Premise: A new handheld device purports to determine the severity of concussions by reading the brain's electrical signals and comparing them to a database of 15,000 scans compiled at a brain research lab.

Conclusion: The new handheld device is intended to help doctors decide whether an athlete who has received a blow in the head during competition should be sent back into the game.

Option A - Incorrect:
The brain's electrical signals were to be compared to a database of 15,000 scans, and not to be regularly updated with new scans as this will not serve the intended purpose of the device. Therefore, this option doesn't hold any claim to be correct.

Option B - Correct:
This is accurate. According to the passage, the device was intended to determine the severity of concussions, and this will further help doctors to make a sound decision about whether an athlete who has received a blow in the head during competition should be sent back into the game".

Option C - Incorrect:
This argument is not concerned about the endorsement of the device by a large number of medical professionals rather it is concerned about the device's effectiveness and ease of helping doctors make a good decision.

Option D - Incorrect:
We can't know from the passage if the database of the 15,000 scans includes the scans of both non-injured and injured athletes. So, this option is not valid.

Option E - Incorrect:
According to the passage, no assessment was carried out by team doctors. Hence, we can't know whether team doctors have until now been mistaken in their assessments of whether an athlete can safely continue to play.