Iterative optimization method for design of quantitative magnetization transfer imaging experiments

Quantitative magnetization transfer imaging (QMTI) using spoiled gradient echo sequences with pulsed off‐resonance saturation can be a time‐consuming technique. A method is presented for selection of an optimum experimental design for quantitative magnetization transfer imaging based on the iterative reduction of a discrete sampling of the Z‐spectrum. The applicability of the technique is demonstrated for human brain white matter imaging at 1.5 T and 3 T, and optimal designs are produced to target specific model parameters. The optimal number of measurements and the signal‐to‐noise ratio required for stable parameter estimation are also investigated. In vivo imaging results demonstrate that this optimal design approach substantially improves parameter map quality. The iterative method presented here provides an advantage over free form optimal design methods, in that pragmatic design constraints are readily incorporated. In particular, the presented method avoids clustering and repeated measures in the final experimental design, an attractive feature for the purpose of magnetization transfer model validation. The iterative optimal design technique is general and can be applied to any method of quantitative magnetization transfer imaging. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Optimized double inversion recovery for reduction of T1 weighting in fluid‐attenuated inversion recovery

Fluid‐attenuated inversion recovery (FLAIR) is a routinely used technique in clinical practice to detect long T₂ lesions by suppressing the cerebrospinal fluid. Concerns remain, however, that the inversion pulse in FLAIR imparts T₁ weighting that can decrease the detectability and mischaracterize some lesions. Hence, FLAIR is usually acquired in conjunction with a standard T₂ to guard against these concerns. Recently, double inversion recovery (DIR) preparations have highlighted certain types of lesions by suppressing both cerebrospinal fluid and white matter but produce even stronger T₁ contrast than FLAIR. This work shows that the inversion times in a DIR sequence can be optimized to minimize unwanted T₁ weighting, enabling the acquisition of cerebrospinal fluid‐suppressed images with pure T₂ weighting. This technique is referred to as T₁‐nulled DIR. The theory to determine the optimized inversion times is discussed and the results are shown by simulations, normal volunteer studies, and multiple sclerosis patient studies. T₁‐nulled DIR provides equivalent or superior contrast between gray and white matters as well as white matter and multiple sclerosis lesion at the same repetition time. Multiple sclerosis lesions appeared sharper on T₁‐nulled DIR compared to FLAIR. T₁‐nulled DIR has the potential to replace the combination of standard T₂ and FLAIR acquisitions in many clinical protocols. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Regional variations in normal brain shown by quantitative magnetization transfer imaging.

A quantitative magnetization transfer imaging (qMTI) study, based on a two-pool model of magnetization transfer, was performed on seven normal subjects to determine, on a regional basis, normal values for the pool sizes, exchange, and relaxation parameters that characterize the MT phenomenon. Regions were identified on high-resolution anatomical scans using a combination of manual and automatic methods. Only voxels identified as pure tissue at the resolution of the quantitative scans were considered for analysis. While no left/right differences were observed, significant differences were found among white-matter regions and gray-matter regions. These regional differences were compared with existing cytoarchitectural data. In addition, the pattern and magnitude of the regional differences observed in white matter was found to be different from that reported previously for an alternative putative MRI measure of myelination, the 10-50-ms T2 component described as myelin water.     

Multiple 3D inversion recovery imaging for volume T1 mapping of the heart

In this article, a three‐dimensional inversion recovery sequence was optimized with the aim of generating in vivo volume T₁ maps of the heart using a 1.5‐T MR system. Acquisitions were performed before and after gadolinium diethylenetriamine penta‐acetic acid (Gd‐DTPA) administration in one patient with hypertrophic cardiomyopathy and in two healthy volunteers. Data were acquired with a multishot fast field echo readout using both ECG and respiratory triggers. A dedicated phantom, composed of four solutions with different T₁ values, was positioned on the subjects' thoracic region to perform patient‐specific calibration. Pixel based T₁ maps were calculated with a custom Matlab^® code. Phantom measurements showed a good accuracy of the technique and in vivo T₁ estimation of liver, skeletal muscle, myocardium, and blood resulted in good agreement with values reported in the literature. Multiple three‐dimensional inversion recovery technique is a feasible and accurate method to perform T₁ volume mapping. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Comparative evaluation of magnetization transfer contrast and fluid attenuated inversion recovery sequences in brain tuberculoma

AIM: To compare T1-weighted magnetization transfer (MT) with fluid attenuated inversion recovery (FLAIR) imaging for evaluating conspicuity and number of lesions in individuals with brain tuberculoma. MATERIALS AND METHODS: In all 28 patients with brain tuberculoma underwent MR examination using fast spin-echo (FSE) T2, spin-echo (SE) T1, T1-weighted MT and FLAIR imaging. Post-contrast T1-weighted MT imaging was taken as the gold standard for assessing the number of lesions. Tuberculomas detected both on T1-weighted MT and FLAIR imaging were examined for the wall to be defined, and were divided into two groups on the basis of presence (group 1) or absence (group 2) of perilesional oedema visible on FLAIR imaging. The mean signal intensity of the wall of the lesions and adjacent oedema or brain parenchyma was analyzed qualitatively and quantitatively. RESULTS: The number of lesions detected on T1-weighted MT was higher than on FLAIR imaging (209 versus 163). Conspicuity in both groups was better on T1-weighted MT images qualitatively as well as quantitatively. The difference in the signal intensity of the wall of the lesion and perilesional oedema was statistically significant only on T1-weighted MT images in group 1 (p=0.0003 versus 0.3), whereas in group 2 it was statistically significant both on T1-weighted MT and FLAIR imaging (p=0.009 versus 0.05). CONCLUSION: FLAIR imaging is not helpful in the examination of brain tuberculomas compared with T1-weighted MT imaging, as it neither contributes to the characterization of lesion nor assesses the true disease load.     

T1 discrimination contributions to proton magnetization transfer in heterogeneous biological systems

Water proton spin-lattice relaxation times are commonly used as a guide in establishing the off-resonance irradiation time as well as the repetition time of the magnetization transfer experiment. T₁ discrimination effects occur if the motionally restricted spin bath longitudinal magnetization does not reach thermal equilibrium. In this study we developed the formalism necessary for the evaluation of T₁ discrimination contributions to proton magnetization transfer arising from the use of a short repetition time relative to the spin-lattice relaxation time of the motionally restricted spin bath. The results of computer simulation indicate that T₁, discrimination contributions occur when the repetition time is small relative to the spin-lattice relaxation time of the motionally restricted spin bath, and when the off-resonance irradiation is weak and far off-resonance. For somewhat longer repetition times, T₁ discrimination contributions become important only when the cross relaxation rate is small, and the fractional amount of motionally restricted component large. The occurrence of T₁ discrimination effects results in distortion of water proton intensity ratio dispersion curves thereby resulting in the estimation of erroneous magnetization transfer parameters, whereas in magnetization transfer contrast enhanced imaging, such contributions are manifested by a decrease in image contrast.     

Optimal acquisition schemes for in vivo quantitative magnetization transfer MRI.

This paper uses the theory of Cramer-Rao lower bounds (CRLB) to obtain optimal acquisition schemes for in vivo quantitative magnetization transfer (MT) imaging, although the method is generally applicable to any multiparametric MRI technique. Quantitative MT fits a two-pool model to data collected at different sampling points or settings of amplitude and offset frequency in the MT saturation pulses. Here we use simple objective functions based on the CRLB to optimize sampling strategies for multiple parameters simultaneously, and use simulated annealing to minimize these objective functions with respect to the sampling configuration. Experiments compare optimal schemes derived for quantitative MT in the human white matter (WM) at 1.5T with previously published schemes using both synthetic and human-brain data. Results show large reductions in error of the fitted parameters with the new schemes, which greatly increases the clinical potential of in vivo quantitative MT. Since the sampling-scheme optimization requires specific settings of the MT parameters, we also show that the optimum schemes are robust to these settings within the range of MT parameters observed in the brain.     

T(1) quantification with inversion recovery TrueFISP.

A snapshot FLASH sequence can be used to acquire the time course of longitudinal magnetization during its recovery after a single inversion pulse. However, excitation pulses disturb the exponential recovery of longitudinal magnetization and may produce systematic errors in T(1) estimations. In this context the possibility of using the TrueFISP sequence to detect the recovery of longitudinal magnetization for quantitative T(1) measurements was examined. Experiments were performed on different Gd-doped water phantoms and on humans. T(1) values derived from inversion recovery TrueFISP were in excellent agreement with the single-point method even for flip angles up to 50 degrees. In terms of T(1) accuracy and SNR, the proposed method seems to be superior to the conventional inversion recovery snapshot FLASH technique. Magn Reson Med 45:720-723, 2001.     

Comparison of the magnetization transfer ratio and fluid-attenuated inversion recovery imaging signal intensity in differentiation of various cystic intracranial mass lesions and its correlation with biological parameters

PURPOSE: To compare the signal intensity on the fluid attenuated inversion recovery (FLAIR) sequence and magnetization transfer ratios (MTRs) for the differentiation of abscesses from non-abscess cystic brain lesions, and to correlate these MR parameters with the viscosity, viable cell density and total protein concentration of the cystic fluid. MATERIALS AND METHODS: Signal intensity on FLAIR and MTRs from the cystic cavity of lesions were calculated from 33 patients (brain abscess (N = 12) and non-abscess (N = 21)). The fluid from the lesion was aspirated at the time of surgery, and the viscosity, viable cell density, and total protein concentration were measured. RESULTS: Signal intensity on FLAIR correlated significantly with the total protein concentration in abscess (r = 0.60, P < 0.05) and non-abscess lesions (r = 0.41, P < 0.05). However, there was no significant difference (P > 0.05) in the FLAIR signal intensity of the abscess (318.8 +/- 75) and non-abscess group (258 +/- 47). The MTR of the brain abscesses (13 +/- 0.95) was significantly higher (P < 0.05) than that of the non-abscess group (3.5 +/- 0.3). A significant correlation was observed between MTR and viscosity (r = 0.75, P < 0.05), total protein concentration (r = 0.60, P < 0.05), and cell density (r = 0.70, P < 0.05) in brain abscess, and viscosity (r = 0.81, P < 0.05) and total protein concentration (r = 0.41, P < 0.05) in non-abscess lesions. CONCLUSION: It is possible to differentiate brain abscesses from non-abscess cystic lesions using MT imaging. The MTR correlates significantly with the viscosity, viable cell density, and total protein concentration in brain abscess, and with viscosity and total protein concentration in non-abscess lesions. FLAIR signal intensity correlates significantly only with the total protein concentration in abscess and non-abscess lesions.     

The role of edema and demyelination in chronic T1 black holes: a quantitative magnetization transfer study

PURPOSE: To use quantitative magnetization transfer imaging (qMTI) in an investigation of T1-weighted hypointensity observed in clinical magnetic resonance imaging (MRI) scans of multiple sclerosis (MS) patients, which has previously been proposed as a more specific indicator of tissue damage than the more commonly detected T2 hyperintensity. MATERIALS AND METHODS: A cross-sectional study of 10 MS patients was performed using qMTI. A total of 60 MTI measurements were collected in each patient at a resolution of 2 x 2 x 7 mm, over a range of saturation pulses. The observed T1 and T2 were also measured. qMT model parameters were estimated using a voxel-by-voxel fit. RESULTS: A total of 65 T2-hyperintense lesions were identified; 53 were also T1 hypointense. In these black holes, the qMTI-derived semisolid pool fraction F correlated negatively with T(1,obs) (r2 = 0.76; P < 0.0001). The water pool absolute size (PDf) showed a weaker correlation with T(1,obs) (positive, r2 = 0.53; P < 0.0001). The magnetization transfer ratio (MTR) showed a similarly strong correlation with F and a weaker correlation with PDf (r2 = 0.18; P < 0.04). CONCLUSION: T1 increases in chronic black holes strongly correlated with the decline in semisolid pool size, and somewhat less to the confounding effect of edema. MTR was less sensitive than T(1,obs) to liquid pool changes associated with edema.     

Optimization of T2-selective binomial pulses for magnetization transfer

Accurate rules have been established to build binomial pulses up to fifth order, for selectively saturating protons at any given T₂ with minimum power deposition. Pulse performance and sensitivity to experimental defects have been evaluated; the third order is generally found to be best suited. It is shown, by combining theory and experiment performed at 0.1 T, that matching the saturation pulse to T₂ of the motionally restricted pool is essential to reveal exchange with free water protons. It is emphasized that, to date, lack of magnetization transfer contrast with binomial pulses is due mainly to insufficient RF level available with most MR imaging systems, especially at high magnetic field.     

Pulsed magnetization transfer imaging with body coil transmission at 3 Tesla: feasibility and application.

Pulsed magnetization transfer (MT) imaging has been applied to quantitatively assess brain pathology in several diseases, especially multiple sclerosis (MS). To date, however, because of the high power deposition associated with the use of short, rapidly repeating MT prepulses, clinical application has been limited to lower field strengths. The contrast-to-noise ratio (CNR) of MT is limited, and this method would greatly benefit from the use of higher magnetic fields and phased-array coil reception. However, power deposition is proportional to the square of the magnetic field and scales with coil size, and MT experiments are already close to the SAR limit at 1.5T even when smaller transmit coils are used instead of the body coil. Here we show that these seemingly great obstacles can be ameliorated by the increased T(1) of tissue water at higher field, which allows for longer maintenance of sufficiently high saturation levels while using a reduced duty cycle. This enables a fast (5-6 min) high-resolution (1.5 mm isotropic) whole-brain MT acquisition with excellent anatomical visualization of gray matter (GM) and white matter (WM) structures, and even substructures. The method is demonstrated in nine normal volunteers and five patients with relapsing remitting MS (RRMS), and the results show a clear delineation of heterogeneous lesions.     

Modified cine inversion recovery pulse sequence for the quantification of myocardial T1 and gadolinium partition coefficient

Purpose:

To optimize and validate a modified cine inversion recovery sequence (MCine‐IR) for myocardial T1 quantification and gadolinium partition coefficient (λ_Gd) estimation at 1.5 T.

Materials and Methods:

The original version of the cine inversion recovery sequence was modified to allow fully transverse magnetization recovery between two successive inversion pulses. Sixty heart phases were acquired from a number of heart cycles determined on a patient heart rate basis. Phantom studies were carried out to find the optimal effective TR for myocardial and blood pool T1 quantifications in pre‐ and postcontrast studies. Four patients with myocardial infarct (MI) and 22 dilated cardiomyopathy (DCM) were investigated, as well as 11 healthy subjects used as controls.

Results:

Effective TR was identified to be 5000 msec and 2000 msec, respectively, for pre‐ and postcontrast studies. A longer precontrast (948 ± 102 msec) and shorter postcontrast (348 ± 27 msec) T1 in ischemic patients relative to DCM (815 ± 98 msec, P = 0.03 and 409 ± 42 msec, P = 0.001) were noted in delayed enhancement (DE) areas. In MI patients λ_Gd resulted higher than in DCM in DE areas (609 ± 167 vs. 422 ± 52, P = 0.01) but lower in segments not exhibiting DE (355 ± 100 vs. 398 ± 54, P = 0.02).

Conclusion:

It was feasible to measure T1 and λ_Gd with MCine‐IR and the results were in good agreement with the literature. J. Magn. Reson. Imaging 2013;37:109–118. © 2012 Wiley Periodicals, Inc.     

Quantitative magnetization transfer and myelin water imaging of the evolution of acute multiple sclerosis lesions

Quantitative magnetization transfer imaging provides in vivo estimates of liquid and semisolid constituents of tissue, while estimates of the liquid subpopulations, including myelin water, can be obtained from multicomponent T₂ analysis. Both methods have been suggested to provide improved myelin specificity compared to conventional MRI. The goal of this study was to investigate the sensitivity of each technique to the progression of acute, gadolinium‐enhancing regions of multiple sclerosis. Magnetization transfer and T₂ relaxometry data were acquired longitudinally over the course of 1 year in five relapsing‐remitting multiple sclerosis patients and in five healthy controls. Parametric maps were analyzed in enhancing lesions and normal‐appearing white matter regions. Quantitative magnetization transfer parameters in lesions were most abnormal at the time of enhancement and followed a pattern of recovery over subsequent months. Lesion myelin water fraction was abnormal but did not show a significant trend over time. Quantitative magnetization transfer was able to track the degree and timing of the partial recovery in enhancing multiple sclerosis lesions in a small group of patients, while the recovery was not detected in myelin water estimates, possibly due to their large variability. Our data suggest the recovery is characterized by quick resolution of inflammation and a slower remyelination process. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.