High-resolution MR imaging of the rat spinal cord in vivo in a wide-bore magnet at 17.6 Tesla

The objective was to demonstrate the feasibility and to evaluate the performance of high-resolution in vivo magnetic resonance (MR) imaging of the rat spinal cord in a 17.6-T vertical wide-bore magnet. A probehead consisting of a surface coil that offers enlarged sample volume suitable for rats up to a weight of 220 g was designed. ECG triggered and respiratory-gated gradient echo experiments were performed on a Bruker Avance 750 wide-bore spectrometer for high-resolution imaging. With T^*₂ values between 5 and 20 ms, good image contrast could be obtained using short echo times, which also minimizes motion artifacts. Anatomy of healthy spinal cords and pathomorphological changes in traumatically injured rat spinal cord in vivo could be visualized with microscopic detail. It was demonstrated that imaging of the rat spinal cord in vivo using a vertical wide-bore high-magnetic-field system is feasible. The potential to obtain high-resolution images in short scan times renders high-field imaging a powerful diagnostic tool.Volker C. Behr and Thomas Weber contributed equally to this work.     

Quantitative in vivo 1H spectroscopic imaging of metabolites in the early postnatal mouse brain at 17.6 T

Object  Early postnatal brain maturation is closely connected to local changes of metabolite levels. Spatially resolved in vivo ¹H NMR spectroscopic imaging is applied to follow absolute changes of brain metabolites in early postnatal mouse brain. Materials and methods  A short echo time semi LASER (localization by adiabatic selective refocusing) chemical shift imaging (CSI) sequence incorporating weighted k-space averaging was implemented at high magnetic field (17.6 T). In vivo measurements were carried out on postnatal days 5, 8, 12, 16, and 20. In vivo relaxation times T ₁ and T ₂ were measured using variable repetition times or a CPMG sequence, respectively, combined with LASER single voxel localization. Results  Spectra were obtained with a spatial resolution of (1 × 1) mm² in a 1.5 mm slice as early as postnatal day 5. Maturational changes of absolute metabolite concentrations of major metabolites were calculated in four different brain regions. A significant increase of N-acetylaspartate (NAA), total creatine (tCr), and glutamate/glutamine (Glx) concentration was paralleled by a decrease of taurine (Tau) concentration with age (P < 0.05). Differences between brain regions were found for NAA, tCr, and Tau (P < 0.05). Furthermore, in vivo T ₁ and T ₂ of the four major brain metabolites in adult mice are reported. Conclusion  The implemented semi LASER CSI sequence allows following regional changes of metabolite levels. It is suitable for investigation of local differences in brain metabolism and development.     

Contrast-enhanced abdominal echo planar imaging: Dynamic signal-intensity changes following intravenous injection of gadolinium-DOTA

Objectives: After I.V. administration of gadolinium-DOTA, the early contrast enhancement pattern and related signal-intensity (SI) changes in normal abdominal organs (kidney, spleen, liver) are evaluated over the first 4 min by using ultrafast spin-echo echo planar imaging (SE-EPI). Methods: On a 1.5-T magnetic resonance unit ultrafast EPI of the upper abdomen was performed in 12 patients in order to show the contrast enhancement pattern and related measurable SI changes onT ₁ andT ₂-weighted (w) images over the first 4 min after I.V. bolus injection of 0.1 mmol kg^–1 gadolinium (Gd)-DOTA in the spleen, liver, renal cortex, and renal medulla. A TR/TE of 500/44 or 45 ms inT ₁w SE-EPI and a TR/TE of 2000/80 or 100 ms inT ₂-w SE-EPI were used. Results: Typical time-dependent SI changes were noticed onT ₁w images: Subsequent to a SI increase in the renal cortex (starting 7 s after the I.V. injection of Gd-DOTA) SI increased first in the outer renal medulla (6 s later) and then in the inner renal medulla (21 s later). A SI increase was observed in the spleen (starting after 15 s) and in the liver (starting 7 s later). OnT ₂-w images, a SI decrease in the renal cortex (starting after 14 s) was followed by migration of a dark band from the outer (after 46 s) to the inner medulla (after 70 s). Only minimal changes were noticed in the spleen and liver. Conclusions: Ultrafast SE-EPI following I.V. bolus injection of Gd-DOTA enables the observation of the very early contrast agent kinetics in various abdominal organs. The associated SI changes onT ₁- andT ₂- SE EPI are related to organ perfusion and contrast agent tissue concentration and biodistribution.Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Penn Plaza, New York, NY 10119.     

Fast three-dimensional sodium imaging of human brain

A three-dimensional sodium imaging technique with a minimum echo time of 0.9 ms is described in a 2.0 Tesla whole-body system. The relaxation behaviour in vivo of sodium was analysed: a lastT ₂ ^* relaxation component between 1.2 and 1.6 ms and a slowT ₂ ^* relaxation component between 7.1 ms and 8.4 ms were quantified in brain tissue of three volunteers. Three-dimensional sodium images of the human brain were acquired in 8.5 min with a resolution of 4.7 × 4.7 × 10 mm (0.2 cc voxel size) and a signal-to-noise ratio of 20 in brain tissue and 30 in cerebrospinal fluid.     

Dual-contrast single breath-hold 3D abdominal MR imaging

Object: Multiple contrasts are often helpful for a comprehensive diagnosis. In 3D abdominal MRI, breath-hold techniques are preferred for single contrast acquisitions to avoid respiratory artifacts. In this paper, highly accelerated parallel MRI is used to acquire large 3D abdominal volumes with two different contrasts within a single breath-hold. Material and methods: In vivo studies have been performed on six healthy volunteers, combining T ₁- and T ₂-weighted, gradient- or spin-echo based scans, as well as water/fat resolved imaging in a single breath-hold. These 3D scans were acquired with an acceleration factor of six, using a prototype 32-element receive array. Results: The presented approach was tested successfully on all volunteers. The whole liver area was covered by a FOV of 350 × 250 × 200 mm³ for all scans with reasonable spatial resolution. Arbitrary scan protocols generating different contrasts have been shown to be combinable in this single breath-hold approach. Good spatial correspondence with negligible spatial offset was achieved for all different scan combinations acquired in overall breath-hold times between 15 and 25 s. Conclusion: Enabled by highly parallel imaging technology, this study demonstrates the technical feasibility and the promising image quality of single breath-hold dual contrast MRI.     

Diffusion-weighted imaging of the spine using radialk-space trajectories

Introduction Diffusion-weighted MR imaging (DWI) of the spine requires robust imaging methods, that are insensitive to susceptibility effects caused by the transition from bone to soft tissue and motion artifacts due to breathing, swallowing, and cardiac motion. The purpose of this study was to develop a robust imaging method suitable for DWI of the spine. Methods and subjects A radialk-space spin echo sequence has been implemented, which is sell-navigating because each acquisition line passes through the origin ofk-space. Influence of cardiac motion and associated flow of cerebrospinal fluid is minimized by cardiac gating with a finger photoplethysmograph. The sequence has been tested on a 1.5T system. Diffusion-weighted images of six normal volunteers were acquired in the sagittal plane with 4b values between 50 and 500 s mm⁻². Because of the symmetries of the cord, diffusion measurements in the head-foot (HF) or left-right (LR) directions were sufficient to measure the dominant effects of anisotropy. Results The apparent diffusion coefficients (ADCs) measured, respectively, in the LR and HF directions were (0.699 ± 0.050) × 10⁻³ and (1.805 ± 0.086) × 10⁻³ mm² s⁻¹ in the spinal cord. (1.588 ± 0.082) × 10⁻³ and (1.528 ± 0.052) × 10⁻³ mm² s⁻¹ in the intervertebral disks, and (0.346 ± 0.047) × 10⁻³ and (0.306 ± 0.035) × 10⁻³ mm² s⁻¹ in the vertebrae of the cervicothoraeic spine. Conclusion Diffusion-weighted spin echo sequences with radial trajectories ink-space provide a means of achieving robust, high quality diffusion-weighted imaging and measuring ADCs in the spine. The application of the diffusion-weighting gradients in different directions allows diffusion anisotropy to be measured.     

The measurement of gastric motor function and transit in man by echo planar magnetic resonance imaging

Echo-planar imaging (EPI) can be used to produce snapshot images of the human stomach and antro-pyloro-duodenal segment in real time as an alternative technique to intubation and exposure to ionizing radiation. The method has been further developed to monitor simultaneous gastric motility and gastric emptying of liquid and solid meals. The model has been utilized to study the effects of pharmacological agents on gastric function.Eight normal subjects were imaged in a 0.5-T superconducting magnet for up to 6 h following ingestion of 800 ml tap water, followed by 500 ml porridge test meal + 500 ml tap water. A rapid multislice technique was adopted to image adjacent transverse slices (10 mm thick) through the gastric region. In addition, three subjects were orally dosed with 20 mg of the prokinetic agent Cisapride. Gastric volumes for each slice were calculated and summed to produce a measure of total gastric volume and gastric emptying. Contractile activity at the level of the antro-pyloric segment was detected using sequential 128 ms images at 3 s intervals. Alternate measurements of gastric volume and motility were made for the duration of the study.Gastric emptyingT _1/2's (times to empty 50% of the gastric contents) of 12.9 min for water and 116 min for porridge were in agreement with results obtained by the traditional techniques of gamma scintigraphy and impedance imaging. The frequency of gastric contractions increased from 2.4 contractions per minute (cpm) to 3.2 cpm following water and from 2.9 to 3.2 cpm following porridge. The prokinetic effect of enhanced coordination of antroduodenal contractions was also observed. These studies have demonstrated that EPI can be used to detect and image gastroduodenal function in man, totally noninvasively, and can be used to study the effects of drugs acting on the gastrointestinal tract.     

Human rapid acquisition with relaxation enhancement imaging at 8 T without specific absorption rate violation

A standard fast imaging sequence, rapid acquisition with relaxation enhancement (RARE), has been applied to human magnetic resonance at 8 T. RARE is known for its speed, good contrast and high RF power content. HighlyT ₂ weighted images, the hallmark of RARE imaging, were acquired from the human brain. It is demonstrated that whileT ₂ values may be reduced at 8 T, high quality RARE images could still be acquired at this field strength. Most importantly however, it is demonstrated that RARE images could be acquired without violating specific absorption rate (SAR) guidelines. Since it is well known thatT ₂ weighted images are of significant value in clinical diagnosis, the implementation of RARE at this field strength will provide ultra high field MRI (UHFMRI) with a valuable imaging protocol at this field strength without exceeding SAR limitations.     

Low-field magnetic resonance imaging of the pelvis in patients with anal dynamic graciloplasty: initial experience

The aim of this study was to determine whether low-field magnetic resonance (MR) imaging can safely and accurately depict inflammatory changes in patients with anal dynamic graciloplasty, in whom high-field MR imaging is contraindicated and ultrasonography and computed tomography are inadequate. A 0.2-T field-strength MR examination was performed in six patients with anal dynamic graciloplasty malfunction in whom reoperation was contemplated. The following sequences were applied:T ₂-weighted turbo spinecho with fat saturation,T ₁-weighted conventional spin-echo, and contrastenhancedT ₁-weighted conventional spin-echo with fat saturation. Results indicated that none of the patients experienced relevant discomfort, pacemaker malfunction, or electrode dislocation with low-field MR imaging. Inflammatory pelvic changes were visualized in four patients and atrophy of the transposed gracilis muscle in another. Surgery was thus avoided in the four, who underwent conservative treatment for their pelvic inflammation. It was concluded that these prelininary results demonstrate the feasibility of MR imaging with a low field strength in patients with anal dynamic graciloplasty. In such patients, in whom diagnostic imaging had been problematic, the potential for safe and accurate visualization will be a boon to treatment planning.     

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.     

Fluorine-19 MRI at 21.1 T: enhanced spin–lattice relaxation of perfluoro-15-crown-5-ether and sensitivity as demonstrated in ex vivo murine neuroinflammation

Objective

Fluorine MR would benefit greatly from enhancements in signal-to-noise ratio (SNR). This study examines the sensitivity gain of ¹⁹F MR that can be practically achieved when moving from 9.4 to 21.1 T.

Materials and methods

We studied perfluoro-15-crown-5-ether (PFCE) at both field strengths (B₀), as a pure compound, in the form of nanoparticles (NP) as employed to study inflammation in vivo, as well as in inflamed tissue. Brains, lymph nodes (LNs) and spleens were obtained from mice with experimental autoimmune encephalomyelitis (EAE) that had been administered PFCE NPs. All samples were measured at both B₀ with 2D-RARE and 2D-FLASH using ¹⁹F volume radiofrequency resonators together. T₁ and T₂ of PFCE were measured at both B₀ strengths.

Results

Compared to 9.4 T, an SNR gain of > 3 was observed for pure PFCE and > 2 for PFCE NPs at 21.1 T using 2D-FLASH. A dependency of ¹⁹F T₁ and T₂ relaxation on B₀ was demonstrated. High spatially resolved ¹⁹F MRI of EAE brains and LNs at 21.1 T revealed signals not seen at 9.4 T.

Discussion

Enhanced SNR and T₁ shortening indicate the potential benefit of in vivo ¹⁹F MR at higher B₀ to study inflammatory processes with greater detail.

     

Characterization of late radiation effects in the rat thoracolumbar spinal cord by MR imaging using USPIO

The aim of this study was to detect late radiation effects in the rat spinal cord using MR imaging with ultra-small particles of iron oxide (USPIO) contrast agent to better understand the development of late radiation damage with emphasis on the period preceding neurological signs. Additionally, the role of an inflammatory reaction was assessed by measuring macrophages that internalized USPIO. T₂-weighted spin echo MR measurements were performed at 7T in six rats before paresis was expected (130–150 days post-irradiation, early group), and in six paretic rats (150–190 days post-irradiation, late group). Measurements were performed before, directly after and, only in the early group, 40 h after USPIO administration and compared with histology. In the early group, MR images showed focal regions in grey matter (GM) and white matter (WM) with signal intensity reduction after USPIO injection. Larger lesions with contrast enhancement were located in and around edematous GM of three animals of the early group and five of the late group. Forty hours after injection, additional lesions in WM, GM and nerve roots appeared in animals with GM edema. In the late paretic group, MR imaging showed WM necrosis adjacent to areas with large contrast enhancement. In conclusion, detection of early focal lesions was improved by contrast administration. In the animals with extended radiation damage, large hypo-intense regions appeared due to USPIO, which might be attributed to blood spinal cord barrier breakdown, but the involvement of blood-derived iron-loaded macrophages could not be excluded.     

Volumetric imaging with homogenised excitation and static field at 9.4 T

Objectives

To overcome the challenges of B₀ and RF excitation inhomogeneity at ultra-high field MRI, a workflow for volumetric B₀ and flip-angle homogenisation was implemented on a human 9.4 T scanner.

Materials and methods

Imaging was performed with a 9.4 T human MR scanner (Siemens Medical Solutions, Erlangen, Germany) using a 16-channel parallel transmission system. B₀- and B₁-mapping were done using a dual-echo GRE and transmit phase-encoded DREAM, respectively. B₀ shims and a small-tip-angle-approximation kT-points pulse were calculated with an off-line routine and applied to acquire T₁- and T ₂ ^* -weighted images with MPRAGE and 3D EPI, respectively.

Results

Over six in vivo acquisitions, the B₀-distribution in a region-of-interest defined by a brain mask was reduced down to a full-width-half-maximum of 0.10 ± 0.01 ppm (39 ± 2 Hz). Utilising the kT-points pulses, the normalised RMSE of the excitation was decreased from CP-mode’s 30.5 ± 0.9 to 9.2 ± 0.7 % with all B ₁ ⁺  voids eliminated. The SNR inhomogeneities and contrast variations in the T₁- and T ₂ ^* -weighted volumetric images were greatly reduced which led to successful tissue segmentation of the T₁-weighted image.

Conclusion

A 15-minute B₀- and flip-angle homogenisation workflow, including the B₀- and B₁-map acquisitions, was successfully implemented and enabled us to reduce intensity and contrast variations as well as echo-planar image distortions in 9.4 T images.

     

Magnetic resonance imaging of bovine ovaries in vitro

A sample of 20 bovine ovaries were imaged in vitro using nuclear magnetic resonance (NMR) techniques to determine the visibility of various physiologic structures. In particular, the possibility of using NMR imaging to differentiate atretic follicles from physiologically selected and ovulatory follicles was examined. Five of the 20 ovaries were preserved in formalin, whereas the remaining 15 were preserved in a saline solution and imaged within 18 hours of death. Images weighted by T₁ and T₂ proton spin relaxation rates were obtained along with some three-dimensional (3-D) data sets acquired via a fast imaging with steady-state precession technique. Physiologically different structures were easily identified in the images from their morphology, especially in the 3-D images. Weighting by T₁ and T₂ was able to separate structures in the fresh ovaries in the following manner. Atretic and cohort follicles appear dark in T₁-weighted images and bright in T₂-weighted images. Ovulatory follicles appear bright in both T₁-and T₂-weighted images, whereas prephysiologic selection follicles present an intermediate brightness in T₁-weighted images and appear dark in T₂-weighted images. The corpus luteum appears bright in T₁-weighted images and dark in T₂-weighted images, whereas cysts in the corpus luteum appear dark in T₁-weighted images and bright in T₂-weighted images. The varying brightness of the follicles at different stages of development is hypothesized to be related to different hormone and protein concentrations in the follicular fluid. For example, it is known that physiologically selected preovulatory follicles contain high concentrations of estrogens in a viscous follicular fluid. The increased viscosity may occur only when the follicle fluid contains high concentrations of estrogen and contributes to bright T₁-weighted images. The possibility of using nuclear relaxation-weighted NMR imaging for the study of follicular dynamics and other ovarian biology therefore shows great promise.     

3D <Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript>-weighted imaging at 7T using dynamic k<Subscript>T</Subscript>-points on single-transmit MRI systems

Objectives

For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B ₁ ⁺ inhomogeneity. Previously, it was shown that a single k_T-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static k_T-points). This work demonstrates that the B ₁ ⁺ dependence of T ₂-weighted imaging can be further mitigated by designing a specific k_T-point pulse for each pulse of a 3D TSE sequence (dynamic k_T-points) even on single-channel transmit systems

Materials and methods

By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic k_T-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic k_T-points were inserted into the TSE sequence to acquire in vivo images at 7T.

Results

The improvement provided by the dynamic k_T-points over the static k_T-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic k_T-points showed systematic improvement of signal and contrast at 7T over regular TSE—especially in cerebellar and temporal lobe regions without the need of parallel transmission.

Conclusion

Designing dynamic k_T-points for a 3D TSE sequence allows the acquisition of T ₂-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B ₁ ⁺ inhomogeneity.

     

PRESS-based proton single-voxel spectroscopy and spectroscopic imaging with very short echo times using asymmetric RF pulses

Modified point-resolved spectroscopy (PRESS) sequences for single voxel spetroscopy (MRS) and spectroscopic imaging (SI) with very short echo time (T _E ) are described using asymmetric radio-frequency (RF) pulses as well as an optimized design and timing of the PRESS sequence. The proposed sequences were implemented on a standard 4.7 T imaging system yielding a T _E of 6.0 ms only. Simulations and experimental data measured on phantoms and the rat brain in vivo are presented for MRS and SI showing a high signal-to-noise ratio and hardly any phase distortions caused by J-coupling.     

Low-molecular-weight paramagnetic 19F contrast agents for fluorine magnetic resonance imaging

Objective

¹⁹F MRI requires biocompatible and non-toxic soluble contrast agents with high fluorine content and with suitable ¹⁹F relaxation times. Probes based on a DOTP chelate with 12 magnetically equivalent fluorine atoms (DOTP-tfe) and a lanthanide(III) ion shortening the relaxation times were prepared and tested.

Methods

Complexes of DOTP-tfe with trivalent paramagnetic Ce, Dy, Ho, Tm, and Yb ions were synthetized and characterized. ¹⁹F relaxation times were determined and compared to those of the La complex and of the empty ligand. In vitro and in vivo ¹⁹F MRI was performed at 4.7 T.

Results

¹⁹F relaxation times strongly depended on the chelated lanthanide(III) ion. T₁ ranged from 6.5 to 287 ms, T₂ from 3.9 to 124.4 ms, and T₂* from 1.1 to 3.1 ms. All complexes in combination with optimized sequences provided sufficient signal in vitro under conditions mimicking experiments in vivo (concentrations 1.25 mM, 15-min scanning time). As a proof of concept, two contrast agents were injected into the rat muscle; ¹⁹F MRI in vivo confirmed the in vivo applicability of the probe.

Conclusion

DOTP-based ¹⁹F probes showed suitable properties for in vitro and in vivo visualization and biological applications. The lanthanide(III) ions enabled us to shorten the relaxation times and to trim the probes according to the actual needs. Similar to the clinically approved Gd³⁺ chelates, this customized probe design ensures consistent biochemical properties and similar safety profiles.

     

High-resolution echo-planar imaging at 3.0 T

Echo-planar imaging (EPI) is a snapshot technique, which is useful in a wide range of clinical applications, including the study of physiological function. Over recent years, EPI has found a major new use in functional imaging of the brain. Many EPI experiments can benefit from the increased signal-to-noise ratio (S/N) which results from imaging at high magnetic field. Recently, we have built a 3.0-T EPI scanner at Nottingham University. The low-level radiofrequency and control electronics have been constructed in-house. This, coupled with software written specifically for the system, results in a performance and flexibility exceeding that of a commercial system. A quiet head gradient set produces gradients of up to 30 mT m^–1. It is driven using a series multiresonant filter circuit, which allows the production of high-strength, trapezoidal- or sinusoidal-switched gradients.Using this scanner it has been possible to obtain images comprising 256×256 pixels, with a 2.5-mm slice and 0.75 mm in-plane resolution, in 140 ms. Multislicing allows a volume set of 16,128×128 images to be obtained in 1.6 s. A comparison of tests performed at fields of 0.5 T and 3.0 T on the same phantom indicates a better than linear increase in S/N with field strength. EPI images obtained at 3.0 T have been used in studies of brain activation during visual stimulation and execution of a simple motor task.     

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.