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Page 32 of 43 Wang et al. Soft Sci 2024;4:41 https://dx.doi.org/10.20517/ss.2024.53

effects of prolonged implantation in living organisms have been inadequately addressed in existing studies.
Therefore, further research is essential to evaluate these aspects comprehensively.

Interventional MRI resonant markers
MRI provides 3D visual imaging of the anatomy and surrounding tissues of the human body. The technique
exploits the magnetization and spin alignment of hydrogen nuclei in a strong magnetic field, where an
applied RF pulse alters their spin, and the resulting emitted signal is detected and processed to generate an
image [116,230] . The operation process is shown in Figure 15A. Compared to other medical imaging
[231]
modalities, such as X-ray imaging, MRI offers several advantages, including the absence of ionizing
radiation, superior soft tissue contrast, and the capability of producing unique three-dimensional images.

In interventional procedures, MRI markers are typically placed on the tips of surgical instruments or
catheters to monitor and visualize their positions. Based on their operational principles, MRI markers are
generally classified into three modalities: passive, active, and resonant. Passive methods involve coating or
embedding paramagnetic particles into the tip of the device. For instance, a gadolinium-filled balloon MRI
[232]
catheter has been utilized for procedures such as transfemoral right heart catheterization and
[233]
transcatheter cavopulmonary anastomosis and shunt [Figure 15B]. In addition to paramagnetic particles,
special braided metal structures have been employed for passive MRI labeling . While this approach is
[234]
simple and efficient, it is subject to uncertainties, such as tissue inhomogeneity within the body affecting
position determination.

Active MRI markers use coaxial cables to connect an external magnetic resonance (MR) scanner to a
receiving coil at the instrument tip, which can be powered either by the scanner or a direct current (DC)
power supply . Saikus et al. developed an active MRI probe utilizing coil winding and demonstrated its
[235]
application in jugular vein access in healthy pigs [Figure 15C]. To address the need for high
[236]
customization and improve manufacturing efficiency, Yildirim et al. designed a series of active MRI
markers using inkjet printing technology [39,85] . This active method generates a locally enhanced magnetic
field, providing excellent visualization. However, the use of coaxial cables can occupy valuable cavity space
and may lead to induced RF heating. To address this challenge, an acousto-optic labeling-based MRI
tracking technique has been developed . This approach integrates piezoelectric crystals with fiber Bragg
[117]
gratings to mitigate the risk of RF-induced heating [Figure 15D], offering a novel solution for employing
active MRI markers in catheter detection.

In contrast, MRI marking (also known as semi-active) methods based on wireless resonant circuits ensure
good imaging without the risk of RF heating [13,99,117] . In this approach, the RF circuit, comprising an RF coil
and a capacitor, is tuned to resonate at the Larmor frequency [237,238] . Ellersiek et al. developed a flexible MRI
marker comprising six winding coils and adjustable capacitors on a PI film with a thickness of less than
50 μm using a MEMS method [Figure 15E]. Additionally, a low-profile RF resonance marker was
[239]
fabricated directly on a guiding catheter surface using physical vapor deposition and electroplating
processes . In addition, several novel sensing principles for interventional MRI markers have been applied
[47]
to in vivo real-time imaging. Bilgin et al. designed a self-resonant RF sensor capable of remote in-situ
[45]
temperature sensing during real-time interventional MRI . The effectiveness of this sensor was
experimentally demonstrated by manually operating the catheter tip inside an ex vivo porcine kidney
[Figure 15F]. In conclusion, numerous studies have demonstrated the potential of resonance-based MRI
markers for providing effective and safe imaging guidance during minimally invasive surgical interventions.