Zihao Zhang1,2, Ning Wei1,2, Xiaofeng Deng3, Dehe Weng4, Jing An4, Yan Zhuo1, Xiaohong Joe Zhou5, and Rong Xue1,6
1State Key Laboratory of Brain and Cognitive Science, Beijing MR Center for Brain Research, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China, People's Republic of, 2Graduate School, University of Chinese Academy of Sciences, Beijing, China, People's Republic of, 3Department of Neurosurgery, Beijing Tiantan Hospital, Capital Medical University, Beijing, China, People's Republic of, 4Siemens Shenzhen Magnetic Resonance Ltd., Shenzhen, China, People's Republic of, 5Center for MR Research and Department of Radiology, University of Illinois at Chicago, Chicago, IL, United States, 6Beijing Institute for Brain Disorders, Beijing, China, People's Republic of
Synopsis
Time-of-Flight
MR Angiography (TOF-MRA) can benefit from better contrast and higher spatial resolution
using ultra-high field 7T MRI. In a segmented TOF technique at 7T, the Specific
Absorption Rate (SAR) of saturation pulses was reduced to enable the
suppression of venous blood signal. In this study, the TOF technique was successfully
used to discriminate between the superficial temporal artery (STA) and vein
(STV), and depict blood flow in
tiny vessels, facilitating future applications in pre-operative assessment for STA-MCA
bypass surgery.Purpose
TOF-MRA benefits substantially from increasing the static B0 field
strength from 3T to 7T, because of improved SNR and longer T1
relaxation.1 However, venous saturation (VS) band is typically omitted
at 7T to satisfy the SAR limitation. Segmented TOF2 is an efficient approach
to suppress venous signal with sparse saturation pulses. With this technique, the
flow signal from small arteries can be imaged at 7T without contamination from veins.
The purpose of this study is to apply segmented TOF with VS to imaging
STA while suppressing STV at 7T, and compare the results with 3T to demonstrate
the advantage for depicting small branches of vessels.
Methods
17 volunteers aged 18–27 without cerebral vascular diseases were
recruited. Each participant underwent MRI and whole-brain imaging of segmented
TOF with VS and conventional TOF without VS at 7T (Siemens Healthcare, Erlangen,
Germany) by using a prototype sequence as well as conventional TOF with VS at Prisma
3T (Siemens Healthcare, Erlangen, Germany). A Nova 32-channel phased-array head
coil was used at 7T, and a Siemens 64-channel phased-array head/neck coil at
3T. Extracranial ROIs were manually identified on all images. Maximum Intensity
Projection (MIP) was applied to extracranial TOF images on left and right sides,
respectively. The number of visible vessels in frontal and parietal branches of
STA at 7T and 3T were compared using a paired t-test.
In segmented TOF at 7T with VS, the number of segments was set to 5 to satisfy
the SAR limitation. Common parameters in segmented TOF and conventional TOF at
7T were pre-optimized as: TR=20ms, FA=18°, TE=5.1ms, voxel=0.34x0.34x0.45mm3,
GRAPPA=3, and the time of acquisition (TA)=10:28min. The protocol of
conventional TOF at 3T was optimized to achieve the maximal SNR and resolution:
TR=21ms, FA=18°, TE=3.5ms, voxel= 0.50x0.50x0.50mm3, GRAPPA=2,
and TA=10:22min.
Results
Fig. 1 showed a typical set of unilateral MIP images of extracranial vessels.
The overlap of STA and STV signal was found unilaterally (on left or right
side) in 6 subjects (35%), and bilaterally in 4 subjects (24%).
In Fig. 1, more tiny branches were visible in 7T images than in 3T
images (denoted by yellow arrows). The averaged numbers of vessels in STA were illustrated
in Fig. 2. The numbers of vessels in 7T images were significantly higher in parietal
branches on both left (p<0.01) and right (p<0.001) sides.
Discussion
In
this study, the overlap of STA and STV was observed in 59% of the subjects. This
indicated the necessity of VS for 7T TOF-MRA when STA is concerned in STA-MCA bypass
surgery. Similar to the intracranial TOF-MRA2, segmented TOF at 7T
MRI has an advantage on imaging flow signal of tiny extracranial vessels.
The
superiority of 7T did not reach statistical significance in frontal branches on
either side. The reason for that might be the limited coverage on the frontal
side of the coil in 7T MRI. On the other hand, there were considerable individual
differences of blood flow and visible vessels between subjects (as shown in
Fig. 1). With an increased sample size, this may reach a statistical
significance.
The
depiction of STA and its branches is important for pre-operative planning of STA-MCA
bypass surgery. Presently, the gold
standard for STA imaging is digital subtraction angiography (DSA), which is an
invasive technique and requires local anesthesia. 3T MRA is noninvasive, but it
may not provide adequate information on STA, limiting its use. Segmented TOF at
7T MRI is a promising technique as it depicts STA and its branches much more
clearly compared to 3T MRA. Segmented TOF at 7T MRI can be a strong competitor
to DSA for patients with planned STA-MCA bypass surgery.
Conclusion
We demonstrated for the first time that segmented TOF with venous
saturation is a valuable technique on imaging tiny branches of STA without the
interference of STV at 7T. This sequence is expected to be valuable for pre-operative planning of STA-MCA
bypass surgery.
Acknowledgements
The work is supported in part by Chinese
MOST grant (2012CB825500), CAS grants (XDB02010001 and XDB02050001).References
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