Magnetic Resonance Imaging of Phosphocreatine and Determination of BOLD Kinetics in Lower Extremity Muscles using a Dual-Frequency Coil Array

Abstract

Magnetic resonance imaging (MRI) provides the unique ability to study metabolic and microvasculature functions in skeletal muscle using phosphorus and proton measurements. However, the low sensitivity of these techniques can make it difficult to capture dynamic muscle activity due to the temporal resolution required for kinetic measurements during and after exercise tasks. Here, we report the design of a dual-nuclei coil array that enables proton and phosphorus MRI of the human lower extremities with high spatial and temporal resolution. We developed an array with whole-volume coverage of the calf and a phosphorus signal-to-noise ratio of more than double that of a birdcage coil in the gastrocnemius muscles. This enabled the local assessment of phosphocreatine recovery kinetics following a plantar flexion exercise using an efficient sampling scheme with a 6 s temporal resolution. The integrated proton array demonstrated image quality approximately equal to that of a clinical state-of-the-art knee coil, which enabled fat quantification and dynamic blood oxygen level-dependent measurements that reflect microvasculature function. The developed array and time-efficient pulse sequences were combined to create a localized assessment of calf metabolism using phosphorus measurements and vasculature function using proton measurements, which could provide new insights into muscle function.

Figure 1. Characterization of the eight-channel ³¹P module of the developed array.

The array provided a similar ³¹P SNR in the center and greater than 2-fold gain in the periphery over the birdcage in the 42 mM Pi phantom (a). Similar results were observed in vivo (b). The measurements are summarized in Table 1.

Figure 2. Normalized ¹H SNR maps (first and fourth columns), B₁⁺ maps (second and fifth columns), and TSE images (third and last columns) acquired with the eight-channel ¹H module (top row) of the developed array.

The array showed favorable performance over the dual-frequency birdcage coil (middle) and similar performance to the clinical 15-channel ¹H array (bottom). The measurements are summarized in Table 2.

Figure 3. Dynamic ³¹P PCr (top row) images acquired using the developed coil array in the calf muscle at different time points during the plantar flexion exercise protocol with the PCr signal time course (bottom) from the segmented gastrocnemius muscle (top right).

Figure 4. In vivo fat fraction and mBOLD images (top row) and mBOLD signal evolution in the soleus muscle (bottom row, ROI inset) following maximum voluntary isometric plantar flexions.

Figure 5. Photograph of the developed ³¹P/¹H array with the protective cover removed.

Overlays highlight an interface board that accommodates the cable trap, transmit/receive switch, and the preamplifier for each coil as well as the power dividers, a ³¹P coil (black), and an ¹H coil (blue).

Figure 6. Unrolled schematic diagram of the ³¹P/¹H coil array and interface.

For simplicity, single ³¹P (black) and ¹H (blue) coils of the 16-channel nested array are highlighted whereas neighboring elements are displayed in a semi-transparent fashion. Abbreviations: RFC = radio frequency choke and RFS = radio frequency short.