Phantom and in vivo study of the Look-Locher T1 mapping method

This paper describes and tests the LL-EPI method for obtaining quantitative T₁ estimates in a few seconds thereby allowing dynamic T₁ studies. It is shown that the method works even when there is an inflow into the imaged volume, e.g., in a vessel. No calibration is needed. The method has been tested in a phantom study with several different scan parameter set-ups, with and without inflow. The method shows robustness and individual scan parameters and inflow rates do not influence the ability to calculate the Gd-DTPA concentration. Linearity prevail between the measured 1/T₁ and the Gd-DTPA concentration in the range 150 < T₁ < 2500 ms. In a dynamic Gd-DTPA phantom study, it was shown that the dynamic LL-EPI T₁ mapping technique was three times more sensitive than the signal from a T^*₂-weighted EPI sequence. In an in vivo study, dynamic T₁ mapping of the Gd-DTPA uptake in a meningioma was performed. Inspection of the uptake curves indicates that the method is feasible in clinical perfusion studies.     

Analysis of the Look-Locker T(1) mapping sequence in dynamic contrast uptake studies: simulation and in vivo validation

An alternative to the pulse sequences at present used in dynamic contrast uptake MRI is the dynamic LL-EPI T(1) mapping method. This method generates T(1) estimates in a few seconds, thereby allowing dynamic studies. A particular advantage of the LL-EPI technique is that it provides the opportunity to generate spatial and temporal information about the paramagnetic contrast agent concentration independently of the inflow rate. This paper illustrates, by computer simulations, the accuracy of the estimated 1/T(1) value when using the LL-EPI technique in situations that are not supported by the model. The simulated situations not supported by the model are those in which the longitudinal and transversal relaxation rates change during the T(1) mapping. The most critical moment occurs during a bolus passage of contrast agent when the concentration gradient is large. The computer simulations of the LL-EPI T(1) mapping method in non-supported situations show that in normal perfused capillary tissue the error in the estimated 1/T(1) value is within the absolute error of 0.1 s(-1) in most simulated situations, although in a typical vessel the simulations do indicate that the stated absolute error tolerance of 0.5 s(-1) is exceeded relatively easily. However, this transgression can be rectified by a non-bolus injection of the contrast agent media.     

Optimization of the ultrafast look-locker echo-planar imaging T1 mapping sequence

The Look–Locker echo-planar imaging (LL-EPI) sequence has been numerically optimized in terms of the signal-to-noise ratio in the measured value of T₁, for both single-shot (repetition time (T_R) = ∞), and dynamically repeated T₁ measurements. The sequence is optimized for the normal biologic range of T₁ (0.2 s to 2.0 s) and for a range of sequence parameters found on most magnetic resonance (MR) scanners. Both linearly and geometrically spaced magnetization sample pulse intervals were considered. For single-shot measurements, the sequence with 24 linearly spaced sample pulses, an inversion time of 0.01 s, an inter-sample pulse delay of 0.10 s, and a sample radiofrequency (RF) pulse flip angle of 25^o was found to be optimum. When the number of sample pulses was limited due to hardware limitations, different pulse sequence parameters were indicated. The optimization procedures used are appropriate for any single-shot T₁ mapping sequence variant and for any rapid T₁ mapping application. The use of an optimized Look–Locker echo-planar imaging sequence is demonstrated by an example of dynamic contrast-enhanced scanning in the brain using fast T₁ mapping.     

Quantification of Gd-DTPA concentration in neuroimaging using T₁3D MP-RAGE sequence at 3.0 T

Purpose

To propose a simple and accurate quantitative method based on the linear relationship between magnetic resonance (MR) signal enhancement (ΔSI=SI_postcontrast−SI_precontrast) and gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) concentration (C) by using T₁-weighted three-dimensional magnetization-prepared rapid acquisition gradient-echo (T₁ 3D MP-RAGE) sequence for the in vivo measurement of Gd-DTPA concentration in real-time neuroimaging at 3.0 T.

Methods

Phantom experiment was carried out to study the linear fitting of signal intensity change vs. Gd-DTPA concentration (ΔSI-C) curve. A goodness-of-fit test was performed to compare the accuracy between the proposed method and the conventional method based on longitudinal relaxation rate (R₁=1/T₁) measurement. The influences on the goodness of fit (R²) and the signal-to-noise ratio (SNR) by sequence parameters were explored. Six human subjects with different brain tumors, who underwent a Gd-DTPA-enhanced MRI, were enrolled for in vivo application of the novel method.

Results

A good linear relationship between ΔSI and Gd-DTPA concentration existed over the concentration range of 0-1 mM (R²=0.985). The linearity of the ΔSI-C curve was as good as that of the 1/T₁-C curve (R²=0.988). Concentrations calculated by both methods had a strong correlation (R²=0.920). An improved linearity of the ΔSI-C curve and an increased SNR can be achieved using sequences with a shorter inversion time (TI) and a higher flip angle. The concentration range of Gd-DTPA in human brain tumors was within the quantitative scope of 0-1 mM.

Conclusions

The proposed quantitative method based on ΔSI measurement is accurate and applicable for real-time neuroimaging at 3.0 T.     

Use of a modified polysaccharide gel in developing a realistic breast phantom for MRI

A polysaccharide material, TX-151, has been used together with water, NaCl, and Al powder to create a tissue equivalent gel to make a realistic, inexpensive, conveniently moldable, temporally stable tissue equivalent MRI phantom. Various phantom compositions were studied for variations in gelling time and relaxation times. Gd-DTPA added as a T₁ (and T₂) modifier and aluminum powder added to decrease T₂ permitted phantoms to be made with a range of relaxation times comparable to human tissues. We have used this polysaccharide gel to create breast phantoms for testing breast coils and evaluating different MRI imaging sequences available for diagnosis. The breast phantoms consisted of a layer of Crisco, a good model for adipose tissue, surrounding the TX-151 gel. Some of these phantoms were created with a silicone implant encapsulated in the gel to simulate an augmented breast. More sophisticated phantoms can easily be developed by additions of other materials to this polysaccharide gel.     

Free-running cardiac magnetic resonance fingerprinting: Joint T1/T2 map and Cine imaging

PurposeTo develop and evaluate a novel non-ECG triggered 2D magnetic resonance fingerprinting (MRF) sequence allowing for simultaneous myocardial T₁ and T₂ mapping and cardiac Cine imaging.MethodsCardiac MRF (cMRF) has been recently proposed to provide joint T₁/T₂ myocardial mapping by triggering the acquisition to mid-diastole and relying on a subject-dependent dictionary of MR signal evolutions to generate the maps. In this work, we propose a novel “free-running” (non-ECG triggered) cMRF framework for simultaneous myocardial T₁ and T₂ mapping and cardiac Cine imaging in a single scan. Free-running cMRF is based on a transient state bSSFP acquisition with tiny golden angle radial readouts, varying flip angle and multiple adiabatic inversion pulses. The acquired data is retrospectively gated into several cardiac phases, which are reconstructed with an approach that combines parallel imaging, low rank modelling and patch-based high-order tensor regularization. Free-running cMRF was evaluated in a standardized phantom and ten healthy subjects. Comparison with reference spin-echo, MOLLI, SASHA, T₂-GRASE and Cine was performed.ResultsT₁ and T₂ values obtained with the proposed approach were in good agreement with reference phantom values (ICC(A,1) > 0.99). Reported values for myocardium septum T₁ were 1043 ± 48 ms, 1150 ± 100 ms and 1160 ± 79 ms for MOLLI, SASHA and free-running cMRF respectively and for T₂ of 51.7 ± 4.1 ms and 44.6 ± 4.1 ms for T₂-GRASE and free-running cMRF respectively. Good agreement was observed between free-running cMRF and conventional Cine 2D ejection fraction (bias = −0.83%).ConclusionThe proposed free-running cardiac MRF approach allows for simultaneous assessment of myocardial T₁ and T₂ and Cine imaging in a single scan.     

Multivariate Image Analysis of Magnetic Resonance Images with the Direct Exponential Curve Resolution Algorithm (DECRA): Part 1: Algorithm and Model Study

Antalek and Windig recently presented a fast method to resolve a series of NMR mixture spectra, where the contribution of the components varies with a decaying exponential [B. Antalek and W. Windig,J. Am. Chem. Soc.118, 10,331–10,332 (1996); W. Windig and B. Antalek,Chemom. Intell. Lab. Syst.37, 241–254 (1997)]. The method was called DECRA (direct exponential curve resolution algorithm). In this paper DECRA will be applied to two series of magnetic resonance images. The signal of one series is based uponT₂relaxation, and the other is based uponT₁relaxation. In order to evaluate the technique, the magnetic resonance images of a phantom where used. A transformation is introduced to enable the application of DECRA to aT₁series of magnetic resonance images. A separate paper in this issue will describe the application of the techniques to magnetic resonance images of the human brain.     

Magnetotransport and magnetotunneling in single-layer, two-layer, and three-layer Bi2Sr2Can−1CunOz (n=1,2,3) single crystals

In continuous magnetic fields H up to 28 T, we have studied the out-of-plane transport properties and tunneling characteristics of high-quality nondoped single crystals of the Bi-cuprate family: Bi₂Sr₂CuO_6+δ (Bi2201), Bi₂Sr₂CaCu₂O_8+δ (Bi2212) and Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi2223) grown by an identical method. For all compounds the out-of-plane magnetotransport ρ_c(H) is negative in the temperature region where ρ_c(T) shows in the normal state a semiconducting-like temperature dependence. The negative magnetoresistance of ρ_c corresponds to the suppression of the semiconducting temperature dependence of ρ_c(T) which is found to be isotropic. For the Bi2201 compound, where the normal state can be reached in the available magnetic fields (28 T), a nearly complete suppression of the low-temperature upturn in ρ_c(T) is observed in the highest magnetic fields with a tendency towards a metallic behavior down to the lowest temperatures (0.4 K). Using the break-junction technique, especially for the Bi2212 and Bi2232 compounds, a clear superconducting gap structure can be observed. Both for temperatures above the critical temperature and for magnetic fields above the upper critical field, a pseudogap structure remains present in the tunneling spectra. The applied magnetic fields yield a stronger suppression of the superconducting state compared to that of the normal-state gap structures as manifested in ρ_c(T) transport and tunneling.     

MRI of human tumor xenografts in vivo: Proton relaxation times and extracellular tumor volume

Proton T₁ and T₂ differ substantially between tumors, but the tumor properties causing heterogeneity in T₁ and T₂ have not been fully recognized. The purpose of the study reported here was to investigate whether differences in T₁ and T₂ between tumors are mainly a consequence of differences in the fractional volume of the extracellular compartment. The study was performed using a single human tumor xenograft line showing large naturally occurring intratumor heterogeneity in the size of the extracellular compartment. The size of the extracellular compartment was calculated from the volume and the density of the tumor cells. Cell volume was measured by an electronic particle counter. Cell density was determined by stereological analysis of histological preparations. T₁ and T₂ were measured by MRI in vivo both in the absence and presence of Gd-DTPA. Two spin-echo pulse sequences were used, one with a repetition time (TR) of 600 ms and echo times (TEs) of 20, 40, 60, and 80 ms and the other with a TR of 2,000 ms and TEs of 20, 40, 60, and 80 ms. Measurements of T₁ and T₂ in the presence of Gd-DTPA were performed in a state of semi-equilibrium between uptake and clearance of Gd-DTPA. MR-images and histological preparations of tumor subregions homogeneous in extracellular volume were analysed in pairs. The extracellular volume differed between tumor subregions from 5 to 70%. T₁ and T₂ measured in the absence of Gd-DTPA differed between tumor subregions by a factor of approximately 1.5 and increased with increasing extracellular volume. The relative decrease in T₁ caused by Gd-DTPA, represented by $\text{(T}{}_{\mathrm{<mtext>1\; control</mtext>}}\text{−T}{}_{\mathrm{<mtext>1\; Gd-DTPA</mtext>}}\text{)}\text{T}{}_{\mathrm{<mtext>1\; control</mtext>}}$, also increased with increasing extracellular volume. The relative decrease in T₂ did not change significantly as the extracellular volume increased. These observations strongly suggest that the size of the extra-cellular compartment is a major determinant of proton T₁s and T₂s of tumors, possibly because the ratios of free to structured and free to bound water increase with increasing extracellular tumor volume.     

Unconventional magnetic and transport properties in Gd1−xLaxMn2Ge2 with two-dimensional arrangement of Mn atoms

In this paper, we present our recent experimental results of magnetic and transport properties of Gd_1−xLa_xMn₂Ge₂ intermetallic compounds with the ThCr₂Si₂-type layered structure. The results obtained indicate that, in GdMn₂Ge₂, a first-order transition from a collinear antiferromagnetic to a collinear ferrimagnetic state appears with decreasing temperature at T_t3, below the Néel temperature T_N. In Gd_1−xLa_xMn₂Ge₂ compounds with x=0.05 and 0.075, after ordering ferrimagnetically at T_t1, two kinds of first-order transitions from a canted ferrimagnetic to a non-collinear antiferromagnetic state and from a non-collinear antiferromagnetic to a reentrant canted ferrimagnetic state occur at T_t2 and T_t3. In Gd_0.925La_0.075Mn₂Ge₂, a field-induced metamagnetic transition from non-collinear antiferromagnetism to canted ferrimagnetism occurs at relatively low fields, accompanied by fractal like multi-step transitions, the so called “devil's stair-case”. Furthermore, a negative giant magnetoresistance (GMR) effect (Δρ/ρ15%) was observed at the field-induced metamagnetic transition. The mechanism of this negative GMR was clarified by comprehensive measurements of the resistivity on single crystals Gd_0.925La_0.075Mn₂Ge₂ and TbMn₂Ge₂. With further increasing x, only canted ferrimagnetism appears with a compensation temperature for 0.10<x<0.40, whereas no compensation behavior appears for x>0.50. The phase diagram obtained indicates that the overall magnetism is controlled by the Mn–Mn intralayer distance in the tetragonal c-plane, reflecting the two-dimensional arrangement of Mn atoms.     

Magnetic Resonance Fingerprinting with short relaxation intervals

PurposeThe aim of this study was to investigate a technique for improving the performance of Magnetic Resonance Fingerprinting (MRF) in repetitive sampling schemes, in particular for 3D MRF acquisition, by shortening relaxation intervals between MRF pulse train repetitions.Material and methodsA calculation method for MRF dictionaries adapted to short relaxation intervals and non-relaxed initial spin states is presented, based on the concept of stationary fingerprints. The method is applicable to many different k-space sampling schemes in 2D and 3D. For accuracy analysis, T₁ and T₂ values of a phantom are determined by single-slice Cartesian MRF for different relaxation intervals and are compared with quantitative reference measurements. The relevance of slice profile effects is also investigated in this case. To further illustrate the capabilities of the method, an application to in-vivo spiral 3D MRF measurements is demonstrated.ResultsThe proposed computation method enables accurate parameter estimation even for the shortest relaxation intervals, as investigated for different sampling patterns in 2D and 3D. In 2D Cartesian measurements, we achieved a scan acceleration of more than a factor of two, while maintaining acceptable accuracy: The largest T₁ values of a sample set deviated from their reference values by 0.3% (longest relaxation interval) and 2.4% (shortest relaxation interval). The largest T₂ values showed systematic deviations of up to 10% for all relaxation intervals, which is discussed. The influence of slice profile effects for multislice acquisition is shown to become increasingly relevant for short relaxation intervals. In 3D spiral measurements, a scan time reduction of 36% was achieved, maintaining the quality of in-vivo T1 and T2 maps.ConclusionsReducing the relaxation interval between MRF sequence repetitions using stationary fingerprint dictionaries is a feasible method to improve the scan efficiency of MRF sequences. The method enables fast implementations of 3D spatially resolved MRF.     

Analysis of inelastic scattering processes of electrons by localized electrons in quantum dots

The inelastic Coulomb scattering rate 1/τ_in of conduction electrons has been theoretically evaluated in the presence of localized states such as quantum dots. By a diagrammatical method, we have formulated 1/τ_in and its relation to the conductivity σ_loc(ω) through localized states. The dependence of τ_in on temperature T is examined in the case that σ_loc(ω) follows the Mott's model. It is found that 1/τ_in varies as T²(ln Δ/T)^d+1 where d is the dimensionality and Δ is tunneling energy between the localized states in the asymptonic T = 0 limit, in agreement with Imry's calculation. It is also found that calculated 1/τ_in deviates from T²(ln Δ/T)^d+1 as T increases, suggesting the importance of correction term at high temperature.     

Organ point dose measurements in clinical multi slice computed tomography (MSCT) examinations with the MOSkin™ radiation dosimeter

This study reports on the application of the MOSkin™ dosimeter in MSCT imaging for the real-time measurement of absorbed organ point doses in a tissue-equivalent female anthropomorphic phantom. MOSkin™ dosimeters were placed within the phantom to measure absorbed point organ doses for 2 commonly applied clinical scan protocols, namely the renal calculus scan and the pulmonary embolus scan. Measured organ doses in the imaged field of view were found to be in the dose range 4.7–9.5 mGy and 16.2–27.4 mGy for the renal calculus scan and pulmonary scan protocols respectively. For the derivation of effective dose, using the more recent ICRP 103 tissue weighting factors (w_T) compared to that of the ICRP 60 w_T resulted in a difference in the derived effective dose by up to 0.8 mSv (−20%) in the renal calculus protocol and up to 1.8 mSv (18%) in the pulmonary embolus protocol. This difference is attributed to the reduced radiosensitivity of the gonads and the increased radiosensitivity of breast tissue in the latest ICRP 103 assigned w_T. The results of this study show that the MOSkin™ dosimeter is a useful real-time tool for the direct assessment of organ doses in clinical MSCT examinations.     

Consideration of slice profiles in inversion recovery Look–Locker relaxation parameter mapping

Purpose

To include the flip angle distribution caused by the slice profile into the model used for describing the relaxation curves observed in inversion recovery Look–Locker FLASH T₁ mapping for a more accurate determination of the relaxation parameters.

Materials and methods

For each inversion time, the flip angle dependent signal of the mono-exponential relaxation model is integrated across the slice profile. The resulting Consideration of Slice Profiles (CSP) relaxation curves are compared to the mono-exponential signal model in numerical simulations as well as in phantom and in-vivo experiments.

Results

All measured relaxation curves showed systematic deviations from a mono-exponential curve increasing with flip angle and T₁ but decreasing with repetition time. Additionally, the accuracy of T₁ was found to be largely dependent on the temporal coverage of the relaxation curve. All these systematic errors were largely reduced by the CSP model.

Conclusion

The proposed CSP model represents a useful extension of the conventionally used mono-exponential relaxation model. Despite inherent model inaccuracies, the mono-exponential model was found to be sufficient for many T₁ mapping situations. However, if only a poor temporal coverage of the relaxation process is achievable or a very precise modeling of the relaxation course is needed as in model-based techniques, the mono-exponential model leads to systematic errors and the CSP model should be used instead.     

Methods for correlating T1ρ and FID components in wideline H NMR studies of motionally heterogeneous polymer systems

We address the problem of correlating the observed FID and T_1ρ components in wideline ¹H relaxation measurements of motionally heterogeneous polymers, and show that different methods of data treatment can highlight different aspects of the correlations present. For a sample of polypropylene we find that the T_1ρ relaxation behaviour is driven by relaxation associated with the intermediate FID component, which strongly suggests a motionally inhomogeneous amorphous region in the sample.     

Effect of physiological Heart Rate variability on quantitative T2 measurement with ECG-gated Fast Spin Echo (FSE) sequence and its retrospective correction

Object

Quantitative T₂ measurement is applied in cardiac Magnetic Resonance Imaging (MRI) for the diagnosis and follow-up of myocardial pathologies. Standard Electrocardiogram (ECG)-gated fast spin echo pulse sequences can be used clinically for T₂ assessment, with multiple breath-holds. However, heart rate is subject to physiological variability, which causes repetition time variations and affects the recovery of longitudinal magnetization between TR periods.

Materials and methods

The bias caused by heart rate variability on quantitative T₂ measurements is evaluated for fast spin echo pulse sequence. Its retrospective correction based on an effective TR is proposed. Heart rate variations during breath-holds are provided by the ECG recordings from healthy volunteers. T₂ measurements were performed on a phantom with known T₂ values, by synchronizing the sequence with the recorded ECG. Cardiac T₂ measurements were performed twice on six volunteers. The impact of T₁ on T₂ is also studied.

Results

Maximum error in T₂ is 26% for phantoms and 18% for myocardial measurement. It is reduced by the proposed compensation method to 20% for phantoms and 10% for in vivo measurements. Only approximate knowledge of T₁ is needed for T₂ correction.

Conclusion

Heart rate variability may cause a bias in T2 measurement with ECG-gated FSE. It needs to be taken into account to avoid a misleading diagnosis from the measurements.     

A rapid measurement of : The DECPMG sequence

The Driven-Equilibrium Carr–Purcell Meiboom–Gill (DECPMG) pulse sequence is a rapid method for obtaining the average ratio of longitudinal to transverse relaxation times T₁/T₂ as a function of T₂. Since this is a one-dimensional experiment, the T₁/T_2T2 ratio can be acquired, potentially, in just two scans; the second scan being a reference CPMG measurement. Conventionally, T₁/T₂ is determined from a two-dimensional T₁-T₂ relaxation correlation experiment. The method described here offers a significant reduction in experimental time without a reduction in signal-to-noise. The T₁/T₂ ratio is useful for comparing the behaviour of liquids in porous media. Here we demonstrate the application of the DECPMG sequence to the study of oil-bearing rocks by differentiating oil or water saturated rock cores, and by observing the relative strengths of surface interaction for water in two types of rock by measuring T₁/T₂ as a function of magnetic field strength.     

Improved Estimation of Protein Rotational Correlation Times fromN Relaxation Measurements

In the study of protein backbone dynamics by¹⁵N relaxation measurements, an initial estimation of the isotropic global correlation time, τ_m, is usually obtained from the averageT₁/T₂ratio of nuclear spins that do not exhibit slow internal motion and withT₂values not significantly shortened by chemical or conformational exchange processes. Different methods have been used for identification of the rates of internal motion. However, the number of nuclear spins included in the τ_mestimation is often larger than the number that ultimately can be fitted to a single-order parameter,S², implying that some nuclear spins involved in the initial τ_mestimation actually have an effective internal correlation time, τ_e, not as fast as assumed. As a consequence, τ_mis underestimated, since internal motion reduces theT₁/T₂ratio. This situation becomes more obvious if the molecule has a large τ_mvalue because the reduction inT₁/T₂ratio arising from internal motion is more significant than for molecules with smaller τ_mand the same degree of internal motion. This Communication describes a more reliable method for identifying nuclear spins which should be excluded from the τ_mestimation because of insufficiently rapid internal motion. This results in an improved τ_mvalue, giving a much better agreement between the number of nuclear spins fitted successfully to a single-order parameter,S², and those used in the τ_mestimation.     

Slow Dynamics Mapping of Large-Linewidth Solids by a MARF Spin-Lock Filter

A new method forT_1ρcontrast in solid-state imaging is presented. The method is based upon the addition of aT_1ρspin-lock pulse to the standard MARF pulse sequence and is able to provide relaxation contrast in dependence either on the lock time or on the intensity of the lock field, without having effects related to the line-narrowing procedure. The results on some large-linewidth solids show thatT_1ρvalues measured by the usual spin-lock pulse sequence agree with those measured on space-resolved lines.     

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT₁enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT₁andT^*:₂relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,¹H, were acquired in a phantom study. Sulfur hexafluoride,¹⁹F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.