Fueling the shift in interest from 1.
Ultra-High-Field MRI: A Snapshot of Its Growing Utility in Neuro Care
The driving forces behind this increased penetration of 3T scanners into the clinical setting include reduced concerns over surface coil availability, radiofrequency RF deposition limit, higher ambient noise, system homogeneity, increased magnetic susceptibility and chemical shift effects, and reduced tissue contrast. Also, this shift results from the documentation of incremental benefits of 3T over 1. Specific absorption rate SAR is a measure of energy deposited by an RF field in a given mass of tissue.
The doubling of field from 1. Therefore, SAR considerations effectively limit scanner performance.
Longer echo time acquisitions commonly used with high-per-formance gradient systems and fat-suppression techniques commonly used with musculoskeletal imaging exacerbate the duty cycle load. Reducing duty cycle by using a longer repetition time TR than the minimum necessary building in cooling time, in a sense is an effective technique but comes at the expense of somewhat longer scan times Figure 2.
Parallel imaging is another powerful method of reducing RF exposure as well as scan times by reducing the number of phase-encoding steps that are performed. Innovative methods of reducing SAR without image compromise are, or will soon be, available. New magnet designs now seen in the clinical setting are inherently more SAR-efficient than were earlier generation systems. Clever pulse sequence manipulations such as applying magnetization transfer prepulses only at the center one third of k-space can maintain improved tissue contrast while depositing considerably less RF energy.
Sound pressure levels SPLs increase with field strength.
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The noise levels at 3T approach twice that of 1. Food and Drug Administration limit permissible sound levels to 99 dBA. Higher gradient performance comes at the cost of higher SPL as well.
Clinical applications of ultra high field MRI
Magnet length also influences the gradient noise generated, thus the shorter bore systems sold today are inherently louder. Methods of reducing SPL include passive approaches, such as the routine use of earplugs, as well as active noise cancellation via headphones.
Reduced gradient performance for certain applications is another approach, but by nature, this limits clinical efficacy. Some currently available 3T systems are equipped with advances, such as acoustically shielded vacuum-based bore liners that keep noise levels below certain limits while maintaining full gradient performance.
T1 relaxation times are prolonged at 3T with respect to 1.
3T MRI in clinical practice
These considerations do not plague other methods for obtaining T1 contrast, such as RF-spoiled gra-dient-recalled, magnetization prepared techniques such as inversion recovery IR , or magnetization transfer MT 3-dimensional spoiled gra-dient-recalled SPGR Figures 3 and 4. Inversion recovery techniques that produce superior T1 contrast at 1.
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With parallel imaging techniques, T1 studies are faster and higher in resolution than those obtained at 1. A routine shift to high bandwidth, to a moderate echo-train ET -FSE T1 FLAIR from spin-echo, has the additional benefit of reducing susceptibility artifact, which is a benefit in patients who have had surgery or who have metal implants, and chemical shift effect sensitivity as well. While the relaxivity of gadolinium is not significantly different at 1. Therefore, many sites utilize a lower dose of contrast 0.
T2 values in biological tissues are unchanged or only slightly decreased with increases in field strength. Conversely, susceptibility artifacts are proportionally more problematic at 3T Figures 8 and 9. The greater signal intensity afforded at 3T is particularly enticing for diffusion-weighted imaging DWI needs. Signal-to-noise ratio can be marginal for routine clinical imaging purposes at 1. Diffusion-weighted imaging studies at high field are typically acquired using echoplanar imaging EPI techniques. These single-shot studies are inherently prone to susceptibility artifact, which can limit evaluation of structures in close proximity to the bony skull base and air-filled paranasal sinuses.
Since susceptibility effects scale with field strength, these artifacts are proportionally worse at higher field. Parallel imaging techniques are routinely applied on modern 3T systems equipped with optimized surface coils and broadband reconstruction hardware, effectively balancing these considerations by decreasing the echo-spacing ES and TE of the scan. This reduces susceptibility artifact and ameliorates signal loss due to T2 decay on these long ET acquisitions Figure Scanning techniques employ lower flip angles that reduce SAR deposition as well as pulsation artifacts.
Optimized coils coupled with parallel imaging techniques maintain scan times similar to or shorter than those at 1. Chemical shift doubles when moving from 1. It also makes it easier for neurosurgeons to selectively remove a tumor without damaging surrounding areas," said Zada, an associate professor of neurological surgery clinical scholar at the Keck School. Reconfiguring the 7T Terra for clinical use involved minor hardware and software updates to comply with FDA standards.
For more information about the scanner, visit cia. The school has more than 1, full-time faculty members and voluntary faculty of more than 2, physicians. These faculty direct the education of approximately medical students and 1, students pursuing graduate and postgraduate degrees. The school trains more than resident physicians in more than 50 specialty or subspecialty programs and is the largest educator of physicians practicing in Southern California.
In , U. The institute, which comprises four interdisciplinary centers, the Laboratory of Neuro Imaging, Imaging Genetics Center, Center for Image Acquisition and Center for Integrative Connectomics, is populated by 19 faculty, more than staff and student researchers and thousands of collaborators around the world. In addition to mapping brain structure and activity in health and disease, the institute has emerged as a leader in informatics solutions, providing key data management services including archival, harmonization, quality control, visualization and dissemination for hundreds of studies, thousands of scientists and millions of subjects worldwide.