E.3.6 Retain Longitudinal Temporal Information Options
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### E.3.6 Retain Longitudinal Temporal Information Options

Dates and times are recognized as having a potential for leakage of identity because they constrain the number of possible individuals that could be the imaging subject, though only if there is access to other information about the individuals concerned to match it against.

However, there are applications that require dates and times to be present to able to fulfill the objective. This is particularly true in therapeutic clinical trials in which the objective is to measure change in an outcome measure over time. Further, it is often necessary to correlate information from images with information from other sources, such as clinical and laboratory data, and dates and times need to be consistent.

Two options are specified to address these requirements:

- Retain Longitudinal Temporal Information With Full Dates Option

- Retain Longitudinal Temporal Information With Modified Dates Option

When the Retain Longitudinal Temporal Information With Full Dates Option is specified in addition to an Application Level Confidentiality Profile, any dates and times present in the Attributes shall be retained, as described in Table E.1-1. The Attribute Longitudinal Temporal Information Modified (0028,0303) shall be added to the Dataset with a value of "UNMODIFIED".

When the Retain Longitudinal Temporal Information With Modified Dates Option is specified in addition to an Application Level Confidentiality Profile, any dates and times present in the Attributes listed in Table E.1-1 shall be modified. The modification of the dates and times shall be performed in a manner that:

- aggregates or transforms dates so as to reduce the possibility of matching for re-identification

- preserves the gross longitudinal temporal relationships between images obtained on different dates to the extent necessary for the application

- preserves the fine temporal relationships between images and real-world events to the extent necessary for analysis of the images for the application

The Attribute Longitudinal Temporal Information Modified (0028,0303) shall be added to the Dataset with a value of "MODIFIED".

### Note

1. Aggregation of dates may be performed by various means such as setting all dates to the first day of the month, all months to the first month of the year, etc., depending on the precision required for the application.

2. It is possible to modify all dates and times to dummy values by shifting them relative to an arbitrary epoch, and hence retain the precise longitudinal temporal relationships amongst a set of studies, when either de-identification of the entire set is performed at the same time, or some sort of mapping or database is kept to repeat this process on separate occasions.

3. Transformation of dates and times should be considered together, in order to address studies that span midnight.

4. Any transformation of times should be performed in such a manner as to not disrupt computations needed for analysis, such as comparison of start of injection time to the acquisition time for PET SUV, or extraction of time-intensity values from dynamic contrast enhanced studies.

The manner of date modification shall be described in the Conformance Statement.

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