Improved Speed and Image Quality for Imaging of Liver Lesions with Auto-calibrated Wave Encoded Variable Density Single-Shot Fast Spin Echo.
Jamil Shaikh1, Feiyu S. Chen2, Valentina S. Taviani3, Kim Nhien Vu1, and Shreyas S. Vasanawala1

1Radiology, Stanford University, Stanford, CA, United States, 2Electrical Engineering and Radiology, Stanford University, Stanford, CA, United States, 3Global MR Applications and Workflow, GE Healthcare, Menlo Park, CA, United States


Abdominal T2-weighted imaging is conventionally lengthy, but single shot approaches significantly improve current acquisition times. For single shot fast spin echo (SSFSE), axial imaging speed and sharpness are constrained by limited parallel imaging acceleration. Here, SSFSE technique with wave encoding and variable-density sampling (wSSFSE) was developed to enable higher accelerations and improve overall image quality. The purpose of this study is to assess image quality, delineation of anatomical structures, lesion conspicuity, and speed improvements with wSSFSE.


T2-weighted imaging is a critical component of body MR protocols. Standard fast spin echo (FSE) can produce diagnostic quality T2 weighted images but typically takes several minutes1. In this time, image degradation occurs due to voluntary patient motion, respiratory artifact, and bowel motion. These shortcomings are mitigated by the use of single-shot (SSFSE) methods, but typically with less SNR and image sharpness in comparison to the standard FSE approach2.

Recently, a modified SSFSE approach with variable refocusing flip angles and full-Fourier acquisition was described and shown to lead to higher SNR and image sharpness3-4. However, parallel imaging acceleration is limited to approximately 2-fold for axial imaging due to limited number of sensitive coils in this direction, and thus T2-decay related blurring remains. Wave encoding5 and its analogs6 can leverage coil sensitivities in the readout direction to increase parallel imaging capability, and can be combined with compressed sensing (CS)7. Here we develop and clinically evaluate a wave-encoded single shot technique combined with a CS reconstruction in the clinical setting of neuroendocrine tumor (NET) metastasis to the liver.


A sinusoidal wave-encoding gradient and variable density sampling was added to a SSFSE sequence with variable refocusing flip angles. Acquisition parameters: 5 mm slice thickness, 0.8 phase FOV, ± 244.1 Hz/pixel effective bandwidth, 320 x 224 matrix, 5 cycles, 10 mT/m wave-encoding gradient amplitude, and 3.5-fold acceleration. This sequence (Fig. 1), as further described8, was included in a liver MRI protocol with gadoxetic acid (Eovist) contrast administration.

With IRB approval and waived consent, we retrospectively identified all patients from 9/21/2017 - 10/21/2017 who had a 3T MRI with a 32-channel torso phased-array and Eovist for evaluation of NET metastases. A standard respiratory-triggered FSE-T2-weighted scan (15 echo-train-length, 5 mm slice thickness, ±284.4 Hz/pixel bandwidth, 320 x 224 matrix, no acceleration,), a conventional SSFSE scan (5 mm slice-thickness, 0.8 phase FOV, ±260.4 Hz/pixel bandwidth, 320 x 224 matrix, 2 fold-acceleration), and a wave-encoded single shot (wSSFSE) were all acquired. All three T2-weighted sequences were fat-suppressed. High-resolution delayed hepatobiliary phase navigated 3D SPGR images (2.2 mm slice thickness, 0.8 phase FOV, ±284.4 Hz/pixel bandwidth, 320 x 320 matrix, flip-angle 25 degrees) with intermittent fat suppression served as a gold standard for presence of lesions.

Two radiologists independently assessed the FSE, SSFSE and wSSFSE images for image quality (subjective SNR, contrast, aliasing/motion artifacts, blurring) and anatomic delineation of the liver, bile ducts, kidneys, pancreas, and musculoskeletal structures on a five point scale.

For each subject, the delayed hepatobiliary phase images were used to identify a true lesion. Then for each such lesion, lesion conspicuity was scored in randomized blinded fashion for FSE, SSFSE and wSSFSE. Subsequently, side-by-side blinded evaluations of lesion conspicuity was performed on a 7 point scale (-3 to 3; 0 implying equivalence).

Cohen’s kappa was used to evaluate inter-reader agreement. One way analysis of variance (ANOVA) tests were performed for paired, nonparametric analyses to determine if wSSFSE was comparable to standard FSE and SSFSE. Bonferroni-Holm correction was used for multiple comparisons.


The final cohort of 20 patients had an average age of 53 ± 12 years, with 11 males. Repetition time averaged 646 ms for SSFSE versus 535 ms for wSSFSE. This resulted in breath-holds that were on average 17% shorter for wSSFSE.

Inter-reader agreement (Table 1) was substantial for both general image quality metrics and anatomic delineation. Compared to SSFSE, wSSFSE demonstrated significantly less subjective noise and greater sharpness while maintaining comparable contrast, lesion conspicuity, and artifacts (Fig. 2). Overall, compared to FSE, wSSFSE had no statistically significant difference in image quality or anatomic delineation.

Regarding lesion conspicuity, no difference was noted between wSSFSE and FSE. More importantly, frequency of lesion detection did not differ by technique; every case of metastasis with adequate conspicuity on FSE was also demonstrated to have equivalent conspicuity on wSSFSE. This implies comparability of wSSFSE and FSE for true lesion delineation.

Side-by-side evaluations revealed a preference for wSSFSE over SSFSE (Fig. 3). No statistically significant preference was observed with FSE compared independently to SSFSE or wSSFSE. Representative example images are shown in Fig. 4.


We demonstrate that wSSFSE can significantly improve image sharpness, spatial resolution, and subjective SNR compared to SSFSE while shortening acquisition time compared to standard SSFSE and FSE. Delineation of some structures was improved with wSSFSE compared to SSFSE and FSE. Lesion conspicuity was not statistically different between the methods. A current limitation of the wSSFSE method is a long image reconstruction time.


Variable density wave-encoded single shot fast spin echo enables fast axial abdominal T2 weighted imaging with image quality and lesion conspicuity comparable or better than FSE. This can dramatically speed abdominal MRI exams.


This work was supported by NIBIB R01EB009690 and GE Healthcare.


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SSFSE imaging with wave encoding a: Pulse sequence diagram of wave-encoded SSFSE. Sinusoidal wave-encoding gradient was applied during the readout of each kx encoding line. Over-sampling of the frequency-encoding (FE) direction accounted for voxel spreading effects from wave encoding. Variable refocusing flip angle schedule was controlled by prescribing first, minimum, last flip and center k-space flip angles. 90° minimum flip angle minimized signal loss due to cardiac pulsation over the left hepatic lobe. b: Illustration wave encoding gradient waveform. c: Illustration of a standard sampling pattern and variable-density sampling pattern for wave encoding sequence.

Inter-reader agreement of image quality (left) and anatomic delineation (right). Cohen’s kappa values were interpreted as the following levels of agreement: < 0 None, Slight agreement (<0.01 - 0.20), Fair Agreement (0.21 - 0.40), Moderate Agreement (0.41 - 0.60), Substantial Agreement (0.61 - 0.80), Almost Perfect Agreement (0.81 - 0.99).

Comparing Inter-reader variability of FSE, SSFSE and wSSFSE: Image quality, anatomic delineation and lesion conspicuity. P-values less than 0.05 denoted with asterisks.

Direct paired comparison between FSE with SSFSE and wSSFSE. One-way ANOVA test was performed between two sequences with mean score values between the two readers tabulated. Overall sequence preference was scored -3 to 3 (with 0 meaning no preference to either sequence). Negative values denote preference to the first labeled sequence, while positive values denote preference to the second labeled sequence. Side-by-side evaluations revealed preference for wSSFSE over SSFSE. A slight preference to FSE was observed when compared to SSFSE and similarly between wSSFSE over FSE. However these was not statistically significant.

Three representative cases comparing lesions seen on Eovist (top row) compared to FSE-T2, standard SSFSE and wSSFSE. Standard SSFSE demonstrates poor image quality (reduced SNR), increased noise and decreased conspicuity of the lesion when compared to FSE-T2 and wSSFSE. Note comparable image quality metrics between FSE and wSSFSE images however respiratory motion artifact (manifested as coherent ghosting) severely limits evaluation secondary to non-regular breathing and subsequently poor respiratory triggering. In comparison, the comparable wSSFSE (bottom row) acquired in a single breath hold offers improved diagnostic and perceived image quality in shorter overall acquisition time.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)