A Comparison between the UTE and PETRA Pulse Sequences for Fluorine-19 MRI at 3 Tesla
Roberto Colotti1, Jean Delacoste1, Giulia Ginami1, Maxime Pellegrin2, Tobias Kober1,3,4, Yutaka Natsuaki5, David Grodzki6, Ulrich Flögel7, Matthias Stuber1,8, and Ruud B. van Heeswijk1

1Department of Radiology, University Hospital (CHUV) and University of Lausanne (UNIL), Lausanne, Switzerland, 2Division of Angiology, University Hospital of Lausanne (CHUV), Lausanne, Switzerland, 3Advanced Clinical Imaging Technology, Siemens Healthcare IM BM PI, Lausanne, Switzerland, 4LTS5, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 5Siemens Medical Solutions, NAM USA DI MR COLLAB WE, Los Angeles, CA, United States, 6Siemens Healthcare GmbH, HC DI MR R&D PLH, Erlangen, Germany, 7Department of Cardiovascular Physiology, Heinrich Heine University, Düsseldorf, Germany, 8Center for Biomedical Imaging (CIBM), Lausanne, Switzerland

Synopsis

Fluorine-19 (19F) MRI of perfluorocarbon emulsions (PFCs) with multiresonant spectra is challenging due to destructive phase interference that leads to short T2 relaxation times (<10 ms). Pulse sequences with very short echo times (≤ 100 µs) can be used to overcome this challenge. In this study, in vitro and in vivo 19F MRI obtained with both UTE and PETRA were acquired and quantitatively compared.

Introduction

Fluorine-19 (19F) MRI of perfluorocarbon emulsions (PFCs) is increasingly used for inflammation studies due to its high specificity and non-toxicity [1]. The PFC perfluorooctyl bromide (PFOB) has already been used in clinical trials as a blood volume expander [2] and oxygen carrier [3]. However, PFOB has a complex multiresonant spectrum (Fig.1), in which the different resonances J-couple to one another, which causes their apparent T2 relaxation time to shorten significantly. This imaging hurdle can be overcome by acquiring the signal before destructive spin interference occurs, for example with ultra-short echo (UTE) and pointwise encoding time reduction with radial acquisition (PETRA) pulse sequences. In this study, we compared the signal to noise ratio (SNR) and the image quality of a UTE pulse sequence that consists of a 3D radial trajectory with balanced steady-state free precession (bSSFP) acquisition [4], with PETRA, which consists of a gradient echo (GRE) 3D radial half-projection outer k-space combined with a pointwise-acquired Cartesian inner k-space.

Methods

For the phantom study, a 50mL tube was filled with 2% agar and with five 1mL syringes with 2% agar gel and PFOB [5] at different concentrations (0-5.9 M). All experiments were performed on a 3T clinical system (Magnetom Prisma, Siemens Healthcare, Germany). A 35-mm-diameter volume transmit/receive coil tuneable on both 19F/1H resonances (Rapid Biomedical, Rimpar, Germany) was used for excitation and signal detection. For 19F MRI, the excitation frequency was set to the CF2(δ-ε) resonance, while the 3D radial bSSFP-UTE pulse sequence was used with the following parameters: field of view (FOV)=160×160×160mm3, repetition time TR=1.94ms, echo time TE=50µs, pixel bandwidth BW=1202Hz/pixel, RF excitation angle 30°, isotropic voxel volume=1mm3, radial views=125952, acquisition time=4min10s. A prototype 3D GRE PETRA pulse sequence was tested with the following parameters: TR=3.29ms, TE=40µs, BW=1838Hz/pixel, RF excitation angle 6°, effective voxel volume=0.1 mm3, reconstructed voxel volume=1 mm3, radial views=126000, acquisition time=7min14s. 1H images were acquired with the same respective pulse sequences and resolution as the 19F images. The SNR of the 19F images of the different PFOB tubes was then determined and normalized to the voxel volume and acquisition time. Here, the SNR of a region of interest (ROI) was defined as the ratio between the average signal in the ROI and the standard deviation in a background ROI of at least 20x20 pixels outside the object of interest. In both cases, a linear fit of the 19F concentration versus the SNR was performed, and the quality of the fit was compared. For a quantitative assessment of image quality, the edge blurring (EB) [6] of the tube with highest concentration was calculated. For in vivo validation, three 13-week-old apolipoprotein-E-knockout (ApoE-/-) C57BL/6 mice (well-established model for vascular inflammation) were intravenously injected with 300µL of PFOB. Seven days after the injection, MRI was performed. After 1H anatomical reference scans, 19F UTE and PETRA images centered on the abdomen were acquired with the same parameters as the phantom scans. 1H and 19F images were acquired with the same respective pulse sequences and resolution. All animal studies were approved by the local animal ethics committee. The normalized SNR from 19F images was compared between the two pulse sequences.

Results

For 19F images, the normalized SNR versus concentration linear fit for the UTE pulse sequence was characterized by a 1.7 times higher slope (and thus sensitivity) than that from the PETRA pulse sequence (Fig.3). The EB was 2.50±0.12 pixels for PETRA and 2.91±0.16 pixels for UTE pulse sequence (p<0.001). In the phantoms, UTE showed more ringing artifacts than PETRA for both 1H and 19F images (Fig.2). The goodness of the linear fit was R2=0.95 for both UTE and PETRA. 19F signal was detected in vivo in the spleen of all animals with both pulse sequences (Fig.4), and the normalized SNR was slightly higher for PETRA when compared to the UTE pulse sequence (31.2±4.9 vs. 29.6±5.3).

Discussion and Conclusions

Both the UTE and PETRA pulse sequences have ultra-short echo times that helped suppress the destructive dephasing arising from the coupled PFOB resonances. In vitro, the sensitivity was 1.7 times higher for UTE, while PETRA allowed for a higher pixel bandwidth and this resulted in higher image quality as evidenced by the lower edge blurring. However, in vivo, PETRA demonstrated slightly higher normalized SNR, which might be due to better robustness to motion of the animal and to different physiological environment of PFOB.

Acknowledgements

Swiss National Science Foundation Grant PZ00P3_154719 (RBvH),

Fondation Pierre Mercier (RBvH)

References

[1] Ruiz-Cabello et al., NMR Biomed 24(2):114-29, 2011

[2] Riess et al., Artif Cells Blood Substit Immobil Biotechnol 34(6):567-580, 2006

[3] Riess et al., Chem Rev 101(9):2792-2919, 2001

[4] Goette et al., Magn Reson Med 74(2):537-543, 2014

[5] Jacoby et al., NMR Biomed 27(3):261-271, 2014

[6] Kording et al., Magn Reson Med 74(5):1257-1265, 2014.

Figures

Figure1. The molecular structure and MR spectrum of perfluorooctyl bromide (PFOB). Two single resonances (CF2Br(α) and CF3(θ)) can be observed in addition to the J-coupled CF2 spectral group that is used for signal generation. The horizontal scale is centered on the CF2(η) peak.

Figure2. 1H PETRA and 19F UTE and PETRA images of the PFOB phantom. a) The five syringes with 2% agar and PFOB at different concentrations can be observed in the larger tube. b) 19F UTE is characterized by ringing artifacts. c) 19F PETRA has higher image quality. The 1H and 19F images were acquired with the same respective pulse sequences and resolution.

Figure3. A comparison of the SNR in the PFOB phantom with multiple concentrations. The SNR as a function of 19F concentration shows that UTE is characterized by 1.7 times higher sensitivity than PETRA. The similarly high R2 value for both linear relationships confirms the fit quality.

Figure4. Axial in vivo 1H and 19F images of the abdomen in ApoE-/- mice. The 1H and 19F images were acquired with the same respective pulse sequences and resolution. The left panel shows the 1H anatomic images of the mouse abdomen with the water tubes, the middle panel shows the unprocessed 19F images and in the right panel the 19F image is overlaid in color on the grayscale anatomic 1H images. The PETRA image has lower SNR due to its smaller effective voxel size.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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