Interventional MR: vascular applications

Three strategies for visualisation of MR-dedicated guidewires and catheters have been proposed, namely active tracking, the technique of locally induced field inhomogeneity and passive susceptibility-based tracking. In this article the pros and cons of these techniques are discussed, including the development of MR-dedicated guidewires and catheters, scan techniques, post-processing tools, and display facilities for MR tracking. Finally, some of the results obtained with MR tracking are discussed.     

Interventional MR: interstitial therapy

The rationale and results for interstitial therapies via interventional MRI in the treatment of tumors in various regions are presented. Different interstitial treatment techniques are presented based on varying technologies both for tumor ablation and treatment monitoring. Data are presented based on 335 patients, 29–84 years of age (mean age 59 years, 196 men and 139 women) with a total of 932 liver tumors, 16 head and neck tumors and 14 abdominal recurrent pelvic and lymphatic tumors. All lesions had been treated with MR-guided laser-induced interstitial thermotherapy (LITT) via 2516 laser applications and 1856 cannulations. Data in the literature are extremely varying depending on author experience, treatment technique, and the included patient material. In our patient material we were able to achieve a local tumor control of 96.7 % depending on the size of the tumorous lesion, the topographical relationship, and the applied laser parameters. The overall cumulative survival rate of patients with liver metastases was 45.74 months (median 40.97 months, 95 % confidence interval 31.42–50.52). The cumulative survival rate of the patient group with hepatic metastases of colorectal carcinoma was 42.71 months (median 39.33 months, 95 % confidence interval 33.26–45.37). In patients with head and neck tumors a relevant reduction in clinically relevant symptoms such as pain, swallowing disorders, or nervous compression was achieved in 11 of 15 patients treated with LITT. In 14 soft tissue tumors, such as pelvic tumor recurrence and lymph node metastases, a local tumor control was obtained in 68 % of lesions. Interstitial therapies under interventional MRI guidance, such as LITT, results in a high local tumor control with an improved survival rate.     

Interventional computed tomography

Due to the development and refinement of computed tomography (CT), sonography, and interventional techniques, the field of interventional radiology has seen tremendous growth in recent years. In particular, the precise anatomic detail provided by CT and sonography has allowed percutaneous biopsies and abscess drainages to be performed safely and effectively. Percutaneous biopsies are now becoming the most common interventional radiographic procedures in many institutions. The usual indications for a biopsy are to determine the etiology of a mass, neoplasm, or inflammation, and to determine whether masses in known oncologic patients represent scarring or residual viable tumor. Accuracy rates for most percutaneous CT-directed biopsies are well over 90%, and the complication rate is very low. CT-directed percutaneous abscess drainages are also safe and effective and, in most cases, will be preferable to surgical drainage. The initial indications for percutaneous drainage (single, unilocular fluid collections) have been greatly expanded to include multiloculated collections, interloop abscesses, periappendiceal abscesses, and even percutaneous cholecystotomies. Biopsy and drainage procedures, together with their accuracy rates, indications and complications, are reviewed in this monograph.     

Indirect MR arthrography: concepts and controversies

Indirect MR arthrography involves intravenous injection of a standard dose of Gadolinium contrast followed, in delayed fashion, by MR imaging. Contrast in taken up by the joint at a rate dependent on a variety of factors including synovial area, vascularity, permeability, and pre-existing joint effusion. Patient acceptance is higher than with direct intra-articular injection, and logical considerations (e.g., not needing a radiologist present) make this an attractive alternative to direct MR arthrography. At best, an indirect MR arthrography exam can look virtually identical to a direct MR arthrogram. However, the radiologist should be aware that vascular tissue inside and outside the joint will enhance, which may be considered either an advantage or disadvantage. Additionally, since all compartments of the joint enhance, information regarding abnormal communication of contrast material is absent. Suboptimal exams occur due to the need for diffusion of contrast into the joint. Exercise can help improve the quality of exams. Using an adequate time delay is essential for optimizing indirect MR arthrography.     

Interventional MR angiography with a floating table

A floating table was integrated into a setup for performance of interventional magnetic resonance (MR) angiography procedures with actively visualized catheters and biplanar real-time image fusion. The setup was evaluated by performing catheterizations in eight pigs. The floating table enabled the authors to follow actively visualized instruments in the pigs' vasculature during MR imaging-guided interventional angiography procedures while performing real-time biplanar MR imaging. Interventional MR angiography with a floating table enables the field of view to be moved along with the instrument tip to the region of interest and thus enhances the usability and flexibility of the interventional MR imaging setup.     

Navigation concepts for MR image-guided interventions

The ongoing development of powerful magnetic resonance imaging techniques also allows for advanced possibilities to guide and control minimally invasive interventions. Various navigation concepts have been described for practically all regions of the body. The specific advantages and limitations of these concepts largely depend on the magnet design of the MR scanner and the interventional environment. Open MR scanners involve minimal patient transfer, which improves the interventional workflow and reduces the need for coregistration, ie, the mapping of spatial coordinates between imaging and intervention position. Most diagnostic scanners, in contrast, do not allow the physician to guide his instrument inside the magnet and, consequently, the patient needs to be moved out of the bore. Although adequate coregistration and navigation concepts for closed-bore scanners are technically more challenging, many developments are driven by the well-known capabilities of high-field systems and their better economic value. Advanced concepts such as multimodal overlays, augmented reality displays, and robotic assistance devices are still in their infancy but might propel the use of intraoperative navigation. The goal of this work is to give an update on MRI-based navigation and related techniques and to briefly discuss the clinical experience and limitations of some selected systems.     

Interventional MR imaging at 1.5 T: quantification of sound exposure

Sound pressure levels (SPLs) during interventional magnetic resonance (MR) imaging may create an occupational hazard for the interventional radiologist (ie, the potential risk of hearing impairment). Therefore, A-weighted and linear continuous-equivalent SPLs were measured at the entrance of a 1.5-T MR imager during cardiovascular and real-time pulse sequences. The SPLs ranged from 81.5 to 99.3 dB (A-weighted scale), and frequencies were from 1 to 3 kHz. SPLs for the interventional radiologist exceeded a safe SPL of 80 dB (A-weighted scale) for all sequences; therefore, hearing protection is recommended.     

Interventional MR imaging: state of the art and future perspectives

Summary The concept of MR guidance of invasive diagnostic and minimally invasive therapeutic procedures is based on the excellent morphologic and functional properties of MR imaging. Prerequisites are adequate patient monitoring and adherence to safety guidelines. Fast and ultrafast sequences, temperature quantification, visualization of intravascular devices, thermal stability of contrast media and thermosensitive contrast media are discussed. The spectrum of clinical applications includes biopsies, thermal ablation modalities, vascular applications, MR endoscopy and intraoperative MR imaging. The development of interventional MR imaging is still in its infancy. In the future, MR imaging may play an important role in interventional radiology and minimally invasive therapy.      

MR tomography in parenchymatous neurocysticercosis

MRI-findings in a case of parenchymal neurocysticercosis are presented. The changes of the lesions as a response to chemotherapy were monitored by MRI and CT. Problems of sensitivity (MRI vs. CT) and MRI differential diagnoses are discussed.     

Magnetic Catheter Manipulation in the Interventional MR Imaging Environment

PurposeTo evaluate deflection capability of a prototype endovascular catheter, which is remotely magnetically steerable, for use in the interventional magnetic resonance (MR) imaging environment.Materials and MethodsCopper coils were mounted on the tips of commercially available 2.3–3.0-F microcatheters. The coils were fabricated in a novel manner by plasma vapor deposition of a copper layer followed by laser lithography of the layer into coils. Orthogonal helical (ie, solenoid) and saddle-shaped (ie, Helmholtz) coils were mounted on a single catheter tip. Microcatheters were tested in water bath phantoms in a 1.5-T clinical MR scanner, with variable simultaneous currents applied to the coils. Catheter tip deflection was imaged in the axial plane by using a “real-time” steady-state free precession MR imaging sequence. Degree of deflection and catheter tip orientation were measured for each current application.ResultsThe catheter tip was clearly visible in the longitudinal and axial planes. Magnetic field artifacts were visible when the orthogonal coils at the catheter tip were energized. Variable amounts of current applied to a single coil demonstrated consistent catheter deflection in all water bath experiments. Changing current polarity reversed the observed direction of deflection, whereas current applied to two different coils resulted in deflection represented by the composite vector of individual coil activations. Microcatheter navigation through the vascular phantom was successful through control of applied current to one or more coils.ConclusionsControlled catheter deflection is possible with laser lithographed multiaxis coil-tipped catheters in the MR imaging environment.     

Truly Hybrid Interventional MR/X-Ray System: Investigation of in Vivo Applications

RATIONALE AND OBJECTIVES: The purpose of this study was to provide in vivo demonstrations of the functionality of a truly hybrid interventional x-ray/magnetic resonance (MR) system. MATERIALS AND METHODS: A digital flat-panel x-ray system (1,024(2) array of 200 microm pixels, 30 frames per second) was integrated into an interventional 0.5-T magnet. The hybrid system is capable of MR and x-ray imaging of the same field of view without patient movement. Two intravascular procedures were performed in a 22-kg porcine model: placement of a transjugular intrahepatic portosystemic shunt (TIPS) (x-ray-guided catheterization of the hepatic vein, MR fluoroscopy-guided portal puncture, and x-ray-guided stent placement) and mock chemoembolization (x-ray-guided subselective catheterization of a renal artery branch and MR evaluation of perfused volume). RESULTS: The resolution and frame rate of the x-ray fluoroscopy images were sufficient to visualize and place devices, including nitinol guidewires (0.016-0.035-inch diameter) and stents and a 2.3-F catheter. Fifth-order branches of the renal artery could be seen. The quality of both real-time (3.5 frames per second) and standard MR images was not affected by the x-ray system. During MR-guided TIPS placement, the trocar and the portal vein could be easily visualized, allowing successful puncture from hepatic to portal vein. CONCLUSION: Switching back and forth between x-ray and MR imaging modalities without requiring movement of the patient was demonstrated. The integrated nature of the system could be especially beneficial when x-ray and MR image guidance are used iteratively.     

Interventional and intraoperative MR: review and update of techniques and clinical experience

The concept of interventional magnetic resonance imaging (MRI) is based on the integration of diagnostic and therapeutic procedures, favored by the combination of the excellent morphological and functional imaging characteristics of MRI. The spectrum of MRI-assisted interventions ranges from biopsies and intraoperative guidance to thermal ablation modalities and vascular interventions. The most relevant recently published experimental and clinical results are discussed. In the future, interventional MRI is expected to play an important role in interventional radiology, minimal invasive therapy and guidance of surgical procedures. However, the associated high costs require a careful evaluation of its potentials in order to ensure cost-effective medical care.