Shimming with permanent magnets for the x-ray detector in a hybrid x-ray/
Departments of Radiology and Physics, Stanford University, Stanford, California 94305
Rebecca Fahrig and Scott T. Williamsb?
Department of Radiology, Stanford University, Stanford, California 94305
Norbert J. Pelc
Departments of Radiology and Bioengineering, Stanford University, Stanford, California 94305
?Received 16 November 2007; revised 9 June 2008; accepted for publication 2 July 2008;
published 7 August 2008?
In this x-ray/MR hybrid system an x-ray flat panel detector is placed under the patient cradle, close
to the MR volume of interest ?VOI?, where the magnetic field strength is ?0.5 T. Immersed in this
strong field, several electronic components inside the detector become magnetized and create an
additional magnetic field that is superimposed on the original field of the MR scanner. Even after
linear shimming, the field homogeneity of the MR scanner remains disrupted by the detector. The
authors characterize the field due to the detector with the field of two magnetic dipoles and further
show that two sets of permanent magnets ?NdFeB? can withstand the main magnetic field and
compensate for the nonlinear components of the additional field. The ideal number of magnets and
their locations are calculated based on a field map measured with the detector in place. Experimen-
tal results demonstrate great promise for this technique, which may be useful in many settings
where devices with magnetic components need to be placed inside or close to an MR
scanner. © 2008 American Association of Physicists in Medicine. ?DOI: 10.1118/1.2963994?
Key words: XMR hybrid system, magnetic field shimming, permanent magnet
Our hybrid system provides the synergic image guidance of
two powerful imaging modalities, x-ray fluoroscopy and
magnetic resonance imaging ?MRI?.1–3Clinical trials have
shown that the hybrid x-ray/MR system can greatly benefit a
number of interventional procedures.4,5In our system, an
x-ray tube and a flat panel detector were placed inside the
magnetic field of an open-bore MR system. Effects of the
magnetic field on the x-ray tube have been studied and pos-
sible solutions proposed.6–8The x-ray detector, being close
to the MR imaging volume under the patient cradle, experi-
ences a magnetic field approximately equal to the system
field strength of 0.5 T. The effect of the magnetic field on the
performance of the x-ray detector was evaluated and no ap-
preciable image degradation was observed.9However, the
effect of the detector on the MR system has not heretofore
been fully explored. The detector can affect the MR system if
it has magnetic components. When placed in the main mag-
netic field, these components can be magnetized and degrade
the homogeneity of the magnetic field of the MR scanner. In
order to make the detector MR compatible, we have tried our
best to replace magnetic components with nonmagnetic sub-
stitutes. Nonetheless, it is difficult to make the detector com-
pletely nonmagnetic and thus the residual field from the de-
tector can still degrade the field homogeneity of the MR
Our MR scanner is able to produce a linear “shim” field to
make the main field more homogenous, i.e., a gradient field
to compensate for the linear spatial components of the inho-
mogeneous field in the imaging volume. Due to the proxim-
ity of the detector to the MR imaging volume, however, the
detector produces a field that contains considerable nonlinear
spatial components. As a result, even after linear shimming
the MR image quality with the detector in position still suf-
fers, especially with the steady-state free precession ?SSFP?
imaging that is frequently used in our interventional
procedures.10,11Our system does not have resistive higher
order shim coils12,13and the superconductive higher-order
shims cannot be easily adjusted. Even if high-order shim-
ming coils could be used, they are designed to produce fields
with spatial variation comparable to the bore size and may
not be an optimal choice for the effects of the x-ray detector.
Passive shimming can also provide nonlinear field compo-
nents to improve the system field homogeneity by strategi-
cally placing pieces of ferromagnetic material in the
magnet.14,15The sizes and locations of the ferromagnetic ma-
terial need to be carefully calibrated. Since our x-ray detector
is not permanently mounted in the MR system, it would be
difficult to remove and replace the passive shimming accord-
ing to the presence of the detector. Therefore, a more conve-
nient and efficient method is very desirable.
Local shimming could be implemented with electric coils
that can be tailored to specific situations. This active shim-
ming scheme is especially well-suited in situations where the
field inhomogeneity is slight and the imaging volume is lo-
calized, e.g., in fMRI studies.16For interventional proce-
dures, on the other hand, the imaging volume can be large
and the deleterious field created by the magnetic material in
38953895Med. Phys. 35 „9…, September 20080094-2405/2008/35„9…/3895/8/$23.00© 2008 Am. Assoc. Phys. Med.
Daniel, F. Laerum, and N. J. Pelc, “Truly hybrid interventional MR/X-ray
system: investigation of in vivo applications,” Acad. Radiol. 8, 1200–
3A. Ganguly, Z. Wen, B. L. Daniel, K. Butts, S. T. Kee, V. Rieke, H. M.
Do, N. J. Pelc, and R. Fahrig, “Truly hybrid X-ray/MR imaging: toward a
streamlined clinical system,” Acad. Radiol. 12, 1167–1177 ?2005?.
4R. Fahrig, G. Heit, Z. Wen, B. L. Daniel, K. Butts, and N. J. Pelc, “First
use of a truly-hybrid X-ray/MR imaging system for guidance of brain
biopsy,” Acta Neurochir. 145, 995–997; discussion 997 ?2003?.
5S. T. Kee, A. Ganguly, B. L. Daniel, Z. Wen, K. Butts, A. Shimikawa, N.
J. Pelc, R. Fahrig, and M. D. Dake, “MR-guided transjugular intrahepatic
portosystemic shunt creation with use of a hybrid radiography/MR sys-
tem,” J. Vasc. Interv. Radiol. 16, 227–234 ?2005?.
6Z. Wen, R. Fahrig, S. Conolly, and N. J. Pelc, “Investigation of electron
trajectories of an x-ray tube in magnetic fields of MR scanners,” Med.
Phys. 34, 2048–2058 ?2007?.
7Z. Wen, N. J. Pelc, W. R. Nelson, and R. Fahrig, “Study of increased
radiation when an x-ray tube is placed in a strong magnetic field,” Med.
Phys. 34, 408–418 ?2007?.
8Z. Wen, R. Fahrig, and N. J. Pelc, “Robust x-ray tubes for use within
magnetic fields of MR scanners,” Med. Phys. 32, 2327–2336 ?2005?.
9R. Fahrig, Z. Wen, A. Ganguly, G. DeCrescenzo, J. A. Rowlands, G. M.
Stevens, R. F. Saunders, and N. J. Pelc, “Performance of a static-anode/
flat-panel x-ray fluoroscopy system in a diagnostic strength magnetic
field: a truly hybrid x-ray/MR imaging system,” Med. Phys. 32, 1775–
10K. Scheffler, O. Heid, and J. Hennig, “Magnetization preparation during
the steady state: fat-saturated 3D TrueFISP,” Magn. Reson. Med. 45,
11M. A. Bernstein, K. F. King, and Z. J. Zhou, Handbook of MRI Pulse
Sequences ?Academic, Amsterdam, 2004?.
12D. H. Kim, E. Adalsteinsson, G. H. Glover, and D. M. Spielman, “Regu-
larized higher-order in vivo shimming,” Magn. Reson. Med. 48, 715–722
13F. Romeo and D. I. Hoult, “Magnet field profiling: analysis and correcting
coil design,” Magn. Reson. Med. 1, 44–65 ?1984?.
14L. Brateman, T. T. Bowman, S. Donstrup, and K. N. Scott, “Magnetic
field shimming by Fourier analysis,” Magn. Reson. Med. 6, 459–473
15B. Dorri, B. Dorri, M. E. Vermilyea, and W. E. Toffolo, “Passive shim-
ming of MR magnets: algorithm, hardware, and results,” IEEE Trans.
Appl. Supercond. 3, 254–257 ?1993?.
16J. J. Hsu and G. H. Glover, “Mitigation of susceptibility-induced signal
loss in neuroimaging using localized shim coils,” Magn. Reson. Med. 53,
17R. C. O’Handley, Modern Magnetic Materials: Principles and Applica-
tions ?Wiley, New York, 2000?.
18J. D. Jackson, Classical Electrodynamics, 3rd ed. ?Wiley, New York,
19M. Prammer, J. Haselgrove, M. Shinnar and J. Leigh, “A new approach to
automatic shimming,” J. Magn. Reson. ?1969-1992? 77, 40–52 ?1988?.
3902Wen et al.: Shimming magnets for the x-ray detector in XMR system3902
Medical Physics, Vol. 35, No. 9, September 2008