High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T1 relaxation obtained from 3D FLASH MRI

An empirical equation for the magnetization transfer (MT) FLASH signal is derived by analogy to dual‐excitation FLASH, introducing a novel semiquantitative parameter for MT, the percentage saturation imposed by one MT pulse during TR. This parameter is obtained by a linear transformation of the inverse signal, using two reference experiments of proton density and T₁ weighting. The influence of sequence parameters on the MT saturation was studied. An 8.5‐min protocol for brain imaging at 3 T was based on nonselective sagittal 3D‐FLASH at 1.25 mm isotropic resolution using partial acquisition techniques (TR/TE/α = 25ms/4.9ms/5° or 11ms/4.9ms/15° for the T₁ reference). A 12.8 ms Gaussian MT pulse was applied 2.2 kHz off‐resonance with 540° flip angle. The MT saturation maps showed an excellent contrast in the brain due to clearly separated distributions for white and gray matter and cerebrospinal fluid. Within the limits of the approximation (excitation <15°, TR/T₁ ? 1) the MT term depends mainly on TR, the energy and offset of the MT pulse, but hardly on excitation and T₁ relaxation. It is inherently compensated for inhomogeneities of receive and transmit RF fields. The MT saturation appeared to be a sensitive parameter to depict MS lesions and alterations of normal‐appearing white matter. Magn Reson Med 60:1396–1407, 2008. © 2008 Wiley‐Liss, Inc.     

Quantitative description of the asymmetry in magnetization transfer effects around the water resonance in the human brain.

Magnetization transfer (MT) imaging provides a unique method of tissue characterization by capitalizing on the interaction between solid-like tissue components and bulk water. We used a continuous-wave (CW) MT pulse sequence with low irradiation power to study healthy human brains in vivo at 3 T and quantified the asymmetry of the MT effects with respect to the water proton frequency. This asymmetry was found to be a difference of approximately a few percent from the water signal intensity, depending on both the RF irradiation power and the frequency offset. The experimental results could be quantitatively described by a modified two-pool MT model extended with a shift contribution for the semisolid pool with respect to water. For white matter, this shift was fitted to be 2.34 +/- 0.17 ppm (N = 5) upfield from the water signal.     

In vivo T(1rho) mapping in cartilage using 3D magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (3D MAPSS)

For T(1rho) quantification, a three-dimensional (3D) acquisition is desired to obtain high-resolution images. Current 3D methods that use steady-state spoiled gradient-echo (SPGR) imaging suffer from high SAR, low signal-to-noise ratio (SNR), and the need for retrospective correction of contaminating T(1) effects. In this study, a novel 3D acquisition scheme-magnetization-prepared angle-modulated partitioned-k-space SPGR snapshots (3D MAPSS)-was developed and used to obtain in vivo T(1rho) maps. Transient signal evolving towards the steady-state were acquired in an interleaved segmented elliptical centric phase encoding order immediately after a T(1rho) magnetization preparation sequence. RF cycling was applied to eliminate the adverse impact of longitudinal relaxation on quantitative accuracy. A variable flip angle train was designed to provide a flat signal response to eliminate the filtering effect in k-space caused by transient signal evolution. Experiments in phantoms agreed well with results from simulation. The T(1rho) values were 42.4 +/- 5.2 ms in overall cartilage of healthy volunteers. The average coefficient-of-variation (CV) of mean T(1rho) values (N = 4) for overall cartilage was 1.6%, with regional CV ranging from 1.7% to 8.7%. The fitting errors using MAPSS were significantly lower (P < 0.05) than those using sequences without RF cycling and variable flip angles.     

Reversed laminar appearance of articular cartilage by T1‐weighting in 3D fat‐suppressed spoiled gradient recalled echo (SPGR) imaging

Purpose:

To investigate the reversed intensity pattern in the laminar appearance of articular cartilage by 3D fat‐suppressed spoiled gradient recalled echo (FS‐SPGR) imaging in magnetic resonance imaging (MRI).

Materials and Methods:

The 3D SPGR experiments were carried out on canine articular cartilage with an echo time (TE) of 2.12 msec, a repetition time (TR) of 60 msec, and various flip angles (5° to 80°). In addition, T1, T2, and T2* in cartilage were imaged and used to explain the laminar appearance in SPGR imaging.

Results:

The profiles of T2 and T2* in cartilage were similar in shape. However, the T2 values from the multigradient‐echo imaging sequence were about 1/3 of those from single spin‐echo sequences at a pixel resolution of 26 μm. While the laminar appearance of cartilage in spin‐echo imaging is caused mostly by T2‐weighting, the laminar appearance of cartilage in fast imaging (ie, short TR) at the magic angle can have a reversed intensity pattern, which is caused mostly by T1‐weighting.

Conclusion:

The laminar appearance of articular cartilage can have opposite intensity patterns in the deep part of the tissue, depending on whether the image is T1‐weighted or T2‐weighted. The underlying molecular structure and experimental protocols should both be considered when one examines cartilage images in MRI. J. Magn. Reson. Imaging 2010;32:733–737. © 2010 Wiley‐Liss, Inc.     

Quantitation of brain tissue changes associated with white matter hyperintensities by diffusion‐weighted and magnetization transfer imaging: The LADIS (leukoaraiosis and disability in the elderly) study

Purpose

To explore the value of diffusion‐weighted imaging (DWI) and magnetization transfer imaging (MTI) for the improved detection and quantification of cerebral tissue changes associated with ageing and white matter hyperintensities (WMH).

Materials and Methods

DWI (n = 340) and MTI (n = 177) were performed in nine centers of the multinational Leukoaraiosis And DISability (LADIS) study investigating the impact of WMH on 65‐ to 85‐year‐old individuals without prior disability. We assessed the apparent diffusion coefficient (ADC) and magnetization transfer ratio (MTR) of normal appearing brain tissue (NABT) and within WMH and related them to subjects' age and WHM severity according to the Fazekas score.

Results

ADC and MTR values showed a significant inter‐site variation, which was stronger for the MTR. After z‐transformation multiple regression analysis revealed WMH severity and age as significant predictors of global ADC and MTR changes. Only lesional ADC, but not MTR was related to WMH severity.

Conclusion

ADC and MTR are both sensitive for age and WMH related changes in NABT. The ADC is more sensitive for tissue changes within WMH and appears to be more robust for multicenter settings. J. Magn. Reson. Imaging 2009;29:268–274. © 2009 Wiley‐Liss, Inc.     

Increased SNR and reduced distortions by averaging multiple gradient echo signals in 3D FLASH imaging of the human brain at 3T

Purpose

To demonstrate how averaging of multiple gradient echoes can improve high‐resolution FLASH (fast low angle shot) magnetic resonance imaging (MRI) of the human brain.

Materials and Methods

3D‐FLASH with multiple bipolar echoes was studied by simulation and in three experiments on human brain at 3T. First, the repetition time (TR) was increased by the square of the flip angle to maintain contrast as derived by theory. Then the number of echoes was increased at constant TR with bandwidths between 110 and 1370 Hz/pixel. Finally, signals of a 12‐echo acquisition train (echo times 4.9–59 msec) were averaged consecutively to study the increase in SNR.

Results

At unchanged contrast, the signal increased proportionally with flip angle and sqrt(TR). Increasing the bandwidth improved delineation of the basal cortex and vessels, while most of the loss in the signal‐to‐noise ratio (SNR) was recovered by averaging. Consecutive averaging increased the SNR to reach maximum efficiency at an echo train length corresponding roughly to T.

Conclusion

SNR is gained efficiently by acquiring additional echoes and increasing TR (and flip angle accordingly to maintain contrast) until the associated T loss in the averaged signal consumes the sqrt(TR) increase in the steady state. A bandwidth of 350 Hz/pixel or higher and echo trains shorter than T are recommended. J. Magn. Reson. Imaging 2009;29:198–204. © 2008 Wiley‐Liss, Inc.     

Optimized high‐resolution mapping of magnetization transfer (MT) at 3 Tesla for direct visualization of substructures of the human thalamus in clinically feasible measurement time

Purpose

To optimize contrast‐to‐noise and spatial resolution of a FLASH‐based magnetization transfer (MT) protocol for visualization of substructures in human thalamus.

Materials and Methods

Healthy adults were examined at 3 Tesla with a three‐dimensional (3D) spoiled gradient‐echo sequence. The signal‐to‐noise ratio (SNR) was increased by averaging eight bipolar echo acquisitions (mean echo time = 12.3 ms; bandwidth = 370 Hz/pixel). Three isotropic datasets with different weighting (proton density: flip angle/repetition time = 7°/30 ms; T₁: 20°/30 ms and MT: 10°/48 ms, Gaussian MT prepulse) yielded maps of T₁, signal amplitude, MT ratio and MT saturation for comparison to MP‐RAGE images. Measuring time was 23 min using partial k‐space acquisition. First, the SNR of MT saturation maps in thalamus was optimized by means of the excitation flip angle. Then, noise and partial volume effects were traded off by means of the resolution. Finally, the contrast within the thalamus and to adjacent structures was compared between different maps.

Results

The optimized MT saturation maps at 0.95 mm isotropic resolution provided the highest contrast. It was most prominent between structures of high axonal content (internal medullary lamina, ventral nuclei) and those containing predominantly neuronal somata (pulvinar, mediodorsal thalamus, geniculate bodies).

Conclusion

Semiquantitative MT saturation maps provide an enhanced intra‐thalamic contrast. The borders and nuclear groups of the thalamus are reliably delineated; individual assignment of singular nuclei seems feasible. J. Magn. Reson. Imaging 2009;29:1285–1292. © 2009 Wiley‐Liss, Inc.     

The effect of blood inflow and B1‐field inhomogeneity on measurement of the arterial input function in axial 3D spoiled gradient echo dynamic contrast‐enhanced MRI

A major potential confound in axial 3D dynamic contrast‐enhanced magnetic resonance imaging studies is the blood inflow effect; therefore, the choice of slice location for arterial input function measurement within the imaging volume must be considered carefully. The objective of this study was to use computer simulations, flow phantom, and in vivo studies to describe and understand the effect of blood inflow on the measurement of the arterial input function. All experiments were done at 1.5 T using a typical 3D dynamic contrast‐enhanced magnetic resonance imaging sequence, and arterial input functions were extracted for each slice in the imaging volume. We simulated a set of arterial input functions based on the same imaging parameters and accounted for blood inflow and radiofrequency field inhomogeneities. Measured arterial input functions along the vessel length from both in vivo and the flow phantom agreed with simulated arterial input functions and show large overestimations in the arterial input function in the first 30 mm of the vessel, whereas arterial input functions measured more centrally achieve accurate contrast agent concentrations. Use of inflow‐affected arterial input functions in tracer kinetic modeling shows potential errors of up to 80% in tissue microvascular parameters. These errors emphasize the importance of careful placement of the arterial input function definition location to avoid the effects of blood inflow. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Measurement of glycine in the human brain in vivo by 1H‐MRS at 3 T: application in brain tumors

Glycine is a key metabolic intermediate required for the synthesis of proteins, nucleic acids, and other molecules, and its detection in cancer could, therefore, provide biologically relevant information about the growth of the tumor. Here, we report measurement of glycine in human brain and gliomas by an optimized point‐resolved spectroscopy sequence at 3 T. Echo time dependence of the major obstacle, myo‐inositol (mI) multiplet, was investigated with numerical simulations, incorporating the 3D volume localization. The simulations indicated that a subecho pair (TE₁, TE₂) = (60, 100) ms permits detection of both glycine and mI with optimum selectivity. In vivo validation of the optimized point‐resolved spectroscopy was conducted on the right parietal cortex of five healthy volunteers. Metabolite signals estimated from LCModel were normalized with respect to the brain water signal, and the concentrations were evaluated assuming the total creatine concentration at 8 mM. The glycine concentration was estimated as 0.6 ± 0.1 mM (mean ± SD, n = 5), with a mean Cramér‐Rao lower bound of 9 ± 1%. The point‐resolved spectroscopy sequence was applied to measure the glycine levels in patients with glioblastoma multiforme. Metabolite concentrations were obtained using the water signal from the tumor mass. The study revealed that a subset of human gliomas contains glycine levels elevated 1.5–8 fold relative to normal. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

B1 transmission‐field inhomogeneity and enhancement ratio errors in dynamic contrast‐enhanced MRI (DCE‐MRI) of the breast at 3T

Purpose:

To quantify B₁ transmission‐field inhomogeneity in breast imaging of normal volunteers at 3T using 3D T₁‐weighted spoiled gradient echo and to assess the resulting errors in enhancement ratio (ER) measured in dynamic contrast‐enhanced MRI (DCE‐MRI) studies of the breast.

Materials and Methods:

A total of 25 volunteers underwent breast imaging at 3T and the B₁ transmission‐fields were mapped. Gel phantoms that simulate pre‐ and postcontrast breast tissue T₁ were developed. The effects of B₁‐field inhomogeneity on ER, as measured using a 3D spoiled gradient echo sequence, were investigated by computer simulation and experiments on gel phantoms.

Results:

It was observed that by using the patient orientation and MR scanner employed in this study, the B₁ transmission‐field field is always reduced toward the volunteer's right side. The median B₁‐field in the right breast is reduced around 40% of the expected B₁‐field. For some volunteers the amplitude was reduced by more than 50%. Computer simulation and experiment showed that a reduction in B₁‐field decreases ER. This reduction increases with both B₁‐field error and contrast agent uptake.

Conclusion:

B₁ transmission‐field inhomogeneity is a critical issue in breast imaging at 3T and causes errors in quantifying ER. These errors would be sufficient to reduce the conspicuity of a malignant lesion and could result in reduced sensitivity for cancer detection. J. Magn. Reson. Imaging 2010;31:234–239. © 2009 Wiley‐Liss, Inc.