
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