Flow-weighted MRI of the lungs with the ECG-gated half-Fourier FSE technique: evaluation of the effect of the cardiac cycle.

We investigated temporal MR signal changes in the peripheral lung and proximal pulmonary vessels during the entire cardiac cycle in order to evaluate the characteristics of the diastolic-systolic subtraction method in the lung. In eight healthy volunteers free of lung diseases, changes in the MR signal during one breath-hold were investigated with the multiple ECG-triggered half-Fourier single-shot fast-spin echo (SS-FSE) technique. The signal intensity-time course curve in the lung showed that biphasic signals decreased 20% to 47% at systole and 5% to 33% at mid-diastole, measured against the maximum signals at late diastole. This signal decrease in the peripheral lung was correlated to that in the proximal pulmonary vessels during an entire cardiac cycle (r=0.667 to 1.000). The best visualization of the lung was obtained at late diastole, when the intra-vascular flow in the lung was expected to be stagnant. Compared with the late diastolic SS-FSE images, the late diastolic-systolic subtracted SS-FSE images improved the signal-to-noise ratio in the lung as well as the signal-intensity ratio of the peripheral lung to surrounding tissues. Although the flow-induced signal dephasing in the lung was completely unavoidable and its amount was unpredictable even at late diastole, the diastolic-systolic subtracted SS-FSE images showed the relative differences in flow alteration during the cardiac cycle between the images at diastole and those at systole. The main characteristic of diastolic-systolic subtracted SS-FSE was the enhancement of visibility of cardiac-dependent signal changes in the lung due to the alteration in pulsatile flow.