Johnathan S. Lewin, MD
Martin W. Donner Professor and Chairman
The Russell H. Morgan Department of Radiology and Radiological Science
Johns Hopkins School of Medicine
Radiologist-in-Chief, The Johns Hopkins Hospital

As the practice of Radiology has evolved during the past decade, there has been increasing emphasis on intervention. In parallel, open surgical procedures have demonstrated a trend toward image-based navigation, endoscopic approaches, and other methods to decrease the invasiveness of more traditional surgery. Radiological interventional skills and surgical minimally-invasive trends have converged to create a burgeoning interest in the use of magnetic resonance imaging for guidance of radiological and surgical procedures.

There are many procedures in which the information provided by MR imaging can be used to monitor a therapeutic intervention. Examples include thermal ablation, in which thermal energy is deposited and the resulting tissue changes are continuously or intermittently observed, or surgical intervention, in which tumor resection or cyst aspiration may be intermittently examined. These forms of interventional MRI require much less modification to standard imaging systems because access to the patient is not necessarily required during the monitoring process. The use of MR imaging for interventional procedure guidance includes its use by interventionalists during manipulation of needles, electrodes, catheters, or laser fibers, and its application by Surgeons to guide endoscopes, scalpels, or curettes. This form of more active intervention requires a significant departure from conventional diagnostic concepts and traditional imaging systems.

There are several recent technical developments that have facilitated the guidance phase of interventional procedures in a time-efficient manner: 1) the construction of higher-quality low-noise receiver chains has allowed lower field systems to provide relatively high signal-to-noise ratio images; 2) the development of rapid gradient-echo pulse sequences for use in interactive guidance during device placement; 3) the development of an interface between optically-linked 3D digitizer and the measurement control software of the MR imager; 4) the ability to view images in the magnetic field through the development of in-room high-resolution RF-shielded monitors.

One of the most straightforward interventional applications for a cross-sectional imaging modality is percutaneous biopsy and aspiration. The tissue contrast, spatial resolution, and multiplanar capabilities of MR have obvious benefits for guidance of biopsy and aspiration applications, and this application was the first reported use of MR imaging to guide intervention. A number of investigators have described the use of MR imaging for guidance of biopsy and aspiration over the last decade and a half. The benefits of multiplanar image acquisition were quickly discovered by Lufkin and colleagues, who first reported the advantages derived from going beyond the single imaging plane, usually axial, available in CT-guided procedures. Furthermore, the high vascular conspicuity due to flow-related enhancement effects inherent in two-dimensional gradient-echo imaging is an additional benefit of MR-guidance for lesions in regions of complex vascular anatomy.

Minimally Invasive Percutaneous Therapy

Much of the excitement surrounding MR imaged guided procedures has been due to the tremendous sensitivity of MR imaging for temperature change and tissue damage. In these areas, MR imaging provides an unequivocal advantage over the often subtle and indistinct changes noted on CT and ultrasound. Many of the studies looking at chemical and thermal ablation under MR image guidance are preliminary, but have shown very encouraging initial results. Thermal ablation has been advocated using several different energy sources, including radiofrequency, laser, cryotherapy, and focused ultrasound. The key advantage of MR that has been driving the development of these techniques has been the ability to monitor tissue destruction in near real-time during the procedure using a variety of MR imaging techniques, allowing interactive repositioning of the interventional device and tailoring of the thermal lesion in order to provide adequate destruction of the targeted tumor and surrounding margin of normal tissue. The ultimate cost-effectiveness of these procedures remains to be proven, although the remarkably lower cost of MR image-guided tumor ablation as compared to open surgical excision provides a strong rationale for the further development of these techniques. In addition, these minimally-invasive techniques allow life-prolonging treatment in a population of patients with localized tumor recurrence or metastasis that are not surgical candidates, and can result in a significant improvement in the quality of life in this cohort.

Catheter-Based Procedures

During the past five years, tremendous advancement has been made in the technical aspects of intravascular MR imaging and catheter-based interventional MR methodology. Many catheter-based techniques used in vascular intervention have been adapted to intravascular MRI. In addition, intravascular MR vessel wall imaging presents a new dimension in the evaluation and potential treatment of cardiovascular disease. Along with the many roles that MR imaging currently plays in diagnosis, the ability to combine MR image-guided vascular intervention with more conventional MR imaging methods such as functional, perfusion, and diffusion-weighted imaging holds great promise. Current techniques include passive vs. active catheter tracking methods, real-time rapid MR fluoroscopic techniques, and intravascular interventional MR vessel wall imaging developments. In addition, newer concepts in improving the functionality of these techniques, such as automated slice definition and adaptive tracking, have also been developed.

Intraoperative MR

The technical requirements for intraoperative MR imaging are slightly different than those necessary for minimally invasive percutaneous procedures, and include the need for sufficient signal-to-noise and spatial resolution to allow accurate identification of residual tumor or hemorrhage in addition to patient access during the procedure. Many MR image-guided surgical interventions have been performed at mid- or high- field, but 0.2 Tesla can also provide the signal-to-noise and spatial resolution necessary for intracranial intraoperative imaging. Intraoperative techniques can be used to localize and target a lesion during the initial phase of surgery, monitor the degree of resection and any collateral damage during lesion resection, and document complete excision or the need for further resection prior to closure.