Time‐interleaved acquisition of modes: An analysis of SAR and image contrast implications

Abstract

As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called timeinterleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B þ 1 modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in B þ 1 homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T 2 , and T 2 *-weighted imaging and, if a specified homogeneity criterion is met, in Key words: 7 Tesla; ultra-high field; body imaging; parallel transmission; RF shimmingSince the early days of magnetic resonance imaging, a steady drive to higher field strengths has been apparent. At higher field strengths, an increased signal-to-noise ratio and new contrasts can be obtained; however, high static field strengths of 7 T and above lead to severe radiofrequency (RF) homogeneity problems (1,2), because the operational frequency is proportional to the static field strength, and hence, the wavelength is shortened. Especially, whole-body imaging at 7 T and higher is therefore challenging (3). Multichannel transmit approaches to tackle these problems have been proposed in the literature. The most notable are static RF shimming (4,5) and Transmit SENSE (6).In RF shimming, each element in an array is driven with its own constant amplitude and phase, while the pulse profile is identical for all elements. By choosing a suitable set of amplitudes and phases, the resulting B þ 1 can be shaped within certain limitations to achieve a more homogeneous field excitation in an extended area or a localized field of view (5,7).Transmit SENSE and equivalent approaches (8) use spatially tailored RF pulses. The amplitude and phase of each element in a transmit array are varied during transmission, leading to element-dependent pulse profiles. This approach yields excellent results but presupposes precise knowledge of element transmission profiles as well as expensive and complicated hardware.Recently, an acquisition scheme called time-interleaved acquisition of modes (TIAMO) (9) has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B þ 1 modes using static RF shimming in an interleaved acquisition, whe...

Nonenhanced Magnetic Resonance Angiography of the Lower Extremity Vessels at 7 Tesla

“…By forming virtual receive elements, both images were reconstructed together as a 32-channel data set using the generalized autocalibrating partially parallel acquisitions (GRAPPA) algorithm to achieve a more homogeneous image. 30,31 For MR imaging of the lower extremity vessels, a 2D nonenhanced T1-weighted spoiled gradient echo sequence (FLASH) was acquired with and without a venous saturation pulse. Both sequence types were acquired consecutively for each imaging station before the AngioSURF table was moved to the subsequent station.…”

Sequence Comparison for Non-Enhanced MRA of the Lower Extremity Arteries at 7 Tesla

“…In those volunteers where the aforementioned problem was encountered, dedicated TIAMO shims [16] were calculated based on acquired B 1 + maps in the same imaging position. The two shims were calculated to adjust the phases of the 8 transmit channels in such a way that in one shim the B 1 + was maximized in a circular region of interest around the right superficial femoral artery, while in the other shim the B 1 + in a circular region of interest around the left superficial femoral artery was maximized.…”

“…An RF pulse of nominal 90° for the suppression of the veins was applied every TR with a thickness of 116 mm and positioned at a distance of 10 mm below the imaging slice, travelling with the current slice position. TIAMO imaging [15], [16] used CP + and CP 2+ except for the stations of the upper thigh in volunteers where dedicated TIAMO shims had to be calculated. Hence, every slice was acquired twice.…”

“…But in order to perform non-contrast-enhanced MRA at 7T, one has to counter challenging issues such as B 0 and B 1 inhomogeneities [11], [12], particularly when considering body imaging [13], [14]. To tackle the latter issue, a recently published technique, Time-Interleaved Acquisition of Modes (TIAMO) [15], [16], was used for this study. The principle is to excite at least two different B 1 transmission modes using static radiofrequency (RF) shimming in an interleaved acquisition.…”

Comparison of Fat Saturation Techniques for Single-Shot Fast Spin Echo Sequences for 7-T Body Imaging

“…We designate this technique time-interleaved acquisition of modes (TIAMO) FS because it is similar to the technique that we generally use for the imaging portion of sequences in abdominal imaging protocols. The principle of TIAMO 22,23 is to excite at least 2 different B 1 transmission modes using static RF shimming in an interleaved acquisition. Overall signal homogeneity can be improved by exploiting the complementary RF patterns of the different transmission modes.…”