When Brock is in the MRI, the sound frequency emitted by the machine huts the symbiote. MRIs do not use sound frequencies. There is a loud knocking noise when a test is performed, but is not a sound that emits a high frequency. It is the equivalent of someone knocking loudly on a door. MRIs use high powered magnets when they perform their tests. An ultrasound, on the other hand, would probably produce the high frequency sound that would hurt the symbiote.
And, speaking of magnets, the room in which the MRI is installed has metal objects all over the place. You can not have metal objects in a room with an MRI installed, unless said objects are tightly fastened down. There have been deaths from metal being "sucked" into the machine with a patient being tested. A young boy was recently killed from a fire extinguisher being "sucked" into the machine at high velocity and striking the boy in the head.