Applied Radiology

Extensive research has been conducted in the field of interventional magnetic resonance imaging. There are several factors driving this research, including the possibility that it may replace traditional fluoroscopy. There are many advantages and disadvantages of the modalities currently used for image guidance. This paper will address the various guidance methods and discuss the advantages and disadvantages of each, as well as current research efforts and developments.

Significant advances in the field of interventional magnetic resonance (MR) imaging have resulted from an interdisciplinary approach involving radiologists, physicists, surgeons, and computer scientists. ¹ According to Gunther et al, ² the most interesting question in conjunction with interventional MR imaging is whether or not it will replace standard X-ray fluoroscopy. The current clinical applications are numerous; however, many of the procedures are performed efficiently and with relatively low risk using the standard methods. The clear challenges of vascular interventions include accurate and rapid visualization and sequence acquisition in a safe and reproducible manner. In situations in which the targeted lesions are seen only with MR imaging, such as breast lesions, MR-guided intervention is the ideal technique. This review addresses the potential advantages and limitations of interventional MR imaging, discusses catheter tracking and visualization techniques, reviews present device development, and assesses current investigational and clinical applications.

Potential advantages of interventional MR imaging
versus conventional fluoroscopy

Interventional radiology originated with fluoroscopically guided percutaneous biopsies and angiography. These X-ray­guided procedures were soon coupled with, and thereafter often replaced by, computed tomography (CT)-guided and ultrasound-guided procedures. Both of these modalities offer the advantage of improved visualization as well as a reduction in radiation exposure. CT guidance provides greater anatomic detail and imaging in the axial plane; ultrasound allows for variation in imaging planes, lower cost, and no exposure to ionizing radiation. In addition to these advantages, both of these techniques offer the radiologist real-time image guidance similar to fluoroscopy.

As the field of interventional radiology progressed, MR imaging was incorporated as a method of guidance. Several contributing factors to the use of MR as a modality for imaging guidance included multiplanar imaging capabilites and a decrease in the radiation exposure to patients and personnel. ^3,4 Additionally, MR imaging, exclusively and in combination with high-performance CT, permitted the integration of three-dimensional (3D) volume rendering and interactive localization. MR imaging also provided optimal tissue contrast, thus allowing the visualization of diseased vessels and their relationship to the surrounding parenchyma without the use of intravenous contrast material. ^2-6

MR guidance also offers the radiologist rapid access to physiologic information, including quantification of flow, perfusion, diffusion, and temperature. ² Real-time information is available to the physician, illustrating the immediate effect of the procedure or intervention. ³ Such immediate feedback may allow for prompt decision-making and consideration of further treatment options that may not have been assessable with conventionally available fluoroscopic procedures.

The combination of fluoroscopy and MR imaging in a tandem system using MRI-compatible catheters and guidewires has provided an alternative to single modalities. This tandem system consists of a fluoroscopy unit, complete with digital subtraction angiography and road mapping, as well as a closed, short-bore 1.5 T magnet. The patient is placed on a table that is MR compatible and transparent to X-rays, therefore, there is no need to reposition the patient. ⁴ The fluoroscopic system may be used for guidewire and catheter placement, and the magnet can be used for multiplanar imaging, anatomic visualization, tissue characterization, and physiologic flow measurements. ⁴ This combined technique incorporates the advantages of both imaging modalities and avoids some of the difficulties associated with a solely MR-guided procedure. This system also provides a significant financial advantage over a dedicated interventional magnet because each of the imaging suites can be used independently.

Using a somewhat different approach, Wen and colleagues ⁷ have developed a method of modifying a real-time fluoroscopic unit for use in conjunction with an interventional magnet. This system consists of a standard X-ray source (X-ray tube and housing), a high-voltage generator, an X-ray detector, a detector power supply, data acquisition electronics, and a display. The X-ray fluoroscopy unit is placed completely within the bore of an interventional system. Both X-ray fluoroscopy and MR images are available to the interventionalist; however, one system is inactive while the other is active. This technique provides an adjunct method that is particularly useful as a bridging point to entirely MR-guided procedures.

Current limitations of interventional MR imaging

Despite the numerous advantages of interventional MR imaging, interventional radiologists are still constrained by the limitations of current equipment and devices. The current MR imaging systems for interventional procedures may be classified as either dedicated open systems, open or closed standard imaging systems, or hybrid units consisting of MR imaging combined with standard X-ray fluoroscopy. ² Because open-bore magnets are limited by lower field strengths and weaker gradients, present research efforts are focused on the development of open magnets with higher field strengths. ^5,6 An innovative development was the design of a magnet that allowed relatively unrestricted access to the patient, similar to that provided in a CT scanner or fluoroscopy suite. ⁸ This dedicated open magnet is a GE Signa 0.5 T (MRT) "Double Doughnut" (GE Medical Systems, Waukesha, WI) that is open vertically and offers relatively unhindered access to the patient (Figure 1). A lower field-strength magnet results in decreased susceptibility artifact from interventional devices; however, a standard closed high-field strength magnet offers the advantage of rapid image acquisition with higher temporal and spatial resolution. Several alternative design options have been considered, including shortening the magnet to provide access to the patient from either end of the table.

Although new magnet designs have resulted in improved image quality and increased accessibility, one of the most challenging aspects of interventional MR imaging involves the development and manufacture of interventional catheters, needles, and guidewires. Standard interventional guidewires are composed of stainless steel and other ferromagnetic materials that are incompatible with the MR environment. Some devices may cause significant artifact and obscure the field of interest. Additionally, metallic devices may heat up secondary to current induction. To overcome the problem of heating, several decoupling methods that are frequently used in surface coils can be implemented. ⁹ Similarly, to address the concerns of ferromagnetic properties and artifacts, several alloys and ceramic materials have been developed. ^5,10,11

As researchers design new catheters and interventional devices, methods of image localization and guidance techniques must be developed concurrently. Researchers have focused on the development of rapid imaging sequences for the standard MR image acquisition and interventional procedures. ⁴ Presently, experimental and clinically investigative devices are visualized using passive and active methods. ⁴ There are three methods of passive visualization used to localize catheters and guidewires. One method involves filling the catheter lumen with contrast agents and causing the catheter to appear brighter than the surrounding unenhanced tissues. ^4,6 This technique was used by Strother and colleagues ¹² in the endovascular treatment of experimental canine aneurysms. Catheters were placed using fluoroscopic guidance and subsequently tracked and depicted with MR imaging using commercially available catheters filled with a gadopentetate dimeglumine solution. Catheter movement was visualized when either a catheter or the coaxial space between a catheter and a guidewire was filled with contrast. Superimposing the reconstructed images obtained during catheter manipulation onto a previously acquired MR angiogram localized the catheter. ¹²

Unal and colleagues ¹¹ used gadolinium-coated catheters and guidewires for passive tracking and visualization in phantoms and animals. This coating technology, in combination with a passive tracking technique, offered several advantages over other active and passive tracking techniques. These advantages included visualization of the entire device independent of its orientation in the magnetic field. These preliminary results demonstrated the potential use of MR-visible coated devices. ¹¹

Another method of passive visualization involves capitalizing on the differences in magnetic susceptibility of the catheter and the surrounding tissue, which results in intravoxel dephasing. This method does not require contrast; however, the geometric distortion that accompanies this effect can limit the practicality of this technique. ^4,6

Active tracking is an additional method of catheter localization. It is based on imaging with either a local coil or the "loopless" antenna of the guidewire. In active tracking, a small receiver coil in the tip of the catheter or guidewire generates the signal. The signal is then created and actively detected or emitted by a device to identify its spatial location. The position of the coil is shown in three dimensions by the depiction of a small colored dot on a previously acquired image. During the procedure, the coil may be tracked up to 20 times per second. ³

As methods of visualization, both passive and active tracking have limitations when compared with conventional fluoroscopy and digital subtraction angiography. These methods of visualization and navigation require substantial deviation from the accustomed fluoroscopic methods. As previously mentioned, passive visualization may be limited by geometric distortion and may require special coating. The constraints of active tracking include the ability to identify the coil position only; thus, the entire length of the device cannot be visualized unless multiple coils are implemented.

Device development

Significant progress toward clinical applications has been made as a result of early experiments. The first human interventional MR imaging experiment, conducted in 1996 by Smits and colleagues, ³ showed clear delineation and visualization of movement of a solid catheter with 5 dysprosium markers. ³ The authors described the early endovascular experiments showing the feasibility of placement of nitinol stents in an arterial graft model under MR imaging guidance. Standard catheters with the tip markers removed were visualized in a phantom. Further iterations involved attaching the flow phantoms to human volunteers for further imaging sequences. ²

As equipment and tracking methods improve, the development of MR-compatible catheters, guidewires, and stents becomes integral to the continued advancement of MR imaging guidance. Considerable research efforts have focused on MR-guided cardiovascular and stenting procedures. Currently, cardiovascular MR imaging is performed using external receivers in external, phased-array radiofrequency (RF) and surface coils that are typically placed in the chest and back of the patient. Coronary atherosclerotic plaques have been visualized using this technique; however, characterization of plaque composition and vascular wall detail is not optimal. In order to achieve a desirable level of detail, a high signal-to-noise ratio and high spatial resolution is required. To this end, Hurst et al ¹³ developed intravascular receiver coils. Endovascular placement of these devices improves the spatial resolution by generating an extremely high signal-to-noise ratio in the voxel immediately adjacent to the coil. According to Lardo, ⁶ most of these early designs were limited by their rapid radial signal intensity fall-off, sensitivity to motion, and poor flexibility. A recent design by Ocali and Atalar consists of a loopless (dipole) antenna. ⁶ This design offers a smaller profile and a higher degree of flexibility than previous designs. The loopless catheter antenna is used as a transmit/receive probe for transmitting RF pulses and receiving MR signal. It can also be used as a receive-only signal probe. The signal intensity is also high enough to allow remote placement of the associated electronics. ⁶

In parallel with the loopless catheter antenna, alternative catheter designs incorporating balloon technology have been developed. Ladd and colleagues ¹⁴ describe a new intravascular design consisting of a loop of wire encased by a polymeric water-filled balloon. Expansion of the balloon increases the coil diameter and improves imaging capabilities. Using an ex vivo technique in a rabbit model, the authors demonstrated enhanced visualization of vessel wall layers and plaque structure. Other investigators have used either a loopless or balloon-expandable transesophageal probe to obtain high-resolution vascular images (Figure 2). ⁶ One promising design, a vascular 0.030-inch internal MR coil (Intercept Vascular 0.030" Internal MR Coil, SurgiVision, Inc., Columbia, MD) that is similar to a guidewire in construction, has recently received Food and Drug Administration approval for high-resolution vascular imaging (Figure 3) (Ingmar Viohl, personal communication, June 26, 2002). The recent developments of small intravascular coils, guidewires, and active and passive methods of visualization have led to the investigation of monitoring balloon angioplasty under MR guidance. Yang et al ¹⁵ recently reported the feasibility of MR-guided balloon angioplasty in an animal model of aortic stenosis. This study was performed entirely under MR guidance with a 1.5F intravascular loopless antenna guidewire as described by Ocali and Atalar in conjunction with a standard 4F, 4-cm balloon angioplasty catheter. ⁶ The balloon catheter was inserted into the vessel over a 0.018-inch guidewire, which was subsequently removed during MR imaging. Passive tracking was used to monitor the margin of the balloon with tantulum markers placed proximal and distal to the balloon. Vessel dilatation was monitored with an intravascular antenna as a high-resolution probe using an MR fluoroscopy sequence. ¹⁵

In conventional fluroscopically guided procedures, balloon angioplasty is often followed by placement of endovascular stents. The design and development of endovascular stents is an active area of research in interventional MR imaging. A new MR-guided stent has been tested in in vitro and sheep models. ⁶ The ingenuity of this stent design allows it to serve not only as a mechanical device to maintain vessel patency, but also as a receiving antenna capable of generating the 3D coordinates of the stent coupled with high-resolution images of the vessel wall. ¹³

Current investigational and clinical applications

Newer techniques have shown promise as alternatives to traditional invasive surgical procedures and current interventional methods. Jolesz and colleagues ⁵ have integrated MR image guidance with computer-assisted surgery. This system allows the surgeon to use the magnet while in the operating room to allow accurate active localization and anatomical definition. The first open brain surgery using intraoperative MR imaging was performed in 1996. Lesions currently treated with this method include hemorrhage, neoplasms, cavernous hemangiomas, and arteriovenous malformations. ⁵ A significant benefit of intraoperative MR imaging is the immediate evaluation of the effectiveness of resection with specific sequences and spectroscopic curves, which may help to prevent the need for a second surgery. This same group has performed >300 craniotomies, >130 cases of MR-guided prostate brachytherapy, and >20 cryosurgical procedures for tumor ablation with preoperative diagnostic imaging and intraoperative therapy monitoring. ⁵

Kee and colleagues ¹⁶ showed successful hepatic-to-portal vein puncture in a swine model using MR and fluoroscopic guidance. A nitinol guidewire was used to access the right hepatic vein using a right jugular vein approach under C-arm guidance. The pigs were then moved into an open configuration Signa SP/i 0.5T MR imaging unit (GE Medical Systems). A nitinol transjugular intrahepatic portosystemic shunt (TIPS) set was subsequently advanced into the right hepatic vein over a guidewire. Multishot echoplanar images were obtained and used to guide puncture of the portal vein by passive artifact from the TIPS set. Successful puncture of the portal vein was confirmed by MR imaging followed by portal venogram and fluoroscopy for additional confirmation. This study concluded that this technique "may in the future prove the standard method of performing what is now a 'blind' access procedure." ¹⁶

Clinical MR guidance was first applied to needle aspirations that were subsequently followed by MR monitoring of thermal ablations. ¹⁷ Breast biopsies and lesion localizations are currently performed clinically. Daniel and colleagues ¹⁸ described a freehand interactive MR imaging technique for preoperative localization of breast lesions (Figure 4). Nineteen MR image-guided breast localization procedures were performed in 17 patients using an open platform breast coil in either a closed-bore 1.5 T magnet or an open-bore 0.5 T magnet. Rapid imaging sequences (fast-spin echo, water selective fast-spin echo, or water-specific three-point Dixon gradient echo) were used to localize the lesion. The needle was manipulated between sequences to the desired location. Accurate localization of the lesion was achieved within 9 mm of the target in all cases and 10 lesions were confirmed with mammography or ultrasound. The remaining 9 lesions were visible only with MR imaging. This study showed a simple and accurate technique for interactive MR imaging-guided localization of breast lesions without the need for stereotactic equipment. This method offers the additional advantage of providing access to lesions close to the chest wall and in the anterior portion of the breast. ¹⁸

Clinical work conducted by Roberts et al ¹⁹ recently showed successful placement of carotid stents in 7 patients using a combined MR imaging/X-ray angiography suite. Patients underwent a preprocedural MR imaging including MR angiography, phase-contrast flow assessment, and perfusion and diffusion imaging. Stents were then placed using standard fluoroscopic methods and patients were imaged after the procedure. ¹⁹ MR imaging provides immediate evaluation of intimal detail and flow, which allows for relatively immediate feedback of the success of the initial procedure and the evaluation for the need of further intervention.

Conclusion

When contrasting interventional MR with the traditional methods of image guidance, relevant issues include: imaging plane capabilities, exposure to ionizing radiation, and available devices and equipment. The question as to whether interventional MR will routinely replace or be combined with standard imaging guidance methods remains. The few clinical trials discussed here have proven successful. Further clinical progress can be anticipated as new devices and methods of imaging localization and guidance evolve.

Acknowledgments

The author wishes to thank Dr. Robert Herkfens and Dr. Bruce Daniel for their advice and guidance, and Dr. Viohl for kindly allowing the use of his images. The author also wants to thank Mark Riesenberger for his images. A special thanks to Kristina Benjamin for her help.