Navigational Guidance and Ablation Planning Tools for Interventional Radiology

Current Problems in Diagnostic Radiology ] (2016) ]]]–]]]

Current Problems in Diagnostic Radiology journal homepage: www.cpdrjournal.com

Navigational Guidance and Ablation Planning Tools for Interventional Radiology Yadiel Sánchez, BAa, Arash Anvari, MDb, Anthony E. Samir, MD, MPHb, Ronald S. Arellano, MDb, Anand M. Prabhakar, MD, MBAc, Raul N. Uppot, MDb,n a

Department of Radiology, Harvard Medical School, Massachusetts General Hospital, Boston, MA Division of Interventional Radiology, Department of Radiology, Harvard Medical School, Massachusetts General Hospital, Boston, MA c Division of Cardiovascular Imaging, Department of Radiology, Harvard Medical School, Massachusetts General Hospital, Boston, MA b

Image-guided biopsy and ablation relies on successful identification and targeting of lesions. Currently, image-guided procedures are routinely performed under ultrasound, fluoroscopy, magnetic resonance imaging, or computed tomography (CT) guidance. However, these modalities have their limitations including inadequate visibility of the lesion, lesion or organ or patient motion, compatibility of instruments in an magnetic resonance imaging field, and, for CT and fluoroscopy cases, radiation exposure. Recent advances in technology have resulted in the development of a new generation of navigational guidance tools that can aid in targeting lesions for biopsy or ablations. These navigational guidance tools have evolved from simple hand-held trajectory guidance tools, to electronic needle visualization, to image fusion, to the development of a body global positioning system, to growth in cone-beam CT, and to ablation volume planning. These navigational systems are promising technologies that not only have the potential to improve lesion targeting (thereby increasing diagnostic yield of a biopsy or increasing success of tumor ablation) but also have the potential to decrease radiation exposure to the patient and staff, decrease procedure time, decrease the sedation requirements, and improve patient safety. The purpose of this article is to describe the challenges in current standard image-guided techniques, provide a definition and overview for these next-generation navigational devices, and describe the current limitations of these, still evolving, next-generation navigational guidance tools. & 2016 Elsevier Inc. All rights reserved.

Introduction Image-guided interventional radiology (IR) is a dynamic subspecialty that has revolutionized the diagnosis and treatment of many medical conditions. The modern IR suite developed during the late 1980s, has evolved from the use of fluoroscopy, to the use of ultrasonography (US), computed tomography (CT) scan, and even magnetic resonance imaging (MRI), and has profoundly expanded the capabilities of an IR practice.1,2 Central to all image-guided procedures is successful targeting of a lesion.3 Although CT, US, and less commonly MRI have been used to target lesions for biopsy or ablation for many years, there are still challenges in using each of these modalities (Table 1).

Challenges of CT-Guided Procedures CT is widely used at many institutions to perform many interventional procedures including biopsies, drainages, and ablations. Many institutions now have dedicated interventional CT scanners that are used only for interventional procedures, and n Reprint requests: Raul Uppot, MD, Division of Interventional Radiology, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 55 Fruit St, Gray 290, Boston, MA. E-mail address: [email protected] (R.N. Uppot).

http://dx.doi.org/10.1067/j.cpradiol.2016.11.002 0363-0188/& 2016 Elsevier Inc. All rights reserved.

some of these have larger gantry diameters to accommodate the patient and the interventional equipment. Standard CT-guided procedures are performed either using a grid (Fig 1) or using an electronic grid and CT gantry laser light to localize the entrance site to the target, with subsequent careful advancement of a needle guided by intermittent multiple localized scans. Alternatively, by using CT fluoroscopy (Fig 2), the needle can be advanced under real-time or near real-time guidance. Standard procedural steps for a CT-guided biopsy are outlined in Table 2. Currently, for all CT-guided procedures, the objective information that can be gathered from the preliminary scan for planning include (1) lesion location (if lesion can be visualized), (2) needle entry site, and (3) depth of the target. A critical missing component is the trajectory angle to the lesion. Although the trajectory angle to the lesion can be measured objectively on the preliminary CT scans, it is dependent on the subjective sense of spatial orientation of the interventional radiologist at the point of care and is dependent on the experience of the interventional radiologist.4 If the interventional radiologist is not certain of the path of needle trajectory and what structures can be injured by needle transversal, he/she would need multiple confirmatory CT scans to carefully advance the needle. This uncertainty increases procedure time, increases radiation dose, and risks injury to adjacent vulnerable structures. Although CT fluoroscopy can decrease procedure time by allowing needle advancement to occur real time or near real time, the trade-off is increased radiation dose to the

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Table 1 Description of the current strengths and limitations of current imaging modalities Modality

Strength

Weakness

US

Real-time guidance No ionizing radiation Portable Inexpensive

2D, lack of multiplanar information Not practical for small or isoechoic lesions Microbubble formation during an ablation can cause artifacts with posterior shadowing Operator dependency

Fluoroscopy

Real-time feedback

Limited ability to easily navigate out of its plain Increased radiation exposure to the patient and staff

CT

Excellent spatial resolution

The iterative nature for guidance that causes increasing radiation to the patient and staff and limits the speed of the procedure Lack of functional activity information High costs

May be real time with CT fluoroscopy

MRI

Excellent contrast resolution No ionizing radiation

Need MR safety monitoring and MR-compatible procedural equipment Interference of MR radiofrequency (RF) signal with ablation RF High costs

interventional radiologist (who must now wear leaded protective gear) and potential increase in radiation dose to the patient5,6; current research and advancements in interventional robotics may solve this problem of excess radiation exposure to the physician and staff.7 Additional challenges in CT-guided procedure include (1) potential poor visibility of the lesion which may only be seen on arterial or portal venous or delayed-phase imaging or as an fluorodeoxyglucose avid PET-CT lesion8 that is not seen on CT images, (2) challenging access to structures such as a high dome liver lesion requiring a transpleural approach, (3) the trajectory to the target blocked by critical structures, and (4) motion (patient motion, breathing motion, bowel peristalsis, and large vessel pulsation).9,10 Challenges of Ultrasound-Guided Procedures At many institutions, ultrasound is routinely used for biopsies, ablations, drainages, and fluid aspirations (among others). Ultrasound guidance offers the benefits of real-time imaging without ionizing radiation (Table 3). Ultrasound, however, is limited by the clinical experience of the interventional radiologist. Also, some lesions are not visible on sonography because of the inherent characteristics of the lesion itself or the surrounding tissue consistency, lesion location (intervening bowel loops or bone), or patient body habitus. In addition, respiratory and patient motion

Fig. 1. Axial CT scan with grid. Radio-opaque grids are routinely used to identify the skin entrance site to a lesion (green arrows). The selected slice provides craniocaudal entry site, and grid provides mediolateral entry point. (Color version of figure is available online.)

may make visualization of the needle tip and the lesion challenging (Fig 3). These limitations can increase procedure time and sedative doses and lower the number of elective procedures that can be performed in a given time. Challenges of Interventional MRI Interventional MRI is not widely available. Interventional MRI is occasionally used at institutions for image-guided ablations.11-13 The benefits include no radiation and good tissue contrast, likely resulting in great visibility of the targeted lesion. MRI also offers the advantage of being able to detect lesions that are not that conspicuous on either CT or US. Another advantage is for complexangled cases not just medial-lateral angles but off angles combined with cranial-caudal angles. The limitations include challenging access to the patient, not compatible in a patient with pacemakers or other metallic implants, ensuring that all interventional equipment is magnetic resonance (MR) compatible, and higher costs. In addition, many of the same limitations described for CT-guided procedures are also true for interventional MRI, including the limited number of scanners in a regular hospital setting. Other Challenges of Current Image-Guidance Tools In recent years, there has been an explosion in the development of new diagnostic imaging tools to better identify and characterize lesions including PET-CT, Eovist MRI, and PET-MRI. Although these new diagnostic imaging tools are more sensitive at detecting and characterizing lesions, often a biopsy is still required for pathologic confirmation, and the sensitivity in detecting these lesions on new, advanced diagnostic tools is not translated to a preprocedure CT or US. Many times if findings on a diagnostic PETCT or Eovist MRI are not seen on the preprocedure planning CT or ultrasound, the interventional radiologists have to visually fuse preplanning CT images with diagnostic scans with a side-by-side comparison and identify landmarks that can offer clues as to the true location of a lesion otherwise invisible on preprocedure planning images (Fig 4). Addressing these challenges of current imaging-guidance equipment, the development of next-generation navigational guidance tools offers the potential to more accurately and efficiently target a lesion with less radiation dose. We now review some of these navigational guidance systems, show they are potential at aiding current techniques, and describe some of their limitations in today's clinical practice.

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Fig. 2. CT fluoroscopy screen shot. CT fluoroscopy allows near real-time advancement of needle to its target. Large middle image represents the centered entry site. On the right panel, the second image represents a slice 5 mm cranial, and the third (lowest) right panel image represents a slice 5 mm inferior to centered site. (Color version of figure is available online.)

Definition and Background “Navigational guidance” means the ability to guide an instrument (needle, applicator, or catheter) from one point to another within the patient's body. They are novel and evolving technologies that can be used in conjunction with current imaging modalities to improve and expedite targeting. Although the focus of this review is on the use of these devices to assist with biopsies and ablations, Table 2 Procedural steps for standard CT-guided biopsy or ablation. Items in red font represent challenges for the interventional radiologist

these devices may be used for other IR procedures including abscess drainages, targeting biliary ducts, or renal calyces. Indications Navigational guidance tools can aid current image-guided procedures. They can help in cases where the lesion is not well seen on ultrasound, in cases where the lesion is only seen on fleeting arterial phase of imaging, lesions that are fluorodeoxyglucose avid on positron emission tomography (PET) but not seen on CT, and to ensure complete ablation volume coverage (Table 4).

Table 3 Procedural steps for standard US-guided biopsy or ablation. Items in red font represent challenges for the interventional radiologist

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Y. Sánchez et al. / Current Problems in Diagnostic Radiology ] (2016) ]]]–]]] Table 4 Potential benefit of navigational guides Clinical scenarios where a navigation system may be helpful in image-guided interventions Poor target visibility

Fig. 3. Ultrasound image needle tip targeting a hypodense lesion. The technical challenge in ultrasound is to align lesion with thin ultrasound beam with thin needle in a moving (breathing or patient or organ motion) lesion.

Evolving Components Navigational guidance tools can be divided into 5 components that have evolved over time, from simple hand-held tools to incorporation of technological hardware and software advances. These components include (1) trajectory guidance, (2) electronic needle visualization, (3) image fusion, (4) body global positioning system (GPS), and (5) ablation volume planning. We review each of these components and describe their strengths and limitations. Some of these tools and devices are available for clinical use in the United States, whereas others have only been used for research purposes and may only be available for clinical use in other countries. A few have been tested at our institution but are no longer clinically available. Trajectory Guidance A basic challenge in image-guided procedures is trajectory guidance to the lesions. Although the precise entrance site and depth to the lesion can objectively be identified and measured, the angle and trajectory to reach the target are subjective assessments that must be made at the point of care. Navigational tools have been developed to aid in trajectory guidance and have evolved from simple hand-held trajectory tools to electromagnetic and optical guidance devices. Ultrasound Probe Guides Historically, the earliest trajectory guidance tools were simple ultrasound probe guides for trajectory guidance. These probe

Small lesions Lesions under bone or calcifications Lesions not seen on US Lesions or its margins only on enhanced CT/MR, PET-CT Overlapping ablation zones During “gas-out” from tissue ablation

Challenging access to target Complex angle of insertion Liver dome and renal hilum Complex anatomical path Critical adjacent structures Challenging intervention plan

Complex geometry of lesion Composite ablations Avoiding necrotic area in biopsy Ablation of residual or recurrence of lesions

Target requiring precision access

Motion compensation

guides can be snapped onto to the side of a transducer and can guide needle along a predefined angle that intersects with the ultrasound beam. Although beneficial in its simplicity and portability, its limitation is that one is locked to a predefined angle that may not be the most suitable trajectory to the lesion. Also, it is difficult to secure in place with probe covers in place as well. Hand-Held Trajectory Guidance for CT For CT, a simple hand-held device (Fig 5) can allow the measured angle on CT to be reproduced at the point of care. The measured angle on the planning CT is manually set on this device. The hand-held device can be placed in a sterile cover at the entrance site and the needle guided from entrance site to target guided by the device to the measured depth. The benefits of this device are its simplicity and portability. Its limitations include manual operation that can be prone to operator errors in positioning and angulation at the entry site, limitations of patient, and breathing motion that can change the internal structures along the pathway of the planned trajectory.

Image-Based Navigational Tools in CT-Guided IR Image-based tracking is the most commonly used tracking modality in vascular applications. It can also be used in needlebased interventions. CT Fluoroscopy CT fluoroscopy is a navigational system in IR procedures that provides a real-time visualization of a wide field of view in anatomy, but it gives only a 2D view with poor soft-tissue contrast resolution. The most obvious application of CT fluoroscopy is imaging-guided biopsy (Fig 6A). Advantages of CT Fluoroscopy Fig. 4. CT-guided procedure planning using prior pelvic MRI. Preplanning a CTguided procedure often requires comparison to a prior diagnostic CT or MRI. Interventional radiologist often has to mentally compare and fuse 2 images on 2 different screens to plan for the procedure. (Color version of figure is available online.)

(1) Faster procedural times, particularly useful for more lengthy procedures that require general anesthesia. (2) Lessen targeting errors due to breathing and patient motion.

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Advantages of CBCT (1) Increase the spatial resolution owing to its real-time fluoroscopic and 3D image reconstructions. (2) Temporal resolution and dynamic range.

Disadvantages of CBCT (1) Motion: because of the relatively long acquisition times of Carm CBCT systems compared to multislice helical CT, motion artifacts can arise. (2) The significant radiation burden to patients and operators. Fig. 5. Hand-held trajectory-guidance device for CT-guided procedures. (Color version of figure is available online.)

Electronic Needle Visualization Disadvantages of CT Fluoroscopy (1) A high amount of radiation both to patients and to operators. (2) 2D image reconstruction.

Rotational Angiography–Based Tools: CBCT Cone-beam CT (CBCT) combines real-time fluoroscopic image with the ability to get a cross-sectional CT.14-19 Cone-beam CT has been used extensively for angiographic procedures such as transarterial chemoembolization but is now slowly growing in its use for nonvascular interventional procedures as well. CBCT technology has been developed and evolved since late 1990s. This technology has the potential to significantly affect the practice of IR, as it can perform near real-time 3D imaging in the interventional suite, in hopes of improving or facilitating needle, catheter, wire, or device placement. CBCT provides a 23- or 46-cm field of view “CT-like” image from an angiography machine after rotation of the C-arm around the patient. It is a modality that enables the generation of entire volumetric dataset in a single gantry rotation by using a 2-dimensional detector system compared to 1-dimensional detector systems in conventional CT (Fig 6B). Abi-Jaoudeh et al evaluated the use of CBCT in 16 patients who underwent 20 ablations procedures for a total of 29 lesions in different areas of the body, which included the liver, the lungs, the psoas muscle, and the intercostal muscles. Of the 29 procedures performed, 28 were technically successful.20 Follow-up at 1 month and a second follow-up at approximately 20 months showed a technical success of 96.1% and 84.6%, respectively, for procedures performed with curative intent.

Evolving from trajectory guidance, the next evolutionary step was electronic needle visualization. Tiny sensors embedded in the tip of the needle can be electronically tracked and displayed on a screen allowing for real-time visualization of the needle tip (Fig 7). This technology can aid ultrasound and CT-guided procedures and may be used in conjunction with body GPS described below.

Image Fusion Image fusion is the process of superimposing different modality (MRI, PET, CT, and US) datasets into 1 frame that was acquired at different times and from different viewpoints. Image fusion was first used in diagnostic radiology to combine functional imaging (such as PET) with anatomical imaging (such as CT or MR) to increase accuracy of diagnosis.21 Recently, image fusion has also been introduced in percutaneous image-guided procedures, initially for biopsies and ablations, and then in other vascular and nonvascular procedures.22,23 Image fusion often allows combining the strength of modalities with higher spatial resolution (CT or MRI) than those with higher temporal resolution (ultrasound) for real-time feedback. Image fusion represents the following 2 components: image co-registration (alignment) and image fusion (overlay). Image co-registration consists of aligning or matching the 2 imaging datasets spatially to each other. It can be automated, semiautomated, or manual. Automated co-registration has the following 2 steps: the first is volume (voxel) reformatting of one image dataset with the other image dataset; the second entails the matching of the reformatted image spatially with the other image. Co-registration can be rigid or flexible. Rigid co-registration matches 2 image dataset just by panning and rotation in each

Fig. 6. (A) Images from arterial embolization of a tumor showing overlay and fusion of a contrast-enhanced 3D image of a vessel with real-time fluoroscopic images. (B) C-arm cone-beam CT suite. CBCT is an image-based tracking system, which can facilitate trajectory guidance. (Color version of figure is available online.)

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(2) A computer workstation and user interface that allows one to load preprocedure scans and selects a target, entry site, and trajectory. (3) An electromagnetic generator that is placed near the intervention site that allows tracking of the fiducials and needle. (4) A tiny sensory coil that is usually embedded at the tip of the needle, which sends localizing data to tracking software.

Fig. 7. Components of electronic needle tracking system. (Color version of figure is available online.)

voxel. Flexible registration matches 2 image dataset by stretching (besides panning and rotation), which improves matching of anatomical landmarks. Flexible registration can improve accuracy in procedures where organ deformation is caused by rigid needle placement. Flexible co-registration can be very time consuming as initial registration can take up to 20 minutes and any additional registration needs 10 minutes. Semiautomated co-registration allows for manual editing of the automated results. Manual methods require landmarks that are matched point to point. It may be more accurate but are more time consuming, which may not be an appropriate method during the procedure. Semiautomatic method uses rough automatic matching between 2 datasets followed by manual matching for more accuracy. Image fusion (co-display) is superimposing of co-registered images acquired from different modalities (US, CT, MRI, or PET) into 1 frame. Limitations of Fusion The accuracy of the image fusion depends completely on the fidelity of the registration. The most important challenges are organ motion, which is associated with patients positioning; organ deformation during the procedure; extrinsic compression by pneumothorax; hematoma; and respiratory movement.24

Body Gps Evolving from electronic needle visualization are newer devices that not only track the needle but also give a simulated view of internal organ structures—a body GPS. Body GPS is achieved by placing fiducials on preprocedure images or by co-registering anatomical landmarks such as the bifurcation of the portal vein on 2 different imaging modalities. By providing an internal simulated view of the body and identifying the exact needle position, these devices potentially increase confidence in needle placement and potentially decrease CT radiation dose. Two such devices exist; one uses electromagnetic fields for tracking and the other uses optical tracking.

The electromagnetic generator creates a low-frequency differential electromagnetic field that is transmitted through the patient's body. This field provides 3D spatial data about internal organs that are sent to the tracking workstation. The 3D spatial data combined with real-time needle tracking (with a sensor embedded in the needle) allow direct guidance to the target.27 After placing fiducials, co-registering images, and identifying entrance site and target, interventional radiologists using this device would just have to line up cross hairs to advance the needle or probe to the target. Procedural imaging may also be co-registered to previous imaging modalities such as a prior diagnostic PET-CT or MRI where the lesion or target is better visualized. Krücker et al evaluated the spatial accuracy provided by electromagnetic needle tracking in 20 patients who underwent either biopsy or radiofreguency ablation. The authors found no limitations in the usual workflow and reported no complications using this tracking method. The authors also reported successful image fusion of CT and US. Needle tracking errors (defined as the virtual position of the needle provided by the tracking software compared to the actual needle position via confirmation imaging with CT) were found to be negative correlated with the distance to the CT table, although the results were not statistically significant. Adding additional fiducial devices helped minimize tracking errors.28 At our institution, we performed a right renal cell carcinoma ablation using this navigational device. Postcontrast CT images were used to identify the lesion and as a baseline for preprocedure planning and probe placement. Confirmatory CT was used to confirm the proper needle placement, and thermal ablation was carried out with no complications (Fig 8). Follow-up CT at 1 and 19 months showed successful tumor ablation (Fig 9). Optical Tracking Technique Optical tracking is another technique used for navigational guidance.27,29 Instead of using an electromagnetic detector field, it uses a small optical camera to aid with trajectory guidance (Fig 10).

Electromagnetic Tracking Electromagnetic tracking allows the creation of a body GPS and can also electronically track the needle.23,25,26 It has 4 components: (1) Electromagnetic fiducial patches placed on a patient's body before preprocedure scan.

Fig. 8. Confirmatory CT was used to confirm proper probe placement for lesion targeting.

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Fig. 9. Follow-up at 1 month (left image) and at 19 months (right image) after right renal tumor ablation.

This optical tracking method has 3 components: (1) An optical fiducial patch, which is placed on the patient's skin overlying the needle entry. It has colored radio-opaque reference markers that are used by navigational software to register the optical sensor position into the CT image dataset. (2) An optical sensor, which is miniature video camera that is clipped on the needle outside of the patient's body. The camera is connected to a computer with a navigational software. (3) A computer workstation and user interface loaded with the tracking software. Using such a device, interventional radiologists would identify a target, an entrance site, and trajectory on the planning workstation, and then by lining up the needle with a camera onto a fiducial patch, they can direct the needle to the target.22 Optical tracking has being used in MRI-guided percutaneous abscess drainage of the brain with encouraging results.30 It has also been found that combining optical navigational systems with low-dose CT may also reduce the radiation burden of patients when performing certain procedures such as lung biopsies.31 The electromagnetic tracking technique has been studied more than the optical tracking technique for percutaneous image-guided procedures. Each device has its own advantages. The fiduciary sensor in the electromagnetic method is generally located at the tip of the needle, which makes it less vulnerable to registration and placement errors because of needle deflection. In optical tracking, the camera is clipped on the handle of the needle outside of the patient, which is vulnerable to needle deflection unless a rigid needle is used. Strategies to compensate for organ deformation caused by breathing using optical tracking device have been tested and have demonstrated to be a rapid and feasible

alternative, although more studies are needed to confirm these results.32 The accuracy of all tracking systems is in the range of 1-5 mm.11 The optical tracking method with a rigid needle usually has the highest accuracy (o 1 mm), and metallic environments do not affect its accuracy. The accuracy of electromagnetic tracking systems is less than 5 mm but because the tip of the needle can be tracked directly, they are still accurate enough for many applications.26,33 Limitations of Electromagnetic and Optical Tracking The biggest limitation of electromagnetic and optical tracking is motion.22 Images used for needle placement are not real-time images but simulated images from procedure planning scans, and any intervening motion (breathing, organ motion, and patient motion) would not be reflected in the needle placement images. Although fiducials placed on the patient can perform respiratory gating (and indicate motion changes on the display), ultimately, the interventional radiologist relies on a virtual image obtained a few minutes prior and not a true real-time image. This limitation may be overcome by fusing CT images with real-time ultrasound and then using these ultrasound images to also help guide the needle. Other limitations of electromagnetic tracking include limited volume coverage of the electromagnetic detector (between 40  40 and 70  70 cm2), which requires that the electromagnetic generator be placed close to intervention area.24 Also, large metallic objects or other magnetic material in the field of procedure can cause interference with the electromagnetic field and reduce its accuracy.23,26 Also, electromagnetic equipment cannot be used for MR-guided procedures.

Fig. 10. (A and B) Components of optical needle tracking system. (Color version of figure is available online.)

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Fig. 11. Postablation bed segmentation with sagittal and 3D view demonstrates residual tumor burden (red) outside the postablation zone (blue). (Color version of figure is available online.)

Other limitations of optical tracking include the camera's need for a clear and direct pathway (a clear “line-of-sight”) between the fiducial marker and the camera clipped on the needle. It is also dependent on rigid device because it tracks the handle of the needle (not the tip), so even small bends of needle can induce placement error.27 Finally a limitation for both of these devices is the setup time, which can add 15-60 minutes for fiducial placement, co-registration, and trajectory planning. Use of these devices for simple lesions may decrease the CT radiation dose but may add overall time to the procedure.

Ablation Volume Planning Image-guided ablation not only requires accurate targeting of the tumor but also requires understanding the entire 3 dimensional volume of the tumor and surrounding structures to insure adequate ablation coverage of the tumor plus a safe 5-10 mm margin. Currently, targeting of the tumor can be performed with ultrasound and under CT or MR guidance; treatment monitoring can also be performed with CT or MR. With the exception of cryoablation, where the actual ablated volume can be visualized in real time as a growing low-density “ice ball,” for all other ablation modalities (radiofrequency, microwave, and irreversible electroporation) the ablated volume is estimated based on manufacturer provided estimates from in vivo studies or from the personal clinical experience of the interventional radiologist. Therefore, the “holy grail” in image-guided ablation has been to develop software that can accurately identify in 3 dimensions the tumor volume and the ablated volume. Such software can provide the interventional radiologist an intraprocedural confidence that the entire volume of

Fig. 12. 3D segmentation image showing segmented tumor bed (red) and segmented ablated tumor bed (blue). (Color version of figure is available online.)

the tumor and margin have been ablated. This software requires automated segmentation of the visible tumor volume, automated segmentation of the ablated bed, and co-registration of the 2 images separated in time (equivalent to the procedure time) to ensure that all the margins of the tumor have been covered by the ablated bed. Although software by different manufacturers has been in testing phase, none are currently clinically available for ablation planning. We had the opportunity to test similar software at our institution for a microwave ablation of a left hepatic lobe hepatocellular carcinoma. Tumor segmentation software was used to identify the tumor and the ablation zone volume. Postablation bed segmentation with sagittal and 3D view was acquired and demonstrated residual tumor burden outside the postablation zone (Fig 11). Subsequently, a new ablation zone was designed to cover the residual tumor burden, and postablation bed segmentation confirmed a complete ablation of the tumor (Fig 12). Current limitation of the tested software include inaccurate segmentation of the tumor or the ablated bed, difficulties with coregistering 2 images due to breathing motion, patient motion, changes in patient positioning between 2 scans (supine vs lateral decubitus vs prone), requirements that axial slices obtained for planning be 1.25 mm axial sections (thereby increasing radiation dose), and changes required to workflow (contrast for planning CT to identify arterially enhancing lesion vs contrast at the conclusion of the procedure to more clearly define ablated bed).

Conclusion The high demand for minimal invasive techniques has surge over the last decade, and IR has been at the forefront of this revolution. Navigational guidance tools are evolving technologies that can have an important role in the future of IR. They can potentially enhance procedure accuracy, efficacy, and safety while reducing other factors such as procedure time, radiation dose, and complications; it can also help decrease the long learning curve for trainees in IR. There is little doubt that these tools will continue to evolve and may someday become the standard of care for minimal invasive treatments. Although many institutions in the United States have been testing and using some of these devices (many even used clinically at European and Asian institutions), there has yet to be widespread adoption of these newer tools owing to some limitations. One limitation of all these devices is per case setup time. Until these devices can be incorporated into the current workflow of a CT or US-guided procedure, the extra steps for registering and planning may not provide overall savings in procedure time. For simple biopsies, most well trained interventional radiologists can perform the procedures safely and effectively, and use of such devices can actually slow down the case.

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A second limitation in the adoption of these devices is cost. Costs for these devices range from a few hundred dollars to a few thousand dollars and varies based on the capabilities of the technology. Although a detailed cost analysis is beyond the scope of this article, purchasing these tools can result in additional costs to standard image-guidance tools (ultrasound, CT, and fluoroscopy) and can only be recouped with potential savings in procedure times. However, potential nonmonetary added benefits include quality and safety improvements for patients, decreased complication rates (although currently not yet proven in the literature), and decreased radiation exposure, especially for the operator if combined with modern robotic systems. In the future, there is a potential in paying for some of these systems by adding current procedural terminology “planning codes” as those codes used in radiation planning by radiation oncologists. Despite these limitations, previous use of these devices have shown a lot of potential, and there is no doubt that these technologies will find their way into the armamentarium of interventional radiologists in the near future. References 1. Bakal CW. Advances in imaging technology and the growth of vascular and interventional radiology: A brief history. J Vasc Interv Radiol 2003;14 (7):855–60. 〈http://www.ncbi.nlm.nih.gov/pubmed/12847193〉 Accessed June 1, 2014. 2. Rösch J, Keller FS, Kaufman JA. The birth, early years, and future of interventional radiology. J Vasc Interv Radiol 2003;14(7):841–53. 〈http://www.ncbi.nlm. nih.gov/pubmed/12847192〉 Accessed June 1, 2014. 3. Goldberg SN, Grassi CJ, Cardella JF, et al. Image-guided tumor ablation: Standardization of terminology and reporting criteria. Radiology 2005;235 (3):728–39, http://dx.doi.org/10.1148/radiol.2353042205. 4. Maybody M, Stevenson C, Solomon SB. Overview of navigation systems in image-guided interventions. Tech Vasc Interv Radiol 2013;16(3):136–43, http: //dx.doi.org/10.1053/j.tvir.2013.02.008. 5. Kloeckner R, dos Santos DP, Schneider J, et al. Radiation exposure in CT-guided interventions. Eur J Radiol 2013;82(12):2253–7, http://dx.doi.org/10.1016/j. ejrad.2013.08.035. 6. Figueira C, Becker F, Blunck C, et al. Medical staff extremity dosimetry in CT fluoroscopy: An anthropomorphic hand voxel phantom study. Phys Med Biol 2013;58(16):5433–48, http://dx.doi.org/10.1088/0031-9155/58/16/ 5433. 7. Kassamali RH, Ladak B. The role of robotics in interventional radiology: Current status. Quant Imaging Med Surg 2015;5(3):340–3, http://dx.doi.org/10.3978/ j.issn.2223-4292.2015.03.15. 8. Giesel FL, Mehndiratta A, Locklin J. Image fusion using CT, MRI and PET for treatment planning, navigation and follow up in percutaneous RFA. Exp Oncol 2009;31(2):106–14. 〈http://www.pubmedcentral.nih.gov/articlerender.fcgi? artid=2850071&tool=pmcentrez&re ndertype=abstract〉 Accessed June 1, 2016. 9. Srimathveeravalli G, Leger J, Ezell P, et al. A study of porcine liver motion during respiration for improving targeting in image-guided needle placements. Int J Comput Assist Radiol Surg 2013;8(1):15–27, http://dx.doi.org/10.1007/ s11548-012-0745-y. 10. Najmaei N, Mostafavi K, Shahbazi S, et al. Image-guided techniques in renal and hepatic interventions. Int J Med Robot 2013;9(4):379–95, http://dx.doi.org/ 10.1002/rcs.1443. 11. Clasen S, Pereira PL. Magnetic resonance guidance for radiofrequency ablation of liver tumors. J Magn Reson Imaging 2008;27(2):421–33.

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