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Effect of radiation induced current on the quality of MR images in an integrated linac-MR system
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10.1118/1.4752422
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    Affiliations:
    1 Department of Physics, University of Alberta, 11322 – 89 Avenue, Edmonton, Alberta T6G 2G7, Canada and Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    2 Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    3 Department of Physics, University of Alberta, 11322 – 89 Avenue, Edmonton, Alberta T6G 2G7, Canada; Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada; and Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    4 Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada
    a) Author to whom correspondence should be addressed. Electronic mail: blburke@ualberta.ca
    Med. Phys. 39, 6139 (2012); http://dx.doi.org/10.1118/1.4752422
/content/aapm/journal/medphys/39/10/10.1118/1.4752422
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/39/10/10.1118/1.4752422

Figures

Image of FIG. 1.
FIG. 1.

Schematic diagram on linac-MR system showing the split solenoid MR magnet, the linear accelerator and the rotational gantry.

Image of FIG. 2.
FIG. 2.

Signal-to-noise ratio loss due to RIC in 3 cm solenoid coil. The solid line shows the SNR when the radiation beam is blocked, the dotted line shows the SNR loss when the radiation beam is incident on the RF coil, and the dashed line shows the SNR after the use of a postprocessing algorithm.

Image of FIG. 3.
FIG. 3.

Sample images acquired with 10 cm solenoid coil. The images were acquired with the linac not producing radiation (left) and with linac producing radiation and RF coil unblocked at 250 MU/min (right). The RIC artefact is not visible.

Image of FIG. 4.
FIG. 4.

Sample images acquired with 3 cm solenoid coil. The images were acquired with the linac not producing radiation (left) and with linac producing radiation and RF coil unblocked at 250 MU/min (right). The RIC artefact is not visible.

Image of FIG. 5.
FIG. 5.

k-space data from images acquired with linac dose rates of 0 and 250 MU/min. The top image was acquired with the radiation not pulsing. The middle image was acquired with a linac dose rate of 250 MU/min but the radiation beam was blocked from reaching the coil; it shows no RIC effects. The bottom image was acquired with a linac dose rate of 250 MU/min and the radiation beam incident on the RF coil; it clearly shows the RIC artefact, which presents itself as near vertical lines in k-space.

Image of FIG. 6.
FIG. 6.

k-space data for gradient echo images taken with the 3 cm solenoid coil with TR = 300, 300.1, and 301 ms with a linac dose rate of 250 MU/min and the radiation beam incident on the RF coil. In the top image, TR = 300 ms, the linac pulses an integer number (54) of times during this TR so the lines seen in k-space due to RIC are nearly vertical. In the middle image, TR = 300.1 ms, there is no longer an integer number of linac pulses during this TR, resulting in timing shifts between successive horizontal (read encode) lines in k-space. Therefore, the lines seen in k-space due to RIC are now slanted from left to right. In the bottom image, TR = 301 ms, the shift between RIC noise pixels in subsequent horizontal (read encode) lines in k-space is now so large that the RIC artefact appears to be random; however, closer inspection shows that it is still regularly spaced on each read encode line.

Image of FIG. 7.
FIG. 7.

k-space data from 256 × 128 gradient echo image acquired with 10 cm coil. TR = 300 ms, linac dose rate = 250 MU/min. Top image is original k-space data and bottom image is k-space data after RIC removal algorithm has been applied. The RIC noise is effectively removed from the k-space data and SNR is recovered. SNR of top image = 16.2, SNR of bottom image = 17.3.

Image of FIG. 8.
FIG. 8.

MR images resulting from application of various filters to image k-space data to remove RIC noise: (a) original MR data with RIC not removed; (b) custom postprocessing algorithm applied; (c) median filter applied; (d) adaptive median filter applied.

Tables

Generic image for table
TABLE I.

SNR for images acquired with 10 cm coil using a gradient echo sequence. SNR was calculated by taking the mean of the signal in the magnitude image divided by the standard deviation of the noise in the real image.

Generic image for table
TABLE II.

SNR for images acquired with 3 cm coil using a gradient echo sequence. SNR was calculated by taking the mean of the signal in the magnitude image divided by the standard deviation of the noise in the real image.

Generic image for table
TABLE III.

SNR for images acquired with 3 cm coil using a spin echo sequence. SNR was calculated by taking the mean of the signal in the magnitude image divided by the standard deviation of the noise in the real image.

Generic image for table
TABLE IV.

SNR for images acquired with 3 cm coil using a bSSFP sequence. SNR was calculated by taking the mean of the signal in the magnitude image divided by the standard deviation of the noise in the real image.

Generic image for table
TABLE V.

Effectiveness of four filtering algorithms at removing RIC noise from MR images acquired on a linac-MR system.

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/content/aapm/journal/medphys/39/10/10.1118/1.4752422
2012-09-21
2014-04-23
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Effect of radiation induced current on the quality of MR images in an integrated linac-MR system
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/39/10/10.1118/1.4752422
10.1118/1.4752422
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