Efficient high‐frequency body coil for high‐field MRI

Abstract

The use of body coils is favored for homogeneous excitation, and such coils are often paired with surface coils or arrays for sensitive reception in many MRI applications. While the body coil's physical size and resultant electrical length make this circuit difficult to design for any field strength, recent efforts to build efficient body coils for applications at 3T and above have been especially challenging. To meet this challenge, we developed an efficient new transverse electromagnetic (TEM) body coil and demonstrated its use in human studies at field strengths up to 4T. Head, body, and breast images were acquired within peak power constraints of <8 kW. Bench studies indicate that these body coils are feasible to 8T. RF shimming was used to remove a high-field-related cardiac imaging artifact in these preliminary studies. Whole-body imaging at field strengths up to 4T was investigated by MR system manufacturers in the late 1980s (1,2). However, it was thought that the RF power requirements were too high, and RF penetration was too low for successful body imaging above 100 MHz (3). As with most "first" results, the earliest images acquired at higher fields were not the "best" results. Skepticism about high-field body imaging followed, and persisted for nearly a decade (4). Not until recently have technology, support, and interest coalesced to produce encouraging results at fields higher than 1.5 T. Preliminary body imaging results acquired with 55-60-cm inner diameter (i.d.) birdcage transmit coils together with phased-array receivers indicate that 3T whole-body systems may have a clinical role (5). However, these developments are still in their infancy, and there are remaining problems to be solved and performance parameters to be improved. Commercial 128-MHz body coils are usually paired with 35-kW RF power amplifiers to compensate for increased RF losses to the patient and the coil. This is a significant peak power increase compared to 1.5 T MRI, where 15-20-kW amplifiers are more typical. Accordingly, the scope and scale of image protocols operating within FDA specific absorption rate (SAR) guidelines for 3T are highly constrained compared to those for 1.5T.By this trend, body imaging at 4T would be more limited still and would require amplifiers approaching a 50-kW peak. Thus, if the human body provided the only significant loss mechanism for imaging at 3T with coil designs requiring 35 kW of peak RF power, body coils and body imaging above 3T would be improbable. However, if it can be shown that RF coil losses contribute a significant fraction of the total loss, then the use of more efficient coil circuits might lessen the total RF losses and make human imaging feasible at 4T and higher.

Coaxial waveguide MRI

“…At 7 Tesla, the proton Larmor frequency is 297 MHz, which corresponds to a wavelength of 14 cm in tissue. When conventional transmit RF coils (2,3) are used, standing wave patterns with local signal maxima and minima are observed (4,5). The patterns cause an inhomogeneous RF excitation field and thus lead to a spatially variable image contrast that bears the risk to obscure anatomical or pathological details.…”

B compensation in 3T cardiac imaging using short 2DRF pulses

“…B + 1 nonuniformity is influenced by several factors including the distance from the RF transmit coil, conductivity, tissue dielectric constant, and factors related to the body size and RF wavelength. Methods for reducing B + 1 inhomogeneity are highly desirable in high field imaging (1)(2)(3)(4) and imaging with surface coil transmission (5). In high-field cardiac imaging (≥ 3T), B + 1 inhomogeneity on the order of 30-50% across the imaging volume has been predicted and observed (1,(6)(7)(8)(9).…”

RF Resonators Using Microstrip Transmission Line at 3 T MRI

“…도구로 사용되기 위하여 현재 많은 연구가 이루어지고 있다 [1]~ [3] . 그러나 고자기장 자기공명영상시스템에서는 RF 공진 기에서 발생하는 자기장(   )의 불균일성으로 인하여 임상 진단 및 분석을 위한 영상을 획득하기가 힘들다.…”