Elwin Bassett1, Ganesh Adluru2, Brent D. Wilson3, Cory Nitzel3, Tobias Block4, Hassan Haji-Valizadeh5, Edward VR DiBella2, and Daniel Kim2
1Physics, University of Utah, Salt Lake City, UT, United States, 2Radiology, UCAIR, University of Utah, Salt Lake City, UT, United States, 3Internal Medicine, Division of Cardiology, University of Utah, Salt Lake City, UT, United States, 4School of Medicine, Radiology, New York University, New York, NY, United States, 5Bioengineering, University of Utah, Salt Lake City, UT, United States
Synopsis
To date, no study has compared 12-fold
accelerated real-time cine MRI with compressed sensing (CS) between Cartesian
and radial k-space sampling schemes. We sought to compare their performance in
patients and volunteers. We compared point spread functions (PSF) to determine
which sampling pattern generates more incoherent aliasing artifacts. We also
compared their performance in a group of 15 patients and one volunteer, where
3-fold accelerated product real-time MRI was used as reference. Two
cardiologists independently graded images from each subject.
PSF analysis showed that radial produces
more incoherent aliasing artifacts. Image quality was better for radial than
Cartesian sampling schemes.
Introduction
ECG gated breath-hold cine MRI with b-SSFP (1,2) readout is considered the gold standard
test for assessment of cardiac function. However, it has two noted limitations:
(i) breath-holding requirement and (ii) sensitivity to arrhythmia. Real-time cine
MRI is a solution to these challenges, but commercially available methods are
still limited by poor data acquisition efficiency. To date, a variety of
acceleration methods have been proposed, including dynamic parallel imaging (3,4), k-t parallel imaging methods such as
BLAST/SENSE (5), and compressed
sensing (CS) (6-9), with each method having advantages and
disadvantages.
In this study, we explore CS as a vehicle to
accelerate real-time cine MRI and compare its performance between Cartesian and
radial k-space sampling schemes. Cartesian acquisition has several advantages
such as a reduced sensitivity to gradient imperfections and capability to use an
asymmetric field of view (FOV) adopted to the human chest. Radial has its own
advantages, including better motion properties (10), capability to retrospectively
determine temporal resolution when using non-repeated sampling schemes such as golden
angle ratio (11), and higher signal
to noise ratio (SNR) due to higher sampling of the center of k-space. To date,
no study has systematically compared real-time cine MRI between Cartesian and
radial k-space sampling schemes. We sought to compare their performance in
patients and volunteers.
Methods
(Theoretical Evaluation) Radial and Cartesian sampling patterns were
compared using analysis of point spread functions (PSF). For a fair comparison,
both radial and Cartesian sampling patterns were made with 144
frequency-encoding points and 12 phase-encoding lines per cardiac phase, and
over 30 cardiac frames, as shown in Figure 1. Fourier transforms were performed
to obtain PSFs in each 2D space (x-y, y-f, and x-f) at midpoint of the third
dimension, for ease in evaluation. PSFs were analyzed to calculate the relative
energy of the central spire to the side lobes (energy ratio = central spire /
side lobes, where higher number translates to more incoherent aliasing
artifacts).
(Prospective
Evaluation) We compared the performance of three free
breathing real-time cine MRI pulse sequences with b-SSFP readouts in a group of
15 patients (9 male, average age 65±14, six female, average age 58±10)
diagnosed with arrhythmias and 1 healthy volunteer: (i) threefold accelerated using
TGRAPPA (4); (ii) twelvefold-accelerated using CS (12,13) with Cartesian sampling, and (iii)
twelvefold-accelerated using CS with radial sampling with tiny golden angles (14). Patients and
volunteer were scanned on a combination of 3T (Verio, Trio) and 1.5T (Espree,
Avanto) whole-body MRI scanners (Siemens). Table 1 summarizes relevant imaging
parameters for the 3 pulse sequences. Note that the radial acquisition had
slightly higher spatial resolution because it enables smaller field of view (i.e.,
oversampling in the frequency-encoding direction) while avoiding considerable fold-over
aliasing artifacts.
(Image
Analysis) The diagnostic quality of the three different
cine MRI methods was graded by two non-invasive cardiologists. A mid-ventricular
short-axis slice was chosen from each pulse sequence for visual assessment of
diagnostic quality. Forty-two (14 from each pulse sequence) cine datasets were
pooled and randomized for blinded analysis. The readers independently scored
the following four categories using a Likert scale: image quality (e.g., endocardial-edge
definition), temporal fidelity of wall motion, artifact level, and apparent
noise level. Averaging over two readers, a score of ≥ 3.0 was considered to be
diagnostically acceptable.
Results
Energy ratio calculations are summarized in Figure
1. The radial sampling pattern had a larger energy ratio in every 2D space
except for x-f, where the Cartesian sampling pattern was fully sampled. When
the energy ratio values of all three 2D spaces were averaged, the radial
pattern had a higher mean energy ratio (1.65) when compared to the mean
Cartesian energy ratio (1.03).
Figure 2 shows representative images of two
different patients diagnosed with atrial fibrillation on 1.5 and 3T scanners, where
radial images consistently produced better image quality than Cartesian CS and
TGRAPPA images. The mean scores are summarized in Table 2. According to the Kruskal-Wallis
test, all 4 categories were significantly different (p < 0.05). With TGRAPPA
as the control, Cartesian CS was significantly different only for the temporal
fidelity score, whereas radial CS was significantly different for all 4
categories. Only radial CS resulted in acceptable scores for all 4 categories.
Conclusions
This study demonstrates that accelerated real-time
cine MRI with radial k-space sampling produces superior image quality than accelerated
real-time cine MRI with Cartesian k-space sampling. Future studies could
evaluate the clinical utility of accelerated real-time cine MRI with radial
k-space sampling for assessment of cardiac function in pediatric patients or
patients with arrhythmia.
Acknowledgements
This work was supported in part by the following grants:
NIH- 5R01HL116895-02
AHA - 14GRNT18350028
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