High-resolution assessment of blood flow in murine RIF-1 tumors by monitoring uptake of H(2)(17)O with proton T(1rho)-weighted imaging

Quantification of H217O by 1H-MR imaging at 3 T: a feasibility study

Background

Indirect ¹H-magnetic resonance (MR) imaging of ¹⁷O-labelled water allows imaging in vivo dynamic changes in water compartmentalisation. Our aim was to describe the feasibility of indirect ¹H-MR methods to evaluate the effect of H₂¹⁷O on the MR relaxation rates by using conventional a 3-T equipment and voxel-wise relaxation rates.

Methods

MR images were used to calculate the R1, R2, and R2* relaxation rates in phantoms (19 vials with different H₂¹⁷O concentrations, ranging from 0.039 to 5.5%). Afterwards, an experimental animal pilot study (8 rats) was designed to evaluate the in vivo relative R2 brain dynamic changes related to the intravenous administration of ¹⁷O-labelled water in rats.

Results

There were no significant changes on the R1 and R2* values from phantoms. The R2 obtained with the turbo spin-echo T2-weighted sequence with 20-ms echo time interval had the higher statistical difference (0.67 s⁻¹, interquartile range 0.34, p < 0.001) and Spearman correlation (rho 0.79). The R2 increase was adjusted to a linear fit between 0.25 and 5.5%, represented with equation R2 = 0.405 concentration + 0.3215. The highest significant differences were obtained for the higher concentrations (3.1–5.5%). The rat brain MR experiment showed a mean 10% change in the R2 value after the H₂¹⁷O injection with progressive normalisation.

Conclusions

Indirect ¹H-MR imaging method is able to measure H₂¹⁷O concentration by using R2 values and conventional 3-T MR equipment. Normalised R2 relative dynamic changes after the intravenous injection of a H₂¹⁷O saline solution provide a unique opportunity to map water pathophysiology in vivo, opening the analysis of aquaporins status and modifications by disease at clinically available 3-T proton MR scanners.

Indirect Proton MR Imaging and Kinetic Analysis of 17O-Labeled Water Tracer in the Brain

Purpose:

The feasibility of steady-state sequences for ¹⁷O imaging was evaluated based on a kinetic analysis of the brain parenchyma and cerebrospinal fluid (CSF).

Materials and Methods:

The institutional review board approved this prospective study with written informed consent. Dynamic 2D or 3D steady-state sequences were performed in five and nine participants, respectively, with different parameters using a 3T scanner. During two consecutive dynamic scans, saline was intravenously administered for control purposes in the first scan, and 20% ¹⁷O-labeled water (1 mL/Kg) was administered in the second scan. Signal changes relative to the baseline were calculated, and kinetic analyses of the curves were conducted for all voxels. Region of interest analysis was performed in the brain parenchyma, choroid plexus, and CSF spaces.

Results:

Average signal drops were significantly larger in the ¹⁷O group than in the controls for most of the imaging parameters. Different kinetic parameters were observed between the brain parenchyma and CSF spaces. Average and maximum signal drops were significantly larger in the CSF spaces and choroid plexus than in the brain parenchyma. Bolus arrival, time to peak, and the first moment of dynamic curves of ¹⁷O in the CSF space were delayed compared to that in the brain parenchyma. Significant differences between the ventricle and subarachnoid space were also noted.

Conclusion:

Steady-state sequences are feasible for indirect ¹⁷O imaging with reasonable temporal resolution; this result is potentially important for the analysis of water kinetics and aquaporin function for several disorders.

Quantitative 17O imaging towards oxygen consumption study in tumor bearing mice at 7 T

(17)O magnetic resonance imaging (MRI) using a conventional pulse sequence was explored as a method of quantitative imaging towards regional oxygen consumption rate measurement for tumor evaluation in mice. At 7 T, fast imaging with steady state (FISP) was the best among gradient echo, fast spin echo and FISP for the purpose. The distribution of natural abundance H2(17)O in mice was visualized under spatial resolution of 2.5 × 2.5mm(2) by FISP in 10 min. The signal intensity by FISP showed a linear relationship with (17)O quantity both in phantom and mice. Following the injection of 5% (17)O enriched saline, (17)O re-distribution was monitored in temporal resolution down to 5 sec with an image quality sufficient to distinguish each organ. The image of labeled water produced from inhaled (17)O2 gas was also obtained. The present method provides quantitative (17)O images under sufficient temporal and spatial resolution for the evaluation of oxygen consumption rate in each organ. Experiments using various model compounds of R-OH type clarified that the signal contribution of body constituents other than water in the present in vivo(17)O FISP image was negligible.

Quantitative T(1)(ρ) imaging using phase cycling for B0 and B1 field inhomogeneity compensation

T(1)(ρ) imaging is useful in a number of clinical applications. T(1)(ρ) preparation methods, however, are sensitive to non-uniformities of the B0 magnetic field and the B1 RF field. These common system imperfections can result in image artifacts and quantification errors in T(1)(ρ) imaging. We report on a phase-cycling method which can eliminate B1 RF inhomogeneity effects in T(1)(ρ) imaging. This method does not only correct for image artifacts but also for T(2)(ρ) contamination caused by B1 RF inhomogeneity. The presence of B0 magnetic field inhomogeneity can compromise the effectiveness of this method for B1 RF inhomogeneity correction. We demonstrate that, by combining the spin-locking scheme reported by Dixon et al. (Myocardial suppression in vivo by spin locking with composite pulses. Magn Reson Med 1996; 36:90-94) with phase cycling, we can simultaneously correct B0 magnetic field inhomogeneity effects and B1 RF inhomogeneity effects in T(1)(ρ) imaging. Phantom and in vivo data sets are used to demonstrate the proposed methods and to compare them with other existing T(1)(ρ) preparation methods.

Mapping of cerebral oxidative metabolism with MRI

Using a T(1rho) MRI based indirect detection method, we demonstrate the detection of cerebral oxidative metabolism and its modulation by administration of the mitochondrial uncoupling agent 2,4-dinitrophenol (DNP) in a large animal model with minimum utilization of gas. The study was performed by inhalation in swine during imaging on clinical MRI scanners. Metabolic changes in swine were determined by two methods. First, in a series of animals, increased metabolism caused by DNP injection was measured by exhaled gas analysis. The average whole-body metabolic increase in seven swine was 11.9% + or - 2.5% per mg/kg, stable over three hours. Secondly, hemispheric brain measurements of oxygen consumption stimulated by DNP injection were made in five swine using T(1rho) MRI following administration of gas. Metabolism was calculated from the change in the T(1rho) weighted MRI signal due to H(2)(17)O generated from inhalation before and after doubling of metabolism by DNP. These results were confirmed by direct oxygen-17 MR spectroscopy, a gold standard for in vivo H(2)(17)O measurement. Overall, this work underscores the ability of indirect oxygen-17 imaging to detect oxygen metabolism in an animal model with a lung capacity comparable to the human with minimal utilization of expensive gas. Given the demonstrated high efficiency in use of and the proven feasibility of performing such measurements on standard clinical MRI scanners, this work enables the adaption of this technique for human studies dealing with a broad array of metabolic derangements.

T1rho-prepared balanced gradient echo for rapid 3D T1rho MRI

PURPOSE

To develop a T1rho-prepared, balanced gradient echo (b-GRE) pulse sequence for rapid three-dimensional (3D) T1rho relaxation mapping within the time constraints of a clinical exam (<10 minutes), examine the effect of acquisition on the measured T1rho relaxation time and optimize 3D T1rho pulse sequences for the knee joint and spine.

MATERIALS AND METHODS

A pulse sequence consisting of inversion recovery-prepared, fat saturation, T1rho-preparation, and b-GRE image acquisition was used to obtain 3D volume coverage of the patellofemoral and tibiofemoral cartilage and lower lumbar spine. Multiple T1rho-weighted images at various contrast times (spin-lock pulse duration [TSL]) were used to construct a T1rho relaxation map in both phantoms and in the knee joint and spine in vivo. The transient signal decay during b-GRE image acquisition was corrected using a k-space filter. The T1rho-prepared b-GRE sequence was compared to a standard T1rho-prepared spin echo (SE) sequence and pulse sequence parameters were optimized numerically using the Bloch equations.

RESULTS

The b-GRE transient signal decay was found to depend on the initial T1rho-preparation and the corresponding T1rho map was altered by variations in the point spread function with TSL. In a two compartment phantom, the steady state response was found to elevate T1rho from 91.4+/-6.5 to 293.8+/-31 and 66.9+/-3.5 to 661+/-207 with no change in the goodness-of-fit parameter R2. Phase encoding along the longest cartilage dimension and a transient signal decay k-space filter retained T1rho contrast. Measurement of T1rho using the T1rho-prepared b-GRE sequence matches standard T1rho-prepared SE in the medial patellar and lateral patellar cartilage compartments. T1rho-preparedb-GRE T1rho was found to have low interscan variability between four separate scans. Mean patellar cartilage T1rho was elevated compared to femoral and tibial cartilage T1rho.

CONCLUSION

The T1rho-prepared b-GRE acquisition rapidly and reliably accelerates T1rho quantification of tissues offset partially by a TSL-dependent point spread function.

Murine liver implantation of radiation-induced fibrosarcoma: characterization with MR imaging, microangiography and histopathology

We sought to establish and characterize a mouse liver tumor model as a platform for preclinical assessment of new diagnostics and therapeutics. Radiation-induced fibrosarcoma (RIF-1) was intrahepatically implanted in 27 C3H/Km mice. Serial in vivo magnetic resonance imaging (MRI) with a clinical 1.5-T-magnet was performed using T1- (T1WI), T2- (T2WI), and diffusion-weighted sequences (DWI), dynamic contrast-enhanced MRI (DCE-MRI), and contrast-enhanced T1WI, and validated with postmortem microangiography and histopathology. Implantation procedure succeeded in 25 mice with 2 deaths from overdosed anesthesia or hypothermia. RIF-1 grew in 21 mice with volume doubling time of 2.55+/-0.88 days and final size of 216.2+/-150.4 mm(3) at day 14. Three mice were found without tumor growth and one only with abdominal seeding. The intrahepatic RIF-1 was hypervascularized with negligible necrosis as shown on MRI, microangiography and histology. On DCE-MRI, maximal initial slope of contrast-time curve and volume transfer constant per unit volume of tissue, K, differed between the tumor and liver with only the former significantly lower in the tumor than in the liver (P<0.05). Liver implantation of RIF-1 in mice proves a feasible and reproducible model and appears promising for use to screen new diagnostics and therapeutics under noninvasive monitoring even with a clinical MRI system.

Detection of 17O-tagged phosphate by (31)P MRS: a method with potential for in vivo studies of phosphorus metabolism

We present a method for MR detection of (17)O-labeled phosphate groups. The method employs the T(2) relaxivity effect of (17)O on (31)P nuclei to distinguish between (17)O-labeled and unlabeled phosphate groups, and uses spin-echo (SE) acquisitions with RF decoupling at the (17)O frequency to generate (31)P spectra that show only (17)O-labeled phosphate groups. The method provides an alternative to spin-labeling experiments, which are limited to the study of rapidly exchanging phosphate groups by the T(1) relaxation rates of phosphorus nuclei. We demonstrate separation of MR signals from labeled and unlabeled phosphate-containing compounds, and characterization of the T(2) effect of (17)O on phosphate nuclei in (17)O-labeled phosphate groups. Previous (17)O and (18)O phosphate-labeled studies used mass spectrometry or high-resolution MR spectroscopy (MRS) to detect the presence of an isotopic label, which requires ex vivo sample preparation. In our method the detection of (17)O-labeled phosphate is manifested as a large change in (31)P T(2), and thus allows in vivo detection using simple MR methods. Thus this method may have potential for in vivo studies of bioenergetics and the metabolism of phosphate-containing compounds.

Artifacts in T1 rho-weighted imaging: compensation for B(1) and B(0) field imperfections

The origin of spin locking image artifacts in the presence of B(0) and B(1) magnetic field imperfections is shown theoretically using the Bloch equations and experimentally at low (omega(1) << Delta omega(0)), intermediate (omega(1) approximately Delta omega(0)) and high (omega(1) >> Delta omega(0)) spin locking field strengths. At low spin locking fields, the magnetization is shown to oscillate about an effective field in the rotating frame causing signature banding artifacts in the image. At high spin lock fields, the effect of the resonance offset Deltao mega(0) is quenched, but imperfections in the flip angle cause oscillations about the omega(1) field. A new pulse sequence is presented that consists of an integrated spin echo and spin lock experiment followed by magnetization storage along the -z-axis. It is shown that this sequence almost entirely eliminates banding artifacts from both types of field inhomogeneities at all spin locking field strengths. The sequence was used to obtain artifact free images of agarose in inhomogeneous B(0) and B(1) fields, off-resonance spins in fat and in vivo human brain images at 3 T. The new pulse sequence can be used to probe very low frequency (0-400 Hz) dynamic and static interactions in tissues without contaminating B(0) and B(1) field artifacts.

Compensation for spin-lock artifacts using an off-resonance rotary echo in T1rhooff-weighted imaging

The origin of image artifacts in an off-resonance spin-locking experiment is shown to be imperfections in the excitation flip angle. A pulse sequence for off-resonance spin locking is implemented that compensates for imperfections in the excitation flip angle through an off-resonance rotary echo. The off-resonance rotary echo alternates the frequency offset and phase of the RF transmitter during two spin-locking pulses of equal duration. The underlying theory is detailed, and MR images demonstrate the effectiveness of the technique in agarose gel phantoms and in in vivo human brain at 3T.

A pulse sequence for rapid in vivo spin-locked MRI

PURPOSE

To develop a novel pulse sequence called spin-locked echo planar imaging (EPI), or (SLEPI), to perform rapid T1rho-weighted MRI.

MATERIALS AND METHODS

SLEPI images were used to calculate T1rho maps in two healthy volunteers imaged on a 1.5-T Sonata Siemens MRI scanner. The head and extremity coils were used for imaging the brain and blood in the popliteal artery, respectively.

RESULTS

SLEPI-measured T1rho was 83 msec and 103 msec in white (WM) and gray matter (GM), respectively, 584 msec in cerebrospinal fluid (CSF), and was similar to values obtained with the less time-efficient sequence based on a turbo spin-echo readout. T1rho was 183 msec in arterial blood at a spin-lock (SL) amplitude of 500 Hz.

CONCLUSION

We demonstrate the feasibility of the SLEPI pulse sequence to perform rapid T1rho MRI. The sequence produced images of higher quality than a gradient-echo EPI sequence for the same contrast evolution times. We also discuss applications and limitations of the pulse sequence.

In vivo 17O NMR approaches for brain study at high field

17O is the only stable oxygen isotope that can be detected by NMR. The quadrupolar moment of 17O spin (I = 5/2) can interact with local electric field gradients, resulting in extremely short T1 and T2 relaxation times which are in the range of several milliseconds. One unique NMR property of 17O spin is the independence of 17O relaxation times on the magnetic field strength, and this makes it possible to achieve a large sensitivity gain for in vivo 17O NMR applications at high fields. In vivo 17O NMR has two major applications for studying brain function and cerebral bioenergetics. The first application is to measure the cerebral blood flow (CBF) through monitoring the washout of inert H2 17O tracer in the brain tissue following an intravascular bolus injection of the 17O-labeled water. The second application, perhaps the most important one, is to determine the cerebral metabolic rate of oxygen utilization (CMRO2) through monitoring the dynamic changes of metabolically generated H2 17O from inhaled 17O-labeled oxygen gas in the brain tissue. One great merit of in vivo 17O NMR for the determination of CMRO2 is that only the metabolic H2 17O is detectable. This merit dramatically simplifies both CMRO2 measurement and quantification compared to other established methods. There are two major NMR approaches for monitoring H2 17O in vivo, namely direct approach by using 17O NMR detection (referred as direct in vivo 17O NMR approach) and indirect approach by using 1H NMR detection for measuring the changes in T2- or T1rho-weighted proton NMR signals caused by the 17O-1H scalar coupling and proton chemical exchange (referred as indirect in vivo 17O NMR approach). Both approaches are suitable for CBF measurements. However, recent studies indicated that the direct in vivo 17O NMR approach at high/ultrahigh fields appears to offer significant advantages for quantifying and imaging CMRO2. New developments have further demonstrated the feasibility for establishing a completely noninvasive in vivo 17O NMR approach for imaging CMRO2 in a rat brain during a brief 17O2 inhalation. This approach should be promising for studying the central role of oxidative metabolism in brain function and neurological diseases. Finally, the similar approach could potentially be applied to image CMRO2 noninvasively in human brain.

Proton MRI of metabolically produced H2 17O using an efficient 17O2 delivery system

In vivo detection of H(2)(17)O produced via metabolic reduction of inhaled (17)O-enriched gas is demonstrated using proton magnetic resonance imaging (MRI). Specifically, (1)H T(1rho)-weighted MRI, which may be readily implemented on any MRI scanner, is applied as an indirect (17)O imaging method to quantitatively monitor the distribution of metabolically produced (17)O water (mpH(2)(17)O) in the rat brain. The delivery of (17)O(2) to rats is conducted via a specially designed closed respiration circuit that conserves the expensive gas. Quantitative mapping of H(2)(17)O performed via (1)H T(1rho)-weighted MRI is validated by direct (17)O-magnetic resonance spectroscopy. The MRI data show that a steady-state H(2)(17)O concentration of 25.7 +/- 1.66 mM (n = 4) is achieved in the rat brain within approximately 30 min under the (17)O inhalation paradigm used. From the first minute of the mpH(2)(17)O time courses, cerebral metabolic rate of oxygen (CMRO(2)) is estimated to be 2.10 +/- 0.44 micromol g(-1) min(-1) (n = 4), a value that is consistent with the literature.

Mouse MRI: Concepts and Applications in Physiology

The purpose of this review is to provide an introduction to the rapidly expanding field of mouse magnetic resonance imaging (MRI). It is by no means meant to be all-inclusive but rather to provide a brief introduction to the basics of MRI theory, provide some insight into the basic experiments that can be performed in mice by using MRI, and bring to light some factors to consider when planning a mouse MRI experiment.

In vivo measurement of T1rho dispersion in the human brain at 1.5 tesla

PURPOSE

To measure T1rho relaxation times and T1rho dispersion in the human brain in vivo.

MATERIALS AND METHODS

Magnetic resonance imaging (MRI) was performed on a 1.5-T GE Signa clinical scanner using the standard GE head coil. A fast spin-echo (FSE)-based T1rho-weighted MR pulse sequence was employed to obtain images from five healthy male volunteers. Optimal imaging parameters were determined while considering both the objective of the study and the guarantee that radio-frequency (RF) power deposition during MR did not exceed Food and Drug Administration (FDA)-mandated safety levels.

RESULTS

T1rho-weighted MR images showed excellent contrast between different brain tissues. These images were less blurred than corresponding T2-weighted images obtained with similar contrast, especially in regions between brain parenchyma and cerebrospinal fluid (CSF). Average T1rho values for white matter (WM), gray matter (GM), and CSF were 85 +/- 3, 99 +/- 1, and 637 +/- 78 msec, respectively, at a spin-locking field of 500 Hz. T1rho is 30% higher in the parenchyma and 78% higher in CSF compared to the corresponding T2 values. T1rho dispersion was observed between spin-locking frequencies 0 and 500 Hz.

CONCLUSION

T1rho-weighted MRI provides images of the brain with superb contrast and detail. T1rho values measured in the different brain tissues will serve as useful baseline values for analysis of T1rho changes associated with pathology.

In vivo quantification ofT1? using a multislice spin-lock pulse sequence

A multislice spin-lock (MS-SL) pulse sequence is implemented on a clinical scanner to acquire multiple images with spin-lock-generated contrast of the knee joints of six healthy human subjects. The MS-SL sequence produces images with T1rho contrast with an additional factor of intrinsic T2rho weighting, which hinders direct measurement of T1rho. A method is presented to compensate the MS-SL-generated data with regard to T2rho in an effort to accurately calculate multislice T1rho maps in a feasible experimental time. The T2rho-compensated multislice T1rho maps produced errors in the measurement of T1rho in healthy patellar cartilage of approximately 5% compared to the gold standard measurement of T1rho acquired with single-slice spin-lock pulse sequence. The MS-SL sequence has potential as an important clinical tool for the acquisition of multislice T1rho-weighted images and/or quantitative multislice T1rho maps.

Pulse sequence for multisliceT1?-weighted MRI

A 2D multislice spin-lock (MS-SL) MR pulse sequence is presented for rapid volumetric T1rho-weighted imaging. Image quality is compared with T1rho-weighted data collected using a single-slice (SS) SL sequence and T2-weighted data from a standard MS spin-echo (SE) sequence. Saturation of longitudinal magnetization by the application of nonselective SL pulses is experimentally measured and theoretically modeled as T2rho decay. The saturation data is used to correct the image data as a function of the SL pulse duration to make quantitative measurements of T1rho. Measurements of T1rho using the saturation-corrected MS-SL data are nearly identical to those measured using an SS-SL sequence. The MS-SL sequence produces quantitative T1rho maps of an entire sample volume with the high-SNR advantages conferred by SE-based sequences.

Method for reduced SART1?-weighted MRI

A reduced specific absorption rate (SAR) version of the T(1rho)-weighted MR pulse sequence was designed and implemented. The reduced SAR method employs a partial k-space acquisition approach in which a full power spin-lock pulse is applied to only the central phase-encode lines of k-space, while the remainder of k-space receives a low-power spin-lock pulse. Acquisition of high- and low-power phase-encode lines are interspersed chronologically to minimize average power deposition. In this way, the majority of signal energy in the central portion of k-space receives full T(1rho)-weighting, while the average SAR of the overall acquisition can be reduced, thereby lowering the minimum safely allowable TR. The pulse sequence was used to create T(1rho) maps of a phantom, an in vivo mouse brain, and the brain of a human volunteer. In the images of the human brain, SAR was reduced by 40% while the measurements of T(1rho) differed by only 2%. The reduced SAR sequence enables T(1rho)-weighted MRI in a clinical setting, even at high field strengths.