Feasibility of a single limb potassium (K40) counter to determine regional skeletal muscle quality

Abstract

PURPOSE: To develop a single sodium iodide (NaI) scintillation detector technique for assessing regional respiratory cell mass via potassium-40 breakdown. METHODS: The apparatus consists of one 4” NaI detector in an open-top lead box. Validation studies consisted of determinations of the day-to-day and within-day variability of the unit, its resolution, sensitivity to nearby muscle tissue not directly contacting the detector and ability to detect differences between beef standards (73, 85, and 97% muscle tissue, balance fat). Beef phantoms were 284g of beef in 4.5 cm tall X 10.1 cm round plastic containers. Spectra were collected (3 replicates) on live human calves of healthy subjects in two age groups (20–31 yrs., n=7; 63–74 yrs., n=6) and normalized to dual-energy x-ray absorptiometry (DEXA)-derived muscle mass. Collections were for 30 minutes duration. Comparisons of in vivo limbs of the two age groups were made using a 2-sample t-test. Bonferroni's test was used to compare muscle concentrations and to test spatial and minimum detectable limit of resolution of the apparatus. RESULTS: Background radiation was highly stable between-days and with time of day (2778.5 ± 9.93; mean ± SE; p=1.00 for day and time of day) with an overall coefficient of variation of 2.2%. Aqueous phantoms (1L) containing 0, 3, 4, and 5g of K+ showed reproducible between-day results (p=0.54). The resolution was found to be at least 0.5g K+. In the plane of the detector, there was a significant (73% muscle, p<0.01), or nearly significant (85% muscle, p=0.18; 97% muscle, p=0.12), difference between 4.5 and 9 cm thick phantoms, but not between 9 and 13.5 cm. In the plane parallel to the face of the detector, there was no statistically significant effect of muscle tissue (85% muscle) outside the edges of the detector. There was a significant effect of muscle concentration on the K40 detected (p<0.01), however the 73% versus the 85% muscle tissue indicated a nonsignificant trend (p= 0.14). Comparison of the two age groups studied also indicated a nonsignificant trend (p=0.16). In vivo studies are currently being conducted to determine the ability to detect age-related changes in muscle quality. CONCLUSION: This technique appears to be reproducible and adequately sensitive to detect relevant differences in skeletal muscle quality while remaining relatively insensitive to interference from nearby biological tissue. Further, these data indicate age-related differences in skeletal muscle quality, possibly due to intramuscular fat and fibrosis, are detectable with this technique.