b) Center for X-Ray Optics, University at Albany SUNY, Albany,. NY. ABSTRACT. The ability of polycapillary optics to reject radiation incident outside the critical ...
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
POLYCAPILLARY OPTICS FOR ANGULAR FILTERING OF X-RAYS AND NEUTRONS IN TWO DIMENSIONS W.M. GibsonaTb, H. HuahgaYb, J. Nicolich”, P. Klein”, and C.A. MacDonaldb a) X-Ray Optical Systems, Inc., Albany, NY, b) Center for X-Ray Optics, University at Albany SUNY, Albany,. NY ABSTRACT The ability of polycapillary optics to reject radiation incident outside the critical angle for total external reflection while efficiently transmitting radiation within the critical angle make them ideal two-dimensional angular filters for X-rays and neutrons. The use of such angular filters is shown to be important for medical imaging to give contrast and resolution improvement, for improved spatial resolution and contrast for radiological in-raging, as two-dimensional collimators for diffraction measurements, for X-ray source size and shape measurements, and for x-ray and neutron angular distribution measurements. 1. INTRODUCTION Since the refractive index of X-rays in glass is slightly less than unity, total reflection occurs when an X-ray is incident on a smooth glass surface at a small incident angle. This is shown schematically in Fig. 1. The critical angle for total external reflection as shown in Fig. 1 is inversely proportional to the x-ray energy and for 30 keV x-rays is about one milliradian (-0.05’). X-rays incident at angles less than the critical angle can then be transmitted through hollow glass capillaries or even deflected if the capillary is curved gently enough that each scattering occurs at less than the critical angle. These properties can be used to produce collimating or focusing optics or to produce angular filters as shown in Fig. 2.
Glass Capillaries 0,
zz
32
Energy [IceV]
mrad
R-500 mm
&!?i
0, =3.8mrad 2
d< 5 pm
Figure 1. Schematic representation of the principles behind the use of capillary fibers.
Figure 2. Some examples of linear polycapillary angular filters.
Figure 3 shows measurements of the transmission of a straight polycapillary fiber as a function of the lateral displacement of the X-ray sourcel. When the displacement is such that the x rays are incident at larger than the critical angle, the transmission drops rapidly to zero. The transmission shown at larger angles for X-ray energies larger than 50 keV is due to penetration of X-rays through the 12.5 cm long borosilicate glass fiber.
313
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
314
Normalized counts
1.0 30 KeV 40 KeV 50 KeV 59 KeV 68 KeV 75 KeV 80 KeV
0.8 0.6 0.4 0.2 0.0
-2
0
-1
1
2
Source Displacement, mm Figure 3. Lateral source scans for a 12.5 cm long borosilicate glass fiber (ref. 1). The energy dependence of the critical angle for total external reflection is also apparent from this figure. The length necessary to completely absorb X-rays incident at angles larger than the critical angle depends on the glass used. For some applications such as mammography, borosilicate glass can be used since the length of the optic is large enough for complete absorption even for relatively high energy X-rays. For other applications it is necessary to use higher density glass with significant content of lead, tungsten or other heavy metals2. This is illustrated in Fig. 4. Borosilicate,14 cm Lead B30, 3 cm Lead B60, 6 cm theory
Lead Glass
0.20
B 3 0: 0 .6
B 6 0:
0.15
0 .5 Transm ission
Figure 4. Large angle transmission through borosilicate and lead glass. B30 and B60 refer the length in mm of the fiber.
High Angle Transmission
Data:
0.10 0.05
d a ta s im d a ta s im
0 .4 0 .3
to
0 .2 0 .1
0.00 20
30
40 50 60 Energy (keV)
70
80
0
10 20 30 40 50 60 70 80 90 E n e rg y (k e V )
2. MEDICAL IMAGING 2.1. Mammography 2.1.1. Contrast One of the most important applications under development for polycapillary angular filters is for contrast and resolution enhancement for breast cancer imaging3. As much as one half of the radiation incident on the detector in a standard mammography system comes from scattered radiation even when a conventional antiscatter grid is used. This scattered radiation reduces the contrast by producing a diffuse background. With a polycapillary angular filter the background due to scattered X-rays is reduced to less that one percent. This is illustrated schematically in Fig. 5 and the effect is shown in Fig. 6.
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
315
POST PATIENT USE OF OPTICS: Contrast No Optic: scattered fog X-ray Source
Patient
Projected Image
CONTRAST Image of Lucite Phantom
Detector
capillary optic entrance plane
With Optic: No fog
Conventional Grid
With Scanned Optic
Figure 5. Schematic of mammography setup.
Figure 6. A Lucite phantom comparison (ref. 3).
The X-ray energy typically used for mammography is 20 keV, chosen to maximize the absorption contrast of the primary beam and to minimize the scattered X-ray background. At higher energy the contrast enhancement is even higher since the scatter fractions increases with x-ray energy2. This is illustrated for a Lucite phantom in Table I. D e p th (cm )
Table I. Contrast improvement for 20 keV and 40 keV X-rays for a 5 cm thick Lucite phantom for different hole depths.
0 .4 0 .5 1 1 .5
C o n tra s t I m p ro v e m e n t 20 keV 40 k eV 1 .7 3 .5 1 .9 3 .2 1 .3 3 .0 1 .6 2 .5
2.1.2 Resolution By using a tapered polycapillary angular filter between the patient and the detector, desired magnification of the image can be obtained without geometric blurring, increasing the effective resolution. This is shown in Fig. 7 and Fig. 8. The modulation transfer function is increased at all spatial frequencies3. 2.2. Imaging Detector for Radioscintigraphy As a collimator in front of a digital X-ray imaging detector, a polycapillary annular filter can be used to produce a compact detector for determining the location and shape of radiolabeled cancers with high resolution and high sensitivity. Such a system is under development for prostate cancer. It is expected that 0.1 mm resolution can be obtained (compared to ~ 3mm resolution for current “gamma camera” radioscintigraphy instruments). The principle of such a detector is shown in Fig. 9 and its use as a segmented or scanned detector to obtain stereoscopic images is shown in Fig. 10. In this application lead glass is used so that the thickness of the angular filter can be as small as a few mm.
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
Grid
316
Scanned optic
Resolution No Optic: geometric blur X-ray Source
Patient
Detector
Projected Image
microcalcifications capillary optic With Optic: No blur entrance plane
Figure 7. Schematic of mammography Figure 8. Image of RMI-156 mammography Setup phantom with grid and with polycapillary optic A Single channel acceptance cone ~ 1.5Ψc ~ 3 mrad ~3 cm
CCD Imaging detector
Parallel Beam
Spatial acceptance area ~0.1mm
Polycapillary collimating/scatter rejection optic
Imbedded scintillator
Figure 9. High-resolution imaging detector.
Figure 10. Scanning or segmentation of detector for stereoscopic imaging.
3. X-RAY DIFFRACTION In X-ray diffraction measurements it is customary to use a soller slit, consisting of a stack of thin metal plates separated by spacers to define the direction of the diffracted beam. The soller slit can be used with a monochromator crystal as shown in Fig.11 or, if an energy dispersive detector is used, be placed directly between the sample and the detector as shown in Fig. 12. It is this configuration that is used in the example given here. In processing steel, hot dipped galvanannealed coatings are frequently used for corrosion resistance, paintability and weldability This is especially important for the automotive industry. The phase composition of the coating is a very important parameter and is directly related to flaking and the press formabillity of the galvanannealed steel sheet. A thick ξ phase remaining on the surface of the coating indicates poor alloying with the substrate which leads to poor drawability and rupture of steel sheets. Measurements on line, as the steel sheets emerge from the annealing furnace, requires high efficiency and sensitivity with a portable, reliable, low power X-ray source. Polycapillary angular filters together with a polycapillary collimating optic between the X-ray source and the sample make such in-line measurements possible. Fig. 13 shows measurements of the diffraction pattern for three steel samples with varying ξ phase
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
317
compositions and Fig. 14 shows an increased sensitivity with polycapillary annular filters compared to a soller slit4. The inset shows the channel diameter of the two annular filters used. Polycapillary annular filters provide two-dimensional angular filtration compared to the onedimensional filtration of a soller slit and also simplify and shorten the alignment procedure. X-Ray Source
Oxford 5011 Cr source Collimating
optic
Coupling system :.
Sample
::
Steel sample
Soiler Slii
Soller slit Amp&k
Detector
detector
8-28 diffraction-meter
Figure 11. Schematic of diffraction setup showing polycapillary collimator and normal soller slit configuration.
Figure 12. Experimental arrangement used for the steel studies. A twodimensional polycapillary angular filter replaced the soller slit.
Figure 13. Diffraction spectra for different steel samples (ref. 4).
Figure 14. 5 phase peak scans with different angular filters (ref 4).
4. MEASUREMENT OF X-RAY SOURCE SIZE AND SHAPE If it is possible to get within a few mm of the source spot of an x-ray source, a convenient and simple way to measure the source spot size and shape is shown in Fig. 15. The image can be made with photographic film examined in a microscope , a digital image plate, or an imaging detector as shown on Fig. 9. This can be done for high energy x rays for which the normal pinhole technique is difficult. 5. SOURCE ANGULAR
DISTRIBUTION
MEASUREMENT
a
Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45.
318
A common way to measure the angular distribution from an x-ray source is to make an image in a normal plane at some distance from the source by use of film, an image plate or an imaging detector, or by scanning a detector with a small aperture in the plane. Such a measurement is usually adequate if the x-ray source spot is small. It is also possible to measure the angular distribution by using an angular filter and scanning it in vertical and horizontal tilt directions as shown in Fig. 16. This is particularly useful if the source is large or inaccessible. .It is difficult for example to measure the angular distribution of neutrons since neutron sources are usually
Source
Source
Optic
Optic
Figure 16. The use of an optic as an angular filter.
@I
Figure 17. Horizontal (a) and vertical (b) angular distributions of neutrons from BT-8 beam line on’NIST research reactor (ref 5). at least several cm2 and often much farther away. A monolithic polycapillary focusing optic was used as an angular filter to measure the angular distribution of cold neutrons from the BT-8 crystal monochromator beam line at the National Institute of Science and Technology (NIST) research reactor5. Horizontal and vertical angular scans for neutrons with h= 3.1 A are shown in Fig.17. The angular distribution was expected to be Gaussian. The observed distribution is believed to be caused by mosaic structure in the Cu diffraction crystal on the monochromator beam line. This is a valuable use of polycapillary annular filters. 6. CONCLUSIONS Polycapillary optics make effective angular filters and can be used for medical imaging for contrast and resolution improvement, scatter rejection for radiological imaging detectors, twodimensional collimation for diffraction measurements, x-ray source size and shape measurements, and x-ray and neutron angular distribution measurements. 7. REFERENCES 1. L. Wang, B.K. Rath, W.M. Gibson, J.C. Kimball, and C.A. MacDonald, “Performance Study of Polycapillary Optic Performance for Hard X-rays,” J. Appl. Phys., 80, 3628-3638 (1996) 2. Cari, Suparmi, W.M. Gibson, and C.A. MacDonald, Contrast Enhancement Mesurementss Usiing Polycapillary X-ray Optics at 20-40 keV”, SPIE Proc., vol. 4320, 163-170 (2001) 3. D.G. Kruger, C.C. Abreu, E.G. Hendee, A. Kocharian, W. W. Peppler, C.A. Mistretta, and C.A. MacDonald, “Imaging Characteristics of X-ray Capillary Optics in Mammography,” Med. Phys., 23, 187196 (1966). 4. H. Huang, C.A. MacDonald, W.M. Gibson, J.X. Ho, J. Chik, A Parsegian, and I Ponomarev, “Focusing Polycapillary Optics for Diffraction”, (these proceedings). 5. W.M. Gibson, H.H. Chen-Mayer, D.F.R. Mildner, H.J. Prask, A.J. Schultz, R. Youngman, T. GnaupelHerold, M.E. Miller, and R. Vitt, “Polycapillary Optics Based Neutron Focusing for Small Sample Neutron Crystallography”, (these proceedings).
