![]() ( A) Used Doppler ultrasound (DUS) system with the ultrasound transducer (1) connected to the transmission line containing the cable traps, which are covered by a surrounding isolating sleeve (2). Four cable traps were required for each cable with a distance of 30 cm for 1.5T and a distance of 20 cm at 3T. As the DUS device was used at 1.5T and 3T, two different cable setups were used. In order to ensure patient safety and sufficient MRI compatibility, the cable was decoupled with respect to the B 1 field using cable traps. The connection between the transducer and the DUS electronic was realized using a 7 m long cable shielded with aluminum. The piezoelectric elements were made of non-magnetic lead zirconate titanate with a diameter of 1 cm. All components of the transducer were replaced by non-magnetic materials or materials with a low magnetic permeability. Ultrasound pulses of 1 MHz were transmitted with a repetition frequency of 3.2 kHz resulting in an acoustic intensity of 4.6 mW/cm 2 at the ultrasound transducer (HP 15245A Hewlett Packard, Palo Alto, CA, USA). ![]() The DUS device was battery powered by lithium battery of 10.4 V with dimensions of 20 × 10 × 15 cm 3. The electronics of the DUS device was a custom build and was placed inside the MRI room during data acquisition. The DUS gating setup is schematically shown in Fig. The purpose of this work was to develop and evaluate an MRI conditional and portable DUS device for 1.5T and 3T that can be operated within the MRI room during image acquisition with regard on compatibility, functionality, and reliability in order to enable fetal cardiac gating for cine MRI. Second, the hardware is normally not designed for the hazardous MRI environment, e.g., regarding safety concerns by placing the transducer and connecting cable directly under the Receive (RX) coil. First, the software is not designed to detect every single heartbeat of the fetus, which is implicitly necessary for cardiac cine MRI. However, using a commercially available CTG is accompanied by two major drawbacks. The method was applied to gate the image acquisition for adult cardiac MRI 19, 20 and for fetal MRI in a sheep model 21 using a standard cardiotocograph (CTG). 16– 18 Thus, DUS represents an ideal method to synchronize the MRI data acquisition with the fetal cardiac cycle. 15 Doppler ultrasound is capable of recording the fetal heart rate in real-time and is in theory not influenced by the electromagnetic field of the MRI. A well-established technique to acquire the fetal heart rate is based on Doppler ultrasound (DUS). ![]() In order to employ clinically proven cardiac imaging methods in conjunction with an immediate image reconstruction, a direct gating method is necessary. 12– 14 Although post-processing methods are capable of producing a good image quality, their time-consuming image reconstruction in combination with fetal movement represents a crucial disadvantage and prevents the possibility to adapt imaging planes in real time. 11 However, the ECG cannot be applied for fetal cardiac MRI and several methods were developed to assign the acquired image data to the correct cardiac cycle using retrospective image reconstruction based on post-processing techniques. 3 One of the main challenges for high-quality fetal cardiac images using conventional cine reconstruction methods is accurate synchronization of the cardiac cycle with the MRI data acquisition, 10 usually achieved using the spatial information of an electrocardiogram (ECG). 8 Fetal cardiovascular magnetic resonance (CMR) may be especially helpful for the detection of duct-dependent lesions, such as coarctation of the aorta 9 and to recognize CHD in late gestation. 3 Fetal cardiac MRI may provide a valuable adjunct to fetal echocardiography as it provides the ability to image the fetal heart with excellent tissue contrast in arbitrary planes independent of the above-mentioned limitations of fetal echocardiography. 6įetal MRI is increasingly used as a second-line imaging tool for prenatal evaluation of the fetal brain 7 and other organs including the fetal heart. 5 Moreover, discordances of up to 29% between pre- and post-natal diagnoses of CHD and a limited ability to measure blood flow using echocardiography encourages the development of new techniques to improve prenatal cardiovascular imaging. Prenatal detection of CHD using echocardiography varies widely from 13% to 82%. 1, 2 However, fetal echocardiography is occasionally limited by the acoustic window, fetal position, maternal adipose tissue, gestational age, abdominal wall scars, and the level of training of the operator. The imaging modality of choice for prenatal diagnosis of congenital heart disease (CHD) is due to its ease of use, high availability, and high diagnostic accuracy fetal echocardiography.
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