UTE imaging in the musculoskeletal system

Magnetic Resonance Imaging of Hard Tissues and Hard Tissue Engineered Bio-substitutes

Magnetic resonance imaging (MRI) is a non-invasive diagnostic imaging tool based on the detection of protons into the tissues. This imaging technique is remarkable because of high spatial resolution, strong soft tissue contrast and specificity, and good depth penetration. However, MR imaging of hard tissues, such as bone and teeth, remains challenging due to low proton content in such tissues as well as to very short transverse relaxation times (T₂). To overcome these issues, new MRI techniques, such as sweep imaging with Fourier transformation (SWIFT), ultrashort echo time (UTE) imaging, and zero echo time (ZTE) imaging, have been developed for hard tissues imaging with promising results reported. Within this article, MRI techniques developed for the detection of hard tissues, such as bone and dental tissues, have been reviewed. The main goal was thus to give a comprehensive overview on the corresponding (pre-) clinical applications and on the potential future directions with such techniques applied. In addition, a section dedicated to MR imaging of novel biomaterials developed for hard tissue applications was given as well.

Water proton density in human cortical bone obtained from ultrashort echo time (UTE) MRI predicts bone microstructural properties

PURPOSE

To investigate the correlations between cortical bone microstructural properties and total water proton density (TWPD) obtained from three-dimensional ultrashort echo time Cones (3D-UTE-Cones) magnetic resonance imaging techniques.

MATERIALS AND METHODS

135 cortical bone samples were harvested from human tibial and femoral midshafts of 37 donors (61 ± 24 years old). Samples were scanned using 3D-UTE-Cones sequences on a clinical 3T MRI and on a high-resolution micro-computed tomography (μCT) scanner. TWPD was measured using 3D-UTE-Cones MR images. Average bone porosity, pore size, and bone mineral density (BMD) were measured from μCT images at 9 μm voxel size. Pearson's correlation coefficients between TWPD and μCT-based measures were calculated.

RESULTS

TWPD showed significant moderate correlation with both average bone porosity (R = 0.66, p < 0.01) and pore size (R = 0.57, p < 0.01). TWPD also showed significant strong correction with BMD (R = 0.71, p < 0.01).

CONCLUSIONS

The presented 3D-UTE-Cones imaging technique allows assessment of TWPD in human cortical bone. This quick UTE-MRI-based technique was capable of predicting bone microstructure differences with significant correlations. Such correlations highlight the potential of UTE-MRI-based measurement of bone water proton density to assess bone microstructure.

AcidoCEST-UTE MRI for the Assessment of Extracellular pH of Joint Tissues at 3 T

OBJECTIVES

The goal of this study was to demonstrate feasibility of measuring extracellular pH in cartilage and meniscus using acidoCEST technique with a 3-dimensional ultrashort echo time readout (acidoCEST-UTE) magnetic resonance imaging (MRI).

MATERIALS AND METHODS

Magnetization transfer ratio asymmetry, radiofrequency (RF) power mismatch, and relative saturation transfer were evaluated in liquid phantoms for iopromide, iopamidol, and iohexol over a pH range of 6.2 to 7.8, at various agent concentrations, temperatures, and buffer concentrations. Tissue phantoms containing cartilage and meniscus were evaluated with the same considerations for iopamidol and iohexol. Phantoms were imaged with the acidoCEST-UTE MRI sequence at 3 T. Correlation coefficients and coefficients of variations were calculated. Paired Wilcoxon rank-sum tests were used to evaluate for statistically significant differences.

RESULTS

The RF power mismatch and relative saturation transfer analyses of liquid phantoms showed iopamidol and iohexol to be the most promising agents for this study. Both these agents appeared to be concentration independent and feasible for use with or without buffer and at physiologic temperature over a pH range of 6.2 to 7.8. Ultimately, RF power mismatch fitting of iohexol showed the strongest correlation coefficients between cartilage, meniscus, and fluid. In addition, ratiometric values for iohexol are similar among liquid as well as different tissue types.

CONCLUSIONS

Measuring extracellular pH in cartilage and meniscus using acidoCEST-UTE MRI is feasible.

Rotator Cuff Tendon Assessment in Symptomatic and Control Groups Using Quantitative MRI

BACKGROUND

Relatively weak correlations between patient symptoms and rotator cuff tendon (RCT) tearing have been reported; however, the relationship between symptoms and tendinosis has been less well-studied.

PURPOSE/HYPOTHESIS

To use quantitative MRI to assess the bilateral RCTs in shoulders of both patients with unilateral symptomatic tendinopathy and control subjects. We hypothesized that quantitative MRI measures would differ between symptomatic patients and controls.

STUDY TYPE

Prospective imaging study.

POPULATION/SUBJECTS

In all, 48 shoulders from 24 subjects (mean age, 32.8 years), including 14 patients with unilateral symptomatic tendinopathy and 10 asymptomatic controls.

FIELD STRENGTH/SEQUENCE

3T/3D ultrashort echo time Cones sequence with magnetization transfer preparation (UTE-Cones-MT) and Carr-Purcell-Meiboom-Gill.

ASSESSMENT

Macromolecular fraction (MMF) and T₂ relaxation were measured in four regions of the superior RCT, including all-segments, and lateral-third, bursal-sided, and articular-sided segments. The Western Ontario Rotator Cuff (WORC) index and visual analog scale were assessed.

STATISTICAL TESTS

Three shoulder groups were evaluated, including symptomatic shoulders, contralateral asymptomatic shoulders in patients, and asymptomatic controls. MMF and T₂ values were compared between groups using a bootstrap-based comparison of means.

RESULTS

Significant differences were found in both MMF and T₂ values between symptomatic and control RCTs when analyzing all-segments (P = 0.027 and P = 0.006, respectively) and articular-sided segments (both P = 0.001). Significant differences between asymptomatic RCTs in patients and control RCTs were also found, including MMF in all four anatomic regions analyzed (P = 0.024-0.044), as well as T₂ in all-segments (P = 0.003), bursal-sided segments (P = 0.021), and articular-sided segments (P = 0.002). No significant differences in MMF (P = 0.420-0.950) or T₂ (P = 0.380-0.910) were seen between ipsilateral symptomatic and contralateral asymptomatic RCTs in patients.

DATA CONCLUSION

Symptomatic RCTs showed significantly lower MMF values and higher T₂ values compared with control RCTs. In patients with unilateral symptomatic tendinopathy, the contralateral shoulder can demonstrate asymptomatic tendinopathy, which can be quantified using MMF or T₂ .

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2. J. Magn. Reson. Imaging 2020;52:864-872.

Correlations of cortical bone microstructural and mechanical properties with water proton fractions obtained from ultrashort echo time (UTE) MRI tricomponent T2* model

Mechanical and microstructural evaluations of cortical bone using ultrashort echo time magnetic resonance imaging (UTE-MRI) have been performed increasingly in recent years. UTE-MRI acquires considerable signal from cortical bone and enables quantitative bone evaluations. Fitting bone apparent transverse magnetization (T2*) decay using a bicomponent model has been regularly performed to estimate bound water (BW) and pore water (PW) in the quantification of bone matrix and porosity, respectively. Human cortical bone possesses a considerable amount of fat, which appears as MRI T2* signal oscillation and can subsequently lead to BW overestimation when using a bicomponent model. Tricomponent T2* fitting model has been developed to improve BW and PW estimations by accounting for fat contribution in the MRI signal. This study aimed to investigate the correlations of microstructural and mechanical properties of human cortical bone with water pool fractions obtained from a tricomponent T2* model. 135 cortical bone strips (~4 × 2 × 40 mm³ ) from tibial and femoral midshafts of 37 donors (61 ± 24 years old) were scanned using ten sets of dual-echo 3D-UTE-Cones sequences (TE = 0.032-24.0 ms) on a 3 T MRI scanner for T2* fitting analyses. Average bone porosity and pore size were measured using microcomputed tomography (μCT) at 9 μm voxel size. Bone mechanical properties were measured using 4-point bending tests. Using a tricomponent model, bound water fraction (Frac_BW ) showed significant strong (R = 0.70, P < 0.01) and moderate (R = 0.58-0.62, P < 0.01) correlations with porosity and mechanical properties, respectively. Correlations of bone microstructural and mechanical properties with water pool fractions were higher for tricomponent model results compared with the bicomponent model. The tricomponent T2* fitting model is suggested as a useful technique for cortical bone evaluation where the MRI contribution of bone fat is accounted for.

Knee menisci segmentation and relaxometry of 3D ultrashort echo time cones MR imaging using attention U-Net with transfer learning

PURPOSE

To develop a deep learning-based method for knee menisci segmentation in 3D ultrashort echo time (UTE) cones MR imaging, and to automatically determine MR relaxation times, namely the T1, T1ρ , and T 2 ∗ parameters, which can be used to assess knee osteoarthritis (OA).

METHODS

Whole knee joint imaging was performed using 3D UTE cones sequences to collect data from 61 human subjects. Regions of interest (ROIs) were outlined by 2 experienced radiologists based on subtracted T1ρ -weighted MR images. Transfer learning was applied to develop 2D attention U-Net convolutional neural networks for the menisci segmentation based on each radiologist's ROIs separately. Dice scores were calculated to assess segmentation performance. Next, the T1, T1ρ , T 2 ∗ relaxations, and ROI areas were determined for the manual and automatic segmentations, then compared.

RESULTS

The models developed using ROIs provided by 2 radiologists achieved high Dice scores of 0.860 and 0.833, while the radiologists' manual segmentations achieved a Dice score of 0.820. Linear correlation coefficients for the T1, T1ρ , and T 2 ∗ relaxations calculated using the automatic and manual segmentations ranged between 0.90 and 0.97, and there were no associated differences between the estimated average meniscal relaxation parameters. The deep learning models achieved segmentation performance equivalent to the inter-observer variability of 2 radiologists.

CONCLUSION

The proposed deep learning-based approach can be used to efficiently generate automatic segmentations and determine meniscal relaxations times. The method has the potential to help radiologists with the assessment of meniscal diseases, such as OA.

Tissue-Specific T2 * Biomarkers in Patellar Tendinopathy by Subregional Quantification Using 3D Ultrashort Echo Time MRI

Background

Quantitative MRI of patellar tendinopathy (PT) can be challenging due to spatial variation of T₂* relaxation times.

Purpose

1) To compare T₂* quantification using a standard approach with analysis in specific tissue compartments of the patellar tendon. 2) To evaluate test–retest reliability of different methods for fitting ultrashort echo time (UTE)‐relaxometry data.

Study Type

Prospective.

Subjects

Sixty‐five athletes with PT.

Field Strength/Sequence

3D UTE scans covering the patellar tendon were acquired using a 3.0T scanner and a 16‐channel surface coil.

Assessment

Voxelwise median T₂* was quantified with monoexponential, fractional‐order, and biexponential fitting. We applied two methods for T₂* analysis: first, a standard approach by analyzing all voxels covering the proximal patellar tendon. Second, within subregions of the patellar tendon, by using thresholds on biexponential fitting parameter percentage short T₂* (0–30% for mostly long T₂*, 30–60% for mixed T₂*, and 60–100% for mostly short T₂*).

Statistical Tests

Average test–retest reliability was assessed in three athletes using coefficients‐of‐variation (CV) and coefficients‐of‐repeatability (CR).

Results

With standard image analysis, we found a median [interquartile range, IQR] monoexponential T₂* of 6.43 msec [4.32–8.55] and fractional order T₂* 4.39 msec [3.06–5.78]. The percentage of short T₂* components was 52.9% [35.5–69.6]. Subregional monoexponential T₂* was 13.78 msec [12.11–16.46], 7.65 msec [6.49–8.61], and 3.05 msec [2.52–3.60] and fractional order T₂* 11.82 msec [10.09–14.44], 5.14 msec [4.25–5.96], and 2.19 msec [1.82–2.64] for 0–30%, 30–60%, and 60–100% short T₂*, respectively. Biexponential component short T₂* was 1.693 msec [1.417–2.003] for tissue with mostly short T₂* and long T₂* of 15.79 msec [13.47–18.61] for mostly long T₂*. The average CR (CV) was 2 msec (15%), 2 msec (19%) and 10% (22%) for monoexponential, fractional order and percentage short T₂*, respectively.

Data Conclusion

Patellar tendinopathy is characterized by regional variability in binding states of water. Quantitative multicompartment T₂* analysis in PT can be facilitated using a voxel selection method based on using biexponential fitting parameters.

Level of Evidence

1

Technical Efficacy Stage

1 J. Magn. Reson. Imaging 2020;52:420–430.

An Update in Qualitative Imaging of Bone Using Ultrashort Echo Time Magnetic Resonance

Bone is comprised of mineral, collagenous organic matrix, and water. X-ray-based techniques are the standard approach for bone evaluation in clinics, but they are unable to detect the organic matrix and water components in bone. Magnetic resonance imaging (MRI) is being used increasingly for bone evaluation. While MRI can non-invasively assess the proton pools in soft tissues, cortical bone typically appears as a signal void with clinical MR techniques because of its short T2*. New MRI techniques have been recently developed to image bone while avoiding the ionizing radiation present in x-ray-based methods. Qualitative bone imaging can be achieved using ultrashort echo time (UTE), single inversion recovery UTE (IR-UTE), dual-inversion recovery UTE (Dual-IR-UTE), double-inversion recovery UTE (Double-IR-UTE), and zero echo time (ZTE) sequences. The contrast mechanisms as well as the advantages and disadvantages of each technique are discussed.

T1- and T2*-Mapping for Assessment of Tendon Tissue Biophysical Properties: A Phantom MRI Study

OBJECTIVES

The aim of this study was to quantitatively assess changes in collagen structure using MR T1- and T2*-mapping in a novel controlled ex vivo tendon model setup.

MATERIALS AND METHODS

Twenty-four cadaveric bovine flexor tendons underwent MRI at 3 T before and after chemical modifications, representing mechanical degeneration and augmentation. Collagen degradation (COL), augmenting collagen fiber cross-linking (CXL), and a control (phosphate-buffered saline [PBS]) were examined in experimental groups, using histopathology as standard of reference. Variable echo-time and variable-flip angle gradient-echo sequences were used for T2*- and T1-mapping, respectively. Standard T1- and T2-weighted spin-echo sequences were acquired for visual assessment of tendon texture. Tendons were assessed subsequently for their biomechanical properties and compared with quantitative MRI analysis.

RESULTS

T1- and T2*-mapping was feasible and repeatable for untreated (mean, 545 milliseconds, 2.0 milliseconds) and treated tendons. Mean T1 and T2* values of COL, CXL, and PBS tendons were 1459, 934, and 1017 milliseconds, and 5.5, 3.6, and 2.5 milliseconds, respectively. T2* values were significantly different between enzymatically degraded tendons, cross-linked tendons, and controls, and were significantly correlated with mechanical tendon properties (r = -0.74, P < 0.01). T1 values and visual assessment could not differentiate CXL from PBS tendons. Photo-spectroscopy showed increased autofluorescence of cross-linked tendons, whereas histopathology verified degenerative lesions of enzymatically degraded tendons.

CONCLUSIONS

T2*-mapping has the potential to detect and quantify subtle changes in tendon collagen structure not visible on conventional clinical MRI. Tendon T2* values might serve as a biomarker for biochemical alterations associated with tendon pathology.

Assessing cortical bone mechanical properties using collagen proton fraction from ultrashort echo time magnetization transfer (UTE-MT) MRI modeling

Cortical bone shows as a signal void when using conventional clinical magnetic resonance imaging (MRI). Ultrashort echo time MRI (UTE-MRI) can acquire high signal from cortical bone, thus enabling quantitative assessments. Magnetization transfer (MT) imaging combined with UTE-MRI can indirectly assess protons in the organic matrix of bone. This study aimed to examine UTE-MT MRI techniques to estimate the mechanical properties of cortical bone. A total of 156 rectangular human cortical bone strips were harvested from the tibial and femoral midshafts of 43 donors (62 ± 22 years old, 62 specimens from females, 94 specimens from males). Bone specimens were scanned using UTE-MT sequences on a clinical 3 T MRI scanner and on a micro-computed tomography (μCT) scanner. A series of MT pulse saturation powers (400°, 600°, 800°) and frequency offsets (2, 5, 10, 20, 50 kHz) was used to measure the macromolecular fraction (MMF) utilizing a two-pool MT model. Failure mechanical properties of the bone specimens were measured using 4-point bending tests. MMF from MRI results showed significant strong correlations with cortical bone porosity (R = -0.72, P < 0.01) and bone mineral density (BMD) (R = +0.71, P < 0.01). MMF demonstrated significant moderate correlations with Young modulus, yield stress, and ultimate stress (R = 0.60-0.61, P < 0.01). These results suggest that the two-pool UTE-MT model focusing on the organic matrix of bone can potentially serve as a novel tool to detect the variations of bone mechanical properties and intracortical porosity.

Age-related decrease in collagen proton fraction in tibial tendons estimated by magnetization transfer modeling of ultrashort echo time magnetic resonance imaging (UTE-MRI)

Clinical magnetic resonance imaging (MRI) sequences are not often capable of directly visualizing tendons. Ultrashort echo time (UTE) MRI can acquire high signal from tendons thus enabling quantitative assessments. Magnetization transfer (MT) modeling combined with UTE-MRI—UTE-MT-modeling—can indirectly assess macromolecular protons in the tendon. This study aimed to determine if UTE-MT-modeling is a quantitative technique sensitive to the age-related changes of tendons. The legs of 26 young healthy (29 ± 6 years old) and 22 elderly (75 ± 8 years old) female subjects were imaged using UTE sequences on a 3T MRI scanner. Institutional review board approval was obtained, and all recruited subjects provided written informed consent. T1 and UTE-MT-modeling were performed on anterior tibialis tendons (ATT) and posterior tibialis tendons (PTT) as two representative human leg tendons. A series of MT pulse saturation powers (500–1500°) and frequency offsets (2–50 kHz) were used to measure the macromolecular fraction (MMF) and macromolecular T2 (T2_MM). All measurements were repeated by three independent readers for a reproducibility study. MMF demonstrated significantly lower values on average in the elderly cohort compared with the younger cohort for both ATT (decreased by 16.8%, p = 0.03) and PTT (decreased by 23.0%, p < 0.01). T2_MM and T1 did not show a significant nor a consistent difference between the young and elderly cohorts. For all MRI parameters, intraclass correlation coefficient (ICC) was higher than 0.98, indicating excellent consistency between measurements performed by independent readers. MMF serving as a surrogate measure for collagen content, showed a significant decrease in elderly leg tendons. This study highlighted UTE-MRI-MT techniques as a useful quantitative method to assess the impact of aging on human tendons.

Improved volumetric myelin imaging in human brain using 3D dual echo inversion recovery-prepared UTE with complex echo subtraction

PURPOSE

Inversion recovery-based UTE (IR-UTE) sequences have been proposed to directly image myelin with extremely short T 2 ∗ (~0.3 ms). In this study, we demonstrate the feasibility of complex echo subtraction to improve 3D IR-UTE imaging of myelin in white matter of the brain in vivo.

METHODS

In IR-UTE imaging, long T₂ components in white matter (i.e., water) are suppressed using an adiabatic inversion recovery preparation pulse. Dual echo UTE data acquisition and magnitude echo subtraction are used to suppress the residual white matter and gray matter signals, providing high myelin contrast. Complex echo subtraction may further improve the myelin contrast by reducing the residual long T₂ water signal contamination caused by regional T₁ variations. To verify the efficacy of the complex subtraction technique, in vivo experiments were performed with 5 non-symptomatic healthy volunteers and 5 multiple sclerosis patients on a 3T clinical MR system. Signal enhancement between the complex subtraction and the magnitude subtraction was introduced to evaluate the improvement.

RESULTS

The complex subtraction improved myelin contrast over the magnitude subtraction in both healthy and patient groups, with more fine myelin structures being revealed. The foci of the demyelinated lesion were more clearly detected by complex subtraction. An average signal enhancement of up to 135.9% was achieved with the complex subtraction over the magnitude subtraction.

CONCLUSION

The complex echo subtraction improves 3D IR-UTE morphologic imaging of myelin in white matter of the brain.

Cardiovascular ultrashort echo time to map fibrosis-promises and challenges

Increased collagen, or fibrosis, is an important marker of disease and may improve identification of patients at risk. In addition, fibrosis imaging may play an increasing role in guiding therapy and monitoring its effectiveness. MRI is the most frequently used modality to detect, visualize and quantify fibrosis non-invasively. However, standard MRI techniques used to phenotype cardiac fibrosis such as delayed enhancement and extracellular volume determination by T1 mapping, require the administration of gadolinium-based contrast and are particularly difficult to use in patients with cardiac devices such as pacemakers and automatic defibrillators. Therefore, such methods are limited in the serial evaluation of cardiovascular fibrosis as part of chronic disease monitoring. A method to directly measure collagen amount could be of great clinical benefit. In the current review we will discuss the potential of a novel MR technique, ultrashort echo time (UTE) MR, for fibrosis imaging. Although UTE imaging is successfully applied in other body areas such as musculoskeletal applications, there is very limited experience so far in the heart. We will review the established methods and currently available literature, discuss the technical considerations and challenges, show preliminary in vivo images and provide a future outlook on potential applications of cardiovascular UTE.

Magnetic resonance imaging (MRI) studies of knee joint under mechanical loading: Review

Osteoarthritis (OA) is a very common disease that affects the human knee joint, particularly the articular cartilage and meniscus components which are regularly under compressive mechanical loads. Early-stage OA diagnosis is essential as it allows for timely intervention. The primary non-invasive approaches currently available for OA diagnosis include magnetic resonance imaging (MRI), which provides excellent soft tissue contrast at high spatial resolution. MRI-based knee investigation is usually performed on joints at rest or in a non-weight-bearing condition that does not mimic the actual physiological condition of the joint. This discrepancy may lead to missed detections of early-stage OA or of minor lesions. The mechanical properties of degenerated musculoskeletal (MSK) tissues may vary markedly before any significant morphological or structural changes detectable by MRI. Recognizing distinct deformation characteristics of these tissues under known mechanical loads may reveal crucial joint lesions or mechanical malfunctions which result from early-stage OA. This review article summarizes the large number of MRI-based investigations on knee joints under mechanical loading which have been reported in the literature including the corresponding MRI measures, the MRI-compatible devices employed, and potential challenges due to the limitations of clinical MRI sequences.

Assessment of an in vitro model of rotator cuff degeneration using quantitative magnetic resonance and ultrasound imaging with biochemical and histological correlation

PURPOSE

Quantitative imaging methods could improve diagnosis of rotator cuff degeneration, but the capability of quantitative MR and US imaging parameters to detect alterations in collagen is unknown. The goal of this study was to assess quantitative MR and US imaging measures for detecting abnormalities in collagen using an in vitro model of tendinosis with biochemical and histological correlation.

METHOD

36 pieces of supraspinatus tendons from 6 cadaveric donors were equally distributed into 3 groups (2 subjected to different concentrations of collagenase and a control group). Ultrashort echo time MR and US imaging measures were performed to assess changes at baseline and after 24 h of enzymatic digestion. Biochemical and histological measures, including brightfield, fluorescence, and polarized microscopy, were used to verify the validity of the model and were compared with quantitative imaging parameters. Correlations between the imaging parameters and biochemically measured digestion were analyzed.

RESULTS

Among the imaging parameters, macromolecular fraction (MMF), adiabatic T1ρ, T2*, and backscatter coefficient (BSC) were useful in differentiating between the extent of degeneration among the 3 groups. MMF strongly correlated with collagen loss (r=-0.81; 95% confidence interval [CI]: -0.90,-0.66), while the adiabatic T1ρ (r = 0.66; CI: 0.42,0.81), T2* (r = 0.58; CI: 0.31,0.76), and BSC (r = 0.51; CI: 0.22,0.72) moderately correlated with collagen loss.

CONCLUSIONS

MMF, adiabatic T1ρ, and T2* measured and US BSC can detect alterations in collagen. Of the quantitative MR and US imaging measures evaluated, MMF showed the highest correlation with collagen loss and can be used to assess rotator cuff degeneration.

Significant correlations between human cortical bone mineral density and quantitative susceptibility mapping (QSM) obtained with 3D Cones ultrashort echo time magnetic resonance imaging (UTE-MRI)

PURPOSE

Quantitative susceptibility mapping (QSM) MRI is a tool that can characterize changes in susceptibility, an intrinsic property which is associated with compositional changes in the tissue. Current QSM estimation of cortical bone is challenging because conventional clinical MRI cannot acquire signal in cortical bone. This study aimed to implement Cones 3D ultrashort echo time MRI (UTE-MRI) for ex vivo QSM measurements in human tibial cortical bone, investigating the correlations of QSM with volumetric intracortical bone mineral density (BMD).

MATERIALS AND METHODS

Nine tibial midshaft cortical bone specimens (25 mm long specimens cut at the mid-point of tibial shaft, 67 ± 20 years old, 5 women and 4 men) were scanned on a clinical 3 T MRI scanner for QSM measurement. The specimens were also scanned on a high-resolution micro-computed tomography (μCT) scanner for volumetric BMD estimation. QSM and μCT results were compared at approximately nine regions of interest (ROIs) per specimen.

RESULTS

Average 3D UTE-MRI QSM showed significantly strong correlation with volumetric BMD (R = -0.82, P < 0.01) and bone porosity (R = 0.72, P < 0.01). Combining all data points together (77 ROIs), QSM showed significant moderate to strong correlation with volumetric BMD after correction for interdependencies in specimens (R = -0.70, P < 0.01). The corrections were required because the data points were not independent in each specimen. Similarly, the correlation between QSM and porosity was significant (R = 0.68, P < 0.01).

CONCLUSIONS

These results suggest that the Cones 3D UTE-MRI QSM technique can potentially serve as a novel and accurate tool to assess intracortical bone mineral density whilst avoiding ionizing radiation.

Volumetric mapping of bound and pore water as well as collagen protons in cortical bone using 3D ultrashort echo time cones MR imaging techniques

Cortical bone assessment using magnetic resonance imaging (MRI) has recently received great attention in an effort to avoid the potential harm associated with ionizing radiation-based techniques. Ultrashort echo time MRI (UTE-MRI) techniques can acquire signal from major hydrogen proton pools in cortical bone, including bound and pore water, as well as from the collagen matrix. This study aimed to develop and evaluate the feasibility of a technique for mapping bound water, pore water, and collagen proton densities in human cortical bone ex vivo and in vivo using three-dimensional UTE Cones (3D-UTE-Cones) MRI. Eight human tibial cortical bone specimens (63 ± 19 years old) were scanned using 3D-UTE-Cones sequences on a clinical 3 T MRI scanner and a micro-computed tomography (μCT) scanner. Total, bound, and pore water proton densities (TWPD, BWPD, and PWPD, respectively) were measured using UTE and inversion recovery UTE (IR-UTE) imaging techniques. Macromolecular proton density (MMPD), a collagen representation, was measured using TWPD and macromolecular fraction (MMF) obtained from two-pool UTE magnetization transfer (UTE-MT) modeling. The correlations between proton densities and μCT-based measures were investigated. The 3D-UTE-Cones techniques were further applied on ten young healthy (34 ± 3 years old) and five old (78 ± 6 years old) female volunteers to evaluate the techniques' feasibility for translational clinical applications. In the ex vivo study, PWPD showed the highest correlations with bone porosity and bone mineral density (BMD) (R = 0.79 and - 0.70, p < 0.01). MMPD demonstrated moderate to strong correlations with bone porosity and BMD (R = -0.67 and 0.65, p < 0.01). MMPD showed strong correlation with age in specimens from female donors (R = -0.91, p = 0.03, n = 5). The presented comprehensive 3D-UTE-Cones imaging protocol allows quantitative mapping of protons in major pools of cortical bone ex vivo and in vivo. PWPD and MMPD can serve as potential novel biomarkers to assess bone matrix and microstructure, as well as bone age- or injury-related variations.

In vivo assessment of extracellular pH of joint tissues using acidoCEST-UTE MRI

Background

Degradation of cartilage and meniscus may be mediated by changes in extracellular pH. The purpose of this study was to optimize saturation powers used with the acidoCEST magnetic resonance imaging (MRI) technique with a 3D ultrashort echo time readout (acidoCEST-UTE) and to demonstrate feasibility of the method for measuring pH in cartilage and meniscus in vivo.

Methods

Magnetization transfer ratio asymmetry and ratio of radiofrequency (RF) power mismatch at different powers were evaluated in cartilage and meniscus tissue phantoms for iopamidol and iohexol. Using optimized RF powers, the acidoCEST-UTE MRI sequence was used to assess pH of joint fluid and tissues in four patients after direct intra-articular administration of iodinated contrast.

Results

In the phantoms, the ratio of powers 0.54/1.10 µT showed the strongest correlation with pH. In vivo acidoCEST-UTE pH measurements of intra-articular fluid were similar to electrode measurements of the contrast agent (7.22 vs. 7.1 for iopamidol, respectively; 7.65 vs. 7.5 for iohexol, respectively). As measured with the acidoCEST-UTE technique, overall mean cartilage pH was significantly lower than overall mean meniscus pH (6.60 vs. 6.72, respectively; P=0.043).

Conclusions

AcidoCEST-UTE MRI after direct intra-articular administration of either iopamidol or iohexol can be used to measure cartilage and meniscus pH in vivo.

Short-T2 MRI: Principles and recent advances

Among current modalities of biomedical and diagnostic imaging, MRI stands out by virtue of its versatile contrast obtained without ionizing radiation. However, in various cases, e.g., water protons in tissues such as bone, tendon, and lung, MRI performance is limited by the rapid decay of resonance signals associated with short transverse relaxation times T₂ or T₂*. Efforts to address this shortcoming have led to a variety of specialized short-T₂ techniques. Recent progress in this field expands the choice of methods and prompts fresh considerations with regard to instrumentation, data acquisition, and signal processing. In this review, the current status of short-T₂ MRI is surveyed. In an attempt to structure the growing range of techniques, the presentation highlights overarching concepts and basic methodological options. The most frequently used approaches are described in detail, including acquisition strategies, image reconstruction, hardware requirements, means of introducing contrast, sources of artifacts, limitations, and applications.

Inversion recovery UTE based volumetric myelin imaging in human brain using interleaved hybrid encoding

PURPOSE

Direct myelin imaging can improve the characterization of myelin-related diseases such as multiple sclerosis. In this study, we explore a novel method to directly image myelin using inversion recovery-prepared hybrid encoding (IR-HE) UTE MRI.

METHODS

The IR-HE sequence uses an adiabatic inversion pulse to suppress the long T₂ white matter signal, followed by 3D dual-echo HE utilizing both single point imaging and radial frequency encoding, for which the subtraction image between 2 echoes reveals the myelin signal with high contrast. To reduce scan time, it is common to obtain multiple spokes per IR. Here, we invented a novel method to improve the HE, adapted for the multi-spoke IR imaging-termed interleaved HE-for which single point imaging encoding is interleaved between radial frequency encodings near nulling point to allow more efficient IR-signal suppression. To evaluate the proposed approach, a computer simulation, myelin phantom experiment, an ex vivo experiment with a cadaveric multiple sclerosis brain, and an in vivo experiment with 8 healthy volunteers and 13 multiple sclerosis patients were performed.

RESULTS

The computer simulation showed that IR-interleaved HE allows for improved contrast of myelin signal with reduced imaging artifacts. The myelin phantom experiment showed IR-interleaved HE allows direct imaging of myelin lipid with excellent suppression of water signal. In the ex vivo and in vivo experiments, the proposed method demonstrated highly specific imaging of myelin in white matter of the brain.

CONCLUSION

IR-interleaved HE allows for time-efficient, high-contrast direct myelin imaging and can detect demyelinated lesions in multiple sclerosis patients.

Abbreviated quantitative UTE imaging in anterior cruciate ligament reconstruction

BACKGROUND

Existing ultrashort echo time magnetic resonance imaging (UTE MRI) methods require prohibitively long acquisition times (~ 20-40 min) to quantitatively assess the clinically relevant fast decay T₂* component in ligaments and tendons. The purpose of this study was to evaluate the feasibility and clinical translatability of a novel abbreviated quantitative UTE MRI paradigm for monitoring graft remodeling after anterior cruciate ligament (ACL) reconstruction.

METHODS

Eight patients who had Graftlink™ hamstring autograft reconstruction were recruited for this prospective study. A 3D double-echo UTE sequence at 3.0 Tesla was performed at 3- and 6-months post-surgery. An abbreviated UTE MRI paradigm was established based on numerical simulations and in vivo validation from healthy knees. This proposed approach was used to assess the T₂* for fast decay component ([Formula: see text]) and bound water signal fraction (f_bw) of ACL graft in regions of interest drawn by a radiologist.

RESULTS

Compared to the conventional bi-exponential model, the abbreviated UTE MRI paradigm achieved low relative estimation bias for [Formula: see text] and f_bw over a range of clinically relevant values for ACL grafts. A decrease in [Formula: see text] of the intra-articular graft was observed in 7 of the 8 ACL reconstruction patients from 3- to 6-months (- 0.11 ± 0.16 ms, P = 0.10). Increases in [Formula: see text] and f_bw from 3- to 6-months were observed in the tibial intra-bone graft ([Formula: see text]: 0.19 ± 0.18 ms, P < 0.05; Δf_bw: 4% ± 4%, P < 0.05). Lower [Formula: see text] (- 0.09 ± 0.11 ms, P < 0.05) was observed at 3-months when comparing the intra-bone graft to the graft/bone interface in the femoral tunnel. The same comparisons at the 6-months also yielded relatively lower [Formula: see text] (- 0.09 ± 0.12 ms, P < 0.05).

CONCLUSION

The proposed abbreviated 3D UTE MRI paradigm is capable of assessing the ACL graft remodeling process in a clinically translatable acquisition time. Longitudinal changes in [Formula: see text] and f_bw of the ACL graft were observed.

Ultrashort Time-to-Echo Magnetic Resonance Imaging at 3 T for the Detection of Spondylolysis in Cadaveric Spines: Comparison With CT

OBJECTIVES

The objective of this study was to compare the diagnostic performance and confidence of conventional, optimized, and ultrashort time to echo (UTE) magnetic resonance (MR) protocols for detection of simulated lumbar spondylolysis in human cadavers. In addition, we sought to demonstrate the feasibility of the UTE technique in subjects with and without spondylolysis.

MATERIALS AND METHODS

Four human lumbar spine specimens with 46 individual pars interarticularis were randomly left intact (n = 26) or received experimental osteotomy (n = 20) using a microsurgical saw to simulate spondylolysis. The specimens were imaged using a computed tomography (CT) scan along with 3 "Tiers" of MR protocols at 3 T: Tier 1, conventional lumbar MR protocol; Tier 2, optimized conventional protocol consisting of a sagittal oblique spoiled gradient recall echo and axial oblique T1 and short tau inversion recovery sequences; and Tier 3, a sagittal UTE MR sequence. Two blinded readers evaluated the images using a 4-point scale (1 = spondylolysis certainly absent, 2 = probably absent, 3 = probably present, 4 = certainly present) at each individual pars. For each imaging protocol, diagnostic performance (sensitivity, specificity, and area under the receiver operating characteristic curve, using the surgical osteotomy as the reference) and confidence were assessed and compared using the McNemar test. Furthermore, 2 human subjects were imaged with the conventional and UTE MR protocols to demonstrate feasibility in vivo.

RESULTS

Diagnostic performance was moderate for Tiers 1 and 2, with a moderate sensitivity (0.70 to 0.75) and high (1.00) specificity. In contrast, CT and Tier 3 UTE MR imaging had both high sensitivity (1.00) and specificity (1.00). The sensitivities of CT or Tier 3 were statistically greater than Tier 1 sensitivity (P = 0.041) and neared statistical significance when compared with Tier 2 sensitivity (P = 0.074). Area under the receiver operating characteristic curve was also significantly greater for CT and Tier 3 (each area = 1.00), compared with the areas for Tier 1 (0.89, P = 0.037) or Tier 2 (0.873, P = 0.024). Diagnostic confidences of CT or Tier 3 were much greater than other Tiers: Both Tiers 1 and 2 had a large percentage of uncertain (>60%, P < 0.001) or wrong interpretations (>10%, P < 0.001), unlike CT or Tier 3 (0% uncertain or wrong interpretations). Preliminary in vivo UTE images clearly depicted intact and fractured pars.

CONCLUSIONS

Our study demonstrated that the detection of pars fractures using a single sagittal UTE MR sequence is superior in performance and confidence to conventional and optimized MR protocols at 3 T, whereas matching those from CT evaluation. Furthermore, we demonstrated the feasibility of in vivo application of the UTE sequence in subjects with and without spondylolysis.

Effect of Autogenous Bone Marrow Aspirate Treatment on Magnetic Resonance Imaging Integration of Osteochondral Allografts in the Knee: A Matched Comparative Imaging Analysis

PURPOSE

To accurately evaluate the effects of bone marrow aspirate (BMA) augmentation on osteochondral allograft (OCA) integration on early postoperative magnetic resonance imaging (MRI) using the comprehensive Osteochondral Allograft MRI Scoring System (OCAMRISS).

METHODS

This imaging study compared patients who underwent OCA transplantation with and without BMA augmentation for the treatment of focal osteochondral defects in the knee performed by a single surgeon between July 2013 and July 2017. Patients were excluded if they underwent implantation of premade plugs, had an overlapping OCA configuration ("snowman" technique), or did not undergo MRI at 6 months postoperatively. Patients were matched by lesion location, lesion size, age, and body mass index, as well as whether they underwent previous surgical procedures. Data were analyzed using descriptive statistics, Spearman correlation, the independent t test, the Mann-Whitney U test, and the χ² test.

RESULTS

A total of 58 patients (29 per group) were included in this study, with an average age of 36.4 ± 10.1 years and mean body mass index of 28.6 ± 5.1. The mean size of the analyzed OCA plugs was 3.3 ± 1 cm². At an average imaging follow-up of 5.6 ± 1 months, 86.2% of the grafts had achieved osseous integration at the graft-host junction and 75.9% did not show any cystic changes in the subchondral bone. No difference in any OCAMRISS subscale was seen comparing OCAs with and without BMA augmentation (P > .05). Specifically, osseous integration and subchondral cyst formation were comparable between groups (P = .128 and P = .539, respectively).

CONCLUSIONS

OCAs showed excellent osseous integration at the graft-host junction on 6-month postoperative MRI. The treatment of OCAs with autogenous BMA did not result in superior imaging outcomes when analyzed using the OCAMRISS.

LEVEL OF EVIDENCE

Level III, case-control study.

3D printing with MRI in pediatric applications

3D printing (3DP) applications for clinical evaluation, preoperative planning, patient and trainee education, and simulation has increased in the past decade. Most of the applications are found in cardiovascular, head and neck, orthopedic, neurological, urological, and oncological surgical cases. This review has three parts. The first part discusses the technical pathway to realizing a physical model, 3DP considerations in pediatric MRI image acquisition, data and resolution requirements, and related structural segmentation and postprocessing steps needed to generalize both virtual and physical models. Standard practices and processing software used in these processes will be assessed. The second part discusses complementary examples in pediatric applications, including cases from cardiology, neuroradiology, neurology, and neurosurgery, head and neck, orthopedics, pelvic and urological applications, oncological applications, and fetal imaging. The third part explores other 3D printing applications and considerations such as using 3DP to develop tissue-specific phantoms and devices for testing in the MR environment, to educate patients and their families, to train clinicians and students, and facility requirements for building a 3DP program. Level of Evidence: 5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2020;51:1641-1658.

Decreased Graft Thickness Is Associated With Subchondral Cyst Formation After Osteochondral Allograft Transplantation in the Knee

BACKGROUND

Subchondral changes, specifically cyst formation, are a known finding after osteochondral allograft (OCA) transplantation.

PURPOSE/HYPOTHESIS

The purpose was to determine potential predictive associations between preoperative patient characteristics or OCA morphology and postoperative OCA appearance as assessed by the osteochondral allograft magnetic resonance imaging scoring system (OCAMRISS) at 6-month follow-up. It was hypothesized that preoperative patient factors or OCA morphology is associated with postoperative OCAMRISS scores.

STUDY DESIGN

Cross-sectional study; Level of evidence, 3.

METHODS

This study evaluated 74 OCAs that were implanted in the femoral condyles of 63 patients for the treatment of symptomatic osteochondral defects in the knee. Postoperative magnetic resonance imaging was obtained at a mean ± SD follow-up of 5.5 ± 1.0 months. A musculoskeletal radiologist scored all grafts according to the OCAMRISS. Point biserial correlation, Mann-Whitney U test, Fisher exact test, and chi-square test were used to distinguish associations between OCAMRISS subscales and age, sex, smoker status, body mass index, previous surgery, concomitant surgery, bone marrow augmentation, graft location, graft size, and bony graft thickness.

RESULTS

OCA bony thickness showed significant correlation with cystic changes at the graft-host junction (P = .019). Grafts with cystic formation were significantly thinner than grafts without cystic changes (P = .008). The odds ratio for grafts with <5-mm bony thickness demonstrating cystic changes was 4.9 (95% CI, 1.5-16.1; P = .009). Bony graft thickness was not associated with graft integration, but 40% of grafts with a bony thickness >9 mm presented with a residual osseous cleft, as opposed to 11.3% of thinner grafts (P = .1). The augmentation with bone marrow aspirate did not affect osseous graft integration or subchondral cystic formation (P = .375 and P = .458, respectively).

CONCLUSION

Osteochondral allograft thickness is associated with subchondral cyst formation at short-term follow-up. Thin grafts demonstrate a substantially increased risk of developing subchondral cysts at the graft-host junction after OCA transplantation. Conversely, thicker grafts may negatively affect osseous graft integration. Hence, surgeons should be aware of the potential pitfalls of transplanting thin or thick grafts regarding cystic formation and delay of osseous integration after cartilage resurfacing.

Advanced magnetic resonance imaging of cartilage components in haemophilic joints reveals that cartilage hemosiderin correlates with joint deterioration

INTRODUCTION

Evidence suggests that toxic iron is involved in haemophilic joint destruction.

AIM

To determine whether joint iron deposition is linked to clinical and imaging outcomes in order to optimize management of haemophilic joint disease.

METHODS

Adults with haemophilia A or haemophilia B (n = 23, ≥ age 21) of all severities were recruited prospectively to undergo assessment with Hemophilia Joint Health Scores (HJHS), pain scores (visual analogue scale [VAS]) and magnetic resonance imaging (MRI) at 3T using conventional MRI protocols and 4-echo 3D-UTE-Cones sequences for one affected arthropathic joint. MRI was scored blinded by two musculoskeletal radiologists using the International Prophylaxis Study Group (IPSG) MRI scale. Additionally, UTE-T2* values of cartilage were quantified. Correlations between parameters were performed using Spearman rank correlation. Two patients subsequently underwent knee arthroplasty, which permitted linking of histological findings (including Perl's reaction) with MRI results.

RESULTS

MRI scores did not correlate with pain scores or HJHS. Sixteen joints had sufficient cartilage for UTE-T2* analysis. T2* values for cartilage correlated inversely with HJHS (r_s  = -0.81, P < 0.001) and MRI scores (r_s  = -0.52, P = 0.037). This was unexpected since UTE-T2* values decrease with better joint status in patients with osteoarthritis, suggesting that iron was present and responsible for the effects. Histological analysis of cartilage confirmed iron deposition within chondrocytes, associated with low UTE-T2* values.

CONCLUSIONS

Iron accumulation can occur in cartilage (not only in synovium) and shows a clear association with joint health. Cartilage iron is a novel biomarker which, if quantifiable with innovative joint-specific MRI T2* sequences, may guide treatment optimization.

Ultrashort echo time magnetic resonance imaging (UTE-MRI) of cortical bone correlates well with histomorphometric assessment of bone microstructure

Ultrashort echo time magnetic resonance imaging (UTE-MRI) techniques have been increasingly used to assess cortical bone microstructure. High resolution micro computed tomography (μCT) is routinely employed for validating the MRI-based assessments. However, water protons in cortical bone may reside in micropores smaller than the detectable size ranges by μCT. The goal of this study was to evaluate the upper limit of UTE-MRI and compare its efficacy to μCT at determining bone porosity ex vivo. This study investigated the correlations between UTE-MRI based quantifications and histomorphometric measures of bone porosity that cover all pores larger than 1 μm. Anterior tibial midshaft specimens from eleven donors (51 ± 16 years old, 6 males, 5 females) were scanned on a clinical 3 T-MRI using UTE magnetization transfer (UTE-MT, three power levels and five frequency offsets) and UTE-T2* sequences. Two-pool MT modeling and bi-component exponential T2* fitting were performed on the MRI datasets. Specimens were then scanned by μCT at 9 μm voxel size. Histomorphometry was performed on hematoxylin and eosin (H&E) stained slides imaged at submicron resolution. Macromolecular fraction from MT modeling, bi-component T2* fractions, and short component T2* showed strong correlations (R > 0.7, p < 0.01) with histomorphometric total and large-pores (>40 μm) porosities as well as with μCT-based porosity. UTE-MRI could also assess small pores variations with moderate correlations (R > 0.5, p < 0.01). The UTE-MRI techniques can detect variations of bone porosity comprised of pores below the range detectable by μCT. Such fine pore variations can contribute differently to the development of bone diseases or to the bone remodeling process, however, this needs to be investigated. In scanned specimens, major porosity changes were from large pores, therefore the μCT employment was likely adequate to validate UTE-MRI biomarkers.

Ultrashort echo time imaging of the lungs under high-frequency noninvasive ventilation: A new approach to lung imaging

Background

Although ultrashort echo time (UTE) sequences allow excellent assessment of lung parenchyma, image quality remains lower than that of computed tomography (CT).

Purpose

To investigate a high‐frequency noninvasive ventilation (HF‐NIV) technique allowing a stabilized inspiration and to compare image quality with current dedicated MR sequences.

Study Type

Prospective.

Population

Ten healthy volunteers.

Field Strength/Sequence

3D radial UTE sequence at 1.5T.

Assessment

UTE‐HF‐NIV sequence was compared with UTE‐free‐breathing (UTE‐FB), reconstructed at end expiration (UTE‐Exp) and average (UTE‐Avg), and breath‐hold VIBE sequences. The distance from lung apex to the dome of the right hemidiaphragm was measured. Visual assessment of the visibility and sharpness of normal anatomical structures was carried out. Dedicated software also quantitatively evaluated vessel–lung and right lung–liver interface sharpness. Apparent signal ratio (Sr) and contrast ratios (Cr) were quantitatively evaluated.

Statistical Tests

Wilcoxon signed rank test for visual scores, paired t‐test for continuous variables, significance at P < 0.05.

Results

The distance between apex and the right hemidiaphragmatic dome was significantly larger (P < 0.001) with UTE‐HF‐NIV compared with UTE‐FB and VIBE acquisitions. Vessel and airway visibility had identical median visual scores with all UTE methods. Median visual scores for sharpness of vessels and airways were significantly higher (P < 0.001) with HF‐NIV (vessels = 3; airways = 2) than in UTE‐FB (vessels = 2; airways = 1) and VIBE (vessels = 1; airways = 1). Software‐based vessel sharpness evaluation resulted in larger values in 8/10 volunteers with UTE‐HF‐NIV (67.3 ± 9.8) compared with UTE‐Avg (62.3 ± 12.6) but the average difference was not significant (P = 0.28). The sharpness of the lung–liver interface was significantly higher (P < 0.001) with HF‐NIV (17.3 ± 5.3) compared with UTE‐Avg (14.1 ± 3.9). Significantly higher values (P < 0.01) of Sr and Cr were observed with UTE‐HF‐NIV compared with UTE‐FB and VIBE.

Data Conclusion

HF‐NIV allowing acquisition at full inspiration significantly improves image quality for lung imaging. This could offer the option to alternate some follow‐up CT studies by using this technique.

Level of Evidence: 2

Technical Efficacy: Stage 1

J. Magn. Reson. Imaging 2019;50:1789–1797.

Ultrashort echo time magnetic resonance fingerprinting (UTE-MRF) for simultaneous quantification of long and ultrashort T2 tissues

PURPOSE

To demonstrate an ultrashort echo time magnetic resonance fingerprinting (UTE-MRF) method that allows quantifying relaxation times for muscle and bone in the musculoskeletal system and generating bone enhanced images that mimic CT scans.

METHODS

A fast imaging steady-state free precession MRF sequence with half pulse excitation and half projection readout was designed to sample fast T₂ decay signals. Varying echo time (TE) of a sinusoidal pattern was applied to enhance sensitivity for tissues with short and ultrashort T₂ values. The performance of UTE-MRF was evaluated via simulations, phantom, and in vivo experiments.

RESULTS

A minimal TE of 0.05 ms was achieved. Simulations indicated the sinusoidal TE sampling increased T₂ quantification accuracy in the cortical bone and tendon but had little impact on long T₂ muscle quantifications. For the rubber phantom, the averaged relaxometries from UTE-MRF (T₁ = 162 ms and T₂ = 1.07 ms) compared well with the gold standard (T₁ = 190 ms and T 2 ∗ = 1.03 ms). For the long T₂ agarose phantom, the linear regression slope between UTE-MRF and gold standard was 1.07 (R² = 0.991) for T₁ and 1.04 (R² = 0.994) for T₂ . In vivo experiments showed the detection of the cortical bone (averaged T₂ = 1.0 ms) and Achilles tendon (averaged T₂ = 15 ms). Scalp structures from the bone enhanced image show high similarity with CT.

CONCLUSION

The UTE-MRF with sinusoidal TEs can simultaneously quantify T₁ , T₂ , proton density, and B₀ in long, short, even ultrashort T₂ musculoskeletal structures. Bone enhanced images can be achieved in the brain with UTE-MRF.

Detection of maturity and ligament injury using magic angle directional imaging

PURPOSE

To investigate whether magnetic field-related anisotropies of collagen may be correlated with postmortem findings in animal models.

METHODS

Optimized scan planning and new MRI data-processing methods were proposed and analyzed using Monte Carlo simulations. Six caprine and 10 canine knees were scanned at various orientations to the main magnetic field. Image intensities in segmented voxels were used to compute the orientation vectors of the collagen fibers. Vector field and tractography plots were computed. The Alignment Index was defined as a measure of orientation distribution. The knees were subsequently assessed by a specialist orthopedic veterinarian, who gave a pathological diagnosis after having dissected and photographed the joints.

RESULTS

Using 50% less scans than reported previously can lead to robust calculation of fiber orientations in the presence of noise, with much higher accuracy. The 6 caprine knees were found to range from very immature (< 3 months) to very mature (> 3 years). Mature specimens exhibited significantly more aligned collagen fibers in their patella tendons compared with the immature ones. In 2 of the 10 canine knees scanned, partial cranial caudal ligament tears were identified from MRI and subsequently confirmed with encouragingly high consistency of tractography, Alignment Index, and dissection results.

CONCLUSION

This method can be used to detect injury such as partial ligament tears, and to visualize maturity-related changes in the collagen structure of tendons. It can provide the basis for new, noninvasive diagnostic tools in combination with new scanner configurations that allow less-restricted field orientations.

Imaging of the region of the osteochondral junction (OCJ) using a 3D adiabatic inversion recovery prepared ultrashort echo time cones (3D IR-UTE-cones) sequence at 3 T

The purpose of this study is to develop a 3D adiabatic inversion recovery prepared ultrashort echo time Cones (3D IR-UTE-Cones) sequence for high resolution and contrast imaging of the region of osteochondral junction (OCJ) of human knee joint using a clinical 3 T scanner. A feasibility study on direct imaging of the OCJ region was performed on a human patellar cartilage sample and on eight cadaveric knee joints using T₁ -weighted, proton density (PD)-weighted and short-T₂ -weighted 3D IR-UTE-Cones sequences. Contrast to noise ratio was measured to evaluate the effectiveness of the 3D IR-UTE-Cones sequences for selective imaging of the OCJ region. Computed tomography imaging was performed in parallel for the cadaveric knee joints. The optimized T₁ -weighted 3D IR-UTE-Cones sequence was used to image the knee joints of eight healthy volunteers and six patients with osteoarthritis (OA) to evaluate morphological changes in the OCJ region. Clinical PD- and T₂ -weighted FSE sequences were also performed for comparison. The T₁ -weighted 3D IR-UTE-Cones sequence showed high resolution and contrast bright band of the normal OCJ region in the cadaveric joints. Normal OCJ appearances were also seen in healthy volunteers. Abnormal OCJ regions, manifested as ill-defined, focal loss or non-visualization of the high intensity band adjacent to the subchondral bone plate, were observed in the knee joints of both ex vivo and in vivo OA patients. The 3D IR-UTE-Cones sequence can image OCJ regions ex vivo and in vivo, with abnormalities depicted with high resolution and contrast. The technique may be useful for demonstrating involvement of OCJ regions in early OA.

T2 mapping of cartilage and menisci at 3T in healthy subjects with knee malalignment: initial experience

PURPOSE

To assess the relationship between knee alignment and T2 values of femorotibial cartilage and menisci in healthy subjects at 3 T.

MATERIALS AND METHODS

Thirty-six healthy subjects divided into three subgroups of 12 neutral, 12 varus, and 12 valgus alignment of the femorotibial joint were investigated on 3-T MR scanner using a 2D multi-echo turbo spin-echo (TSE) sequence for T2 mapping. Wilcoxon signed-rank test and analysis of covariance (ANCOVA) were performed to determine any statistically significant differences in subregional T2 values of femorotibial cartilage and menisci among the three subgroups of healthy subjects.

RESULTS

Lateral femoral anterior cartilage subregion (52 ± 3 ms, mean ± standard deviation; 53 ± 2 ms) had significantly higher T2 values (p < 0.05) than medial femoral anterior cartilage subregion (51 ± 2 ms; 51 ± 2 ms) in varus and valgus groups, respectively. There were statistically significant differences (p < 0.05) in T2 values of tibial central cartilage subregion between lateral and medical compartment among varus, valgus, and neutral subgroups. Lateral body segment of meniscus (41 ± 3 ms) had significantly higher (p < 0.05) T2 values than medial body segment (40 ± 2 ms) in the varus subgroup.

CONCLUSIONS

Some degree of correlation between knee alignment and subregional T2 values of femorotibial cartilage and menisci exists in healthy subjects. These findings indicate that T2 mapping may be sensitive in assessing the load distribution pattern of human cartilage and menisci with knee alignment abnormality, which may be used as reference baseline when understanding the occurrence and progression of knee osteoarthritis.

Quantitative MRI Musculoskeletal Techniques: An Update

OBJECTIVE. For many years, MRI of the musculoskeletal system has relied mostly on conventional sequences with qualitative analysis. More recently, using quantitative MRI applications to complement qualitative imaging has gained increasing interest in the MRI community, providing more detailed physiologic or anatomic information. CONCLUSION. In this article, we review the current state of quantitative MRI, technical and software advances, and the most relevant clinical and research musculoskeletal applications of quantitative MRI.

Radiofrequency phase encoded half-pulses in simultaneous multislice ultrashort echo time imaging

PURPOSE

To describe a simultaneous multislice (SMS) ultrashort echo time (UTE) imaging method using radiofrequency phase encoded half-pulses in combination with power independent of number of slices (PINS) inversion recovery (IR) pulses to generate multiple-slice images with short T₂ * contrasts in less than 3 min with close to an eightfold acceleration compared with a standard 2D approach.

THEORY AND METHODS

Radiofrequency phase encoding is applied in an SMS (N_SMS = 4) excitation scheme using "sinc" half-pulses. With the use of coil sensitivity encoding (SENSE) and controlled aliasing in parallel imaging (CAIPI) in combination with a gradient echo 2D spiral readout trajectory and IR PINS pulses for contrast enhancement a fast UTE sequence is developed. Images are obtained using a model-based reconstruction method. Sequence details and performance tests on phantoms as well as the heads of healthy volunteers at 3T are presented.

RESULTS

An SMS UTE sequence with an undersampling factor of 4 is developed using radiofrequency phase encoded half-pulses and PINS IR pulses which enables the acquisition of 8 slices at 0.86² mm² resolution in less than 3-min scan time. UTE images of the head are obtained showing highlighted signal of cortical bone. Image quality and T₂ contrast are comparable to the one obtained by corresponding single slice acquisitions with only minor deviations.

CONCLUSIONS

The method combining radiofrequency phase encoded SMS half-pulses and PINS IR pulses presents a suitable approach to SMS UTE imaging. The usage of coil sensitivity information and increased sampling density by means of interleaved slice group acquisitions allows to reduce the total scan time by a factor close to 8.

Validity and reliability of measurements of aponeurosis dimensions from magnetic resonance images

Muscle performance is closely related to the structure and function of tendons and aponeuroses, the sheet-like, intramuscular parts of tendons. The architecture of aponeuroses has been difficult to study with magnetic resonance imaging (MRI) because these thin, collagen-rich connective tissues have very short transverse relaxation (T2) times and therefore provide a weak signal with conventional MRI sequences. Here, we validated measurements of aponeurosis dimensions from two MRI sequences commonly used in muscle-tendon research (mDixon and T1-weighted images), and an ultrashort echo time (UTE) sequence designed for imaging tissues with short T2 times. MRI-based measurements of aponeurosis width, length, and area of 20 sheep leg muscles were compared to direct measurements made with three-dimensional (3D) quantitative microdissection. The errors in measurement of aponeurosis width relative to the mean width were 1.8% for UTE, 3.7% for T1, and 18.8% for mDixon. For aponeurosis length, the errors were 7.6% for UTE, 1.9% for T1, and 21.0% for mDixon. Measurements from T1 and UTE scans were unbiased, but mDixon scans systematically underestimated widths, lengths, and areas of the aponeuroses. Using the same methods, we then found high inter-rater reliability (intraclass correlation coefficients >0.92 for all measures) of measurements of the dimensions of the central aponeurosis of the human tibialis anterior muscle from T1-weighted scans. We conclude that valid and reliable measurements of aponeurosis dimensions can be obtained from UTE and from T1-weighted scans. When the goal is to study the macroscopic architecture of aponeuroses, UTE does not hold an advantage over T1-weighted imaging.

Collagen proton fraction from ultrashort echo time magnetization transfer (UTE-MT) MRI modelling correlates significantly with cortical bone porosity measured with micro-computed tomography (μCT)

Intracortical bone porosity is a key microstructural parameter that determines bone mechanical properties. While clinical MRI visualizes the cortical bone with a signal void, ultrashort echo time (UTE) MRI can acquire high signal from cortical bone, thus enabling quantitative assessments. Magnetization transfer (MT) imaging combined with UTE-MRI can indirectly assess protons in the bone collagenous matrix, which are inversely related to porosity. This study aimed to examine UTE-MT MRI techniques to evaluate intracortical bone porosity. Eighteen human cortical bone specimens from the tibial and fibular midshafts were scanned using UTE-MT sequences on a clinical 3 T MRI scanner and on a high-resolution micro-computed tomography (μCT) scanner. A series of MT pulse saturation powers (500°, 1000°, 1500°) and frequency offsets (2, 5, 10, 20, 50 kHz) were used to measure the macromolecular fraction (MMF) and macromolecular T2 (T2MM ) using a two-pool MT model. The measurements were made on 136 different regions of interest (ROIs). ROIs were selected at three cortical bone layers (from endosteum to periosteum) and four anatomical sites (anterior, mid-medial, mid-lateral, and posterior) to provide a wide range of porosity. MMF showed moderate to strong correlations with intracortical bone porosity (R = -0.67 to -0.73, p < 0.01) and bone mineral density (BMD) (R = +0.46 to +0.70, p < 0.01). Comparing the average MMF between cortical bone layers revealed a significant increase from the endosteum towards the periosteum. Such a pattern was in agreement with porosity reduction and BMD increase towards the periosteum. These results suggest that the two-pool UTE-MT technique can potentially serve as a novel and accurate tool to assess intracortical bone porosity.

3D Cones Acquisition of Human Extremity Imaging Using a 1.5T Superconducting Magnet and an Unshielded Gradient Coil Set

We developed ultrashort echo-time (UTE) imaging sequences with 3D Cones trajectories for a home-built compact MRI system using a 1.5T superconducting magnet and an unshielded gradient coil set. We achieved less than 7 min imaging time and obtained clear in vivo images of a human forearm with a TE of 0.4 ms. We concluded that UTE imaging using 3D Cones acquisition was successfully implemented in our 1.5T MRI system.

Conical ultrashort echo time (UTE) MRI in the evaluation of pediatric acute appendicitis

PURPOSE

Magnetic resonance imaging (MRI) sequences with conical k-space trajectories are able to decrease motion artifacts while achieving ultrashort echo times (UTE). We assessed the performance of free-breathing conical UTE MRI in the evaluation of the pediatric pelvis for suspected appendicitis.

METHODS

Our retrospective review of 84 pediatric patients who underwent MRI for suspected appendicitis compared three contrast-enhanced sequences: free-breathing conical UTE, breath-hold three-dimensional (3D) spoiled gradient echo (BH-SPGR), and free-breathing high-resolution 3D SPGR (FB-SPGR). Two radiologists performed blinded and independent evaluations of each sequence for image quality (four point scale), anatomic delineation (four point scale), and diagnostic confidence (five point scale). Subsequently, the three sequences were directly compared for overall image quality (- 3 to + 3 scale). Scores were compared using Kruskal-Wallis and Wilcoxon signed-rank tests.

RESULTS

UTE demonstrated significantly better perceived signal-to-noise ratio (SNR) and fewer artifacts than BH-SPGR and FB-SPGR (means of 3.6 and 3.4, 3.4 and 3.2, 3.1 and 2.7, respectively; p < 0.0006). BH-SPGR and FB-SPGR demonstrated significantly better contrast than UTE (means of 3.6, 3.4, and 3.2, respectively; p < 0.03). In the remaining categories, UTE performed significantly better than FB-SPGR (p < 0.00001), while there was no statistical difference between UTE and BH-SPGR. Direct paired comparisons of overall image quality demonstrated the readers significantly preferred UTE over both BH-SPGR (mean + 0.5, p < 0.00001) and FB-SPGR (mean + 1.2, p < 0.00001).

CONCLUSIONS

In the evaluation of suspected appendicitis, free-breathing conical UTE MRI performed better in the assessed metrics than FB-SPGR. When compared to BH-SPGR, UTE demonstrated superior perceived SNR and fewer artifacts.

Comparison of T2* mapping between regular echo time and ultrashort echo time with 3D cones at 3 tesla for knee meniscus

The objectives of this study were to compare the ultrashort T2* relaxation time with the T2* relaxation time using the 3 dimensional (3D) cones sequence in 3 groups of patients with normal, degenerated, and torn knee menisci, and to demonstrate the additional effect of the ultrashort echo time (UTE) signal intensity.Following institutional review board approval, 42 knee magnetic resonance imaging (MRI) scans of 42 patients who presented with knee pain and underwent knee MRIs, with the 3D Cones of UTE sequence (minimum TEs: 32 μs) and a 3T MRI scanner (Discovery 750, GE Healthcare, Waukesha, WI), were analyzed. The enrolled patients were classified into 3 subgroups:normal meniscus on conventional MRI, with no positive meniscus-related physical examination in medical records;meniscal degeneration with signal changes on conventional MRI; andmeniscal tear.For the quantitative assessment, the mean values inside user-drawn regions of interest (ROIs) of the medial menisci were drawn on UTE T2* map and T2* map. For statistical analyses, 1-way analysis of variance (ANOVA) with post-hoc analysis using the Tukey HSD test was conducted to compare groups, and effect size was used to compare the discrimination power.The ultrashort T2* relaxation times were higher in patients with meniscal tear than in those with normal and degeneration groups (P <.05, respectively) whereas T2* relaxation times were not statistically significantly different. The ultrashort T2* relaxation times showed higher effect sizes than the T2* times between tear and normal/degeneration.The ultrashort T2* relaxation times showed better delineation of meniscal degeneration or tears than T2* relaxation times. The ultrashort T2* relaxation times could be more sensitive at differentiating between normal and pathologic meniscal conditions in patients.

Detecting stress injury (fatigue fracture) in fibular cortical bone using quantitative ultrashort echo time-magnetization transfer (UTE-MT): An ex vivo study

Bone stress injury (BSI) incidents have been increasing amongst athletes in recent years as a result of more intense sporting activities. Cortical bone in the tibia and fibula is one of the most common BSI sites. Nowadays, clinical magnetic resonance imaging (MRI) is the recommended technique for BSI diagnosis at an early stage. However, clinical MRI focuses on edema observations in surrounding soft tissues, rather than the injured components of the bone. Specifically, both normal and injured bone are invisible in conventional clinical MRI. In contrast, ultrashort echo time (UTE)-MRI is able to detect the rapidly decaying signal from the bone. This study aimed to employ UTE-MRI for fatigue fracture detection in fibula cortical bone through an ex vivo investigation. Fourteen human fibular samples (47 ± 20 years old, four women) were subjected to cyclic loading on a four-point bending setup. The loading was displacement controlled to induce -5000 ± 1500 μ-strain at 4 Hz. Loading was stopped when bone stiffness was reduced by 20%. Fibula samples were imaged twice, using UTE-MRI and micro-computed tomography (μCT), first pre-loading and second post-loading. After loading, the macromolecular fraction (MMF) from UTE-MT modeling demonstrated a significant decrease (12% ± 20%, P = 0.02) on average. Single-component T2 * also decreased significantly by BSI (12% ± 11%, P = 0.01) on average. MMF reduction is hypothesized to be a result of collagenous matrix rupture and water increase. However, faster T2 * decay might be a result of water shifts towards newly developed microcracks with higher susceptibility. Despite this good sensitivity level of the UTE-MRI technique, the μCT-based porosity at a voxel size of 9 μm was not affected by loading. UTE-MRI shows promise as a new quantitative technique to detect BSI.

Ultrashort time-to-echo quantitative magnetic resonance imaging of the triangular fibrocartilage: differences in position

PURPOSE

To compare T2* values of the triangular fibrocartilage (TFC) obtained by ultrashort time-to-echo (UTE) techniques at the neutral position, ulnar flexion of the wrist, and pronation of the forearm.

MATERIALS AND METHODS

MR imaging was performed in ten healthy volunteers with a 3-T MR system by using an eight-channel knee coil. Coronal wrist T2* maps from three-dimensional cone UTE pulse sequences were obtained at the neutral, ulnar flexion, and pronation positions (TR: 19 ms, TE: 0.032 ms/4 ms/8 ms/12 ms, FOV: 18 cm, matrix: 430 × 430, section thickness: 1.5 mm, scan time: 8 min 31 s). UTE-T2* maps were calculated on a pixel-by-pixel basis for all structures of the wrist visualized in the coronal planes. The entire region of interest (ROI) for TFC was manually delineated, and the average T2* value was calculated for each ROI by three radiologists. The Kruskal-Wallis test, Wilcoxon signed-rank test, or intraclass correlation coefficients (ICC) were used for statistics.

RESULTS

The difference in the average T2* value among the three groups according to the forearm/wrist position was significant (p < 0.001). The T2* value of the TFC at pronation (mean ± 2 SD: 7.92 ± 1.37 ms) was significantly lower than those at the neutral (10.08 ± 1.90 ms) and ulnar flexion positions (9.15 ± 1.03 ms) (p < 0.017). The ICC showed a substantial interobserver agreement in the T2* value measurements of the TFC (ICC = 0.986).

CONCLUSION

T2* relaxation time measurement of the TFC using UTE may be useful for assessing the loading effect by the forearm/wrist position.

KEY POINTS

• The T2* value of the TFC may reflect the biomechanics of the wrist joint. • Acute loading at pronation results in a decrease in the T2* value of the TFC. • Quantitative wrist UTE MRI was successfully performed in vivo.

Feasibility of quantitative ultrashort echo time (UTE)-based methods for MRI of peripheral nerve

Peripheral nerves are a composite tissue consisting of neurovascular elements packaged within a well-organized extracellular matrix. Their composition, size, and anatomy render nerves a challenging medical imaging target. In contrast to morphological MRI, which represents the predominant approach to nerve imaging, quantitative MRI sequences can provide information regarding tissue composition. Here, we applied standard clinical Carr-Purcell-Meiboom-Gill (CPMG) and experimental three-dimensional (3D) ultrashort echo time (UTE) Cones sequences for quantitative nerve imaging including T₂ measurement with single-component analysis, T₂ * measurement with single-component and bi-component analyses, and magnetization transfer ratio (MTR) analysis. We demonstrated the feasibility and the high quality of single-component T₂ *, bi-component T₂ *, and MTR approaches to analyze nerves imaged with clinically deployed 3D UTE Cones pulse sequences. For 24 single fascicles from eight nerves, we measured a mean single-component T₂ * of 22.6 ±8.9 ms, and a short T₂ * component (STC) with a mean T₂ * of 1.7 ±1.0 ms and a mean fraction of (6.74 ±4.31)% in bi-component analysis. For eight whole nerves, we measured a mean single-component T₂ * of 16.7 ±2.2 ms, and an STC with a mean T₂ * of 3.0 ±1.0 ms and a mean fraction of (15.56 ±7.07)% in bi-component analysis. For nine fascicles from three healthy nerves, we measured a mean MTR of (25.2 ±1.9)% for single fascicles and a mean MTR of (23.6 ±0.9)% for whole nerves. No statistically significant correlation was observed between any MRI parameter and routine histological outcomes, perhaps due to the small sample size and lack of apparent sample pathology. Overall, we have successfully demonstrated the feasibility of measuring quantitative MR outcomes ex vivo, which might reflect features of nerve structure and macromolecular content. These methods should be validated comprehensively on a larger and more diverse set of nerve samples, towards the interpretation of in vivo outcomes. These approaches have new and broad implications for the management of nerve disease, injury, and repair.

Understanding Magnetic Resonance Imaging of Knee Cartilage Repair: A Focus on Clinical Relevance

The aims of this review article are (a) to describe the principles of morphologic and compositional magnetic resonance imaging (MRI) techniques relevant for the imaging of knee cartilage repair surgery and their application to longitudinal studies and (b) to illustrate the clinical relevance of pre- and postsurgical MRI with correlation to intraoperative images. First, MRI sequences that can be applied for imaging of cartilage repair tissue in the knee are described, focusing on comparison of 2D and 3D fast spin echo and gradient recalled echo sequences. Imaging features of cartilage repair tissue are then discussed, including conventional (morphologic) MRI and compositional MRI techniques. More specifically, imaging techniques for specific cartilage repair surgery techniques as described above, as well as MRI-based semiquantitative scoring systems for the knee cartilage repair tissue-MR Observation of Cartilage Repair Tissue and Cartilage Repair OA Knee Score-are explained. Then, currently available surgical techniques are reviewed, including marrow stimulation, osteochondral autograft, osteochondral allograft, particulate cartilage allograft, autologous chondrocyte implantation, and others. Finally, ongoing research efforts and future direction of cartilage repair tissue imaging are discussed.

Feasibility of using an inversion-recovery ultrashort echo time (UTE) sequence for quantification of glenoid bone loss

OBJECTIVE

To utilize the 3D inversion recovery prepared ultrashort echo time with cones readout (IR-UTE-Cones) MRI technique for direct imaging of lamellar bone with comparison to the gold standard of computed tomography (CT).

MATERIALS AND METHODS

CT and MRI was performed on 11 shoulder specimens and three patients. Five specimens had imaging performed before and after glenoid fracture (osteotomy). 2D and 3D volume-rendered CT images were reconstructed and conventional T1-weighted and 3D IR-UTE-Cones MRI techniques were performed. Glenoid widths and defects were independently measured by two readers using the circle method. Measurements were compared with those made from 3D CT datasets. Paired-sample Student's t tests and intraclass correlation coefficients were performed. In addition, 2D CT and 3D IR-UTE-Cones MRI datasets were linearly registered, digitally overlaid, and compared in consensus by these two readers.

RESULTS

Compared with the reference standard (3D CT), glenoid bone diameter measurements made on 2D CT and 3D IR-UTE-Cones were not significantly different for either reader, whereas T1-weighted images underestimated the diameter (mean difference of 0.18 cm, p = 0.003 and 0.16 cm, p = 0.022 for readers 1 and 2, respectively). However, mean margin of error for measuring glenoid bone loss was small for all modalities (range, 1.46-3.92%). All measured ICCs were near perfect. Digitally registered 2D CT and 3D IR-UTE-Cones MRI datasets yielded essentially perfect congruity between the two modalities.

CONCLUSIONS

The 3D IR-UTE-Cones MRI technique selectively visualizes lamellar bone, produces similar contrast to 2D CT imaging, and compares favorably to measurements made using 2D and 3D CT.

Nonoperative Management of a Severe Proximal Rectus Femoris Musculotendinous Injury in a Recreational Athlete: A Case Report

This report describes a severe injury to the proximal rectus femoris (RF) muscle in a 37-year-old recreational athlete. This injury is a relatively rare occurrence in both the general and elite athletic populations. Acute and long-term imaging and functional outcomes are described. This athlete was able to return to full activity without surgical intervention. Follow-up imaging demonstrated gross healing of both complete (or near complete) muscle and tendon tears. LEVEL OF EVIDENCE: V.