IN-VIVO PERFUSION MEASUREMENTS IN THE HUMAN PLACENTA USING ECHO-PLANAR IMAGING AT 0.5 T

This paper presents the first in vivo measurements of perfusion in the human placenta from 20 weeks gestational age until term, using the no n-selective/selective inversion recovery echo-planar imaging sequence, in which data is alternately acquired following a selective and non-s elective inversion pulse. Twenty pairs of images were collected, two e ach at the following inversion times: 20, 310, 610, 910, 1110, 1410, 1 910, 2810, 3310, and 4510 ms with the sequence being repeated with a r epetition time (TR) of 10 s, The results of these measurements were us ed to suggest the optimum sequence for future work in terms of the sig nal to noise ratio in the measured perfusion rate in a given measureme nt time. The sequence was also analyzed to determine the expected vari ability in the measurements. In normal pregnancies the average value o f perfusion rate was found to be 176 (standard error = +/-24) ml/100 m g/min, (n = 16, standard deviation = 96 ml/100 mg/min), The expected v ariability in the measured parameters due to signal to noise ratio con siderations alone was calculated to be 71%, For a maximum scanning tim e of 400 s, the optimum sequence for measuring placental perfusion was found to require 8 repetitions at each of 10 inversion times which we re geometrically spaced (given by a(o), a(o)r, a(o)r(2), a(o)r(3),...) , with a(o) = 850 ms, r = 1.073 and TR = 5 s, giving a pixel variabili ty of 38%, Other timing schemes are recommended for measuring perfusio n in other anatomical regions with different values of perfusion rate and longitudinal relaxation time.