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A technique suitable for diffusion tensor imaging (DTI) at high field strengths is presented in this work. The method is based on a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) k-space trajectory using EPI as the signal readout module, and hence is dubbed PRO-PELLER EPI. The implementation of PROPELLER EPI included a series of correction schemes to reduce possible errors associated with the intrinsically higher sensitivity of EPI to off-resonance effects. Experimental results on a 3.0 Tesla MR system showed that the PROPELLER EPI images exhibit substantially reduced geometric distortions compared with single-shot EPI, at a much lower RF specific absorption rate (SAR) than the original version of the PROPELLER fast spin-echo (FSE) technique. For DTI, the self-navigated phase-correction capability of the PROPELLER EPI sequence was shown to be effective for in vivo imaging. A higher signal-to-noise ratio (SNR) compared to single-shot EPI at an identical total scan time was achieved, which is advantageous for routine DTI Key words: PROPELLER imaging; EPI; geometric distortions; specific absorption rate; diffusion tensor imagingThe importance of diffusion tensor imaging (DTI) in white matter diseases is now well recognized by the clinical neurology community. It has several applications, including the detection of pathologically induced alterations in neural fiber architecture resulting from multiple sclerosis (1), traumatic axonal injury (2), adrenoleukodystrophy (3), or tumors (4,5). The current implementation of DTI often uses single-shot echo-planar imaging (EPI) as the signal readout module following diffusion-weighted (DW) magnetization preparation. Due to strong susceptibility effects from the air-tissue interface, however, the EPI images show severe geometric distortions that are prominent especially near the skull base (6,7). As a consequence, image mapping methods based on DTI, such as fractional anisotropy maps or neural fiber tractograms, are inherently prone to errors in regions such as the frontal lobe near the frontal sinus and optic chiasm in the central brain base.Reductions in geometric distortions can be accomplished via a decrease in the total data acquisition time following the RF excitation pulse, so as to reduce influences from off-resonance spins. A typical method to achieve this purpose is the multishot EPI or segmented EPI technique, which splits the series of gradient-echo acquisitions into several TRs, at the expense of possible motion artifacts (8). In DW imaging (DWI) using multishot EPI, navigator phase correction is further needed because of phase inconsistencies in the presence of involuntary motion sensitized by the DW gradients (6,7,9). Alternatively, imaging methods based on spin-echo acquisitions are intrinsically immune to off-resonance effects due to the refocusing functions of the 180°pulses. One way to achieve distortion-free DTI images is to use a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELL...

Purpose Whole-brain cerebral blood flow (CBF) measured by phase-contrast MRI (PC-MRI) provides an important index for brain function. This work aimed to optimize the PC-MRI imaging protocol for accurate CBF measurements. Materials and Methods Two studies were performed on a 3 Tesla system. In Study 1 (N=12), we optimized in-plane resolution of PC-MRI acquisition for CBF quantification by considering accuracy, precision, and scan duration. In Study 2 (N=7), we assessed the detrimental effect of non-perpendicular imaging slice orientation on CBF quantification. Both One-way ANOVA with repeated measurement and Friedman test were used to examine the effects of resolution and angulation on CBF quantification. Additionally, we evaluated the inter-rater reliability in PC-MRI data processing. Results Our results showed that CBF measurement with 0.7mm resolution could be overestimated by up to 13.3% when compared to 0.4mm resolution. Moreover, CBF could also be overestimated by up to 18.8% when the slice orientation is deviated by 30° from the ideal angulation. However, within 10° of the ideal slice orientation, estimated CBF was not significantly different from each other “(p=0.23 and 0.45 for internal carotid artery and vertebral artery, respectively). Inter-rater difference was <3%. Conclusion For fast and accurate quantification of whole-brain CBF with PC-MRI, we recommend the use of an imaging resolution of 0.5mm and a slice orientation that is less than 10° from vessel’s axial plane.

The advantage of increased signal-to-noise ratio (SNR) efficiency in balanced steady-state free precession (SSFP) imaging (also denoted as true fast imaging in steady-state precession (TrueFISP), balanced fast field-echo (FFE), or fast imaging employing steady-state acquisition (FIESTA) by various manufacturers) has made this technique attractive for clinical applications. Examples of such applications include (but certainly are not limited to) cardiac imaging (1,2), angiography (3,4), gastrointestinal imaging (5,6), and fetal imaging (7). In certain situations, the signal from fat protons is a major source of interference that hinders our ability to interpret the image unambiguously. This is understood because fat has a higher T 2 /T 1 value compared to parenchymal tissues, which corresponds to bright steady-state signals on SSFP images (8). Therefore, for SSFP imaging applications intended to highlight fluids with large T 2 /T 1 values, such as angiography, myelography, and MR cholangiopancreatography (MRCP), it is essential to eliminate the fat signals.Fat suppression in SSFP imaging can be accomplished by using frequency-selective RF pulses in every TR, similarly to the conventional approach used in spin-echo imaging (5). This method increases TRs that are ordinarily short in generic SSFP sequences, and thus increases total scan time by a noticeable factor. Alternatively, magnetization preparation during the steady state, which refers to the addition of one fat-suppression pulse every several TRs, has also been shown to be effective (9). The latter method is advantageous in that the scan time is not significantly increased, which is beneficial for 3D examinations.Other methods, such as linear combination SSFP (10) and fluctuating equilibrium MR (11), have been proposed that make use of the SSFP spectral profiles manipulated by different RF phase schemes to selectively reconstruct different spectral species. For 2D imaging, the use of a single fat-suppression RF pulse followed by a centric-ordered SSFP readout should serve the same purpose well, with the exception that the resulting image contrast is inevitably altered to proton-density weighting due to the transient-state signal behavior (12).In a recent work, it was shown that SSFP images exhibit spin-echo-like behavior, such that spin isochromats at similar resonant frequencies show phase coherence at either 0°or 180°relative to the RF pulses at the time TR/2, the nominal TE in SSFP imaging (13). For off-resonance species, such as fat relative to water, the SSFP angle (i.e., the precession phase angle for the spin isochromats within one TR in the rotating frame) can be manipulated by adjusting the center reference frequency, which in turn determines the directional location for phase coherence in the rotating frame (13). This property leads naturally to the use of in-phase and out-of-phase images for Dixon addition/subtraction to achieve fat-water separation in SSFP imaging (14). In this study, we demonstrate the feasibility of separating fat and water sig...