Trabecular bone volume fraction mapping by low-resolution MRI

Photoacoustic Imaging and Characterization of Bone in Medicine: Overview, Applications, and Outlook

Photoacoustic techniques have shown promise in identifying molecular changes in bone tissue and visualizing tissue microstructure. This capability represents significant advantages over gold standards (i.e., dual-energy X-ray absorptiometry) for bone evaluation without requiring ionizing radiation. Instead, photoacoustic imaging uses light to penetrate through bone, followed by acoustic pressure generation, resulting in highly sensitive optical absorption contrast in deep biological tissues. This review covers multiple bone-related photoacoustic imaging contributions to clinical applications, spanning bone cancer, joint pathologies, spinal disorders, osteoporosis, bone-related surgical guidance, consolidation monitoring, and transsphenoidal and transcranial imaging. We also present a summary of photoacoustic-based techniques for characterizing biomechanical properties of bone, including temperature, guided waves, spectral parameters, and spectroscopy. We conclude with a future outlook based on the current state of technological developments, recent achievements, and possible new directions.

B1 mapping using an EPI-based double angle approach: A practical guide for correcting slice profile and B0 distortion effects

PURPOSE

Aim of this study was to develop a reliable B₁ mapping method for brain imaging based on vendor MR sequences available on clinical scanners. Correction procedures for B₀ distortions and slice profile imperfections are proposed, together with a phantom experiment for deriving the approximate time-bandwidth-product (TBP) of the excitation pulse, which is usually not known for vendor sequences.

METHODS

The double angle method was used, acquiring two gradient echo echo-planar imaging data sets with different excitation angles. A correction factor C (B₁ , TBP, B₀ ) was derived from simulations for converting double angle method signal quotients into bias-free B₁ maps. In vitro and in vivo tests compare results with reference B₁ maps based on an established in-house sequence.

RESULTS

The simulation shows that C has a negligible B₁ dependence, allowing for a polynomial approximation of C (TBP, B₀ ). Signal quotients measured in a phantom experiment with known TBP reconfirm the simulation results. In vitro and in vivo B₁ maps based on the proposed method, assuming TBP = 5.8 as derived from a phantom experiment, match closely the reference B₁ maps. Analysis without B₀ correction shows marked deviations in areas of distorted B₀ , highlighting the importance of this correction.

CONCLUSION

Double angle method-based B₁ mapping was set up for vendor gradient echo-echo-planar imaging sequences, using a correction procedure for slice profile imperfections and B₀ distortions. This will help to set up quantitative MRI studies on clinical scanners with release sequences, as the method does not require knowledge of the exact RF-pulse profiles or the use of in-house sequences.

Characterization of Structural Bone Properties through Portable Single-Sided NMR Devices: State of the Art and Future Perspectives

Nuclear Magnetic Resonance (NMR) is a well-suited methodology to study bone composition and structural properties. This is because the NMR parameters, such as the T2 relaxation time, are sensitive to the chemical and physical environment of the 1H nuclei. Although magnetic resonance imaging (MRI) allows bone structure assessment in vivo, its cost limits the suitability of conventional MRI for routine bone screening. With difficulty accessing clinically suitable exams, the diagnosis of bone diseases, such as osteoporosis, and the associated fracture risk estimation is based on the assessment of bone mineral density (BMD), obtained by the dual-energy X-ray absorptiometry (DXA). However, integrating the information about the structure of the bone with the bone mineral density has been shown to improve fracture risk estimation related to osteoporosis. Portable NMR, based on low-field single-sided NMR devices, is a promising and appealing approach to assess NMR properties of biological tissues with the aim of medical applications. Since these scanners detect the signal from a sensitive volume external to the magnet, they can be used to perform NMR measurement without the need to fit a sample inside a bore of a magnet, allowing, in principle, in vivo application. Techniques based on NMR single-sided devices have the potential to provide a high impact on the clinical routine because of low purchasing and running costs and low maintenance of such scanners. In this review, the development of new methodologies to investigate structural properties of trabecular bone exploiting single-sided NMR devices is reviewed, and current limitations and future perspectives are discussed.

Single-sided NMR to estimate morphological parameters of the trabecular bone structure

PURPOSE

Single-sided ¹ H-NMR is proposed for the estimation of morphological parameters of trabecular bone, and potentially the detection of pathophysiological alterations of bone structure. In this study, a new methodology was used to estimate such parameters without using an external reference signal, and to study intratrabecular and intertrabecular porosities, with a view to eventually scanning patients.

METHODS

Animal trabecular bone samples were analyzed by a single-sided device. The Carr-Purcell-Meiboom-Gill sequence of ¹ H nuclei of fluids, including marrow, confined inside the bone, was analyzed by quasi-continuous T₂ distributions and separated into two ¹ H pools: short and long T₂ components. The NMR parameters were estimated using models of trabecular bone structure, and compared with the corresponding micro-CT.

RESULTS

Without any further assumptions, the internal reference parameter (short T₂ signal intensity fraction) enabled prediction of the micro-CT parameters BV/TV (volume of the trabeculae/total sample volume) and BS/TV (external surface of the trabeculae/total sample volume) with linear correlation coefficient >0.80. The assignment of the two pools to intratrabecular and intertrabecular components yielded an estimate of average intratrabecular porosity (33 ± 5)%. Using the proposed models, the NMR-estimated BV/TV and BS/TV were found to be linearly related to the corresponding micro-CT values with high correlation (>0.90 for BV/TV; >0.80 for BS/TV) and agreement coefficients.

CONCLUSION

Low-field, low-cost portable devices that rely on intrinsic magnetic field gradients and do not use ionizing radiation are viable tools for in vitro preclinical studies of pathophysiological structural alterations of trabecular bone.

A practical method for post-acquisition reduction of bias in fast, whole-brain B1-maps

Large consistent differences have been observed between maps of the flip angle correction factor (commonly called "B₁-maps") produced with different fast methods in the human brain. We present an empirical procedure for first-order multiplicative bias correction that can be applied when more than one B₁-mapping method is available. We use a B₁-map measurement in a calibration phantom as a reference and the voxel-wise histogram mode between ratios of B₁-maps produced from different methods to calculate determine the bias as a multiplicative correcting scale factor. Institutional implementations of four common methods of B₁-mapping were assessed: Method of Slopes, FSE and EPI double angle methods (DAM), and Bloch-Siegert. In human subjects, the multiplicative bias used to correct for each of the four methods was: Method of Slopes = 1.005, FSE-DAM = 0.956, EPI-DAM = 1.080, and Bloch-Siegert = 1.128. Scaling to remove this bias between methods produces more consistent B₁-maps which enable more consistent values for any computations requiring flip angle correction. In addition, we present evidence that the corrected B₁ maps, using our calibration method, are also more accurate.

Bone volume-to-total volume ratio measured in trabecular bone by single-sided NMR devices

PURPOSE

Reduced bone strength is associated with a loss of bone mass, usually evaluated by dual-energy X-ray absorptiometry, although it is known that the bone microstructure also affects the bone strength. Here, a method is proposed to measure (in laboratory) the bone volume-to-total volume ratio by single-sided NMR scanners, which is related to the microstructure of the trabecular bone.

METHODS

Three single-sided scanners were used on animal bone samples. These low-field, mobile, low-cost devices are able to detect the NMR signal, regardless of the sample sizes, without the use of ionizing radiations, with the further advantage of signal localization offered by their intrinsic magnetic field gradients.

RESULTS

The performance of the different single-sided scanners have been discussed. The results have been compared with bone volume-to-total volume ratio by micro CT and MRI, obtaining consistent values.

CONCLUSIONS

Our results demonstrate the feasibility of the method for laboratory analyses, which are useful for measurements like porosity on bone specimens. This can be considered as the first step to develop an NMR method based on the use of a mobile single-sided device, for the diagnosis of osteoporosis, through the acquisition of the signal from the appendicular skeleton, allowing for low-cost, wide screening campaigns. Magn Reson Med 79:501-510, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Cylinders or walls? A new computational model to estimate the MR transverse relaxation rate dependence on trabecular bone architecture

OBJECTIVE

Bone density is distributed in a complex network of interconnecting trabecular plates and rods that are interspersed with bone marrow. A computational model to assess the dependence of the relaxation rate on the geometry of bone can consider the distribution of bone material in the form of two components: cylinders and open walls (walls with gaps). We investigate whether the experimentally known dependence of the transverse relaxation rate on the trabecular bone structure can be usefully interpreted in terms of these two components.

MATERIALS AND METHODS

We established a computer model based on an elementary computational cell. The model includes a variable number of open walls and infinitely long cylinders as well as multiple geometric parameters. The transverse relaxation rate is computed as a function of these parameters. Within the model, increasing the trabecular spacing with a fixed trabecular radius is equivalent to thinning the trabeculae while maintaining constant spacing.

RESULTS

Increasing the number of cylinder and wall gap elements beyond their nearest neighbors does not change the transverse relaxation rate. Although the absolute contribution to the relaxation due to open walls is on average more important than that due to cylinders, the latter drops off rapidly. The change on transverse relaxation rate is larger for changing cylinder geometry than for changing wall geometry, as it can be seen from the effect on the relaxation rate when trabecular spacing is varied, compared to varying the size of wall gaps.

CONCLUSION

Our results provide strong evidence that trabecular thinning, which is associated with increasing age, decreases the relaxation rates. The effect of thinning plates and rods on the transverse relaxation can be understood in terms of simple cylinders and open walls. A reduction in the relaxation rate can be seen as an indication of thinning cylinders, corresponding to reduced bone stability and ultimately, osteoporosis.

DREAM--a novel approach for robust, ultrafast, multislice B₁ mapping

A novel multislice B₁-mapping method dubbed dual refocusing echo acquisition mode is proposed, able to cover the whole transmit coil volume in only one second, which is more than an order of magnitude faster than established approaches. The dual refocusing echo acquisition mode technique employs a stimulated echo acquisition mode (STEAM) preparation sequence followed by a tailored single-shot gradient echo sequence, measuring simultaneously the stimulated echo and the free induction decay as gradient-recalled echoes, and determining the actual flip angle of the STEAM preparation radiofrequency pulses from the ratio of the two measured signals. Due to an elaborated timing scheme, the method is insensitive against susceptibility/chemical shift effects and can deliver a B₀ phase map and a transceive phase map for free. The approach has only a weak T₁ and T₂ dependence and moreover, causes only a low specific absorption rate (SAR) burden. The accuracy of the method with respect to systematic and statistical errors is investigated both, theoretically and in experiments on phantoms. In addition, the performance of the approach is demonstrated in vivo in B₁-mapping and radiofrequency shimming experiments on the abdomen, the legs, and the head on an eight-channel parallel transmit 3 T MRI system.

Measuring the absolute water content of the brain using quantitative MRI

Methods for quantitative imaging of the brain are presented and compared. Highly precise and accurate mapping of the absolute water content and distribution, as presented here, requires a significant number of corrections and also involves mapping of other MR parameters. Here, either T(1) and T(2)(*) or T(2) is mapped, and several corrections involving the measurement of temperature, transmit and receive B(1) inhomogeneities and signal extrapolation to zero TE are applied. Information about the water content of the whole brain can be acquired in clinically acceptable measurement times (10 or 20 min). Since water content is highly regulated in the healthy brain, pathological changes can be easily identified and their evolution or correlation with other manifestations of the disease investigated. In addition to voxel-based total water content, information about the different environments of water can be gleaned from qMRI. The myelin water fraction can be extracted from the fit of very high-SNR multiple-echo T(2) decay curves with a superposition of a large number of exponentials. Diseases involving de- or dysmyelination can be investigated and lead to novel observations regarding the water compartmentalisation in tissue, despite the limited spatial coverage. In conclusion, quantitative MRI is emerging as an unparalleled tool for the study of the normal and diseased brain, replacing the customary time-space environment of the sequential mixed-contrast MRI with a multi-NMR-parametric space in which tissue microscopy is increasingly revealed.

Structural and mechanical parameters of trabecular bone estimated from in vivo high-resolution magnetic resonance images at 3 tesla field strength

PURPOSE

To evaluate the performance of a new 3 Tesla (T) high-resolution trabecular bone (TB) imaging technique at two resolution regimens in terms of serial reproducibility and sensitivity.

MATERIALS AND METHODS

The left distal tibial metaphysis of seven healthy volunteers was imaged at three time-points using a FLASE (fast large-angle spin-echo) pulse sequence at 137 x 137 x 410 mum(3) and (160 mum)(3) voxel sizes. Image artifacts, motion degradation, and serial image volume misalignments were controlled to maximize reproducibility of image-derived measures of scale, topology, orientation in terms of structural anisotropy, and finite-element derived Young's and shear moduli. Coefficients of variation (CV) and intraclass correlation coefficients (ICC) for structural and mechanical parameters were evaluated as measures of reproducibility and reliability. The ability of structural and mechanical parameters to distinguish between subjects was tested by analysis of variance.

RESULTS

Reproducibility was generally higher in the anisotropic data (CVs 1-5% versus 1-9% for isotropic images). Anisotropic voxel size yielded greater measurement reliability (ICCs 0.75-0.99, mean = 0.92 versus 0.62-0.99, mean = 0.86) and better discrimination of the seven subjects (75% versus 50% of the possible comparisons were significantly different [P < 0.05]) except for measures of structural anisotropy and topology. Isotropic resolution improved detection of structural orientation and permitted visualization of small perforations in longitudinal trabecular plates not detected at anisotropic resolution.

CONCLUSION

Improved image acquisition and processing techniques enhance reproducibility of structural and mechanical parameters derived from high-resolution MRI of metaphyseal bone in the distal tibia.

Correlation of signal attenuation-based quantitative magnetic resonance imaging with quantitative computed tomographic measurements of subchondral bone mineral density in metacarpophalangeal joints of horses

OBJECTIVE

To evaluate the ability of signal attenuation-based quantitative magnetic resonance imaging (QMRI) to estimate subchondral bone mineral density (BMD) as assessed via quantitative computed tomography (QCT) in osteoarthritic joints of horses.

SAMPLE POPULATION

20 metacarpophalangeal joints from 10 horse cadavers.

PROCEDURES

Magnetic resonance (MR) images (dorsal and transverse T1-weighted gradient recalled echo [GRE] and dorsal T2*-weighted GRE fast imaging employing steady-state acquisition [T2*-FIESTA]) and transverse single-slice computed tomographic (CT) images of the joints were acquired. Magnetic resonance signal intensity (SI) and CT attenuation were quantified in 6 regions of interest (ROIs) in the subchondral bone of third metacarpal condyles. Separate ROIs were established in the air close to the joint and used to generate corrected ratios and SIs. Computed tomographic attenuation was corrected by use of a calibration phantom to obtain a K(2)HPO(4)-equivalent density of bone. Correlations between QMRI performed with different MR imaging sequences and QCT measurements were evaluated. The intraobserver repeatability of ROI measurements was tested for each modality.

RESULTS

Measurement repeatability was excellent for QCT (R(2) = 98.3%) and QMRI (R(2) = 98.8%). Transverse (R(2) = 77%) or dorsal (R(2) = 77%) T1-weighted GRE and QCT BMD measurements were negatively correlated, as were dorsal T2*-FIESTA and QCT (R(2) = 80%) measurements. Decreased bone SI during MR imaging linearly reflected increased BMD.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of this ex vivo study suggested that signal attenuation-based QMRI was a reliable, clinically applicable method for indirect estimation of subchondral BMD in osteoarthritic metacarpophalangeal joints of horses.

Quantitative MRI of parallel changes of articular cartilage and underlying trabecular bone in degeneration

OBJECTIVE

To determine the interrelations between degenerative changes in articular cartilage and underlying trabecular bone during development of osteoarthritis and to test the ability of quantitative magnetic resonance imaging (MRI) to detect those changes.

METHODS

Human cadaver patellae were investigated with quantitative MRI methods, T(2) and dGEMRIC, at 1.5T. Same measurements for isolated cartilage samples were performed at 9.4T. Bone samples, taken at sites matched with cartilage analyses, were measured with MRI and peripheral quantitative computed tomography (pQCT). Mechanical and quantitative microscopic methods were also utilized for both cartilage and bone samples.

RESULTS

Significant differences were found between the samples with different stages of degeneration in mechanical properties, T(2) at 1.5T and proteoglycan (PG) content of articular cartilage. dGEMRIC at 9.4T discerned samples with advanced degeneration from the others. Bone variables measured with pQCT discerned samples with no or minimal and advanced degeneration, and mechanical properties of trabecular bone discerned samples with no or minimal degeneration from the others. Significant linear correlations were found between the bone and cartilage parameters. Characteristically, associations between variables were stronger within the samples with no or minimal degeneration compared to all samples.

CONCLUSIONS

Quantitative MRI variables, especially T(2) relaxation time of articular cartilage, may be feasible surrogate markers for early and advanced osteoarthritic changes in joint tissues, including decreased elastic moduli, PG and collagen contents of cartilage and mineral density and volume fraction of trabecular bone. Further work is required to resolve the relaxation mechanisms at clinically applicable field strengths.

Structural and functional assessment of trabecular and cortical bone by micro magnetic resonance imaging

Osteoporosis is a multifactorial disorder of bone mineral homeostasis affecting the elderly. It is a major public health issue with significant socioeconomic consequences. Recent findings suggest that bone loss-the key manifestation of the disease-is accompanied by architectural deterioration, both affecting the bone's mechanical competence and susceptibility to fracture. This article reviews the potential of quantitative micro MRI (mu-MRI), including a discussion of the technical requirements for image acquisition, processing, and analysis for assessing the architectural implications of osteoporosis and as a means to monitor the response to treatment. With current technology, the resolution achievable in clinically acceptable scan times and necessary signal-to-noise ratio (SNR) is comparable to trabecular thickness. This limited spatial resolution regime demands processing and analysis algorithms designed to operate under such limiting conditions. It is shown that three different classes of structural parameters can be distinguished, characterizing scale, topology, and orientation. There is considerable evidence that osteoporotic bone loss affects all three classes but that topological changes, resulting from conversion of trabecular plates to rods, with the latter's eventual disconnection, are particularly prominent. Clinical applications discussed can be divided into those dealing with assessment of osteoporotic fracture risk as opposed to the study of the effect of disease progression and regression in response to treatment. Current data suggest that noninvasive assessment of cortical and trabecular bone (TB) architecture by mu-MRI may provide new surrogate endpoints to assess the efficacy of intervention in osteoporosis treatment and prevention.

Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field

A new method has been developed for fast image-based measurements of the transmitted radiofrequency (RF) field. The method employs an actual flip-angle imaging (AFI) pulse sequence that consists of two identical RF pulses followed by two delays of different duration (TR(1) < TR(2)). After each pulse, a gradient-echo (GRE) signal is acquired. It has been shown theoretically and experimentally that if delays TR(1) and TR(2) are sufficiently short and the transverse magnetization is completely spoiled, the ratio r = S(2)/S(1) of signal intensities S(1) and S(2), acquired at the beginning of the time intervals TR(1) and TR(2), depends on the flip angle (FA) of applied pulses as r = (1 + n * cos(FA))/(n + cos(FA)), where n = TR(2)/TR(1). The method allows fast 3D implementation and provides accurate B(1) measurements that are highly insensitive to T(1). The unique feature of the AFI method is that it uses a pulsed steady-state signal acquisition. This overcomes the limitation of previous methods that required long relaxation delays between sequence repetitions. The method has been shown to be useful for time-efficient whole-body B(1) mapping and correction of T(1) maps obtained using a variable FA technique in the presence of nonuniform RF excitation.

Prediction of mechanical properties of trabecular bone using quantitative MRI

Techniques for quantitative magnetic resonance imaging (MRI) have been developed for non-invasive estimation of the mineral density and structure of trabecular bone. The R*(2) relaxation rate (i.e. 1/T*(2)) is sensitive to bone mineral density (BMD) via susceptibility differences between trabeculae and bone marrow, and by binarizing MRI images, structural variables, such as apparent bone volume fraction, can be assessed. In the present study, trabecular bone samples of human patellae were investigated in vitro at 1.5 T to determine the ability of MRI-derived variables (R*(2) and bone volume fraction) to predict the mechanical properties (Young's modulus, yield stress and ultimate strength). Further, the MRI variables were correlated with reference measurements of volumetric BMD and bone area fraction as determined with a clinical pQCT system. The MRI variables correlated significantly (p < 0.01) with the mechanical variables (r = 0.32-0.46), BMD (r = 0.56) and bone structure (r = 0.51). A combination of R*(2) and MRI-derived bone volume fraction further improved the prediction of yield stress and ultimate strength. Although pQCT showed a trend towards better prediction of the mechanical properties, current results demonstrate the feasibility of combined MR imaging of marrow susceptibility and bone volume fraction in predicting the mechanical strength of trabecular bone and bone mineral density.

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.

A new method for fast quantitative mapping of absolute water content in vivo

The presence of brain edema, in its various forms, is an accompanying feature of many diseased states. Although the localized occurrence of brain edema may be demonstrated with MRI, the quantitative determination of absolute water content, an aspect that could play an important role in the objective evaluation of the dynamics of brain edema and the monitoring of the efficiency of treatment, is much more demanding. We present a method for the localized and quantitative measurement of absolute water content based on the combination of two fast multi-slice and multi-time point sequences QUTE and TAPIR for mapping the T(2)* and T(1) relaxation times, respectively. Incorporation of corrections for local B(1) field miscalibrations, temperature differences between the subject and a reference probe placed in the FOV, receiver profile inhomogeneities and T(1) saturation effects are included and allow the determination of water content with anatomical resolution and a precision >98%. The method was validated in phantom studies and was applied to the localized in vivo measurement of water content in a group of normal individuals and a patient with brain tumor. The results demonstrate that in vivo measurement of regional absolute water content is possible in clinically relevant measurement times with a statistical and systematic measurement error of <2%.

Trabecular bone volume fraction measurements of a large number of subjects using a compact MRI

Trabecular bone volume fraction (TBVF) and speed of sound (SOS) were measured for the right calcanei of 416 female volunteers. The TBVF was measured with a compact MRI developed in our laboratory. The SOS was measured with a commercial quantitative ultrasound system. It was observed that the correlation coefficient between TBVF and SOS and that between TBVF and age varied depending on the location of region of interest (ROI) in the calcaneus. As a result, an optimum circular ROI with a diameter of 20 mm was determined so that the correlation coefficients were maximized. In the optimum ROI, transverse relaxation time (T2) of the bone marrow protons of the calcaneus was found to be concentrated in a narrow range over the subjects. This result suggested that a 50% scan time reduction in the TBVF measurements could be made by skipping the T2 correction procedure.

Musculoskeletal MRI at 3.0 T: Relaxation Times and Image Contrast

OBJECTIVE

The purpose of our study was to measure relaxation times in musculoskeletal tissues at 1.5 and 3.0 T to optimize musculoskeletal MRI methods at 3.0 T.

MATERIALS AND METHODS

In the knees of five healthy volunteers, we measured the T1 and T2 relaxation times of cartilage, synovial fluid, muscle, marrow, and fat at 1.5 and 3.0 T. The T1 relaxation times were measured using a spiral Look-Locker sequence with eight samples along the T1 recovery curve. The T2 relaxation times were measured using a spiral T2 preparation sequence with six echoes. Accuracy and repeatability of the T1 and T2 measurement sequences were verified in phantoms.

RESULTS

T1 relaxation times in cartilage, muscle, synovial fluid, marrow, and subcutaneous fat at 3.0 T were consistently higher than those measured at 1.5 T. Measured T2 relaxation times were reduced at 3.0 T compared with 1.5 T. Relaxation time measurements in vivo were verified using calculated and measured signal-to-noise results. Relaxation times were used to develop a high-resolution protocol for T2-weighted imaging of the knee at 3.0 T.

CONCLUSION

MRI at 3.0 T can improve resolution and speed in musculoskeletal imaging; however, interactions between field strength and relaxation times need to be considered for optimal image contrast and signal-to-noise ratio. Scanning can be performed in shorter times at 3.0 T using single-average acquisitions. Efficient higher-resolution imaging at 3.0 T can be done by increasing the TR to account for increased T1 relaxation times and acquiring thinner slices than at 1.5 T.

Analysis of 1H-NMR relaxation time distributions in L1 to L6 rat lumbar vertebrae

A better knowledge of the NMR relaxation behavior of bone tissue can improve the definition of imaging protocols to detect bone diseases like osteoporosis. The six rat lumbar vertebrae, from L1 to L6, were analyzed by means of both transverse (T(2)) and longitudinal (T(1)) relaxation of (1)H nuclei at 20 MHz and 30 degrees C. Distributions of relaxation times, computed using the multiexponential inversion software uniform penalty inversion, extend over decades for both T(2) and T(1) relaxation. In all samples, the free induction decay (FID) from an inversion-recovery (IR) T(1) measurement shows an approximately Gaussian (solid-like) component, exp[-1/2(t/T(GC))2], with T(GC) approximately 12 micros (GC for Gaussian component) and a liquid-like component (LLC) with initially simple-exponential decay. Averaging and smoothing procedures are adopted to obtain the ratio alpha between GC and LLC signals and to get separate T(1) distributions for GC and LLC. Distributions of T(1) for LLC show peaks centered at 300-500 ms and shoulders going down to 10 ms, whereas distributions of T(1) for GC are single broad peaks centered at roughly 100 ms. The T(2) distributions by Carr-Purcell-Meiboom-Gill at 600 micros echo spacing are very broad and extend from 1 ms to hundreds of ms. This long echo spacing does not allow one to see a peak in the region of hundreds of micros, which is better seen by single spin-echo T(2) measurements. Results of the relaxation analysis were then compared with densitometric data. From the study, a clear picture of the intratrabecular and intertrabecular (1)H signals emerges. In particular, the GC is presumed to be due to (1)H in collagen, LLC due to all the fluids in the bone including water and fat, and the very short T(2) peak due to the intratrabecular water. Overall, indications of some trends in composition and in pore-space distributions going from L1 to L6 appeared. Published results on rat vertebrae obtained by fitting the curves by discrete two-component models for both T(2) and T(1) are consistent with our results and can be better interpreted in light of the shown distributions of relaxation times.

Development of a compact MRI system for trabecular bone volume fraction measurements

A compact MRI system for measuring trabecular bone volume fraction (TBVF) of the calcaneus was developed with the use of a 0.21 T permanent magnet and portable MRI console. The entire system weighed < 600 kg and was installed in a 2 m x 2 m space. Two cross-sectional spin-echo images of a heel acquired with external reference phantoms (total measurement time = 5 min) were used to quantify the TBVF of the calcaneus. The linearity and reproducibility of the measurements were evaluated by means of proton density-adjusted phantoms. Comparative measurements with quantitative ultrasound (QUS) in groups of healthy female volunteers showed a relatively high positive correlation (R(2) = 0.4539, 0.2693) between TBVF and the speed of sound (SOS). These results demonstrate the potential of this new system for measuring bone density. Magn Reson Med 52:440-444, 2004.

Improved pore space structure characterization by fusion of relaxation tomography maps

Quantitative Relaxation Tomography in porous media furnishes maps of internal sections where each pixel represents T1 or T2 of water 1H in the corresponding voxel, so that quantitative information on the pore space structure can be obtained. The porosity can be determined at different length scales by correcting pixel by pixel the signal intensity for T2 decay. Moreover, on the basis of the distribution of T1, the microporosity fraction can be computed, as well as several voxel-average porosities. Since T1 and T2 encode different pieces of information, fusion image techniques can improve the characterization of the pore space, showing simultaneously, on the same image, maps of the two parameters. Examples are given of application to a water-saturated travertine core and to a pig femur. Different kinds of look-up tables were tried by varying two of the three dimensions of the HSV color space in such a way as to optimize both the T1 and T2 contrasts simultaneously.

Quantitative magnetic resonance imaging in the calcaneus and femur of women with varying degrees of osteopenia and vertebral deformity status

Quantitative magnetic resonance imaging (QMRI) allows measurement of two parameters that are related to the integrity of the trabecular bone: R2*, the rate constant of the free induction signal, and trabecular bone volume fraction (BVF), the counterpart of apparent density. In this work, R2* and BVF were measured in 68 women (mean age, 58.2 +/- 9.5 years) of varying spinal bone mineral density (BMD) T scores (mean, -1.37 +/- 1.54) and vertebral fracture status on a commercial 1.5 T whole-body imager using customized image acquisition and processing techniques. Twenty-five of the patients had vertebral fractures, characterized by the total cumulative deformity burden exceeding 200%. R2* was measured in the calcaneus and proximal femur and BVF could be measured in the calcaneus only. On a pixel-by-pixel basis, calcaneal R2* and BVF within each subject were highly positively correlated (r2 = 0.61 +/- 0.11) but the correlation of region-of-interest (ROI) means for different calcaneal sites among patients was weaker (r2 = 0.34; p < 0.0001). The strongest discriminator of vertebral deformity was R2* of the calcaneus, which was lower in the fracture group, consistent with lower trabecular density. Among the calcaneal sites examined, the subtalar region, a location characterized by dense nearly horizontal trabeculae that transmit the stresses imparted by body weight from the tibia to the heel, best discriminated the two groups (p = 0.0001), with 77% diagnostic accuracy as determined from the area under the receiver operating characteristic (ROC) curve (compared with 66% for vertebral BMD). The cavum calcanei, an anterior site of low trabecular density, and the tuber calcanei (the location ordinarily used for ultrasound measurements) also had significantly reduced R2* in the fracture group (p < 0.005 and p = 0.01, respectively). The R2av*, computed as the average of all pixels in the calcaneus, was a strong discriminator as well (p < 0.005). On the other hand, calcaneal BVF was only marginally discriminating (p = 0.05). Among the BMD sites examined, the lumbar spine (average L1-L4) was significant (p = 0.005, 66% diagnostic accuracy), as was the femoral neck (p = 0.01). The data suggest the calcaneus to be suited as a surrogate site to assess vertebral osteoporosis and that R2* is sensitive to alterations in bone quality not captured by density.