U.S. patent number 3,927,318 [Application Number 05/467,446] was granted by the patent office on 1975-12-16 for cross-sectional fluorescent imaging system.
Invention is credited to Albert Macovski.
United States Patent |
3,927,318 |
Macovski |
December 16, 1975 |
Cross-sectional fluorescent imaging system
Abstract
A narrow x-ray beam is scanned across a cross section containing
a relatively high atomic number material. When excited by the x-ray
beam the material fluoresces and emits its characteristic radiation
lines. The emission information along the entire line is collected
to create a signal indicative of the line integral of the emission
information. This line integral information, taken from many
positions and angles in the cross section, is applied to a computer
which reconstructs a cross-sectional image of the fluorescing
material. The transmitted narrow beam, which represents the line
integral of the density information, can simultaneously be used to
create a reconstruction of the cross-sectional density pattern.
Inventors: |
Macovski; Albert (Palo Alto,
CA) |
Family
ID: |
23855734 |
Appl.
No.: |
05/467,446 |
Filed: |
May 6, 1974 |
Current U.S.
Class: |
378/6; 378/5;
378/44; 378/98.5 |
Current CPC
Class: |
A61B
6/483 (20130101); A61B 6/485 (20130101); G01N
23/223 (20130101); G01N 2223/076 (20130101) |
Current International
Class: |
A61B
6/02 (20060101); G01N 23/223 (20060101); G01N
23/22 (20060101); G01N 023/20 () |
Field of
Search: |
;250/272,273,274,275,445T |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Fluorescent Thyroid Scanning Without Radioisotopes," Hoffer, et
al., Radiology, Vol. 99, Apr., 1971, p. 117..
|
Primary Examiner: Church; Craig E.
Claims
What is claimed is:
1. Apparatus for imaging fluorescent radiation from a specific
material in a cross section of an object comprising:
an x-ray beam directed through the cross section of the object;
means for translating the x-ray beam so that all regions of the
cross section are excited at a plurality of angles;
means for detecting the integrated fluorescent radiation emitted
along the entire x-ray beam at each position of the x-ray beam and
forming a plurality of integrated emission signals;
a computer for reconstructing the fluorescent radiation information
of the cross section of the object from the plurality of integrated
emission signals; and
means for displaying the reconstructed fluorescent radiation
information.
2. Apparatus as recited in claim 1 including means for correcting
the plurality of integrated emission signals for the attenuation of
the fluorescent radiation in the object before reaching the
detector means.
3. Apparatus as recited in claim 2 wherein the means for correcting
for attenuation includes an energy spectrum analyzer for separately
measuring the K.sub..alpha. and K.sub..beta. components of the
fluorescent radiation and means for comparing their amplitudes to
their initial emitted relative amplitudes to compute the required
correction.
4. Apparatus as recited in claim 2 wherein the means for correcting
for attenuation includes a pre-programmed compensator based on an
object having an assumed size and absorption where the
pre-programmed compensator is varied according to the distance of
the x-ray beam to the edge of the object.
5. Apparatus as recited in claim 1 wherein the means for detecting
the fluorescent radiation includes a plurality of gamma-ray
detectors whereby a large percentage of the radiation is
received.
6. Apparatus as recited in claim 1 wherein the means for detecting
the fluorescent radiation includes an energy spectrum analyzer for
isolating the energy level of the fluorescent radiation from the
specific material.
7. Apparatus as recited in claim 6 wherein the spectrum analyzer is
a pulse-height discriminator.
8. Apparatus as recited in claim 1 including:
means for detecting the x-ray beam after transmission through the
object at each position of the x-ray beam to form a plurality of
transmission signals;
means for computing the density information of the cross section of
the object from the plurality of transmission signals; and
means for displaying the reconstructed density information.
9. Apparatus as recited in claim 8 wherein the means for displaying
the reconstructed density information includes a composite display
of the density and fluorescent radiation information.
10. Apparatus as recited in claim 9 wherein the composite display
is a color display having the density and fluorescent radiation
information in different colors.
11. Apparatus as recited in claim 1 including means for correcting
the fluorescent radiation information for the attenuation of the
x-ray beam in the object.
12. Apparatus as recited in claim 11 wherein the means for
correcting for the attenuation of the x-ray beam includes a
pre-programmed compensator based on an object having an assumed
absorption where the attenuation at each position of the x-ray beam
to each region of the cross section is calculated.
13. Apparatus as recited in claim 11 wherein the means for
correcting for the attenuation of the x-ray beam comprises:
means for detecting the x-ray beam after transmission through the
object to form a plurality of transmission signals;
means for computing the attenuation of the x-ray beam to each
region of the cross section from the plurality of transmission
signals; and
means for correcting the fluorescent radiation information at each
region of the cross section using the computed attenuation of the
x-ray beam to that region.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to systems for selectively imaging trace
amounts of specific materials. In a primary application the
invention relates to systems for imaging the fluorescent radiation
from materials which are excited by x-rays.
2. Description of Prior Art
Existing systems for imaging fluorescent radiation from excited
materials usually include a collimated x-ray source for producing
an x-ray beam. A selective detector is used having a pulse-height
analyzer which is tuned to the energy of the particular material
under study. This detector includes a collimator or focusing
structure for examining each region along the x-ray beam. The
detector and its collimator are normally scanned along the x-ray
beam so as to sequentially measure the emission from each region
along the beam. This is an inefficient process since, while
emissions are occurring along the entire beam, only one portion at
a time is being collected. In addition, the focusing collimators
used to collect the radiation from a specific region are relatively
inefficient. A system of this type is described by P. B. Hoffer and
A. Gottschalk in a paper entitled, "Fluorescent Thyroid Scanning
Without Radioisotopes" appearing in Radiology, Vol. 99, p. 117,
April, 1971.
The imaging of fluorescent radiation is used in medicine for making
images of the natural iodine in the thyroid or for imaging the
selective uptake of administered materials in diseased areas such
as tumors. Another important system for the diagnosis of diseased
regions is computerized tomography which provides an accurate
cross-sectional density image. In this system accurate x-ray
projections of a particular cross section are taken at many angles.
This projection information, which is a number of line integrals of
the density information, is applied to a computer which
reconstructs the desired density image. A system of this type is
presently manufactured by EMI in England and is described by J.
Ambrose in the British Journal of Radiology, Vol. 46, p. 1016,
1973. Although this instrument provides an accurate cross-sectional
density image, many disease processes do not result in a
significant density change and are thus better diagnosed by
detecting the selective uptake of a material into the diseased
region.
SUMMARY OF THE INVENTION
An object of this invention is to provide an efficient system for
imaging specific materials by their fluorescent radiation.
It is also an object of this invention to provide an efficient
system for imaging fluorescent emissions by simultaneously
collecting the integrated emission information along the entire
x-ray beam.
It is a further object of this invention to provide a combined
display indicating the density information and the amounts of a
specific material.
Briefly, in accordance with this invention, one or more unfocused
detectors are used to collect the fluorescent radiation emitted
along the entire x-ray beam. This information is collected for
large numbers of positions and angles of the beam. This
line-integral fluorescent information is applied to a computer to
reconstruct a cross-sectional image of the fluorescing regions. The
transmitted x-ray beam can also be collected and used to provide a
cross-sectional density image of the same area.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete disclosure of the invention, reference may be
made to the following detailed description of an illustrative
embodiment thereof which is given in conjunction with the
accompanying drawing, of which FIG. 1 is a schematic diagram
illustrating an embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An understanding of the broad aspects of the invention may best be
had by reference to FIG. 1 of the drawings. A collimated x-ray
source 10 can be derived using a conventional x-ray tube or an
isotope source. These divergent sources are collimated using one or
more metal absorbers having appropriate apertures to form a
collimated beam 11. This beam is projected through the object under
study 12 which would normally be the human anatomy. The object is
known to contain amounts of a relatively high atomic number
material which will fluoresce when excited by the x-ray beam 11 and
emit its characteristic radiation. The material should have a
relatively high atomic number so that the fluorescent emission will
be sufficiently high energy to escape the object without excessive
attenuation. The materials used are similar to those employed in
nuclear medicine, except non-radioactive, which are selectively
taken up in diseased areas such as tumors. Structures 13 in the
object 12 represent regions which have selectively taken up the
material and will thus fluoresce at their characteristic radiation
energy when excited.
In present fluorescent imaging systems, a focused collimator would
be used to collect the radiation from a specific region along beam
11. This is an inefficient process since only a small portion of
the total fluorescent radiation is detected at any one time. This
inefficiency results in excessive radiation to the patient and
insufficient photons to provide a good image. In the system
described here, the radiation is collected along the entire x-ray
beam. No focusing structures are required. One or more gamma-ray
detectors, such as 14 and 15, can be used to capture photons
emitted along the length of the x-ray beam and create signal 21
representing the integrated emission along the beam. Ordinarily, an
image could not be formed in this manner since the integrated
emission along the entire line is being measured rather than the
emission from a small region. An image can be formed, however, by
providing these integrated measurements at a variety of angles.
Thus the x-ray source 10 is linearly scanned, as shown by the
dotted line, over an entire cross section of object 10. This same
scan is repeated at a plurality of angles with the integrated
fluorescent emission detected at every position for every angle.
Depending on the configuration of gamma-ray detectors, 14 and 15,
they can also be moved to aid in the photon collection. Thus, for
each region in a cross section of object 10, a number of integrated
emission measurements are made for lines which go through that
region at different angles.
The general mathematical problem of reconstructing a
cross-sectional image from line-integral information taken at many
angles has been studied for many years in connection with many
applications. A comprehensive bibliography on this subject is
available from Richard Gordon of the National Institutes of Health,
Building 31, Bethesda, Maryland. An example of some of the
procedures which can be used is given in a paper by G. T. Herman
in, "Two Direct Methods for Reconstructing Pictures from Their
Projections: A Comparative Study," in Computer Graphics and Image
Processing, Vol. 1, p. 123-144, 1972. A general computer technique
known as ART (Algebraic Reconstruction Technique) has received wide
usage in this field. The most recent use of reconstructions of this
type is in a recently released brain-scanning instrument known as
the EMI-Scanner. In this instrument the x-ray transmission through
a cross section of the brain at a number of angles are recorded and
applied to a computer. The computer reconstructs the
cross-sectional density image, with great accuracy, using the ART
technique. This system is described in a paper by J. Ambrose in the
British Journal of Radiology, Vol. 46, 1973. Any one of the many
reconstruction algorithms can be applied to signal 21 to create a
reconstructed cross-sectional image of fluorescent radiation.
Leaving optional boxes 19 and 20 for subsequent discussion, the
signal 21 can be applied directly to reconstruction computer 16
where the cross section is reconstructed from the integrated
measurements by one of many known systems. The resultant
fluorescent radiation information 23, indicating the presence of
the materials 13, is displayed on display 17.
In the system described thusfar the x-ray beam transmitted through
the object is not measured or used in any way since only the
resultant fluorescent radiation was detected. This transmitted beam
can be detected, however, and used to form a cross-sectional
density image as does the EMI-scanner. This density information
would be very valuable in medical diagnosis for accurately defining
the anatomy so that the relative positions of the diseased areas
which take up the administered material can be well-defined. To
create this cross-sectional density display, detector 18 is used to
measure the transmitted beam at every position and angle of x-ray
beam 10 to form transmission signal 22. The detector 18 can either
be mechanically scanned in synchronism with the source 10 or an
array of detectors can be used to collect the transmitted beam at
all of its positions. Transmission signal 22 is applied to the same
reconstruction computer 16 to reconstruct the density image signal
24 in one of the many known ways. The reconstructed density image
signal 24 can be displayed in display 17 either separately or in
conjunction with the fluorescent radiation signal 23. A number of
combinational display configurations can be used including having
the fluorescent radiation information displayed as a color overlay
on the black and white density information. In this way the
diseased areas, which have selectively taken up the administered
material, will become very apparent and be readily localized.
The detectors 14 and 15 which measure the fluorescent radiation can
be one of a variety of gamma-ray detectors including scintillating
crystals, proportional counters or solid state detectors. For
improved discrimination they can employ energy spectrum analyzers
for extracting the energy region corresponding to the
characteristic fluorescent emission energy of the material being
studied. The energy spectrum analyzer can be a pulse-height
discriminator where the size of the detected pulse associated with
each received photon indicates its energy. The detector 18 which
measures the transmitted x-ray beam can also be any of the
gamma-ray detectors previously listed.
The system has two potential sources of error; the attenuation of
the fluorescent radiation through the object 12, and the
attenuation of the x-ray beam 10 as it transverses the object. The
fluorescent attenuation is determined by the energy of the
radiation and the absorption coefficient of the various materials
in the object. If a very large number of fluorescent radiation
detectors, such as 13 and 14, are used the effect of this
attenuation will be minimized since there will usually be some path
or combination of paths of relatively low attenuation. A
compensation system 19 can be used to minimize the effect of the
absorption. The object can be assumed to have a specific absorption
coefficient, for example that of soft tissue or water. The specific
compensation would then be based on the geometry of the scanning
and detecting configuration. For example, as shown in FIG. 1, the
distance from the beam to the top edge of object 12 is y while the
distance to the bottom edge is d-y. Thus the transmission of the
radiation to detector 14 will be e.sup..sup.-.sup..mu.y while that
to detector 15 will be e.sup. .sup.-.sup..mu.(d-y) where .mu. is
the linear absorption coefficient of the assumed material. Thus the
effect of the attenuation can be minimized if compensation system
19 has a gain function [e.sup..sup.-.sup..mu.y +
e.sup..sup.-.sup..mu.(d-y) ].sup..sup.-1. The size of the object d
can be initially provided to the system while the distance y is
made available by the mechanical scanning system. The variable
compensating gain function can be accomplished by a small analog or
digital circuit arrangement.
A more exact compensation for fluorescent attenuation can be
achieved through the separation of the various components of the
fluorescent radiation. For example, excited materials produce
K.sub..alpha. and K.sub..beta. emissions at specific energies
having specific relative amplitudes when emitted. Compensation
system 19 can include a pulse-height discriminator which separates
and measures the K.sub..alpha. and K.sub..beta. components. The
measured intensity at each energy region, I.sub..alpha. and
I.sub..beta., are given by
where .mu..sub..alpha. and .mu..sub..beta. are the absorption
coefficients of the object at the two energies, Z is the effective
path length, I is the desired initial fluorescent output at the
K.sub..alpha. energy and C is known initial ratio of the
K.sub..beta. emission energy to that of the K.sub..alpha. emission.
Eliminating Z in the two equations and solving for I we obtain
##EQU1## which is an expression for the desired emitted intensity
independent of the path length. Thus compensator 19 can be
pre-programmed with the known values of .mu..sub..alpha.,
.mu..sub..beta., and C to calculate the desired I signal from the
measured values of I.sub..alpha. and I.sub..beta..
The second source of error is the attenuation that x-ray beam 11
receives before exciting the fluorescent structures 13. One method
of correcting this error is to again assume that the object
consists of a given material such as soft tissue which is
equivalent to water. The attenuation of each beam after
transversing a distance S is thus e.sup..sup.-.sup..mu.S where .mu.
is the assumed absorption coefficient of the object. Thus the
calculated fluorescent emission can be corrected at each region by
correcting for the beam attenuation to that region. Thus
compensator 20 will modify the value of each region by dividing the
calculated value by ##EQU2## where n is the number of beams going
through each region in the cross section and S.sub.n is the path
length through the object to the particular region. This summation
is calculated for every region and used to compensate the
reconstructed fluorescent emission signal to obtain corrected
signal 23 which is applied to display 17.
A more accurate correction for the x-ray beam attenuation can be
obtained by using the actual attenuation or density values in the
cross section rather than assumed values. The actual density values
have been calculated in the reconstruction computer 16 and appear
in signal 24. The attenuation of each beam to a given region is
given by ##EQU3## where g(S.sub.1) is the attenuation to the region
at S.sub.1 and f(S) is the calculated density values along the
path. Thus the total attenuation to point S.sub.1 is given by
##EQU4## where g.sub.n (S.sub.1) represents the attenuation of each
of the n beams which intersect point S.sub.1. The value of ##EQU5##
is calculated for every region i in the cross section and becomes
correction signal 25. Compensator 20 divides each calculated
fluorescent emission signal by the correction signal 25 to obtain
the corrected fluorescent emission signal 23 which is applied to
display 17.
While particular embodiments of the invention have been shown and
described, it will of course be understood that the invention is
not limited thereto since many modifications in the x-ray scanning
arrangements and electronic processing can be made. It is
contemplated that the appended claims will cover any such
modifications as fall within the true spirit and scope of the
invention.
* * * * *