Vijay Rana

Verification of the performance accuracy of a real-time skin-dose tracking system for interventional fluoroscopic procedures

A tracking system has been developed to provide real-time feedback of skin dose and dose rate during interventional fluoroscopic procedures. The dose tracking system (DTS) calculates the radiation dose rate to the patients skin using the exposure technique parameters and exposure geometry obtained from the x-ray imaging system digital network (Toshiba Infinix) and presents the cumulative results in a color mapping on a 3D graphic of the patient. We performed a number of tests to verify the accuracy of the dose representation of this system. These tests included comparison of system-calculated dose-rate values with ionization-chamber (6 cc PTW) measured values with change in kVp, beam filter, field size, source-to-skin distance and beam angulation. To simulate a cardiac catheterization procedure, the ionization chamber was also placed at various positions on an Alderson Rando torso phantom and the dose agreement compared for a range of projection angles with the heart at isocenter. To assess the accuracy of the dose distribution representation, Gafchromic film (XR-RV3, ISP) was exposed with the beam at different locations. The DTS and film distributions were compared and excellent visual agreement was obtained within the cm-sized surface elements used for the patient graphic. The dose (rate) values agreed within about 10% for the range of variables tested. Correction factors could be applied to obtain even closer agreement since the variable values are known in real-time. The DTS provides skin-dose values and dose mapping with sufficient accuracy for use in monitoring diagnostic and interventional x-ray procedures.

A tracking system to calculate patient skin dose in real-time during neurointerventional procedures using a biplane x-ray imaging system

PURPOSE Neurovascular interventional procedures using biplane fluoroscopic imaging systems can lead to increased risk of radiation-induced skin injuries. The authors developed a biplane dose tracking system (Biplane-DTS) to calculate the cumulative skin dose distribution from the frontal and lateral x-ray tubes and display it in real-time as a color-coded map on a 3D graphic of the patient for immediate feedback to the physician. The agreement of the calculated values with the dose measured on phantoms was evaluated. METHODS The Biplane-DTS consists of multiple components including 3D graphic models of the imaging system and patient, an interactive graphical user interface, a data acquisition module to collect geometry and exposure parameters, the computer graphics processing unit, and functions for determining which parts of the patient graphic skin surface are within the beam and for calculating dose. The dose is calculated to individual points on the patient graphic using premeasured calibration files of entrance skin dose per mAs including backscatter; corrections are applied for field area, distance from the focal spot and patient table and pad attenuation when appropriate. The agreement of the calculated patient skin dose and its spatial distribution with measured values was evaluated in 2D and 3D for simulated procedure conditions using a PMMA block phantom and an SK-150 head phantom, respectively. Dose values calculated by the Biplane-DTS were compared to the measurements made on the phantom surface with radiochromic film and a calibrated ionization chamber, which was also used to calibrate the DTS. The agreement with measurements was specifically evaluated with variation in kVp, gantry angle, and field size. RESULTS The dose tracking system that was developed is able to acquire data from the two x-ray gantries on a biplane imaging system and calculate the skin dose for each exposure pulse to those vertices of a patient graphic that are determined to be in the beam. The calculations are done in real-time with a typical graphic update time of 30 ms and an average vertex separation of 3 mm. With appropriate corrections applied, the Biplane-DTS was able to determine the entrance dose within 6% and the spatial distribution of the dose within 4% compared to the measurements with the ionization chamber and film for the SK150 head phantom. The cumulative dose for overlapping fields from both gantries showed similar agreement. CONCLUSIONS The Biplane-DTS can provide a good estimate of the peak skin dose and cumulative skin dose distribution during biplane neurointerventional procedures. Real-time display of this information should help the physician manage patient dose to reduce the risk of radiation-induced skin injuries.

Updates in the real-time Dose Tracking System (DTS) to improve the accuracy in calculating the radiation dose to the patients skin during fluoroscopic procedures.

We have developed a dose-tracking system (DTS) to manage the risk of deterministic skin effects to the patient during fluoroscopic image-guided interventional cardiac procedures. The DTS calculates the radiation dose to the patients skin in real-time by acquiring exposure parameters and imaging-system geometry from the digital bus on a Toshiba C-arm unit and displays the cumulative dose values as a color map on a 3D graphic of the patient for immediate feedback to the interventionalist. Several recent updates have been made to the software to improve its function and performance. Whereas the older system needed manual input of pulse rate for dose rate calculation and used the CPU clock with its potential latency to monitor exposure duration, each x-ray pulse is now individually processed to determine the skin-dose increment and to automatically measure the pulse rate. We also added a correction for the table pad which was found to reduce the beam intensity to the patient for under-table projections by an additional 5-12% over that of the table alone at 80 kVp for the x-ray filters on the Toshiba system. Furthermore, mismatch between the DTS graphic and the patient skin can result in inaccuracies in dose calculation because of inaccurate inverse-square-distance calculation. Therefore, a means for quantitative adjustment of the patient-graphic-model position and a parameterized patient-graphic library have been developed to allow the graphic to more closely match the patient. These changes provide more accurate estimation of the skin-dose which is critical for managing patient radiation risk.

SU‐F‐BRA‐09: Comparison of Skin‐Dose Distributions Calculated by a Real‐Time Dose‐Tracking System with That Measured by Gafchromic Film for a Fluoroscopic C‐Arm Unit

Purpose: To assess the ability of a real‐time dose‐tracking system (DTS) to accurately represent the skin dose distribution for fluoroscopic interventional procedures by comparison to that measured using Gafchromic film (XR‐RV3, ISP, Wayne, NJ).Methods: We have developed a dose‐tracking system that calculates the radiation dose to the patients skin in real‐time using the exposure parameters and imaging‐system‐geometry obtained from the digital bus on a Toshiba Infinix C‐arm unit. The DTS presents the cumulative dose values in a color mapping on a 3D graphic of the patient for immediate feedback to the interventionalist. Gafchromic film was used to verify the spatial correspondence of the mapped distribution and the accuracy of the dose accumulation on the graphic. The film was calibrated against the readings of a 6 cc ionization chamber (PTW‐Freiburg GmbH, Freiburg, Germany) over a range of exposure values from 0 to 1500 R using cine‐radiographic exposures on the same C‐arm system. A simulated cardiac‐catheterization procedure was performed with the film wrapped around an Alderson torso phantom; the density distribution and converted dose values on the film were compared to that of the DTS graphic.Results: The DTS and film distributions were compared and excellent agreement was obtained within the cm‐sized surface elements used for the patient model demonstrating proper geometric scaling of the graphic. The dose values for individual points on the phantom surface agreed within 10% between the film and DTS even with inexact contouring of the film with the phantom in the measurement Conclusions: As shown by the agreement with Gafchromic film, the DTS provides skin‐dose distribution mapping with sufficient accuracy for use in monitoring interventional fluoroscopic procedures. NIH Grants R01‐EB002873, R01‐EB002873, R43‐FD0158401, R44‐FD0158402, and Toshiba Medical Systems Corporation.

A real-time skin dose tracking system for biplane neuro-interventional procedures

A biplane dose-tracking system (Biplane-DTS) that provides a real-time display of the skin-dose distribution on a 3D-patient graphic during neuro-interventional fluoroscopic procedures was developed. Biplane-DTS calculates patient skin dose using geometry and exposure information for the two gantries of the imaging system acquired from the digital system bus. The dose is calculated for individual points on the patient graphic surface for each exposure pulse and cumulative dose for both x-ray tubes is displayed as color maps on a split screen showing frontal and lateral projections of a 3D-humanoid graphic. Overall peak skin dose (PSD), FOV-PSD and current dose rates for the two gantries are also displayed. Biplane- TS uses calibration files of mR/mAs for the frontal and lateral tubes measured with and without the table in the beam at the entrance surface of a 20 cm thick PMMA phantom placed 15 cm tube-side of the isocenter. For neuro-imaging, conversion factors are applied as a function of entrance field area to scale the calculated dose to that measured with a Phantom Laboratory head phantom which contains a human skull to account for differences in backscatter between PMMA and the human head. The software incorporates inverse-square correction to each point on the skin and corrects for angulation of the beam through the table. Dose calculated by Biplane DTS and values measured by a 6-cc ionization chamber placed on the head phantom at multiple points agree within a range of -3% to +7% with a standard deviation for all points of less than 3%.

SU-E-I-22: Dependence On Calibration Phantom and Field Area of the Conversion Factor Used to Calculate Skin Dose During Neuro-Interventional Fluoroscopic Procedures

PURPOSE To determine the appropriate calibration factor to use when calculating skin dose with our real-time dose-tracking system (DTS) during neuro-interventional fluoroscopic procedures by evaluating the difference in backscatter from different phantoms and as a function of entrance-skin field area. METHODS We developed a dose-tracking system to calculate and graphically display the cumulative skin-dose distribution in real time. To calibrate the DTS for neuro-interventional procedures, a phantom is needed that closely approximates the scattering properties of the head. We compared the x-ray backscatter from eight phantoms: 20-cm-thick solid water, 16-cm diameter water-filled container, 16-cm CTDI phantom, modified-ANSI head phantom, 20-cm-thick PMMA, Kyoto-Kagaku PBU- 50 head, Phantom-Labs SK-150 head, and RSD RS-240T head. The phantoms were placed on the patient table with the entrance surface at 15 cm tube-side from the isocenter of a Toshiba Infinix C-arm, and the entrance-skin exposure was measured with a calibrated 6-cc PTW ionization chamber. The measurement included primary radiation, backscatter from the phantom and forward scatter from the table and pad. The variation in entrance-skin exposure was also measured as a function of the skin-entrance area for a 30×30 cm by 20-cm-thick PMMA phantom and the SK-150 head phantom using four different added beam filters. RESULTS The entranceskin exposure values measured for eight different phantoms differed by up to 12%, while the ratio of entrance exposure of all phantoms relative to solid water showed less than 3% variation with kVp. The change in entrance-skin exposure with entrance-skin area was found to differ for the SK-150 head compared to the 20-cm PMMA phantom and the variation with field area was dependent on the added beam filtration. CONCLUSION To accurately calculate skin dose for neuro-interventional procedures with the DTS, the phantom for calibration should be carefully chosen since different phantoms can contribute different backscatter for identical exposure parameters. Research supported in part by Toshiba Medical Systems and NIH Grants R43FD0158401, R44FD0158402 and R01EB002873.

Significance of including field non-uniformities such as the heel effect and beam scatter in the determination of the skin dose distribution during interventional fluoroscopic procedures

The current version of the real-time skin-dose-tracking system (DTS) we have developed assumes the exposure is contained within the collimated beam and is uniform except for inverse-square variation. This study investigates the significance of factors that contribute to beam non-uniformity such as the heel effect and backscatter from the patient to areas of the skin inside and outside the collimated beam. Dose-calibrated Gafchromic film (XR-RV3, ISP) was placed in the beam in the plane of the patient table at a position 15 cm tube-side of isocenter on a Toshiba Infinix C-Arm system. Separate exposures were made with the film in contact with a block of 20-cm solid water providing backscatter and with the film suspended in air without backscatter, both with and without the table in the beam. The film was scanned to obtain dose profiles and comparison of the profiles for the various conditions allowed a determination of field non-uniformity and backscatter contribution. With the solid-water phantom and with the collimator opened completely for the 20-cm mode, the dose profile decreased by about 40% on the anode side of the field. Backscatter falloff at the beam edge was about 10% from the center and extra-beam backscatter decreased slowly with distance from the field, being about 3% of the beam maximum at 6 cm from the edge. Determination of the magnitude of these factors will allow them to be included in the skin-dose-distribution calculation and should provide a more accurate determination of peak-skin dose for the DTS.

Lens of the eye dose calculation for neuro-interventional procedures and CBCT scans of the head

The aim of this work is to develop a method to calculate lens dose for fluoroscopically-guided neuro-interventional procedures and for CBCT scans of the head. EGSnrc Monte Carlo software is used to determine the dose to the lens of the eye for the projection geometry and exposure parameters used in these procedures. This information is provided by a digital CAN bus on the Toshiba Infinix C-Arm system which is saved in a log file by the real-time skin-dose tracking system (DTS) we previously developed. The x-ray beam spectra on this machine were simulated using BEAMnrc. These spectra were compared to those determined by SpekCalc and validated through measured percent-depth-dose (PDD) curves and half-value-layer (HVL) measurements. We simulated CBCT procedures in DOSXYZnrc for a CTDI head phantom and compared the surface dose distribution with that measured with Gafchromic film, and also for an SK150 head phantom and compared the lens dose with that measured with an ionization chamber. Both methods demonstrated good agreement. Organ dose calculated for a simulated neuro-interventional-procedure using DOSXYZnrc with the Zubal CT voxel phantom agreed within 10% with that calculated by PCXMC code for most organs. To calculate the lens dose in a neuro-interventional procedure, we developed a library of normalized lens dose values for different projection angles and kVp’s. The total lens dose is then calculated by summing the values over all beam projections and can be included on the DTS report at the end of the procedure.

Incorporating Corrections for the Head-Holder and Compensation Filter when Calculating Skin Dose during Fluoroscopically-Guided Interventions

The skin dose tracking system (DTS) that we developed provides a color-coded illustration of the cumulative skin dose distribution on a 3D graphic of the patient during fluoroscopic procedures for immediate feedback to the interventionist. To improve the accuracy of dose calculation, we now have incorporated two additional important corrections (1) for the holder used to immobilize the head in neuro-interventions and (2) for the built-in compensation filters used for beam equalization. Both devices have been modeled in the DTS software so that beam intensity corrections can be made. The head-holder is modeled as two concentric hemi-cylindrical surfaces such that the path length between those surfaces can be determined for rays to individual points on the skin surface. The head-holder on the imaging system we used was measured to attenuate the primary x-rays by 10 to 20% for normal incidence, and up to 40% at non-normal incidence. In addition, three compensation filters of different shape are built into the collimator apparatus and were measured to have attenuation factors ranging from 58% to 99%, depending on kVp and beam filtration. These filters can translate and rotate in the beam and their motion is tracked by the DTS using the digital signal from the imaging system. When it is determined that a ray to a given point on the skin passes through the compensation filter, the appropriate attenuation correction is applied. These corrections have been successfully incorporated in the DTS software to provide a more accurate determination of skin dose.

TH-AB-201-01: A Real-Time Skin-Dose Mapping System for Region-Of-Interest (ROI) Fluoroscopy

Purpose: The real-time dose-tracking system (DTS) which we developed for fluoroscopically–guided procedures has been upgraded so that it can track the patient skin dose when using a region-of-interest (ROI) beam attenuator. Methods: ROI fluoroscopy is a method for greatly reducing patient dose by inserting an attenuator with an aperture into the x-ray beam. The aperture defines an ROI with normal dose and image quality, while under the attenuator the dose and quality are reduced. A moveable copper ROI attenuator has been installed in the collimator assembly of a Toshiba Infinix system. The DTS provides a 3D-dose mapping on a patient graphic and, to track dose for ROI fluoroscopy, the software now models the attenuator shape and position. A calibration file provides values for the transmission of the attenuator as a function of kVp and beam filtration and dose corrections are performed if a ray to a given point on the patient graphic is determined to intersect the attenuator. Backscatter is included in the skin-dose determination and is dependent on the ROI beam size projected onto the patient. In the current version, a spatially invariant factor corrects for backscatter within the ROI and from the ROI region into the attenuated region. The agreement of the dose distribution calculated by the DTS with the actual distribution was verified by measurement with calibrated radiochromic film and a 20-cm PMMA phantom. Results: The DTS is able to accurately model the size and position of the ROI attenuator as projected onto the patient graphic. Measurements with radiochromic film show the skin-dose estimation agreement to be within +/−12%, with most error arising from the spatial variation of backscatter. Conclusion: The dose distribution on the patient can now be tracked in real-time with acceptable accuracy when the new dose-saving feature of ROI fluoroscopy is used. This research was supported in part by Toshiba Medical Systems Corporation and NIH Grant R01EB002873.