A MEDLINE search for "high-field MRI"  retrieves mainly studies performed at 1.5 tesla. That may soon change. In 1998, the US Food and Drug Administration gave marketing clearance for scanners operating at up to 4T, and in 2002, the agency approved some 3T scanners for the brain or the whole body. More than 100 of these machines are now installed, and they are likely to become the standard.

But when? Do you need a 3T scanner today? There are clearly studies that can be performed at 3T that are not possible, or at least not as easy, at lower field strengths, and many studies can be accomplished faster. The clinical role of these scanners vis-à-vis those of lower field strengths, however, is not yet defined. Although active shielding of the scanners minimizes the architectural problems, permitting installation in facilities now equipped with 1.5T scanners, acquisition of a higher-field machine mandates considerable adjustment of an MRI practice, including revision of many imaging protocols. Radiologists who use such scanners also stress the need for close cooperation with a physicist to get good results.

Is it worth it?

The Promise

The great appeal of 3T MRI is improvement in image quality and resolution. Because the signal-to-noise ratio (SNR) correlates in approximately linear fashion with field strength, it is roughly twice as great at 3T as at 1.5T. The time necessary to acquire satisfactory images can be substantially reduced, minimizing motion artifacts and possibly speeding patient turnover.  Alternatively, the same acquisition time can be used to obtain images at higher resolution. Functional MRI (fMRI) and MR spectroscopy (MRS) benefit significantly. Also, greater contrast is available at higher field strength, a fact already well known from comparisons of images obtained at 0.5T, 1T, and 1.5T.^1,2 Among other benefits, higher contrast may permit reduction of gadolinium doses and, in some cases, earlier detection of disease, a possible stimulus for more patient referrals.

But Wait a Minute&

Of course, the greater SNR and contrast come at a price. First, tissues differ in their magnetic susceptibility, and this effect is exacerbated at higher field strengths. Indeed, it was long thought that these differences would preclude clinical use of ultra-high-field systems. In the brain, the effects are particularly noticeable on T2-weighted images of iron-containing tissues and at tissue borders and airbone interfaces such as around the paranasal sinuses.³ On T1-weighted images, susceptibility effects can create bright areas that lead to confusion on postcontrast images if a precontrast view is not available for comparison. However, this problem is not insurmountable: the undesirable effects can be reduced by measures such as increasing the spatial resolution or acquiring thinner sections.

Second, there are safety concerns. The energy deposited in the patient's tissues is fourfold higher at 3T than at 1.5T.  Particularly with radiofrequency-intensive pulse sequences such as fast spin echo and fluid attenuation inversion recovery (FLAIR), the limit on the specific absorption rate (SAR) power deposition prescribed by the FDA can easily be reached using protocols that are satisfactory at 1.5T.³ Scanner manufacturers are incorporating modified pulse sequences to avoid this problem, which can also be solved by restricting the volume of tissue that is studied in detail. Another safety issue is the virtual absence of data on scanning of patients having various types of implants at 3T.

Third, 3T images are more subject to flow artifacts. Although these can be reduced by such measures as cardiac gating, there may remain studies such as routine brain imaging where 1.5T is superior.

Two other effects of higher field strengths have both drawbacks and benefits. One is the doubling of the chemical shift when the field strength is doubled. This effect can cause misregistration of fat and water, especially after gadolinium administration, leading to erroneous diagnoses, but it improves the analysis of MR spectra by increasing the resolution of different metabolites. The second is the longer and more variable T1 relaxation time. For example, at 3T, relaxation time is as much as 40% greater for brain parenchyma, whereas there is little change for cerebrospinal fluid.⁴ A potentially important consequence is a reduction in the contrast between the white and gray matter in the brain. On the other hand, the effect can enable better background suppression and reduction of the in-plane saturation effect for time-of-flight (TOF) magnetic resonance angiography (MRA).

Finally, there are some practical disadvantages of 3T scanners. At present, these machines cost at least $1 million more than a 1.5T scanner, although this is likely to change as more 3T systems are installed.  Present magnets also have smaller bores than those on 1.5T systems, making them too small to image some patients and increasing the likelihood that an examination will be limited by patient claustrophobia.

What Is Being Done at 3T?

Success with cochlear implants requires high-resolution imaging to determine the correct length and dimensions.⁵ At the Mayo Clinic in Rochester, Minn, the modality of choice is a three-dimensional (3D) fast spin echo MRI scan at 3T with a hybrid array coil that provides as much as a 250% improvement in SNR compared with a standard head coil.⁶ In this way, it is possible to distinguish the acoustic from the vestibular branches of the VIIIth cranial nerve, and the anatomy of the labyrinth can be examined with maximum intensity projection reformatting. The Clinic is correlating audiometric data with the extent of cochlear nerve atrophy depicted by MRI in the hope of being able to determine which ear would benefit most from an implant.

Higher-field scanners promise to deliver further improvements in MRA by creating greater vessel conspicuity and reducing artifacts by shortening the acquisition time.⁷ A team at the Mayo Clinic compared 3D TOF 1.5T and 3T for MRA of the intracranial and cervical vessels in 20 patients, with two radiologists interpreting the studies independently.⁸ Voxels as small as 0.62 mm³ were readily obtained at 3T, and the images were rated significantly better. In the neck, it was possible to identify carotid dissections and ulcerations of atherosclerotic plaque.

Of course, the greatest demand for MRA is in the coronary system, which has resisted noninvasive angiography because of the small size and nearly constant motion of the vessels. There is some evidence that 3T will make such studies possible. In a feasibility study, a team from Beth Israel Deaconess Medical Center and Harvard Medical School scanned nine healthy adult volunteers.⁹ Two scout scans were acquired, and a navigator echo was positioned at the liverheart boundary to allow subsequent images to be acquired during free breathing. A 3D segmented k-space gradient echo sequence was then used to acquire 10 slices with an effective thickness of 3 mm and an in-plane resolution of 0.7-1.0 mm over an average time per scan of about 7 minutes and a total session of no more than 1 hour. A series of 20 1.5-mm slices were then interpolated, and the 3D images were analyzed.  The images were of high quality in all subjects, and it was possible to see small branching vessels as well as proximal and mid coronary arteries.

The musculoskeletal system is one of the most common applications of MRI, and there is evidence that 3T scanners will find a place there. At the 2003 Scientific Meeting of the Radiological Society of North America, a research team from Zurich, Switzerland, reported on a direct comparison of 1.5T and 3T imaging for the wrist.¹⁰ The higher field strength provided superior bonemuscle and bonecartilage contrast and better visibility of the intercarpal ligaments and cartilage and the median and ulnar nerves. The principal drawback was the much longer scan time required: an average of 56.2 vs 27.7 minutes.

Numerous applications are being developed for the brain, and some clinicians believe 3T should always be used for this organ if available. A German group compared 1.5T and 3T in 11 consecutive patients with multiple sclerosis, with the latter field strength being exploited to obtain more and thinner slices in the same scan time required at 1.5T.¹¹ Images acquired at 3T were rated significantly superior in lesion conspicuity and diagnostic value, although artifacts were more common. In these investigators' view, 3T MRI "may further strengthen the role of MRI as the most sensitive paraclinical test available for the early diagnosis of multiple sclerosis." Researchers at the University of Cambridge in the UK used diffusion tensor imaging (DTI) at 3T in 20 patients with known primary brain tumors or metastases and calculated relative anisotropy indexes, a measure of tissue disorganization.¹² In 10 of the 13 patients with high-grade gliomas, the lesions measured by DTI were larger than those seen on T2-weighted images, and previously unrecognized contralateral abnormalities were found by DTI in the white matter in four of the 13 patients. The team suggested that such abnormalities indicated infiltration by tumor and that DTI "may provide a useful method of detecting occult white matter invasion by gliomas."

The superiority of 3T imaging for MRS has resulted in its use in numerous clinical and investigational studies of the brain. At Albert Einstein College of Medicine in New York, proton MRS at 3T is used to define the site and extent of temporal lobe epilepsy foci for the surgeons. Investigators at the National Cardiovascular Center in Osaka, Japan, have applied proton MRS to studies of the metabolic abnormalities of Alzheimer's disease.¹³ A particularly interesting application was described at the RSNA by Richard Frey, MD, and coworkers from Vienna, Austria.¹⁴ In a double-blind study, those investigators used single-voxel proton MRS at 3T in 22 depressed patients and 22 age- and sex-matched control subjects, with the patients being imaged before and after treatment with citalopram, a specific serotonin-reuptake inhibitor.  Concentrations of several compounds, including creatine, choline, and myoinositol, were measured. Before treatment, depressed patients had significantly higher concentrations of creatine in the frontal lobes than did the control subjects; with drug treatment, their creatine concentrations became indistinguishable from those of the controls. The investigators concluded that MRS at 3T can be used to study the metabolic changes of depression and the effects of treatment.

Conclusion

Although it may be true, as some have asserted, that the question about installing 3T MRI is not if but when, each department will have to consider whether it needs the higher-field scanners enough to make the required adjustments. The new scanners are expected to have their greatest utility in applications such as MRS, TOF MRA, fMRI, and DTI. Until the role of 3T for other purposes is better defined, departments that do not perform these high-end studies may prefer to wait.

Judith Gunn Bronson, MS, is a contributing writer for Decisions in Imaging Economics.

References:

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