Refraction-enhanced x-ray imaging of mouse lung using synchrotron radiation source

Structural properties in ruptured mitral chordae tendineae measured by synchrotron-based X-ray phase computed tomography

The link between the structural properties and the rupturing of chordae tendineae in the mitral valve complex is still unclear. Synchrotron-radiation-based X-ray phase computed tomography (SR-XPCT) imaging is an innovative way to quantitatively analyze three-dimensional morphology. XPCT has been employed in this study to evaluate the chordae tendineae from patients with mitral regurgitation and to analyze structural changes in the ruptured chordae tendineae in patients with this condition. Six ruptured mitral chordae tendineae were obtained during surgical repairs for mitral regurgitation and were fixed with formalin. In addition, 12 healthy chordae tendineae were obtained from autopsies. Employing XPCT (effective pixel size, 3.5 µm; density resolution, 1 mg cm-3), the density of the chordae tendineae in each sample was measured. The specimens were subsequently analyzed pathologically. The mean age was 70.2 ± 3.0 in the rupture group and 67.2 ± 14.1 years old in the control group (p = 0.4927). All scans of chorda tendineae with SR-XPCT were performed successfully. The mean densities were 1.029 ± 0.004 in the rupture group and 1.085 ± 0.015 g cm-3 in the control group (p < 0.0001). Density based on SR-XPCT in the ruptured mitral chordae tendineae was significantly lower compared with the healthy chorda tendinea. Histological examination revealed a change in the components of the connective tissues in ruptured chorda tendinea, in accordance with the low density measured by SR-XPCT. SR-XPCT made it possible to measure tissue density in mitral chordae tendineae. Low density in mitral chordae tendineae is associated with a greater fragility in ruptured mitral chordae tendineae.

High-energy X-ray micro-laminography to visualize microstructures in dense planar objects

High-energy X-ray micro-laminography has been developed to observe inner- and near-surface structures in dense planar objects that are not suitable for observation by X-ray micro-tomography. A multilayer-monochromator-based high-intensity X-ray beam with energy of 110 keV was used for high-energy and high-resolution laminographic observations. As a demonstration of high-energy X-ray micro-laminography for observing dense planar objects, a compressed fossil cockroach on a planar matrix surface was analyzed with effective pixel sizes of 12.4 µm and 4.22 µm for wide field of view and high-resolution observations, respectively. In this analysis, the near-surface structure was clearly observed without undesired X-ray refraction-based artifacts from outside of the region of interest, a problem typical in tomographic observations. Another demonstration visualized fossil inclusions in a planar matrix. Micro-scale features of a gastropod shell and micro-fossil inclusions in the surrounding matrix were clearly visualized. When observing local structures in the dense planar object with X-ray micro-laminography, the penetrating path length in the surrounding matrix can be shortened. This is a significant advantage of X-ray micro-laminography where desired signals generated at the region of interest including optimal X-ray refraction effectively contribute to image formation without being disturbed by undesired interactions in the thick and dense surrounding matrix. Therefore, X-ray micro-laminography allows recognition of the local fine structures and slight difference in the image contrast of planar objects undetectable in a tomographic observation.

Functional lung imaging with synchrotron radiation: Methods and preclinical applications

Many lung disease processes are characterized by structural and functional heterogeneity that is not directly appreciable with traditional physiological measurements. Experimental methods and lung function modeling to study regional lung function are crucial for better understanding of disease mechanisms and for targeting treatment. Synchrotron radiation offers useful properties to this end: coherence, utilized in phase-contrast imaging, and high flux and a wide energy spectrum which allow the selection of very narrow energy bands of radiation, thus allowing imaging at very specific energies. K-edge subtraction imaging (KES) has thus been developed at synchrotrons for both human and small animal imaging. The unique properties of synchrotron radiation extend X-ray computed tomography (CT) capabilities to quantitatively assess lung morphology, and also to map regional lung ventilation, perfusion, inflammation and biomechanical properties, with microscopic spatial resolution. Four-dimensional imaging, allows the investigation of the dynamics of regional lung functional parameters simultaneously with structural deformation of the lung as a function of time. This review summarizes synchrotron radiation imaging methods and overviews examples of its application in the study of disease mechanisms in preclinical animal models, as well as the potential for clinical translation both through the knowledge gained using these techniques and transfer of imaging technology to laboratory X-ray sources.

Synchrotron Radiation-based X-ray phase-contrast imaging of the aortic walls in acute aortic dissection

Objective

Synchrotron radiation-based X-ray phase-contrast tomography (XPCT) imaging is an innovative modality for the quantitative analysis of three-dimensional morphology. XPCT has been used in this study to evaluate ascending aorta specimens from patients with acute type A aortic dissection (ATAAD) and to analyze the morphologic structure of the aortic wall in patients with this condition.

Methods

Aortic specimens from 12 patients were obtained during repairs for ATAAD and were fixed with formalin. Five patients had Marfan syndrome (MFS), and seven did not. In addition, six normal aortas were obtained from autopsies. Using XPCT (effective pixel size, 12.5 μm; density resolution, 1 mg/cm³), the density of the tunica media (TM) in each sample was measured at eight points. The specimens were subsequently analyzed pathologically.

Results

The density of the TM was almost constant within each normal aorta (mean, 1.081 ± 0.001 g/cm³). The mean density was significantly lower in the ATAAD aortas without MFS (1.066 ± 0.003 g/cm³; P < .0001) and differed significantly between the intimal and adventitial sides (1.063 ± 0.003 vs 1.074 ± 0.002 g/cm³, respectively; P < .0001). The overall density of the TM was significantly higher in the ATAAD aortas with MFS than those without MFS (1.079 ± 0.008 g/cm³; P = .0003), and greater variation and markedly different distributions were observed in comparison with the normal aortas. These density variations were consistent with the pathologic findings, including the presence of cystic medial necrosis and malalignment of the elastic lamina in the ATAAD aortas with and without MFS.

Conclusions

XPCT exhibited differences in the structure of the aortic wall in aortic dissection specimens with and without MFS and in normal aortas. Medial density was homogeneous in the normal aortas, markedly varied in those with MFS, and was significantly lower and different among those without MFS. These changes may be present in the TM before the onset of aortic dissection.

MÖNCH detector enables fast and low-dose free-propagation phase-contrast computed tomography of in situ mouse lungs

Due to the complexity of the underlying pathomechanism, in vivo mouse lung-disease models continue to be of great importance in preclinical respiratory research. Longitudinal studies following the cause of a disease or evaluating treatment efficacy are of particular interest but challenging due to the small size of the mouse lung and the fast breathing rate. Synchrotron-based in-line phase-contrast computed tomography imaging has been successfully applied in lung research in various applications, but mostly at dose levels that forbid longitudinal in vivo studies. Here, the novel charge-integrating hybrid detector MÖNCH is presented, which enables imaging of mouse lungs at a pixel size of 25 µm, in less than 10 s and with an entrance dose of about 70 mGy, which therefore will allow longitudinal lung disease studies to be performed in mouse models.

X-ray specks: low dose in vivo imaging of lung structure and function

Respiratory health is directly linked to the structural and mechanical properties of the airways of the lungs. For studying respiratory development and pathology, the ability to quantitatively measure airway dimensions and changes in their size during respiration is highly desirable. Real-time imaging of the terminal airways with sufficient contrast and resolution during respiration is currently not possible. Herein we reveal a simple method for measuring lung airway dimensions in small animals during respiration from a single propagation-based phase contrast x-ray image, thereby requiring minimal radiation. This modality renders the lungs visible as a speckled intensity pattern. In the near-field regime, the size of the speckles is directly correlated with that of the dominant length scale of the airways. We demonstrate that Fourier space quantification of the speckle texture can be used to statistically measure regional airway dimensions at the alveolar scale, with measurement precision finer than the spatial resolution of the imaging system. Using this technique we discovered striking differences in developmental maturity in the lungs of rabbit kittens at birth.

Phase-contrast computed tomography for quantification of structural changes in lungs of asthma mouse models of different severity

Lung imaging in mouse disease models is crucial for the assessment of the severity of airway disease but remains challenging due to the small size and the high porosity of the organ. Synchrotron inline free-propagation phase-contrast computed tomography (CT) with its intrinsic high soft-tissue contrast provides the necessary sensitivity and spatial resolution to analyse the mouse lung structure in great detail. Here, this technique has been applied in combination with single-distance phase retrieval to quantify alterations of the lung structure in experimental asthma mouse models of different severity. In order to mimic an in vivo situation as close as possible, the lungs were inflated with air at a constant physiological pressure. Entire mice were embedded in agarose gel and imaged using inline free-propagation phase-contrast CT at the SYRMEP beamline (Synchrotron Light Source, `Elettra', Trieste, Italy). The quantification of the obtained phase-contrast CT data sets revealed an increasing lung soft-tissue content in mice correlating with the degree of the severity of experimental allergic airways disease. In this way, it was possible to successfully discriminate between healthy controls and mice with either mild or severe allergic airway disease. It is believed that this approach may have the potential to evaluate the efficacy of novel therapeutic strategies that target airway remodelling processes in asthma.

Phase contrast portal imaging using synchrotron radiation

Microbeam radiation therapy is an experimental form of radiation treatment with great potential to improve the treatment of many types of cancer. We applied a synchrotron radiation phase contrast technique to portal imaging to improve targeting accuracy for microbeam radiation therapy in experiments using small animals. An X-ray imaging detector was installed 6.0 m downstream from an object to produce a high-contrast edge enhancement effect in propagation-based phase contrast imaging. Images of a mouse head sample were obtained using therapeutic white synchrotron radiation with a mean beam energy of 130 keV. Compared to conventional portal images, remarkably clear images of bones surrounding the cerebrum were acquired in an air environment for positioning brain lesions with respect to the skull structure without confusion with overlapping surface structures.

Feasibility study of propagation-based phase-contrast X-ray lung imaging on the Imaging and Medical beamline at the Australian Synchrotron

Propagation-based phase-contrast X-ray imaging (PB-PCXI) using synchrotron radiation has achieved high-resolution imaging of the lungs of small animals both in real time and in vivo. Current studies are applying such imaging techniques to lung disease models to aid in diagnosis and treatment development. At the Australian Synchrotron, the Imaging and Medical beamline (IMBL) is well equipped for PB-PCXI, combining high flux and coherence with a beam size sufficient to image large animals, such as sheep, due to a wiggler source and source-to-sample distances of over 137 m. This study aimed to measure the capabilities of PB-PCXI on IMBL for imaging small animal lungs to study lung disease. The feasibility of combining this technique with computed tomography for three-dimensional imaging and X-ray velocimetry for studies of airflow and non-invasive lung function testing was also investigated. Detailed analysis of the role of the effective source size and sample-to-detector distance on lung image contrast was undertaken as well as phase retrieval for sample volume analysis. Results showed that PB-PCXI of lung phantoms and mouse lungs produced high-contrast images, with successful computed tomography and velocimetry also being carried out, suggesting that live animal lung imaging will also be feasible at the IMBL.

Measurement of absolute regional lung air volumes from near-field x-ray speckles

Propagation-based phase contrast x-ray (PBX) imaging yields high contrast images of the lung where airways that overlap in projection coherently scatter the x-rays, giving rise to a speckled intensity due to interference effects. Our previous works have shown that total and regional changes in lung air volumes can be accurately measured from two-dimensional (2D) absorption or phase contrast images when the subject is immersed in a water-filled container. In this paper we demonstrate how the phase contrast speckle patterns can be used to directly measure absolute regional lung air volumes from 2D PBX images without the need for a water-filled container. We justify this technique analytically and via simulation using the transport-of-intensity equation and calibrate the technique using our existing methods for measuring lung air volume. Finally, we show the full capabilities of this technique for measuring regional differences in lung aeration.

Dose optimization approach to fast X-ray microtomography of the lung alveoli

A basic prerequisite for in vivo X-ray imaging of the lung is the exact determination of radiation dose. Achieving resolutions of the order of micrometres may become particularly challenging owing to increased dose, which in the worst case can be lethal for the imaged animal model. A framework for linking image quality to radiation dose in order to optimize experimental parameters with respect to dose reduction is presented. The approach may find application for current and future in vivo studies to facilitate proper experiment planning and radiation risk assessment on the one hand and exploit imaging capabilities on the other.

Simple solution to the Fresnel-Kirchoff diffraction integral for application to refraction-enhanced radiography

We present a simple solution to the Fresnel-Kirchoff diffraction integral that is appropriate for x-ray radiography of strongly absorbing and phase-shifting objects in the geometrical optics regime, where phase contrast enhancements can be considered to be caused by refraction by a semi-opaque object. We demonstrate its accuracy by comparison to brute-force numerical ray trace and diffraction calculations of a representative simulated object, and show excellent agreement for spatial scales corresponding to Fresnel numbers greater than unity. The result represents a significant improvement over approximate formulas typically used in analysis of refraction-enhanced radiographs, particularly for radiography of transient phenomena in objects that strongly refract and show significant absorption.

Characterization of speckle in lung images acquired with a benchtop in-line x-ray phase-contrast system

We investigate the manifestation of speckle in propagation-based x-ray phase-contrast imaging of mouse lungs in situ by use of a benchtop imager. The key contributions of the work are the demonstration that lung speckle can be observed by use of a benchtop imaging system employing a polychromatic tube-source and a systematic experimental investigation of how the texture of the speckle pattern depends on the parameters of the imaging system. Our analyses consists of image texture characterization based on the statistical properties of pixel intensity values.

Refraction-enhanced backlit imaging of axially symmetric inertial confinement fusion plasmas

X-ray backlit radiographs of dense plasma shells can be significantly altered by refraction of x rays that would otherwise travel straight-ray paths, and this effect can be a powerful tool for diagnosing the spatial structure of the plasma being radiographed. We explore the conditions under which refraction effects may be observed, and we use analytical and numerical approaches to quantify these effects for one-dimensional radial opacity and density profiles characteristic of inertial-confinement fusion (ICF) implosions. We also show how analytical and numerical approaches allow approximate radial plasma opacity and density profiles to be inferred from point-projection refraction-enhanced radiography data. This imaging technique can provide unique data on electron density profiles in ICF plasmas that cannot be obtained using other techniques, and the uniform illumination provided by point-like x-ray backlighters eliminates a significant source of uncertainty in inferences of plasma opacity profiles from area-backlit pinhole imaging data when the backlight spatial profile cannot be independently characterized. The technique is particularly suited to in-flight radiography of imploding low-opacity shells surrounding hydrogen ice, because refraction is sensitive to the electron density of the hydrogen plasma even when it is invisible to absorption radiography. It may also provide an alternative approach to timing shockwaves created by the implosion drive, that are currently invisible to absorption radiography.

Murine pulmonary acinar mechanics during quasi-static inflation using synchrotron refraction-enhanced computed tomography

We visualized pulmonary acini in the core regions of the mouse lung in situ using synchrotron refraction-enhanced computed tomography (CT) and evaluated their kinematics during quasi-static inflation. This CT system (with a cube voxel of 2.8 μm) allows excellent visualization of not just the conducting airways, but also the alveolar ducts and sacs, and tracking of the acinar shape and its deformation during inflation. The kinematics of individual alveoli and alveolar clusters with a group of terminal alveoli is influenced not only by the connecting alveolar duct and alveoli, but also by the neighboring structures. Acinar volume was not a linear function of lung volume. The alveolar duct diameter changed dramatically during inflation at low pressures and remained relatively constant above an airway pressure of ∼8 cmH2O during inflation. The ratio of acinar surface area to acinar volume indicates that acinar distension during low-pressure inflation differed from that during inflation over a higher pressure range; in particular, acinar deformation was accordion-like during low-pressure inflation. These results indicated that the alveoli and duct expand differently as total acinar volume increases and that the alveolar duct may expand predominantly during low-pressure inflation. Our findings suggest that acinar deformation in the core regions of the lung is complex and heterogeneous.

The role of lung inflation and sodium transport in airway liquid clearance during lung aeration in newborn rabbits

BACKGROUND

Recent phase-contrast X-ray imaging studies suggest that inspiration primarily drives lung aeration and airway liquid clearance at birth, which questions the role of adrenaline-induced activation of epithelial sodium channels (ENaCs). We hypothesized that pressures generated by inspiration have a greater role in airway liquid clearance than do ENaCs after birth.

METHODS

Rabbit pups (30 d of gestation) were delivered and sedated, and 0.1 ml of saline (S) or amiloride (Am; an ENaC inhibitor) was instilled into the lungs before mechanical ventilation. Two other groups (30 d of gestation) were treated similarly but were also given adrenaline (S/Ad and Am/Ad) before mechanical ventilation.

RESULTS

Amiloride and adrenaline did not affect functional residual capacity (FRC) recruitment (P > 0.05). Amiloride increased the rate of FRC loss between inflations (Am: -5.2 ± 0.6 ml/kg/s), whereas adrenaline reduced the rate of FRC loss (S/Ad: -1.9 ± 0.3 ml/kg/s) as compared with saline-treated controls (S: -3.5 ± -0.6 ml/kg/s; P < 0.05).

CONCLUSION

These data indicate that inspiration is a major determinant of airway liquid clearance and FRC development during positive pressure ventilation. Although ENaC inhibition and adrenaline administration had no detectable effect on FRC development, ENaC may help to prevent liquid from re-entering the airways during expiration.

Phase contrast imaging reveals low lung volumes and surface areas in the developing marsupial

Marsupials are born with immature lungs when compared to eutherian mammals and rely, to various extents, on cutaneous gas exchange in order to meet metabolic requirements. Indeed, the fat-tailed dunnart is born with lungs in the canalicular stage of development and relies almost entirely on the skin for gas exchange at birth; consequently undergoing the majority of lung development in air. Plane radiographs and computed tomography data sets were acquired using phase contrast imaging with a synchrotron radiation source for two marsupial species, the fat-tailed dunnart and the larger tammar wallaby, during the first weeks of postnatal life. Phase contrast imaging revealed that only two lung sacs contain air after the first hour of life in the fat-tailed dunnart. While the lung of the tammar wallaby was comparatively more developed, both species demonstrated massive increases in air sac number and architectural complexity during the postnatal period. In addition, both the tammar wallaby and fat-tailed dunnart had lower lung volumes and parenchymal surface areas than were expected from morphometrically determined allometric equations relating these variables to body mass during the neonatal period. However, lung volume is predicted to scale with mass as expected after the neonatal marsupial reaches a body mass of ∼1 g and no longer relies on the skin for gas exchange. Decreased lung volume in the marsupial neonate further supports the maxim that cutaneous gas exchange occurs in the marsupial neonate because the respiratory apparatus is not yet capable of meeting the gas exchange requirements of the newborn.

Development of a multilayer mirror for high-intensity monochromatic x-ray using lab-based x-ray source

A parabolic, multilayer x-ray mirror, which can be used with a general lab-based x-ray source, was designed and fabricated. A glass substrate for the mirror was fabricated. Its surface was determined by following the rotation of a parabolic curve and was polished precisely. On the substrate surface, six W/Al bilayers were deposited to form the multilayer mirror. The effects of the mirror on x-ray images were investigated based on the calculated modulation transfer function (MTF) and image intensity values. Higher MTF and intensity values of an x-ray image were obtained using the mirror.

Synchrotron-radiation phase-contrast imaging of human stomach and gastric cancer: in vitro studies

The electron density resolution of synchrotron-radiation phase-contrast imaging (SR-PCI) is 1000 times higher than that of conventional X-ray absorption imaging in light elements, through which high-resolution X-ray imaging of biological soft tissue can be achieved. For biological soft tissue, SR-PCI can give better imaging contrast than conventional X-ray absorption imaging. In this study, human resected stomach and gastric cancer were investigated using in-line holography and diffraction enhanced imaging at beamline 4W1A of the Beijing Synchrotron Radiation Facility. It was possible to depict gastric pits, measuring 50-70 µm, gastric grooves and tiny blood vessels in the submucosa layer by SR-PCI. The fine structure of a cancerous ulcer was displayed clearly on imaging the mucosa. The delamination of the gastric wall and infiltration of cancer in the submucosa layer were also demonstrated on cross-sectional imaging. In conclusion, SR-PCI can demonstrate the subtle structures of stomach and gastric cancer that cannot be detected by conventional X-ray absorption imaging, which prompt the X-ray diagnosis of gastric disease to the level of the gastric pit, and has the potential to provide new methods for the imageology of gastric cancer.

In vivo physiological saline-infused hepatic vessel imaging using a two-crystal-interferometer-based phase-contrast X-ray technique

Using a two-crystal-interferometer-based phase-contrast X-ray imaging system, the portal vein, capillary vessel area and hepatic vein of live rats were revealed sequentially by injecting physiological saline via the portal vein. Vessels greater than 0.06 mm in diameter were clearly shown with low levels of X-rays (552 µGy). This suggests that in vivo vessel imaging of small animals can be performed as conventional angiography without the side effects of the presently used iodine contrast agents.

Phase-contrast X-ray microtomography of mouse fetus

A phase-contrast X-ray microtomography system using the Talbot imaging has been built at the SPring-8 synchrotron radiation facility. This system has much higher density resolution than absorption-based X-ray microtomography. The tomographic sections of formalin-fixed mouse fetuses obtained with this method clearly depict various organs without any staining at a pixel resolution of up to 5 µm. Since this technique allows us to obtain three-dimensional structural information without sectioning, it will be particularly useful to examine anomalies that take place during development. It can be also used to quantitatively measure volume and mass of organs during development.

The projection approximation and edge contrast for x-ray propagation-based phase contrast imaging of a cylindrical edge

We examine the projection approximation in the context of propagation-based phase contrast imaging using hard x-rays. Specifically, we consider the case of a cylinder or a rounded edge, as a simple model for the edges of many biological samples. The Argand-plane signature of a propagation-based phase contrast fringe from the edge of a cylinder is studied, and the evolution of this signature with propagation. This, along with experimental images obtained using a synchrotron source, reveals how propagation within the scattering volume is not fully described in the projection approximation's ray-based approach. This means that phase contrast fringes are underestimated by the projection approximation at a short object-to-detector propagation distance, namely a distance comparable to the free-space propagation within the volume. This failure of the projection approximation may become non-negligible in the detailed study of small anatomical features deep within a large body. Nevertheless, the projection approximation matches the exact solution for a larger propagation distance typical of those used in biomedical phase contrast imaging.

Lung cancer and angiogenesis imaging using synchrotron radiation

Early detection of lung cancer is the key to a cure, but a difficult task using conventional x-ray imaging. In the present study, synchrotron radiation in-line phase-contrast imaging was used to study lung cancer. Lewis lung cancer and 4T1 breast tumor metastasis in the lung were imaged, and the differences were clearly shown in comparison to normal lung tissue. The effect of the object-detector distance and the energy level on the phase-contrast difference was investigated and found to be in good agreement with the theory of in-line phase-contrast imaging. Moreover, 3D image reconstruction of lung tumor angiogenesis was obtained for the first time using a contrast agent, demonstrating the feasibility of micro-angiography with synchrotron radiation for imaging tumor angiogenesis deep inside the body.

High-resolution visualization of airspace structures in intact mice via synchrotron phase-contrast X-ray imaging (PCXI)

Anatomical visualization of airspace-containing organs in intact small animals has been limited by the resolution and contrast available from current imaging methods such as X-ray, micro-computed tomography and magnetic resonance imaging. Determining structural relationships and detailed anatomy has therefore relied on suitable fixation, sectioning and histological processing. More complex and informative analyses such as orthogonal views of an organ and three-dimensional structure visualizations have required different animals and image sets, laboriously processed to gather this complementary structural information. Precise three-dimensional anatomical views have always been difficult to achieve in small animals. Here we report the ability of phase-contrast synchrotron X-ray imaging to provide detailed two- and three-dimensional visualization of airspace organ structures in intact animals. Using sub-micrometre square pixel charge-coupled device array detectors, the structure and anatomy of hard and soft tissues, and of airspaces, is readily available using phase-contrast synchrotron X-ray imaging. Moreover, software-controlled volume-reconstructions of tomographic images not only provide unsurpassed image clarity and detail, but also selectable anatomical views that cannot be obtained with established histological techniques. The morphology and structure of nasal and lung airways and the middle ear are illustrated in intact mice, using two- and three-dimensional representations. The utility of phase-contrast synchrotron X-ray imaging for noninvasively localizing objects implanted within airspaces, and the detection of gas bubbles transiting live airways, are other novel features of this visualization methodology. The coupling of phase-contrast synchrotron X-ray imaging technology with software-based reconstruction techniques holds promise for novel and high-resolution non-invasive examination of airspace anatomy in small animal models.

Positive end-expiratory pressure enhances development of a functional residual capacity in preterm rabbits ventilated from birth

The factors regulating lung aeration and the initiation of pulmonary gas exchange at birth are largely unknown, particularly in infants born very preterm. As hydrostatic pressure gradients may play a role, we have examined the effect of a positive end-expiratory pressure (PEEP) on the spatial and temporal pattern of lung aeration in preterm rabbit pups mechanically ventilated from birth using simultaneous phase-contrast X-ray imaging and plethysmography. Preterm rabbit pups were delivered by caesarean section at 28 days of gestational age, anesthetized, intubated, and placed within a water-filled plethysmograph (head out). Pups were imaged as they were mechanically ventilated from birth with a PEEP of either 0 cmH2O or 5 cmH2O. The peak inflation pressure was held constant at 35 cmH2O. Without PEEP, gas only entered into the distal airways during inflation. The distal airways collapsed during expiration, and, as a result, the functional residual capacity (FRC) did not increase above the lung's anatomic dead space volume (2.5 ± 0.8 ml/kg). In contrast, ventilation with 5-cmH2O PEEP gradually increased aeration of the distal airways, which did not collapse at end expiration. The FRC achieved in pups ventilated with PEEP (19.9 ± 3.2 ml/kg) was significantly greater than in pups ventilated without PEEP (−2.3 ± 3.5 ml/kg). PEEP greatly facilitates aeration of the distal airways and the accumulation of FRC and prevents distal airway collapse at end expiration in very preterm rabbit pups mechanically ventilated from birth.

Imaging lung aeration and lung liquid clearance at birth using phase contrast X-ray imaging

The transition to extra-uterine life at birth is critically dependent on airway liquid clearance to allow the entry of air and the onset of gaseous ventilation. We have used phase contrast X-ray imaging to identify factors that regulate lung aeration at birth in spontaneously breathing term and mechanically ventilated preterm rabbit pups. Phase contrast X-ray imaging exploits the difference in refractive index between air and water to enhance image contrast, enabling the smallest air-filled structures of the lung (alveoli; < 100 microm) to be resolved. Using this technique, the lungs become visible as they aerate, allowing the air-liquid interface to be observed as it moves distally during lung aeration. Spontaneously breathing term rabbit pups rapidly aerate their lungs, with most fully recruiting their functional residual capacity (FRC) within the first few breaths. The increase in FRC occurs mainly during individual breaths, demonstrating that airway liquid clearance and lung aeration is closely associated with inspiration. We suggest that transpulmonary pressures generated by inspiration provide a hydrostatic pressure gradient for the movement of water out of the airways and into the surrounding lung tissue after birth. In mechanically ventilated preterm pups, lung aeration is closely associated with lung inflation and a positive end-expiratory pressure is required to generate and maintain FRC after birth. In summary, phase contrast X-ray imaging can image the air-filled lung with high temporal and spatial resolution and is ideal for identifying factors that regulate lung aeration at birth in both spontaneously breathing term and mechanically ventilated preterm neonates.

Investigation of physical image characteristics and phenomenon of edge enhancement by phase contrast using equipment typical for mammography

A technique called phase contrast mammography (PCM) has only recently been applied in clinical examination. In this application, PCM images are acquired at a 1.75 x magnification using an x-ray tube for clinical use, and then reduced to the real size of the object by image processing. The images showed enhanced object edges; reportedly, this enhancement occurred because of the refraction of x rays through a cylindrical object. The authors measured the physical image characteristics of PCM to compare the image characteristics of PCM with those of conventional mammography. More specifically, they measured the object-edge-response characteristics and the noise characteristics in the spatial frequency domain. The results revealed that the edge-response characteristics of PCM outperformed those of conventional mammography. In addition, the characteristics changed with the object-placement conditions and the object shapes. The noise characteristics of PCM were better than those of conventional mammography. Subsequently, to verify why object edges were enhanced in PCM images, the authors simulated image profiles that would be obtained if the x rays were refracted and totally reflected by using not only a cylindrical substance but also a planar substance as the object. So, they confirmed that the object edges in PCM images were enhanced because x rays were refracted irrespective of the object shapes. Further, they found that the edge enhancements depended on the object shapes and positions. It was also proposed that the larger magnification than 1.75 in the commercialized system might be more suitable for PCM. Finally, the authors investigated phase-contrast effects to breast tissues by the simulation and demonstrated that PCM would be helpful in the diagnoses of mammography.

Anniversary paper. Development of x-ray computed tomography: the role of medical physics and AAPM from the 1970s to present

The AAPM, through its members, meetings, and its flagship journal Medical Physics, has played an important role in the development and growth of x-ray tomography in the last 50 years. From a spate of early articles in the 1970s characterizing the first commercial computed tomography (CT) scanners through the "slice wars" of the 1990s and 2000s, the history of CT and related techniques such as tomosynthesis can readily be traced through the pages of Medical Physics and the annals of the AAPM and RSNA/AAPM Annual Meetings. In this article, the authors intend to give a brief review of the role of Medical Physics and the AAPM in CT and tomosynthesis imaging over the last few decades.

Benchtop phase-contrast X-ray imaging

Clinical radiography has traditionally been based on contrast obtained from absorption when X-rays pass through the body. The contrast obtained from traditional radiography can be rather poor, particularly when it comes to soft tissue. A wide range of media of interest in materials science, biology and medicine exhibit very weak absorption contrast, but they nevertheless produce significant phase shifts with X-rays. The use of phase information for imaging purposes is therefore an attractive prospect. Some of the X-ray phase-contrast imaging methods require highly monochromatic plane wave radiation and sophisticated X-ray optics. However, the propagation-based phase-contrast imaging method adapted in this paper is a relatively simple method to implement, essentially requiring only a microfocal X-ray tube and electronic detection. In this paper, we present imaging results obtained from two different benchtop X-ray sources employing the free space propagation method. X-ray phase-contrast imaging provides higher contrast in many samples, including biological tissues that have negligible absorption contrast.

Polychromatic phase-contrast computed tomography

Polychromatic phase-contrast radiography differs from traditional (absorption-only) radiography in that the method requires at least a partially coherent x-ray source and the resulting images contain information about the phase shifts of x-rays in addition to the traditional absorption information. In a typical embodiment, this effect results in a measurable enhancement in image contrast at the edges of objects. In this study, a phase-contrast imaging system was adapted to allow an object to be imaged at multiple projections, and these projections were used to generate phase-contrast computed tomography images. The images obtained with this technique show edge enhancements surrounding the objects within the image.

Angular resolution in magnification radiography and the observation of x-ray wave interaction signatures

High resolution x-ray imaging studies have demonstrated significant radiographic contrast enhancements that are attributed to wave interactions within the sample. This paper reviews diffraction and refraction in the context of medical radiography, describing signatures produced by each process and the necessary experimental conditions for observing them. The concept of angular resolution is introduced and applied to current x-ray source and detector configurations, testing their ability to record these features. It is difficult to record interference patterns arising from refractive phase shifts because their formation requires a mono-energetic beam. The refraction of x-rays across boundaries, as described by Snell's law, produces strong contrast enhancements when they are struck at close to the glancing incidence. Deflections are proportional to the change in electron density (at energies above the K-edge) and square root of the wavelength, so they can be observed with a poly-energetic beam. Diffraction can also be observed with white radiation, but produce fringes with far narrower separation under the same irradiation conditions. In both cases, the observation of wave interaction signatures requires a propagation distance between the sample and detector, and selection of an appropriate geometric magnification, which can be estimated using a simple model presented here.

Imaging lung aeration and lung liquid clearance at birth

Aeration of the lung and the transition to air-breathing at birth is fundamental to mammalian life and initiates major changes in cardiopulmonary physiology. However, the dynamics of this process and the factors involved are largely unknown, because it has not been possible to observe or measure lung aeration on a breath-by-breath basis. We have used the high contrast and spatial resolution of phase contrast X-ray imaging to study lung aeration at birth in spontaneously breathing neonatal rabbits. As the liquid-filled fetal lungs provide little absorption or phase contrast, they are not visible and only become visible as they aerate, allowing a detailed examination of this process. Pups were imaged live from birth to determine the timing and spatial pattern of lung aeration, and relative levels of lung aeration were measured from the images using a power spectral analysis. We report the first detailed observations and measurements of lung aeration, demonstrating its dependence on inspiratory activity and body position; dependent regions aerated at much slower rates. The air/liquid interface moved toward the distal airways only during inspiration, with little proximal movement during expiration, indicating that trans-pulmonary pressures play an important role in airway liquid clearance at birth. Using these imaging techniques, the dynamics of lung aeration and the critical role it plays in regulating the physiological changes at birth can be fully explored.

Refraction-enhanced tomography of mouse and rabbit lungs

In order to evaluate the effectiveness of edge enhancement by refraction in computed tomography, images of a cross section of a euthanized mouse thorax were recorded at low (20 keV) and high (72 keV) x-ray energies at a spatial resolution of about 40 microm. Compared with the images obtained with the detector at 30 cm from an object, when the object was located at 113 cm from the detector, the contrast between tissues and air was improved at both energies. The improvement was more pronounced at 72 keV where the absorption contrast was weaker. This effect was due to refraction at the surfaces of alveolar membranes and small airways which creates areas with apparently high and low linear attenuation coefficients within tissues. The edge enhancement by refraction was also effective in images of a euthanized rabbit thorax at x-ray energies of 40 and 70 keV at a spatial resolution of about 0.15 mm. These results raise the possibility that the refraction contrast may be utilized to obtain a high-resolution tomographic image of human lung and bone with low dose.

High-resolution X-ray refraction imaging of rat lung and histological correlations

This study was performed to observe microstructures of the rat lung, using a synchrotron radiation beam and to compare findings with histological observations. X-ray refraction images from ex-vivo ventilating rat lung were obtained with an 8 KeV monochromatic beam and 20-mum thick CsI(Tl) scintillation crystal. The visual image was magnified using a 20x microscope objective and captured using an analog CCD camera. Obtained images were compared with conventional light microscopic findings from the same tissue. Pulmonary microstructures, including alveolar ducts, alveolar sacs, alveoli, alveolar walls, and perialveolar capillary networks were clearly identified with spatial resolution of as much as 1.2 mum and had good correlation with conventional light microscopic findings. The shape of alveoli appeared more round in SR images than in the light microscopic images. The results suggest that X-ray microscopy study of the lung using synchrotron radiation demonstrates the potential for clinically relevant microstructure of lung tissue without sectioning and fixation.

Phase contrast X-ray imaging of mice and rabbit lungs: a comparative study

The significant degree of X-ray phase contrast created by air-tissue interfaces, coupled with the poor radiographic contrast of conventional chest radiographs, makes the inflated lung an ideal candidate for investigating the potential diagnostic improvement afforded by phase contrast X-ray imaging. In small animals these methods highlight the lung airways and lobe boundaries and reveal the lung tissue as a speckled intensity pattern not seen in other soft tissues. We have compared analyser-based and propagation-based phase contrast imaging modalities, together with conventional radiographic imaging, to ascertain which technique shows the greatest image enhancement for various lung sizes. The conventional radiographic image of a mouse was obtained on a Siemens Nova 3000 mammography system, whilst phase contrast images of mice and rabbit chests were acquired at the medical imaging beamline (20B2) at the SPring-8 synchrotron radiation research facility in Japan. For mice aged 1 day, 1 week and 1 month old it was determined that analyser-based imaging showed the greatest overall image contrast, however, for an adult rabbit both techniques yielded excellent contrast. The success of these methods in creating high quality images for rabbit lungs raises the possibility of improving human lung imaging using phase contrast techniques.

Dynamic imaging of the lungs using x-ray phase contrast

High quality real-time imaging of lungs in vivo presents considerable challenges. We demonstrate here that phase contrast x-ray imaging is capable of dynamically imaging the lungs. It retains many of the advantages of simple x-ray imaging, whilst also being able to map weakly absorbing soft tissues based on refractive index differences. Preliminary results reported herein show that this novel imaging technique can identify and locate airway liquid and allows lung aeration in newborn rabbit pups to be dynamically visualized.

Evaluation of edge effect due to phase contrast imaging for mammography

It is well-known that the edge effect produced by phase contrast imaging results in the edge enhancement of x-ray images and thereby sharpens those images. It has recently been reported that phase contrast imaging using practical x-ray tubes with small focal spots has improved image sharpness as observed in the phase contrast imaging with x-ray from synchrotron radiation or micro-focus x-ray tubes. In this study, we conducted the phase contrast imaging of a plastic fiber and plant seeds using a customized mammography equipment with a 0.1 mm focal spot, and the improvement of image sharpness was evaluated in terms of spatial frequency response of the images. We observed that the image contrast of the plastic fiber was increased by edge enhancement, and, as predicted elsewhere, spectral analysis revealed that as the spatial frequencies of the x-ray images increased, so did the sharpness gained through phase contrast imaging. Thus, phase contrast imaging using a practical molybdenum anode tube with a 0.1 mm-focal spot would benefit mammography, in which the morphological detectability of small species such as microcalcifications is of great concern. And detectability of tumor-surrounded glandular tissues in dense breast would be also improved by the phase contrast imaging.

[Application of phase contrast imaging to mammography]

Phase contrast images were obtained experimentally by using a customized mammography unit with a nominal focal spot size of 100 microm and variable source-to-image distances of up to 1.5 m. The purpose of this study was to examine the applicability and potential usefulness of phase contrast imaging for mammography. A mammography phantom (ACR156 RMI phantom) was imaged, and its visibility was examined. The optical density of the phantom images was adjusted to approximately 1.3 for both the contact and phase contrast images. Forty-one observers (18 medical doctors and 23 radiological technologists) participated in visual evaluation of the images. Results showed that, in comparison with the images of contact mammography, the phantom images of phase contrast imaging demonstrated statistically significantly superior visibility for fibers, clustered micro-calcifications, and masses. Therefore, phase contrast imaging obtained by using the customized mammography unit would be useful for improving diagnostic accuracy in mammography.

On the origin of speckle in x-ray phase contrast images of lung tissue

Phase contrast x-ray imaging of small animal lungs reveals a speckled intensity pattern not seen in other tissues, making the lungs highly visible in comparison to other organs. Although bearing a superficial resemblance to alveoli, the cause of this speckle has not been established. With a view to determining the mechanism for the formation of speckle, this paper details the results of propagation-based phase contrast experiments performed on mice lungs, together with packed glass microspheres used to emulate lung tissue. These experimental studies are compared to numerical simulations, based on wave propagation techniques. We find that speckle arises from focusing effects, with multiple alveoli acting as aberrated compound refractive lenses. Both experiments and modelling suggest that this speckle-formation phenomenon may lead to better screening methods for human lungs than conventional radiography.

Quantitative characterization of edge enhancement in phase contrast x-ray imaging

The aim of this study was to model the edge enhancement effect in in-line holography phase contrast imaging. A simple analytical approach was used to quantify refraction and interference contrasts in terms of beam energy and imaging geometry. The model was applied to predict the peak intensity and frequency of the edge enhancement for images of cylindrical fibers. The calculations were compared with measurements, and the relationship between the spatial resolution of the detector and the amplitude of the phase contrast signal was investigated. Calculations using the analytical model were in good agreement with experimental results for nylon, aluminum and copper wires of 50 to 240 microm diameter, and with numerical simulations based on Fresnel-Kirchhoff theory. A relationship between the defocusing distance and the pixel size of the image detector was established. This analytical model is a useful tool for optimizing imaging parameters in phase contrast in-line holography, including defocusing distance, detector resolution and beam energy.