Breath-held high-resolution cardiac T2 mapping with SKRATCH

Background Several cardiac T2 mapping techniques with varying T2 preparation (T2Prep) times have been proposed for the quantification of cardiac edema [1-3]. Among these, radial T2 mapping, which is robust to motion artifacts, suffers from a low signal-to-noise ratio (SNR) caused by the undersampling of the k-space periphery and by its density compensation function (DCF) (Fig. 1a). However, since the contrast of an image is mainly determined by the center of its k-space, the T2-weighted images can share their k-space periphery using the KWIC (K-space Weighted Image Contrast) filter (Fig. 1b) to reduce undersampling artifacts [4]. This allows for higher undersampling (Fig. 1c) and thus for a decrease in acquisition time [5]. We demonstrated that navigator-gated KWIC-filtered cardiac T2 mapping (Shared K-space RAdial T2 Characterization of the Heart, SKRATCH) enables a considerable decrease in acquisition time while maintaining the T2 precision [5]. The goal of this study was to extend this approach to a short breath-held high-resolution T2 map acquisition and to compare its performance to navigatorgated T2 mapping.


Background
Several cardiac T 2 mapping techniques with varying T 2 preparation (T 2 Prep) times have been proposed for the quantification of cardiac edema [1][2][3]. Among these, radial T 2 mapping, which is robust to motion artifacts, suffers from a low signal-to-noise ratio (SNR) caused by the undersampling of the k-space periphery and by its density compensation function (DCF) (Fig. 1a). However, since the contrast of an image is mainly determined by the center of its k-space, the T 2 -weighted images can share their k-space periphery using the KWIC (K-space Weighted Image Contrast) filter (Fig. 1b) to reduce undersampling artifacts [4]. This allows for higher undersampling (Fig. 1c) and thus for a decrease in acquisition time [5].
We demonstrated that navigator-gated KWIC-filtered cardiac T 2 mapping (Shared K-space RAdial T 2 Characterization of the Heart, SKRATCH) enables a considerable decrease in acquisition time while maintaining the T 2 precision [5]. The goal of this study was to extend this approach to a short breath-held high-resolution T 2 map acquisition and to compare its performance to navigatorgated T 2 mapping. 1 University Hospital (CHUV) and University of Lausanne (UNIL), Lausanne, Switzerland Full list of author information is available at the end of the article Figure 1 Schematic overview of the KWIC filter. a. A radial k-space sampling pattern shown below its DCF along one radial line. The DCF is used to weigh the k-space points. b. Three similar k-spaces that share their periphery through the KWIC filter, thus increasing the local sampling density and decreasing the local weight attributed by the DCF. The radii outside of which data were added were defined through the Nyquist criterion. c. An undersampled KWIC-filtered k-space. While the number of lines has decreased, the periphery of k-space still has a higher sampling density than the standard radial k-space in a.

Methods
The novel breath-held SKRATCH protocol consisted of a GRE sequence with a continuously increasing goldenangle radial acquisition. This ensured a unique k-space trajectory for all 64 lines of each of the 4 T 2 Prep durations (0/30/45/60 ms), pixel size of 1.2 × 1.2 × 8 mm 3 and a total duration of 7 heartbeats. As reference, a navigatorgated radial cardiac T 2 mapping GRE sequence was acquired with 3 T 2 Prep durations (0/30/60 ms), 308 lines/ image and a pixel size of 1.25 × 1.25 × 5 mm 3 [3]. Images were acquired at 3T (Magnetom Prisma, Siemens Healthcare) in 17 healthy volunteers at the same midventricular short-axis orientation with both protocols. The T 2 maps were segmented according to the AHA guidelines [6]. The mean T 2 value (μ T2 ) and the relative standard deviation (σ R = standard deviation/ μ T2 ) of each segment as well as the myocardial area were calculated and tested for significant differences. The SKRATCH T 2 map was acquired twice in 11 of the volunteers for Bland-Altman reproducibility analysis.

Results
The SKRATCH T 2 maps had average values of 39.9 ± 4.4 ms, while those of the reference T 2 maps were 39.1 ± 3.1 ms (p = 0.04, Fig. 2a-c). σ R increased from 8 ± 2% for the standard T 2 maps to 11 ± 2% for the SKRATCH T 2 maps (p < 0.001). The myocardial area decreased from 643 ± 155 to 585 ± 121 pixels for the SKRATCH T 2 maps (a 10% decrease, p = 0.008). The repeatability analysis resulted in a confidence interval of ± 3.09 ms (Fig. 2d).

Conclusions
The SKRATCH T 2 maps were highly similar to the reference high-resolution T 2 maps, while the shortening to breath-hold duration came at the cost of an acceptably small increase in standard deviation and decrease in  http://www.jcmr-online.com/content/18/S1/P27 myocardial area. These encouraging results will need to be validated in future high-resolution studies in patients.