Spin-lock imaging of intrinsic susceptibility gradients in tumors.
ABSTRACT: PURPOSE:Previous studies have shown that diffusion of water through intrinsic susceptibility gradients produces a dispersion of the spin-lattice relaxation rate in the rotating frame (R1 ? ) over a low range of spin-locking amplitudes (0 < ?1 < 100 Hz), whereas at higher ?1 and high magnetic fields, a second dispersion arises due to chemical exchange. Here, we separated these different effects and evaluated their contributions in tumors. METHODS:Maps of R1 ? and its changes with locking field were acquired on intracranial 9-L tumor models. The R1 ? changes due to diffusion ( R1?Diff ) were calculated by subtracting maps of R1 ? at 100 Hz (R1 ? [100 Hz]) from those at 0 Hz (R1 ? [0 Hz]). The R1 ? changes due to exchange ( R1?Ex ) were calculated by subtracting maps of R1 ? at 5620 Hz (R1 ? [5620 Hz]) from those of R1 ? at 100 Hz (R1 ? [100 Hz]). Measurements of vascular dimensions and spacing were performed ex vivo using 3D confocal microscopy. RESULTS:The R1 ? changes at low ?1 in tumors (5.24 ± 1.78 s-1 ) are substantially (p = 3.76 ) greater than those in normal tissues (1.36 ± 0.70 s-1 ), which we suggest are due to greater contributions from diffusion through susceptibility gradients. Tumor vessels were larger and spaced less closely compared with normal brain, which may be 1 factor contributing the susceptibility within 9-L tumors. The contrast between tumor and normal tissues for R1?Diff is larger than for R1?Ex and for the apparent R2w . CONCLUSION:Images that are sensitive to the variations of spin-lock relaxation rates at low ?1 provide a novel form of contrast that reflects the heterogeneous nature of intrinsic variations within tumors.
Project description:We investigated whether oscillating gradient diffusion MRI (dMRI) can provide information on brain microstructural changes after formaldehyde fixation and after hypoxic-ischemic (HI) injury beyond that provided by conventional dMRI.Pulsed gradient spin echo (PGSE) and oscillating gradient spin echo (OGSE) dMRI of the adult mouse brain was performed in vivo (50-200 Hz, b?=?600 mm(2)/s), and a similar protocol was applied to neonatal mouse brains at 24 h after unilateral hypoxia-ischemia. Animals were perfusion fixed with 4% paraformaldehyde for ex vivo dMRI and histology.Apparent diffusion coefficients (ADCs) measured in the live adult mouse brain presented tissue-dependent frequency-dependence. In vivo OGSE-ADC maps at high oscillating frequencies (>100 Hz) showed clear contrast between the molecular layer and granule cell layer in the adult mouse cerebellum. Formaldehyde fixation significantly altered the temporal diffusion spectra in several brain regions. In neonatal mouse brains with HI injury, in vivo ADC measurements from edema regions showed diminished edema contrasts at 200 Hz compared with the PGSE results. Histology showed severe tissue swelling and necrosis in the edema regions.The results demonstrate the unique ability of OGSE-dMRI in delineating tissue microstructures at different spatial scales.
Project description:An approach for the quantitative description of the kinetics of very fast exchange processes (?ex < 50-100 ?s) associated with transient, reversible protein oligomerization, is presented. We show that on-resonance 15N-R1? measurements conducted as a function of protein concentration at several spin-lock radio frequency field strengths are indispensable for unambiguous determination of the rate constants for interconversion between monomeric and higher order oligomeric species. The approach is experimentally demonstrated on the study of fast, reversible tetramerization of the full-length Huntingtin exon 1 protein, httex1, responsible for Huntington's disease. Incorporation of concentration-dependent 15N-R2,eff data, obtained from on-resonance R1? measurements performed at three spin-lock field strengths, into analysis of the kinetic scheme describing reversible tetramerization of httex1 allowed us to uniquely determine the rate constants of interconversion between the various species. This approach serves as a valuable complement to the existing array of NMR techniques for studying early, transient oligomerization events in protein aggregation pathways.
Project description:While diffusion-tensor-imaging tractography provides remarkable in vivo anatomical connectivity of the central nervous system, the majority of DTI studies to date are predominantly limited to tracking large white-matter fibers. This study investigated DTI tractography using long diffusion time (t(diff)) to improve tracking of thinner fibers in fixed rhesus monkey brains. Stimulated Echo Acquisition Mode (STEAM) sequence on a 3T Siemens TRIO was modified to include a diffusion module. DTI was acquired using STEAM with t(diff) of 48 and 192 ms with matched signal-to-noise ratios (SNR). Comparisons were also made with the conventional double-spin echo (DSE) at a short t(diff) of 45 ms. Not only did the fractional anisotropy increase significantly with the use of long diffusion time, but directional entropy measures indicated that there was an increased coherence amongst neighboring tensors. Further, the magnitude of the major eigenvector was larger at the t(diff) = 192 ms as compared to the short t(diff). Probabilistic connectivity maps at long t(diff) showed larger areas of connectivity with the use of long diffusion time, which traversed deeper into areas of low anisotropy. With tractography, it was found that the length of the fibers, increased by almost 10% in the callosal fibers that branch into the paracentral gyrus, the precentral gyrus and the post central gyrus. A similar increase of about 20% was observed in the fibers of the internal capsule. These findings offer encouraging data that DTI at long diffusion time could improve tract tracing of small fibers in areas of low fractional anisotropy (FA), such as at the interfaces of white matter and grey matter.
Project description:Spin relaxation in the rotating frame (R1?) is a powerful NMR technique for characterizing fast microsecond timescale exchange processes directed toward short-lived excited states in biomolecules. At the limit of fast exchange, only k(ex)=k(1)+k(-1) and ?ex=p(G)p(E)(??)(2) can be determined from R1? data limiting the ability to characterize the structure and energetics of the excited state conformation. Here, we use simulations to examine the uncertainty with which exchange parameters can be determined for two state systems in intermediate-to-fast exchange using off-resonance R1? relaxation dispersion. R1? data computed by solving the Bloch-McConnell equations reveals small but significant asymmetry with respect to offset (R1? (??)?R1? (-??)), which is a hallmark of slow-to-intermediate exchange, even under conditions of fast exchange for free precession chemical exchange line broadening (k(ex)/??>10). A grid search analysis combined with bootstrap and Monte-Carlo based statistical approaches for estimating uncertainty in exchange parameters reveals that both the sign and magnitude of ?? can be determined at a useful level of uncertainty for systems in fast exchange (k(ex)/??<10) but that this depends on the uncertainty in the R1? data and requires a thorough examination of the multidimensional variation of ?(2) as a function of exchange parameters. Results from simulations are complemented by analysis of experimental R1? data measured in three nucleic acid systems with exchange processes occurring on the slow (k(ex)/??=0.2; pE=?0.7%), fast (k(ex)/??=?10-16; p(E)=?13%) and very fast (k(ex)=39,000 s(-1)) chemical shift timescales.
Project description:To examine the potential use of blood oxygenation level dependent (BOLD) and tissue oxygenation level dependent (TOLD) contrast MRI to assess tumor oxygenation and predict radiation response.BOLD and TOLD MRI were performed on Dunning R3327-AT1 rat prostate tumors during hyperoxic gas breathing challenge at 4.7 T. Animals were divided into two groups. In Group 1 (n = 9), subsequent (19) F MRI based on spin lattice relaxation of hexafluorobenzene reporter molecule provided quantitative oximetry for comparison. For Group 2 rats (n = 13) growth delay following a single dose of 30 Gy was compared with preirradiation BOLD and TOLD assessments.Oxygen (100%O2 ) and carbogen (95%O2 /5%CO2 ) challenge elicited similar BOLD, TOLD and pO2 responses. Strong correlations were observed between BOLD or R2* response and quantitative (19) F pO2 measurements. TOLD response showed a general trend with weaker correlation. Irradiation caused a significant tumor growth delay and tumors with larger changes in TOLD and R1 values upon oxygen breathing exhibited significantly increased tumor growth delay.These results provide further insight into the relationships between oxygen sensitive (BOLD/TOLD) MRI and tumor pO2 . Moreover, a larger increase in R1 response to hyperoxic gas challenge coincided with greater tumor growth delay following irradiation.
Project description:There is intense interest in developing non-invasive prognostic biomarkers of tumor response to therapy, particularly with regard to hypoxia. It has been suggested that oxygen sensitive MRI, notably blood oxygen level-dependent (BOLD) and tissue oxygen level-dependent (TOLD) contrast, may provide relevant measurements. This study examined the feasibility of interleaved T2*- and T1-weighted oxygen sensitive MRI, as well as R2* and R1 maps, of rat tumors to assess the relative sensitivity to changes in oxygenation. Investigations used cohorts of Dunning prostate R3327-AT1 and R3327-HI tumors, which are reported to exhibit distinct size-dependent levels of hypoxia and response to hyperoxic gas breathing. Proton MRI R1 and R2* maps were obtained for tumors of anesthetized rats (isoflurane/air) at 4.7 T. Then, interleaved gradient echo T2*- and T1-weighted images were acquired during air breathing and a 10 min challenge with carbogen (95% O2 -5% CO2). Signals were stable during air breathing, and each type of tumor showed a distinct signal response to carbogen. T2* (BOLD) response preceded T1 (TOLD) responses, as expected. Smaller HI tumors (reported to be well oxygenated) showed the largest BOLD and TOLD responses. Larger AT1 tumors (reported to be hypoxic and resist modulation by gas breathing) showed the smallest response. There was a strong correlation between BOLD and TOLD signal responses, but ?R2* and ?R1 were only correlated for the HI tumors. The magnitude of BOLD and TOLD signal responses to carbogen breathing reflected expected hypoxic fractions and oxygen dynamics, suggesting potential value of this test as a prognostic biomarker of tumor hypoxia.
Project description:To develop a magnetic resonance (MR) imaging approach to noninvasively image quantitative Po(2) in the human vitreous.Human studies were approved by the institutional review board with informed consent obtained from all subjects and were HIPAA compliant. Animal studies were performed with animal care committee approval. An MR imaging method to measure the longitudinal relaxation rate, or R1, of water was implemented with a 3.0-T MR imager. R1 was calibrated in water phantoms at multiple Po(2) and temperature conditions (n = 10) and in ex vivo animal vitreous (n = 2). Vitreous Po(2) was imaged in three human volunteers (age range, 26-28 years) in multiple sessions on separate days to evaluate reproducibility. The effects of temperature and ambient air were evaluated by acquiring data with the eye open and closed. Statistical analysis consisted of t tests, with P less than .05 indicating significant difference.Calibrations of phantoms and ex vivo vitreous yielded an R1 association with oxygen of 0.209 sec(-1) + Po(2) ⋅ 2.07 × 10(-4) sec(-1)/mm Hg at 37°C, and an association with temperature (Δ[1/R1]/ΔTemperature) of 0.106 sec/°C ± 0.009 (standard deviation). A difference in R1 was found between the phantoms and vitreous. If uncorrected, vitreal Po(2) would be significantly overestimated (P < .001). In vivo human vitreous Po(2) maps were spatially heterogeneous, with a whole vitreous Po(2) of 16.7 mm Hg ± 6.5 (eye closed). Measurements between open and closed eyes showed spatially dependent R1 differences, which translated to temperature differences of 0.34°-0.83°C across the eye.This study established an MR imaging protocol to image quantitative vitreous Po(2) noninvasively and evaluated effects from vitreal macromolecules, temperature gradients, and ambient air on vitreal Po(2) values. Measurement of vitreous Po(2) with MR imaging has the potential to be used to study eye diseases noninvasively.
Project description:We have previously reported that the dispersion of spin-lattice relaxation rates in the rotating frame (R1? ) of tissue water protons at high field can be dominated by chemical exchange contributions. Ischemia in brain causes changes in tissue pH, which in turn may affect proton exchange rates. Amide proton transfer (APT, a form of chemical exchange saturation transfer) has been shown to be sensitive to chemical exchange rates and able to detect pH changes non-invasively following ischemic stroke. However, the specificity of APT to pH changes is decreased because of the influence of several other factors that affect magnetization transfer. R1? is less influenced by such confounding factors and thus may be more specific for detecting variations in pH. Here, we applied a spin-locking sequence to detect ischemic stroke in animal models. Although R1? images acquired with a single spin-locking amplitude (?1 ) have previously been used to assess stroke, here we use ?R1? , which is the difference in R1? values acquired with two different locking fields to emphasize selectively the contribution of chemical exchange effects. Numerical simulations with different exchange rates and measurements of tissue homogenates with different pH were performed to evaluate the specificity of ?R1? to detect tissue acidosis. Spin-lock and APT data were acquired on five rat brains after ischemic strokes induced via middle cerebral artery occlusions. Correlations between these data were analyzed at different time points after the onset of stroke. The results show that ?R1? (but not R1? acquired with a single ?1 ) was significantly correlated with APT metrics consistent with ?R1? varying with pH.