Applied Radiology

Dr. Bloomgarden received his MD and PhD from the University of Pennsylvania, Philadelphia, PA. He completed his residency in Diagnostic Radiology at Beth Israel Deaconess Medical Center in Boston. He is currently completing his fellowship in Interventional Radiology at the University of Pennsylvania Medical Center and affiliated hospitals. He will be taking a position in Cardiovascualr Imaging and Interventional Radiology at St. Luke's Medical Center, Milwaukee, WI, where he plans to perform Interventional Procedures on their 0.7T open MR scanner.

The recent development of the use of magnetic resonance imaging (MRI) to guide and monitor medical procedures holds much promise. At present, MR-guided biopsies, wire localizations, and chemical ablations can be performed on both open and closed MRI systems with little or no specialized hardware. Thermal ablations require specialized hardware, much of which already exists, although this procedure needs to be further studied and developed before it is accepted for widespread use. Finally, intravascular procedures utilizing MR guidance are still in preliminary research and development, but offer the potential for vascular imaging and treatments not available with any other modality.

As the revolution in minimally invasive surgery progresses, the need for more advanced and accurate methods for guiding and evaluating procedures is growing. Most intravascular radiological procedures are performed under fluoroscopy and X-ray guidance. Cross-sectional imaging, including ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI), is used for most percutaneous procedures, such as biopsies and ablations. As these cross-sectional imaging modalities have become faster and easier to use, they have become more available to aid interventional radiologists in these procedures.

Each modality has its own advantages and disadvantages. Although the advantages of ultrasound (particularly real-time rapid imaging) are frequently exploited, the disadvantages of poor tissue resolution and poor penetration can limit the ability to perform certain procedures. CT has better penetration and good contrast between air, fat, and tissue. However, CT often has difficulty in distinguishing lesions without the use of exogenous contrast agents. Such agents, however, only aid conspicuity during a transient imaging interval. In addition, CT provides only axial (or limited off-axial) imaging guidance and can not be performed in real time.

MRI, a newer modality for procedure guidance, has many potential advantages that may make it more attractive than other currently available modalities. One such advantage is the lack of radiation exposure. Radiologists are subject to high levels of radiation during their lifetime career; use of non­X-ray modalities such as MRI will lower this risk. In addition, the multiplanar imaging capabilities of MRI permit non-axial needle tracking, such as is desired in reaching liver lesions from a subcostal approach. These areas and lesions are often not seen well with ultrasound, and tracks are difficult to follow with axial CT images. MRI is often the only imaging modality that affords the inherent tissue contrast needed to visualize lesions in steady state (ie, not during transient contrast phases). MRI, with its high inherent tissue contrast, can visualize different structures within an organ, including vessels and ducts, without exogenous contrast agents. Finally, and probably most importantly, MRI permits monitoring of treatment, such as thermal ablation, directly and in real time while the procedure is under way.

The recent advances in scanner design, MR-compatible instruments, and rapid imaging have propelled MR into the vanguard of imaging. This article will review the scanner designs, current applications, and recent developments that will enable MR guidance to be a reality in the next few years. This article will focus on applications of MRI in abdominal and vascular procedures, while briefly mentioning some of the neurointerventional, breast, and other applications.

Magnet designs

The ideal MR system for interventional and surgical procedures provides adequate access to the patient from all directions, yet maintains a good signal to noise ratio, rapid imaging, and low artifacts. As yet, there is no one perfect scanner system, but there are many system designs, and many commercially available and prototype systems in current use.

The first system design used for biopsy and neurosurgical procedures in the United States is still used today at the Brigham and Women's Hospital in Boston, MA. Their design, by GE Medical Systems (Waukesha, WI), is called the "double donut" and consists of a cylindrical scanner with 54 inches of the center portion removed. This center area provides access to the patient at the scanner's isocenter for the surgeon or interventionalist. Disadvantages of this design include lower magnetic field strength at the center of the bore (0.5 T) and lower field homogeniety. This costly system may not suitable for most hospitals, but similar systems have recently been installed in other large research institutions.

Several commercially available systems also permit access to the patient in the scanner. Such open systems, which can also be used for imaging larger or claustrophobic patients, generally use biplanar horizontal magnets. These systems come in varying field strengths. Permanent or resistive magnets, the least expensive, have low field strength (<0.1 T to 0.3 T) and therefore have limited tissue contrast, temporal, and spatial resolution. The more expensive "mid-field" magnets are usually superconducting types and have main magnet strengths of 0.5 T to 0.7 T. These are the highest field strengths designed with open configuration. The C-arm designs (produced by Seimens Medical Solutions, Erlangen, Germany) look like conventional fluoroscopic C-arms and permit the widest access to the patient but have the lowest field strength. Scanners by GE Medical Systems, Hitachi Medical Systems America (Twinsburg, OH) and Toshiba America Medical Systems (Tustin, CA) have wider magnets with higher field strength, but require 2 to 4 supporting posts, decreasing access to the patient (Figure 1A). A new prototype design by Fonar (Melville, NY) incorporates the magnet hardware into the ceiling and floor, thereby providing the widest space between the magnets with 360š access (Figure 1B). This scanner is still in the development stages.

Alternative ideas for MR-guided procedures include use of a combination fluoroscopy/MRI suite. These rooms, such as that developed by Phillips Medical Systems (Bothell, WA) and installed at the University of Minnesota, Minneapolis, MN, center around a custom table that slides a patient from a C-arm fluoroscope into a short bore, high-field magnet. ¹ This flexible design permits high-resolution rapid imaging, and is suitable for monitoring procedures after instrument placement using fluoroscopy or ultrasound. It can also be used for intravascular procedures, whereby both MR and X-ray guidance can be used for catheter and guidewire positioning, imaging, and treatment guidance.

Hardware developments continue to occur rapidly, and new designs and advances will continue to appear as the technology matures. Recent developments include X-ray systems incorporated into mid-field MR systems. ^2,3

Biopsies and localization procedures

Early MR pioneers first reported on the use of MR-compatible devices for percutaneous biopsies and fluid drainages in the late 1980s. ^4,5 Following these early proofs of principle papers, there has been a very slow translation of these methods into clinical practice. Limitations include limited access to the patient and the time-consuming nature of these procedures due to closed bore systems. In closed-bore magnets, as in CT-guided procedures, the following steps are performed: 1) imaging the lesion (with or without intravenous contrast); 2) marking the location on the skin surface; 3) advancing a needle toward the lesion; 4) securing the needle; and 5) re-imaging to correct the position. Steps 3, 4, and 5 are repeated until the needle is visualized at the periphery of the lesion. Each step requires the operator to move the patient in and out of the magnet bore and view the images on the monitor outside of the scanner room. In this manner, lesions in the liver, kidney, adrenal glands, prostate, and breast have been biopsied successfully when the lesions were not detected on any other imaging modality.

The most extensive experience in MR-guided procedures has come from non-abdominal applications. In 2001, Salomonowitz ⁶ published his experience with 361 MR-guided procedures on 250 patients over 3 years. Procedures included spinal nerve root injections, intrathoracic and vertebral body biopsies, intervertebral joint blockades, and intra-abdominal (mostly liver, pancreas, and lymph node) biopsies. He reported no complications.

MR guidance has also been used for procedures on other organ systems. Specialized neurosurgical procedures are performed in MR suites like the one found at Brigham and Women's Hospital in Boston. ⁷ At the Hospital of the University of Pennsylvania (Philadelphia, PA), breast biopsies are the most commonly performed procedures. Most of these lesions are only detected on contrast-enhanced MRI. These procedures require custom designed grids and compression devices, which will not be reviewed here.

Reports have described MR-guided prostate biopsies for lesions detected with endorectal prostate MRI. ^8,9 Recent experiments with MR-guided radioactive seed placement and cryotherapy for prostate cancer have also been reported. ^10-12

In general, at the Hospital of the University of Pennsylvania, MR is still reserved for biopsy and treatment procedures for lesions not visualized with other modalities. We do not have an open configuration scanner, and therefore use the algorithm previously outlined: Imaging followed by needle manipulation with the patient outside the magnet bore, then re-imaging. Procedures can take hours, but often could not be done with any other modality, other than open surgical biopsy.

Imaging is usually a combination of breath-hold (usually in expiration) T1-weighted gradient-echo and/or single-shot fast spin-echo T2-weighted images of the region of interest. If the lesion is not well visualized, a contrast-enhanced three-dimensional (3D) gradient-echo image with fat saturation is obtained. After localizing the lesion, axial, sagittal, or coronal thin-section images are obtained. An entry site is chosen from the images, and that site on the skin is marked, using the axial position LASER and the right/left distance from isocenter.

Following appropriate preparation, draping, and use of local anesthesia, the needle is advanced toward the lesion from the predetermined orientation, as calculated from the images. As the needle is advanced, serial in-plane imaging can be acquired to verify orientation and position. Needle artifacts are related to pulse sequence and orientation of the needle with respect to the main magnetic field. To minimize artifacts, imaging with highly refocused pulse sequences such as fast- (turbo-) spin echo can be used. ^13-15 After advancing to the correct position, perpendicular images along the track of the needle can be obtained to verify position. Figure 2 shows two images obtained during needle placement in a liver lesion near the inferior vena cava (IVC).

When the needle is at the appropriate position, fine needle aspirates, core biopsy samples, or instillation of chemical ablation agents can be performed. Following ablation or biopsy, post-procedure imaging (T1-weighted or contrast-enhanced images) are obtained to confirm treatment and/or evaluate for complications.

The risk of complications and the length of time required for such procedures could be lessened significantly with open configuration systems; in-room monitors; and rapid, user-friendly image prescription software. Over the last few years, such systems have been demonstrated at research conferences. They continue to be improved and are slowly being installed at many hospitals and medical centers.

Ablations

One of the largest areas of growth in interventional radiology over the last few years is in the percutaneous treatment of cancer. Cancers­­including hepatocellular carcinoma, lung cancer, and metastatic colon cancer­­have been treated in the liver, lung, and bones. The indications for regional or local ablation continue to grow, but currently include nonresectable hepatoma, colon cancer metastases to the liver or bone, metastatic disease to the lung, and nonresectable primary lung cancers. MRI is best suited for liver lesions, as other modalities can be used to visualize lung and bone tumors.

The incidence of hepatoma continues to increase as a result of increases in hepatitis C in the United States. Although the only definitive therapy is surgical resection, many patients are not candidates due to liver dysfunction or high surgical risk. Percutaneous procedures aimed at destroying the tumor and a small margin of normal tissue can be performed in a number of ways. Thermal therapy, with heat (radiofrequency [RF] ablation, lasers, or focused ultrasound) or cold (cryotherapy), has been used for a number of years. Chemical ablation with alcohol or acetic acid has also been shown to be effective in the treatment of hepatomas. However, the methods and doses for delivery of these agents have not been defined completely, therefore more accurate monitoring of the ablation procedure should be done to maximize efficacy and minimize complications.

RF ablation is the delivery of alternating electric current, modulating in the low radiofrequency range of <1 MHz. This electric current results in rapid ion movements in the adjacent tissue that cause heating. The heat induces cellular damage resulting in tumor necrosis. Necrosis can occur instantaneously at temperatures >60šC, but can also occur using lower temperatures for prolonged periods of time. MR is the only imaging modality with the potential to directly measure tissue temperature to determine when a lesion is fully treated.

There are multiple approaches to measuring temperature. The three most practical methods are described below. The most straightforward approach is the use of T1 maps to follow temperature changes. T1 measures the longitudinal relaxation time and is commonly evaluated qualitatively with T1-weighted images. T1 temperature images are easy to implement, provide relatively high temporal and spatial resolution, but suffer from poor linearity with temperature change and a somewhat poor signal-to-noise ratio.

Diffusion-weighted images are also temperature sensitive with good sensitivity and similar temporal and spatial resolution to T1 images. However, factors other than temperature affect diffusion weighting, including ischemia and changes in tissue characteristics that occur during the coagulation process. In addition, diffusion images are direction dependent, requiring additional images to measure the diffusion tensor. Finally, fat can cause artifacts in diffusion images acquired near fat/water interfaces.

The third and most promising method of measuring temperature is based on the chemical shift of the water/proton frequency with changes in temperature (proton resonance frequency method). Using fast spoiled gradient-echo sequences that correct for most of the local magnetic field inhomogenieties, the proton frequency shift is related linearly to the temperature change. These PRF sequences have high temporal resolution, good sensitivity and spatial resolution, and have been shown to be more precise than T1- or diffusion-weighted image sequences. Several published studies describe the use of MR thermometric maps during tumor ablation. ^16-19 Recent experience on low-field (0.2 T) systems show that both conventional T2-weighted imaging and turbo spin-echo imaging sequences provide conspicuity of the treated region similar to that of contrast-enhanced imaging, permitting continuous monitoring during treatment. ¹³ Further research is necessary to validate these findings before this method can be put into widespread clinical use.

Other delivery systems can also be monitored with MR thermometry. Cryoablation probes are MR compatible and can be positioned with MR guidance, and monitored using similar pulse sequences. Other heat delivery systems include focused ultrasound technology in which arrays of high-frequency ultrasound transducers placed outside the body deliver energy resulting in heating below the skin surface. Integrated 3D imaging with computer tracking and MR thermometric monitoring allows for the automated determination of appropriate heat delivery. ²⁰ This treatment has been applied in the brain, breast, and uterus (leiomyomas). ^7,19,21,22 Early experience with totally noninvasive MR-guided focused ultrasound therapy of benign fibroadenomas has been shown to be possible, ²² although preliminary results suggest more work is needed in monitoring and targeting lesions.

While thermal therapies require specialized and expensive equipment that must be MR compatible, chemical ablation, on the other hand, requires only a thin needle (or possibly one with multiple side holes). Research in Asia, and more recently in Europe and the United States, has shown efficacy and increased survival with both alcohol and acetic acid instillations as treatment for hepatocellular carcinoma and liver metastases. ^23-27

Alcohol causes decreased T1-, T2-, and diffusion-weighted signal on rapid imaging sequences. Following ablation with alcohol, the low signal intensity on the T2-weighted images corresponds with the area of nonenhancement on the post-gadolinium images and represents the area of cell death. Immediately following instillation, contrast-enhanced images can be obtained to verify the size of the treatment area. Figure 3 shows some edema surrounding a lesion recently treated with ethanol injection. The treated area appears low in signal intensity, corresponding to the area of cell necrosis. A small amount of subcapsular fluid is also noted.

An advantage of using acetic acid is the difference in the resonant frequency of acetic acid versus water. Selective chemical shift imaging can be performed to suppress the water during acetic acid instillation. Thus the treated area will appear bright on an acetic acid selective image. Figure 4 shows an image acquired during treatment of a nonresectable hepatoma in a patient with poor liver function and high surgical risk. The series of images shows the increasing size of the treated region using chemical selective imaging.

Intravascular MRI

The rationale for the use of MRI for interventional procedures is based on recent studies describing atherosclerotic plaques with luminal appearances that do not necessarily correspond with the potential for harm. These vulnerable plaques--in which a thin fibrous layer covers a large lipid core--have the potential to rupture, leading to vessel occlusion even though the lesion may not cause significant luminal narrowing. This phenomenon has been seen in carotid and coronary vessels. MRI that permits visualization of the vessel wall with a catheter in place will reveal the characteristics of the plaque and vessel more clearly.

Additionally, MR-guided procedures can be performed without the use of ionizing radiation or nephrotoxic contrast agents. Current advancements in this field include development of intravascular catheters and wires that can be tracked (ie, visualized) with the MR scanner and that act as antennae for high-resolution vascular imaging. Because synthetic catheter material lacks any MR signal, various strategies have been developed for making them visible. These strategies can be divided into passive or active catheter tracking.

Passive tracking uses material or contrast agents added to the catheter to make them visible on MR sequences. Some commercially made catheters incorporate dysprosium rings near the end of the catheter, which appear as focal signal voids in bright blood sequences. ^28,29 Such catheters were used by Bucker et al ²⁸ to deploy IVC filters in pigs using real-time imaging in a combined MR-fluoroscopy system. Omary et al filled catheters with dilute GD-DTPA and imaged using two-dimensional (2D) and 3D spoiled gradient-echo imaging sequences. ³⁰ They successfully tracked catheter manipulation during renal artery angioplasty in pigs (Figure 5A and 5B). ³¹ Passive devices can be tracked with conventional or fast imaging, and/or background suppressed images, or can be superimposed upon angiographic sequences.

Passive catheters, however, can have poor conspicuity within certain images, especially those with large section thickness or low in-plane resolution.

Alternatively, active catheters produce inherent signal, eliminating the problem of poor conspicuity. Active catheter tracking uses catheters or guidewires embedded with an RF coil directly attached to the scanner. The coil causes a peak in the Fourier-transformed images corresponding to the position of the tip of the coil. ³² After appropriate three-plane imaging, the position of the tip of the coil is known and can be superimposed on any image. To visualize more than just the catheter tip, modifications can be made to the catheters. These modifications include using multiple coils along the catheter length or using short linear segment coils. The limitations of active catheter tracking include catheter expense, local heating, and poor tracking if the patient moves. However, these same catheters can be used for imaging the vessel wall. This dual function is exemplified in Figure 6, which shows an active catheter being tracked (Figure 6A) in a rabbit aorta and the corresponding high-resolution vessel wall image (Figure 6B).

Semi-active catheters provide MR circuitry that enhances imaging without direct electrical connections with the scanner. ^15,32 This type of system reduces the risks of coil heating and artifacts from standing waves. In addition, wireless inductive coupling can result in signal enhancement when imaging the tissue immediately adjacent to the coil. The coil can be mounted on an inflatable balloon at the end of a catheter. Alternative strategies include the use of a laser-controlled inductive coil that can be tuned or detuned with photo-optics, for use in tracking or imaging without risks of additional heating. ³³ The preliminary research on this method is a very exciting beginning to the nascent field of intravascular MRI. However, before it becomes standard in clinical use, further developments in active and passive catheter and guidewires are required.

Conclusion

Interventional MRI is a rapidly developing area. Cancer treatment represents the fastest growing area among many interventional radiology practices. MR currently represents the most accurate modality for biopsies and cancer treatments. Improvements in open MR systems, device technology, and image prescription software will make these procedures easier and faster in the next few years. Finally, although intravascular MRI is still in its infancy, the possibilities inherent in this technology offer a new paradigm for intravascular therapy based on direct vessel-wall pathology, rather than luminal irregularity.

Acknowledgments

The author would like to thank Dr. David Roberts (University of Pennsylvania Medical Center, Philadelphia, PA) and Dr. Reed Omary (Northwestern Memorial Hospital, Chicago, IL) for their helpful discussions and images and Dr. Albert Lardo (Johns Hopkins Medical Center, Baltimore, MD) for his images.