Fully balanced steady-state 3D-spin-echo (bSSSE) imaging at 3 Tesla

7 Tesla and Beyond: Advanced Methods and Clinical Applications in Magnetic Resonance Imaging

ABSTRACT

Ultrahigh magnetic fields offer significantly higher signal-to-noise ratio, and several magnetic resonance applications additionally benefit from a higher contrast-to-noise ratio, with static magnetic field strengths of B0 ≥ 7 T currently being referred to as ultrahigh fields (UHFs). The advantages of UHF can be used to resolve structures more precisely or to visualize physiological/pathophysiological effects that would be difficult or even impossible to detect at lower field strengths. However, with these advantages also come challenges, such as inhomogeneities applying standard radiofrequency excitation techniques, higher energy deposition in the human body, and enhanced B0 field inhomogeneities. The advantages but also the challenges of UHF as well as promising advanced methodological developments and clinical applications that particularly benefit from UHF are discussed in this review article.

Accuracy of MRI-based finite element assessment of distal tibia compared to mechanical testing

High-resolution MRI-derived finite element analysis (FEA) has been used in translational research to estimate the mechanical competence of human bone. However, this method has yet to be validated adequately under in vivo imaging spatial resolution or signal-to-noise conditions. We therefore compared MRI-based metrics of bone strength to those obtained from direct, mechanical testing. The study was conducted on tibiae from 17 human donors (12 males and five females, aged 33 to 88years) with no medical history of conditions affecting bone mineral homeostasis. A 25mm segment from each distal tibia underwent MR imaging in a clinical 3-Tesla scanner using a fast large-angle spin-echo (FLASE) sequence at 0.137mm×0.137mm×0.410mm voxel size, in accordance with in vivo scanning protocol. The resulting high-resolution MR images were processed and used to generate bone volume fraction maps, which served as input for the micro-level FEA model. Simulated compression was applied to compute stiffness, yield strength, ultimate strength, modulus of resilience, and toughness, which were then compared to metrics obtained from mechanical testing. Moderate to strong positive correlations were found between computationally and experimentally derived values of stiffness (R²=0.77, p<0.0001), yield strength (R²=0.38, p=0.0082), ultimate strength (R²=0.40, p=0.0067), and resilience (R²=0.46, p=0.0026), but only a weak, albeit significant, correlation was found for toughness (R²=0.26, p=0.036). Furthermore, experimentally derived yield strength and ultimate strength were moderately correlated with MRI-derived stiffness (R²=0.48, p=0.0022 and R²=0.58, p=0.0004, respectively). These results suggest that high-resolution MRI-based finite element (FE) models are effective in assessing mechanical parameters of distal skeletal extremities.

Micro-MR imaging-based computational biomechanics demonstrates reduction in cortical and trabecular bone strength after renal transplantation

PURPOSE

To examine the ability of three-dimensional micro-magnetic resonance (MR) imaging-based computational biomechanics to detect mechanical alterations in trabecular bone and cortical bone in the distal tibia of incident renal transplant recipients 6 months after renal transplantation and compare them with bone mineral density (BMD) outcomes.

MATERIALS AND METHODS

The study was approved by the institutional review board and complied with HIPAA guidelines. Written informed consent was obtained from all subjects. Micro-MR imaging of distal tibial metaphysis was performed within 2 weeks after renal transplantation (baseline) and 6 months later in 49 participants (24 female; median age, 44 years; range, 19-61 years) with a clinical 1.5-T whole-body imager using a modified three-dimensional fast large-angle spin-echo pulse sequence. Micro-finite-element models for cortical bone, trabecular bone, and whole-bone section were generated from each image by delineating the endosteal and periosteal boundaries. Mechanical parameters (stiffness and failure load) were estimated with simulated uniaxial compression tests on the micro-finite-element models. Structural parameters (trabecular bone volume fraction [BV/TV, bone volume to total volume ratio], trabecular thickness [TbTh], and cortical thickness [CtTh]) were computed from micro-MR images. Total hip and spine areal BMD were determined with dual-energy x-ray absorptiometry (DXA). Parameters obtained at the follow-up were compared with the baseline values by using parametric or nonparametric tests depending on the normality of data.

RESULTS

All mechanical parameters were significantly lower at 6 months compared with baseline. Decreases in cortical bone, trabecular bone, and whole-bone stiffness were 3.7% (P = .03), 4.9% (P = .03), and 4.3% (P = .003), respectively. Decreases in cortical bone, trabecular bone, and whole-bone failure strength were 7.6% (P = .0003), 6.0% (P = .004), and 5.6% (P = .0004), respectively. Conventional structural measures, BV/TV, TbTh, and CtTh, did not change significantly. Spine BMD decreased by 2.9% (P < .0001), while hip BMD did not change significantly at DXA.

CONCLUSION

MR imaging-based micro-finite-element analysis suggests that stiffness and failure strength of the distal tibia decrease over a 6-month interval after renal transplantation.

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.

3D fast spin echo with out-of-slab cancellation: a technique for high-resolution structural imaging of trabecular bone at 7 Tesla

Spin-echo-based pulse sequences are desirable for the application of high-resolution imaging of trabecular bone but tend to involve high-power deposition. Increased availability of ultrahigh field scanners has opened new possibilities for imaging with increased signal-to-noise ratio (SNR) efficiency, but many pulse sequences that are standard at 1.5 and 3 T exceed specific absorption rate limits at 7 T. A modified, reduced specific absorption rate, three-dimensional, fast spin-echo pulse sequence optimized specifically for in vivo trabecular bone imaging at 7 T is introduced. The sequence involves a slab-selective excitation pulse, low-power nonselective refocusing pulses, and phase cycling to cancel undesired out-of-slab signal. In vivo images of the distal tibia were acquired using the technique at 1.5, 3, and 7 T field strengths, and SNR was found to increase at least linearly using receive coils of identical geometry. Signal dependence on the choice of refocusing flip angles in the echo train was analyzed experimentally and theoretically by combining the signal from hundreds of coherence pathways, and it is shown that a significant specific absorption rate reduction can be achieved with negligible SNR loss.

In vivo high-resolution magnetic resonance skin imaging at 1.5 T and 3 T

As a noninvasive modality, MR is attractive for in vivo skin imaging. Its unique soft tissue contrast makes it an ideal imaging modality to study the skin water content and to resolve the different skin layers. In this work, the challenges of in vivo high-resolution skin imaging are addressed. Three 3D Cartesian sequences are customized to achieve high-resolution imaging and their respective performance is evaluated. The balanced steady-state free precession (bSSFP) and gradient echo (GRE) sequences are fast but can be sensitive to off-resonance artifacts. The fast large-angle spin echo (FLASE) sequence provides a sharp depiction of the hypodermis structures but results in more specific absorption rate (SAR). The effect of increasing the field strength is assessed. As compared to 1.5 T, signal-to-noise ratio at 3 T slightly increases in the hypodermis and almost doubles in the dermis. The need for fat/water separation is acknowledged and a solution using an interleaved three-point Dixon method and an iterative reconstruction is shown to be effective. The effects of motion are analyzed and two techniques to prevent motion and correct for it are evaluated. Images with 117 x 117 x 500 microm(3) resolution are obtained in imaging times under 6 min.

In vivo evaluation of the presence of bone marrow in cortical porosity in postmenopausal osteopenic women

This is the first observational study examining cortical porosity in vivo in postmenopausal osteopenic women and to incorporate data from two different imaging modalities to further examine the nature of cortical porosity. The goal of this study was to combine high-resolution peripheral computed tomography (HR-pQCT) images, which contain high spatial resolution information of the cortical structure, and magnetic resonance (MR) images, which allow the visualization of soft tissues such as bone marrow, to observe the amount of cortical porosity that contains bone marrow in postmenopausal osteopenic women. The radius of 49 and the tibia of 51 postmenopausal osteopenic women (age 56 ± 3.7) were scanned using both HR-pQCT and MR imaging. A normalized mutual information registration algorithm was used to obtain a three-dimensional rigid transform which aligned the MR image to the HR-pQCT image. The aligned images allowed for the visualization of bone marrow in cortical pores. From the HR-pQCT image, the percent cortical porosity, the number of cortical pores, and the size of each cortical pore was determined. By overlaying the aligned MR and HR-pQCT images, the percent of cortical pores containing marrow, the number of cortical pores containing marrow, and the size of each cortical pore containing marrow were measured. While the amount of cortical porosity did not vary greatly between subjects, the type of cortical pore, containing marrow vs. not containing marrow, varied highly between subjects. The results suggest that cortical pore spaces contain components of varying composition, and that there may be more than one mechanism for the development of cortical porosity.

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.

Spin-echo micro-MRI of trabecular bone using improved 3D fast large-angle spin-echo (FLASE)

Fast large-angle spin echo (FLASE) is a common pulse sequence designed for quantitative imaging of trabecular bone (TB) microarchitecture. However, imperfections in the nonselective phase-reversal pulse render it prone to stimulated echo artifacts. The problem is further exacerbated at isotropic resolution. Here, a substantially improved RF-spoiled FLASE sequence (sp-FLASE) is described and its performance is illustrated with data at 1.5T and 3T. Additional enhancements include navigator echoes for translational motion sensing applied in a slice parallel to the imaging slab. Whereas recent work suggests the use of fully-balanced FLASE (b-FLASE) to be advantageous from a signal-to-noise ratio (SNR) point of view, evidence is provided here that the greater robustness of sp-FLASE may outweigh the benefits of the minor SNR gain of b-FLASE for the target application of TB imaging in the distal extremities, sites of exclusively fatty marrow. Results are supported by a theoretical Bloch equation analysis and the pulse sequence dependence of the effective T(2) of triglyceride protons. Last, sp-FLASE images are shown to provide detailed and reproducible visual depiction of trabecular networks in three dimensions at both anisotropic (137 x 137 x 410 microm(3)) and isotropic (160 x 160 x 160 microm(3)) resolutions in the human distal tibia in vivo.

Assessment of cartilage-dedicated sequences at ultra-high-field MRI: comparison of imaging performance and diagnostic confidence between 3.0 and 7.0 T with respect to osteoarthritis-induced changes at the knee joint

OBJECTIVE

The objectives of the study were to optimize three cartilage-dedicated sequences for in vivo knee imaging at 7.0 T ultra-high-field (UHF) magnetic resonance imaging (MRI) and to compare imaging performance and diagnostic confidence concerning osteoarthritis (OA)-induced changes at 7.0 and 3.0 T MRI.

MATERIALS AND METHODS

Optimized MRI sequences for cartilage imaging at 3.0 T were tailored for 7.0 T: an intermediate-weighted fast spin-echo (IM-w FSE), a fast imaging employing steady-state acquisition (FIESTA) and a T1-weighted 3D high-spatial-resolution volumetric fat-suppressed spoiled gradient-echo (SPGR) sequence. Three healthy subjects and seven patients with mild OA were examined. Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), diagnostic confidence in assessing cartilage abnormalities, and image quality were determined. Abnormalities were assessed with the whole organ magnetic resonance imaging score (WORMS). Focal cartilage lesions and bone marrow edema pattern (BMEP) were also quantified.

RESULTS

At 7.0 T, SNR was increased (p < 0.05) for all sequences. For the IM-w FSE sequence, limitations with the specific absorption rate (SAR) required modifications of the scan parameters yielding an incomplete coverage of the knee joint, extensive artifacts, and a less effective fat saturation. CNR and image quality were increased (p < 0.05) for SPGR and FIESTA and decreased for IM-w FSE. Diagnostic confidence for cartilage lesions was highest (p < 0.05) for FIESTA at 7.0 T. Evaluation of BMEP was decreased (p < 0.05) at 7.0 T due to limited performance of IM-w FSE.

CONCLUSION

Gradient echo-based pulse sequences like SPGR and FIESTA are well suited for imaging at UHF which may improve early detection of cartilage lesions. However, UHF IM-w FSE sequences are less feasible for clinical use.

Bone mass and architecture determination: state of the art

Bone fracture occurs when the bone strength (i.e. the ability of the bone to resist a force) is less than the force applied to the bone. In the elderly, falls represent the more severe forces applied to bone. Bone density is a good marker of bone strength, and has been used widely in this respect. Nevertheless, many aspects of bone strength cannot be explained by bone density alone. For this reason there has been increasing interest in studying architectural parameters of bone, beyond bone density, which may affect bone strength. Macro-architectural parameters include e.g. bone size and geometry assessed with techniques such as radiography, dual-energy x-ray absorptiometry (DXA), peripheral quantitative computed tomography (QCT), computed tomography (CT) and magnetic resonance imaging (MRI). Micro-architectural parameters include fine cortical and trabecular structural detail which can be evaluated using high-resolution imaging techniques such as multidetector CT, MRI, and high-resolution peripheral QCT. These techniques are providing a great deal of new information on the physiological architectural responses of bone to aging, weightlessness, and treatment. This will ultimately lead to the prediction of fracture risk being improved through a combined assessment of bone density and architectural parameters.

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.

T1rho, T2 and focal knee cartilage abnormalities in physically active and sedentary healthy subjects versus early OA patients--a 3.0-Tesla MRI study

(1) To assess the degree of focal cartilage abnormalities in physically active and sedentary healthy subjects as well as in patients with early osteoarthritis (OA). (2) To determine the diagnostic value of T2 and T1rho measurements in identifying asymptomatic physically active subjects with focal cartilage lesions. Thirteen asymptomatic physically active subjects, 7 asymptomatic sedentary subjects, and 17 patients with mild OA underwent 3.0-T MRI of the knee joint. T1rho and T2 values, cartilage volume and thickness, as well as the WORMS scores were obtained. Nine out of 13 active healthy subjects had focal cartilage abnormalities. T1rho and T2 values in active subjects with and without focal cartilage abnormalities differed significantly (p < 0.05). T1rho and T2 values were significantly higher (p < 0.05) in early OA patients compared to healthy subjects. T1rho measurements were superior to T2 in differentiating OA patients from healthy subjects, yet T1rho was moderately age-dependent. (1) Active subjects showed a high prevalence of focal cartilage abnormalities and (2) active subjects with and without focal cartilage abnormalities had different T1rho and T2 composition of cartilage. Thus, T1rho and T2 could be a parameter suited to identify active healthy subjects at higher risk for developing cartilage pathology.

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.

In vivo determination of bone structure in postmenopausal women: a comparison of HR-pQCT and high-field MR imaging

Bone structural measures obtained by two noninvasive imaging tools-3T MRI and HR-pQCT-were compared. Significant but moderate correlations and 2- to 4-fold discrepancies in parameter values were detected, suggesting that differences in acquisition and analysis must be considered when interpreting data from these imaging modalities.

INTRODUCTION

High-field MRI and high resolution (HR)-pQCT are currently being used in longitudinal bone structure studies. Substantial differences in acquisition and analysis between these modalities may influence the quantitative data produced and could potentially influence clinical decisions based on their results. Our goal was to compare trabecular and cortical bone structural measures obtained in vivo by 3T MRI and HR-pQCT.

MATERIALS AND METHODS

Postmenopausal osteopenic women (n = 52) were recruited for this study. HR-pQCT imaging of the radius and tibia was performed using the XtremeCT scanner, with a voxel size of 82 x 82 x 82 microm(3). MR imaging was performed on a 3T Signa scanner using SSFP imaging sequences, with a pixel size of 156 x 156 microm(2) and slice thickness of 500 microm. Structure parameters were calculated using standard HR-pQCT and MRI analysis techniques. Relationships between measures derived from HR-pQCT, MRI, and DXA were studied.

RESULTS

Significant correlations between HR-pQCT and MRI parameters were found (p < 0.0001) and were strongest for Tb.N (r(2) = 0.52), Ct.Th (r(2) = 0.59), and site-specific Tb.Sp (r(2) = 0.54-0.60). MRI and HR-pQCT provided statistically different values of structure parameters (p < 0.0001), with BV/TV and Tb.Th exhibiting the largest discrepancies (MR/HR-pQCT = 3-4). Although differences in the Tb.N values were statistically significant, the mean differences were on the order of our reproducibility measurements. Systematic differences between MRI and HR-pQCT analysis procedures leading to discrepancies in cortical thickness values were observed, with MRI values consistently higher. Minimal correlations were found between MRI or HR-pQCT parameters and DXA BMD or T-score, except between HR-pQCT measures at the radius and the ultradistal radius T-scores, where moderate correlations were found (r(2) = 0.19-0.58).

CONCLUSIONS

This study provides unique insight into two emerging noninvasive tools for bone structure evaluation. Our findings highlight the significant influence of analysis technique on results of in vivo assessment and underscore the importance of accounting for these differences when interpreting results from these modalities.