Cardiac Gating Parameters

How do you set the various parameters for a cardiac gated study, such as repetition time (TR), trigger delay, and trigger window? 
Cardiac gating is typically performed using detection of the R-wave since this is usually the most prominent feature of the EKG. The R-wave coincides with the beginning of ventricular systole. In prospective cardiac gating, MR data acquisition begins after being triggered by the first R-wave. The acquisition is completed before the next R-wave in a period known as the R-R interval.  The cycle repeats with the next R-wave.
The R-R interval in milliseconds (msec) and the heart rate in beats per minute (bpm) are inversely related by the formula:
R-R (msec) = 60,000 ÷ heart rate (bpm) 
Heart rates of 60−100 bpm therefore correspond to R-R intervals of 1000−600 msec.
The repetition time (TR) in a prospectively gated sequence cannot be freely set but must be a multiple of the average R-R interval. Thus for a patient with heart rate = 60, the only choices for TR would be 1000 ms, 2000 ms, 3000 ms, etc. The R-R multiple chosen is sometimes called the gating factor.
Trigger delay is the time interval between the first R-wave and the beginning of data acquisition. For systolic imaging the trigger delay should be set to in the range of 0−50 msec, but for diastolic imaging delays on the order of 150−250 msec are typical.
The acquisition window occupies the bulk of the R-R interval and is the time when MR data is collected. For most prospective cardiac MR studies, the acquisition window occupies the middle 85-90% of the R-R interval.
The final 10-15% of each R-R interval is typically reserved as the trigger window. The trigger window serves as a buffer period to allow for slight variations in heart rate. That way, if the next R-wave occurs slightly earlier or later than expected, it will still be detected. R-waves occurring outside the defined trigger window are rejected.
Note that the above discussion applies to prospective cardiac gating only. In retrospective gating data is acquired continuously along with the EKG then resorted and assigned various cardiac phases at a later time. The TR in retrospective gating is thus not restricted to a multiple of the R-R interval but can have any value compatible with the pulse sequence chosen. Data can still be rejected if arrhythmias occur after this retrospective analysis.

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References
      Chia JM, Fischer SE, Wickline SA, Lorenz CH. Performance of QRS detection for cardiac MRI with a novel vectorcardiographic triggering method. J Magn Reson Imaging 2000; 12:678-688.    
      Comeau CR. Introduction to cardiovascular MR imaging. Milwaukee, GE Medical Systems, 1999. (An old application manual from GE that still contains lots of good basic information).
      Lanzer P, Barta C, Botvinick EH, et al. ECG-synchronized cardiac MR imaging: method and evaluation. Radiology 1985;155:681–686.
      Niendorf T, Winter L, Frauenrath T. Electrocardiogram in an MRI environment: clinical needs, practical considerations, safety implications, technical solutions, and future directions. In: Mills RM (ed). Advances in electrocardiograms - methods and analysis. InTech Open Access Publishing (www.intech.com), 2012.
Related Questions
      Is cardiac gating the same as triggering?