Highly-accelerated volumetric brain examination using optimized wave-CAIPI encoding

Background: Rapid volumetric imaging protocols could better utilize limited scanner resources.

Purpose: To develop and validate an optimized 6-minute high-resolution volumetric brain MRI examination using Wave-CAIPI encoding.

Study type: Prospective.

Population/subjects: Ten healthy subjects and 20 patients with a variety of intracranial pathologies.

Field strength/sequence: At 3 T, MPRAGE, T₂ -weighted SPACE, SPACE FLAIR, and SWI were acquired at 9-fold acceleration using Wave-CAIPI and for comparison at 2-4-fold acceleration using conventional GRAPPA.

Assessment: Extensive simulations were performed to optimize the Wave-CAIPI protocol and minimize both g-factor noise amplification and potential T₁ /T₂ blurring artifacts. Moreover, refinements in the autocalibrated reconstruction of Wave-CAIPI were developed to ensure high-quality reconstructions in the presence of gradient imperfections. In a randomized and blinded fashion, three neuroradiologists assessed the diagnostic quality of the optimized 6-minute Wave-CAIPI exam and compared it to the roughly 3× slower GRAPPA accelerated protocol using both an individual and head-to-head analysis.

Statistical test: A noninferiority test was used to test whether the diagnostic quality of Wave-CAIPI was noninferior to the GRAPPA acquisition, with a 15% noninferiority margin.

Results: Among all sequences, Wave-CAIPI achieved negligible g-factor noise amplification (g_avg ≤ 1.04) and burring artifacts from T₁ /T₂ relaxation. Improvements of our autocalibration approach for gradient imperfections enabled increased robustness to gradient mixing imperfections in tilted-field of view (FOV) prescriptions as well as variations in gradient and analog-to-digital converter (ADC) sampling rates. In the clinical evaluation, Wave-CAIPI achieved similar mean scores when compared with GRAPPA (MPRAGE: Ø_W = 4.03, Ø_G = 3.97; T₂ w SPACE: Ø_W = 4.00, Ø_G = 4.00; SPACE FLAIR: Ø_W = 3.97, Ø_G = 3.97; SWI: Ø_W = 3.93, Ø_G = 3.83) and was statistically noninferior (N = 30, P < 0.05 for all sequences).

Data conclusion: The proposed volumetric brain exam retained comparable image quality when compared with the much longer conventional protocol.

Level of evidence: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:961-974.