Phase Contrast Method Research Articles

Artifact Halo Reduction in Phase Contrast Microscopy Using Apodization

A new phase contrast microscopy technique for halo reduction is proposed. This technique is based on an apodization method combined with the Zernike phase contrast method. Although it has been a diflicult theoretical problem, the proposed technique achieves halo reduction by considering angles of diffraction and phase differences. The technique utilizes an apodized phase plate consisting of a quarter wave phase shift ring with a 250/0 transmittance, and a pair of adjacent rings, which have 500/0 transmittance. This element is placed at the back focal plane of the objective. The result is startling, halo reduced images of phase objects providing enhanced inner details. Key word~: phase contrast microscopy, apodization, halo, apodized phase plate, angle of diffraction, phase difference

Off-centered spiral trajectories.

The quality of spiral images depends on the accuracy of the k-space sampling locations. Although newer gradient systems can provide more accurate gradient waveforms, the sampling positions can be significantly distorted by timing misregistration between data acquisition and gradient systems. Even after the timing of data acquisition is tuned, minor residual errors can still cause shading artifacts which are problematic for quantitative MR applications such as phase-contrast flow quantitation. These timing errors can ideally be corrected by measuring the actual k-space trajectory, but trajectory measurement requires additional data acquisition and scan time. Therefore, off-centered spiral trajectories which are more robust against timing errors are proposed and applied to the phase-contrast method. The new trajectories turn shading artifacts into a slowly varying linear phase in reconstructed images without affecting the magnitude of images.

Reconfigurable ternary-phase array illuminator based on the generalised phase contrast method

A new technique is presented for implementing a dynamic array illuminator scheme based on the generalised phase contrast imaging method. It is shown that by using a ternary-phase rather than a binary-phase approach for encoding input patterns a larger range of array compression factors can be achieved. We demonstrate how a graphical phase chart can be used for the optimisation of the array illumination. Due to the low requirements for both spatial and temporal bandwidth products the encoding method is ideal for implementation on reconfigurable phase-only spatial light modulating devices with diverse applications such as dynamic laser tweezer arrays and generation of structured light for machine vision.

Portal venous blood flow while breath-holding after inspiration or expiration and during normal respiration in controls and cirrhotics.

In this study, we used magnetic resonance (MR) imaging to measure portal blood flow in 12 healthy controls and 17 cirrhotics while they were breath-holding after inspiration and after expiration. We then compared the results with measurements made during normal respiration in the healthy controls and cirrhotics. Blood flow in the main portal vein under basal fasting conditions was quantitated using the cine phase-contrast MR velocity mapping method. Three measurements were made on one occasion, as follows: (1) throughout the cardiac cycle during normal respiration, (2) with the subject breath-holding after maximal inspiration, and (3) with the subject breath-holding after maximal expiration. During normal respiration, portal blood flow was 1.3 +/- 0.21/min in controls vs 1.0 +/- 0.11/min in cirrhotics (P < 0.0001); while subjects were breath-holding after inspiration, portal blood flow was 1.0 +/- 0.21/min in controls vs 0.9 +/- 0.11/min in cirrhotics; and while subjects were breath-holding after expiration, portal blood flow was 1.5 +/- 0.21/min in controls vs 1.1 +/- 0.21/min in cirrhotics (P < 0.0001). The differences were primarily due to changes in flow velocity. When the magnitude of these hemodynamic changes in the three respiratory conditions was compared in controls and cirrhotics, analysis of variance (ANOVA) showed a significant difference (P < 0.0001). In controls, portal blood flow decreased during maximal inspiration relative to flow during normal respiration (-24.6 +/- 8.3%). Changes in portal blood flow in controls were greater than in cirrhotics (-13.5 +/- 4.5%) (P < 0.0001); however, the difference in blood flow increase associated with maximal expiration between the two groups (+11.8 +/- 9.4% vs +5.9 +/- 11.5%) was not significant. We found that the respiration-induced hemodynamic variation in portal blood flow was less in cirrhotics than in the healthy controls. Portal blood flow measurements made during normal respiration using MR imaging closely reflect nearly physiologic conditions.

The van Cittert-Zernike Theorem

The beam of light emanating from a quasi-monochromatic point source (or a sufficiently distant extended source) is said to be spatially coherent: the reason is that, at any two points on a given cross-section of the beam, the oscillating electromagnetic fields maintain their relative phase at all times. If an opaque screen with two pinholes is placed at such a cross-section, Young's interference fringes will form, and the observed fringe contrast will be 100% (at and around the center of the fringe pattern). This is the sense in which the fields at two points are said to be spatially coherent relative to each other. If the relative phase of the fields at the two points varies randomly with time, the pair of point sources will fail to produce Young's fringes and, therefore, the fields are considered to be incoherent. In practice there is a continuum of possibilities between the aforementioned extremes, and the resulting fringe contrast may fall anywhere between zero and 100%. The fields at the two points are then said to be partially coherent with respect to one another, and the properly defined fringe contrast in Young's experiment is used as the measure of their degree of coherence. Optical systems involving partially coherent illumination are explored in several other chapters of this book; see, for example, “Coherent and incoherent imaging” (Chapter 5), “Michelson's stellar interferometer” (Chapter 35), “Zernike's method of phase contrast” (Chapter 38), and “polarization microscopy” (Chapter 39).

Inverse problem in the X-ray phase contrast method

An analysis is made of the possibility of solving the inverse problem of reconstructing the internal structure of an object using a set of experimental distributions measured in an x-ray beam for various orientations of the object.

New possibilities for x-ray diagnosis of atherosclerosis using a phase contrast method

The internal structure of materials with a low scattering power and small density gradient was investigated using a new method of phase contrast x-ray introscopy. Model calculations were made of the contrast image using dynamic x-ray diffraction theory. The object of the investigation was a capillary containing a blood clot, which modeled a blood vessel with a thrombus. As a result, it was shown that the new method has distinct advantages over the conventionally used absorption method in obtaining completely satisfactory images.

Pattern generation by inverse phase contrast

The beam shaping properties of the so-called inverse phase contrast (IPC) method are investigated. The importance of including the effects of the complex average value of the object phase mask is stressed.

Lossless light projection

A new technique for energy-preserving phase-only light projection is demonstrated. The phase-only encoding is based on an extension of the Zernike phase-contrast method into the domain of full-range [0; 2pi ] phase modulation, breaking the usual small-phase-angle limitation. Controlling the spatial average value of the input-phase pattern and choosing appropriate phase retardation at the phase-contrast filter yield pure-phase-based image formation. Experimental results demonstrate close to 90% energy efficiency. Output intensity levels with magnitudes more than 3.5times that of the input intensity level were measured in the brightest regions of the projected images.

Velocity encoding using both phase and magnitude.

Imaging time constitutes a major limitation of phase-contrast (PC) angiography. It is possibly the main disadvantage of PC methods over the time-of-flight (TOF) methods that actually are used clinically. This relatively long imaging time comes from the fact that conventional PC methods require the acquisition of at least four images with different velocity sensitization to reconstruct a single angiogram (1, 2). However, more than one-half of the information gathered through the acquisition of these four images is either redundant or simply discarded. We propose a faster approach to making PC angiograms in which the quantity of data acquired is diminished by as much as a factor 2. This is made possible by encoding velocity information in both the phase and magnitude of the image. Due to the use of extra radiofrequency (RF) and gradient waveforms, decreases in data requirements do not translate in a direct manner into decreases in imaging time. Nevertheless, significant reductions in imaging time are achieved with the present approach.

Pattern generation by inverse phase contrast

Beam shaping is usually achieved by placing a phase mask at the input end of an optical system. The shaped beam is formed at its optical Fresnel or Fourier plane. Designing the phase mask is often a computationally intensive task. An alternative approach is presented, exploiting the concept of phase contrast imaging. In the inverse phase contrast (IPC) method the design of the mask is simple and direct.

Noninvasive analysis of renal artery blood flow dynamics with MR cine phase-contrast flow measurements.

It was the aim of this study to quantify the measurement of pulsatile flow in the renal artery with the noninvasive magnetic resonance cine phase-contrast (MRCPC) method and combine it with the simultaneous assessment of pulsatile flow with a transit-time ultrasound (TTUS) flowmeter. In seven foxhounds with a chronically implanted precalibrated TTUS flow probe, MRCPC flow measurements were made in the renal artery with a temporal resolution of 32 ms. Mean and pulsatile flow signal were compared by the simultaneous ipsi- or contralateral measurement of the renal blood flow signal by both methods (TTUS and MRCPC). In addition, comparative MRCPC and TTUS flow measurements were made with artificial renal artery stenosis and after the administration of angiotensin II. The mean flow data assessed by the noninvasive MRCPC flow measurements showed an excellent correlation with corresponding TTUS recording (r = 0.99). The MRCPC flow signal displayed a waveform of the renal artery flow profile that was very similar to the TTUS flow pulse. The hemodynamic changes induced by angiotensin II or due to renal artery stenosis were also reliably detected by MRCPC. MRCPC provides a reliable noninvasive method for the quantification of mean blood flow and the assessment of the pulsatile flow signal in the renal artery and proves to be sensitive to hemodynamic changes of pathophysiological importance. Alternatively, the method may be used for studies in physiology that demand a noninvasive approach.

Density fluctuations in Heliotron E measured using C0 2 laser phase contrast method

Density fluctuations in NBI (neutral beam injection) plasmas in Heliotron E were studied using the laser phase contrast (LPC) method. A counter-propagating pair of waves, electron mode and ion mode, was observed in the edge region. Their dependences on the ion and electron temperatures are discussed in an experiment where ECH (electron cyclotron heating) pulses were superposed on the NBI plasmas.

Microstructure of diesel soot particles probed by electron microscopy: First observation of inner core and outer shell

It is well known that soot particles emitted from a diesel engine are usually observed as chain-like aggregates (secondary particles) composed of several tens to hundreds of primary spherical particles. The mature aggregates are not easily eliminated by in-cylinder oxidation. Thus, lowering particulate emission from a diesel engine is difficult. In order to determine whether or not there are effective solutions for reducing soot, the early stage of the formation process before the aggregation needs to be clarified. The authors carried out an ultra-high-resolution electron microscopic observation to analyze the microstructure of diesel soot in further detail. In the present study, the authors adopted two types of TEM techniques to make the microstructure of diesel soot clear: one is a phase-contrast method, and another is a hollow-cone beam method. It was revealed that a primary particle of diesel soot has two distinct parts, each with different structures. The details of these structures are discussed in the Results.

Magnetic resonance angiography.

Magnetic resonance angiography (MRA) permits the non-invasive visualization of blood flow through the effects of moving spins on the magnetic resonance signal. MRA techniques can be divided into two main classifications depending upon the primary effect responsible for contrast in the image. Angiograms can be produced using either the time-of-flight (TOF) or phase contrast (PC) methods. Each method has particular advantages and limitations as an angiographic imaging technique and these are reflected in their respective applications. This review article is intended to outline the scientific and technical development of MRA from its basis in the earliest in vitro nuclear magnetic resonance (NMR) experiments, through the implementation of in vivo angiographic techniques on whole body MRI systems, to the recent rapid expansion in MRA acquisition and processing techniques.

Enhancement and Suppression of Density Fluctuations around Electron Drift Frequency in Heliotron E Plasmas Measured UsingCO2Laser Phase Contrast Method

Enhancement and suppression of electron density fluctuations around the electron drift frequency was observed in Heliotron E. This behavior corresponded to changes in the density profiles. Density fluctuations around this frequency were enhanced when the density profile was peaked in the inner region, and suppression was associated with the flattening of the density profile. An examination of the frequency, wavenumber, and collisionality regimes suggested that the most likely candidate for the instability responsible for driving these fluctuations is the collisionless circulating electron mode.

Phase contrast image synthesis

A new method is presented for synthesizing arbitrary intensity patterns based on phase contrast imaging. The concept is grounded on an extension of the Zernike phase contrast method into the domain of full range [0; 2π] phase modulation. By controlling the average value of the input phase function and by choosing appropriate phase retardation at the phase contrast filter, a pure phase to intensity imaging is accomplished. The method presented is also directly applicable in dark field image synthesis.

158. fMRIにおけるPhase contrast法を用いた流速測定の基礎的検討

158. fMRIにおけるPhase contrast法を用いた流速測定の基礎的検討

MR Angiography of Abdominal and Peripheral Arteries

This review article deals with MR angiography (MRA) of abdominal and peripheral arteries. Pulsatile flow, respiratory motion and peristalsis impose difficulties in imaging the vascular structures in the abdomen and the lower extremities. Development of new techniques, such as segmentation of the data acquisition, using specific acquisition windows in relation to a cardiac trigger, magnetization preparation of the tissue and phase-encoding re-ordering or sorting, have reduced the artifacts associated with abdominal and peripheral MRA. Clinical MR investigations of the arteries branching from the abdominal aorta such as the renal and mesenteric arteries and arteries in the lower extremities have revealed that severe stenoses or occlusions can be diagnosed accurately while the grading of less severe stenosis is more difficult. The phase-contrast method has been used to quantify blood flow and study the hemodynamics in abdominal and peripheral vessels. Quantitative flow information can be used to diagnose vascular disease and provides important physiological information. More prospective clinical studies, in which recently developed MRA techniques are compared with conventional angiography, are necessary before conclusive decisions can be made as to whether MRA may replace these methods.

MR Angiography of Abdominal and Peripheral Arteries

This review article deals with MR angiography (MRA) of abdominal and peripheral arteries. Pulsatile flow, respiratory motion and peristalsis impose difficulties in imaging the vascular structures in the abdomen and the lower extremities. Development of new techniques, such as segmentation of the data acquisition, using specific acquisition windows in relation to a cardiac trigger, magnetization preparation of the tissue and phase-encoding re-ordering or sorting, have reduced the artifacts associated with abdominal and peripheral MRA. Clinical MR investigations of the arteries branching from the abdominal aorta such as the renal and mesenteric arteries and arteries in the lower extremities have revealed that severe stenoses or occlusions can be diagnosed accurately while the grading of less severe stenosis is more difficult.The phase-contrast method has been used to quantify blood flow and study the hemodynamics in abdominal and peripheral vessels. Quantitative flow information can be used to diagnose vasc...