Three-T evolves to meet speed vs. quality balance

April 1, 2007

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.

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.

QUALITY VERSUS THROUGHPUT

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:

  • S: MR signal

  • N: noise amplitude

  • B0: magnetic induction

  • Delta w: bandwidth

  • T1, T2: relaxation times

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.

FINDING A BALANCE

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).

INVESTING FOR THE FUTURE

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.

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