FFF.2.3.2 Mask Subtraction

This section provides information on the usage of attributes to encode an image acquisition in subtracted display mode.

FFF. User Scenario

A straightforward DSA acquisition is performed. The first few frames do not contain contrast, then the rest of frames contain contrast. An “averaged mask” may be selected to average some of the first frames without contrast.

A peripheral stepping DSA acquisition is performed. The acquisition is running in N steps and is timed to perform a mask run (e.g., from feet to abdomen) and then perform contrast runs at the positions of each mask, as triggered by the user.

One or more ranges of contrast frames will be used for subtraction from the mask for loop display. During the display, some ranges are to be fully subtracted, some others may be partially subtracted allowing a certain degree of visibility of the anatomical background visible on the mask, and finally some ranges are to be displayed un-subtracted.

FFF. Encoding Outline

The Enhanced XA SOP Class allows the encoding of the mask attributes similar to what the XA SOP Class provides.

The Enhanced XA SOP Class allows defining of specific display settings to be applied to a subset of frames, for instance the recommended viewing mode and the degree of visibility of the mask.

FFF. Encoding Details

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


IE Module PS 3.3 Reference Usage
Image Mask C.7.6.10 Specifies the subtraction parameters.
XA/XRF Multi-frame Presentation C.8.19.7 Specifies display settings of the groups of frames.

FFF. Mask Module Recommendations

This module is used to specify the subtraction parameters. The number of items depends on the number of Subtractions to be encoded. Typically, in case of AVG_SUB, the number of items is at least the number of ranges of contrast frames to be subtracted from a different mask.

Table FFF.2. 3-5 MASK MODULE Recommendations

Attribute Name Tag Comment
Recommended Viewing Mode (0028,1090) Recommended in this scenario, a value of “SUB” is used in this case.
Mask Subtraction Sequence (0028,6100) Recommended in this scenario. Items can be used to specify: - A range of contrast frames is to be subtracted from a generated mask; - A different set of pixel-shift pairs is to be applied to a range of contrast frames.

FFF. XA/XRF Multi-frame Presentation Module Recommendations

The frame ranges of this module typically include all the masks and contrast frames defined in the Mask Module, and their presentation settings are consistent with the Mask Module definitions.

The mask frames are typically displayed non-subtracted, i.e. Recommended Viewing Mode (0028,1090) equals NAT.

If there is a frame range without mask association, the value “NAT” is used for Recommended Viewing Mode (0028,1090) in the item of the Frame Display Sequence (0008,9458) of that frame range.

In case where Recommended Viewing Mode (0028,1090) equals “NAT”, the display is expected to be un-subtracted even if the Recommended Viewing Mode (0028,1090) of the Mask module equals “SUB”.

FFF. Examples

The user performs an X-Ray acquisition in three steps:

  1. First step of 5 frames for mask acquisition, without contrast agent injection;

  2. Second step of 20 frames to assess the arterial phase, with contrast agent injection, to be subtracted to the average of the 5 mask frames acquired in the first phase;

  3. Third step of 10 frames to assess the venous phase, without further contrast agent injection, to be subtracted to a new mask related to that phase and with a 20% of mask visibility.

In the three steps, the system automatically identifies the mask frame(s) to be associated with the contrast frames.

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

Recommended Viewing Mode (0028,1090) = SUB
Mask Subtraction Sequence (0028,6100)
Item 1
>Mask Operation (0028,6101) = AVG_SUB
>Subtraction Item ID (0028,9416) = 1
>Applicable Frame Range (0028,6104) = 6\25
>Mask Frame Numbers (0028,6110) = 1\2\3\4\5
>Mask Selection Mode (0028,9454) = SYSTEM
Item 2
>Mask Operation (0028,6101) = AVG_SUB
>Subtraction Item ID (0028,9416) = 1
>Applicable Frame Range (0028,6104) = 27\35
>Mask Frame Numbers (0028,6110) = 26
>Mask Selection Mode (0028,9454) = SYSTEM
Frame Display Sequence (0008,9458)
Item 1
>Start Trim (0008,2142) = 1
>Stop Trim (0008,2143) = 5
>Skip Frame Range Flag (0008,9460) = DISPLAY
>Recommended Viewing Mode (0028,1090) = NAT
Item 2
>Start Trim (0008,2142) = 6
>Stop Trim (0008,2143) = 25
>Skip Frame Range Flag (0008,9460) = DISPLAY
>Recommended Viewing Mode (0028,1090) = SUB
>Mask Visibility Percentage (0028,9478) = 0
Item 3
>Start Trim (0008,2142) = 26
>Stop Trim (0008,2143) = 26
>Skip Frame Range Flag (0008,9460) = SKIP
>Recommended Viewing Mode (0028,1090) = NAT
Item 4
>Start Trim (0008,2142) = 27
>Stop Trim (0008,2143) = 35
>Skip Frame Range Flag (0008,9460) = DISPLAY
>Recommended Viewing Mode (0028,1090) = SUB
>Mask Visibility Percentage (0028,9478) = 20

Figure FFF.2.3-2Attributes of Mask Subtraction and display