The overall objective of this protocol is to acquire a better understanding of the normal physiological responses of the tongue to contraction tasks. Specifically, our goals are: (1) to quantify 3D volumetric changes of the tongue as a function of maximal voluntary contraction tasks, (2) to examine task-induced changes in blood flow and how tongue vessels and muscles interact during graded lingual contractions, and (3) to determine task-induced variations in the diffusion properties of water molecules in lingual tissue. To date we have recruited a total of 25 healthy volunteers, with whom we conducted on-site tongue task training and MRI screening. Eighteen of these individuals qualified; 15/18 subjects were studied between 11/23/01 and 8/26/02, and 3/18 will be studied after 10/7/02. Experimental tasks included resting (R) and bolus-holding (H) postures, and oral (O) and oropharyngeal (P) maximum voluntary isometric contractions. From the MRI data, we have computed and normalized the mean difference scores in tongue volume by subject across tasks. No apparent differences were observed for H-R and O-R. Statistical analysis of P-R (Range=5.42-10.06% difference, M=7.98, SD=1.77) and P-H (Range=4.20-11.76%, M=7.69, SD=2.19) with alpha at .025 showed significant difference from a hypothetical mean of zero (p<.0001). Intra-subject variations in calculated volume for each task were small (<1%), reflecting high subject consistency and intra-rater reproducibility. Preliminary analysis of regional volume changes in the lingual vascular bed in two subjects also showed significant task-induced differences. Analysis of data on lingual blood flow waveform (velocity, resistive index, etc.) and inner vessel diameters during post-contraction reperfusion as a function of task and by vessel site revealed: (1) similar baseline arterial diameters (p=0.542) at all recording sites (main, sub-, and deep lingual arteries); (2) consistent increase in vessel diameters at start of reperfusion (p<0.001), but no difference in diameter across contraction regions (p=0.106); (3) increase in volume flow following both Oral and Orop contractions (p = 0.002) and across all arterial sites (p=0.002) with main lingual artery having the highest reperfusion volume (p=0.005). This MVIC profile differed from that of dry swallows in reperfusion rate, return-to-baseline rate, volume flow, and velocity, but not in vessel diameter. Our findings indicate MRI to be a viable means for studying lingual volumetrics, and show a consistent trend toward task-induced changes in the in vivo tongue volume. The mechanisms for such change may lie in the interactions among various types of intrinsic and extrinsic tissues (e.g., muscular, vascular, connective). Through further investigation and extensive 3D modeling, we will be able to test the muscular hydrostat concept and elucidate the biomechanical intricacies of the in vivo human tongue. Evidence from the ultrasound blood flow studies suggests that intramuscular pressures, in response to the degree of muscle contraction, directly influence hyperemia patterns, and that different functional tongue segments may have differing intramuscular demands. We believe that continued examination and correlation of MRI and ultrasound data will enable us to gain insights into task-induced interactions between lingual musculature and vasculature, as well as the biomechanical elements and limitations in lingual strength training through resistance exercises in patients with weakened tongue or compromised lingual function.
