Материал: part17

X-Ray Incidence

Receptor Plane

Figure FFF.1.2-8. Projection of a Point of the Positioner Coordinate System​

A point (P Xp , P Yp , P Zp ) in the positioner coordinate system (Op, Xp, Yp, Zp) can be expressed as a point (P u , Pv) in the image​ coordinate system by applying the following transformation:​

Pu = (SID / (ISO - PYp) ).PXp​

Pv = (SID / (ISO - PYp) ).PZp​

The ratio SID / (ISO - PYp) is also called magnification ratio of this particular point.​

FFF.1.2.5 Field of View Transformations​

FFF.1.2.5.1 Detector​

The following concepts illustrate the model of the X-Ray detector:​

Physical detector array (or physical detector matrix) is the matrix composed of physical detector elements.​

Note​

NotallthedetectorelementsareactivatedduringanX-Rayexposure.Theactivedetectorelementsareinthedetectoractive​ area, which can be equal to or smaller than the physical detector area.​

Physical detector element coordinates represented as (idet, jdet) are columns and rows of the physical detector element in the​ physical detector array.​

Detector TLHC element is the detector element in the Top Left Hand Corner of the physical detector array and corresponds to (idet,​ jdet) = (0,0).​

The Attribute Detector Element Physical Size (0018,7020) represents the physical dimensions in mm of a detector element in the​ row and column directions.​

The Attribute Detector Element Spacing (0018,7022) contains the two values Djdet and Didet, which represent the physical distance​ in mm between the centers of each physical detector element:​

•​Didet = detector element spacing between two adjacent columns;​

•​Djdet = detector element spacing between two adjacent rows.​

The Attribute Detector Element Physical Size (0018,7020) may be different from the Detector Element Spacing (0018,7022) due to​ the presence of spacing material between detector elements.​

TheAttributePosition of Isocenter Projection (0018,9430)containsthepoint(ISO_Pidet, ISO_Pjdet),which represents theprojection​ of the Isocenter on the detector plane, measured as the offset from the center of the detector TLHC element. It is measured in phys-​

ical detector elements.​

- Standard -​

The Attribute Imager Pixel Spacing (0018,1164) contains the two values Dj and Di, which represent the physical distance measured​ at the receptor plane between the centers of each pixel of the FOV image:​

•​Di =imager pixel spacing between two adjacent columns;​

•​Dj =imager pixel spacing between two adjacent rows.​

The zoom factor represents the ratio between Imager Pixel Spacing (0018,1164) and Detector Element Spacing (0018,7022). It may​ be different from the detector binning (e.g., when a digital zoom has been applied to the pixel data).​

•​Zoom factor (columns) = Di / Didet;​

•​Zoom factor (rows) = Dj / Djdet.​

∆idet

Figure FFF.1.2-9. Physical Detector and Field of View Areas​

FFF.1.2.5.2 Field of View​

The following concepts illustrate the model of the field of view:​

The field of view (FOV) corresponds to a region of the physical detector array that has been irradiated.​

The field of view image is the matrix of pixels of a rectangle circumscribing the field of view. Each pixel of the field of view image​ may be generated by multiple physical detector elements.​

The Attribute FOV Origin (0018,7030) contains the two values (FOV idet, FOV jdet ), which represent the offset of the center of the​ detector element at the TLHC of the field of view image, before rotation or flipping, from the center of the detector TLHC element. It​

is measured in physical detector elements. FOV Origin = (0,0) means that the detector TLHC element is at the TLHC of a rectangle​ circumscribing the field of view.​

TheAttributeFOVDimension(0018,9461)containsthetwovaluesFOVrowdimensionandFOVcolumndimension,whichrepresent​ the dimension of the FOV in mm:​

•​FOV row dimension =dimension in mm of the field of view in the row direction;​

•​FOV column dimension = dimension in mm of the field of view in the column direction.​

FOV pixel coordinates represented as (i, j) are columns and rows of the pixels in the field of view image.​

FOV TLHC pixel is the pixel in the Top Left Hand Corner of the field of view image and corresponds to (i, j) = (0,0).​

As an example, the point (ISO_Pi, ISO_Pj) representing the projection of the Isocenter on the field of view image, and measured in​ FOV pixels as the offset from the center of the FOV TLHC pixel, can be calculated as follows:​

- Standard -​

ISO_Pi = (ISO_Pidet - FOVidet).Didet / Di - (1 - Didet / Di) / 2​

ISO_Pj = (ISO_Pjdet - FOVjdet).Djdet / Dj - (1 - Djdet / Dj) / 2​

Figure FFF.1.2-10. Field of View Image​

FFF.1.2.5.3 Field of View Rotation and Flip​

The Attribute FOV Rotation (0018,7032) represents the clockwise rotation in degrees of field of view relative to the physical detector.​

The Attribute FOV Horizontal Flip (0018,7034) defines whether or not a horizontal flip has been applied to the field of view after ro-​ tation relative to the physical detector.​

The Attribute Pixel Data (7FE0,0010) contains the FOV image after rotation and/or flipping.​

Pixel data coordinates is the couple (c,r) where c is the column number and r is the row number.​

Figure FFF.1.2-11. Examples of Field of View Rotation and Horizontal Flip​

- Standard -​

FFF.1.3 Calibration​

The X-Ray Projection Pixel Calibration Macro of the Section C.8.19.6.9 “X-Ray Projection Pixel Calibration Macro” in PS3.3 specifies​ the Attributes of the image pixel size calibration model in X-Ray conic projection, applicable to the Enhanced XA SOP Class.​

In this model, the table plane is specified relative to the Isocenter. As default value for the Attribute Distance Object to Table Top​ (0018,9403), the half distance of the patient thickness may be used.​

Oblique projections are considered in this model by the encoding of the Attribute Beam Angle (0018,9449), which can be calculated​ from Positioner Primary Angle (0018,1510) and Positioner Secondary Angle (0018,1511) as follows:​

For Patient Positions HFS, FFS, HFP, FFP:Beam Angle = arcos( |cos(Positioner Primary Angle) | * |cos(Positioner Secondary Angle)​ | ).​

For Patient Positions HFDR, FFDR, HFDL, FFDL:Beam Angle = arcos( |sin(Positioner Primary Angle) | * |cos(Positioner Secondary​ Angle) | ).​

The resulting pixel spacing, defined as D Px * SOD / SID, is encoded in the Attribute Object Pixel Spacing in Center of Beam​ (0018,9404). Its accuracy is practically limited to a beam angle range of +/- 60 degrees.​

FFF.1.4 X-Ray Generation​

This chapter illustrates the relationships between the X-Ray generation parameters:​

Figure FFF.1.4-1. Example of X-Ray Current Per-Frame of the X-Ray Acquisition​

Values per frame are represented by the following symbols in this section:​

In the Frame Content Sequence (0020,9111):​

· Frame Acquisition Duration (0018,9220) in ms of frame « i » =Dti​

In the Frame Acquisition Sequence (0018,9417):​

·KVP (0018,0060) of frame « i » = kVpi​

·X-Ray Tube Current in mA (0018,9330) of frame « i » = mAi​

The following shows an example of calculation of the cumulative and average values per image relative to the values per-frame:​

•​Number of Frames (0028,0008) = N​

•​Exposure Time (0018,9328) in ms = SUMN (Dti)​

•​X-Ray Tube Current (0018,9330) in mA = 1/N * SUMN (mAi)​

•​Average Pulse Width (0018,1454) in ms = 1/N * SUMN (Dti)​

- Standard -​

•​KVP (0018,0060) = 1/N * SUMN (kVpi)​

•​Exposure (0018,9332) in mAs = SUMN (Dti * mAi / 1000)​

FFF.1.5 Pixel Data Properties and Display Pipeline​

This chapter describes the concepts of the display pipeline.​

TheX-Rayintensity(I)attheimagereceptorisinverselyproportionaltotheexponentialfunctionoftheproductoftheobject'sthickness​

(x) traversed by the X-Ray beam and its effective absorption coefficient (m): I ~ e- m x.​

The X-Ray intensity that comes into contact with the image receptor is converted to the stored pixel data by applying specific signal​ processing. As a first step in this conversion, the amplitude of the digital signal out of the receptor is linearly proportional to the X-Ray​ intensity. In further steps, this digital signal is processed in order to optimize the rendering of the objects of interest present on the​ image.​

The Enhanced XA IOD includes Attributes that describe the characteristics of the stored pixel data, allowing to relate the stored pixel​ data to the original X-Ray intensity independently from the fact that the image is "original" or "derived".​

When the Attribute Pixel Intensity Relationship (0028,1040) equals LIN:​

•​P ~ I: The pixel values (P) are approximately proportional to X-Ray beam intensity (I).​

When the Attribute Pixel Intensity Relationship (0028,1040) equals LOG:​

•​P ~ x: The pixel values (P) are approximately proportional to the object thickness (x).​

In order to ensure consistency of the displayed stored pixel data, the standard display pipeline is defined.​

On the other side, the stored pixel data is also used by applications for further analysis like segmentation, structure detection and​ measurement, or for display optimization like mask subtraction. For this purpose, the Pixel Intensity Relationship LUT described in​ Section C.7.6.16.2.13.1 “Pixel Intensity Relationship LUT” in PS3.3 defines a transformation LUT enabling the conversion from the​ stored pixel data values to linear, logarithmic or other relationship.​

For instance, if the image processing applied to the X-Ray intensity before storing the Pixel Data allows returning to LIN, then a Pixel​ IntensityRelationshipLUTwiththefunction"TO_LINEAR"isprovided.Thefollowingfigureshowssomeexamplesofimageprocessing,​ and the corresponding description of the relationship between the stored pixel data and the X-Ray intensity.​

Figure FFF.1.5-1. Examples of Image Processing prior to the Pixel Data Storage​

No solution is proposed in the Enhanced XA SOP Class to standardize the subtractive display pipeline. As the Enhanced XA image​ is not required to be stored in a LOG relationship, the Pixel Intensity Relationship LUT may be provided to convert the images to the​ logarithmic space before subtraction. The creation of subtracted data to be displayed is a manufacturer-dependent function.​

As an example of subtractive display, the pixel values are first transformed to a LOG relationship, and then subtracted to bring the​ background level to zero and finally expanded to displayable levels by using a non-linear function EXP similar to an exponential.​

- Standard -​