Usually PET MR Scans are used to acquire detailed three dimensional images of the cells functionality inside the body which also help detect a wide selection of cancers, their stages and their response to treatment.   

Additionally PET MR Scans can serve to identify a number of neurological conditions related to the brain.
 

Magnetic resonance imaging utilizes strong magnetic fields & radio waves to produce detailed images of the inside of the body. MR is unique in that it uses a combination of high field strength magnets & radio waves, as well as the magnetic properties of hydrogen in water, to create detailed pictures.  

And while the MR scan is going on, the PET scanner is collecting information at the same time, hence the compound nature of the device.

• Less Radiation: being a hybrid system exposes the patient to less radiation compared to separate traditional PET/CT scans.
• Detection Accuracy: Ideal for intervention planning since it accurately detects the tumor which makes it valuable for cancer patients.
• Better Patient Experience: Since the system is hybrid, patients only need one appointment to go through scans. This method is basically time efficient.
• Efficiency: It detects diseases that do not actually appear on other imaging methods and technology.