Measuring inorganic phosphate and intracellular pH in the healthy and hypertrophic cardiomyopathy hearts by in vivo 7T 31P-cardiovascular magnetic resonance spectroscopy

Figures

• 

  Fig. 1. A photograph of our custom-built dual-tuned (31P/1H) surface RF-coil, comprising a quadrature 31P coil (two 15 cm loops, with overlap decoupling) and a single 1H loop (10 cm in diameter) described in detail in Schaller et al. [15]

• 

  Fig. 2. Simulations of the Bloch Equations for potential off-resonance errors in T1 measured using the dual TR technique with AHP excitation, at the lowest B1+ expected in vivo at the depth of the heart, i.e. ~ 10 cm from the coil. The simulations were performed for resonance offsets ranging from − 180 Hz to 180 Hz from the centre frequency and for the T1 values ranging from 1 s to 10 s. The 20% error margin for T1 estimation is depicted as a grey dashed line

• 

  Fig. 3. Spectra acquired with the interleaved TR sequence using a single AHP excitation frequency centred at the 2,3-DPG resonance (marked with a black arrow near the x-axis in the red voxel spectra). Spectra are shown for voxels in the chest wall muscle down to the intraventricular septum. Note the increased Pi amplitude in all voxels acquired at longer TR, demonstrating the improved detectability of Pi, which allows for reliable cardiac pHi determination. The effective voxel size is depicted using a dashed white line

• 

  Fig. 4. 31P-CMRS spectra acquired in the chest muscle (blue voxel - upper row) and human heart (green voxel - bottom row) with TR1 = 3 s (left) and TR2 = 6 s (right). Spectra were acquired with a single AHP excitation frequency centred on Pi (marked with a black arrow near the x-axis). Please note that while spectra are scaled equally for TR1 and TR2, the heart spectra are scaled differently than the skeletal muscle spectra. A 30 Hz Lorentzian filter was applied to all spectra for better visualization. A pronounced Pi signal can be seen in TR2 spectra both in chest muscle and in the heart. Due to the limited excitation bandwidth of the AHP pulse, β-ATP peak is not excited, and thus, not shown

• 

  Fig. 5. A representative result of the fitting procedure. A mid-septal spectrum measured during an even acquisition overlaid by the final spectral fit (top); individual peaks fitting (middle) and the residual (bottom) is depicted

• 

  Fig. 6. Bland-Altman plots of the in vivo test-retest quantification of pH (a) and Pi/PCr ratio (b) in the human heart septum using long TR 31P-CMRS with adiabatic excitation at 7 T. The mean absolute differences between the two scans were 0.007 for pH and 0.003 for Pi/PCr, showing very low bias. The estimated repeatability coefficients were 0.052 for pH and 0.027 for the Pi/PCr ratio

• 

  Fig. 7. Representative mid-septal spectra with corresponding localizers acquired in a healthy subject (a, c) and an HCM patient (b, d). The spectra shown are a combination of the two interleaved AHP frequency data sets: one acquired with the excitation centred between PCr and γ-ATP (black) and the other with the excitation centred between the 2,3-DPG resonances (grey). The excitation frequencies are marked with arrows of corresponding colour near the x-axis. Spectra are scaled equally for the healthy subject and the HCM patient. Please note the more dominant cardiac Pi signal and ATP signals in the patient’s spectrum and the reduced 2,3-DPG reflecting that more of the voxel is filled with myocardium due to LV wall thickening