Download X-Ray Tube for Use in Magnetic Fields

X-RAY TUBE FOR USE IN MAGNETIC FIELDS
Zhifei Wen, Rebecca Fahrig, Nianxiang Sun, ShanXiang Wang and Norbert Pelc
Stanford University, Stanford, CA
Abstract
In the hybrid interventional W x - r a y system’ an x-ray tube is
placed in a high magnetic field. X-rays are produced in the x-ray
tube by bombarding a target with high-energy electrons. Ideally,
the MRI system’s magnetic field and the x-ray tube’s electric field
should be aligned, but in practice the electron beam may be
deflected by a misaligned magnetic field*.Here we propose putting
well-aligned permanent magnets inside the x-ray tube to minimize
undesired deflection due to the unknown external field.
Electron trajectory projections under different conditions:
I
A
Br
0.3T, Bx = 0.03T
,
111
Dr * 0.3T, Rx = 0 031
Introduction
Both x-ray fluoroscopy
_ _ and MRI are powerful tools for
guiding interventional procedures. Dual-modality imaging would
combine, into the same gantry, the high spatial and temporal
resolution of x-ray fluoroscopy for placement of catheters with the
soft-tissue contrast, 3D visualization and physiological information
of MRI.
Figure 1 shows the electron trajectories in a computer model
of our x-ray tube with no magnetic field present. The trajectories
were generated by a finite-element program (Opera-3d). One half
of a cylinder with radius of O.3mm and length of 3mm was used to
represent the filament. Thermionic emission was modeled using
Child’s law current limit model (temperature: 2500K, work
function: 4.5ev; total current: 7.5mA).
In our W x - r a y hybrid system, the x-ray tube is placed
inside the magnet, in a high but uniform magnetic field (-0.3T).
With a small misalignment angle 0 between cathode-anode axis
(therefore electric field E) and magnetic field B, the electrons
principally spiral along the B field so the deflection is A=L*tanB in
the direction of the magnetic field component BLperpendicular to
E, where L(=lcm) is the cathode-anode distance (Fig 2).
In other applications, the x-ray tube may be placed outside
but close to the magnet, where the field may be lower (less than
0.02T) but where we may have little knowledge of its direction. In
a weak B field, the deflection is approximately proportional to BI
in the direction of the Lorenz force q,Vx B because the velocity of
the electrons V is mainly in the direction of cathode-anode axis
(Fig 3)
100
Z(mm)
Figure 1 Electron trajectory projection (in x-zplane) with no B field
Methods
To reduce deflection of the focal spot by the magnetic field,
one possible scheme is to create a high magnetic field Bmagin the
cathode-anode axis direction in the volume that the electrons pass
through, for example using permanent magnets. For the first case
where the x-ray tube is in the high field, the angle between E and
B+B,,, is lower (tan0’= BI /(BI1+Bmag)),
so the deflection should
be reduced. In the second case, with the presence of Bmag,the
scenario has changed to that of the first case: the deflection
becomes A=L*tan0’ in the direction of the transverse component
of the weak field B.
© Proc. Intl. Soc. Mag. Reson. Med. 10 (2002)
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Simulations
Two simulations were done without permanent magnets
present: one with a magnetic field B=0.3T but misaligned with the
electric field by 5.7”, which would normally cause the focal spot to
be deflected in the x direction by -0.9mm (Fig 2); the other
simulation had only a transverse magnetic field B, of 0.02T
causing the beam to be deflected in y by 1.8mm (Fig 3).
In the next two simulations two cylindrical (diameter 1.5cm,
length 2cm) SmCo permanent magnets were put 0.5cm behind the
filament and the anode respectively, with axes aligned with the
cathode-anode axis. In the volume close to the cathode-anode axis,
the magnetic field from the magnets is mostly in the +z direction,
varying fiom -0.5T at the magnet surface to -0.2T at the center
between the magnets. Because the external field is less than the
intrinsic coercivity of the magnets, the external field should not
affect the magnets and the total magnetic field is simply the vector
sum of both fields. In the misaligned external field, the focal spot
deflection in x was reduced to 0.5mm (Fig 4), which agreed with
A’=L*tan0’=Bl/(BI1+Bm,J and an average B,
-0.3T. In the
transverse magnetic field Bx=0.02T, the former deflection in y was
mostly removed, but there was a deflection of 0.7mm in x (Fig 5).
This is consistent with A’=L*tanB’=BI/B,,=0.02/0.3=0.67mm,
further supporting the assumption that it was converted to the highfield scenario.
Conclusions
Placing properly aligned permanent magnets inside the x-ray
tube is a promising way to make the tube more immune to an
unknown or misaligned external magnetic field.
Acknowledgments
This work was supported by GE Medical Systems, NIH grant
P41 RR09784, and the Lucas Foundation.
References
1. R. Fahrig, K. Butts, J.A. Rowlands, et al: JMRI 13,294-300,
2000.
2. Z. Wen, R. Fahrig, andN.J. Pelc: Proc. ISMRM, p. 2188,2001.