De-identification of Facial Images by Use of Composites

 

*Mark E. Engelstad MD, DDS, MHIOregon Health & Science UniversityDept of Oral and Maxillofacial SurgeryDept Medical Informatics & Clinical Epidemiology

Genevieve B. Melton, MD, MAUniversity of MinnesotaInstitute for Health InformaticsDepartment of Surgery

Medbiquitous Annual Symposium, Baltimore MD May 10, 2011

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Pre-op De-identification

Original injuryPeriorbital area

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Post-op De-identification

The Questions:

Do composites de-identify faces?

Even those that are well-known to an observer?

Are facial composites realistic in appearance?

Figure 2: A comparison of two techniques for facial image de-identification. The middle image (B) is the original image. (A) black boxes only. (C) a facial composite, altered in the area of eyes and eyebrows only.

This is a PRE-operative patient

This is a POST-operative patient

I recognize this patient

Me

Faces

Subjects viewed the composite faces twice—first unaware that the faces were composites, and then primed to the presence of composites.

Subjects viewed 20 composite faces

• Subjects viewed 20 composite faces

• 10/20 had a third of a familiar face (test face)

Test Face

Test Face

Results

Subject Response Unprimed (1st Viewing)

Primed (2nd Viewing)

Facial CompositesTotal = 20

Composites of Unfamiliar

Faces Total = 10

Did Not Identify (True Neg)

100% (120/120 ) *

42% (50/120)

Identified Wrongly(False Pos)

0% (0/120) 58% (70/120)

Composites with Familiar (Test) Faces

Total = 10

Identified Correctly (True Pos)

0% (0/120) 62% (74/120)

Identified Wrongly (False Pos)

0% (0/120) 19% (23/120)

Failed to Identify (False Neg)

100% (120/120 ) *

19% (23/120)

No subjects identified test faces unless they were primed to their presence (* p < 0.001).

Results

Familiar Face Composite A Region Visible

Familiar Face Composite B Region Visible

Faces A and B Views by region (n)

42%(5/12)

79% *(19/24)

67%(24/36)

Upper

36

71% †(17/24)

38% †9/24

54%(26/48)

MidFace

48

67%(8/12)

67%16/24

67%(24/36)

Lower

36

Total Face A

63%(30/48)

Total Face B

61%(44/72)

Total Faces A and B62%

(74/120)

Total

120

Table 2: Identification of Test faces after priming--compared by facial region. Percentages of subjects who correctly identified a familiar face when regions of that face were visible in the composite image are shown (true positives). In Test Face B, a significant difference (* p<0.01) in identification rate existed between Upper Face and Midface. Test Face A Midface was recognized correctly more often than Test Face B Midface (†p<0.01)

OriginalBlack Boxes Composite, Eyes only

Making a Facial Composite

1: Photoshop

2: A Library

Step 2,3: Remove Background, Change laterality

4: Size all images to a standard (800 x1200)

5: Align the facial features

6: Create a Layer Mask

7: Use a Brush to reveal deeper layer

8: Blend the edges between the two layers

9: Correct Color Tones

Show Simulation/ Example