Synopsis
For longitudinal investigations reproducibility and
accuracy of ASL measurements are of essence. The purpose of this study is
to investigate how magnet design and labeling position within the magnet
impact CBF quantification. Our results indicate that
CBF values are not always reproducible in our ultra-short wide bore scanner. Great
variability may be introduced by the actual position of the labeling slice with
respect to the magnet isocenter. The exact cause of this difference requires
further investigation. However, positioning the labeling slice in isocenter
provided a simple solution to overcome this issue and to measure reproducible
CBF values.Purpose
Cerebral
blood flow (CBF) is an important measure of brain function, often applied to
diagnose and evaluate cerebral disorders
1. Arterial spin labeling
(ASL) is widely used for serial, noninvasive quantification of CBF to assess treatment
response or disease progression
2. For such longitudinal
investigations reproducibility and accuracy of ASL measurements are of essence.
Past ASL studies have shown reproducibility of CBF estimates
3–5, but did not investigate how
magnet design and labeling position with respect to the isocenter impact
quantification. The purpose of this study is to investigate these potential
issues.
Methods
Fifteen healthy volunteers enrolled in an IRB approved protocol were scanned
back-to-back on two 3T systems, a standard bore Siemens Trio (M1), and an ultra-short
wide bore Skyra (M2). Scanner software
versions were B17 and D13, and respective product Q2TIPS
6 ASL sequences were
applied with the following parameters: TR=2280ms, TE=23ms, TI=1400ms, TI1=700ms,
FOV=210×210mm
2, matrix 64×64, 11 slices, thickness 5mm, and 1mm gap.
The labeling slice was 100mm thick and placed 20mm inferior to the imaging
block. Sequence code and parameters were reviewed by an experienced programmer
and the timing parameters were found to be identical. ASL scans for each magnet were performed at three
specific table positions: ISO, where the center of the imaging block was aligned
with the magnet’s isocenter, ISO-50 and ISO-100, where the center of the
imaging block was placed 50mm and 100mm superior from isocenter. In ISO-100
position, the center of the labeling slice coincided with the magnet’s
isocenter. A 3D T1-weighted full brain volume was acquired and used
for segmentation and registration using the FSL toolbox
7. CBF was calculated
8 using Matlab, assuming
identical label efficiency independent of magnet type and table position. Two-way
repeated measurement ANOVA was performed in SigmaPlot to test for difference in
CBF quantification between MR systems and table position. To test the
homogeneity of the main magnetic field, a B0 field map was obtained
from a series of 3D multi-gradient echo phase images
9,10 (FOV=256×256mm
2,
matrix size =128×128, slice thickness 5mm, 32 slices, TR=50ms, 3 TEs of 6, 11, 16ms
and FA=25°).
Results
Figure 1 shows representative
gray matter (GM) CBF maps of the same volunteer at the three table positions acquired
on both magnets. On M1, CBF maps appear approximately equivalent and
consistently show the same results independent of the table position. M2 maps, however, demonstrate decreased CBF
values when the labeling block moves away from isocenter. Only at ISO-100, where
the labeling block is aligned with the isocenter, were CBF values comparable between
M1 and M2. The results of the two-way repeated
measure ANOVA (Figure 2) emphasize a difference in CBF quantification between
magnets dependent on table position (p<0.01). The individual magnet analysis
found that on M2, CBF values measured in ISO-100 are significantly different
from those in ISO-50 and ISO (p<0.001).
In
contrast, CBF values from M1 were reproducible independent of table position. Further,
we found a significant difference between M1 and M2 for CBF values measured at
ISO and ISO-50 (p<0.01); however, no difference was observed for CBF values
obtained at ISO-100 (p=0.349).
Discussion
Our results indicate that CBF values are not always reproducible. Great
variability may be introduced by the actual position of the labeling slice with
respect to the isocenter in our ultra-short wide bore M2 scanner. As shown in Figure
3, B0 field maps acquired at ISO and ISO-100 on M1 are virtually
identical while the ones acquired on M2 show visible differences. This could indicate
a less spatially homogenous field which decreases the labeling efficiency and results
in a lower CBF. ASL images are typically acquired in ISO position which, in
this study, produced the largest CBF bias on the short bore magnet (20% lower
than M1). Moving the tagging slice into isocenter (ISO-100), where the field is
most homogenous, resulted in the highest CBF values that are consistent between
M1 and M2. Our results indicate that the labeling position in ASL studies should
be fixed at isocenter to assure comparable CBF values for cohort studies or
longitudinal exams that involve short bore magnets.
Conclusion
We noticed systematic differences in CBF values between a long and short
bore magnet. The exact cause of this difference requires further investigation.
However, positioning the labeling slice in isocenter provided a simple solution
to overcome this issue and to measure reproducible CBF values on both magnets.
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