Spiral Scanning Mode Research Articles

Ion Beam Figuring Method for Optical Mirrors with Spiral Scan Mode

The ion beam figuring method for optical mirrors with spiral scan mode is discussed.With the near-Gaussian property of the removal function,the figuring process can be simplified to a linear one and described by conventional computer controlled optics shaping(CCOS) principle.The modified Richard-Lucy iterative algorithm based on the Bayesian principle is adopted for the solution of dwell time.On the basis of analysis of the continuous velocity of dwell time,the criteria for determining the spiral path parameters are given.With these methods,the ion beam figuring technology with spiral scan mode is established.The figuring experiments on planar and aspheric samples are made,the final surface errors are both below 1/100λ.The experimental results show that spiral figuring is a deterministic method of high efficiency for optical mirrors,and can be used as the final figuring process,moreover the processing cost can be reduced.

Imaging of Porous Polyethylene Implant by Using Multidetector Spiral Computed Tomography

The porous polyethylene implant (Medpor; Porex Surgical, Inc., Newnan, GA) is one of the most usable implants in clinical practice. Because it is radiolucent on both direct radiography and conventional computed tomography, visualization of the implant becomes an important issue. In this clinical study, availability of multidetector spiral computed tomography for visualization of the implant was evaluated. The technique was used in 15 patients who had Medpor reconstructions on their faces, including frontal (seven), malar (five), and mandibular (five) regions. A 16-channel multislice computed tomography scanner was used in spiral scanning mode. Sagittal, coronal, and transverse multiplanar reformation and volume-rendered images were reformatted using a PC-based three-dimensional reconstruction program (Extended Brilliance Workspace, Release 1.0.1.1, Philips Medical Systems, Amsterdam, The Netherlands). The three-dimensional images along the course of the implant were additionally reformatted. Window width and level were adjusted for viewing the bone as respective values of 1100 HU and 450 HU in contrast to the values of 800 HU and 200 HU, respectively, for the implant. The detailed contour of the implants could be easily traced on the multidetector computed tomographic images, including either the concave or convex areas. The calcifications and cystic formations around the implants and the spatial relationship between the defects and the implants were also shown. We believe that the present multidetector computed tomographic technique offers direct visualization of the Medpor implant in the body. It has many advantages over the current magnetic resonance imaging techniques that need tissue in growth for visualization.

Filtered backprojection formula for exact image reconstruction from cone‐beam data along a general scanning curve

Recently, Katsevich proved a filtered backprojection formula for exact image reconstruction from cone-beam data along a helical scanning locus, which is an important breakthrough since 1991 when the spiral cone-beam scanning mode was proposed. In this paper, we prove a generalized Katsevich's formula for exact image reconstruction from cone-beam data collected along a rather flexible curve. We will also give a general condition on filtering directions. Based on this condition, we suggest a natural choice of filtering directions, which is more convenient than Katsevich's choice and can be applied to general scanning curves. In the derivation, we use analytical techniques instead of geometric arguments. As a result, we do not need the uniqueness of the PI lines. In fact, our formula can be used to reconstruct images on any chord as long as a scanning curve runs from one endpoint of the chord to the other endpoint. This can be considered as a generalization of Orlov's classical theorem. Specifically, our formula can be applied to (i) nonstandard spirals of variable radii and pitches (with PI- or n-PI-windows), and (ii) saddlelike curves.

Fast volume scanning approaches by X-ray-computed tomography

X-ray-computed tomography (CT) successfully underwent a transition from slice-by-slice imaging to volume imaging in the decade after 1990 due to the introduction of spiral scan modes. Later, the transition from single-slice to multislice scanning followed. With the advent of new detector technologies we are now looking forward to circular and spiral scanning using area detectors and the respective reconstruction approaches. This review treats the basic technological and algorithmic prerequisites and approaches. It goes into detail with respect to the achievable performance and image quality characteristics and discusses the range of applications. Scanning of the complete body trunk with submillimeter isotropic resolution in less than 30 s is possible today. Further extensions to new medical applications, for example, cardiac CT, but also to nonmedical applications, for example, nondestructive material testing, make CT an emerging imaging modality again.

New low-contrast resolution phantoms for computed tomography.

Computed tomography (CT) has been established as a major imaging modality in diagnostic radiology. Accordingly, acceptance testing and quality control of CT scanners is of great importance. While most procedures and phantoms for testing are widely accepted, there is still discussion and uncertainty about low-contrast (LC) sensitivity. In our opinion this unsatisfactory situation is caused at least in part by the lack of suitable phantoms for LC resolution measurements. We investigated the commonly used phantoms for LC detectability, the Catphan, and for LC resolution, the ATS phantom. While the Catphan showed stable object contrasts, the ATS phantom's measured contrast exhibited a strong dependence on temperature and x-ray quality. Based on newly developed polyurethane resin materials, we designed and tested a LC resolution phantom with several different contrast steps. The object contrasts showed no dependence on temperature and beam quality. The new LC resolution phantom proved to be very suitable for measuring a scanner's low-contrast sensitivity in the image plane, one of the most important image quality parameters. To assess LC resolution in three dimensions we designed an additional phantom with rows of spherical objects. A first prototype was evaluated in a multicenter study. The setup proved to be very helpful to quantify the in-plane and axial LC sensitivity of spiral CT scan modes.

A microtomographic system for the nondestructive evaluation of bone architecture.

Microtomography (micro-computed-tomography, mu-CT) is a method to image and quantify trabecular bone. It has the capability to address the role of trabecular architecture on the mechanical properties of bone and to study trabecular bone remodeling. The system described in this work is based on a compact fan-beam type tomograph that can work in spiral scanning or multislice mode. An X-ray tube with a microfocus is used as a source, a CCD-array as a detector. Samples with diameters from a few millimeters to a maximum of 14 mm can be measured, typically, bone biopsies with a diameter of 8 mm and a length of approximately 10 mm are measured. Spatial resolution is 28 microns. Usually the volume of interest contains 4 x 4 x 4 mm3 and is represented in 14 x 14 x 14 microns3 voxels. 3D stereological indices are extracted according to the standard definitions used in histomorphometry. Triangular surface representation is effected with an extended marching cube algorithm and forms a convenient basis for finite element analysis. Microtomographic measurements may be employed to "calibrate" lower-dose, lower-resolution images in vivo as well as to nondestructively assess unprocessed surgical bone biopsy specimens. These specimens remain intact for mechanical or histological testing.

Evaluation of spiral CT on a fourth-generation system

Spiral scanning performance characteristics of a fourth-generation CT scanner are reviewed and compared with axial CT. Slice sensitivity profiles (SSPs) were obtained for a 5 mm collimator setting and compared with the corresponding dose profile. Various pitch factors were used along with four different spiral interpolators: 180° linear (180L), 360° linear (360L), single sidelobe (SSL) high-order and dual sidelobe (DSL) high-order. A noise factor equation is presented that can be used to predict the noise level for any valid spiral interpolator. Noise measurements for the four spiral interpolators were compared with the predicted values. Spatial (x-y) resolution was measured for both axial and spiral scan modes. As expected, no change in spatial resolution was observed. On the basis of our full-width-at-half-maximum (FWHM) results, z-axis resolution was maintained using the high order interpolators with spiral pitch factors up to 1.5. Finally, a set of chest scans was obtained to provide a direct comparison of clinical performance between axial and spiral CT.

Spiral CT of the liver.

The usefulness of CT in the detection and characterization of hepatic disease is well established. Requirements for high-quality examinations include rapid bolus infusion of iodinated contrast materials, rapid acquisition of axial images, and minimal respiratory motion in the patient. These requirements are satisfied in spiral or helical CT. Spiral CT is the latest advance in rapid scanning techniques. In the spiral scan mode, image acquisition is continuous, as patients are advanced at a constant rate through the CT gantry. This eliminates the interscan delay and the gap between scan slices. Because the scan can be completed within a single breath-hold, respiratory misregistration of adjacent scan slices is eliminated. These factors also contribute to high-quality multidimensional displays (e.g., coronal and sagittal views). In this pictorial essay, we illustrate the advantages and applications of spiral CT scanning for imaging the liver.