The exciting alternative option of using Doppler ultrasound for MR cardiac gating allows to synchronize MR image acquisition with the fetal cardiac rhythm in a way that was previously not possible. Without this synchronization, motion artifacts typically render cardiac MR images useless for diagnosis.
Typically, cardiac MR utilizes either electrocardiography (ECG) or vectorcardiography (VCG) techniques to synchronize MR acquisition with the heart rhythm to “freeze” heart motion. Both methods require the attachment of electrodes to the patient’s skin, which makes the approach unsuitable for fetal cardiac imaging.
The recently introduced Doppler ultrasound device for gating, smart-sync, was developed by northh medical GmbH in Germany, in collaboration with Philips and clinical collaborators. This device can detect the heartbeat during MRI acquisition, and sends a gating signal to the scanner. This allows for the synchronization of MR data acquisition with the fetal heartbeat, resulting in fetal cardiac MR images of diagnostic quality. It allows for the use of standard cardiac sequences, so that acquired images instantly appear on the screen, allowing for immediate evaluation of slice positioning and quality – an important feature that allows for adjustments should the fetus change position.
High image quality can be obtained for morphological images as well as 2D and 4D blood flow measurements, in a normal clinical setting. Together with the acquisition speed of the Philips Compressed SENSE method, fast and well-directed imaging of the fetal heart and flow can be performed, opening up a new avenue for congenital heart disease diagnosis before birth.
The ultrasound-based trigger signal is reliable and easy to use due to its wireless connection. The signal also remains undistorted by the MR field, unlike ECG and VCG methods, and eliminates the need to attach electrodes to the skin. The positioning of the smart-sync probe is quite simple, only requiring the proper placement above the fetal heart.
Typically, prenatal cardiac examinations are performed with ultrasound echocardiography. However, this technique can be limited in cases of late gestational age, due to calcification of the bone, fetal position, or a lack of amniotic fluid. In addition, echocardiography is highly observer-dependent and with no alternative methods for assessing inconclusive results.
However, since accurate prenatal diagnosis is closely associated with optimal postnatal outcomes, a first-time right diagnosis is highly desirable, especially in identifying cases of congenital heart defects. Therefore, the use of cardiac MR with smart-sync represents a long-awaited alternative for fetal ECG.
These sequences allow for immediate assessment of acquired images:
Smart-sync gating can also be used in cardiac MR examinations for adults and children. It offers an undisturbed trigger signal, even at high fields, and eliminates the need for the unwieldly positioning of the adhesive electrodes on the patient’s chest. Conversely, conventional ECG/VCG is patient-dependent, is prone to electric field errors, and susceptible to arrhythmia, which particularly affects image acquisition in diastole, where the required delay can only be predicted and not measured, resulting in inaccurate predictions of arrhythmic heart beats.
In essence, smart-sync solves these issues by measuring actual blood flow and cardiac motion via ultrasound. This allows for the acquisition of high quality cardiac images, as well as use of high temporal and spatial resolution, with notable benefits for quantification and longitudinal studies. Trigger signals based on ultrasound can diminish the number of incorrect signals, and circumvent the need for repeating scans due to motion artifacts.
These 1.5T and 3.0T ExamCards include basic MR sequences for studying the morphology and function of a fetal heart, based on cardiac gating via the smart-sync Doppler ultrasound device. The ExamCards may be used to help users familiarize themselves with the technique, and can be easily integrated into clinical procedures.
Results from case studies are not predictive of results in other cases. Results in other case may vary.
