Synopsis
It has been reported
that T1 in skeletal muscles is proportional to the percentage of
typeⅠfibers which would be a determinant for athletic performance. However,
the precision of the measured T1 values is unclear due to various
pulse sequences used for the measurement. In this study, we evaluated the reliability of T1 measured
by pulse sequences with inversion recovery technique that is believed to be
most accurate in T1 measurement. A value of goodness of fitting was
employed for the quantitative evaluation of the obtained data. The influence of
SNR on the T1 measurement was also examined.Purpose
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 T
1 is proportional to the percentage of typeⅠfibers in human skeletal muscles [1], the T
1 measurement in
skeletal muscles has been tried. However, the precision of the measured T
1 values is unclear due to various pulse sequences for the measurement. In this study, we evaluated the reliability of T
1 measurements of
pulse sequences with inversion recovery technique that is believed to be most
accurate in T
1 measurement. A theoretical fitting parameter was
employed for the quantitative evaluation of the obtained data. The influence of
SNR on the T
1 measurement was also examined.
Materials and Methods
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)
= MTI=0×ABS[1 – 2NRF×exp(-TI/T1)]
where the factor NRF took into account the
effect of an incomplete inversion. NRF is related to the flip angle θ of the preparation pulse by: NRF=(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 NRF,
the differences of over 5 percent of parameter Q were assumed to be significant
in each sequence by unpaired t-test.
Results and Discussion
Both fitting
evaluation values Q and NRF 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 NRF 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.
Conclusions
Among pulse sequences
for the T
1 measurement using IR technique, IR-TF is the best. Moreover, IR-TF can
be applied for rapid T
1 measurement.
Acknowledgements
This
work was supported by a Grant-in-Aid for Scientific Research (B) from the
Ministry of Education, Culture, Sports, Science, and Technology (25282170). Furthermore,
we thank Japan Institute of Sports Sciences (JISS) for only assistance with
data collection.References
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