Thomas Lindner1, Naomi Larsen1, Olav Jansen1, and Michael Helle2
1Clinic for Radiology and Neuroradiology, University Hospital Schleswig-Holstein, Kiel, Germany, 2Philips GmbH Innovative Technologies, Research Laboratories, Hamburg, Germany
Synopsis
In this
study, different approaches for obtaining selective Arterial Spin Labeling
(ASL) angiograms are presented. Conventionally, the label image of each artery
has to be matched with a separate control image. In this study, the number of
control acquisitions is reduced, thus, making it possible to reduce acquisition
times considerably. In one approach, a shared control condition is used for
three selectively labeled arteries to obtain the final images (“cycled
super-selective ASL”). This means that only one control image is used for
subsequent data processing with three images of different arteries. In the
second approach, no control image is required at all and the angiography
information can be obtained from the label images only (“self-control ASL”). Image
quality appeared similar in all approaches. Compared to super-selective ASL,
image acquisition times are reduced in the presented approaches.Introduction
Arterial Spin Labeling (ASL) techniques, commonly used
for cerebral perfusion imaging, can also be used for the visualization of
intracranial arteries. These techniques have been modified in recent years to
perform selective imaging of individual brain feeding arteries, which are
mostly the internal carotid arteries (ICA) as well as the vertebral arteries (VAs).
In general, ASL relies on two acquisitions, namely a “label” (inversion) and a
“control” (without inversion) image, which are subsequently subtracted to remove
the static background tissue. Usually, multiple corresponding label and control
images are required, which can prolong the scan time and make ASL prone to motion
artifacts. In this study, super-selective pCASL was used to tag individual
arteries of interest [1]. In a first approach, only one non-selective control
acquisition is performed to be shared by three label images from the ICAs and
the VAs (“cycled super-selective ASL”). Similar approaches for perfusion
territory imaging with pulsed ASL methods have already been proposed [2]. In
addition, in this study, a new approach is explored taking advantage of the
fact that the blood spin magnetization of contralateral (non-tagged) arteries is
in a different (pseudo-randomized) state in superselective ASL. We hypothesize that
this pseudo-randomized magnetization state can be used as an intrinsic control
condition for subsequent image processing without the need of acquiring a
control image (“self-control ASL”).
Materials and Methods
Thirteen healthy volunteers (8 females, mean age 28.2 years)
underwent MR scanning under the general protocol for sequence development,
approved by the local ethics committee. Imaging was performed on a Philips 3T
Achieva (Philips, Best, The Netherlands) scanner using a standard 32 channel
SENSE Head coil. Super-selective ASL parameters: were: 400ms labeling and 50ms
labeling delay. Image acquisition consisted of a 3D T1-TFE readout with 10°
Flip Angle, 0.9*0.9*0.9mm³ voxel size, 120 slices and 6 acquisition time-points
with a temporal resolution of 150ms after labeling. The calculations of the
final angiographic images were performed using Matlab R2013a (The Mathworks,
Natick, MA). Selective labeling was achieved using extra gradient moments in
the Gx and Gy direction of 1.08 mT/m [1]. To selectively label the arteries, a
low-resolution time-of-flight scan was performed covering the neck. The
location of the labeling focus was planned manually. In cycled super-selective
ASL and self-control ASL, the labeling focus positions for each of the labeled
arteries were entered before the scan. During scanning the individual labeling
focus positions for each vessel were cycled periodically so that all data
acquisition was performed in a single measurement. Total acquisition times were
5:08 min for non-selective ASL, 15:24 min for super-selective ASL (three
individual acquisitions of 5:08min), 10:17 min for cycled super-selective ASL with
only a single non-selective control condition, and 7:43 min for self-control
ASL, for which no control condition was acquired at all. The resulting images
were compared with non-selective ASL and conventional super-selective ASL in
terms of signal-to-noise ratio (SNR) measured at specific locations in each
artery.
Results and Discussion
Image
acquisition was successfully performed in all volunteers (Fig. 1). All measurements
present similar SNR compared with non-selective ASL angiography (not shown). In
super-selective ASL, the additionally employed gradients perpendicular to the blood
flow direction cause the magnetization in the label and control experiments to
oscillate in antiphase, resulting in fluctuations of the labeling efficiency
outside the labeling focus [1]. The magnetization in a non-selective control
experiment does not perfectly correspond to the super-selective label
experiment, thus, might result in decreased image quality. However, the
acquisition of a single non-selective control image (without additional
transversal gradients) appears sufficient to process super-selectively labeled
images of different arteries. Furthermore, it was also possible to only use the
label images for the generation of angiograms as the magnetization in
contralateral arteries can be used as intrinsic control condition. Compared with
non-selective ASL acquisitions, some selective ASL methods increase the total
imaging time with respect to the number of tagged arteries. This can also
increase the appearance of potential artifacts, e.g. when a patient moves in-between
different scans or during the image acquisitions. For super-selective ASL, the
presented approaches demonstrate a promising alternative for scan time
reduction without the loss of image quality, especially, when the aim is to
label several small intracranial arteries.
Conclusion
Super-selective ASL can be performed using a shared
control condition as well as without acquiring a control image and without
notable loss of SNR. This can accelerate the total image acquisition time compared
with conventional super-selective ASL.
Acknowledgements
This work was supported by funding of the German Research Foundation (DFG), grant number JA 875/4-1.References
[1] Helle M et. al. Magn Reson Med 2010;64:777-86
[2] Günther M. Magn
Reson Med 2006;56:671-5