Phase Contrast Imaging - Thesis Example

Phase Contrast Imaging Chapter 1 Introduction Ever since the detection of X-rays over a century ago, radiology has been one of the key research and analysis technologies for material science, life science and medical application subjects (Margaritondo, 2005). Until lately, however, it was only associated with the absorption of X-rays by the sample which is quite susceptible, this makes it likely to browse through the internal attributes of the sample, and at the same time limits the contrast. It is quite a known fact that such a limit has a significant influence on issues for instance, the mass screening of the number of individuals suffering from breast cancer or cardiovascular maladies (Margaritondo, 2005). As suggested by Fitzgerald, the fundamental principles of X-ray image formation as well as interpretation in radiography have stayed necessarily unaffected ever since Rontgen first discovered X-rays over a century ago (Fitzgerald, 2000). The traditional approach is reliant on X-ray absorption as a mere source of contrast, and also outlines chiefly on ray optics to define and interpret the formation of image. As suggested by Yacobi et al, Phase contrast is the most challenging and complicated mechanism for a beginner to imagine, however, at the same time, it is the most powerful mechanism for generating images with ultra-high resolution (Yacobi et al, 1994). Phase Contrast imaging, which is informally known as High Resolution or HR imaging, is a process of imaging in Transmission Electron Microscopy, and is one of the chief components that discriminates Transmission Electron Microscopy from traditional optical microscopy. Nevertheless, phase contrast imaging is often interpreted as synonymous to high-resolution TEM (Williams and Carter, 1996). Moreover, phase contrast microscopy produces high-contrast images of transparent samples such as cells or micro-organisms (Murphy, 2002). This ability commences from the fact that the atoms in a substance disseminate electrons as they pass through them, thereby, giving rise to diffraction in contrast, along with the distinction that is already present in the transmitted beam. Phase-contrast imaging contributes to the maximum imaging technology that has ever developed, and can also enable for resolutions ranging less than one angstrom, thus, allowing the straight viewing of lines of atoms in a crystalline substance. As suggested by Wilkinson and Schut, in phase contrast microscopy, the differences in refractive index are converted into differences in the image intensity (Wilkinson and Schut, 1998). The explanation of phase-contrast images is usually not a clear-cut task by any means. As viewed by Zhang, phase contrast images usually exhibit periodic contrast transformations or reversals (Zhang, 2001). The uncoiling of the differences viewed in the High Resolution image in order to identify the features as a result of which the atoms in the substance can hardly be performed with the naked eye. As an alternative, for the reason that the merger of contrasts as a reason of the multiple diffracting constituents as well as planes and the transmitted beam is diverse, the computer replications are brought in to use so as to identify what kind of distinct disparate structures may create in a phase-contrast image. As a point in fact, a sensible amount of information regarding the sample is required to be comprehended prior to the interpretation of a phase contrast image, for example a speculation about the constituents of the substance of the crystalline structure. Phase contrast takes place when a sample causes a phase shift from some of the electrons relative to other electrons (Immergut and Trigg, 1993). Phase contrast images are produced by the eradication of the objective opening totally, or by bringing in to use a substantial objective opening which makes sure that not only the transmitted shaft of light, but also the diffracted ones are enabled to add to the image. The implements which are particularly patterned for phase contrast imaging are usually referred to as high resolution transmission electron microscopes or HRTEMs, and vary from analytic Transmission Electron Microscopy, chiefly in the pattern of the electron beam support. Where on one hand, investigative TEMs involve supplementary detectors associated to the column for spectroscopic measurements, HRTEMs has little or no supplementary additions for assuring a moderate electro-magnetic atmosphere throughout the column for each of the beams, thereby, leaving the sample which can be either transmitted or diffracted. For the fact that phase-contrast imaging is reliant of the disparities in phases amongst the electrons that abandon the sample, any supplementary phase shift which takes place between the sample and the screen tends to make the image unlikely to construe, as a result of which, a considerably low grade of lens deviation is quite a prerequisite for HRTEMs, and developments in the spherical abnormality alteration have tended to allow an all new generation of HRTEMs to attain levels which were once interpreted as impossible. 1.2 Significance of Phase Contrast Imaging in Absorption Methods The utilization of phase-contrast imaging seems to provide with the tormenting possibilities of producing the largest transformation in medical X-ray imaging from the times since the discovery of calculated tomography (Lewis, 2004). A large number of experiments carried out by analysts across all the four continents have generated some unusual and extraordinary images which have representd tremendously improvements contrast over the conventional approaches, thereby, revealing the soft tissue distinguishing at micron range resolutions. Contrast enhancements are easy to attain at doses, and not less than the ones called for by traditional X-ray imaging. The use of synchrotrons has unleashed the possibilities presented by such technologies, however, remorsefully, the implementation of such ideas in a clinical context needs that technology be propelled to its constraints in a number of fields that are inclusive of c-ray reserves, optics and detectors. As is a known fact, X-rays are extensively used for analyzing the internal anatomy of several objects and proves to be a subject of tremendous global interest due to its capability of high penetrability into animal soft tissues that is associated with the short wavelength of x-rays (Davidson, 2003). Traditional radiographic imaging approaches are based upon the disparities between the photo-electric absorption of x-rays amongst the soft-tissue and bones which can also be interpreted as contrast media. Sadly, in cases where high energy is exploited to reflect deep body tumors, the image contrast of soft-tissues as a result of absorption reduces noticeably. As Davidson noticed, this takes place because low-Z elements such as Carbon-based organic soft-tissue with a standard atomic number of Z equalizing 7.64 fail to considerably absorb high-energy medical X-rays which happen to be ranging between 15Kev and 100Kev. Soft-tissues are usually apparent to such hard x-ray photons and moreover, the calcium in bones possesses a much higher Z-value of 20, whereas, iodine in contrast media possesses a Z-value of 53 (Davidson, 2003). Phase contrast imaging, with the help of ability of non-destructive measuring of abstract samples in situations, has proven to be a meticulously significant and usual technology for experiments associated with beams of light. For the reason that, phase contrast imaging has developed in to the plodder for abstract assessment in various experiments, a large amount of evaluations is devoted all over examinations for outlining an appropriate quantitative model of the techniques. Phase contrast imaging is essentially dissimilar from traditional X-ray imaging methods for the reason that the mechanism of image formation does not usually depend upon the discrepant absorption by tissues. Instead, X-ray beams experience discrepant phase shifts in passing through an appendage, thereby, consequently intervening either constructively or destructively at the X-ray camera (Christoph et al, 2007). As a result, tissues are discriminated by their different indices of refraction, and no by their absortive characteristics. The imaging approach is largely vulnerable towards density changeableness of the tissues than traditional absorption approaches and allows imaging of soft tissue with high contrast with no absence of the usage of contrast agents (Christoph et al, 2007). In comparison to X-ray absorption imaging, phase contrast imaging is better appropriated for the illustration of soft-tissue structures which do not considerably tend to absorb x-rays, however, may comprise of various non-absorptive structural features along with dimensions between one micron and one millimeter. Phase contrast imaging contributes to a technology which provides with changeableness in the refractive index of a non-absorbing object which is quite visible. A shift in the phase of x-ray photons is characterized by slight inclinations from their incident course as they pass through an object such as animal soft-tissues that takes place post the brief and elastic interaction of the photons with the atoms in their course. A phase-shift contributes to a type of deflection of the incident beam within a substance which is typically in the range of 1 to 10 micro-radians (Davidson, 2003). When sufficiently understood, the phase-shift transfers the intensity of the deflected beam of light to an all new different position on the detector, for instance, an adjacent pixel which is either in the x- or y-direction. Rational or coherent light is a prerequisite for phase-contrast imaging, and may be demonstrated as a collection of beams which appear to be parallel to the optical axis. A rational ray of light may be generated by lasers as obvious, UV or IR frequencies. However, currently a coherent or rational beam of light can be produced merely by bringing into use the synchrotron undulators. The x-ray phase shifts that are experienced by an incident beam of light can be viewed as a micro-radian deflection just in case of implementation of a coherent beam of incident light, where there occurs no sloping ray divergence for illuminating the object under analysis (Davidson, 2003). Various radiographic tasks that rely on absorption effects merely result in a deficiency of image contrast where Phase Contrast imaging resolves this difficulty as it is based on the phase shift attribute which is a different physical effect represented by "d" or delta in the image below. In case of human tissues and in the energy range of radiology, d proves to be much bigger than "b" or beta (Research Avtivities, 2005). d and b as a function of energy in KeV for biological tissue (Research Activities, 2005) As a result, the effects as a result of d are considerably pertinent than the ones as a consequence to b. The relevance and significance of a clinical application of phase contrast imaging in quite obvious, for the reason that various radiological fields will tend to seek advantage from such novelty. Mere instances are inclusive of previous tumor detection in mammography, improved imaging of bone anatomy, improved lung-imaging of blood-vessels in the absence of contrast media (Research Activities, 2005). Following is an example of phase contrast imaging in comparison to the traditional imaging: Figure: Conventional Imaging (Research Activites, 2005) Figure: Phase-Contrast Imaging (Research Activities, 2005) Revolutionary investigations which have been carried out bringing into use synchrotron radiation have representd that such a technique of phase-contrast imaging results in a considerable increase in mage contrast and featured visibility, thereby, enabling the investigation and identification of structures which are invisible by means of conventional technologies (Research Activities, 2005). Post the preliminary investigations that were carried out on thin and ordinary samples, human tissue samples have appeared to be analyzed delineating promising outcomes. However, the chief restraint to the extensive implementation of phase contrast imaging is the requirement of exploiting a coherent X-ray source along with a detector which possesses high spatial resolution. Nevertheless, it has been illustrated that pertinent edge improvement effects are yet attained if only the collaborative influence of source and recognizer is positioned within specific constraints (Research Activities, 2005). This new technology of phase contrast imaging, as a result, illustrates that considerable enhancements in image quality can be attained with the use of "off the shelf" implementation similar to the traditional mammography bases and flat panel recognizers. Furthermore, it resolves a number of difficulties associated with the previous conventional techniques. Firstly, it is able of use phase effects in two-dimension with the help of creating arrays of square apertures, and not elongated slits. Secondly, it performs with entirely polychromatic radiation. Thirdly, it carries out its function with diverging rays of light. Fourthly, it is insensate to vicinal vibrations and can be elongated to large areas of view. Lastly, it pays no attention to the inessential dose delivery to the previous sample coded aperture process. There is yet another attribute that adds to the clear detailing and enhancement in contrast as a result of phase contrast imaging is the fascinating feature, which lay in the recognition of a number of faint and blur structures that brim a tumour and are rarely visible in the absorption image (Research Activities, 2005). This denotes the likeliness of betterment in analysis, description of the tumour constraints along with discrimination between gentle and rough formations. More to it, it proposes that tumours which are at a very early phase of formation and are still imperceptible to traditional techniques can be easily recognized by phase contrast imaging. Chapter 2 2.1 The Methods used for Phase contrast Imaging 2.1.1 X-ray Phase Contrast Imaging using Interferometer In traditional X-ray imaging, contrast is attained by means of the difference in the absorption cross section of the elements of the object (Pfeiffer et al, 2005). The technology gives away supreme outcomes where tremendously absorbing structures for example, bones, that are implanted in a grid of relatively susceptibly absorbing substance. Nevertheless, in such cases where diversified forms of tissues with like absorption attributes are under analysis, the absorption of x-ray contrast is associatively pitiable. As a result, the differentiation of pathologic from non-pathologic tissue by means of absorption radiograph as attained with a present hospital-based X-ray system still stays practically unlikely for many of the tissue compositions (Pfeiffer et al, 2005). As a consequence, in order to overcome these constraints, there have been generated various methods and approaches to produce radiographic contrast by means of the phase shift of X-rays that pass through a sample, as investigated in recent years (Fitzgerald, 2000). They can well be categorized in to crystal interferometer methods, analyzer techniques, and free-space proliferation techniques. Even though, some of them tend to give out better outcomes for particular difficulties, none of them is extensively brought into use. In precise terms, none of them is capable of having found out medical analyses applications which call for a substantial area of view of many centimetres, the efficacious exploitation of broadband radiation, as facilitated by the laboratory X-ray producers and a considerably compact set-up. As an optional methodology, differential phase contrast setup can efficaciously be brought into use in order to recover quantitative phase images possessing polychromatic X-ray sources of low brilliancy (Pfeiffer et al, 2006). The consequences lately represent a chief step forward in radiography with average X-ray tube sources which are capable of facilitative with all of the information communicated by traditional radiography, along with supplementary knowledge on soft-tissues. This tends to prove its significance for clinical purposes, specifically in the recognition of soft-tissue pathologies. This anticipation is extraordinarily held up by the actuality that differential phase contrast or DPC approaches are usually capable of illustrating benefits in the imaging of tumour masses with associatively steady inconsistencies of the integrated phase shift, in case of comparison with other phase contrast imaging approaches and methods, for example, free space propagation, hybrid, diffraction enhanced imaging, etc (Pagot et al, 2005). As a result of the outcomes which have been attained with a standard, associatively low-cost and economically accessible X-ray tube producer, a widespread implementation of the interferometer methods is envisaged where phase imaging would come up to be desirable, however, is presently inaccessibly. For instance, it is convicted that such an approach of interferometer usage can easily be applied without key transformations to presently prevailing medical imaging systems, meticulously in the view of the ease of manufacturing large-area gratings by bringing into use the average photolithography, the high confrontation of the approach alongside mechanical unsteadiness and the likeliness of using the recognizers with considerably large pixels and a large area of view (Pfeiffer et al, 2005). For the fact that phase contrast imaging does not internally depend upon the absorption of rays in a substance, the radiation dose tends to potentially degrade with the usage of higher X-ray synergies. Moreover, there are perceived various applications of this method in the non-destructive analyses, for example, the classification of surface non-uniformities of reflective X-ray optics (Weitkamp et al, 2005). Eventually, these outcomes have introduced the ways for phase imaging investigations by using other radiation appearances, for which sources of associatively low brilliancy are existent, for example, beams of neutrons or atoms (Keith et al, 1991). 2.1.2 Diffraction Enhanced Imaging The technique called Diffraction Enhanced Imaging facilitates with all of the knowledge communicated by the traditional X-rays along with detailed information on the soft-tissue attributes that were previously accessible only with supplementary scanning approaches for example, ultrasound or magnetic resonance imaging (Li et al, 2003). This technique brings into use the intense beams of X-rays that are accessible at synchrotron sources such as the NSLS, and are substantially brighter than the ones generated by traditional X-ray tubes, thereby, facilitating with enough monochromatic X-ray flux for the sake of imaging post the selection of a single wavelength (Li et al, 2003). Where on one hand, in traditional X-ray images, the several shades of grey are generated for the reason that different tissues tend to absorb different amounts of X-ray synergy, it works appreciably in imaging the bones and other calcified tissues, however, less adequately in the imaging of soft-tissues which possess like and low X-ray absorption. On the other hand, in Diffraction Enhanced Imaging, the science-workers are quite interested and curious about the X-rays which pass through the tissues and the way they incline and scatter as they perform, for the reason that these attributes differ more faintly between different types of tissues (Li et al, 2003). (a) A conventional synchrotron radiograph of a foot (Li et al, 2003). (b) The same foot in Diffraction Enhanced Imaging (Li et al, 2003). In order to analyze a sample with Diffraction Enhanced Imaging, the science-workers position a perfect silicon crystal between the sample and the image recognizer. In procedure, as X-rays from the synchrotron pass through the sample, they turn or refract or scatter different amounts, relying upon the composition and microscopic anatomy of the tissue in the sample. Further, when the severally reflected rays abandon the sample, thereby, striking the silicon crystal, they diffract by varied amounts in accordance with their angular spread. As a result, the silicon crystal aid in the conversion of subtle disparities in scattering the angles as generated by the disparate tissues into concentration dissimilarities, which can further be swiftly recognized by a traditional X-ray recognizer or detector. This results in tremendously detailed icons or images which are susceptible to soft tissue kinds (Li et al, 2003). The Diffraction Enhanced Images have been generated with a lower X-ray dose than the one used for diagnostic X-rays without any requirement of contrast agent, thereby, making the technique quite practicable as a potential screening implement (Li et al, 2003). The scientists and researchers are still carrying out their investigations on how to scale down the diffraction Enhanced Imaging design so that it can be brought into use in a clinical setting. However, they claim that this is likely to be possible and that the method may lately tremendously improve mammography and develop into an increasingly significant attribute in the detection of other soft tissue pathologies, for instance, osteoarthritis, lung and breast cancer, etc (Li et al, 2003). 2.1.3 Phase-contrast X-ray Imaging Combining Free-space Propagation and Bragg Diffraction Regardless of their common evolvement, X-ray phase contrast imaging techniques are construed separately in the literature studies, both in theoretical and practical analyses. However, with the help of using a coherent X-ray beam and opting for a suitable setup, two techniques that are propagation based imaging and analyzer based imaging can be united together so as to produce images which are capable of delineating original attributes. This technique is referred to as Hybrid Phase Contrast Imaging (Coan et al, 2005). The propagation based method comprises of recording the intervention pattern that is generated by opting for one or various sample-to-recognizer distances, and the theoretical definition of which is based on Fresnel diffraction (Cloetens et al, 1996). On the other hand, the analyzer based technology comprises of positioning a perfect crystal in Bragg geometry between the sample as well as the recognizer (Coan et al, 2005). The crystal imitates as a filter for the radiation that is refracted and spread inside the sample, for the reason that it merely accepts a narrowed sequence of angles of the incident ray positioned at the Bragg angle for the particular synergy (Bravin, 2003). As a result, Hybrid Imaging is a combination of both of the above stated techniques that are carried out practically by analyzing the wave transmitted by the sample post propagation in air by means of a perfect crystal substance. The practical statistics are obtained by using 25 KeV X-rays, along with a Si(111) analyzer crystal, as well as pure phase objects, and thin cylindrical polymer fibres (Coan et al, 2005). The Hybrid Imaging technique unites the propagation and diffraction effects such as the Propagation based signal that is identified in first estimate by the Laplacian of the phase as prefaced by the object and by the sample-recognizer distance. The Propagation-based signal passes through the Analyzer based crystal that acts as if it is a band-pass filter, as a result, making only a few frequencies to reach the recognizer. The hybrid images represent a feeble reliance on the angular positioning of the analyzer, thereby, alleviating the limitations in terms of optical alignment as well as the steadiness of the crystal which contribute to some serious drawbacks of the Analyzer-Based technique of Phase Contrast Imaging. In order to entirely comprehend the strange attributes of the Hybrid Imaging signal, further theoretical as well as practical analyses are a call for. This technique tends to find implementation in various areas that are already developed by hard X-ray phase-contrast imaging, along with bringing the advantages of the superior refraction sensitivity of the Analyzer-based technique. Chapter 3 3.1 Phase Retrieval Phase retrieval is the process of attempting to regain the wave-front error or fault, provided a measurement of the PSF (White, 1994). Phase retrieval methods have been brought into use ever since the discovery of the deviation in the Hubble Space Telescope primary errors so as to classify the HST system (Fienup et al, 1993). The efforts associated with phase retrieval which were carried out previously had been targeted chiefly at an exact measurement of the spherical abnormality. Hence, today scientists use similar technologies along with some innovatively developed systems for enhancing their comprehension of the various optical systems. Phase retrieval has got a lot of attributes in common with deconvolution, and many of the methods and approaches that are being brought in to use for image restoration possess counterparts for phase retrieval (White, 1994). Nevertheless, the equation associating the phase of the viewed PSF comes out to be non-linear in the phase retrieval difficulty, which makes the procedure of retaining substantially more complicated and challenging than image restoration. A specific problem that arises in phase retrieval is that maximum possibility approaches for finding the phase appear to get wedged at the local maxima of the possibility and not in finding the internationally finest solution (White, 1994). According to Combettes, in its general form, the problem of signal recovery is to estimate the original form of a signal 'x' in an operational space 'L' from the measurements of physically associated signals and Priori information (Combettes, 1996). In problems associated with phase-retrieval, the measurements comprise of the modulus of 'm' of the Fourier change bx of x. It can be interpreted that the imaging model is defined by the relationship |bx|= m, (Bauschke et al, 2002) where x refers to the object of the imaging model. A general signal space which suitably mocks up the fundamental physics is the complex Hilbert space whose equation is L= L2[RN,C] (Bauschke et al, 2002) As a result, a signal 'x' in L refers to a square integrable function mapping a ceaseless variable t RN to a complex number x(t) C (Bauschke et al, 2002). Along with the imaging model, a significant piece of information which is ideally accessible in phase retrieval problems is that the support of x is held in some set D C RN. In case 1E denotes the characteristic function of a set E C RN, the object domain constraint impounds x to the set S= {y L : y 1CD = 0} The phase retrieval problem can be proposed as that of locating a function x L which is capable of satisfying the following two constraints: X S M The above formulation denotes the phase retrieval problem as an inability of finding a point in the junction of constraint sets that is a set theoretic approximation problem in the sense of equation of object domain constraint. Such mproblems are referred to as feasibility problems (Bauschke et al, 2002). Phase retrieval has developed in to a significant implement in many different fields of optics. Its usage in recognition of the abnormalities present in the Hubble Space Telescope has helped in the deblurring of images which were already taken, thereby, producing corrective optics to fix the Hubble in Space. More to it. NASA has been planning on the exploitation f phase retrieval so as to align the James Webb Space Telescope (Brady and Fienup, 2004). Lately, the implementations may prove to be of enormous help in the changing of optical metrology, along with remote sensing fields. With the present computer proficiencies, phase retrieval algorithms may prove to be more challenging, which in turn may enable all these other applications and systems to be possible. 4. Conclusions The X-ray radiographic absorption imaging has proved to be a precious implement in medical analyses and material science titles, particularly for biological tissue samples, polymers or fibre composites (Pfeiffer et al, 2006). Nevertheless, the usage of traditional or conventional X-ray radiography is restrained for the reason that they facilitate with weak absorption. This is resolved at tremendously excellent X-ray synchrotron sources by bringing into use phase-sensitive imaging methods which tend to enhance the contrasts (Momose, 2003). Still, the prerequisites of the illuminating radiation construe to the fact that, till date, hard X-ray phase sensitive imaging has been theoretical with swiftly accessible X-ray sources such as X-ray tubes. Hence, this dissertation has reported a set-up which represents all about phase contrast imaging along with its significance in the absorption methods. Moreover, it explains all the advantages of contrast-enhanced phase sensitive imaging, and is entirely well-matched with the traditional absorption radiography techniques. It is effectually applicable to the X-ray medical imaging, non-destructive industry analyses, along with other low-brilliancy radiation techniques, such as those of the neutrons and atoms (Pfeiffer et al, 2006). The phase contrast imaging method can swiftly be implemented with least or no key transformations to the currently prevailing medical imaging systems, chiefly in view of the ease of manufacturing substantial gratings by using standard photolithography, along with the likeliness of using recognizers or detectors having large pixel attributes and large field of view (Pfeiffer et al, 2006). Such efficacious outcomes open the course for further phase-imaging experiments using other forms of radiation, for which there are sources of associatively low brilliancy presently are existent, such as neutrons or atoms (Keith et al, 1991). References 1. Bauschke, H. H. Combettes, P. L. and Luke, D. 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