Hyperemic flow heterogeneity within the calf, foot, and forearm measured with continuous arterial spin labeling MRI

Abstract

Arterial spin labeling (ASL) is a noninvasive magnetic resonance imaging (MRI) technique for microvascular blood flow measurement. We used a continuous ASL scheme (CASL) to investigate the hyperemic flow difference between major muscle groups in human extremities. Twenty-four healthy subjects with no evidence of vascular disease were recruited. MRI was conducted on a 3.0 Tesla Siemens Trio whole body system with a transmit/receive knee coil. A nonmagnetic orthopedic tourniquet system was used to create a 5-min period of ischemia followed by a period of hyperemic flow (occlusion pressure = 250 mmHg). CASL imaging, lasting from 2 min before cuff inflation to 3 min after cuff deflation, was performed on the midcalf, midfoot, and midforearm in separate sessions from which blood flow was quantified with an effective temporal resolution of 16 s. When muscles in the same anatomic location were compared, hyperemic flow was found to be significantly higher in the compartments containing muscles known to have relatively higher slow-twitch type I fiber compositions, such as the soleus muscle in the calf and the extensors in the forearm. In the foot, the plantar flexors exhibited a slightly delayed hyperemic response relative to that of the dorsal compartment, but no between-group flow difference was observed. These results demonstrate that CASL is sensitive to flow heterogeneity between diverse muscle groups and that nonuniform hyperemic flow patterns following an ischemic paradigm correlate with relative fiber-type predominance.

Comparison of oxidative capacity among leg muscles in humans using gated 31P 2‐D chemical shift imaging

“…For example, previous studies (8)(9)(10)(11)(12)(13) have reported a higher percentage of type I fibers in the soleus (69-88%) than the gastrocnemius (43-60%) in humans. Consistent with a typical phenotype profile of a muscle composed primarily of type I fibers, the human soleus has been associated with slow neuronal firing rates during contractions (14), long contraction times (15), enhanced microvascular functioning (16,17), and low glycolytic enzyme activity (8). However, succinate dehydrogenase (SDH) activity, an in vitro marker of oxidative potential, was observed to be similar in the lateral gastrocnemius and soleus (11).…”

Measurement of skeletal muscle perfusion dynamics with pseudo‐continuous arterial spin labeling (pCASL): Assessment of relative labeling efficiency at rest and during hyperemia, and comparison to pulsed arterial spin labeling (PASL)

“…In skeletal muscle, this can be accomplished by measuring the response during or following exercise (functional hyperemia), 7–9 or following induced ischemia (reactive hyperemia). 7,10,11 Functional vascular impairment manifests as a blunting and delay of reperfusion in response to the stressor. 1 Even though it is not physiologically analogous to the demands of daily living, reactive hyperemia provides a more reproducible and subject effort-independent stimulus compared to functional hyperemia, and thus has been purported as the preferred stressor for studying peripheral vascular function.…”

“…This signal can be converted into perfusion in units of mL/min/100g through application of various models. 7,11,15,16 …”

“…The PASL method employed for this study is a flow-alternating inversion recovery variant developed by Raynaud et al 7 in which slice-selective and nonselective adiabatic inversion RF pulses are used for label and control conditions, respectively. Using a similar one-compartment model and assuming the T 1 of blood and tissue can be approximated as equal, perfusion can be quantified as: $$f=\frac{\lambda }{\mathit{\text{PLD}}}\cdot \text{ln}\left[\frac{M_{\mathit{\text{label}}}-M_{\mathit{\text{control}}}}{M_{\mathit{\text{label}}}+M_{\mathit{\text{control}}}}\cdot \left(1-e^{\frac{\text{italicPLD}}{T_{1,\text{italicblood}}}}\right)+1\right]$$Skeletal muscle perfusion during hyperemia has been quantified with PASL, 7 continuous ASL (CASL), 11 or, more recently, pCASL. 2 Each of these methods has benefits and drawbacks.…”

“…7 On the other hand, pCASL and CASL utilize a series of pulses to achieve inversion, thereby reducing labeling efficiency 17 and limiting temporal resolution to 6–8 seconds. 1,2,11 CASL or pCASL methods do offer a theoretical increase in signal-to-noise ratio (SNR) compared to PASL as the labeled blood water spins accumulate in the tissue throughout the longer labeling duration. 19 …”