Fabrication of 3D Printed Hollow Spheres for QC and Feasibility for Use With xSPECT Bone

doi:10.21203/rs.3.rs-3126362/v1

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Fabrication of 3D Printed Hollow Spheres for QC and Feasibility for Use With xSPECT Bone

https://doi.org/10.21203/rs.3.rs-3126362/v1

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Quantitative accuracy and constancy of Siemens xSPECT Bone quantitative reconstruction algorithm (xBone) can be monitored using activity-filled hollow spheres, which could be 3D printed (3DP-S) and increases accessibility to phantoms. One concern is that 3D prints can have air gaps in the walls which may pose issues for attenuation correction and xBone tissue zone mapping. This study assessed the feasibility of using 3DP-S with materials PLA, PETG and Resin as substitutes for commercial hollow spheres (C-S). Phantom preparation and acquisition parameters were based on the white paper. A Jaszczak phantom was fitted with six ^99mTc- and contrast-filled 3DP-S. SPECT/CT acquisitions were performed on the Siemens Intevo T6 and reconstructed with xBone. Regions-of-interest for activity concentration measurements were drawn to the internal diameter of the spheres. PLA and PETG printed via filament freeform fabrication resulted in minute air gaps mainly at steep overhang however did not impact xBone zone maps. Activity concentration recovery of the 3DP-S were within +/-5% of C-S when sufficient projection angles are used. Resin printed via masked stereolithography experienced minor resin pooling and increased wall thickness – the smallest sphere was not usable. Resin printing achieved the best watertightness and transparency. PLA and PETG were most affordable but was labour intensive in construction. PLA performed best overall in print reproducibility and quantitative accuracy. Similarly printed hollow spheres can be used for quality control of xBone accuracy where C-S are not available. While 3D printing increases accessibility to phantoms, close oversight is required of printing conditions.

3D Printing

hollow spheres

xSPECT

xBone

quality assurance

nuclear medicine

3D printing has increasingly been used in nuclear medicine predominantly for manufacturing organ structures for dosimetry and image quality optimisation, with driving factors relating to costs, accessibility, lack of variety and versatility. Alssabbagh et al [1] have demonstrated the use of 3D Printed (3DP) thyroid phantoms for optimisation of image quality to address the lack of realistic thyroid phantoms as early proof of concept. Single compartment kidney phantoms have been fabricated for assessing accuracy of quantitation for dosimetry by Tran-Gia et al [2] addressing the lack of anatomical variety of commercially available kidney phantoms. Cardiac phantoms have been developed by many groups to address the costs and impracticalities of current commercially available phantoms [3] and to create myocardial left ventricle phantoms with a variety of defects to assess image quality for different pathology-mimicking scenarios [4]. A moving 3D printed liver phantom was designed to characterise the effect of liver motion and impact on defect detection in myocardial perfusion imaging at various liver-to-myocardial proximities [5].

3D printing has the potential to be used for standard quality control (QC) tests, in addition to dosimetry and image quality optimisation. As with anatomical phantoms, commercial QC equipment may not be universally or financially accessible. With decreasing costs of 3D printing equipment, offerings of small-scale commercial 3D printing services are growing in cities, libraries, community makerspaces, and tertiary educational institutions [6–12]. Consequently, a nuclear medicine imaging site may be able to model and print QC equipment for tests that would otherwise not be performed due to lack of access to equipment.

Across positron emission tomography (PET) and gamma cameras, hollow spheres are key pieces of equipment for quality assurance (QA). These are used for various core qualitative and quantitative system performance and image quality tests. Ideally, QA of quantitative imaging should also entail monitoring the activity concentration recovery of the hollow spheres.

3D printing hollow spheres may be a solution where a site may not possess or have access to a hollow sphere set. The most affordable and available form of small-scale 3D printing is typically fused filament fabrication (FFF) – which may also be referred to as fused deposition modelling – usually using plastic filament.

Nuclear medicine QC equipment that contains liquid or that is submerged under water is typically made from plastic that has been moulded solid and uniform without air gaps (injection moulding). An issue of 3D printing NM equipment is the potential for minute air gaps due to the manufacturing nature of FFF. In the printing process, lines of plastic filament are extruded from a nozzle and deposited in the x-y axis, layer by layer. Objects can be printed solid (100% infill) or some percentage hollow (< 100% infill). Depending upon the geometry of the object, printer nozzle size and overall printing parameters, small air gaps may still occur when 100% infill is specified. Small air gaps in the walls of a 3DP hollow sphere may potentially pose an issue for attenuation correction hence impacting accuracy of reconstruction. This may be exaggerated if 3DP hollow spheres are used for QC monitoring of the Siemens quantitative reconstruction algorithm xSPECT Bone (xBone) [13].

The xBone algorithm is a Siemens-specific acquisition and reconstruction methodology used for quantitative SPECT/CT of bone scans [14, 15]. Accuracy has been assessed [16] to be consistent with evaluations conducted by Siemens [17, 18], with some depth dependency [19] and for various phantoms configurations [20, 21]. The xBone algorithm is designed such that SPECT reconstruction is informed by taking the CT as the frame of reference for SPECT reconstruction. In addition to an attenuation map, the CT is also used to generate a ‘zone’ map that delineates five groups of material based on CT number (HU): air and lung, cortical bone, soft bone, adipose tissue, and soft tissue. These groups define five material-specific zones for SPECT reconstructions for the purpose of achieving greater spatial resolution and decreasing partial volume effects as a way to enable quantitative reconstruction [13].

The majority of 3DP research in nuclear medicine has been centred on the FFF printing modality, using thermoplastic filament materials such as polylactic acid (PLA), polyethylene terephthalate glycol (PETG) and acrylonitrile butadiene styrene (ABS) as they are commonly available and for their low cost.

Using the FFF modality, air gaps that may be present in the walls of the hollow sphere between each extruded line of filament. This may be bypassed through an alternative modality of printing called stereolithography where objects are made from liquid resin. One method of inexpensive stereolithography is masked stereolithography (mSLA), where there is a UV LED screen which lights up a cross-section of the object to only cure the pool of resin which is to form the object such that each whole layer is cured simultaneously.

The aims of this study were to:

Compare the fabrication of hollow spheres using PLA and PETG via the FFF modality and resin via mSLA modality in terms of reproducibility, cost, transparency to water, watertightness, labour intensiveness in its preparation and use;

Assess its impact on quantitative accuracy for activity recovery as it pertains to xBone; and

Determine the more suitable material for 3D printing for QC of xBone.

2.1 PRINTING MATERIAL AND SETUP

2.1.1 3D Design and Prints

The hollow spheres were designed in FreeCAD (version 0.2) [22]. Each sphere was modelled as two hemispheres with a top half and the bottom half from which the sphere will be filled. The spheres were designed with internal volumes 0.5, 1, 2, 4, 8, and 16 mL with wall thickness of 1.5 mm.

Figure 1 shows the bottom half consisting of the hemisphere with a screw neck (UNC ¼” thread) and filling hole (1.2 mm diameter) tapered to guide air bubbles to escape. A plate insert was modelled with 2 concentric rings of 12 threaded holes allowing two sets of hollow spheres to be installed and attached into a Jaszczak phantom.

A nylon UNC ¼” threaded rod was purchased and cut into approximately 6 cm length rods used to cap the hollow spheres. This size also allowed it to be attach directly to a Data Spectrum Jaszczak phantom.

2.1.2 Printing and Construction

The final hollow sphere sets and the plate insert were printed at a local 3D printing service. Two printing modalities were used for the hollow spheres: a Prusa MK3, for FFF printing with eSun clear PLA + and Polymake Polylite clear PETG filaments; and a Prusa SL1 mSLA printing with clear resin.

For FFF printing, a nozzle of 0.4 mm was used, set to a layer height of 0.2 mm and at 100% infill. All other printing parameters were set by the company as required for printing spherical overhang.

The printed hemispheres objects were finished in-house. The PLA and PETG were made watertight. A thin layer of UV resin was used to fill layer gaps on the interior layers and excess resin was wiped away prior to curing. To attach the hemispheres together, a thin layer of UV resin was used as an adhesive between the two hemispheres. Minor sanding of the opposing hemisphere contact surfaces was required to level the surface prior to this.

The wall thickness was verified using a digital calliper. The internal volume was verified in two ways: (1) by weighing each sphere on a microgram scale before and after filling with water and (2) recording the volume injected using clinical graduated syringes.

2.2 Phantom Setup

The hollow spheres were filled with an aqueous solution of ^99mTc (250 kBq/mL) and iodinated contrast media (350 HU). The iodine was required for simulating a distinct, high-density zone for xBone reconstruction [13]. The Jaszczak phantom was configured with a plate insert. One commercial hollow sphere set and one 3DP hollow sphere set were installed concentrically into the 3DP plate (Fig. 2). The plate was screwed into the Jaszczak phantom with nylon screws.

The phantom was filled with a background activity of ^99mTc 50 kBq/mL for a ratio of 5:1.

This configuration was repeated for each printed material. Sets were also alternated between being installed in the exterior and interior rings of the plate insert.

2.3 Acquisition and Setup

The phantom was mounted upon a 3DP jig for reproducible positioning. This procedure was performed for each phantom configuration and repeated for a minimum of three times for statistical purposes.

The acquisition and reconstruction parameters used are outlined in Table 1. In the literature, the projection angle is typically 60 degrees. However, local clinical protocol uses 64 degrees and to maintain consistency with clinical protocol this research used 64 degrees projection angle.

One set combination of acquisitions resulted in 68 individual printed sphere configurations (17 spheres x 2 angles of projection x 2 reconstruction parameters). This was repeated multiple times for statistical purposes.

Table 1

Acquisition parameters comparing two projections angles (64 and 120 degrees), and two xBone reconstruction parameters ‘Best’ and ‘Fast’ for each projection angle.
Acquisition	64P	120P	Reconstruction	Best	Fast
Matrix	256 x 256		Mode	xBone
Projections Angles	64	120	Zone	Phantom with Contrast
Time Per Projection	12 sec		Updates	1 iteration 72 subset	2 iteration 24 subset
Orbit	Continuous Auto-Contouring		Post-Filtering	Gaussian 10 mm
CT	CareDose OFF 110 kV, 100 mAs 3 mm slice		Corrections	DEW Scatter CTAC

3.1 3D Design

A summary of the results of the printed hollow spheres are outlined for PLA PETG, and resin materials in Table 2 and Fig. 3.

Table 2

Summary and comparison of the assessment categories for the PLA, PETG, resin and commercial spheres
Material	PLA	PETG	Resin	Commercial Spheres
Watertightness	Fine acrylic sealing coat		✓✓✓	✓✓✓
Water Visibility	✓	✓	✓✓	✓✓✓
Defects	-	-	Vol ≤ 1 mL	-
Reproducibility	✓✓	✓	Vol ≥ 2 mL	-
Local Cost – 1 Set	~$50	~$50	~$100	$1800

Watertightness

Inherent to FFF printing are small gaps between layers at large angles of overhang. These gaps resulted in imperceptible water leakage thus required waterproofing with UV resin. Waterproofing was not required for the mSLA prints as the printing layers are completely solid.

Despite the miniscule air gaps with FFF printing, there was no air zone present in the hollow sphere plastic zone. There was no impact on zone mapping at reconstruction.

Transparency

While clear filament was selected, the PLA and PETG prints were almost opaque. The water level was easily visible however there was difficulty seeing air bubbles. The resin spheres printed via mSLA were less opaque than the PLA or PETG, however were also not fully transparent without additional processing of the resin.

Print Reproducibility

Multiple sets of the PLA hollow spheres printed with ease whereas two objects of the PETG prints were defective and required reprinting (Fig. 4). This was expected as PETG is generally known to be somewhat more sensitive to printing settings and environmental factors compared to PLA [23, 24].

The resin hemispheres printed via mSLA had the slightest increase in wall thickness located towards the top of the printing direction. This may be due to liquid resin pooling during curing. This degree of minute increase did not perceptibly impact the results.

The two smallest resin hemispheres were defective at printing (Fig. 5). The 1 mL sphere printed with resin pooling on the top half that turned internal geometry into a conical frustum. This decreased the total internal volume to approximately 0.7 mL. The 0.5 mL sphere printed with thickened walls up to 4 mm at its widest point from the designed 1.5 mm, rendering this sphere unusable. These issues were not able to be rectified by the printing company. A potential cause of this issue may be due to excess layer curing time which could result in UV light bleeding.

Any differences in the printed wall thickness of non-defective hemispheres not measurable by a calliper measured < ± 0.3 mm. Internal volume was verified in two ways: (1) recording the volume of water injected into the spheres using syringes, and (2) using a microgram scale to weight the spheres before and after filling with water.

Cost

At the local printing service, the cost of printing PLA or PETG were on par at $50 AUD and resin printing was approximately $100 AUD.

3.2 Reconstructed Activity Concentration

To compare the performance of 3DP hollow spheres to the commercial hollow spheres, the activity concentration recovery to a known concentration was assessed. Analysis was based the similarity between the values of the 3D prints to the commercial spheres.

With the greater boundary delineation afforded by xBone, a circular region of interest (ROI) was drawn to the known internal diameter on the CT image at the central axial slice of each sphere. The ROI was duplicated onto the sphere in the SPECT slices (Fig. 6). Background ROIs of corresponding diameters were also obtained from the uniformity region in the phantom. From these ROIs, activity concentration measurements were extracted.

The results for the 3DP spheres are reported as relative to the commercial spheres of the corresponding sizes (Fig. 7). The first parameter is the relative percentage of the activity concentration achieved, at which 100% would mean it achieved the same activity concentration as commercial spheres.

With the acquisitions using 120 projections, all three materials achieved mean activity concentration recovery percentage (ACR%) within ± 5% of the commercial spheres of the respective sizes. Of the 68 configurations, 7 configurations (2 from PETG, 4 from PLA, and 1 from resin spheres) resulted in an ACR% that exceeded ± 5% of the commercial spheres – all occurring with acquisitions using 64 projections.

Under all acquisition and reconstruction methods, PLA and resin spheres had the overall lowest and comparable standard deviation. The PETG material exhibited the largest variability in ACR%.

The magnitude of variability between the three material types indicates correlation to the reproducibility, uniformity, and filling density of the walls of the spheres. That is, the miniscule air gaps inherent to the FFF printing modality may impact the attenuation correction, whereas the lower variability of resin prints may be attributed to the full solidity afforded by the mSLA modality.

The accuracy and precision of the ACR% of the 3DP hollow spheres is determined more by the choice of acquisition and reconstruction parameters as opposed to its material type. The result of this study demonstrates that the feasibility of using 3DP hollow spheres is not limited by its construction but is instead limited by standard sampling requirements. This is particularly important for xBone as it requires a matrix size of 256 x 256.

With the comparable performance between PLA and resin spheres, the choice of material may be selected based on other factors. The printing modality for PLA is more widely accessible and print reproducibility is greater than resin printing. Meanwhile, resin spheres require less manual construction as it does not require waterproofing and filling is easier as it has greater material transparency.

The smallest resin sphere (0.5 mL) was not able to be printed by the local printing company, therefore limits this study to comment on the quantitative accuracy or feasibility for this sized resin sphere. This is a useful reminder that the printing refinement can be dependent upon the extent of collaboration between the user, the printing company and their available equipment.

Between the two best performing in terms of ACR% and comparable materials PLA and resin, PLA was the most suitable material locally on the basis of the ability of print a full set.

The ACR% of the total volume of each sphere was not assessed with the current model of greater wall thickness. The thicker walls may potentially result in exaggerated PVE. However, ACR% of total volume is not a parameter that is typically assessed in standard QC monitoring.

As 3DP hollow spheres are used for the purpose of long-term QC monitoring, longevity assessment of the spheres is underway. This will examine the structural integrity, such as potential swelling, of the spheres after extended duration of being submerged in water and assess quantitative accuracy compared to unsubmerged spheres. Should swelling occur after prolonged submersion, a new set can be easily reproducibly printed using PLA or resin. The current recommendation is that the 3DP hollow sphere should be emptied and stored in a dry location in between use.

Future works may investigate: (1) whether the two smallest size resin spheres can be flawlessly printed and then be assessed as the potential ideal 3DP material; or (2) whether use of a smaller nozzle size in FFF printing hence smaller air gaps will cause less variance in PETG and PLA results.

Hollow spheres can be 3DP for use as quality control phantoms provided there is sufficient attention to printing parameters. While 3DP printing can create phantoms that a site would not otherwise be able to access, it must be cautioned that there can potentially be a moderate to significant time cost in designing, testing, and manufacturing the phantom. In the situation where digital 3D print files of phantoms may be obtained or purchased, the time cost of designing may be avoided. However, it is still not as simple as “print and play” as optimisation of printing parameters is necessary for each design for each printer. Guidelines to fabrication and community troubleshooting forums are widely available online, as well as in the literature on development and pitfalls of printing and its QC [25–28].

3DP hollow spheres printed with the FFF and mSLA modalities and these PLA, PETG and resin materials are able to be used for QC monitoring of quantitative xBone acquisition and reconstruction algorithm. When acquisition parameters have been optimised to incorporate a sufficient number of projection angles and counts, sufficient accuracy has been obtained in comparison to the results from commercial spheres. This suggests that 3DP hollow spheres may be an appropriate alternative where commercial spheres are not accessible. The full range of sizes of hollow spheres were most reliably constructed via FFF using PLA filament in this study. Despite that, the potential of resin printing via SLA with further optimisation of printing parameters must not be discounted. The work performed here is in agreement with current literature [29, 30] that 3DP can broaden accessibility to tools and phantoms, however it must be acknowledged that there will be limitations for researchers without access to 3D printers or otherwise do not have full control of the printing procedure.

Acknowledgements

I would like to acknowledge Mr Siddharth Mehta for his support as the co-supervising radiology medical physics specialist, fellow medical physics registrar Dr Nathan Thorpe for his advice on 3D printing, and the nuclear medicine department at Gosford Hospital for their support and help throughout this study.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Competing Interest

The authors have no relevant financial or non-financial interests to disclose.

Conflict of Interest

Authors declare that they have no conflict of interest.

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Tuyet Oanh Lam. The first draft of the manuscript was written by Tuyet Oanh Lam and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Ethical Approval

This article does not contain any studies with human participants or animals performed by any of the authors.

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You are reading this latest preprint version

Fabrication of 3D Printed Hollow Spheres for QC and Feasibility for Use With xSPECT Bone

Status:

Version 1

Abstract

Figures

1 INTRODUCTION

2 METHODS AND MATERIALS

2.1 PRINTING MATERIAL AND SETUP

2.1.1 3D Design and Prints

2.1.2 Printing and Construction

2.2 Phantom Setup

2.3 Acquisition and Setup

3 RESULTS

3.1 3D Design

3.2 Reconstructed Activity Concentration

4 DISCUSSION

5 CONCLUSION

Declarations

References

Status:

Version 1