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Simultaneous Measurement of Perfusion and T2* in Calf Muscle at 7T with Dynamic Exercise using Radial Acquisition.
Sultan Zaman Mahmud1, Bruce Gladden2, Andreas Kavazis2, Robert Motl3, Thomas Denney1, and Adil Bashir1

1Department of Electrical and Computer Engineering, Auburn University, Auburn, AL, United States, 2School of Kinesiology, Auburn University, Auburn, AL, United States, 3Department of Physical Therapy, University of Alabama at Birmingham, Birmingham, AL, United States

Synopsis

Impairments in oxygen delivery and consumption can lead to reduced muscle endurance and physical disability. Perfusion, a measure of microvascular blood flow, provides information on nutrient delivery. T2* provides information about relative tissue oxygenation. Changes in these parameters following stress, such as exercise, can yield important information about imbalance between delivery and consumption. In this study we implemented golden angle radial MRI acquisition technique to simultaneously quantify muscle perfusion and T2* at 7T, and demonstrate assessment of spatial and temporal changes in these parameters within calf muscles during recovery from plantar flexion exercise.

Introduction

Reduced muscle endurance is a leading cause of disability in a host of conditions such as neuromuscular disease, heart failure, aging, etc. Reduced blood flow, delivery to, and consumption by the working muscle are likely to cause decline in muscle ability to sustain workloads 1. Thus, understanding of spatial and temporal aspects of muscle perfusion is important for our understanding of sustained muscle performance. Muscle perfusion can be measured using arterial spin labeling and provides information about microvascular blood flow. T2* is related to the total amount of deoxyhemoglobin in the tissue and, by extension, is an estimate of blood oxygen saturation, an indirect measurement of oxygen extraction by the tissue. The methods for measurement of perfusion and T2* have been previously reported 2. However these methods have not been demonstrated with dynamic exercise at 7T. Herein, we implemented novel interleaved golden angle radial MRI acquisition technique to simultaneously quantify muscle perfusion and T2* We further demonstrated the assessment of spatial and temporal changes in perfusion and T2* in calf muscle during recovery from plantar flexion exercise.

Methods

Pulse sequence was developed to acquire data simultaneously from two different slices. Arterial spin labeling sequence SATIR 3 was implemented using hyperbolic secant inversion pulse for spin tagging and golden angle radial readout (Fig 1 - slice 1). Time delay between perfusion tagging and acquisition was used for interleaved acquisition of T2* data for a slice located 3 cm distally from the perfusion slice (Fig 1 – slice 2). A multi-echo radial GRE sequence with radial acquisition was used for T2* mapping. Temporal resolution for perfusion and T2* was approximately 1.3 seconds. MR imaging was performed on a Siemens 7T system (Erlangen, Germany) using a surface coil. Six subjects participated in the study. Quantitative perfusion and T2* maps were acquired at rest. Common acquisition parameters: FOV = 192 mm, slice = 5 mm, TR = 1.28 s, Flip angle = 15⁰ and 64 radial acquisition. Resting perfusion measurements were acquired with a slice selective and nonselective tagging pulse, tagging time = 1 sec. T2* was acquired with TEs = 2.2, 5.0, 7.8, 10.6 and 13.4 ms. The subjects then performed 2 minutes of plantar flexion at 0.5 Hz against a resistance of 40% of MVC. Data was acquired for 3 minutes in recovery. Quantitative perfusion maps were determined as described before3. T2* was calculated by fitting a mono-exponential function to magnitude signal intensity.

Results

A representative slice selective perfusion weighted image and T2* map at rest are shown in Figure 2. Blood vessels on perfusion images show high signal due to flow. At rest the perfusion in calf muscle was 5 ± 2 mL/min/100g. Figure 3 shows changes in perfusion and T2* immediately after exercise indicating regions of muscle activation. These overlay maps are changes from the resting state. A region of interest (ROI) corresponding to the activated muscle regions identified on the perfusion maps was manually traced on the images. ROI analysis showed that perfusion was significantly increased reaching 70 ± 10 mL/min/100g immediately after exercise. Perfusion recovered slowly during post-exercise rest period and average time to return to baseline was approximately 100 s. Shim difference between experiments affects baseline (resting) T2* therefore post exercise T2* normalized to the resting map from each individual. T2* in the selected ROI decreased by 15 % immediately after exercise from the resting value. T2* recovery showed exponential behavior; i.e., a fast recovery followed by much slower recovery to resting values.

Discussions

This study demonstrates the ability to simultaneously quantify skeletal muscle perfusion and T2*, both at rest and dynamically, for post exercise recovery in calf muscle at 7T. An interleaved golden angle radial acquisition pulse sequence was implemented for the study which helps reduce bulk motion artifacts. The temporal and spatial resolution of the protocol was sufficient to measure changes in metabolism-related parameters post exercise. Dynamic plantar flexion exercise isolates the calf muscles and might provide valuable insight into pathophysiological processes independent of impaired heart function. Accordingly, the information provided by this technique may prove to be very valuable in understanding muscle metabolism in healthy subjects as well as patient populations who experience mobility disability.

Acknowledgements

No acknowledgement found.

References

  1. Jacobi B, et al. Skeletal muscle bold MRI: from underlying physiological concepts to its usefulness in clinical conditions. J Magn Reson Imaging. 2012; 35(6):1253-65.
  2. Englund EK, et al. Combined measurement of perfusion, venous oxygen saturation and skeletal muscle T2* during reactive hyperemia in the leg. J Cardiovasc Magn Reson. 2013; 15(1):70.
  3. Raynaud JS, et al. Determination of skeletal muscle perfusion using arterial spin labeling NMRI: validation by comparison with venous occlusion plethysmography. Magn Reson Med. 2001; 46(2):305-11.

Figures

Figure 1: Slice locations for perfusion and T2* imaging. Slice-1 is perfusion slice and slice-2 is T2* slice.

Figure 2: (a) Transverse anatomical image of slice 1 and (b) perfusion weighted image of the same slice at rest. (c) Transverse anatomical image of slice 2 and (d) T2* map of the same slice at rest.

Figure 3: (a) Overlay showing region of increased perfusion due to exercise and (b) dynamic perfusion during recovery from plantar flexion from selected ROI. (c) Overlay showing region of decreased T2* immediately after the exercise and (d) dynamic changes in T2* during recovery from exercise from selected ROI.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
0414