Cp-1044 Date: 2010/08/19




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CP-1044 Date: 2010/08/19
FOV clarifications for XA Derived images Status: Letter Ballot

DICOM Correction Item

Correction Number CP-1044

Log Summary: FOV clarifications for XA Derived images.

Type of Modification

Addition


Name of Standard

PS 3.3, PS 3.6 – 2009



Rationale for Correction:

Based on the discussions of Field Of View Origin in Supplement 139 (Enhanced XA Informative Annex), the Figure C.8-15 of PS3.3 is confusing because the FOV Origin seems to be related to the center of the TLHC Pixel Data instead of the center of the TLHC Detector Element. This figure should be modified, and a clarifying note should be added.

The PS3.3 states that the FOV is the area encoded in the Pixel Data. This is true in case of original XA images. However, this statement prevents any implementation of a DICOM XA derived image encoding in the Pixel Data a region smaller than the FOV. The PS3.3 should be enhanced to:


  • state that for Original images, the FOV is the area encoded in the Pixel Data

  • to enable the case of encoding a region smaller than the FOV, a new attribute should be defined in order to relate the origin of the Pixel Data Area to the Detector Area. This attribute (type 3) would allow to continue relating the Pixel Data to the geometry of the acquisition system (in particular, the relationship to the Isocenter Projection)




Sections of documents affected

PS 3.3 C.8.7.2.1.2, C.8.11.4.1.1 and C.8.19.6

PS 3.6


Correction Wording:



Amend PS 3.3 as follows:

Extend the definition of Field of View in C.8.7.2.1.2 to Digital Detector


C.8.7.2.1.2 Field of View

The Field of View Attribute describes the shape and dimensions of the Image Receptor Intensifier Field of View (zoom mode), applicable to both Image Intensifier and Digital Detector. This could be further restricted by the Collimator. See Section C.8.7.3.

Relax the constraint of Pixel Data area being the Field of View, to allow cropping, padding and/or resizing. Add note to clarify that the Field of View Origin is from the center of the detector element at the TLHC of the Field of View .


C.8.11.4.1.1 Physical, Active, Field of View, Exposed and Displayed Areas

The relationship between the Physical Detector Area, the Active Detector Area, the Field of View (what is stored in the Pixel Data (7FE0,0010)), the Exposed Area (after X-Ray Collimation) and the Displayed Area is illustrated in the following diagrams.

Note: Some of these Attributes relate the image data to manufacturer specific characteristics of the detector that may be used for quality control purposes, e.g. correlation of image artifacts with a detector defect map, analysis of noise performance, etc.

The Displayed Area is defined in pixel coordinates relative to the stored image pixel values by the Attributes of the Display Shutter Module (see section C.7.6.11). If this Module is not present or supported, then the Displayed Area is equal to the Field of View.

The Exposed Area is defined in pixel coordinates relative to the stored image pixel values by the Attributes of the X-Ray Collimator Module (see section C.8.7.3).

The size and the spacing of the pixels of the stored Pixel Data (7FE0,0010) are defined by the following Attributes:

- Imager Pixel Spacing (0018,1164),

- Rows (0028,0010),

- Columns (0028,0011)

For the Digital X-Ray IODs, the Field of View is usually rectangular in shape. For images with an Image Type (0008,0008) Value 1 equal to ORIGINAL, the Field of View describes an area with the same shape and the same size (rows and columns multiplied by corresponding values of Imager Pixel Spacing) as the stored Pixel Data (7FE0,0010).

The shape and size of the Field of View and the relationship with the spacing of the pixels are defined by the following Attributes:

- Field of View Shape (0018,1147),

- Field of View Dimensions (0018,1149),

- Imager Pixel Spacing (0018,1164),

- Rows (0028,0010),

- Columns (0028,0011)

If the Field of View Shape (0018,1147) attribute equals RECTANGLE:

- Field of View Dimensions row dimension = Imager Pixel Spacing row spacing * Rows

- Field of View Dimensions column dimension = Imager Pixel Spacing column spacing * Columns

If the Field of View Shape (0018,1147) attribute equals ROUND or HEXAGONAL:

- Field of View Dimensions diameter = Imager Pixel Spacing row spacing * Rows

- Field of View Dimensions diameter = Imager Pixel Spacing column spacing * Columns

The following Attributes define the relationship of the Field of View to the Physical Detector Area:

- Field of View Origin (0018,7030),

- Field of View Rotation (0018,7032),

- Field of View Horizontal Flip (0018,7034).

For DERIVED images, the stored Pixel Data (7FE0,0010) may be the result of cropping, resizing and/or padding of the Field of View, thus describing a different area. The following Attribute specifies in this case the relationship of the Pixel Data area to the Physical Detector Area:

- Pixel Data Area Origin Relative to FOV (xxxx,xxxx)

For the Digital X-Ray IODs, the Active Area, i.e. that part of the detector matrix that was activated for this exposure, is usually rectangular in shape. The shape and size of the Active Area and the size and spacing of the detectors are defined by the following Attributes:

- Detector Active Shape (0018,7024),

- Detector Active Dimensions (0018,7026),

- Detector Element Physical Size (0018,7020),

- Detector Element Spacing (0018,7022).

Notes: 1. The Detector Element Physical Size (0018,7020) and Detector Element Spacing (0018,7022) may be different if there are insensitive regions between each detector.

2. This model of description is not able to accurately describe multiple matrices of detectors that are “tiled” to produce a single image.

The following optional Attribute defines the relationship of the Active Area to the Physical Detector Area:

- Detector Active Origin (0018,7028).



The For ORIGINAL images the Field of View describes an area equal to the size (rows and columns) of the stored Pixel Data (7FE0,0010), then the relationship between detector elements and stored image pixels is defined by Detector Binning (0018,701A) which specifies how many detectors, in each of the row and column directions, contribute to (are pooled or averaged to form) a single stored image pixel.

Notes: 1. Detector Binning (0018,701A) may have values less than one if sub-sampling is used to derive an image with higher spatial resolution than the detector matrix.



2. The detector binning results in a field of view pixel size different from the physical detector pixel. In this case it is important to notice that the Field Of View Origin (0018,7030) is defined with respect to the center of the detector element located at the TLHC of the Field of View.

3. For those DERIVED images where the Field of View describes an area not equal to the size (rows and columns) of the stored Pixel Data (7FE0,0010), the Detector Binning (0018,701A) no longer specifies the relationship between detector elements and stored image pixels.



Figure C.8-15


Explanation of DX Digital Detector Attributes



Figure C.8-xx
Explanation of Pixel Data Area Origin





Physical Area


TLHC



(0018,702

8)



TLHC


Active Area


(0018,7030)



Field of View


(before rotation/flip)


TLHC






Pixel Data Area


(xxxx,xxxx)

TLHC

Add Pixel Data Area Origin attribute to Frame Pixel Data Properties Sequence


C.8.19.6.4 XA/XRF Frame Pixel Data Properties Macro

Table C.8.19.6-4 specifies the attributes of the Frame Pixel Data Properties Functional Group macro.

Table C.8.19.6-4


XA/XRF FRAME PIXEL DATA PROPERTIES MACRO ATTRIBUTES

Attribute Name

Tag

Type

Attribute Description

Frame Pixel Data Properties Sequence

(0028,9443)

1

Sequence containing the pixel data properties for this frame.

Only a single Item shall be permitted in this sequence.



>Pixel Intensity Relationship

(0028,1040)

1

The relationship between the Pixel and the X-Ray beam intensity. See C.8.19.6.4.1.1.

>Pixel Intensity Relationship Sign

(0028,1041)

1

The sign of the relationship between the Pixel sample values stored in Pixel Data (7FE0,0010) and the X-Ray beam intensity.

Enumerated Values:

1 = Lower pixel values correspond to less X-Ray beam intensity

-1 = Higher pixel values correspond to less X-Ray beam intensity

See C.8.11.3.1.2 for further explanation.


>Imager Pixel Spacing

(0018,1164)

1C

Physical distance measured at the receptor plane of the detector between the centers of each pixel specified by a numeric pair – row spacing value (delimiter) column spacing value in mm. See 10.7.1.3 for further explanation of the value order.

The value of this attribute shall never be adjusted to account for calibration against an object of known size; Pixel Spacing (0028,0030) is specified for that purpose.

It is only allowed to be adjusted to compensate for the change of the Field of View Dimension(s) in Float (0018,9461) attribute. See C.8.19.6.4.1.2

Required if Image Type (0008,0008) Value 1 equals ORIGINAL. May be present otherwise.



Note: These values are the actual pixel spacing distances of the stored pixel values of an image.

>Pixel Data Area Origin

(xxxx,xxxx)

3

Offset of the center of the TLHC pixel of the image stored in Pixel Data (7FE0,0010) before rotation or flipping, from the center of the TLHC detector element of the Physical Detector Area. It is measured in physical detector pixels as a row offset followed by a column offset.

Note: Due to the differences in imager pixel and detector element spacing, one may expect this attribute to have non-integer values.

See C.8.11.4.1.1.












Amend PS 3.6 as follows:

6 Registry of DICOM data elements


Tag

Name

Keyword

VR

VM

(xxxx,xxxx)

Pixel Data Area Origin

PixelDataAreaOrigin

FL

2





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