1.5T MRI: Friend to Physician and Patient Alike

Utilization of magnetic resonance imaging is on the rise and 1.5T systems remain one of the strongest players in the marketplace. An estimated 24.2 million MRI procedures were performed in the United States in 2003 in facilities using fixed and mobile MRI systems - which shows a 10 percent+ jump from the 21.9 million procedures performed in 2002, according to IMV Medical Information Division Inc.'s 2004 MRI Market Summary Report. High-field 1.5T MRI systems accounted for more than two-thirds of MRI units installed in 2004, replacing older units in both hospitals and non-hospital settings, the report found.

MRI's strength in the diagnostic imaging community is attributed to a number of key elements, including its ability to acquire images without the use of ionizing radiation. MR imaging provides clinicians with unprecedented soft-tissue contrast, which is valuable in determining certain diseases and conditions.

Imaging at 1.5T has been done in for a number of years. "Because of its maturity, there has been more application development," says Vickey Hanson, America's 1.5T MR marketing manager for GE Healthcare. "There are larger surface coil portfolios. There are larger application portfolios - from very broad applications to very specific applications." These range from the bread-and-butter MRI applications, such as brain, spine and neck and musculoskeletal imaging, to emerging applications such as vascular, cardiac, breast and pediatric imaging, as well as functional MRI.

"There are more 1.5T scanners out there than any other [type of MR system]," says Edmond Knopp, MD, associate professor of radiology and neurosurgery, section chief of neuroradiology at New York University School of Medicine. "At the same time, they are capable of doing everything, whereas the low-field scanners are not capable of doing everything and their image resolution is very poor. Also, there are not many 3T systems [installed clinically] and they are somewhat more difficult to use."

The customer base for high-field 3T MRI systems has transitioned from research institutes to clinical settings as the technology touts increased signal to noise (SNR) ratio and faster scanning with higher resolution. However, 3T systems carry price tags of $700,000 to $1.2 million more than 1.5T systems. The 1.5T systems also are less sensitive to motion artifacts and do not have to deal with SAR (specific absorption rate) issues that plague 3T.


And patient friendly, too

Besides being workhorse scanners and accommodating a variety of emerging applications, the newer generation of 1.5T systems have designs that are more patient friendly. "The most prominent recent trend in the 1.5T market has been to introduce MRI scanners that are more patient focused," says Mark Totina, product manager for Toshiba America Medical Systems Inc. "If you were to analyze the latest MRI scanners released by the major vendors, you would find the common theme of shorter bores, wider apertures, quieter scanners and niche patient-focused features."

Toshiba's patient friendly applications for its Vantage 1.5T ultra-short MRI system include a noise reduction technology called Pianissimo. The technology decreases gradient acoustic noise up to 90 percent, which is equivalent to the noise reduction of being 40 meters away from a point source of noise.

Health Scan Imaging in Murrieta, Calif., recently installed Toshiba's Vantage 1.5T MR system. "MRI exams are noisy," says Bill Kelly, MD, neuroradiologist, founder and director of Health Scan Imaging. "This has become in many cases just as intimidating to the patient as the confinement [issues]. Now when patients come in for an exam and hear how quiet the magnet is, it's another factor that increases their likelihood of tolerating the exam."

Newer 1.5T designs also include shorter bores. Raleigh MRI in North Carolina utilizes Siemens Medical Solutions' Avanto 1.5T MRI scanner that has a compact 60-centimeter magnet (150 cm length). "You can actually stand next to the magnet and outstretch your arms and touch both ends," explains Margaret King, RT, director of MRI services at Raleigh MRI, an outpatient imaging center affiliated with Wake Radiology Diagnostic Imaging Inc. King says that lumbar spine exams are completed with some portion of the patient's head outside the bore.  

In August 2004, Siemens launched the open bore Magnetom Espree 1.5T MR system. The system's 70 cm bore (175 cm length) allows for 60 percent of exams to be completed with the patient's head outside the bore.

In addition to smaller bores that achieve patient comfort, King says that today's 1.5T scanners image faster and that the gradients are more efficient. "Equipment gets more and more expensive as it gets faster and faster," says King. "But the more patients I can get through here with an optimal exam, the better I can accommodate our backlog."

The system is used for conventional MR imaging - brain, spine and orthopedic imaging - as well as for more advanced procedures, such as contrast-enhanced angiographies, body MR, cardiac procedures and brain spectroscopy.

The Avanto 1.5T system is Siemens' first Tim (total imaging matrix) scanner. Tim technology incorporates a whole body surface coil design combining up to 76 integrated coil elements with up to 32 RF channels. Tim technology eliminates steps in workflow since the technologist does not have to move the patient on and off the table during a procedure. King uses a brain and spine exam to explain how the technology improves productivity: "The patient is positioned on the table and the technologist places the coil for both the spine and neck. The entire head exam is performed. But instead of moving the patient off the table and putting on the neck coil, the technologist can actually move the table x number of millimeters after the head procedure is completed and begin the spine exam. This saves so much time and workflow is definitely impacted."


Advanced imaging

Another reason high-field MRI has become the dominant once again is the introduction of newer software technologies that accommodate a number of emerging MR applications. NYU's Knopp says their 1.5T MRI scanner used at the medical center addresses challenges specific to brain imaging. "We can get very high resolution [images] without any image distortion in short scan times," says Knopp. "We can see lesions and abnormalities that we could not otherwise see on conventional 1.5T scanners. The system is used for preoperative surgical planning and there are significantly fewer artifacts, which is important is localizing lesions."

The ability to do very good brain imaging is an important application of 1.5T MRI systems. Challenges include tremor patients such as those who have Parkinson's disease. These patients cannot be sedated and that means the images may be compromised.

The MRI department at St. Francis Hospital in Beech Grove, Ind., is using GE's Signa Excite 1.5T MRI system equipped with Propeller imaging. Propeller is a motion reduction sequence used for all brain imaging, says Paul Minnis, MR team leader at St. Francis. "The patient can physically be moving in the scanner and we will still get a clear image," says Minnis. "It also cuts down on pulsatility in the blood vessels. The technology also clears up the shadows created by the orbits when the eyes are moving." Propeller can be used as a corrective technique for uncooperative elderly and confused patients. Either voluntary or subtle physiological motions can be removed for the images.

The system also is equipped with GE's Vibrant technology, which Minnis describes as a breast imaging technique that allows high-quality images of both breasts to be captured at shorter scanning times. "We are currently doing 80 breast MRI studies per month," says Minnis. "We are at the point now where specific time slots two days a week are dedicated to our breast surgeons. There have been cases where the MR scanner found an additional lump in a woman's breast that both the ultrasound and x-rays studies missed."

While Minnis says that future enhancements to the system include incorporating GE's high definition (HD) technology (released at RSNA 2004), St. Francis's MR department also is looking into MR-focused ultrasound. The technology integrates focused ultrasound thermal ablation with MR imaging to provide a non-invasive method for destroying tumors. It can be used for the non-invasive treatment of uterine fibroids.

"For example, MR data are used to locate and find the uterine fibroids," says Minnis. "A built-in ultrasonic probe focuses on the tumor, heats it up and kills it. It can take up to three to four hours to do depending on the severity of the case. The big thing is that these patients do not need to take any time off work other than the one day for the MRI procedure. Typically, they have to take six weeks off of work for a conventional hysterectomy."

GE's Hanson says that breast imaging is the most explosive use of MR yet. "The next step that MR has to take [in breast MR] is the addition of more specificity," says Hanson. "Right now there is a belief that spectroscopy will play a very key role in the ability to bring specificity up and prevent needless biopsies. Diffusion imaging is another key technique in breast and body imaging. Diffusion imaging that is done without contrast can be done in 40 seconds, and the results you get equate to a contrast-enhanced exam."

Another area of growth in the 1.5T MRI arena is cardiac imaging. Cardiac MR has tremendous potential, says Hanson, but its downside is that it is a disruptive technology. "The adoption curve has been very slow but cardiologists are beginning to recognize the value of cardiac MR," says Hanson. "The challenge of cardiac MR imaging is that you have so much moving so fast that needs to be imaged. You have both the breathing motion and the heart moving. Many times you are imaging patients for cardiac disease, which often means that they do not have a good electrical signal and that poses a challenge. Many patients cannot catch their breath, much less hold their breath. A technique really needs to be developed that allows for real-time imaging that does not rely on cardiac gating nor requires the patient to hold their breath."


A robust scanner

According to Health Scan Imaging's Kelly, 1.5T scanners are more capable of consistently providing useful results for emerging applications, such as breast and cardiac studies. However, these still account for a minority of exams. "The bulk of MRI exams still reflect the prevalence of disease," he says. "MRI is the appropriate modality for most significant complaints related to the brain and spine, as well as dysfunctional conditions of the joints."

Kelly says that speed is the biggest component that 1.5T systems bring to brain imaging studies. "If you have a faster system, you can apply more pulse sequences, says Kelly. "For instance, if you are going to allow 30-minutes for brain MRI exams, and a patient had a closed head injury, you want to add additional brain sequences that augment sensitivity for abnormalities specific to that brain injury. By speeding up the efficiency of the exam, you can increase the number of pulse sequences and in turn, increase both the detection sensitivity and completeness of that examination."

Napa Valley Imaging Center in California recently installed GE's 1.5T Excite HD MR system. According to Christopher Schultz, MD, director of MRI at the imaging center, the scanner has had a dramatic impact on throughput and productivity. "There also is the ability to acquire high-quality, high-definition images, which makes it satisfying for the radiologists as the interpreter of the image," indicates Schultz.

One of the new applications of the high-definition 1.5T scanner is a technique called LAVA (liver acquisition with volume acceleration). "It allows for three-dimensional abdominal imaging that is 25 percent faster than previous techniques," says Schultz. "At the same time, it allows for 25 percent better noise resolution and coverage. The technique is usually used on problem solving cases in the liver, pancreas or kidney. They are generally survey evaluations for patients who cannot receive contrast for a CT study."

Down the road, the clinicians at Napa Valley will be commencing a breast MRI program. "This is another area where having the new high-definition coils and multiple arrays available on the new breast coils is going to make an impact on imaging of the breast," explains Schultz. "GE's Vibrant allows for imaging of both breasts simultaneously in one exam. Previously, women would only have one breast exam done one day and have to come back for a second appointment to examine the second breast. Breasts can be imaged in either the sagittal or axial plane with true resolution. For women who are having MRI evaluation for breast cancer or staging to be completely imaged in one setting, it is very beneficial from their [psychological] standpoint."


Conclusion

1.5T MRI systems provide a good balance between the cost, performance and practicality. Bread-and-butter MR imaging is getting even better at 1.5T and manufacturers of these systems are making sure end-users can accommodate emerging applications ranging from breast and cardiac MR, whole body angiography and functional MRI. In addition, it is important that customers consider their upgrade path upfront, how amenable the system is to handling more advanced applications as well as how costly it is to acquire these features.