E1232. Fast MRI Techniques For Abdominal Imaging Simplified: Making Sense of it All
Authors
Ranu Taneja;
Changi General Hospital
Winfred Goh Xi Tai;
Changi General Hospital
Background
Motion is a major limiting factor in Abdominal MRI. This exhibit aims to provide a simplified explanation of various techniques for fast abdominal MRI.
Educational Goals / Teaching Points
Simplify physics behind fast MR techniques and enable Radiologists to apply fast sequences to improve quality in abdominal MRI.
Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Each MR signal generates a line in k-space (raw data matrix) that is Fourier transformed into an image. So rapid signal acquisition, reduced k-space filling and partial Fourier techniques can reduce scan time. Gradient Recalled Echo (GRE) sequences use flip angles < 90°and gradient fields instead of 180° refocusing pulse of Spin Echo to accelerate signal acquisition. Two fast GRE sequences are spoiled GRE (eg FLASH, SPGR, T1-FFE), where residual transverse magnetization (TM) is destroyed after each cycle, and steady-state GRE (eg TRU-FISP, T2-FFE), where TM is refocused to contribute to steady-state. Modified FLASH sequences eg VIBE/ THRIVE/ LAVA are breath hold (BH) 3D-T1FS sequences using zero filling in k-space for improved resolution. Fast GRE can be combined with inversion prepulse to improve signal-to-noise ratio (SNR) for T1WI (eg TURBO FLASH, FSPGR). RARE (Rapid Acquisition with Relaxation Excitement) uses multiple refocusing pulses to generate echo train, with faster filling of k-space (eg. Fast SE (FSE) or Turbo SE). When single 90° pulse is followed by echo train it is called Single Shot (SS) FSE or HASTE (phase-conjugate symmetry, a partial Fourier technique, enables further acceleration). SE based EPI (Echo Planar Imaging) uses rapidly oscillating frequency encoding gradients to generate echo train, enabling subsecond T2W imaging, though with limited SNR (acceptable for DWI). Simultaneous data acquisition via multiple phased array coil elements results in data redundancy, enabling partial sampling of k-space to generate images, the basis of parallel imaging (PI) and compressed sensing (CS). For conventional PI, k-space is undersampled by skipping phase encoding lines in equidistant steps, causing aliasing artifacts, mitigated by reconstruction, done in k-space domain (GRAPPA) or post Fourier transformed/ image domain (SENSE). PI can be used for Cartesian (regularly spaced) or non-Cartesian (such as radial or spiral) data acquisition. Latter is motion-resistant, and data can be acquired free breathing eg BLADE and STAR VIBE. CAIPIRINHA is a PI technique used for 3D BH abdominal imaging that offsets aliasing through phase shifts. CS relies on sparsity, incoherence (pseudorandom sampling) and nonlinear image reconstruction. Combination of CS and PI (eg GRASP) results in greater acceleration of MRI and CS GRASP VIBE has allowed free breathing 3D-T1WI. Simultaneous multi-slice (SMS) techniques reduce scan time by exciting several slices simultaneously using multiband pulse and are useful where CS is not available.
Conclusion
MR techniques have evolved to speed up acquisition, reduce motion artifacts. A combination of these techniques has revolutionized abdominal imaging and allowed for 3D-MRCP, static and dynamic 2D and 3D-T1 and T2WI with excellent quality in all patients.
