Muscle-associated triglyceride measured by computed tomography and magnetic resonance spectroscopy

Abstract

Objective: Muscle triglyceride can be assessed in vivo using computed tomography (CT) and 1H magnetic resonance spectroscopy (MRS), two techniques that are based on entirely different biophysical principles. Little is known, however, about the cross-correlation between these techniques and their test-retest reliability.

Research methods and procedures: We compared mean muscle attenuation (MA) in soleus and tibialis anterior (TA) muscles measured by CT with intra- and extramyocellular lipids (IMCL and EMCL, respectively) measured by MRS in 51 volunteers (26 to 72 years of age, BMI = 25.5 to 39.3 kg/m2). MA of midthighs was also measured in a subset (n = 19). Test-retest measurements were performed by CT (n = 6) and MRS (n = 10) in separate sets of volunteers.

Results: MA of soleus was significantly associated with IMCL (r = -0.64) and EMCL, which by multiple regression analysis was explained mostly by IMCL (p < 0.001) rather than EMCL (beta = -0.010, p = 0.94). Muscle triglyceride was lower in TA than in soleus, and MA of TA was significantly correlated with EMCL (r = -0.40) but not IMCL (r = -0.16). By CT, MA of midthighs was correlated with MA in soleus (r = 0.40, p = 0.07) and whole calf (r = 0.62, p < 0.05). Finally, both MA and IMCL were highly reliable in soleus (coefficient of variation = < 2% and 6.7%, respectively) and less reliable in TA (4% and 10%, respectively).

Discussion: These results support the use of both CT and MRS as reliable methods for assessing skeletal muscle lipid.

Figure 1

Water-suppressed PRESS boxes (7.5 × 7.5 × 10 mm³ voxels) were collected from the largest volume of the calf muscle (TE = 35 ms; TR = 2 seconds). The figure shows four possible positions in the soleus and one position in the TA muscle in a cross-sectional scout image (TE = 9.7 ms; TR = 500 ms; field of view = 10 × 7.5 mm). IMCL and EMCL were determined from the average of the sum of three PRESS boxes in the soleus and one to two PRESS boxes in the TA muscles. Typical ¹H MRS obtained from a single PRESS box in the soleus and TA muscles is shown. The spectrum shows the CH₂ and CH₃ peaks of EMCL and IMCL that are shifted in frequency by 0.2 ppm and the residual water trimethyl amine (A) and creatine peaks.

Figure 2

MA was measured from a single slice cross-sectional CT image (120 kv, 310 mA, 40-cm field of view, 512 × 512 pixel matrix) by windowing the area of the desired muscle. This figure shows windowing of the soleus muscle from a scan obtained at midcalf.

Figure 3

Muscle-associated lipid storage measured by ¹H MRS (A and B) and CT (C) in the soleus (n = 51) and TA (n = 48) muscles. EMCL and IMCL measured by MRS are in arbitrary units relative to a peanut oil phantom positioned external to the calf (A) or relative to the internal water peak (B). CT data are the mean attenuation values in HUs. Values are means ± SD.

Figure 4

Relationship between IMCL and EMCL CH₂ peak measured by ¹H MRS and mean MA determined by CT in the soleus muscle (r = −0.64 for IMCL and −0.37 for EMCL; p < 0.01; n = 0.51). IMCL and EMCL are expressed in arbitrary units relative to a peanut oil phantom that is positioned external to the calf. A low mean MA reflects a higher fat content.

Figure 5

The cross-correlation between the IMCL CH₂ peak set relative to an external oil phantom and the internal water peak (r = 0.98; p < 0.01; n = 0.51).

Figure 6

Test—retest results of muscle lipid storage measured by ¹H MRS and CT in the soleus muscle. (Top) Test—retest data for mean MA measured by CT in six individuals (ICC = 0.96; CV = 1.96%). (Bottom) Test—retest data for IMCL measured by MRS (sum of three voxels) in 10 individuals (ICC = 0.98; CV = 7.25%). In both cases, volunteers exited the patient bed after scan 1 and were repositioned for scan 2.