H. Douglas Morris1 and J. Andrew Derbyshire2
1NIH Mouse Imaging Facility, National Institutes of Health, Bethesda, MD, United States, 2Functional MRI Facility, National Institutes of Health, Bethesda, MD, United States
Synopsis
Rapid
imaging of endogenously labeled neuroprogenitor cells in the rat brain is shown
using high-field MRI and efficient Steady-State Free Precession sequences. A RF-phase cycle progression based on the
Golden Angle is used to produce Linear Combination SSFP (LCSSFP)
images without banding artifacts. The method yields high resolution images with
few global distortions suitable for cell tracking and calculating relaxation
images.Purpose
MRI
is an excellent tool for cell tracking via magnetic micro-particles. It is possible to track individual single
cells via magnetic particles greater than 1 micron diameter using high
resolution MR microscopy. This technique
relies on the magnetic field distortion of the particle to produce a local artifact
in the image many times larger that the particle itself.[1] Consequently, cells can be tracked in tissue
in 3D and
in vivo.
In vivo cell labeling techniques are possible
using i
n situ label reservoirs.[2] High
concentrations of the magnetic label in the sub-ventricular zone will label
neuroprogenitor cells yet generate global magnetic field distortions in
high-SNR efficiency techniques such as steady-state free precession (SSFP).[3]
The field artifacts can be suppressed by a weighted linear combination
of SSFP images with RF cycling
(LCSSFP).[4] Quantitative relaxation
imaging of labeled tissues is possible using a LCSSP corrected version of the
DESPOT method. [5] We describe a technique based on the Golden Angle to parse
the offset bandwidth without a fixed set of acquisitions as applied to the
LCSSFP technique. Using the Golden angle
increment in the RF phase, the banding artifacts do not appear in the same
position twice no matter how long the cycle is projected forward. This is analogous to the use of Golden Angle
selected projections in back-projection MRI.[6]
We demonstrate the utility of this method for rapidly obtaining MR DESPOT
images of a rat brain labeled with micron-sized magnetic particles.
Methods
MR images were
acquired on a Bruker Avance III 14.1T NMR spectrometer with microimaging
apparatus (Bruker Biospin, Inc., Billerica MA, USA) using a standard Avance RF III
console, Micro-5 gradients (G=1.5 T/m, SR=2500 T/m/s) and 25 mm ID SAW RF
coil. The 3D SSFP imaging parameters are
TR/TE=2.0/1.0ms, pulse angle 20° or 60°, 1 NEX, FOV 30x18x13 mm, 512x340x168
for a resolution of 50x50x75 µm, with a total acquisition time of 1minute 45
seconds per image. The LCSSFP images
were formed by the complex summation of the individual images with a minimum of
3 phase precession sequences. The
minimum time required for each LCSSFP image is 3 times that of a
single SSFP image. Six-week-old Sprague-Dawley rats (Charles River
Laboratories, Inc., Wilmington, MA) were stereotactically injected with 1.4x10
8
MPIOs (Bangs Laboratories, Inc., Fishers, IN).
Two-weeks post injection, animals were transcardially perfused with PBS
followed by 10% buffered formalin solution.
Intact brains were removed and stored in PBS.
Results
Figure 1 shows
representative saggital and transverse slices for a conventional SSFP (A) and
LCSSFP (B) sequences through the midplane of a normal rat brain. Figure 1A images demonstrates the banding artifact observed in SSFP images where the high
concentration of magnetic particles in the ventricles produce local field
gradients and move the resonant frequency out of the passband for the pulse
sequence. Figure 1B images are formed of three
separate SSFP images with different RF pulse phase progression cycles: (0-0-0-0),
(0-137.51-275.02-52.53), and (0-275.02-190.04-105.06). The
acquired images are squared, summed, and scaled. The banding artifacts are greatly reduced in the brain tissue away from the large distortions of the high concentration of magnetic
particles in the ventricles and produces a high quality image. DESPOT1 or DESPOT2 analysis is performed on
the resultant two excitation angle images. Figure
1C is a R1 map calculated image. Total acquisition time is approximately
10:30s.
Conclusion
High-resolution (50x50x75
µm) images of a rat brain injected with iron oxide particles were acquired
using SSFP sequence at 14T under 2 minutes. Field artifacts were successfully
suppressed using golden angle LCSSFP with 3 phase cycles, with a 3-fold
increase in acquisition time. SNR and artifact suppression can be enhanced by
incremented the phase cycles and acquiring the additional images independently
due to the Golden Angle (GA) progression. The sliding window nature of the GA can
be exploited to provide rapid MR microscopy for investigating the position of
magnetic particles in the rodent brain even with local field distortions. The resultant images use DEPOT1/2 relaxation analysis
to create R1/R2 maps in addition to cell tracking images.
Acknowledgements
We acknowledge the support of NIH, NINDS, NIMH, and the NIH Mouse Imaging Facility and their staff.References
1) Shapiro et al. MRI detection of single particles for cellular imaging.
Proc Natl Acad Sci USA 101(30)
10901-6 (2004).
2) Sumner et al. In vivo labeling of adult neural progenitors for
MRI with micron sized particles of iron oxide: Quantitation of labeled cell
phenotype. Neuroimage 44(3) 671-8
(2009)
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eine neue schenell Pulssequenxae fur die Kernspintomographie., Electromedica
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