Frank A. DiBianca

Performance analysis of a CsI-based flat panel detector in a cone beam variable resolution X-ray system

A new Cone-Beam CT (CBCT) system is introduced that uses the concept of Variable Resolution X-ray (VRX) detection, which has previously been demonstrated to significantly increase spatial resolution for small objects. An amorphous silicon Flat Panel Detector (FPD) with a CsI scintillator (PaxScan 2020, Varian, Salt Lake City, UT) is coupled with a micro-focus x-ray tube (35 - 80 kVp, 10 - 250 μA) to form a CBCT. The FPD is installed on a rotating arm that can be adjusted to any angle θ, called the VRX angle, between 90° and 0° with respect to the x-ray direction. A VRX angle of 90° for the detector corresponds to a conventional CBCT whereas a VRX angle of 30° means that the detector is tilted 90° - 30° = 60° from its perpendicular position. Tilting the FPD in this manner reduces both the line-spread function width and the sampling distance by a factor of sin(&thgr;), thereby increasing detector spatial resolution proportionately. An in-house phantom is used to measure the MTF of the reconstructed CT images using different VRX angles. An increase by a factor of 1.67 ± 0.007 is observed in the MTF cutoff frequency at 30° compared to 90° in images acquired at 75 kVp. Expected theoretical value for this case is 2.0. The new Cone-Beam Variable Resolution X-ray (CB-VRX) CT system is expected to significantly improve the images acquired from small objects - such as small animals - while exploiting the opportunities offered by a conventional CBCT.

Comparison of multi-arm VRX CT scanners through computer models

Variable Resolution X-ray (VRX) CT scanners allow imaging of different sized anatomy at the same level of detail using the same device. This is achieved by tilting the x-ray detectors so that the projected size of the detecting elements is varied producing reconstructions of smaller fields of view with higher spatial resolution.1 The detector can be divided in two or more separate segments, called arms, which can be placed at different angles, allowing some flexibility for the scanner design. In particular, several arms can be set at different angles creating a target region of considerably higher resolution that can be used to track the evolution of a previously diagnosed condition, while keeping the patient completely inside the field of view (FOV).2 This work presents newly-developed computer models of single-slice VRX scanners that allow us to study and compare different configurations (that is, various types of detectors arranged in any number of arms arranged in different geometries) in terms of spatial and contrast resolution. In particular, we are interested in comparing the performance of various geometric configurations that would otherwise be considered equivalent (using the same equipment, imaging FOVs of the same sizes, and having a similar overall scanner size). For this, a VRX simulator was developed, along with mathematical phantoms for spatial resolution and contrast analysis. These tools were used to compare scanner configurations that can be reproduced with materials presently available in our lab.

Analysis of axial spatial resolution in a variable resolution x-ray cone beam CT (VRX-CBCT) system

The Variable Resolution X-ray (VRX) technique has been successfully used in a Cone-Beam CT (CBCT) system to increase the spatial resolution of CT images in the transverse plane. This was achieved by tilting the Flat Panel Detector (FPD) to smaller vrxy angles in a VRX Cone Beam CT (VRX-CBCT) system. In this paper, the effect on the axial spatial resolution of CT images created by the VRX-CBCT system is examined at different vrxx angles, where vrxx is the tilting angle of the FPD about its x-axis. An amorphous silicon FPD with a CsI scintillator is coupled with a micro-focus x-ray tube to form a CBCT. The FPD is installed on a rotating frame that allows rotation of up to 90° about x and y axes of the FPD. There is no rotation about the z-axis (i.e. normal to the imaging surface). Tilting the FPD about its x-axis (i.e. decreasing the vrxx angle) reduces both the width of the line-spread function and the sampling distance by a factor of sin vrxx, thereby increasing the theoretical detector pre-sampling spatial resolution proportionately. This results in thinner CT slices that in turn help increase the axial spatial resolution of the CT images. An in-house phantom is used to measure the MTF of the reconstructed CT images at different vrxx angles.

A backscattered x-ray imager for medical applications

Conventional X-ray radiographic systems rely on transmitted photons for the production of images. Backscatter imaging makes use of the more abundant scattered photons for image formation. Specifically, incoherently (Compton) scattered X-ray photons are detected and used for image formation in this modality of medical imaging. However, additional information is obtained when the transmitted X-ray photons are also detected and used. Transmission radiography produces a two-dimensional image of a three dimensional system, therefore image information from a shallower object is often contaminated by image information from underlying objects. Backscattered x-ray imaging largely overcomes this deficiency by imaging depth selectively, which reduces corruption of shallow imaging information by information from deeper objects lying under it. Backscattered x-ray imaging may be particularly useful for examining anatomical structures at shallow depths beneath the skin. Some typical applications for such imaging might be breast imaging, middle ear imaging, imaging of skin melanomas, etc. Previous investigations, by way of theoretical calculations and computational simulations into the feasibility of this kind of imaging have uncovered high-contrast and SNR parameters. Simulations indicate that this method can be used for imaging relatively high-density objects at depths of up to approximately five centimeters below the surface. This paper presents both theoretical and experimental SNR results on this new medical imaging modality.

Reconstruction artifacts in VRX CT scanner images

Variable Resolution X-ray (VRX) CT scanners allow imaging of different sized anatomy at the same level of detail using the same device. This is achieved by tilting the x-ray detectors so that the projected size of the detecting elements is varied to produce reconstructions of smaller fields of view with higher spatial resolution. As with regular CT scanners, the images obtained with VRX scanners are affected by different kinds of artifacts of various origins. This work studies some of these artifacts and the impact that the VRX effect has on them. For this, computational models of single-arm single-slice VRX scanners are used to produce images with artifacts commonly found in routine use. These images and artifacts are produced using our VRX CT scanner simulator, which allows us to isolate the system parameters that have a greater effect on the artifacts. A study of the behavior of the artifacts at varying VRX opening angles is presented for scanners implemented using two different detectors. The results show that, although varying the VRX angle will have an effect on the severity of each of the artifacts studied, for some of these artifacts the effect of other factors (such as the distribution of the detector cells and the position of the phantom in the reconstruction grid) is overwhelmingly more significant. This is shown to be the case for streak artifacts produced by thin metallic objects. For some artifacts related to beam hardening, their severity was found to decrease along with the VRX angle. These observations allow us to infer that in regular use the effect of the VRX angle artifacts similar to the ones studied here will not be noticeable as it will be overshadowed by parameters that cannot be easily controlled outside of a computational model.

An automated image evaluation procedure for computed tomography systems

A procedure is developed that enables comprehensive and automatic image quality evaluation of computed tomography (CT) systems. This procedure includes custom-designed software and an image quality phantom composed of subsections with regional test objects. The phantom is designed so that the maximum amount of information concerning image quality and system performance can be obtained in a single scan. The software automatically analyzes phantom images and generates measurements of image quality that are both quantitative and objective. The image quality parameters that will be attained from a single scan of the phantom include: spatial resolution, contrast, contrast signal-to-noise ratio, linearity, uniformity, slice thickness, temporal resolution, and dose. This evaluation procedure provides a simple, automated method of quality control. The phantom and procedure can also be used as a research tool for studying modifications of CT system components. In this study, we present results from a mathematical model of the phantom. We discuss the design and validation of the phantom and accompanying software.

Study of the detective quantum efficiency for the kinestatic charge detector as a megavoltage imaging device

Megavoltage x-ray imaging suffers from relatively poor contrast and spatial resolution compared to diagnostic kilovoltage x-ray imaging due to the dominant Compton scattering in the former. Recently available amorphous silicon/selenium based flat-panel imagers overcome many of the limitations of poor contrast and spatial resolution that affect conventional video based electronic portal imaging devices (EPIDs). An alternative technology is presented here: kinestatic charge detection (KCD). The KCD uses a slot photon beam, high-pressure gas (xenon, 100 atm) and a multi-ion rectangular chamber in scanning mode. An electric field is used to regulate the cation drift velocity. By matching the scanning speed with that of the cation drift, the cations remain static in the object frame of reference, allowing temporal integration of the signal. KCD imaging is characterized by reduced scatter and a high signal-to-noise ratio. Measurements and Monte Carlo simulations of modulation transfer function (MTF), noise power spectrum (NPS) and the detective quantum efficiency (DQE) of a prototype small field of view KCD detector (384 channels, 0.5 mm spacing) were carried out. Measurements yield DQE[0]=0.19 and DQE[0.5cy/mm]=0.01. KCD imaging is compared to film and commercial EPID systems using phantoms, with the KCD requiring an extremely low dose (0.1 cGy) per image. A proposed cylindrical chamber design with a higher ion-collection depth is expected to further improve image quality (DQE[0]>0.25).

Volumetric analysis of tumors in rodents using the variable resolution x-ray (VRX) CT-scanner

The Variable Resolution X-ray (VRX) CT system, developed at the UTHSC, Memphis, has the potential for use in animal imaging. Animal models of tumor progression and pharmacological impact are becoming increasingly important in understanding the molecular and mechanistic basis of tumor development. In general, CT-imaging offers several advantages in animal research: a fast throughput of seconds to minutes reducing the physiological stress animals are exposed to, and it is an inexpensive modality affordable to many animal laboratories. We are developing the VRX CT scanner as a non-invasive imaging modality to measure tumor volume, progression, and metastasis. From the axial images taken by the VRX CT-scanner, tumor area was measured and the tumor volume was calculated. Animals were also imaged using an optical liquid nitrogen-cooled CCD camera to detect tumor fluorescence. A simple image fusion with a planner x-ray image was used to ascertain the position of the tumors, animals were then sacrificed the tumors excised, and the tumor volume calculated by physical measurements. Furthermore, using a specially designed phantom with three spheres of different volumes, we demonstrated that our system allowed us to estimate the volume with up to 10% accuracy; we expect this to increase dramatically in the next few months.