Optimized three-dimensional sodium imaging of the human heart on a clinical 3T scanner

Abstract

Purpose: Optimization of sequence and sequence parameters to allow three-dimensional (3D) sodium imaging of the entire human heart in vivo in a clinically reasonable time.

Theory and methods: A stack of spirals pulse sequence was optimized for cardiac imaging by considering factors such as spoiling, nutation angles, repetition time, echo time, T1/T2 relaxation, off-resonance, data acquisition window, motion, and segmented k-space acquisition. Simulations based on Bloch equations as well as the exact trajectory used for data acquisition provided the basis for choice of parameter combinations for sodium imaging. Sodium phantom scanning was used to validate the choice of parameters and for corroboration with simulations. In vivo cardiac imaging in six volunteers was also performed with an optimized sequence.

Results: Phantom studies showed good correlation with simulation results. Images obtained from human volunteers showed that the heart can be imaged with a nominal resolution of 5 × 5 × 10 mm(3) and with a signal-to-noise ratio >15 (in the septum) in about 6-10 minutes. Long axis views of the reformatted human heart show true 3D imaging capability.

Conclusion: Optimization of the sequence and its parameters allowed in vivo 3D sodium imaging of the entire human heart in a clinically reasonable time.

Keywords: 3D imaging; heart; sodium MRI.

Figure 1

The signal resulting from sequence A (SSFP-FID) with three different TR times: (A1): TR = 20 ms, flip = 55°; (A2) TR = 30 ms, flip = 60°; and (A3) TR = 40 ms, flip = 65°. The square boxes near the corresponding peaks mark the measured SNR scaled to the value obtained from the simulated signal for sequence B3(opt) (see Figure 2).

Figure 2

A: The signal resulting from sequence B (SPGR) with an optimized flip angle scheme. Simulated signal from different TR times are shown: (B1): TR = 20 ms, flip = 45° (max); (B2) TR = 30 ms, flip = 50° (max); and (B3) TR = 40 ms, flip = 50° (max). The square boxes near the corresponding peaks mark measured SNR from the phantom scaled to the value obtained from the simulated signal for sequence B3(opt). B: The signal resulting from sequence B (SPGR) with a constant flip angle scheme. Simulated signal from different TR times are shown: (B1): TR = 20 ms, flip = 35°; (B2) TR = 30 ms, flip = 40°; and (B3) TR = 40 ms, flip = 40°. The square boxes near the corresponding peaks mark measured SNR from the phantom normalized to the value obtained from the simulated signal for sequence B3.

Figure 2

A: The signal resulting from sequence B (SPGR) with an optimized flip angle scheme. Simulated signal from different TR times are shown: (B1): TR = 20 ms, flip = 45° (max); (B2) TR = 30 ms, flip = 50° (max); and (B3) TR = 40 ms, flip = 50° (max). The square boxes near the corresponding peaks mark measured SNR from the phantom scaled to the value obtained from the simulated signal for sequence B3(opt). B: The signal resulting from sequence B (SPGR) with a constant flip angle scheme. Simulated signal from different TR times are shown: (B1): TR = 20 ms, flip = 35°; (B2) TR = 30 ms, flip = 40°; and (B3) TR = 40 ms, flip = 40°. The square boxes near the corresponding peaks mark measured SNR from the phantom normalized to the value obtained from the simulated signal for sequence B3.

Figure 3

Simulated change in signal as a function of a change in the excitation angle as would be expected due to B1 inhomogeneity. A polynomial of order three was fit to get a smooth variation. α_max is the optimal flip angle as determined through simulations. TR = 40 ms for both sequences. The RF spoiled gradient echo sequence shows better B1 robustness compared with sequence A (SSFP-FID).

Figure 4

Measured B1 inhomogeneity characteristics for the three sequences for TR = 40 ms. Measurements were taken in coronal slices parallel to the sodium coil. From the above, it’s apparent that sequence B3 (constant excitation or optimized train) provides better signal in the presence of RF inhomogeneity.

Figure 5

Simulated k-space data for the exact spiral trajectory used for data acquisition. TR = 20 ms, TE = 0.74 ms, spiral arms = 36, echo train length = 18. Two shot acquisition and T2 relaxation effects can be easily perceived as the signal evolves from the center of k-space.

Figure 6

(A) PSF resulting from data acquired with sequence B1: TR = 20ms, flip angle = 35°, 36 spiral arms, echo train length = 18. Peak value of the PSF was 1.29. FWHM was 6.8 mm. (B) Line profile through PSF (at y = 150 mm) resulting from data acquired with the three SPGR sequences B1: TR = 20 ms, flip angle = 35°, 36 spiral arms, echo train length (etl) = 18; B2: TR = 30 ms, flip angle = 40°, 24 spiral arms, etl = 12 and B3: TR = 40 ms, flip angle = 40°, 18 spiral arms, etl = 9. Peak values for PSF were 1.72, 1.59 and 1.29, respectively. FWHM was 6.8 mm for all three cases. Only the central 100 mm of FOV (= 300 mm) is shown.

Figure 6

(A) PSF resulting from data acquired with sequence B1: TR = 20ms, flip angle = 35°, 36 spiral arms, echo train length = 18. Peak value of the PSF was 1.29. FWHM was 6.8 mm. (B) Line profile through PSF (at y = 150 mm) resulting from data acquired with the three SPGR sequences B1: TR = 20 ms, flip angle = 35°, 36 spiral arms, echo train length (etl) = 18; B2: TR = 30 ms, flip angle = 40°, 24 spiral arms, etl = 12 and B3: TR = 40 ms, flip angle = 40°, 18 spiral arms, etl = 9. Peak values for PSF were 1.72, 1.59 and 1.29, respectively. FWHM was 6.8 mm for all three cases. Only the central 100 mm of FOV (= 300 mm) is shown.

Figure 7

Line profile through image 5 (of 16 slices) for a NaCl/agar phantom obtained from three SPGR sequences B1, B2 and B3 with a constant flip angle train. Increased TR from B1 (TR=20ms) to B3 (TR = 40ms) does not result in increased blurring from T2 effects. Line profile was obtained by taking the mean of 20 line profiles (to reduce noise variation) around the center of the image.

Figure 8

Eight contiguous slices (out of 16 slices) of the sodium phantom obtained using a 3D spiral sequence with optimized sequence values: TR/TE = 40/0.77 ms, 18 spiral arms with etl = 9, res: 4 × 4 × 8 mm³, NSA = 16. Total scan time: 5 min 22 s. All images with constant window/level = 1200/600.

Figure 9

(A): Six (of 18) slices obtained along the short axis from a human volunteer (57 kg female) using the optimized 3D spiral sequence. True nominal resolution was 5 × 5 × 10 mm³ (reconstructed: 5 × 5 × 5 mm³). Total imaging time: 8:50. The images have been thresholded to reduce background noise. (B): Reformatted view (long-axis) of the 18 slices (sodium imaging) shows the LV and RV. (Location of short axis images of figure 9(A) are shown as dashed lines on the long axis image.) The entire volume (of 90 mm from apex to base) was acquired in 8 min 50s. On right is the ¹H long axis image of a single slice (long axis view) acquired with the same volunteer. Ruler shows dimension in cm for both images.

Figure 9

(A): Six (of 18) slices obtained along the short axis from a human volunteer (57 kg female) using the optimized 3D spiral sequence. True nominal resolution was 5 × 5 × 10 mm³ (reconstructed: 5 × 5 × 5 mm³). Total imaging time: 8:50. The images have been thresholded to reduce background noise. (B): Reformatted view (long-axis) of the 18 slices (sodium imaging) shows the LV and RV. (Location of short axis images of figure 9(A) are shown as dashed lines on the long axis image.) The entire volume (of 90 mm from apex to base) was acquired in 8 min 50s. On right is the ¹H long axis image of a single slice (long axis view) acquired with the same volunteer. Ruler shows dimension in cm for both images.