Embossed radiography utilizing energy subtraction
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- Osawa, A., Watanabe, M., Sato, E. et al. Radiol Phys Technol (2009) 2: 77. doi:10.1007/s12194-008-0048-8
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Currently, it is difficult to carry out refraction-contrast radiography by using a conventional X-ray generator. Thus, we developed an embossed radiography system utilizing dual-energy subtraction for decreasing the absorption contrast in unnecessary regions, and the contrast resolution of a target region was increased by use of image-shifting subtraction and a linear-contrast system in a flat panel detector (FPD). The X-ray generator had a 100-μm-focus tube. Energy subtraction was performed at tube voltages of 45 and 65 kV, a tube current of 0.50 mA, and an X-ray exposure time of 5.0 s. A 1.0-mm-thick aluminum filter was used for absorbing low-photon-energy bremsstrahlung X-rays. Embossed radiography was achieved with cohesion imaging by use of the FPD with pixel sizes of 48 × 48 μm, and the shifting dimension of an object in the horizontal direction ranged from 100 to 200 μm. At a shifting distance of 100 μm, the spatial resolutions in the horizontal and vertical directions measured with a lead test chart were both 83 μm. In embossed radiography of non-living animals, we obtained high-contrast embossed images of fine bones, gadolinium oxide particles in the kidney, and coronary arteries approximately 100 μm in diameter.
KeywordsEmbossed radiographyDigital subtractionEnergy subtractionContrast resolutionPolychromatic X-rays
Extremely clean monochromatic parallel X-ray beams have been formed by use of synchrotrons and single silicon crystals, and these beams have been applied to carrying out enhanced iodine K-edge angiography [1–3], phase-contrast radiography [4–6], and topography [7, 8]. For performing K-edge angiography, monochromatic X-rays with energies just beyond the iodine K-edge of 33.2 keV have been used, because these rays are absorbed effectively by iodine-based contrast media. Currently, phase-contrast radiography is based primarily on X-ray refraction in objects, and soft tissues, such as breast cancers , can be imaged with high contrast. However, it is difficult to carry out phase-contrast imaging of contrast media for medical angiography, delivered nano-particles, and hard tissues. Two-dimensional X-ray topography also is a method for imaging defects in crystals by utilizing X-ray diffraction.
Without the use of synchrotrons, several different monochromatic X-ray sources [9–12] have been developed, and K-edge angiography, phase radiography, and topography have been carried out. In particular, a cerium X-ray generator [13, 14] has been developed and has been applied to carrying out cone-beam K-edge angiography because cerium Kα rays (34.6 keV) are absorbed effectively by iodine media. Next, phase radiography for edge enhancement of objects has been performed by magnification radiography with a microfocus X-ray generator and polychromatic X-rays . However, this conventional phase radiography is usable for imaging only of soft tissues, and a microfocus X-ray tube is necessary. Therefore, we are very interested in the development of a novel radiography system instead of the phase imaging by use of a generalized digital X-ray image sensor and a conventional large-focus tungsten tube that produces polychromatic X-rays.
Energy subtraction radiography [16, 17] is an important technique for imaging target regions in vivo by removing muscle and bone regions from radiograms. Currently, two different energy radiograms are obtained with two different tube voltages, and dual-energy subtraction radiography is carried out by digital subtraction between the high- and low-energy images. Furthermore, embossed radiography (ER)  is a novel technique for constructing concavoconvex radiography, such as phase-differential imaging, and is realizable with digital image subtraction after image shifting with an optimal dimension between two images. The image shifting is carried out by moving an X-ray source , by moving an object precisely, and by shifting image pixels in a flat panel detector (FPD) with use of a computer program before subtraction. By use of this radiography, the target region in various objects can be imaged with embossing, and the maximum contrast resolution is achieved without a drop in spatial resolution. In addition, our ultimate goal of embossing is the development of a novel computed tomography (CT) system with a high contrast resolution for cancer diagnosis that utilizes a drug delivery system.
In our research, major objectives are as follows: construction of edge enhancement radiography such as phase-differential imaging utilizing embossing, an increase in the contrast resolution of the target region, and energy subtractions of contrast media and nano-particles. Therefore, we carried out preliminary experiments for ER utilizing two-shot dual-energy subtraction by shifting objects.
Experimental setup for ER
Measurement of X-ray intensity
Measurement of X-ray spectra
The image contrast varies corresponding to the X-ray spectra for radiography, and we have to measure the spectral distributions before energy subtraction. In order to measure X-ray spectra, we employed a cadmium telluride (CdTe) detector (XR-100T, Amptek, Bedford, MA) and measured spectra at 1.0 m from the X-ray source. The cooled detector unit with a charge amplifier was set at 1.0 m from the X-ray source, and event signals from the detector unit were amplified again by a shaping amplifier unit. A 0.1-mm-diameter lead pinhole was set in front of the detector facing the X-ray source to decrease the photon count rate. The photon energy was discriminated by a multichannel analyzer (MCA-8000A, Amptek, Bedford, MA), and the X-ray spectra were observed on a personal computer monitor. Measurement results for the X-ray spectra are important for carrying out dual-energy subtraction because the spectral distribution is the X-ray intensity as a function of the photon energy. In particular, when we perform K-edge subtraction, we have to confirm the spectra with energies below and beyond the K-edge energy, and the spectra should be controlled to optimal distributions by selection of the tube voltage, the element of the filter, and its filter thickness.
Method for ER utilizing single-energy subtraction
The X-ray exposure time was 5.0 s, and two embossed radiograms were obtained with K values of 1.0 and 0.7. At a K of 1.0, the absorption contrast is removed, and the embossed effect is confirmed easily. When K is decreased, the absorption contrast increases, and the embossed effect decreases. Because the absorption contrast seldom varied in a K range from 0.5 to 0.7, K was determined as 0.7 for performance of single-energy ER with absorption contrast.
Method for ER utilizing dual-energy subtraction
Measurement of spatial resolution
Making of gadolinium oxide particles
A gadolinium oxide suspension for injection was made in a high-pressure machine (Starburst Mini, Sugino Machine, Namerikawa, Japan) for dispersing micro-particles. The high-pressure dispersing was carried out five times with use of physical saline, and the average particle diameter and the density of gadolinium oxide were 700 nm and 20%, respectively.
Objects for ER
For performing ER, we used five objects as follows: a nonliving nude mouse with a weight of 15 g, a lead test chart (PTW, L659035), an extracted rabbit kidney (8 g), an extracted rabbit heart (11 g), and a dried pig vertebra (22 g). The mouse, kidney, and heart were preserved in 10% formalin solution. The kidney and heart were extracted from two different rabbits with weights of approximately 2 kg after anesthetic (5% Nembutal) injection of 1.8 ml in an ear. A renal pelvis in the kidney was filled with gadolinium oxide suspension of 0.3 ml as described above by injection, and coronary arteries were filled with iodine-based microspheres. When a 20%-microsphere suspension by use of physical saline was injected into the arteries, the saline penetrated capillaries with diameters below 10 μm and reached the veins. Thus, the 37%-iodine microspheres 15 μm in diameter remained in the arteries.
ER utilizing single-energy subtraction
ER utilizing dual-energy subtraction
Conclusion and outlook
To image a target region in vivo, we have to confine the X-ray spectra to optimal distributions for energy subtraction, and unnecessary regions for diagnosis should be deleted from radiograms. However, in cases where we carry out only embossment, single energy subtraction is usable. The tube voltage should be selected corresponding to radiographic objectives, and we have to use an optimal-element filter and to determine its effective thickness.
With this FPD, the exposure time was 5.0 s because the exposed dose rate from the 100-μm-focus tube was low owing to the maximum tube current of 0.5 mA. In addition, focal spot diameters below 1.0 mm are usable, and the exposure time decreases substantially with increasing spot diameter. Recently, a program for two-dimensional pixel shifting has been developed, and one-shot ER is realizable with single-energy subtraction.
Magnification radiography [19, 20] is useful for improving the spatial resolution in digital radiography. The spatial resolution improves with increasing magnification ratio, and the maximum ratio increases with decreasing focus diameter. Therefore, magnification ER utilizing energy subtraction can be performed, and a program for digital subtraction including a pixel-shifting function is useful for composing embossed images.
As compared with conventional radiography, the advantages of dual-energy ER are as follows: (1) contrast resolution can be increased up to approximately 1.0. (2) Edge-enhancement radiography such as phase-differential imaging is realizable. (3) Hard tissues are imaged effectively as compared with phase imaging. (4) Nano-particles and liquid contrast media in medical angiography can be observed as concavoconvex images with high contrast.
In human imaging, a computed radiography system is usable, and various applications will become possible as follows: angiography with iodine and gadolinium media, cancer diagnosis with nano-particles, hard-tissue (bone) imaging, and various radiographies with high contrast resolutions. In addition, an energy discriminating FPD system that can get spectral data is very useful for carrying out single-shot dual-energy subtraction with short exposure times.
This ER utilizing dual-energy subtraction is usable in various digital radiography systems, including a CR system. Therefore, we developed an ER program for CR utilizing a two-dimensional image-shifting function. However, it was difficult to carry out dual-energy ER precisely by using two imaging plates. We are currently developing a multi-slice mini-focus X-ray CT system utilizing a 100-μm-focus tube, and embossed tomography with use of single- and dual-energy subtractions would be employed for increasing the contrast resolution of the target region. In our research, energy subtraction has been effective for imaging iodine contrast media and gadolinium oxide nano-particles, and ER is useful for edge enhancement radiography like phase imaging and for increasing the contrast resolution without a decrease in the spatial resolution.
We would like to express our thanks to the reviewers for giving very helpful advice concerning our paper. This work was supported by Grants-in-Aid for Scientific Research and Advanced Medical Scientific Research from MECSST, Health and Labor Sciences Research Grants, Grants from the Keiryo Research Foundation, Promotion and Mutual Aid Corporation for Private Schools of Japan, the Japan Science and Technology Agency (JST), and the New Energy and Industrial Technology Development Organization (NEDO).