Time‐resolved three‐dimensional (3D) phase‐contrast (PC) balanced steady‐state free precession (bSSFP)

Abstract

In this study the feasibility of a time-resolved, three-dimensional (3D), three-directional flow-sensitive balanced steadystate free precession (bSSFP) sequence is demonstrated. Due to its high signal-to-noise ratio (SNR) in blood and cerebrospinal fluid (CSF) this type of sequence is particularly effective for acquisition of blood and CSF flow velocities. Flow sensitivity was achieved with the phase-contrast (PC) technique, implementing a custom algorithm for calculation of optimal gradient parameters. Techniques to avoid the most important sources of bSSFP-related artifacts (including distortion due to eddy currents and signal voids due to flow-related steady-state disruption) are also presented. Magnetic resonance imaging (MRI) is a widely used technique for noninvasive flow quantification, useful in the evaluation of both anatomy and function within the human circulatory system; e.g., the cardiovascular system and the cerebrospinal fluid (CSF) circulation system. A traditional method of flow quantification is the phasecontrast (PC) technique, which was first introduced in 1960 by Hahn (1). PC-MRI is based on the encoding of the mean velocity of an isochromat into the corresponding voxel location of the reconstructed phase image. An additional reference image is acquired to remove background effects via phase subtraction and the underlying fluid velocity or tissue motion is represented by the pixel intensity in the resulting phase difference image.The traditional approach consists of the acquisition of two-dimensional (2D) slices with a modified radio frequency (RF)-spoiled sequence that is sensitive to flow in the throughplane direction (2). More recently, time-resolved three-dimensional (3D) sequences, encoding flow in three directions, were proposed and used in research studies of thoracic and brain vessels (3-8). Such data acquisition strategies allow for improved quantification of velocities, compared to singledirection flow encoding, which is biased if the flow is not completely parallel to the encoding direction. Furthermore, the acquisition of 3D volumes in combination with threedirectional velocity encoding permits the visualization of complex flow patterns, and the estimation of other parameters such as wall shear stress or vorticity (9) Although 3D methods suffer from longer scan times, 3D acquisitions also provide improved and potentially isotropic resolution related to the higher signal-to-noise ratio (SNR) of a thick 3D slab compared to the signal obtained from a thin 2D slice.One major disadvantage of typical flow quantification methods based on RF-spoiled gradient-echo (GRE) sequences is tissue saturation that occurs when the repetition time TR is significantly shorter than T 1 , as in the case of blood and CSF. This disadvantage is particularly noticeable for 3D acquisitions of large slabs or in cases of slow flow because of reduced in-flow enhancement. For this reason, and particularly for CSF flow, which is more prone to tissue saturation, through-slice flow-sensitive balanced steady-state ...

Magnetic resonance 4D flow characteristics of cerebrospinal fluid at the craniocervical junction and the cervical spinal canal

“…Further improvements in 4D PC imaging can be expected as new techniques such as accelerated Fourier velocity encoding [25] and balanced SSFP PC imaging, which has been suggested by Santini et al [27] for CSF flow analysis, become more broadly available and postprocessing software tools become more sophisticated. Such advances in PC sequence techniques may help to reduce acquisiton time and further improve spatial resolution of 4D PC imaging.…”

Time-Resolved MRI After Ingestion of Liquids Reveals Motility Changes After Laparoscopic Sleeve Gastrectomy—Preliminary Results

“…Although mostly used to acquire static anatomical images in clinical routine, MRI offers a wide range of dynamical applications. The bSSFP sequence family is characterized by very short repetition times, thus allowing for imaging with a high temporal resolution [18]. Variants of the sequences used in this study are, for example, commonly used for cardiac imaging or MR defecography [19,20].…”

Characterization of the Discrepancies Between Four-Dimensional Phase-Contrast Magnetic Resonance Imaging and In-Silico Simulations of Cerebrospinal Fluid Dynamics

“…Application of 2D PCMRI for this purpose would require an impractical amount of imaging time. Time-resolved 3D phasecontrast MRI (4D PCMRI) has been increasingly appreciated as a rapid and novel approach to investigate complex CSF flow phe nomena [11][12][13][14][15] by quantification of three-directional velocity in formation within a volume of interest [16].…”