Autocalibrating parallel imaging of in vivo trabecular bone microarchitecture at 3 Tesla

Six-minute, in vivo MRI quantification of proximal femur trabecular bone 3D microstructure

BACKGROUND

Assessment of proximal femur trabecular bone microstructure in vivo by magnetic resonance imaging has recently been validated for acquiring information independent of bone mineral density in osteoporotic patients. However, the requisite signal-to-noise ratio (SNR) and resolution for interrogation of the trabecular microstructure at this anatomical location prolongs the scan duration and renders the imaging protocol clinically infeasible. Parallel imaging and compressed sensing (PICS) techniques can reduce the scan duration of the imaging protocol without substantially compromising image quality. The present work investigates the limits of acceleration for a commonly used PICS technique, ℓ1-ESPIRiT, for the purpose of quantifying measures of trabecular bone microarchitecture. Based on a desired error tolerance, a six-minute, prospectively accelerated variant of the imaging protocol was developed and assessed for intersession reproducibility and agreement with the longer reference scan.

PURPOSE

To investigate the limits of acceleration for MRI-based trabecular bone quantification by parallel imaging and compressed sensing reconstruction, and to develop a prototypical imaging protocol for assessing the proximal femur microstructure in a clinically practical scan time.

METHODS

Healthy participants (n = 11) were scanned by a 3D balanced steady-state free precession (bSSFP) sequence satisfying the Nyquist criterion with a scan duration of about 18 min. The raw data were retrospectively undersampled and reconstructed to mimic various acceleration factors ranging from 2 to 6. Trabecular volumes-of-interest in four major femoral regions (greater trochanter, intertrochanteric region, femoral neck, and femoral head) were analyzed and six relevant measures of trabecular bone microarchitecture (bone volume fraction, surface-to-curve ratio, erosion index, elastic modulus, trabecular thickness, plates-to-rods ratio) were obtained for images of all accelerations. To assess agreement, median percent error and intraclass correlation coefficients (ICCs) were computed using the fully-sampled data as reference. Based on this analysis, a prospectively 3-fold accelerated sequence with a duration of about 6 min was developed and the analysis was repeated.

RESULTS

A prospective acceleration factor of 3 demonstrated comparable performance in reproducibility and absolute agreement to the fully-sampled scan. The median CoV over all image-derived metrics was generally <6 % and ICCs >0.70. Also, measurements from prospectively 3-fold accelerated scans demonstrated in general median percent errors of <7 % and ICCs >0.70.

CONCLUSION

The present work proposes a method to make in vivo quantitative assessment of proximal femur trabecular microstructure with a clinically practical scan duration of about 6 min.

Relationship between radiation dose and microbleed formation in patients with malignant glioma

BACKGROUND

Cranial irradiation is associated with long-term cognitive changes. Cerebral microbleeds (CMBs) have been identified on susceptibility-weighted MRI (SWI) in patients who have received prior cranial radiation, and serve as radiographic markers for microvascular injury thought to contribute to late cognitive decline. The relationship between CMB formation and radiation dose has not previously been quantified.

METHODS

SWI was performed on 13 patients with stable WHO grade III-IV gliomas between 2 and 4 years after chemoradiotherapy to 60 Gy. The median age at the time of treatment was 41 years (range 25 - 74 years). CMBs were identified as discrete foci of susceptibility on SWI that did not correspond to vessels. CMB density for low (<30 Gy), median (30-45 Gy), and high (>45 Gy) dose regions was computed.

RESULTS

Twelve of 13 patients exhibited CMBs. The number of CMBs was significantly higher for late (>3 years from treatment) compared to early (<3 years) timepoints (early median 6 CMBs; late median 27 CMBs; p = 0.001), and there were proportionally more CMBs at lower doses for late scans (p = 0.006). 88% of all CMBs were observed in regions receiving at least 30 Gy, but the CMB density within medium and high dose regions was not significantly different (p = 0.33 and p = 0.9, respectively, for early and late time points).

CONCLUSIONS

CMBs predominantly form in regions receiving at least 30 Gy, but form in lower dose regions with longer follow-up. We do not observe a clear dose-response relationship at doses above 30 Gy. These findings provide important information to assess the risk of late microvascular sequelae from cranial irradiation.

Multi-reception strategy with improved SNR for multichannel MR imaging

A multi-reception strategy with extended GRAPPA is proposed in this work to improve MR imaging performance at ultra-high field MR systems with limited receiver channels. In this method, coil elements are separated to two or more groups under appropriate grouping criteria. Those groups are enabled in sequence for imaging first, and then parallel acquisition is performed to compensate for the redundant scan time caused by the multiple receptions. To efficiently reconstruct the data acquired from elements of each group, a specific extended GRAPPA was developed. This approach was evaluated by using a 16-element head array on a 7 Tesla whole-body MRI scanner with 8 receive channels. The in-vivo experiments demonstrate that with the same scan time, the 16-element array with twice receptions and acceleration rate of 2 can achieve significant SNR gain in the periphery area of the brain and keep nearly the same SNR in the center area over an eight-element array, which indicates the proposed multi-reception strategy and extended GRAPPA are feasible to improve image quality for MRI systems with limited receive channels. This study also suggests that it is advantageous for a MR system with N receiver channels to utilize a coil array with more than N elements if an appropriate acquisition strategy is applied.

MRI of trabecular bone using a decay due to diffusion in the internal field contrast imaging sequence

PURPOSE

To characterize the DDIF (Decay due to Diffusion in the Internal Field) method using intact animal trabecular bone specimens of varying trabecular structure and porosity, under ex vivo conditions closely resembling in vivo physiological conditions. The DDIF method provides a diffusion contrast which is related to the surface-to-volume ratio of the porous structure of bones. DDIF has previously been used successfully to study marrow-free trabecular bone, but the DDIF contrast hitherto had not been tested in intact specimens containing marrow and surrounded by soft tissue.

MATERIALS AND METHODS

DDIF imaging was implemented on a 4.7 Tesla (T) small-bore, horizontal, animal scanner. Ex vivo results on fresh bone specimens containing marrow were obtained at body temperature. Control measurements were carried out in surrounding tissue and saline.

RESULTS

Significant DDIF effect was observed for trabecular bone samples, while it was considerably smaller for soft tissue outside the bone and for lipids. Additionally, significant differences were observed between specimens of different trabecular structure.

CONCLUSION

The DDIF contrast is feasible despite the reduction of the diffusion constant and of T(1) in such conditions, increasing our confidence that DDIF imaging in vivo may be clinically viable for bone characterization.

7-Tesla susceptibility-weighted imaging to assess the effects of radiotherapy on normal-appearing brain in patients with glioma

PURPOSE

To evaluate the intermediate- and long-term imaging manifestations of radiotherapy on normal-appearing brain tissue in patients with treated gliomas using 7T susceptibility-weighted imaging (SWI).

METHODS AND MATERIALS

SWI was performed on 25 patients with stable gliomas on a 7 Tesla magnet. Microbleeds were identified as discrete foci of susceptibility that did not correspond to vessels. The number of microbleeds was counted within and outside of the T2-hyperintense lesion. For 3 patients, radiation dosimetry maps were reconstructed and fused with the 7T SWI data.

RESULTS

Multiple foci of susceptibility consistent with microhemorrhages were observed in patients 2 years after chemoradiation. These lesions were not present in patients who were not irradiated. The prevalence of microhemorrhages increased with the time since completion of radiotherapy, and these lesions often extended outside the boundaries of the initial high-dose volume and into the contralateral hemisphere.

CONCLUSIONS

High-field SWI has potential for visualizing the appearance of microbleeds associated with long-term effects of radiotherapy on brain tissue. The ability to visualize these lesions in normal-appearing brain tissue may be important in further understanding the utility of this treatment in patients with longer survival.

On the significance of motion degradation in high-resolution 3D μMRI of trabecular bone

RATIONALE AND OBJECTIVES

Subtle subject movement during high-resolution three-dimensional micro-magnetic resonance imaging of trabecular bone (TB) causes blurring, thereby rendering the data unreliable for quantitative analysis. In this work, the effects of translational and rotational motion displacements were evaluated qualitatively and quantitatively.

MATERIALS AND METHODS

In experiment 1, motion was induced by applying various simulated and previously observed in vivo trajectories as phase shifts to k-space or rotation angles to k-space segments of a virtually motion-free data set. In experiment 2, images that were visually free of motion artifacts from two groups of 10 healthy individuals, differing in age, were selected to probe the effects of motion on TB parameters. In both experiments, images were rated for motion severity, and the scores were compared to a focus criterion, the normalized gradient squared.

RESULTS

Strong correlations were observed between the motion quality scores and the corresponding normalized gradient squared values (R(2) = 0.52-0.64, P < .01). The results from experiment 1 demonstrated consistently lower image quality and alterations in structural parameters of 9% to 45% with increased amplitude of displacements. In experiment 2, the significant differences in structural parameter group means of the motion-free images were lost upon motion degradation. Autofocusing, a postprocessing correction method, partially recovered the sharpness of the original motion-free images in 13 of 20 subjects.

CONCLUSIONS

Quantitative TB structural measures are highly sensitive to subtle motion-induced degradation, which adversely affects precision and statistical power. The results underscore the influence of subject movement in high-resolution three-dimensional micro-magnetic resonance imaging and its correction for TB structure analysis.

Magnetic resonance imaging evaluation of weight-bearing subchondral trabecular bone in the knee

OBJECTIVE

Changes in weight-bearing subchondral bone are central to osteoarthritis (OA) pathophysiology. Using MR, knee trabecular bone is typically assessed in the axial plane, however partial volume artifacts limit the utility of MR methods for femorotibial compartment subchondral bone analysis. Oblique-coronal acquisitions may enable direct visualization and quantification of the expected increases in femorotibial subchondral trabecular bone.

METHODS

MR acquisition parameters were first optimized at 3 Tesla. Thereafter, five volunteers underwent axial and coronal exams of their right knee. Each image series was evaluated visually and quantitatively. An anatomically standardized region-of-interest was placed on both the medial and lateral tibial plateaus of all coronal slices containing subchondral bone. Mean and maximum marrow signal was measured, and "bone signal" was calculated.

RESULTS

The MR acquisition had spatial resolution 0.2 × 0.2 × 1.0 mm and acquisition time 10.5 min. The two asymptomatic knees exhibited prominent horizontal trabeculae in the tibial subchondral bone, while the one confirmed OA knee had disorganized subchondral bone and absent horizontal trabeculae. The subchondral bone signal was 8-14% higher in both compartments of the OA knee than the asymptomatic knees.

CONCLUSION

The weight-bearing femorotibial subchondral trabecular bone can be directly visualized and changes quantified in the coronal-oblique plane. Qualitative and quantitative assessments can be performed using the resultant images and may provide a method to discriminate between the healthy and OA knees. These methods should enable a quantitative evaluation of the role of weight-bearing subchondral bone in the natural history of knee OA to be undertaken.

High-resolution imaging techniques for the assessment of osteoporosis

The importance of assessing the bone's microarchitectural make-up in addition to its mineral density in the context of osteoporosis has been emphasized in several publications. The high spatial resolution required to resolve the bone's microstructure in a clinically feasible scan time is challenging. At present, the best suited modalities meeting these requirements in vivo are high-resolution peripheral quantitative imaging (HR-pQCT) and magnetic resonance imaging (MRI). Whereas HR-pQCT is limited to peripheral skeleton regions like the wrist and ankle, MRI can also image other sites like the proximal femur but usually with lower spatial resolution. In addition, multidetector computed tomography has been used for high-resolution imaging of trabecular bone structure; however, the radiation dose is a limiting factor. This article provides an overview of the different modalities, technical requirements, and recent developments in this emerging field. Details regarding imaging protocols as well as image postprocessing methods for bone structure quantification are discussed.

Windows on the human body--in vivo high-field magnetic resonance research and applications in medicine and psychology

Analogous to the evolution of biological sensor-systems, the progress in "medical sensor-systems", i.e., diagnostic procedures, is paradigmatically described. Outstanding highlights of this progress are magnetic resonance imaging (MRI) and spectroscopy (MRS), which enable non-invasive, in vivo acquisition of morphological, functional, and metabolic information from the human body with unsurpassed quality. Recent achievements in high and ultra-high field MR (at 3 and 7 Tesla) are described, and representative research applications in Medicine and Psychology in Austria are discussed. Finally, an overview of current and prospective research in multi-modal imaging, potential clinical applications, as well as current limitations and challenges is given.

Local bone enhancement fuzzy clustering for segmentation of MR trabecular bone images

PURPOSE

Segmentation of trabecular bone from magnetic resonance (MR) images is a challenging task due to spatial resolution limitations, signal-to-noise ratio constraints, and signal intensity inhomogeneities. This article examines an alternative approach to trabecular bone segmentation using partial membership segmentation termed fuzzy C-means clustering incorporating local second order features for bone enhancement (BE-FCM) at multiple scales. This approach is meant to allow for a soft segmentation that accounts for partial volume effects while suppressing the influence of noise.

METHODS

A soft segmentation method was developed and evaluated on three different sets of data; interscan reproducibility was evaluated on six test-retest in vivo MR scans of the proximal femur, correlation between MR and HR-pQCT measurements was evaluated on 49 in vivo scans from the distal tibia, and the potential for fracture discrimination was evaluated using MR scans of calcaneus specimens from 15 participants with and 15 participants without vertebral fracture. The algorithm was compared to fuzzy clustering using the intensity as the only feature (I-FCM) and a dual thresholding algorithm. The metric evaluated was bone volume over total volume (BV/TV) within user-defined regions of interest.

RESULTS

BE-FCM had a higher interscan reproducibility (rms CV: 2.0%) compared to I-FCM (5.6%) and thresholding (4.2%), and expressed higher correlation to HR-pQCT data (r = 0.79, p < 10(-11)) compared to I-FCM (r = 0.74, p < 10(-8)) and thresholding (r = 0.70, p < 10(-6)). BE-FCM was also the method that was best able to differentiate between a control and a vertebral fracture group at a 95% significance level.

CONCLUSIONS

The results suggest that trabecular bone segmentation by BE-FCM can provide a precise BV/TV measurement that is sensitive to pathology. The segmentation method may become useful in MR imaging-based quantification of bone microarchitecture.

Imaging of the musculoskeletal system in vivo using ultra-high field magnetic resonance at 7 T

Recently, great progress has been made in particularly in the imaging of cartilage and bone structure. Increased interest has focused on high-field (3 Tesla) imaging and more recently on ultra-high field (UHF) magnetic resonance imaging (MRI) at 7 T for in vivo imaging. Because the signal-to-noise ratio (SNR) scales linearly with field strength, a substantial increase in SNR is expected compared with lower field strengths. This gain in SNR can be used to increase spatial resolution or reduce imaging time. The goal of this review was to highlight recent developments and challenges in in vivo musculoskeletal (MSK) imaging using UHF-MRI at 7 T. One focus of this review is on the emerging methodology of quantitative MRI for the assessment of trabecular bone structure at the tibia, wrist, and knee. In particular for this application, susceptibility effects between the bone and bone marrow transitions that scale with field strength have to be considered. Another important MSK application is the characterization of knee cartilage morphology. The higher SNR provided by UHF-MRI is a potential advantage for visualizing, segmenting, and analyzing cartilage. Standard clinical MSK imaging relies heavily on T1, T2, and proton density weighted fast spin echo sequences. However, fast spin echo imaging has proven to be very challenging at higher fields because of very high specific absorption rates, using multiple pulses in a short time frame; thus the imaging protocols have to be adapted and gradient echo sequences may be more beneficial. Imaging of more central body parts such as the spine at 7 T is still in its infancy and dedicated coils have to be developed.

Advances in osteoporosis imaging

In the assessment of osteoporosis, the measurement of bone mineral density (BMD(a)) obtained from dual energy X-ray absorptiometry (DXA; g/cm(2)) is the most widely used parameter. However, bone strength and fracture risk are also influenced by parameters of bone quality such as micro-architecture and tissue properties. This article reviews the radiological techniques currently available for imaging and quantifying bone structure, as well as advanced techniques to image bone quality. With the recent developments in magnetic resonance (MR) techniques, including the availability of clinical 3T scanners, and advances in computed tomography (CT) technology (e.g. clinical Micro-CT), in-vivo imaging of the trabecular bone architecture is becoming more feasible. Several in-vitro studies have demonstrated that bone architecture, measured by MR or CT, was a BMD-independent determinant of bone strength. In-vivo studies showed that patients with, and without, osteoporotic fractures could better be separated with parameters of bone architecture than with BMD. Parameters of trabecular architecture were more sensitive to treatment effects than BMD. Besides the 3D tomographic techniques, projection radiography has been used in the peripheral skeleton as an additional tool to better predict fracture risk than BMD alone. The quantification of the trabecular architecture included parameters of scale, shape, anisotropy and connectivity. Finite element analyses required highest resolution, but best predicted the biomechanical properties of the bone. MR diffusion and perfusion imaging and MR spectroscopy may provide measures of bone quality beyond trabecular micro-architecture.

Evaluation of correction methods for coil-induced intensity inhomogeneities and their influence on trabecular bone structure parameters from MR images

Magnetic resonance (MR) imaging-based quantitative trabecular bone structure analysis has gained increasing interest in osteoporotic fracture risk assessment and treatment evaluation related to osteoporosis. In vivo MR images of anatomic regions such as the proximal femur and distal tibia are generally acquired with a surface coil in order to obtain sufficient sensitivity and resolution for quantification of the trabeculae. However, these coils introduce intensity inhomogeneities which affect the trabecular bone structure analysis. This work evaluates the applicability of a fully automatic coil correction by nonparametric nonuniform intensity normalization (N3) in the analysis of trabecular bone parameters. The ability to correct for coil-induced intensity inhomogeneity was evaluated ex vivo on proximal femur specimens scanned with both a surface coil and a volume coil, which allowed for a direct evaluation of the performance of the coil correction methods without any major confounding factors. In addition, trabecular bone parameter values were correlated with values from high-resolution peripheral computed tomography (HR-pQCT) scans, and the reproducibility of trabecular bone parameters was evaluated in an in vivo study of repeat hip MR scans. The trabecular bone parameters determined from MR surface coil scans processed with the N3 coil correction method showed significant correlation (p < 0.05) with corresponding values from homogeneous intensity data in the ex vivo study. This can be compared to the correlation without coil correction (p < 0.5), and coil correction using low-pass filtering (LPF) (p < 0.53). The in vivo interscan variability was reduced from 8.9% to 12.8% using LPF-based to 3.6%-8.4% (CV) using N3 coil correction; hence the results showed that N3 is advantageous to LPF-based coil correction. No significant differences in correlation to HR-pQCT data were found for the coil correction methods. The significant correlations with volume coil data and high reproducibility of the N3 processed data imply that N3 coil correction preserve image information while accurately correcting for coil-induced intensity inhomogeneities, which makes it suitable for quantitative analysis of trabecular bone structure from MR images acquired with surface coils.

Regionally optimized reconstruction for partially parallel imaging in MRI applications

Based on the conventional SENSE and GRAPPA, a regionally optimized reconstruction method is developed for reduced noise and artifact level in partially parallel imaging. In this regionally optimized reconstruction, the field-of-view (FOV) is divided into a number of small regions. Over every small region, the noise amplification and data fitting error can be balanced and minimized locally by taking advantage of spatial correlation of neighboring pixels in reconstruction. The full FOV image can be obtained by "region-by-region" reconstruction. Compared with the conventional SENSE, this method gives better performance in the regions where there are pixels with high SENSE g-factors. Compared with GRAPPA, it is better in the regions where all the pixels have low SENSE g-factors. In this work, we applied the regionally optimized reconstruction in four important imaging experiments: brain, spine, breast, and cardiac. It was demonstrated in these experiments that the overall image quality using this regionally optimized reconstruction is better than that using the conventional SENSE or GRAPPA.

A local mutual information guided denoising technique and its application to self-calibrated partially parallel imaging

The application of Partially Parallel Imaging (PPI) techniques to regular clinical Magnetic Resonance Imaging (MRI) studies has brought about the benefit of significantly faster acquisitions but at the cost of amplified and spatially variant noise, especially, for high parallel imaging acceleration rates. A Local Mutual Information (LMI) weighted Total Variation (TV) based model is proposed to remove non-evenly distributed noise while preserving image sharpness. For self-calibrated PPI, such as GeneRalized Auto-calibration Partially Parallel Acquisition (GRAPPA) and modified SENSitivity Encoding (mSENSE), a low spatial resolution high Signal to Noise Ratio (SNR) image is available besides the reconstructed high spatial resolution low SNR image. The LMI between these two images is used to detect the noise distribution and the location of edges automatically, and is then applied as guidance for denoising. To better preserve sharpness, Bregman iteration scheme is utilized to add the removed signal back to the denoised image. Entropy of the residual map is used to automatically terminate iteration without using any information of the golden standard or real noise. Results of the proposed algorithm on synthetic and in vivo MR images indicate that the proposed technique preserves image edges and suppresses noise well in the images reconstructed by GRAPPA. The comparison with some existing techniques further confirms the advantages. This algorithm can be applied to enhance the clinical applicability of self-calibrated PPI. Potentially, it can be extended to denoise general images with spatially variant noise.

Three-dimensional image registration of MR proximal femur images for the analysis of trabecular bone parameters

This study investigated the feasibility of automatic image registration of MR high-spatial resolution proximal femur trabecular bone images as well as the effects of gray-level interpolation and volume of interest (VOI) misalignment on MR-derived trabecular bone structure parameters. For six subjects in a short-term study, a baseline scan and a follow-up scan of the proximal femur were acquired on the same day. For ten subjects in a long-term study, a follow-up scan of the proximal femur was acquired 1 year after the baseline. An automatic image registration technique, based on mutual information, utilized a baseline and a follow-up scan to compute transform parameters that aligned the two images. In the short-term study, these parameters were subsequently used to transform the follow-up image with three different gray-level interpolators. Nearest-neighbor interpolation and B-spline approximation did not significantly alter bone parameters, while linear interpolation significantly modified bone parameters (p<0.01). Improvement in image alignment due to the automatic registration for the long-term and short-term study was determined by inspecting difference images and 3D renderings. This work demonstrates the first application of automatic registration, without prior segmentation, of high-spatial resolution trabecular bone MR images of the proximal femur. Additionally, inherent heterogeneity in trabecular bone structure and imprecise positioning of the VOI along the slice (anterior-posterior) direction resulted in significant changes in bone parameters (p<0.01). Results suggest that automatic mutual information registration using B-spline approximation or nearest neighbor gray-level interpolation to transform the final image ensures VOI alignment between baseline and follow-up images and does not compromise the integrity of MR-derived trabecular bone parameters used in this study.

Relationship between trabecular bone structure and articular cartilage morphology and relaxation times in early OA of the knee joint using parallel MRI at 3 T

OBJECTIVE

To evaluate trabecular bone structure in relationship with cartilage parameters in distal femur and proximal tibia of the human knee at 3Tesla (3T) using high-resolution magnetic resonance imaging (MRI) with parallel imaging.

METHOD

Sixteen healthy controls and 16 patients with mild osteoarthritis (OA) were studied using a 3T magnetic resonance (MR) scanner and an eight-channel phased-array knee coil. Axial 3D GeneRalized Autocalibrating Partially Parallel Acquisition (GRAPPA)-based phase cycled Fast Imaging Employing Steady State Acquisition (FIESTA-c) images were acquired in order to quantify the trabecular bone structure. For assessing cartilage morphology (thickness, volume), sagittal high-resolution 3D spoiled gradient echo (SPGR) images were acquired. In a subset of the subjects, sagittal images were acquired for measuring T1rho and T2 relaxation times, using 3D T1rho and T2 mapping techniques.

RESULTS

Good measurement reproducibility was observed for bone parameters, the coefficients of variations (CVs) ranging from 1.8% for trabecular number (app. Tb.N) to 5.5% for trabecular separation (app. Tb.Sp). Significant differences between control and OA groups were found for bone volume fraction bone volume over total volume (app. BV/TV) and app. Tb.Sp in all compartments. Significantly increased values in T1rho and T2 were demonstrated in OA patients compared with controls at the femur, but not at the tibia. T1rho was negatively correlated with app. BV/TV, app. Tb.N and app. Tb.Sp both at the medial femoral condyle (MFC) and lateral tibia (LT), while T2 was only correlated at the LT. Also, medial tibia (MT) T1rho was negatively correlated with app. BV/TV (R(2)=-0.49, P<0.05) and app. Tb.N (R(2)=-0.42, P<0.05) from the opposite side of lateral femoral condyle (LFC). Significant correlations were found between trabecular bone parameters and cartilage thickness and normalized volume, mainly at LT, tibia (T) and femur (F).

CONCLUSION

At this early stage of OA, an overall decrease in bone structure parameters and an increase in cartilage parameters (T1rho, T2) were noticed in patients. Trabecular bone structure correlated with articular cartilage parameters suggesting that loss of mineralized bone is associated with cartilage degeneration.

GRAPPA-based susceptibility-weighted imaging of normal volunteers and patients with brain tumor at 7 T

Susceptibility-weighted imaging (SWI) is a valuable technique for high-resolution imaging of brain vasculature that greatly benefits from the emergence of higher field strength MR scanners. Autocalibrating partially parallel imaging techniques can be employed to reduce lengthy acquisition times as long as the decrease in signal-to-noise ratio does not significantly affect the contrast between vessels and brain parenchyma. This study assessed the feasibility of a Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA)-based SWI technique at 7 T in both healthy volunteers and brain tumor patients. GRAPPA-based SWI allowed a twofold or more reduction in scan time without compromising vessel contrast and small vessel detection. Postprocessing parameters for the SWI needed to be modified for patients where the tumor causes high-frequency phase wrap artifacts but did not adversely affect vessel contrast. GRAPPA-based SWI at 7 T revealed regions of microvascularity, hemorrhage and calcification within heterogeneous brain tumors that may aid in characterizing active or necrotic tumor and monitoring treatment effects.

In vivo ultra-high-field magnetic resonance imaging of trabecular bone microarchitecture at 7 T

PURPOSE

To investigate the feasibility of 7 T magnetic resonance imaging (MRI) to visualize and quantify trabecular bone structure in vivo by comparison with 3T MRI and in vivo three-dimensional (3D) high-resolution peripheral quantitative computed tomography (HR-pQCT).

MATERIALS AND METHODS

The distal tibiae of 10 healthy volunteers were imaged. Therefore, fully balanced steady state free precession (bSSFP) and spin-echo (bSSSE) pulse sequences were implemented and optimized for 7 T. Structural bone parameters, such as apparent bone-volume over total-volume fraction (app.BV/TV), apparent trabecular plate separation (app.TbSp), apparent trabecular plate thickness (app.TbTh), and apparent trabecular plate number (app.TbN), were derived.

RESULTS

All structural trabecular bone parameters correlated well (r > 0.6) between 7T and 3T, and between 7 T and HR-pQCT (r > 0.69), with the exception of app.TbTh, which correlated modestly (r = 0.41) between field strengths and very low with HR-pQCT (r < 0.16). Regarding absolute values, app.TbN varied only 4% between field strengths, and only 0.6% between 7 T and HR-pQCT. App.TbSp correlated best between 7 T and HR-pQCT (r = 0.89). Using bSSSE, significant smaller trabecular thickness and significant higher trabecular number were found compared to bSSFP.

CONCLUSION

We concluded that imaging and quantification of the trabecular bone architecture at 7 T is feasible and preferably done using bSSSE. There exists great potential for ultra-high-field (UHF) MRI applied to trabecular bone measurements.

Assessment of trabecular bone structure comparing magnetic resonance imaging at 3 Tesla with high-resolution peripheral quantitative computed tomography ex vivo and in vivo

In vivo high-resolution peripheral quantitative micro-CT (HR-pQCT) is a new modality for imaging peripheral sites like the distal tibia and the distal radius, providing structural bone parameters. Comparing HR-pQCT with MRI, we found that both modalities are capable of offering meaningful information on trabecular structure.

BACKGROUND

Magnetic resonance imaging (MRI) has emerged as the leading in vivo method for measuring trabecular bone micro-architecture and providing structural information. Recently, an in vivo HR-pQCT modality was introduced for imaging peripheral sites like the distal tibia and the distal radius, providing structural bone parameters. The goal of this work was to compare and evaluate the performances and in vivo capabilities of HR-pQCT in comparison with MRI at 3 Tesla.

METHODS

To this end images of 8 human specimens (5 tibiae and 3 radii) and 11 participants (6 tibia and 5 radii) were acquired with both modalities. Additionally, the radius specimens were scanned with micro-CT (muCT), which was used as a standard of reference. Structural parameters calculated from MRI were compared with results from HR-pQCT images and additionally muCT for the radii specimens.

RESULTS

High correlations (r > 0.7) were found for trabecular number and trabecular spacing between the two modalities in vivo and ex vivo. 2D and 3D analysis revealed high correlations (r > 0.8) in structural bone parameters for all measurements. Using micro-CT as standard of reference both results from QCT and MRI correlated well.

CONCLUSION

Both imaging modalities were found to perform equally well regarding trabecular bone measurements.

Reconstruction in image space using basis functions for partially parallel imaging

General theory of a new reconstruction technique for partially parallel imaging (PPI) is presented in this study. Reconstruction in Image space using Basis functions (RIB) is based on the general principle that the PPI reconstruction in image space can be represented by a pixel-wise weighted summation of the aliased images directly from undersampled data. By assuming that these weighting coefficients for unaliasing can be approximated from the linear combination of a few predefined basis functions, RIB is capable of reconstructing the image within an arbitrary region. This paper discusses the general theory of RIB and its relationship to the classical reconstruction method, GRAPPA. The presented experiments demonstrate RIB with several MRI applications. It is shown that the performance of RIB is comparable to that of GRAPPA. In some cases, RIB shows advantages of increasing reconstruction efficiency, suppressing artifacts and alleviating the nonuniformity of noise distribution. It is anticipated that RIB would be especially useful for cardiac and prostate imaging, where the field of view during data acquisition is required to be much larger than the region of interest.

Fast High-Spatial-Resolution MRI of the Ankle with Parallel Imaging Using GRAPPA at 3 T

OBJECTIVE

The purpose of our study was to compare an autocalibrating parallel imaging technique at 3 T with standard acquisitions at 3 and 1.5 T for small-field-of-view imaging of the ankle.

MATERIALS AND METHODS

MRI of the ankle was performed in three fresh human cadaver specimens and three healthy volunteers. Axial and sagittal T1-weighted, axial fat-saturated T2-weighted, and coronal intermediate-weighted fast spin-echo sequences, as well as a fat-saturated spoiled gradient-echo sequence, were acquired at 1.5 and 3 T. At 3 T, reduced data sets were reconstructed using a generalized autocalibrating partially parallel acquisition (GRAPPA) technique, with a scan time reduction of approximately 44%. All images were assessed by two radiologists independently concerning image quality. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured in every data set. In the cadaver specimens, macroscopic findings after dissection served as a reference for the pathologic evaluation.

RESULTS

SNR and CNR in the GRAPPA images were comparable to the standard acquisition at 3 T. The image quality was rated significantly higher at 3 T with both normal and parallel acquisition compared with 1.5 T. There was no significant difference in ligament and cartilage visualization or in image quality between standard and GRAPPA reconstruction at 3 T. Ankle abnormalities were better seen at 3 T than at 1.5 T for both normal and parallel acquisitions.

CONCLUSION

Using higher field strength combined with parallel technique, MR images of the ankle were obtained with excellent diagnostic quality and a scan time reduction of about 44%. In addition, parallel imaging can provide more flexibility in protocol design.