Virtual Monochromatic Research Articles

Oncologic applications of dual-energy CT in the abdomen.

Dual-energy computed tomographic (DECT) technology offers enhanced capabilities that may benefit oncologic imaging in the abdomen. By using two different energies, dual-energy CT allows material decomposition on the basis of energy-dependent attenuation profiles of specific materials. Although image acquisition with dual-energy CT is similar to that with single-energy CT, comprehensive postprocessing is able to generate not only images that are similar to single-energy CT (SECT) images, but a variety of other images, such as virtual unenhanced (VUE), virtual monochromatic (VMC), and material-specific iodine images. An increase in the conspicuity of iodine on low-energy VMC images and material-specific iodine images may aid detection and characterization of tumors. Use of VMC images of a desired energy level (40-140 keV) improves lesion-to-background contrast and the quality of vascular imaging for preoperative planning. Material-specific iodine images enable differentiation of hypoattenuating tumors from hypo- or hyperattenuating cysts and facilitate detection of isoattenuating tumors, such as pancreatic masses and peritoneal disease, thereby defining tumor targets for imaging-guided therapy. Moreover, quantitative iodine mapping may serve as a surrogate biomarker for monitoring effects of the treatment. Dual-energy CT is an innovative imaging technique that enhances the capabilities of CT in evaluating oncology patients.

Generation of virtual monochromatic CBCT from dual kV/MV beam projectionsa)

To develop a novel on-board imaging technique which allows generation of virtual monochromatic (VM) cone-beam CT (CBCT) with a selected energy from combined kilovoltage (kV)∕megavoltage (MV) beam projections. With the current orthogonal kV∕MV imaging hardware equipped in modern linear accelerators, both MV projections (from gantry angle of 0°-100°) and kV projections (90°-200°) were acquired as gantry rotated a total of 110°. A selected range of overlap projections between 90° to 100° were then decomposed into two material projections using experimentally determined parameters from orthogonally stacked aluminum and acrylic step-wedges. Given attenuation coefficients of aluminum and acrylic at a predetermined energy, one set of VM projections could be synthesized from two corresponding sets of decomposed projections. Two linear functions were generated using projection information at overlap angles to convert kV and MV projections at nonoverlap angles to approximate VM projections for CBCT reconstruction. The contrast-to-noise ratios (CNRs) were calculated for different inserts in VM CBCTs of a CatPhan phantom with various selected energies and compared with those in kV and MV CBCTs. The effect of overlap projection number on CNR was evaluated. Additionally, the effect of beam orientation was studied by scanning the CatPhan sandwiched with two 5 cm solid-water phantoms on both lateral sides and an electronic density phantom with two metal bolt inserts. Proper selection of VM energy [30 and 40 keV for low-density polyethylene (LDPE), polymethylpentene, 2 MeV for Delrin] provided comparable or even better CNR results as compared with kV or MV CBCT. An increased number of overlap kV and MV projection demonstrated only marginal improvements of CNR for different inserts (with the exception of LDPE) and therefore one projection overlap was found to be sufficient for the CatPhan study. It was also evident that the optimal CBCT image quality was achieved when MV beams penetrated through the heavy attenuation direction of the object. A novel technique was developed to generate VM CBCTs from kV∕MV projections. This technique has the potential to improve CNR at selected VM energies and to suppress artifacts at appropriate beam orientations.

Generation of Virtual Monochromatic (VM) CBCT from Dual kV/MV Beam Projections

Generation of Virtual Monochromatic (VM) CBCT from Dual kV/MV Beam Projections

Implementation of dual‐energy technique for virtual monochromatic and linearly mixed CBCTs

To implement dual-energy imaging technique for virtual monochromatic (VM) and linearly mixed (LM) cone beam CTs (CBCTs) and to demonstrate their potential applications in metal artifact reduction and contrast enhancement in image-guided radiation therapy (IGRT). A bench-top CBCT system was used to acquire 80 kVp and 150 kVp projections, with an additional 0.8 mm tin filtration. To implement the VM technique, these projections were first decomposed into acrylic and aluminum basis material projections to synthesize VM projections, which were then used to reconstruct VM CBCTs. The effect of VM CBCT on the metal artifact reduction was evaluated with an in-house titanium-BB phantom. The optimal VM energy to maximize contrast-to-noise ratio (CNR) for iodine contrast and minimize beam hardening in VM CBCT was determined using a water phantom containing two iodine concentrations. The LM technique was implemented by linearly combining the low-energy (80 kVp) and high-energy (150 kVp) CBCTs. The dose partitioning between low-energy and high-energy CBCTs was varied (20%, 40%, 60%, and 80% for low-energy) while keeping total dose approximately equal to single-energy CBCTs, measured using an ion chamber. Noise levels and CNRs for four tissue types were investigated for dual-energy LM CBCTs in comparison with single-energy CBCTs at 80, 100, 125, and 150 kVp. The VM technique showed substantial reduction of metal artifacts at 100 keV with a 40% reduction in the background standard deviation compared to a 125 kVp single-energy scan of equal dose. The VM energy to maximize CNR for both iodine concentrations and minimize beam hardening in the metal-free object was 50 keV and 60 keV, respectively. The difference of average noise levels measured in the phantom background was 1.2% between dual-energy LM CBCTs and equivalent-dose single-energy CBCTs. CNR values in the LM CBCTs of any dose partitioning are better than those of 150 kVp single-energy CBCTs. The average CNR for four tissue types with 80% dose fraction at low-energy showed 9.0% and 4.1% improvement relative to 100 kVp and 125 kVp single-energy CBCTs, respectively. CNRs for low-contrast objects improved as dose partitioning was more heavily weighted toward low-energy (80 kVp) for LM CBCTs. Dual-energy CBCT imaging techniques were implemented to synthesize VM CBCT and LM CBCTs. VM CBCT was effective at achieving metal artifact reduction. Depending on the dose-partitioning scheme, LM CBCT demonstrated the potential to improve CNR for low contrast objects compared to single-energy CBCT acquired with equivalent dose.

TH-E-218-01: Dual-Energy CBCT Imaging for Metal Artifact Reduction and Contrast Enhancement

Purpose: To develop dual-energy CBCT imaging techniques for potential applications in metal artifact reduction and contrast enhancement. Methods: Metal artifact reduction using virtual monochromatic (VM) technique was evaluated with a bench-top CBCT system. Low- and high-energy projections were acquired at 80kVp and 150kVp (with an additional 0.8mm tin filtration). These projections were then decomposed into acrylic and aluminum basis material projections to synthesize VM projections. VM projections were then reconstructed to generate VM CBCTs. The effect of VM CBCT on the metal artifact reduction was evaluated with an in-house titanium-BB phantom. Optimal energy to maximize contrast-to-noise ratio (CNR) in VM CBCT was determined using a water phantom containing two iodine concentrations.The effect of contrast enhancement using linearly-mixed-image technique was evaluated using a kV on-board imager at 80kVp and 125kVp. The dose partitioning between low- and high-energy CBCTs was varied while keeping total dose approximately equal to single-energy CBCTs, measured using an ion chamber. Noise level and CNR for six tissue types were investigated for dual-energy CBCTs in comparison with single energy CBCTs at 100kVp. Results: Metal artifacts were substantially reduced using VM CBCT and quantitative analysis showed about 70% improvement in CNR in the titanium-BB phantom study. Optimal energy selection to maximize CNR was 50keV for both iodine concentrations. A maximum noise reduction of 4% was achieved for linearly-mixed dual-energy CBCTs compared with that of equivalent-dose single-energy CBCTs. Linearly-mixed CBCTs showed an improvement of CNR ranging from 3-14% when 60% and 80% dose was partitioned to low-energy projections. Conclusions: Dual-energy CBCT imaging techniques were developed and shown to be effective in metal artifact reduction. Linearly-mixed dual-energy CBCT has the potential to improve noise and CNR depending on the dose-partitioning scheme compared with single-energy CBCT with equivalent dose. This work is partially supported by a research grant from Varian Medical Systems.