A/an is a sudden worsening in the severity of the symptoms or signs.

Exacerbation is the term which is referred to a sudden worsening in the severity of the symptoms or signs.
For example, an asthma.
It may occur or worsen through different factors, like air pollution, smoke, dusts, etc. This is called exacerbation.

Unlike  ct scans, which use x-rays,  mri scans  use powerful magnetic fields and radio frequency pulses to produce detailed pictures of organs, soft tissues, bone and other internal body structures.  differences between  normal and abnormal tissue is often clearer on an  mri  image than a  ct.you can see more with a mri scan. for example on an mri you can see internal bleeding.

Because it spreads quickly