Migration and biodegradation of free silicone from silicone gel-filled implants after long-term implantation

In vivo¹H NMR chemical shift imaging (CSI), ¹H NMR localized spectroscopy (STEAM) and multinuclear NMR spectroscopy ²⁹sl, ¹³C, ¹H) were used to characterize the aging process of silicone gel-filled implants in a rat model after long-term implantation. Although no significant changes could be observed in the implants or surrounding tissue by in vivo ¹H chemical shift imaging, in vivo ¹H localized spectroscopy of the livers from the longer term population revealed the presence of silicone. Ex vivo ²⁹Sl spectroscopy of the liver, spleen, and the capsule formed around the 9 and 12 month implants clearly demonstrated and confirmed for the first time that a significant amount of free silicone migrates from silicone gel-filled implants. Also, these results show that silicones are not metabolically inert, and their biodegradation in tissue and within the implant can be monitored after 9 and 12 months by changes in the ²⁹Sl chemical shifts seen in corresponding ex vivo spectra. The NMR findings are supported by those obtained by atomic absorption spectroscopy. Silicone aging changes not only the chemical composition of the gel, but also its proton T₂ relaxation times, which increase with long implantation times. The three dimensional structure of the gel disintegrates (i.e., polymer chain rupture), increasing the molecular mobility of the polymer and, consequently, its protons T₂ values. The relaxation data we obtained reflect this in vivo degradation, especially in the case of implant rupture. Additionally, small concentrations of fat in the silicone gel were found within the implants. The presence of these lipophilic substances also might increase the T₂ values (plasticizer effect). These findings may assist in evaluating the implant integrity and disease symptoms related to their presence in humans.     

Dynamic 13C−1H nuclear polarization of lipid methylene resonances applied to broadband proton-decoupled in vivo 13C MR spectroscopy of human breast and calf tissue

Dynamic nuclear polarization of the coupled ¹³C?^lH spin system was studied for optimizing the signal-to-noise ratio of in vivo ¹³C MR spectra. In particular, the truncated driven and transient nuclear Overhauser effect (NOE) of the proton-decoupled ¹³C resonances from methylene carbons in vegetable oil and in human calf tissue was observed. Maximum in vivo NOE enhancements n = 1.5 and 0.9 were found, respectively. Theoretical fits to the data yield ¹³C?¹ cross-relaxation times in the order of 0.6 s. Significant signal enhancement over the whole in vivo ¹³C chemical shift range is obtained with minimum expense utilizing the NOE of the dipolar coupled ¹³C?¹ spin system in addition to proton-decoupling. NOE-enhanced proton-decoupled in vivo ¹³C MR spectra were acquired within 17 min in volunteer examinations from the human breast and the calf. These spectra show well-resolved resonances of carbons in lipids and several other cellular compounds.     

NOE Enhancements and T1 Relaxation Times of Phosphorylated Metabolites in Human Calf Muscle at 1.5 Tesla

Nuclear Overhauser effect (NOE) enhancements and relaxation times of ³¹P metabolites in human calf were measured in 12 volunteers (4 men and 8 women) at 1.5 T using a dual tuned four-ring birdcage. The NOE enhancements of inorganic phosphate (P₁), phosphocreatine (PCr), γ-, α-, and β-nucleoside triphosphate (NTP) from 19 measurements were 0.51 ± 0.10, 0.64 ± 0.03, 0.53 ± 0.03, 0.56 ± 0.08, and 0.47 ± 0.05, respectively. The relaxation times were independent of proton irradiation and from 23 measurements were 3.49 ± 0.35, 4.97 ± 0.58, 4.07 ± 0.36, 2.90 ± 0.25, and 3.61 ± 0.25 s for P₁, PCr, γ-, α-, and β-NTP, respectively. No significant differences between gender and age were observed for either NOE enhancements or relaxation times. Also, among nine volunteers, we observed no significant differences in T₁ between the coupled and decoupled cases.     

Proton MR spectroscopy of the normal human prostate with an endorectal coil and a double spin-echo pulse sequence

This report describes the use of an endorectal coil and a double spin-echo pulse sequence for localized ¹H MR spectroscopy of the normal prostate in volunteers. The spectra showed well-resolved signals for citrate, (phospho)choline, and creatine protons. Additional signals were assigned to taurine and myoinositol protons. J modulation of the main and outer peaks of citrate could be monitored in vivo. Apparent relaxation times T₁ and T₂ have been estimated for the methyl protons of cholines and creatine. An effective T₁ relaxation time was estimated for the main peaks of the citrate multiplet. Ratios of the integrals of these resonances have been evaluated, and tissue contents of choline and creatine were estimated using the H₂O signal as an internal reference. Spectro-scopic imaging experiments revealed a lower relative citrate signal in central parts of the prostate than in peripheral parts.     

Detection of 13C-labeled metabolites in the in vivo canine heart by B1 insensitive heteronuclear coherent polarization transfer and comparison of signal enhancement with NOE

A recently developed adiabatic coherent polarization transfer enhancement technique [H. Merkle, H. Wei, M. Garwood, K. U?urbil. J. Magn. Reson. 99, 480–494 (1992)] was employed to perform ¹³C spectroscopy in the intact canine heart in vivo during [2-¹³C]-acetate infusion into the left descending coronary artery; the results were compared with ¹³C spectra obtained with conventionally employed nuclear Overhauser enhancement. The results demonstrate that both methods can be performed by using surface coils to obtain in vivo ¹³C spectra and that coherent polarization transfer provides better enhancement than NOE for [2-¹³C]-acetate but not for short T₂ compounds.     

The use of a priori knowledge to quantify short echo in vivo 1h mr spectra

In vivo ¹H MR spectra of the prefrontal cortex acquired with the stimulated echo acquisition mode (STEAM) TE = 20 ms sequence were quantified to determine relative levels of cerebral metabolites. A priori knowledge of spectra from individual metabolites in aqueous solution was incorporated into a frequency domain quantification technique. The accuracy and precision of modeling these metabolites were investigated with simulated spectra of varying signal-to-noise ratios (SNRs) and relative metabolite levels. The efficacy of modeling in vivo data was tested by quantifying 10 repeated measures of two consecutively acquired in vivo spectra (an 8?cm³ volume of interest (VOI) and a 4?cm³ VOI positioned within the 8?cm³ VOI) on the same normal subject. The differences in levels of glutamate (Glu), phosphocreatine plus creatine (PCr+Cr) and choline-containing compounds (Cho₁ between spectra from the 8? and 4?cm³ VOIs corresponded with the expected differences observed in the proportions of gray matter within the VOIs (estimated from ¹H images). Correcting for the T₁ and T₂ relaxation, the estimated concentrations of N-acetylaspartate, PCr+Cr, Cho₁, Glu, and glutamine were consistent with previous in vivo and in vitro reports.     

13C-NMR relaxation in glycogen

This study is the first report on the multiexponential T₂ relaxation of the ¹³C-1 carbon of glycogen. In contrast to T₁ relaxation, which does not display observable multiexponential decay behavior, T₂ relaxation is described by a continuous distribution of T₂ times. Changes in molecular weight and sample viscosity, which affect the overall mobility of the glycogen particle have little influence on T₁ and T₂ relaxation times. This is in contradiction with earlier results that T₂ is dominated by the overall motion of the glycogen particles [L.-H. Zang Biochemistry 29, 6815–6820 (1990)]. T₁ depends strongly on the external field B_o and is almost temperature independent in the range 23–37°C whereas T₂ is field independent and varies appreciably with temperature. The experimental T₁ and T₂ relaxation data are shown to be consistent with existing theoretical models for relaxation, suitably modified to include a distribution of correlation times for the internal motions. The presence of fast decaying components (short T₂) in the FID implies broad line components in the frequency spectrum and the corresponding need to appropriately set the integration limits for the quantification of the glycogen peak.     

Dipolar resonance frequency shifts in 1H MR spectra of skeletal muscle: Confirmation in rats at 4.7 T in Vivo and observation of changes postmortem

Non-isotropic contributions to ¹H MR spectra from human skeletal muscle in vivo have recently been observed in the 0-to 5-ppm region. One pair of peaks has been identified to be subject to dipolar couplings. The corresponding changes in resonance frequency are related to the orientation of muscle fibers with respect to the external magnetic field and are analogous to the behavior of small molecules dissolved in liquid crystals. Image-guided localized spectroscopy based on the STEAM method has been applied to verify these phenomena in rat skeletal muscle in vivo and to investigate the effect postmortem. Residual dipolar couplings and anisotropic contributions to ¹H MR spectra of skeletal muscle have been confirmed in animals and at a higher field strength—albeit with a slightly different spectral pattern compared to the human study. The most prominent dipolar doublet due to creatine and/or phosphocreatine vanishes postmortem with a rate similar to the disappearance of phosphocreatine, and is no longer observable 2 h postmortem.     

Analysis of macromolecule resonances in 1H NMR spectra of human brain

Macromolecule resonances underlying metabolites in ¹H NMR spectra were investigated in temporal lobe biopsy tissue from epilepsy patients and from localized ¹H spectra of the brains of healthy volunteers. The ¹H NMR spectrum of brain tissue was cornpared with that of cytosol and dialyzed cytosol after removal of low molecular weight molecules (4500 daltons) at 8.4 and 2.1 Tesla. The assignment of specific resonances to macromolecules in 2.1 Tesla, short- TE, localized human brain ¹H NMR spectra in vivo was made on the basis of a J-editing method using the spectral parameters (δ, J) and connectivities determined from 2D experiments in vitro. Two prominent corinectivities associated with macromolecules in vitro (0.93–2.05 δ and 1.6–3.00 δ) were also detected in vivo by the J-editing method. Advantage was taken of the large difference in measured T₁ relaxation times between macromolecule and metabolite resonances in the brain spectrum to acquire ‘metabolite-nulled’ macromolecule spectra. These spectra appear identical to the spectra of macromolecules isolated in vitro.     

Anomalous Transverse Relaxation in 1H Spectroscopy in Human Brain at 4 Tesla

Longitudinal (T₁) and apparent transverse relaxation times (T₂) of choline-containing compounds (Cho), creatine/phospho-creatine (Cr/PCr), and N-acetyl aspartate (NAA) were measured in vivo in human brain at 4 Tesla. Measurements were performed using a water suppressed stimulated echo pulse sequence with complete outside volume presaturation to improve volume localization at short echo times. T₁-values of Cho (1.2 ± 0.1 s), Cr (1.6 ± 0.3 s), and NAA (1.6 ± 0.2 s) at 4 Tesla in occipital brain were only slightly larger than those reported in the literature at 1.5 Tesla. Thus, TR will not adversely affect the expected enhancement of signal-to-noise at 4 Tesla. Surprisingly, apparent T₂-values of Cho (142 ± 34 ms), Cr (140 ± 13 ms), and NAA (185 ± 24 ms) at 4 Tesla were significantly smaller than those at 1.5 Tesla and further decreased when increasing the mixing interval TM. Potential contributing factors, such as diffusion in local susceptibility related gradients, dipolar relaxation due to intracellular paramagnetic substances and motion effects are discussed. The results suggest that short echo time spectroscopy is advantageous to maintain signal to noise at 4 Tesla.     

Quantitative tissue sodium concentration mapping of normal rat brain

A quantitative in vivo method for obtaining maps of tissue sodium concentration (TSC) by MRI is compared to the invasive, global ²²Na radionuclide dilutional technique in the normal rat brain. The MR method uses a three-dimensional projectional acquisition scheme to minimize signal losses from transverse relaxation. Internal calibration standards are used to convert the signal intensity into TSC after correction for B₁ inhomogeneities by using the ratio of ²³Na and ¹H images obtained with identical B₁ distributions and sensitivities at the two frequencies. Over the biological range of concentrations, the TSC, measured as the ratio of MR signals of ²³Na and ¹H, gives a linear response with concentration. In the normal rat brain, the mean TSC measured using the MRI method (TSC = 45 ± 4 mM, animals = 5) is not significantly different from the global ²²Na radionuclide method (TSC = 49 ± 6 mM, animals = 7).     

In vivo 19F spin relaxation and localized spectroscopy of fluoxetine in human brain

Fluorine-19 NMR spectroscopy was used to monitor the anti-depressant drug fluoxetine (and its metabolite norfluoxetine) in vivo in human brain. A quadrature birdcage head coil, developed for operation at 60.1 MHz, yielded a signal from the head 2 to 4 times stronger than for surface coils. It was used to measure the in vivo ¹⁹F spin-lattice relaxation time (T₁) of fluoxetine for five patients by the inversion-recovery technique. The individual T₁s varied from 149 to 386 ms, which was attributed in part to interindividual differences based on the reproducibility of a phantom T₁. The individual T₁ correlated weakly with approximate brain concentration. A lower limit of 3 to 4 ms was found for the spin-spin relaxation time from line width measurements. Low resolution 4-dimensional spectroscopic imaging confirmed that the single in vivo ¹⁹F resonance for fluoxetine arose primarily from brain. The spectrum of a cerebral hemisphere (in formalin) obtained at autopsy from a patient on 40 mg/day of fluoxetine for 19 weeks was comparable with that seen for patients in vivo. The in vivo signal arose about equally from fluoxetine and the active me tabolite norfluoxetine, as demonstrated by the in vitro ¹⁹F NMR spectrum of the lipophilic extract of a small section of brain. In virto quantitation of frozen samples from three brain regions yielded combined fluoxetine/norfluoxetine concentrations of 12.3 to 18.6 μ/ml, which is higher than typically determined in vivo, and suggests that the fluorinated drugs may not be 100% visible in vivo.     

Measurement of transverse relaxation times of J‐coupled metabolites in the human visual cortex at 4 T

Accurate quantification of ¹H NMR spectra often requires knowledge of the relaxation times to correct for signal losses due to relaxation and saturation. In human brain, T₂ values for singlets such as N‐acetylaspartate, creatine, and choline have been reported, but few T₂ values are available for J‐coupled spin systems. The purpose of this study was to measure the T₂ relaxation times of J‐coupled metabolites in the human occipital lobe using the LASER sequence. Spectra were acquired at multiple echo times and were analyzed with an LCModel using basis sets simulated at each echo time. Separate basis spectra were used for resonances of protons belonging to the same molecule but having very different T₂ values (e.g., two separate basis spectra were used for the singlet and multiplet signal in N‐acetylaspartate). The T₂ values for the N‐acetylaspartate multiplet (149 ± 12 ms), glutamate (125 ± 10 ms), myo‐inositol (139 ± 20 ms), and taurine (196 ± 28 ms) were successfully measured in the human visual cortex at 4 T. These measured T₂ relaxation times have enabled the accurate and absolute quantification of cerebral metabolites at longer echo times. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

MRI contrast from relaxation along a fictitious field (RAFF)

A new method to measure rotating frame relaxation and to create contrast for MRI is introduced. The technique exploits relaxation along a fictitious field (RAFF) generated by amplitude‐ and frequency‐modulated irradiation in a subadiabatic condition. Here, RAFF is demonstrated using a radiofrequency pulse based on sine and cosine amplitude and frequency modulations of equal amplitudes, which gives rise to a stationary fictitious magnetic field in a doubly rotating frame. According to dipolar relaxation theory, the RAFF relaxation time constant (T_RAFF) was found to differ from laboratory frame relaxation times (T₁ and T₂) and rotating frame relaxation times (T_1ρ and T_2ρ). This prediction was supported by experimental results obtained from human brain in vivo and three different solutions. Results from relaxation mapping in human brain demonstrated the ability to create MRI contrast based on RAFF. The value of T_RAFF was found to be insensitive to the initial orientation of the magnetization vector. In the RAFF method, the useful bandwidth did not decrease as the train length increased. Finally, as compared with an adiabatic pulse train of equal duration, RAFF required less radiofrequency power and therefore can be more readily used for rotating frame relaxation studies in humans. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Validation of 13C NMR measurements of liver glycogen in vivo

The natural abundance ¹³C NMR intensity of the glycogen C1 resonance was measured in the surgically exposed liver of rabbits in vivo (n = 17) by integration from 98 to 104 ppm and compared double blindedly to the subsequent biochemical measurement. Coil loading was measured each time from a reference sphere at the coil center and the NMR Intensity was normalized accordingly. For quantification, the normalized NMR intensity was calibrated using aqueous glycogen solutions ranging from 110 to 1100 μmol glucosyl units/g (n = 14). An in vivo range from 110 to 800 pmol glucosyl unit/g wet weight was measured with a highly linear correlation with concentration (r = 0.85, P < 0.001). The in vivo NMR concentration was 0.95 ± 0.05 (mean ± standard error, n = 17) of the concomitant enzymatic measurement of glycogen content. We conclude that the ¹³C NMR signal of liver glycogen C1 is essentially 100% visible in vivo and that natural abundance ¹³C NMR spectroscopy can provide reliable noninvasive estimates of in vivo glycogen content over the physiological range of liver glycogen concentrations when using adequate localization and Integration procedures.     

Quantifying hepatic glycogen synthesis by direct and indirect pathways in rats under normal ad libitum feeding conditions

Hepatic glycogen synthesis from intact hexose (direct pathway) relative to that from gluconeogenic precursors (indirect pathway) was quantified in ad libitum‐fed rats. Following ²H₂O administration and overnight feeding, the livers were removed and glycogen ²H‐enrichment was measured by ²H NMR. Six controls and six rats rendered hyperglycemic by streptozotocin (STZ; fasting blood glucose = 385 ± 31 mg/dl) were studied. The indirect pathway contribution, estimated as glycogen hydrogen 5 relative to hydrogen 2 enrichment, was 54% ± 4% for control rats—similar to values from healthy, meal‐fed humans. In STZ‐treated rats, the indirect pathway contribution was significantly higher (68% ± 4%, P < 0.05 vs. controls), similar to that of Type 1 diabetic (T1D) patients. In conclusion, sources of hepatic glycogen synthesis in rats during ad libitum nocturnal feeding were quantified by analysis of glycogen enrichment from ²H₂O. STZ caused alterations resembling the pathophysiology of hepatic glycogen synthesis in T1D patients. Magn Reson Med 61:1–5, 2009. © 2008 Wiley‐Liss, Inc.     

Observation of a 1H double quantum filtered signal of water in biological tissues

The observation of a ¹H double quantum filtered (DQF) NMR signal of water in bovine sciatic nerve, bovine articular cartilage, rat tail tendon, and rat brain is reported. The origin of this signal in rat tail tendon was found to be a result of residual dipolar interaction between water protons and macromolecular protons. The dependence of the width of the ¹H DQF spectra on the orientation indicated that in rat tail tendon the effective director of the residual dipolar interaction is parallel to the collagen fibers. ¹H DQF NMR may be applied in imaging where the contrast obtained is related to the degree of order in the tissue.     

Quantitative spectroscopic imaging with in situ measurements of tissue water T1, T2, and density

The use of tissue water as a concentration standard in proton magnetic resonance spectroscopy (¹H‐MRS) of the brain requires that the water proton signal be adjusted for relaxation and partial volume effects. While single voxel ¹H‐MRS studies have often included measurements of water proton T₁, T₂, and density based on additional ¹H‐MRS acquisitions (e.g., at multiple echo or repetition times), this approach is not practical for ¹H‐MRS imaging (¹H‐MRSI). In this report we demonstrate a method for using in situ measurements of water T₁, T₂, and density to calculate metabolite concentrations from ¹H‐MRSI data. The relaxation and density data are coregistered with the ¹H‐MRSI data and provide detailed information on the water signal appropriate to the individual subject and tissue region. We present data from both healthy subjects and a subject with brain lesions, underscoring the importance of water parameter measurements on a subject‐by‐subject and voxel‐by‐voxel basis. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Theoretical basis for sodium and potassium MRI of the human heart at 1.5 T

Knowledge of the extent and location of viable tissue is important to clinical diagnosis. In principle, sodium (²³Na) and potassium (³⁹K) MRI could noninvasively provide information about tissue viability. In practice, imaging of these nuclei is difficult because, compared with water protons (¹H), ²³Na and ³⁹K have lower MR sensitivities (9.2 and 0.051%, respectively), and lower in vivo concentrations (ca. 1000-fold). On the other hand, the relatively short T₁ relaxation times of ²³Na and ³⁹K (ca. 30 and 10 ms, respectively) suggest that optimized imaging pulse sequences may in part alleviate the weak signal of these nuclei. In this study, numerical simulations of high-speed imaging sequences were developed and used to maximize ²³Na and ³⁹K image signal-to-noise ratio (SNR) per unit time within the constraints of existing gradient hardware. The simulation demonstrated that decreasing receiver bandwidth at the expense of echo time (TE) results in a substantial increase in ²³Na and ³⁹K image SNR/time despite the short T₂ and T₂* of these nuclei. Referenced to the available ¹H signal on existing 1.5 T scanners, the simulation suggested that it should be possible to acquire three-dimensional ²³Na images of the human heart with 7 × 7 × 7 mm resolution and ³⁹K images with 26 × 26 × 26 mm resolution in 30 min. Experimentally in humans at 1.5 T, three-dimensional ²³Na images of the heart were acquired in 15 min with 6 × 6 × 12 mm resolution and signal-to-noise ratios of 11 and 7 in the left ventricular cavity and myocardium, respectively, which is very similar to the predicted result. The results demonstrate that by choosing imaging pulse sequence parameters that fully exploit the short relaxation times of ²³Na and ³⁹K, potassium MRI is improved but remains impractical, whereas sodium MRI improves to the point where ²³Na imaging of the human heart may be clinically feasible on existing 1.5 T scanners.     

In vivo NMR imaging and spectroscopic investigation of renal pathology in lean and obese rat kidneys

Diabetic nephropathy is a major cause of end-stage renal failure. While our understanding of the pathogenesis of nephropathy is incomplete, progressive glomerular injury appears to play a significant role in the decline of renal function. Proton NMR spectroscopy and imaging techniques were used to address changes in renal pathology associated with glomerular mesangial expansion in vivo in kidneys from spontaneously obese and lean (control) littermate Zucker rats. Fully functioning rat kidneys were surgically exposed and externalized for direct NMR signal detection via a coil placed around the organ. High-resolution (78 μm in plane) proton images were obtained at 4.7 T magnetic field strength revealing fine structure within the well-defined cortical and medullary regions. The obese rat kidney images were distinct in appearance from the lean kidney images and exhibited marked cortical expansion as well as increased overall kidney size. Enlargement of mean glomerular diameter was verified histologically in the obese kidneys as compared with the lean kidneys. Proton T₁ and T₂ relaxation times were determined from the entire kidney using standard spectroscopic techniques, and from specific regions within the kidney from multiple T₁ and T₂-weighted images. Additionally, image contrast enhancement resulting from saturation transfer between protons in restricted-mobility environments and mobile water protons within the kidney was investigated in the lean and obese rat kidneys using magnetization-transfer imaging techniques. At the early stage of renal injury examined in this study, diseased and healthy kidneys could not be differentiated on the basis of relaxation times alone. The magnitude of saturation transfer obtained in cortical tissue in the lean and obese kidneys was also not statistically significantly different. However, the magnitude of saturation transfer achieved in the medullary tissue of obese kidneys was statistically significantly less than that achieved in lean kidneys.