High-field scanners assume routine clinical caseloads

April 14, 2006

The market for 3T MRI is undoubtedly growing. Five years ago, few sites were using a 3T scanner for anything other than research. Now the higher field strength devices can be found in many clinical radiology departments and outpatient imaging clinics across the U.S. Clinical 3T is making inroads in Europe, too, though at a pace commensurate with its smaller MR base.

The market for 3T MRI is undoubtedly growing. Five years ago, few sites were using a 3T scanner for anything other than research. Now the higher field strength devices can be found in many clinical radiology departments and outpatient imaging clinics across the U.S. Clinical 3T is making inroads in Europe, too, though at a pace commensurate with its smaller MR base.

The majority of centers using 3T for clinical work purchased the higher field strength system as a second or third unit. Hardware and software advances have secured a firm foothold for 1.5T MRI in clinical radiology, and successive upgrades continue to push the technology past early limitations. The relatively novel 3T scanners simply can't compete with state-of-the-art 1.5T systems in certain areas.

But 3T MRI excels in a number of applications. Evidence is also amassing that 3T can at least match 1.5T in a large number of clinical radiology examinations. So while few if any sites are sending their 1.5T machines to the scrapheap, they are keen to see how 3T capability can complement their existing MR service.

The setup at Duke University Medical Center in North Carolina shows one way clinical 3T is being put to work in routine practice. The center has seven MR scanners devoted to clinical work, two of which are 3T units. Triaging of patients between 1.5T and 3T depends in part on clinical indication, and in part on which scanner has a free scheduling slot.

"For most indications, we are not too concerned about which scanner the patients go on," said Dr. Elmar Merkle, director of body MRI at Duke.

Merkle shares the view of most radiologists that 3T's superiority shines brightest in neuroimaging and musculoskeletal applications. Both areas generally involve imaging with a small field-of-view. He has also opted to conduct all prostate MR work at 3T.

Any studies in which significant abdominal fluid is expected are performed at 1.5T, which will yield better quality T2-weighted images, he said. This includes all fetal MR and cases of suspected ascites. Problems can arise if patients with ascites are inadvertently scheduled on a 3T unit and then have to be squeezed into the already-full 1.5T list.

"If a patient arrives for an MR exam at 10 a.m., you can't say come back at 8 p.m. That's not acceptable. But this pushes everything back," he said.

Radiology staff at Yale-New Haven Hospital in Connecticut acquired a clinical 3T scanner in June 2005, after housing a 3T research system for two years. The new unit works alongside four 1.5T scanners to accommodate Yale's busy caseload.

"We make a habit of using it for virtually everything. We don't discriminate," said Dr. Jeffrey Weinreb, director of medical imaging at Yale University.

A number of neurological applications have proven their mettle at 3T. These will be scheduled preferentially on the higher field scanner, he said. Applications include examinations of seizure patients, especially children. Visualization of epileptogenic foci is generally easier with 3T MRI owing to higher spatial resolution (Figure 1).

Patients who are pregnant, obese, or known to have ascites will be imaged only at 1.5T. Cardiac imaging cases are kept off the clinical 3T unit because of difficulties with artifacts.

Clinical cardiac imaging is not yet on the 3T agenda at Johns Hopkins University in Baltimore, Maryland, either, unlike most other applications. The large university healthcare practice now has two working 3T systems and another two under installation. Along with eight 1.5T scanners, these are distributed between the main hospital and an adjacent outpatient center. Because the 3T units tend to have a longer bore, patients who are nervous about their scan or likely to be claustrophobic will probably be assigned to a 1.5T unit, said Dr. David Bluemke, director of MRI. Patients with implants or medical devices are unlikely to be scanned at 3T.

"A lot of devices have not been validated at 3T," Bluemke said. "Also, you cannot guarantee that what a patient tells you about their device is 100% accurate. Using 1.5T gives you a wider safety margin."

The 3T scanners are used preferentially for examinations involving small fields-of-view, such as musculoskeletal radiology, and for brain scans. Abdominal MRI requests are split between the scanners, though results are better at 1.5T, Bluemke said.

Texas A&M University Health Science Center in College Station and its affiliate, the Scott and White Clinic, now operate six MR scanners, one of which is a 3T unit. No clinical applications are considered off limits for the higher field strength system. Patients needing brain scans or cervical spine imaging are most likely to be examined using the 3T scanner. Early experience of contrast-enhanced MR angiography at 3T is also showing the value of higher signal-to-noise ratio

to visualization of vessel architecture, especially in the carotids, said Dr. Val Runge, who is based at the Scott and White Clinic (Figure 2). The nature of clinical practice, however, means that the ideal scenarios are not always possible.

"I wouldn't want to see any patient with small brain metastases or acoustic neuroma not be scanned with 3T," Runge said. "But in a busy clinical practice, you don't always have that choice. The reality is that people get scheduled on the machines that are available."

Dr. Pavan Punukollu, a radiologist at St. Rita's Medical Center in Lima, Ohio, similarly strives to ensure that brain MR requests are prioritized to the center's 3T scanner. Visualization of demyelineated plaques in multiple sclerosis patients is especially impressive, he said.

St. Rita's is a community practice that accommodates outpatient and inpatient referrals. The center has three 1.5T scanners and an open 0.23T MR unit in addition to its 3T system. MR examinations of the cervical spine, knee, and wrist have all turned out well at 3T, Punukollu said (Figure 3). Some abdominal examinations, such as MR cholangiopancreatography, also seem sharper with the higher signal, although priority is given to neuroimaging cases.

"You do get some gain with body MR work, as things are just crisper. The problem is, we have so many brain MR examinations ordered on some days that it is hard to bump patients off the 3T scanner to do a body MR," Punukollu said.

RESEARCH AGENDA

Space for clinical 3T at academic sites can depend on who owns the scanner. When a 3T system has been purchased with a scientific grant, radiology researchers may not be keen to give up time slots for routine imaging. Clinical cases may be triaged onto the higher field unit only if they fit within a research focus.

The 3T scanner at the University of California, Los Angeles, for example, was installed primarily for cardiovascular MR research and applications. Consequently, the majority of clinical work performed on the system is contrast-enhanced MRA, said Dr. Paul Finn, a professor of radiology at UCLA's cardiovascular research laboratory. Patients presenting for carotid, renal, or abdominal MRA will most likely be examined with the higher field strength unit. But if no slot is free, the department's 1.5T scanner will provide excellent quality images as well.

"The 3T system can probably do MRA a bit better in certain areas, but our 1.5T system does a terrific job," he said.

Juggling the demands of clinical radiology with research commitments is a common issue in Europe, where most 3T scanners have been bought by university hospitals. At Ghent University Hospital in Belgium, for example, funding for a 3T MR system was secured in December 2004 with a scientific grant. This means that, strictly speaking, the scanner should be reserved for research purposes, said Dr. Rik Achten, director of the Ghent Institute of fMRI. But given the prowess of 3T in certain applications, he has opted to use two half-days each week for routine studies.

"We do routine 3T imaging as part of our presurgical epilepsy evaluation program because we found that the 3T system sees more small lesions, such as cortical dysplasia. You can miss those on 1.5T," Achten said.

The 3T scanner is currently equipped with coils only for neuroimaging. Coils for abdominal and cardiac imaging are expected to arrive later this year. Colleagues working in cardiology and a prostate cancer clinic are already expressing interest in transferring their workload to 3T, Achten said (Figure 4). He envisions that in another two years, the scanner will be overloaded by clinical demands.

The two 3T scanners housed at the University Hospital of Tubingen, Germany, are also used primarily for research at present. Investigations are focused on small field-of-view applications, such as cardiac MR, brain studies, and small joint imaging. Patients presenting with indications that would mesh with research interests may be scanned at 3T. All other patients requiring an MR examination are scheduled on one of the hospital's six 1.5T scanners. Prof. Heinz-Peter Schlemmer, an associate professor of radiology, feels under no pressure to alter this status quo.

"At the moment, 1.5T is the best MR system that is available worldwide. It allows us to get beautiful images," he said.

Radiologists at University Medical Center St. Raboud in Nijmegen, the Netherlands, have access to just one 3T scanner. The time on this system is split into one day pure research, one day clinical research, and three days clinical MRI. The routine clinical work is further subdivided into half a day for neuroradiological applications, half a day for breast MR, and two days for body imaging.

"I much prefer doing body imaging at 3T, and now I would not want to go back to 1.5T," said Prof. Jelle Barentsz, chair of radiology research at the Nijmegen center. "The problems, such as field inhomogeneities that sometimes cause darkening and limits on specific absorption rate, are minimal. You can really use the gain in signal-to-noise to get better images."

The mix of patients sent to the University of Bonn's radiology department for 3T MRI is broader still. The two 3T systems are used for "virtually everything," according to Dr. Christiane Kuhl, a professor of radiology. The only exceptions are contrast-enhanced MRA of the liver and pancreas, which are performed exclusively at 1.5T, owing to artifacts at 3T.

"You are able to acquire pretty images with 3T. The question is, as always, is this necessary to come up with more accurate diagnoses? This is much more difficult to prove," she said.

Scan time on the 3T system is prioritized according to clinical indication. Neuroimaging is no longer performed at 1.5T, and cardiac studies will likely be prioritized for 3T if perfusion imaging is required. Otherwise, any patient could, in theory, receive a 3T examination.

"It would make no sense to buy a 3T system as a sole magnet and then end up not being able to do an abdominal study," Kuhl said. "So our approach has been to show that 3T can be used for areas that were initially rated difficult."

Radiology staff at Queen's Medical Centre in Nottingham, the U.K., are already exploring the capabilities of 3T MRI in a clinical setting. Their 3T scanner, operational since November 2005, is running alongside two 1.5T systems. It is being used to scan all patients who would previously have been examined on a now-decommissioned 10-year-old 1.5T unit, said Andrew Cooper, MRI unit manager.

"The neuroradiologists would like to have more control, but we are finding that it works very well for other body and musculoskeletal applications too," he said. "The pelvic imaging we have done is absolutely stunning."

A 3T system at the Paul Strickland Scanner Centre in Middlesex, the U.K., has also been purchased with clinical imaging in mind. The scanner has been running in tandem with a 1.5T unit since December 2005. Both systems serve patients from the adjacent publicly funded Mount Vernon Cancer Centre, as well as referrals from nearby private practices. Most musculoskeletal imaging requests are split evenly between the two. Pelvic imaging, brain scans, and cervical spines are directed toward 3T, while upper abdominal imaging and metastases surveys are performed at 1.5T.

About 10% of scanning time is set aside for research. This is currently conducted on the 1.5T unit, though in two or three years, the balance will likely shift to 3T, said Dr. Anwar Padhani, consultant radiologist and head of imaging research at the center.

The decision to use 3T as a second unit was partly made to give the center an edge over its competitors, according to Padhani. Although the center is a charitable trust, with equipment financed through fundraising, it must still cover its operating costs. Private MRI clinics nearby have already invested in state-of-the-art 1.5T systems, but they are unlikely to move to 3T as long as they are operating just one scanner.

MARKETING MRI

But centers operating a 3T system may still have a marketing advantage. In applications where 3T truly excels, such as brain MR, the images speak for themselves. Referring clinicians can see the difference for themselves in the comfort of their own offices, thanks to PACS, Punukollu said.

"One neurosurgeon, who has now left town, refused to use anything other than 3T MRI. He would ship patients who had been going to another hospital over to our unit for imaging," he said.

Kuhl agrees that the message about 3T is filtering through to referring physicians. A growing number of referrals for MRA and cardiac perfusion are now requesting that patients should be examined at 3T, she said.

The selling power of higher field MRI is being tested in the U.S. at 3T Imaging, a private diagnostic imaging center in Morton Grove, Illinois. The multimodality imaging center has just one MR scanner and has opted for a 3T system. This decision should help distinguish the center from the 20 or so other MRI units already operating within a five-mile radius, said Dr. Richard Kim, an interventional radiologist and managing director of 3T Imaging.

The center, housed in a building shared by a large orthopedic group, is picking up a large number of requests for musculoskeletal imaging. It is also targeting neurology work and has secured a steady stream of abdominal imaging referrals from a nearby group of gastroenterologists.

"We wanted to take outpatient imaging to another level," Kim said. "From a marketing standpoint, there is nothing better than to say, 'We have a 3T magnet,' and then show the superior images that 3T generates."

Having a 3T magnet can make an impact on patients, too, according to Dr. Linda Moy, an assistant professor at New York University Medical Center. Patients who book breast MR workup at NYU are assigned randomly to one of six 1.5T or two 3T scanners. A majority of these patients are newcomers to the practice (Figure 5).

"New York City is a very competitive environment," Moy said. "As more patients request high-field magnets, it makes sense that we consider this when making our next purchase."

Bluemke is less convinced of the impact of 3T marketing on the general population. A few patients are, admittedly, requesting 3T for their scans at Johns Hopkins' hospitals and clinics. Schedulers will generally go along with these requests. But in the main, patients are more concerned about their illness and their comfort during imaging than the field strength of the magnet that will be used for their exam, he said.

Patients who insist on 3T over 1.5T, regardless of the indication, may not be doing the right thing, according to Runge. Just because an imaging practice has purchased a 3T system doesn't mean that staff members are getting the best out of the machine. Users need to know their own limitations and be prepared to adapt their 1.5T protocols to suit the higher field strength scanner.

"The educated patients are beginning to ask for 3T, which is an interesting situation. If you have someone who is not paying attention to detail, you may have a terribly running 3T, so they may be requesting a bad exam," he said.

EFFICIENCY DRIVE

Patients scanned at 3T could, however, spend less time in the imaging suite. Working with higher SNR means that acquisition times can be shortened without adversely affecting spatial resolution. This philosophy is being put into practice at the Scott and White Clinic. Time for a sagittal T1-weighted brain MRI with 3-mm slices has been reduced from four minutes to just one. Addition of axial diffusion T2-weighted MRI and FLAIR imaging takes the complete screening protocol to just seven minutes. Image quality is also better because patients find it easier to stay still during the shorter acquisition period.

"If you choose to use your higher SNR to increase speed of scanning, any modern integrated 3T system is going to beat any 1.5T system in terms of throughput," Runge said.

Patients may appreciate the benefit of shorter scanning for longer protocols. A standard rectal MRI workup at Queen's Medical Centre, for example, has been cut from 40 minutes to 25 minutes by moving patients to 3T. Because candidates for this examination are generally elderly and find it difficult to be still for long periods, the reduction makes a real difference.

But efficiency gains are not guaranteed. Many radiologists prefer to use the higher SNR to push the quality of images generated, Barentsz said.

"If I have to choose between an examination of two minutes and four minutes, and the four-minute exam will give me a better image, I go for that. In oncology, an important decision will often be made on the basis of a slightly better image," he said.

Rapid image acquisition may also have little impact on the overall study time. Reducing a body MRI breath-hold sequence from 20 seconds to 14 seconds, for instance, is not going to alter throughput. Radiology staff will still spend the same amount of time getting patients into the imaging suite, prepared, and on and off the table.

Scheduling periods at 3T Imaging are intentionally set to run long. Each examination is scheduled for approximately 45 minutes, with the exception of stroke packages that are allocated one hour. The generous time slot allows sequences to be honed, given that 3T is not yet "plug and play," Kim said. Unexpected referrals from the neighboring orthopedic practice can also be accommodated.

At NYU, Moy allocates a longer time period for breast MR examinations performed at 3T. This is to perform additional imaging sequences as part of research protocols. But the potential of 3T to provide shorter exams is exploited if extra cases have to be squeezed in at short notice.

"If our schedule is completely full, I'd rather have an add-on patient scanned on the 3T unit, because the study can be done in about 15 minutes instead of 30 minutes," she said.

This special section was compiled by Paula Gould, contributing editor of Diagnostic Imaging Europe.