Alternating repetition time balanced steady state free precession

Abstract

A novel balanced SSFP technique for the separation or suppression of different resonance frequencies (e.g., fat suppression) is presented. The method is based on applying two alternating and different repetition times, TR 1 and TR 2 . This RF scheme manipulates the sensitivity of balanced SSFP to offresonance effects by a modification of the frequency response profile. Starting from a general approach, an optimally broadened stopband within the frequency response function is designed. This is achieved with a TR 2 being one third of TR 1 and an RF-pulse phase increment of 90°. With this approach TR 2 is too short (ϳ1 ms) to switch imaging gradients and is only used to change the frequency sensitivity. Without a significant change of the spectral position of the stopband, TR 1 can be varied over a range of values (ϳ2.5-4.5 ms) while TR 2 and phase cycling is kept constant. On-resonance spins show a magnetization behavior similar to balanced SSFP, but with maximal magnetization at flip angles about 10°lower than in balanced SSFP. The total scan time is increased by about 30% compared to conventional balanced SSFP. Balanced SSFP (TrueFISP, FIESTA, b-FFE, bSSFP) shows high SNR at short repetition times (1,2) and is frequently used in applications that require high temporal resolution, such as in cardiac imaging or perfusion studies. The T 2 / T 1 -weighted contrast of bSSFP generates a bright fat signal (T 2 /T 1 ϳ0.3) comparable to the signal intensity of liquids or fluids. A separation or suppression of fat signals is thus essential for, e.g., coronary artery imaging (3) or cartilage imaging (4), or in general, to separate different resonances or metabolites.Several methods have been proposed to generate frequency selective bSSFP images. A simple and robust approach is FS-SSFP (spectral or fat saturated SSFP) (5), where magnetization is longitudinally stored after an ␣/2 flipback pulse, thereby offering an interruption of the steady state and making the insertion of a spectral selective saturation pulse possible. A different method is FEMR (fluctuating equilibrium MR) (6), which is based on a modification of the frequency response function of bSSFP generated by modified phase cycling schemes. Two resonances, such as fat and water images, can be acquired simultaneously in an interleaved manner. In LC-SSFP (linear combination SSFP) (7,8), several images with different frequency responses are combined to generate a shaped frequency sensitivity. A similar approach is based on binominal excitation patterns (9) or on the direct synthesis of shaped frequency response function (10). A further possibility is to use the phase difference of the fat and water spins for separation, either by direct phase detection (11) or Dixon-type post processing (12-15).An intrinsic drawback of FEMR, LC-SSFP, binominal excitation patterns (9), and similar variants (16) is the large increase (ϳ100%) in total acquisition time to generate the required frequency response function, as well as a restriction of possible repetition times TR. FS-SSFP...

Linear Response Equilibrium versus echo-planar encoding for fast high-spatial resolution 3D chemical shift imaging

“…To overcome the 1/T R periodicity of the excitation function alternating T R 's may be used following up on previous work concerning balanced SSFP sequences [32][33][34]. Here, we present a modified sequence pattern for LRE with two alternating repetition times T R1 and T R2 for suppressing larger spectral bands referred to as dual-T R LRE (dT R LRE) hereafter.…”

“…In analogy to imaging sequences with varying T R [31][32][33][34] the new sequence aims at increasing water suppression.…”

Optimization of alternating TR‐SSFP for fat‐suppression in abdominal images at 3T

“…In (15), the solution for fat suppression at 3T consisted of TR 1 ¼ 3.46 ms, TR 2 ¼ 1.15 ms, 1/t ¼ 3, and a phase cycling of 0 180 180 0. In our hands the implementation of this setting was not possible due to SAR limitations, which led to a modification and optimization of ATR-SSFP.…”

“…Alternating TR-SSFP (ATR-SSFP) (15) is similar to fluctuating equilibrium-SSFP in the sense that the equilibrium magnetization also oscillates between two different steady states. However, these are obtained by alternating the duration of TR between TR 1 and TR 2 , where the ratio TR 1 /TR 2 is equal to a constant integer 1/t.…”

“…At 3T however, the increased field inhomogeneity poses more challenges to the effectiveness of ATR-SSFP for fat suppression. A recent approach, yielding a FRF with a very broad stopband, has been proposed in (16) whereby the original approach in (15) is extended such that the sequence consists of several alternating TRs instead of only two. The drawback of this approach for abdominal imaging at 3T is that the increase in the number of alternating TRs to as much as four not only increases the scan time beyond what is possible for a breath-hold, but it also increases the number of RF pulses and SAR deposition.…”