Method and apparatus for irradiation field detection in digital radiographic images

An automated method and apparatus are disclosed for detecting the irradiation field of a radiographic image, wherein digital image data is acquired and subjected to multiple phases of digital imaging processes. The image is first acquired by a radiation image recording apparatus through a landmark-synthesizing device that generates landmark patterns on the acquired image. These patterns are indicative of where the irradiation field is located, and are subsequently detected by digital image processing techniques. Next, the irradiation field is robustly and efficiently identified through these landmark patterns. An irradiation map is then created that functions as a template for any further image processing to be done on the irradiation field. Once the irradiation field is identified, the landmark patterns are removed from the acquired image using digital image processing techniques if desired. Finally, a mask image is generated to reduce the flare effect of the output image in areas that have received very little radiation, making the output image both diagnostically useful and aesthetically pleasing.

FIELD OF THE INVENTION 
The present invention relates in general to digital image processing for 
medical systems, and, more particularly, to an automated method and 
apparatus for detecting the irradiation field of a digital radiographic 
image for various applications related to diagnostic imaging. 
BACKGROUND OF THE INVENTION 
In traditional (analog) screen/film radiography, the film functions both as 
a recording medium, converting light from the screen into a latent image 
that is rendered visible after chemical processing, and as a display 
medium on a viewbox. Therefore, the characteristic curve of the 
screen/film system largely determines the contrast with which small and 
low-contrast details are displayed in the output image (film). In order to 
provide the best output image, the radiographer must try to control the 
amount of scatter radiation reaching the film. The principal factors that 
affect the amount of scatter produced are the kilovoltage applied to the 
radiation generator and the irradiated material (e.g., tissue). As 
kilovoltage increases, the percentage of primary photons that will undergo 
scattering also increases. As the volume of irradiated tissue increases, 
the amount of scatter produced is increased. Volume will increase as the 
irradiation field size increases or as the patient thickness increases. 
The atomic number of the irradiated material also has an impact on the 
amount of scatter produced. Higher atomic number materials have a greater 
number of electrons within each atom, and therefore photons have a greater 
chance of interacting with these materials. 
To control the amount of scatter and reduce patient dose, one of the most 
common methods is the use of some beam-restricting devices such as 
aperture diaphragms, cones/cylinders and collimators. Of them, the 
collimator is the most commonly employed beam restrictor in radiography 
because it permits an infinite number of field sizes using only one 
device. It also has the advantage of providing a light source for the 
radiographer as an aid in properly placing the x-ray source tube. Accurate 
collimation of the x-ray beam to the region of interest reduces the area 
and volume of tissue irradiated and provides for a reduced amount of 
scatter reaching the film, resulting in better image contrast. 
In digital radiography, on the other hand, storage phosphors are used for 
the digital acquisition of projection radiography. These phosphors offer a 
very wide exposure latitude (10.sup.4 :1) compared with that (40:1) of the 
traditional screen/film radiography. Because of this wide range of 
detectable exposures, the necessity of re-exposing a patient due to 
improper selection of exposure factors is virtually eliminated. Moreover, 
the separation of image acquisition and display stages provides 
opportunities for the electronic processing, storage, and transmission of 
radiographic images. 
In the descriptions that follow, the irradiation field will be used to 
denote the image area containing the body part and the direct x-ray 
exposed region which has received unattenuated x-rays. The non-irradiation 
field will be used to denote the very low intensity area, wherein a highly 
absorbent beam restrictor is used to "frame" the irradiation field in the 
acquired image. 
Although image processing techniques are allowed to be applied in digital 
radiography, the effectiveness of such techniques depends on the careful 
choice of the various parameters that control their performance. For 
example, histogram-based tone-scale transformation is a simple and 
effective way of adjusting the contrast of an image. However, the 
histogram is a global characteristic of the image, and therefore does not 
distinguish between the anatomically important regions of the image (e.g., 
the body part) and the unimportant regions of the image (e.g., the 
non-irradiation field). Thus, a tone-scale transformation based on such a 
histogram will be suboptimum if it is unduly influenced by the unimportant 
regions of the image. 
Due to the importance of the information from the irradiation field, a 
variety of prior art methods have been proposed to detect the irradiation 
field of a radiographic image. For example, U.S. Pat. No. 4,731,863 
teaches a technique for finding gray level thresholds between anatomical 
structures and image regions based on zero-crossings of a peak detection 
function derived from application of smoothing and differencing operators 
to the image histogram. This method produces a series of peaks by 
analyzing the histogram at several different signal resolutions. According 
to this method, the peaks need to be interpreted for each exam type and 
exposure technique. That is, for one exam type or exposure technique a 
low-signal peak could correspond to a non-irradiation field, but for 
another type or technique it could also correspond to a body part if no 
non-irradiation field is present in the image. Thus, some additional 
information may be needed to complete the analysis. 
Other methods of histogram analysis have also been proposed. U.S. Pat. No. 
4,952,805 teaches an irradiation field finding technique based on dividing 
the histogram into several sections with an intensity thresholding 
procedure and then doing a statistical shape analysis (discriminate 
analysis) of the section believed to correspond to an irradiation field. A 
decision about the presence of an irradiation field is made based on the 
shape of this section in the histogram. However, the large variety of 
histogram shapes that can occur with different exam types and different 
input modalities (such as magnetic resonance imaging (MRI), computed 
tomography (CT), ultrasound (US), nuclear medicine, digital subtraction 
angiography (DSA), and computed radiography (CR)) make this type of 
analysis subject to error. In addition, since a single threshold is chosen 
to represent the transition from a non-irradiation field to an irradiation 
field, this method does not perform well when the transition is fairly 
wide, such as when x-ray scatter is present. 
European Patent Application 288,042, published Oct. 26, 1988, proposes an 
irradiation field finding method using the image histogram. In this 
method, the histogram is again divided into a number of sections by an 
automatic thresholding procedure. Then a statistical analysis 
(discriminate analysis), combined with information about the exam type, 
exposure technique, and desired body portion to be displayed, is used to 
adjust the separation points between the sections until desired ranges for 
the irradiation field are found. This method is less prone to variations 
in exam type and input modality because this information is incorporated 
into the decision process. However, the se of fixed thresholds still poses 
problems if there is nonuniformity in the non-irradiation field. 
A more effective way of detecting the irradiation field of a radiographic 
image is to include spatial information in the analysis, in addition to 
the intensity information provided by the histogram. Several methods have 
been described for doing this. U.S. Pat. Nos. 4,804,842 and 5,028,782 
disclose a method for detecting the irradiation field in an image based on 
calculating derivatives of the input image and then identifying those 
points whose derivatives are higher than a threshold value identified with 
edge points. Then a new histogram of the input image is done using only 
the points identified as edge points, and from this histogram another 
threshold value is chosen to represent the boundary of the irradiation 
field. This method is claimed to provide a more accurate measure of the 
field than a simple histogram method. However, it still requires a priori 
knowledge that a collimator or field stop was in fact used to define the 
irradiation field, otherwise low-signal portions of the image inside the 
body part may be clipped by the intensity thresholding that defines the 
boundary. Furthermore, if image pixels inside the body part have a signal 
value comparable to or lower than those in the non-irradiation field (as 
when there is significant x-ray scatter), the edge of the irradiation 
field may not even be found with this method. Finally, if the region in 
the non-irradiation field is nonuniform in intensity, which is frequently 
the case when there is scatter present, there will not be a strong edge at 
the boundary of the irradiation field, and the derivative at the edge 
points may not have a high enough value to pass the threshold, leading to 
inaccuracies in finding the edge points. 
Other irradiation field detection methods have been described that use one 
dimensional edge detection along arbitrary lines drawn across the image. 
For example, U.S. Pat. No. 4,967,079 discloses a method for storage 
phosphor digital radiography systems that uses derivatives along radial 
lines from the image center, followed by a thresholding operation to 
detect potential edge points of the irradiation field. The boundary of the 
field is recognized by testing the colinearity of the found edge points. 
In order to be effective, this method requires a strong edge transition 
from the non-irradiation field to the irradiation field. However, the 
transition can sometimes be very weak and even inverted (body part with a 
lower signal than foreground, due to scatter). Furthermore, if the image 
involves multiple smaller images recorded on a large detector (so-called 
subdivision or multiple exposure recording), there will be many edges 
detected along radial lines from the image center, possibly leading to the 
detection of false boundaries. 
An alternate approach to irradiation field detection has been disclosed in 
U.S. Pat. No. 4,859,850. In this case, lines are extended from the edge of 
the image towards the center and, for each line, the transition regions 
from low signal to high signal at the edge of an irradiation field are fit 
with a linear or nonlinear equation. When the differences between the 
extrapolated fitted values (calculated from the equation) and the actual 
image values inside the field become too large, or when the extrapolated 
values reach a threshold signal level, the edge of the field is assumed to 
have been found. One problem with this method is that it assumes that a 
non-irradiation field has to be present (i.e., a priori knowledge of the 
exposure technique is required). A second problem has already been 
mentioned above, namely, that the method assumes that the signal values 
inside the irradiation field are always larger than those immediately 
outside it, which is not always the case when scatter is present. A third 
problem is that if subdivision recording has been used, the method may not 
find all of the necessary edges to define each irradiation subfield within 
the image. Finally, the use of multiple linear or nonlinear fits on 
multiple lines across the image is an inefficient, time-consuming way to 
find the irradiation field boundaries. 
A possible solution to the previously mentioned problem of detecting edges 
in subdivision recording has been proposed in U.S. Pat. No. 4,851,678. In 
this method, designed for storage phosphor digital radiography, potential 
edge points can still be found using the above method of differentiation 
along lines, but other possibilities are also disclosed. For example, once 
a few candidate edge points have been found, a boundary tracking 
procedure, based on following the likeliest edge points around the 
boundary from nearest neighbor to nearest neighbor (using a 
ridge-following algorithm) until they close on themselves again, is used 
to find the irradiation field. This method purports to handle multiple 
exposure subfields as well because multiple starting edge points can be 
followed around their respective irradiation subfields. Since the method 
of finding prospective edge points is similar to those above, similar 
potential problems exist, namely, that the method can break down when the 
edge transition from the irradiation field to the non-irradiation field is 
weak. Also the ridge-following algorithm can be very sensitive to noise, 
so the image data must be smoothed before analysis. 
As indicated above, the presence of multiple smaller images recorded on a 
single larger recording medium (i.e., subdivision recording) can create 
problems in locating all of the irradiation subfields in the image. 
Sometimes a preprocessing stage can be used to identify the use of 
subdivision recording and also the format of the image (2-on-1, 4-on-1, 
etc.). For example, U.S. Pat. No. 4,829,181 teaches a method of 
recognizing a subdivision pattern in a storage phosphor system using 
differentiation to detect prospective edge points, followed by a 
colinearity test to see if the edge points lie on straight lines. If the 
edge points lie on straight lines, subdivision recording is judged to be 
present. A limitation of this method is that it can detect only 
rectilinear patterns, i.e., patterns with essentially horizontal or 
vertical linear separations. 
Another approach to detecting such subdivision patterns is the use of 
pattern matching. U.S. Pat. No. 4,962,539 discloses a method that uses a 
set of binary, stored masks representing typical subdivision recording 
patterns. The input digital image is converted into a binary image by 
thresholding, and the resulting binary image is statistically compared 
with each of the masks in the stored set. The stored mask with the highest 
degree of matching is judged to be the recording pattern on the input 
image. While this method can handle a wider variety of patterns than the 
one above, it is still limited to the stored library of patterns for 
matching. Any irregular patterns not included in the library may not have 
a high degree of matching, and may therefore be chosen incorrectly. 
Furthermore, statistical matching is complex and time consuming. 
In another prior art method, U.S. Pat. No. 5,268,967 discloses a method 
that first analyzes the edge content of the image, and then breaks the 
image into a set of nonoverlapping, contiguous blocks of pixels. The edge 
density in each block is computed as one indicator of the level of detail 
or "busyness" in various locations of the image, and, therefore, an 
indicator of whether these locations are more likely to belong to the 
irradiation or non-irradiation field. 
Further analysis of the image and classification into the various regions 
take place on a block-by-block basis. Although the reliability of this 
method is good, in particular for single exposure images, the level of 
complexity is high and the speed is prohibitive for real-time 
applications. 
Thus a need remains for an automated method and apparatus for digital image 
processing to perform the detection of an irradiation field and multiple 
irradiation subfields. Such a method and apparatus would allow the 
parameters of subsequent image processing techniques to be calculated more 
robustly and efficiently, leading to better image quality and more 
accurate diagnosis. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an automated method and apparatus 
for digital image processing to perform the detection of an irradiation 
field and multiple irradiation fields. 
A further object of the invention is to allow the parameters of subsequent 
image processing techniques to be calculated more robustly and 
efficiently, leading to better image quality and more accurate diagnosis. 
A further object of the invention is to generate a mask image from a given 
radiographic image to make the output image both diagnostically useful and 
more visually pleasing. 
According to the preferred embodiment of the present invention as directed 
to irradiation field detection, the foregoing problems in the prior art 
are overcome by practice of a method comprising the steps of: 
(a) acquiring a landmarked digital image of the object of interest; 
(b) detecting a landmark pattern in the acquired image; and 
(c) generating an irradiation map that identifies the location of the 
irradiation field in relation to the landmark patterns, said irradiation 
map functioning as a template for further image processing of the 
radiographic image. 
In addition, the present invention discloses a method that generates a mask 
image to make the output image both diagnostically useful and 
aesthetically pleasing. 
The technical advantage of the present invention for irradiation field 
detection is that it overcomes the problems associated with the prior art 
while, at the same time, performing well: 
(a) when there is severe nonuniformity in the non-irradiation field; 
(b) without a priori information about the presence of a collimator or 
about its shape and size; 
(c) even if the transition from the non-irradiation field to the 
irradiation field has a fairly wide intensity range; 
(d) for multiple exposure images; and 
(e) for real-time applications. 
Moreover, the present invention does not require the computation of edge 
points and therefore, being insensitive to noise, does not require an 
edge-tracking procedure. Furthermore, it provides an automatic and 
efficient means for reducing flare effect on the output images.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown a block diagram of an imaging system 
including the automated method and system of the present invention. The 
imaging system comprises an image acquisition device 10 for acquiring an 
input image 12, a digital irradiation field processor 14, a digital image 
signal processor 16, and an image display device 18 for producing an 
output image 20. Although the digital irradiation field processor 14 is 
illustrated as a separate system, it can reside within the digital image 
processor 16. 
The digital image signals to be processed utilizing the described system 
can be acquired in a variety of ways. For example, such images are 
typically derived from storage phosphor (computed radiography) systems, 
film digitizer systems, image intensifier systems, and the like. 
Additionally, images acquired via computed tomography (CT) and magnetic 
resonance imaging (MRI) systems can be utilized. The type of image signal 
processor 16 utilized in the imaging system will be dependent upon the 
type of digital image being processed and the ultimate use of the results 
of said processing. Finally, any number of display devices can be used 
with the imaging system. For example, the most common display devices are 
film writers (either laser printers or cathode ray tube (CRT) printers) 
and CRT soft displays. Additionally, the output image can be captured for 
permanency on photographic film. 
A radiation image recording apparatus is illustrated in FIG. 2. The 
radiation is emitted from an x-ray tube 30 through a beam restrictor, such 
as a collimator 34, to an object 36 lying an object support 38. The 
radiation then irradiates a stimulable phosphor plate 40 having a surface 
composed of excitable phosphors. The collimator 34, which employs movable 
lead shutters, is the most commonly employed beam restrictor in the image 
recording apparatus, and it permits an infinite number of field sizes 
using only one device. The collimator 34 also offers a radiographer a 
light field which outlines the exposure field and provides a cross hair to 
identify the center of the x-ray beam. The light field is provided by 
mounting a mirror 32 supported in the path of the x-ray beam at a 
45.degree. angle. A light source 42 is then placed opposite the mirror and 
the light is projected through the collimator during a set-up procedure 
prior to capturing the radiographic image. The light source 42 and the 
x-ray source 30 must be equidistant from one another to ensure that the 
light field and the x-ray irradiation field are the same size. 
When the stimulable phosphor plate 40 is placed and secured in position, 
sensing devices activate an electric motor 44 which is coupled to the 
collimator 34 and drives the lead shutters of the collimator 34 into 
proper position. When operating properly, automatic collimators should 
leave an unexposed border (non-irradiation field) on all four sides of the 
exposed phosphor plate 40. However, the field size can also be manually 
controlled by the radiographer. By so doing, image quality can be improved 
and patient dose can be minimized. 
To be useful according to the subsequent description of the present 
invention, the collimator 34, as shown in FIG. 3, consists of two sets of 
lead shutters. A set of lower lead shutters 46 are mounted at right angles 
to a set of upper lead shutters 48, each set moving in opposing pairs as 
illustrated. Each set moves symmetrically from the center of the 
irradiation field to define an irradiation field 50 within the area of the 
plate 40. These lead shutters 46, 48 can be adjusted to correspond to an 
infinite number of square or rectangular field sizes. The two sets of 
shutters serve to regulate the field size and, in addition, have two other 
purposes. The lower shutters 46 reduce penumbra along the periphery of the 
beam because of their greater distance from the focal spot. The upper 
shutters 48 help in reducing the amount of off-focus (stem) radiation 
reaching the film by absorbing this radiation before it exits. 
As the radiation energy strikes the surface of the plate 40, a portion of 
the energy is stored by the phosphors. Upon subsequent stimulation by 
visible light or other stimuli, the storage phosphors give off light in 
direct proportion to the amount of energy stored therein. Areas of the 
phosphors receiving unattenuated radiation absorb the most energy and thus 
produce the most light when subsequently stimulated. Areas in which lesser 
amounts of radiation energy are absorbed, due to the presence of the 
object (e.g., body region), produce a proportionately lesser amount of 
light when subsequently stimulated. 
FIG. 4 illustrates a digital radiographic image acquired from the radiation 
image recording apparatus described in FIGS. 2 and 3. The acquired image 
contains the irradiation field 50 consisting of the body part 52 being 
imaged and the direct x-ray exposed region 54 (high intensity region), as 
well as a non-irradiation field 56 (low intensity region). 
In general, as shown in FIG. 5, the irradiation field detection method of 
the present invention includes the following steps: (1) landmark 
generation 58; (2) landmark detection 60; (3) irradiation map generation 
62; and (4) landmark removal 64. For purposes of illustration only, the 
operation of the present invention will be described with reference to a 
digital hand radiographic image, as shown in FIG. 4. 
Landmark Generation 
The landmark generation step 58 in the disclosed method of the present 
invention is implemented in the image acquisition device in which the 
above-described radiation image recording apparatus (FIG. 2) is used to 
acquire the landmarked image. The device that generates landmark 
pattern(s) in the acquired image is referred herein as a 
landmark-synthesizing device (LSD). In the preferred embodiment of the 
present invention, as shown in FIG. 6, the LSD 66 is simply a modified 
collimator 66 that has a small circular hole 68 close to the inner edge 
center of each lead shutter. When the radiation is irradiated through the 
modified collimator, some photons will pass through the small holes 68 and 
project onto the stimulable phosphor plate 40. After the phosphor plate 40 
is stimulated by visible light or other stimuli, the acquired digital 
radiographic image, as shown in FIG. 7, contains the irradiation field 50 
including the body part 52 being imaged and the direct x-ray exposed 
region 54 receiving unattenuated x-rays, as well as the non-irradiation 
field 56 including well-aligned landmark patterns 70a . . . d around the 
irradiation field. The alignment of these landmark patterns 70a . . . d in 
the acquired digital image is ensured by the facts that the two sets of 
lead shutters 46, 48, that are at right angles to one another, move in 
opposing pairs, and each set moves symmetrically from the center of the 
irradiation field 50. This implies that detecting two landmark patterns, 
for example, 70a and 70c or 70d, rendered from any two lead shutters 46 
and 48 which are perpendicular to each other, suffices to provide the 
identification of the other two landmark patterns. 
It is important to note that not every landmark pattern has the same 
intensity value because of the scatter radiation and the possibility that 
some landmark patterns are rendered through the body part. For example, as 
shown in FIG. 7, the bottom landmark pattern 70d has a lower intensity 
value than those of the other three patterns because the radiation is 
emitted through the wrist. Nevertheless, the intensity values of the 
pixels in each individual landmark pattern are much more uniformly 
distributed and greater than those of their surrounding pixels in the 
nonirradiated field. Therefore, it becomes much easier for subsequent 
image processing techniques to detect these patterns, which in turn 
identifies the irradiation field. Moreover, the proportion of the landmark 
area to the irradiation field area is very small, and hence the potential 
extra exposure through the modified collimator should not be a problem for 
diagnostic radiology. If desired, this proportion can be further reduced 
by adjusting the exposure conditions and the size of the holes in the 
modified collimator 66. 
For example, in one experimental study, a hand phantom was placed on the 
object support 38 of the radiation image recording apparatus and the 
modified collimator 66 was provided with a circular hole 68 in each of its 
lead shutters. The following describes the exposure conditions and the 
acquired image. The diametric size of the hole 68 and the distance from 
the hole to the corresponding shutter's edge are 1 mm and 3 mm, 
respectively. The exposure time, x-ray tube voltage, and current are 1.25 
sec, 83 kVp, and 9.4 mAs, respectively. The distance from the x-ray tube 
30 to the stimulable phosphor plate 40 is 100 cm, and the distance from 
the collimator 66 to the stimulable phosphor plate 40 is 72 cm. The 
irradiation field 50 rendered in the acquired image has a rectangular 
shape with 267 mm in length and 226 mm in width. Moreover, the landmark 
patterns 70a . . . d rendered in the acquired image have an elliptic shape 
with 11 mm in length and 5 mm in width. In terms of digital code values 
for a 12-bit digital image, the intensity values of the one landmark 
pattern rendered through the wrist range from 3562 to 3610. The intensity 
values of the other three landmark patterns range from 4065 to 4095, the 
maximum code value for a 12-bit digital image. The intensity values of the 
other pixels in the non-irradiation field range from 274 to 2875. The 
distance from these landmark patterns to the irradiation field is 7 mm in 
average. As a result, the proportion of the landmark area to the 
irradiation field area is approximately 
EQU (.pi..times.11/2.times.5/2)/(267.times.226).apprxeq.0.072%. 
Although the LSD illustrated in the landmark generation step of the present 
invention uses a collimator 66 with four small circular holes 68, it 
should be noted that any type(s) and/or number of LSD(s) can be designed 
and utilized where those used provide robust and efficient methods for the 
irradiation field detection in the subsequent processing. In other words, 
if a LSD is capable of generating landmark patterns providing the ability 
of detecting the irradiation field and/or any other anatomically or 
physically meaningful field of the image, such a LSD can be used, if 
desired. 
It should also be noted that a LSD can be made in such a way that the 
landmark patterns are activated by some automatic and/or manual control 
unit(s). For example, when a stimulable phosphor plate is placed in 
position, a sensing device may activate an electric switch which makes 
holes appear in the collimator lead shutters. Thus the LSD can be used in 
a more general fashion for both screen/film radiography and digital 
radiography. 
Once the landmark patterns are rendered in the acquired digital 
radiographic image, image processing techniques can be applied to identify 
these patterns, which in turn leads to the detection of the irradiation 
field. 
Landmark Detection 
Since the intensity values of the pixels in each individual landmark 
pattern of the acquired digital image are uniformly distributed and much 
greater than those of their surrounding pixels, a simple local 
thresholding technique suffices to detect these patterns. For example, by 
applying the well-known local thresholding method disclosed in Niblack (W. 
Niblack, An Introduction to Digital Image Processing, pp. 115-116, 
Englewood Cliffs, N.J.: Prentice Hall, 1986), the acquired digital image 
shown in FIG. 7 is binarized. The resulting binary image consists of all 
the landmark patterns plus other possible direct x-ray exposed regions in 
the irradiation field, as shown in FIG. 8(a). To discriminate between the 
landmark patterns and the direct x-ray exposed regions, well-known 
mathematical morphology techniques may be applied (see, e.g., P. Maragos, 
"Pattern spectrum and multiscale shape representation," IEEE Trans. 
Pattern Anal. Mach. Intell., vol. PAM-11, pp. 701-716, 1989; and J. Serra, 
Image Analysis and Mathematical Morphology, Academic Press, New York, 
1982), which include many efficient algorithms for identifying the 
landmark patterns by their shapes and sizes. An example is shown in FIG. 
8(b) where the landmark patterns are identified using morphological 
techniques. 
Moreover, if the landmark patterns are designed in such a way that 
different shapes and/or sizes are used for different exam types (e.g., 
circle for chests and square for elbows), one can again apply 
morphological techniques to differentiate the desirable landmark patterns 
from the others, rendering a recognition capability of the disclosed 
method. 
Although the landmark detection step of the present invention illustrated 
above uses morphological techniques, it should be noted that any other 
image processing technique(s) can be utilized or developed where those 
used provide the detection capability of the landmark patterns in the 
acquired digital image. 
Irradiation Map Generation 
Once the landmark patterns are detected, the irradiation field can be 
easily identified in many ways. For example, as shown in FIG. 9, let 
(x.sub.i,y.sub.i) (1.ltoreq.i.ltoreq.4) denote the center coordinates of 
the four well-aligned landmark patterns. The center of the irradiation 
field (x.sub.c,y.sub.c) is thus obtained by x.sub.c =x.sub.2 and y.sub.c 
=y.sub.1 because of x.sub.2 =x.sub.4, y.sub.1 =y.sub.3, and each set of 
lead shutters moving symmetrically from the center of the irradiation 
field. A rectangular region R centered at (x.sub.c,y.sub.c) with its 
boundary passing through (x.sub.i,y.sub.i), (1.ltoreq.i.ltoreq.4) is 
defined and shown by the dashed lines in FIG. 9. The region R is normally 
larger than that of the irradiation field, therefore, one can shrink the 
region R down by the amount 2 .DELTA.x in width and the amount 2 .DELTA.y 
in height to obtain the true irradiation field. The values .DELTA.x and 
.DELTA.y, depending on the exposure conditions, are determined in advance 
by experiments. 
Subsequently, a two-valued (binary) irradiation map that functions as a 
template for any further image processing to be done on the irradiation 
field is created. In general, if N multiple exposure images are detected, 
a (N+1)-valued irradiation map is created for any further image processing 
to be done on the N irradiation subfields of the image. FIG. 12(b) 
illustrates an example where the four irradiation subfields 72a . . . d 
and the one non-irradiation field 72e are detected and represented by five 
different intensity values i.sub.1 . . . i.sub.5, respectively. As a 
result, if a histogram of the desired exam type is required, only those 
pixels with the correct value in the (N+1)-valued irradiation field are 
included in the calculation. If edge enhancement is being performed, 
pixels in the undesired regions (e.g., the non-irradiation field 72e and 
non-desired subfields) are not included in the calculation of enhancement 
parameters. In this way, only the relevant information in the image is 
included in subsequent image processing of the image, leading to images 
with high quality and high diagnostic utility. 
Although the detection of the irradiation field illustrated in the 
irradiation map generation step of the present invention uses a cue from 
the generated landmark patterns, it should be noted that many other cues 
and/or methods can be developed and utilized where those used identify the 
accurate irradiation field in the acquired image. 
Landmark Removal 
If desired, the landmark patterns can be removed from the acquired image 
once the irradiation field has been identified in the disclosed method of 
the present invention. For example, as shown in FIG. 10(a), the digital 
image contains a sequence of N horizontal, non-overlapping blocks 80 where 
the top landmark pattern is enclosed in the ith block. As shown in the 
enlarged portion in FIG. 10(b), the landmark pattern 70c in the block 
image f.sub.i (x,y) is then smoothed out by taking the intensity average 
between its neighboring block images, f.sub.i-1 (x,y) and f.sub.i+1 (x,y). 
Subsequently, to avoid possible edge artifacts from one block to another, 
a simple one-dimensional (1D) averaging operation using 1.times.5 kernel 
is applied to the pixels in the block images f.sub.i-1 (x,y), f.sub.i 
(x,y) and f.sub.i+1 (x,y). Another efficient way to remove the landmark 
pattern is the use of a curve interpolation technique (e.g., cubic spline 
interpolation) in which the value f.sub.i (x,y) is considered unknown and 
interpolated by the values f.sub.1 (x,y), f.sub.2 (x,y), . . . , f.sub.i-1 
(x,y), f.sub.i+1 (x,y), . . . f.sub.N (x,y) extracted from the sequence of 
block images. By decomposing the acquired digital image into a sequence of 
vertical, non-overlapping blocks, the left and right landmark patterns can 
be processed in the similar way. 
Although the landmark removal step of the present invention uses simple 
smoothing operations and/or a curve interpolation technique, it should be 
noted that any other image processing methods can be developed and 
utilized where those used remove landmark pattern(s) in the acquired 
image. 
Mask Generation 
Today most radiologists interpret the final output images printed on films 
and displayed on viewboxes. These printed and displayed films should be 
interpreted under conditions that provide good visibility, comfort, and 
minimal fatigue. When the films are placed and the light source is on, the 
light emitting from the viewboxes through the non-irradiation field (i.e., 
very low intensity areas) of the displayed images produces a fair amount 
of "flare". However, the contrast sensitivity of the eye (the ability to 
distinguish small luminance differences) is greatest when the surroundings 
are of about the same brightness as the area of interest. Therefore, to 
see detail in radiographic images, it is important to reduce flare to a 
minimum. Flare can be reduced by using manually-driven masks to cover 
unused portions of a viewbox or to cover the non-irradiation field in the 
output images being examined. In the present invention, we disclose an 
automatic and efficient means to mask out the non-irradiation fields of 
output digital images such that these images can be interpreted in more 
pleasant conditions. 
Consequently, another object of the present invention is to automatically 
generate mask images from which final output images can be made more 
diagnostically useful and aesthetically pleasing. As shown in FIG. 11(a), 
a mask image is generated wherein a non-irradiation mask field 82, 
detected at the previous irradiation map generation step 62 of the 
disclosed method, is given by a very high intensity value (e.g., 4095), 
and an open mask field 84 is given a very low intensity value (e.g., 0). 
If any subsequent image processing performed on the irradiation field is 
done, the enhanced irradiation field is patched back to the open mask 
image 84, as shown in FIG. 11(b). Therefore, flare of the output image 
viewed by radiologists can be reduced to a minimum. 
In addition, the intensity values assigned to the pixels in the 
non-irradiation field may vary according to our particular need. For 
example, as shown in FIG. 11(c), the pixels composing the "frame" 86 
around the irradiation field can be given by an intensity value (say, 
2000) different from that of the other pixels in the non-irradiation 
field. The frame 86 distinguishably separates the non-irradiation field 
being masked and the direct x-ray exposed regions of the acquired image, 
making the output image more pleasing. This is more evident when applied 
to multiple exposure images, as shown in FIG. 12(d). 
Although the mask generation of the present invention uses the intermediate 
results from the irradiation map generation step of the disclosed method 
in the present invention, it should be noted that the mask generation idea 
can be utilized and independently applied to any other types of image 
processing methods which identify the irradiation field and/or regions of 
interest in the acquired image. Although the non-irradiation mask field is 
given by a high intensity value in the present invention to reduce the 
flare effect, it should be noted that any number and types of intensity 
values and/or image patterns can be utilized and applied to the 
non-irradiation mask field, according to the specific application(s) at 
hand. Consequently, the density and texture of the mask may be dependent 
upon the application. 
FIG. 12 illustrates an example where a plurality of irradiation subfields 
on a single image are detected by the disclosed method of the present 
invention. FIG. 12(a) shows the acquired digital radiographic image 
containing four different exam types, each with well-aligned landmark 
patterns around its irradiation subfield. The four irradiation subfields 
72a . . . 72d and the one non-irradiation field 72e are detected and 
represented by five different intensity values, as shown in FIG. 12(b). 
The associated mask image resulting from the disclosed method is given in 
FIG. 12(c). The subsequent image processing algorithms can then be applied 
to each subfield of the acquired image, leading to an output image with 
high quality and high diagnostic utility in each subfield. These enhanced 
subfields are then combined with the mask image, as shown in FIG. 12(d), 
wherein the frames are used to make the output image more pleasing. 
Although a preferred embodiment of the present invention has been 
illustrated in the accompanying drawings and described in the foregoing 
detailed description, it will be understood that the invention is not 
limited to the embodiment disclosed, but is capable of numerous 
rearrangements and modifications of parts and elements without departing 
from the spirit of the invention. For example, instead of modifying a 
collimator shutter with a small hole to provide a landmark image, it is 
also possible to add a landmark attachment to an existing collimator. As 
shown in FIG. 13, an attachment 90 with a hole 92 can be attached (by 
suitable means, such as slip-on flanges 93) to each existing collimator 
shutter 94 (only one shown). Furthermore, the invention can be used with 
regard to any type of application involving radiographic imaging, for 
example, industrial applications as well as medical applications. In 
particular, instead of being an anatomical region, the object of interest 
may be a machine part or a product part subject to inspection, such as an 
airplane wing. 
TS LIST 
10 image acquisition device 
12 input image 
14 digital irradiation field processor 
16 digital image signal processor 
18 image display device 
20 output image 
30 x-ray tube 
32 mirror 
34 collimator 
36 object 
38 object support 
40 stimulable phosphor plate 
42 slight source 
44 electric motor 
46 lower set of lead shutters 
48 upper set of lead shutters 
50 irradiation field 
52 body part 
54 x-ray exposed region 
56 non-irradiation field 
58 landmark generation 
60 landmark detection 
62 irradiation map generation 
64 landmark removal 
66 modified collimator 
68 hole 
70a . . . d landmark patterns 
72a . . . d irradiation subfields 
72e non-irradiation field 
80 sequence of blocks 
82 non-irradiation mask field 
84 open mask field 
86 frame 
90 attachment 
92 hole 
93 flange 
94 existing collimator shutter