DICOM PS3.17 2019a - Explanatory Information |
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This Annex contains examples of how to encode diffusion models and acquisition parameters within the Quantity Definition Sequence of Parametric Maps and in ROIs in Measurement Report SR Documents.
The approach suggested is to describe that an ADC value is being measured by using ADC (generic) as the concept name of the numeric measurement, and to add post-coordinated concept modifiers to describe:
the model (e.g., mono-exponential, bi-exponential or other multi-compartment models) (drawn from CID 7273 “MR Diffusion Models”)
the method of fitting the data points to that model (e.g., for mono-exponential models, log of ratio of two samples, linear least-squares for log-intensities of all b-values) (drawn from CID 7274 “MR Diffusion Model Fitting Methods”)
relevant numeric parameters, such as the b-values used during acquisition of the source images (drawn from CID 7275 “MR Diffusion Model Specific Methods”)
The model and method of fitting are encoded separately since even though the method of fitting is sometimes dependent on the model, the model may be known but not the method of fitting, or there may be no code for the method of fitting.
The generic concept of ADC, (113041, DCM, "Apparent Diffusion Coefficient"), is used, rather than the specific concept of ADC_{m}, (113290, DCM, "Mono-exponential Apparent Diffusion Coefficient"), since the model is expressed in a post-coordinated manner. Most clinical users will not be concerned with which model was used, and so the ability to display and query for a single generic concept is preferred. However, model-specific pre-coordinated concepts for ADC are provided, as are concepts for other model parameters when a single ADC concept is inappropriate, e.g., for the fast and slow components of a bi-dimensional model.
The generic concept of (G-C036, SRT, "Measurement Method") is used to describe the model, rather than being used to described the fitting method, since the model is the more important aspect of the measurement to distinguish. This pattern is consistent with historical precedent (e.g., in Section RRR.3 the model (Extended Tofts) for DCE-MR measurements is described using the Measurement Method and the fitting method is not described).
Also illustrated is how the (121050, DCM, "Equivalent Meaning of Concept Name") can be used to communicate a single human readable textual description for the entire concept.
This example shows how to use the Table C.7.6.16-12b “Real World Value Mapping Item Macro Attributes” in PS3.3 to describe pixel values of an ADC parametric map obtained from a pair of B0 and B1000 images fitting the log ratio ot two samples to a mono-exponential function (single compartment model). It elaborates on the simple example provided in Section C.7.6.16.2.11.1.2 “Real World Values Mapping Sequence Attributes” by adding coded concepts that describe the model, the method of fitting and listing the b-values used.
Real World Value Mapping Sequence (0040,9096)
LUT Explanation (0028,3003) = "ADC mm2/s mono-exponential log ratio B0 and B1000"
Measurement Units Code Sequence (0040,08EA) = (mm2/s, UCUM, "mm2/s")
Quantity Definition Sequence (0040,9220):
CODE (G-C1C6, SRT, "Quantity") = (113041, DCM, "Apparent Diffusion Coefficient")
CODE (G-C036, SRT, "Measurement Method") = (113250, DCM, "Mono-exponential ADC model")
CODE (113241, DCM, "Model fitting method") = (113260, DCM, "Log of ratio of two samples")
NUMERIC (113240, DCM, "Source image diffusion b-value") = 0 (s/mm2, UCUM, "s/mm2")
NUMERIC (113240, DCM, "Source image diffusion b-value") = 1000 (s/mm2, UCUM, "s/mm2")
TEXT (121050, DCM, "Equivalent Meaning of Concept Name") = "ADC mono-exponential log ratio B0 and B1000"
In this usage, the text of the (121050, DCM, "Equivalent Meaning of Concept Name") is redundant with the value of LUT Explanation (0028,3003); either or both could be omitted.
The parameter describing a b-value of 0 is expected to be sent, and one should not assume that a b-value of 0 is used if it is absent, since some methods may use a low b-value (e.g., 50), which is not 0.
There is no consensus in the MR community or scientific literature as to the appropriate units to use to report diffusion coefficient values to the user, nor amongst the MR vendors as to how to encode them. In this example, the units are specified as "s/mm2". If the diffusion coefficient pixel values were encoded as integers with such a unit, they could then be encoded with a Rescale Slope of 1E-06, given the typical range of values encountered. Alternatively, the pixel values could be encoded as floating point pixel data values with identity rescaling. Or, if the units were specified "um2/s" (or "10-6.mm2/s", which is the same thing), then integer pixels could be used with a Rescale Slope of 1. Application software can of course rescale the values for display and convert the units as appropriate to the user's preference, as long as they are unambiguously encoded.
DICOM PS3.17 2019a - Explanatory Information |
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