  Background: Several studies have shown that both respiratory and cardiac
motion can be extracted from the Pilot Tone (PT) signal successfully. However,
most of these studies were performed in healthy volunteers. In addition,
validating PT using ECG as a reference can be problematic because both PT and
ECG tend to be unreliable in patients with arrhythmias. Purpose: We seek to
evaluate the accuracy and reliability of the cardiac and respiratory signals
extracted from PT in patients clinically referred for cardiovascular MRI with
the image-derived signals as the reference. Methods: Twenty-three patients were
scanned on a 1.5 T scanner using balanced steady-state free-precession
real-time (RT) cine sequence. The PT signal was generated by a built-in PT
transmitter integrated within the body array coil. For comparison, commercial
ECG and BioMatrix (BM) respiratory sensor signals were synchronously recorded.
Results: The respiratory motion extracted from PT correlated positively with
the image-derived respiratory signal in all cases and showed a stronger
correlation (absolute coefficient: 0.95+-0.09) than BM (0.72+-0.24). For the
cardiac signal, PT trigger jitter (standard deviation of PT trigger locations
relative to ECG triggers) ranged from 6.6 to 83.3 ms, with a median of 21.8 ms.
The mean absolute difference between the PT and corresponding ECG cardiac cycle
duration was less than 5% of the averaged ECG RR interval for 21 out of 23
patients. Overall, the performance of PT-based trigger extraction was
comparable to that of ECG. We did not observe significant linear dependence
(p>0.28) of PT delay and PT jitter on the patients' BMI or cardiac cycle
duration. Conclusions: This study demonstrates the potential of PT to monitor
both respiratory and cardiac motion in patients clinically referred for
cardiovascular MRI.
