FFF.2.1.5 Image Detector and Field of View

This section provides information on the encoding of the image detector parameters and field of view applied during the X-Ray acquisition.

FFF.2.1.5.1 User Scenario

The user selects a given size of the field of view before starting the acquisition. This size can be smaller than the size of the Image Detector.

The position of the field of view in the detector area changes during the acquisition in order to focus on an object of interest.

Acquired image is networked or stored in offline media, then the image is:

  1. Displayed and reviewed in cine mode, and the field of view area needs to be displayed on the viewing screen;

  2. Used for quality assurance, to relate the pixels of the stored image to the detector elements, for instance to understand the image artifacts due to detector defects;

  3. Used to measure the dimension of organs or other objects of interest;

  4. Used to determine the position in the 3D space of the projection of the objects of interest.

FFF.2.1.5.2 Encoding outline

The XA SOP Class does not encode some information to fully characterize the geometry of the conic projection acquisition, such as the position of the Positioner Isocenter on the FOV area. Indeed, the XA SOP Class assumes that the isocenter is projected in the middle of the FOV.

The Enhanced XA SOP Class encodes the position of the Isocenter on the detector, as well as specific FOV attributes (origin, rotation, flip) per-frame or shared. It encodes some existing attributes from DX to specify information of the Digital Detector and FOV. It also allows differentiating the image intensifier vs. the digital detector and then defines conditions on attributes depending on image intensifier or digital detector.

FFF.2.1.5.3 Encoding details

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

Table FFF.2. 1-40 ENHANCED X-RAY ANGIOGRAPHIC IMAGE IOD MODULES

IE Module PS 3.3 Reference Usage
Image XA/XRF Acquisition C.8.19.3 Specifies the type of detector.
X-Ray Image Intensifier C.8.19.4 Conditional to type of detector. Applicable in case of IMG_INTENSIFIER.
X-Ray Detector C.8.19.5 Conditional to type of detector. Applicable in case of DIGITAL_DETECTOR.

Table FFF.2. 1-41 ENHANCED XA IMAGE FUNCTIONAL GROUP MACROS

Functional Group Macro PS 3.3 Reference Usage
X-Ray Field of View C.8.19.6.2 Specifies the field of view.
XA/XRF Frame Pixel Data Properties C.8.19.6.4 Specifies the Imager Pixel Spacing.

FFF.2.1.5.3.1 XA/XRF Acquisition Module Recommendations

The usage of this module is recommended to specify the type and details of the receptor.

Table FFF.2.1-42 XA/XRF ACQUISITION MODULE Recommendations

Attribute Name Tag Comment
X-Ray Receptor Type (0018,9420) Two values are applicable to this scenario: IMG_INTENSIFIER or DIGITAL_DETECTOR
Distance Receptor Plane to Detector Housing (0018,9426) Applicable to this scenario, regardless the type of receptor.

Distance Receptor Plane to Detector Housing (0018,9426) is a positive value except in the case of an image intensifier where the receptor plane is a virtual plane located outside the detector housing, which depends on the magnification factor of the intensifier.

The Distance Receptor Plane to Detector Housing (0018,9426) may be used to calculate the pixel size of the plane in the patient when markers are placed on the detector housing.

FFF.2.1.5.3.2 X-Ray Image Intensifier Module Recommendations

When the X-Ray Receptor Type (0018,9420) equals “IMG_INTENSIFIER” this module specifies the type and characteristics of the image intensifier.

[pic]

Figure FFF.2.1-23Schema of the Image Intensifier

The Intensifier Size (0018,1162) is defined as the physical diameter of the maximum active area of the image intensifier. The active area is the region of the input phosphor screen that is projected on the output phosphor screen. The image intensifier device may be configured for several predefined active areas to allow different levels of magnification.

The active area is described by the Intensifier Active Shape (0018,9427) and the Intensifier Active Dimension(s) (0018,9428).

The field of view area is a region equal to or smaller than the active area, and is defined as the region that is effectively irradiated by the X-Ray beam when there is no collimation. The stored image is the image resulting from digitizing the field of view area.

There is no attribute that relates the FOV origin to the intensifier. It is commonly assumed that the FOV area is centered in the intensifier.

The position of the projection of the isocenter on the active area is undefined. It is commonly understood that the X-Ray positioner is calibrated so that the isocenter is projected in the approximate center of the active area, and the field of view area is centered in the active area.

FFF.2.1.5.3.3 X-Ray Detector Module Recommendations

When the X-Ray Receptor Type (0018,9420) equals “DIGITAL_DETECTOR” this module specifies the type and characteristics of the image detector.

The size and pixel spacing of the digital image generated at the output of the digital detector are not necessarily equal to the size and element spacing of the detector matrix. The detector binning is defined as the ratio between the pixel spacing of the detector matrix and the pixel spacing of the digital image.

If the detector binning is higher than 1.0 several elements of the detector matrix contribute to the generation of one single digital pixel.

The digital image may be processed, cropped and resized in order to generate the stored image. The schema below shows these two steps of the modification of the pixel spacing between the detector physical elements and the stored image:

[pic]

Figure FFF.2.1-24Generation of the Stored Image from the Detector Matrix

Table FFF.2.1-43 X-RAY DETECTOR MODULE Recommendations

Attribute Name Tag Comment
Detector Binning (0018,701A) The ratio between the pixel spacing of the detector matrix and the pixel spacing of the digital image. It does not describe any further post-processing to resize the pixels to generate the stored image.
Detector Element Spacing (0018,7022) Pixel spacing of the detector matrix.
Position of Isocenter Projection (0018,9430) Relates the position of the detector elements to the isocenter reference system. It is independent from the detector binning and from the field of view origin. This attribute is defined if the Isocenter Reference System Sequence (0018,9462) is present.

FFF.2.1.5.3.4 X-Ray Field of View Macro Recommendations

The usage of this macro is recommended to specify the characteristics of the field of view.

When the field of view characteristics change across the multi-frame image, this macro is encoded on a per-frame basis.

The field of view region is defined by a shape, origin and dimension. The region of irradiated pixels corresponds to the interior of the field of view region.

When the X-Ray Receptor Type (0018,9420) equals “IMG_INTENSIFIER”, the intensifier TLHC is undefined. Therefore the field of view origin cannot be related to the physical area of the receptor. It is commonly understood that the field of view area corresponds to the intensifier active area, but there is no definition in the DICOM standard that forces a manufacturer to do so. As a consequence, it is impossible to relate the position of the pixels of the stored area to the isocenter reference system.

Table FFF.2.1-44 X-RAY FIELD OF VIEW MACRO Recommendations

Attribute Name Tag Comment
Field of View Sequence (0018,9432)
>Field of View Shape (0018,1147) Applicable in this scenario.
>Field of View Dimension(s) in Float (0018,9461) Applicable in this scenario.
>Field of View Origin (0018,7030) Applicable only in the case of digital detector.
>Field of View Rotation (0018,7032) Applicable regardless the type of receptor.
>Field of View Horizontal Flip (0018,7034) Applicable regardless the type of receptor.
>Field of View Description (0018,9433) Free text defining the type of field of view as displayed by the manufacturer on the acquisition system. For display purposes.

FFF.2.1.5.3.5 XA/XRF Frame Pixel Data Properties Macro Recommendations

The usage of this macro is recommended to specify the Imager Pixel Spacing.

When the field of view characteristics change across the multi-frame image, this macro is encoded on a per-frame basis.

Table FFF.2.1-45 XA/XRF FRAME PIXEL DATA PROPERTIES MACRO Recommendations

Attribute Name Tag Comment
Frame Pixel Data Properties Sequence (0028,9443)
>Imager Pixel Spacing (0018,1164) Applicable regardless the type of receptor.

In case of image intensifier, the Imager Pixel Spacing (0018,1164) may be non-uniform due to the pincushion distortion, and this attribute corresponds to a manufacturer-defined value (e.g., average, or value at the center of the image).

FFF.2.1.5.4 Examples

FFF.2.1.5.4.1 Field of View on Image Intensifier

This example illustrates the encoding of the dimensions of the intensifier device, the intensifier active area and the field of view in case of image intensifier.

In this example, the diameter of the maximum active area is 410 mm. The image acquisition is performed with an electron lens that focuses the photoelectron beam inside the intensifier so that an active area of 310 mm of diameter is projected on the output phosphor screen.

The X-Ray beam is projected on an area of the input phosphor screen of 300 mm of diameter, and the corresponding area on the output phosphor screen is digitized on a matrix of 1024 x1024 pixels. This results on a pixel spacing of the digitized matrix of 0.3413 mm.

The distance from the Receptor Plane to the Detector Housing in the direction from the intensifier to the X-Ray tube is 40 mm.

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

         
Columns   (0028,0011) = 1024
X-Ray Receptor Type (0018,9420) = IMG_INTENSIFIER
Distance Receptor Plane to Detector Housing (0018,9426) = 40.0
Intensifier Size (0018,1162) = 410.0
Intensifier Active Shape (0018,9427) = ROUND
Intensifier Active Dimension(s) (0018,9428) = 310.0
     
  Item 1  
         
    >Field of View Sequence (0018,9432)  
      Item 1    
        >>Field of View Shape (0018,1147)
      Item 1    
        >>Imager Pixel Spacing (0018,1164)

Figure FFF.2.1-25 Attributes of the example of Field of View on Image Intensifier

FFF.2.1.5.4.2 Field of View on Digital Detector

The following examples show three different ways to create the stored image from the same detector matrix.

In the figures below:

Note that the detector active dimension is not necessarily the FOV dimension.

In all the examples,

In the first example, there is neither binning nor resizing between the detector matrix and the stored image.

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

Rows       (0028,0010) = 8
Columns   (0028,0011) = 8
X-Ray Receptor Type (0018,9420) = DIGITAL_DETECTOR
Detector Binning (0018,701A) = 1.0\1.0
Detector Element Spacing (0018,7022) = 0.2\0.2
Position of Isocenter Projection (0018,9430) = 5\7
Per-Frame Functional Groups Sequence (5200,9230)    
           
  Item i    
    >Field of View Sequence (0018,9432)    
      Item 1      
        >>Field of View Shape (0018,1147)
      Item 1      
        >>Imager Pixel Spacing (0018,1164) =

[pic]

Figure FFF.2.1-26Attributes of the first example of Field of View on Digital Detector

In the second example, there is a binning factor of 2 between the detector matrix and the digital image. There is no resizing between the digital image (binned) and the stored image.

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

Rows       (0028,0010) = 4
Columns   (0028,0011) = 4
X-Ray Receptor Type (0018,9420) = DIGITAL_DETECTOR
Detector Binning (0018,701A) = 2.0\2.0
Detector Element Spacing (0018,7022) = 0.2\0.2
Position of Isocenter Projection (0018,9430) = 5\7
Per-Frame Functional Groups Sequence (5200,9230)  
         
  Item i  
    >Field of View Sequence (0018,9432)
      Item 1  
        >>Field of View Shape (0018,1147)
      Item 1    
        >>Imager Pixel Spacing (0018,1164) =

[pic]

Figure FFF.2.1-27 Attributes of the second example of Field of View on Digital Detector

In the third example, in addition to the binning factor of 2 between the detector matrix and the digital image, there is a resizing of 0.5 (downsizing) between the digital image (binned) and the stored image.

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

Rows       (0028,0010) = 2
Columns   (0028,0011) = 2
X-Ray Receptor Type (0018,9420) = DIGITAL_DETECTOR
Detector Binning (0018,701A) = 2.0\2.0
Detector Element Spacing (0018,7022) = 0.2\0.2
Position of Isocenter Projection (0018,9430) = 5\7
Per-Frame Functional Groups Sequence (5200,9230)    
           
  Item i    
    >Field of View Sequence (0018,9432)    
      Item 1      
        >>Field of View Shape (0018,1147)
      Item 1    
        >>Imager Pixel Spacing (0018,1164) =

[pic]

Figure FFF.2.1-28 Attributes of the third example of Field of View on Digital Detector

Note that the description of the field of view attributes (dimension, origin) is the same in these three examples. The field of view definition is independent from the binning and resizing processes.