This section provides information on the encoding of the projection pixel size calibration and the underlying geometry.
The user wants to measure the size of objects in the patient with a default system calibration based on the acquisition geometry and the default distance from the table to the object. In order to have more accurate measurements than this default calibration, the user may provide information of the distance from the table to the object to be measured.
The image is stored in an archive system and retrieved by a second user who wants to re-use the calibration and needs to know which object this calibration applies to.
This second user may need to re-calibrate based on another object at a different geometry.
In conic projection imaging, the pixel size in the patient is not constant. If a value of Pixel Spacing (0028,0030) is provided, it is best appropriate at a given distance from the X-Ray source to the object of interest in the patient (patient plane). It is less exact for other objects at other distances.
In addition, the distance from the X-Ray source to the object of interest may change per frame in case of gantry or table motion. In this case the Enhanced XA SOP Class allows the pixel size in the patient to be defined per-frame.
A macro provides a compound set of all relevant Attributes.
The value "Table to Object Height" can be used for individual patient plane definition.
Automatic isocenter calibration method is supported.
Values of gantry and table positions are provided to complete all necessary Attributes for a later re-calibration.
This section provides detailed recommendations of the key Attributes to address this particular scenario. See Section C.8.19.6.9.1 in PS3.3 for detailed description of the Attributes involved in the calculation of the calibration.
Table FFF.2.4-1. Enhanced X-Ray Angiographic Image IOD Modules
| C.8.19.3 |
Specifies system characteristics relevant for this scenario. |
In order to check if a calibration is appropriate, certain values have to be set in the XA/XRF Acquisition Module.
Table FFF.2.4-3. XA/XRF Acquisition Module Recommendations
This macro is recommended to provide the Pixel Spacing in the receptor plane. Typically the Image Pixel Spacing is identical for all frames. Future acquisition system techniques may result in per frame individual values.
This macro contains the core inputs and results of calibration.
When there is no movement of the gantry and table, the macro is typically used in shared functional group context.
The Attribute Beam Angle (0018,9449) is supplementary for the purpose of calibration; it is derived from the Primary and Secondary Positioner Angles but is not intended to replace them as they provide information for other purposes.
Table FFF.2.4-5. X-Ray Projection Pixel Calibration Macro Recommendations
The user performs an X-Ray acquisition with movement of the positioner during the acquisition. The patient is in Head First Supine position. During the review of the Multi-frame Image, a measurement of the object of interest in the frame "i" needs to be performed, which requires the calculation of the pixel spacing at the object location for that frame.
For the frame "i", the Positioner Primary Angle is -30.0 degrees, and the Positioner Secondary Angle is 20.0 degrees. According to the definition of the positioner angles and given the patient position, the Beam Angle is calculated as follows:
Beam Angle = arcos( |cos(-30.0) | * |cos(20.0) | ) = 35.53 degrees
The value of the other Attributes defining the geometry of the acquisition for the frame "i" are the following:
ISO = 750 mm SID = 983 mm TH = 187 mm
ΔPx (Imager Pixel Spacing) = 0.2 mm/pix
The user provides, via the application interface, an estimated value of the distance from the object of interest to the tabletop: TO = 180 mm. This value can be encoded in the Attribute Distance Object to Table Top (0018,9403) of the Projection Pixel Calibration Sequence (0018,9401) for further usage.
This results in an SOD of 741.4 mm (according to the equation SOD = 750mm - [(187mm-180mm) / cos(35.53°) ] ), and in a magnification ratio of SID/SOD of 1.32587.
The resulting pixel spacing at the object location and related to the center of the X-Ray beam is calculated as ΔPx * SOD / SID = 0.150844 mm/pix. This value can be encoded in the Attribute Object Pixel Spacing in Center of Beam (0018,9404) of the Projection Pixel Calibration Sequence (0018,9401) for further usage.
The encoded values of the key Attributes of this example are shown in Figure FFF.2.4-1.
