Evaluation of cardiac function: Technical requirements

Cardiac function parameters are of utmost importance in the detection, characterization, and prognosis of acquired and congenital heart diseases. Cine and phase-contrast imaging are the basic imaging techniques for assessment of cardiac function. Beside a field strength of at least 1T or more, basic requirement for sufficient signal detection are the use of multi-element surface coils as they not only significantly improve signal-to-noise-ratios but also are a prerequisite for the use of parallel imaging techniques. In most applications, Cine and phase-contrast imaging are performed with use of ECGtriggering: it helps to reduce motion artifacts and enables acquisition of images in different cardiac phases. This ECG synchronization may be prospective or retrospective. In prospective triggering, data acquisition is initiated by the detection of the R-wave of the QRS-complex, the last 10% of the RR-interval are excluded from the data acquisition window to avoid artifacts due to the variable physiological length of the RR-interval. In retrospective gating, untriggered MR data and ECG signals are acquired continuously throughout the RR-interval. The phaseencoding gradient is incremented each time an R wave is detected. Prior to Fourier transformation, the k-space lines are assigned to the correct phases in the cardiac cycle, thus covering the entire RR-interval. If an adequate trigger signal is not recorded with the chest wall lead, peripheral pulse oximetry using a photopletysmograph delivers information about the cardiac cycle. The delay between the R wave and maximum peripheral pulse wave ranges from 150 to 500 ms, depending on the heart rate. Images acquired immediately after the peak in the peripheral pulse wave are assigned to diastole. Systole is located at the end of the trigger period. Systolic images can be acquired only when retrospective gating is used.