Sugar free tissue-mimicking MRI phantoms for improved signal-to-noise ratio
Carlotta Ianniello1,2, Ryan Brown1, Martijn Cloos3, Qi Duan4, Jerzy Walczyk3, Graham Wiggins3, Daniel K Sodickson2,3, and Riccardo Lattanzi2,3

1Radiology, Center for Advanced Imaging Innovation and Research (CAI2R) and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States, 2The Sackler Institute of Graduate Biomedical Science, New York University School of Medicine, New York, NY, United States, 3Center for Advanced Imaging Innovation and Research (CAI2R) and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States, 4Laboratory of Functional and Molecular Imaging, NINDS, National Institutes of Health, Bethesda, MD, United States

Synopsis

We investigated Polyvinylpyrrolidone (PVP) as an alternative to sugar to control relative permittivity in tissue-mimicking MR phantoms. We constructed a two-compartment phantom filled with water solutions of PVP and NaCl, the latter used to control conductivity. A lower amount of PVP than sugar is required, allowing low permittivity materials to be realized. While signal decreases rapidly in sugar-based phantoms, PVP materials have long T2*/T2, making PVP-based phantoms suitable for the validation of MR-based electrical properties mapping techniques that rely on high SNR of signal and B1+ maps. PVP solutions are relatively inexpensive, easy to mix and do not require preservatives.

TARGET AUDIENCE

Researchers interested in the construction of tissue-mimicking MR phantoms.

PURPOSE

Tissue-mimicking magnetic resonance (MR) phantoms with known electrical properties (EP) can be useful tools for many applications including SAR calibration[1] and to validate EP mapping techniques[2][3]. Sugar and NaCl are inexpensive and accessible ingredients that are commonly used in water solutions to control relative permittivity (εr) and conductivity (σ) of phantoms, respectively[4]. However, the large quantities of sugar needed to reduce εr have the undesirable effects of introducing unwanted phase gradients and shortening the T2*, which can result in impractically low SNR[5]. This can be especially troublesome for EP mapping techniques that rely on local derivatives of signal and B1+ maps[2][3]. In this work, a water-soluble polymer Polyvinylpyrrolidone (PVP), previously introduced for diffusion phantoms[6][7], is investigated as substitute for sugar to control εr in a two-compartment MR phantom.

METHODS

Tissue-mimicking materials. We used NaCl (Sodium Chloride – anhydrous, free-flowing, Redi-DriTM, ACS reagent, ≥ 99%, Sigma-Aldrich®) to control σ, PVP (Polyvinylpyrrolidone - average mol wt 40,000, Sigma-Aldrich®) to vary εr and distilled water as solvent. The target EP for the two phantom compartments at 300 MHz were εr = 53.5 and 69.3, σ = 0.6 and 0.9 S/m, to mimic liver (Material A) and heart (Material B) tissue, respectively[8]. Water, PVP and NaCl (Table 1) were mixed together at room temperature and stirred in order to facilitate complete dissolution of the PVP powder. Then, we let the solution stand for about 48 hours. EP measurements. The EP of the two materials were measured with a dielectric probe (Agilent 85070E slim-probe kit) in the frequency range 200-400 MHz (801 data points). The measurements were performed at the temperature of our 7T MR scanner room (16°C). Phantom construction. We designed and 3D printed an empty cylinder (diameter=12.5 cm, length=15 cm) composed of two identical halves that were glued together with a 0.5 mm plastic layer in the middle to insulate the two compartments (Fig. 1). The two materials were poured through two ports simultaneously to avoid deformation of the separation layer. Experiments. We acquired gradient-echo (GRE) MR images of the phantom on a 7T full-body scanner (Magnetom, Siemens Medical Solutions) equipped with an 8-channel pTx system, using 64x64x64 matrix size, FA=10, TE/TR = 2.5/50 ms, 0.3x0.3x0.3 mm3 voxel size, and an 8-element Tx-Rx array[9]. We calculated B1+ maps using the Actual Flip-angle Imaging (AFI) technique[10]. We measured T1 using an inversion-recovery pulse sequence (TI=25-3600 ms) and acquired a GRE slice (TR=100 ms, 1.0x1.0x5.0 mm3 voxel size) repeatedly for varying TE (3-60 ms) to estimate transverse relaxation time, for both the PVP-based phantom and a previously constructed sugar-based phantom with similar EP[11].

RESULTS

The measured EP for both materials are plotted vs. frequency in Figure 2. At 300 MHz, we measured εr=55.3 and σ=0.45 S/m, and εr=74,9 and σ=0.89 S/m, for Material A and B, respectively. Figure 3 shows that signal decays rapidly for the sugar phantom as TE increases and is completely dephased for TE=20 ms, whereas only approximately 50% of the signal is lost for the PVP phantom. The short T2* of the sugar-based phantom can be explained by the fact that a large amount of sugar was used (~1,5kg per one liter of dH2O to achieve εr~50) and sugar creates a multi-peak spectra[12] near the proton resonance that interferes with the signal. To confirm that the PVP-phantom provided suitable SNR for EP mapping techniques, we tested it with the Local Maxwell Tomography (LMT) technique[3]. Figure 4 shows the reconstructed maps for εr (left) and σ (right). Mean values, measured in the central region of the two compartments to avoid edge artifacts, were in relatively good agreement with the true EP: εr,LMT=57 σLMT=0.59 S/m for Material A, εr,LMT=78 σLMT=1.04 S/m for Material B.

DISCUSSION

We built a two-compartment PVP-based MR phantom with EP that mimic human tissues. The SNR was higher than for a sugar-based phantom with similar EP, indicating that PVP allows low relative permittivity phantoms without compromising the SNR. PVP is relatively inexpensive, easy to dissolve in water, does not require any preservative and smaller amounts are needed to achieve low values of εr compared to sugar (PVP[g]~100g vs. Sugar[g]~520g to achieve εr~69). Future work will include the characterization of a general recipe for PVP phantoms for ranges of EP, with corresponding measures of T1, T2 and T2*. We also plan to construct a four-compartments phantom with EP of healthy and cancerous tissues for validation of EP mapping techniques.

Acknowledgements

NSF CAREER 1453675, NIH P41 EB017183, NIH R01 EB002568

References

[1] “Specific Absorption Rate (SAR) Estimation for Cellular Phone” Association of Radio Industries and Businesses (ARIB), January 27, 1998.

[2] U. Katscher, T. Voigt, C. Findeklee, P. Vernickel, K. Nehrke, and O. Dossel, “Determination of electrical conductivity and local SAR via B1 mapping” IEEE Transactions on Medical Imaging, vol. 28, pp. 1365–1374, 2009.

[3] D. K. Sodickson, L. Alon, C. M. Deniz et al., “Local Maxwell tomography using transmit-receive coil arrays for contact-free mapping of tissue electrical properties and determination of absolute RF phase” in Proceedings of the 20th Scientific Meeting of the International Society of Magnetic Resonance in Medicine (ISMRM ’12), vol. 387, 2012.

[4] Q. Duan, J. H. Duyn, N. Gudino, J. A. de Zwart , P. van Gelderen, D. K. Sodickson, R. Brown “Characterization of a dielectric phantom for high-field magnetic resonance imaging applications” Med. Phys. 41 (10), October 2014.

[5] C. Leuze “Construction and testing of a realistic head phantom for assessment of radiofrequency power deposition in MRI”, September 2008.

[6] C. Pierpaoli , J. Sarlls, U. Nevo, P. J. Basser, and F. Horkay, “Polyvinylpyrrolidone (PVP) water solutions as isotropic phantoms for diffusion MRI studies”, Proc. Intl. Soc. Mag. Reson. Med. 17 (2009)

[7] J. E. Sarlls, C. Pierpaoli, Q. Duan, W. Devine and H. Merkle, “Calibrated Diffusion Phantom for 7T MRI”, Proceedings of the 22th Scientific Meeting of the International Society of Magnetic Resonance in Medicine (ISMRM ’14)

[8] http://niremf.ifac.cnr.it/tissprop/ - An Internet resource for the calculation of the dielectric properties of body tissues in the frequency range 10 Hz - 100 GHz.

[9] G. C. Wiggins, B. Zhang, G. Chen, and D. Sodickson, “A Highly Decoupled 8 Channel Transmit-Receive Loop Array for 7T with Diverse B1 Profiles”, Proceedings of the 20th Scientific Meeting of the International Society of Magnetic Resonance in Medicine (ISMRM), 2012, p.309.

[10] V. L. Yarnykh “Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field”, Magnetic Resonance in Medicine Volume 57, Issue 1, pages 192–200, January 2007.

[11] C. Ianniello, R. Brown, V. Angellotti, B. Grivo, D. K. Sodickson, R. Massa, R. Lattanzi “Design and construction of a tissue-mimicking phantom to validate electrical properties mapping techniques based on magnetic resonance” 1st URSI Atlantic Radio Science Conference (URSI AT-RASC), 18-22 May 2015, ExpoMeloneras Convention Centre, Gran Canaria.

[12] http://www.hmdb.ca/spectra/nmr_one_d/1293

Figures

Table 1: The phantom recipes.

Figure 1: The phantom (diameter=12.5 cm, length=15 cm). Two half-cylinder plastic shells (A) of equal internal volume (B) were glued together with a 0.5 mm plastic layer between them (C). Two holes, closed with screw tops, were made to fill each half separately.

Figure 2: Measured EP of material A and B in the frequency range 200-400 MHz (801 data points). At 300 MHz (7T) εr=55.3 and σ=0.45 S/m for material A and εr=74.9 and σ=0.89 S/m for material B. The error bars represent the standard error across 30 measurements.

Figure 3: GRE images of an axial slice in the sugar-based phantom (top row) and PVP-based phantom (bottom row) with similar electrical properties, for increasing TE values. Normalized average signal in an ROI for each compartment vs. TE (rightmost column) shows that signal rapidly decays for the sugar-based phantom.

Figure 4: Electrical properties of the PVP-based phantom reconstructed using the Local Maxwell Tomography (LMT) technique: permittivity (left) and conductivity (right). The mean and the standard deviation of the EP in artifact-free regions of the two compartments (blue ROI’s) were similar to the expected values.



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