Why does it take longer to get the maximum health benefit from a leisurely walk than from swimming laps?

Because the water has more pressure and it takes more effort to move therefore your muscles are working harder than you would if you were just walking. 

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.