A Dose Tracking System for real-time feedback to the physician during image-guided neurointerventional procedures using a biplane x-ray imaging system
Rana, Vijay Kumar
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Use of Biplane fluoroscopic image guided interventional procedures to treat cerebrovascular anomalies has increased over the past years. Physicians use Biplane imaging systems to get two, almost orthogonal projections of the head, and thus get a better perception of the 3D location of the anomaly and improved access for treatment. These procedures often take a long time during which the patient is exposed to high amounts of radiation, and thus lead to increased risk of radiation induced injuries to the patient, specially to the patient's skin which gets the highest amount of radiation dose. No method exists which can be efficiently used to calculate patient skin dose and its spatial distribution in real-time for immediate feedback to the physician. A Biplane Dose Tracking System (Biplane-DTS) compatible with the Toshiba Infinix Biplane imaging system, has been developed that can be used to calculate patient skin dose and display it on a monitor as a color-coded map on a 3D human graphic, for immediate feedback to the physician. The Biplane-DTS system consists of multiple components such as simulated parts of the imaging system as 3D graphics, a user interactive GUI to display skin dose as a color map, a data acquisition part to collect geometry and exposure parameters from a digital CAN bus on the Biplane imaging system, a 3D human graphic for simulating the patient skin surface, functions for copying data to and from the computer GPU memory for performing a large number of calculations simultaneously, functions for determining which parts of the patient graphic skin surface are within the beam and calculating dose and color for these surface parts by using the appropriate calibration and corrections factors. A humanoid graphic library was developed by using a third-party software, MakeHuman, that can be used to simulate the patient graphic surface of a human population of different genders and with a range of weights, heights, and ages. The patient graphic from the original cardiac DTS was modified to improve the visualization of the graphic, and to improve the patient graphic resolution without compromising the real-time performance speed of the system. The accuracy of the Biplane-DTS to calculate the patient skin dose and the spatial distribution of the skin dose was tested in 2D and 3D by performing tests using different phantoms such as 20 cm PMMA, 16 cm diameter CTDI, and SK-150 head phantoms. Dose values calculated by Biplane-DTS were compared to the measurements taken with different dosimeters such as a calibrated 6 cc PTW ionization chamber and radiosensitive Gafchromic film. Biplane-DTS was found to calculate skin dose with less than 3% error and the spatial distribution of the dose with less than 2% error. A study was performed to investigate the significance of factors that contribute to non-uniformity such as the heel effect and backscatter from the patient to areas of the skin inside and outside the collimated beam. With a solid-water phantom and with the collimator opened completely for the 20 cm FPD 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. Another program 'Dose Management Utility (DMU)' has been developed that helps post-procedure review of the patient skin dose map in 3D as saved by the Biplane-DTS at the end of a procedure. The DMU allows the user to look at the 3D graphic from different orientations, and also to read the patient skin dose at individual points on the colored patient graphic, to help in analyzing the dose map. The work presented shows the capability of the Biplane-DTS for use in the clinic during neuro-interventional procedures and its potential to help reduce the risk of radiation induced skin injuries to the patient. Biplane-DTS also has the potential to be extended to calculating dose to different organs, such as the lens of the eyes, during image-guided procedures. Our Biplane-DTS system can be used in conjunction with other techniques such as ROI-fluoroscopy to minimize the risk of radiation induced skin injuries to the patient during fluoroscopic interventional procedures.