Robert R. Edelman1,2, Shivraman Giri3, and Ioannis Koktzoglou4,5
1Radiology, NorthShore University HealthSystem, Evanston, IL, United States, 2Radiology, Northwestern University, Chicago, IL, United States, 3Siemens HealthCare, 4Radiology, NorthShore University HealthSystem, 5Radiology, University of Chicago
Synopsis
QISS is a robust method for
nonenhanced MRA that conventionally uses a TrueFISP readout. At 3 Tesla, use of TrueFISP is challenging
due to high RF power deposition and sensitivity to off-resonance artifacts. In order to overcome these limitations, we
implemented a QISS technique that incorporates a new type of readout that is a hybrid
of TrueFISP and FLASH pulse sequences, which we call “Fast Interrupted Steady-State”
(FISS). A pilot study was conducted to
evaluate the potential benefits of QISS FISS at 3 Tesla for evaluation of the
peripheral arteries and great vessels of the chest.
Purpose:
The QISS (quiescent-interval
slice-selective) technique is a robust approach for nonenhanced MR angiography
of the peripheral arteries (1), which also shows promise for evaluation of
other vascular territories such as the great vessels of the chest and
extracranial carotid arteries. Compared
with 1.5 Tesla, QISS MRA at 3 Tesla benefits from an improved signal-to-noise
ratio (SNR) and vessel-to-background contrast.
However, QISS conventionally uses a balanced steady-state
free-precession or TrueFISP readout, which poses significant challenges when
applied at high field. For optimal
results, the TrueFISP readout requires a large flip angle and short echo
spacing, which at 3 Tesla may necessitate a large reduction in flip angle due
to SAR limitations. At high field, TrueFISP
images can be degraded by off-resonance and out-of-slice flow artifacts (e.g.
banding and hot spots), which are often severe in the chest region. One option is to use a FLASH readout, which is
less sensitive to off-resonance effects than TrueFISP and is not SAR-limited. However, a FLASH readout has other drawbacks
including inferior SNR and saturation of slowly moving arterial spins.
In order to overcome these challenges,
we developed a novel pulse sequence which we call “FISS" (Fast Interrupted Steady-State). The FISS pulse sequence is essentially a
hybrid of TrueFISP and FLASH pulse sequences that maintains the benefits of a
TrueFISP readout while avoiding its limitations. In order to determine its potential benefits
and limitations, we conducted a pilot study of QISS using a FISS readout for
evaluation of the peripheral arteries and great vessels of the chest at 3 Tesla,
and compared it with QISS using TrueFISP and FLASH readouts.Methods:
The QISS FISS pulse sequence
consists of an initial in-plane saturation or inversion RF pulse, optional
tracking saturation, quiescent interval, followed by the FISS
readout. The sequence is ECG-gated such
that the quiescent interval spans systole while the readout is timed to occur
during slow diastolic flow. The FISS
readout consists of repeated modules of the form:
[+alpha/2,(-alpha,read,+alpha)n,-alpha/2,spoil],
in which the imaging gradients are balanced aside from the final spoiler. The RF pulses applied after the readout
function to tip the magnetization vector back along the longitudinal axis,
thereby reducing saturation effects akin to a TrueFISP readout. For this study, n was set to a value of 1 with echo spacing of ~9-13 ms. This IRB-approved study was conducted on a 3
Tesla scanner (MAGNETOM Skyra-fit, Siemens Healthcare, Erlangen, Germany). A prototype QISS FISS sequence was compared
with QISS TrueFISP and QISS FLASH. All
sequences used a radial k-space trajectory.
Typical slice thickness was 3-mm with in-plane resolution on the order
of 1-mm. For peripheral MRA, 40 slices
were acquired at each table station. For
chest MRA, ~5-10 slices were acquired per breath-hold. Excitation flip angles ranged from 15 to 90
degrees. Because the echo spacing for
both QISS FISS and QISS FLASH was longer than for a TrueFISP readout, 2 or 3
shots were typically acquired. Results:
QISS FISS showed markedly
improved image quality, SNR and vessel-to-background contrast compared with a
FLASH readout, plus a substantial reduction in image artifacts compared with a TrueFISP
readout (Figure 1). Image quality for QISS FISS was much better
than QISS FLASH for larger flip angles.
For instance, using a flip angle of 89 degrees, QISS FISS provided
excellent demonstration of through-plane arterial flow, whereas QISS FLASH
showed severe image degradation and near complete suppression of arterial
signal. Compared with QISS TrueFISP,
QISS FISS showed more uniform vascular and background signal as well as less
severe flow and off-resonance artifacts.
The image quality of the QISS FISS technique was maintained through the
entire image stack (e.g. 40 slices for peripheral vascular imaging), indicating
that the method is stable away from the magnet isocenter. Discussion and Conclusion:
QISS FISS is a new imaging
technique that provides major benefits for nonenhanced MRA at 3 Tesla. It largely avoids the image artifacts and SAR
limitations that are encountered with a TrueFISP readout, while improving image
quality and arterial enhancement compared with a FLASH readout. There are no steady-state signal contributions
from vascular spins that have moved out of the slice, which avoids a common
source of flow artifacts with TrueFISP (2).
Initial results in the peripheral arteries and chest vessels appear
promising. Further work will be directed
towards sequence optimization and clinical validation. We also plan to explore the use of the FISS technique
for cine imaging of the heart at 3 Tesla, where it has the potential to greatly
improve image quality compared with cine TrueFISP.Acknowledgements
Study
was funded by NIH grants R01 HL130093 and R21 HL126015.References
References. 1. Edelman et al. Magn Reson Med 2010; 63: 951. 2. Markl
M, et al. Magn Reson Med. 2003;50:892.