Intraprocedural imaging: Flat panel detectors, rotational angiography, FluoroCT, IVUS, or still the portable C-arm?

Abstract

Fluoroscopy is the current standard by which intraoperative imaging is performed during endovascular aortic interventions. The goal of the imaging system is to provide adequate visualization of the aorta and its branches throughout the entire range of the treatment area. This may range from the femoral arteries through the ascending aorta. While the endovascular treatment of descending thoracic aortic aneurysm and abdominal aortic aneurysms (AAA) is becoming commonplace, the evolution of this specialty is toward providing less invasive approaches to increasingly complex disease processes. The imaging systems that allow us to perform these procedures through small incisions, or needle punctures, must evolve with the device technology to facilitate this growing complexity. In addition to providing adequate visualization of the aorta, its branches, and the tools that are used to place them, the technology needs to be safe, for both the patient and the operative team. The imaging system should allow the operator to perform complex procedures with limited use of radiation and contrast agents. Limiting radiation exposure is also important for the operative team, as radiation effects are cumulative and will become compounded over a career of fluoroscopy-directed procedures. While it is easy to stay up-to-date with the latest endovascular therapies that may be offered to our patients, as vascular surgeons it is often more difficult to keep apprised of the advancements in imaging technology that will allow us to provide therapies in a more efficient and safe fashion. This review will highlight some of the current advances in imaging that are available. The basic design of current fluoroscopic imaging systems used in vascular angiography (with both fixed imaging systems and portable C-arm machines) involves an image intensifier (II) in combination with camera optics, a pickup tube or charge-couple device (CCD)-based camera, an analog to digital converter, and image processors for the creation of the angiographic images. The system possesses the ability to update an image at rapid rates, and must be able to do so with clarity and as little radiation as possible. Modern-day image intensifiers function when X-rays are detected and converted into light photons by the cesium iodide (CI) input phosphor scintillator (Fig 1). A corresponding number of electrons are released by the photocathode, and the electrons are accelerated (in a vacuum) and focused to an output phosphor using a cylindrical electronic focusing gradient. Light photons are produced at the output phosphor. Optical lenses couple the light emitted from the output phosphor to the TV camera, creating a video signal subsequently re-rendered on a video monitor in a closed-circuit configuration for viewing the X-ray fluoroscopy sequence. The image intensifier performs this conversion process at required repetition rates using relatively small numbers of incident X-ray photons – thus occurring quickly with small amounts of radiation. This sequence of events occurs in analog, and as such it is subject to image degradation. This occurs due to deviation of the process from perfect proportionality or spatial linearity, or through the introduction of noise at one or more of the stages that leads to a departure from the assumed direct relationships between the stages.1Holmes Jr, D. Laskey W. Wondrow M. Cusma J. Flat-panel detectors in the cardiac catheterization laboratory: revolution or evolution - what are the issues?.Catheter Cardiovasc Interv. 2004; 63: 324-330Crossref PubMed Scopus (22) Google Scholar These potential sources of error highlight the limitations of the standard imaging equipment that may be overcome with newer technologies. Sources of error that are attributable to the standard image intensifier include scatter radiation, spatial distortion, input phosphor blurring, contrast variation, veiling glare, and quantum noise.1Holmes Jr, D. Laskey W. Wondrow M. Cusma J. Flat-panel detectors in the cardiac catheterization laboratory: revolution or evolution - what are the issues?.Catheter Cardiovasc Interv. 2004; 63: 324-330Crossref PubMed Scopus (22) Google Scholar, 2van der Zwet P. Meyer D. Reiber J. Automated and accurate assessment of the distribution, magnitude, and direction of pincusion distortion in angiographic images.Invest Radiol. 1995; 30: 204-213Crossref PubMed Scopus (18) Google Scholar Scatter radiation is due to a reduction in image contrast that is common to all systems that utilize a broad-beam imaging geometry and is a by-product of the X-ray source used. Spatial distortion is due to the fact that the image surface of the image intensifier is curved and the resulting image is mapped onto a flat surface. This leads to pin cushion distortion (Fig 2). This distortion increases the further one gets from the center of the image. Input phosphor blurring is due to propagation of light in the transverse direction within the phosphor layer. X-ray photons from one location cause electrons to be emitted from another, adjacent location. As a result, the image blurs. There is a trade-off between the thickness of the phosphor layer, which makes it more efficient, and a lower degree of blurring. The absorption characteristics of the input phosphor determine how much variation occurs from location to location within an image. The efficiency for absorbing X-rays depends on their energy, and variations result in differing amounts of contrast in the corresponding image locations. Veiling glare is defined as the image degradation that results from the scattering process that occurs within the image intensifier. Quantum noise is due to the finite number of X-rays used to form the image. As the number of X-rays used to produce a signal at a particular location is reduced, the relative amount of noise increases, limiting the ability to discriminate low contrast signals in the resulting image. Another limitation of conventional imaging systems is the video camera systems. Video cameras convert the image intensifier output image into a format that can be displayed on a television monitor. Two major categories of video cameras are used in these systems, pickup tube and CCD cameras. Pickup tube cameras use an electron beam to scan the camera target in a continuous pattern, which generates a voltage for each location on the target. A CCD camera has a target that is composed of a rectangular array of discrete cells. Each CCD cell captures the corresponding portion of the light image and resulting in the production of a voltage signal, which is read and processed in a manner identical to that of the pickup tube's output signal. As with the image intensifier, the cameras add a layer of limitations to the imaging system. The cameras have limits with regard to spatial resolution and dynamic range. More unique to the camera system, however, is the lag that can occur. This is the result of information from the previous image remaining after the target has been scanned. This results in either a blurring of the image, or an exposure shadow of a rapidly moving object such as a wire or catheter. Flat panel detectors (FPD) have begun to replace the standard imager intensifier and video camera systems in order to overcome some of their limitation. Flat panel technology is based on the use of a thin film transistor (TFT) that utilizes an amorphous silicon array. This system is arranged as a row and column array of detector elements (Fig 3). Within each detector element are the TFT, a charge collection electrode, and a charge collection capacitor. Interconnecting each element via the TFT and capacitor are “gate” and “drain” lines.3Seibert J. Flat-panel detectors: how much better are they?.Pediatr Radiol. 2006; 36: 173-181Crossref PubMed Scopus (86) Google Scholar The TFT switch is closed during the exposure, and incident X-rays interact with the converter producing a corresponding charge that is stored in the local capacitor. When the X-ray exposure is terminated, one gate line at a time is set high to activate all connected TFTs along the row, where the charge flows from the local capacitors through the transistors and down the drain lines in parallel to the output charge amplifiers.3Seibert J. Flat-panel detectors: how much better are they?.Pediatr Radiol. 2006; 36: 173-181Crossref PubMed Scopus (86) Google Scholar The output signal is digitized and the digital image is constructed one row at a time. Deactivating the gate line resets the TFTs for the next exposure, and the adjacent gate line is activated for the next row of data. The process continues until the whole array is analyzed. For real-time fluoroscopic imaging, the readout procedure must occur fast enough to acquire data from all detector elements over a period of 33 ms or 30 frames per second.3Seibert J. Flat-panel detectors: how much better are they?.Pediatr Radiol. 2006; 36: 173-181Crossref PubMed Scopus (86) Google Scholar This speed places high demands on the switching characteristics of the TFTs, charge/discharge rate of the capacitors, and the ultimate speed of the output phase. Significant differences exist in FPDs with regard to X-ray detection and signal conversion – and represent the difference between indirect and direct conversion. Indirect detectors use a phosphor material that absorbs X-rays and produces a proportionate number of light photons that subsequently interact with a photodiode electrode on the TFT array, producing a corresponding charge in the detector element capacitor. The phosphors are typically characterized as unstructured or structured. Unstructured phosphors are typically lower-cost than structured phosphors, but there is a significant tradeoff between spatial resolution and absorption efficiency. A structured phosphor, however, is able to confine light photons, thus limiting lateral light spread and providing high resolution despite a thick phosphor layer that provides good absorption efficiency. Given these qualities, structured phosphors are more widely used despite their increased fragility. Direct detectors use a semiconductor material sandwiched between two electrodes that absorb and convert X-ray energy directly into ion pairs. Amorphous selenium is the current material used for this. A large voltage bias is placed between the electrodes in order to keep the charge confined to the detector and to prevent ion pairs from recombining.3Seibert J. Flat-panel detectors: how much better are they?.Pediatr Radiol. 2006; 36: 173-181Crossref PubMed Scopus (86) Google Scholar This provides a simpler TFT structure and allows for high intrinsic spatial resolution. It is less efficient, however, and has a greater lag than indirect detectors. Clinical radiography results have demonstrated the clear superiority of FPD systems over conventional radiography devices.4Rong X. Shaw C. Liu X. Lemacks M. Thompson S. Comparison of an amorphous silicon/cesium iodide flat-panel digital chest radiography system with screen/film and computed radiography systems - a contrast detail phantom study.Med Phys. 2001; 28: 2328-2335Crossref PubMed Scopus (81) Google Scholar, 5Floyd C.J. Warp R. Dobbins 3, J. Chotas H. Baydush A. Vargas-Voracek R. et al.Imaging characteristics of an amorphous silicon flat-panel detector for digital chest imaging.Radiology. 2001; 218: 683-688Crossref PubMed Scopus (126) Google Scholar There are several characteristics of the FPD that make them more advantageous than conventional image intensifier/TV display.3Seibert J. Flat-panel detectors: how much better are they?.Pediatr Radiol. 2006; 36: 173-181Crossref PubMed Scopus (86) Google Scholar With the FPD there is less image distortion, better image uniformity and flat field capabilities, no veiling glare, and no vignetting as is seen with conventional systems. FPD have a rectangular field of view and use the entire image monitor. In addition, FPD have a small compact design allowing for improved patient access (Fig 4). Detective quantum efficiency (DQE), a measure of a detector's ability to preserve information in the image relative to the incident X-ray information presented at the phosphor, is higher for the FPD relative to the image intensifier except at low exposures.3Seibert J. Flat-panel detectors: how much better are they?.Pediatr Radiol. 2006; 36: 173-181Crossref PubMed Scopus (86) Google Scholar At higher exposure levels typical of digital subtraction angiography (DSA), the large signal produced by the absorbed X-rays, allows the gain of the output charge amplifies of the FPD to be low. At fluoroscopy exposure levels, however, the necessary low exposure per image requires significant gain amplification to achieve a reasonable signal level for digitization. In addition, other noise sources from the FPD are increased, resulting in an image with low signal to noise ratio, causing low contrast resolution and reduced image quality. Furthermore, at low exposure levels, detector lag is seen. This can be overcome in the manufacturing process by providing detector backlighting. With current FPD imaging systems, proper dose adjustment per frame and appropriate frame rates are essential to ensure that the optimal image quality is achieved. Alternatively, a hybrid approach has been developed to overcome some of these problems. This approach is based on coupling a thick CsI structured scintillator to a thin photosensitive (a-Se) semiconductor.6Zhao W. Li D. Reznik A. Lui B. Hunt D. Rowlands J. et al.Indirect flat panel detector with avalanche gain: fundamental feasibility investigtion for SHARP-AMFPI (scintillator HARP active matrix flat panel imager).Med Phys. 2005; 32: 2954-2966Crossref PubMed Scopus (70) Google Scholar The thin semiconductor layer produces photo-induced electron-hole pairs at the top interface from incident light photons and these propagate to the bottom surface. Under higher voltage, the holes undergo multiplication and create more holes and electrons. This allows for improved noise performance at lower radiation levels, better temporal performance, and less lag. Rotational angiography is obtained by performing a motorized movement at constant speed of the C-arm around the patient during continuous contrast injection and fluoroscopy. C-arm cone-beam computed tomography (CT) is an advanced imaging capability that uses C-arm flat panel fluoroscopy systems to acquire and display three-dimensional (3D) images. The flat panel detector functions much like the multiline detectors used in multi-detector CT (MDCT). It provides high- and low-contrast soft tissue “CT-like” images in multiple viewing planes (Fig 5). This provides a significant improvement over conventional single-planar digital subtraction angiography (DSA) and fluoroscopy. While the technology has been available for many years, its emergence in the clinical arena did not accelerate until the introduction of FPDs, as this provided images that were far superior to the data acquired using conventional image intensifier systems, with much improved spatial resolution and imaging of soft tissue structures. It is not yet known what the long-term effect of employing this technology will have on the ability to perform routine interventional procedures; it is clear that it provides more confidence about both vascular and soft tissue anatomy to those operators currently performing complex endovascular procedures. In the United States, three C-arm cone-beam CT systems (FluoroCT) are commercially available: DynaCT (Siemens Medical Solutions, Forchheim, Germany), XperCT (Phillips Medical Systems, Eindhoven, The Netherlands), and InnovaCT (GE Healthcare, Waukesha, Wisc). Each manufacturer has its own imaging protocol that is tailored to each system's different rotation time, number of projections acquired, image quality, and time required for reconstruction. The two main factors that will drive use in the operating room are the time for set up, image acquisition, and image reconstruction and the quality of the images produced.7Wallace M. Kuo M. Glaiberman C. Binkert C. Orth R. Soulez G. Three-dimnesional C-arm cone-beam CT: applications in the interventional suite.J Vasc Interv Radiol. 2009; 20: S523-S537Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar To obtain 3D images from a conventional rotational angiographic run, two methods exist. The first consists of an examination in two phases. The C-arm rotates in a continuous 200° to 270° arc (varying based on the actual manufacturer) around the patient's torso for an acquisition period of 5 or 8 seconds. The initial acquisition phase has two actions. The first sweep of the C-arm acts as the mask for the subsequent data acquisition during the injection of the contrast. The return sweep of the C-arm in an arc is performed while contrast is injected during the entire period of data acquisition. For aortography, contrast is injected at rates of 10 cc/s for up to 8 seconds, leading to a total volume delivered of 80 cc. Frequently, a mixture of 50% contrast is used, resulting in a total additional contrast dose of 40 cc. Once the images are obtained, the data are transferred to a work station where specific algorithms are performed to correct for image intensifier and contrast distortion. Data are reconstructed using a volume-rendered technique. Other methods can be employed to visualize the images including surface-shaded display (SSD), maximum intensity projection (MIP), or multiplanar reformatting (MPR). Current software allows for 1-minute reconstruction time for the initial 3D image; real-time reconstruction of the 3D volume will become feasible in the future. The user can then “page through” image sections and reformat them for viewing in various slab thicknesses and projections. Accurate length and diameter measurements can be made based on these reconstructions, which may aid in procedure planning and sizing. In addition, the information may allow the physician to determine the optimal projection (angulation and skew) of the fluoroscopic tube for endovascular intervention. The advantage of this technique is that it can be performed in the angiography suite for immediate pre- or postoperative evaluation. While C-arm cone-beam CT (FluoroCT) provides improved imaging compared with conventional 3D angiography, it is not without some drawbacks. There are a number of imaging artifacts that can occur during the acquisition or reconstruction of the obtained image. Two potential sources of geometric distortion may impact the spatial accuracy of the 3D reconstruction. These include the image intensifier-television system distortion and the gantry “wobble” during image acquisition. Gantry wobble is visualized as a vertical displacement between successive 2D projection images. A calibration procedure is carried out by the manufacturer, and this is automatically applied to all images prior to 3D reconstruction, which corrects for this wobble artifact. Another potential artifact is movement artifact, which causes blurring of the image. This occurs predominantly due to patient movement. While this is less of an issue with imaging of the neurovascular system, it becomes a larger issue while imaging the thoracic or abdominal arterial system as patients must hold their breath during image acquisition. Metal artifacts produce star-like artifacts in the reconstructed image and are due to scatter caused from metal objects in situ. In addition, truncation artifacts lead to strip-like artifacts in the lateral projections. This is due to insufficient power of the X-ray to penetrate the target, which is thicker at this location. Placing the patient's arms over the head can help to reduce this artifact. Other problems include difficulty with contrast differentiations, particularly in areas of low radiographic contrast.8Irie K. Murayama Y. Saguchi T. Ishibashi T. Ebara M. Takao H. et al.DynaCT soft-tissue visualization using an angiographic C-arm system: initial clinical experience in the operating room.Oper Neurosurg. 2007; 62: 266-272Crossref Scopus (38) Google Scholar Of some concern is the additional radiation dose provided. The radiation dose for a 14-second acquisition is similar to that of a biplane digital subtraction acquisition during a routine cerebral angiogram.9Klucznik R. Current technology and clinical applications of three-dimensional angiography.Radiol Clin North Am. 2002; 40: 711-728Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar Radiation dose, however, is higher in FPD-based C-arm cone-beam CT (236 mGy) compared with traditional 3D-DSA with a standard image intensifier (50 mGy).8Irie K. Murayama Y. Saguchi T. Ishibashi T. Ebara M. Takao H. et al.DynaCT soft-tissue visualization using an angiographic C-arm system: initial clinical experience in the operating room.Oper Neurosurg. 2007; 62: 266-272Crossref Scopus (38) Google Scholar One final drawback of FluoroCT is the limited data acquisition in the Z axis (approximately 18 cm of imaging depending on the manufacturer). A number of preliminary experiences have been reported documenting the application of FluoroCT to the treatment of vascular-based pathologies including aortic interventions, hepatic arterial interventions, portal vein embolization, transjugular intrahepatic portosystemic shunts, and peripheral vascular interventions.7Wallace M. Kuo M. Glaiberman C. Binkert C. Orth R. Soulez G. Three-dimnesional C-arm cone-beam CT: applications in the interventional suite.J Vasc Interv Radiol. 2009; 20: S523-S537Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar Potential applications include its use for preprocedure anatomic diagnosis and treatment planning, device implantation, and postprocedural assessment of successful therapy. Little research has been presented regarding the application of FluoroCT during endovascular aneurysm repair. Technical image quality is inferior to MDCT, especially in terms of low contrast resolution.10Smyth J. Sutton D. Huston J. Evaluation of the quality of CT-like images obtained during a commercial flat panel detector system.Biomed Imagin Interv J. 2006; 2: e48Google Scholar Eide et al evaluated 20 patients with infrarenal abdominal aortic aneurysms and compared images from FluoroCT with MDCT-derived images. Endovascular aneurysm repair relevant arterial diameters and lengths were assessed, as were nine anatomic areas regarding visibility. There were no significant differences in the measured arterial diameters and lengths. MDCT, however, had a significantly higher visibility score. Using a higher contrast dose during FluoroCT, however, acceptable diagnostic quality was found in 78% to 94% of the cases for eight of the nine investigated anatomic areas. Published outcomes evaluating clinical applications to vascular surgery and aneurysm repair remain limited.11Biasi L. Ali T. Thompson M. Intra-operative dynaCT in visceral-hybrid repair of an extensive thoracoabdominal aortic aneurysm.Eur J Cardiothorac Surg. 2008; 34: 1251-1252Crossref PubMed Scopus (20) Google Scholar It has been demonstrated, however, that its use can be helpful in the successful sizing and placement of stent for the treatment of peripheral artery stenoses and peripheral artery aneurysms pathology.12Unno N. Mitsuoka H. Takei Y. Igarashi T. Uchiyama T. Yamamoto N. et al.Virtual angioscopy using 3-dimensional rotational digital subtraction angiography for endovascular assessment.J Endovasc Ther. 2002; 9: 529-534Crossref PubMed Scopus (33) Google Scholar, 13van den Berg J. Overtoom T. de Valois J. Moll F. Using three-dimensional rotational angiography for sizing of covered stents.Am J Radiol. 2002; 178: 149-152Google Scholar, 14van den Berg J. Moll F. Three-dimensional rotational angiograph in peripheral endovascular interventions.J Endovasc Ther. 2003; 10: 595-600Crossref PubMed Scopus (16) Google Scholar The addition of this imaging technique was performed with little increase in the duration of the procedure and with small volumes of contrast used and radiation exposure to the patient. At the Cleveland Clinic, we have begun to use this technology to evaluate the adequacy of aneurysm repair after fenestrated and branched aortic endografting. FluoroCT has successfully demonstrated the presence and location of endoleaks not initially detected on conventional angiography (Fig 6). Others have demonstrated that FluoroCT has been capable of identifying the source of an endoleak when the location was not clear on conventional MDCT.15Biasi L. Ali T. Hinchcliffe R. Morgan R. Intraoperative DynaCT detection and immediate correction of a Type 1a endoleak following endovascular repair of abdominal aortic aneurysm.Cardiovasc Intervent Radiol. 2009; 32: 535-538Crossref PubMed Scopus (46) Google Scholar Used in this fashion, however, FluoroCT may prove overly sensitive with a high false-positive rate. This may be particularly true in the identification of type II endoleaks. Data acquisition occurs during continuous injection of intra-arterial contrast, providing imaging during both the early arterial and later venous filling phases. MDCT data acquisition occurs at a shorter time point and is performed well after the patient has left the operating room. This may allow time for these branch vessels to occlude, thus resolving the endoleak. Angiography alone cannot provide all information necessary for endovascular treatment of aortic disease. Assessment of vessel diameter and the condition of the vessel wall require complementary CT examination. Despite the development of C-arm cone-beam CT, contrast-enhanced spiral CT is the main tool for preoperative evaluation and planning before treatment with a stent graft. Our ability to incorporate these images into the operating room has improved significantly. It is now more common to view these images on monitors within the interventional suite or operating room rather than view cut films on a light box. More helpful, however, is the integration of the CT images directly into the live fluoroscopically-derived images. This is akin to a familiar technique known as “roadmapping.” Traditional roadmapping involves the superimposition of digital subtraction of the contrast-filled vessel lumen on the live fluoroscopic image. This technique provides good spatial resolution and contrast for real-time images. It also lowers the risk of inadvertent event when compared with the direct catheterization of vessels in which manipulation of wires and catheters distally sometimes causes vasospasm, dissection, and dislodgement of plaques because of uncertain spatial relationship between the devices and the vessel walls. An inherent limitation of this process, however, is the static nature of the projection. Differential projections, however, are usually mandatory for a better delineation of tortuous vessels, requiring additional contrast dose and radiation exposure. To overcome this, fusion imaging utilizing a preoperative CT angiogram and an intraoperative non-contrasted FluoroCT can be performed. The basic method for this begins with the preprocedural attainment of a contrast-enhanced spiral MDCT. For optimal reconstruction, the recommended slice thickness is 1 mm. Once in the operating room, patients undergo a preprocedural, non-contrasted FluoroCT with either a 5 or 8 second acquisition time. If there are significant calcifications present within the arterial images being evaluated, or previous placement of a stent graft, then a shorter acquisition time can be used. This scan is then registered to the available MDCT using mainly bony structures as landmarks. To assist in increasing accuracy, areas of calcification from within the aorta or its branches can aid in alignment. Once registration is complete, the MDCT image can be superimposed on the live fluoroscopic image (Fig 7) . Alternatively, a graphic outline can be of the aorta, its branches, or any area of interest can be highlighted using computer-generated graphics (Fig 7). The superimposed image (or graphics) will then update in the correct projection depending on the arc-angle of the C-arm. One of the down-sides of this tool is that spatial displacement can occur with the introduction of stiff wires, catheters, and the stent graft delivery system (Fig 8). This is particularly true in patients with tortuous anatomy. In our experience, this discrepancy is minimal, and once the potential offset is identified, it can be easily compensated for.Fig 8Image from intra-operative use of fusion of preoperative CT and with the live fluoroscopic image. There is an overlay of the aortic aneurysm with a catheter traversing the aorta and entering the left renal artery (arrow). The catheter was easily advanced to this location based solely on the fusion of the two images. Note how the wire in the renal artery does not exactly follow the course of the projected path (double arrow) due to displacement of the artery by the wire and catheter.View Large Image Figure ViewerDownload Hi-res image Download (PPT) There is little information about the usefulness of this technology in the treatment of aortic aneurysms or complex aortic disease. It has been shown to accurately outline the coronary sinus anatomy and assist in guiding cardiac resynchronization therapy (CRT) placement.16Auricchio A. Sorgent A. Soubelet E. Regoli F. Spinucci G. Vaillant R. et al.Accuracy and usefulness of fusion imaging between three-dimensional coronary sinus and coronary veins computed tomographic images with projection images obtained using fluoroscopy.Europace. 2009; 11: 1483-1490Crossref PubMed Scopus (26) Google Scholar The technique is accurate in displaying CT-derived anatomy on fluoroscopy and, by doing so, facilitated the location of the carotid sinus and its branches – a shortcoming in CRT implantation. In addition, this technology has been shown to assist in more accurately delivering therapy during atrial fibrillation ablation.17Knecht S. Skali H. O'Neill M. Wright M. Matsuo S. Chaudhry G. et al.Computed tomography-fluoroscopy overlay evaluation during catheter ablation of left atrial arrhythmia.Europace. 2008; 10: 931-938Crossref PubMed Scopus (70) Google Scholar We have used this technology to assist in placing fenestrated and branched aortic endografts at the Cleveland Clinic. Review of our experience demonstrates that the use of fusion imaging was able to decrease contrast dose use by nearly 25%, without a significant increase in radiation dose delivered or overall operative time (unpublished data). In addition, we have begun to use this technology to direct translumbar aortic sac puncture for endoleak embolization. As experience with this imaging process grows, its application will clearly become more widespread. Intravascular ultrasound (IVUS) is a critical technology that is extraordinarily useful during thoracic aortic stent graft surgery. IVUS technology provides detailed information about lesion morphology and precise visualization of vessel wall anatomy. This provides useful diagnostic information as well as aiding in treatment of aortic lesions. Current IVUS catheters operate in a high-resolution B-mode. They operate at frequencies that range from 10 to 40 MHz. In the thoracoabdominal aorta, imaging of the larger diameter vessel walls requires low-frequency catheters (ie, 8-10 MHz). Catheter designs that enable delivery of the ultrasound elements without monorail delivery wire eliminate wire artifact and enhance 360° degree views of the arterial anatomy.18Song T. Donayre C. Kopchok G. White R. Intravascular ultrasound use in the treatment of thoracoabdominal dissections, aneurysms, and transections.Semin Vasc Surg. 2006; 19: 145-149Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar Given this, coaxial phased array systems that produce images utilizing electronically rotating signals rather than mechanical rotating systems over eccentric wire delivery are used to provide improved imaging characteristics. To obtain the best aortic wall visualization, the IVUS catheter is directed perpendicular to the luminal surface. The practitioner must become familiar with interpretation of the IVUS signals in order to best understand what is underlying aortic pathology and what is merely artifact. Arterial calcification and prosthetic graft material can cause attenuation of the IVUS signal. In addition, catheters and wires can introduce noise into the system obscuring the surrounding field of view. IVUS introduction is applied through an arterial sheath – the size of which is dependent upon the specific IVUS catheter used but ranges between 5 and 10 Fr. The catheter is inserted over a wire, and imaging is typically performed during withdrawal of the catheter as this provides a higher quality assessment. IVUS can provide information about the arterial wall morphology, branch vessel origins, and lengths can be assessed using a pull-back method. In addition, IVUS can be used to measure luminal dimensions, which may assist in planning for stent graft selection. It can assist in better assessing landing zones for the presence of thrombus. And in patients in whom contrast doses need to be limited, IVUS, in combination with fluoroscopy, can be used to mark sites of specific landing zones and aid in prevention of the unintentional coverage of a vessel such as the celiac or renal arteries. IVUS is particularly useful in the treatment of aortic dissections. While CT scans can give some assessment as to the patency or compression of the true lumen at a specific moment in time, IVUS evaluation allows real-time assessment of true lumen expansion (or collapse) and the physiologic effects on this. When treating aortic dissections it is often difficult to traverse the true lumen only, and IVUS is nearly essential at providing the necessary information to verify catheter and wire location, as well as providing diagnostic information about the location of fenestrations along the length of the intimal flap (Fig 9). IVUS is helpful at assessing adequacy of true lumen treatment of dissections. When flow is redirected to the true lumen, expansion of the lumen occurs, and this can immediately be visualized by IVUS. This is complemented by the use of contrast angiography and intra-arterial pressure measurements. IVUS has proven itself a very useful tool in the successful endovascular treatment of a variety of aortic pathologies including aortic aneurysms, dissections, coarctations, penetrating ulcers, and traumatic aortic transections.19Kpodonu J. Ramaiah V. Dietrich E. Intravascular ultrasound imaging as applied to the aorta: a new tool for the cardiovascular surgeon.Ann Thorac Surg. 2008; 86: 1391-1398Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar As with conventional angiography, IVUS technology continues to evolve. While standard IVUS offers a somewhat restricted tissue penetration and no Doppler or color Doppler capabilities, recent advancements have overcome this. A phase-array IVUS probe with Doppler, pulsed wave Doppler, and color-Doppler capabilities (AcuNav; Siemens, Mountain View, Calif) has been developed, mainly for intra-cardiac use.20Nyman R. Eriksson M. The future of imaging in the management of abdominal aortic aneurysm.Scand J Su. 2008; 97: 110-115PubMed Google Scholar, 21Liu Z. McCormick D. Dairywala I. Surabhi S. Goldberg S. Turi Z. et al.Catheter-based intracardiac echocardiography in the interventional cardiac laboratory.Catheter Cardiovasc Interv. 2004; 63: 63-71Crossref PubMed Scopus (15) Google Scholar This system provides both anatomic and physiologic data with good penetration. The probe may be beneficial for evaluating larger diameter arteries such as the aorta. The probe has a diameter of 8 to 10 Fr, and scanning is possible percutaneously through an introducer sheath. The 64-element transducer (5-10 MHz) has a tissue penetration ability of up to 15 cm, and it is mounted on the tip of a steerable catheter 90 cm long. A 60 cm-long introducer sheath is used to make it possible to maneuver the probe inside the introducer sheath at all times and to avoid vascular trauma. The phase-array elements of the probe have a characteristic fluoroscopic appearance that allows determination of the exact position of the probe. Initial analysis of the use of this catheter during endovascular aneurysm repair demonstrated that intra-arterial use of the catheter provided aortic dimensions equivalent to those determined by preoperative CT.22Eriksson M.-O. Wanhainen A. Nyman R. Intravascular ultrasound with a vector phased-array probe (AcuNav) is feasible in endovascular abdominal aortic aneurysm repair.Acta Radiol. 2009; 8: 870-875Crossref Scopus (9) Google Scholar Color Doppler was able to facilitate the identification of vessel origins (Fig 10). Endoleak identification, however, was not possible with this technology and is likely due to the direction of flow relative to the IVUS probe (due to an unfavorable Doppler angle) and due to movement artifacts in the aneurysm sac. Future use of this tool in treating complex aortic pathologies without the use of contrast agents warrants investigation. As technology advances and our ability to treat complex vascular disease in an endovascular fashion expands, we will rely more and more on concurrent improvements in our ability to image the vasculature, its pathology, and the tools we use to treat them. Many new advances are improving the ability to image the aorta and other arterial branches, allowing us to provide endovascular care with lower radiation dose, lower contrast doses, and hopefully overall fewer complications. It is clear the further advancements in this area will continue to significantly improve our ability to image the disease we are treating, make better intra-operative decisions, and improve patient care.