Wolfgang Drahanowsky, MD

The feasibility of 3T MR as a general-purpose clinical imaging modality is an ongoing controversy. Although whole-body 3T MRI has been available for several years, most systems to date have been sold to university hospitals and research facilities. Relatively few have gone to sites performing routine work, such as general imaging centers and private practices.

At the Urania Diagnostic Center, a private imaging center in downtown Vienna, we have been using 3T MRI in clinical routine since October 2004. More than 28,500 patients have been scanned on our 3T system (Philips Achieva 3.0 quasar dual) as of February 2007. We achieved this, with an average throughput of 45 patients per day, by operating the system for 13 to 15 hours daily.

Limited coil availability largely prevented the use of 3T scanners in general imaging centers until about 2004. Most private imaging centers cover a broad range of applications in all body regions, and many systems are operated as stand-alone units. Just a few coils were available during the early 3T years, and these were mostly for neuroradiological applications. Some of these coils also suffered from problems such as peripheral image distortion and limited craniocaudal coverage.1 Most coil problems have, thankfully, been resolved. Manufacturers can now offer complete coil sets for all body regions, overcoming this initial limitation.

Concerns over safety have also been raised in debates about the general usability of 3T. Early systems often required "patient cooling" pauses between sequences to stay within legal limits of radiofrequency deposition in the body.2 Scan parameters were restricted to keep within RF limits, for instance, by reducing the number of slices for a given repetition time.

These restrictions initially made it impractical to use 3T in commercial, high-throughput settings. Modern designs are significantly more efficient than first-generation units in terms of RF deposition. Hardware design and the quality of routines intended to reduce the specific absorption rate of RF energy are of great importance. New pulse sequences with modified RF and gradient waveforms, and the increasing use of parallel imaging techniques such as SENSE, ASSET, or GRAPPA, can decrease patients' RF load significantly.3-5 SAR limitations still mean that new protocols and sequences must be adapted for the higher field strength, however.

The sensitivity toward SAR errors during routine imaging strongly depends on system-specific factors such as transmit body coil design and SAR modeling. This may vary significantly from system to system. We have found that SAR errors and the need for cooling pauses are virtually nonexistent during routine imaging after the initial phase of protocol development. This finding has also been reported by other sites.6

Concerns over possible harmful effects from the static magnetic field were greatly reduced when the U.S. Food and Drug Administration extended its nonsignificant risk status for clinical fields to 8T in July 2003 and approved the clinical use of ultrahigh-field scanners from several different manufacturers.7 It seems likely that the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the European Union will adopt higher static field limits in 2008, but the situation is unclear.

With these initial, fairly prohibitive limitations largely overcome, 3T systems began slowly to become more prevalent outside the scientific world. Several private sites now perform routine clinical MRI at 3T.

But what is the rationale for buying a 3T system for a private practice, if proven 1.5T technology is available for 50% to 70% of the capital cost? The answer depends on individual circumstances, though a number of common factors might well influence the purchasing decision.

Declining reimbursement and the high capital cost of MR systems put private operators under considerable pressure to optimize scanning efficiency and patient throughput. However, the demand for high-end imaging outside hospitals is increasing. Patients are well informed about the latest diagnostic modalities, thanks to the media and the Internet, and their choice of imaging center is influenced by this information. Public health providers in many European countries are increasingly outsourcing imaging services to the private sector.

Successful private operators can deliver the same image quality as their rivals in university hospitals. Practices must also be able to handle complex cases and relatively long examinations, such as diagnosis of epilepsy or abdominal studies. This ability must be balanced against the commercial requirements of running a business. Using 3T technology helped us to achieve a better balance in this respect.

The most important argument for using a higher field strength is the inherently better signal-to-noise ratio, following the formula:

Doubling the field strength causes the SNR to double. SNR is also proportional to the square of the scanning time. This means that in certain situations, keeping the SNR constant, it should be possible to scan up to four times faster at 3T than at 1.5T.

SNR is also a function of T1 and T2, however, which causes it to vary significantly depending on the type of examination and tissues examined.10,11 Yet even an increase in SNR of only 30% when moving from 1.5T to 3T is still a significant improvement.12 This gain can be used for faster sequences, better image quality, or some combination of the two.

The use of parallel imaging has particularly helped us to obtain an excellent balance between good image quality and speed in various applications. These include musculoskeletal imaging (Figures 1 and 2) and abdominal MRI (Figures 3 and 4). Abdominal studies were initially considered difficult or even impossible to perform on 3T systems, but most problems have been addressed.

The dielectric effects sometimes seen at higher field strengths can, at times, cause partial darkening on upper abdominal MR studies. The severity of these artifacts is extremely variable, and they are mostly a problem in thin patients. Diagnostic-quality images can usually be obtained by changing the acquisition plane. We never place dielectric pads on the patients. Problems with SAR are very rare for abdominal imaging at 3T, in our experience.

The combination of dedicated multichannel phased-array coils with parallel imaging allows us to acquire excellent musculoskeletal images within exceptionally short imaging times (Figure 5). We can perform complex multiseries examinations, such as temporal lobe evaluation (Figure 6), and obtain equal or better image quality compared with 1.5T, while still keeping within narrow time constraints (Figure 6).

Improved image quality continues to be the primary motivation for changing an MR imaging system in both clinics and radiology departments. The power of 3T allows practitioners to create recognizably better images in the same time as a 1.5T system, or even faster, leading to greater diagnostic confidence in many body regions.

Emerging applications, such as functional MRI, MR spectroscopy, diffusion tensor imaging, and high-resolution MR angiography, clearly benefit from the increased signal available at 3T. Researchers from Fox Chase Cancer Center in Philadelphia reported in 2001 an average voxel SNR increase in proton MRS of 23% to 46%, when comparing 3T with 1.5T. They observed better delineation of peaks at the higher field strength, with a 50% shorter acquisition time.11 Stanford University investigators, in the same year, reported a significant increase in detected activation with blood oxygen level-dependent imaging when using 3T instead of 1.5T. 13

Both fMRI and DTI are arousing significant interest in neurosurgery as tools for noninvasive surgical planning. The inherent advantages of high-field MRI in these applications may lead to robust methods that can be performed routinely. This advance could, in turn, open up possible new business opportunities for private imaging centers.

A new MRI system is a long-term investment. Competition among private imaging centers is growing. Three-T MRI is, without doubt, the current cutting-edge technology in clinical MRI. It may soon become the field strength of choice for clinical MRI, well within the typical life cycle of a clinical scanner. Prospective buyers should include 3T technology in their purchase considerations now.

Three-T appears to be the next logical step in the evolution of MRI. Scientific data demonstrate significant advantages in many applications. Most initial problems have been tackled, and 3T is no longer just a research tool. Our personal experience of more than 28,500 routine 3T MR examinations in all body regions shows clearly that modern 3T systems are sufficiently versatile and stable to cover all requirements of a general imaging center.

For a private imaging center that is seeing significant demand for complex examinations but that still covers a sizable number of routine examinations and seeks to offer its customers cutting-edge MRI technology, a 3T system can be a viable option.

DR. DRAHANOWSKY is a senior radiologist and technical director, DR. ROBINSON is a senior radiologist, DR. PRAYER is medical director, and DR. BARTON is managing director, all at the Urania Diagnostic Center in Vienna.

Ross JS. The high-field-strength curmudgeon. AJNR 2004;25(9):1455-1456.

International Electrotechnical Commission. Medical electrical equipment-part 2: particular requirements for the safety of magnetic resonance equipment for medical diagnosis. Geneva, Switzerland: IEC; 2002;601-602,633.

Heidemann RM, Griswold MA, Muller M, et al. [Feasibilities and limitations of high field parallel MRI]. Radiologe 2004;44(1):49-55. German.

Pruessmann KP, Weiger M, Scheidegger MB, Boesiger P. SENSE: sensitivity encoding for fast MRI. Magn Reson Med 1999;42(5):952-962.

Griswold MA, Jakob PM, Heidemann RM, et al. Generalized autocalibrating partially parallel acquisitions (GRAPPA). Magn Reson Med 2002;47(6):1202-1210.

Shapiro MD, Magee T, Williams D, et al. The time for 3T clinical imaging is now. AJNR 2004;25(9):1628-1629; author reply 1629.

U.S. Food and Drug Administration. Criteria for significant risk investigations of magnetic resonance diagnostic devices. FDA Center for Devices and Radiological Health. 2003. July 14. www.fda.gov/ cdrh/ode/guidance/793.pdf.

Bottomley PA, Foster TH, Argersinger RE, Pfeifer LM. A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from 1-100 MHz: dependence on tissue type, NMR frequency, temperature, species, excision, and age. Med Physics 1984;11(4):425-448.

Schmitt F, Grosu D, Mohr C, et al. [3 Tesla MRI: successful results with higher field strengths]. Radiologe 2004;44(1):31-47. German.

Schindera ST, Merkle EM, Dale BM, et al. Abdominal magnetic resonance imaging at 3.0 T what is the ultimate gain in signal-to-noise ratio? Acad Radiol 2006;13(10):1236-1243.

Gonen O, Gruber S, Li BS, et al. Multivoxel 3D proton spectroscopy in the brain at 1.5 versus 3.0 T: signal-to-noise ratio and resolution comparison. AJNR 2001;22(9):1727-1731.

Frayne R, Goodyear BG, Dickhoff P, et al. Magnetic resonance imaging at 3.0 Tesla: challenges and advantages in clinical neurological imaging. Invest Radiol 2003;38(7):385-402.

Kruger G, Kastrup A, Glover GH. Neuroimaging at 1.5T and 3.0T: Comparison of oxygenation-sensitive magnetic resonance imaging. Magn Reson Med 2001;45(4):595-604.

Articles

The feasibility of 3T MR as a general-purpose clinical imaging modality is an ongoing controversy. Although whole-body 3T MRI has been available for several years, most systems to date have been sold to university hospitals and research facilities.

Three-T evolves to meet speed vs. quality balance

Most 3T MR systems are sold to university hospitals and research facilities, and relatively few go to private practices. This is perhaps understandable. Choosing 3T over 1.5T means opting for a novel technology instead of a well-established solution. So why take the risk?

Private practices have to take many factors into account when considering whether to invest in 3T. Improved image quality continues to be the primary motivation for changing systems. The big promise of 3T is doubled signal compared with 1.5T. In reality, the signal-to-noise ratio varies depending on the type of examination performed and the tissues examined. An increase of 50% to 90% is common, and in certain applications, SNR may increase by only 30%. But this is still a significant advantage over 1.5T.

Most scientific 3T work to date has concentrated on using the increased signal to achieve higher resolution images. This ability has proven clinical advantages in several areas, such as the detection of small cartilage lesions in musculoskeletal examinations or visualization of epileptogenic foci on brain scans. It is easy to create striking, high-resolution images in many body regions with 3T. But is this enough to justify the acquisition of a much more expensive system? Hardly.

Increased signal, however, brings another important benefit: the possibility of reducing scanning time. Declining reimbursement poses great pressure upon private operators to optimize efficiency and throughput. Meanwhile, the demand for high-end imaging outside the hospital increases. In Europe, even complex examinations are increasingly being outsourced to private sites.

Balancing good image quality and high patient throughput is consequently of utmost importance to private sites. In our experience, 3T is able to achieve a better balance in this respect.

We started using 3T MRI at the Urania Diagnostic Center, a private imaging practice in Vienna, in October 2004. Our full set of dedicated coils allows us to examine every body region that we used to examine on our 1.5T system. Patient comfort in our short-bore scanner is identical to that for our 1.5T unit, and patient throughput is high. By June 2006, we had performed more than 20,000 routine clinical examinations at 3T, averaging 45 patients per day.

Parallel imaging is our essential tool for achieving an optimum balance between speed and quality. We can perform a high-quality knee MRI in six minutes, with excellent cartilage depiction. A complete temporal lobe epilepsy protocol with six sequences, including 1024 matrix high-resolution T2-weighted and inversion recovery sequences, takes just over 15 minutes. And our fat-saturated 3D sequence for shoulder arthrography-120 reformatable slices at 0.8 mm-is complete in just over four minutes.

The increased signal available at 3T offsets the major drawback of parallel imaging: decreased SNR. Parallel imaging also helps us to manage susceptibility artifacts and increased radiofrequency deposition, two inherent problems of high-field MRI.

The beauty of 3T MRI for us is that we can cover routine examinations quickly and produce high-quality images. But if the case requires that extra bit, then we have additional imaging power. We can bump up the resolution, do excellent MR angiography, and use more sophisticated techniques, such as diffusion tensor imaging and fiber tracking, real-time functional MRI, and spectroscopy. Many of these newer MR applications may open up further business opportunities, such as use of MR as a tool for presurgical planning. Being one of just a few sites to have the latest MR technology also leads to a competitive advantage.

Of course, 3T still has its weak points. I see little advantage in doing spinal imaging at 3T, for example. Sensitivity toward artifacts is also greater, especially in the upper abdomen.

I believe that most private sector MR imaging will be performed at 1.5T for a few more years yet. But if a private practice expects state-of-the-art quality, requires high patient throughput, has significant demand for complex examinations, and wants to keep within time constraints, it has to at least consider buying a 3T scanner.

Is it worth it, economically? Time will tell. It could be said that deciding between 1.5T and 3T is similar to choosing between a Volkswagen and a Mercedes and is thus a question of individual circumstances and personal taste. Many radiologists tend to choose the latter, don't they?

DR. DRAHANOWSKY is a senior radiologist and technical director at the Urania Diagnostic Center in Vienna.

Most 3T MR systems are sold to university hospitals and research facilities, and relatively few go to private practices.

Wolfgang Drahanowsky, MD

Articles

Three-T evolves to meet speed vs. quality balance

3T in private practice: Why take the risk?