Volume 12 Supplement 1

# Analysis of the transient phase of balanced SSFP with non-continuous RF for cardiac imaging

## Introduction

The transient phase of balanced SSFP (bSSFP) is the period during which magnetization approaches steady state. The transient phase of non-ECG-gated, continuous-RF bSSFP has been characterized by a simple exponential decay with a time constant that is a flip-angle-weighted average of T1 and T2 . Cardiac imaging applications, however, often utilize bSSFP with non-continuous RF excitation. The example considered here, Look-Locker-based T1 mapping, begins with an ECG trigger, and is followed by magnetization preparation, a bSSFP imaging segment, and a recovery time prior to the subsequent ECG trigger. Multiple time points are acquired, separated by the R-R interval TRR. The description of the continuous-RF transient phase is not applicable in this case.

## Purpose

The goal of this work was to develop an analytical expression for the transient phase of bSSFP with non-continuous RF excitation. The resulting equation can be applied to Look-Locker acquisitions to provide true quantification of T1 (and T2), rather than an "apparent" T1 (T1*).

## Methods

The pulse sequence is shown in Figure 1 and is periodic, beginning with data acquisition (a segment of N views) and ending with a recovery time Trec = TRR-N × TR before the next segment. Let MT(n) be the transient magnetization prior to time point n, and assume the magnetization at the subsequent time point is reduced to λMT(n) [1, 2]. The transient response may then be written This work will show that where Rx, z are rotation matrices for RF excitation/alternation, E t represents relaxation during time t, and B denotes steady-state catalyzation. The equation AMT(n) = λMT(n) can be solved for the real eigenvalue λ eig (T1, T2) of A, which is a function of T1, T2, and known imaging parameters. Because it describes the exponential evolution of the transient magnetization, λ can also be written T1* can be determined from fitting the time point images acquired during the transient phase. With an appropriate pulse sequence, λ eig (T1, T2) = λ image can be solved for T1 and T2.

## Results

Bloch simulation of the pulse sequence in Figure 1 was performed, and T1* was determined by curve fitting. T1, T2, and T1* were then calculated using λ eig (T1, T2) and showed perfect agreement.

## Conclusion

Previously reported cardiac T1 mapping techniques using bSSFP have employed various assumptions and approximations to estimate T1. This work presents an analytical expression for the transient phase of non-continuous-RF bSSFP. It provides the ability to directly quantify T1&T2 for cardiac imaging while obviating such assumptions in acquisition or post-processing.

## References

1. 1.

Scheffler : MRM. 2003, 49: 781

2. 2.

Hargreaves : MRM. 2001, 46: 149

## Author information

Authors

### Corresponding author

Correspondence to Glenn S Slavin.

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Reprints and Permissions

Slavin, G.S. Analysis of the transient phase of balanced SSFP with non-continuous RF for cardiac imaging. J Cardiovasc Magn Reson 12, P230 (2010). https://doi.org/10.1186/1532-429X-12-S1-P230 