Adiabatic multi‐echo ³¹P spectroscopic imaging (AMESING) at 7 T for the measurement of transverse relaxation times and regaining of sensitivity in tissues with short T₂* values

Abstract

An adiabatic multi-echo spectroscopic imaging (AMESING) sequence, used for (31) P MRSI, with spherical k-space sampling and compensated phase-encoding gradients, was implemented on a whole-body 7-T MR system. One free induction decay (FID) and up to five symmetric echoes can be acquired with this sequence. In tissues with low T2 and high T2 , this can theoretically lead to a potential maximum signal-to-noise ratio (SNR) increase of almost a factor of three, compared with a conventional FID acquisition with Ernst-angle excitation. However, with T2 values being, in practice, ≤400 ms, a maximum enhancement of approximately two compared with low flip Ernst-angle excitation should be feasible. The multi-echo sequence enables the determination of localized T2 values, and was validated with (31) P three-dimensional MRSI on the calf muscle and breast of a healthy volunteer, and subsequently applied in a patient with breast cancer. The T2 values of phosphocreatine, phosphodiesters (PDE) and inorganic phosphate in calf muscle were 193 ± 5 ms, 375 ± 44 ms and 96 ± 10 ms, respectively, and the apparent T2 value of γ-ATP was 25 ± 6 ms. A T2 value of 136 ± 15 ms for inorganic phosphate was measured in glandular breast tissue of a healthy volunteer. The T2 values of phosphomonoesters (PME) and PDE in breast cancer tissue (ductulolobular carcinoma) ranged between 170 and 210 ms, and the PME to PDE ratios were calculated to be phosphoethanolamine/glycerophosphoethanolamine = 2.7, phosphocholine/glycerophosphocholine = 1.8 and PME/PDE = 2.3. Considering the relatively short T2 values of the metabolites in breast tissue at 7 T, the echo spacing can be short without compromising spectral resolution, whilst maximizing the sensitivity.

Whole-body radiofrequency coil for31P MRSI at 7 T

“…For instance, multiple refocusing pulses can be implemented, enabling the optimization of SNR per unit of scan time and T 2 * refocusing; this may be crucial as B 0 shimming over the entire body may be challenging, as can be observed from the highly variable linewidths in the large FOV scan of Figure . Short‐TR multi‐echo sequences also make it possible to measure even short transverse relaxation times T 2 , and can improve SNR in poor shimming situations . Polarization transfer sequences with rectangular pulses for 31 P can be performed with shorter TR, gaining SNR per unit of time .…”

In-vivo 31P-MRS of skeletal muscle and liver: A way for non-invasive assessment of their metabolism

“…This can be partially compensated by the use of short TRs, utilizing the large T 1 differences between 31 P and 1 H spins [34]. Similarly, adiabatic multi echo spectroscopic imaging (AMESING [35]) could be used to regain the sensitivity lost due to T 2 ∗ losses [36].…”

1H/31P Polarization Transfer at 9.4 Tesla for Improved Specificity of Detecting Phosphomonoesters and Phosphodiesters in Breast Tumor Models

“…Only differences in T2 relaxation will attribute to an unknown variance in polarization transfer efficiency. However, recent innovations in pulse sequence design may even recover this missing information in T2 [23].…”

“…PE, PC, GPE, and GPC levels from the BINEPT spectra were corrected for polarization transfer efficiency (about 50%) and for 1 H T1, 1 H T2, and 31 P T2 relaxation with estimates based on literature values. 1 H relaxation times of tCho were based on values obtained at 7 and 9.4 T [21], [22], and 31 P T2 relaxation times were based on values obtained at 3 and 7 T [10], [23], [24] (Table 1). …”