Margarita Gorodezky^{1,2}, Andrew David Scott^{1,2}, Pedro F Ferreira^{1,2}, Sonia Nielles-Vallespin^{1,2,3}, Peter D Gatehouse^{1,2}, Dudley J Pennell^{1,2}, and David N Firmin^{1,2}

The spatial resolution of DT-CMR
STEAM acquisitions was increased by implementing an interleaved variable
density spiral readout. Bulk motion
during STEAM diffusion encoding is unavoidably encoded in the image phase which
can result in signal loss for multi-shot acquisition when the multiple
interleaves are combined. A phase correction was implemented using the fully
sampled centres of k-space to calculate the differences in phase between
interleaves. In 7 volunteers we show improved data quality at 2.0x2.0mm^{2}
using interleaved spirals compared to single-shot EPI and we obtain similar
DT-CMR parameters.

**Methods:**

A spiral STEAM sequence (1)
with a 2D reduced field-of-view (FOV) and asymmetric RF-pulses was modified to
use interleaved variable-density spirals acquired over 4 cardiac cycles (figure
1a/b). Data were acquired on a Siemens Skyra 3T scanner in 7 healthy volunteers
27 [19-53] years, in a single mid-ventricular slice in systole with both spiral
sequences (resolution: 2.8x2.8x8mm^{3} with single interleave spiral and
2.0x2.0x8mm^{3} with two interleaved spirals, TE=11ms) and a single-shot
EPI sequence (resolution: 2.8x2.8x8mm^{3} and 2.0x2.0x8mm^{3} with
TE=24ms and TE=34ms respectively) (figure 1c). Each EPI breath hold was 18
cardiac cycles long and acquired 1 average of each of 6 diffusion weighted
images. Each spiral breath hold was 16 cardiac cycles and acquired 1 average of
either 6 diffusion weighted images for the single shot acquisition or 3
diffusion weighted images for the interleaved acquisition. For each sequence 10
averages of b_{main}=600s/mm^{2} (20 breath-holds
interleaved-spiral/10 breath-holds single shot acquisitions) and 1 average b_{ref}=150s/mm^{2}
(2/1 breath-holds) in each of 6 diffusion directions were acquired.

The spiral images were reconstructed off-line and, for the interleaved acquisitions, a phase correction was performed (3-5) to correct for motion induced phase differences between the interleaves. Both interleaves corresponding to each image were windowed in k-space (0.25x field of view Gaussian) leaving the low-frequency motion-induced phase data. This was used to match the phase of the second interleave to that of the first (figure 2). The multiple receive coil spiral data was combined using an optimal SNR coil-weighted reconstruction (6). For the EPI sequence a product SENSE x2 reconstruction was used.

The diffusion tensor (DT) was
calculated pixel-wise with in-house MATLAB software (7)
using all averages for all sequences (matched number of averages) and then with
5 averages of b_{main}=600s/mm^{2} and 1 average and 3
diffusion directions of b_{ref}=150s/mm^{2} for the interleaved
spiral sequence to match the number of breath-holds to the single shot
spiral/EPI acquisitions.

**Conclusion:**

1. Gorodezky M, Ferreira P, Scott AD, Nielles-Vallespin S, Pennell D, Firmin D. A comparison of spiral and EPI trajectories for in-vivo cardiac DTI. Society for Cardiovascular Magnetic Resonance 20th Annual Scientific Sessions; 2017; Washington DC, USA.

2. Gorodezky M, Scott AD, Ferreira P, Nielles-Vallespin S, Khalique Z, Pennell D, Firmin D. In-vivo comparison of STEAM EPI and STEAM spiral diffusion-weighted sequences. International Society for Magnetic Resonance in Medicine Annual Meeting; 2017; Honolulu, USA.

3. Scott AD, Nielles‐Vallespin S, Ferreira PF, McGill LA, Pennell DJ, Firmin DN. The effects of noise in cardiac diffusion tensor imaging and the benefits of averaging complex data. NMR in Biomedicine. 2016;29(5):588-99.

4. Pipe JG, Farthing VG, Forbes KP. Multishot diffusion‐weighted FSE using PROPELLER MRI. Magnetic resonance in medicine. 2002;47(1):42-52.

5. Pipe JG. Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging. Magnetic resonance in medicine. 1999;42(5):963-9.

6. Roemer PB, Edelstein WA, Hayes CE, Souza SP, Mueller O. The NMR phased array. Magnetic resonance in medicine. 1990;16(2):192-225.

7. Ferreira PF, Kilner PJ, McGill L-A, Nielles-Vallespin S, Scott AD, Ho SY, et al. In vivo cardiovascular magnetic resonance diffusion tensor imaging shows evidence of abnormal myocardial laminar orientations and mobility in hypertrophic cardiomyopathy. Journal of Cardiovascular Magnetic Resonance. 2014;16(1):1.

Figure 1: A single-shot spiral
(a), variable density interleaved spiral (b) and EPI (c) STEAM sequences. The field
of view was 120x120mm^{2} for the single-shot spiral, 110x110-120x120mm^{2}
in the centre linearly decreasing to 60x60mm^{2} for the VD interleaved
spiral and 360x135mm^{2} for the EPI sequence. Since half of the
readout has to precede the TE for the EPI readout, the TE has to be longer than
for the spiral readout. The readout durations were 15 and 17ms for the single-shot
and interleaved spiral, and 13 and 19ms for the EPI sequence at low and high
resolutions respectively.

Figure 2: Magnitude and phase
images of the individual interleaves and their combinations before (top) and
after (bottom) the phase correction. Differences in phase of the interleaves
can lead to signal loss when the interleaves are combined in complex space (summing
the image magnitude would lead to an SNR reduction). To perform the phase
correction, low resolution phase images were calculated for both interleaves.
The difference of these images was calculated and the phase of the second
interleave was corrected by this phase difference. The figure shows high-resolution
images from a single receive coil for both phase and magnitude.

Figure 3: For a typical volunteer, a calculated magnitude image at b=0s/mm^{2}, DT parameter maps for FA, MD, helical
angle (HA), E2A and a map of the number of negative eigenvalues are shown for
both sequences and at both resolutions. For the interleaved spiral, two
reconstructions are shown, one with a matched number of averages to the EPI
acquisition (10 averages, 20 breath holds) and one with a matched acquisition duration
(5 averages, 10 breath-holds).

Figure 4: A subject wise comparison
of the DT parameters for both sequences at both resolutions is shown. For the
interleaved spiral two reconstructions are compared. In (a) the mean fractional
anisotropy (FA) is increased when increasing the EPI spatial resolution
(p=0.03), but not for the spiral acquisition (p>0.05). The mean diffusivity
(MD) (b) is similar between all sequences. The median secondary eigenvector
angulation (E2A) (c) over the left ventricle is similar between spiral
acquisitions but marginally significantly reduced when increasing the
resolution of the single-shot EPI acquisition.

Figure 5: A subject wise
comparison of the SNR (a) and the data quality parameters: standard deviation
of transverse angle (stdTA) (b); and the number of negative eigenvalues
(%neg.EV) (c). SNR is higher using the spiral sequence than the equivalent
resolution EPI acquisition, but is naturally reduced for both sequences when
the resolution is increased. While the standard deviation of the transverse
angle and the number of negative eigenvalues increases with increased
single-shot EPI resolution it is similar between standard and high resolution
spiral acquisitions.