## 1 Introduction  
The average cost for a magnetic resonance imaging (MRI) scan has been suggested to be in excess of $2500 [1]. In comparison to the average cost of an ultrasound at $263 [2], from a cost perspective alone it is unsurprising that the routine monitoring of placental health is undertaken throughout a pregnancy using ultrasound. Routine ultrasound, however, reveals little about many diseases of the placenta until they have progressed. MRI is generally thought to be safe after the end of the first trimester. As early as 1998 [3], it had been reported that the nuclear magnetic resonance (NMR) relaxation parameter, known as the transverse relaxation time of the placenta decreases through pregnancy. Furthermore, for cases of pre-eclampsia this value fell below the 'healthy' trend line with other later studies confirming these findings [4; 5; 6]. Ultrasound scans have become routine during pregnancy in many countries around the world, as they are affordable in most hospitals and can be purchased in some quantity in an average obstetrics department. If a simple add-on to the ultrasound could be developed to provide a measure of the transverse relaxation time of the placenta, then it should be possible to provide an early prediction of mothers at high risk of pre-eclampsia who could be more closely monitored.  
Unilateral magnetic resonance systems, often termed inside-out, usually use an array of permanent magnets to generate a uniform magnetic field, satisfying the requirements of a magnetic resonance experiment, some distance away from a surface. The earliest unilateral systems were designed for well logging, and consequently much of the technology was not published in the academic literature [7]. The introduction of unilateral systems to the mainstream came in the late 1990's with the development of the NMR-MoUSE [8; 9]. This bench top system had a magnet array that produced a uniform magnetic field parallel to, and a few mm away from, the surface of the device. Paired with a radio frequency (RF) surface coil, MR is performed in the small region where an RF field and static magnetic fields overlap approximately perpendicularly. This is known as the sensitive volume.  
The NMR-MoUSE has been a commercial product for nearly two decades and has evolved into a suite of instruments offering different distances of sensitive regions above the surface from 2 mm with 5 uM resolution for the PM2 up to 25 mm away with 100 uM resolution for the PM25 [10]. Medical applications of this unilateral system have been appearing in the literature, including profiling of human skin [11], articular cartilage [12], bone volume-to-total volume ratio [13], T\({}_{1}\)-based mammographic density [14] and T\({}_{2}\)-based mammographic density [15].  
In our work, we report the development of a low-field unilateral system, which we call the NMR CAPIBarA-Clinical Assessment of Patients Implemented with Bar magnet Arrays (because it is like a very large NMR-MoUSE), which operates at a magnetic field of only 18 mT. The instrument is capable of measuring transverse relaxation times over the range of values presented in the literature for the developing placenta and at a distance sufficiently far away to reach an anterior placenta for an average pregnancy.