Axial Collimation Research Articles

Two-dimensional numerical study of effect of magnetic field on evolution of laser-driven jets

Astrophysical jets are highly collimated supersonic plasma beams distributed across various astrophysical backgrounds. The triggering mechanism, collimation transmission, and stability of jets have always been a research hotspot of astrophysics. In recent years, observations and laboratory research have found that the magnetic field plays a crucial role in jet collimation, transmission, and acceleration. In this work, the two-dimensional numerical simulation of the jet in front of the CH plane target driven by an intense laser is carried out by using the open-source MHD FLASH simulation program. We systematically investigate the dynamic behaviors of jet evolution caused by the Biermann self-generated magnetic field, the external magnetic field with different directions and initial strengths and compare them with each other. Simulation results show that the Biermann self-generated magnetic field does not affect the jet interface dynamics. The external magnetic field has a redirecting effect on the plasma outflow. The external magnetic field, which is parallel to the direction of the plasma outflow center in front of the target, is conducive to the generation and collimation of the jet. The evolution of the jet goes through three stages: antimagnetic ellipsoid cavity, conical nozzle, and collimated jet. Its formation process and evolution process result from competition among plasma thermal, magnetic, and ram pressure. In terms of force, plasma thermal pressure gradient and magnetic pressure forces play a decisive role in the jet evolution process. The presence of magnetic pressure significantly limits the radial expansion of the jet to achieve axial collimation transmission. The length-diameter ratio of the jet is positively correlated with the initial axial applied magnetic field intensity. In addition, we observe in the simulation that there are many node-like structures in the jet evolution zone, similar to the jet node in YSO. The results provide a reference for future experimental research related to jets and contribute to a more in-depth understanding of the evolution of celestial jets.

CT dosimetry at the Australian Synchrotron for 25-100 keV photons and 35-160 mm-diameter biological specimens.

The dose length product (DLP) method for medical computed tomography (CT) dosimetry is applied on the Australian Synchrotron Imaging and Medical Beamline (IMBL). Beam quality is assessed from copper transmission measurements using image receptors, finding near 100% (20 keV), 3.3% (25 keV) and 0.5% (30-40 keV) relative contributions from third-harmonic radiation. The flat-panel-array medical image receptor is found to have a non-linear dose response curve. The amount of radiation delivered during an axial CT scan is measured as the dose in air alone, and inside cylindrical PMMA phantoms with diameters 35-160 mm for mono-energetic radiation 25-100 keV. The radiation output rate for the IMBL is comparable with that used for medical CT. Results are presented as the ratios of CT dose indices (CTDI) inside phantoms to in air with no phantom. Ratios are compared for the IMBL against medical CT where bow-tie filters shape the beam profile to reduce the absorbed dose to surface organs. CTDI ratios scale measurements in air to estimate the volumetric CTDI representing the average dose per unit length, and the dose length product representing the absorbed dose to the scanned volume. Medical CT dose calculators use the DLP, beam quality, axial collimation and helical pitch to estimate organ doses and the effective dose. The effective dose per unit DLP for medical CT is presented as a function of body region, beam energy and sample sizes from neonate to adult.

Magnetically Driven Flows in Arched Plasma Structures

Laboratory experiments demonstrate high-speed plasma flows from both footpoints of arched magnetic flux tubes, resulting in bulk plasma transport into the flux tube and persistent axial collimation even as the flux tube lengthens and kinks. The measured flows are in agreement with the predictions of hoop force and collimation models involving fundamental MHD forces. These forces are expected to drive plasma acceleration in other open flux configurations with arched geometries, such as those found on the solar surface.

The stranded stone: Relationship between acute appendicitis and appendicolith

Background/Aim:To examine the relationship between acute appendicitis and the presence of an appendicolith in abdominal CT scans of patients attending emergency services.Materials and Methods:Abdominal CT scan reports were retrospectively reviewed for 267 patients through the PACS database. A 16-slices MDCT GE Light Speed scanner (Milwaukee WI) was used with a scanning protocol of 5 mm axial collimation and a pitch of 1.0, along with oral contrast material (Gastrografin 3.7% diatrizoate meglumine) and 140 mL of intravenous (IV) nonionic contrast material (Omnipaque). Particular attention was given to the study protocol, patients' age, and gender.Statistical Analysis:We used MS-EXCEL and SPSS version 12.0 to perform chi-square and Fisher's exact tests. Bookends and Papers, components in Mac OS X software, were used for literature reviews and the organization of results.Results:Two hundred and sixty-seven abdominal CT scan reports were examined along side their respective images on a GE Centricity workstation. Thirty-four (12.7%) were labeled as acute appendicitis cases based on the CT findings and the rest were assigned other diagnoses. Twenty-six of the 267 CT scan reports were plain studies and 241 were contrast-enhanced scans. Less than half of the patients (123, 46.1%) were males and 144 (53.9%) were females. Thirteen males (48.1%) and 14 (51.9%) females were found to have an appendicolith. Only 3% in the ≤ 11 years' age group, in contrast to 40% in the 11-20 years' age group, was diagnosed with appendicitis. The incidence in other age groups was as follows: 19% in the 21-30, 14% in the 31-40, 2.5% in the 41-50, 8% each in the 51-60 and 61-70, and none in the ≥71 years' age groups.Conclusions:We conclude that the presence of an appendicolith i) has no particular predilection for gender or age, and ii) is not associated with a diagnosis of appendicitis.

Computed tomography imaging of the paranasal sinuses: Direct versus reformatted coronal images

To evaluate the image quality of coronal reformatted CT images compared with direct coronal CT images. Cross-sectional. Patients who underwent paranasal sinus CT scans using both direct coronal images (DC) and axially acquired images with coronal reconstructions (CR) were consecutively enrolled. Two reconstruction groups were evaluated: group CR1 with 1-mm axial collimation and group CR2 with 0.625-mm axial collimation. Scans were presented in a randomized, blinded fashion to two sinus surgeons and a neuroradiologist. When compared with CR1, DC images had higher image quality for each of five key anatomic landmarks (P < .0001). DC images were more likely to be adequate preoperative imaging (P < .0001). When compared with CR2, improved DC image quality was noted in the majority of key anatomic landmarks. No difference was noted in the "adequacy rating" between the DC and CR2 (0.625 mm) protocols. The 0.625-mm reformatted images were judged to be adequate preoperative imaging. The additional information provided by multiple-image planes make reformatted 0.625-mm axial images an acceptable approach.

Derivation and Validation of a Sensitivity Formula for Slit-Slat Collimation

An analytic formula is derived for the sensitivity of collimators achieving transverse collimation with a slit and axial collimation with a slat assembly whose septa may be parallel or focus on a line. The formula predicts sin(3) phi dependence on the incidence angle and, in the particular case of parallel slats, 1/h dependence on the distance from the slit. More complex expressions for sensitivity that do not diverge at points near the slit or the focal line of the slat assembly are also derived. The predictions of the formulas are checked against simple cases for which solutions are available from direct calculation as well as against Monte Carlo simulation and published experimental data. Agreement is good in all cases analyzed. An approximate penetration model is also introduced: it involves the use of a sensitivity-effective slit width and septal length. Its predictions are compared to simulation results. Agreement was found to be compatible with statistical fluctuation (+/- 0.3%) for geometric sensitivity and better than 3% of total sensitivity in the worst case of septa designed for high-energy (364.5 keV) photons.

Practice Management Guidelines for the Screening of Thoracolumbar Spine Fracture

Fractures to the thoracolumbar spine (TLS) commonly occur because of major trauma mechanisms. In one series, 4.4% of all patients arriving at a Level I trauma center were diagnosed as having TLS fracture. Approximately 19% to 50% of these fractures in the TLS region will be associated with neurologic damage to the spinal cord. To date there are no randomized studies and only a few prospective studies specifically addressing the subject. The Eastern Association for the Surgery of Trauma organization Practice Management Guidelines committee set out to develop an EBM guideline for the diagnosis of TLS fractures. A computerized search of the National Library of Medicine and the National Institutes of Health MEDLINE database was undertaken using the PubMed Entrez (www.pubmed.gov) interface. The primary search strategy was developed to retrieve English language articles focusing on diagnostic examination of potential TLS injury published between 1995 and March 2005. Articles were screened based on the following questions. (1) Does a patient who is awake, nonintoxicated, without distracting injuries require radiographic workup or a clinical examination only? (2) Does a patient with a distracting injury, altered mental status, or pain require radiographic examination? (3) Does the obtunded patient require radiographic examination? Sixty-nine articles were identified after the initial screening process, all of which dealt with blunt injury to the TLS, along with clinical, radiographic, fluoroscopic, and magnetic resonance imaging evaluation. From this group, 32 articles were selected. The reviewers identified 27 articles that dealt with the initial evaluation of TLS injury after trauma. Computed tomography (CT) scan imaging of the bony spine has advanced with helical and currently multidetector images to allow reformatted axial collimation of images into two-dimensional and three-dimensional images. As a result, bony injuries to the TLS are commonly being identified. Most blunt trauma patients require CT to screen for other injuries. This has allowed the single admitting series of CT scans to also include screening for bony spine injuries. However, all of the publications fail to clearly define the criteria used to decide who gets radiographs or CT scans. No study has carefully conducted long-term follow-up on all of their trauma patients to identify all cases of TLS injury missed in the acute setting.

TH‐E‐330A‐06: Scatter Characterization in Cone‐Beam CT Systems with Offset Flat Panel Imagers

Purpose: X‐ray scatter significantly degrades the quality of cone‐beam CT (CBCT) reconstructions by introducing cupping and streaking artifacts. Simple correction techniques, based on subtracting a constant value across a projection, fail when the flat‐panel detector is transaxially offset as is required to increase the reconstruction field‐of‐views for body scans. The purpose of this study was to measure x‐ray scatter profiles for transaxially offset detectors and to characterize the resulting artifacts. Method and Materials: Data were collected on a table‐top CBCT system. A pelvic phantom was imaged with a Varian 4030CB imager offset by 16 cm. Scatter was estimated by subtracting a nearly “scatter‐free” projection data set, obtained by narrowing the axial collimator blades, from the full CBCT data set obtained with the blades in their fully open position. The resulting scatter estimate, valid in the narrow region of overlap, was extrapolated to generate a scatter estimate across the entire axial extent of the detector. This scatter estimate was then subtracted from the original CBCT data to generate scatter‐corrected images. Results: The scatter profile in full‐fan projections is relatively flat and symmetric. In contrast, in the half‐fan configuration the measured scatter profile was asymmetric decreasing monotonically from the phantom‐air boundary through the phantom center to the imager edge. The slope of this profile varied smoothly from the AP to lateral views resulting in reconstructions with abnormally bright and dark regions. Scatter correction using the measured profile proved effective. Cupping and doming amplitudes were reduced by 2/3. The average reconstruction error in the prostate region was reduced from over 120 HU to less than 40 HU. Conclusions: The asymmetries introduced by an offset detector result in non‐uniform scatter profiles that generate unusual cupping artifacts. Our technique provides a means of characterizing these profiles. Conflict of Interest: Funding provided by Varian Medical Systems.

Using the bootstrap method to evaluate image noise for investigation of axial collimation in hybrid PET

The purpose of this paper was to investigate the use of the bootstrap method to evaluate noise in three-dimensional (3-D) ordered-subsets expectation-maximization list-mode iterative reconstructions from data simulated on a hybrid positron emission tomography (PET) system. In addition, image noise from reconstructions acquired with various levels of axial collimation was investigated. A warm cylinder (20-cm diameter and 70-cm long) was simulated using the GEANT 3.21 Monte Carlo simulation code for different collimator designs: open-frame (OPEN), parallel (PAR) axial, and hybrid parallel fan-beam (HPF) axial collimators. Twenty noisy sets of list-mode data were simulated for each collimator. In addition, events from the first noisy list-mode data set of each category (OPEN, PAR axial, and HPF axial collimators) were chosen at random with replacement to produce twenty new sets of the same size. List-mode data were reconstructed using a 3-D ordered-subsets expectation-maximization (OSEM) list-mode iterative reconstruction algorithm using two subsets and up to 10 iterations. The standard deviations within volumes of interest (VOIs) were computed, and the mean value of this estimate over noise realizations was evaluated. The results indicate that there is no statistical significant difference in the standard deviation of VOI estimates from noisy realizations and bootstrap samples when using the OPEN, the PAR axial, and the HPF axial collimators. The results showed that the precision of VOI estimates varies with the axial spatial location within the object and the axial collimator. In addition, noise variation within list-mode reconstructed images appears to be related to the 3-D sensitivity map.

A Monte Carlo simulation study to evaluate septal spacing using triple-head hybrid PET imaging

The low coincidence fraction (ratio of true coincidences to single events detected) and a limited count rate capability reduce performance of hybrid PET systems as compared with dedicated PET. Axial collimation (i.e., using septa oriented perpendicular to the axis of rotation) has been used to reduce acceptance of photons from outside the field of view, reduce scattered events, and allow the use of 2-D image reconstruction algorithms. The major drawback of axial collimation is a reduced coincidence detection efficiency. The goal of this study was to evaluate the effect of axial collimator septal spacing, as well as to evaluate a new hybrid parallel-fan beam axial collimator design which may better shield out of field-of-view (FOV) photons, reduce the number of scattered events, and increase the sensitivity to the central FOV. The GEANT 3.21 Monte Carlo (MC) simulation code was modified to model a Philips IRIX triple-head hybrid PET system in the 0/spl deg/-90/spl deg/-270/spl deg/ geometry and used to investigate different axial collimator designs. Both lead and tungsten septa were investigated. Basic PET system parameters such as scatter fraction, global true coincidences to single events (T/S) ratio, axial profiles of the T/S ratio, and energy spectra were examined. Noise equivalent count (NEC) rate curves were plotted. Noise and contrast were evaluated for different collimator designs and reconstruction strategies. Comparisons to an experimentally acquired line source phantom showed that the GEANT 3.21 MC simulation code accurately modeled the Philips IRIX camera. The T/S ratio was observed to increase with increasing septal spacing from 0.77 to 2.27 cm and was approximately constant for larger septal spacing. Widening the septal spacing modified the T/S axial profiles making them more nonuniform with a strong peak toward the axial center of the FOV. Using the hybrid parallel-fan beam axial collimator provided a more uniform axial T/S profile for some septal spacings and improved contrast and decreased the noise in the reconstructed slices. In addition, for a given single events rate, a larger proportion of single events were measured within the photopeak energy window as septal spacing decreased. This observation can have implications on the recorded random rate for some PET systems.

Determination of three-dimensional voxel sensitivity for two- and three-headed coincidence imaging

Recently, SPECT and positron emission tomography imaging modalities have been hybridized so that positron coincidence detection can be accomplished with SPECT systems. Originally, only systems with two opposing camera heads were employed. Recent developments to improve sensitivity include the addition of a third camera head. Several authors have developed methods to calculate line-of-response and voxel sensitivities, known as rotational and geometric weights, respectively. These weights are important for use as normalization factors in iterative image reconstruction, as well as to provide insight into the nonuniformity of voxel sensitivity across the reconstructed field-of-view. Although their formulations use analytic expressions, the equations derived for the voxel sensitivities involve an integral which cannot be computed in closed form; that is, one must use a numerical approximation. This may affect the voxel sensitivity in that the accuracy and speed of such a discrete calculation are heavily dependent on the mesh size used. The authors' alternative approach, that does not rely on the numerical approximation, is to directly calculate the solid angle subtended by each voxel with the detectors over all detector positions. They include results for two camera heads 180/spl deg/ apart (DUAL), three camera heads 120/spl deg/ apart (TRI), and three camera heads 90/spl deg/ and 180/spl deg/ apart (C-SHAPE), with and without axial collimation.

Optimization of septal spacing in hybrid PET using estimation task performance

Hybrid PET imaging, a low cost alternative to dedicated PET, is now commonly used in many clinics. The major factors that limit image quality of hybrid positron emission tomography (PET) systems are a limited count-rate capability, and a low coincidence fraction (i.e., trues to singles ratio). Axial collimation or slats are typically used to reduce single (unpaired) photons from impinging on the detector face. Unfortunately, axial collimation also prevents a number of true coincidences from being detected. In this study, we have used a multi-parameter estimation task to study quantitation performance resulting from projection data obtained using a triple-head system with different levels of axial collimator septal spacing. Specifically, the task is the estimation of amplitude (A) and size (R) of a three-dimensional Gaussian function embedded in a uniform cylindrical background. To evaluate task performance, the bias and variance of maximum-likelihood estimates of the parameters are studied by forming an approximate joint probability density function for the estimates. It was observed that as projection data noise increases; joint PDFs of the estimates become more skewed, with a larger difference between the variance of estimates and their minimum variance bound (Cramer-Rao bound). Estimator performance varied as a function of lesion location, which was expected since coincidence sensitivity for rotating hybrid PET systems with axial collimation is position dependent. In general, results showed that approximately 2.0-cm septal spacing provided projection data with the best overall estimator performance for lesions located within the central 20-cm axial field-of-view.

Gamma camera PET with low energy collimators: characterization and correction of scatter

Imaging of myocardial viability with fluorodeoxyglucose (FDG) is possible with positron emission tomography (PET) and with SPECT. The image resolution from SPECT is poor but the data is reported to provide clinical information comparable to PET. Studies have just begun to appear using gamma camera PET and either axial slat collimators or open frame graded absorbers to image myocardial viability. Alternatively, it may be possible to use standard low energy collimators en detecting coincidences. Although image quality may suffer, it may be possible to devise methods such that no clinical information is lost. Such an approach also paves the way for dual isotope sequential or simultaneous imaging of coincidence and single photons. Here, we characterize the scatter fraction and scatter distribution of gamma camera PET with low energy collimators, and investigate the improvements possible with a convolution-subtraction scatter correction scheme. Monte Carlo simulations, line sources, a realistic phantom, and a human study were used. The scatter fraction was found to be almost identical to that obtained with axial slat collimators on a triple head gamma camera hybrid PET scanner. Images acquired with low energy collimators were degraded but still of good quality compared to acquisitions using axial collimation. The scatter correction scheme showed a degree of improvement over reconstructions without scatter correction. This approach is useful not only toward making sequential or simultaneous dual isotope imaging possible, but may also be useful to save time in a busy clinic that does both SPECT scans and cardiac FDG studies, since collimators would not need to be changed.

Influence of the angle of incidence on the sensitivity of gamma camera based PET

Thicker crystals have been used to increase the detection efficiency of gamma cameras for coincidence imaging. This results in a higher detection probability for oblique incidences than for perpendicular incidences. As the point sensitivity at different radial distances is composed of coincidences with different oblique incidences, the thickness of the crystal will have an effect on the sensitivity profiles. To correct this non-uniform sensitivity, a sensitivity map is needed which can be measured or calculated. For dual- or triple-head gamma camera based positron emission tomography (PET) a calculated sensitivity map is preferable because the radius and the head orientation often change between different acquisitions. First, these sensitivity maps are calculated for 2D acquisitions by assuming a linear relationship between the detection efficiency and the crystal thickness. The 2D approximation is reasonable for gamma cameras with a small axial acceptance angle. The results of the 2D approximation show a good agreement with the results of Monte Carlo simulations of different realistic gamma camera configurations. For dual-head gamma cameras the influence on the sensitivity profile is limited. Greater variation of the sensitivity profile is seen on three-headed gamma cameras and correction of this effect is necessary to obtain uniform reconstruction. To increase the sensitivity of gamma cameras, axial collimators with larger acceptance angles are used. To obtain a correct sensitivity for these cameras a sensitivity calculation in 3D is needed. For a fixed camera position the sensitivity is obtained by integrating the detection efficiency over the solid angle formed by the voxel and the intersection of the first detector with the projection of the second detector on the plane of the first detector. The geometric sensitivity is obtained by averaging this for all camera angles. The values obtained show a good agreement with the Monte Carlo simulations for different points in the field of view. Both 2D and 3D sensitivity profiles show the highest influence of the detector thickness on the radial profiles of the U-shape configuration. Taking the detector thickness into account also has an influence on the axial profiles. This influence is maximal in the centre where more oblique coincidences are present. This method is not limited to gamma camera based PET scanners but can be used to calculate the sensitivity of any PET camera with continuous detector blocks.

Optimisation of noise equivalent count rates for brain and body FDG imaging using gamma camera PET

Describes the use of noise-equivalent count (NEC) rates to optimise the clinical use of a modified dual-headed gamma camera for positron emission tomography (PET). Phantoms were used to simulate the distribution and uptake for brain and body /sup 18/F-fluorodeoxyglucose (FDG) imaging, including the presence of activity outside the imaging field of view. Camera count rates were recorded as a function of activity in the field of view in the presence and absence of lead septa. The use of variable numbers and widths of energy windows was also investigated. The results indicate that acquisitions without axial collimators is optimal for brain imaging, while a restriction of 12/spl deg/ in the axial acceptance angle leads to superior NEC rates for torso imaging. The most significant factor affecting the NEC rates for body imaging with no axial collimation is the detection of random coincidences from out-of-field activity. Under most imaging conditions, the use of a Compton-photopeak energy window in addition to the traditional photopeak-photopeak appears to lead to increased NEC rates, although an accurate scatter correction would be necessary to compensate for the associated increase in the scatter fraction.

Jet Dynamics in Black Hole Fields: A Collimation Mechanism

We study the axial collimation of the vortical geodesics in the field of a rotating black hole. This effect manifests itself when the energy and angular momenta of the particles that move on those geodesics vary slowly in dynamical time. In this paper we deduce the constraints that ensure collimation and show how the latter arises when the orbital parameters are perturbed by a given electromagnetic field or by pressure gradients.

Axial collimation in the Kerr metric (astrophysical jets)

Under suitable physical conditions, the vortical geodesics in the Kerr metric can collimate towards the symmetry axis. The authors study the constraints on the rate of change of the parameters which identify the vortical orbits and which guarantee the most efficient collimation process.