Center for Functional MRI In the Department of Radiology


The scanners can output TTL (Tansistor-Trasistor Logic) pulses during a MRI scan. In fMRI, TTL pulses are often used to trigger the start of a user-defined task.  On GE scanners, two types of TTL pulses are provided during an fMRI EPI scan:  TTL/ slice and TTL/epoch.

  • TTL/slice: a TTL is generated every time a slice is acquired. This is used when a time reference is needed for processing fMRI data. For example, TTL/slice should be recorded for physiological noise correction using RETROICOR.
  • TTL/epoch: a TTL is generated at the start of every epoch  in a paradigm.  The paradigm can be defined by users in the fMRI window by clicking the fMRI button on the scanner GUI.  This TTL is usually used for triggering the start of stimulus. 
Both types of TTL pulses are available inside the projection cabinet and clearly labled. Please make sure to use the correct TTL type by connecting to the corresponding TTL BNC port.