Материал: part03

Note​

1.​This example assumes a scaling of 0.01 for the Effective Atomic Number which would be reasonable for images for​ which the effective atomic number was not greater than 40 for any pixels.​

2.​The real-world value mapped pixels in the image may have been adjusted to represent the attenuation as if the pixel​ was filled with the remaining materials to preserve the relationship between the HU value of the pixel and the materials​ contained (shown as HU in the first row), or they may have not been adjusted (shown as HU_MOD).​

C.11.2 VOI LUT Module​

Table C.11-2 specifies the Attributes that describe the VOI LUT.​

Table C.11-2. VOI LUT Module Attributes​

Attribute Name​ Tag​ Type​ Attribute Description​

Include Table C.11-2b “VOI LUT Macro Attributes”​

Table C.11-2b. VOI LUT Macro Attributes​

(0028,1050) and Window Width (0028,1051).​ See Section C.11.2.1.3 for further explanation.​

Defined Terms:​

LINEAR​

LINEAR_EXACT​

SIGMOID​

When this Attribute is not present, the interpretation of the values of​ Window Center (0028,1050) and Window Width (0028,1051) is linear​ as in Section C.11.2.1.2.​

- Standard -​

C.11.2.1 LUT Attribute Descriptions​

C.11.2.1.1 LUT Descriptor​

The three values of LUT Descriptor (0028,3002) describe the format of the LUT Data in the corresponding LUT Data (0028,3006)​ Attribute.​

The first value is the number of entries in the lookup table. When the number of table entries is equal to 216 then this value shall be​ 0.​

The second value is the first input value mapped. The Value Representation of the second value (US or SS) depends on the source​ of the input to the VOI LUT, and shall be:​

•​the same as specified by Pixel Representation (0028,0103), if there is no Modality LUT or Rescale Slope and Intercept specified;​

•​SS if the possible output range after application of the Rescale Slope and Intercept may be signed;​

Note​

This is always the case for the CT Image IOD in which the Rescale Type is specified to be Hounsfield Units, which are​ always signed.​

•​US otherwise.​

This input value is mapped to the first entry in the LUT. All input values less than the first value mapped are also mapped to the first​ entry in the LUT Data. An input value one greater than the first value mapped is mapped to the second entry in the LUT Data. Sub-​ sequent input values are mapped to the subsequent entries in the LUT Data up to an input value equal to number of entries + first​ value mapped - 1 that is mapped to the last entry in the LUT Data. Input values greater than or equal to number of entries + first value​ mapped are also mapped to the last entry in the LUT Data.​

The third value specifies the number of bits for each entry in the LUT Data. If the VOI LUT is included in an Image IOD, the third value​ of LUT Descriptor (0028,3002) shall be 8 or 16 bits, unless otherwise specialized. If the VOI LUT is included in a Presentation State​ IOD, the third value of LUT Descriptor (0028,3002) shall be between 8 and 16 inclusive. The LUT Data shall be stored in a format​ equivalent to 8 bits allocated when the number of bits for each entry is 8, and 16 bits allocated when the number of bits for each entry​ is 16, where in both cases the high bit is equal to bits stored - 1, and where bits stored is the third value.​

Note​

1.​Since LUT Descriptor (0028,3002) is multi-valued, in an Explicit VR Transfer Syntax, only one value representation (US​ or SS) may be specified, even though the first and third values are always by definition interpreted as unsigned. The​ explicit VR actually used is dictated by the VR needed to represent the second value.​

2.​Some implementations have encoded 8 bit entries with 16 bits allocated, padding the high bits; this can be detected by​ comparing the number of entries specified in the LUT Descriptor with the actual value length of the LUT Data entry. The​ value length in bytes should equal the number of entries if bits allocated is 8, and be twice as long if bits allocated is​ 16.​

The LUT Data contains the LUT entry values.​

The output range is from 0 to 2n-1 where n is the third value of LUT Descriptor. This range is always unsigned.​

C.11.2.1.2 Window Center and Window Width​

C.11.2.1.2.1 Default LINEAR Function​

If VOI LUT Function (0028,1056) is absent or has a value of LINEAR, Window Center (0028,1050) and Window Width (0028,1051)​ specify a linear conversion from stored pixel values (after any Modality LUT or Rescale Slope and Intercept specified in the IOD have​ been applied) to values to be displayed. Window Center contains the input value that is the center of the window. Window Width​ contains the width of the window.​

- Standard -​

Note​

Theterms"windowcenter"and"windowwidth"arenotconsistentlyusedinpractice,norweretheydefinedinpreviouseditions​ oftheStandard.Thedefinitionsherearepresentedforthepurposeofdefiningconsistentmeaningsforidentityandthreshold​ transformations while preserving the common practice of using integer values for center and width.​

Window Width (0028,1051) shall always be greater than or equal to 1.​

When Window Width (0028,1051) is greater than 1, these Attributes select the range of input values that are to be mapped to the full​ range of the displayed output.​

When Window Width (0028,1051) is equal to 1, they specify a threshold below which input values will be displayed as the minimum​ output value.​

Note​

Whether the minimum output value is rendered as black or white may depend on the value of Photometric Interpretation​ (0028,0004) or the presence of a Presentation LUT Module.​

These Attributes are applied according to the following pseudo-code, where x is the input value, y is an output value with a range​ from ymin to ymax, c is Window Center (0028,1050) and w is Window Width (0028,1051):​

if (x <= c - 0.5 - (w-1) /2), then y = ymin​

else if (x > c - 0.5 + (w-1) /2), then y = ymax​

else y = ((x - (c - 0.5)) / (w-1) + 0.5) * (ymax- ymin) + ymin​

Note​

1.​For the purpose of this definition, a floating point calculation without integer truncation is assumed, though the manner​ of implementation may vary as long as the result is the same.​

2.​Thepseudo-codefunctioncomputesacontinuousvalueovertheoutputrangewithoutanydiscontinuityattheboundaries.​ Thevalueof0forwisexpresslyforbidden,andthevalueof1forwdoesnotcausedivisionbyzero,sincethecontinuous​ segment of the function will never be reached for that case.​

3.​For example, for an output range 0 to 255:​

c=2048, w=4096 becomes:​

if (x <= 0) then y = 0​

else if (x > 4095) then y = 255​

else y = ((x - 2047.5) / 4095 + 0.5) * (255-0) + 0​

c=2048, w=1 becomes:​

if (x <= 2047.5) then y = 0​

else if (x > 2047.5) then y = 255​

else /* not reached */​

c=0, w=100 becomes:​

if (x <= -50) then y = 0​

else if (x > 49) then y = 255​

else y = ((x + 0.5) / 99 + 0.5) * (255-0) + 0​

c=0, w=1 becomes:​

- Standard -​

if (x <= -0.5) then y = 0​

else if (x > -0.5) then y = 255​

else /* not reached */​

4.​A Window Center of 2n-1 and a Window Width of 2n selects the range of input values from 0 to 2n-1. This represents a​ mathematical identity VOI LUT transformation over the possible input values (whether used or not) in the case where​ no Modality LUT is specified and the stored pixel data are n bit unsigned integers.​

In the case where x1 is the lowest input value actually used in the Pixel Data and x2 is the highest, a Window Center​ of (x1+x2+1)/2 and a Window Width of (x2-x1+1) selects the range of input values from x1 to x2, which represents the​ full range of input values present as opposed to possible. This is distinct from the mathematical identity VOI LUT​ transformation, which instead selects the full range of input values possible as opposed to those actually used. The​ mathematical identity and full input range transformations are the same when x1 = 0 and x2 is 2n-1 and the input values​ are n bit unsigned integers. See also Note 7.​

5.​A Window Width of 1 is typically used to represent a "threshold" operation in which those integer input values less than​ the Window Center are represented as the minimum displayed value and those greater than or equal to the Window​ Center are represented as the maximum displayed value. A Window Width of 2 will have the same result for integral​ input values.​

6.​The application of Window Center (0028,1050) and Window Width (0028,1051) may select a signed input range. There​ is no implication that this signed input range is clipped to zero.​

7.​The selected input range may exceed the actual range of the input values, thus effectively "compressing" the contrast​ range of the displayed data into a narrower band of the available contrast range, and "flattening" the appearance. There​ are no limits to the maximum value of the window width, or to the minimum or maximum value of window level, both of​ which may exceed the actual or possible range of input values.​

8.​Input values "below" the window are displayed as the minimum output value and input values "above" the window are​ displayed as the maximum output value. This is the common usage of the window operation in medical imaging. There​ is no provision for an alternative approach in which all values "outside" the window are displayed as the minimum output​ value.​

9.​TheoutputoftheWindowCenter/WidthorVOILUTtransformationiseitherimplicitlyscaledtothefullrangeofthedisplay​ device if there is no succeeding transformation defined, or implicitly scaled to the full input range of the succeeding​ transformation step (such as the Presentation LUT), if present. See Section C.11.6.1.​

10.​FractionalvaluesofWindowCenterandWindowWidtharepermitted(sincetheVRoftheseAttributesisDecimalString),​ and though they are not often encountered, applications should be prepared to accept them.​

C.11.2.1.2.2 General Requirements for Window Center and Window Width​

The Window Center (0028,1050), Window Width (0028,1051) and VOI LUT Function (0028,1056) Attributes shall be used only for​ Images with Photometric Interpretation (0028,0004) values of MONOCHROME1 and MONOCHROME2. They have no meaning for​ other Images.​

If multiple values are present, both Attributes shall have the same number of values and shall be considered as pairs. Multiple values​ indicate that multiple alternative views may be presented.​

IfanyVOILUTTableisincludedbyanImage,aWindowWidthandWindowCenterortheVOILUTTable,butnotboth,maybeapplied​ to the Image for display. Inclusion of both indicates that multiple alternative views may be presented.​

If multiple Items are present in VOI LUT Sequence (0028,3010), only one may be applied to the Image for display. Multiple Items in-​ dicate that multiple alternative views may be presented.​

If the VOI LUT Module is defined in an IOD and if neither a VOI LUT Sequence nor a Window Width and Window Center are present,​ then the VOI LUT stage of the grayscale pipeline is defined to be an identity transformation.​

- Standard -​

Note​

1.​This requirement is specified so that IODs that define a particular output space for the grayscale pipeline, such as P-​ Values, are not in an undefined state when no VOI LUT Sequence or Window Width and Window Center are present.​

2.​Despite the Type 3 requirement for VOI LUT Sequence and Window Center, implementations that render images are​ expected to implement and apply these transformations when they are present in the image, unless overridden by the​ user, a presentation state, or a hanging protocol, and to allow the user to select which transformation to apply when​ multiple transformations are present.​

C.11.2.1.3 VOI LUT Function​

TheVOILUTFunction(0028,1056)specifiesapotentiallynon-linearconversionfortheoutputofthe(conceptual)ModalityLUTvalues​ to the input of the (conceptual) Presentation LUT.​

The behavior for the value LINEAR is defined in Section C.11.2.1.2.1. For all other values, the VOI LUT Function (0028,1056) shall​ include a unique descriptor of the LUT function to be used. Each descriptor is associated with a bivariate function of Window Center​ (0028,1050) and Window Width (0028,1051).​

If the VOI LUT Function (0028,1056) is present with a value other than LINEAR, the values provided in Window Center (0028,1050)​ and Window Width (0028,1051) shall not be interpreted as a linear conversion of the (conceptual) Modality LUT values to the input​ to the (conceptual) Presentation LUT - but as parameters for the function defined by the VOI LUT Function descriptor in (0028,1056).​

When defined, each descriptor must provide the functional relationship between the output of the (conceptual) Modality LUT values​ to the input of the (conceptual) Presentation LUT.​

C.11.2.1.3.1 SIGMOID Function​

If the value of VOI LUT Function (0028,1056) is SIGMOID, the function to be used to convert the output of the (conceptual) Modality​ LUT values to the input of the (conceptual) Presentation LUT is given by​

Window Width (0028,1051) shall always be greater than 0.​

Note​

WindowWidth(0028,1051)isrequiredtobegreaterthanzerotopreventdivisionbyzero(quiteapartfrombeingmeaningless).​

C.11.2.1.3.2 LINEAR_EXACT Function​

If the value of VOI LUT Function (0028,1056) is LINEAR_EXACT, the function to be used to convert the output of the (conceptual)​ Modality LUT values to the input of the (conceptual) Presentation LUT is given by the following pseudo-code, where x is the input​ value, y is an output value with a range from ymin to ymax, c is Window Center (0028,1050) and w is Window Width (0028,1051):​

if (x <= c - w/2), then y = ymin​

- Standard -​