July 28, 2008

Open MRI: Improving Interventional Access
By Kathy Hardy
Radiology Today
Vol. 9 No. 15 P. 10

Open MR alleviates the fears of claustrophobic patients who cannot tolerate the closed tube magnet, yet the images taken via open MR are often not of the same quality as those taken with closed MR. These open MRI pros and cons are well known.

However, according to study results published in the May issue of Radiology, a recently introduced open MR scanner with a field strength of 1.0T can effectively remove visual barriers to localizing lesions and provide better resolution images, combining high-field image quality with a greater viewing angle and flexibility. With this study, radiologists evaluated the technique and time factors for real-time, MR-guided wire localization of suspicious breast lesions by using this higher field strength imager.

Radiologist Axel Gossmann, MD, conducted the study, which is the first involving a 1.0T open MR system, along with two other radiologists and three gynecologists, while he was with the University of Cologne in Germany. With Gossmann’s 11 years of experience in breast MR and eight years experience in MR-guided breast intervention, he says this study involved a faster, more precise scanner than older, low-field open MR systems on the market.

Improved Workflow
“The advantages of real-time, open MR-guided wire localization are a simplified workflow due to ease of operation on the part of the radiologist and efficiency for the patient,” he says. “Even lesions that are difficult to localize or are not approachable, such as those close to the chest wall, are amenable to this technique. You can use this system to detect nearly any lesion within the breast.”

Less-invasive image-guided procedures such as wire localization, which involves inserting a hollow needle into a suspicious area of the breast, may benefit from this MR technology. Wire localization is one possible MR-guided biopsy procedure and is performed to help physicians locate lesions during surgical biopsy.

The University of Cologne was the first European site to install a Philips Panorama 1.0T in June 2005. Gossmann says patient access is a major advantage with the open MR, and this scanner’s open architecture and moveable patient table allowed them to easily access patients during the study, even while they were immobilized in the treatment position.

“This open system is unique,” he says. “When we place a patient in the scanner, we have access to them. We can do any kind of interventional procedure while they’re in there.”

As noted in the study, MR imaging of the breast locates lesions that often cannot be detected by other imaging methods. With that, an MR-guided intervention may be required to conduct tissue diagnosis, and MR-guided wire localization followed by a biopsy of the lesion is still the standard for MR-guided intervention.

Gossmann notes that this standard process conducted with a traditional cylindrical system MR is fraught with technical challenges due to the closed nature of the device. Limited access to the patient, particularly with breast procedures, typically requires repeated transfer of the patient in and out of the imager during the interventional procedure, leading to increased exam times, he says. With the increased time, you run the risk of contrast material washout, which can diminish the visibility of the lesion relative to the guide wire. Any reduction in time between when the contrast is administered and when the images are taken reduces the likelihood of the contrast dissipating.

Contrast Washout
“It’s the wash in, wash out phenomenon,” Gossmann says. “You can lose contrast agent if the procedure takes too long. It takes a lot of time to move the patient in and out of the cylindrical magnet, and you lose contrast during that time. If you lose too much contrast, then you can’t see the lesion.”

MR-guided wire localization performed in a traditional closed-bore imager requires different techniques, he says, such as stereotactic and freehand methods. With needle insertion occurring outside the magnet, radiologists may miss shifting breast tissue or a needle deviation occurring inside the tube, which therefore cannot be corrected at that time. In addition, breast lesions situated close to the chest wall, within the axillary tail, or close to implants are difficult to localize or may be inaccessible with standard techniques.

“This process is very demanding on the patient and more time consuming,” he says. “With closed MR, you don’t see what you’re doing. Working outside the scanner, you don’t have control of the needle. That makes it difficult to get close to the chest wall, risking possible injury to the lung. When working outside, you’re working blind. The procedure is more safe with the open scanner.”

With open MR, Gossmann says, the patient, can remain positioned in the center of the magnet throughout the procedure, from the time contrast agent is administered until the localization is completed.

“Repeated transfer in and out of the magnet, as happens with closed-bore MR, is not necessary,” he says.

Interventional Access
However, the success of MR-guided procedures not only depends on the ease of access to the patient but also on how clearly lesions appear. Dependence on MR to provide high-resolution images has been the sticking point with open MR. The construction of a traditional closed-bore MR unit is conducive to acquiring stronger magnet signals and therefore higher quality images. Traditional open MR scanners have field strengths of 0.2 to 0.7T. High-field systems are generally considered to operate at or above 1.0T. Consequently, closed MR became synonymous with high-field MR and open MR synonymous with low field.

When open MR was introduced, the magnets often did not provide the same quality of images as the closed-tube type units did, Gossmann says. This meant some exams could not be performed with the open MR. If the open MR was used, longer scan times were required, which compromised image quality.

However, with this particular 1.0T imager, patients experience fast, dynamic imaging in near real-time, as well as good spatial resolution, Gossman says. During the course of the study, this enabled visualization of the needle during the interventional procedure.

“With the open system, you leave the patient inside the scanner,” Gossman says. “You can stick the needle in and keep the MR running. You can check the monitor to see what you’re doing so that you can control the tip of the needle.”

The open MR also improves the chances that the contrast agent will maintain its strength throughout the course of the procedure, Gossmann says.

Study activities began between January 2006 and July 2006, when participating radiologists performed 531 MR breast imaging examinations, 131 of which involved women with breast cancer. Within that group, 30 women with a total of 31 MR-detected lesions underwent MR-guided wire localization followed by surgery. In all the patients, diagnostic MR helped detect suspicious lesions that were not visible with mammography. MR-guided wire localization was performed, rather than core-needle biopsy, because surgery was planned.

Using MR-guided needle localization, the investigators identified 12 malignant lesions, two high-risk lesions, and 17 benign findings. Gossmann reports that total preoperative procedure time ranged from 16 to 36 minutes, with the time after contrast agent injection running from 5 to 14 minutes. The interventional procedure time, as monitored via MR imaging, ranged from 8 to 38 seconds. The mean total magnet time for patients in this study was 21 minutes.

“The reduction in time is important,” Gossman says. “That means less discomfort for the patient and better opportunity for the contrast agent to show where the lesions are located.”

Patient Alignment
Using the open MRI improved patient alignment and immobilization, with patients placed on a flat and rigid table fitted with MR-compatible immobilization devices. Patients remained prone while a dedicated breast array coil conducted the imaging. An immobilization and positioning device, complete with pillar system, was used to reduce motion in the breast, which was placed in the isocenter of the magnet, and to allow for accurate needle guidance. Gentle compression was used to hold the breast in place.

Once the contrast-enhanced lesions were located and marked, the guide wire was inserted through the needle and real-time MR imaging was utilized to help visualize the needle placement and wire release. Assisting with this process is a monitor located close to the magnet for convenient display of the MR images during the procedure, Gossman says. Any shift of the breast tissue or needle deviation can be corrected, if necessary, without removing the patient from the magnet.

“This allows the radiologist to stay in the imaging room with the patient, while at the same time watching the placement of the needle and release of the guide wire during the procedure.”

Overall, the study results showed that real-time, MR-guided wire localization of suspicious breast lesions by using the open 1.0T MR imager allowed for correction of the needle position during placement and reduced the interventional procedure time. With that, contrast-enhanced lesions showed only a minimal decrease in visibility during the procedure, Gossmann says.

“This real-time, MR-guided localization process provides a unique combination of clinical excellence and patient acceptance,” he says. “It’s unique to have this high resolution with the open design.”

The open MR 1.0T is not just for breast lesions, Gossmann adds. The system is applicable to scans of any soft tissue areas, including the brain, he says.

“This is just the beginning,” Gossmann says. “There are many other ways improvements can be made in open MR imaging. In the future, there may be work with the needles and guide wires that are currently just being adapted to real-time techniques. There may be other new materials that will help refine the process.”

— Kathy Hardy is freelance writer based in Phoenixville, Pa., and a frequent contributor to Radiology Today.