# Development of a cost-effective 3D-printed MRI phantom for enhanced teaching of system performance and image quality concepts

Habeeb Yusuff, Pierre-Emmanuel Zorn, Céline Giraudeau, Benoit Wach, Philippe Choquet, Simon Chatelin, Jean-Philippe Dillenseger

► To cite this version:

Habeeb Yusuff, Pierre-Emmanuel Zorn, Céline Giraudeau, Benoit Wach, Philippe Choquet, et al. Development of a cost-effective 3D-printed MRI phantom for enhanced teaching of system performance and image quality concepts. Magnetic Resonance Materials in Physics, Biology and Medicine, 2024, 10.1007/s10334-024-01217-z. http://dx.doi.org/10.1007/s10334-024-01217-z

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# Development of a cost-effective 3D-printed MRI phantom for enhanced teaching of system performance and image quality concepts

Habeeb Yusuff¹ · Pierre-Emmanuel Zorn² · Céline Giraudeau³ · Benoît Wach¹ · Philippe Choquet¹,²,⁴,⁵ · Simon Chatelin¹ · Jean-Philippe Dillenseger¹,²,⁴,⁵

Received: 19 August 2024 / Revised: 8 November 2024 / Accepted: 21 November 2024
© The Author(s), under exclusive licence to European Society for Magnetic Resonance in Medicine and Biology (ESMRMB) 2024

## Abstract

**Purposes** This research highlights the need for affordable phantoms for MRI education. Current options are either expensive or limited. A phantom, easy to manufacture and distribute, is proposed to demonstrate various pedagogical concepts, aiding students in understanding MRI image quality concepts.

**Methods** We designed a cylindrical MRI phantom that comprises sections that can be filled with chosen liquids and gels. The dimensions were chosen to fit most consumer-grade 3D printers, facilitating widespread dissemination. It includes five modular sections for evaluating spatial resolution, geometrical accuracy, slice thickness accuracy, homogeneity, and contrast.

**Results** The modular cylindrical MRI phantom was successfully fabricated. Each section of the phantom was tested to ensure it met the specified pedagogical needs. The spatial resolution section provided clear images for evaluating fine details. The geometrical accuracy section allowed for precise measurement of distortions. The slice thickness accuracy section confirmed the consistency of slice thickness across different MRI sequences. The homogeneity section demonstrated uniform signal distribution, and the contrast section effectively displayed varying contrast levels.

**Conclusions** This modular MRI phantom offers a cost-effective tool for educational purposes in MRI. Its design enables educators to demonstrate multiple pedagogical scenarios with a single object. The phantom's compatibility with consumergrade 3D printers and its modularity makes it accessible and adaptable to various educational settings. Future work could explore further customization and enhancement of the phantom to cover additional educational needs. This tool represents a significant step toward improving MRI education and training by providing a practical, hands-on learning experience.

**Keywords** MRI · Phantom · 3D printing · Education · Radiology

## Introduction

Numerous phantoms are available for conducting quality controls or sequence evaluations in MRI [1–17]. Quality assurance (QA) in MRI is increasingly imperative, particularly concerning distortion assessments, notably when images are employed for surgical [9] or radiotherapy treatment planning [12]. Furthermore, the development of acquisitions that generate quantitative data requires the use of QA phantoms. A number of commercially available phantoms, such as the ACR phantoms [1–3], provide consensus on device and sequence qualification standards. Nonetheless, commercial phantoms often incur significant costs and are limited in availability [1–3, 6]. In the realm of research, numerous laboratories develop their own phantoms to address specific questions related to their research activities [4–8]. These laboratory-developed phantoms vary in size, design, and are less accessible due to the need for specialized equipment and professional fabrication methods (e.g., laser cutter, and drill). There are also low-cost design phantoms made from commonly available objects (e.g., LEGO™ and syringes) [13, 14] or objects printed in 3D [15–17], which currently offer numerous opportunities. Teaching concepts of MR system performance and image quality is a complex art, particularly given the diverse backgrounds of students (engineers, physicists, radiographers, physicians, etc.) who possess varying levels of knowledge (e.g., radiologists have a clinical approach, radiographers a practical one, and physicists a more fundamental approach). We believe that it is essential to convey to the new generation of students that an MRI is not merely a device for producing simple images but increasingly serves as a tool for extracting quantitative measurements from acquired data.

To teach these complex concepts, appropriate tools are necessary. A recognized pedagogical approach involves supplementing theoretical teachings with dedicated practical exercises on systems. Therefore, access to clinical devices for teaching is a condition that should be prioritized. However, access to these environments remains limited and depends on local possibilities and organizations. Even if access to an MRI machine is available for practical sessions, constructing a
pedagogical session is not straightforward. In fact, not all organizations have a wide range of phantoms to offer various pedagogical scenarios explaining fundamental concepts (e.g., spatial resolution, contrast studies, deformations, artifacts, and antenna qualities).

The objective of this work is to propose a dedicated phantom that enables educators to offer various pedagogical demonstrations with a single object. This phantom aims to satisfy the following specifications:

- Economical (low-cost)

- Simple to fabricate and disseminate (open source)

- User-friendly and modular

- Ability to elicit multiple pedagogical scenarios

To the best of our knowledge, no existing tools align with these specifications. This phantom should not be construed as a quality control instrument but rather as a pedagogical aid for the MRI teaching community at large. It will streamline the orchestration of pedagogical scenarios, affording students a visual comprehension of the qualitative aspects inherent in MRI acquisitions.

# Materials and methods

## Phantom design

### Global design and approach

Utilizing SolidWorks software (2022 SP0.0, Dassault Systèmes, France), we have engineered a modular cylindrical MRI phantom comprising multiple sections, each capable of accommodating a chosen liquid or gel. To ensure compatibility with various coil sizes, the dimensions were tailored to fit common knee and head coils. In addition, consideration was given to conforming to the dimensions compatible with most consumer-grade 3D printer surfaces (typically ranging from 20 x 20 to 30 x 30 mm).

This phantom comprises five modular sections, each evaluating spatial resolution, geometrical accuracy, slice thickness accuracy, homogeneity, and contrasts (Fig. 1). All sections are filled with liquid Dota Gd³⁺ solution (0.1% w/w Gd³⁺ in water) to generate high signal (in T1<sub>w</sub> and T2<sub>w</sub> both) and as a consequence high contrast with the solid printed parts. External markers (ridges) aid in centering the phantom within the system (B₀) or the coil. All CAD files have been exported in Stereo-lithography (.stl) format and are proposed as open-source files for the community (see supplemental data).

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Global view of the phantom. CAD (a), photography (b)</figcaption></figure>
**Spatial resolution section**

Designed to demonstrate and assess spatial resolution, this section features a support structure capable of accommodating multiple inserts with concentric alternations of known spatial frequencies (Fig. 2). Seven resolutions are provided with corresponding single-line thicknesses:

- Module #10: 1.0 mm alternations (5.0 lines pairs per centimeter, lp/cm)

- Module #9: 0.9 mm alternations (5.56 lp/cm)

- Module #8: 0.8 mm alternations (6.25 lp/cm)

- Module #7: 0.7 mm alternations (7.14 lp/cm)

- Module #6: 0.6 mm alternations (8.33 lp/cm)

- Module #5: 0.5 mm alternations (10.0 lp/cm)

- Module #4: 0.4 mm alternations (12.5 lp/cm)

Resolution values are visibly marked in the center of the modules on MRI slices (e.g., '10' for 1.0 mm alternations, '4' for 0.4 mm alternations) (Fig. 2a). The aim of this section is to show that spatial resolution in MRI is not necessarily isotropic (according to each phase or frequency encoding axis direction); and this is why we have proposed a concentric line design. This section could also illustrate that some options allowing access to smaller pixels (e.g., zero-filling process) do not quantitatively improve spatial resolution. These are important points to teach students (see results Section Spatial resolution).

<figure><img src="image_3.png" /><figcaption>**Fig. 2** Spatial resolution section: resolution inserts (a) and support (b)</figcaption></figure>
## Distortion and geometrical accuracy section

The geometric accuracy section (Fig. 3a, b) assesses how accurately the image represents lengths within the imaged subject. This involves measuring lengths on the images between easily identifiable locations in the phantom and comparing these measurements to the known values of those lengths. This section comprises a grid of 11 rows and 11 columns, forming squares with sides of 10 mm. This odd-numbered grid has an origin corresponding to the center of the phantom (in the x, y axes) and follows the rounded shape of the cylinder.

The most common causes of failure in this test could be:

- Miscalibration of one or more gradients. A miscalibrated gradient causes its associated image dimension (x, y, or z) to appear longer or shorter than it actually is.

- Use of too low acquisition bandwidth. It is common practice on some scanners and at some facilities to reduce acquisition bandwidth to increase the signal-to-noise ratio. This can be pushed to the point where normal inhomogeneities in B0 manifest as spatial distortions in the image. On most scanners, the default bandwidth for T1 acquisitions is set high enough to avoid this problem.

- High B0 inhomogeneities. Such inhomogeneities could be caused by improper adjustment of the gradient offsets, improper adjustment of passive or active magnet shims, or a ferromagnetic object such as a hair clip lodged in the magnet bore.

## The slice thickness section

The slice thickness accuracy test assesses how accurately a slice of specified thickness is achieved. The prescribed slice thickness is compared with the measured slice thickness by measuring the lengths of two pairs of ramps. The two pairs of ramps (12° and 25°) are crossed (Fig. 3c, d): one ramp has a negative slope and the other a positive slope with respect to the slice plane. As a result, the signal ramps will appear larger in the slice image than the actual slice thickness. To calculate the slice thickness, the distances D of the signal ramps must be multiplied by a factor tanθ (D*tanθ) where θ is the ramp angle, i.e., a factor 0.466 and 0.5 for the 25° and 12° ramp, respectively. If the phantom is tilted in the rightleft direction, one ramp will appear longer than the other. The use of crossed ramps allows for the correction of errors introduced by right-left tilt, and the slice thickness formula provided in the next section accounts for this. The signal ramp distance is extracted from the plot profile (Fig. 4a).

<figure><img src="image_4.png" /><figcaption>**Fig. 3** CAD of the grid (a), slice thickness (c), homogeneity (e) and contrasts (g) sections. Pictures of the grid (b), slice thickness (d), homogeneity (f) and contrasts (h) sections</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 4** MRI of the spatial resolution section on 1.5 T and 0.31 T systems using spin echo T1-weighted sequences. The visualization of the 0.4 mm lines insert (blue) depends on the acquisition matrix. White arrowheads illustrate situations where zero-filling methods impact spatial resolution in the phase encoding direction (red arrows)</figcaption></figure>

The choice of two angles (12° and 25°) provides double verification of the measurements. In addition, the 12° angle is suitable for small matrices as it allows for greater sampling of the ramp signal. To guide the user (e.g., student), the formula for calculating the slice thickness for each ramp (12° and 25°) is inscribed directly on the phantom, and directly visible in the MR slice (Fig. 3d).

## Homogeneity section (Fig. 3e, f)

The image intensity uniformity section (Fig. 3e, f) evaluates the uniformity of image intensity over a large, liquid-only region of the phantom located near the center of the imaged volume, and consequently, near the center of the coil. Coils designed for clinical use exhibit relatively uniform spatial sensitivity near their center when loaded typically with human body parts (e.g., knee, head). A lack of image intensity uniformity indicates deficiencies in B0 or B1 (RF pulse) homogeneity, non-uniformity due to the multi-element coil design, or, in cases of significant non-uniformity, a defective coil. Other elements may be the source of inhomogeneities and can be illustrated with this phantom (eddy currents effects, radiofrequency, as well as GRE imaging especially bSSFP at low-field strength versus high field strength, etc.).
It is up to the teacher to choose which aspect he/she wishes to discuss during these practical sessions.

Qualitative assessment of image homogeneity can be achieved by different methods. The method used with ACR phantoms [1–3] can be performed by calculating the percent integral uniformity (PIU). This method, employed in the ACR phantom, can also be applied to our phantom, although it will not be detailed in this study.

## Contrasts section

The contrast section (Fig. 3g, h) can be used for the evaluation of the MRI scanner's capability to discern materials (tissues) with varying levels of contrast. This section comprises a central element, a cylinder with a 26 mm inner diameter, divided into four portions. Primarily, this central element is designed to be filled with gels to demonstrate chemical displacement. In addition, the section includes ten peripheral cylinders (15 mm inner diameter), numbered from 1 to 10, which can be filled with gels possessing known properties (e.g., gels with known T1 and T2 relaxation times). The selection of gels depends on the pedagogical requirements of the user. Numerous studies detail the fabrication and relaxometric properties of specific gels [18–26]. In our study (Fig. 3g, h), we utilized 2% agar-agar to create an aqueous medium and 70% PolyVinyl Chloride Plastisol (PVCp) to simulate a fatty component (from a chemical shift perspective). The production process of PVCp gels is well described [25]. Consequently, in this instance, our phantom facilitated the illustration of chemical shift artifacts and fat-suppression methods (Fig. 3g, h).3D-printing process

## FDM printing

STL models were printed using PETG 1.75 mm filaments on an FDM printer (K1max, Creality 3D Technology Co., Shenzhen, China) from Prusament PETG (Prusa Polymers, Prague, Czech Republic). G-code files, used to control the FDM 3D printer, were generated using Creality-print slicer (v5.3.8.6984, Creality 3D Technology Co., Shenzhen, China), and the printing parameters are outlined in Table 1. All 3D-printing processes were followed by post-processing activities which involves simple remover of supports using pliers and knives. The dimensions of PETG elements were verified using a clinical multislice CT scanner (Drive, Siemens Healthineers, Forchheim, Germany). The CT scanner undergoes regular quality control checks and is calibrated to minimize geometric distortions. The acquisition was performed at 140 kV, reconstructed with a bone filter (Ur70) with 0.4 slice thickness / 0.2 mm increment, and pixel dimensions of 0.23 x 0.23 mm².

## SLA printing

Spatial resolution elements STL models were printed on a SLA printer (Halot-One, Creality 3D technology Co., Shenzhen, China) with a UV curable resin (Creality Standard Rigid blue-resin Plus, Creality 3D technology Co., Shenzhen, China). We choose this technology because this type of 3D printer can achieve high printing resolutions (<0.1 mm) that are not achievable with FDM 3D printers. Printed files (.cx3d format) were programmed on Halot-Box slicer (v3.5.5, Creality 3D technology Co., Shenzhen, China), and the printing parameters are listed in Table 1.

To ensure spatial frequency values of the different spatial resolution elements, we decided to scan these parts with a nano-CT (RX Solutions EasyTom 150–160). The small voxel dimensions of 0.020x0.020x0.020mm³ allow us to measure spatial alternances with adequate precision. Spatial alternances were measured on each spatial resolution elements (n = 30 measures per element) using Horos opensource code software (Horosproject.org).
<table><caption>Table 1 FDM and SLA printing parameters</caption><thead><tr><th></th><th></th></tr></thead><tbody><tr><td>FDM printer model</td><td>K1max (Creality)</td></tr><tr><td>Nozzle</td><td>0.4 mm/520</td></tr><tr><td>Material/nozzle temperature/bed temperature</td><td>PETG/220 °C/60 °C</td></tr><tr><td>Infill</td><td>100%</td></tr><tr><td>Layer height</td><td>0.2 mm</td></tr><tr><td>Wall line count</td><td>4</td></tr><tr><td>Print/travel speed</td><td>300/500 mm/s</td></tr><tr><td>Total printing duration</td><td>26 h 05 min</td></tr><tr><td>Specific parts printing duration</td><td></td></tr><tr><td>House/cover/contrast section/</td><td>13 h 08 min/2 h<br>18 min/2 h 26 min</td></tr><tr><td>Resolution section/slice thickness section/</td><td>2 h 19 min/2 h 18 min</td></tr><tr><td>Grid section/homogeneity section/</td><td>2 h 48 min/48 min</td></tr><tr><td>SLA printer model</td><td>Halot-one (creality)</td></tr><tr><td>Layer thickness/total floors</td><td>0.050 mm/520</td></tr><tr><td>Initial exposure/exposure time</td><td>40 s/3 s</td></tr><tr><td>Resin volume</td><td>49 mL</td></tr><tr><td>Print time</td><td>2 h 23 min</td></tr></tbody></table>

# MRI acquisitions

## Proposed learning situations

For each section, we propose a learning situation to illustrate how this phantom could be used. The learning situations presented here are not exhaustive; we consider that these situations are interesting for beginner MR students/users. The aim of these tests was to illustrate the educational use of this phantom.

- Education case #1: The impact of zero-filling process on spatial resolution (1.5 T vs 0.31 T).

- Education case #2: Geometrical distortion comparisons (1.5 T vs 0.31 T).

- Education case #3: Homogeneity comparisons (1.5 T vs 0.31 T).

- Education case #4: How to evaluate slice thickness with an angle-shaped phantom at 1.5 T?

- Education case #5: Contrast analysis, chemical artifact, and fat-suppression efficiency evaluation at 1.5 T.

## MRI systems and images processing

These experiments were conducted using two distinct systems. The aim here is not to be exhaustive, nor to compare these two devices, but simply to show that this phantom can be used in very different MRI environments. We use a whole-body MRI at 1.5 T (Magnetom Aera, Siemens Healthineers, Erlangen, Germany), and an extremity-dedicated MRI at 0.31 T (O-Scan, Esaote, Italy). Acquisitions at 1.5 T were performed using a 24-channel head receive coil (Siemens, Erlangen, Germany). Acquisitions at 0.31 T were obtained using a 2-channel knee receive coil (Kneecoil, Esaote, Italy).

The main acquisition parameters will be directly noted in the figures in "Results".

All quantitative data extracted from the acquired MRI images (distance, plot profile, ROI, etc.) were obtained using the tools available on Horos open-source code software (Horosproject.org) without any use of automatic data extraction software.

# Results

## 3D-printing manufacturing accuracy

Table 2 compares the measurements obtained via computeraided design (CAD) with the actual measurements derived from MSCT images for FDM prints and nano-CT images for SLA prints. The measurements correspond closely to theoretical dimensions, demonstrating high accuracy (less than 0.5 mm error for FDM prints and less than 0.05 mm for SLA prints).
<table><caption>**Table 2** Geometrics characteristics of the phantoms</caption><thead><tr><th>Part (printing process)<br>Measurement method (number of measurements)</th><th>CAD distances (in mm)</th><th>Real measured distances (mean ± SD)</th></tr></thead><tbody><tr><td><strong>Cover (FDM)</strong><br>MSCT control (n=30 measurements)</td><td>Height (extern): 19 mm<br>Height (intern): 132 mm<br>Diameter (extern): 132.5 mm<br>Diameter (intern): 123.5 mm</td><td>Height (extern): 18.9±0.23 mm<br>Height (intern): 131.8±0.32 mm<br>Diameter (extern): 131.8±0.32 mm<br>Diameter (intern): 123.1±0.25 mm</td></tr><tr><td><strong>Housing (FDM)</strong><br>MSCT control (n=30 measurements)</td><td>Height (extern):115 mm<br>Height (intern): 110 mm<br>Diameter (extern):124 mm<br>Diameter (intern): 114 mm</td><td>Height (extern): 115.6 mm±0.41 mm<br>Height (intern): 110.7 mm±0.53 mm<br>Diameter (extern):123.9 mm±0.35 mm<br>Diameter (intern): 114.1 mm±0.26 mm</td></tr><tr><td><strong>Slice thickness section (FDM)</strong><br>MSCT control (n=30 measurements)</td><td>Ramp angle 1: 12°<br>Ramp angle 2: 25°</td><td>Ramp angle 1: 12.1±0.1°<br>Ramp angle 2: 25.0±0.1°</td></tr><tr><td><strong>Geometrical accuracy section (FDM)</strong><br>MSCT control (n=30 measurements)</td><td>Square size x: 10.0 mm<br>Square size y: 10.0 mm</td><td>Square size x: 9.96±0.21 mm<br>Square size y: 9.98±0.23 mm</td></tr><tr><td><strong>Spatial resolution inserts (SLA)</strong><br>NanoCT control (n=30 measurements)</td><td>1.0 mm<br>0.9 mm<br>0.8 mm<br>0.7 mm<br>0.6 mm<br>0.5 mm<br>0.4 mm</td><td>0.99±0.04 mm<br>0.89±0.05 mm<br>0.78±0.05 mm<br>0.68±0.04 mm<br>0.58±0.04 mm<br>0.49±0.02 mm<br>0.39±0.02 mm</td></tr></tbody></table>

## Spatial resolution

Figure 4 illustrates one use of the spatial resolution section. In this example, we used different acquisition matrices (256 × 256, 512 × 512, 256 × 512, 512 × 256) for a fixed square FOV of 140 mm at 1.5 and 0.31 T. This experiment enabled us to show students that:

- The effect of the zero-filling method on spatial resolution.

- The importance of linking spatial resolution to the dimensions of the acquired pixels rather than the reconstructed pixel size.

- That the intensity of the magnetic field does not affect spatial resolution performance in this case.

Overall, the students agreed that evaluating performance in terms of spatial resolution can only be effectively illustrated using appropriate phantoms.

## Homogeneity, distortion and geometrical accuracy

Figure 5 illustrates the use of the homogeneity (Fig. 5a), distortion and geometrical accuracy (Fig. 5b) sections. Figure 5a shows, using a deliberately reduced window width, that homogeneity is not perfect and depends on the position of the antenna elements (coils). When the phantom was positioned along the z-axis (cross-section view), signal homogeneity was observed from the periphery towards the center, indicating centripetal signal loss. In contrast, when the phantom was positioned along the y-axis (coronal view), a signal loss was observed at the edges along the z-axis and a signal increase at the edges along the y-axis. Different methods exist to allow a relative evaluation (in %) of homogeneity [1–3] but will not be described in this work.

Figure 5b shows, with the help of a grid, that distortions are more pronounced at the edges of the section (transverse plane; 140 mm FOV) on the low-field MRI (0.31 T) compared to the acquisition made at 1.5 T. This can be attributed to the fact that we are at the limit of the maximum FOV of this device dedicated to joints (FOV max = 16 cm), whereas we are in the center of the effective zone of the 1.5 T MRI (FOV max = 50 cm). The deformations illustrated at 0.31 T may be linked both to the peripheral inhomogeneity of $B_0$ and to a loss of gradient linearity at the periphery.
<figure><img src="image_6.png" /><figcaption>**Fig. 5** MRI results of the homogeneity section (a) and the distortion section (b). In the homogeneity section (a), we observed that homogeneity depends on the orientation of the phantom within the coil. This can be explained by the design of the coil and the geometric distribution of its elements. The green insert shows the profile plot along the z and x direction (green line), which is another way of illustrating inhomogeneity (along a given direction). In the geometrical accuracy section (b), we observed that distortions are clearly visible (black arrowheads) on the 0.31 T MRI system. Bottom two panels in section (b) show the true grid dimensions in red overlaid with the MRI images.</figcaption></figure>

## Slice thickness

Figure 6 shows the one method to evaluate slice thicknesses of 3 mm (Fig. 6a) and 5 mm (Fig. 6b). These examples illustrates that the slice profiles (3 mm and 5 mm) appear to be well calibrated on this 1.5 T MRI system. The objective here is to show students the relationship between the length of the grayscale (visible on the plot profile), the slopes of the phantom (12° or 25°), and the calculated slice thickness. This relationship is directly observable in the acquired images which is interesting from a pedagogical point of view when you want students to discover the method on their own.

## Contrasts

Figure 7 presents standard T1, T2, and Rho weighted images (Fig. 7b, c, and d) and illustrates chemical shift artifacts (Fig. 7d and e) as well as the efficiency of fat-suppression methods (Fig. 7f). This case study demonstrates the following:

- Different weighting protocols (TE and TR) result in varying contrasts between different elements of the phantom (water vs. fat vs. water + gadolinium).

- Chemical shift artifacts occur along the axis of the readout gradient.

- Increasing the matrix size (i.e., readout bandwidth) reduces chemical shift artifacts (e.g., comparison of element number 3 in Fig. 7d and e).

- The fat-saturation (FS) method is effective in attenuating the PVCp signal, which mimics fat chemical shift properties, thereby eliminating chemical shift artifacts.
<figure><img src="image_7.png" /><figcaption>**Fig. 6** Evaluation of slice thickness on a 1.5 T MRI system: 3 mm (a) and 5 mm (b). For each example (a and b), 30 measurements were taken (15 on 25° slopes and 15 on 12° slopes) to obtain a mean value and a standard deviation</figcaption></figure>

## Student first feedback

The students' comments on the use of this phantom were collected orally at the end of the sessions. The aim was to find out how they felt about the phantom's ease of use and educational value. The students' reactions were very positive and we noted a great deal of interest during these practical sessions. However, no questionnaire-based study has been conducted to date, and the impact of this tool on student learning will be the subject of specific work with educational experts.

## Discussion

In this section, we will discuss the main points that have been raised after more than a year of using this tool during practical work sessions with different types of students (engineers, radiologists, radiographers, and physicists). We will first discuss the design and manufacture of this phantom, and then explore its potential applications in teaching.
<figure><img src="image_8.png" /><figcaption>**Fig. 7** Analysis of T1, Rho, T2, and T1-fat-saturation (FS) contrasts, along with chemical shift artifact (yellow arrowhead), at 1.5 T. The readout encoding direction is indicated by the red arrows and associated with the number of encoding steps. We demonstrate that the chemical shift artifact is reduced when the readout encoding matrix is increased (e), and it disappears when using fat suppression (f)</figcaption></figure>

**Added values of this phantom concept**

Although other commercial phantoms exist, we decided to compare our design with the ACR phantoms, which are a well-established reference in the clinical field. As indicated in Table 3, many elements are comparable to ACR standards. With an internal diameter of 123.5 mm, our phantom is positioned between the medium and small versions of the ACR phantom, making it suitable for use in brain or knee-dedicated coils. Our phantom offers several technical advantages over the ACR phantoms:

- Enhanced spatial resolution: it enables a more detailed study of spatial resolution, with 0.4 mm alternations compared to the 0.7 mm limit of the ACR Small version.

- Modularity and flexibility: unlike sealed ACR phantoms, our phantom is modular, allowing the integration of different gels based on the user's needs and available equipment.

- Cost-effective and accessible fabrication: it can be produced using widely available consumer-grade 3D printers (FDM and SLA), which are increasingly accessible in laboratories, clinical technical platforms, or health science schools (e.g., pedagogical fab labs).

- Educational benefits: the modular design allows different sections to be handled separately, helping students to better understand the specific properties being studied in each section.

This design facilitates widespread dissemination to clinical services or medical schools equipped with standard 3D printers, aligning with the outlined specifications in the introduction. The objective is to offer modularity conducive to pedagogical needs. In addition, our dissemination method is advantageous. We will make the STL and STEP files for the various phantom components available, enabling any reader of this article with access to entry-level 3D printers to print them directly.
<table><caption>**Table 3** Synthetic comparisons between ACR phantoms and our custom phantom</caption><thead><tr><th>Criteria</th><th>ACR phantom</th><th>Our phantom</th></tr></thead><tbody><tr><td>Price</td><td>Commercial prices (www.acrphantom.com)<br/>- Large (L) ≈2630$<br/>- Medium (M) ≈2460$<br/>- Small (S) ≈2930$</td><td>Materials prices<br/>- FDM filaments &lt;25€<br/>- SLA resin &lt;5€<br/>- Gels 5–20€ (choice depending)</td></tr><tr><td>Weight</td><td>≈ 6 kg (L); ≈3.6 kg (M); ≈1.5 kg (S)</td><td>≈1.5 kg</td></tr><tr><td>Size</td><td>L: 190 mm (inside diameter)/148 mm (inside length)<br/>M: 165 mm (inside diameter)/134 mm (inside length)<br/>S: 100 mm (inside diameter)/100 mm (inside length)</td><td>123.5 mm (inside diameter)<br/>110 mm (inside length)</td></tr><tr><td>Material/design</td><td>Acrylic plastic<br/>sealed at both ends</td><td>PETG, Resin<br/>not sealed</td></tr><tr><td>Filing solution</td><td>10 mM NiCl<sub>2</sub> and 75 mM NaCl<br/>Permanent</td><td>Water + 1/1000 Dota Gd<sup>3+</sup><br/>Not permanent<br/>(choice dependent)</td></tr><tr><td>Spatial resolution (high-contrast)<br/>demonstration</td><td>4×4 pairs of hole arrays geometry<br/>1.1 mm (L, M)<br/>1.0 mm (L, M)<br/>0.9 mm (L, M, S)<br/>0.8 mm (M, S)<br/>0.7 mm (S)</td><td>Cylindric concentric lines geometry<br/>1.0 mm (5.0 lp/mm)<br/>0.9 mm (5.56 lp/mm)<br/>0.8 mm (6.25 lp/mm)<br/>0.7 mm (7.14 lp/mm)<br/>0.6 mm (8.33 lp/mm)<br/>0.5 mm (10.0 lp/mm)<br/>0.4 mm (12.5 lp/mm)</td></tr><tr><td>Homogeneity demonstration</td><td>L: 190 mm (Inside diameter)<br/>M: 165 mm (Inside diameter)<br/>S: 100 mm (Inside diameter)</td><td>123.5 mm (inside diameter)</td></tr><tr><td>Distortion demonstration</td><td>L: 10×10 grid (1.5 cm square)<br/>M: 8×8 grid (1.5 cm square)<br/>S: 5×6 grid (1.5 cm square)</td><td>10×10 grid (1.0 cm square)</td></tr><tr><td>Slice thickness evaluation</td><td>1 dual-ramps of 5.71°</td><td>2 dual-ramps of 12° and 25°</td></tr><tr><td>Contrasts illustration</td><td>Fixed contrast values<br/>at 1.4%, 2.5%, 3.6%, and 5.1% (independent of pulse sequence parameters)</td><td>modular (gels choice depending)<br/>Not permanent</td></tr><tr><td>Chemical shift illustration</td><td>Not available</td><td>Available in this work with fat chemical shift mimicking gel (PVCp)</td></tr></tbody></table>

## Limitations of this phantom concept

Despite its advantages, this phantom has certain limitations:

- **Manufacturing and material durability:** the manufacturing process is not industrial, and the materials have not yet been tested for long-term durability (e.g., aging and deformation of plastics). Periodic re-printing of components will likely be necessary. We have chosen FDM printing with PETG for its sealing properties; however, other materials (e.g., polypropylene, ABS, and polycarbonate) could potentially enhance this properties. Achieving a fully waterproof structure is also possible with SLA printing, but large-format SLA printers are currently expensive and not widely accessible as consumer tools. Using solid phantoms (Phantom filled with gels instead of solution) is an attractive and practical approach, but it requires further investigation to estimate how long such phantoms can be preserved and used in good condition, as a further perspective for this study. In addition, the small size of the resolution insert component in our phantom, particularly the alternations in the 0.4 mm and 0.5 mm inserts, is prone to blockages during the printing process. Significant post-processing efforts, such as using thin syringes to clear the blockages, were required to ensure proper function. To minimize these issues and achieve more accurate and smooth prints, the use of higher-precision 3D printers is highly recommended.

- **Modularity and stability:** while the modular design is advantageous, it also presents challenges. The stability of the gels and filling liquid will depend on the user's choices, potentially making the phantom unstable over time. In addition, the sealing is not perfect. To minimize leakage risk, we systematically treat the housing and cover with a commercially available sealing spray (Vernis Universal, Akzo Nobel Decorative Paints, France) and reinforce the cover's seal using thermoplastic film (M parafilm, Bemis Company, Inc, Neenah, Wisconsin, USA) preferring a vertical setup in the coil (Fig. 5, left).
Given these considerations, our phantom cannot yet be considered suitable for rigorous quality control purposes, but it is interesting for teaching purpose.

Indeed, we were able to carry out over 15 practical teaching sessions, concerning radiographer students, over one academic year using this tool. We filled and emptied it at each session. This constraint was judged to be minimal compared to the pedagogical possibilities offered by the tool. In addition, we always used two identical versions: a “filled” version which was positioned in the device, and an “exploded” version where students could take charge of each module of the phantom. Feedback from students was very positive, and we noted a high level of interest during these practical sessions (see Section Student first feedback). We will also be offering this practical work to other students (radiologists, engineers) in the near future. The impact of this tool on student learning will be the subject of specific research work with pedagogical experts.

## Educational phantom versus quality assurance phantom: is there a difference?

As we indicated from the outset, this phantom is primarily intended for practical MRI teaching sessions on-site. We observed that ACR-type phantoms are not available at all clinical or research sites hosting students or practical course sessions. In addition, the ACR phantom has limitations. As previously mentioned, the spatial resolution patterns of ACR phantoms, limited to 0.7 mm (accessible only in its small version), no longer correspond to the resolutions currently achievable in MRI. It is quite common to reach acquired pixel sizes of 0.5 mm in the slice plane (excluding interpolation or zero filling). Furthermore, the ability to place specific gels in the “contrast” section is beneficial, allowing students to work on weighted contrasts and relaxometry mapping. By positioning gels with chemical shift properties (e.g., PVCp), it is possible to evaluate spectral fat-suppression methods. The educational applications are numerous and depend to some extent on the teaching needs and creativity of the instructors. The pedagogical scenarios presented in this work are not exhaustive; the number of possibilities is greater and depends on the instructor’s creativity. Of course, this can be considered a drawback, as the preparation and filling time for the phantom is significant (approximately 20 mins to fill and seal the phantom with parafilm) and must be anticipated. Although it cannot be considered a quality control phantom (see Section Limitations of this phantom concept Limitations of this phantom concept), it can still be useful for objectively evaluating some sequence parameter modifications for optimization purposes before applying them to human subjects. The concept of this phantom is evolutionary, and we do not rule out making new developments and corrections available as it evolves. The data (stl and step files) that support the findings of this study are available as a supplementary data.

The primary distinction between an educational phantom and a quality assurance (QA) phantom lies in their intended use and design criteria.

### Educational phantom

- Purpose: designed primarily for teaching and training purposes. It allows students and trainees to understand and practice MRI techniques, including spatial resolution, contrast weighting, relaxometry mapping, and fatsuppression methods.

- Design flexibility: typically, modular and customizable. Users can adjust components and integrate various gels to mimic different tissue properties and chemical shifts.

- Accessibility: emphasis on ease of fabrication and cost-effectiveness. Our phantom, for instance, can be produced using consumer-grade 3D printers, making it widely accessible to educational institutions.

- Educational value: offers hands-on learning experiences. The modularity helps students grasp specific MRI properties and techniques by allowing them to manipulate individual sections.

### Quality assurance phantom

- Purpose: designed to ensure the consistent performance and calibration of MRI systems. It is used for routine checks, system validation, and compliance with industry standards.

- Design precision: must adhere to strict manufacturing standards to ensure reproducibility and accuracy. Typically, QA phantoms are produced using industrial processes and high-quality materials.

- Durability and stability: made from durable materials to withstand frequent use and long-term storage. Sealed designs ensure stability of the internal components and prevent leaks.

- Regulatory compliance: often required to meet specific standards set by regulatory bodies such as the American College of Radiology.
of Radiology (ACR). This ensures they provide reliable and standardized measurements for system performance evaluation.

## Conclusion

This work presents a low-cost, in-house-build phantom, useful for practical MRI teaching. It represents a good alternative for teachers who do not have access to the full range of ACR phantoms. Although it cannot be considered as a quality control phantom, our experience has shown that it is useful for teaching fundamental notions (spatial resolution, homogeneity, etc.) to all MRI users. The choice of opensource distribution is motivated by the need to bring together the community of MRI teachers, whether clinicians (radiologists, radiographers), engineers, physicists or researchers.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10334-024-01217-z.

**Acknowledgements** The authors would like to thank M Damien FAVIER, from the Institut Charles Sadron, for his advice on nano-CT acquisitions

**Funding** This work of the Interdisciplinary Thematic Institute HealthTech, as part of the ITI 2021–2028 program of the University of Strasbourg, CNRS and Inserm, was supported by IdEx Unistra (ANR10-IDEX-0002) and SFRI (STRAT’US project, ANR-20-SFRI-0012) under the framework of the French Investments for the Future Program.

**Data Availability** The data (stl and step files) that support the findings of this study are available as supplementary data.

## Declarations

**Conflict of interest** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Ethical statement** Not applicable (no humans or animals findings).

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