Collimator Width Research Articles

CT evaluation of bone dehiscence of the superior semicircular canal as a cause of sound- and/or pressure-induced vertigo.

To describe the computed tomographic (CT) findings at different collimation widths associated with superior semicircular canal (SSC) dehiscence syndrome and to determine the frequency of these findings in a control population. Temporal bone CT scans with 1.0-mm and/or 0.5-mm collimation were obtained in 50 patients with sound- and/or pressure-induced vestibular symptoms. The control population consisted of 50 patients undergoing CT at 1.0-mm collimation and 57 patients undergoing CT at 0.5-mm collimation for other reasons. SSC dehiscence was documented on CT scans in all 36 patients with the clinical syndrome, with bilateral findings in six patients. Six other patients without specific clinical signs appeared to have dehiscence on 1.0-mm-collimated scans. Intact bone overlaying the SSC was subsequently identified with 0.5-mm-collimated CT in each case. On the 1.0-mm-collimated scans in 50 control patients, an area judged as possible or definite dehiscence was identified in 18 of 100 ears. The bone overlaying the SSC was intact in each of the 114 control ears evaluated with 0.5-mm-collimated CT. CT findings from the patients with vestibular symptoms combined with those in the control population indicated that the positive predictive value of an apparent dehiscence in the diagnosis of SSC dehiscence syndrome improved from 50% with 1.0-mm-collimated CT with transverse and coronal images to 93% with 0.5-mm-collimated CT with reformation in the plane of the SSC. The positive predictive value of CT in identification of SSC dehiscence syndrome improves with 0.5-mm-collimated helical CT and reformation in the SSC plane.

Influence of Anatomical, Scanning, and Reconstructing Parameters on Image Quality of CT Angiography: Vascular Phantom Study

Purpose: To investigate the image quality of CT angiograms obtained with various anatomical, scanning, and reconstruction parameters using a phantom with small vessels, and to determine the technique appropriate to a specific vascular anatomy. Materials and Methods: The vascular phantom consisted of three pairs of parallel tubes with a luminal size of 1, 2, and 3 mm. Using the phantom, CT scans were obtained at three inter-tubal distances (1, 2, and 3 mm), three angles of inclination relative to the z-axis (0 degree, 45 degree, and 90 degree), three collimation widths (1, 3, and 5 mm) and two pitch factors (1 and 2). Using axial images obtained at 1-, 2-, and 3-mm reconstruction intervals, two types of three-dimensional images were rendered, and the quality of 972 such images was subjectively graded in terms of edge definition and artifact by three radiologists. Results: All vessels parallel to the z-axis showed good image quality irrespective of other factors. A pair of vessels separated by 1 mm appeared to be stuck to each other. As collimation width and reconstruction interval increased, image quality decreased significantly and artifact increased. The quality of images obtained at 3 mm collimation and with a 6 mm increment was significantly better where collimation and increment were both 5 mm (p=0.001). Vessels 1 mm in size suffered artifactual degradation, especially in the case of SSD images. Most obliquely oriented vessels showed stair-step artifact, which tended to be severe when the reconstruction interval was large. Conclusion: In small vessels perpendicular to or inclined relative to the z-axis, a smaller collimation width and reconstruction interval lessens image degradation. When wider scan coverage is intended, a larger pitch rather than a larger collimation width is recommended. To lessen the artifacts occurring where vessels are obliquely oriented, the reconstruction interval should be reduced.

Effect of Radiographic Contrast Material Exposure on Spiral CT Attenuation of Renal Calculi

Rationale and Objectives The authors performed this study to determine whether exposure of renal calculi to radiographic contrast material has an effect on the attenuation values at computed tomography (CT) performed with varying collimation widths. Materials and Methods Renal calculi (23 stones of various composition) were scanned with 1-, 3-, and 10-mm collimation. Stones were then exposed to a solution of radiographic contrast material for 5 minutes, washed with water, and rescanned 36 hours later. The reproducibility of the CT attenuation measurements on different days was evaluated by obtaining measurements in a subset of 16 renal stones on 4 different days. Results There was no statistically significant change in attenuation after contrast material exposure at narrow collimation. At wider collimation, statistically significant increases were noted in both attenuation and standard deviation. A small amount of variability between readings was noted on different days, with a minimal increase in attenuation each day. Correlation between readings remained very high. Conclusion Exposure of stones to a radiographic contrast material had a statistically significant effect on CT attenuation values only at wide collimation. This may be related to technical factors including volume averaging. Absence of an effect at narrow collimation suggests that the attenuation values of renal stones do not significantly change after exposure to contrast material.

Correction of Helical CT Attenuation Values with Wide Beam Collimation: In Vitro Test with Urinary Calculi

Rationale and Objectives Urinary calculi are now commonly detected with helical computed tomography (CT), and it has been proposed that stone composition can be determined from CT attenuation values. However, typical scans are made with a beam collimation of 5 mm or more, resulting in volume averaging and reduction in accuracy of attenuation measurement. The authors tested a model for correction of errors in attenuation values, even at section widths larger than the width of the object. Materials and Methods Human urinary stones were scanned with helical CT at different beam collimation widths. A computer model was used to predict the effect of beam width and stone size on accuracy of measured attenuation. Results At 3-mm collimation, the model corrected the attenuation readings with an underestimation of 12% ± 1 (compared with values at 1-mm collimation; 127 stones; diameters of 1.7–11.3 mm). With attenuation measured at 10-mm collimation, the model underestimated the true value by 34% ± 3 (103 stones), with a significant negative correlation with stone diameter on magnitude of error (diameters of 3.0–11.3 mm). Correlation of data from patient scans with subsequent in vitro scanning of the same stones confirmed the validity of the model, but corrected in vivo scans consistently yielded lower values for the stones than in vitro. Conclusion Volume averaging effects on attenuation in helical CT are predictable in vitro for urinary calculi—and presumably for other roughly spherical structures—as long as section width does not excessively exceed the diameter of the structure.

Practical aspects of Compton scatter densitometry

A technique for determining the physical density distribution within an object using Compton scattered radiation is described. The technique is based on measuring the variation with the depth of 90° scattered radiation flux from an external monoenergetic photon source using a high-resolution solid-state detector. The measured scattered signal from an isolated volume element of the interrogated object is proportional to its electron density. For a definite region within the scanned object, the variation of the scattered yield with depth, along the direction of the incident beam, will give a counting curve with slope proportional to the density of the examined region. The spatial resolution of the system, determined by the collimator width, is 2 mm. The mathematical aspects of the technique are presented and the experimental data obtained in line scans of representative tissue equivalent materials are also shown to demonstrate the validity of the method.

Effects of scanning and reconstruction parameters on image quality in electron-beam CT angiography: Coronary artery phantom study

This study compared the image quality obtained with different scanning and reconstruction parameters for electron-beam computed tomographic (CT) angiography and sought optimal methods for visualizing the coronary artery lumen. Electron-beam CT angiography with contrast material enhancement was used to image 35 branches of fresh postmortem swine coronary arteries. Different collimation widths, fields of view (FOVs), reconstruction kernels, and algorithms were employed to reconstruct the acquired raw data into CT angiographic images. Image quality was compared and analyzed. The contrast-to-noise ratios (C/Ns) for 1.5-, 2-, and 3-mm section thickness were 28.4 +/- 15.2, 31.9 +/- 9.3, and 33.8 +/- 14.5, respectively (P < .05). The lengths of visualized coronary artery lumina were significantly longer for 1.5-mm scanning (71.6 mm +/- 4.3) than for 2-mm (58.3 mm +/- 5.5) and 3-mm scanning (59.0 mm +/- 8.0) (P < .01). The C/Ns for 12.7-, 18.0-, and 26.0-cm FOV reconstruction were 32.8 +/- 9.9, 28.9 +/- 8.2, and 27.1 +/- 8.2, respectively (not significant), and the visualized luminal lengths were 76.1 mm +/- 12.5, 71.7 mm +/- 14.6, and 65.4 mm +/- 13.1, respectively (not significant). The highest C/N (48.2 +/- 13.3) was achieved with smooth kernels and a cone-beam algorithm, and the lowest (14.7 +/- 3.4) with very sharp kernels and a normal algorithm. Cone-beam algorithm images had significantly higher C/Ns than did normal algorithm images (P < .001), and they demonstrated longer coronary artery lumina (P < .01). Collimation width, FOV, reconstruction kernels, and algorithms are important in the processing of high-quality electron-beam coronary angiograms. A 1.5-mm collimation width, 12.7-cm FOV, cone-beam reconstruction algorithm, and very sharp kernels should help in obtaining the best image quality and depicting the longest segments of coronary artery lumen.

Helical CT of urinary calculi: effect of stone composition, stone size, and scan collimation.

Helical CT has become the preferred methodology for identifying urinary calculi. However, the ability to predict stone composition, which influences patient treatment, depends on the accurate measurement of the radiographic attenuation of stones. We studied the effects of stone composition, stone size, and scan collimation width on the measurement of attenuation in vitro. One hundred twenty-seven human urinary calculi of known composition and size were scanned at 120 kVp, 240 mA, and a 1:1 pitch at different collimations. A model, based on the physics of helical CT, was used to predict the effect of scan collimation width and stone size on measured attenuation. At a 1-mm collimation, stone groups could be differentiated by attenuation: the attenuation of uric acid was less than that of cystine or struvite, which overlapped; these were less than the attenuation of calcium oxalate monohydrate, which was in turn lower than that of brushite and hydroxyapatite, which overlapped and showed the highest values. At a wider collimation, attenuation was lower and the ability to differentiate stone composition was lost. Attenuation also decreased with smaller stones. At a 10-mm collimation, some uric acid stones (<approximately 6 mm) and other stones (< approximately 4 mm) had very low attenuation, so low that they could remain undetected on helical CT. The model predicted well the degree that attenuation was affected by stone size and collimation width. Stone composition and stone size, relative to CT collimation, independently influenced CT attenuation. The effect of stone size and collimation generally conformed to the model's predictions. We determined that small stones with low attenuation can be overlooked on helical CT.

Calcium stone fragility is predicted by helical CT attenuation values.

Helical CT has become the preferred method for imaging urinary calculi, and so it would be useful if data from helical CT could also be used to predict the number of shockwaves (SWs) needed to break a given stone. We measured the number of SWs required to comminute calcium stones in vitro. The SW requirement correlated with stone size (volume, weight, diameter) and with helical CT attenuation values when the scans were performed at 3-mm collimation. When CT scans were performed at 1-mm collimation, the number of SWs needed for comminution did not correlate with helical CT attenuation values. This result indicates that the correlation with 3-mm scans was attributable to volume-averaging effects, in which smaller stones yield smaller attenuation values. That is, attenuation values from helical CT at larger beam collimation widths contain information about stone size that can be exploited to predict the fragility of calcium stones. We observed that for calcium stones, the number of SWs to comminution was generally less than half the stone CT attenuation value in Hounsfield units. This "half-attenuation rule" predicted the number of SWs needed to complete fragmentation for 95% of calcium stones (24/24 calcium oxalate monohydrate, 13/13 hydroxyapatite, 8/10 brushite stones). This in vitro study suggests that it may be possible to predict effective SW dose using helical CT prior to lithotripsy.

Spiral CT optimization for measurement of bronchial lumen diameter using an experimental model.

The aim of the study was to determine optimal parameters for demonstrating sublobar bronchi on spiral CT. Measurements were obtained from five parallel polyethylene tubes embedded in foam matrix with similar radiographic characteristics to segmental and subsegmental bronchi and to lung parenchyma, respectively. Collimation widths of 1.5, 2, 3 and 4 mm were used, with a pitch of 1 or 1.5 and a reconstruction interval of 1 mm or 2 mm. Various slice planes were used. Images acquired orthogonally were viewed normally. Images acquired in planes oblique or parallel to the long axes of the tubes were reformatted into a plane orthogonal to the long axes of the tubes to be comparable with the directly acquired orthogonal images. Tube diameters were measured at lung window settings (L, -400; W, 1300) and compared with known true inner and outer tube diameters. Measurements from images acquired orthogonal to the tube long axes were accurate regardless of slice thickness. Images acquired obliquely or parallel only produced accurate measurements at the lowest slice thickness (1.5 mm). Pitch and reconstruction interval had no effect on measurement error in any scan plane. It is concluded that a slice thickness of 1.5 mm or less, with a pitch of 1.5, should be used when acquiring images at angles other than orthogonal to the long axes of experimental tubes equivalent to the segmental and subsegmental bronchi. It is suggested that similar parameters should be used in vivo and that the examination should be targeted to the area of the bronchial tree in question to reduce patient dose and length of breath-hold.

Experimental determination of section sensitivity profiles and image noise in electron beam computed tomography.

To determine the effect of continuous-volume scanning (CVS) on z-axis resolution, section sensitivity profiles were measured on an electron beam computed tomography (CT) scanner and compared with those obtained using the step-volume scanning (SVS) mode. A steel bead was imaged using different scan parameters, and the mean CT number over the bead was plotted against the z-axis position to determine section sensitivity profiles. From these profiles, full width at half maximum (FWHM), full width at tenth maximum (FWTM), and full width at tenth area (FWTA) were calculated. A uniform water phantom was imaged to measure noise. To determine the visual significance of changes in the section sensitivity profile, a section thickness and contiguity phantom was imaged. All section sensitivity profiles measured had an FWHM value within 0.5 mm of the nominal scan width. The FWTM and FWTA values increased with the CVS mode compared with the SVS mode. This broadening of the section sensitivity profiles was most significant with larger collimator widths. However, use of smaller collimator widths increased image noise. When all other parameters remained constant, increasing the exposure time to reduce image noise did not affect the section sensitivity profile. The CVS mode produced wider section sensitivity profiles than the SVS mode. This effect was minimized when the smallest collimator width was used, but at the expense of increased image noise.

A spherical dose model for radiosurgery plan optimization

Conventional 3D dose calculations for stereotactic radiosurgery involve integration of individual static beams comprising a set of arcs. For iterative optimization of multiple isocentre treatment, which requires repetitive dose calculations at a large number of sample points, the conventional method is too slow. To overcome this problem spherically symmetric dose distributions are assumed. The authors describe a spherical dose model derived from a parametrized convolution of the collimator width and a dose spread kernel. The method is fast and easy to implement requiring just a single empirically derived value. Furthermore, the model is differentiable with respect to the parameters to be optimized. This property is useful when the optimization strategies rely on gradient information.

CT in vascular pathologies.

Since the introduction of helical scanners, CT angiography (CTA) has achieved an essential role in many vascular applications that were previously managed with conventional angiography. The performance of CTA is based on the accurate selection of collimation width, pitch, reconstruction spacing and scan delay, which must be modulated on the basis of the clinical issue. However, the major improvement of CT has been provided by the recent implementation of many post-processing techniques, such as multiplanar reformatting, shaded surface display, maximum intensity projections, 3D perspectives of surface and volume rendering, which simulate virtual intravascular endoscopy. The integration of the potentialities of the scanner and of the image processing techniques permitted improvement of: (a) the evaluation of aneurysms, dissection and vascular anomalies involving the thoracic aorta; (b) carotid artery stenosis; (c) aneurysms of abdominal aorta; (d) renal artery stenosis; (e) follow-up of renal artery stenting; and (f) acute or chronic pulmonary embolism. Our experience has shown that the assessment of arterial pathologies with CTA requires the integration of 3D post-processing techniques in most applications.

The effect of helical pitch and beam collimation on the lesion contrast and slice profile in helical CT imaging.

A model is presented in this paper to describe how the contrast of a reconstructed object and slice sensitivity profile are affected by (1) the table speed or helical pitch, (2) the x-ray collimations, (3) the size of the object, (4) the alignment between the reconstructed slice and the object, (5) the distance of the object from the axis of rotation, and (6) the helical CT reconstruction algorithm employed. This contrast model is validated by both computer simulations and experiments. With this model, the contrast of a reconstructed object, slice sensitivity profile, and the longitudinal MTF can be accurately predicted. The optimal scan strategy and the point of diminishing returns can be determined prior to scanning. Several conclusions can be drawn from this model. First, overlapping reconstruction significantly improves overall scan contrast sensitivity of helical CT. Second, with a given x-ray collimation, low pitch helical scans provide better longitudinal resolutions. Third, with a given volume coverage rate (i.e., a given table speed), narrow collimation high pitch helical scans provide better longitudinal resolutions than wide collimation low pitch ones and therefore are recommended for high-contrast thin-slice applications. A lesion conspicuity model is also established.

Blurring of Vessels in Spiral CT Angiography: Effects of Collimation Width, Pitch, Viewing Plane, and Windowing in Maximum Intensity Projection

Our goal was to examine the effects of collimation width (CW), pitch, viewing plane, and windowing on the display of in-plane vessels in maximum intensity projection (MIP). A theoretical concept based on partial volume averaging of vessels was developed to describe the contents of voxels (densities) in MIP and to derive cross-sectional vessel diameters and blurring. To validate the concept and to describe the influence of pitch, a Plexiglas cone submerged in water was scanned with varying CW and pitch. Binary MIP with three representative window levels was chosen so that definitive vessel diameters could be quantitated. The theoretical concept correctly predicted voxel contents and blurring for CW > or = 3 mm and low pitch. For high pitch, actual blurring was larger; however, for a given table speed, blurring of the cone decreased with pitch while increasing with CW. Overall blurring was most effectively reduced by using a thin CW and the transverse viewing plane. In the transverse viewing plane, the least blurring was found using binary MIP with a low window level. On the contrary, in the longitudinal viewing plane, blurring was minimized using a window level halfway between the density of the cone and that of the surrounding water. For CW > or = 3 mm, blurring of in-plane vessels can be explained with a simple geometrical concept based on partial volume. For accurate display, the transverse viewing plane should be used, a proper windowing must be chosen, and the CW should be kept below vessel size while raising the pitch to cover a reasonable volume.

HELICAL (SPIRAL) CT OF THE RETROPERITONEUM

Helical CT promises to improve imaging of the retroperitoneum through the elimination of respiratory misregistration and the minimization of partial volume averaging. The speed of acquisition enables the entire abdomen and pelvis to be imaged in less than 2 minutes with standard collimator widths. This is a distinct advantage in critically ill patients, particularly in the case of trauma. Selective intravenously administered contrast enhancement is made possible with helical CT. Rapid scanning allows imaging in an arterial, renal cortical, renal medullary, or delayed collecting system phase. This property of helical CT enables CT angiography to emerge as a powerful minimally invasive alternative to conventional arteriography for imaging the abdominal aorta and its branches. After 4 years of investigating the use of volumetric CT, the clinical settings described herein remain the only clear indications for using helical CT in the kidney and retroperitoneum. These studies took advantage of the single most important feature of helical CT--the elimination of respiratory misregistration. Two broad diagnostic advantages result: (1) the ability to search and characterize a region thoroughly, such as in the evaluation of a small renal mass, or to search for the needle-tip during an interventional procedure; and (2) the ability to create multiplanar and 3-D images that are of sufficient quality to be useful in clinical practice, such as in imaging the vasculature (CT angiogram), and in surgical planning.

Radiation dose distributions for electron arc therapy using electrons of 6-20 MeV

Radiation dose distributions in electron arcs depend in a complicated manner on electron energy, collimation width and depth of isocentre and cannot easily be deduced from the dose distributions in stationary beams. In the present work the method of measuring dose distributions for electron arcs using film is discussed. The changes in the arc dose distribution produced by changes in electron energy, radiation field width and radius of curvature (i.e., depth of isocentre) are investigated. The change in radial percentage depth-dose with changes in radius of curvature or source-surface distance of the target volume is minimal whereas changes in radiation field size produce changes in percentage depth-dose and bremsstrahlung contamination at the isocentre. Several options available for treatment planning are presented and discussed.

Comparison of highly oriented pyrolytic and ordinary graphite as polarizers of Mo Kα radiation in EDXRF

AbstractHighly oriented pyrolytic graphite (HOPG) and ordinary graphite were compared with respect to detection limits as excitation targets in a triaxial EDXRF set‐up. HOPG as a Bragg polarizer of Mo Kα x‐rays did not show appreciable advantages over ordinary graphite as a Barkla scatterer. Under relatively wide collimation conditions necessary to obtain a sufficient count rate in analytical practice, HOPG acts mainly as a Barkla scatterer.

Design criteria and performance parameters of an alpha irradiation device for cell studies

An alpha irradiation device is described that utilises a commercially available disc-shaped americium-241 source of 8 cm diameter. The alpha particles traverse a moving collimator and the source is rotated to reduce the influence of source inhomogeneities. Source, collimator and a shutter disc are mounted in a container which is flushed with helium to reduce energy losses of the alpha particles before reaching the exit foil. The shutter disc is activated by a computer-controlled step motor. The broad beam of alpha particles emerges from the exit window of the container with a remaining range in tissue of about 15 mu m. An intermittent computer-controlled use of a preabsorber makes it possible to reduce dose differences within a depth up to 12.5 mu m to not more than +or-3%. With the commercially available americium sources a dose rate of 0.2 Gy min-1 is reached; this can be increased by utilising a somewhat wider collimator.

Optimization of electron arc therapy doses by multi-vane collimator control

Retrospective computer simulations, based on clinical treatment planning data available from over 50 patients treated by electron arc radiotherapy to the chestwall following mastectomy, show that a dramatic improvement in dose uniformity can, in many clinical situations, be achieved by dynamic shaping of the electron arc collimator, under computer control, as a function of gantry angle and distance superior or inferior to the central plane. The greatest improvement in dose uniformity is seen in calculational planes in which the patient contour has the greatest departure from a circular shape. Dosimetric studies demonstrate this improvement. Indicators for use of variable-width multi-vane electron arc collimators include the following: (1) Mechanical constraints of the therapy equipment may limit the placement of isocenter to an inadequate depth which causes large variation in the SSD around the arc; (2) Out of the central plane, the shape of the chest wall may change dramatically across the limits of the arc, creating large variations in the dose distribution; (3) Clinical definition of the treatment surface to include surgical scars or other at-risk volume may create an irregularly shaped treatment surface, thereby changing the fraction of the arc included in the treatment surface from one plane to the next. Electron arc collimator shape determines both the dose rate and the electron arc beam profile. Both the dose rate and the beam profile must be included in the integration of dose to a point within the arc. The dose to a point within the arc can be modified by as much as a factor of 1.5 to 2.0 by increasing the collimator width from 3 cm to 7 cm. A multi-vane collimator allows these changes to be made in each specific plane to compensate for changes in patient contour.

Optimization of Electron Arc Therapy Doses by Multi-Vane Collimator Control

Abstract Retrospective computer simulations, based on clinical treatment planning data available from over 50 patients treated by electron arc radiotherapy to the chestwall following mastectomy, show that a dramatic improvement in dose uniformity can, in many clinical situations, be achieved by dynamic shaping of the electron arc collimator, under computer control, as a function of gantry angle and distance superior or inferior to the central plane. The greatest improvement in dose uniformity is seen in calculational planes in which the patient contour has the greatest departure from a circular shape. Dosimetric studies demonstrate this improvement. Indicators for use of variablewidth multi-vane electron arc collimators include the following: (1) Mechanical constraints of the therapy equipment may limit the placement of isocenter to an inadequate depth which causes large variation in the SSD around the arc; (2) Out of the central plane, the shape of the chest wall may change dramatically across the limits of the are, creating large variations in the dose distribution; (3) Clinical definition of the treatment surface to include surgical scars or other at-risk volume may create an irregularly shaped treatment surface, thereby changing the fraction of the arc included in the treatment surface from one plane to the next. Electron arc collimator shape determines both the dose rate and the electron arc beam profile. Both the dose rate and the beam profile must be included in the integration of dose to a point within the arc. The dose to a point within the arc can be modified by as much as a factor of 1.5 to 2.0 by increasing the collimator width from 3 cm to 7 cm. A multi-vane collimator allows these changes to be made in each specific plane to compensate for changes in patient contour.