Performance of an aerosol jet-deposited wireless resonant marker: in vitro temperature measurements and in vivo visualization
Caroline D. Jordan1, Bradford R. H. Thorne1, Arjun Wadhwa2, Eugene Ozhinsky1, Vincent Fratello2, Sravani Kondapavulur1,3,4, Aaron D. Losey1, Teri Moore1, Carol Stillson1, Colin Yee1, Ronald D. Watkins5, Greig C. Scott6, Alastair J. Martin1, Xiaoliang Zhang1, Mark W. Wilson1, and Steven W. Hetts1

1Radiology and Biomedical Imaging, University of California, San Francisco, San Francisco, CA, United States, 2Quest Integrated, Kent, WA, United States, 3Graduate Program in Bioengineering, UC Berkeley-UCSF, Berkeley, CA, United States, 4Bioengineering and Therapeutic Sciences, University of California, Berkeley, Berkeley, CA, United States, 5Radiology, Stanford University, Stanford, CA, United States, 6Electrical Engineering, Stanford University, Stanford, CA, United States


Endovascular catheter-based procedures under MRI can be challenging as standard fabrication methods for markers are rigid and bulky, and new microfabrication methods need more analysis on their tracking characteristics. We analyzed a wireless resonant circuit tracking marker that was printed using aerosol jet deposition on a polymer catheter. In a phantom, we acquired bSSFP sequences and a B1+ map, and measured temperature using probes and MR thermometry. In vivo, in the carotid arteries, we acquired GRE sequences and a B1+ map. The marker demonstrated good signal, with minimal temperature increases, suggesting that these markers have good tracking characteristics.


Endovascular embolectomy under X-ray guidance is an effective treatment for patients suffering an acute ischemic stroke (Fig. 1a); however, fewer than 10% of patients receive such treatments due to the limited time window1. MRI could extend this window since diffusion weighted imaging (DWI), in combination with perfusion imaging, is the gold standard in determining if brain tissue is still viable. Interventional MRI has not been adapted, and there are a lack of tools. One method to track markers uses a wireless RLC circuit,2 (Fig. 1b) which inductively couples with the magnetic flux from the RF, resulting in induced current, and a local magnetic field3 (Fig. 1c). Recent work implemented an inductor geometry of a double helix4,5 (Fig. 1d), using aerosol jet deposition6 (Fig. 1e, f), a new additive manufacturing process7; however, characterization of heating and in vivo performance is limited. We demonstrate analysis of the tracking characteristics of an aerosol jet-deposited resonant marker by measuring temperature in vitro, and testing the markers in a preclinical embolectomy scenario in vivo.


In vitro experiment

An aerosol jet-deposited marker catheter6 was placed in a rectangular water phantom doped with CuSO4 and the phantom was oriented either parallel or perpendicular to B0. Images were acquired in both orientations at 3.0T (Discovery MR 750w, GE Healthcare) using an 8-channel cardiac coil. A balanced steady state free precession (bSSFP) sequence, and a 5° Bloch-Siegert B1+ map8,9 were acquired (Table 1). An ROI was manually drawn around the entire marker to measure the mean signal, and then the ratio with the nearby background water signal was calculated for the relative amplification factor. Image processing was performed in MATLAB 2016a (MathWorks Inc., Natick, MA).

Temperature measurements

A Hydroxyethyl cellulose gel was prepared10, and four temperature probes (Luxtron, LumaSense Technologies, Inc. Santa Clara, CA) were placed in the gel (Fig. 3a). Temperature was recorded every second during a fast spin echo (FSE) sequence, as well as two minutes before and after the scan, for a total scan time of 15:02 min (Table 1). The baseline temperature of each probe at room temperature was subtracted for comparison. We then acquired a set of MR thermometry images in an oblique scan plan, parallel to the catheter (FSPGR, Tacq = 16 sec), interleaved with the periods of FSE acquisition with the number of averages of 1, 2, and 3 (Table 1) for a total scan time of 4:37 min. Temperature maps were generated offline with proton resonance frequency shift (PRF) technique.

In vivo experiment

The study protocol was approved by the university’s committee. A catheter was placed into each common carotid artery of a single swine (43.2 kg, female), using a 30 × 30 cm2 flat panel C-arm X-ray system guidance (Cios Alpha, Siemens Healthineers, Munich, Germany). A gradient recalled echo (GRE) sequence was acquired for visualization, and a 5° B1+ map using the double angle method11 (Table 1), with the same coil. An ROI was manually drawn around the entire marker to measure mean signal, and the relative amplification factor was calculated.


In vitro results

The low flip angle bSSFP sequences (5° and 15°) show the signal amplification of the markers relative to the background signal, in comparison with the high flip angle (90°) sequence (Fig. 2a). For the 5° B1+ map, the relative amplification factor measured 1.63 in the parallel orientation, and 1.77 in the perpendicular orientation (Fig. 2b).

Temperature results

No appreciable heating was measured by the temperature probes (Fig. 3b). MR thermometry images confirmed this by demonstrating no detectable heating (Fig. 3c).

In vivo results

The coronal GRE sequence shows especially good signal and contrast for the catheter on the left side (Fig. 4a), and the two catheter placements can be seen on X-ray (Fig. 4b). The 10° B1+ map had an amplification factor of 5.3 (Fig. 4c). Using a lower flip angle of 5° demonstrates the low background signal, and the signal amplification of the marker (Fig. 4d). As the flip angle increases to 20°, there is more signal in the surrounding tissue, and the contrast decreases (Fig. 4e). With a higher flip angle of 45°, there is over-flipping of the marker signal, causing a decreased signal, while the background tissue signal increases (Fig. 4f).


B1+ mapping, imaging, and temperature measurement experiments demonstrated that the wireless resonant circuit showed sufficient local signal amplification, while demonstrating no appreciable heating. We demonstrated good tracking characteristics of a complete LC circuit micro-fabricated using aerosol deposition, both in vitro and in vivo at 3T, allowing for a low profile guide catheter that can be tracked under MRI for endovascular neurointerventional applications.


The authors gratefully acknowledge Andrew Chu and Dave Barry at Penumbra, Inc., for in-kind provision of catheters, Steven Conolly, PhD and Lucas Carvajal for engineering discussions, Labonny Biswas, PhD and Graham Wright, PhD for discussions about real-time imaging, and Jay F. Yu, Galen Reed, PhD, and R. Reeve Ingle, PhD for assistance with RTHawk. Funding supported included NIH grants UL1 TR001872, R01 EB012031, R21 EB020283, TL1 TR001871, T32 GM007618.


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Table 1. Acquisition Parameters for the Datasets in this Study

Figure 1. An MRI-visible catheter could be used to perform an embolectomy under MRI. The wireless resonant marker is used as a marker for a) guidance to the clot, and the marker can be modeled as a b) parallel LC circuit, which c) inductively couples with the RF transmit coil. d) The double helix inductor and the twin-lead transmission line capacitor are shown using red (dielectric ink) and blue (conductive ink) traces. e) A realization example is shown, along with the f) full length catheter, and the marker print is shown in comparison with a hand-wound marker, demonstrating the low profile.

Figure 2. Phantom results at 3T demonstrate good signal amplification. The aerosol jet deposition prints and bSSFP sequences of the double helix resonant marker are shown in a water phantom a) parallel to and perpendicular to B0. The sequences with low flip angles show the surrounding signal amplification of the circuits, which can be compared with a 90° flip angle, showing signal drop-out due to over-flipping. b) The 5° bSSFP sequence, and 5° B1+ map are shown for the full-length marker.

Figure 3. Temperature measurements and MR thermometry in a phantom at 3T demonstrate minimal heating. a) The set up of the temperature probes in the gel is shown. b) Compared to the reference probe, no substantial heating was observed. c) This was confirmed by MR thermometry maps after three FSE sequences, increasing in time.

Figure 4. The wireless resonant markers demonstrate good signal in an in vivo preclinical scenario at 3T. The markers placed in the carotid arteries of a single swine are shown in a coronal scan plane under a) MRI and b) X-ray. c) In a sagittal plane, the B1+ map quantifies signal amplification. d) Using a lower flip angle (5°) demonstrates low background signal and marker signal amplification. e) As the flip angle increases (20°), there is more signal in the surrounding tissue. f) With a higher flip angle (45°), there is over-flipping near the marker, causing a decreased signal, while the background signal increases.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)