Limits of a localized magnetic resonance spectroscopy assay for ex vivo myocardial triacylglycerol

Abstract

Localized magnetic resonance spectroscopy (LMRS) promises a powerful noninvasive means to determine myocardial triacylglycerol (TAG) in a clinical setting. Here, the linearity, specificity, robustness, precision, and accuracy, of an ex vivo mouse-heart LMRS TAG assay are assessed by quantifying the spatial, spectral, and relaxation induced uncertainties. The protocol, which is based on localization by adiabatic selective refocusing (LASER) using frequency offset corrected inversion pulses (FOCI), alternating gradient polarity, and simple post-processing, is shown to have good characteristics. The presented protocol has a benchmark, phantom-based, accuracy of 3%, and when applied to ex vivo mouse-hearts the accuracy is 6%, making the LMRS assay comparable to the typical destructive bioanalytical assay.

Intramyocardial triglyceride quantification by magnetic resonance spectroscopy: In vivo and ex vivo correlation in human subjects

“…The small biopsy size and resulting large surface to volume ratio made scaling the area of the TG resonance by either the H 2 O resonance area or wet weight inconsistent. Thus, after the HR acquisition, the dry weight of the biopsy was determined via TGA analysis in a nitrogen atmosphere with the temperature increased to and left at 90 C until a constant weight was achieved, typically 2 to 4 h. The ex vivo lipid concentration is presented as the area of the TG methylene resonance from 1.15 to 1.25 ppm, determined as reported previously, scaled by the dry weight of the sample (14). All of the MAS areas were additionally scaled by a common factor to approach the HR scale.…”

“…MRS measurements of myocardial TG levels have been shown to be an independent predictor of diastolic function in patients with diabetes and are responsive to changes in fatty acid delivery to the heart (12, 13). The validity of the method in the human heart is based on nonlocalized MRS measurements from excised rodent heart that correlated with biochemical measurements (9, 14). However, the specificity of the method in human heart is unproven.…”

“…Chemical extracts for biochemical or mass spectrometry TG assays were not possible due to the small biopsy size (∼2 mg). Therefore, we used magic angle spinning (MAS) and high‐resolution MRS, which have been validated against biochemical measurements, to quantify TG in ex vivo samples (9, 14, 15).…”

1H‐Magnetic Resonance Spectroscopy for Quantifying Myocardial Lipid Content in Humans With the Cardiometabolic Syndrome

“…The MRS protocol for measuring myocardial IMCL is based on a free‐breathing, point‐resolved spectroscopy (PRESS) sequence with alternating gradient polarity every average, optimized crushers, COG5(2,3,2:0) phase cycling, and both electrocardiographic (ECG) and 2‐dimensional prospective acquisition correction respiratory gating 8–11 . As no water suppression is used, the alternating gradient polarity minimizes spectral distortions and generally provides more robust quantification; the optimized crushers minimize the echo time (Te) while ensuring suppression of outer volume signal from pulse imperfections; and, considering motion, the reduced 5 step (10 with alternating gradient polarity) phase cycling similarly minimizes outer volume signal by reducing the cycle time.…”

“…After the voxel location and trigger delay have been determined, the transmitter power and frequency are optimized and an automatic shim is performed on a 50% enlarged voxel; the voxel is enlarged because these procedures are not gated. Ten PRESS averages are then collected and Fourier transformed to verify that the H 2 O full‐width half‐height line width is below approximately 0.3 ppm 10 …”