Typical human skeletal muscle is composed of typeⅠand typeⅡmuscle fibers. The relative percentage of typeⅠfibers would be a determinant for athletic performance in either endurance or sprint events. Since it has been reported that T₁ is proportional to the percentage of typeⅠfibers in human skeletal muscles [1], the T₁ measurement in skeletal muscles has been tried. However, the precision of the measured T₁ values is unclear due to various pulse sequences for the measurement. In this study, we evaluated the reliability of T₁ measurements of pulse sequences with inversion recovery technique that is believed to be most accurate in T₁ measurement. A theoretical fitting parameter was employed for the quantitative evaluation of the obtained data. The influence of SNR on the T₁ measurement was also examined.

Polyvinyl alcohol (PVA) gel (T₁ = 850 ms, NIKKO FINES INDUSTRIES Co.,Ltd., Tokyo, Japan) was examined on 1.5T whole-body MRI units (Magnetom Symphony or Aera, Siemens AG, Erlangen, Germany) using an extremity coil (Ex) or a built-in body coil (Bo). Temperature was 27℃. An inversion recovery pulse sequence (IR) used Spin-Echo sequence (SE; IR-SE), TurboFLASH (TF; IR-TF), and Spin-Echo Echo-Planar-Imaging (EPI; IR-EPI). The echo time (TE) of this acquisition scheme was 10 ms (SE and TF) and 30 ms (EPI), respectively. The other protocols of SE measurements were performed with TR 5000 ms, TE 10 ms, voxel size 0.93×0.93×10 mm, matrix 256×256, field of view (FOV) 240 mm×240 mm, and NEX 1. Thirteen inversion times (TI) were used, varying from 50 to 4500 ms (50, 100, 125, 170, 200, 250, 500, 1000, 1500, 2000, 3000, 4000, 4500). Total acquisition time was 3 h 24 min for 13 slices. The condition of TF were with TE 1.29 ms, matrix 128×128, and thirteen inversion times (TI) were used, varying from 50 to 4500 ms (110, 125, 150, 170, 200, 250, 500, 1000, 1500, 2000, 3000, 4000, 4500). Total acquisition time was 1 m 30 s for all slices. TR and FOV was the same as SE. The condition of EPI was with TE 30 ms, and a total acquisition time of 26 s. The other condition was same as TF. However, EPI-images were only acquired by the extremity coil.

Assuming full relaxation of the spin system before each excitation, the absolute MR signal (M) as a function of inversion time TI is given by

M(TI) = M_TI=0×ABS[1 – 2N_RF×exp(-TI/T₁)]

where the factor N_RF took into account the effect of an incomplete inversion. N_RF is related to the flip angle θ of the preparation pulse by: N_RF=(1-cosθ)/2.

Five ROI were placed in physical object from each image. Imperfections of the inversion pulses are accounted by factor according to Blüml S et al [2]. T₁ relaxation time was calculated by Levenberg-Marquardt method for non-linear regression analysis. All analysis was performed in Matlab R2014b programming environment (The Mathworks, Natick, MA). The estimation of fitting calculating results was used the parameter Q that provides a stringent measure of goodness of fit using chi-square [3]. If the results of fitting were good, the values of chi-square close to zero, and the parameter Q close to one.

Evaluation of significant difference was as follows. Highest accuracy T₁ calculated by each pulse sequence was set as the standard value. Differences between the standard T₁ and the others value exceeding 10 percent of the T₁ value were assumed to be significant. About the difference in the parameter Q about N_RF, the differences of over 5 percent of parameter Q were assumed to be significant in each sequence by unpaired t-test.

Both fitting evaluation values Q and N_RF for the data obtained by using IR-TF were the best and showing the value of T₁ is around 880 ms as the most reliable value in this experiment (Figure 1). In general, when TR is greater than 5T₁, the calculated T₁ is insensitive to the effect of RF inhomogeneity and giving rise to yield a reliable T₁ [4]. Although TR (5000 ms) is greater enough in our experiment, the obtained T₁ depends on the used pulse sequence. There should exist unideal flip angle portion especially in the vicinity of the edge plane of the slice depending on the slice profile. The dispersion of the longitudinal magnetization after read out of the MR signal affects the T₁ measurement. When the longitudinal magnetization after inversion time is Mz(TI), the longitudinal magnetization after read out pulse are expressed as Mz(TI)cosαat the position of flip angle α for gradient echo pulse sequence. For the SE pulse sequence, that is expressed as Mz(TI)cosαcos2α. The sinusoidal part at the edge of the slice where α is less than 90° becomes minus for the SE pulse sequence. This may deteriorate the accuracy of T₁ measurement. Although the reason of the worst T₁ accuracy for IR-EPI (Figure 1) is unknown, this may be caused by broad slice profile of soft RF pulses to reduce SAR. The parameter Q affected by N_RF was significant, especially the value of SE had weight changes by using RF coil.

The results of different T₁ versus each sequence condition of IR sequences studies summarized in Figure 1. The parameter Q of all sequence conditions using Ex except EPI was higher than the value using Bo. These results indicate that T₁ measurement is strongly susceptible to SNR. It is suggested that the image noise has an influence on underestimate in calculating T₁. Figure 2 shows typical evaluated and fitted T₁ relaxation curve from pixels in PVA-gel on IR-TF and IR-EPI images. The trend of SE agrees with the result of TF.

Regarding rapid imaging for T₁ measurement, the weak points of EPI are in harmony with these results [5]. Likewise, these results about useful TF are consistent with the previous studies. Therefore, there is every possibility that IR-TF could be applied for rapid T₁ measurement.

Among pulse sequences for the T₁ measurement using IR technique, IR-TF is the best. Moreover, IR-TF can be applied for rapid T₁ measurement.