C.7.6.10.1.1 Mask Operation

Mask Operation (0028,6101) specifies a type of mask operation to be performed. The Defined Terms identifying the mask operation to be performed are as follows:

NONE (No Subtraction) No mask subtraction operation is specified;

AVG_SUB (Average Subtraction) The frames specified by the Mask Frame Numbers (0028,6110) are averaged together, shifted by the amount specified in the Mask Sub-pixel Shift (0028,6114), then subtracted from the contrast frames in the range specified in the Applicable Frame Range (0028,6102) . Contrast Frame Averaging (0028,6112) number of frames starting with the current frame are averaged together before the subtraction. If the Applicable Frame Range is not present in this sequence item, the Applicable Frame Range is assumed to end at the last frame number of the image minus Contrast Frame Averaging (0028,6112) plus one;

TID (Time Interval Differencing) The mask for each frame within the Applicable Frame Range (0028,6102) is selected by subtracting TID Offset (0028,6120) from the respective frame number. If the Applicable Frame Range is not present in this sequence item, the Applicable Frame Range is assumed to be a range where TID offset subtracted from any frame number with the range results in a valid frame number within the Multi-frame image.

Note: A positive value for TID Offset (0028,6120) means that the mask frame numbers are lower than the subtracted frame numbers. A negative TID Offset means that the mask frame numbers are higher than the subtracted frame numbers.

REV_TID (Reversed Time Interval Differencing) The number of the mask frame for each contrast frame within the Applicable Frame Range (0028,6102) is calculated by subtracting the TID Offset (0028,6120) from the first frame within the Applicable Frame Range, the TID Offset (0028,6120) +2 from the second frame within the Applicable Frame Range, the TID Offset (0028,6120) +4 from the third frame and so on. The Applicable Frame Range (0028,6102) shall be present.

When multiple pairs of frame numbers are specified in the Applicable Frame Range attribute, the beginning frame numbers (i.e. the first frame number in each pair) shall be in increasing order.

Algorithm to calculate the Mask Frame Number:

MFN = (FCFN - TID Offset) - (CFN - FCFN)

In which:

MFN = Mask Frame Number

CFN = Contrast Frame Number

FCFN = First Contrast Frame Number, the first frame number of the first pair in the Applicable Frame Range

Note: A positive value for TID Offset (0028,6120) means that the mask frame numbers are lower than the subtracted frame numbers. A negative TID Offset means that the mask frame numbers are higher than the subtracted frame numbers.

Note: Example of TID Offset, see Figure C.7.6.10-1:

[pic]

Figure C.7.6.10-1

Number of Frames: 32

Applicable Frame Range: 20 to 30

TID Offset: 5

For Calculating the TID Offset for Mask Operation REV_TID see table C.7.6.10-1:

Table C.7.6.10-1 Example Mask Frame Numbers for Mask Operation REV_TID

Contrast Frame Number (CFN) (Absolute value) Mask Frame Number (MFN) (Absolute value)
20 15
21 14
22 13
28 7
29 6
30 5

In this example the acquisition of the mask frames starts with frame 5 and ends with frame 15. The acquisition of the contrast frames starts with frame 20 and ends with frame 30 (Applicable Frame Range). The number 5 for TID Offset indicates a gap between “end of mask frames” and “begin of contrast frames” of 4 frames, e.g. injection phase and/or time needed to drive C-arm in reverse. Additionally, in this example, the first 4 frames and the last two frames are not used for this Reversed Time Interval Differencing loop.