Steady‐state preparation for spoiled gradient echo imaging

143

227

Variable flip angle T 1 mapping and actual flip-angle imaging B 1 mapping are widely used quantitative MRI methods employing radiofrequency spoiled gradient-echo pulse sequences. Incomplete elimination of the transverse magnetization in these sequences has been found to be a critical source of T 1 and B 1 measurement errors. In this study, comprehensive theoretical analysis of spoiling-related errors in variable flip angle and actual flip-angle imaging methods was performed using the combined isochromat summation and diffusion propagator model and validated by phantom experiments. The key theoretical conclusion is that correct interpretation of spoiling phenomena in fast gradient-echo sequences requires accurate consideration of the diffusion effect. A general strategy for improvement of T 1 and B 1 measurement accuracy was proposed based on the strong spoiling regimen, where diffusion-modulated spatial averaging of isochromats becomes a dominant factor determining magnetization evolution. Practical implementation of strongly spoiled variable flip angle and actual flip-angle imaging techniques requires sufficiently large spoiling gradient areas (A G ) in combination with optimal radiofrequency phase increments (f 0 ). Optimal regimens providing <2% relative T 1 and B 1 measurement errors in a variety of tissues were theoretically derived for prospective in vivo variable flip angle (pulse repetition time 5 (1) is a widely used method allowing generation of T 1 -weighted contrast in gradient echo (GRE) sequences with short pulse repetition time (TR). Technically, RF spoiling is achieved by linearly incrementing the phase of an RF pulse between successive TR with a specific value of the phase increment. After the initial publication (1) suggested optimal phase increments of 117 or 123 , several other values were proposed (2-4). Default settings of the RF phase increment also vary between MRI equipment manufacturers (5). While all these approaches generally allow acquisition of heavily T 1 -weighted GRE images in the steady state, special attention should be given to the role of RF spoiling in quantitative imaging methods.One of most widely used quantitative applications of the RF spoiled GRE sequence is the variable flip angle (VFA) method for T 1 mapping (6,7). In modern implementations, the VFA method takes advantage of a very short TR achievable due to RF spoiling, which allows time-efficient three-dimensional implementation with large anatomic coverage (8-10). Accuracy of VFA T 1 measurements was extensively studied in aspects of optimal flip-angle sampling (7-10) and correction of amplitude of RF field (B 1 ) nonuniformities (4,9,10). Recently, incomplete spoiling was identified as a critical source of errors in the VFA method (4,5,11). While data processing in this method relies on the ideally spoiled signal equation (12), the use of fast RF spoiled sequences may result in partial preservation of transverse coherences and, therefore, deviation of the signal behavior from the idealized theoretical model. Techn...

Section: Methodsmentioning

confidence: 99%

Section: Optimization Of T 1 Measurement Accuracymentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Optimal radiofrequency and gradient spoiling for improved accuracy of T₁ and B₁ measurements using fast steady‐state techniques

Yarnykh¹

2010

143

227

“…This may not be very surprising, as we saw that the transient phase of the latter is more complicated by itself and in its dependence on the initial conditions. Furthermore, existing techniques, such as presented in (27) for RF-spoiled sequences, usually ignore transverse coherence effects and have their limitations, if these cannot be neglected. It is not within the scope of this article, to answer the question, whether and how an effective preparation scheme should look like, but a few general aspects can be discussed.…”

Section: Steady-state Preparationmentioning

confidence: 99%

Analytical solution to the transient phase of steady‐state free precession sequences

Ganter

2009

The transient phase of short-TR steady-state free precession (SSFP) sequences exhibits an often striking complexity and is not only important for nonequilibrium applications (e.g., rapid T 1 -measurements), but can also cause severe artifacts in conventional imaging. In both cases, balanced SSFP sequences are practically (with regard to preparation efficiency) and conceptually (concerning the theoretical understanding of the decay) easier to handle their unbalanced counterparts, for which currently no theory is available. Based on a decomposition of coherence pathways into irreducible subpaths, an exact mathematical solution to the transient phase of unbalanced SSFP sequences is presented in this article, which also includes the known results for balanced SSFP and the steady state of arbitrary SSFP sequences as special cases. As an application, it is shown that the familiar Look-Locker expression for the accelerated magnetization recovery in RFspoiled sequences is only valid for T 2 → 0. In addition to oscillatory perturbations, systematic deviations from the monoexponential decay are observed for T 2 > 0 as a consequence of memory effects. Magn Reson Med 62:149-164, 2009.

“…The first published value was 117°(4), but different values are also in use, partly because of better convergence properties (10,11).…”

mentioning

confidence: 99%

Steady state of gradient echo sequences with radiofrequency phase cycling: Analytical solution, contrast enhancement with partial spoiling

Ganter

2005

Spoiled gradient echo sequences can only reach a homogeneous steady state if sufficiently strong crusher gradients are used in combination with RF phase cycling (RF spoiling). However, the signal depends quite sensitively on the chosen phase increment and-lacking analytical solutions-numerical simulations must be used to study the transient and steady-state magnetization. For the steady state an exact analytical solution is derived, which holds for arbitrary sequence and tissue parameters. Besides a considerably improved computation performance, the analytical approach enables a better understanding of the complicated dependence on . For short repetition times (TR) the regime of small turns out to be particularly interesting: It is shown that the typical c , where RF spoiling starts to become effective, is essentially inversely proportional to T 2 . This tissue dependence implies that contrasts can be considerably larger with partial spoiling ( Ϸ c ) than with conventional RF spoiling ( Ͼ Ͼ c ). As an example, the uptake of contrast agents in tissues is investigated.