Time‐efficient fast spin echo imaging at 4.7 T with low refocusing angles

Abstract

An implementation of fast spin echo at 4.7 T designed for versatile and time-efficient T 2 -weighted imaging of the human brain is presented. Reduced refocusing angles (␣ < 180°) were employed to overcome specific absorption rate (SAR) constraints and their effects on image quality assessed. Image intensity and tissue contrast variations from heterogeneous RF transmit fields and incidental magnetization transfer effects were investigated at reduced refocusing angles. We found that intraslice signal variations are minimized with refocusing angles near 180°, but apparent gray/white matter contrast is independent of refocusing angle. Incidental magnetization transfer effects from multislice acquisitions were shown to attenuate white matter intensity by 25% and gray matter intensity by 15% at 180°; less than 5% attenuation was seen in all tissues at flip angles below 60°. We present multislice images acquired without excess delay time for SAR mitigation using a variety of protocols. Fast spin echo (FSE) imaging, also known as RARE (1) and TSE, is a robust imaging sequence with numerous clinical and research applications. Prototypical FSE uses a 90°e xcitation pulse and a train of 180°refocusing pulses repeated every TR to form a single image. These refocusing pulses render FSE largely insensitive to static field inhomogeneities. High-field imaging systems-loosely defined here as those exceeding 3 T-promise exceptional capabilities including faster or higher-resolution imaging with altered, and possibly enhanced, tissue contrast mechanisms. Unfortunately, there are impediments to high-field imaging (2): First, static magnetic field heterogeneities scale linearly with main field strength and exacerbate signal losses and geometric distortions. Second, short RF wavelengths cause nonuniform sensitivity profiles for excitation and reception (3). Third, RF power deposition scales approximately quadratically with static field strength (4); patient safety requirements can limit successive high flip angle pulses, as required by FSE.Nevertheless, high-resolution FSE images of the human brain have previously been obtained at 4.7 T (5,6) using long interecho spacings (22 ms) to reduce the temporal density of RF pulses, short echo trains (eight refocusing pulses), slightly reduced refocusing flip angles (162°), and likely also long RF pulses, to constrain specific absorption rate (SAR) within safety regulations (4 W/kg for short exposure times). These works demonstrate that FSE and high field are not mutually exclusive; however, this implementation is ill-suited for variants, such as half Fourier acquisition single-shot turbo spin echo (HASTE), where short echo spacings and long echo trains are required and would produce extreme SAR levels due to a large number of brief, high-angle pulses per unit time. SAR mitigation strategies such as RF pulse elongation and bandwidth reduction provide modest power savings and can be applied to high-resolution imaging, but are, in general, of limited versatility and are insufficient to compensa...

Applications of stimulated echo correction to multicomponent T2 analysis

“…The ability of the stimulated echo‐corrected method to perform well in the presence of low refocusing pulse angles also presents the opportunity to intentionally reduce the applied flip angle, providing a simple method for reducing the specific absorption rate of multiecho scans while not sacrificing accuracy. Lebel has demonstrated the viability of this method using fast spin echo at high field strength (4.7 T) (25).…”

Detecting lesions in multiple sclerosis at 4.7 tesla using phase susceptibility‐weighting and T2‐weighting

“…RF heating scales with the square of the magnetic field and the square of the RF pulse amplitude. To control RF heating at 4.7T, the RF refocusing pulses were lengthened in time and reduced in flip angle by using a variable refocusing train (21, 27). For this implementation, we used 6‐ms pulse duration refocusing pulses (480 Hz pulse bandwidth) with variable flip angle trains of 7 or 8 echoes as listed.…”

“…Certainly, it is well recognized that higher spatial resolution, in particular thinner slices, will result in higher lesion counts (20). Recently several groups (21–24) have demonstrated high definition T2WFSE of the human brain at 4.7T. This 4.7T field strength is also well positioned for excellent susceptibility weighting.…”

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