Measurements of T1 and T2 relaxation times of colon cancer metastases in rat liver at 7 T

The purpose of this study was to investigate the magnetic resonance imaging (MRI) characteristics of colon cancer metastases in rat liver at 7 T. A dedicated RF microstrip coil of novel design was built in order to increase the signal-to-noise ratio and, in combination with respiratory triggering, to minimize motion artifacts. T₁- and T₂-weighted MR imaging was performed to follow tumor growth. T₁-weighted images provided a good anatomical delineation of the liver structure, while the best contrast between metastases and normal liver tissue was achieved with T₂-weighted images.Measurements of T₁ and T₂ relaxation times were performed with inversion recovery FLASH and Carr–Purcell–Meiboom–Gill and inversion recovery FLASH imaging sequences, respectively, for quantitative MR characterization of metastases. Both the T₁ and T₂ of the metastases were significantly higher than those of normal liver tissue. Further, an increase in the T₁ relaxation time of the metastases was observed with tumor growth. These findings suggest that quantitative in vivo MR characterization provides information on tumor development and possibly response to therapy, though additional studies are needed to elucidate the correlation between the changes in relaxation times and tumor microenvironment.     

Estimation of metabolite <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> relaxation times using tissue specific analysis,signal averaging and bootstrapping from magnetic resonance spectroscopic imaging data

Object A novel method of estimating metabolite T ₁ relaxation times using MR spectroscopic imaging (MRSI) is proposed. As opposed to conventional single-voxel metabolite T ₁ estimation methods, this method investigates regional and gray matter (GM)/white matter (WM) differences in metabolite T ₁ by taking advantage of the spatial distribution information provided by MRSI. Material and methods The method, validated by Monte Carlo studies, involves a voxel averaging to preserve the GM/WM distribution, a non-linear least squares fit of the metabolite T ₁ and an estimation of its standard error by bootstrapping. It was applied in vivo to estimate the T ₁ of N-acetyl compounds (NAA), choline, creatine and myo-inositol in eight normal volunteers, at 1.5 T, using a short echo time 2D-MRSI slice located above the ventricles. Results WM-T _1,NAA was significantly (P < 0.05) longer in anterior regions compared to posterior regions of the brain. The anterior region showed a trend of a longer WM T ₁ compared to GM for NAA, creatine and myo-Inositol. Lastly, accounting for the bootstrapped standard error estimate in a group mean T ₁ calculation yielded a more accurate T ₁ estimation. Conclusion The method successfully measured in vivo metabolite T ₁ using MRSI and can now be applied to diseased brain.     

Changes in proton transverse relaxation times of rat myocardium that has suffered a previous oxidative insult

An oxidative insult can induce severe damage, as in the phenomenon of myocardial ischemia and reperfusion. However, there are situations in which the damage is not so obvious (e.g., silent ischemia or aging), and the negative effects will be seen only in time. Our aim was to reveal these small changes in the myofilaments by using the nuclear magnetic resonance (NMR) technique. We used Wistar rat hearts in a constant-pressure Langendorff system, perfused with oxygenated Krebs-Henseleit buffer at 37°C. After 15 minutes of stabilization, the hearts were perfused with buffer supplemented with H₂O₂ at 50, 75, or 100 μmol/L for 15 or 30 minutes. Fifteen-minute and 45-minute perfusion controls and unperfused hearts were also collected. Heart rate (HR) and left ventricular developed pressure (LVDP) were determined with the help of a latex balloon, inserted in the left ventricle and connected with a pressure transducer. Proton transverse relaxation times (T ₂) were determined at the end of the experiment.T ₂ values were measured again in the same tissue fragments after they had been glycerinated and incubated in relaxation and rigor media. The functional parameters (HR, LVDP, coronary flow) were not significantly changed in control and 50 μmol/L H₂O₂ groups but were increased in the 75 μmol/L H₂O₂ group and significantly decreased in the 100 μmol/L H₂O₂ group.T ₂ is significantly decreased in rigor media starting with 50 μmol/L H₂O₂ administrated for 30 minutes and does not correlate with dose and duration of the oxidative insult.T ₂ in rigor is shorter than in relaxation media within the groups, and this difference is increased in the treated hearts.     

Quantitative proton MRS of brain tumors reveals increased cholineT 2 in meningiomas

Quantitative proton spectroscopy was performed on 26 volunteers and 9 patients using STEAM. Voxels (8 ml) were localized within white matter or meningioma and water-suppressed spectra acquired with TR=2 s at three echo times. Concentrations were calculated using individual relaxation parameter values with tissue water as an internal reference. Compared to white matter, meningiomas were characterized by an increased choline/creatine ratio, the absence ofN-acetylasparate, and the presence of alanine. Further, theT ₂ of choline and its concentration were both significantly greater in meningiomas than white matter (p<0.01). Thus, the high choline/creatine ratio seen in meningioma spectra is the consequence of a greater concentration and a longerT ₂. The longerT ₂ may reflect differences in the relative proportions of choline-containing compounds. Our data demonstrate that individual measurements of relaxation parameters are important for long echo spectra and may reveal important metabolic information.     

Aqueous urea as a model system for bi-exponential relaxation

Object To evaluate the utility of aqueous urea, doped inner- and outer-sphere relaxation agents, as an adjustable two-component model system. Materials and Methods T₂ was measured from 12 molal urea mixtures at pH 7.0 with varying amounts of the MnCl₂ and FeO_1.44 (Feridex^®, Berlex Inc, Montville, NJ). Results Bi-exponential relaxation was observed, with rates that were bilinearly related to [MnCl₂] and [FeO_1.44]. FeO_1.44 had comparable relaxivities on both urea and water, while MnCl₂ relaxivity was >  15x larger for water than for urea. Conclusion Aqueous urea, doped with inner- and outer-sphere contrast agents, is a two-compartment model system, which can be exhibit a wide range of different T₂s and signal fractions.     

Contrast-modified gradient echo imaging using rotary echo preparatory pulses

The use of on-resonance binomial composite pulses in two- or three-dimensional magnetization-prepared gradient-recalled echo magnetic resonance imaging experiments generates rotary echoes, leading to an increase in contrast range that is, in part, determined by the ratio ofT ₂ toT ₁. In comparison with other fast gradient-recalled echo imaging techniques designed for enhancedT ₂ contrast, this method is more robust with respect to radiofrequency field inhomogeneity and less sensitive with respect to motion artifacts. Three-dimensional parametric images may be calculated using least-squares fitting based on a simple model for steady-state longitudinal magnetization during the imaging sequences.     

NMR microscopy of single neurons at 500 MHZ

Previous NMR microimaging studies at 360 MHz have demonstrated a clear differentiation between the nucleus and cytoplasm in isolated single neurons. In particular, theT ₂ of the cell nucleus is 2.5 times larger than that of the cytoplasm. In order to determine the magnitude of possibleT ₂ ^* influences on these observations, images of single cells have been obtained at 500 MHz using spin-echo and line-narrowing sequences. Comparison of the images acquired by the two sequences, and of the spin-echo images at 360 and 500 MHz, imply that anyT ₂ ^* contributions are relatively small. Consequently, the measuredT ₂ differences in spin-echo imaging represent a true difference in theT ₂ relaxation in the two cellular compartments.     

T2 Relaxation time study of the pig renal allografts

A previous study showed the possibility of the dependence of theT ₂ maps measured by Siemens Magnetom MR imager 1.5 T on the viability of the cadaveric kidney. For theT ₂ relaxometry study the pig model was used. Ten pig renal allografts were examined during 68 h after removal. Significant differences were found in the change of the initialT ₂ ⁱⁿⁱ values in the histogram within the first 20 h [₁=T ₂ ⁱⁿⁱ (2)–T ₂ ⁱⁿⁱ (20)] or 8 h [₂=T ₂ ⁱⁿⁱ (2)–T ₂ ⁿⁱ (8)] after removal between the groups of the damaged kidneys (₁=20.2±6.8 ms, ₂=19.4±3.0) and the kidneys without apparent damage (₁=9.4±8.5 ms, ₂=5.4±5.8).     

A continuous-flow perfusion system for the maintenance and NMR study of small tissue samples in vitro

To describe and evaluate a novel perfusion system developed to maintain excised tissue in a flowing, oxygenated bathing solution during acquisition of nuclear magnetic resonance (NMR) data, and in addition allow precise data to be acquired continuously while altering the composition of the bathing solution surrounding the tissue. A chamber to house the tissue sample was constructed of interlocking sections of polyethylene tubing, and had approximate internal dimensions of 4 mm in diameter and 4 mm in height. Temperature-controlled, physiologically appropriate buffer solution was pumped via an infusion pump through the chamber, entering and exiting by way of small openings on either end. Immediately surrounding the polyethylene chamber was a tight-fitting four-loop solenoid RF coil. Measured proton NMR parameters were found to be fairly insensitive to the flow rate of the buffer if this coil was used only for reception and a larger-volume transmit-only coil was used for excitation. Temperature control of the sample was successfully implemented between 25 and 40°C. The perfusion system was found to be resistant to the effects of flow rate, as well as a useful tool for the administration of drugs or agents to the tissue. Changes in buffer composition could be performed on the fly without the need to reposition the sample each time a change was made. This avoidance of repositioning was found to yield a fivefold improvement in the precision of T₂ spectral parameters (using frog sciatic nerve as a sample).The authors wish to acknowledge the Canadian Institutes of Health Research (CIHR) for funding and the In Vivo NMR Facility of the University of Alberta for infrastructural support     

DynamicT 1 studies of gadolinium uptake in brain tumors using LL-EPI

Using ultrafastT ₁ mapping (LL-EPI), the uptake from a bolus injection of Gd-DO3A (ProHance) into the sagittal sinus and a brain tumor has been monitored. The measurement of absoluteT ₁ removes the possible error in uptake curves created fromT ₁-weighted sequences caused by changes inT ₂ ^* and simplifies the calculation of ProHance concentration. The LL-EPI sequence has an acquisition time of 1.2 s and is repeated every 4 s to obtain uptake curves with a high temporal and spatial resolution. Optimization of the LL-EPI sequence has been performed to obtain a precision of 5% over theT ₁ range 0.3–1.2 s.     

Fast MR-imaging of porous media

The study of fluid displacements in porous media is of considerable interest for the oil industry as it provides a better understanding of the oil-recovery process. Consequently, fast MR imaging techniques for routine applications of sequencial analysis of large samples would be valuable. The performances of a variant of the Fast Low Angle Shot (FLASH) imaging technique called SNAP with a very short acquisition time of 144 ms per slice were evaluated on water and/or oil-filled core samples such as chalk and limestone whose linewidths were smaller than 800 Hz. The SNAP signal intensity was theoretically reviewed and inversion-recovery preweighted SNAP was used for fastT ₁ measurements in porous media. The images so acquired were processed using a pixel clusterization technique to segment images and roughly estimate oil and water content. This imaging method opens a new field of investigation such as the study of rapid fluid ingress or displacements.Address for correspondence: Catholic University of Louvain, Bâtiment Lavoisier (Bte 3A), Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Penn Plaza, New York, NY10119.     

Relative assessment of brain iron levels using MRI at 3 tesla

High field MR (magnetic resonance) images can be made sensitive to the relative concentration of tissue iron through the use ofT*₂-weighted contrast. This has enabled tissue iron levels to be assessed noninvasively by quantification of transverse relaxation rates. High field MRI may provide a new method to investigate neurological diseases which result in alteration of brain iron levels in specific areas of the human brain. Parkinson's disease (PD) results in an increase in iron concentration within the lateral region of the substantia nigra (SN), and provides one potential application of this methodology. Preliminary results of our findings are that there is a significant difference in SN iron levels in PD patients compared with age-matched controls, assessed by quantification of the reversible line-broadening component of transverse relaxation rate,R₂.     

An iron-based T 1 contrast agent made of iron-phosphate complexes: In vitro and in vivo studies

A new iron-based T ₁ contrast agent consisting of a complex of iron ions coordinated to phosphate and amine ligands (Fe_(phos) in short) has been characterized by spectroscopic and magnetic measurements. NMR relaxation studies showed r ₁ values to be dependent on the phosphate salt concentration, K₂HPO₄, present in the medium. r ₁ reaches a maximum value of 2.5 mM^?1 s^?1 for measurements carried out at 7 T and 298 K. ³¹P MRS, Mössbauer spectroscopy and magnetic measurements of Fe_(phos) solutions suggest paramagnetic Fe³⁺ ions present in the studied iron–phosphate complex. In vitro and in vivo toxicity experiments with C6 cells and CD1 mice, respectively, demonstrated lack of toxicity for Fe_(phos) at the highest dose tested in the MRI experiments (12 mM iron for C6 cells and 0.32 mmol iron/kg for mice). Finally, T ₁ weighted images of brain tumours in mice have shown positive contrast enhancement of Fe_(phos) for tumour afflicted regions in the brain.     

Scaffold-aided repair of articular cartilage studied by MRI

OBJECTIVE: The objective of the study was to evaluate the ability of the noninvasive magnetic resonance techniques to monitor the scaffold-aided process of articular cartilage repair. MATERIALS AND METHODS: Defects of 4 mm in diameter and 3 mm in depth were created in right knees of 30 adolescent white New Zealand rabbits. Fourteen rabbits were implanted with poly(lactide-co-glycolic acid) (PLGA) scaffold trimmed to match the size and the shape of the defect (PLGA+ group). No procedure was applied to the remaining 16 animals (PLGA- group). Animals were sacrificed sequentially at 4, 12, and 24 weeks after the surgery and magnetic resonance T (2)-weighted images (400 MHz) of the dissected bone plugs at eight different echo times were taken to derive T (2) relaxation time. The images and the T (2) time dependencies versus the tissue depth were statistically analyzed. Histological results of bone plugs were evaluated using semiquantitative histological scales. RESULTS: The results obtained for PLGA repair tissue were evaluated versus the PLGA- group and the healthy tissue harvested from the opposite knee (reference group), and compared with histological results (hematoxylin and eosin staining). The magnetic resonance images and T (2) relaxation time profiles taken 4 weeks after surgery for both the PLGA- and PLGA+ group did not reveal the tissue reconstruction. After 12 weeks of treatment T (2) time dependence indicates a slight reconstruction for PLGA+ group. The T (2) time dependence obtained for PLGA+ samples taken after 24 weeks of treatment resembled the one observed for the healthy cartilage, indicating tissue reconstruction in the form of fibrous cartilage. The tissue reconstruction was not observed for PLGA- samples. CONCLUSION: The study revealed correlation between magnetic resonance and histology data, indicating the potential value of using MRI and spatial variation of T (2) as the noninvasive tools to evaluate the process of articular cartilage repair. It also suggested, that the PLGA scaffold-aided treatment could help to restore the proper architecture of collagen fibrils.     

T2 relaxation time study of iron overload in b-thalassemia

Myocardial iron deposition occurs as a result of blood transfusion therapy in b-thalassemia major patients. Since this deposition causes various cardiac complications, it is of interest to assess the iron content of the myocardium in relation to the clinical picture of the patients. Two different MRI indices were used to achieve this purpose: the T2 relaxation time and the heart/skeletal muscle signal intensity ratio. ECG gated spin echo images were obtained from 54 adult thalassemic patients, with a mean age of 26 (18–44) years, at TE = 22 ms and 60 ms, using a 1.5 T system. Patients were divided into 2 groups (A and B), according to their serum ferritin levels (> or < 2000 ng ml^-1). Results were compared with nine controls, with a mean age of 25 (18–43) years. Heart T2 relaxation time in controls (44.3 ± 3.5 ms) was higher than in group A (29.9 ± 5.7 ms,P< 0.001) and group B (33.4 ± 6.8 ms,P < 0.01). T2 was measurable in 66% of group A and 83% of group B patients. The heart/muscle signal intensity ratio in group A (0.45 ± 0.27) was lower than in group B (0.82 ± 0.33,P < 0.001) and the controls (1.15 ± 0.20,P < 0.001). The heart/muscle signal intensity ratio was measurable in 94% of the patients and demonstrated an inverse relationship with the serum ferritin levels(r = - 0.52, P<0.01). This study indicates that the heart/muscle ratio is a sensitive index of iron overload and it can be measured in the majority of patients, irrespective of tissue iron concentration, thereby offering an advantage over the use of T2 relaxation time. © 1998 Elsevier Science B.V. All rights reserved.     

Data disturb characteristics of 1T2C ferroelectric memory array

Abstract

The data disturb characteristics of a newly proposed 1T2C-type ferroelectric memory with an array structure were investigated, in which one ferroelectric capacitor C_fa was used for storing data and the other capacitor C_fb was used for reading out the data. It was found that the data disturb problem in write operation was much improved by using a lower write voltage and a compensation pulse. It was also demonstrated that the use of a thicker film in C_fa was a very effective approach for realizing reliable disturb-free read-out operation.     

Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle with respect to B₀. In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 m). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at =0°, 45°, and 90°. As increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B₀ without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.     

Hybrid sequences for rapidT 1 imaging

This article generalizes the concept of the Look-LockerT ₁-measurement sequence to include both EPI-like and Snapshot FLASH-like elements and it provides a bridge between a number of previously demonstrated methods of quantitativeT ₁ imaging. It is shown that a segmentedk-space acquisition provides numerous advantages if sufficient time is available.     

31P MR spectroscopy and in vitro markers of oxidative capacity in type 2 diabetes patients

Background: Skeletal muscle mitochondrial function in type 2 diabetes (T2D) is currently being studied intensively. In vivo ³¹P magnetic resonance spectroscopy (³¹P MRS) is a noninvasive tool used to measure mitochondrial respiratory function (MIFU) in skeletal muscle tissue. However, microvascular co-morbidity in long-standing T2D can interfere with the ³¹P MRS methodology. Aim: To compare ³¹P MRS-derived parameters describing in vivo MIFU with an in vitro assessment of muscle respiratory capacity and muscle fiber-type composition in T2D patients. Methods: ³¹P MRS was applied in long-standing, insulin-treated T2D patients. ³¹P MRS markers of MIFU were measured in the M. vastus lateralis. Muscle biopsy samples were collected from the same muscle and analyzed for succinate dehydrogenase activity (SDH) and fiber-type distribution. Results: Several ³¹P MRS parameters of MIFU showed moderate to good correlations with the percentage of type I fibers and type I fiber-specific SDH activity (Pearson’s R between 0.70 and 0.75). In vivo and in vitro parameters of local mitochondrial respiration also correlated well with whole-body fitness levels (VO _2peak) in these patients (Pearson’s R between 0.62 and 0.90). Conclusion: Good correlations exist between in vivo and in vitro measurements of MIFU in long-standing insulin-treated T2D subjects, which are qualitatively and quantitatively consistent with previous results measured in healthy subjects. This justifies the use of ³¹P MRS to measure MIFU in relation to T2D.     

Imaging lung function using rapid dynamic acquisition of <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript>-maps during oxygen enhancement

This paper describes imaging of lung function with oxygen-enhanced MRI using dynamically acquired T ₁ parameter maps, which allows an accurate, quantitative assessment of time constants of T ₁-enhancement and therefore lung function. Eight healthy volunteers were examined on a 1.5-T whole-body scanner. Lung T ₁-maps based on an IR Snapshot FLASH technique (TE = 1.4 ms, TR = 3.5 ms, FA = 7 ^∘) were dynamically acquired from each subject. Without waiting for full relaxation between subsequent acquisition of T ₁-maps, one T ₁-map was acquired every 6.7 s. For comparison, all subjects underwent a standard pulmonary function test (PFT). Oxygen wash-in and wash-out time course curves of T ₁ relaxation rate (R ₁)-enhancement were obtained and time constants of oxygen wash-in (w _in) and wash-out (w _out) were calculated. Averaged over the whole right lung, the mean w _out was 43.90 ± 10.47 s and the mean (w _in) was 51.20 ± 15.53 s, thus about 17% higher in magnitude. Wash-in time constants correlated strongly with forced expired volume in one second in percentage of the vital capacity (FEV₁ % VC) and with maximum expiratory flow at 25% vital capacity (MEF25), whereas wash-out time constants showed only weak correlation. Using oxygen-enhanced rapid dynamic acquisition of T ₁-maps, time course curves of R ₁-enhancement can be obtained. With w _in and w _out two new parameters for assessing lung function are available. Therefore, the proposed method has the potential to provide regional information of pulmonary function in various lung diseases.