feat(ExtensionManager): add ability to save current frame index with no annotation present

[deepakri201]

Context

Using the usAnnotation extension, clinicians often first perform a rough assessment of a particular ultrasound clip by visually identifying the frame with the maximum number of B-lines. Later, the clinician or a fellow may add in pleural line and B-line annotations. However, in the current extension, the user cannot save an annotation JSON file without adding the pleural lines and B-lines.

Changes & Results

Therefore, we added the ability to save out a JSON annotation file, even if no pleural lines or B-lines are present in the frame. The JSON file will contain an entry with the current frame number, and an empty list for both the pleural lines and B-lines.

A sample resulting JSON file is as follows:

{
  "SOPInstanceUID": "1.2.840.113663.1500.1.248223208.3.1.20110323.110042.375",
  "GrayscaleConversion": false,
  "mask_type": "fan",
  "angle1": 47.37832997335552,
  "angle2": 132.7093899573615,
  "center_rows_px": 419.9508448540707,
  "center_cols_px": 94.38658474142345,
  "radius1": 26.654419138056458,
  "radius2": 469.68489213060354,
  "AnnotationLabels": [],
  "frame_annotations": {
    "24": {
      "pleura_lines": [],
      "b_lines": []
    }
  }
}

Testing

This change can be tested by:

• searching for US images
• selecting a study (e.g cardiac)
• scrolling to a particular frame
• clicking the download JSON button in the "Annotated Frames" section

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• OS: macOS 14.7.3
• Node version: 25.5.0
• Browser: Chrome 142.0.7444.60

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