Self-navigated three-dimensional cardiac T2 mapping at 3T
Journal of Cardiovascular Magnetic Resonance volume 15, Article number: P51 (2013)
Cardiac T2 mapping using a variable T2 preparation module (T2Prep) has recently gained attention for its ability to quantify the extent of edema (Giri, JCMR 2009). Due to time constraints, the T2 maps are commonly acquired as one or several two-dimensional slices, while the underlying pathology has a three-dimensional (3D) structure. The next logical step would therefore be to exploit recent hardware and software advances to directly acquire 3D T2 maps. To this end, we tested the feasibility of using a self-navigated 3D radial acquisition with a variable T2Prep for 3D T2 mapping at 3T.
Approval was obtained from the institutional review board. A 3D self-navigated undersampled balanced steady-state free precession (bSSFP) sequence (TR/TE=2.6/1.33ms, matrix 1283, flip angle 70°) with a spiral phyllotaxis radial 3D trajectory (Piccini, MRM 2011) was implemented on a 3T clinical system (Skyra, Siemens AG). This self-navigated pulse sequence allows free breathing acquisitions with 100% scan efficiency, while ECG triggering every 2 heartbeats and TET2Prep=60/30/0ms allow for a total acquisition time of ~18min with an isotropic spatial resolution of (1.7mm)3. The datasets were registered using 3D affine registration (Studholme, Med Image Anal 1996). Through Bloch equation simulations, the heart-rate-dependent T1 -relaxation-related offset in the T2-fitting equation was ascertained. Subsequently, the validity and accuracy of the T2 fitting was tested in a phantom whose "true" T2 values were previously determined. The in vivo robustness of the T2 determination was then tested in 9 healthy adult subjects. Finally, the sequence was applied for the detection of edema in a 75-year-old male infarct patient after revascularization of his proximal left circumflex.
The Bloch equation simulations of the pulse sequence demonstrated that the input T2 value could be accurately fitted from the magnetization M with the equation [M=M 0e -TET2Prep/T2 +0.08M0], while the fitted T2 had only a ~3% variation over the common range of heart rates (Fig.1A). The phantom T2 maps demonstrated high homogeneity and fitting accuracy with the 3D sequence matching the ‘true' value to within 1% (Fig.1B). The volunteer study (Fig.2A-C) suggested good agreement with previously reported T2 values at T2=39.3±3.9ms (Van Heeswijk, JACC Imaging 2012, in press). A region of significantly elevated T2 (60.4±9.1 vs. 41.0±4.5ms) was identified in the patient in the infero-lateral myocardium of the left ventricle (Fig.2D,E), consistent with the findings on X-ray coronary angiography.
The proposed technique provides an easy and time-efficient way to obtain accurate isotropic T2 maps of the whole heart. Accurate T2 values were obtained in the phantom, while those in volunteers are consistent with previously reported values. The preliminary patient study demonstrated elevated T2 in the infarcted region as expected.
Foundation Emma Muschamp
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van Heeswijk, R.B., Piccini, D., Feliciano, H. et al. Self-navigated three-dimensional cardiac T2 mapping at 3T. J Cardiovasc Magn Reson 15 (Suppl 1), P51 (2013). https://doi.org/10.1186/1532-429X-15-S1-P51