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 the
presented study, selective Arterial Spin Labeling and Phase encoded information are obtained in a single acquisition, thereby allowing
for artery-selective velocity and flow imaging. From the data, several image
contrasts can be derived. First, anatomical images are available. From these, static
artery-selective angiograms are obtained, which are processed as binary image
masks and applied to the phase encoded images. Thereby, vessel-selective Phase
encoded angiograms are created. Furthermore, it is possible to calculate
venous-only images by subtraction of the arterial information derived from the
ASL measurements from the phase-encoded images.Introduction
Arterial Spin Labeling (ASL) can be used for
morphological imaging of the arterial architecture. To perform artery-selective
imaging, the label process has to be modified so that only a single artery of
interest is tagged, e.g. using super-selective ASL [1]. The feasibility of
super-selective ASL for static and time-resolved angiographic acquisitions was
already presented [2, 3]. However, the measurement of blood flow velocity and
direction with ASL-based methods is limited with respect to spatial and time
resolutions. Phase-contrast angiography measurements provide higher precision
regarding the quantification of hemodynamic blood flow parameters and can add
important information for a differential diagnosis [4]. In this study, we present
an approach based on ASL that allows selectively visualizing individual
arteries of the brain in conjunction with phase-encoded information in order to
derive detailed information about blood flow direction and velocity, which may
be important for the evaluation of e.g. aneurysms, arterio-venous malformations
and stenotic arteries [4].
Materials and Methods
Four healthy volunteers and a patient suffering from an
aneurysm in the bifurcation of the right arteria cerebri media underwent MR
scanning under the general protocol for sequence development, approved by the
local ethical committee. Imaging was performed on a Philips 3T Achieva
(Philips, Best, The Netherlands) scanner using a standard 32 channel SENSE Head
coil. For tagging, a labeling duration of 1000ms was used in order to ensure
sufficient filling of all intracranial arteries [5]. Image acquisition started
immediately after labeling using a 3D T1-TFE readout with 0.7mm³ isotropic
voxel size and 120 slices. Phase-encoding was performed in right-left,
anterior-posterior and feet-head direction. The gradient strengths were chosen
to resolve flow velocities of up to 100 cm/s. The acquisition time for a scan
of a single artery was 5 min. 49sec. After acquisition of the images, further
processing has to be performed. A flow-chart how to perform processing is shown
in Fig. 1. First, the magnitude images (label and control) are subtracted to obtain
the selective angiograms of the arteries (Fig. 1, left column). From these
images, a binary mask is created by means of signal treshholding. Subsequently,
the mask is applied to the phase-encoded images representing flow velocities in
different directions (Fig. 1, right column). Merging and color-encoding of the
masked images results in selective phase-contrast angiograms (Fig. 1, 2a). Additionally,
the individual arteries are merged into a single frame, each presenting a
single color (Fig. 2b). The signal intensity is used to represent different
flow velocities. All calculations were performed using Matlab R2013a (The
Mathworks, Natick, MA).
Results and Discussion
Image acquisition
was successfully performed in all volunteers and in the patient. Multiple image
contrasts were obtained. For instance, the unsubtracted ASL magnitude images
might be useful for the evaluation of brain structure due to the high isotropic
spatial resolution (Fig. 1). By processing the data, selective phase-encoded
angiograms could be obtained (Fig. 2). The high SNR of the subtracted ASL
magnitude images allowed creating a binary mask by simple treshholding of the
signal intensity. The final post-processed images show high resolution static angiograms
of an individual selected artery in conjunction with directional and velocity blood
flow information. In the case of the patient scan, the aneurysm could be visualized
as projection angiogram without unwanted superposition of contralateral
arteries similar to images generated with digital subtraction angiography (Fig.
3). In addition, hemodynamic parameters were quantified which may be important
for an advanced diagnosis. Usually, such information cannot be obtained from a single
MRI measurement. At least two acquisitions are required, a selective ASL acquisition
and a phase-contrast angiography have to be performed individually, which can prolong
scan times significantly and make the measurements prone to subject motion in
between the acquisitions. Furthermore, it is also possible to obtain a
venous-only image from the phase-encoded images (not shown here). For this
purpose, a mask of the arterial vessels can be applied to the phase-encoded
images, leaving only the signal from the veins in the final images.
Conclusion
The presented
method makes it possible to gather artery-selective information in conjunction
with important hemodynamic parameters like blood flow direction and velocity in
the cerebrovascular system within a single acquisition. Further studies in
patients suffering from vascular diseases are required to prove the applicability
of this method in a clinical setting.
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] Jensen-Kondering
U et. al. Eur J Radiol.
2015;84:1758-67
[3] Lindner
T et. al. Magn Reson Imaging 2015;33:840-6
[4] MacDonald E and
Frayne R. J NMR in Biomedicine 2015;7:767-91
[5] Robson PM et.
al. Radiology 2010;257:507-15