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Dual-energy projection radiography: Initial clinical experience



Selective removal of substances of a particular mean atomic number in projection radiographs has been accomplished using an experimental system for line-scanned digital radiography. X-ray images were produced with two different X-ray spectra by modulating the kilovoltage applied to the X-ray source and the filtration between the source and the patient. A Compton/photoelectric decomposition algorithm provided subtraction images with bone or soft-tissue (water) shadows removed. Applications to chest, abdominal, and skeletal imaging in vivo are demonstrated.
Dual-Energy Projection
Radiography: Initial Clinical
William A. Brody1
Douglas M. Cassel1
F. Graham Sommer1
Leonard A. Lehman&
Albert Macovski1
Robert E. Alvarez1
Norbert J. Pelc
Stephen J. Riederer2
Anne L. Hall2
Received March 26, 1981; accepted May 12,
Presented at the annual meeting of the Ameri-
can Roentgen Ray Society, San Francisco, March
This work was supported by the General Elec-
tric Company (Medical Systems Division), the Na-
tional Science Foundation, and Established Inves-
tigator Award (W. R. Brody) from the American
Heart Association and the Central Mission Trails
‘Department of Radiology S-072, Stanford
University School of Medicine, Stanford, CA
94305. Address reprint requests to W. R. Brody.
2 Medical Systems Division, General Electric
Co., Milwaukee, WI 53201.
AJR 137:201-205, August 1981
0361 -803X/81 I 1372-0201 $00.00
© American Roentgen Ray Society
Selective removal of substances of a particular mean atomic number in projection
radiographs has been accomplished using an experimental system for line-scanned
digital radiography. X-ray images were produced with two different x-ray spectra by
modulating the kilovoltage applied to the x-ray source and the filtration between the
source and the patient. A Compton/photoelectric decomposition algorithm provided
subtraction images with bone or soft-tissue (water) shadows removed. Applications to
chest, abdominal, and skeletal imaging in vivo are demonstrated.
Existing radiographic systems do not make explicit use of the energy infor-
mation present in the transmitted x-ray spectrum. Manipulation of the energy
spectrum in film radiography has been quite limited, for example, high kilovoltage
(kVp) for minimizing bone shadows in chest radiographs, and low kilovoltage
spectra for maximizing soft-tissue contrast in mammography. True energy-de-
pendent radiographic imaging methods have been applied to projection radiog-
raphy for dual-beam and three-beam K-edge detection of iodine [1 , 2] using
quasi monoenergetic x-ray spectra with subtraction fluoroscopy.
The electronic x-ray detectors for computed tomography (CT) have made
possible the use of dual-energy methods for identification of material atomic
number and density. A general method for dual energy CT developed by Alvarez
and Macovski [3] is based on a Compton/photoelectric decomposition of x-ray
attenuation. While the concept of dual-energy CT has become familiar to radiol-
ogists, it has not been generally recognized that these same dual energy methods
can be applied to projection x-ray imaging as well [4]. For projection radiography,
these dual energy techniques provide selective subtraction or cancellation of
materials of certain average atomic number. This report describes the initial
application of a prototype imaging system for dual-energy line-scanned radiog-
raphy for chest and abdominal radiography in vivo.
Materials and Methods
An experimental system for scanned projection radiography was developed using a
General Electric 8800 CT/T scanner extensively modified for dual-energy imaging. A line-
scanned radiograph is produced by mechanically translating the patient through the
narrowly collimated (1 .9 mm) x-ray fan beam. A 511-element linear-array xenon detector
collects the x-rays transmitted through the patient and produces an electrical signal which
is converted into digital format. A minicomputer performs the subsequent normalization,
calibration, and logarithmic compression of the intensity signal similar to CT.
As the patient is translated through the gantry, the x-ray generator is pulsed, each pulse
yielding one line of x-ray transmission information. An image is constructed by the
juxtaposition of such lines. For dual-energy imaging, the generator produces, at 60 Hz, x-
ray pulses that alternate between 135 kVp, 250 mA, 5.5 msec pulse width and 85 kVp,
1000 mA, and 3.3 msec. In synchrony with the x-ray pulses, a rotating filter assembly
placed between the tube and the patient filters the low and high kilovoltage spectra with
Fig. 1 -Dual-energy radiography for oral cholecystography. A, 85 kVp effectively removes bowel gas, allowing higher contrast image of gallbladder.
(unsubtracted) line-scanned radiograph. Gallbladder opacified, but partially Extrahepatic common bile duct faintly seen.
obscured by superimposed bowel gas. B, Soft-tissue subtraction image
AJR:137, August 1981
Fig. 2.-Dual-energy excretory urogram. A. Line-scanned radiograph at
85 kvp. Kidneys and collecting systems faintly visualized. B, Soft-tissue
subtraction image removes superimposed bowel gas, allowing enhancement
of kidneys, collecting systems, and descending colon, which contains dilute
0.1 mm erbium and 1 .52 mm bronze, respectively. The combination
of kilovoltage and filtration yields x-ray spectra with average ener-
gies of about 45 and 85 keV.
Because the x-ray table is translated continuously and alternate
lines are generated at low and high kilovoltage, subtraction imaging
requires the low kilovoltage image to be shifted by one line with
respect to the high kilovoltage image. Before subtraction, the low
and high energy images are interpolated so that they will be exactly
water-soluble contrast medium from previous CT scan. Retroperitoneal fat
planes, which appear as low density (negative) shadows, are enhanced by
soft-tissue subtraction process.
The manner in which the low and high kilovoltage images are
combined has been discussed in detail elsewhere [5, 6]. Briefly,
the algorithm converts the high and low kilovoltage images into the
aluminum and Lucite components of the scanned object. These
aluminum and Lucite ‘basis” images can in turn be combined
linearly to form a tissue cancellation image. Images in which the
bone is cancelled, or the bone shadows are removed by substituting
the bone with an equivalent amount of soft-tissue, can be similarly
produced. In fact, material of any desired atomic number can be
Fig. 3.-Metastatic prostate carcinoma. A, Diffuse rib metastases from (bone mimic tissue subtraction) shows pleural or extrapleural soft-tissue
prostate carcinoma. B, 85 kVp scanned radiograph (unsubtracted). C, Soft- shadows associated with rib metastases. This image enhances airlsoft-tissue
tissue subtraction shows diffuse metastases to ribs and spine with complete interfaces: pleura, airways, pulmonary vessels.
removal of heart, pulmonary vascularity, and airways. D, Bone subtraction
AJR:137, August 1981
cancelled by an appropriate linear combination of the two basis
images. With monoenergetic low and high energy beams the algo-
rithm would simply perform a weighted linear subtraction to produce
the aluminum and Lucite images (or any other desired subtraction
image). However, with the broad spectra resulting from x-ray tubes,
the effects of beam hardening introduce significant nonlinearities
that require more complex image combination [5, 6].
A scanning speed of 35 mm/sec was used for these dual energy
radiographs. The radiation exposure, measured at the surface of a
chest phantom using thermoluminescent dosimeters, was 130 mR
(33.5 x 106 C/kg).
Dual-energy technique has been used to image the chest, skel-
eton, kidney, liver, gallbladder, bowel, and vascular systems. De-
tailed evaluation of the diagnostic effectiveness of dual-energy
radiography for different disease states is underway. Informed
consent was obtained for all examination under protocols approved
by the Stanford Human Subjects Committee.
For abdominal imaging, selective soft-tissue cancellation
provides effective removal of bowel gas shadows as dem-
onstrated in figures 1 and 2 of the gallbladder and kidney.
Visualization of the gallbladder in the unsubtracted image is
limited by the superimposed bowel gas shadows as shown
in the 85 kVp line-scanned radiograph (fig. 1A). With the
use of soft-tissue subtraction, the bowel gas shadows are
removed, allowing the gallbladder to be displayed at much
BRODY ET AL. AJR:137, August 1981
higher contrast (fig. 1 B). The extrahepatic common duct
can be seen on this subtraction image.
An excretory urogram (fig. 2) of a patient with lymphoma
shows how soft-tissue subtraction (removal of water-density
variations) eliminates the confusing bowel gas shadows that
obscure the contrast-filled kidneys and collecting systems.
This particular patient had water-soluble contrast in the
gastrointestinal tract for a prior CT scan. While this contrast
is not particularly apparent on the unsubtracted image, the
distal colon is well demonstrated on the soft-tissue-can-
celled image (fig. 2B). Also the perirenal and other retro-
peritoneal fat planes are enhanced by the subtraction pro-
cess. This enhancement is achieved because of the differ-
Fig. 4.-Soft-tissue subtraction provides improved skeletal detail by re-
moving interfering superimposed structures. Soft-tissue-cancelled left ante-
rior oblique projection shows sternum, ribs, and other skeletal structures.
ence in chemical composition (average atomic number)
between fat and lean body tissues.
In the chest, dual-kilovoltage imaging allows selective
cancellation of either bone or soft-tissue shadows. A film
chest radiograph on a patient with prostate carcinoma and
known bony metastases shows some of the rib lesions (fig.
3A), but obscures the diffuse involvement of the spine. The
unsubtracted scanned projection radiograph at 85 kVp (fig.
3B) shows similar features. However, the selective soft-
tissue subtraction image (fig. 3C) shows the bony metas-
tases clearly with rib and spine involvement. The alternative,
the bone-cancellation image, highlights the extraosseous
soft-tissue shadows associated with some of the rib metas-
tases (fig. 3D). It displays the soft tissues of the lungs and
mediastinum without the usual thoracic cage image.
The improved skeletal detail that results from soft-tissue
subtraction is illustrated by figure 4, a soft-tissue-cancelled
image of the human chest obtained with a shallow left
anterior oblique projection. Note the clarity with which the
sternum is demonstrated.
Finally, application of soft-tissue subtraction to determine
calcification of pulmonary nodules is shown in figure 5. A
large pulmonary nodule located adjacent to the left hemidia-
phragm is seen with the line-scanned system at 85 kVp (fig.
5A). Calcification was suspected on plain film chest radi-
ography, but could not be confirmed with this unsubtracted
image, even when edge enhancement algorithms were ap-
plied. On the other hand, the soft-tissue subtraction image
(fig. SB) shows unequivocally the crescentic calcification
around the periphery of the nodule.
Dual-energy scanned projection radiography provides se-
lective cancellation (or enhancement) of materials of any
specified average atomic number. While non K-edge dual-
energy techniques have been proposed before, attempts to
produce subtraction images have been impaired for two
Fig. 5.-Calcification in chest nodule
can be detected using dual-energy sub-
traction. A. Large nodule adjacent to left
hemidiaphragm on unsubtracted 85 kVp
film; calcification cannot be demon-
strated. B. Soft-tissue subtraction image
unequivocally shows rim of calcification
surrounding periphery of lesion (arrows).
AJR:137, August 1981
reasons: (1 ) the film/screen systems for x-ray detection
may have been limited by the nonlinearities and limited
dynamic range of film detectors, and by the energy depen-
dent scatter accepted by these large area detectors [7, 8];
and (2) the lack of rapid energy switching makes the sub-
traction method highly susceptible to motion artifacts for
application in vivo [5]. With the rapid switching generator
used in the current study, the low and high kilovoltage
images are in perfect spatial registration, so that motion
effects are minimized.
While any desired atomic number material may be can-
celled by this technique, in our limited experience with the
method, the selective soft-tissue (water) subtraction and the
bone subtraction were most useful. The soft-tissue subtrac-
tion images remove soft-tissue variations and, in the chest
or abdomen, isolate the skeletal structures from confusing
superimposed shadows. Abdominal and retroperitoneal fat
planes, having a lower average atomic number than water,
become negative shadows on the tissue-cancelled image.
The soft-tissue subtraction also seems useful for the detec-
tion of calcification in soft-tissue lesions, such as pulmonary
With the administration of urographic or cholecysto-
graphic contrast media, the soft-tissue subtractions en-
hance kidney and gallbladder visualization. The removal of
confusing bowel gas shadows may alleviate the need for
conventional tomography which is so often required for
satisfactory studies. In addition, soft-tissue subtraction may
be useful in small and large bowel imaging either with water-
soluble contrast media or with barium.
The bone subtraction images, especially those with the
bone space filled by an equivalent thickness of soft tissue
(bone mimic tissue subtraction), were found most useful in
the chest, where the confusing rib shadows are eliminated
for unimpaired view of the pulmonary parenchyma. Because
of the obscuration of pulmonary nodules lying under the ribs
[9], we postulate that the bone removal will improve nodule
detection. In addition, we learned from these studies that
the bone mimic tissue image enhances the visualization of
air-tissue interfaces, such as the mediastinal pleural reflec-
tions, the pleural surfaces, the larynx, trachea, and bronchi.
Although the x-ray exposure for dual energy imaging may
be higher than that of a single energy image, this is not a
requirement. Moreover, the use of tissue or bone subtraction
with dual energy leads to images in which lesion detection
is limited by signal-to-noise ratio and hence by dose. In the
unsubtracted images as with conventional radiography, le-
sion detection is more often limited by interference from
superimposed high contrast structures than by dose. With
subtraction, these interfering tissues can often be removed,
allowing one to see low contrast lesions by increasing the
contrast of the displayed image, up to the limits imposed by
signal-to-noise ratio.
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... Keywords: dual layer, flat panel detector, dual energy imaging, material decomposition, scatter correction [1,2] . ...
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... Dual-Energy (DE) radiography has found a variety of application in the medical imaging space with well-known clinical benefits [1][2][3][4][5][6][7][8][9] . The recent widespread adoption of digital X-ray detectors, together with advances in X-ray generators and a reduction in component cost, has resulted in an increased number of DE offerings by a variety of equipment manufacturers [10][11][12] . ...
We propose a new objective numerical figure of merit to aid in the evaluation and comparison of tissue-selective images generated from dual-energy radiography systems. A metric is developed through identification of the requirements of a successful objective metric and analyzed in a variety of scenarios. The proposed Dual-Energy Subtraction Efficiency (DSE) metric adequately describes a multitude of image properties for all simulated image scenarios, indicating its ability to accurately and objectively describe image quality. The DSE and its measurement method described can become a tool to characterize dual-energy radiographic image quality objectively and quantitatively, allowing for improved system comparison, development, and optimization.
... Dual-energy radiography using either different kVps or dual-layer detector is 5 able to obtain two radiographs or images, one at higher energy and the other at lower energy. By making weighted subtraction of these two images [1,2] or by performing a two-material decomposition [3], elimination of overlaying materials and enhancement of the selective target material can be achieved. ...
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Dual-energy X-ray radiography has been commonly used for materials separation. However, its performance is limited, especially for the separation of close materials or the identification of multiple materials. To cope with this problem, we propose to investigate the material decomposition ability of spectral radiography based on photon-counting detector. The latter having energy resolving capability can provide spectral information of several energy bins and thus enables selective imaging of multiple materials. In this framework, a classification-based patchwise regularized decomposition method was proposed to gain better differentiation between materials. It consists of performing several decompositions with reduced number of materials in the basis and classifying these decompositions using their cost function values. The results on simulations showed that, in the presence of Poisson noise, the method without classification can separate acrylonitrile-butadiene-styrene (ABS) from three kinds of flame retardants (FRs: brominated FR, chlorinated FR and phosphorus FR), but that the type of FR cannot be identified. With the classification technique, ABS and three kinds of FRs can be both separated and identified at the same time when the thickness was as large as 2 mm or 4 mm. The results on real data from physical photon-counting detector further confirm that the ABS, Br and Cl can be separated from each other.
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Computed tomographic (CT) is a fundamental imaging modality to generate cross-sectional views of internal anatomy in a living subject or interrogate material composition of an object, and it has been routinely used in clinical applications and nondestructive testing. In a standard CT image, pixels having the same Hounsfield Units (HU) can correspond to different materials, and it is therefore challenging to differentiate and quantify materials. Dual-energy CT (DECT) is desirable to differentiate multiple materials, but the costly DECT scanners are not widely available as single-energy CT (SECT) scanners. Recent advancement in deep learning provides an enabling tool to map images between different modalities with incorporated prior knowledge. Here we develop a deep learning approach to perform DECT imaging by using the standard SECT data. The end point of the approach is a model capable of providing the high-energy CT image for a given input low-energy CT image. The feasibility of the deep learning-based DECT imaging method using a SECT data is demonstrated using contrast-enhanced DECT images and evaluated using clinical relevant indexes. This work opens new opportunities for numerous DECT clinical applications with a standard SECT data and may enable significantly simplified hardware design, scanning dose and image cost reduction for future DECT systems.
... In 1977, Riederer et al. presented their work of selective iodine imaging by CT scans using three heavily ltered X-ray beams [Riederer and Mistretta, 1977]. Material selective imaging using dual-energy scan also extended to X-ray radiographic imaging [Brody et al., 1981. ...
X-ray computed tomography (X-ray CT) plays an important part in non-invasive imaging since its introduction. During the past few years, numerous technological advances in X-ray CT have been observed, including spectral CT, which uses photon counting detectors (PCDs) to discriminate transmitted photons corresponding to selected energy bins in order to obtain spectral information with one single acquisition.Spectral CT enables us to overcome many limitations of the conventional CT techniques and opens up many new application possibilities, among which quantitative material decomposition is the hottest topic. A number of material decomposition methods have been reported and different experimental systems are under development for spectral CT. According to the type of data on which the decomposition step operates, we have projection domain method (decomposition before reconstruction) and image domain method (decomposition after reconstruction). The commonly used decomposition is based on least square criterion, named proj-LS and ima-LS method. However, the inverse problem of material decomposition is usually ill-posed and the X-ray spectral CT measurements suffer from Poisson photon counting noise. The standard LS criterion can lead to overfitting to the noisy measurement data. In the present work, we have proposed a least log-squares criterion for projection domain method to minimize the errors on linear attenuation coefficient: proj-LLS method. Furthermore, to reduce the effect of noise and enforce smoothness, we have proposed to add a patchwise regularization term to penalize the sum of the square variations within each patch for both projection domain and image domain decomposition, named proj-PR-LLS and ima-PR-LS method.The performances of the different methods were evaluated by spectral CT simulation studies with specific phantoms for different applications: (1) Medical application: iodine and calcium identification. The decomposition results of the proposed methods show that calcium and iodine can be well separated and quantified from soft tissues. (2) Industrial application: ABS-flame retardants (FR) plastic sorting. Results show that 3 kinds of ABS materials with different flame retardants can be separated when the sample thickness is favorable.Meanwhile, we simulated spectral CT imaging with a PMMA phantom filled with Fe, Ca and K solutions. Different acquisition parameters, i.e. exposure factor and number of energy bins were simulated to investigate their influence on the performance of the proposed methods for iron determination.
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Recent research and advanced investigation on advanced sensors and transducers
The advent of new X-ray detectors, i.e. photon-counting detectors, introduced in spectral photon-counting computed tomography (SPCCT) systems allow analysis of the energy composition of the transmitted X-ray spectrum. By dividing the spectrum into well-chosen energy-based datasets, the specific and quantitative « multicolor » imaging of materials (such as gadolinium and gold), also known as K-edge imaging, is possible with such systems. Meanwhile, the field of nanoparticle contrast agents for CT has expanded rapidly over the past decade, with the greatest number of publications focusing on gold nanoparticles (AuNP). It turns out that AuNP are a good candidate for K-edge imaging, du to their high K-edge energy (~80.3 keV). In addition, they have the potential to circulate longer than iodinated contrast agents for improved blood pool imaging and to be highly biocompatible. The main objective of this thesis work is to evaluate in vitro and in vivo K-edge imaging using a preclinical prototype SPCCT system in combination with pegylated AuNP and clinically approved contrast agents based on iodine and gadolinium for imaging of the lumen and arterial wall in healthy and atherosclerotic rabbits. Our main contribution is to demonstrate that K-edge imaging is feasible in vivo in combination with AuNP to improve the specific assessment of the atherosclerotic arterial wall in comparison to conventional imaging via the quantification of its macrophage burden, as well as to perform « bicolor » imaging (i.e. simultaneous imaging of two agents) for specific differentiation between enhancement of the lumen with one iodinated contrast agent and enhancement of the aortic wall with K-edge AuNP.
Dual-energy mammography and the tissue-cancellation algorithm were applied to the problem of enhancing the detectability of calcifications and lesions in cluttered mammogram background. A multicomponent (plexiglass (PMMA), polypropylene (PP), water) tissue-equivalent breast phantom and a standard full-field digital mammography system operating in two of its standard operating modes (26 kVp, 40 mAs, Mo/Mo and 49 kVp, 6.3 mAs, Rh/Rh target/filter) were used. We studied to what extent the contrast between the tissue substitutes can be eliminated depending on the difference in their attenuation properties, and whether the obtained level of contrast cancellation is sufficient to unambiguously discriminate microcalcifications and inserts from the background. Mixed results were obtained. It was shown that the tissue-cancellation method can provide nearly perfect elimination of the background clutter (17-fold decrease in standard deviation) for homogeneous materials with close attenuation properties (PMMA and water) while increasing the contrast of a third material (PP, 1.7 times increase in contrast-to-noise ratio), thus facilitating its detection and delineation. However, cancelling the contrast between materials with more different attenuation properties (PP and water) does not simultaneously lead to any significant enhancement of the contrast of a third material (PMMA), if its attenuation properties are close to those of either PP or water. Moreover, intrinsic inhomogeneity of the phantom constituents becomes important as it results in incomplete contrast cancellation and preservation of visually significant background clutter. Although contrast cancellation increased the signal-to-noise ratio of microcalcifications by about 33%, the relative contrast calcifications–background can diminish, making it possible for the remaining clutter to obscure the microcalcifications.
Purpose: To improve dose reporting of CT scans, patient-specific organ doses are highly desired. However, estimating the dose distribution in a fast and accurate manner remains challenging, despite advances in Monte Carlo methods. In this work, we present an alternative method that deterministically solves the linear Boltzmann transport equation (LBTE), which governs the behavior of x-ray photon transport through an object. Methods: Our deterministic solver for CT dose (Acuros CTD) is based on the same approach used to estimate scatter in projection images of a CT scan (Acuros CTS). A deterministic method is used to compute photon fluence within the object, which is then converted to deposited energy by multiplying by known, material-specific conversion factors. To benchmark Acuros CTD, we used the AAPM Task Group 195 test for CT dose, which models an axial, fan beam scan (10 mm thick beam) and calculates energy deposited in each organ of an anthropomorphic phantom. We also validated our own Monte Carlo implementation of Geant4 to use as a reference to compare Acuros against for other common geometries like an axial, cone beam scan (160 mm thick beam) and a helical scan (40 mm thick beam with table motion for a pitch of 1). Results: For the fan beam scan, Acuros CTD accurately estimated organ dose, with a maximum error of 2.7% and RMSE of 1.4% when excluding organs with <0.1% of the total energy deposited. The cone beam and helical scans yielded similar levels of accuracy compared to Geant4. Increasing the number of source positions beyond 18 or decreasing the voxel size below 5 × 5 × 5 mm3 provided marginal improvement to the accuracy for the cone beam scan but came at the expense of increased run time. Across the different scan geometries, run time of Acuros CTD ranged from 8 to 23 s. Conclusions: In this digital phantom study, a deterministic LBTE solver was capable of fast and accurate organ dose estimates.
A study was undertaken in an attempt to describe causes of detection errors in quantitative terms. Multiple chest radiographs of patients with reported pulmonary nodules were collected, as were earlier films in which the patients' lesions had been overlooked (these lesions were visible in retrospect). With use of a parameter (the conspicuity of the lesion) calculated from densitometric measurements the lesions were sorted into two groups that correlated well with detection or miss by radiologists. Conspicuity was defined as a quantity that is directly proportional to the lesion's contrast and inversely proportional to the complexity of the background.
Information contained in the x-ray energy spectrum can be used to produce selective radiographic images of bone or soft tissue. A method has been devised to separate bone and soft tissue based upon differences in photoelectric absorption and Compton scattering using an appropriate combination of images obtained with radiographic exposures at 70 KVP and 140 KVP. Since photoelectric absorption is highly dependent upon atomic number, high atomic number materials such as calcium can be easily separated from water density substances. Using a prototype system for line-scanned radiography, selective subtraction of bone or soft-tissue has been implemented. Because this method uses a conventional broad-spectrum x-ray source, it was necessary to develop a nonlinear polynomial approximation to estimate tissue and bone thickness. The model was verified with phantom studies using water and aluminum. The application of this dual-energy bone and soft-tissue separation to chest radiography is demonstrated. This method allows accurate estimation of tissue and bone thickness and should find application to chest radiography for improved lesion detection and for bone mineral assessment.
Dual energy basis decomposition techniques apply to single projection radiographic imaging. The high and low energy images are non-linearly transformed to generate two energy-independent images characterizing the integrated Compton/photoelectric attenuation components. Characteristic linear combinations of these two basis images identify unknown materials, cancel known materials, and generate synthesized monoenergetic images. The problems of intervening materials and material displacement are solved in general for a wide class of clinical imaging tasks. The basis projection angle identifies one from a family of energy selective imaging tasks, and such performance measures as the contrast enhancement factor (CEF) and signal to noise ratio (SNR) are expressed as functions of this angle. Algorithms for the decomposition of high and low energy measurements are compared and experimental images are included.
A system is described for measuring the amount of bone and soft tissue in a path through any point in a radiograph. The basic information required is the transmitted intensity with two different source spectra. It is shown experimentally that this data may be used to accurately calculate the bone and soft tissue thicknesses.