- Oral presentation
Quantitative free-breathing 3T T2-mapping of the heart designed for longitudinal studies
Journal of Cardiovascular Magnetic Resonancevolume 14, Article number: O51 (2012)
Recently, T2-weighted MRI for the characterization of edema after myocardial infarction has attracted considerable attention (Friedrich, NatRevCardiol2010). Furthermore, the recently proposed combination of bSSFP imaging and T2Prep for T2-mapping at 1.5T has enabled a rapid quantitative cardiac T2 estimation (Huang et al., MRM2007). However, the accuracy of this method may still be limited due to the complex T2/T1 signal weighting. Especially for longitudinal studies designed for monitoring and/or guiding therapy, accurate and reproducible T2 measurements will be critical. A novel quantitative 3T T2-mapping protocol was therefore developed and tested in both healthy volunteers and patients.
An adiabatic T2prep with 3 incremental TE values, affine coregistration, a navigator and 2D radial gradient echo imaging were combined for free-breathing T2-mapping at 3T with a spatial resolution of 1.25mm. Bloch equation simulations of this sequence were used to optimize scan parameters and to determine an empirical equation that compensates for T1 relaxation and which returns the “true” T2. The T2-mapping sequence and empirical equation were then validated in a series of 15 phantoms in which the true T2 was determined with a 9-TE spin-echo sequence. Next, the myocardial short axis T2 of 8 healthy volunteers was mapped in two different scan sessions while a reference phantom (T2=43.1±0.7ms) was placed next to the thorax. The average myocardial T2 for both sessions was computed with and without correction with the “true” reference phantom T2. Finally, this validated protocol was used in 5 patients in the subacute phase after revascularization of acute ST-elevation myocardial infarctions and compared to T2-weighted TSE imaging.
As a result of both the simulations and phantom scans, optimized sequence parameters included: TET2prep=60/30/0ms, TRR=3 heartbeats, TR/TE=5.3/2.4ms. The empirical equation to determine T2 was S=S0[exp(-TET2prep/T2)+0.06], where S and S0 are the measured and steady-state signal (Fig. 1a). Scans of the phantoms with known T2 confirmed a 12±2%(p<0.001) improvement in T2 estimation with the empirical equation as compared to the standard T2 decay measurements (Fig. 1b). The myocardial T2 in the volunteers was homogeneous (42±5ms over all volunteers) and on average showed a 5±2% difference between the two scan sessions. When compensated with the T2 from the reference phantom, this difference decreased to 2±1% (p=0.02). In all patients, T2maps could successfully be obtained and a clear demarcation of zones with elevated T2 values was consistent with the findings on T2-weighted MRI and X-ray coronary angiography as shown in the example in Fig. 2.
The methodology presented in this study enables robust and accurate cardiac T2-mapping at 3T, while the addition of a reference phantom improves reproducibility. Therefore, it may be well-suited for longitudinal studies in patients with ischemic heart disease.