It is an interesting appearance and you have eagle eyes to find it.
The para spinal muscles are ugly on ultrasound and look very fibrous, but you have done well.
Anatomically it is in the multifidus muscle.
I am not sure what the appearance represents,
Could it be some oedema in the muscle, partial tear injury ?
What was the clinical scenario (acute onset, trauma, stretching injury, sport, occupation etc)
I will ask Ray Chen to take a look at this case as he is the master of spine ultrasound.
I will be interested in his comments.
Steve.