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Felix W Wehrli

Publications and source records attributed to Felix W Wehrli.

At least 19 recordsLinked to original sources
PubMed2006

Experimental and computational analyses of the effects of slice distortion from a metallic sphere in an MRI phantom.

Susceptibility artifacts due to metallic prostheses are a major problem in clinical magnetic resonance imaging. We theoretically and experimentally analyze slice distortion arising from susceptibility differences in a phantom consisting of a stainless steel ball bearing embedded in agarose gel. To relate the observed image artifacts to slice distortion, we simulate images produced by 2D and 3D spin-echo (SE) and a view angle tilting (VAT) sequence. Two-dimensional SE sequences suffer from extreme slice distortion when a metal prosthesis is present, unlike 3D SE sequences for which--since slices are phase-encoded--distortion of the slice profile is minimized, provided the selected slab is larger than the region of interest. In a VAT sequence, artifacts are reduced by the application of a gradient along the slice direction during readout. However, VAT does not correct for the excitation slice profile, which results in the excitation of spins outside the desired slice location and can lead to incorrect anatomical information in MR images. We propose that the best sequences for imaging in the presence of a metal prosthesis utilize 3D acquisition, with phase encoding replacing slice selection to minimize slice distortion, combined with excitation and readout gradient strengths at their maximum values.

Artifacts↗
PubMed2006

MR susceptometry for measuring global brain oxygen extraction.

Monitoring of oxygen saturation in jugular venous blood gives an estimate of the balance of global oxygen delivery and cerebral oxygen consumption. We present a noninvasive approach to measure oxygen saturation in vivo in the internal jugular vein using MR susceptometry by exploiting the characteristic susceptibility of deoxyhemoglobin, and demonstrate the feasibility of performing such measurements in a group of subjects. We assessed the sensitivity of the method for detecting small changes in oxygen saturation by monitoring the variations observed during breath-holding and hypoventilation experiments. Unlike alternative methods, the susceptometric technique does not require calibration.

Brain↗
PubMed2006

Quantitative MRI for the assessment of bone structure and function.

Osteoporosis is the most common degenerative disease in the elderly. It is characterized by low bone mass and structural deterioration of bone tissue, leading to morbidity and increased fracture risk in the hip, spine and wrist-all sites of predominantly trabecular bone. Bone densitometry, currently the standard methodology for diagnosis and treatment monitoring, has significant limitations in that it cannot provide information on the structural manifestations of the disease. Recent advances in imaging, in particular MRI, can now provide detailed insight into the architectural consequences of disease progression and regression in response to treatment. The focus of this review is on the emerging methodology of quantitative MRI for the assessment of structure and function of trabecular bone. During the past 10 years, various approaches have been explored for obtaining image-based quantitative information on trabecular architecture. Indirect methods that do not require resolution on the scale of individual trabeculae and therefore can be practiced at any skeletal location, make use of the induced magnetic fields in the intertrabecular space. These fields, which have their origin in the greater diamagnetism of bone relative to surrounding marrow, can be measured in various ways, most typically in the form of R2', the recoverable component of the total transverse relaxation rate. Alternatively, the trabecular network can be quantified by high-resolution MRI (micro-MRI), which requires resolution adequate to at least partially resolve individual trabeculae. Micro-MRI-based structure analysis is therefore technically demanding in terms of image acquisition and algorithms needed to extract the structural information under conditions of limited signal-to-noise ratio and resolution. Other requirements that must be met include motion correction and image registration, both critical for achieving the reproducibility needed in repeat studies. Key clinical applications targeted involve fracture risk prediction and evaluation of the effect of therapeutic intervention.

Animals↗
PubMed2006

Skeletal effects of short-term exposure to dexamethasone and response to risedronate treatment studied in vivo in rabbits by magnetic resonance micro-imaging and spectroscopy.

Supraphysiological levels of glucocorticoids (GC) cause bone atrophy and growth retardation. In this study, we examine whether these adverse structural effects are reversible and whether treatment with bisphosphonate is protective by in vivo serial micro-magnetic resonance (micro-MR) imaging and spectroscopy. Rabbits (n = 16) were divided into four groups, a control group (n = 4) and three groups (n = 4 each) receiving GC in the form of dexamethasone (0.45 mg/kg/day). In one of the GC groups dexamethasone exposure was discontinued after 2 weeks, the other two groups were maintained on dexamethasone, with one receiving risedronate (5 microg/kg twice per week) from the beginning of treatment, the other one receiving risedronate after 2 weeks. Animals were imaged at baseline and at 2, 4, and 8 weeks. Trabecular bone volume fraction (trabecular bone volume/tissue volume, TB/TV), trabecular bone thickness (Tb.Th), and topological structural parameters were measured in the distal femoral epiphysis, as was the epiphyseal growth plate (EGP), and marrow fat fraction, which was computed from the integrated proton spectra. Two weeks of dexamethasone exposure substantially reduced TB/TV, along with thinning of the epiphyseal growth plate and conversion of hematopoietic to adipocytic marrow. However, TB/TV recovered to normal levels within 2 weeks after cessation of GC exposure and remained constant throughout the remainder of the protocol. In contrast, neither EGP thickness nor marrow composition returned to control levels after cessation of GC exposure as rapidly as did TB/TV. Besides increased TB/TV, bisphosphonate treatment resulted in a more connected platelike network than observable at baseline, but GP atrophy and marrow conversion caused by GC exposure were not affected. The data lend support to a protective effect of risedronate on trabecular architecture during GC exposure. The improvement in trabecular network parameters beyond baseline values further suggest a therapeutic effect of risedronate.

Animals↗
PubMed2006

Noninvasive assessment of bone microarchitecture by MRI.

In determining fracture risk, it has become apparent that bone mineral density accounts for only a portion of bone strength, with the remainder being determined by the material and structural properties of the bone tissue. Over the past 15 years, high-resolution MRI has provided a window into the structural nature of bone disease. Cross-sectional studies imaging the trabecular bone in patients with conditions ranging from postmenopausal osteoporosis to organ transplantation to renal osteodystrophy have all demonstrated a correlation of microarchitecture with fracture burden and have done so at a variety of anatomic sites. Recently, the utility of longitudinal studies for monitoring treatment in vivo has been demonstrated. This technique is noninvasive, involving no contrast or ionizing radiation, and provides useful clinical information independent of bone mineral density, thereby allowing for better classification of those at high risk for fracture.

Animals↗
PubMed2006

Quantification of the roles of trabecular microarchitecture and trabecular type in determining the elastic modulus of human trabecular bone.

UNLABELLED: The roles of microarchitecture and types of trabeculae in determining elastic modulus of trabecular bone have been studied in microCT images of 29 trabecular bone samples by comparing their Young's moduli calculated by finite element analysis (FEA) with different trabecular type-specific reconstructions. The results suggest that trabecular plates play an essential role in determining elastic properties of trabecular bone. INTRODUCTION: Osteoporosis is an age-related disease characterized by low bone mass and architectural deterioration. Other than bone volume fraction (BV/TV), microarchitecture of bone is also believed to be important in governing mechanical properties of trabecular bone. We quantitatively examined the role of microarchitecture and relative contribution of trabecular types of individual trabecula in determining the elastic property of trabecular bone. MATERIALS AND METHODS: Twenty-nine human cadaveric trabecular bone samples were scanned at 21-mum resolution using a microCT system. Digital topological analysis (DTA) consisting of skeletonization and classification was combined with a trabecular type-specific reconstruction technique to extract the skeleton and identify topological type of trabeculae of the original trabecular bone image. Four different microCT-based finite element (FE) models were constructed for each specimen: (1) original full voxel; (2) skeletal voxel; (3) rod-reconstructed, preserving rod volume and plate skeleton; and (4) plate-reconstructed, preserving plate volume and rod skeleton. For each model, the elastic moduli were calculated under compression along each of three image-coordinate axis directions. Plate and rod tissue fractions directly measured from DTA-based topological classification were correlated with the elastic moduli computed from full voxel model. RESULTS: The elastic moduli of skeleton models were significantly correlated with those of full voxel models along all three coordinate axes (r(2) = 0.38 approximately 0.53). The rod-reconstructed model contained 21.3% of original bone mass and restored 1.5% of elastic moduli, whereas the plate-reconstructed model contained 90.3% of bone mass and restored 53.2% of elastic moduli. Plate tissue fraction showed a significantly positive correlation (r(2) = 0.49) with elastic modulus by a power law, whereas rod tissue fraction showed a significantly negative correlation (r(2) = 0.42). CONCLUSIONS: These results quantitatively show that the microarchitecture alone affects elastic moduli of trabecular bone and trabecular plates make a far greater contribution than rods to the bone's elastic behavior.

Aged↗
PubMed2005

Effect of testosterone replacement on trabecular architecture in hypogonadal men.

UNLABELLED: We evaluated the effect of testosterone treatment on trabecular architecture by microMRI in 10 untreated severely hypogonadal men. After 2 years, microMRI parameters of trabecular connectivity improved significantly, suggesting the possibility that testosterone improves trabecular architecture. INTRODUCTION: Osteoporosis, characterized by low BMD and diminished bone quality, is a significant public health problem in men. Hypogonadal men have decreased BMD and deteriorated trabecular architecture compared with eugonadal men, and testosterone treatment improves their BMD. We tested the hypothesis that testosterone replacement in hypogonadal men would also improve their trabecular architecture. MATERIALS AND METHODS: We selected 10 untreated severely hypogonadal men and treated them with a testosterone gel for 24 months to maintain their serum testosterone concentrations within the normal range. Each subject was assessed before and after 6, 12, and 24 months of testosterone treatment by magnetic resonance microimaging (microMRI) of the distal tibia and by DXA of the spine and hip. The microMRI parameters reflect the integrity of the trabecular network and include the ratio of all surface voxels (representing plates) to curve voxels (representing rods) and the topological erosion index, a ratio of topological parameters expected to increase on trabecular deterioration to those expected to decrease. The higher the surface-to-curve ratio and the lower the topological erosion index, the more intact the trabecular network. RESULTS: Serum testosterone concentrations increased to midnormal after 3 months of treatment and remained normal thereafter. After 24 months of testosterone treatment, BMD of the spine increased 7.4% (p<0.001), and of the total hip increased 3.8% (p=0.008). Architectural parameters assessed by microMRI also changed: the surface-to-curve ratio increased 11% (p=0.004) and the topological erosion index decreased 7.5% (p=0.004). CONCLUSIONS: These results suggest the possibility that testosterone replacement of hypogonadal men improves trabecular architecture.

Adolescent↗
PubMed2005

Mineral volume and morphology in carotid plaque specimens using high-resolution MRI and CT.

OBJECTIVE: High-resolution MRI methods have been used to evaluate carotid artery atherosclerotic plaque content. The purpose of this study was to assess the performance of high-resolution MRI in evaluation of the quantity and pattern of mineral deposition in carotid endarterectomy (CEA) specimens, with quantitative micro-CT as the gold standard. METHODS AND RESULTS: High-resolution MRI and CT were compared in 20 CEA specimens. Linear regression comparing mineral volumes generated from CT (VCT) and MRI (VMRI) data demonstrated good correlation using simple thresholding (VMRI=-0.01+0.98VCT; R2=0.90; threshold=4xnoise) and k-means clustering methods (VMRI=-0.005+1.38VCT; R2=0.93). Bone mineral density (BMD) and bone mineral content (BMC [mineral mass]) were calculated for CT data and BMC verified with ash weight. Patterns of mineralization like particles, granules, and sheets were more clearly depicted on CT. CONCLUSIONS: Mineral volumes generated from MRI or CT data were highly correlated. CT provided a more detailed depiction of mineralization patterns and provided BMD and BMC in addition to mineral volume. The extent of mineralization as well as the morphology may ultimately be useful in assessing plaque stability.

Adult↗
PubMed2005

Spatially-confined arterial spin-labeling with FAIR.

PURPOSE: To investigate the effectiveness of slab-selective inversion in pulsed arterial spin labeling with body coil excitation as a means to reduce large vessel contamination of the perfusion signal. MATERIALS AND METHODS: Studies were conducted by varying the tagging width in multislice flow-sensitive alternating inversion recovery (FAIR) in conjunction with body coil excitation on a Siemens Sonata whole-body 1.5-T scanner. The results of spatially-confined tagging were then compared with conventional nonselective tagging in the presence and absence of a bipolar gradient crusher pair in order to determine the effectiveness of suppressing vascular signal and to estimate the bolus width that reaches the capillary bed. RESULTS: It is shown in five volunteers, ages 23-38 years, that depending on the average velocity of the arterial blood flow in the tagging region, a bolus of 6-8 cm in width reaches the capillary bed at a fixed inversion time TI of 1.4 seconds, while a bolus of 11.2-16.5 cm in width enters the imaging region. Further, noticeable velocity differences have been found among the participating subjects, with averages ranging from 10.1 to 13.9 cm/second. CONCLUSION: The data suggest that it is advantageous to replace nonselective global tagging in FAIR perfusion imaging with body coil excitation by spatially-confined tagging to reduce undesired residual tagged blood in large vessels.

Adult↗
PubMed2005

Implications of pulse sequence in structural imaging of trabecular bone.

PURPOSE: To investigate the SNR and image properties of 3D steady-state free precession (SSFP), fast large-angle spin echo (FLASE), gradient-recalled acquisition in steady state (GRASS), and spoiled GRASS (SPGR) for structural imaging of trabecular bone (TB). MATERIALS AND METHODS: SNR was examined theoretically and experimentally on phantoms, bone specimens, and in vivo. The bone volume fraction, TB thickness, and echo time (TE) dependence of the thickness were compared. The trabecula was modeled as a cylinder in simulations to examine the intra-voxel spin-dephasing in SSFP and GRASS. Images were acquired on a 1.5 T Siemens Sonata system (40 mT/m maximum gradient, 200 T/m/s peak slew rate). RESULTS: Within the hardware and safety limit constraints, SNR of FLASE was superior, followed by SSFP, GRASS, and SPGR. The trabecular thickness derived from gradient-echo images was 10-45% greater than that obtained with FLASE. Conversely, SSFP images delineated partial volume trabeculae better than FLASE. Simulations indicated that the artifactual thickening was more severe in SSFP than in GRASS, which was attributed to off-resonance effects from the induced gradients at the bone/marrow interface. CONCLUSION: FLASE had the highest SNR and was insensitive to susceptibility dephasing. Although SSFP has superior SNR compared to GRASS, off-resonance effects and duty cycle limitations may compromise its practicality in this application. Inc.

Algorithms↗
PubMed2005

Nuclear magnetic resonance studies of bone water.

Mineralized bone tissue has a significant water component. Bone water is associated with the collagen fibers or mineral fraction or occurring as pore water of the Haversian and lacunocanalicular system. Among the multiple physiologic functions that include signaling and providing to bone its viscoelastic properties, bone water enables the transport of ions and nutrients to and waste products from the cells. In addition, it plays a key role during mineralization whereby collagen-bound water is gradually replaced by calcium apatite-like mineral. In this review it is shown how nuclear magnetic resonance (NMR) allows the study of various physiologically relevant properties of bone water nondestructively. Isotope exchange experiments are described from which the apparent water diffusion coefficient can be calculated. The method is based on monitoring the migration of H2O into the D2O after immersion of the specimen in heavy water. Data obtained from rabbit cortical bone in the normal and mineral-depleted skeleton provide evidence for the hypothesized reciprocal relationship between bone water and mineral. Further, from the diffusion coefficient (Da = (7.8+/-1.5) x 10(-7) cm2/s) measured at 40 degrees C it can be inferred that diffusive transport of small molecules from the bone's microvascular system to the osteocytes occurs within minutes. Finally, whereas isotope exchange is not feasible in vivo, it is shown that bone water can be imaged by proton MRI.

Animals↗
PubMed2005

Method for cortical bone structural analysis from magnetic resonance images.

RATIONALE AND OBJECTIVES: Quantitative evaluation of cortical bone architecture as a means to assess bone strength typically is accomplished on the basis of images obtained by means of dual-energy X-ray absorptiometry (DXA) or computed tomography. Magnetic resonance (MR) imaging has potential advantages for this task in that it allows imaging in arbitrary scan planes at high spatial resolution. However, several hurdles have to be overcome to make this approach practical, including resolution of issues related to nonlinear receive coil sensitivity, variations in marrow composition, and the presence of periosteal isointense tissues, which all complicate segmentation. The aim of this study is to develop MR acquisition and analysis methods optimized for the detection of cortical boundaries in such complex geometries as the femoral neck. MATERIALS AND METHODS: Cortical boundary detection is achieved by radially tracing intensity profiles that intersect the periosteal and endosteal boundaries of bone. Profiles subsequently are normalized to the intensity of the marrow signal, processed with morphologic image operators, and binarized. The resulting boundaries are mapped back onto the spatial image, and erroneous boundary points are removed. From the detected cortical boundaries, cortical cross-sectional area and thickness are computed. The method was evaluated on cortical bone specimens and human volunteers on the basis of high-resolution images acquired at a 1.5-Tesla field strength. To assess whether the method is sensitive to detect the expected dependencies of cortical parameters in weight-bearing bone on overall habitus, 10 women aged 46-73 years (mean age, 56 years) underwent the cortical imaging protocol in the proximal femur, and results were compared with DXA bone mineral density parameters of the hip and spine. RESULTS: Reproducibility was approximately 2%. Double oblique images of the femoral neck in the 10 women studied showed that cortical cross-sectional area correlated strongly with height (r = 0.88; p = .0008), whereas cortical diameter versus age approached significance (r = 0.61; p = .06). Measurements in specimens of some cortical parameters indicated resolution dependence. However, note that specimen ranking within each parameter remained constant across all resolutions studied. CONCLUSION: Data suggest the new method to be robust and applicable on standard clinical MR scanners at arbitrary anatomic locations to yield clinically meaningful quantitative results.

Absorptiometry, Photon↗
PubMed2005

High-resolution black-blood MRI of the carotid vessel wall using phased-array coils at 1.5 and 3 Tesla.

RATIONALE AND OBJECTIVES: The aim of this report is to investigate the magnetic field dependence of the signal-to-noise ratio (SNR) for carotid vessel wall magnetic resonance imaging using phased-array (PA) surface coils by comparing images obtained at 1.5 and 3 Tesla (T) and determine the extent to which the improved SNR at the higher field can be traded for improved spatial resolution. MATERIALS AND METHODS: Two pairs of dual-element PA coils were constructed for operation at the two field strengths. The individual elements of each PA were matched to 50 Omega impedance on the neck and tuned at the respective frequencies. The coils were evaluated on a cylindrical phantom positioned with its axis parallel to the main field and the coils placed on either side of the phantom parallel to the sagittal plane. In vivo magnetic resonance images of the carotid arteries were obtained in five subjects at both field strengths with a fast spin-echo double-inversion black-blood pulse sequence with fat saturation. SNR was measured at both field strengths by using standard techniques. RESULTS: At a depth corresponding to the average location of the carotid arteries in the study subjects, mean phantom SNR for the two coils was higher at 3 T by a factor of 2.5. The greater than linear increase is caused by only partial coil loading of these relatively small coils. The practically achievable average SNR gain in vivo was 2.1. The lower in vivo SNR gain is attributed to a reduction in T2 and prolongation of T1 at the higher field strength and, to a lesser extent, the requirement for a decreased refocusing pulse flip angle to operate within specific absorption rate limits. The superior SNR at 3 T appears to provide considerably improved vessel-wall delineation. CONCLUSIONS: Carotid artery vessel-wall magnetic resonance imaging using PA surface coils provides a considerable increase in SNR when field strength is increased from 1.5 to 3 T. This increase can be traded for enhanced in-plane resolution.

Adult↗
PubMed2005

A new approach for simultaneous measurement of ADC and T2 from echoes generated via multiple coherence transfer pathways.

The study of rotational and translational diffusion requires the measurement of both T2 and apparent diffusion coefficient (ADC), quantities that are typically measured in separate experiments. The exploitation of echoes generated via multiple coherence transfer pathways offers an opportunity for measuring T2 and ADC values simultaneously in a single experiment. A series of RF pulses can generate multiple echoes via different coherence pathways with each one being uniquely encoded. Here, we demonstrate one pulse sequence that uses an initial theta; RF pulse to generate three coherence orders (C = 0, -1, +1). In the particular version of the method discussed here only two are used (C = 0, +1). Each order is encoded with a different b value from which the ADC is derived. The coherence order echo C = 0 is refocused to quantify T2. The performance of the method--dubbed simultaneous measurement of ADC and relaxation time (SMART)--is demonstrated on a set of samples differing in T2 and ADC achieved by varying the relative volume fractions in mixtures of gadolinium-doped H2O and D2O. The regional SMART derived T2 and ADC agree well with those obtained with conventional double-spin-echo and pulsed gradient spin-echo methods.

Contrast Media↗
PubMed2005

Structural anisotropy and internal magnetic fields in trabecular bone: coupling solution and solid dipolar interactions.

We investigate the use of intermolecular multiple-quantum coherence to probe structural anisotropy in trabecular bone. Despite the low volume fraction of bone, the bone-water interface produces internal magnetic field gradients which modulate the dipolar field, depending on sample orientation, choice of dipolar correlation length, correlation gradient direction, and evolution time. For this system, the probing of internal magnetic field gradients in the liquid phase permits indirect measurements of the solid phase dipolar field. Our results suggest that measurements of volume-averaged signal intensity as a function of gradient strength and three orthogonal directions could be used to non-invasively measure the orientation of structures inside a sample or their degree of anisotropy. The system is modeled as having two phases, solid and liquid (bone and water), which differ in their magnetization density and magnetic susceptibility. A simple calculation using a priori knowledge of the material geometry and distribution of internal magnetic fields verifies the experimental measurements as a function of gradient strength, direction, and sample orientation.

Anisotropy↗
PubMed2005

Correcting bulk in-plane motion artifacts in MRI using the point spread function.

A technique is proposed for correcting both translational and rotational motion artifacts in magnetic resonance imaging without the need to collect additional navigator data or to perform intensive postprocessing. The method is based on measuring the point spread function (PSF) by attaching one or two point-sized markers to the main imaging object. Following the isolation of a PSF marker from the acquired image, translational motion could be corrected directly from the modulation transfer function, without the need to determine the object's positions during the scan, although the shifts could be extracted if desired. Rotation is detected by analyzing the relative displacements of two such markers. The technique was evaluated with simulations, phantom and in vivo experiments.

Algorithms↗
PubMed2005

A novel local thresholding algorithm for trabecular bone volume fraction mapping in the limited spatial resolution regime of in vivo MRI.

Recent advances in micro-magnetic resonance imaging have shown the possibility of in vivo assessment of trabecular bone architecture. However, the small feature size and relatively low signal-to-noise ratio (SNR) achievable in vivo cause the intensity histogram to be unimodal. The critical first step in the processing of these images is the extraction of bone volume fraction for each voxel. Here, we propose a local threshold algorithm (LTA) that determines the marrow intensity value in the neighborhood of each voxel based on nearest-neighbor statistics. Using the local marrow intensities we threshold the image and scale the intensities of voxels partially occupied by bone to produce a marrow volume fraction map of the trabecular bone region. We show that structural parameters derived with the LTA are highly correlated with those obtained with the previously published histogram deconvolution algorithm (HDA) and that the LTA is robust to image noise corruption. The LTA is found to correctly identify trabeculae with a significantly higher reliability than HDA. Finally, we demonstrate that the LTA is superior in preserving connectivity by showing for 75 in vivo images that the genus of the trabecular bone surface is always higher than when processed with the HDA.

Algorithms↗