Interventional radiology (IR) is a subspecialty of radiology in which radiologists perform minimally invasive operations to diagnose, treat, and cure a variety of conditions. As compared to traditional surgeries, IR procedures can reduce surgical risks, operating and recovery time, costs, and at times lead to improved patient outcomes. The range of diseases and organ systems amenable to IR procedures are extensive and include vascular, oncologic, hepatobiliary, gastrointestinal, genitourinary, pulmonary, musculoskeletal, and neurologic intervention. IR procedures broadly involve angioplasty and stenting, thrombolysis, embolization, ablation, biopsy, drainage, injection, and retrieval.
Three-dimensional (3D) printing technologies are already well established in the surgical domain. In fact, even before the advent of this technology, researchers across the world were creating 3D objects for surgical planning by milling structures from foam, plastic, and other materials using a subtractive approach. , A glimpse into its role in surgical fields may substantiate parallel use cases within IR. Complex procedures require preoperative evaluation, and often, practice, to ensure a successful outcome. The role of 3D printing in surgery is usually for the purpose of illustrating anatomy in a relatable 3D method to surgeons, to create an anatomically accurate environment for hands-on simulation of a procedure, to serve as an intraoperative reference tool, to create customized equipment, and to develop tailored devices for a certain patient or procedure. This technology has been shown to reduce surgical time, increase operator confidence, and lead to improved operative results.
There is growing evidence that physical 3D printed models aid clinicians in improving patient management and allow for improved patient outcomes. These models can add value to clinical practice by allowing preprocedural planning or fabrication of custom devices and can have a large impact on trainee education and patient understanding. These use cases lay the foundation for several applications of 3D printing within IR. In this chapter, we will address printing techniques and workflow relevant to IR, use cases of this technology in IR, and the future of 3D printing in IR.
3D Printing Workflow
The general workflow to fabricate a 3D printed anatomic model involves image acquisition, segmentation, post-processing with computer-aided design (CAD) software, printing, and model post-processing, as exemplified in Fig. 11.1 . As these topics have been discussed in detail in previous chapters, we will not go into depth here. However, it must be noted that a model’s accuracy depends on how well the targeted structures can be clearly distinguished from surrounding tissues on the initial imaging. The study of choice should provide the maximum contrast differentiation between the anatomy of interest and surrounding structures, which depends on size, shape, density, and magnetic resonance characteristics of tissue.
IR procedures vary widely by organ system, specific intervention, and age group, warranting unique imaging needs which should be considered when printing a model. For example, computed tomography (CT) may be the preferred modality when characterizing a complex inferior vena cava (IVC) filter retrieval due to the spatial resolution and ability to delineate metallic material. Certain tumors may have borders which are better defined on magnetic resonance imaging (MRI) and can aid in planning approach to a biopsy or complex ablation. Technical factors such as signal-to-noise ratio and contrast-to-noise ratio can, respectively, impact the ability to resolve fine structures such as small vessels and the ability to distinguish different materials such as pleural effusion from adjacent atelectasis.
Once an anatomic model has been segmented and prepared for printing using CAD software, the many different options for 3D printing must be considered. The International Organization for Standardization and American Society of Testing and Materials have categorized these techniques under seven standardized headings including vat photopolymerization, material extrusion, directed energy deposition, powder bed fusion, binder jetting, material jetting, and sheet lamination. The unique aspects of each of these techniques are discussed separately in Chapter 5 entitled “3D Printing Principles and Technologies.” Techniques most relevant to use cases that arise in IR include vat photopolymerization, material extrusion, binder jetting, and material jetting. Once printing is complete, the part and the build plate are removed. The part is cleaned and support structures, if present, are removed ( Fig. 11.2 ). For IR planning purposes, hollowed vasculature models may be required. It is important to note that for these models, technologies with dissolvable support materials are preferred.
Clinical Use Cases of 3D Printing in IR
Deciding when a 3D printed model can be beneficial in clinical practice based on patient outcomes is an area that is still under active investigation. Multiple studies have suggested that 3D printed models in the fields of vascular and nonvascular IR and neurointerventional radiology can bolster provider confidence and be useful for procedural planning. , These models can be valuable for procedural planning when multiple approaches are possible, rehearsal for high-risk procedures where the margin for error is low, or even for routine procedures where operative time can be reduced. Printed models of the preoperative anatomy afford the ability to create a sophisticated procedural plan, rehearse to prevent and manage complications and reduce intraoperative radiation and anesthesia, particularly desirable in the pediatric population and in long complex cases. While multiple other modalities of advanced 3D image visualization and simulation exist, including virtual reality (VR) and augmented reality (AR), human cadaver, and live animals, 3D printing is a low-risk modality that offers unique advantages in terms of patient specificity, minimal risks to the user, and ability to integrate haptic feedback. Printed models may also be used in conjunction with imaging techniques and can accommodate use of actual interventional devices.
Vascular and Nonvascular General Interventional Procedures
Interventional treatments address a variety of pathologies in multiple organ systems through vascular, percutaneous, or natural-orifice directed routes. Printed anatomic vascular models in IR may be used for in vivo device testing, flow simulations, or presurgical planning.
A survey-based study of common IR procedures showed that the use of 3D models for preprocedural and intraoperative guidance was feasible and low cost with promising utility in planning and execution. These procedures included transarterial chemoembolization, percutaneous ablation, and splenic artery aneurysm repair. Models of relevant anatomy were printed using an affordable consumer-grade liquid resin desktop 3D printer. These included clear hollow vascular models of the aorta and target arteries and models of target organs for ablation with relevant surrounding anatomy manually denoted by painting them in different colors. The models were available prior to procedures as well as during procedures, where they were accessible in sterile bags for easy intraoperative manipulation. Interventional radiologists who performed procedures unanimously recommended the use of such models for similar cases. They rated the models favorably in terms of utility, ability to enhance spatial understanding, and ability to increase confidence in treatment approach. It should be noted that biocompatible and sterilizable surgical grade resins are available, and therefore if models are printed using these materials and appropriately sterilized preoperatively, they may be brought into the interventional suite without requiring placement in a sterile bag.
Hepatobiliary and portal vein intervention is common in IR and is an area where 3D printing may be particularly beneficial to facilitate access to the biliary or portal systems and to avoid vascular injury. For example, printed models of livers can assist in planning transjugular intrahepatic portosystemic shunt (TIPS) placement. One study postulated that reliance on 2D imaging alone may incur risk of complications such as extracapsular hemorrhage and nontarget puncture. By having a model that displays the locations of hepatic and portal vessels preoperatively, a radiologist can plan the ideal path for tract creation. In this study, hepatic parenchyma was printed with translucent acrylate polymer, containing hepatic and portal veins which were hollow and could accommodate catheters. The study noted that the complexity of printing could be reduced with a method to color vascular structures postprinting, rather than incorporating multiple colors into the raw print materials. Similar models may be advantageous in planning approach to percutaneous or transvenous liver biopsy in order to avoid complications related to vascular injury.
Portal vein stenosis is a common complication of liver transplant and often requires endovascular treatment, including balloon angioplasty and stent placement. One study used portal-phase contrast-enhanced CT data to create hollow models of portal vein stenosis for preoperative simulation of endovascular treatment and demonstrated that creating this type of 3D printed model was feasible. Ten hollow models were printed using fused deposition modeling, a widely used and relatively inexpensive printing technique. The study also assessed models for reproducibility by filling them with water and obtaining T2-weighted MR images. After coregistration and binarization of the images, they were combined to create an overlap map, which demonstrated sufficient accuracy and precision in size and shape when compared to the CT mask images. Further work is required to determine the actual clinical utility of 3D printed models for portal vein stenosis.
The precision and accuracy of 3D printed models relative to the patient’s anatomy is especially important for IR modeling, since many vascular structures are miniscule with intricacies and tortuosities that must be well visualized on a 3D model. Such research on model accuracy has been performed with multiple organ systems. One study used 3D CT angiography data of a splenic artery aneurysm to create 10 hollow vascular models using a fused deposition modeling-type desktop 3D printer. Models were filled with water and scanned with T2-weighted MRI for evaluation of the lumen. Cross-sectional areas were similar between models, reflecting high precision, and mean cross-sectional areas of the afferent artery were the same as those calculated from the original mask images, reflecting high accuracy.
3D printing has also been shown to assist with the treatment of visceral aneurysms including splenic, hepatic, gastric, epigastric, gastroduodenal, and posterior superior pancreaticoduodenal aneurysms. Applications for 3D printing also abound in aortic and cardiovascular intervention, such as in abdominal aortic aneurysm repair , including aortic arch repair, , aortic valve replacement, mitral valve replacement, and pulmonary valve stent implantation. , Flow models of the abdominal vasculature may also be created to assess flow dynamics and appropriately size devices preoperatively ( Fig. 11.3 ).
While applications for 3D printing in preoperative planning and simulation abound in IR, 3D printing also affords the creation of tools that can be used to increase efficiency and easy during IR procedures. One example is the F-Spoon, a handheld external compression device that was created to facilitate CT-guided and fluoroscopy-guided percutaneous abdominal intervention such as biopsies, drainages, and ablations. The device was designed with the goal of facilitating access to targets and minimizing radiation exposure to radiologists. The design, created to accommodate a sterile cover, included a handgrip and curved armrest to allow steady control while applying continuous pressure to the abdomen. The tip of the device included a keyhole cutout which could slide around a needle embedded in the abdominal wall.
Various 3D printing techniques can be used to highlight intraoperative vascular anatomic relationships in neurointerventional radiology and neurosurgery. Multiple reports in the literature have demonstrated the feasibility of 3D printing neurovascular models using these technologies for diagnosis, preoperative planning, simulation, trainee education, and patient counseling
At this time, the most widely utilized indication for 3D printed neurointerventional models is for the treatment of intracranial (cerebral) aneurysms. Intracranial aneurysms carry risk of rupturing, therefore must be treated using methods such as surgical clipping or endovascular coiling to seal off the aneurysm. The reported rate of intraprocedural aneurysmal rupture during coil embolization varies from 1% to 5%, significantly increasing risk of periprocedural death and disability. The use of 3D printed models to guide approach and device selection may improve chances of successful embolization.
Several studies have shown how 3D printed intracranial aneurysm models can be useful for intervention planning. First, in 2015, Mashiko et al. created a 3D printed vessel model which was first coated with liquid silicone and then melted leaving an outer layer as a hollow elastic model; and simulation using the elastic model was thought to be useful to understand the 3D aneurysm structure. In 2015, Namba et al. also created hollowed patient-specific 3D printed aneurysm models for 10 consecutive patients undergoing endovascular coiling and these models were used for preoperative microcatheter shaping, a key factor for successful coil embolization of cerebral aneurysms which can be difficult to achieve. They found that the preplanned microcatheter shapes demonstrated stability in 9 of 10 cases. A 2016 study also investigated the use of 3D printed models of intracranial arterial aneurysms to produce optimally shaped microcatheters for coil embolization. Twenty-seven aneurysms were treated using a total of 48 microcatheters shaped while referring to the 3D printed vessel model. Of the 48 catheters, only 9 (19%) required modification of the initial shape due to inappropriate positioning of the catheter and only 14 (29%) of catheter placements required repositioning due to catheter kick back. There were no procedure-related complications, including aneurysm rupture. A post-procedural questionnaire on the usefulness of the technique indicated favorability. In another study, rehearsing on 3D printed models reduced the time of operations for arteriovenous malformations (AVMs) and vein of Galen malformations. It is reasonable to believe that optimal catheter shaping may be related to decreased procedure time and radiation.
3D printed negative molds typically filled with silicone can also be used to create models of aneurysms. In a Japanese study, preoperative simulation of endovascular treatment for cerebral aneurysms was performed using patient-specific distensible vascular silicone models generated from 3D rotational angiographic images. They demonstrated wide necks, tortuous routes of access, and hypoplastic segments. Interventions were simulated, including attempted possible methods for coil embolization, and aided in finalizing an approach and choosing the appropriate devices. The simulations were particularly useful in navigating microcatheters during the actual procedure by facilitating their shaping beforehand. One limitation noted of the silicone models was that the insertion of a catheter or guidewire did not alter vessel shapes the way they do in actuality.
Similar to above, 3D printing technology has also been used to create realistic hollowed, neurovascular models for preoperative flow simulation using clinical devices such as catheters or stents. For example, printed models of cerebral aneurysms may be used to determine which flow diverter device is best suited for treatment and to accurately predict post-treatment flow alterations. , A case report by Sullivan et al. described rehearsal on a 3D printed model prior to treating an 8-year-old boy with a fusiform aneurysm of the supraclinoid segment of the left internal carotid artery (ICA) with a saccular component and documented growth on serial imaging. Due to an increase in size of the parent vessel, attempts to use an off-label Pipeline Embolization Device (Medtronic, Dublin, Ireland) were aborted. The parent vessel was too large for any flow diverter available in the United States (US), and clip placement was not ideal due to the likely dissecting nature of the lesion. The team believed that the patient would benefit from the SILK flow diverter device (Balt Extrusion, Montmorency, France) and US Food and Drug Administration (FDA) and local Institutional Review Board (IRB) approvals were granted. The most recent cerebral angiogram was used to 3D print the patient’s cerebral vasculature. Two strategies were rehearsed using the model, including deployment of a standalone construct with the SILK device extending from the ICA terminus to the ophthalmic segment, versus a dual-device construct using the Leo + stent (Balt Extrusion, Montmorency, France) and the SILK flow diverter. The latter approach failed during simulation due to lack of ideal expansion of the Leo + device in the ICA terminus due to the sharp curvature and diameter mismatch of that segment to the more proximal carotid artery. Because of the simulation results, the patient was treated with the single construct method using the SILK flow diverter only, beginning proximal to the tortuous segment. The device was deployed uneventfully and crossed the entirety of the aneurysm neck with cone-beam CT angiogram demonstrating good wall apposition of the device without parent vessel compromise and favorable 6 month follow-up.
3D printing can also be applied to cerebral AVMs. Printed models have been found to be a useful tool for presurgical planning, allowing for shorter patient consultation time, increased acceptance of the procedure by patients and relatives, as well as shorter time between intraoperative digital subtraction angiography and start of endovascular treatment. Due to the urgent nature of treatment for patients with strokes, 3D printing is not commonly utilized for preprocedure planning for thrombectomy. However, 3D printed flow models have been used to compare thrombectomy approaches and stent retriever performance in stroke models.
3D Printing for IR Training
3D printing has the potential to play a large role in IR education and training for students, residents, fellows, and any physicians looking to adopt a new procedure. The creation of 3D printed anatomical models and training phantoms can prove useful in teaching anatomy relevant to IR procedures, access techniques, and handling of wires and other devices.
Multiple studies have demonstrated 3D printed anatomy to be a more effective teaching model than the conventional cadaver model, with fewer limitations related to cost, reproducibility, and accessibility. Repeated use of a cadaver can also destroy the normal anatomy. Meanwhile, the manufacturing of current traditional noncadaveric medical models is also costly, as well as time-consuming and complex. Models of anatomical structures can be printed with high accuracy and attention to tissue structural detail. A prospective study on veterinary students showed that students who studied using physical 3D printed models outperformed those who used textbooks or 3D computer models in aptitude tests of lower extremity anatomy. In another study, a cohort of 29 medical students demonstrated significant improvement in knowledge acquisition, knowledge reporting, and structural conceptualization of ventricular septal defects after using high-fidelity 3D printed models of ventricular septal defects from MRI data. Physical 3D printed liver models proved to be more effective than a traditional anatomic atlas in teaching hepatic segmental anatomy to medical students in another study. VR technologies may also be useful for IR education. , Table 11.1 describes the pros and cons of each method of teaching for IR education.