Li Zhao^{1} and David Alsop^{1}

Conventional 3D arterial spin labeling images suffer from pronounced T2 blurring. In this work, new variable flip angle schemes, which can provide a Hann or Fermi window response across the slice direction, or which can be easily corrected to the designed window response and provide optimal SNR, were evaluated. Volunteers’ results show reduced blurring, improved SNR and contrast with the proposed methods.

Since an algorithm for simultaneous global optimization of flip angle train and filter based correction was not available, optimization was divided into two sections. First, variable flip angle trains were designed to achieve signal amplitudes, including T2 decay, matched to a number of different window functions. The extended phase map algorithm [6] was used. Echo trains were iteratively calculated as the first echo amplitude was gradually increased from zero until the target shape of response could not be achieved. Standard constant asymptotic flip angle schemes were also implemented as a benchmark. The second stage of the optimization was selection of the echo train with optimal SNR for the desired window response. In general, the optimal echo train doesn’t need to be the one that exactly produces the window response. Instead, a different echo train in combination with a correction filter may achieve a better SNR.

Five volunteers were imaged by 3D pseudo continuous ASL on a 3T GE scanner. 32 slices with slice thickness 4mm covered the majority of the brain. Five interleaved spirals with 4.1ms readout provided 4.5x4.5mm in-plane resolution for a 24cm FOV. Other parameters were labeling duration 2s, post labeling delay 1.8s, 6 repetitions, TR 4.5s, TE 9.8ms and echo spacing 9.8ms.

ASL images were processed in MATLAB. To quantify the SNR of ASL more accurately, the noise was measured by the difference from the first and last three pairs of ASL images [7]. Image quality was measured by a non-reference blurring metric [8], where large value indicated more blurred image. The contrast between grey and white matters was measured with 3D mask generated by segmenting the T1 weighted images with a probability map in SPM12. Nonparametric Friedman's test was used to compare each method with the standard flip angle scheme without correction.

The flip angle designs and resulted echo signal are shown in Figure 1. Variable trains and filter corrections were able to achieve clear improvements in image resolution. Figure 2 shows results from one selected volunteer. With optimal flip angle and filter correction, the sharpness of ASL images was improved at the expense of SNR.

Quantitative results, Figure 3, show fairly equivalent
performance for different optimization strategies: (1) The original standard constant asymptotic flip
angle method resulted in the highest SNR (9.3±1.9), but also the most blurred images (33.2±1.5) and the lowest
contrast (2.1±0.2).
(2) When corrected to Hann window, the standard sequence resulted in significantly
higher contrast (2.3±0.2 p<0.05). The optimized
constant asymptotic flip angle for Hann windows provided the highest SNR as expected. The sequence designed for direct Hann response provided the least blurred images. (3) When corrected to
Fermi window, the constant flip angle optimized for Fermi window provided the
highest SNR, but generally the SNR of corrected image was lower than the
original ones (the standard method with Fermi window correction 4.9±0.7 p<0.005
and direct Fermi response 4.4±0.9, p<0.01). When corrected to Fermi window, all
methods showed reduced blurring (standard 27.3±1.3 p<0.001, optimal constant for Fermi 28.4±0.8 p<0.01 and
direct Fermi 28.1±1.9 p<0.01) and standard method (2.4±0.2 p<0.05)
and the constant flip angle optimal for Fermi (2.4±0.2 p<0.05) showed improved contrast.

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Figure 1
variable flip angle and resulted response signal. Standard: the standard
sequence with constant asymptotic flip angle. C-Hann/Fermi: the constant
asymptotic flip angle sequence with optimal SNR for Hann/Fermi post correction.
D-Hann/Fermi: the variable flip angle designed to provide Hann/Fermi window
response directly without filter correction.

Figure 2 Reduced T2
blurring with variable flip angle and/or filter correction. (a) standard
constant asymptotic flip angle scheme. (b) standard method filter corrected to
Hann window. (c) standard method filter corrected to Fermi window. (d) optimal
constant flip angle for Hann with post correction. (e) optimal constant flip
angle for Fermi with post correction. (f) variable flip angle with Hann
response. (g) variable flip angle with Fermi response.

Figure 3
Quantification of SNR, blurring and contrast in five volunteers. The labels of
x-axis indicate the correction method. The error bars show the standard
deviation across subjects. C-Hann/Fermi: the constant asymptotic flip angle sequence with optimal SNR for Hann/Fermi post correction. D-Hann/Fermi: the variable flip angle designed to provide Hann/Fermi window response directly without filter correction.