Editorial on “Human Placenta Blood Flow During Early Gestation With Pseudocontinuous Arterial Spin Labeling MRI”

The umbrella terms ischemic placenta disease (IPD) and ischemic placenta syndrome (IPS) refer to very serious complications of pregnancy that encompass preeclampsia/ eclampsia, intrauterine growth retardation (IUGR), or small for gestational age (SGA) and placental abruption. Eclampsia affects 6.0 per 1000 pregnancies, while IUGR incidence is reported to be between 3% and 7% of the total population. The consequences of IPS can be severe. IPS is thought to be responsible for half of preterm labors and is the main cause of fetal deaths during pregnancy. Improved methods are needed to predict and assess IPS. Currently, data on maternal history, uterine artery pulsatility index, arterial pressure, maternal serum pregnancy-associated plasma protein-A, placental growth factor, etc, are used in this assessment. However, none of these methods demonstrate what is actually happening at the placenta level. Placental development and function are difficult to measure during pregnancy. In clinical practice, indirect placental function is inferred via fetal growth and fetal Doppler ultrasound (US), which has been shown to be useful in assessing some aspects of IPS in severe preeclampsia, but specificity is limited. Magnetic resonance imaging (MRI) has been considered a potentially useful method for the purpose of detecting and monitoring IPS. Some MRI techniques that have been advocated for this purpose utilize contrast agents. However, the recognition of gadolinium-associated nephrogenic systemic fibrosis and newer concerns about gadolinium accumulation in the brain have prompted caution in the use of MRI contrast agents unless the benefits clearly outweigh the risks. Approaches that do not employ gadolinium are therefore attractive not the least from the cost and time perspectives. A promising tool for MRI-based assessment of perfusion without the use of contrast material is arterial spin labeling (ASL). Water protons in blood are labeled by exposure to radiofrequency pulses and arrival in tissue as perfusion is typically detected by subtracting images obtained without and with labeling. Motion between subtracted images can degrade the results and poses a challenge. ASL has many advantages for the study of placental perfusion during pregnancy compared to other functional MRI techniques. ASL is the only functional (f)MRI technique that gives a quantitative measurement of placental blood flow without any invasiveness. Labeling can be achieved in different ways. One way is to apply a series of short pulses, such as 1000 pulses each msec (hence, pseudocontinuous ASL or pCASL) in the presence of a magnetic field, which is recommended by an ISMRM perfusion study group and the European consortium for ASL in dementia. It is becoming more common for some authors to simply refer to pCASL as CASL, as true CASL is becoming less common. Positioning the labeling slice is very important for obtaining a correct evaluation of placenta perfusion. Arterial transit time governs how long it takes for labeled blood to travel to the tissue of interest. If the delay is too short, the labeled blood may not have reached the tissue, while too long a delay will decrease the signal-to-noise ratio (SNR). It is well known from the pathophysiology that the blood injected into the intervillous space is marginalized and the flow is very slow and it takes about 10–20 seconds until it is drained in the uterine veins. Based on this fact, to minimize this problem the labeling plane of the ASL sequence should be positioned close to the level of the decidual plate, where the injection of the spiral arteries takes place. In this issue of JMRI, Liu et al applied advanced ASL methods to assess human placental blood flow in a series of patients with normal pregnancies and patients with IPS. The authors assessed these patients at two timepoints during pregnancy and studied pCASL prospectively in the expectant mothers. The study is impressive, as it was a prospective study including 118 mothers. It is also noteworthy that the authors imaged women relatively early in pregnancy and two times. There is a need for methods that can predict later complications when there is still time to act. This is one of the first MRI-based studies to take on this problem. The authors found that two variables, hPBF (high placental blood flow) and relative hPBF, showed