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Shimming with permanent magnets for the x-ray detector in a hybrid x-ray/MR system
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10.1118/1.2963994
/content/aapm/journal/medphys/35/9/10.1118/1.2963994
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/35/9/10.1118/1.2963994

Figures

Image of FIG. 1.
FIG. 1.

Flowchart of the optimization process for shim magnets: is the baseline field, is the field measured with detector in place. is fitted with field of dipoles, and the fitting result is the targeted dipoles for shim magnets. is the field measured with shim magnets, and is the dipole fitting result of the net field produced by shim magnets . The final shimming efficacy is tested by measuring the field with detector and shim magnets .

Image of FIG. 2.
FIG. 2.

Experimental setup for field map measurement: the x-ray detector sits underneath a phantom inside an open-bore MR scanner.

Image of FIG. 3.
FIG. 3.

The field of a small magnet can be approximated with the field of a magnetic dipole ; the main MR field is in the direction.

Image of FIG. 4.
FIG. 4.

Two sets of magnets are mounted at the bottom of the detector with two pairs of slotted bars allowing separate 2D location adjustment for each of the two sets of magnets.

Image of FIG. 5.
FIG. 5.

With no detector inside the MR scanner, axial images of the phantom (, ) were obtained with (a) GRE (, slice thickness: , flip angle: 50°) and (b) SSFP (, slice thickness: , flip angle: 60°). (c) Measured field map. (d) Histogram of the field sampled at voxels in the phantom (offset and linear spatial terms removed).

Image of FIG. 6.
FIG. 6.

With the detector inside the scanner, images of the phantom were obtained with (a) GRE and (b) SSFP sequences. (c) Measured field map. (d) Histogram of the field sampled in the phantom. Imaging parameters were the same as in Fig. 5.

Image of FIG. 7.
FIG. 7.

With the detector and shimming magnets inside the scanner, images of the phantom were obtained with (a) GRE and (b) SSFP sequences. (c) Measured field map. (d) Histogram of the field sampled in the phantom. Imaging parameters were the same as in Fig. 5.

Tables

Generic image for table
TABLE I.

Results of field measurement and dipole fitting.

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/content/aapm/journal/medphys/35/9/10.1118/1.2963994
2008-08-07
2014-04-24
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Shimming with permanent magnets for the x-ray detector in a hybrid x-ray/MR system
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/35/9/10.1118/1.2963994
10.1118/1.2963994
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