A Simulation Tool to Visualize Patient Peak Skin Dose with Fluoroscopic and Interventional Procedures Using DICOM
 
Authors:
Yasaman Khodadadegan, Arizona State University; Muhong Zhang, PhD; Teresa Wu, PhD; William Pavlicek, PhD; Beth A. Schueler, PhD; Kenneth A. Fetterly, PhD; Brian Chong, PhD
 
Background:
Numerous episodes of skin damage and harm to patients have occurred as a consequence of elevated x-ray exposures during fluoroscopically-guided interventional procedures. As a consequence, the Joint Commission, as well as a majority of states, has specified that skin exposures from fluoroscopy be routinely monitored as a quality assurance metric. X-ray exposure to a localized region of the skin (i.e., peak skin dose) that exceeds 15,000 mGy is certain to cause skin damage and, as a sentinel event, it is reportable to the Joint Commission. Nonetheless, peak skin dose is rarely monitored because of the inability to localize the exposure to the patient. Most commonly, monitoring is accomplished via a paper log entry of the end of examination readout of cumulative air kerma (mGy) to the Interventional Reference Point (IRP), Dose Area Product, and elapsed fluoroscopic time (minutes). Monitoring of patients using the log is, of itself, an impediment due to the complexities surrounding the use of radiation units and their meaning.

We report our development of a simulation tool to visualize an ongoing or completed interventional procedure that results in a calculated estimate of localized skin dose which reports the exposure in units of mGy. The primary source for these estimates is using DICOM Dose Structured Report data. This tool is comprised of three parts.

First, a suite of simulated fluoroscopic x-ray equipment configurations were fabricated to match those commonly found in use for interventional procedures (Fig 1). These include conventional image intensification and digital detection systems, single and bi-plane gantry configurations, and ceiling and floor mounted C-arms with left and right facing tables. The software simulation display rotates the displayed gantry following keyboard data entry (to compare with the actual gantry) as an aid in validation of DICOM primary and secondary gantry angles during physics calibration.

Figure 1

Secondly, a library of 3-dimensional mathematical male and female phantoms is constructed from defined elliptical cylinders and cones. These simulations allow standardized models (specific size) for routine or regulatory-based quality assurance monitoring, yet also permits semi-customization to an individual patient (height and weight) to closely match the actual patient being examined. The technologist enters the height and weight estimates, along with patient orientation (supine/prone, feet or head first, left or right decubitis), and distance to the table end at the beginning of the procedure. The technologist selected height and weight values that are automatically matched to the nearest sized library phantom.

Table 1

Thirdly, an Excel macro automatically reads the DICOM tags recorded during the examination including: primary and secondary angles, distance from source to detector, and DAP and air kerma at IRP for each irradiation event (fluoroscopy footswitch or fluoroscopic image acquisition). We identified the input parameters required to calculate a skin dose map (Table 1). Of those, the DICOM Dose Structured Report (DSR) includes primary and secondary angles, distance from source to detector, DAP and air kerma at IRP, patient position as required tags, and table position as optional tag. Additionally, input parameters are needed that are not included in the DSR. These include vertical distance from isocenter to table, horizontal distance from isocenter to the head of table, and distance from patient’s head to table (Fig 2). In our current work, these parameters are manually inserted by the user. The physicist (during calibration) enters specified data in the Excel macro for backscatter factors, table attenuation correction, and those parameters mentioned above which are not included in the DSR yet. The physicist can choose to accept the vendors reported exposure value or perform a personalized calibration. The Excel macro identifies the irradiated skin of the patient phantom from the inputs of source to table distance, field size (square for digital, circular for image intensifiers), DAP, air kerma at interventional reference point (IRP), and descritizes the irradiated field into 1 x 1 cm^2 regions. The macro calculates the dose to each 1 x1 cm^2 point on the skin surface. As each “irradiation event” is recorded (DICOM Structured Dose syntax), the cumulative exposures are added, and when “points” on the skin overlap the dose is summed. At the completion of the procedure, a peak skin dose is determined and an area and mGy skin dose is shown on the patient phantom (Fig 3).

Figure 2

Figure 3

Figure 4
 
Evaluation:
A blinded comparison with independent measurements of exposures were compared using DICOM developed calculations and the Skin Dose tracking Excel macro. In these tests, physical measurements were performed using calibrated ionization chambers and Lucite phantoms. An arbitrary sequence of primary and secondary gantry angles were chosen, using both cardiac cine and radiology serial imaging, and the total exposure was measured. The image header data was collected and used in blinded fashion with the described Excel macro procedure and reported values from DICOM tags. The results demonstrated an overall agreement of ~12% (See Table 2).

Table 2
 
Discussion:
The shortcomings of paper logs for recording patient exposures are significant in that, not only are such records oftentimes incomplete, but they also fail to provide insight to the estimate of peak skin dose. The impediments to recording the peak skin dose value are founded in the need for having a reasonable representation of the patient and the equipment that is used. X-ray systems have complicated geometrical movements which are more easily understood when using graphical models. Since patients are variously positioned on the equipment, it is helpful to have a patient simulation that is under the control of the technologist for an episode of care. A computed peak skin dose, when recorded following a procedure, can be electronically archived and monitored for QA purposes, thereby reducing the need for paper log records of dose.
 
Conclusion:
We report on our experience using a Peak Skin Dose Simulation system that provides user inputs for physics calibrations of radiation output, F-factor, and table attenuation. Peak skin dose estimates are calculated using specified DICOM tags, including primary and secondary angles and exposure at the IRP. Technologist inputs provide equipment choices, patient size, orientation selection, and QA monitoring.
 
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