Magnetization transfer effects in multislice RARE sequences

A 2D multislice spin-lock (MS-SL) MR pulse sequence is presented for rapid volumetric T 1 -weighted imaging. Image quality is compared with T 1 -weighted data collected using a singleslice (SS) SL sequence and T 2 -weighted data from a standard MS spin-echo (SE) sequence. Saturation of longitudinal magnetization by the application of nonselective SL pulses is experimentally measured and theoretically modeled as T 2 decay. The saturation data is used to correct the image data as a function of the SL pulse duration to make quantitative measurements of T 1 . Measurements of T 1 using the saturation-corrected MS-SL data are nearly identical to those measured using an SS-SL sequence. The MS-SL sequence produces quantitative T 1 maps of an entire sample volume with the high-SNR advantages conferred by SE-based sequences.Magn In this study, spin-lock (SL) MRI was used to generate an alternative contrast to conventional proton density, T 1 -, or T 2 -weighted MRI methods. In SL-MRI, a long-duration, low-power SL pulse is applied to "lock" the spins in the transverse plane. During the SL pulse, the transverse magnetization decays according to T 1 , the spin-lattice relaxation in the rotating frame of reference. The amplitude of the SL pulse is commonly referenced in terms of the nutation frequency (␥B 1 ), which is typically in the range of a few hundred hertz to a few kilohertz. T 1 relaxation phenomena are sensitive to physicochemical processes with inverse correlation times on the order of the nutation frequency of the SL pulse. By setting the amplitude of the SL pulse to coincide with the frequency of the molecular processes of interest, the signal from the SL-MRI sequence becomes heavily weighted by the T 1 parameter according to Eq. [1], where TSL is the SL pulse duration, and S is the signal intensity as a function of TSL.By acquiring a series of T 1 -weighted images at varying TSL durations with constant SL pulse amplitude, and using Eq.[1], the T 1 parameter can be measured on a pixelby-pixel basis using linear regression to create a quantitative spatial map of T 1 values. SL pulse sequences may be applicable for imaging multiple systems, such as muscle (1,2), breast (3), liver (4), brain (5,6), and tumors (7,8). T 1 relaxation has been used as an indicator of proteoglycan content in articular cartilage, and is being developed as a diagnostic tool for the detection of osteoarthritis (9 -12). In addition, T 1 -weighted MRI of H 2 17O was recently used to measure cerebral and tumor perfusion (13,14). The sensitivity of T 1 to a variety of physiological parameters has also been demonstrated (15).T 1 weighting can be added to most MRI sequences by including an SL pulse cluster at the beginning of the pulse sequence. The magnetization thereby becomes "T 1 -prepared" and results in a T 1 -weighted signal. A 2D singleslice (SS) SL sequence based on a spin-echo (SE) sequence has been successfully implemented to produce a single 2D T 1 -weighted slice (16). The schematic for the SS-SL sequence is shown in Fig. 1. The nut...

mentioning

confidence: 89%

Pulse sequence for multislice T_1ρ‐weighted MRI

Wheaton

Borthakur

Charagundla

et al. 2004

“…Magnetization transfer (MT) effects significantly alter tissue contrast in multislice FSE (24), are enhanced at high field, and depend on the refocusing flip angles (25), and make accurate contrast predictions difficult (11). We demonstrate MT effects in 2D FSE at various refocusing flip angles by plotting ratios of multislice to single-slice image intensities in regions of CSF (body of the left lateral ventricle), GM (left posterior cingulate gyrus), and WM (splenium of the corpus callosum).…”

Section: Incidental Magnetization Transfer Contrastmentioning

confidence: 99%

“…Magnetization transfer effects are well known to alter contrast with multislice FSE (24). A recent work at 4.7 T demonstrated signal differences of 20 -25% in edge slices when using refocusing angles of 162°, and attributed these differences to MT signal attenuation (6).…”

Section: Incidental Magnetization Transfer Contrastmentioning

confidence: 99%

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

Wilman

2009

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...

“…Since MT is expected to play a large role in determining the contrast of FSE (9,14), the change in contrast with TE is not necessarily straightforward to predict. We assessed the magnitude of the MT effect in the multislice FSE sequence used in this study by acquiring image data sets with reversed slice order (i.e., the same axial slices acquired in both the inferior-superior direction and the superior-inferior directhird ventricle), rather than at the same position relative to the RF coil.…”

Section: Effect Of Te and Slice Acquisition Order On Snr And Contrastmentioning

confidence: 99%

High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

Thomas

Vita

Roberts³

et al. 2004

In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162°) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B 1 ) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B 1 inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness ‫؍‬ 2 Over recent years, the use of high-field MR scanners for imaging the human body has become increasingly widespread, due to the improvements they offer in signal-tonoise ratio (SNR) and susceptibility contrast (1,2). These benefits can be exploited to allow 1) the acquisition of MR images with higher spatial resolution and thus greater anatomical detail, 2) more robust detection of susceptibility-related changes in signal intensity (e.g., BOLD contrast in functional MRI (fMRI)), and 3) easier discrimination of metabolite signals in MR spectra, due to increased dispersion as well as higher SNR.Along with the advantages of high-field MR systems, there are also associated problems, ranging from technical challenges to safety concerns. At higher fields, the wavelength of the RF excitation pulses approaches the dimensions of the object being imaged, causing inhomogeneities in the RF (B 1 ) field via RF focusing effects (3). As a result, the RF flip angle varies over the imaging field of view (FOV), potentially leading to variations in both signal intensity and contrast across images. In addition, the RF power required for spin excitation increases with the magnetic field strength. This means, for some RF-intensive pulse sequences, that the power deposition may exceed permissible specific absorption rate (SAR) safety limits.In this work, we describe the implementation and application of a standard clinical MRI pulse sequence on a high-field 4.7-T whole-body system. The pulse sequence we implemented is fast spin echo (FSE) (4,5). FSE is used clinically as an efficient method for obtaining high-quality T 2 -weighted images. However, the extension to high field is not necessarily straightforward, due to the aforementioned issues of B 1 inhomogeneity and SAR power restrictions. In a recent study, we presented our initial results concerning FSE imaging at 4.7 T (6). Here we investigate in more detail the factors involved in using FSE as a technique for imaging the human brain at 4.7 T. To do this, we assess the extent of B 1 variation within the brain and its effect on image signal in...