Why Buy an Expensive $7 Million) 7T MRI System for Biomedical Research?
Magnetic resonance imaging (MRI) is a versatile technique to image changes in the morphological, structural, metabolic, and functional status of the human body in vivo. Since its inception in the beginning of the 1980s, magnetic field strength has emerged from <0.5T to a wide use of 3.0T today. Interestingly, most MRI systems are used purely for clinical purposes and operate at low field 1.5T) while only a fraction is at field strengths of 3T or above. One of the main reasons for this is the increased cost of higher field systems, which needs to be justified. There are currently 40 ultrahigh field (UHF) MRI systems 7T) worldwide that are being used primarily for research applications, and this number is steadily increasing. There are research systems at 9.4T being used for primarily for neuroscience imaging (Minnesota, Tubingen, and Chicago) and even higher UHF systems up to 14T are in the planning stages (USA, France, South Korea) For a given image contrast, the three intrinsic factors that determine the efficiency of MR imaging are signal to noise ratio (SNR) imaging speed, and spatial/spectral resolution. These factors are interrelated and this provides flexibility to use the increased intrinsic SNR at UHF. The UHF systems 7T) will provide some excellent new opportunities such as X nuclei spectroscopy and imaging, improved parallel transmit and imaging capability, enhanced lesion detection sensitivity, improved chemical exchange saturation transfer (CEST) improved ultrashort echo time (UTE) imaging, electrical property mapping, improved spectral resolution, improved blood oxygen level dependent (BOLD) functional MRI, quantitative susceptibility mapping (QSM) time of flight angiography, arterial spin labeling (ASL) etc. and flexibility for improved understanding of pathophysiology and disease mechanisms/process rather than anatomy. At the same time, throughout the history of MRI an increase in field strength has always been associated with some technical challenges in terms of manufacturing, siting, and optimization. Sometimes the potential improvements in image quality may be counterbalanced by these increased artifacts, bright hot spots, increased physiological noise, increased tissue specific absorption rate (SAR) radiofrequency (RF) inhomogeneity, geometric distortion, and intravoxel dephasing. However, some of these challenges have been mitigated with appropriate hardware (eg, novel RF coil development, parallel transmit and receiving capability, higher order dynamic shimming, etc. software (improved adiabatic RF pulse sequences, omission of refocusing pulses, etc. and image postprocessing. Despite these challenges, recent pilot studies at 7T provided excellent improvements in brain imaging (1 10) spectroscopy (9,11 13) and musculoskeletal imaging (14 17) body imaging (18,19) X nuclei imaging (20 26) cardiovascular imaging (27,28) breast imaging (29,30) prostate imaging (31) and zero echo time imaging (32) These initial pilot studies demonstrated the clinical potential of UHF imaging at 7T in general and the advantages of improved sensitivity for X nucleus spectroscopy and imaging of tissue metabolism and function, CEST, QSM, and BOLD fMRI applications in particular. 7T has evolved rapidly over the past decade and will continue to grow steadily for new innovative potential applications across a broad spectrum of biomedical research. For example, recently the CEST methodology revolutionized the endogenous MRI contrast enhancement method that indirectly detects the number of chemically exchangeable water protons with higher sensitivity and suitable exchange rate at 7T. These include high resolution imaging of proteins and peptides with amide protons, glycosaminoglycans, glycogen, glucose, myo inositol, high resolution imaging of tissue pH, creatine, and neurotransmitters like glutamate. These endogenous metabolites can be potentially exploited for improved understanding of disease processes such as cancer, neurological, psychiatric disease, stroke, and musculoskeletal diseases. The second area would be X nuclei MR spectroscopy and imaging applications at UHF include Na, K, Li P, C, O, etc. The in vivo MRS and MRI techniques provide vital metabolic information for noninvasively evaluating cellular processes, metabolic turnovers, energy metabolism, oxygen consumption rate, neurotransmitters function in number of neurodegenerative diseases as well as other organs. The applicability of in vivo X nuclei MRS and MRI is in general limited at standard clinical field systems due to low gyromagnetic ratio as compared to H spin, resulting in poor detection sensitivity, spectral DOI 10.1002/jmri.24444 View this article online at wileyonlinelibrary.com. JOURNAL OF MAGNETIC RESONANCE IMAGING 40:280 282 (2014)
Journal of magnetic resonance imaging JMRI