Full Projections Research Articles

Improved compressed sensing-based cone-beam CT reconstruction using adaptive prior image constraints

Volumetric cone-beam CT (CBCT) images are acquired repeatedly during a course of radiation therapy and a natural question to ask is whether CBCT images obtained earlier in the process can be utilized as prior knowledge to reduce patient imaging dose in subsequent scans. The purpose of this work is to develop an adaptive prior image constrained compressed sensing (APICCS) method to solve this problem. Reconstructed images using full projections are taken on the first day of radiation therapy treatment and are used as prior images. The subsequent scans are acquired using a protocol of sparse projections. In the proposed APICCS algorithm, the prior images are utilized as an initial guess and are incorporated into the objective function in the compressed sensing (CS)-based iterative reconstruction process. Furthermore, the prior information is employed to detect any possible mismatched regions between the prior and current images for improved reconstruction. For this purpose, the prior images and the reconstructed images are classified into three anatomical regions: air, soft tissue and bone. Mismatched regions are identified by local differences of the corresponding groups in the two classified sets of images. A distance transformation is then introduced to convert the information into an adaptive voxel-dependent relaxation map. In constructing the relaxation map, the matched regions (unchanged anatomy) between the prior and current images are assigned with smaller weight values, which are translated into less influence on the CS iterative reconstruction process. On the other hand, the mismatched regions (changed anatomy) are associated with larger values and the regions are updated more by the new projection data, thus avoiding any possible adverse effects of prior images. The APICCS approach was systematically assessed by using patient data acquired under standard and low-dose protocols for qualitative and quantitative comparisons. The APICCS method provides an effective way for us to enhance the image quality at the matched regions between the prior and current images compared to the existing PICCS algorithm. Compared to the current CBCT imaging protocols, the APICCS algorithm allows an imaging dose reduction of 10–40 times due to the greatly reduced number of projections and lower x-ray tube current level coming from the low-dose protocol.

Method for removing longitudinal chromatism in full color holographic projection system

Hologram, as a type of diffractive optical element, is sensitive to wavelength in the process of optoelectronic reconstruction. Due to the different wavelengths of three prime colors used in full color holographic projections, there are chromatisms which badly spoil the reconstructed image. The chromatisms are composed of transverse and longitudinal chromatisms. For a computer generated hologram, transverse chromatism can be compensated by resampling the object information. However, it becomes more complex for longitudinal chromatism. This paper analyzes how the image is reconstructed from the phase-type hologram in a holographic projection system and the causations of longitudinal chromatism. To remove it, this paper proposes loading a specially designed phase distribution on a phase-type hologram. The advantage of this method is that it can be achieved by computer calculation and without adding any hardware such as achromatic optical element. A time-sharing system for a full-color hologram projection is developed in this paper. Comparisons have been made between the reconstructed images with and without chromatism. The experimental result shows that the method is effective in removing longitudinal chromatism and the quality of the reconstructed image is improved.

Effective dose for scoliosis patients undergoing full spine radiography

Scoliotic patients underwent many radiological examinations during their control and treatment periods. Nowadays, few studies have calculated effective dose which is the primary indicator of radiation risk. In this study, the PCXMC program is used to calculate the effective doses associated with scoliosis radiography. Five age groups of patients, proposed by the National Radiological Protection Board, have been chosen: <1, 1-4, 5-9, 10-15 and ≥16 y (adult patients). Patient and radiographic data were collected from 99 patient examinations for both anteroposterior and lateral full spine X-ray projections. Results showed the effective dose ranged from 118 to 1596 μSv for the frontal projection and from 97 to 1370 μSv for the lateral projection, with patient age varying from 3 months to 22 y. This study presents the effective dose against patient age and demonstrates the necessity to optimise patient protection for this type of examination.

SU-GG-I-12: Low Dose and Edge-Preserving CBCT Reconstruction by Compressed Sensing with Anisotropic Diffusion Filter Utilizing Prior Images in IGRT

Purpose: Radiation dose incurred to patients during repeated CBCT scans has caused great concerns. Decreasing the number of projections reduces the radiation dose, but this leads to significantly degraded image quality. In this work, we propose a method to solve the sparse data CBCTreconstruction problem by compressed sensing (CS) technique with anisotropicdiffusion filter (ADF) utilizing prior images. Method and Materials: The reconstructed images from full projections taken in the first day of the patient’s treatment were used as prior knowledge. In a subsequent scan of the same patient on a different day, the prior images were utilized as an initial guess in the iterative reconstruction process with limited projections. Our CS method minimized ADF of image subject to the data fidelity by using a constrained optimization strategy. The data fidelity constraint is enforced by the simultaneous algebraic reconstruction technique and the image constraint is minimized by the standard steepest gradient descent method. A series of phantom and patient studies were carried out. Results and discussion: A CS‐based CBCTreconstruction technique with ADF has been established. The technique allows us to effectively take advantage previously obtained CBCTimages and provides high quality CBCTimages with undersampled projections. With the prior knowledge assisted CS‐ADT method, we were able to use as little as 36 projections to obtain high contrast‐to‐noise images with spatial resolution comparable to imagesreconstructed with fully sampled data. With such sparse projection data, the traditional FDK‐only reconstruction failed to yield meaningful images. View aliasing artifacts seen in FDK images were greatly suppressed in imagesreconstructed by the prior knowledge assisted CS‐ADT method. Conclusions: Use of prior knowledge and CS method provides an effective way for sparse data CBCTreconstruction. It reduces radiation dose in the repeated CBCT scans and greatly enhances image quality.

Metal Artifact Reduction by Using Limited EPID Projections and Deformable Registration in Prostate Seed Localization

Metal Artifact Reduction by Using Limited EPID Projections and Deformable Registration in Prostate Seed Localization Jina Chang, Tae-Suk Suh and Won-Gyun Jung Department of Biomedical Engineering, College of Medicine, Catholic University, Seoul 137-701 Sung-Ho Park Department of Radiation Oncology, Asan Medical Center, University of Ulsan, Ulsan 138-736 Siyong Kim Department of Radiation Oncology, Mayo Clinic, Jacksonville, FL 32224 (Received 23 December 2008) A novel metal artifact reduction technique has been introduced. It uses image deformable registration between a kilovoltage cone-beam computed tomography (kV-CBCT) image set and a megavoltage cone-beam CT (MV-CBCT) image set. The kV-CBCT image set (IS-1) is obtained with full projections, but the MV-CBCT image set (IS-2) is reconstructed with a limited number of projected views by using an electronic portal imaging device (EPID). The overall procedure consists of four steps. First, for each image set, an intensity threshold value is applied to extract images having artifact streaks from the IS-1 and seed only images from the IS-2. Second, seed clusters in the IS-2 that do not have clear corresponding clusters in the IS-1 are eliminated. Third, both IS-1 and IS-2 are registered with deformation to generate a deformation grid. Finally, the original kV-CBCT image is re-sampled according to the deformation grid. The e ectiveness of this method was studied using a test phantom designed to simulate various complex seed positions. A total of 91 dummy seeds (diameter of 1 mm and length of 5 mm) were placed for this test. Using the current method, the metal artifact was greatly reduced. Quantitatively, the ratio of the artifactincluded seed volume to the physical seed volume was reduced from 9.72 to 6.45. The results of this study demonstrated the e ectiveness of this procedure for metal artifact reduction. This improvement may help to increase the accuracy of implant dosimetry. The proposed method can be easily implemented in most clinics. PACS numbers: 87.53.Jw

SU‐FF‐J‐78: Assessment of An Interpolated Ray Trace Algorithm for Deriving Motion From CBCT Scans

Purpose: To asses the accuracy of an interpolated ray‐trace algorithm for deriving respiratory motion from megavoltage cone beam computed tomography (MVCBCT) scans. Method and Materials: 21 Siemens Oncor MVCBCT scans were taken of a tungsten pin attached to a motion phantom positioned with various orientations and table shifts to create motion with variable amplitudes of up to 1 cm along posterior‐anterior and right‐left axes and 3 cm in the cranio‐caudal axis. Edge detection found the pin in all 200 CBCT raw data projections. The intersection of ray‐traces, from the pin's image to the source, for two full exhale projections defined the exhale bounding point, while using two full inhale projections yields the inhale bounding point. These bounding points were ray‐traced into all projections. The position of the pin's image, relative to the projected bounding points, was used to interpolate between bounding points to find the pin's room coordinates for all projections. A plot of pin position versus projection time stamp was compared to the expected motion. Results: A bounding box, whose opposite corners are the projected bounding points, will contain the tungsten pin only if angular separations between projections used for bounding point determination are between 15 and 163 degrees, and differences in full inhale amplitudes are accounted for. Under these conditions, deviations between expected and calculated cranio‐caudal motion are primarily due to flat panel geometry error. The errors are worst at the edges of the CBCT reconstruction volume, with maximum and mean errors of 1.06 and 0.34 mm, respectively. When flat panel errors are corrected, the worst case mean error is 0.16 mm.Conclusion: For bounded motion of up to 3 cm, the interpolated ray‐trace algorithm will contribute little to the overall error for applications that determine diaphragm motion from patient CBCT scans.

Efferent retinal projections visualized by immunohistochemical detection of the estrogen-related receptor beta in the postnatal and adult mouse brain

Recently, a new nuclear receptor subfamily has been identified and referred to as estrogen-related receptors. This new group shares sequence similarity, target genes, co-regulatory proteins, and action sites with the estrogen receptors; however, natural estrogens are not estrogen-related receptors ligands. One of the receptors belonging to this group, estrogen-related receptor beta (ERRbeta), is essential for embryo development and is believed to be involved in estrogen-regulated pathways. In this study, we analyzed the presence of the ERRbeta protein in the mouse brain by means of immunohistochemistry, using a commercial polyclonal antibody against ERRbeta (Sigma, E0156). This study represents the first description dealing with the immunolocalization of ERRbeta in a mammalian brain. Our results revealed numerous ERRbeta immunoreactive fibers in the retinal efferent projections in the brain, which was in agreement with the presence of intense ERRbeta immunoreactivity in the cell bodies and axonal processes of the retinal ganglion cells. In both postnatal and adult brains, ERRbeta immunoreactive fibers were distributed in a pattern which perfectly matched the retinal efferent projections: optic tract, supraoptic commissure, hypothalamic suprachiasmatic nucleus, ventral and dorsal geniculate nuclei, pretectal nuclei, and superior colliculus. Due to reliable, fine, and complete staining of the retinal axons obtained with the anti-ERRbeta antibody (E0156), we suggest that this antibody could be used as a valuable tool for labeling the full retinofugal projections in postnatal or adult brains.

First super-high-resolution model projection that the ancient "Fertile Crescent" will disappear in this century

The first full projections of rainfall and streamflow in the “Fertile Crescent” of Middle East are presented in this paper. Up until now, this has not been possible due to the lack of observed data and the lack of atmospheric models with sufficient resolution. An innovative super-high-resolution (20-km) global climate model is employed, which accurately reproduces the precipitation and the streamflow of the present-day Fertile Crescent. It is projected that, by the end of this century, the Fertile Crescent will lose its current shape and may disappear altogether. The annual discharge of the Euphrates River will decrease significantly (29-73%), as will the streamflow in the Jordan River. Thus countermeasures for water shortages will become much more difficult.

Full 3-D treatment verification using an aggregated kV/MV cone-beam CT technique

Full 3-D treatment verification is desirable and yet to be achieved for conformal radiation therapy, especially for IMRT. Conventionally, treatment portals are verified with 2-D portal images which lack of 3-D anatomical information around the treatment volume. The use of MV portal CT for 3-D treatment verification is limited due to excessive radiation. While the gantry-mounted kV CT delivers much less dose for 3-D verification, it lacks the treatment portal information. This dilemma could be resolved by using both kV and MV imaging devices, orthogonally mounted on the linac gantry. The purpose of this study is to develop an aggregated adaptive cone-beam CT technique for full 3-D treatment verification. In this study, we took an aggregated approach to effectively reconstruct CT images with the combination of full kV projections and truncated MV projections. Cone-beam reconstruction was based on adaptive multilevel algebraic technique (MLS-ART). The projections were acquired using a Varian CL21EX accelerator with both a kV imaging device and an aSi500 portal imager mounted orthogonally on the gantry. At present, each MV projection image was acquired using one monitor unit which could be potentially reduced. The low-exposure kV projections were acquired to reconstruct anatomical CT images and the MV projections were added to enhance the anatomical information around the treatment volume in the reconstructed CT images. To minimize the radiation dose, only limited numbers of kV and MV projections were acquired and only the truncated MV projections were acquired to cover the treatment volume. Phantoms for different anatomical sites were used in this study. The effects of total number of projections, the combination of numbers of kV and MV projections, different anatomical sites, and the attenuation coefficient conversion between the MV and kV projections on the quality of reconstructed adaptive 3-D cone-beam CT were also investigated. Preliminary results from phantom studies indicated that adaptive kV/MV cone-beam CT images generated using as low as 9 truncated MV projections (<10 MUs) and 18 full kV projections provide reasonable estimation of 3-D treatment anatomy for treatment verification. The details of the treatment anatomy improve as the projection number increases. Cross-calibration using a CT phantom with multiple density inserts provides proper conversion between kV and MV projections for kV/MV cone-beam reconstruction. Orthogonal dual-image acquisition could also reduce the acquisition time by about 30–50%. The dual-energy adaptive cone-beam reconstruction introduced a new 3-D verification technique with clinically acceptable dosage. It provides not only 3-D anatomy but also the treatment volume information. Based on the 3-D anatomical structures effectively reconstructed using limited number of projections, dose verification and gated cone-beam CT may become possible for real-time 3-D IMRT dose and anatomy verification

REGION-OF-INTEREST TOMOGRAPHY USING A COMPOSITE FOURIER-WAVELET ALGORITHM

ABSTRACT In the present work, we develop an algorithm for the reconstruction of ROI tomography from local projection data. The algorithm uses the localization properties of the wavelets after ramp filtering, where the wavelets used are one dimensional symmetric wavelets with sufficient number of vanishing moments. The proposed composite algorithm involving Fourier as well as wavelet expansions does not use full length projections even for the reconstruction of low resolution image. It differs from the other algorithms by its hybrid nature and applicability to both parallel beam as well as fan beam geometries. Besides, the algorithm has the added advantage of parallelizability and the economic use of data for local image reconstruction.

New sampling scheme for region-of-interest tomography

The wavelet localization technique was previously applied to the study of region-of-interest (ROI) tomography. It achieves a significant saving in the required projections if only a small region of a tomographic image is of interest. In this paper, we first show that with the same sampling scheme, a simple interpolation applied to the samples can give a result at least as good as that using the original wavelet localization approach. It implies that the use of the wavelet transform is not the key to the reduction of the sampling requirement. In fact, the quality of the reconstructed ROI is largely determined by the structure of the sampling scheme. Rather than directly reducing the projection number, the use of the wavelet theory permits a clear understanding of how to achieve a good sampling pattern. Based on an error analysis using the wavelet theory, we further suggest a new sampling scheme such that the number of required projections in each angle is reduced in a multiresolution form. A new multiresolution interpolation algorithm is then used to interpolate the missing samples to obtain the full projections. As a result, more than 84% of projections are saved, as compared with the traditional approach, in reconstructing an ROI of 32/spl times/32 pixels in an image of 256/spl times/256 pixels. A series of simulations was performed to reconstruct different sizes of the ROI. All results show that the signal-to-error ratios of the reconstructed ROI are comparable with that using full projection data set.