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.     

Autonomous magnetic resonance imaging

Access to Magnetic Resonance Imaging (MRI) across developing countries ranges from being prohibitive to scarcely available. For example, eleven countries in Africa have no scanners. One critical limitation is the absence of skilled manpower required for MRI usage. Some of these challenges can be mitigated using autonomous MRI (AMRI) operation. In this work, we demonstrate AMRI to simplify MRI workflow by separating the required intelligence and user interaction from the acquisition hardware. AMRI consists of three components: user node, cloud and scanner. The user node voice interacts with the user and presents the image reconstructions at the end of the AMRI exam. The cloud generates pulse sequences and performs image reconstructions while the scanner acquires the raw data. An AMRI exam is a custom brain screen protocol comprising of one T₁-, T₂- and T₂*-weighted exams. A neural network is trained to incorporate Intelligent Slice Planning (ISP) at the start of the AMRI exam. A Look Up Table was designed to perform intelligent protocolling by optimizing for contrast value while satisfying signal to noise ratio and acquisition time constraints. Data were acquired from four healthy volunteers for three experiments with different acquisition time constraints to demonstrate standard and self-administered AMRI. The source code is available online. AMRI achieved an average SNR of 22.86 ± 0.89 dB across all experiments with similar contrast. Experiment #3 (33.66% shorter table time than experiment #1) yielded a SNR of 21.84 ± 6.36 dB compared to 23.48 ± 7.95 dB for experiment #1. AMRI can potentially enable multiple scenarios to facilitate rapid prototyping and research and streamline radiological workflow. We believe we have demonstrated the first Autonomous MRI of the brain.     

Three-dimensional black-blood T2 mapping with compressed sensing and data-driven parallel imaging in the carotid artery

PurposeTo develop a 3D black-blood T₂ mapping sequence with a combination of compressed sensing (CS) and parallel imaging (PI) for carotid wall imaging.Materials and methodsA 3D black-blood fast-spin-echo (FSE) sequence for T₂ mapping with CS and PI was developed and validated. Phantom experiments were performed to assess T₂ accuracy using a Eurospin Test Object, with different combination of CS and PI acceleration factors. A 2D multi-echo FSE sequence was used as a reference to evaluate the accuracy. The concordance correlation coefficient and Bland-Altman statistics were calculated. Twelve volunteers were scanned twice to determine the repeatability of the sequence and the intraclass correlation coefficient (ICC) was reported. Wall-lumen sharpness was calculated for different CS and PI combinations. Six patients with carotid stenosis > 50% were scanned with optimised sequence. The T₂ maps were compared with multi-contrast images.ResultsPhantom scans showed good correlation in T₂ measurement between current and reference sequence (r = 0.991). No significant difference was found between different combination of CS and PI accelerations (p = 0.999). Volunteer scans showed good repeatability of T₂ measurement (ICC: 0.93, 95% CI 0.84–0.97). The mean T₂ of the healthy wall was 48.0 ± 9.5 ms. Overall plaque T₂ values from patients were 54.9 ± 12.2 ms. Recent intraplaque haemorrhage and fibrous tissue have higher T₂ values than the mean plaque T₂ values (88.1 ± 6.8 ms and 62.7 ± 9.3 ms, respectively).ConclusionThis study demonstrates the feasibility of combining CS and PI for accelerating 3D T₂ mapping in the carotid artery, with accurate T₂ measurements and good repeatability.     

Bi-phase age-related brain gray matter magnetic resonance T1ρ relaxation time change in adults

ObjectivesTo investigate normative value and age-related change of brain magnetic resonance T1ρ relaxation at 1.5 T.MethodsThis study was approved by the local ethical committee with participants' written consent obtained. There were 42 adults healthy volunteers, including 20 males (age: 41 ± 16 (mean ± standard deviation) years, range: 22–68 years,) and 22 females (age: 39 ± 15 years, range: 21–62 years). MRI was performed at 1.5 T using 3D fluid suppressed turbo spin echo sequence. Regions-of-interests (ROIs) were obtained by atlas-based tissue segmentation and T1ρ was calculated by fitting the mean value to mono-exponential model. Correlation between T1ρ relaxation of brain gray matter regions and age was investigated.ResultsA regional difference among individual gray matter areas was noted; the highest values were observed in the hippocampus (98.37 ± 5.37 ms, median: 97.88 ms) and amygdala (94.95 ± 4.34 ms, median: 94.73 ms), while the lowest values were observed in the pallidum (83.81 ± 5.49 ms, median: 83.77 ms) and putamen (83.93 ± 4.76 ms, median: 83.99 ms). Gray matter T1ρ values decreased slowly (mean slope: − 0.256) and significantly (p < 0.05) with age in gray matter for subjects younger than 40 years old, while for subjects older than 40 years old there was no apparent correlation between T1ρ relaxation and age. Global white matter measured T1ρ value of 88.65 ± 3.47 ms (median: 87.86 ms), and the correlation with age was not significant (p = 0.18).ConclusionGray matter T1ρ relaxation demonstrates a bi-phase change with age in adults of 22–68 years.     

Rapid high-resolution volumetric T1 mapping using a highly accelerated stack-of-stars Look Locker technique

PurposeTo develop a fast volumetric T₁ mapping technique.Materials and methodsA stack-of-stars (SOS) Look Locker technique based on the acquisition of undersampled radial data (>30× relative to Nyquist) and an efficient multi-slab excitation scheme is presented. A principal-component based reconstruction is used to reconstruct T₁ maps. Computer simulations were performed to determine the best choice of partitions per slab and degree of undersampling. The technique was validated in phantoms against reference T₁ values measured with a 2D Cartesian inversion-recovery spin-echo technique. The SOS Look Locker technique was tested in brain (n = 4) and prostate (n = 5). Brain T₁ mapping was carried out with and without k_z acceleration and results between the two approaches were compared. Prostate T₁ mapping was compared to standard techniques. A reproducibility study was conducted in brain and prostate. Statistical analyses were performed using linear regression and Bland Altman analysis.ResultsPhantom T₁ values showed excellent correlations between SOS Look Locker and the inversion-recovery spin-echo reference (r² = 0.9965; p < 0.0001) and between SOS Look Locker with slab-selective and non-slab selective inversion pulses (r² = 0.9999; p < 0.0001). In vivo results showed that full brain T₁ mapping (1 mm³) with k_z acceleration is achieved in 4 min 21 s. Full prostate T₁ mapping (0.9 × 0.9 × 4 mm³) is achieved in 2 min 43 s. T₁ values for brain and prostate were in agreement with literature values. A reproducibility study showed coefficients of variation in the range of 0.18–0.2% (brain) and 0.15–0.18% (prostate).ConclusionA rapid volumetric T₁ mapping technique was developed. The technique enables high-resolution T₁ mapping with adequate anatomical coverage in a clinically acceptable time.     

Magnetic resonance T21 measurements of the normal human lung in vivo with ultra-short echo times

The objective of this study was to measure T₂1 values of the normal human lung in vivo during breathhold using a rapid gradient-echo sequence with ultra-short echo times (TE). A sagittal slice of the right lung was imaged in six volunteers with various TE ranging from 0.5 ms to 5 ms using a clinical 1.5 Tesla MR scanner. T₂1 values were calculated in a region of interest in the dependent and non-dependent lung. In the dependent lung, T₂1 values of 1.1 ms ± 0.15 ms were measured, and in the non-dependent lung, 0.86 ms ± 0.11 (p < 0.01). T₂1 measurements of the normal human lung during breathhold are feasible with a clinical MR unit. The short T₂1 values require the use of very short TE times (<2.5 ms) in gradient-echo sequences to obtain adequate signal intensity from lung tissue.     

Flip Angle for the Optimal <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>-weighted Image in the 3-D Volumetric Interpolated Breath-hold Examination Abdominal Magnetic Resonance Imaging Technique

The purpose of this study was to determine an optimal flip angle for T ₁-weighted images on abdominal examination by comparing the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) depending on the change in flip angle based on the volumetric interpolated breath-hold examination (VIBE) technique. The subjects in this study included 50 patients (20 men and 10 women; average age, 60 years) who visited the hospital between October 2009 and March 2010 to receive an abdominal magnetic resonance imaging (MRI) examination. Among the 50 patients, there were 27 patients with hypervascular hepatocellular carcinomas (HCCs) and 23 patients with hemorrhagic HCCs. A 3 T MR scanner (Magnetom Tim Trio; Siemens, Germany) with a 12-channel body coil was used. For the pulse sequence, the VIBE (time of repetition TR, 3.18 ms; time of echo TE, 1.16 ms; matrix, 384 × 307; slice thickness, 3 mm; field of view FOV, 380 mm; bandwidth BW, 720 Hz) and breath-hold examination with an examination time of 19 s were used. Images of the axial and coronal planes at three flip angles (10°, 25°, and 35°) were obtained. Based on the images obtained, the signal intensities of the liver, lesions, and background noise were measured and the SNR and CNR were calculated. For evaluation of the optimal flip angle, SPSS for Windows (version 17.0) was used to conduct the non-parametric Kruskal–Wallis test. The SNRs for hypervascular and hemorrhagic HCCs, depending on changes in flip angle of the VIBE, were 11.12 ± 0.98, 10.83 ± 1.44, and 9.61 ± 1.66, and 76.00 ± 6.43, 43.32 ± 5.89, and 30.45 ± 4.27 at angles of 10°, 25°, and 35°, respectively. The CNRs were 14.83 ± 0.12, 7.38 ± 0.41, and 5.70 ± 0.66, and 3.95 ± 0.21, 2.42 ± 0.58, and 1.69 ± 0.93, respectively (p < 0.05). At a flip angle of 10°, the SNR and CNR were the highest. When the flip angles were 25° and 35°, the contrast of the image, as well as the SNR, were shown to have a downward trend (p < 0.05). A flip angle of 10° is considered to be useful for the optimal T ₁-weighted image to detect HCC in the three-dimensional VIBE abdominal MRI technique.     

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.     

Imaging patients pre and post deep brain stimulation: Localization of the electrodes and their targets

PurposeDeep brain stimulation (DBS) has become a widely performed surgical procedure for patients with medically refractory movement disorders and mental disorders. It is clinically important to set up a MRI protocol to map the brain targets and electrodes of the patients before and after DBS and to understand the imaging artifacts caused by the electrodes.MethodsFive patients with DBS electrodes implanted in the habenula (Hb), fourteen patients with globus pallidus internus (GPi) targeted DBS, three pre-DBS patients and seven healthy controls were included in the study. The MRI protocol consisted of magnetization prepared rapid acquisition gradient echo T1 (MPRAGE T1W), 3D multi-echo gradient recalled echo (ME-GRE) and 2D fast spin echo T2 (FSE T2W) sequences to map the brain targets and electrodes of the patients. Phantom experiments were also run to determine both the artifacts and the susceptibility of the electrodes. Signal to noise ratio (SNR) on T1W, T2W and GRE datasets were measured. The visibility of the brain structures was scored according to the Rose criterion. A detailed analysis of the characteristics of the electrodes in all three sequence types was performed to confirm the reliability of the postoperative MRI approach. In order to understand the signal behavior, we also simulated the corresponding magnitude data using the same imaging parameters as in the phantom sequences.ResultsThe mean ± inter-subject variability of the SNRs, across the subjects for T1W, T2W, and GRE datasets were 20.1 ± 8.1, 14.9 ± 3.2, and 43.0 ± 7.6, respectively. High resolution MPRAGE T1W and FSE T2W data both showed excellent contrast for the habenula and were complementary to each other. The mean visibility of the habenula in the 25 cases for the MPRAGE T1W data was 5.28 ± 1.11; and the mean visibility in the 20 cases for the FSE T2W data was 5.78 ± 1.30. Quantitative susceptibility mapping (QSM), reconstructed from the ME-GRE sequence, provided sufficient contrast to distinguish the substructures of the globus pallidus. The susceptibilities of the GPi and globus pallidus externa (GPe) were 0.087 ± 0.013 ppm and 0.115 ± 0.015 ppm, respectively. FSE T2W sequences provided the best image quality with smallest image blooming of stimulator leads compared to MPRAGE T1W images and GRE sequence images, the measured diameters of electrodes were 1.91 ± 0.22, 2.77 ± 0.22, and 2.72 ± 0.20 mm, respectively. High resolution, high bandwidth and short TE (TE = 2.6 ms) GRE helped constrain the artifacts to the area of the electrodes and the dipole effect seen in the GRE filtered phase data provided an effective mean to locate the end of the DBS lead.ConclusionThe imaging protocol consisting of MPRAGE T1W, FSE T2W and ME-GRE sequences provided excellent pre- and post-operative visualization of the brain targets and electrodes for patients undergoing DBS treatment. Although the artifacts around the electrodes can be severe, sometimes these same artifacts can be useful in identifying their location.     

Fast T1 mapping of the brain at high field using Look-Locker and fast imaging

This study aims to develop and evaluate a new method for fast high resolution T1 mapping of the brain based on the Look-Locker technique. Single-shot turboflash sequence with high temporal acceleration is used to sample the recovery of inverted magnetization. Multi-slice interleaved acquisition within one inversion slab is used to reduce the number of inversion pulses and hence SAR. Accuracy of the proposed method was studied using simulation and validated in phantoms. It was then evaluated in healthy volunteers and stroke patients. In-vivo results were compared to values obtained by inversion recovery fast spin echo (IR-FSE) and literatures. With the new method, T₁ values in phantom experiments agreed with reference values with median error < 3%. For in-vivo experiments, a T1 map was acquired in 3.35 s and the T1 maps of the whole brain were acquired in 2 min with two-slice interleaving, with a spatial resolution of 1.1 × 1.1 × 4 mm³. The T₁ values obtained were comparable to those measured with IR-FSE and those reported in literatures. These results demonstrated the feasibility of the proposed method for fast T1 mapping of the brain in both healthy volunteers and stroke patients at 3T.     

Synthetic T2-weighted images of the lumbar spine derived from an accelerated T2 mapping sequence: Comparison to conventional T2w turbo spin echo

ObjectivesTo evaluate the diagnostic usefulness of synthetic T₂-weighted images of the lumbar spine derived from ten-fold undersampled k-space data using GRAPPATINI, a combination of a model-based approach for rapid T₂ and M₀ quantification (MARTINI) extended by generalized autocalibrating partial parallel acquistion (GRAPPA).Materials and methodsOverall, 58 individuals (26 female, mean age 23.3 ± 8.1 years) were examined at 3 Tesla with sagittal and axial T₂w turbo spin echo (TSE) sequences compared to synthetic T₂₋weighted contrasts derived at identical effective echo times and spatial resolutions. Two blinded readers graded disk degeneration and evaluated the lumbar intervertebral disks for present herniation or annular tear. One reader reassessed all studies after four weeks. Weighted kappa statistics were calculated to assess inter-rater and intra-rater agreement. Also, all studies were segmented manually by one reader to compute contrast ratios (CR) and contrast-to-noise ratios (CNR) of the nucleus pulposus and the annulus fibrosus.ResultsOverall, the CR_T2w was 4.45 ± 1.80 and CR_T2synth was 4.71 ± 2.14. Both correlated (rsp = 0.768;p < 0.001) and differed (0.26 ± 1.38;p = 0.002) significantly. The CNR_T2w was 1.73 ± 0.52 and CNR_T2synth was 1.63 ± 0.50. Both correlated (rsp = 0.875;p < 0.001) and differed (−0.10 ± 0.25;p < 0.001) significantly. The inter-rater agreement was substantial to almost perfect (κ = 0.808–0.925) with the intra-rater agreement also substantial to almost perfect (κ = 0.862–0.963). The area under the curve of the receiver operating characteristics assessing disk herniation or annular tear ranged from 0.787 to 0.892.ConclusionsThis study concludes that synthetic images derived by GRAPPATINI can be used for clinical routine assessment with inter-rater and intra-rater agreements comparable to conventional T₂w TSE.     

Development of high resolution 3D hyperpolarized carbon-13 MR molecular imaging techniques

The goal of this project was to develop and apply techniques for T₂ mapping and 3D high resolution (1.5 mm isotropic; 0.003 cm³) ¹³C imaging of hyperpolarized (HP) probes [1-¹³C]lactate, [1-¹³C]pyruvate, [2-¹³C]pyruvate, and [¹³C,¹⁵N₂]urea in vivo. A specialized 2D bSSFP sequence was implemented on a clinical 3T scanner and used to obtain the first high resolution T₂ maps of these different hyperpolarized compounds in both rats and tumor-bearing mice. These maps were first used to optimize timings for highest SNR for single time-point 3D bSSFP acquisitions with a 1.5 mm isotropic spatial resolution of normal rats. This 3D acquisition approach was extended to serial dynamic imaging with 2-fold compressed sensing acceleration without changing spatial resolution. The T₂ mapping experiments yielded measurements of T₂ values of > 1 s for all compounds within rat kidneys/vasculature and TRAMP tumors, except for [2-¹³C]pyruvate which was ~ 730 ms and ~ 320 ms, respectively. The high resolution 3D imaging enabled visualization the biodistribution of [1-¹³C]lactate, [1-¹³C]pyruvate, and [2-¹³C]pyruvate within different kidney compartments as well as in the vasculature. While the mouse anatomy is smaller, the resolution was also sufficient to image the distribution of all compounds within kidney, vasculature, and tumor. The development of the specialized 3D sequence with compressed sensing provided improved structural and functional assessments at a high (0.003 cm³) spatial and 2 s temporal resolution in vivo utilizing HP ¹³C substrates by exploiting their long T₂ values. This 1.5 mm isotropic resolution is comparable to ¹H imaging and application of this approach could be extended to future studies of uptake, metabolism, and perfusion in cancer and other disease models and may ultimately be of value for clinical imaging.     

Diffusion Tensor Magnetic Resonance Imaging in Ring-Enhancing Cerebral Lesions

The objective of this study is to determine differential diagnostic value of diffusion tensor imaging (DTI) in high-grade brain astrocytomas, brain solitary metastases and brain abscesses. 53 patients with cerebral solitary lesions which showed ring enhancement on contrast-enhanced T ₁-weighted images were enrolled in this study. Brain tissues were examined pathologically from 49 patients to confirm the cerebral occupational diseases. Four patients have been diagnosed with primary cancer plus brain solitary metastasis. DTI measurements were obtained from regions of interest placed on central cavity, white matter of the immediate peritumoral region (IPR) and cerebral white matter of the normal side. The cavity of high-grade astrocytoma and brain metastases displayed hypointense signals; most of the brain abscess cavities displayed high signal intensity except for one case with uneven signal intensity. Mean diffusivity (MD) and fractional anisotropy (FA) values could be used for differentiation between tumor and abscess in brain. The brain abscess cavities showed restricted diffusion and anisotropy [MD = (0.604 ± 0.13) × 10⁻³ mm²/s, FA = 0.185 ± 0.03], whereas the central portion of high-grade astrocytoma [MD = (2.76 ± 0.26) × 10⁻³ mm²/s, FA = 0.069 ± 0.02] and solitary brain metastases [MD = (2.82 ± 0.29) × 10⁻³ mm²/s, FA = 0.064 ± 0.02] showed unrestricted diffusion and isotropy. Brain abscess could be differentiated by MD and FA values in their cavities from brain tumors (P < 0.01). The IPRs were all depicted as hyperintense or isointense signals on diffusion-weighted imaging. The difference between FA values in the IPR of high-grade brain astrocytomas and other groups was statistically significant (P < 0.01). In conclusion, our results suggested the potential role of the cavity MD and FA values in the differential diagnoses of brain tumors and brain abscesses; meanwhile, high-grade astrocytomas could be distinguished from solitary metastases and abscesses by evaluating their corresponding FA values in the IPR on brain magnetic resonance imaging (MRI). Combined with conventional MRI, DTI may help radiologists to facilitate the differential diagnosis of ring-enhancing cerebral lesions in clinical practice.     

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.     

Characterization of parotid gland tissue: A description of an MRI protocol set-up and results of in-vivo applications

A reliable protocol for proton T₂ mapping of the parotid region was set up for future characterization of parotid gland disease. A Carr-Purcell-Meiboom-Gill sequence, phase compensated, available on our 1.5 T imager, was selected and acquisition parameters were chosen on the basis of tests performed on phantoms (agarose-doped gels with T₂ in the physiological range). Some experiments were carried out to evaluate the accuracy of T₂ calculations for selective and nonselective refocussing pulses, for image uniformity corrections, and for different situations of slice shift and repetition times. The chosen protocol was then applied to in vivo evaluations to check the long-term precision by means of repeated measurements performed on the same subject over a 2-month period. Two or more reference gels were positioned both in the phantom and volunteer at the edge of the field-of-view (FOV). Image postprocessing consisted of an automatic procedure, written by the authors in Fortran 77, that selected the best fit for each pixel between mono- and biexponential decay models, and prepared four parametric images (T₂ and Rho slow and fast contribution, Rho being a function of proton density and of T₁) that may be used for future elaborations. The phantom experiment results showed an accuracy of 2.5% if a linear correction was performed using the reference gels at the edge of the FOV. No significant differences in accuracy were found between selective and nonselective refocussing pulse, and a homogeneity correction was not demonstrated necessary. The measurements performed on four volunteers showed that the best decaying model for healthy parotid tissue was monoexponential. Evaluated T₂ resulted 80.18 ± 6.11 ms (72.96 ± 4.97 ms for uncorrected results). Long-term reproducibility of the group of measurements from one volunteer, summarizing all the measurement errors, ranged from 0.9 to 8.5%. The two-way ANOVA that was carried out considering the two classes of volunteers and of parotid positions (right or left) showed that differences found between the two parotids were not significant, while T₂ differences among individuals are significant if a probability level higher than 1.1% is accepted. As in this cases, the main source of error can be attributed to the biological variations among individuals. Future statistics collected on patients for the T₂ evaluations of the pathologic tissue will clarify whether the T₂ relaxation is a sufficient parameter for T₂ discrimination of healthy and pathologic tissue.     

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.     

The charge radii of198Pt-183Pt

The reactions⁵⁸Ni+¹⁰²Pd→¹⁶⁰W and⁵⁸Ni+¹⁰⁶Cd→¹⁶⁴Os were investigated to search for new decay data of neutron deficient nuclei. Excitation energies of the compound nuclei covered a range from 47 to 89 MeV. Velocity separation of the evaporation residues and position time correlations with the a decays of the implanted nuclei were used. The following new decay data were measured:¹⁶²Os (E_α=(6611 ±30) keV, T_1/2=(1.9±0.7) ms);¹⁵⁸W (T_1/2=(0.9±0.3) ms);¹⁵⁸m_W (E=1.88 MeV, E_α=(8280±30) keV, T_1/2=(0.01-1) ms);^155mLu (E_α=(5575±10) keV); β decay of¹⁵⁶Ta (T_1/2 > 10 ms) to the 8⁺ yrast isomer in¹⁵⁶Hf. A cross section of 5μb was measured for the new isotope¹⁵⁶Ta produced in a p3n evaporation channel from¹⁶⁰W at 64 MeV excitation energy.     

Knight shifts for short-lived β emitters in Pt

The Knight shifts K for short-lived β emitters ¹²N and ²⁷Si implanted in Pt have been measured by means of β-NMR technique. The results were K(¹²N in Pt)= +(5.8 ± 2.1)× 10^-4 and K(²⁷Si in Pt)= +(1.4 ± 0.8)× 10^-3. The spin–lattice relaxation time T₁ was measured for ¹²N in Pt. The result was T₁(¹²N in Pt, T=300 K)= 66 ± 8 ms, thus, T₁T= 20 ± 2 Ks. The present Knight shifts are in good agreement with the KKR band structure calculation with local lattice relaxation determined theoretically. This revised version was published online in August 2006 with corrections to the Cover Date.     

Proton nuclear magnetic resonance spectroscopy of normal and edematous brain tissue in vitro: Changes in relaxation during tissue storage

Proton nuclear magnetic resonance relaxation times, T₁ and T₂, of water in unfixed gray and white matter from normal and edematous rabbit brain tissues were measured in vitro at 23°C and 100 MHz to evaluate the effects of the temperature (?25°C to 37°C) and duration (0 to 96 h) of tissue storage on relaxation times. T₁ and T₂ tended to decrease during storage, probably from slow dehydration of the tissue. This effect was greatest in tissues stored at 37°C and least in those stored at 4 and ?25°C; decreases in T₁ and T₂ were greater in white matter than in gray matter. Freezing brain tissue to ?25°C caused a sudden decrease in the T₂ of normal white matter. Relaxation times were constant for 5 h in tissues stored at 23°C and for 40 h at 4°C. These results correlated well with corresponding tissue water loss.     

Specific heat measurement of stable and metastable liquid Ti–Al alloys

The specific heats of liquid Ti–20at.%Al and Ti–51at.%Al alloys are determined to be 33.01±2.75 and 31.27±2.91 J mol⁻¹ K⁻¹ in the stable superheated and metastable undercooled states by using an electromagnetic levitation drop calorimeter. The experimental temperature ranges are 1733–2133 K and 1511–1948 K, and maximum undercoolings of 230 (0.12 T _L) and 242 K (0.14 T _L) are achieved, respectively. On the basis of the experimental results, the specific heat dependence on the composition is obtained for binary Ti–Al alloys.