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A multiparametric (1H, 23Na, diffusion, flow) anthropomorphic abdominal phantom for multimodal MR and CT imaging
Wiebke Neumann*1, Tanja Uhrig*1, Nadia K. Paschke1, Marius Siegfarth2, Andreas J. Rothfuss2, Gordian Kabelitz1, Khanlian Chung1, Alena-Kathrin Schnurr1, Lothar R. Schad1, Jan L. Stallkamp2,3, and Frank G. Zöllner1

1Computer Assisted Clinical Medicine, Heidelberg University, Mannheim, Germany, 2Fraunhofer Institute for Manufacturing Engineering and Automation, Project Group for Automation in Medicine and Biotechnology, Mannheim, Germany, 3Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany

Synopsis

Anthropomorphic phantoms are essential for the evaluation of image registration algorithms in multimodal imaging, quantification experiments in multinuclear MR imaging, and verification of diffusion and flow measurements. A human-like abdominal phantom incorporating a liver with lesions, a rib cage, vessels, and a lung was developed. Its tissue-mimicking characteristics were evaluated with 1H and 23Na MR and CT imaging as well as functional flow and diffusion MR imaging. The phantom exhibited morphological and functional parameters comparable to corresponding human values. It is suitable for a quantitative evaluation of a clinical workflow ranging from diagnostics to interventional procedures.

Introduction

Image guided interventions are based on real-time imaging. A quantitative evaluation of a clinical workflow, ranging from diagnostics to interventional procedures, is an important step towards clinical implementation. To exploit the benefits of various imaging modalities, the image fusion of data sets with previously acquired images from computed tomography (CT) and magnetic resonance imaging (MRI) is object of recent research. Especially in MRI, this includes morphological and functional imaging providing complementary information about tumor characteristics. Anthropomorphic phantoms can mimic complex anatomy and physiology while providing an essential ground-truth for the verification of quantitative imaging. Image registration algorithms face a variety of challenges, such as different voxel sizes, geometric deformations and changing patient positions between modalities. Here, anthropomorphic phantoms serve as an additional level between geometric phantoms and in vivo studies for the validation of algorithms under practical conditions and enable the design of a feasible imaging protocol for an optimized workflow in terms of time, costs and patients comfort.

Methods

The abdominal phantom was created from several modules that were designed as close to human anatomy as possible (Fig. 1). A rib cage consisting of a spinal section with vertebral bodies and ribs surrounds a liver and lung module. It was 3D printed and made of polylactide (PLA). The lung module was also 3D printed using polystyrene (PS), splitting into the left and right lobe and acts as a geometric stabilizer of the liver. Based on a CT scan, a human liver was contoured and a mold was created. Then, silicone mixed with 20% silicone oil was poured into the mold (2.5 liter) and left to cure. Further, three oval inclusions (each 37ml, 3cm diameter) were incorporated and represented tumors with different morphological and functional characteristics. Inclusion A was made of silicone and 5% CaCO3 to induce variations in the Hounsfield units and a coloring agent (green) for visibility during biopsy. Inclusion B was only made of silicone and a coloring agent (blue). Inclusion C was made of 154 mmol NaCl solution for physiological Na+ concentration and 2% agarose acting as a stabilizer and 0.1 ml Dotarem (0.5 mmol/ml) to reduce relaxivity [1].

MR images were acquired with a 3T whole-body scanner (Magnetom Skyra, Siemens Healthineers, Germany) and a double resonant 1H/23Na transmit receive array (Rapid Biomedical GmbH, Germany). T1 and T2 weighted images were acquired (TSE sequence with TE=[8,17,34,42,51]ms, TI=[25,50,100,200,400,600,800]ms, 1.4mm isometric pixel resolution). Sodium images were acquired by using a radial density adapted 3D UTE sequence [2]. Parameters: TE/TR/FA=0.6ms/30ms/54°, 11000 projections, FoV=(350mm)3, resolution=(6mm)3, TRO=20ms, full sampling, TA=11:00min. Reconstructions of the channels were performed using adaptive combination [3]. Diffusion-weighted (EPI sequence with TR=3700ms and TE=50ms, b-value=[50,200,800]) as well as flow imaging (3D-TWIST sequence: TE/TR/FA=1.44ms/15ms/15°, temporal resolution of 2 s, total acquisition time of 100 s) was performed. For flow measurements, water served as a blood substitute and was pumped through an artificial artery. Contrast agent was injected automatically to simulate a bolus. CT images were acquired with a Somatom Force (Siemens Healthineers, Germany) with kVp=110 kV and a smoothing convolution kernel.

Results

Morphological images (Fig. 2A) were obtained. All components as well as inclusions were clearly distinguishable in 1H MR imaging. The NaCl-based inclusion C was visualized through 23Na (Fig. 3A) and diffusion imaging (Fig. 3B). The flow measurement showed a typical bolus distribution (Fig. 3C) for varying flow velocities. Besides MR imaging, the structures were well detectable in CT imaging (Fig. 2B) and ranged between 62 HU and 123 HU, which was in the range of human soft tissue values [4].

Discussion & Conclusion

A human-like abdominal phantom incorporating a liver with lesions, a rib cage, vessels, and a lung was developed that met the requirements of a simple, cost-effective and reproducible system. Its tissue-mimicking characteristics were evaluated with 1H and 23Na MR and CT imaging as well as functional flow and diffusion MR imaging. The phantom exhibited morphological and functional parameters comparable to corresponding human values. It is suitable for the evaluation of a clinical workflow ranging from diagnostics to interventional procedures. In the future, the phantom setup could be complemented by additional modules focusing on specific research objectives such as tissue-like capillaries for perfusion studies, 23Na MR quantification experiments, inclusion of PET tracers for realistic registration scenarios, and cone beam CT controlled biopsies.

Acknowledgements

This research project is part of the Research Campus M²OLIE and funded by the German Federal Ministry of Education and Research (BMBF) within the Framework “Forschungscampus: public-private partnership for Innovations” under the funding code 13GW0092D.

* shared first authorship

References

[1] Hattori et al. Development of a MRI phantom equivalent to human tissues for 3.0-T MRI. Medical Physics 2013; 40(3):032303

[2] Nagel AM, et al. Sodium MRI using a density-adapted 3D radial acquisition technique. MRM 2009;62:1565-1573.

[3] Walsh DO, et.al. Adaptive reconstruction of phased array MR imagery. Magn Reson Med. 2000;43(5):682-690.

[4] Kalender, W. A. Computertomographie. Publicis Corporation Publ. 2006

Figures

Figure 1: The design of the phantom. A: CAD rendering of the phantom. The liver (red), lung (blue), blood vessels (red & blue) and rib cage (white) are visible. B: Photograph of the manufactured phantom. The filling material was a translucent silicone. A coloring agent (blue) was added to the liver. The inclusions in the liver are not visible in this depiction.

Figure 2: Multi-modal morphological MR and CT image of a transverse slice of the phantom. A: T2 weighted proton MR image (TSE sequence, TR=2500ms, TE=51ms, isometric pixel resolution of 1.4mm, FA=160°). All lesions are visible. Reference probes for sodium imaging are also shown. B: CT image. Only lesion B and C are visible.

Figure 3: Multi-nuclear MR imaging of a transverse slice of the phantom. A: 23Na imaging. Lesion C containing NaCl (red arrow) and reference probes are visible. B: The same lesion is also visible during diffusion imaging (red arrow). C: Graph of contrast agent bolus for varying flow velocities. The artificial artery is indicated in the MR image (red arrow).

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