The complete system is comprised of simulated male and female patients, simulated and configurable C-arms, a robust database that automatically accepts DICOM tags including DICOM Structured Report Dose tags (11 are required) ,and the four additional system parameters that are necessary for peak dose estimation (these are entered via a technologist and physicist screen). One additional tag is read from DICOM header file to determine whether collimator shape is round or rectangle. Two-way exchange of data from all XA systems with the database are needed to complete the computation, recording, and archiving of patient dose and location.
The approach taken to estimate skin dose is to first locate and properly position the virtual patient on the table. With the center top of the patients head as the origin, the air kerma to each 0.01
region at the surface of the patient likely to be struck by the X-ray beam is computed. Knowledge of the shape of the surface and the distance from each region to the origin permits trigometric determination of the 0.01
regions exact location in space. Knowledge of table positions and gantry angulations (validated data from DICOM and system parameters) specifies the field size on the patient surface. Knowledge of each 0.01
of the patient surface relative to PERP permits appropriate corrections for inverse square law. Other required corrections also apply backscatter for determination of skin dose (since both the kVp and the field size on the patient surface are determined). Corrections also can be applied for table and pad attenuation and calibration factors for the exposure (corrections to the vendors specified value at PERP).
Computed peak skin dose (Gy) and location recorded in the database after each exam can automatically initiate email or pager alerts based upon predetermined threshold values for peak skin dose (Action Levels).
A schematic of data flow is summarized as shown in Figure .
Fig 1 Skin dose tool. (1) A physicist validates and enters system parameters once per system. (2) The technologist enters modality, station ID, and date to retrieve DICOM data from DIT after each patient exam is finished. (3) Technologist checks patient position (more ...)
Male and Female Virtual Patients
SolidWorks (2009 x64 Edition SP5.0 Concord, MA, USA) simulation software was used to construct standardized male and female phantoms using biometric values using NASA, Man-Systems Integration Standards.10
While multiple phantom options exist,11,12
mathematical models enable the straightforward location of any point on the surface of the model because the geometry, shape, and dimensions are known. This was considered advantageous in this application. The height of the standard adult male phantom is approximately 1.86 m and the weight is ~90 kg, while the height for adult female phantom is ~1.67 m and weight is 72 kg. While the arms (Figure ) are included for completeness, they are not used in the calculation of skin dose. In clinical practice, the arms are carefully avoided during any XA procedure and would be extended away from the region of X-ray interest. The female phantom breast is included in this model, and skin dose to the breast is computed. Because these three-dimensional mathematical male and female models are constructed from defined elliptical cylinders and cones, coordinates of the surface of the body are derived using geometric computations. Upon completion of each procedure, the technologist enters gender, height, and weight estimates, along with patient orientation on the table (supine/prone, feet or head first, and left or right lateral), and a corresponding phantom is automatically loaded. Thus, calculation of all skin dose values are performed after the exam is finished.
Fig 2 Standard male and standard female phantoms. Sizes are based on NASA, Man-Systems Integration Standards.10
The surface of the phantom is reduced to regions of 0.01
(Figure ). Knowing the geometry of the phantom (surface) relative to the placement of the patient on the table permits the calculated (x
)) location for each point of the skin (center of the 0.01
square), where f
) maps x
coordinate of each point to corresponding z
coordinate based on mathematical geometry used in phantom construction.
Fig 3 Standard male phantom. The surface area of the model (except for arms) is localized to each 0.01×0.01 m2. These regions are referenced to the origin (top center of the head).
Virtual Equipment Configuration and Geometry
SolidWorks simulation software was used to depict conventionally available angiographic C-arm equipment including XRII (X-ray image intensification) and solid state (flat panel digital) X-ray receptors. The patient can be positioned on the table with the patient head to the right (Figure ) as with head first, or feet to the right (feet first). The patient can also be positioned as prone, supine, or left or right side. Software provides visual confirmation of each episode positioning for technologist and physicist validation. Figure is a typical single C-arm digital detector configuration with the patient positioned head first and supine. The direction of +x
axis is the longitude (moving cranial–caudal), while the direction of +y
axis is to the patient left. The +z
axis directs to the front of the patient.13
The software simulation image display actually rotates the displayed gantry following keyboard data entry (to visualize in comparison with the actual gantry). This was found particularly useful as an aid in validating DICOM tag geometry values during initial physicist acceptance when the patient can be switched from prone to supine as an example.
Displayed gantry angle of −30° for an image intensifier C-arm. Male patient is supine, centered in y, and head first.
Primary and Secondary Gantry Angles
The primary gantry angle (Figure ) is when a patient is left anterior oblique–right anterior oblique (LAO-RAO) (y–z plane). At 0°, the X-ray tube is beneath the patient and is pointed vertically upwards. A positive primary angle corresponds to an X-ray beam that enters the patient from the patient’s right posterior and exits to the patients left anterior. The secondary angle is the cranial–caudal (x–z plane). At 0° for secondary angulations, the X-ray tube is beneath the patient; a positive secondary angle corresponds with the tube arcing to the patient toes and detector moving toward the patient head. Movement of the C-arm or table horizontally or vertically is discussed below. Note: Importantly, the technologist enters the patient positioning (i.e., supine or prone) at the XA operator console, which alters (as validated) the DICOM tag values recorded in the database at the end of the exam for primary and secondary angles. These values are considered patient centric when matching with position automatically occurs.
Simulated fluoroscopic X-ray equipment configurations and system coordination.
Definition of System Origin and PERP
Regardless of recorded values of table movement (longitudinal, lateral, and up–down), we assume that the origin be always set at the center of the top of the head of the phantom (Figure ), and the coordinate system for all 0.01
regions is referenced to this point location. The PERP is a point in space located 0.15 m toward the X-ray source from the isocenter of rotation of the C-arm.7
The program calculates the skin exposure that occurs to each 0.01
from the reported exposure to the PERP using knowledge of distances.
DICOM Tags: Type I and II Equipment
XA equipment for localized skin dose computations are classified into two groups: type I and type II.
Type I equipment are provided with DICOM Structured Report Dose (DSR). Each footswitch activation provides data for both acquisitions (digital spots, cine, and DSA) and fluoroscopy. DSR provides 11 of the 16 necessary data to calculate exposure to each 0.01
areas. Four additional input parameters are needed (see Table , type I column). An XA technologist must enter one distance measurement once per procedure, and three entries are needed to be entered by the physicist once per system during validation to calculate localized skin dose.
Input data for type I and type II equipment
Type II XA equipment are not provided with DSR but do provide a number of useful DICOM tags, which permit estimation of peak skin dose. Type II equipment operator consoles may be provided with a printable dose summary (PDS) designed for paper printing. It is possible to print these data to a virtual printer and electronically capture dose descriptive information to the DICOM Index Tracker (DIT) database. DICOM tag and PDS information are footswitch specific for acquisition, and therefore, peak skin dose can be directly calculated for type II equipment, except for the fluoroscopy component. Fluoroscopy data in type II equipment are only provided in summary form. In our implementation, the total (reported) fluoroscopy exposure is distributed to peak skin dose locations in an amount proportional to the dose received from acquisitions. The localized skin dose and locations with acquisition is first calculated, and the total skin dose is then scaled prorated to these locations. With type II equipment, default values are used for lateral and longitudinal table position. As with type I equipment, an entry of a distance measure by the technologist and three one-time specifications by the physicist are required.
Physicist System Validation
Comprehensive reviews of XA equipment geometry, radiation dose measurement, and DICOM tag inspection are necessary to implement this approach for localizing peak skin dose. As an aid to these tasks, a physicist screen (Figure ) provides some assistance in validating the system. Screen data fields are available for attenuation corrections for table and pad, separate calibration corrections for fluoroscopy and acquisitions KAP at PERP, and desired back scatter factor use. Choice of preferred default values may be selected. DICOM tag values must be reviewed and validated, including gantry angulations, table positions, C-arm movement if possible, and correctness of a patient centric use of distances and equipment type, including bi-plane and detector type. The equipment chosen is displayed, and by clicking on Display, the positional information on the C-arm image is updated so that the physical gantry, virtual C-arm, and tag data are co-registered. Also specified (keystroke entered) are the list of physician authorized users and ALARA values.
Physicist validation interface once per system. Copyrighted and used with permission from Mayo Foundation for Medical Education and Research. All rights reserved.
Computation of Peak Skin Dose
A “point” within a 0.01
area is assigned the computed value. Each representing point on the discretized surface has the coordinate (m
0.5)) assuming m
as integers. To expedite the computation, the irradiated regions (projected X-ray field on the surface of the patient) are initially determined using the positioning of the C-arm. Other patient geometries are discussed below.
HCH is the horizontal distance from the C-arm current (radiation event) position to C-arm home position (home positions for both table and C-arm are defined by the system (0, 0)). HTC is the horizontal distance from table home position to C-arm home position and is specified by the physicist once during validation. The notations in Table are used in the calculation of specified locations.
List of required distances for calculating coordinates of isocenter, source, and PERP
The following notation is used to represent indices for calculation.
|IS||Index identifying isocenter|
|RF||Index identifying PERP|
|Spot||Index identifying X-ray source|
|c1, c2, c3, c4||Corners of area at patient entrance reference point for rectangular cone|
The coordinates of isocenter (xIs
) can be computed as follows:
A general formulation corresponding to the coordinates of X-ray source (xspot
) and PERP (xRF
Note: In Eqs. 7
, and 9
, the length 0.15 m is the distance from PERP to the X-ray source.
Depiction of certain needed distances (listed in Table ) based on our proposed coordinate system.
Calculation of Air Kerma to Each Region of Skin (0.01 × 0.01 m2)
The method of how to isolate the irradiated points of the patient surface struck by the X-ray beam is discussed in the Appendix
. For each point of (x
) of the radiated area, we calculate the distance from the X-ray source to each 0.01
point of the applicable area using Eq. 26
of the Appendix
Since PERP is defined as 0.15 m from isocenter toward the X-ray source.
Here, Kskin and KPERP are the air kerma at the patient skin and PERP, respectively.
Each of the irradiated points at the surface of the virtual patient for each run is calculated, and inverse square law corrections are made for individually differing distances to the surface of the patient.
Tables and list the needed corrections when the patient is optionally positioned on the table.
Vertical distance from origin (top center of patient head) to the table top for standard phantom
Adjustment required for primary and secondary angles for different patient positions
An adjustment must be applied when patient positioning is feet first; then, distance in Eqs. 1
, and 7
will be represented as Eq. 12
These adjustments would be applied to the above equations as applicable.
Back Scatter Factor
Table is a partial listing of back scatter factors that are used, a required and significant correction to the calculated air kerma.14
A back scatter factor for a tube voltage of 80 kV, filter 3.0, HVL 3.04, and field size 0.2
, is 1.40, which is the default value. Calculated air kerma (Eq. 11
) values are corrected using back scatter factor, thereby converting air kerma to skin dose.
Table 5 Partial table of back scatter factors reproduced from ICRU14 for body tissue
Upon summing the skin dose received by each 0.01
regions of skin (when overlap occurs with each fluoroscopic and acquisition event of one exam), the skin dose at each 0.01
region is computed.