Alterations in MR relaxation of normal canine gallbladder bile during fasting

The correlation between the concentration and proton relaxation of bile was studied by examining sequential changes in the MR image appearance and relaxation times of gallbladder bile during a 24-h fasting period in dogs. Bile relaxation times computed from images showed progressive shortening during the first 4–8 h of fasting: T₁ decreased from 500–900 ms to 250–400 ms and T₂ from 130–190 ms to 70–100 ms at 0.15 T. Similar in vitro results at 0.47 T were obtained on aspirated bile samples. Relaxation times of gallbladder bile remained longer than those of the liver, and we conclude that in general the gallbladder will appear less intense than the liver on T₁-weighted images (with short enough TE) and more intense on T₂-weighted images regardless of the bile concentration. The liver/gallbladder contrast may reverse in a normal subject during fasting for pulse sequences combining both T₁ and T₂ effects, which may be explored for the possible visual detection of abnormal gallbladder function on an image.     

Activation of area V5 by visual perception of motion demonstrated with echoplanar MR imaging

Cortical activation in visual association areas known to be responsible for the perception of motion was investigated in two volunteers who viewed a projected animated cartoon periodically “run” and “frozen” during collection of echoplanar MR images. Ten axial, contiguous, 5 mm thick, T₂-weighted, gradient-echo images (TE 40 ms, TR 3000 ms) depicting BOLD contrast were acquired through the occipital lobe using a GE Signa 1.5 T system with an advanced NMR operating console. Images were analysed by time series regression modelling estimating power in the MR signal at the ON-OFF frequency of motion. Highly significant activation in response to motion perception was identified in both subjects bilaterally in area V5.     

Assessment of T1, T1ρ, and T2 values of the ulnocarpal disc in healthy subjects at 3 tesla

Objective

The purpose of this study was to implement clinically feasible imaging techniques for determination of T₁, T_1ρ, and T₂ values of the ulnocarpal disc and to assess those values in a cohort of asymptomatic subjects at 3 tesla. Resulting values were compared between different age groups, since former histological findings of the ulnocarpal disc indicated frequent early degenerative changes of this tissue starting in the third decade of life, even in asymptomatic subjects.

Materials and methods

Twenty-seven healthy subjects were included in this study. T₁ measurements were performed using 3D spoiled gradient-echo (GRE) sequence with variable flip angle. A series of T_1ρ and T₂-weighted images was acquired by a 3D GRE sequence after suitable magnetization preparation. T_1,T_1ρ, and T₂ maps of the ulnocarpal disc were calculated pixel-wise. Representative mean values from extended regions were analysed.

Results

Mean T₁ values of the ulnocarpal disc ranged from 722 ms in a 39 year-old subject to 1264 ms in a 65 year-old subject, T_1ρ ranged from 9.2 ms (26 year-old subject) to 25.9 ms (65 year-old subject). Calculated T₂ values showed a large range from 4.1 ms to 22.3 ms. T_1ρ and T₁ values tended to increase with age (p < 0.05), whereas T₂ did not.

Conclusions

MR relaxometry of the ulnocarpal disc is feasible, and T_1,T_1ρ, and T₂ values show modest variance in asymptomatic subjects. The potential of relaxation mapping to reveal relevant structural changes in patients has to be investigated in further studies.     

Large angle spin-echo imaging

A study was undertaken to assess the use of excitation flip angles greater than 90° for T₁ weighted spin-echo (SE) imaging with a single 180° refocusing pulse and short TR values. Theoretical predictions of signal intensity for SE images with excitation pulse angles of 90–180° were calculated based on the Bloch equations and then measured experimentally from MR images of MnCl₂ phantoms of various concentrations. Liver signal-to-noise ratios (SNR) and liver-spleen contrast-to-noise ratios (CNR) were measured from breathhold MR images of the upper abdomen in 16 patients using 90 and 110° excitation flip angles. The theoretical predictions showed significant improvements in SNR with excitation flip angles >90°, which were more pronounced at small TR values. The phantom studies showed reasonably good agreement with the theoretical predictions in correlating the excitation pulse angle with signal intensity. In the human imaging studies, the 110° excitation pulse angle resulted in a 7.4% (p < .01) increase in liver SNR and an 8.2% (p = .2) increase in liver-spleen CNR compared to the 90° pulse angle at TR = 275 ms. Increased signal intensity resulting from the use of large flip angle excitation pulses with a single echo SE pulse sequence was predicted and confirmed experimentally in phantoms and humans.     

Magnetic resonance imaging of cortical bone with ultrashort TE pulse sequences

PurposeNormal adult cortical bone has a very short T₂ and characteristically produces no signal with pulse sequence echo times (TEs) routinely used in clinical practice. We wished to determine whether it was possible to use ultrashort TE (UTE) pulse sequences to detect signal from cortical bone in human subjects and use this signal to characterise this tissue.Subjects and MethodsSeven volunteers and 10 patients were examined using ultrashort TE pulse sequences (TE=0.07 or 0.08 ms). Short and long inversion as well as fat suppression pulses were used as preparation pulses. Later echo images were also obtained as well as difference images produced by subtracting a later echo image from a first echo image. Saturation pulses were used for T₁ measurement and sequences with progressively increasing TEs for T₂* measurement. Intravenous gadodiamide was administered to four subjects.ResultsSignal in cortical bone was detected with UTE sequences in children, normal adults and patients. This signal was usually made more obvious by subtracting a later echo image from the first provided that the signal-to-noise ratio was sufficiently high.Normal mean adult T₁s ranged from 140 to 260 ms, and mean T₂*s ranged from 0.42 to 0.50 ms. T₁ increased significantly with age (P<.01).Increased signal was observed after contrast enhancement in the normal volunteer and the three patients to whom it was administered.Reduction in signal from short T₂ components was seen in acute fractures, and increase in signal in these components was seen with new bone formation after fracture malunion. In a case of osteoporosis, bone cross-sectional area and signal level appeared reduced.ConclusionSignal can be detected from normal and abnormal cortical bone with UTE pulse sequences, and this can be used to measure its T₁ and T₂* as well as observe contrast enhancement. Difference images are of value in increasing the conspicuity of cortical bone and observing abnormalities in disease.     

High field NMR microscopic imaging of cultivated strawberry fruit

The experimental conditions required for discrimination of various types of tissue in fruits of cultivated strawberry (Fragaria × Ananassa) at high fields (ca. 7 T) have been investigated. In marked contrast to soft fruits of other species, from which informative images have been derived at high fields using a variety of pulse sequences and acquisition parameters, appreciable image intensities from parenchymal and vascular tissues in healthy strawberry fruits were obtained only with a spin-echo imaging sequence using large sweep widths (ca. 100,000 Hz), and consequently small values for TE (<5 ms), indicating predominantly short T₂ values for these tissues. Damage caused by infection by the fungal pathogen Botrytis cinerea is readily seen as a result of a large increase in T₂ in the infected tissue, whereas ripening processes appear to be characterized primarily by small variations in the T₂-weighted contrast and in the relative magnitudes of T₁ between vascular and parenchymal tissue. In addition, it was possible selectively to enhance the contributions to images from the achenes (“seeds”) by using very short relaxation delays, thereby enhancing T₁-dominated contrast mechanisms.     

Determination of T1ρ values for head and neck tissues at 0.1 T: a comparison to T1 and T2 relaxation times

In order to optimize head and neck magnetic resonance (MR) imaging with the spin-lock (SL) technique, the T₁ρ relaxation times for normal tissues were determined. Furthermore, T₁ρ was compared to T₁ and T₂ relaxation times. Ten healthy volunteers were studied with a 0.1 T clinical MR imager. T₁ρ values were determined by first measuring the tissue signal intensities with different locking pulse durations (TL), and then by fitting the signal intensity values to the equation with the least-squares method. The T₁ρ relaxation times were shortest for the muscle and tongue, intermediate for lymphatic and parotid gland tissue and longest for fat. T₁ρ demonstrated statistically significant differences (p < 0.05) between all tissues, except between muscle and tongue. T₁ρ values measured at locking field strength (B1L) of 35 μT were close to T₂ values, the only exception being fat tissue, which showed T₁ρ values much longer than T₂ values. Determination of tissue relaxation times may be utilized to optimize image contrast, and also to achieve better tissue discrimination potential, by choosing appropriate imaging parameters for the head and neck spin-lock sequences.     

Proton relaxation times in human red bone marrow by volume selective magnetic resonance spectroscopy

In hematological diseases the composition of red bone marrow shows alterations. The relaxation timesT ₁ andT ₂ of water and lipids in human hemopoietic bone marrow of 14 normal volunteers and 10 patients with acute leukemia and bone marrow carcinosis are determined using a double spin echo spectroscopy sequencein vivo. The volumes of interest (VOI) of (13 mm)³ in the center of vertebral bodies are examined using different measurement parameters. ForT ₁ measurements an inversion-recovery method is used.T ₂ is evaluated from spectra with differentTE. T ₁ (water) is found in a range between 1000 and 1700 ms,T ₁ (lipids) in a range between 260 and 320 ms in healthy volunteers.T ₂ (water) is determined between 32 and 65 ms. In some cases phase distortions of the water signals occur in the spectra. Water flow within the VOI may be a possible reason.T ₂ (lipids) is evaluated between 73 and 91 ms. The patients with acute leukemia exhibit clearly reduced lipid signals in their spectra. Lipid relaxation times could not be determined in these cases.T ₂ (water) is prolonged in acute leukemia to 51–98 ms.T ₁ (water) was not significantly different from values of healthy volunteers in our measurements. Results are discussed in comparison to relaxometric data from imaging and STEAM spectroscopic methods of other authors.     

Bi-Exponential T2 Decay in Dairy Cream Phantoms

MRI phantoms are an important part of any experiment because they provide a reference of known parameters. There are many choices of mono-exponential T₂ phantoms, but few choices for bi-exponential T₂ phantoms. We have found that dairy cream provides an excellent bi-exponential T₂ model with similar relaxation times to those found in white matter. Five cream phantoms of different milk fat percentages (2, 6, 10, 18 and 35%) were imaged with an optimized Carr–Purcell–Meiboom–Gill sequence. The decay curves for each of the phantoms were fit using Non-Negative Least Squares. We found that the short T₂ component fraction relative to the total energy in the distribution correlated linearly (r = 0.9973) with the milk fat percentage. The short T₂ time was 38 ± 4 ms and the long T₂ time was 135 ± 4 ms.     

T1ρ mapping improvement using stretched-type adiabatic locking pulses for assessment of human liver function at 3 T

PurposeThe purpose of this study is to investigate the performance of stretched-type adiabatic spin lock pulses for homogeneous spin locking with a flexible spin lock time (TSL) setting.MethodsT_1ρ values were obtained from 61 patients and five normal volunteers who were categorized using the Child–Pugh classification and scanned using each spin lock pulse type. The pulses used were the block and two kinds of hyperbolic secant (HS); HS8_10, and HS8_5. Visual scoring was categorized using a four point scale (1:Severe, 2:Moderate, 3:Mild and 4:None) to evaluate the homogeneity of the T_1ρ map and the source images obtained by each spin lock pulse. Mean T_1ρ values among the patient groups with different Child–Pugh classification were compared.ResultsThe visual assessment scores were 1.98 ± 1.05 for block pulse locking, 3.87 ± 0.39 for HS8_10 pulse locking, and 3.83 ± 0.45 for HS8_5 pulse locking, respectively. The scores between block pulse and HS8_10 were significantly different (p < 0.001), as were those between block pulse and HS8_5 (p < 0.001).The median T_1ρ values of normal liver function, Child–Pugh A, and Child–Pugh B or C were 37.00 ms, 40.77 ms, and 42.20 ms for block pulse, 46.75 ms, 50.78 ms, and 55.60 ms for HS8_10, and 48.80 ms, 55.42 ms, and 57.80 ms for HS8_5, respectively.ConclusionThe spin locking sequence using stretched-type adiabatic pulses provides homogeneous liver T_1ρ maps with reduced artifact and is necessary for a robust evaluation of liver function using T_1ρ.     

Assessment of acute myocardial infarction in man with magnetic resonance imaging and the use of a new paramagnetic contrast agent gadolinium-bopta

To assess the feasibility of and characterize the new paramagnetic contrast agent gadolinium-BOPTA/dimeglumine (Gd-BOPTA) to detect acute myocardial infarctions with MR imaging, 24 patients (53.3 ± 8.3 yr) were examined 9.3 ± 3.6 days after a first myocardial infarction. Short-axis T₁-weighted and T₂-weighted MR imaging was performed at three slice levels. T₁-weighted images were obtained before, immediately after, 15, 30, and 45 min after injection. Patients received either 0.05 or 0.1 mmol/kg body weight Gd-BOPTA. Images were qualitatively and quantitatively analyzed. Two patients showed no signs of infarction on T₂-weighted images as opposed to contrast-enhanced T₁-weighted images. Contrast-to-noise ratio was not affected by the dosage level. Signal intensity (SI) of normal to infarcted myocardium was significantly improved by both dosages (p < .0005) but a dosage of 0.05 mmol/kg produced significantly higher SI inf/norm (1.42 ± 0.07 vs. 1.34 ± 0.06, respectively, p = .015). SI of normal and infarcted myocardium enhanced immediately after administration of 0.05 mmol/kg (29.3 ± 5.1% and 53.8 ± 9.6% respectively), which decreased thereafter to 5.3 ± 4.8% and 40.2 ± 8.5% respectively, at 45 min (p < .002 for normal myocardium). SI enhancement immediately after 0.1 mmol/kg Gd-BOPTA showed no decrease within the first 45 min. Gd-BOPTA enables the detection of myocardial infarction. Optimal infarct delineation is achieved from 15 to 45 min after administration of 0.05 mmol/kg body weight Gd-BOPTA. Gd-BOPTA at 0.05 mmol/kg does improve image quality as measured by contrast-to-noise ratio and SI enhancement as compared to 0.10 mmol/kg.     

A comparative study at 3 T of sequence dependence of T2 quantitation in the knee

Objective

T₂ mapping has been used widely in detecting cartilage degeneration in osteoarthritis. Several scanning sequences have been developed in the determination of T₂ relaxation times of tissues. However, the derivation of these times may vary from sequence to sequence. This study seeks to evaluate the sequence-dependent differences in T₂ quantitation of cartilage, muscle, fat and bone marrow in the knee joint at 3 T.

Methods

Three commercial phantoms and 10 healthy volunteers were studied using 3 T MR. T₂ relaxation times of the phantoms, cartilage, muscle, subcutaneous fat and marrow were derived using spin echo (SE), multiecho SE (MESE), fast SE (FSE) with varying echo train length (ETL), spiral and spoiler gradient (SPGR) sequences. The differences between these times were then evaluated using Student's t test. In addition, the signal-to-noise ratio (SNR) efficiency and coefficient of variation of T₂ from each sequence were calculated.

Results

The average T₂ relaxation time was 36.38±5.76 ms in cartilage and 34.08±6.55 ms in muscle, ranging from 27 to 45 ms in both tissues. The times for subcutaneous fat and marrow were longer and more varying, ranging from 41 to 143 ms and from 42 to 160 ms, respectively. In FSE acquisition, relaxation time significantly increases as ETL increases (P<.05). In cartilage, the SE acquisition yields the lowest T₂ values (27.52±3.10 ms), which is significantly lower than those obtained from other sequences (P<.002). T₂ values obtained from spiral acquisition (38.27±6.45 ms) were higher than those obtained from MESE (34.35±5.62 ms) and SPGR acquisition (31.64±4.53 ms). These differences, however, were not significant (P>.05).

Conclusion

T₂ quantification can be a valuable tool for the diagnosis of degenerative disease. Several different sequences exist to quantify the relaxation times of tissues. Sequences range in scan time, SNR efficiency, reproducibility and two- or three-dimensional mapping. However, when choosing a sequence for quantitation, it is important to realize that several factors affect the measured T₂ relaxation time.     

Retrospective correction for T1-weighting bias in T2 values obtained with various spectroscopic spin-echo acquisition schemes

Localized tissue transverse relaxation time (T₂) is obtained by fitting a decaying exponential to the signals from several spin-echo experiments at different echo times (TE). Unfortunately, time constraints in magnetic resonance spectroscopy (MRS) often mandate in vivo acquisition schemes at short repetition times (TR), that is, comparable with the longitudinal relaxation constant (T₁). This leads to different T₁-weighting of the signals at each TE. Unaccounted for, this varying weighting causes systematic underestimation of the T₂'s, sometimes by as mush as 30%. In this article, we (i) analyze the phenomenon for common MRS spin-echo T₂ acquisition schemes; (ii) propose a general post hoc T₁-bias correction for any (TR, TE) combination; (iii) show that approximate knowledge of T₁ is sufficient, since a 20% uncertainty in T₁ leads to under 3% bias in T₂; and consequently, (iv) efficient, precision-optimized short TR spin-echo T₂ measurement protocols can be designed and used without risk of accuracy loss. Tables of correction for single-refocusing (conventional) spin-echo and double refocusing, such as, PRESS acquisitions, are provided.     

Diffusion measurements using oscillating gradients

A novel method for the measurement of self-diffusion coefficients employing oscillating gradients is presented. The method used has advantages over conventional techniques and will allow measurements to be made at very short diffusion times (<1 ms) and should prove particularly useful for shortT ₂ materials.     

High resolution neuroimaging at 4.1 T

In this article we report on acquisition of high resolution 512 × 512 images at 4.1 T using an inversion recovery gradient-echo sequence and a volume head coil developed for high field applications. The T₁ values for cerebral white and grey matter were measured to be 834 and 1282 ms, respectively. The partial saturation inversion recovery sequence (T_ir 800 ms and TR 2500 ms) provided excellent contrast-to-noise for white to grey matter. Consequently, the images consistently visualized the thalamic nuclear groups, hippocampal fine structure, as well as small draining vessels of the white matter.     

Preparation and low field magnetoresistance of double layered manganite La1.4Ca1.6Mn2O7

Double layered manganite of La_1.4Ca_1.6Mn₂O₇ (DLCMO) was prepared using solid state reaction method and had a metal-insulator transition temperature (T_MI) of 125 K. The short range 2D-feerromagnetic ordering (T_C2) starts growing when T<168 K and it gets converted into 3D-ferromagnetic ordering (T_C1) at 114 K. Low field magnetoresistance (MR) behaviour of the DLCMO was investigated and compared with an infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). For DLCMO, in the temperature range between T_C1 and T_C2, the MR showed a gradual increase with the magnetic field. The observed MR and R-T behaviour of double layered manganite for T_C1<T<T_C2 has been explained in the frame work of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions] and percolative behaviour of transport in FM-PM mixture.     

Simultaneous measurements of diffusion and transverse relaxation in exercising skeletal muscle

The aim of this study was to compare proton T₂ and apparent diffusion coefficient (ADC) variations induced by exercise in skeletal muscle, to provide some more information on the source of their variations. T₂ and ADC were measured in the forearm flexor digitorum muscles in 12 healthy volunteers at rest and after an exercise, using a sequence allowing simultaneous measurements of both parameters. At rest, T₂ was 30.6 ± 1.8 ms (mean ± 1 SD) and ADC was 1.82 ± 0.11 × 10⁻⁹ m²/s. With exercise, T₂ varied by +28 ± 12% (p < .001 vs. rest) and ADC varied by +12 ± 3% (p < .001). The recovery of T₂ after exercise was faster than that of ADC, with half-times of 7 ± 2 min and of 15 ± 8 min (p < .01), respectively. It is concluded that both T₂ and ADC increase with exercise. However, the mechanisms of variation of T₂ and ADC with exercise are probably different, T₂ mostly reflecting changes in water content and ADC reflecting temperature variations.     

T2 Relaxation of peripheral nerve measured in vivo

It is demonstrated that multi-exponential transverse (T₂) relaxation components can be estimated from multi-echo images of peripheral nerve. Three T₂-relaxation components with T₂ values ± standard deviations (populations ± standard deviations) of 19 ± 7 ms (26 ± 9%), 63 ± 31 ms (29 ± 11%) and 241 ± 24 ms (45 ± 7%) have been identified in vivo in the sciatic nerve of the amphibian Xenopus laevis. The longer-lived component, not identified previously in vivo, provides a significant contrast-to-noise ratio (CNR) between nerve and muscle in the latter-echo images. It is shown that the CNR can be further improved by the averaging of selected images from the multi-echo set.     

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.     

Hepatocellular lesions with increased iron uptake on superparamagnetic iron oxide-enhanced magnetic resonance imaging in cirrhosis or chronic hepatitis: comparison of four magnetic resonance sequences for lesion conspicuity

Purpose

The aim of this study was to determine the adequate MR sequence for the lesion conspicuity of hepatocellular lesions with increased iron uptake on superparamagnetic iron oxide (SPIO)-enhanced MRI.

Materials and Methods

SPIO-enhanced MRI was performed using a 1.5-T system. Among 25 patients with hypovascular hepatocellular nodules on contrast-enhanced dynamic CT (no early enhancement at arterial phase and hypoattenuation at equilibrium phase), 39 lesions with increased iron uptake on SPIO-enhanced MRI were evaluated. SPIO-enhanced MRI included (1) T1-weighted in-phase gradient recalled echo (GRE) images, (2) T2-weighted fast spin echo (FSE) images, (3) T2*-weighted GRE with moderate TE (7 ms) and (4) long TE (12 ms). The lesion-to-liver contrast-to-noise ratios of the hepatocellular nodule and the signal-to-noise ratio (SNR) of the hepatic parenchyma were calculated by one radiologist for a quantitative assessment. MR images were reviewed retrospectively by two independent radiologists to compare the subjective lesion conspicuity in each image set based on a four-point rating scale.

Result

The mean lesion-to-liver contrast-to-noise ratios with T2*-weighted GRE with moderate TE (7 ms) was highest (5.79±3.71) and was significantly higher than those with T1-weighted, in-phase images (3.79±3.23, P<.01), T2-weighted images (2.72±1.52, P<.001) and T2*-weighted GRE with long TE (12 ms) (3.93±2.69, P<.05). The subjective rating of lesion conspicuity was best on the T2*-weighted GRE with moderate TE (7 ms), followed by that on the T2*-weighted GRE with moderate TE (7 ms; P<.05).

Conclusion

T2*-weighted GRE sequence with moderate TE (7 ms) showed high lesion-to-liver contrast-to-noise ratios in hepatocellular lesions with increased iron uptake on SPIO-enhanced MRI, indicating better lesion conspicuity of hypointense hepatocellular nodules in cirrhosis or chronic hepatitis.