Kirpal Kohli

Dosimetric evaluation of Plastic Water Diagnostic Therapy

High‐precision radiotherapy planning and quality assurance require accurate dosimetric and geometric phantom measurements. Phantom design requires materials with mechanical strength and resilience, and dosimetric properties close to those of water over diagnostic and therapeutic ranges. Plastic Water Diagnostic Therapy (PWDT: CIRS, Norfolk, VA) is a phantom material designed for water equivalence in photon beams from 0.04 MeV to 100 MeV; the material has also good mechanical properties. The present article reports the results of computed tomography (CT) imaging and dosimetric studies of PWDT to evaluate the suitability of the material in CT and therapy energy ranges. We characterized the water equivalence of PWDT in a series of experiments in which the basic dosimetric properties of the material were determined for photon energies of 80 kVp, 100 kVp, 250 kVp, 4 MV, 6 MV, 10 MV, and 18 MV. Measured properties included the buildup and percentage depth dose curves for several field sizes, and relative dose factors as a function of field size. In addition, the PWDT phantom underwent CT imaging at beam qualities ranging from 80 kVp to 140 kVp to determine the water equivalence of the phantom in the diagnostic energy range. The dosimetric quantities measured with PWDT agreed within 1.5% of those determined in water and Solid Water (Gammex rmi, Middleton, WI). Computed tomography imaging of the phantom was found to generate Hounsfield numbers within 0.8% of those generated using water. The results suggest that PWDT material is suitable both for regular radiotherapy quality assurance measurements and for intensity‐modulated radiation therapy (IMRT) verification work. Sample IMRT verification results are presented. PACS number: 87.53Dq

On using the dosimetric leaf gap to model the rounded leaf ends in VMAT/RapidArc plans

Partial transmission through rounded leaf ends of Varian multileaf collimators (MLC) is accounted for with a parameter called the dosimetric leaf gap (DLG). Verification of the value of the DLG is needed when the dose delivery is accompanied by gantry rotation in VMAT plans. We compared the doses measured with GAFCHROMIC film and an ionization chamber to treatment planning system (TPS) calculations to identify the optimum values of the DLG in clinical plans of the whole brain with metastases transferred to a phantom. We noticed the absence of a single value of the DLG that properly models all VMAT plans in our cohort (the optimum DLG varied between 0.93±0.15 mm and 2.2±0.2 mm). The former value is considerably different from the optimum DLG in sliding window plans (about 2.0 mm) that approximate IMRT plans. We further found that a single‐value DLG model cannot accurately reproduce the measured dose profile even of a uniform static slit at a fixed gantry, which is the simplest MLC‐delimited field. The calculation overestimates the measurement in the proximal penumbra, while it underestimates in the distal penumbra. This prompted us to expand the DLG parameter from a plan‐specific number to a mathematical concept of the DLG being a function of the distance in the beams eye view (BEV) between the dose point and the leaf ends. Such function compensates for the difference between the penumbras in a beam delimited with a rounded leaf MLC and delimited with solid jaws. Utilization of this concept allowed us generating a pair of step‐and‐shoot MLC plans for which we could qualitatively predict the value of the DLG providing best match to ionization chamber measurements. The plan for which the leafs stayed predominantly at positions requiring low values of the DLG (as seen in the profiles of 1D slits) yielded the combined DLG of 1.1±0.2 mm, while the plan with leafs staying at positions requiring larger values of the DLG yielded the DLG 2.4±0.2 mm. Considering the DLG to be a function of the distance (in BEV) between the dose point and the leaf ends allowed us to provide an explanation as to why conventional single‐number DLG is plan‐specific in VMAT plans. PACS numbers: 87.56.jf, 87.56.nk

Electrical characterization of bolus material as phantom for use in electrical impedance and computed tomography fusion imaging

Abstract Phantoms are widely used in medical imaging to predict image quality prior to clinical imaging. This paper discusses the possible use of bolus material, as a conductivity phantom, for validation and interpretation of electrical impedance tomography (EIT) images. Bolus is commonly used in radiation therapy to mimic tissue. When irradiated, it has radiological characteristics similar to tissue. With increased research interest in CT/EIT fusion imaging there is a need to find a material which has both the absorption coefficient and electrical conductivity similar to biological tissues. In the present study the electrical properties, specifically resistivity, of various commercially available bolus materials were characterized by comparing their frequency response with that of in-vivo connective adipose tissue. It was determined that the resistivity of Gelatin Bolus is similar to in-vivo tissue in the frequency range 10 kHz to 1MHz and therefore has potential to be used in EIT/CT fusion imaging studies.

Applications for Low Frequency Impedance Analysis Systems

We present methodology and instrumentation used to carry out and log automated multiple electrical impedance measurements using multiplexed control. We also address the issue of measurement error introduced by the instrumentation, and demonstrate how we reduce these effects in our experiments. Finally, we present two potential applications for our automated electrical impedance analysis systems: tissue scanning and mapping via impedance measurements between arrays of electrodes, and materials testing of novel conductive polymer materials.

Embedded process for flexible conductive electrodes for applications in tissue electrical impedance scanning (EIS)

We present the fabrication and testing of micro-patternable flexible conductive nanoparticle composite polymer (C-NCP) electrode arrays for electrical impedance scanning (EIS). We attempt to minimize EIS issues of mechanical skin contact and resolution through the use of highly compliant micropatternable elastomeric C-NCPs. We anticipate an increase in spatial resolution as the electrodes can be patterned into high density arrays using a new multi-level process presented here for the first time. We characterize the conductivity of the electrodes (average resistivity of 2.98×10−4 ohm-m +/− 8.3% at 60 wt-% of silver nanoparticles), compare the baseline impedance map with a new circuit phantom, and demonstrate anomaly detection in a gelatin tissue phantom using highly flexible Ag/AgCl C-NCP electrodes.

Initial experiments with flexible conductive electrodes for potential applications in cancer tissue screening

We present initial results on the fabrication and testing of micropatternable conductive nanocomposite polymer (C-NCP) electrodes for tissue impedance measurements. We present these proof-of-concept results as a first step toward the realization of our goal: an improved Electrical Impedance Scanning (EIS) system, whereby tissue can be scanned for cancerous tissue and other anomalies using large arrays of highly flexible microfabricated electrodes. Previous limitations of existing EIS system are addressed by applying polymer based microelectromechanical system (MEMS) technology. In particular, we attempt to minimize mechanical skin contact issues through the use of highly compliant elastomeric polymers, and increase the spatial resolution of measurements through the development of microelectrodes that can be micropatterned into large, highly dense arrays. We accomplish these improvements through the development of C-NCP electrodes that employ silver nanoparticle fillers in an elastomer polymer base that can be easily patterned using conventional soft lithography techniques. These new electrodes are tested on conventional tissue phantoms that mimic the electrical characteristics of human tissue. We characterize the conductivity of the electrodes (average resistivity of 7x10-5 ohm-m +/- 14.3% at 60 wt-% of silver nanoparticles), and further employ the electrodes for impedance characterization via Cole-Cole plots to show that measurements employing C-NCP electrodes are comparable to those obtained with normal macroscopic metal electrodes. We also demonstrate anomaly detection using our highly flexible Ag/AgCl C-NCP electrodes on a tissue phantom.

Feature-space assessment of electrical impedance tomography coregistered with computed tomography in detecting multiple contrast targets.

PURPOSE Fusion of electrical impedance tomography (EIT) with computed tomography (CT) can be useful as a clinical tool for providing additional physiological information about tissues, but requires suitable fusion algorithms and validation procedures. This work explores the feasibility of fusing EIT and CT images using an algorithm for coregistration. The imaging performance is validated through feature space assessment on phantom contrast targets. METHODS EIT data were acquired by scanning a phantom using a circuit, configured for injecting current through 16 electrodes, placed around the phantom. A conductivity image of the phantom was obtained from the data using electrical impedance and diffuse optical tomography reconstruction software (EIDORS). A CT image of the phantom was also acquired. The EIT and CT images were fused using a region of interest (ROI) coregistration fusion algorithm. Phantom imaging experiments were carried out on objects of different contrasts, sizes, and positions. The conductive medium of the phantoms was made of a tissue-mimicking bolus material that is routinely used in clinical radiation therapy settings. To validate the imaging performance in detecting different contrasts, the ROI of the phantom was filled with distilled water and normal saline. Spatially separated cylindrical objects of different sizes were used for validating the imaging performance in multiple target detection. Analyses of the CT, EIT and the EIT/CT phantom images were carried out based on the variations of contrast, correlation, energy, and homogeneity, using a gray level co-occurrence matrix (GLCM). A reference image of the phantom was simulated using EIDORS, and the performances of the CT and EIT imaging systems were evaluated and compared against the performance of the EIT/CT system using various feature metrics, detectability, and structural similarity index measures. RESULTS In detecting distilled and normal saline water in bolus medium, EIT as a stand-alone imaging system showed contrast discrimination of 47%, while the CT imaging system showed a discrimination of only 1.5%. The structural similarity index measure showed a drop of 24% with EIT imaging compared to CT imaging. The average detectability measure for CT imaging was found to be 2.375 ± 0.19 before fusion. After complementing with EIT information, the detectability measure increased to 11.06 ± 2.04. Based on the feature metrics, the functional imaging quality of CT and EIT were found to be 2.29% and 86%, respectively, before fusion. Structural imaging quality was found to be 66% for CT and 16% for EIT. After fusion, functional imaging quality improved in CT imaging from 2.29% to 42% and the structural imaging quality of EIT imaging changed from 16% to 66%. The improvement in image quality was also observed in detecting objects of different sizes. CONCLUSIONS The authors found a significant improvement in the contrast detectability performance of CT imaging when complemented with functional imaging information from EIT. Along with the feature assessment metrics, the concept of complementing CT with EIT imaging can lead to an EIT/CT imaging modality which might fully utilize the functional imaging abilities of EIT imaging, thereby enhancing the quality of care in the areas of cancer diagnosis and radiotherapy treatment planning.

SU‐FF‐I‐14: Investigation of Metallic Filters On the Detectability of Soft Tissues in Cone Beam Computed Tomography Using Flat Panel Detector of Acuity Simulator

Purpose: The purpose of this work is to investigate the image quality with metallic filters partially or completely covering the CBCTdetector.Materials and methods:Aluminum metallic filters of various thicknesses were placed on top of the detector covering them either completely or partially. The influence of these filters on the detectability of soft tissue was evaluated quantitatively. The parameters investigated included signal to noise ratio (SNR),contrast to noise ratio (CNR),image uniformity and CT number accuracy. The studies were carried out using the Catphan phantom imaged in the half fan configuration mode. Assessment was carried using a 36mm2 region of interest (ROI). CNR was determined with ROIs within the insert and adjacent to the insert of the low contrast object. SNR was assessed in the uniform water region. CT numbers were measured for air, PMP, LDPE, polystyrene, acrylic, delrin, Teflon and water. All the measurements were carried out at 125kVp and 80mA with slice thickness of 2.5mm. Results: The SNR for half fan mode improved by 31% for the 0.6mm filter covering the detector uniformly while the 1.5mm filter partially covering the detector (on the non bowtie side) improved SNR by 34%. For the similar setup the CNR improved by 33% and 10% respectively. These improvements could be due to metallic filter selectively absorbing low energy scattered photons. No noticeable improvement in image uniformity was observed. The CT number accuracy was not compromised by the use of filter. Conclusion: The results of our current investigation suggest use of metallic aluminum filter on flat panel detector can improve the ability to detect low contrast objects. A series of experiments are currently underway to evaluate the effect of different filters under different geometries for both full and half fan image acquisitions.

Cancer detection based on electrical impedance spectroscopy: A clinical study

Abstract An electrical Impedance based tool is designed and developed to aid physicians performing clinical exams focusing on cancer detection. Current research envisions improvement in sensor-based measurement technology to differentiate malignant and benign lesions in human subjects. The tool differentiates malignant anomalies from nonmalignant anomalies using Electrical Impedance Spectroscopy (EIS). This method exploits cancerous tissue behavior by using EIS technique to aid early detection of cancerous tissue. The correlation between tissue electrical properties and tissue pathologies is identified by offering an analysis technique based on the Cole model. Additional classification and decision-making algorithm is further developed for cancer detection. This research suggests that the sensitivity of tumor detection will increase when supplementary information from EIS and built-in intelligence are provided to the physician.

MLC‐based penumbra softener of EDW borders to reduce junction inhomogeneities

&NA; Junctions of fields are known to be susceptible to developing cold or hot spots in the presence of even small geometrical misalignments. Reduction of these dose inhomogeneities can be accomplished through decreasing the dose gradients in the penumbra, but currently it cannot be done for enhanced dynamic wedges (EDW). An MLC‐based penumbra softener was developed in the developer mode of TrueBeam linacs to reduce dose gradients across the side border of EDWs. The movement of each leaf was individually synchronized with the movement of the dynamic Y jaw to soften the penumbra in the same manner along the entire field border, in spite of the presence of the dose gradient of the EDW. Junction homogeneity upon field misalignment for side‐matched EDWs was examined with the MV imager. The fluence inhomogeneities were reduced from about 30% per mm of shift of the field borders for the conventional EDW to about 2% per mm for the softened‐penumbra plan. The junction in a four‐field monoisocentric breast plan delivered to the Rando phantom was assessed with film. The dose inhomogeneities across the junction in the superior‐inferior direction were reduced from about 20% to 25% per mm for the conventional fields to about 5% per mm. The dose near the softened junction of the breast plan with no shifts did not deviate from the conventional plan by more than about 4%. The newly‐developed softened‐penumbra junction of EDW (and/or open) fields was shown to reduce sensitivity to misalignments without increasing complexity of the planning or delivery. This methodology needs to be adopted by the manufacturers for clinical use.