Introduction
The present article explores the rapidly developing subject of ultra-high field magnetic resonance spectroscopy (MRS) and imaging (MRI) at seven Tesla and higher magnetic fields. While acknowledging the breadth of this field of study, the review focuses mostly on biological applications, which have been a major force behind the development of ultra-high field magnetic resonance technology. The Center for Magnetic Resonance Scientific (CMRR) has made noteworthy contributions to the study, particularly on neuroimaging and functional brain imaging, a broad area of scientific interest.
How Has 7 Tesla MRI Revolutionized the Study of Human Brain Activity and Biomedical Research?
The development of magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) at ultrahigh magnetic fields (seven Tesla and above) has proven crucial in the nondestructive extraction of complex biological data from living systems. The advancement of functional magnetic resonance imaging (fMRI), which provides previously unattainable resolution, has significantly transformed the in-depth analysis of human brain activity.
Efforts to investigate the application of four Tesla (T) MRIs for human imaging encountered difficulties in the 1980s; preliminary findings from large manufacturers produced less-than-ideal brain images at four T. However, the University of Minnesota's Center for Magnetic Resonance Research (CMRR) made history by presenting better brain images using a sequence called MDEFT (modified driven equilibrium Fourier transform) at 4 T.
Success with the CMRR sparked additional interest in functional imaging at magnetic fields greater than 4 T. An important turning point was reached in 1999 with the development of a 7 T system. The "Lego" 7 T human MR system from CMRR was the first of its kind, but it provided significant research that paved the way for 7 T systems to be widely used worldwide.
Unmatched spatial resolution and specificity were attained by human fMRI at 7 T, and spectroscopy offered insightful information on neurochemistry and metabolism. Shifting the study focus from brain-focused studies to imaging the human torso at 7 T brought problems and opportunities for creative inquiry.
Currently, 7 T is a premier platform for human biomedical research, producing ever more illuminating outcomes as more labs obtain advanced equipment. The sector expects significant breakthroughs with the implementation of 9.4 T scanners and the anticipated 10.5 and 11.7 T systems. The present review offers a thorough investigation of ultrahigh field MRI, emphasizing the significance of 7 T research in promoting human biological investigations.
How Does High Magnetic Field Complexity Affect the MRI Signal-To-Noise Ratio?
The signal-to-noise ratio (SNR), essential for high-quality images in magnetic resonance imaging (MRI), becomes more complicated at high magnetic fields. The radio frequency (RF) wavelength utilized for imaging in conductive human tissues gets shorter than the object's dimensions at higher fields, like 7 Tesla, which results in nonuniform RF behavior. Due to this, there is an ‘attenuated traveling wave,’ which produces an intensely uneven picture signal. The problem is seen in brain imaging at 7 T, where the core has a much higher SNR than the periphery.
The source of this nonuniformity is the human head's constructive and destructive RF traveling wave interference. RF inhomogeneity is addressed with techniques like B1-shimming and strategies like multichannel transmit coils and parallel transmit capabilities. Even with SNR gains at 7 T, a hurdle remains, especially for high gyromagnetic nuclei like protons, such as recurrent RF pulsing and higher bandwidth. Promising SNR improvements at higher fields have been observed for low gyromagnetic nuclei, such as 17O and 23Na, which may increase spatial and temporal resolution in biological studies.
How Does Ultra-High Field Functional Magnetic Resonance Imaging Improve Imaging?
At high magnetic fields, such as 7 Tesla, functional magnetic resonance imaging (fMRI) allows for a thorough investigation of the structure and functioning of the human brain. For accurate functional mapping, these ultrahigh fields improve signal-to-noise ratio (SNR) and spatial precision. Large blood arteries might cause problems with traditional fMRI techniques like T2-weighted gradient recalled echo (GRE) fMRI. However, by lowering these errors, ultrahigh fields are advantageous, particularly for spin echo (SE)-based fMRI, which gains from higher spatial specificity.
The revolutionary technology known as parallel imaging removes obstacles to quick whole-brain coverage, which is essential for functional magnetic resonance imaging (fMRI) research. By using parallel imaging, multiband (MB) imaging improves both acquisition speed and spatial resolution. High-resolution imaging is made possible by slice-accelerated MB methods, which get around the drawbacks of lengthy repetition durations (TRs) for full brain coverage at 7 Tesla. Furthermore, the method of resting-state functional magnetic resonance imaging (rfMRI) unveils resting-state networks (RSNs) by examining spontaneous brain activity. Ultrahigh fields produce greater spontaneous activity signals at 7 Tesla, which permits higher spatial resolution without compromising statistical significance.
Overall, developments in high-field fMRI methods offer a comprehensive understanding of brain organization, connection, and function that may be useful in treating neurological conditions and cognitive processes.
How Does Ultra High Field MRI Enhance Anatomical Imaging and Contrast Mechanisms for Better Pathology Visualization?
An invaluable tool in biomedical research, structural MRI is increasingly used to examine anatomical characteristics in various disorders. It has significantly changed our knowledge of diseases like multiple sclerosis, epilepsy, cerebrovascular illness, and neurological and psychiatric problems. The development of ultrahigh field MRI, which operates at high magnetic fields, is anticipated to improve our capacity to see abnormalities in human illnesses. Improved signal-to-noise ratios, better contrast mechanisms, and the development of novel imaging techniques that provide previously unheard-of anatomical detail are credited with this advancement.
In addition to providing better T1-weighted pictures, ultrahigh fields also present new contrast mechanisms such as susceptibility, phase, and T2-weighted imaging. These developments make it possible to obtain comprehensive anatomical data and may improve the identification of minute anomalies like microbleeds. Furthermore, using ultrahigh fields in vascular imaging without needing contrast agents is beneficial in areas such as clinical medicine, tumor biology, and functional MRI. It makes it possible to see small arterial blood vessels more clearly and helps diagnose disorders associated with blood vessel occlusion.
How Does Multichannel Parallel Transmit Technology Improve Imaging in the Human Torso at Ultrahigh Fields?
Early 4 Tesla (4 T) attempts were hindered by problems such as ‘dielectric resonance,’ which resulted in nonuniform signals. This made imaging the torso difficult, except for employing specialist equipment to image the prostate.
Using multichannel transmitters with independent control is the solution, particularly when local transmit coil arrays are used. They optimized and homogenized the B+1 fields by carefully adjusting the transmit RF, which made it possible to image the targeted organs in the torso more clearly, even at 7 Tesla (7 T). The chapter demonstrates how this method solves the earlier issues and produces high-quality photos, especially of the heart, when paired with techniques like B+1 shimming.
Other research groups have embraced multichannel parallel transmit approaches, improving coil shapes and pulse designs. More coils, sophisticated pulse designs for better uniformity, and quicker picture collection are among the ongoing advancements. The section also discusses the anticipated major advancements in angiography and perfusion imaging methods for torso organs, including the kidneys and heart, using ultrahigh magnetic fields in the future.
How Does Parallel Transmit (pTx) Technology Address Specific Absorption Rate (SAR) Challenges in Ultrahigh Field MRI?
One major obstacle for ultrahigh field MRI is the Specific Absorption Rate (SAR), which is the amount of power injected into the subject during imaging. Higher magnetic fields lead to a large rise in SAR, especially when using multichannel parallel transmit (pTx) technology to address ultrahigh field B+1 uniformity problems. Regulatory agencies, like the FDA, set ‘global’ and ‘local’ SAR limits, which act as operating conditions safety standards. While technology and software on MRI scanners can be used to experimentally estimate and monitor global SAR, local SAR, especially in pTx applications, is of more significance. Currently, it is difficult to quantify local SAR practically, which forces the use of generic models and compromises individual performance because of imaging session limitations.
Notwithstanding these difficulties, parallel transmission technology offers ways around SAR restrictions for energy-intensive imaging techniques. Through pulse design optimization, pTx aims to balance controlling power-related penalty terms, including local or global SAR, and attaining targeted B+1 spatial patterns. This strategy, exemplified by techniques like the L-curve analysis, is especially useful at ultrahigh fields, enabling improved imaging performance even at higher field strengths.
Conclusion
Over the past three decades, developments in ultrahigh magnetic fields have produced more advanced equipment and enhanced imaging techniques, raising the bar for MR imaging regarding biological information. Further development is anticipated with devices like the 11.7 T systems at NIH and NeuroSpin and the 10.5 T system at the University of Minnesota. There are recorded occurrences of 7 T's superiority in pathological visualization, and discussions concerning its application in clinical practice are ongoing. Research is being done on new contrast mechanisms and ultrahigh field measuring capabilities. Crucially, even at lower fields, such as 3 T used in clinical investigations, image quality has improved because of solutions created for ultrahigh field difficulties.
