Author + information
- Published online December 20, 2017.
- Margo Batie, MS,
- Sarah Bitant, MS,
- Janette F. Strasburger, MD,
- Vishal Shah, PhD,
- Orang Alem, PhD and
- Ronald T. Wakai, PhD∗ ()
- ↵∗Department of Medical Physics, Wisconsin Institutes for Medical Research, 1111 Highland Avenue, Madison, Wisconsin 53705-2275
Fetal magnetocardiography (fMCG) is an emerging technology that has provided invaluable insight into the mechanisms of fetal arrhythmia. Its efficacy for diagnosis and management of serious fetal arrhythmia has been acknowledged in the recent American Heart Association Statement on Diagnosis and Treatment of Fetal Cardiac Disease (1).
fMCG is based on the principle that bioelectric currents generate surface magnetic fields, as well as surface potentials, which are proportional to the net current. The main technical requirement for fMCG sensors is a magnetic field resolution of ≤10 fT/(Hz)1/2 (fT= femtotesla) over a bandwidth of 0 to 100 Hz. The number of sensors needed depends on the desired coverage of the maternal surface and the level of interference suppression required of the signal processing.
Using a 2-shell, magnetically shielded room in a typical hospital environment, a minimum of 5 to 10 sensors is required.
A major barrier to clinical adoption of fMCG is the high cost and complexity of superconducting quantum interference device (SQUID) technology (2). Recently, however, a cheaper, more practical type of magnetometer, known as an optically pumped magnetometer (OPM) (3), has become available. In this study, we compared the quality of fMCG recordings made using SQUID with those made with the OPM system.
The subjects were 15 healthy women, 8 with uncomplicated pregnancies and 7 with pregnancies complicated by fetal arrhythmia or a high risk of fetal arrhythmia.
The fMCG recordings were acquired within a 2-shell, magnetically shielded room, using a U.S. Food and Drug (FDA)-approved SQUID magnetometer system (model 624 Biomagnetometer; Tristan Technologies, San Diego, California) (Figure 1A) and an array of OPM sensors (QuSpin Zero field magnetometer; QuSpin, Inc., Louisville, Colorado) (Figure 1B). The OPM sensors are modular. A sensor array was formed by producing a holder by 3D-printing to accommodate up to 8 sensors, arranged in a 3×3 square grid with 3.81-cm grid spacing, with the center location occupied by a support post. The number of OPM sensors deployed increased from 3, initially, to 8, as additional sensors were acquired. At least 10 min of data were recorded with each device, moving the sensor at least once during the session.
Signal processing was used to remove maternal and environmental interference. The quality of the SQUID and OPM recordings were compared by computing the signal-to-noise ratio (SNR) of the rhythm strips, defined as the peak-to-peak amplitude of the fetal QRS complex divided by the root-mean-square noise value.
The quality of the SQUID recordings was good to excellent in all subjects. The SNR ranged from 12.8 to 48.2. Compared to the SQUID data, the OPM data showed significantly lower SNR for the first 11 fetuses studied, with SNR ratios (SNROPM/SNRSQUID) ranging from 0.13 to 0.75. Most of these subjects were studied using 6 or fewer OPM sensors, and noise from the data acquisition system degraded the SNR by a factor of approximately 2. For the last 4 subjects, however, the SNR ratios were much closer to unity, ranging from 0.74 to 1.04. These subjects were studied using 8 OPM sensors and a higher performance data acquisition system.
Five fetuses showed serious sustained arrhythmias. The first fetus had sinus bradycardia with intermittent atrial flutter (not shown). The second fetus was referred with a diagnosis of atrioventricular (AV) block. The fMCG revealed the critical finding that the AV block was secondary to severe QTc prolongation (QTc >700 ms) (Figures 2A and 2B). The fetus was treated with oral magnesium but died suddenly 10 days after the fMCG study. The mother of the third fetus had long QT syndrome (KCNQ1 mutation). The fMCG at 29 weeks’ gestation showed severe QT prolongation suggestive of long QT syndrome (Figures 2C and 2D), which was confirmed postnatally (Figure 2E). The fourth fetus showed a predominant rhythm of ventricular bigeminy (Figures 2F and 2G) with occasional short runs of ventricular tachycardia and a virtual absence of normal sinus rhythm. The fifth fetus had a low atrial rhythm which resulted in a low heart rate and a short PR interval (Figures 2I and 2J). The diagnosis of low atrial rhythm prompted the referring physician to re-examine the fetus for heterotaxy, which was subsequently confirmed. Two other fetuses showed frequent ectopy due to premature ventricular and premature atrial contractions (not shown).
The main finding of this study is that the OPM system can detect fetal rhythm abnormalities with efficacy similar to that of the FDA-approved SQUID magnetometer. Despite the modest number of subjects, we also demonstrated the ability of the OPMs to detect abnormal repolarization, a critical and unique capability of fMCG. Efforts are underway to operate OPMs in small, person-sized magnetic shields, which are less expensive and portable. We are optimistic that the lower cost and practicality of OPMs will enable fMCG to be much more widely available in the near future.
Please note: This study was supported by U.S. National Institutes of Health, grants R44 HL114182 and R01 HL63174 and by Small Business Innovative Research grant R44 HL114182 to QuSpin, Inc. Drs. Shah and Alem are employees of QuSpin Inc. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.
- 2017 American College of Cardiology Foundation