Positive contrast spiral imaging
Here, we have demonstrated spiral GRE imaging for substantially reduced heating of a commercial nitinol guidewire in the scanner. In addition, using positive contrast imaging with real-time color overlay, we have demonstrated improved passive visualization of a commercial nitinol guidewire. Existing nitinol guidewires have a range of mechanical properties (stiffness, curvature, torquability, surface coating, geometry) to address procedural needs, and it would be beneficial to use these existing products for CMR-guided procedures. Thus, the ability to safely and effectively use a commercial nitinol guidewire in the CMR environment could accelerate the clinical translation of CMR-guided cardiovascular catheterization procedures.
Positive contrast spiral imaging was achieved using through-slice dephasing. Positive contrast was observed at a range of dephasing gradient moments, indicating that the choice of desphasing moment does not have to be precisely tuned [
13]. Here, for left-heart catheterization experiments, the slice-refocusing gradient was inverted and reduced the zeroth moment to 25 %. This value was chosen to eliminate most background signal, while maintaining signal the entire guidewire length. Separate anatomical and guidewire images were acquired in an interleaved fashion, reducing the achievable frame rate to 6.25 frames/s using the sequence parameters presented here. This frame rate is equivalent to what is used for CMR-guided right heart catheterization in patients with accelerated bSSFP imaging [
20]. Future work could investigate parallel imaging for improved frame rate or alternate acquisition schemes with the device image updating more frequently than the anatomical image. Reasonable image quality was also observed for anatomical imaging.
Positive contrast has been used for both interventional device imaging and cell tracking using iron oxide nanoparticles [
8‐
13,
21‐
24]. Positive contrast has been achieved using dephasing in the slice-encoding and frequency-encoding directions, using on-resonance signal suppression, exploiting the off-resonance properties of bSSFP and with susceptibility gradient mapping in post-processing. Dephasing-based methods are the most conducive to real-time imaging. To our knowledge, this is the first time that positive contrast methods have been combined with spiral imaging, with the goal of reducing RF-induced heating. Here, because we are using a GRE sequence [
10,
12], all positive contrast signal is a result of local magnetic field gradients counteracting the dephasing gradient.
Real-time color overlay
The real-time color overlay used here permits intuitive visualization of the guidewire, mimicking active device visualization. We have demonstrated similar visualization methods with a dual echo bSSFP sequence previously [
13]. The positive contrast device imaging is non-specific, and other regions of magnetic field inhomogeneity (e.g. air-tissue interfaces) also produce bright signal. Image processing can isolate guidewire signal reliably in most configurations. The air-tissue interface of the lung at the level of the aortic arch is closely neighboring the vasculature, and therefore creates the most challenging region to isolate guidewire signal. Also, the tapered tip of the guidewire is problematic, as the positive contrast signal is reduced at the guidewire tip. Thus, the color overlay is least reliable as the guidewire tip curves around the aortic arch. Additional color overlay artifacts result from background positive contrast signal matching the large/elongated structure criteria. These image processing artifacts mainly appear outside of the vasculature and therefore are not distracting to interventionists performing the procedure.
Furthermore, the guidewire can move out of plane, reducing reliability of color overlay. A thick slice of 12 mm was used for left-heart catheterization to avoid the guidewire moving out of plane during imaging.
Guidewire heating
Heating tests were performed in an acrylic gel phantom, where heat dissipation is minimal, in order to generate worst-case scenario temperature increases. Furthermore, due to increased insulation, heating was found to be maximal when the guidewire was used in combination with a catheter with the tips co-localized. Although this configuration would not be used clinically for an entire procedure, it is plausible that the guidewire and catheter tips would coincide at some point during navigation, and thus it was used to assess worst-case scenario heating in vivo.
The safety improvements enabled by spiral GRE are clearly illustrated from the phantom experiments. In the gel phantom, unsafe levels of heating (50 °C ) were observed using Cartesian bSSFP with standard parameters. In direct comparison, using spiral GRE imaging, temperature increases remained <0.7 °C. Cartesian GRE with 10° flip angle also reduces heating substantially (<1.1 °C), however we have focused on spiral imaging because of the high frame-rate and SNR-efficiency.
Heating regulations have been established for other medical devices. International standard for MRI safety IEC 60601-2-33:2008 [
25] sets a body temperature threshold of 39 °C during non-invasive imaging. Additionally, IEC 60601–1:2005 [
26] section 11.1.2.2 covers invasive devices and sets a body temperature threshold of 43 °C for continuous exposure and proposes “Where 41 °C is not exceeded, no justification is required”. In compliance with this regulation, marketed intracameral ultrasound probes (e.g. Siemens Acuson Acunav 8 Fr intracardiac ultrasound catheter [
27]) will suspend operation if the temperature reaches 43 °C.
In a 60 kg swine, we observed a localized increase of 4 °C above body temperature (local temperature ~41 °C) using standard real-time Cartesian bSSFP imaging, emphasizing the potential safety concerns of metallic guidewire imaging in humans. In comparison, no heating was observed using spiral GRE imaging, indicating the potential safety benefit of moving to RF-efficient sequences.
Heating is very sensitive to a wide variety of factors such as off-isocenter position, guidewire length, insertion length and curvature [
3‐
6]. Here, only a small subset of possible conditions were tested, and further extensive heating tests are required to validate the safety of spiral GRE under all possible configurations used during catheterization procedures.
Limitations
This method is limited by the non-specific nature of the positive contrast signal. This results in less reliable color overlay of the device signal due to errors in image processing. Future work will investigate alternative image processing methods to improve the specific isolation of the guidewire signal. Furthermore, because there is no unique guidewire tip signal, it is impossible to determine if the tip of the guidewire is contained within any given image. Future work will investigate unique tip markers for passive guidewires, as well as automated slice repositioning to maintain the guidewire tip within the imaging frame [
28]. In general, gradient echo imaging has reduced blood-myocardium contrast compared to the bSSFP sequence typically used for real-time imaging [
29]. Spiral imaging is also susceptible to off-resonance blurring and image distortion, which has been corrected here using a real-time framework [
17]. Despite the reduced blood-myocardium contrast and possible spiral imaging artifacts, the inherent safety of spiral GRE imaging makes this tradeoff worthwhile.