Chest scan using GE Healthcare’s Signa HDx 1.5T MRI system.

No longer just “bone-and-joint” imaging systems, 1.5T MRI systems are the field strength of choice these days in the United States—due in part to well-established protocols, imaging sequences and flexibility in applications and patient accommodation. As more clinical protocols are created and vendors add new features and software enhancements, 1.5T MRI installs continue at a steady pace in clinical imaging specialties traditionally dominated by digital x-ray, multidetector CT and nuclear medicine.

To many clinicians, 1.5T MR has become the “meat-and-potatoes” field strength, thanks to well-established protocols and sequences for imaging procedures. With a wide variety of coil options, 1.5T has the potential for better image quality across a variety of imaging applications in comparison to 3T. Imaging departments and imaging centers recognize that 1.5T simply has a more proven track record than its higher field strength cousin 3T.

When adding or replacing MRI, 1.5T is king—approximately 49 percent of respondents to the 2008 Health Imaging & IT Top Trends Survey who planned to buy an MRI system in 2008 said they were purchasing or planning to purchase 1.5T MR versus 38 percent considering a 3T. Another 7 percent are considering a 1T MR purchase, while 3 percent are considering 7T.

Even though 3T systems can scan faster or at higher resolution than 1.5T scanners, the proven, reliable imaging strength and quality of 1.5T are what many medical imaging professionals continue to compare everything else to, according to Alan R. Moody, radiologist in chief, department of diagnostic imaging, at Sunnybrook Health Sciences Centre in Toronto.

He says that two years ago, the overall atmosphere in the marketplace was that 3T was going “to storm the ramparts and replace 1.5T.” However, over time, this has been found to no longer be the case and now the general consensus is that 1.5T is the imaging workhorse now generally accepted as the gold standard in MRI.

“1.5T MRI is basically the standard—it does everything, and to challenge that at 3T you pretty much have to be able to do everything that 1.5T does and then do it better,” Moody says.

While 3T is excellent for neurological and musculoskeletal applications, it lags behind 1.5T in abdominal imaging and cardiac imaging, which are areas in which “we tend to put the majority of our work for the 1.5T,” he notes. Another challenge of higher strength magnets is the interaction with metal implants such as stents, aneurysm clips and even prosthetic devices. With 1.5T, patients who have these safety concerns are not an issue and can be imaged without an increase in metal artifact.

Moody and researchers have used 3D MRI (1.5T TwinSpeed MR system from GE Healthcare) to accurately detect bleeding within the walls of diseased carotid arteries. Their study results suggest that the technique may prove to be a useful screening tool for patients at high risk for stroke, according to a study published in the October issue of Radiology.

“1.5T just works,” he says. The evolution from lower field to 1.5T over time has been a relatively linear and natural progression until the emergence of 3T, when the industry thought 3T images would be better, but 1.5T held fast. “1.5T has now struck upon the highest level upon which you can achieve good imaging for the array of imaging applications, such as abdominal, neuro, cardiac, chest, spine, peripheral, joint imaging—without one of those groups falling off the map,” Moody comments. “The reason 1.5T is so successful is that it works for all of those applications.”

“As you go to the higher field strengths, because of the physics, some of the artifacts that you get start to impact on the image quality that you don’t get at 1.5T and you have to work harder to get your images out at 3T,” he says.

Another limitation of 3T, and thus a benefit of 1.5T, is related to its energy deposition. 3T might enable faster and higher resolution imaging, however, the restriction is in how much energy can be put back into the patient. “With 3T you have to ease back on the throttle, and you potentially lose some of the advantages you had of ramping up to that field strength in the first place,” Moody says, who adds that there seems to be a slight plateauing of that linear progression of the low field strength. Instead of racing to the next level in magnet strength, the imaging community has hit a plateau, staying strong with 1.5T MRI for its applicability across imaging applications.

“If you need high resolution and you can keep things still for long enough, then you will get the benefits of 3T,” he concludes. “If you are dealing with areas where you need less resolution and you have potential movements, like in the abdomen, then you are probably well-off with 1.5T.”

A dedicated 1.5T MR

MR-based breast imaging also is gaining steam, topping the charts as the No. 1 clinical area that facilities are focusing on with MRI in 2008 (at 33 percent of Health Imaging & IT Top Trends Survey respondents). Clinical studies consistently show that breast MRI finds previously undetected lesions and the speed and comfort of the scanners makes the 1.5T imaging workhorse an ideal modality for dedicated breast imaging. Installs are soaring, as are the number of system offerings.

With breast imaging at 1.5T, it isn’t necessary to have a dedicated scanner to understand the clinical value that 1.5T brings, but it does “have its advantages,” says Robin Shermis, MD, medical director of breast imaging at Toledo Hospital Breast Care Center.

The hospital utilizes the 1.5T MR for breast cancer screening of high-risk patients, pre-operative staging in patient diagnosed with breast cancer, to establish recurrent cancer or residuals and as a problem-solving tool to go beyond what conventional imaging provides for breast imaging, Shermis says.

The benefit of using the dedicated 1.5T unit, from Aurora Imaging Technology, is that from the ground up, both the software and hardware are designed around the breast. Using SpiralRodeo software for diagnosing ductal carcinoma in situ allows for better resolution and faster imaging. “While some machines take 45 minutes, we are done in 20 with better resolution to see lesions that other machines can’t,” he adds.

“We get about 160 slices per breast while most machines get about 64 slices,” Shermis says. “The signal-to-noise ratio is basically equivalent to a 4T magnet because SpiralRodeo gets much better fat suppression, enabling us to see lesions much more clearly.”

Regardless of the reasons behind choosing 1.5T MRI, it is evident that the scanners are continuing to meet a pressing need across the spectrum of imaging specialties, in the face of 3T’s gaining popularity—clearly proving that the modality has a secure foothold in the industry and is here to stay.