ASYMMETRIC-ECHO, SHORT TE, RETROSPECTIVELY GATED MR-IMAGING OF THE HEART AND PULMONARY VESSELS

Although retrospectively cardiac-gated (cine) magnetic resonance imagi ng has shown promise for large-vessel pulmonary vascular imaging, it h as not been able to depict the peripheral pulmonary vasculature, where signal is dephased because of susceptibility and/or motion artifacts. The authors developed a cine pulse sequence that uses asymmetric echo es and radio-frequency envelopes to achieve reduced gradient moments a nd a short TE, thereby reducing signal losses due to disordered flow a nd susceptibility effects. The effects of TE (2.8-12 msec) and the deg ree of echo symmetry as measured by the echo symmetry fraction (ESF) ( 0.6-1.0) are considered in the pulmonary vasculature and the heart. In pulmonary vessels, the signal-to-noise ratio nearly doubled as TE was decreased from 12 to 2.8 msec, but there was only about a 15% differe nce as the ESF decreased from 1.0 to 0.6, consistent with T2 losses d ominating gradient moment dephasing. At a TE of 2.8 msec, the sequence improves visualization of pulmonary vessels and may be helpful for di agnosing pulmonary emboli. In the heart, however, the contrast-to-nois e ratio between blood land cardiac tissue decreased by 30% as TE decre ased from 12 to 2.8 msec and was not affected by changes in ESF. Flow artifacts in the cardiac blood pool, including those that can aid in d iagnosis (eg. signal loss due to ''jet'' flow), are much less pronounc ed when a small ESF and short TE are used, making this sequence less a ttractive for investigation of cardiac flow irregularities. The reduce d flow artifacts in this case, however, permit excellent depiction of gross cardiac anatomy.