A preliminary study of real-time imaging for MR guided vascular intervention and image quality assessment

doi:10.21203/rs.3.rs-1956079/v2

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A preliminary study of real-time imaging for MR guided vascular intervention and image quality assessment

https://doi.org/10.21203/rs.3.rs-1956079/v2

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Background

Real-time MRI provides radiation-free alternative to X-ray guided interventions, enables superb tissue imaging without administration of contrast agents. In this study, we proposed an 3T MRI system, and evaluated the image quality of real-time sequences for MR compatible guidewire and balloon.

Methods

MR guided intervention system were setup with 3T MR scanner and MR compatible monitor. Real-time sequences (FLASH and TrueFISP) were optimized and evaluated in the interventional procedure with a 3D printing aortic phantom, MR compatible guidewire and balloon catheter. After the images were obtained, the image quality was evaluated objectively, and subjectively. T tests was used to analyse the results.

Results

The temporal resolution of both sequences were 2 frames/s and spatial resolution was 1.1mm×1.1mm×10mm. For objective assessment, the TrueFISP sequence showed higher SNR and CNR than the FLASH sequence. Image uniformity and geometric distortion did not show significant difference. The subjective scores showed FLASH was higher than TrueFISP, because of moderate artefacts of guidewire and good visualization of balloon catheter.

Conclusions

In conclusion, the study demonstrates the MR real-time imaging, FLAH sequence, is feasible of guiding cardiovascular interventions.

Interventional MRI

real-time imaging sequences

compatible devices

image quality

Endovascular interventional techniques were used for diagnosis and therapy of various diseases, including endovascular angioplasty, stent implantation, intravascular embolization, perfusion, thrombectomy, and transjugular intrahepatic portosystemic shunt (TIPS)[1]. However, current endovascular interventions were mostly performed under X-ray fluoroscopic guidance, where patients and surgeons were exposed to accumulation of radiation[2]. Additionally, fluoroscopy-guided interventional techniques require iodinated contrast agents to visualize blood vessels, and the cumulative application of iodinated contrast media may lead to nephrotoxic effects[3, 4].

In contrast, interventional MRI (iMRI) has significant benefits for image-guided intravascular interventions[5]. MRI provides excellent characteristics of vessel wall and surrounding tissue, multi-parameter imaging, without the use of contrast agents and no ionizing radiation hazards, which is a promising technique in cardiovascular intervention[6]. Real time imaging modality is necessary for image-guided procedure and is key to successfully execute interventional MRI [7]. The fast low angle shot (FLASH) and true fast imaging with steady-state precession (TrueFISP) sequences may provide sufficient temporal and spatial resolution for intervention.

In addition, most commercially available cardiovascular interventional devices, such as catheters, guidewires or implants, are not dedicated designed for iMRI[7]. Accordingly, compatibility of all iMRI devices for this study need to be carefully investigated[8]. Some MR-compatible devices have different imaging effects in different real-time sequences, which need to be studied and compared.

The study was to setup an iMRI system and evaluate the real-time MRI guided intervention using two sequence, FLASH and TrueFISP, as well as MR compatible guidewire and balloon.

2.1. Devices

In this study, an MR-guided intervention system (Fig. 1) was setup, which consisted of a 3.0T MR-scanner (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany) and an MR compatible monitor (Fuqing Medical Technology Co., Ltd, Hefei, Anhui, China).

All experiments were performed using a peripheral balloon catheter (PTA35-6040B, DK MEDTECH, Suzhou, Jiangsu, China) (Fig. 2b) and MR compatible guidewire (EPflex, Dettingen an der Erms, Germany) (Fig. 2c). The balloon catheter was filled with 0.8ml 1% Gadodiamide mixed with normal saline[9]. The guidewire was 0.035in in diameter and 150cm in length, which consisted of a high-strength para-aramid synthetic fiber core, surrounded by a bending-resistant high performance polymer and a hydrophilic coating, the markers in the guidewire were discretely mosaicked for navigation.

2.2. In vitro study

To test the feasibility of the devices and sequences, in vitro experiments were performed. A 3D printed aorta phantom printed by 3D printer (SPS250j, Shaanxi Hengtong Intelligent Machine Co., Ltd, Shaanxi, China) (Fig. 2a) was used in experiments. The aorta phantom was made of transparent resin with CTA data from a normal adult. We put the “aorta” in a plastic box and filled with normal saline.

The intervention process was performed in saline filled 3D printed aortic phantom. Firstly, the guidewire was placed in the thoracic “aorta”. Then, the balloon catheter was advanced in the thoracic aorta. After achieving the targeted vessel, the guidewire was withdrawal[10, 11]. All the procedure were under the guidance of real time MR sequences: FLASH and TrueFISP. The parameters of the real-time imaging sequence were: (1) FLASH, TR/TE = 6.8/2.65 ms, FA = 12º, FOV = 360 × 147mm, slice thickness = 10 mm, phase partial Fourier = 6/8, bandwidth = 230Hz/Px, (2) TrueFISP, TR/TE = 321.35/1.88 ms, FA = 49º, FOV = 360 × 144mm, slice thickness = 10 mm, bandwidth = 1116Hz/Px[12, 13].

We acquired the phantom imaging with commercial 18-channel phase array coil placed superior to the “aorta”. Due to the length of the aorta, two identical phase array coils were used to cover entire aorta and the “aorta” was divided into three segments for imaging during the intervention.

2.3. Data analysis

After all the procedures were finished, the real-time images were evaluated objectively and subjectively. For the objective scoring, the image signal-to-noise ratio (SNR), contrast noise ratio (CNR), geometric distortion, and image uniformity of the two sequences were measured[14]. The signal intensity and noise at different positions of the image were measured and their mean value was taken to calculate the SNR (Equations.1).

The CNR between gadolinium-filled balloon and normal saline in the phantom was measured and calculated (Equations.2).

Regarding image uniformity, most areas of multiple images were measured, and the mean value of their uniformity (U) was calculated (Equations.3).

In terms of geometric distortion, the actual length of the gadolinium balloon and its length in the image was measured, and then the geometric distortion (GD) was calculated (Equations.4)

For subjective assessment, Likert scoring was applied to assess the image sharpness, distortion, and artifacts of all sequences[15]. The images quality was scored subjectively by two experienced radiologists with seventeen years practice (radiologist one) and twenty years practice (radiologist two). The evaluation criteria were as follows: 1 points = poor image sharpness with serious distortion or artifacts, 2 points = medium image sharpness with medium distortion or artifact, 3 points = good image sharpness with slight distortion or artifacts, 4 points = excellent image sharpness with no distortion or artifacts. The results of the objective and subjective analyses were compared between the two sequences using independent sample T-test by SPSS Statistics 26 (IBM).

The temporal resolution was 2 frames/s and spatial resolution was 1.1×1.1×10mm for the both sequences. The artifacts of the guidewire were moderate for FLASH, which did not affect the visualization of both the guidewire and balloon catheter, and the balloon was high signal (Fig. 3a-c). For TrueFISP, the artifacts of the guidewire were severe, the balloon was low signal and blurred by the artifacts of the guidewire (Fig. 3d-f).

The SNR of the two sequences were statistically analysis, as well as CNR. The SNR of TrueFISP was statistically higher than that of FLASH (P < 0.05) as well as CNR (P < 0.05). There was no statistical difference for image uniformity (p = 0.133) as well as geometric distortion (p = 0.717) between the two sequences. The image uniformity of two sequences were both excellent and geometric distortion were mild (Table 1).

Two radiologists reached agreement on subjective scoring. The subjective scores of the two sequences did show significant difference. The score of FLASH was significance higher than that of TrueFISP (p < 0.05).

Table 1

Results of objective evaluation
	FLASH	TrueFISP	P
SNR	102 ± 16	618 ± 137	p < 0.05
CNR	73.5 ± 18	162.2 ± 46	p < 0.05
Image uniformity	80%±6༅	74%±4༅	p = 0.133
Geometric distortion	2%±1༅	2%±1༅	p = 0.717

MRI is a proved diagnostic technique with a growing number of applications in interventional procedures and therapy[16, 17]. The interventions performed by MR include MR-guided biopsy, thermotherapy[18, 19], cryoablation, brachy therapy, and high-intensity focused ultrasound[20] as well as localized delivery of cells, drugs and contrast agents[21, 22]. An important application of iMRI is MR guidance vascular interventions[23].

To achieve MR-guided vascular interventions, many aspects including real-time imaging sequences and MR-compatible devices have been investigated. The imaging sequences used in iMRI are somewhat different from those used in diagnostic MRI, because the real-time imaging sequences require high temporal and high spatial resolution simultaneously. However, the high temporal resolution and high spatial resolution are contradictory. The compromise among temporal resolution, spatial resolution, and SNR should be achieved. Therefore, many real-time imaging sequences are customize developed[4]. Most of the sequences are gradient echo with short repetition time (TR). Metal guidewire induced heating injury should be avoid by adjusting the Gradient echo (GRE) sequence parameters. Campbell-Washburn et al. optimized the low SAR sequence by adjusting the flip angle and TR so that the temperature increase is limited within 0.1°C during MR scan.[24]. In addition, real-time sequences with different k-space filling methods are also compared. Comparison of image quality by Cartesian k-space filled TrueFISP and radial k-space filled TrueFISP revealed that radial k-space filling generally is of less artifacts with high temporal resolution[25]. Real-time MRI enables guidance at 10 frames per second, comparable to current conventional X-ray (7.5-10fps) guidance[26]. In addition to optimizing the sequences, dedicated workstation is also developed. Some studies reported advanced visualization platform in a separated workstation enabling real-time visualization of multiplane images and 3D graphics[15, 27]. This platform requires the installation of a workstation, which is not available for all MR scanner. To achieve real-time imaging on conventional MRI under limited conditions, the two real-time imaging sequences (FLASH and TrueFISP) were optimized, tested and compared in this study.

The development of MR-compatible interventional devices is another important aspect of implementing iMRI in addition to real-time imaging sequences. Several studies tested different balloon catheters filled with diluted gadolinium to improve the success rate of the interventional procedures. However, without the MR-compatible guidewire, the successful rate of interventional procedures is not satisfied[28]. MR-compatible guidewires are currently in development, such as active guidewires with temperature probes[29], and passive guidewire including segmented guidewires[30], and fiberglass-cored guidewires[31]. Through MR-compatible guidewires, successful rate of interventional procedures was improved and complex interventions can be performed. In this study, the fiberglass-core passive guidwire and MR compatible balloon catheter was used. The guidewire consisted of high -intensity fiber cores, hydrophilic polymer tips and iron magnetic markers, which can be visualized distinctly. The balloon catheter was made of nylon with two metal markers at both ends for positioning under X-ray. In order to visualize the balloon catheter under MR guidance, diluted gadolinium was filled.

In this study, the results showed that the two real-time imaging sequences had sufficient SNR and CNR, the uniformity of the image was excellent, and the geometric distortion was also within the acceptable range. The objective evaluation showed that the TrueFISP was better than FLASH, but subjective evaluation, the FLASH was better than the TrueFISP. Although the SNR and CNR of TrueFISP were higher than those of FLASH, the artifact from guidewire markers in TrueFISP were severe during real-time imaging, which affected the balloon positioning. In addition, the gadolinium-filled balloon showed low signal in TrueFISP, which made balloon positioning difficult. In FLASH, the guidewire marker induced artifacts were moderate and the gadolinium-filled balloon showed high signal, which is more obvious facilitated to MR guided interventions (Fig. 3). Even if, the SNR and CNR of FLASH is lower than that of TrueFISP, the visualization of guidewire and balloon catheter is feasible of MR real-time imaging.

MR-guided interventions have many advantages, but real-time imaging sequences and MR compatible devices are in developing, which has not been widely used in clinical practice. Current limitations of iMRI applications include heating of the instruments, MR compatibility of monitoring equipment, cost of build up an iMRI suite. In the future, with the development of devices and the decline of cost, iMRI may be used widely.

Limitations

The limitation of this study was that only intravascular intervention experiments have been carried out on the aorta phantom, with no animal experiments confirmation. Taking into account the effects of breathing and heartbeat in the living body, the real-time imaging sequence needs to be further optimized.

TIPS	transjugular intrahepatic portosystemic shunt
iMRI	interventional magnetic resonance imaging
FLASH	fast low angle shot
TrueFISP	true fast imaging with steady-state precession
TR	repetition time
TE	ehco time
FA	flip angle
FOV	Field of View
SNR	signal-to-noise ratio
CNR	contrast noise ratio
U	uniformity
GD	geometric distortion
GRE	Gradient echo

Ethics approval and consent to participate

The study was conducted in accordance with the Declaration of Helsinki. The institutional review board of Taiyuan Central Hospital of Shanxi Medical University approved this study (2022013-1) and waived the need for informed consent of participant because of the retrospective design.

Consent for publication

Not applicable

Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing Interests

The authors declare that they have no competing interests

Funding

This work was supported by Shanxi Provincial Department of Science and Technology (Grant numbers 202103021224003 and 20210302124402) and Shanxi Provincial Health Commission (Grant numbers 2020XM05 and 2018109). Author Yanfeng Meng, Yueyou Peng has received research support from Shanxi Provincial Department of Science and Technology band Shanxi Provincial Health Commission.

Author Contributions

Conceptualization, Yanfeng Meng and Kunkun Liu; Resources, Yueyou Peng; Investigation, Tingting Liu, Ningning Song and Tianfeng Shi; Software, Yueluan Jiang and Yunhui Kang; Writing – original draft, Kunkun Liu; Supervision, Weibin Dai and Yanfeng Meng; Writing – review & editing, Yanfeng Meng and Kunkun Liu

Acknowledgments

The authors acknowledge EPflex Feinwerktechnik GmbH for MR compatible guidewire providing.

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No competing interests reported.

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A preliminary study of real-time imaging for MR guided vascular intervention and image quality assessment

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Abstract

Background

Methods

Results

Conclusions

Figures

1. Introduction

2. Materials And Methods

2.1. Devices

2.2. In vitro study

2.3. Data analysis

3. Results

4. Discussion

Abbreviations

Declarations

References

Additional Declarations

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