Magnetization Transfer Pulse Research Articles

Post-Gadolinium Evaluation of Intracranial Tumours: A Comparative Study of Spin Echo, Fast Spin Echo, Gradient Echo T1-Weighted Sequences, with and without Magnetization Transfer

This study compares the use of six different post-gadolinium T1-weighted sequences on intracranial tumours with the aim of evaluating their efficacy in terms of lesion and edema contrast resolution and cortex visualization. Forty-eight intracranial neoplastic lesions were evaluated after contrast media administration by means of the following T1-weighted sequences: Fast Spin-Echo (FSE), Spin-Echo (SE) and Gradient-Echo (GE) with and without Magnetization Transfer (MT) pulse. For each technique tumour/background contrast resolution (T/B CR), oedema/background contrast resolution (E/B CR) and white matter/grey matter contrast resolution (WM/GM CR) were calculated. GE sequences, with and without MT, showed the lowest contrast resolution values for all parameters. FSE MT sequences showed the highest T/B CR values. SE sequences, instead, showed the best E/B and WM/GM CR values. FSE and SE MT values were intermediate between FSE MT and SE sequences. No sequence had high values for all contrast resolution parameters. The sequence with the best T/B CR was the FSE MT, but that with the best anatomic peritumoural edema and cortex definition was SE. Both FSE MT and SE T1-weighted sequences may be acquired to obtain accurate post-gadolinium tumour evaluation.

Quantitative Interpretation of Magnetization Transfer in Spoiled Gradient Echo MRI Sequences

A method for analyzing general pulsed magnetization transfer (MT) experiments in which off-resonance saturation pulses are interleaved with on-resonance excitation pulses is presented. We apply this method to develop a steady-state signal equation for MT-weighted spoiled gradient echo sequences and consider approximations that facilitate its rapid computation. Using this equation, we assess various experimental designs for quantitatively imaging the fractional size of the restricted pool, cross-relaxation rate, and T(1) and T(2) relaxation times of the two pools in a binary spin bath system. From experiments on agar gel, this method is shown to reliably and accurately estimate the exchange and relaxation properties of a material in an imaging context, suggesting the feasibility of using this technique in vivo.

Head and neck lesions: determination of an optimal MT technique for prediction of malignancies.

To determine an optimal magnetization transfer (MT) technique for diagnosis of malignancies in the head and neck. Lesion magnetization transfer ratios (MTRs) and lesion-to-muscle MTRs were prospectively measured in 52 head and neck lesions of 52 patients at frequency offsets of 0.3, 0.5, and 1 kHz from water resonance. The diagnostic capability for each MT pulse was calculated by using receiver operating characteristic (ROC) curves, and an optimal MT technique was determined for diagnosis of malignancies. The lesion MTRs and the lesion-to-muscle MTRs in malignant tumors were statistically significantly greater than those in benign lesions at both 0.5- and 1-kHz MT pulses, but no significant differences were noted between them at the 0.3-kHz MT pulse. Diagnosis with the lesion-to-muscle MTRs was better than that with the lesion MTRs at each MT pulse. Among all MTRs, lesion-to-muscle MTRs at the 1-kHz MT pulse showed the highest diagnostic capability for malignancies (area under the ROC curve = 0.82+/-0.06). With a lesion-to-muscle MTR at a 1-kHz MT pulse of 0.61 or greater, the highest accuracy of 85% was attained with 90% sensitivity and 77% specificity. Lesion-to-muscle MTRs at a 1-kHz MT pulse were optimal for diagnosis of malignancies in the head and neck.

Characterization of breast lesion morphology with delayed 3DSSMT: an adjunct to dynamic breast MRI.

The purpose of the study was to determine the sensitivity and specificity of various morphologic criteria in distinguishing malignant from benign breast lesions using a new sequence (3DSSMT) performed immediately after dynamic breast MRI. 3DSSMT combines a water-selective spectral-spatial excitation and an on-resonance magnetization transfer pulse with three-dimensional spoiled gradient-echo imaging. Morphologic features of 87 pathologically confirmed lesions were analyzed. The presence of either skin thickening, or a combination of a spiculated or microlobulated border, with a rim, ductal, linear, or clumped enhancement pattern was 94% specific and 54% sensitive for malignancy. Conversely, the presence of either a perfectly smooth border, a well-defined margin, non-enhancing internal septations, or a macrolobulated border was 97% specific and 35% sensitive for a benign diagnosis. In conclusion, delayed 3DSSMT discriminates a significant number of benign and malignant breast lesions; it has the potential to improve the diagnostic accuracy of dynamic breast MRI.

Magnetic resonance findings in amyotrophic lateral sclerosis using a spin echo magnetization transfer sequence. Preliminary report.

We present the magnetic resonance (MR) findings of five patients with amyotrophic lateral sclerosis (ALS) using a spin-echo sequence with an additional magnetization transfer (MT) pulse on T1-weighted images (T1 SE/MT). These findings were absent in the control group and consisted of hyperintensity of the corticospinal tract. Moreover we discuss the principles and the use of this fast but simple MR technique in the diagnosis of ALS.

Pulsed magnetization transfer imaging: evaluation of technique.

With use of an established physical model, numeric simulations were performed to evaluate current imaging protocols for the two primary applications of magnetization transfer: cerebral magnetic resonance angiography and neuroimaging of white matter disease. The authors found that the current technique is appropriate in the former but suboptimal in the latter. Further clinical investigations could potentially improve magnetization protocols for neuroimaging.

The characterization of human brain tumor using magnetization transfer technique in magnetic resonance imaging

The clinical applicabiity of magnetization transfer (MT) technique in magnetic resonance imaging (MRI) for the estimation of the histological and constitutional feature of brain tumors was investigated. MT effect was evaluated by measuring the MT ratio (MTR). The parameters in 1.5-tesla MRI system were as follows: TR, 50 msec; TE, 5 msec; flip angle, 30 degree; offset frequency of off-resonance MT pulse, 1000 Hz. The sequence was performed in 20 normal volunteers and 45 patients with brain tumors which were characterized histologically and surgically. The MTR for brain tumors was significantly lower than that for normal brain tissue (p < 0.05). The MTR for meningioma was higher than that for the other brain tumors (p < 0.05). In the meningiomas, MTR for fibrous type was higher than that for méningothélial type, but there was no statistical significance. Regarding the physical consistency for the brain tumors, as classified by surgery, there was a statistically significant difference in MTR between the soft tumor group (0.22 ± 0.03, n = 6) and the hard tumor group (0.36±0.04, n=10) (p < 0.01). This study suggested that the MT technique for patients with brain tumor may be useful to understand the characteristics of the tumors presurgically, based on the degree of intermolecular interaction of macromolecule such as protein. [Neurol Res 1999; 21: 250-254]

Lesion load measurements in multiple sclerosis: the effect of incorporating magnetization transfer contrast in a Fast-Flair sequence

We compared the ability and reproducibility of a fast fluid-attenuated inversion recovery (fast-FLAIR) sequence with and without a magnetization transfer (MT) pulse for detecting and measuring multiple sclerosis (MS)-related abnormalities on magnetic resonance imaging (MRI) scans from 20 patients. The Contrast-to-Noise ratios between lesions and normal-appearing white matter, lesion numbers, lesion volumes and the variability of such measurements were similar for the two sequences. This suggests that the addition of MT to FLAIR sequences as currently implemented on standard MRI scanners does not improve the detection of MS lesions.

Magnetization Transfer Ratio in Head and Neck Lymphadenopathy

Purpose: The purpose of this study was to determine whether the magnetization transfer ratio(MTR) differs between malignant and benign cervical lymphadenopathy. Materials and Methods: Magnetization transfer ratios were obtained from 104 lymph nodes of 43 patients. Fifty-five nodes were malignant and 49 were benign. Biopsy or cervical lymph node dissection was performed in 83 nodes, while the remaining 21 were diagnosed clinically or by follow-up imaging studies. Among the 55 malignant nodes, squamous cell carcinomas accounted for 29 cases, lymphomas for 15, undifferentiated carcinomas for four, acute myelogenous leukemia for four, and melanomas for three. The 49 benign nodes comprised 21 cases of reactive hyperplasia, 12 of Kikuchi ’s disease, nine of acute lymphadenitis, and seven of tuberculous lymphadenitis. All scans were performed using a 1.5T Magnetom Vision(Siemens, Erlangen, Germany) with phased-array or Helmholtz-type neck coil. Scanning was performed with and without magneti-zation transfer pulse(MT pulse : 11.2 T, 250 Hz band-width, off-set 2.0 KHz) using FLASH 2D sequencing. The region of interest(ROI) for signal intensity(SI) measurements was sampled at the same nodes by keeping the position, shape and size of the ROI constant for the scans before and after the MT pulse was applied. SI measurements were repeated more than three times in each node and the mean value was used to calculate MTR. In this study, however, corrected MTRs(CoMTRs) were used for correction of the effect of background noise produced by magnetic field inhomogeneity. Results: Mean CoMTRs of malignant and benign nodes were 0.33(SD:0.04) and 0.28(SD:0.05), respectively. This difference was statistically significant. At CoMTR 0.31, the sensitivity and specificity of malignant nodes were 83% and 75%, respectively. Conclusion: A CoMTR of above 0.31 suggests malignant lymphadenopathy. CoMTR is one of the MR criteria which can serve to differentiate between malignant and benign lymphadenopathy.

Magnetic relaxation contrast agents in magnetization transfer imaging.

The effects of magnetic relaxation agents are explored in the context of magnetization transfer pulse sequences using cross-linked protein gels as modeled tissue systems. Magnetization transfer pulse sequences were used to study contrast agents that are designed to bind to rotationally immobilized protein targets. The dynamic range available from contrast agents, used in conjunction with magnetization transfer pulse sequences, is comparable with or better than that based on spin-echo imaging sequences with short repetition times. Furthermore, useful changes in the intensity of water resonances may be achieved by using this combined approach even though the paramagnetic metal center may not have a free coordination position in the chelate complex for water molecule exchange. The inclusion of magnetization transfer acquisition protocols in the context of magnetic imaging with contrast agents presents new opportunities for control of the information content of the image and for new classes of contrast agent structure and delivery.

Role of magnetization transfer imaging in bone tumors

The authors evaluated the role of magnetization transfer imaging in differentiation of bone tumors, with special emphasis on cartilaginous tumors. Fifty-one patients with skeletal tumors or tumor-like lesions who had undergone magnetic resonance (MR) imaging were included. The tumors were divided into three groups according to their gross appearance and the origin of tissue: cyst, cartilaginous tumor, and noncartilaginous solid tumor. A gradient-recalled acquisition in the steady state sequence was used for MR imaging, and examinations were performed both with and without off-resonance magnetization transfer pulses. Magnetization transfer ratios were obtained as an indicator of magnetization transfer effect of the lesions and were compared. Then, the magnetization transfer ratios of all tumors in the cartilaginous tumor group were compared. The magnetization transfer ratios of benign and malignant solid tumors were also compared. The mean magnetization transfer ratio for cartilaginous tumors was 0.31 +/- 0.08 (standard deviation), and that of cysts and noncartilaginous solid tumors was 0.07 +/- 0.03 and 0.40 +/- 0.14, respectively. Comparisons between the three groups showed statistically significant intergroup differences in magnetization transfer ratio (P < .05). In the cartilaginous tumor category, enchondroma and low-grade chondrosarcoma had a lower magnetization transfer effect than chondroblastoma and mesenchymal chondrosarcoma. The mean magnetization transfer ratios of benign (n = 28) and malignant (n = 18) tumors were 0.35 +/- 0.11 and 0.39 +/- 0.15, respectively; there was no statistically significant intergroup difference (P = .14). Magnetization transfer imaging could be useful for categorizing bone tumors as cysts, cartilaginous tumors, or noncartilaginous solid tumors.

Pulsed off-resonance magnetization transfer for brain tumor in patients

We investigated the clinical applicability of pulsed off-resonance magnetization transfer (MT) technique in magnetic resonance (MR) imaging for the estimation of the histological feature of brain tumor. The parameters were optimized using bovine serum albumin (BSA) solutions and gels in 1.5-tesla MR system as follows: TR, 50 msec; TEl 5 msec; flip angle, 30 degree; offset frequency, 1000 Hz. MT effect was evaluated by measuring the MT ratio (MTR) and reconstructing calculated MTR color image. The MTRs of the BSA solutions and gels increased with increasing concentrations of protein, while the MTRs of the gels were remarkably higher than those of the solutions. The optimized sequence was performed in 20 healthy volunteers and 30 patients with brain tumors. In normal brains, the MTRs were significantly higher in white matter than in gray matter (p<0.05). The MTR in brain tumor was significantly lower than that in normal brain tissue (p<0.05) and the calculated MTR image produced contrast between brain tumor and normal brain tissue. There was a statistically significant difference in MTR between benign (0.30±0.05, n= 17) and malignant (0.26±0.04, n= 13) tumors (p< 0. 05). The optimized MT technique is quite sensitive for estimating the degree of intermolecular interaction between macromolecules and water within the central nervous system. [Neural Res 1998; 20: 313-319]

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

The purpose of this work was to develop a method for determining the T1sat and magnetization transfer (MT) rate constants by analyzing the slice-select flip angle dependent MT behavior of normal white and gray matter. The technique uses a high MT power, three-dimensional (3D) gradient-recalled echo (GRE) sequence, with a well chosen MT pulse frequency offset, such that the experimental conditions closely satisfy requisite assumptions for invoking a first order rate process for MT. Integral to this method is that the T1sat and MT ratio values are obtained under explicitly identical MT saturation conditions. The T1sat of white matter was found to be approximately 300 msec, and the MT rate constant was approximately 2.0 sec(-1). The T1sat of gray matter was approximately 500 msec, and the MT rate constant was 1.1 sec(-1). We also found a strong dependence of the MT rate constant on the slice-select flip angle used for the imaging sequence, independent of the MT saturation parameters. Strongly T1-weighted imaging sequences can result in the underestimation of the MT rate constant by 50%. Practical technical suggestions for quantitative MT experiments are put forth.

A General Solution of the Standard Magnetization Transfer Model

The standard model of magnetization transfer consists of six coupled, first-order differential equations which describe a lossless exchange of magnetization between two sites. The system of differential equations is solved semi-analytically in full generality. The solution allows one to model any experiment generating magnetization transfer. It is especially useful in investigation spin systems subjected to pulsed magnetization transfer experiments.

MAGNETIZATION TRANSFER IN NEUROIMAGING

Magnetization transfer (MT) imaging has become a focus of recent basic science and clinical investigations because of its unique physical basis of contrast, and the potential to quantify aspects of the biochemical structure and composition of tissues. In this article, basic theories of MT are discussed, and several practical MT pulse and sequence considerations are addressed. The clinical utility of MT techniques in MR angiography, postgadolinium T1-weighted scanning, and spine scanning are discussed. Highlights of quantitative MT studies for pathologic tissue characterization and for studies of patients with white-matter disorders such as multiple sclerosis are included, giving a comprehensive review of the neuroimaging application of MT.

Effects of brain membranes on 1H nuclear magnetic resonance signal intensity of ethanol in vitro.

In vivo proton nuclear magnetic resonance (1H NMR) studies of ethanol in animal and human brains have shown that only a fraction of ethanol in brain is visible by NMR. The goals of these in vitro 1H NMR experiments were to determine: (1) whether the interaction of ethanol with brain membranes in vitro diminishes ethanol visibility; and (2) if a magnetization transfer (MT) effect can be observed for the interaction of ethanol with brain membranes in vitro. Furthermore, pilot studies were performed to determine if the brain membranes from rats chronically exposed to ethanol had a different effect on ethanol NMR visibility and spin-spin relaxation time (T2) than brain membranes obtained from control rats. Results show that the NMR visibility of ethanol is lower in rat brain membrane suspensions in vitro as compared to ethanol in saline solutions. The factors decreasing ethanol NMR visibility are T2 relaxation, water presaturation time, and off-resonance saturation by a frequency-dependent MT pulse. One-pulse NMR measurements without water presaturation showed that ethanol visibility was significantly increased by 15% in brain membrane suspensions of ethanol-fed rats, suggestive of decreased ethanol partitioning compared to controls. Furthermore ethanol in brain membrane suspensions from ethanol-fed rats showed smaller MT effects than from control rats. These results provide a mechanism for decreased NMR visibility of ethanol in brain, and suggest that chronic exposure to ethanol produces membrane changes which result in increased NMR visibility.

Understanding pulsed magnetization transfer.

Using a two-pool exchange model of magnetization transfer (MT), numeric simulations were developed to predict the time dependence of longitudinal magnetization in both semisolid and liquid pools for arbitrary pulsed radiofrequency (RF) irradiation. Whereas RF excitation of the liquid pool was modeled using the time-dependent Bloch equations, RF saturation of the semisolid pool was described by a time-dependent rate proportional to both the absorption lineshape of the semisolid pool and the square of the RF pulse amplitude. Simulations show good agreement with experimental results for a 4% agar gel aqueous system in which the two-pool kinetics have been well studied previously. These simulations provide a method for interpreting pulsed MT effects, are easily extended to biologic tissues, and provide a basis for optimizing clinical imaging applications that exploit MT contrast.

Analysis of magnetization transfer effects on T1-weighted spin-echo scans using a simple tissue phantom simulating gadolinium-enhanced brain lesions.

The purpose of this study was to analyze the effect of several magnetization transfer (MT) pulse and T1-weighted spin-echo (SE) sequence parameters on lesion-to-background contrast, using a simple tissue phantom emulating the T1 relaxation and MT properties of gadolinium-enhanced brain lesions. Eggbeaters (Nabisco Inc., East Hanover, NJ) liquid egg product was doped with gadolinium in six concentrations from .0 to 1.0 mmol and cooked. The gadolinium-doped egg phantom and normal volunteer brains were studied using an SE sequence with TE = 20 msec and high power, pulsed, off-resonance MT saturation. The effects of MT pulse frequency offset (1,000-6,000 Hz), sequence repetition time (TR = 500-1,000 msec, with MT power held constant), and slice-select flip angle (60-120 degrees) on the magnetization transfer ratio (MTR) and the simulated lesion-to-background contrast were determined at the different "intralesion" gadolinium concentrations. The MTR and lesion-to-background contrast of all materials were greatest at narrow MT pulse frequency offsets. There was in inverse relationship between gadolinium concentration and MTR and a positive correlation between the gadolinium concentration and lesion-to-background (L/B) contrast, a weak negative correlation between slice-select flip angle and L/B, and a negative correlation between TR and L/B. The relaxation properties and MT behavior of the egg phantom are close to that expected for enhancing brain lesions, allowing a rigorous analysis of several variables affecting lesion-to-background contrast for high MT power, T1-weighted SE sequences.

Dispersion in magnetization transfer contrast at a given specific absorption rate due to variations of RF pulse parameters in the magnetization transfer preparation.

The effects of RF pulse parameters on magnetization transfer contrast (MTC) were investigated using a magnetization prepared segmented fast gradient echo sequence. MTC was found not to be uniquely determined by the specific absorption rate (SAR). RF pulse parameters (RF amplitude, number of RF pulses and RF duration) also affect MTC. There can be 40% variation in MTC due to differences in RF parameters at SAR = 1 W/kg for a 70-kg subject. Increasing the number of RF pulses is a more efficient way to increase MTC than increasing RF amplitude. This phenomenon is likely caused by the fact that the time scale for magnetization transfer between the free and restricted proton pools is on the same order or longer than the duration of the MT pulse. Accordingly, increase in the MT pulse duration by increasing the number of RF pulses in the MT pulse allows more effective magnetization transfer. Such information can be used as a guide to select RF pulse parameters for a magnetization transfer (MT) pulse. An off-resonance MT pulse designed under this guide for coronary MR angiography improved the depiction of distal vessels.

Off‐resonance pulsed magnetization transfer in clinical MR imaging: Optimization by an analysis of transients

Significant degrees of magnetization transfer (MT) have been observed in the magnetic resonance imaging of biological materials by conventional clinical imaging sequences, as well as by sequences specifically designed to enhance MT image contrast. Two aspects of these procedures distinguish them from the classic spectroscopic MT experiments using either so-called "hard" radiofrequency (RF) pulses of short duration and high power, or continuous wave RF irradiation of low power. First, clinical sequences must make exclusive use of "soft" pulses of intermediate length and power. Second, biological materials are modeled by a two-spin system involving magnetization transfer between a narrow and a broad homogeneous spectral component. Such materials are a relatively restricted group within the larger family of materials studied with MT by spectroscopy. The current paper addresses these two issues with a theory that gives a new transient analysis of the off-resonance pulsed MT problem for biological materials. It leads to predictions for optimal magnetization transfer in the context of medical imaging that have been verified by computer modeling.