In vivo perfusion measurements in the human placenta using echo planar imaging at 0.5 T

Abstract

This paper presents the first in vivo measurements of perfusion in the human placenta from 20 weeks gestational age until term, using the non-selective/selective inversion recovery echo-planar imaging sequence, in which data is alternately acquired following a selective and non-selective inversion pulse. Twenty pairs of images were collected, two each at the following inversion times: 20, 310, 610, 910, 1110, 1410, 1910, 2810, 3310, and 4510 ms with the sequence being repeated with a repetition time (TR) of 10 s. The results of these measurements were used to suggest the optimum sequence for future work in terms of the signal to noise ratio in the measured perfusion rate in a given measurement time. The sequence was also analyzed to determine the expected variability in the measurements. In normal pregnancies the average value of perfusion rate was found to be 176 (standard error = +/-24) ml/100 mg/min. (n = 16, standard deviation = 96 ml/100 mg/min). The expected variability in the measured parameters due to signal to noise ratio considerations alone was calculated to be 71%. For a maximum scanning time of 400 s, the optimum sequence for measuring placental perfusion was found to require 8 repetitions at each of 10 inversion times which were geometrically spaced (given by a(o), a(o)r, a(o)r2, a(o)r3, . . .), with a(o) = 850 ms, r = 1.073 and TR = 5 s, giving a pixel variability of 38%. Other timing schemes are recommended for measuring perfusion in other anatomical regions with different values of perfusion rate and longitudinal relaxation time.

Advanced MR imaging of the placenta: Exploring the in utero placenta–brain connection

“…To date, only two studies have explored the use of in vivo FAIR imaging in the placenta, likely due to the difficulty in reliably acquiring perfusion data in the placenta. Although FAIR imaging did not reveal differences in perfusion between healthy and compromised pregnancies in one study (80), when the authors generated perfusion maps, they reported decreased perfusion fraction in pregnancies complicated by FGR. The authors speculate that perfusion maps were more representative of overall blood movement, given the multi-directional flow and could be used to distinguish normal and pathologic pregnancies (80).…”

Functional imaging of the human placenta with magnetic resonance

“…70 Human studies ASL MRI was first developed to study perfusion in the brain, kidney, retina, and some tumors. 68 36,38 The perfusion map showed more areas of low perfusion in relation to a decrease in placental perfusion in IUGR pregnancies (923 vs 2359 arbitrary units; P ¼ .003) 38 ; a correlation with the uterine artery pulsatility index was established later (Table 2). 77 Limitations Although ASL MRI can yield a quantitative estimate of placental perfusion without using an exogenous contrast agent, only a few studies have been published since the seminal publications in 1998.…”

“…77 Limitations Although ASL MRI can yield a quantitative estimate of placental perfusion without using an exogenous contrast agent, only a few studies have been published since the seminal publications in 1998. 36,38 The limitations of this technique are a poor signal-to-noise ratio (SNR) with most methods, owing to small measurable differences between tagged and untagged images and an important sensitivity to motion, transit delay, and resonance offset artifacts. 67 Future challenges ASL remains the only fMRI technique that allows the measurement of tissue perfusion with no exogenous contrast agent.…”

SPIO‐enhanced magnetic resonance imaging study of placental perfusion in a rat model of intrauterine growth restriction

“…[9][10][11][12][13] Perfusion can be measured in several ways with MRI, including arterial spin labelling (ASL), echo planar imaging (EPI), and intravoxel incoherent motion (IVIM). [14][15][16] We have previously described a novel MRI-based approach to study placental function in a living mouse model, based on the use of gadolinium contrast agents and dynamic contrast enhancement (DCE) MRI. This has the capacity to measure placental perfusion and permeability in both physiological and pathological settings; 10,[12][13][14][15][16] however, gadolinium is not recommended for use during human pregnancy.…”