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

(Px (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 (Px * 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.

Below are the encoded values of the key attributes of this example:

Positioner Type (0018,1508) = CARM
X-Ray Receptor Type (0018,9420) = DIGITAL_DETECTOR
C-arm Positioner Tabletop Relationship (0018,9474) = YES
Shared Functional Group Sequence (5200,9229)
Item 1 For all frames
Item 1
Item 1
>Projection Pixel Calibration Sequence (0018,9401)
Item 1
Image Enhanced XA/XRF Image C.8.19.2 Specifies the image type: ORIGINAL or DERIVED.

Table FFF.2. 4-7 ENHANCED XA IMAGE FUNCTIONAL GROUP MACROS

Functional Group Macro PS 3.3 Reference Usage
Derivation Image C.7.6.16.2.6 Specifies the different derivation steps (including the latest step) that led to this instance.
Pixel Intensity Relationship LUT C.7.6.16.2.13 Specifies the relationship between the stored pixel data values and the X-Ray intensity of the resulting derived instance.
XA/XRF Frame Characteristics C.8.19.6.1 Specifies the latest derivation step that led to this instance.
XA/XRF Frame Pixel Data Properties C.8.19.6.4 Specifies the characteristics of the derived pixel data, both geometric and densitometric.

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

Attribute Name Tag Comment
Image Type (0008,0008) The first value is DERIVED in this scenario.

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

Attribute Name Tag Comment
Derivation Image Sequence (0008,9124) Contains one item per derivation process that led to this SOP Instance.
>Derivation Description (0008,2111) Free text description of this derivation process, typically for display purposes.
>Derivation Code Sequence (0008,9215) Contains as many items as derivation steps in this derivation process.
>Source Image Sequence (0008,2112) Contains one item per source SOP Instance used in this derivation process.

If this image is not derived from source SOP Instances, the Derivation Image macro is not present, and the XA/XRF Frame Characteristics 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

Attribute Name Tag Comment
XA/XRF Frame Characteristics Sequence (0018,9412)
>Derivation Description (0008,2111) Contains the description of the latest derivation process.
>Derivation Code Sequence (0008,9215) Contains as many items as derivation steps in this derivation process.
>Acquisition Device Processing Description (0018,1400) Specifies the derivation made at the acquisition system.
>Acquisition Device Processing Code (0018,1401) Specifies the derivation made at the acquisition system.

If the image is derived from one or more SOP Instances, the XA/XRF Frame Characteristics Sequence always contains the same values as the last item of the Derivation Image Sequence.

If the image is derived but not from other SOP Instances, it means that the derivation was performed on the Acquisition system, and the Acquisition Device Processing Description (0018,1400) and the Acquisition Device Processing Code (0018,1401) contain the information of that derivation.

An image derived from a derived image will change the Derivation Description but not the Acquisition Device Processing Description.

FFF.2.4.2.3.5 XA/XRF Frame Pixel Data Properties Macro Recommendations

The usage of this macro is recommended to specify the type of processing applied to the stored pixel data of the derived frames.

Table FFF.2.4-11 XA/XRF FRAME PIXEL DATA PROPERTIES MACRO Recommendations

Attribute Name Tag Comment
Frame Pixel Data Properties Sequence (0028,9443) Recommended in this scenario.
>Frame Type (0008,9007) The first value is DERIVED in this scenario
>Image Processing Applied (0028,9446) In case of derivation from a derived image, this attribute contains a concatenation of the previous values plus the new value(s) of the latest derivation process.