Ícaro A F de Oliveira1,2, Thomas Roos1, Serge O Dumoulin1,2,3, and Wietske van der Zwaag1
1Spinoza Centre for Neuroimaging, Amsterdam, Netherlands, 2Experimental and Applied Psychology, VU University, Amsterdam, Netherlands, 3Experimental Psychology, Helmholtz Institute, Utrecht University, Utrecht, Netherlands
Synopsis
We
compared MPRAGE and MP2RAGE at Ultra-High Field (UHF) in terms of
signal separability in gray and white matter. Using a k-space shutter, kt-point
universal pulses for signal excitation, and an efficient TR-FOCI for signal
inversion, we obtained very good signal contrast throughout the brain,
including the cerebellum, for the MPRAGE. Nevertheless, gray-white matter
contrast was larger in the MP2RAGE data, leading to better segmentation
results, especially in areas affected by low B1+. Hence,
MP2RAGE appears more suitable for 7T T1-weighted anatomical data,
despite the longer acquisition times.
Introduction
High
quality anatomical T1-weighted images are important for several MRI
applications, for example, volumetric studies, gray-matter segmentation, and
anatomical reference in fMRI[1]. The MPRAGE family (MPRAGE and
MP2RAGE) [2,3] are the most commonly used sequences at 7T.
Both
MPRAGE and MP2RAGE sequences rely on a uniform RF transmit field (B1+
) during the magnetization preparation, and nearly always use an adiabatic inversion
pulse. One of the ultra-high field (UHF) challenges is that the field
uniformity is often compromised, to the extent that the inversion efficiency of
an adiabatic pulse may also be affected, especially in the cerebellum and
temporal lobes [4]. To achieve more homogeneous contrast, it is important to
ensure an effective inversion independent of B1+
inhomogeneities. This can be achieved using either better adiabatic pulses like
the TR-FOCI pulse [5] or special pulse design for both the inversion and
excitation pulses on parallel transmission (Multix) systems. The required
prescans can be avoided by using Universal Pulses (Ups) [6].
To
acquire the best possible MPRAGE on our 7T, we acquired T1-weighted images with
(1) a k-space shutter to reduce scantime, (2) Universal kt-point pulses to
homogenize signal excitation and (3) a TR-FOCI to uniformly invert the signal.
These data were compared to MP2RAGE data acquired with a standard HS8 inversion
pulse or a TR-FOCI. Methods
Three
healthy volunteers (age 23-40 years, 2 females) participated in
the study. Imaging was performed on a 7T scanner
Philips System using a 32-channel head coil for reception (Nova Medical) and
8-channel for transmission (32Rx8Tx). MPRAGEs and MP2RAGEs were acquired in the
same session. The common parameters for both techniques are; FOV =
230×230×186mm3, 0.8 mm isotropic voxels, 2D SENSE= 1.8×1.8, and
slice oversampling of 20%. For MP2RAGE, Bloch simulations were used to optimize
the inversion time, TRvolume and flip angles [3]. Specific
parameters are given in Table 1.
For MPRAGE1, was the
‘standard’ MPRAGE, with an HS8 inversion, normal excitation, and a fully
sampled matrix. For MPRAGE2 we added a vendor-supplied k-space shutter, reducing
scantime. A similar number of lines per GRE readout block was used (164) as for
the MPRAGE (159), ensuring similar T1-relaxation during the
readouts. For MPRAGE3, we additionally replaced the standard excitation pulses
with UPs, and finally for MPRAGE4
we added the TR-FOCI pulse. For the MP2RAGEs we changed only the inversion
pulse from HS8 to TR-FOCI. All
data were segmented using SPM12.
Results
Figure
2a and b show the sampling pattern in k-space of the k-space shutter and the
resulting point spread function (PSF). PSF changes are seen on the cardinal
axes. To observe the effect in the images acquired with k-space shutter, a line
on the LR axis through the ventricles was plotted for both MPRAGE1 and MPRAGE2.
The observed profiles (Fig 2c-e) show minimal changes, the most noticeable is
the signal intensity reduction related to the reduced scantime.
Figure
3 shows an example slice from all sequences, for all subjects. The contrast in
the MPRAGE3 and MPRAGE4 improved substantially in the low-B1 region
in the cerebellum. The red contours in Figure 4 indicates the gray matter
boundaries of the segmentation. For MPRAGE1 and MPRAGE2 the segmentation fails
in some regions and is generally more correct for MP2RAGE than for MPRAGE data.
The rightmost column shows a zoomed-in area in the cerebellum with all gray
matter boundary segmentations overlaid.
Figure 5 shows
the signal distributions in MPRAGE and MP2RAGE acquisitions for both gray (~100
and ~1300) and white (~200 and ~3000) matter. Better separability of the gray
matter and white matter is confirmed by an ROC curve analysis (panel c and d). Discussion & Conclusion
In
this comparison, the MP2RAGE acquisitions were matched to the MPRAGE in terms
of resolution, FOV, and lines of k-space points per readout block and, hence,
also for the number of readouts/inversion pulses. Consequently, the total
acquisition time differed significantly as the MP2RAGE has to accommodate an
extra readout within the TRMP(2)RAGE.
The
use of a k-space shutter to reduce scantime did not significantly affect the
MPRAGE, no difference in blurring at the high contrast edge of the ventricles
was found for MPRAGE1 and MPRAGE2. The addition of universal kt-points pulses
and a TR-FOCI inversion increased SNR and contrast in the region of the
cerebellum, particularly affected by low B1+. Nevertheless, the whole-brain
signal intensity distributions of gray and white matter did not become better
separable (Figure 5).
Therefore,
we conclude that, at 7T, the image quality of an MP2RAGE is higher than in a
comparable MPRAGE, even if the latter has state-of-the-art excitation and
inversion pulses to combat
signal inhomogeneity. Use of the MP2RAGE merits the investment in additional scantime. Acknowledgements
No acknowledgement found.References
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