Материал: part17

In this example, two Subtraction Item IDs are defined in the Mask Subtraction Sequence.​

The encoded values of the key Attributes of this example are shown in Figure FFF.2.3-5.​

Figure FFF.2.3-5. Example of Per-Frame Frame Pixel Shift Macro for Multiple Shifts​

FFF.2.4 Processing​

FFF.2.4.1 Projection Pixel Calibration​

This section provides information on the encoding of the projection pixel size calibration and the underlying geometry.​

FFF.2.4.1.1 User Scenario​

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.​

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FFF.2.4.1.2 Encoding Outline​

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.​

FFF.2.4.1.3 Encoding Details​

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​

Table FFF.2.4-2. Enhanced XA Image Functional Group Macros​

FFF.2.4.1.3.1 XA/XRF Acquisition Module Recommendations​

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​

FFF.2.4.1.3.2 XA/XRF Frame Pixel Data Properties Macro 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.​

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Table FFF.2.4-4. XA/XRF Frame Pixel Data Properties Macro Recommendations​

FFF.2.4.1.3.3 X-Ray Projection Pixel Calibration Macro Recommendations​

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​

FFF.2.4.1.3.4 X-Ray Geometry Macro Recommendations​

When there is no change of the geometry, the macro is used in shared functional group context.​

FFF.2.4.1.4 Example​

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 magni-​ fication 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.​

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The encoded values of the key Attributes of this example are shown in Figure FFF.2.4-1.​

Figure FFF.2.4-1. Attributes of X-Ray Projection Pixel Calibration​

FFF.2.4.2 Image Derivation and Pixel Data Properties​

This section provides information on the encoding of the derivation process and the characteristics of the stored pixel data.​

FFF.2.4.2.1 User Scenario​

Anacquisitionsystemperformsseveralprocessingstepsonanoriginalimage,andthenitcreatesaderivedimagewiththeprocessed​ pixel data.​

Aviewingapplicationappliespost-processingalgorithmstothatderivedimage,e.g.,measurements,segmentationetc.Thisapplication​ needs to know what kind of post-processing can or cannot be applied depending on the characteristics of the derived image.​

FFF.2.4.2.2 Encoding Outline​

The XA SOP Class does not encode any specific Attribute values to characterize the type of derivation.​

The Enhanced XA SOP Class encodes defined terms for processing applied to the Pixel Data, and allows getting back to linear rela-​ tionshipbetweenpixelvaluesandX-Rayintensity.Viewingapplicationscanconsistentlyinterpretthestoredpixeldataandenable/disable​ applications like edge detection algorithms, subtraction, filtering, etc.​

FFF.2.4.2.3 Encoding Details​

This section provides detailed recommendations of the key Attributes to address this particular scenario.​

Table FFF.2.4-6. Enhanced X-Ray Angiographic Image IOD Modules​

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Table FFF.2.4-7. Enhanced XA Image Functional Group Macros​

FFF.2.4.2.3.1 Enhanced XA/XRF Image Module Recommendations​

The usage of this module is recommended to specify the image type.​

Table FFF.2.4-8. Enhanced XA/XRF Image Module Recommendations​

FFF.2.4.2.3.2 Derivation Image Macro Recommendations​

The usage of this macro is recommended to encode the information of the different derivation processes and steps, as well as the​ source SOP instance(s) when the image or frame are derived from other SOP Instance(s).​

Table FFF.2.4-9. Derivation Image Macro Recommendations​

If this image is not derived from source SOP Instances, the Derivation Image macro is not present, and the XA/XRF Frame Charac-​ teristics macro is used instead.​

FFF.2.4.2.3.3 Pixel Intensity Relationship LUT Macro Recommendations​

The usage of this macro is recommended to enable the applications to get the pixel values back to a linear relationship with the X-​ Ray intensity.​

If readers of the image do not recognize the Pixel Intensity Relationship value, readers can use the value "OTHER" as default.​

The number of bits in the LUT Data Attribute (0028,3006) may be different from the value of Bits Stored Attribute (0028,0101).​

FFF.2.4.2.3.4 XA/XRF Frame Characteristics Macro Recommendations​

The usage of this macro is recommended to specify the derivation characteristics​

Table FFF.2.4-10. XA/XRF Frame Characteristics Macro Recommendations​

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