Материал: part17

Page 526​ DICOM PS3.17 2020a - Explanatory Information​

Table FFF.2.1-42. XA/XRF Acquisition Module Recommendations​

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.​

X-RAY TUBE

INTENSIFIER

SIZE

Figure FFF.2.1-23. Schema 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 intens-​ ifier.​

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.​

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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:​

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

Table FFF.2.1-43. X-Ray Detector Module Recommendations​

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​

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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​

In case of image intensifier, the Imager Pixel Spacing (0018,1164) may be non-uniform due to the pincushion distortion, and this At-​ tribute 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​ focusesthephotoelectronbeaminsidetheintensifiersothatanactiveareaof310mmofdiameterisprojectedontheoutputphosphor​ 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.​

The encoded values of the key Attributes of this example are shown in Figure FFF.2.1-25.​

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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:​

•​The blue dotted-line squares represent the physical detector pixels;​

•​The blue square represents the TLHC pixel of the physical detector area;​

•​The purple square represents the physical detector pixel in whose center the Isocenter is projected;​

•​The dark green square represents the TLHC pixel of the region of the physical detector that is exposed to X-Ray when there is​ no collimation inside the field of view;​

•​The light green square represents the TLHC pixel of the stored image;​

•​The thick black straight line square represents the stored image, which is assumed to be the field of view area. The small thin​ black straight line squares represent the pixels of the stored image;​

•​The blue dotted-line arrow represents Field Of View Origin (0018,7030);​

•​The purple arrow represents the position of the Isocenter Projection (0018,9430).​

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

In all the examples,​

•​The physical detector area is a matrix of 10x10 square detector elements, the TLHC element being the element (1,1);​

•​The detector elements irradiated during this acquisition (defining the field of view) are in a matrix of 8x8 whose TLHC element is​ the element (3,3) of the physical detector area.​

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

The encoded values of the key Attributes of this example are shown in Figure FFF.2.1-26.​

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Figure FFF.2.1-26. Attributes 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.​

The encoded values of the key Attributes of this example are shown in Figure FFF.2.1-27.​

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