The difference is Terahertz radiation is a lot safer and can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. It can also penetrate fog and clouds, but cannot penetrate conductive materials like metal or water.

The key advantage that they got here is that they shrunk the receiver part that actually records the image into something that fits in a phone or similar small device.

While whether they need a source on the other side of what is being scanned depends on the application and the level of detail they would need.

Since like microwaves used for RADAR, Terahertz radiation doesn’t penetrate metal or water.  So could be configured like RADAR for a single hand held device for things like checking out the piping or electrical system of a house for example.

For medical purposes, and other high resolution requirements, though they’ll have to have a dedicated source.  Along with more powerful computers to process the image for finer details.

However, Earth’s atmosphere is a strong absorber of Terahertz radiation.  So range would be limited and “Eerierook’s” idea of ambient source would be limited even with the sun angled behind the object.

Given further advancement though it might be possible to make a basic portable scanner like a Tricorder and that would be pretty cool.