Cardiac triggering is essential for accurately quantifying pulmonary perfusion using arterial spin labeling, and is regularly used in pulsed-ASL methods in the lungs at 1.5T [1,2]. Compared to pulsed ASL, pseudo-continuous ASL (pCASL) offers higher signal to noise ratio (SNR) [3]. A pCASL approach was recently applied to measure pulmonary perfusion [4]. To minimize the sensitivity of pulmonary perfusion to cardiac motion, the beginning of the pCASL pulse sequence was cardiac triggered. However, the prolonged repetition times of pCASL (~5-6 sec), due to long labeling duration and long post-label delay, renders the acquisition sensitive to variations in the heart rate (fig. 1). Thus, the purpose of this work was to investigate methods to reduce cardiac cycle induced signal variations in pulmonary perfusion using pCASL. Pulmonary perfusion weighted images are typically acquired with a single shot turbo spin echo (SShTSE) due to its reduced sensitivity to B₀ inhomogeneities. However, it has been previously shown that the SShTSE signal in the lungs varies throughout the cardiac cycle [5], and that these variations are significant enough to mimic perfusion-weighted images [6]. Similar signal variations were observed in pulmonary perfusion using pCASL, even when the sequence was cardiac triggered, due to slight variations in the cardiac cycle between the control and label acquisitions (fig. 1). Various approaches were explored to overcome this challenge including: 1) pCASL labeling of the right pulmonary artery close to the lungs, 2) cardiac triggering the acquisition after the pCASL labeling, rather than from the beginning of the sequence (fig. 2), and 3) using flow insensitive acquisitions such as T1 fast field echo (T1-FFE) with a projection acquisition. Normal volunteers were scanned on a 3T Ingenia scanner (Philips Healthcare, Best, The Netherlands) with IRB approval and written informed consent. To demonstrate the signal differences that may arise from improper cardiac triggering, systolic and diastolic proton density images (ECG triggered using 100ms and 400ms delays, respectively) were acquired using a SShTSE without labeling. Similar images were also acquired using a T1-FFE with a projection acquisition without labeling. We hypothesized that the projection acquisition will be insensitive to cardiac signal variations due to the acquisition of the center of k-space throughout the cardiac cycle. Subsequently, pCASL perfusion-weighted images of the right lung were acquired using a SShTSE and a T1-FFE with a projection acquisition. Sagittal pCASL labeling was applied perpendicularly to the right pulmonary artery for 2.5 seconds, followed by a 500ms post-label delay to allow labeled blood to perfuse the lung tissue. To ensure that the data acquisition with SShTSE occurred during diastole, the cardiac trigger was placed after the post-label delay (fig. 2). This approach minimized signal variations due to pulsatile flow, even in cases with significantly varied heart rate between control and label acquisitions. Figure 3 shows the difference between images acquired during systolic and diastolic phase using a proton density weighted SShTSE (fig. 3b) and T1-FFE with Projection acquisition (fig. 3c). Even without labeling, SShTSE generates a “perfusion-weighted” image due to its sensitivity to cardiac cycle induced signal variations, while the T1-FFE with projection acquisition is insensitive to such cardiac signal variations. Figure 4 shows the pulmonary perfusion-weighted images acquired with sagittal pCASL labeling of the right pulmonary artery using cardiac triggered SShTSE acquisition (fig. 4b) and T1-FFE with projection acquisition (without triggering) (fig. 4c) showing true perfusion-weighted signal. The sensitivity of SShTSE acquisition to cardiac cycle induced signal variations has been demonstrated previously, to generate “perfusion-weighted” images [6]. However, the variations in signal intensities depending upon the cardiac phase rendered these images inconsistent and challenging to quantify. In this work, we demonstrated a true perfusion-weighted image using pCASL labeling by cardiac triggering the SShTSE data acquisition. This ensured that the only signal variations between control and label images are from spin labeling and not due to cardiac cycle. However, this poses a new challenge of variable post-label delay between the control and label acquisitions that may introduce errors in quantification. As an alternate, our approach with T1-FFE with projection acquisition is insensitive to cardiac induced signal variations (fig. 3c) and thus may be a viable acquisition strategy without cardiac triggering (fig. 4c). Future work will quantify the perfusion maps with both acquisitions and compare the perfusion maps against pulsed-ASL approaches, such as FAIR [7].