Brain and spine imaging benefit from increased field strength

Neuroimaging with MRI at 3T is superior for nearly every application in the brain and spine, and it is certainly inferior for none. The technique has unique strengths for performing vascular work and functional brain imaging, but there is nothing that a 3T MR scanner can't do better than a 1.5T machine.

While 3T poses technical challenges, such as an increased specific absorption rate (SAR), tools are available to master them. The only major deterrent to more widespread 3T use-its fundamentally higher cost-is dissipating as clinical imaging centers make a strong financial case for the technology.

"Over the last four to five years, 3T MRI has become more and more common in the clinical practice setting," said Dr. Lawrence Tanenbaum, director of MRI at Mount Sinai School of Medicine in New York City.

Nearly all academic medical centers have one or more 3T scanners, he said, noting that the trend is toward making 3T MRI the standard of care.

"Although the Deficit Reduction Act has slowed implementation a bit, fortunately for people who want to see 3T succeed, the law came fairly late in the process, after the adaptation of 3T was inherent to what everybody was doing in clinical practice," he said.

Perhaps the biggest emerging area for 3T MRI is the shift toward volumetric imaging. Just as CT went to volumetric imaging with center slices, borne on multichannel scanners, that could be constructed in any plane in 2D and 3D, volumetric imaging is available now for MR scanning.

Volumetric imaging's dependence on the slice thickness and resolution that can be created from source data makes it more feasible at 3T than at lower fields because 3T is more powerful and generates more signal, Tanenbaum said.

With volumetric imaging comes greater efficiency in scan acquisition, reducing the number of scans from three down to one and shaving scanning time. To be able to view anatomy in three planes on T2-weighted images, for example, in the past would require three separate scans and eight to 10 minutes of scanning time. With volumetric techniques, the same data can be acquired in one five-minute scan. Volumetric imaging also offers the opportunity to obtain ever thinner slices.

"Thinner slices mean less averaging of tissue and probably better sensitivity to the smallest of lesions and the earliest stages of disease," he said.

There is every reason to believe that the thinner slices of volumetric techniques will improve sensitivity in detecting subtle neurologic diseases such as multiple sclerosis. Volumetric imaging also should facilitate preoperative planning, especially when done in conjunction with diffusion tensor tractography, which provides high-quality registration of functional diffusion tensor data and anatomical information.

MR SPECTROSCOPY

The higher field strength of 3T MRI improves the reliability of measures of trace amounts of brain chemicals such as gamma-aminobutyric acid (GABA) and glutamate, the two main inhibitory and excitatory neurotransmitters. Measurements of changes in neurotransmitter concentrations have never been reliable, however, because imaging has been done at lower field strengths and in only certain parts of the brain.

"The breakthrough is that at 3T MRI, the high field makes these measurements more reliable and more robust, meaning that pretty much anyone should be able to do this as opposed to only a half dozen big academic medical centers," said Dr. John Gore, director of the Vanderbilt University Institute of Imaging Science in Nashville.

Gore explained that GABA concentrations in the human brain approach the detection limit for MR spectroscopy, which is approximately 1 mM. GABA concentrations also can be hidden by more abundant compounds, such as choline.

Gore and his colleagues have developed a technique for measuring neurotransmitter levels anywhere in the brain. In a paper published last fall, the Vanderbilt researchers reported on the use of an automated time-domain spectral alignment approach that captured high-quality GABA spectra at 3T using a standard 30-cm transmit/receive head coil.

The method corrects for variations in the quality of spectra that are the result of frequency and phase variations from shot to shot. It was able to obtain GABA spectra from the frontal lobe, which is particularly difficult to image because of susceptibility-induced frequency and phase variations.

"The method is being used routinely in clinical research, which means several subjects a week are getting this diagnostic study. It is essentially an add-on to the standard neurological exam on 3T-or it could be," Gore said.

Using MR spectroscopy to measure GABA and glutamate may offer clues to the evaluation of neurological and psychiatric conditions or the effectiveness of treatment.

"Changes in GABA and glutamate levels have been implicated in several neuropsychiatric disorders, and they are a biomarker of treatment response to certain drugs. In the field of alcohol or drug addiction, epilepsy, schizophrenia, and depression, researchers have shown that [measuring] GABA levels can be useful and even diagnostic," he said.

MR spectroscopy at 3T also is opening the door to metabolic profiling in the brain. A study from the Norwegian University of Science and Technology tested the use of proton MR spectroscopy to characterize brain metastases from different primary cancers and evaluate changes in spectra during and after radiotherapy (BMC Cancer 2007;7:141).

Multivariate analysis indicated that clusters of spectra differed in patients with primary lung cancer and patients with primary breast cancer, and there was a significant correlation between MR spectra data and five-month survival. The investigators concluded that metabolic spectra generated by MR spectroscopy may help clinicians plan and monitor the effects of treatment of brain metastases.

TECHNICAL CHALLENGES

The same methods that worked well at 1.5T MRI don't translate directly to higher field imaging because tissue differences and the contrast between gray and white matter change with field strength. So 1.5T parameters and settings will not produce high-quality images at 3T. SAR also is intensified at higher fields.

But manufacturers have devised new pulse sequences to minimize SAR without compromising contrast.

Siemens Medical Solutions uses a special train of radiofrequency pulses that are similar to the echo train of a turbo spin-echo sequence, to reduce SAR by as much as 80%. SPACE (Sampling Perfection with Application Optimized Contrasts using different flip angle Evolutions) flip-angle TSE may reduce SAR by a factor of 6. The methods have been optimized for T2-weighted 3D imaging of the entire head, according to a report by Siemens' staff scientist Dr. John Grinstead.

Spinal imaging, which has been considered particularly difficult at 3T because of increased artifact, has benefited from the development of new coils and sequences.

GE Healthcare's newest head, neck, and spine array is a 29-element coil that fosters full spinal and head imaging without requiring that dedicated coils be repositioned. As a result, radiologists can look for multiple sclerosis lesions or metastases in the spinal cord as well as the brain in a single scan.

Philips Medical Systems' new SENSE NeuroVascular 16 coil, used with the company's large field-of-view Achieva 3T X-series, provides a full view of the back of the brain and the brain stem as well as deep penetration of the neck over the cervical spine, according to a report from the General Hospital of St. Jan in Bruges, Belgium. The SENSE Spine 15 thoracic and lumbar coil expands signal area, allowing radiologists to see myelomalacia in the medulla and the source of neuroforamina in degenerative disease in the spine.

A pair of sequences from GE focus on generating critical image contrast to depict the spinal nerves coming out of the neck. MERGE (a Multiple Echo Recombined Gradient Echo sequence) targets MS plaques, and COSMIC (Coherent Oscillatory State acquisition for the Manipulation of Image Contrast) looks for nerves that may be trapped by a prolapsed disc.

"Those are two sequences that make sure SAR is controlled and translated to a clear clinical benefit," said Dr. Adrian Knowles, MRI clinical development leader for GE.

Ms. Sandrick is a contributing editor to Diagnostic Imaging.