CY Wang1, R Zhang2, L Jiang3, R Wang4, XD Zhang4, H Wang3, K Zhao4, LX Jin3, J Zhang1,2, XY Wang1,4, and J Fang1,2
1Academy for Advanced Interdisciplinary Studies, Peking University, Beijing, China, People's Republic of, 2College of Engineering, Peking University, Beijing, China, People's Republic of, 3Philips Healthcare, Suzhou, China, People's Republic of, 4Department of Radiology, Peking University First Hospital, Beijing, China, People's Republic of
Synopsis
This study demonstrates the feasibility of combining 2D-RF
excitation pulse and ASE sequence (focused ASE sequence, FASE) for single renal
OEF measurement. Comparison between
images acquired with full-FOV ASE and focused ASE was conducted to confirm the
advantages of the focused ASE sequence for single renal imaging.
The new technique could reduce artifacts and
distortion caused by susceptibility differences, and limit spatial blurring due
to T2-decay, which is promising for diagnosis of some renal diseases.Background:
Evaluation
of renal oxygenation plays an important role in the diagnosis of acute renal
ischemia (1) and chronic kidney disease (2). Recently, the asymmetric spin echo
(ASE) EPI technique (3) has been widely used to probe signal alterations induced
by susceptibility effects, which could provide information about renal oxygen
extraction fraction (OEF). However, the single-shot EPI acquisition applied in
the ASE sequence suffers from the problem of severe susceptibility artifacts and distortion due to the relatively
long echo train length (ETL) in renal imaging, even with the advances in
parallel imaging. One solution is to reduce the field of view (FOV) of the single-shot
EPI with a special designed 2D-RF excitation pulse (4). With the introduction of the 2D-RF
pulse, much higher spatial resolution could be achieved for single renal imaging.
Purpose:
This study
demonstrates the feasibility of combining 2D-RF excitation pulse and ASE sequence
(focused ASE sequence, FASE) for single renal OEF measurement. Comparison between images acquired with full-FOV
ASE and focused ASE was conducted to confirm the advantages of the focused ASE
sequence for single renal imaging.
Materials and Methods:
Focused Asymmetric Spin-echo Sequence:
A single-shot triple-echo ASE sequence with 32 varied echo
shifts was
implemented to acquire the source images for renal OEF
quantification. The sequence diagram is shown in Fig.1. The ASE sequence is a
modification of the spin echo EPI sequence in which the 1800
refocusing pulse has a time offset of τ from TE/2. Furthermore, a 2D-RF excitation
pulse was applied to achieve focused renal imaging. The duration of the 2D-RF pulse
is typically between 10 ms to 18 ms depending on the FOV in phase encoding
direction.
Phantom Studies:
To
demonstrate the efficacy of this FASE technique, we performed a study using a
quality control phantom. MRI
measurements were carried out on a 3.0 Tesla MR scanner (Achieva, Philips
Medical Systems, Best, Netherlands) with gradient strength = 80 mT/m and
gradient slew rate = 200 T/m/s. A 32-channel
Cardiac coil was used for signal reception. The FOV for full-FOV acquisition and FASE acquisition were 340 ×
220 mm2 and 200 × 220 mm2, respectively. Other parameters
were: TR = 2000 ms, TE1/TE2/TE3 = 65/93/121
ms, slice thickness = 5 mm, SENSE factor = 2.
Volunteer Studies:
Approved by the local institutional
human study committee, ten healthy volunteers (mean
age 24.5 ± 3 years) participated in this study. Renal images
were acquired with identical scanning parameters as phantom studies. To reduce the impact of the intravascular
signal, a pair of small flow dephasing gradients (b = 40 s/mm2)
was applied. Respiratory triggering was used to reduce respiratory motion artifacts.
The total acquisition time was approximately 3 minutes.
To assess the baseline scan-rescan reproducibilities
of the the proposed method, both FASE and full-FOV ASE sequences were repeated
in a 30 min interval.
Quantitative Analysis:
The measurement of renal OEF was
derived
from a theoretical model proposed by Yablonskiy and Haacke (5). Two experienced
radiologists (with 7 years and 6 years of experience in MRI) scored the images obtaied
from two sequences (τ = 0
ms), blinded to the particular acquisition strategy. Images were scored based
on both the artifact level and image sharpness using a 5-point scoring system,
with 5 as excellent, 4 as good, 3 as fair, 2 as acceptable, and 1 as poor. A
paired Student’s t-test was used to
compare the results calculated from full-FOV ASE and FASE images with
significance level of 0.05.
Results:
Full-FOV and FASE images (τ = 0 ms)
of the phantom are shown in Fig.2. Less image
distortion was observed in the focused ASE image. And the focused ASE sequence
effectively reduced blurring in the PE direction compared to full-FOV ASE.
Similar results were found in renal imaging (Fig.3). FASE based original images
(τ = 0 ms)
and calculated parametrical maps showed significantly better
image quality with sufficient depiction of the cortico-medullary structure of
the kidneys (Table 1). The baseline scan-rescan CVs of renal OEF measurement were
6.65% in the cortex and 8.13% in the medulla.
Conclusions:
Based
on phantom and volunteer studies, we demonstrated the
feasibility of combining 2D-RF excitation pulse with ASE sequence for single
renal OEF measurement. The new technique could reduce artifacts and distortion
caused by susceptibility differences, and limit spatial blurring due to
T2-decay, which is promising for diagnosis of some renal diseases.
Acknowledgements
No acknowledgement found.References
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Yablonskiy DA, Haacke EM. Magn Reson Med 1994;32:749-763.