Synopsis
A non-contrast enhanced intracranial three-dimensional dynamic magnetic
resonance angiography (4D-MRA) based on pseudo-continuous arterial spin
labelling with the keyhole technique
(4D-PACK) was implemented. Images acquired from three volunteers were compared
with the data acquired without the keyhole technique. We show that the 4D-PACK
can accelerate acquisition speed, while keeping flow dynamics information.INTRODUCTION
Arterial
spin labelling (ASL) has shown its capacity for non-contrast enhanced intracranial
three-dimensional (3D) dynamic magnetic resonance angiography (4D-MRA)(1-6). The
challenge in an ASL-based 4D-MRA is to acquire multiple-phase data within a
clinically acceptable scan time, while keeping high spatial resolution,
sufficient anatomical coverage and a high flow signal. Pseudo-continuous
ASL-based 4D-MRA (4D-PCASL) can provide high flow signal, but scan time
prolongation is a problem to be solved. One solution has been proposed
recently, which is to combine the PCASL with a highly undersampled 3D radial
acquisition(3).
In this
study, we propose another acceleration approach, which is to combine the
4D-PCASL with contrast-enhanced timing-robust angiography (CENTRA) k-space sampling
techniques and the keyhole technique(7). We call it the 4D-PACK
(four-dimensional PCASL-based angiography using CENTRA Keyhole) from this point
on. Keyhole is an established technique for contrast-enhanced 4D-MRAs(7-9), but
has been hardly used for ASL-based non-contrast 4D-MRAs. We evaluated the
clinical feasibility of 4D-PACK by comparing it to 4D-PCASL.
MATERIALS AND METHODS
The
4D-PCASL scheme
The
4D-PCASL scheme is described in figure 1. Inflow dynamic data can be acquired
by changing the labelling duration. In this study, we used seven label
durations: 200ms, 400ms, 600ms, 800ms, 1000ms, 1200ms, and 2000ms. Each phase
session consists of label and control imaging. The shot interval varies
according to the labelling duration. The pre-saturation pulse is applied to the
imaging slab just before the labelling, which resets the effects of the
labelling applied in the previous shot so that only fresh spins that flow into
the imaging slab after the saturation pulse are visualised.
The
4D-PACK
The 4D-PACK
scheme is identical to the 4D-PCASL but CENTRA and the keyhole technique are
applied. Multiple reference points, in which high-frequency data are shared by
other time points’ data, are decided upon by regarding the tissue signal level.
Since the pre-saturation pulse is applied just before the labelling, the tissue
signal level depends solely on the labelling duration as the T1 recovery time
varies.
The overall
steps for deciding upon the reference points are described in figure 2. We set
thresholding in the signal ratio between the two images in which high-frequency
data are shared. In detail, the high-frequency data are copied to other data in
the cases in which the signal ratio between two images is less than 1.3;
otherwise new reference data are acquired. Here, the signal level at each time
point is simulated by the equation: Signal = 1-exp (-LD / T1wm),
where LD is labelling duration and T1wm is white matter T1 value
that is set to 1010ms. The signal ratio is then calculated by the simple
division of the signal level between two time points. As a result of this
calculation, the label duration of 1200ms is decided upon as a reference, and
high-frequency data are copied to the data in durations of 800ms, 1000ms, and
2000ms. For the data in the durations of 200ms, 400ms, and 600ms, full data
were acquired without any data sharing.
Subject, equipment and sequence parameters
The 4D-PACK
technique was implemented on a 3.0T scanner (Philips Ingenia R5). Three healthy
subjects were examined after obtaining informed consent as required by the institutional
review board. 4D-PCASL and 4D-PACK with 38% keyhole images were acquired and
compared. The common acquisition parameters were: acquisition sequence,
T1-turbo field echo; TR/TE, 4.9/1.76ms; flip angle, 11°; echo train length, 60;
3D slab thickness, 80mm; voxel size, 1.0*1.4*1.6 (50 partitions); parallel
imaging factor, 3.0.; and half scan factor, 0.8*0.8 (applied to slice and phase encoding
direction). The acquisition time was 8min for the 4D-PCASL, and 5min14sec for
the 4D-PACK.
Feasibility
assessment
In the image evaluation, the arterial transit time (ATT) was measured
for the 4D-PCASL and 4D-PACK at each segment of the middle cerebral artery
(MCA), M1, M2, M3, and M4. Here, the ATT was defined as the labelling duration
for the signal to reach more than half of its maximum value, as described in
figure 3(4)(10). We
then measured the correlation coefficient between the ATTs in the 4D-PCASL and
in the 4D-PACK, and assessed clinical feasibility.
RESULTS
The
representative dynamic data acquired by the 4D-PCASL and 4D-PACK are shown in
figure 4. For all three volunteers, the ATT correlation coefficient between the
4D-PCASL and 4D-PACK was consistently high at more than 0.95.
CONCLUSION
The
PCASL-based 4D-MRA can be accelerated by using the keyhole technique, while
retaining the inflow dynamic information. It indicates that the 4D-PACK is
feasible for 4D-MRA applications.
Acknowledgements
We
thank Tetsuo Ogino for valuable advices with the imaging sequence development.References
REFERENCES
1. Yu S, Yan L, Yao Y,
Wang S, Yang M, Wang B, Zhuo Y, Ai L, Miao X, Zhao J, Wang DJ. Noncontrast
dynamic mra in intracranial arteriovenous malformation (avm), comparison with
time of flight (tof) and digital subtraction angiography (dsa). Magn Reson
Imaging 2012;30:869.
2. Yan L, Wang S, Zhuo Y,
Wolf RL, Stiefel MF, An J, Ye Y, Zhang Q, Melhem ER, Wang DJ,. Unenhanced
dynamic mr angiography: high spatial and temporal resolution by using true
fisp-based spin tagging with alternating radiofrequency. Radiology
2010;256:270.
3. Wu H, Block WF, Turski
PA, Mistretta CA, Rusinak DJ, Wu Y, Johnson KM. Noncontrast dynamic 3d
intracranial mr angiography using pseudo-continuous arterial spin labeling
(pcasl) and accelerated 3d radial acquisition. J Magn Reson Imaging
2014;39:1320.
4. Robson PM, Dai W,
Shankaranarayanan A, Rofsky NM, Alsop DC,. Time-resolved vessel-selective
digital subtraction mr angiography of the cerebral vasculature with arterial
spin labeling. Radiology 2010;257:507.
5. Iryo Y, Hirai T,
Nakamura M, Inoue Y, Watanabe M, Ando Y, Azuma M, Nishimura S, Shigematsu Y,
Kitajima M, Yamashita Y. Collateral circulation via the circle of willis in
patients with carotid artery steno-occlusive disease: evaluation on 3-t 4d mra
using arterial spin labelling. Clin Radiol 2015;70:960.
6. Bi X, Weale P, Schmitt
P, Zuehlsdorff S, Jerecic R. Non-contrast-enhanced four-dimensional (4d)
intracranial mr angiography: a feasibility study. Magn Reson Med 2010;63:835.
7. Hadizadeh DR, Gieseke
J, Beck G, Geerts L, Kukuk GM, Boström A, Urbach H, Schild HH, Willinek WA.
View-sharing in keyhole imaging: partially compressed central k-space
acquisition in time-resolved mra at 3.0 t. Eur J Radiol 2011;80:400.
8. Soize S, Bouquigny F,
Kadziolka K, Portefaix C, Pierot L. Value of 4d mr angiography at 3t compared
with dsa for the follow-up of treated brain arteriovenous malformation. AJNR Am
J Neuroradiol 2014;35:1903.
9. Wu Q LM. A comparison
of 4d time-resolved mra with keyhole and 3d time-of-flight mra at 3.0 t for the
evaluation of cerebral aneurysms. BMC Neurol 2012;12:50:.
10.
Riederer SJ, Haider CR, Borisch EA. Time-of-arrival mapping at
three-dimensional time-resolved contrast-enhanced mr angiography. Radiology
2009;253:532.