What the research is about:
Development of a source of pulsed,
tunable, near-monochromatic X-rays deliverable in a beam geometry large enough
to encompass a sizable body part, would open the door to diagnostic imaging
techniques heretofore unavailable.
The Free Electron Laser
(FEL), it was felt, could be configured to deliver just such a beam, by use of inverse Compton
scattering. The X-rays were created by the counter propagation of the FEL
electron beam and its own infrared beam producing photons with energies from
14-18 keV. After successful demonstration of this technique, as described below,
an even more robust and compact machine was developed, built and tested at the
Vanderbilt MFEL facility.
Initially, sustained/long duration X-ray output was successfully
demonstrated emanating from the monochromatic x-ray beamline of the Vanderbilt
Free Electron Laser in 1998. Tunable, pulsed monochromatic X-rays ranging in energy from
14 - 18 keV were produced by inverse Compton scattering created by the counter
propagation of the FEL e-beam and its own infrared beam. The
electron beam of the FEL was focused to a diameter typically between 100 and 200
microns, at a point designated the interaction zone (IZ), which lies within the
e-beam vacuum line, roughly at the halfway point between the linear accelerator
and the wiggler. Likewise the IR output of the FEL was intercepted in the vault,
where it was simultaneously focused to the same diameter as the electron beam and
reinserted into the electron beam vacuum line. There the beams were counterpropagated
against one another at the IZ point. These beams were brought
into coalignment, first by visualization of the transition radiation produced by
the electron beam hitting a beryllium screen and secondly by focusing HeNe and
diode lasers (that have been pre-aligned through the wiggler cavity) onto an
aluminum screen. The e-beam and IR screens were machined from a single Al plug so
that their surfaces are at 90 degrees to one another and centered to the
electron beam using actuators in the X, Y and Z directions. Both beams were
observed through a common CaF window via a CCD TV camera with a remotely
controlled and adjustable zoom/focus/iris lens. Timing of the arrival of the IR
and e-beam pulses at the IZ point was adjusted using an optical trombone and
settings determined at an earlier time by RF phase Interferometry measurements
using the same IZ screen and each of the beams in turn reflected from the IZ
screen onto a photodiode detector. Beam alignment took approximately three
minutes, following initialization and adjustment of the FEL itself. The X-rays
produced exited the beamline through a beryllium window and were directed onto
mosaic crystals which diverted the beam to an imaging laboratory on the floor
above the vault.
The initial applications of these X-rays were directed toward
human imaging, specifically for the diagnosis of breast diseases including
cancer. Eventual extension to other portions of the body, cell biology and
material sciences are already anticipated. Cancerous breast tissues exhibit
higher linear attenuation characteristics than do normal tissues, when studied
with monochromatic X-rays. This property can be exploited to improve the
sensitivity and specificity of breast imaging in a number of ways. Standard
geometry monochromatic imaging, CT imaging using new X-ray optics made from
mosaic crystals, phase contrast imaging and time-of-flight imaging are just a
few examples of their potential uses. These improvements in imaging are not
restricted to the breast, but apply to any body part and to materials science as
well. The energy of the X-ray beam was not limited to 14-18 keV using this
technique, but rather was restricted in these experiments to that range due to
the large angle at which the beam had to be diverted by the mosaic crystals to
exit the vault. Higher energy beams (up to 40-50 keV) are achievable using
smaller angles of reflection from the mosaics and using light of shorter wavelengths. The
characteristics of the X-rays are such that they can be used in
standard geometry monochromatic imaging, in CT-like images of the breast using a
rotating mosaic crystal "optic", time-of-flight imaging and phase
contrast images.
The monochromaticity and narrow divergence
angle of this X-ray beam not only would allow the mosaic crystals to divert the beam
from the vault to an imaging laboratory above the vault, but also allows the
redirection of the beam in a circular fashion creating CT images using conebeam
backprojection algorithms.
The fact that the X-rays are pulsed in 2
ps bursts permits them to be used for time-of-flight imaging, where data is
collected by imaging only ballistic photons up to 180 ps from the initiation of
the exposure and ignoring scatter exiting over many nanoseconds. This alone can
improve conspicuity by ten times.
The small effective spot size of X-ray
beam enables the performance of phase contrast imaging using information
traditionally discarded in conventional imaging.
All of these techniques can be effected
while reducing radiation dose to a patient and decreasing scatter due to the
tunability of the beam and the limited bandwidth/narrow energy range delivered
to the imaged part.
Further development of small sources is
currently underway in an effort to scale them to the size of a standard X-ray
room in a hospital or clinic. News
acknowledgments
This work was supported by grants from The Office of Naval
Research : ONR-N00014-94-1-1023 and by a grant from the
Eastman Kodak Corporation, Health Sciences Division, Rochester, New York. The
Vanderbilt University Wm. M. Keck Free Electron Laser Center is supported by
Vanderbilt University and by grants from the Office of Naval Research, and the
Wm. M. Keck Foundation.
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Initially, sustained/long duration X-ray output was successfully
demonstrated emanating from the monochromatic x-ray beamline of the Vanderbilt
Free Electron Laser in 1998. Tunable, pulsed monochromatic X-rays ranging in energy from
14 - 18 keV were produced by inverse Compton scattering created by the counter
propagation of the FEL e-beam and its own infrared beam. The
electron beam of the FEL was focused to a diameter typically between 100 and 200
microns, at a point designated the interaction zone (IZ), which lies within the
e-beam vacuum line, roughly at the halfway point between the linear accelerator
and the wiggler. Likewise the IR output of the FEL was intercepted in the vault,
where it was simultaneously focused to the same diameter as the electron beam and
reinserted into the electron beam vacuum line. There the beams were counterpropagated
against one another at the IZ point. These beams were brought
into coalignment, first by visualization of the transition radiation produced by
the electron beam hitting a beryllium screen and secondly by focusing HeNe and
diode lasers (that have been pre-aligned through the wiggler cavity) onto an
aluminum screen. The e-beam and IR screens were machined from a single Al plug so
that their surfaces are at 90 degrees to one another and centered to the
electron beam using actuators in the X, Y and Z directions. Both beams were
observed through a common CaF window via a CCD TV camera with a remotely
controlled and adjustable zoom/focus/iris lens. Timing of the arrival of the IR
and e-beam pulses at the IZ point was adjusted using an optical trombone and
settings determined at an earlier time by RF phase Interferometry measurements
using the same IZ screen and each of the beams in turn reflected from the IZ
screen onto a photodiode detector. Beam alignment took approximately three
minutes, following initialization and adjustment of the FEL itself. The X-rays
produced exited the beamline through a beryllium window and were directed onto
mosaic crystals which diverted the beam to an imaging laboratory on the floor
above the vault.