The relies upon radioactive sugar that is injected into the body to form representations of the brain.

Answer is Positron Emission Tomography.

Positron emission tomography (PET) is an imaging test based on nuclear medicine. In PET imaging a small amount of liquid radioactive material such as simple sugar (glucose) called fluorodeoxyglucose (FDG) is injected into the bloodstream. It is accumulated in the body and emit gamma rays which are  detected by PET. Many diseases such as cancers, brain diseases and heart diseases are diagnosed by the PET imaging.

The brain imaging method called positron-emission tomography (PET) relies upon radioactive sugar that is injected into the body to form representations of the brain.

It is functional imaging technique that is used to observe metabolic processes in the brain (or some other part of the body) in order to the diagnose the disease. The system detects pairs of γ- rays emitted indirectly by a positron-emitting radionuclide, which is introduced into the body on a biologically active molecule called a radioactive tracer. Commonly used radioactive tracer is an analogue of glucose -fludeoxyglucose (FDG).