Year: 2012
Type: article
Abstract: Recently, T 2 -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 T 2 Prep for T 2 -mapping at 1.5T has enabled a rapid quantitative cardiac T 2 estimation (Huang et al., MRM2007). However, the accuracy of this method may still be limited due to the complex T 2 /T 1 signal weighting. Especially for longitudinal studies designed for monitoring and/or guiding therapy, accurate and reproducible T 2 measurements will be critical. A novel quantitative 3T T 2 -mapping protocol was therefore developed and tested in both healthy volunteers and patients. An adiabatic T 2 prep with 3 incremental TE values, affine coregistration, a navigator and 2D radial gradient echo imaging were combined for free-breathing T 2 -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 T 1 relaxation and which returns the “true” T 2 . The T 2 -mapping sequence and empirical equation were then validated in a series of 15 phantoms in which the true T 2 was determined with a 9-TE spin-echo sequence. Next, the myocardial short axis T 2 of 8 healthy volunteers was mapped in two different scan sessions while a reference phantom (T 2 =43.1±0.7ms) was placed next to the thorax. The average myocardial T 2 for both sessions was computed with and without correction with the “true” reference phantom T 2 . Finally, this validated protocol was used in 5 patients in the subacute phase after revascularization of acute ST-elevation myocardial infarctions and compared to T 2 -weighted TSE imaging. As a result of both the simulations and phantom scans, optimized sequence parameters included: TE T2prep =60/30/0ms, T RR =3 heartbeats, TR/TE=5.3/2.4ms. The empirical equation to determine T 2 was S=S0[exp(-TE T2prep /T 2 )+0.06], where S and S0 are the measured and steady-state signal (Fig. 1a ). Scans of the phantoms with known T 2 confirmed a 12±2%(p<0.001) improvement in T 2 estimation with the empirical equation as compared to the standard T 2 decay measurements (Fig. 1b ). The myocardial T 2 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 T 2 from the reference phantom, this difference decreased to 2±1% (p=0.02). In all patients, T 2 maps could successfully be obtained and a clear demarcation of zones with elevated T 2 values was consistent with the findings on T 2 -weighted MRI and X-ray coronary angiography as shown in the example in Fig. 2 . Single pixel T 2 -mapping in a simulation and phantom scan. A) Simulated magnetization (black dots) for myocardium with input T 2 =45ms at the T 2 prep times (60, 30 and 0ms) and fitted curves with the standard (dashed line) and new, empirical (whole line) equation. The new equation leads to more accurate T 2 computations. B) Similar results are obtained in a pixel in a T 2 map of a phantom where the T 2 was determined to be 45ms with a 9-TE spin echo scan. Short-axis T 2 map together with conventional T 2 -weighted turbo spin-echo and X-ray coronary angiogram in a patient with a myocardial infarct. A) A clearly demarcated zone with elevated T 2 can be seen in the region of the black arrow, which might indicate myocardial edema. The non-infarcted tissue has a homogenous T 2 , while the reference phantom adjacent to the thorax appears homogeneous with T 2 values similar to those in healthy tissue. B) The conventional T 2 -weighted TSE image confirms the elevated T 2 in the region of the infarct (arrow). C) Consistent with these findings, the x-ray coronary angiogram shows a severe stenosis in an obtuse marginal artery (arrow). The methodology presented in this study enables robust and accurate cardiac T 2 -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.
Language: English
Subfield: Radiology, Nuclear Medicine and Imaging
Field: Medicine
Domain: Health Sciences
Open Access status: gold
Estimated APC paid: $3,000
OpenAlex ID: W2024434302