Back to EveryPatent.com
United States Patent |
5,299,248
|
Pelc
|
March 29, 1994
|
Reduced field-of-view system for imaging compact embedded structures
Abstract
A CT apparatus for scanning compact structures associated with a larger
body uses radiation source producing a reduced field-of-view to simplify
construction and reduce exposure of the larger body. Truncation artifacts
in the reconstructed image caused by volume elements in the larger body
imaged by the radiation beam only for projections at some angles, are
reduced by acquiring two projections at two different energies and
combining those projections to compensate for the attenuation of the
radiation by the volume elements of the larger body.
Inventors:
|
Pelc; Norbert J. (Los Altos, CA)
|
Assignee:
|
Lunar Corporation (Madison, WI)
|
Appl. No.:
|
052228 |
Filed:
|
April 22, 1993 |
Intern'l Class: |
H05G 001/60 |
Field of Search: |
378/5
|
References Cited
U.S. Patent Documents
3848130 | Nov., 1974 | Macovski | 250/369.
|
3965358 | Jun., 1976 | Macovski | 250/369.
|
4029963 | Jun., 1977 | Alvarez et al. | 250/360.
|
4506327 | Mar., 1985 | Tam | 378/5.
|
4550371 | Oct., 1985 | Glover et al. | 364/414.
|
Other References
Generalized Image Combinations In Dual KVP Digital Radiography, A. L.
Lehmann et al., Med. Phys. 8(5) Sep./Oct. 1981.
|
Primary Examiner: Church; Craig E.
Attorney, Agent or Firm: Quarles & Brady
Parent Case Text
This application is a continuation of application Ser. No. 07/860,818,
filed Mar. 31, 1992.
Claims
We claim:
1. A method for generating a tomographic image of an imaged object having a
compact structure of a first material contained within a larger body of a
second material, the first and second materials having different energy
dependent attenuations, comprising the steps of:
(a) collimating the width of a radiation beam to substantially equal the
width of the compact structure;
(b) projecting radiation at a first and second energy level through the
compact structure;
(c) acquiring a first and second tomographic projection set of attenuations
of the x-rays transmitted through the compact structure and intervening
portions of the body at the first and second energy at a plurality of
angles about the imaged object;
(d) combining the first and second tomographic projection sets to produce a
third tomographic projection set dependent substantially on the
attenuation only of the compact structure; and
(e) reconstructing an image of the compact structure from the third
tomographic projection set.
Description
FIELD OF THE INVENTION
The present invention relates to the field of radiographic analysis of the
human body and, in particular, to a method of measuring and displaying
tomographic views of compact structures embedded in the human body.
BACKGROUND OF THE INVENTION
In a computed tomography system ("CT system"), an x-ray source is
collimated to form a fan beam with a defined fan beam angle and fan beam
width. The fan beam is oriented to lie within the x-y plane of a Cartesian
coordinate system, termed the "imaging plane", and to be transmitted
through an imaged object to an x-ray detector array oriented within the
imaging plane.
The detector array is comprised of detector elements which each measure the
intensity of transmitted radiation along a ray projected from the x-ray
source to that particular detector element. The detector elements can be
organized along an arc each to intercept x-rays from the x-ray source
along a different ray of the fan beam.
The intensity of the transmitted radiation received by each detector
element in the detector array is dependent on the attenuation of the x-ray
beam along a ray by the imaged object. Each detector element .alpha.
produces an intensity signal I.sub..alpha. dependent on the intensity of
transmitted radiation striking that detector element .alpha..
The x-ray source and detector array may be rotated on a gantry within the
imaging plane so that the fan beam intercepts the imaged object at
different angles. At each angle, a projection is acquired comprised of the
intensity signals from each of the detector elements .alpha.. The
projections at each of these different angles together form a tomographic
projection set.
The acquired tomographic projection set is typically stored in numerical
form for computer processing to "reconstruct" a slice image according
reconstruction algorithms known in the art. The reconstructed slice images
may be displayed on a conventional CRT tube or may be converted to a film
record by means of a computer controlled camera.
The volume subtended by the fan beam, as intercepted by the detector
elements during rotation of the gantry, defines the field-of-view of the
CT system.
The amount of data required to reconstruct a CT image is a function of the
CT system's field-of-view, the larger the field-of-view, the more data
that must be collected and processed by the CT system and thus the longer
the time required before an image can be reconstructed. The acquisition of
additional data in each projection also increases the cost and number of
the components of the CT system.
Therefore, for imaging compact structures within the body, it would be
desirable to limit the field-of-view to an angle commensurate with the
cross-sectional area of that compact structure. Such a reduction in
field-of-view, accompanied by a reduction in the size of the fan beam,
would reduce the total dose of x-rays received by the patient. In a CT
machine constructed for only imaging compact structures, a reduced
field-of-view would reduce the cost of the machine and provide increased
image reconstruction speed as a result of the reduced amount of data
required to be processed. Also, as is known in the art, smaller field of
view images may be reconstructed faithfully using fewer projection angles,
thereby further reducing the reconstruction times. The reduced cost of
such a machine would result primarily from the reduced number of detectors
and associated data handling circuitry required, and from the less
powerful image reconstruction processor required to handle the amount of
reduced data. Cost savings from a resulting simplified mechanical
construction might also be achieved.
Unfortunately, for a CT system to accurately reconstruct images of a
compact structure within an attenuating body, it is ordinarily necessary
that the entire body containing the compact structure be within the CT
system's field-of-view. Even when the only structure of interest is
centrally located and its attenuation properties are very different than
those of the rest of the section, such as the spine within an abdominal
section, conventional CT methods require that substantially the entire
object be within the field of view. If the body containing the compact
structure extends beyond the field-of-view of the CT system, then
projections at some gantry angles will include attenuation effects by
volume elements of the body not present in projections at other gantry
angles. For the present discussion, these volume elements present in only
some projections are termed "external volumes".
In the reconstruction process, the attenuation caused by external volumes
is erroneously assigned to other volume elements in the reconstructed
image. This erroneous assignment produces artifacts, manifested as shading
or cupping, and sometimes as streaks, in the reconstructed tomographic
image and are termed "truncation artifacts".
Selective material imaging by use of x-ray transmission measurements at
multiple energies is known. However, when used in a CT mode, prior methods
acquired data for the entire object.
SUMMARY OF THE INVENTION
The present invention provides a method for reducing the effect of external
volumes on the reconstructed image and thus allowing the construction of a
reduced field-of-view CT machine in cases where the goal is to form an
image of a compact structure whose attenuation properties differ from
those of the rest of the section. The different energy dependence of the
attenuation of the compact structure and the body is exploited to produce
a projection set reflecting only the compact structure. This projection
set is created from a combination of two projections sets taken at
different x-ray energies.
Specifically, radiation having first and second energies is projected
though the compact structure and portions of the body over the
field-of-view and a first and second projection set at the first and
second energies is acquired. The first and second projections sets are
then combined to produce a third projection set dependent substantially on
only the attenuation of the compact structure. This third projection set
is reconstructed into a image of the compact structure.
The present invention relies on the realization that external volumes do
not contribute to the values in this third projection set, and therefore
do not detract from the accuracy of the final image.
It is thus one object of the invention to reduce the truncation artifacts
affecting a reduced field-of-view CT machine in applications where the
compact structure to be imaged is embedded in or attached to a second
structure outside of the field-of-view of the CT machine, and has
differential attenuation properties.
It is another object of the invention to reduce the size of the radiation
beam of a conventional CT machine to match the size of a compact structure
of interest, thus reducing total patient exposure, without creating
unacceptable truncation artifacts.
Other objects and advantages besides those discussed above shall be
apparent to those experienced in the art from the description of a
preferred embodiment of the invention which follows. In the description,
reference is made to the accompanying drawings, which form a part hereof,
and which illustrate one example of the invention. Such example, however,
is not exhaustive of the various alternative forms of the invention, and
therefore reference is made to the claims which follow the description for
determining the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view in elevation of the gantry of a reduced
field-of-view CT machine showing "external volumes" within a body
surrounding a contrasting compact structure of interest, said external
volumes not within the field-of-view of the CT machine but nevertheless
attenuating the radiation beam at some gantry angles;
FIG. 2 is a block diagram of a first embodiment of the reduced
field-of-view CT system of FIG. 1 useful for practicing the present
invention;
FIG. 3 is a block diagram of a second embodiment of a reduced field-of-view
CT system of FIG. 1 useful for practicing the present invention; and
FIG. 4 is a block diagram of a third embodiment of a reduced field-of-view
CT system of FIG. 1 useful for practicing the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
I. Selective Imaging with Two Energies
Referring to FIG. 1, a radiation source 10 is mounted on the rim of a
generally circular gantry 12 to generate a diametrically oriented fan beam
14 of radiation with a narrow fan angle .phi.. The gantry 12 is operable
to rotate through angles .theta. about a center of the gantry 16 within an
image plane 18 with the fan beam 14 parallel to the image plane. A patient
20 is positioned at the center of the gantry 16 so that a compact
structure of interest 22, such as the spine, is within the field-of-view
24 defined by the volume irradiated by the fan beam 14 at all of a
plurality of gantry angles .theta..
The fan beam 14 is received by a detector array 26 having a plurality of
detector elements 28 positioned on the gantry 12 opposite to the radiation
source 10 with respect to patient 20 and the gantry center 16. Each
detector element 28, distinguished by index .alpha., measures the
intensity I.sub..alpha. of the fan beam 14 attenuated by the patient 20
along a ray 30 of the fan beam 14 at angle .phi..alpha. extending from the
radiation source 10 to the center of that detector element 28. The
collection of intensity measurements I.sub..alpha., for all detector
elements 28 at a gantry angle .theta. forms a projection and the
collection of projections for all gantry angles .theta. forms a projection
set.
The fan angle .phi. is such as to subtend the compact structure 22 at the
plurality of gantry angles .theta. but is less than that required to
subtend the entire cross section of the patient 20 in the image plane 18.
This limited extent of the fan beam 14 significantly reduces the
complexity and expense of the detector array 26 and the succeeding
processing electronics (not shown in FIG. 1). The limited fan angle .phi.
of the fan beam 14 also causes certain volumes elements 32 ("external
volumes") of the patient 20 to contribute to a projection obtained at a
first gantry angle .theta.=.theta..sub.1 but not to contribute to a
projection at a second gantry angle .theta.=.theta..sub.2. As mentioned,
these external volumes 32 that are present in only some of the projections
of a tomographic projection set create artifacts in the reconstructed
image. Generally, all volumes outside of the field-of view 24 are external
volumes 32.
The acquisition of two projections at two different energies of radiation
from radiation source 10 can be used to eliminate the contribution of
these external volumes 32 to the projections, provided that the
characteristic attenuation function of the material of the external volume
32 are suitably different from those of the material of the compact
structure 22.
Monoenergetic Imaging
If two projections are obtained representing the attenuation of the fan
beam 14 along rays 30 by the patient 20 for two radiations energies, these
projections may be used to distinguish between the attenuation caused by
each of two different basis materials: one material of the external
volumes 32 and one material of the compact structure 22. Thus the
attenuation of the material of the external volumes 32 and of the compact
structure 22 may be determined and the effect of the former eliminated
from the projections. The distinction between radiation energy or
frequency, and intensity or flux is noted.
The intensity measurement I.sub..alpha.1 along a ray .alpha. of a first
high energy of fan beam 14 radiation will be:
I.sub..alpha.1 =I.sub.01.spsb.e -(.mu..sub.e1 M.sub.e +.mu..sub.cl
M.sub.c)(1)
where I.sub.01 is the intensity of the fan beam 14 of radiation absent the
intervening patient 20; .mu..sub.e1 and .mu..sub.c1 are the known values
of the mass attenuation coefficient (cm.sup.2 /gm) of the material of
external volume 32 and of compact structure 22 respectively at this first
radiation energy; and M.sub.e and M.sub.c are the integrated mass
(gm/cm.sup.2) of external volume 32 and of compact structure 22
respectively.
This equation may be simplified as follows:
##EQU1##
The values of M.sub.e1 and M.sub.c1 of equation (1) are dependent on the
energy of the radiation of the fan beam 14 and on the chemical
compositions of the materials 32 and 22.
As is well known in the art, the values of .mu..sub.e1 and .mu..sub.c1 may
be measured, or computed, given the chemical composition of the materials.
A second intensity measurement I.sub..alpha.2 along the same ray 30, at a
second radiation energy, will be given by the following expression:
##EQU2##
where .mu..sub.e3 and .mu..sub.c2 are different from .mu..sub.e1 and
.mu..sub.c1, by virtue of the different photon energy, and I.sub.o2 is the
incident intensity. Again, and M.sub.e2 and M.sub.c2 may be measured or
computed.
Equations 2 and 3 are two independent equations with two unknowns, M.sub.e
and M.sub.c, and may be solved simultaneously to provide values for
M.sub.e and M.sub.c. For example,
##EQU3##
This, in turn, results from the different energies of the two beams and
from the different chemical compositions of the two materials
(fundamentally, different relative contributions of photoelectric
absorption and Compton scattering for the two materials).
With knowledge of M.sub.e and M.sub.c, the contribution of the external
volume 32 may be eliminated by substituting for the intensity measurement
I.sub..alpha.1 the value I.sub.01.sbsp.e -.mu..sub.c1 M.sub.c, or more
simply, by using the calculated value M.sub.c directly in the
reconstruction algorithms as is understood in the art. The creation and
measurement of two monoenergetic radiation beams will be described further
below.
Polyenergetic Imaging
Faster imaging requires a stronger radiation sources 10, which also often
entails an increase in the width of the energy spectrum of the radiation
source 10 at each energy E. For such broadband radiation, equations (2)
and (3) above, become more complex requiring an integration over the
spectrum of the radiation source 10 as follows:
I.sub..alpha. .intg.I.sub.0 (E).sub.e -{M.sub.e .mu..sub.e (E)+M.sub.c
.mu..sub.c (E)}.sub.dE (5)
Such equations do not reduce to a linear function of M.sub.e and M.sub.c
after the logarithm, and hence more complex non-linear techniques must be
adopted to evaluate M.sub.e and M.sub.c.
One such technique, termed the closed form fit approximates the value of
M.sub.c as a polynomial function of the log measurements along ray .alpha.
at a high and low energy, for example:
M.sub.c =k.sub.1 L.sub.1 +k.sub.2 L.sub.2 +k.sub.3 L.sub.1.sup.2 +k.sub.4
L.sub.2.sup.2 +k.sub.5 L.sub.1 L.sub.2 (6)
M.sub.e can similarly be computed.
It will be recognized that polynomials of different orders may be adopted
instead. The coefficients of the polynomial, k.sub.1 through k.sub.5, are
determined empirically by measuring a number of different, calibrated,
superimposed thicknesses of the two materials to be imaged. Alternatively,
it is known that the total measured polyenergetic attenuation can be
treated as if the attenuation had been caused by two dissimilar "basis"
materials. Aluminum and Lucite.TM. have been used as basis materials. The
computed basis material composition is then used to compute M.sub.e and
M.sub.c. The advantage of this approach is that it is easier to build
calibration objects from aluminum and Lucite.TM. than, for example, bone
and soft tissue. The decomposition of an arbitrary material into two basis
materials and further details on selective material imaging are described
in the article "Generalized Image Combinations in Dual KVP Digital
Radiography", by Lehmann et al. Med. Phys. 8(5), Sept/Oct 1981.
The determination of the coefficients of equation (6) is performed with a
radiation source having the same spectral envelope as the radiation source
10 used with the CT apparatus. The coefficients are determined using a
Least Squares fit to the empirical measurements developed with the known
thicknesses of the models.
As indicated by the above discussion, the ability to distinguish between
two materials 32 and 22, and thus the ability to discount the effect of
one such material (32) requires a differential relative attenuation by the
materials caused by photoelectric and the Compton effects. This
requirement will be met by materials having substantially different
average atomic numbers and is enhanced by increased difference in the two
energies.
It is possible that the external volumes 32 of the patient 20 will include
more than one type of material. An examination of the equations (3) and
(4), however, reveals that the above described method will not
unambiguously identify the thicknesses of a material in the presence of
more than two material types within the patient 20. As a result, the above
described method works best when the material of the compact structure 22
and the materials of the external volumes 32 have sufficiently different
attenuation functions so that the variations among tissue types of the
external volumes 32 are small by comparison. Examples are where the
compact structure 22 is bone and the external volumes 32 are muscle, water
or fat; or where the compact structure contains iodinated contrast agent
and the external volumes 32 do not. These limitations are fundamental to
dual energy selective material imaging and are not unique to the present
use. In any case, errors resulting from the simplifying assumption of
their being only two materials in body 20, one for the compact structure
22 and one for the external volumes 32 are low enough to permit the above
method to be used for the intended reduction of image artifacts.
II. Dual Energy Reduced Field-of-View CT Apparatus
Referring now to FIGS. 1 and 2, in a first embodiment CT gantry 12 holds a
radioisotope 34 which produces the fan beam of radiation 14 directed
toward the patient 20. The radioisotope 34 is preferably a radioactive
isotope such as GD.sub.153, which when filtered by filter 36 prior to the
fan beam 14 intercepting the patient 20, produces a fan beam 14 composed
of radiation in one of two distinct and essentially monoenergetic bands.
After passing through the patient 20, this radiation is received by a
detector array 26(a) comprised of a number of detector elements 28 which
together receive and detect radiation along each ray 30 of the fan beam 14
to produce separate signals I.sub..alpha.1 and I.sub..alpha.2 for each
detector element .alpha. and for each energy of radiation.
The detector 26(a) is a scintillating crystal type detector, coupled to a
photomultiplier tube, or alternatively a proportional counter using xenon
or other high atomic weight gas such as is well understood in the art.
With either such detector 26(a), the energy level of the received
radiation of the fan beam 14 is measured by a pulse height analyzer 38
which measures the energy deposited by each quantum of radiation, either
pulses of light detected by the photodetector in the crystal-type detector
26(a) or pulses of charge produced by the proportional counter 26(a). The
pulse height analyzer 38 characterizes each pulse, by its height, as
either high or low energy. The counts of high and low energy pulses for a
fixed period of time become the measures I.sub..alpha.1 and I.sub..alpha.2
respectively. The data for each detector element 28(a) is processed by
selective material computation circuit 40 which performs the calculations
described above (e.g. equation 4), to produce a projection set containing
attenuation information for the compact structure 22 only.
The control system of a CT imaging system suitable for use with the present
invention has gantry motor controller 42 which controls the rotational
speed and position of the gantry 12 and provides information to computer
44 regarding gantry position, and image reconstructor 46 which receives
corrected attenuation data from the selective material computation circuit
40 and performs high speed image reconstruction according to methods known
in the art. Image reconstructor 40 is typically an array processor in a
large field-of-view CT machine, however in the present invention, with a
reduced field-of-view, the image reconstruction may be performed
acceptably by routines running in a general purpose computer.
Electric communication between the rotating gantry 12 and the selective
material computations circuit 40 is via retractable cabling (not shown)
which is paid out for a limited number of gantry rotations and then
returned to a take up spools for the same number of gantry rotations in
the other direction.
The patient 20 rests on a table 48 which is radiotranslucent so as to
minimize interference with the imaging process. Table 48 is controlled so
that its upper surface translates across the image plane 18 and may be
raised and lowered to position the compact structure 32 within the
field-of-view 24 of the fan beam 14. The speed and position of table 14
with respect to the image plane 18 and field-of-view 24, is communicated
to and controlled by computer 44 by means of table motor controller 50.
The computer 44 receives commands and scanning parameters via operator
console 52 which is generally a CRT display and keyboard which allows the
user to enter parameters for the scan and to display the reconstructed
image and other information from the computer 44.
A mass storage device 54 provides a means for storing operating programs
for the CT imaging system, as well as image data for future reference by
the user.
Typically projection data will be acquired over 360.degree. of gantry
rotation each projection including information on the attenuation of the
radiation source for radiation at both of the radiation energies. As is
known in the art, however, images may be reconstructed from projection
data acquired over less than 360.degree. of gantry rotation provided at
least a minimum gantry rotation of 180.degree. plus the fan beam angle is
obtained. Image reconstruction using less than 360.degree. of projection
data can further reduce the data required to be processed by the image
reconstructor 46. The weighting and reconstruction of images from a half
scan data set are discussed in detail in "Optimal Short Scan Convolution
Reconstruction for Fanbeam CT", Dennis L. Parker, Medical Physics 9(2)
March/April 1982.
Referring to FIGS. 1 and 3, in a second embodiment of the invention, an
x-ray tube 56 is held on gantry 12 as the radiation source 10 in place of
the radioisotope 34 of FIG. 2. The dual energies of radiation are produced
by switching the operating voltage of the x-ray tube 56 as is well
understood in the art. Synchronously with the switching of the voltage on
the x-ray tube 56, one of two filter materials of filter wheel 58 is
rotated into the path of the fan beam 14 on a rotating filter wheel, prior
to the beam intercepting the patient 20. The filter materials serve to
limit the bandwidth of the polyenergetic radiation from the x-ray tube 56
for each voltage. The filter wheel 58 and the x-ray tube are controlled by
x-ray control 62.
A single integrating type detector 26(b) employing either a scintillating
crystal type detector or a gaseous ionization type detector coupled to an
electrical integrator is used to produce the intensity signal, and the
integrated signal for each energy level is sampled synchronously with the
switching of the bias voltage of the x-ray tube 56 and the rotation of the
filter wheel 58, by data acquisition system 60 to produce the two
intensity measurements I.sub..alpha.1 and I.sub..alpha.2 used by the
selective material computation circuit 40 employing the polyenergetic
corrections technique previously described (e.g. Equation 6).
In all other respects the CT system in this embodiment is the same as that
described for the first embodiment.
Preferably, two projection sets are acquired, one at high x-ray energy, and
one at low x-ray energy, at each gantry angle .theta. before the gantry 12
is moved to the next gantry angle .theta. in an "interleaved" manner so as
to minimize problems due to possible movement of the patient 20. It will
be apparent to one of ordinary skill in the art, however, that each
projection set may be acquired in separate cycles of gantry rotation, the
advantage to this latter method being that the x-ray tube voltage and the
filter wheel 58 need not be switched back and forth as frequently or as
fast.
Referring to FIGS. 1 and 4, in a third embodiment, the radiation source 10
is an unmodulated x-ray tube 56 producing a polyenergetic fan beam 14 as
controlled by x-ray control 62. This fan beam 14 is filtered by stationary
filter 64 to concentrate the spectral energies of the x-ray radiation into
a high and low spectral lobe. Stationary filter 64 is constructed of a
material exhibiting absorption predominantly in frequencies or energy
between the two spectral lobes. A detector 26(c) is comprised of a primary
and secondary integrating type detector 66 and 68 arranged so that the fan
beam 14, after passing through the patient 20, passes first through
primary detector 66 and then after exiting the primary detector, passes
through the secondary detector 68. Each detector 66 and 68 is a gaseous
ionization detector filled with an appropriate high atomic number gas such
as xenon or a scintillation detector. Relatively lower energy x-ray
photons will give up most of their energy in the primary detector 66 and
be recorded as the low energy signal I.sub..alpha.1 for that ray 30 in fan
beam 14. These lower energy x-ray have a high probability of interacting
in the short distance occupied by the primary detector 66 because the
attenuation of the detector is higher at the lower energies. The higher
energy photons will give up proportionally more of their energy in the
secondary detector 68 and thereby produce the higher energy signal
I.sub..alpha.2. These two signals are collected by a data acquisition
system 70 and used to produce selective material projections by circuit 40
using the polychromatic techniques described above, and reconstructed into
an image as before.
In all other respects the CT system of the third embodiment is the same as
that described for the first embodiment.
It will occur to those who practice the art that many modifications may be
made without departing from the spirit and scope of the invention. For
example, other similar combinations of the detectors and radiations
sources, three of which are described above, may be used to create the
dual energy signals I.sub..alpha.1 and I.sub..alpha.2. The mechanical
structure of the CT apparatus may be based on other well known geometries
such as the "translate/rotate" configuration of CT scanner where the
radiation source 10 and a detector 26 are translated together across the
patient. Also, other energy dependent attenuation effects, for example the
k-edge absorption of certain materials. such as iodine, may be employed.
In order to apprise the public of the various embodiments that may fall
within the scope of the invention, the following claims are made.
Top