Clinical MRI studies are typically performed with 1.5-tesla (T) magnets, though the use of 3-T systems is becoming more common. The use of ultrahigh-field-strength MRI—using 7-T magnets—remains largely the domain of major research facilities such as New York University.
Dr. Yulin Ge, a radiologist at New York University, New York, and his colleagues have been using 7-T MRI to further their understanding of the pathophysiology of multiple sclerosis.
There are two main advantages to high-field MRI over imaging with conventional field-strength magnets. The higher signal intensity-to-noise ratio (SNR) that can be achieved with a stronger magnet strength allows for greater resolution, which in turn can improve the detection of lesions. Lesion counts may be 45% greater using a 4T system with better appreciation of tissue heterogeneity within lesions as compared with a 1.5-T system.
Separately, higher field strength also boosts susceptibility effects. Magnetic susceptibility is the degree of magnetization of a material in response to an applied magnetic field. Oxyhemoglobin in arterial blood has no substantial magnetic properties, but deoxyhemoglobin, which is present in the draining veins after the oxygen has been unloaded in the tissues, is strongly paramagnetic. It can thus serve as an intrinsic paramagnetic contrast agent, the effect of which is increased with greater field strength. This in turn improves the examination of microscopic venous structures, brain iron, and microbleeds. “So venous structures can be shown very well on high-field MRI,” Dr. Ge said in an interview.
The researchers have been able to directly see small vascular abnormalities—mainly small venous structures—at a very early stage of MS. “These findings have never been shown on conventional MR,” he said. High-field MRI detects these changes before the blood-brain barrier breakdown that can be seen on conventional MRI.
The MS lesions appear to be centered on a small venule. “We can see the abnormal signals on and around the venous wall,” Dr. Ge said. “And the lesion developed along the veins—along the venous course in the initial stage of development can be clearly seen on 7T MRI.” These findings may be direct in vivo evidence of the vascular pathogenesis of lesions, he added. While it takes only 6 minutes to perform such susceptibility-sensitive imaging, slices are 2 mm thick with pixel resolution of 0.2x0.2 mm
These vascular abnormalities are “not seen in one or two lesions but in many lesions,” he said (as shown in image A). The number of lesions with vascular abnormalities that can be detected “gives you a very important indication of disease activity beyond the resolution of conventional MRI.”
Gadolinium-enhancing lesions currently are considered the first detectable MR abnormalities in MS patients. However, the fact that vascular abnormalities detected using ultrahigh-field MRI can be seen even earlier “has important implications for treatment,” said Dr. Ge.
High-field-strength MRI might allow physicians to track the early disease activity and the effectiveness of treatments. This is particularly important now that disease-modifying therapies are available.
The immunomodulatory drugs—interferon β-1a IM (Avonex), interferon β-1b SC (Betaseron) glatiramer acetate SC (Copaxone), or interferon β-1a SC (Rebif) are approved for and are currently used in the United States as first-line therapies for MS to prevent relapses or disease progression.
Newer compounds approved by the Food and Drug Administration for use in MS include mitoxantrone (Novantrone) and natalizumab (Tysabri). Last year, natalizumab was reintroduced for the treatment of relapsing forms of MS with a black box warning about the risk of progressive multifocal leukoencephalopathy associated with treatment. The drug had previously been voluntarily suspended from the market following the development of PML in three patients treated with the drug (two for MS and one for Crohn's disease).
Ultra-high-field MRI shows an intimate relationship between lesions and centered small veins (arrows), suggesting a primarily vascular pathogenesis of MS lesions. Photos courtesy Dr. Yulin Ge