Retained products of conception (RPOCs) result from an aborted pregnancy or retained placental remnants, and they sometimes cause life-threatening hemorrhage. Assessment of their vascularity is important for prompt patient management. Arterial spin-labeling (ASL)-MRI enables non-invasive tissue perfusion measurement without administration of contrast materials, and it has now become a robust technique for neuroimaging.¹ However, this technique has not been well evaluated for abdominal organs other than the kidneys so far. In this study, ASL-MRI was used to evaluate the vascularity of RPOCs and to clarify the clinical feasibility of this approach.This study was approved by the Institutional Ethics Committee, and informed consents were obtained from each subject. A clinical 3.0-T MRI scanner was used. ASL-MRI was performed using the in-house-developed pulsed-continuous type ASL sequence with multi-slice 2D echo-planar imaging acquisition (6 slices), with TR/TE=5500/18.2 ms, matrix 96 x 128, and slice thickness 9 mm. Arterial labeling was performed just below the aortic bifurcation with 4.0-s labeling duration. Post labeling time was set to 3.0 s determined by the preliminary examinations with healthy subjects. Total acquisition time of ASL-MRI was approximately 10 min. Seven clinically-diagnosed RPOC patients were enrolled in this study. All patients underwent Doppler-US and ASL-MRI, and 6 patients also underwent dynamic contrast-enhanced MRI. All images were visually evaluated by a board-certified radiologist. For semi-quantitative assessment, signals on ASL-MRI were normalized in proportion to the signals of urine on the reference proton density images.Five RPOC patients (31-73 days postpartum) showed quite high signals over RPOCs, especially in enlarged abnormal vessels, while 2 patients who underwent MRI relatively earlier (8 and 15 days postpartum) showed no abnormal signals. A representative case is shown in Figure 1. All 5 patients showing high signals on ASL-MRI underwent follow-up MRI after therapy. High signals in 4 patients were visually and semi-quantitatively decreased, while 1 patient showed signal increases (Figs. 1, 2). The imaging findings of each study are summarized in Figure 3 (Table). Two cases (cases 1 and 2) showed no flow signals on Doppler US, while ASL-MRI demonstrated a high signal over the RPOCs. The other two cases (case 5 and 7) showed the opposite findings, with no high signals over the RPOCs and the presence of flow signals on Doppler US.Evaluation of RPOCs by ASL-MRI was clinically feasible, and high signals were observed in 5 of 7 cases. Moreover, the therapeutic response can be evaluated by ASL-MRI. However, these ASL-MRI findings were not completely identical to those of other conventional imaging modalities. Although the reason for this discrepancy remains unknown, in the cases in which Doppler-US failed to detect flow signals, ASL-MRI and dynamic contrast-enhanced MRI successfully demonstrated hypervascular lesions. This may be explained by the disadvantages of US, such as operator-dependency and less tissue penetration than MRI. In the other discrepant cases in which ASL-MRI did not show high signals, one possible explanation is a technical problem associated with uterine ASL-MRI, such as unstable labeling efficiency, susceptibility effects from air, and artifacts from bowel movements. Another explanation is that high signals on ASL-MRI might rather represent relatively large arteries compared to other imaging (so-called “vascular artifacts”). Since such a discrepancy was seen in rather early postpartum cases, this might be related to abnormal vessel development in the course of progression, and it might provide additional information compared to the other conventional imaging modalities. From our initial experience, we identified several problems to overcome for uterine ASL-MRI: the low signal-to-noise ratio should be improved to shorten acquisition time; and the strong susceptibility effect from air in bowel should be overcome for reliable quantification and reproducibility. In the present study in particular, the 2D-EPI readout sequence was used for ASL-MRI, which is the most efficient usage of the MR signal available per unit time, but it is more sensitive to susceptibility effects. Other readout sequences, such as fast spin echo or the balanced steady-state free precession sequence,² may be more suitable in such circumstances, because they are less sensitive to susceptibility effects.Evaluation of RPOC by ASL-MRI is clinically feasible and can be used to assess therapeutic response. Although clinical advantages over conventional imaging need to be evaluated, ASL-MRI has clinical potential for non-invasive assessment of RPOCs and might be useful for determining the indications and optimal timing for surgical procedures.