The accuracy of ultrashort echo time MRI sequences for medical additive manufacturing

Abstract

Objectives: Additively manufactured bone models, implants and drill guides are becoming increasingly popular amongst maxillofacial surgeons and dentists. To date, such constructs are commonly manufactured using CT technology that induces ionizing radiation. Recently, ultrashort echo time (UTE) MRI sequences have been developed that allow radiation-free imaging of facial bones. The aim of the present study was to assess the feasibility of UTE MRI sequences for medical additive manufacturing (AM). Methods: Three morphologically different dry human mandibles were scanned using a CT and MRI scanner. Additionally, optical scans of all three mandibles were made to acquire a "gold standard". All CT and MRI scans were converted into Standard Tessellation Language (STL) models and geometrically compared with the gold standard. To quantify the accuracy of the AM process, the CT, MRI and gold-standard STL models of one of the mandibles were additively manufactured, optically scanned and compared with the original gold-standard STL model. Results: Geometric differences between all three CT-derived STL models and the gold standard were ,1.0 mm. All three MRI-derived STL models generally presented deviations ,1.5 mm in the symphyseal and mandibular area. The AM process introduced minor deviations of ,0.5 mm. Conclusions: This study demonstrates that MRI using UTE sequences is a feasible alternative to CT in generating STL models of the mandible and would therefore be suitable for surgical planning and AM. Further in vivo studies are necessary to assess the usability of UTE MRI sequences in clinical settings.

Measuring and Establishing the Accuracy and Reproducibility of 3D Printed Medical Models

“…Comparisons of the bone versus the printed model (both digitized by using optical or CT scanning) also have been performed. These comparisons yielded similar results, with an average difference in dimensions across the entire outer surface of the bone (relative to the 3D model) of less than 1 mm; however, differences can be as high as 5-6 mm in portions of the model (Table 4) (48,51,52,55).…”

“…The basic methodology of comparing cadaveric specimens with 3D printed models (Fig 10) has since been applied to all major 3D printing technologies to assess the accuracy of 3D printing of not only the skull (17,34,35,46,47) but also the mandible (45,46,48), vertebral bodies (49), and pelvis (50) ( Table 3). Comparisons of the bone versus the printed model (both digitized by using optical or CT scanning) also have been performed.…”

“…The chosen imaging modality or protocol also affects accuracy-both directly, in terms of the achieved image contrast, and indirectly, in terms of segmentation. In one study (48), the models of dry mandibles created from CT images were more accurate (<0.9 mm for 95th percentile of measurements) than those created from ultrashort-echo MR images (<1.74 mm for 95th percentile of measurements) ( Table 3). Figure 12 shows a humerus segmented from CT images by using two typically used thresholds: 226 HU for general CT bone segmentation and 400 HU, which is at the lower end of the range of attenuation values typically used for cortical bone segmentation (400-700 HU) (32,46).…”

Cumulative Inaccuracies in Implementation of Additive Manufacturing Through Medical Imaging, 3D Thresholding, and 3D Modeling: A Case Study for an End-Use Implant

“…When a different metal AM machine is used with the manufacturer's recommended process parameters, the reported error may deviate with a fairly good precision of <0.14 mm, excluding the geometries that are prone to support removal error situated on the z-axis [59]. Table 10 lists accuracy measurement studies [81][82][83][84][85][86][87][88][89][90][91][92] of additively manufactured constructs using various anatomical models, imaging medium and modality, measurement principles, AM technologies, and measurement instruments. Most studies have used CT imaging technology to image a dry skull.…”

3D printing from MRI Data: Harnessing strengths and minimizing weaknesses

“…Several studies have compared the accuracy of CT versus MRI for creation of models, with somewhat mixed results. While some authors concluded that CT‐ and MRI‐derived models are sufficiently comparable in accuracy to warrant the use of MRI data over CT (<1 mm difference between CT and MRI), others reported a >2 mm discrepancy of MRI‐derived models, which they concluded made MRI inappropriate for their clinical use (patient‐specific templates for total knee arthroplasty) . It should be noted that the majority of studies to date assessing relative accuracy of CT versus MRI were performed on cadaveric bone models; since cortical bone does not generate much MRI signal due to extremely short transverse relaxation times ( T 2 ), it is not surprising that MRI performed slightly worse in terms of accuracy when compared to CT. Further, differences in MRI acquisition parameters, segmentation technique, and method of accuracy assessment (eg, computer‐based model comparison vs. physical measurement with calipers) all make the current body of work challenging to apply broadly.…”