November 2016, VOLUME 207
NUMBER 5

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November 2016, Volume 207, Number 5

Vascular and Interventional Radiology

Technical Innovation

Computer-Based Vertebral Tumor Cryoablation Planning and Procedure Simulation Involving Two Cases Using MRI-Visible 3D Printing and Advanced Visualization

+ Affiliations:
1Department of Radiology, Brigham & Women's Hospital, 75 Francis St, Boston, MA 02115.

2Medical Imaging of Lehigh Valley, Allentown, PA.

3Department of Radiology, Applied Imaging Science Laboratory, Brigham & Women's Hospital, Boston, MA.

Citation: American Journal of Roentgenology. 2016;207: 1128-1131. 10.2214/AJR.16.16059

ABSTRACT
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OBJECTIVE. We report the development and use of MRI-compatible and MRI-visible 3D printed models in conjunction with advanced visualization software models to plan and simulate safe access routes to achieve a theoretic zone of cryoablation for percutaneous image-guided treatment of a C7 pedicle osteoid osteoma and an L1 lamina osteoblastoma. Both models altered procedural planning and patient care.

CONCLUSION. Patient-specific MRI-visible models can be helpful in planning complex percutaneous image-guided cryoablation procedures.

Keywords: 3D printing, additive manufacturing, rapid prototyping, surgical planning, tumor ablation

Percutaneous image-guided thermal ablation of tumors in the posterior spinal elements is difficult because these tumors often abut major neural structures including the nerve roots, dura, and spinal cord. Both neurologic damage and CSF leak have been reported as complications of these procedures [1, 2]. Three-dimensional procedural planning and simulation may enhance procedure safety and efficacy. Also known as “additive manufacturing” and as “rapid prototyping,” 3D printing has been used for procedural planning in many areas including endovascular aneurysm repair and many types of surgery [35]. It has also been used for spine surgery simulation [68]. We created CT- and MRI-visible models of a cervical osteoid osteoma and a lumbar osteoblastoma and then used those models in conjunction with advanced visualization techniques to plan and simulate percutaneous MRI-guided spine tumor cryoablation.

Materials and Methods
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This study was approved by our institutional review board and was performed in compliance with HIPAA. Patients provided informed consent for the in vivo procedures according to standard institutional protocol. The need for patient informed consent was waived for the research component involving the development of the 3D models and procedure simulations.

Patient 1 was a 17-year-old male subject who presented with decreased but persistent pain after partial cryoablation of a C7 right pedicle osteoid osteoma. The lesion was only partially ablated because the most anteroinferior aspect of the tumor was at the periphery of the ablation zone and the probe could not be repositioned without ice ball extension into the thecal sac. Because the patient was experiencing residual persistent pain, we developed a CT-visible, MRI-compatible, and MRI-visible 3D printed phantom to determine whether there was a viable percutaneous approach that would enable safe cryoablation of the residual lesion.

Raw data from the patient's most recent cervical spine CT study were reconstructed at 1-mm isotropic voxel resolution. Segmentation with isolation of bone and tumor was performed using software (Vitrea 6.7, Vital Images). The segmentation data were exported in a stereolithography (STL) file format and were postprocessed using computer-aided design (CAD) software (3-matic, version 9.0, Materialise) to optimize for printing (remeshing, smoothing, and trimming).

The model was printed on an Objet 500 Connex printer (Stratasys) using a material (Objet RGD525 High Temperature, Stratasys) that has been newly identified as possessing an MRI signal, likely because of the small proportion of petroleum product in the resin formulation [9]. An MRI-visible 3D printing material was not previously known; thus, simulation was not an option at the time of this patient's initial cryoablation, which we also performed.

The print time for the particular printing technology used (i.e., material jetting) is dependent on a number of factors including the desired printing resolution, the specific printing material used, and whether multiple models are printed together. The print time for this particular model was 12.5 hours. If this model were printed on the same printer but using a different material (Vero White, Stratasys), the print time would have been 4.5 hours because of differences in the time required to relieve the internal stresses in the printed parts between the printing of each layer. The cost of printing this model including overnight shipping was $437.

The planned procedure was simulated on the 3D printed model in our combined CT and MRI procedure suite. The model was placed in a position similar to that expected for the patient's spine, and the radiologist performed the procedure under sterile conditions. Drilling was performed under CT guidance (Biograph mCT 64 PET/CT System, Siemens Healthcare). After drilling, the radiologist advanced the same type of cryoprobe to be used in the actual procedure (17-gauge IceSeed, Galil Medical) to the anterior aspect of the visible lesion. The model was then transferred to the MRI scanner, and the cryoablation size and location of the ice ball were monitored with a 3-T unit (Magnetom Verio MRI System, Siemens Healthcare). The simulated procedure took approximately 90 minutes.

The in vivo procedure was performed with technique identical to the simulation in the patient the next day, approximately 9 months after the initial cryoablation, by the same radiologists who performed both the initial cryoablation and the simulation (Fig. 1). The in vivo procedure took approximately 7.5 hours from the first CT image to the final MR image including an intraprocedural delay of approximately 2 hours due to a technical malfunction of the MRI scanner. At our institution, we typically monitor spine cryoablation using MRI, which enables visualization of the ice ball and proactive manipulation of the argon flow rate to avoid ablation of adjacent nerves and of the thecal sac without adjuvant thermoprotective techniques.

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Fig. 1A —17-year-old male patient with C7 osteoid osteoma.

A, Axial CT image of model shows tumor (white arrow) and prior ablation needle track (black arrow).

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Fig. 1B —17-year-old male patient with C7 osteoid osteoma.

B, Axial CT image of model with cryoprobe (black arrow) positioned within tumor (white arrow).

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Fig. 1C —17-year-old male patient with C7 osteoid osteoma.

C, Sagittal T2-weighted turbo spin-echo MR images of model (C) and patient (D). Model is rotated slightly clockwise relative to patient but is otherwise in similar plane. Images show ice ball (white arrows) encompassing entire tumor and sparing neural foramina (black arrows).

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Fig. 1D —17-year-old male patient with C7 osteoid osteoma.

D, Sagittal T2-weighted turbo spin-echo MR images of model (C) and patient (D). Model is rotated slightly clockwise relative to patient but is otherwise in similar plane. Images show ice ball (white arrows) encompassing entire tumor and sparing neural foramina (black arrows).

Patient 2 was a 22-year-old man who also presented with decreased but persistent pain after partial cryoablation of an L1 left lamina osteoblastoma. The L1 lesion was only partially ablated because a conservative approach was taken to ensure the safety of the adjacent nerve root. Given the utility of the 3D model in patient 1, a similar model was created of the L1 tumor and a similar sequence of events was followed. Printing of this model took 15.5 hours and cost $819 including overnight shipping. The simulated procedure took approximately 90 minutes.

Because of a high likelihood of procedure failure identified during the simulation, which we discuss in the Results section, advanced 3D visualization models were created to explore alternative ablation approaches. Specifically, CAD software (3-matic) was used to simulate an approximately ellipsoidal 3 × 2 cm cryoablation zone, the maximum anticipated size of a safe cryoablation zone in this case. The manufacturer-supplied specifications of the ice ball shape for this cryoprobe by laboratory testing in gel are 2.0 × 2.7 cm for −20°C and 3.3 × 3.8 cm for 0°C. The in vivo sizes are typically smaller, and we dynamically adjust the growth of ice balls in vivo by titrating cryoprobe flow rate and freeze times. Simulated ablation zones were placed in multiple orientations, and the track necessary to achieve each zone was simulated with a thin cylindric shape. The in vivo procedure was performed the next day, approximately 9 months after the initial cryoablation, by the same radiologists using an approach identified with the 3D software models (Fig. 2). The in vivo procedure took approximately 5 hours from the first CT image to the final MR image.

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Fig. 2A —22-year-old man with L1 osteoblastoma.

A, Three-dimensional printed model, printed using CT- and MRI-visible material for simulation of dual-track approach, shows inferior palpable tumor lobule (arrows).

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Fig. 2B —22-year-old man with L1 osteoblastoma.

B, Advanced 3D visualization software model developed before in vivo procedure displays residual tumor lobules inferior (white arrows) and deep (black arrows) to ablated tumor nidus. Cryoprobe tracks (blue) used during model simulation and theoretic ablation zones (yellow) are shown. Red = tumor lobules.

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Fig. 2C —22-year-old man with L1 osteoblastoma.

C, Advanced 3D visualization software model developed before in vivo procedure displays theoretic contralateral single-track approach that allowed simultaneous ablation of both lobules. Blue line and blue arrow indicate direction of cryoprobe, and yellow volume indicates projected ice ball contour. Although modeled ice ball contour appears to invade expected thecal space, CSF flow was expected to decrease size of this component of ice ball in vivo. This approach was used in in vivo procedure next day, ice ball was carefully monitored, and there was no ice ball extension into thecal sac. Red and yellow arrows indicate planes orthogonal to cryoprobe.

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Fig. 2D —22-year-old man with L1 osteoblastoma.

D, Reconstructed sagittal double oblique CT image with posterior aspect of patient conventionally oriented at left of image shows residual tumor lobules inferior (white arrows) and deep (black arrows) to ablated tumor nidus. Cryoprobe is positioned through both of these residual lobules.

Results
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Before viewing the 3D model of the C7 right pedicle osteoid osteoma in patient 1, we planned to enter the C7 posterior elements more superiorly than we had during the first ablation to enable a steep approach for easy positioning of the cryoprobe tip within the residual tumor. The 3D model showed that a residual cortical defect from the initial C7 ablation not only was patent and accessible, but also was more prominent and more deformed than was visible on the individual 2D CT slices. The extent of this deformity, likely in part because of slow healing from partial ablation of the track, generated concern that creating a second cortical hole would decrease osseous integrity and result in subsequent posterior element fracture. As a result, we decided to use the simulation as a test to determine whether the residual lesion could be adequately targeted through the initial cortical hole. Toward that end, the initial cortical hole was used for access during the simulation, a new more angulated track was drilled through the model medullary tissue, and the cryoprobe tip was advanced to the anterior edge of the residual tumor. The same approach was taken in vivo the next day. An ice ball covered the residual tumor without compromising the thecal sac or adjacent neural foramina, there were no complications, and the patient experienced complete sustained pain relief to the time of this writing, approximately 8 months after ablation. Follow-up CT from approximately 6 months after ablation showed a nearly healed cortical defect and appropriate sclerosis in the region of ablation without growing residual tumor.

The 3D model of the L1 tumor in patient 2 showed two visible and palpable residual tumor lobules located inferior (i.e., within the foramen) and deep to the initial ablation zone. These lobules were difficult to see as discrete elements of residual tumor on the 2D CT images. During the simulation, the deep lesion was approached directly through the lamina and then a second track was drilled to the inferior lobules through the same cortical hole. Drill shavings from the inferior track partially blocked the superior track, and it proved difficult to reposition the cryoprobe from one track into the other track under MRI guidance. The simulation thus suggested a high likelihood of procedure technical failure.

Because the patient was scheduled to undergo the procedure the next day, there was not adequate time to print a new model and perform a second simulation using a new approach. Thus, that evening an alternative approach was sought by consulting and manipulating the STL model in 3D visualization software. The 3D visualization software showed that there was only a single approach that would allow simultaneous and safe ablation of both lobules. That approach—through the contralateral side of the spinous process—was taken the next day during the in vivo procedure. Again, an ice ball covered the residual tumor without compromising the thecal sac or adjacent neural foramina, there were no complications, and the patient experienced complete sustained pain relief to the time of this writing, approximately 5 months after ablation. There has been no follow-up imaging at the time of this writing.

Discussion
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We report the use of MRI-compatible, CT-visible, and MRI-visible 3D printed models used in conjunction with advanced visualization software methods to plan and simulate safe access routes to achieve a theoretic zone of cryoablation for percutaneous image-guided cryoablation of spine tumors. These models are easily generated using current radiology postprocessing software and are prepared for 3D printing using commercially available CAD software.

We generated 3D printed models from the CT examinations of a patient with a C7 right pedicle osteoid osteoma and a patient with an L1 left lamina osteoblastoma. The models were used for procedural planning and cryoprobe placement simulation before performing the procedures in the patients.

The model of the C7 tumor revealed more extensive cortical deformation than was apparent on the CT slices, which led to an alteration in approach. The simulation proved that an approach through the residual cortical hole allowed appropriate access to the residual tumor without further compromise of the cortex.

The simulated procedure on the L1 tumor model revealed that the planned procedure with two separate pathways to the two residual tumor lobules was technically suboptimal and was likely to fail. Subsequent advanced visualization software models resulted in the use of an otherwise implausible contralateral approach. Both cryoablations were technically successful with complete tumor ablation, complete pain relief, and no complications. In the future, we plan to use advanced visualization models before performing a simulated procedure.

Two limitations of our technique are the cost of printing the models, $437 and $819 for the reported cases—the latter more expensive because of the larger model size due to the larger size of the lumbar versus thoracic vertebral bodies—and the additional procedure room time for simulation. The most important technical limitations are that tumor tissue in the printed model has the same CT attenuation and MRI signal intensity as model bone tissue and that the spinal cord and adjacent nerves are not included in the printed model. We are currently developing techniques to enable MRI signal differentiation of model tumor tissue from model normal tissue and to include additional major structures, such as the nerves, in our printed models. Moreover, because cryoablation is a dynamic process that involves titrating argon gas flow rates and freeze and thaw times, tumor access and cryoprobe positioning are only the first steps of a successful ablation. Factors not encountered during a simulation, such as alterations of the ablation zone due to CSF or vessel flow, could alter procedural management and goals during the process of an in vivo ablation.

Our experience indicates that 3D printed and advanced visualization software models can be helpful in planning and optimizing the approach to complex percutaneous image-guided cryoablation procedures—specifically, when the tumor is in the spinal posterior elements where there is a high risk of injury to adjacent neural structures. Moreover, advanced visualization software models may have the greatest impact if generated before the simulated procedure to ensure that the theoretically optimal approach is used in the simulation. We plan to use advanced visualization software models followed by simulation on 3D printed models on all future complex spine tumor ablation cases because using this approach initially may avoid the need for additional procedures to eliminate residual untreated tumor.

D. Mitsouras received a grant from the National Institute of Biomedical Imaging (grant EB015868) and a grant from Vital Images, a Toshiba Medical Systems Group company.

References
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Address clinical correspondence to J. P. Guenette () and technical correspondence to D. Mitsouras ().

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