Accuracy of 3D MR microscopy for trabecular bone assessment: a comparative study on calcaneus samples using 3D synchrotron radiation microtomography

Magnetic resonance (MR) imaging is attractive for a noninvasive and radiation-free assessment of in vivo trabecular bone architecture. However the quantitative evaluation of architectural parameters could be biased by the limited sensitivity of MR. The aim of this study was to determine the accuracy of trabecular bone architectural parameters obtained from 3D high-resolution MR images, by comparison to reference images obtained by high-resolution X-ray microtomography using synchrotron radiation, from 29 samples of human calcaneus. MR images were obtained with a 66 m×66 m×66 m voxel size, using a 8.5 T MR microscope. Microtomography images were acquired with a 10 m×10 m×10 m voxel size, from the same samples. 3D architectural parameters characterizing the morphometry, topology, anisotropy, and orientation were computed from both modalities and carefully compared. To avoid errors, an identical region of interest was selected in the two corresponding images, and the same algorithms were run at identical spatial resolution. Our results establish that network connectivity, orientation and anisotropy are reliable from the MR data. The bone volume fraction, and morphometric parameters measured from the MR data, were found to be biased with respect to their values from the microtomography data, although there was a significant correlation between the two modalities.An erratum to this article can be found at     

Functional MR with use of FLASH sequences in the evaluation of the phono-articulatory tract

The aims of this study are to determine visualization of normal phonation structures with the use of MR fast sequences and anatomical reference indices which can be used in evaluating upper resonator pathologies. A total of 12 normal subjects were studied, a 1 T system, a volumetric receiving head and neck coil and FLASH sequences for one sagittal and two coronal scans. Each subject was instructed to take a deep breath and then to produce the vowel sounds for the entire duration of the scan. The movement of the following anatomical structures were considered: lips, tongue, soft palate, pharynx and epiglottis. For each subject sagittal scans were used to measure the minimal palate-tongue distance (p−t), the minimal anterio-posterior diameter of the mesopharynx (mp), and the epiglottis excursion angle (ep) with respect to the plane determined by the false vocal cords. Our results were compared with a diagrammed representation of tongue movements (vocal trapezium). This study underlines the validity of MR imaging techniques in the study of the phono-articulatory tract. because MR provided good detail of the phono-articulatory structures enabling an overall functional evaluation. MR would appear useful in evaluating defects in the velum and palate, in staging of oropharyngeal neoplasms and in surgical reconstructions of the tongue.     

Towards 1H-MRSI of the human brain at 7T with slice-selective adiabatic refocusing pulses

Objective To explore the possibilities of proton spectroscopic imaging (¹H-MRSI) of the human brain at 7 Tesla with adiabatic refocusing pulses. Materials and methods A combination of conventional slice selective excitation and two pairs of slice selective adiabatic refocusing pulses (semi-LASER) results in the formation of an echo from a localized volume. Depending on the used radio frequency (rf) coil efficiency and available rf power, the duration of the adiabatic full passage pulses (AFPs) is adapted to enable echo times down to 50 ms (head coil) or 30 ms (local surface coil). Results An AFP duration of 5 ms with a corresponding bandwidth of 5.1 kHz resulted in a chemical shift displacement error of 23% over 3.8 ppm at 7T. Using a local surface coil and an echo time down to 30 ms, we detected not only the three main metabolites (NAA, Cr and Cho), but also coupled signals from myo-inositol and glutamate/glutamine in spectra from 0.14 cc voxels with linewidths down to 10 Hz in 10 min measurement time. Conclusions The semi-LASER pulse sequence enables ¹H-MRSI of the human brain at 7T for larger parts of the brain as well as small localized areas with both a high spectral and spatial resolution. Part of this work has been presented at the ISMRM-ESMRMB joint meeting in 2007 in Berlin, Germany, abstract number 43.     

Spectroscopic imaging of bone marrow composition in vertebral bodies

The proton spectroscopic imaging technique that uses read gradient during acquisition was used for the measurement of the proton spectra in the lumbar and thoracic part of the spine of a patient with breast cancer without known skeletal métastases. The bone marrow fat water ratios were evaluated in the same location before and after chemotherapy treatment. The results were corrected for relaxation effects. The fat/water ratios showed a significant increase as a consequence of the bone marrow degradation process due to chemotherapy. The proposed spectroscopic imaging technique offers rapid acquisition of proton spectra from large volumes of the vertebral bodies.     

7Li 2D CSI of human brain on a clinical scanner

Lithium salts have been widely used in the treatment of mood disorders, but the mechanism of action is still not clear. In this work, a methodology for two-dimensional Lithium-7 imaging on clinical systems is presented. The data were acquired using a phosphorus volume head coil that was re-tuned for the Lithium-7 frequency. A spectroscopic sequence was used to acquire the free induction decay (FID) after volume excitation using a hard pulse. The results obtained on the head of patients undergoing lithium treatment (n = 7, 0.6 mEq/1 average serum level) demonstrate that images of adequate signal to noise ratio (100:1) can be obtained in acceptable imaging times (55 min) using the proposed methodology. The distribution of ’’Li appears uniform in the brains of the patients studied.     

Breath-hold spin-echo MR imaging for evaluation of dynamic enhancement of native and treated hepatocellular carcinoma after intravenous Gd-DTPA administration

Using a simple modification of a standard spin-echo sequence which enables acquisition of three breath-hold images in 15 s, dynamic enhancement of 30 histologically proven hepatocellular carcinomas (17 native tumors, 6 completely necrotic tumors after nonsurgical treatments, and 7 tumors with viable and necrotic portions) after intravenous injection of gadolinium-DTPA was evaluated. Native hepatocellular carcinomas and viable portions in treated nodules showed elective enhancement in images obtained 40 s after contrast injection. Contrast between these lesions and the normal liver decreased thereafter. No contrast uptake was seen in entirely necrotic nodules and necrotic portions of treated nodules. Because of the capability to demonstrate the elective arterial blood supply typical of hepatocellular carcinoma, breath-holdT ₁-weighted spin-echo sequence should replace conventionalT ₁-weighted images for the evaluation of intravenously administered gadolinium-DTPA enhancement of this tumor before and after nonsurgical treatments.     

Long-term stability of cardiac function in normal and chronically failing mouse hearts in a vertical-bore MR system

We previously demonstrated stability of ventricular volumes and cardiac function in normal and in chronically failing mouse hearts in MR systems with a vertical-bore magnet for up to 1 h. However, in order to exploit the benefits of an increased magnetic field strength of these MR systems in more time-consuming studies required by, for example MR spectroscopy, we investigated whether cardiac function and ventricular volumes of healthy and infarcted mice would be affected in vertical position over a prolonged period. We applied high-resolution MR cine imaging on an 11.7 T vertical MR system to monitor cardiac functional parameters of normal and chronically failing mouse hearts over a period of 3 h in an upright position, with a temporal resolution of 15 min. We monitored left-ventricular volumes and cardiac functional parameters in both groups. In normal mice, we detected a decrease of left-ventricular end-systolic volumes by 8 l and an approximately 23% increase of ejection fraction over time indicating a small but detectable degree of orthostatic dysregulation. Observed changes were more pronounced in mice with heart failure. Despite significant changes in left-ventricular volumes and function, absolute values measured for all functional cardiac parameters are consistent with near-physiological conditions. Thus, mice can be studied in high-field MR systems positioned vertically for 3 h.     

MR imaging of degenerative disc disease in the lumbar spine with ultrashort TE pulse sequences

Objective The objective of this study was to assess the feasibility of using ultrashort TE (UTE) pulse sequences to image the lumbar spine.Materials Pulse sequences of TE=0.08 ms were used to image the lumbar spine in 5 normal subjects and 14 patients with degenerative disease. Contrast enhancement was administered in 11 cases.Results The sequences showed high signal in the anterior and posterior longitudinal ligaments, the cartilaginous end plate, the annulus fibrosus, the ligamentum flavum, interspinous ligaments and insertions of ligaments. Normal contrast enhancement was seen in these structures. Enhancement of hypertrophied ligaments and scar tissue was readily identified. Long T2 suppression techniques were useful in distinguishing enhancement of scar tissue from veins. Enhancement in discs was more obvious than with conventional sequences. In a case of thalassaemia bands of high signal were seen in the intervertebral discs parallel to the end plates.Conclusion The UTE sequences offer new options for visualizing discs, scar tissue, ligaments and other structures of the lumbar spine in health and disease.     

Susceptibility-based MRI contrast of the CSF by intravascular superparamagnetic nanoparticles

Endorem^®, a suspension of superparamagnetic iron oxide dextran nanoparticles (NP), have been injected intravenously to healthy anesthetized rats for the purpose of contrast enhancement of brain in gradient-echo imaging at 200 MHz. Not only gray and white matter but also particular regions of the cerebrospinal fluid (CSF) were contrasted in sagittal and transverse images, although samples of this fluid did not contain NP. The selected contrast in the CSF would result from the ability of dense vascular beds containing highly magnetized particles to induce a remote susceptibility effect far beyond the vascular walls into a large fraction of extravascular water.     

Indirect evidence for the potential ability of magnetic resonance imaging to evaluate the myocardial iron content in patients with transfusional iron overload

The purpose of this study was to evaluate the potential ability of magnetic resonance imaging (MRI) for evaluation of myocardial iron deposits. The applied MRI technique has earlier been validated for quantitative determination of the liver iron concentration. The method involves cardiac gating and may, therefore, also be used for simultaneous evaluation of myocardial iron. The tissue signal intensities were measured from spin echo images and the myocardium muscle signal intensity ratio was determined. The SI ratio was converted to tissue iron concentration values based on a modified calibration curve from the liver model. The crucial steps of the method were optimized; i.e. recognition and selection of the myocardial slice for analysis and positioning of the regions of interest (ROIs) within the myocardium and the skeletal muscle. This made the myocardial MRI measurements sufficiently reproducible. We applied this method in 41 multiply transfused patients. Our data demonstrate significant positive linear relationships between different iron store parameters and the MRI-derived myocardial iron concentration, which was significantly related to the serum ferritin concentration (ρ = 0.62.P < 0.0001) and to the MRI-determined liver iron concentration (ρ = 0.36,P = 0.02). The myocardial MRI iron concentrations demonstrated also a significant positive correlation with the number of blood units given (ρ = 0.45,P = 0.005) and the aminotransferase serum concentration (ρ = 0.54,P = 0.0008). Our data represents indirect evidence for the ability of MRI techniques based on myocardium/muscle signal intensity ratio measurements to evaluate myocardial iron overload.     

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.     

Application of two-dimensional CSI for absolute quantification of phosphorus metabolites in the human liver

There have recently been a number of studies dealing with the absolute quantification of concentrations of MR-visible phosphorus compounds in different tissues. The use of absolute values rather than intensity ratios may furnish additional information about metabolic changes associated with different diseases. The purpose of this study was to develop a general procedure for measuring molar metabolite concentrations and to apply it with respect to the evaluation of human liver 31P-MRS data measured using a standard slice-selective two-dimensional CSI sequence and commercial¹H/³¹P surface coil. The experimental determination of all surface coil-related factors influencing signal intensity was undertaken using a gradient echo imaging technique that can be adapted to commercial systems. The resulting values for healthy volunteers (N = 9) showed concentrations of PME = 2.8 ± 1.3 mM, PDE = 9.9 ± 2.7 mM,P _i = 1.7 ± 0.7 mM, and ATP = 3.6 + 0.9 mM in the human liver. The data are quite consistent with published findings.     

T2-weighted breathold imaging of the liver: a quantitative and qualitative comparison of fast spin echo and half Fourier single shot fast spin echo imaging

The imaging characteristics of two EPI-hybrid breath-hold sequences, T2-weighted fast spin-echo [FSE, effective echo time (TE_eff) 138ms] and half Fourier single shot turbo spin-echo (HASTE, TE_eff 60 ms), were compared in hepatic imaging. A total of 111 patients with suspected hepatic disease were studied at 1.5 Tesla using a body phase-array coil. The signal-to-noise (S/N) and contrast-to-noise (C/N) ratios for organs and lesions were calculated and quantitatively compared. Organ delineation, visualization of anatomical structures and pathological lesions, artifacts, and total image quality were qualitatively assessed and statistically compared. The final diagnoses were metastases from colorectal, breast, and pancreatic cancer in 23/111, hepatocellular carcinoma in 15/111, cysts in 19/111, hemangiomas in 9/111, several other lesions in 7/111, and no lesions in 38/111 of the cases. A total of 139 lesion in 73% of the patients were seen while 85% of the lesions were at least 1.5 cm in size. Regarding S/Ns HASTE was significantly (P<0.03) superior to FSE with only minor (P>0.05) differences in C/Ns between the two sequences for anatomical and pathological structures. HASTE demonstrated in almost all (97.3%) of the cases no artifacts, while on fast SE imaging moderate to minor artifacts were present in 23.5–51.7% of the cases. The overall image quality and diagnostic confidence was rated significantly higher (good 43.2%, excellent 53.2%) for HASTE than for fast SE imaging (good 44.8%, excellent 17.6%). Providing comparable C/Ns for anatomical and pathological structures, breatheld HASTE imaging proved to be superior to fast SE in T2-weighted imaging of the upper abdomen regarding general image quality, and, with adequate technical prerequisites, may be a suitable substitute of fast T2-imaging techniques.     

Dynamic snapshot gradient-echo imaging of head and neck malignancies: time dependency and quality of contrast-to-noise ratio

The purpose of this study was to evaluate the time dependency of the contrast-to-noise ratio (CNR) of head and neck malignancies during contrast-enhanced MR imaging. Then we would compare the CNR of dynamic snapshot gradient-echo (SGE) images with conventional spin-echo (SE) and fast spin-echo (FSE) sequences. Fifteen patients with squamous cell carcinomas were examined with T1W-SE, T2W-FSE, contrast-enhanced Gd-TlW-SE, and T1W-SGE sequences, the latter statically and contrast-enhanced dynamically. The CNR for all sequences and adjacent tissues was computed and the time to reach maximal CNR (T_max) was determined for dynamic studies. The CNR was time dependent with two distinct T_max at 6–18 and 60–160 s which corresponded to two different tumor enhancement patterns. Neither enhancement pattern correlated with distinct histologic findings or tumor grading. The CNR improved for the Gd-TlW-SGE images. The improvement was statistically significant in relation to T1W-SE and Gd-TlW-SE images at the floor of the mouth and at the tongue base. The good CNR of the dynamic Gd-TlW-SGE measurements justifies further investigations of this method in order to improve tumor delineation.     

Relaxation rates of blood with osmotically modified red cell volume: Application of the two-compartment fast exchange model

Blood has been considered as a simplified tissue model, both physiologically and physically consisting in two compartments, extra-cellular and intra-cellular. In the physiologic condition (300 mOsm), the relaxation rates of red cell suspensions in saline increased linearly with the hematocrit in the range 0–0.80 according to Fullerton's model of fast proton exchanges between the two compartments (Fullerton GD, Potter JL, Dornbluth NC. NMR relaxation of protons in tissues and other macromolecular water solutions. Magn Reson Imaging 1982;1:209–228). In experiments of osmotic variations, between 200 and 900 mOsm at three constant red cell numbers in the samples, non-linear variations of relaxation rates with red cell volume were observed. In the hyperosmotic domain, the particularly high increase in blood transverse relaxation rate with the decreasing cell volume has been attributed to the progressive water-protein organization in the cellular compartment. A generalised form of the fast exchange model has been applied to extended experimental conditions of red cells, by introducing the red cell volume ratio of modified to iso-osmotic values, and the volume fraction of iso-osmotic red cells.     

ECG-gated <Superscript>23</Superscript>Na-MRI of the human heart using a 3D-radial projection technique with ultra-short echo times

Pathological changes in tissue often manifest themselves in an altered sodium gradient between intra- and extracellular space due to a malfunctioning Na⁺–K⁺ pump, resulting in an increase in total sodium concentration in ischaemic regions. Therefore, ²³Na-MRI has the potential to non-invasively differentiate viable from non-viable tissue by detecting concentration changes of intra- and extracellular sodium. As the in vivo sodium signal shows a bi-exponential T₂ decay, with a short component of less than 1 ms, the accurate quantification of the total sodium content requires imaging techniques with ultra-short echo times (TE) below 0.5 ms. A 3D-radial projection technique has been developed which allows the acquisition of ECG-triggered sodium images of the human heart with a TE of 0.4 ms. With this pulse sequence ²³Na-MRI volunteer measurements of the head or the heart were performed in less than 18 min on a 1.5-T clinical scanner with an isotropic resolution of 10 mm³. The signal to noise ratio of the radial projection technique is twofold higher than that of a Cartesian gradient echo pulse sequence (TE = 3.2 ms). Radial ²³Na-MRI provides a tool for clinical studies, aiming at the differentiation of viable and non-viable tissue.     

A simple method for mapping the B1 field distribution of linear RF coils

Inhomogeneity of the radio frequency (RF) field B₁ leads to intensity variations in MR images and to spatial dependence of spectral line amplitudes. In this paper, a simple method of measuring the B₁ field components of an unsegmented linear coil is described. The method is designed for the coils operating up to 20 MHz. The B₁ field distribution is replaced by the static magnetic field caused by DC current flowing through the coil. The technique involves rotating the coil 90° so that measured B₁ component is aligned with B₀ and measuring the shift of resonance frequency using a spectroscopic imaging sequence. Experimental results were in good agreement with the theoretical computations.     

Development and validation of an algorithm for registration of serial 3D MR breast data sets

We report on the development of an algorithm to improve the registration of serial 3D MR breast images using combined global translation and rotation with locally varying parameters as geometric transformations. Several phantom and volunteer data sets were acquired and registered using mutual information as a similarity measure of the matching process. After applying a global translation by using a rigid matcher, optimum horizontal and vertical rotation angles were determined. In case of the phantom measurements, angle optimization was performed for each slice of the 3D data set of the phantom, which was deliberately shifted and rotated around different axes. In case of registration of volunteer data, optimum rotation parameters were calculated for preselected equidistant slices of the data set to speed up the calculation time. For slices located between and outside these support slices, the rotation angles were calculated by linear interpolation and extrapolation of the slope of the regression determined by the optimized angles of the support slices. The algorithm improves the registration of serial 3D MR data sets and represents a compromise between a rigid and an elastic 3D matching procedure.     

Pharmacokinetics of Gadomer-17, a new dendritic magnetic resonance contrast agent

Rationale and objectives: Gadomer-17 is a new magnetic resonance (MR) contrast medium presently in clinical development. It is a dendritic gadolinium (Gd) chelate carrying 24 Gd ions. This study investigated the pharmacokinetic behavior of this contrast medium. Methods: The pharmacokinetics of Gadomer-17 were investigated in different species (rat, rabbit, dog, monkey) for up to 7 days after intravenous (i.v.) injection of 25–100 μmol/kg body weight. In addition, elimination and biodistribution were evaluated after single i.v. injection of Gadomer-17 in rats. Results: After i.v. injection Gadomer-17 distributes almost exclusively within the intravascular space without significant diffusion into the interstitial space. The volume of distribution (Vc) in the initial or α-phase ranged from 0.04 1/kg (rats, rabbits) to 0.06 1 kg (monkeys) and 0.07 1/kg (dogs), which reflects mainly the plasma volume. The blood/plasma concentration profile was found to be biphasic. The volume of distribution at a steady state is clearly smaller than that of other contrast media, which distribute to the extracellular space. After single i.v. injection in rats, the dendritic contrast medium was rapidly and completely eliminated from the body, mainly via glomerular filtration. No long-term accumulation or retention of the nonmetabolized agent was detectable in organs or tissues. Conclusions: Gadomer-17 is a promising new MR contrast medium that has an intravascular distribution and a rapid renal elimination.