Flip angle dependence of experimentally determined T1sat and apparent magnetization transfer rate constants

Pattern of Altered Magnetization Transfer Rate in Alzheimer's Disease

BACKGROUND

Biomarkers for Alzheimer's disease (AD) are crucial for early diagnosis and treatment monitoring once disease modifying therapies become available.

OBJECTIVE

This study aims to quantify the forward magnetization transfer rate (kfor) map from brain tissue water to macromolecular protons and use it to identify the brain regions with abnormal kfor in AD and AD progression.

METHODS

From the Cardiovascular Health Study (CHS) cognition study, magnetization transfer imaging (MTI) was acquired at baseline from 63 participants, including 20 normal controls (NC), 18 with mild cognitive impairment (MCI), and 25 AD subjects. Of those, 53 participants completed a follow-up MRI scan and were divided into four groups: 15 stable NC, 12 NC-to-MCI, 12 stable MCI, and 14 MCI/AD-to-AD subjects. kfor maps were compared across NC, MCI, and AD groups at baseline for the cross-sectional study and across four longitudinal groups for the longitudinal study.

RESULTS

We found a lower kfor in the frontal gray matter (GM), parietal GM, frontal corona radiata (CR) white matter (WM) tracts, frontal and parietal superior longitudinal fasciculus (SLF) WM tracts in AD relative to both NC and MCI. Further, we observed progressive decreases of kfor in the frontal GM, parietal GM, frontal and parietal CR WM tracts, and parietal SLF WM tracts in stable MCI. In the parietal GM, parietal CR WM tracts, and parietal SLF WM tracts, we found trend differences between MCI/AD-to-AD and stable NC.

CONCLUSION

Forward magnetization transfer rate is a promising biomarker for AD diagnosis and progression.

Characterisation of fibrosis in chemically-induced rat mammary carcinomas using multi-modal endogenous contrast MRI on a 1.5T clinical platform

OBJECTIVES

To determine the ability of multi-parametric, endogenous contrast MRI to detect and quantify fibrosis in a chemically-induced rat model of mammary carcinoma.

METHODS

Female Sprague-Dawley rats (n=18) were administered with N-methyl-N-nitrosourea; resulting mammary carcinomas underwent nine-b-value diffusion-weighted (DWI), ultrashort-echo (UTE) and magnetisation transfer (MT) magnetic resonance imaging (MRI) on a clinical 1.5T platform, and associated quantitative MR parameters were calculated. Excised tumours were histologically assessed for degree of necrosis, collagen, hypoxia and microvessel density. Significance level adjusted for multiple comparisons was p=0.0125.

RESULTS

Significant correlations were found between MT parameters and degree of picrosirius red staining (r > 0.85, p < 0.0002 for ka and δ, r < -0.75, p < 0.001 for T1 and T1s, Pearson), indicating that MT is sensitive to collagen content in mammary carcinoma. Picrosirius red also correlated with the DWI parameter fD* (r=0.801, p=0.0004) and conventional gradient-echo T2* (r=-0.660, p=0.0055). Percentage necrosis correlated moderately with ultrashort/conventional-echo signal ratio (r=0.620, p=0.0105). Pimonidazole adduct (hypoxia) and CD31 (microvessel density) staining did not correlate with any MR parameter assessed.

CONCLUSIONS

Magnetisation transfer MRI successfully detects collagen content in mammary carcinoma, supporting inclusion of MT imaging to identify fibrosis, a prognostic marker, in clinical breast MRI examinations.

KEY POINTS

• Magnetisation transfer imaging is sensitive to collagen content in mammary carcinoma. • Magnetisation transfer imaging to detect fibrosis in mammary carcinoma fibrosis is feasible. • IVIM diffusion does not correlate with microvessel density in preclinical mammary carcinoma.

Alternate ascending/descending directional navigation approach for imaging magnetization transfer asymmetry

A new method for imaging magnetization transfer (MT) asymmetry with no separate saturation pulse is proposed in this article. MT effects were generated from sequential two-dimensional balanced steady-state free precession imaging, where interslice MT asymmetry was separated from interslice blood flow and magnetic field inhomogeneity with alternate ascending/descending directional navigation (ALADDIN). Alternate ascending/descending directional navigation provided high-resolution multislice MT asymmetry images within a reasonable imaging time of ∼ 3 min. MT asymmetry signals measured with alternate ascending/descending directional navigation were 1-2% of baseline signals (N = 6), in agreement with those from the conventional methods. About 70% of MT asymmetry signals were determined by the first prior slice. The frequency offset ranges in this study were >8 ppm from the water resonance frequency, implying that the MT effects were mostly associated with solid-like macromolecules. Potential methods to make alternate ascending/descending directional navigation feasible for imaging amide proton transfer (∼ 3.5 ppm offset from the water resonance frequency) were discussed.

Localized grey matter damage in early primary progressive multiple sclerosis contributes to disability

Disability in primary progressive multiple sclerosis (PPMS) has been correlated with damage to the normal appearing brain tissues. Magnetization transfer ratio (MTR) and volume changes indicate that much of this damage occurs in the normal appearing grey matter, but the clinical significance of this remains uncertain. We aimed to localize these changes to distinct grey matter regions, and investigate the clinical impact of the MTR changes. 46 patients with early PPMS and 23 controls underwent MT and high-resolution T1-weighted imaging. Patients were scored on the Expanded Disability Status Scale (EDSS), Multiple Sclerosis Functional Composite and subtests (Nine-Hole Peg Test, Timed Walk Test, Paced Auditory Serial Addition Test [PASAT]). Grey matter volume and MTR were compared between patients and controls, adjusting for age. Mean MTR for significant regions within the motor network and in areas relevant to PASAT performance were correlated with appropriate clinical scores, adjusting for grey matter volume. Patients showed reduced MTR and atrophy in the right pre- and left post-central gyri, right middle frontal gyrus, left insula, and thalamus bilaterally. Reduced MTR without significant atrophy occurred in the left pre-central gyrus, left superior frontal gyri, bilateral superior temporal gyri, right insula and visual cortex. Higher EDSS correlated with lower MTR in the right primary motor cortex (BA 4). In conclusion, localized grey matter damage occurs in early PPMS, and MTR change is more widespread than atrophy. Damage demonstrated by reduced MTR is clinically eloquent.

3D MTR measurement: From 1.5 T to 3.0 T

This study investigates some of the issues involved in magnetization transfer ratio (MTR) acquisition, and in particular aims to determine whether high quality in vivo MTR measurements can be made at 3.0 T. The dependency of the MTR white-to-grey matter contrast to noise ratio (CNR) on MT pulse characteristics at 1.5 T and at 3.0 T was investigated using an established two-pool model for MT. The simulations showed that MT pulse parameters optimizing the CNR can be derived for both field strengths. Both the SNR and the CNR of MTR maps at 3.0 T were increased compared to 1.5 T. Images obtained using a safe in vivo MTR acquisition protocol based on results of simulations at 3.0 T are presented.

Magnetization transfer ratio measurements of the brain in children with tuberous sclerosis complex

BACKGROUND

Magnetization transfer contrast and magnetization transfer ratio (MTR) in brain are mainly related to the presence of myelin. Neuropathological studies of brain lesions in tuberous sclerosis complex (TSC) have demonstrated disordered myelin sheaths.

OBJECTIVE

To evaluate the MTR of the brain in children with TSC and to compare with that in controls.

MATERIALS AND METHODS

Four patients (aged 0.41-8.4 years, mean 2.5 years) with TSC and four age- and sex-matched controls were evaluated with classic MR sequences and with a three-dimensional gradient-echo sequence without and with magnetization transfer pre-pulse. The MTR was calculated as: (SI(0)-SI(m))/SI(0)x100%, where SI(m) refers to signal intensity from an image acquired with a magnetization transfer pre-pulse and SI(0) the signal intensity from the image acquired without a magnetization transfer pre-pulse.

RESULTS

The MTR values of cortical tubers (44.1+/-4.1), of subependymal nodules (51.6+/-4.8) and of white matter lesions (52.4+/-1.8) were significantly lower than those of cortex (58.7+/-3.53), of basal ganglia (caudate nucleus 58.2+/-2.8, putamen 59.6+/-2.5, thalamus 61.3+/-2.4) and of white matter (64.2+/-2.5) in controls (P<0.001). The MTR of normal-appearing white matter (61.2+/-3.0) in patients was lower than that of white matter in controls (P<0.01). The MTR of cortex and basal ganglia in patients was not significantly different from that in controls.

CONCLUSIONS

MTR measurements not only provide semiquantitative information for TSC lesions but also reveal more extensive disease.

Magnetization transfer ratio in the brain of preterm subjects: age-related changes during the first 2 years of life

To study the progress of myelination in preterm-born subjects by measuring the MT ratio (MTR) from birth, up to 24 months of corrected age. One hundred twenty-five preterm subjects (64 males and 61 females of gestational age 33+/-2.4 weeks with chronologic and corrected age of 9.3+/-5.1 and 7.7+/-5.1 months, respectively) with normal brain MR using classic sequences were further evaluated for MTR by using a three-dimensional gradient-echo sequence (TR=32/TE=8/flip angle=6 degrees 4 mm/2 mm overlapping sections) with and without magnetization transfer prepulse. The magnetization transfer ratio was calculated as: MTR=(SIo-SIm)/SIox100%, where SIm refers to signal intensity from an image acquired with a MT prepulse and SIo the signal intensity from the image acquired without a MT prepulse. MTR increased asymptotically in the genu (R2=0.85) and splenium (R2=0.85) of the corpus callosum, the white matter of the frontal lobe (R2=0.91) and occipital lobe (R2=0.82), thalamus (R2=0.86), caudate nucleus (R2=0.67) and putamen (R2=0.71), reaching the 95% of the final value at the corrected age 18.7, 17.7, 15.6, 12.9, 10.4, 9.2 and 6.4 months, respectively. This study shows age-related changes of the brain MTR and provides data that may be useful to assess disturbances in the progress of myelination.

Quantification of magnetization transfer rate and native T1 relaxation time of the brain: correlation with magnetization transfer ratio measurements in patients with multiple sclerosis

The purpose of this paper is to perform quantitative measurements of the magnetization transfer rate (Kfor) and native T1 relaxation time (T1free) in the brain tissue of normal individuals and patients with multiple sclerosis (MS) by means of multiple gradient echo acquisitions, and to correlate these measurements with the magnetization transfer ratio (MTR). Quantitative magnetization transfer imaging was performed in five normal volunteers and 12 patients with relapsing-remitting MS on a 1.5 T magnetic resonance (MR) scanner. The T1 relaxation time under magnetization transfer irradiation (T1sat) was calculated by means of fitting the signal intensity over the flip angle in several 3D spoiled gradient echo acquisitions (3 degrees , 15 degrees , 30 degrees , and 60 degrees ), while a single acquisition without MT irradiation (flip angle of 3 degrees ) was utilized to calculate the MTR. The Kfor and T1free constants were quantified on a pixel-by-pixel basis and parametric maps were reconstructed. We performed 226 measurements of Kfor, T1free, and the MTR on normal white matter (NWM) of healthy volunteers (n=50), and normal-appearing white matter (NAWM) and pathological brain areas of MS patients (n=120 and 56, respectively). Correlation coefficients between Kfor-MTR, T1free-MTR, and T1free-Kfor were calculated. Lesions were classified, according to their characteristics on T1-weighted images, into isointense (compared to white matter), mildly hypointense (showing signal intensity lower than white matter and higher than gray matter), and severely hypointense (revealing signal intensity lower than gray matter). "Dirty" white matter (DWM) corresponded to areas with diffused high signal, as identified on T2-weighted images. Strong correlation coefficients were obtained between MTR and Kfor for all lesions studied (r2=0.9, p<0.0001), for mildly hypointense plaques (r2=0.82, p<0.0001), and for DWM (r2=0.78, p=0.0007). In contrast, comparison between MTR and T1free values yielded rather low correlation coefficients for all groups assessed. In severely hypointense lesions, an excellent correlation was found between Kfor and T1free measurements (r2=0.98, p<0.0001). Strong correlations between Kfor and T1free were found for the rest of the subgroups, except for the NAWM, in which a moderate correlation was obtained (r2=0.5, p<0.0001). We conclude that Kfor and T1free measurements are feasible and may improve our understanding of the pathological brain changes that occur in MS patients.

Estimation of magnetization transfer rates from PACE experiments with pulsed RF saturation

A new imaging method has been developed for estimating the magnetization transfer rate (MTR) in a biologic two-pool system such as the brain tissue. The transfer rate is calculated from the ratio of the MTR to T(1sat), where T(1sat) is the apparent longitudinal relaxation time under complete saturation of the macromolecular pool. MTR and T(1sat) maps were obtained with a phase acquisition of composite echo (PACE) technique combined with pulsed radiofrequency (RF) saturation. The influences of RF saturation power and frequency offset on quantitative results were investigated with phantom and in vivo measurements. In white matter of seven healthy volunteers we found a mean transfer rate of 1.5 sec(-1), where the highest transfer rate was found in the genu of the corpus callosum (k(f) = 1. 9 sec(-1)). It could be shown that conditions near to complete saturation can also be reached under common restrictions by the specific absorption rate.

Fast multislice T(1) and t(1sat) imaging using a phase acquisition of composite echoes (PACE) technique

An optimized multislice data acquisition scheme for phase acquisition of composite echoes (PACE) imaging is presented. This new scheme uses a repetition time equal to the mixing time and an appropriate phase cycling scheme, which allows for more efficient exploitation of all composite echoes. These modifications provide true multislice capability in parallel to a reduction of the acquisition time by a factor >2 compared with the original PACE method. Moreover, T(1) values can be obtained directly from phase images without the use of a lookup table. Because of the symmetrical application of the radio frequency pulses, this method is also well suited for T(1sat) imaging. Phantom studies showed a significantly better accuracy of the multislice fast PACE technique compared with a conventional two-point method in multislice acquisition mode, although precision was limited at high T(1) values. T(1) and T(1sat) measurements in brain tissue of eight healthy volunteers confirmed the stability of the fast PACE technique in a clinical setting. Magn Reson Med 42:1089-1097, 1999.