3D-MRI versus 3D-CT in the evaluation of osseous anatomy in femoroacetabular impingement using Dixon 3D FLASH sequence

Deep learning and conventional hip MRI for the detection of labral and cartilage abnormalities using arthroscopy as standard of reference

OBJECTIVES

To evaluate the performance of high-resolution deep learning-based hip MR imaging (CSAI) compared to standard-resolution compressed sense (CS) sequences using hip arthroscopy as standard of reference.

METHODS

Thirty-two patients (mean age, 37.5 years (± 11.7), 24 men) with femoroacetabular impingement syndrome underwent 3-T MR imaging prior to hip arthroscopy. Coronal and sagittal intermediate-weighted TSE sequences with fat saturation were obtained using CS (0.6 × 0.8 mm) and high-resolution CSAI (0.3 × 0.4 mm), with 3 mm slice thickness and similar acquisition times (3:55-4:12 min). MR scans were independently assessed by three radiologists and a hip arthroscopy specialist for labral and cartilage abnormalities. Sensitivity, specificity, and accuracy were calculated using arthroscopy as reference standard. Statistical comparisons between CS and CSAI were performed using McNemar's test.

RESULTS

Labral abnormality detection showed excellent sensitivity for radiologists (CS and CSAI: 97-100%) and the surgeon (CS: 81%, CSAI: 90%, p = 0.08), with 100% specificity. Overall cartilage lesion sensitivity was significantly higher with CSAI versus CS (42% vs. 37%, p < 0.001). Highest sensitivity was observed in superolateral acetabular cartilage (CS: 81%, CSAI: 88%, p < 0.001), while highest specificity was found for the anteroinferior acetabular cartilage (CS and CSAI: 99%). Sensitivity was lowest for the assessment of the anteroinferior and posterior acetabular zones, and inferior and posterior femoral zones (CS and CSAI < 6%).

CONCLUSION

CS and CSAI MR imaging showed excellent diagnostic performance for labral abnormalities. Despite CSAI's improved cartilage lesion detection, overall diagnostic performance for cartilage assessment remained suboptimal.

KEY POINTS

Question Accurate preoperative detection of labral and cartilage lesions in femoroacetabular impingement remains challenging, with current MRI protocols showing variable diagnostic performance. Findings High-resolution deep learning-based and standard-resolution compressed sense MRI demonstrate comparable diagnostic performance, with high accuracy for labral defects but limited sensitivity for cartilage lesions. Clinical relevance Current MRI protocols, regardless of resolution optimization, show persistent limitations in cartilage evaluation, indicating the need for further technical advancement to improve diagnostic confidence in presurgical planning.

Radiomics features outperform standard radiological measurements in detecting femoroacetabular impingement on three-dimensional magnetic resonance imaging

Femoroacetabular impingement (FAI) is a cause of hip pain and can lead to hip osteoarthritis. Radiological measurements obtained from radiographs or magnetic resonance imaging (MRI) are normally used for FAI diagnosis, but they require time-consuming manual interaction, which limits accuracy and reproducibility. This study compares standard radiologic measurements against radiomics features automatically extracted from MRI for the identification of FAI patients versus healthy subjects. Three-dimensional Dixon MRI of the pelvis were retrospectively collected for 10 patients with confirmed FAI and acquired for 10 healthy subjects. The femur and acetabulum were segmented bilaterally and associated radiomics features were extracted from the four MRI contrasts of the Dixon sequence (water-only, fat-only, in-phase, and out-of-phase). A radiologist collected 21 radiological measurements typically used in FAI. The Gini importance was used to define 9 subsets with the most predictive radiomics features and one subset for the most diagnostically relevant radiological measurements. For each subset, 100 Random Forest machine learning models were trained with different data splits and fivefold cross-validation to classify healthy subjects versus FAI patients. The average performance among the 100 models was computed for each subset and compared against the performance of the radiological measurements. One model trained using the radiomics features datasets yielded 100% accuracy in the detection of FAI, whereas all other radiomics features exceeded 80% accuracy. Radiological measurements yielded 74% accuracy, consistent with previous work. The results of this preliminary work highlight for the first time the potential of radiomics for fully automated FAI diagnosis.

3.0T magnetic resonance imaging-based hip bone models for femoroacetabular impingement syndrome are equivalent to computed tomography-based models

This study aimed to compare three-dimensional (3D) proximal femoral and acetabular surface models generated from 3.0T magnetic resonance imaging (MRI) to the clinical gold standard of computed tomography (CT). Ten intact fresh-frozen cadaveric hips underwent CT and 3.0T MRI scans. The CT- and MRI-based segmented models were superimposed using a validated 3D-3D registration volume-merge method to compare them. The least surface-to-surface distance between the models was calculated by a point-to-surface calculation algorithm using a custom-written program. The variables of interest were the signed and absolute surface-to-surface distance between the paired bone models. One-sample t-tests were performed using a signed and absolute test value of 0.16 mm and 0.37 mm, respectively, based on a previous study that validated 1.5T MRI bone models by comparison with CT bone models. For the femur, the average signed and absolute surface-to-surface distance was 0.18 ± 0.09 mm and 0.30 ± 0.06 mm, respectively. There was no difference in the signed surface-to-surface distance and the 0.16 mm test value (t = 0.650, p = 0.532). However, the absolute surface-to-surface difference was less than the 0.37 mm test value (t = -4.025, p = 0.003). For the acetabulum, the average signed and absolute surface-to-surface distance was -0.06 ± 0.06 mm and 0.26 ± 0.04 mm, respectively. The signed (t = -12.569, p < 0.001) and absolute (t = -8.688, p < 0.001) surface-to-surface difference were less than the 0.16 mm and 0.37 mm test values, respectively. Our data shows that 3.0T MRI bone models are more similar to CT bone models than previously validated 1.5T MRI bone models. This is likely due to the higher resolution of the 3T data.

Fast field echo resembling CT using restricted echo-spacing (FRACTURE) MR sequence can provide craniocervical region images comparable to a CT in dogs

Magnetic resonance imaging (MRI) is essential for evaluating cerebellar compression in patients with craniocervical junction abnormalities (CJA). However, it is limited in depicting cortical bone because of its short T2 relaxation times, low proton density, and organized structure. Fast field echo resembling a computed tomography (CT) scan using restricted echo-spacing (FRACTURE) MRI, is a new technique that offers CT-like bone contrast without radiation. This study aimed to assess the feasibility of using FRACTURE MRI for craniocervical junction (CCJ) assessment compared with CT and conventional MRI, potentially reducing the need for multiple scans and radiation exposure, and simplifying procedures in veterinary medicine. CT and MRI of the CCJ were obtained from five healthy beagles. MRI was performed using three-dimensional (3D) T1-weighted, T2-weighted, proton density-weighted (PDW), single echo-FRACTURE (sFRACTURE), and multiple echo-FRACTURE (mFRACTURE) sequences. For qualitative assessment, cortical delineation, trabecular bone visibility, joint space visibility, vertebral canal definition, overall quality, and artifacts were evaluated for each sequence. The geometrical accuracy, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) were quantified. Both sFRACTURE and CT images provided significantly higher scores for cortical delineation and trabecular bone visibility than conventional MRI. Joint space visibility and vertebral canal definition were similar to those observed on CT images, regardless of the MR sequence. In the quantitative assessment, the distances measured on T2-weighted images differed significantly from those measured on CT. There were no significant differences between the distances taken using T1-weighted, PD-weighted, sFRACTURE, mFRACTURE and those taken using CT. T1-weighted and sFRACTURE had a higher SNR for trabecular bone than CT. The CNR between the cortical bone and muscle was high on CT and FRACTURE images. However, the CNR between the cortical and trabecular bones was low in mFRACTURE. Similar to CT, FRACTURE sequences showed higher cortical delineation and trabecular bone visibility than T2-weighted, T1-weighted, and PDW CCJ sequences. In particular, sFRACTURE provided a high signal-to-noise ratio (SNR) of the trabecular bone and a high CNR between the cortical bone and muscle and between the cortical and trabecular bones. FRACTURE sequences can complement conventional MR sequences for bone assessment of the CCJ in dogs.

The impact of data augmentation and transfer learning on the performance of deep learning models for the segmentation of the hip on 3D magnetic resonance images

Different pathologies of the hip are characterized by the abnormal shape of the bony structures of the joint, namely the femur and the acetabulum. Three-dimensional (3D) models of the hip can be used for diagnosis, biomechanical simulation, and planning of surgical treatments. These models can be generated by building 3D surfaces of the joint's structures segmented on magnetic resonance (MR) images. Deep learning can avoid time-consuming manual segmentations, but its performance depends on the amount and quality of the available training data. Data augmentation and transfer learning are two approaches used when there is only a limited number of datasets. In particular, data augmentation can be used to artificially increase the size and diversity of the training datasets, whereas transfer learning can be used to build the desired model on top of a model previously trained with similar data. This study investigates the effect of data augmentation and transfer learning on the performance of deep learning for the automatic segmentation of the femur and acetabulum on 3D MR images of patients diagnosed with femoroacetabular impingement. Transfer learning was applied starting from a model trained for the segmentation of the bony structures of the shoulder joint, which bears some resemblance to the hip joint. Our results suggest that data augmentation is more effective than transfer learning, yielding a Dice similarity coefficient compared to ground-truth manual segmentations of 0.84 and 0.89 for the acetabulum and femur, respectively, whereas the Dice coefficient was 0.78 and 0.88 for the model based on transfer learning. The Accuracy for the two anatomical regions was 0.95 and 0.97 when using data augmentation, and 0.87 and 0.96 when using transfer learning. Data augmentation can improve the performance of deep learning models by increasing the diversity of the training dataset and making the models more robust to noise and variations in image quality. The proposed segmentation model could be combined with radiomic analysis for the automatic evaluation of hip pathologies.

Systematic review of computed tomography parameters used for the assessment of subchondral bone in osteoarthritis

OBJECTIVE

To systematically review the published parameters for the assessment of subchondral bone in human osteoarthritis (OA) using computed tomography (CT) and gain an overview of current practices and standards.

DESIGN

A literature search of Medline, Embase and Cochrane Library databases was performed with search strategies tailored to each database (search from 2010 to January 2023). The search results were screened independently by two reviewers against pre-determined inclusion and exclusion criteria. Studies were deemed eligible if conducted in vivo/ex vivo in human adults (>18 years) using any type of CT to assess subchondral bone in OA. Extracted data from eligible studies were compiled in a qualitative summary and formal narrative synthesis.

RESULTS

This analysis included 202 studies. Four groups of CT modalities were identified to have been used for subchondral bone assessment in OA across nine anatomical locations. Subchondral bone parameters measuring similar features of OA were combined in six categories: (i) microstructure, (ii) bone adaptation, (iii) gross morphology (iv) mineralisation, (v) joint space, and (vi) mechanical properties.

CONCLUSIONS

Clinically meaningful parameter categories were identified as well as categories with the potential to become relevant in the clinical field. Furthermore, we stress the importance of quantification of parameters to improve their sensitivity and reliability for the evaluation of OA disease progression and the need for standardised measurement methods to improve their clinical value.

Differences in Kinematic Changes From Self-Selected to Fast Speed Gait in Asymptomatic Adults With Radiological Signs of Femoro-Acetabular Impingement

Femoro-acetabular impingement (FAI) may present as alterations in the skeletal morphology of the hip. Repercussions of FAI can be witnessed in self-selected speed walking as well as physical exercise such as running or fast speed walking. The aim of this study was to investigate changes in kinematics at different gait speeds in subjects presenting with radiological findings invoking FAI. One hundred thirty asymptomatic adults underwent biplanar X-rays with a calculation of 3D hip parameters: acetabular anteversion, abduction and tilt, vertical center edge angle (VCE), femoral anteversion, neck-shaft angle, acetabular coverage of the femoral head, femoral head diameter and neck length. Parameters were classified according to FAI clinical thresholds. Two groups were created: Control group (63 subjects having up to one subnormal hip parameter in favour of FAI) and Radiographic FAI group (67 subjects having ≥2 subnormal hip parameters that might cause FAI). All subjects underwent 3D gait analysis at self-selected and fast speed, from which kinematic parameters were generated. Arithmetic differences between fast and self-selected speed gait were considered as gait changes. Subjects in the Radiographic FAI group had decreased acetabular tilt (24 vs. 19˚), anteversion (19 vs. 16˚), abduction (55 vs. 53˚), femoral anteversion (18 vs. 14˚) and increased VCE (29 vs. 33˚, all p<0.05), compared to controls. Changes from self-selected to fast speed showed that subjects in the Radiographic FAI group had lower range of motion (ROM) pelvic rotation (7 vs. 4˚) and ROM hip flexion/extension (10 vs. 7˚), reduced hip extension (-4 vs. -2˚) and step length (16 vs. 13 cm; all p<0.05). The Radiographic FAI group had decreased acetabular abduction, anteversion and femoral anteversion in favour of FAI. When adapting from self-selected to fast speed gait, the Radiographic FAI group seemed to limit pelvic rotation and hip flexion/extension resulting in a decrease in step length. These kinematic limitations were previously reported in subjects with symptomatic FAI. Gait analysis could be considered as a functional diagnostic tool to assess FAI along with radiological assessment.

Three-Dimensional CT and 3D MRI of Hip- Important Aids to Hip Preservation Surgery

Common hip internal derangements include femoroacetabular impingement (FAI), developmental dysplasia of hip (DDH) dysplasia, and avascular necrosis (AVN) of the femoral head. These are initially screened by radiographs. For preoperative planning of hip preservation, 3-dimensional (3D) CT is commonly performed to assess bony anatomy and its alterations. Magnetic resonance imaging (MRI) is used to evaluate labrum, hyaline cartilage, tendons, synovium, and loose bodies, and provides vital information for surgical decision-making. However, conventional 2D MRI techniques are limited by lack of isotropic multiplanar reconstructions and partial volume artifacts. With advancements in hardware and software, novel isotropic 3D MR Proton Density images are acquired with acceptable acquisition times leading to improved visualization of soft tissue and osseous structures for various hip conditions. Three-Dimensional MRI allows multiplanar non-gap reconstructions along the structures of interest. It results in detection of small, otherwise inconspicuous labral tears without the need for MR arthrogram, which can be subsequently measured. In addition, radial reconstructions of the femoral head can be performed from original 3D volume MR imaging and CT imaging without the need for individual different plane acquisitions. Three-Dimensional MRI thus impacts surgical decision-making for the important common hip derangement conditions. For example, femoral head hyaline cartilage loss may make hip preservation difficult or impossible. In this review, we discuss the advantages and technical details of 3D CT and MRI and their significant role in aiding hip preservation surgery for common hip conditions. The conditions discussed in this article include FAI, DDH, AVN, synovial disorders, cartilaginous tumors, and hip fractures.

[Preoperative MR imaging for hip dysplasia : Assessment of associated deformities and intraarticular pathologies]

BACKGROUND

Developmental dysplasia of the hip (DDH) is a known reason for hip pain for adolescents and young adults. Preoperative imaging is increasingly recognized as an important factor due to the recent advances in MR imaging.

OBJECTIVES

The aim of this article is to give an overview of preoperative imaging for DDH. The acetabular version and morphology, associated femoral deformities (cam deformity, valgus and femoral antetorsion) and intraarticular pathologies (labrum and cartilage damage) and cartilage mapping are described.

METHODS

After an initial evaluation with AP radiographs, CT or MRI represent the methods of choice for the preoperative evaluation of the acetabular morphology and cam deformity, and for the measurement of femoral torsion. Different measurement techniques and normal values should be considered, especially for patients with increased femoral antetorsion because this could lead to misinterpretation and misdiagnosis. MRI allows analysis of labrum hypertrophy and subtle signs for hip instability. 3D MRI for cartilage mapping allows quantification of biochemical cartilage degeneration and yields great potential for surgical decision-making. 3D-CT and, increasingly, 3D MRI of the hip to generate 3D pelvic bone models and subsequent 3D impingement simulation can help to detect posterior extraarticular ischiofemoral impingement.

RESULTS AND DISCUSSION

Acetabular morphology can be divided in anterior, lateral and posterior hip dysplasia. Combined osseous deformities are common, such as hip dysplasia combined with cam deformity (86%). Valgus deformities were reported in 44%. Combined hip dysplasia and increased femoral antetorsion can occur in 52%. Posterior extraarticular ischiofemoral impingement between the lesser trochanter and the ischial tuberosity can occur in patients with increased femoral antetorsion. Typically, labrum damage and hypertrophy, cartilage damage, subchondral cysts can occur in hip dysplasia. Hypertrophy of the muscle iliocapsularis is a sign for hip instability. Acetabular morphology and femoral deformities (cam deformity and femoral anteversion) should be evaluated before surgical therapy for patients with hip dysplasia, considering the different measurement techniques and normal values of femoral antetorsion.

A radiomics approach to the diagnosis of femoroacetabular impingement

Introduction

Femoroacetabular Impingement (FAI) is a hip pathology characterized by impingement of the femoral head-neck junction against the acetabular rim, due to abnormalities in bone morphology. FAI is normally diagnosed by manual evaluation of morphologic features on magnetic resonance imaging (MRI). In this study, we assess, for the first time, the feasibility of using radiomics to detect FAI by automatically extracting quantitative features from images.

Material and methods

17 patients diagnosed with monolateral FAI underwent pre-surgical MR imaging, including a 3D Dixon sequence of the pelvis. An expert radiologist drew regions of interest on the water-only Dixon images outlining femur and acetabulum in both impingement (IJ) and healthy joints (HJ). 182 radiomic features were extracted for each hip. The dataset numerosity was increased by 60 times with an ad-hoc data augmentation tool. Features were subdivided by type and region in 24 subsets. For each, a univariate ANOVA F-value analysis was applied to find the 5 features most correlated with IJ based on p-value, for a total of 48 subsets. For each subset, a K-nearest neighbor model was trained to differentiate between IJ and HJ using the values of the radiomic features in the subset as input. The training was repeated 100 times, randomly subdividing the data with 75%/25% training/testing.

Results

The texture-based gray level features yielded the highest prediction max accuracy (0.972) with the smallest subset of features. This suggests that the gray image values are more homogeneously distributed in the HJ in comparison to IJ, which could be due to stress-related inflammation resulting from impingement.

Conclusions

We showed that radiomics can automatically distinguish IJ from HJ using water-only Dixon MRI. To our knowledge, this is the first application of radiomics for FAI diagnosis. We reported an accuracy greater than 97%, which is higher than the 90% accuracy for detecting FAI reported for standard diagnostic tests (90%). Our proposed radiomic analysis could be combined with methods for automated joint segmentation to rapidly identify patients with FAI, avoiding time-consuming radiological measurements of bone morphology.

Emerging Technology in Musculoskeletal MRI and CT

This article provides a focused overview of emerging technology in musculoskeletal MRI and CT. These technological advances have primarily focused on decreasing examination times, obtaining higher quality images, providing more convenient and economical imaging alternatives, and improving patient safety through lower radiation doses. New MRI acceleration methods using deep learning and novel reconstruction algorithms can reduce scanning times while maintaining high image quality. New synthetic techniques are now available that provide multiple tissue contrasts from a limited amount of MRI and CT data. Modern low-field-strength MRI scanners can provide a more convenient and economical imaging alternative in clinical practice, while clinical 7.0-T scanners have the potential to maximize image quality. Three-dimensional MRI curved planar reformation and cinematic rendering can provide improved methods for image representation. Photon-counting detector CT can provide lower radiation doses, higher spatial resolution, greater tissue contrast, and reduced noise in comparison with currently used energy-integrating detector CT scanners. Technological advances have also been made in challenging areas of musculoskeletal imaging, including MR neurography, imaging around metal, and dual-energy CT. While the preliminary results of these emerging technologies have been encouraging, whether they result in higher diagnostic performance requires further investigation.

Synthetic CT in Musculoskeletal Disorders: A Systematic Review

ABSTRACT

Repeated computed tomography (CT) examinations increase patients' ionizing radiation exposure and health costs, making an alternative method desirable. Cortical and trabecular bone, however, have short T2 relaxation times, causing low signal intensity on conventional magnetic resonance (MR) sequences. Different techniques are available to create a "CT-like" contrast of bone, such as ultrashort echo time, zero echo time, gradient-echo, and susceptibility-weighted image MR sequences, and artificial intelligence. This systematic review summarizes the essential technical background and developments of ultrashort echo time, zero echo time, gradient-echo, susceptibility-weighted image MR imaging sequences and artificial intelligence; presents studies on research and clinical applications of "CT-like" MR imaging; and describes their main advantages and limitations. We also discuss future opportunities in research, which patients would benefit the most, the most appropriate situations for using the technique, and the potential to replace CT in the clinical workflow.

3D-MRI versus 3D-CT in the evaluation of glenoid deformity in glenohumeral arthritis using Dixon 3D FLASH sequence

OBJECTIVE

To compare MRI with 3D reconstructions and 3D-CT with respect to assessment of glenoid wear in osteoarthritic shoulders.

METHODS

3D reconstructions were generated for CT and MR (utilizing the Dixon technique) imaging performed on 29 osteoarthritic shoulders. Two reviewers independently performed glenoid morphometric measurements and evaluated glenoid erosion. Mean differences between the two modalities were calculated. Inter-observer agreement was calculated using kappa coefficient.

RESULTS

The combined mean absolute difference (bias) in glenoid version between 3D-CT and 3D-MRI was 2.7° ± 1.6° (range 0.15-7.85, P value = 0.7). The combined mean absolute difference in glenoid inclination between 3D-CT and 3D-MRI was 6.8° ± 4.1° (range 0.8°-15.75°, P value = 0.17). No significant inter-reader variation in glenoid version and inclination measurements on 3D-CT and 3D-MRI was found (P > 0.05). The inter-reader reliability for both CT and MRI was high for Walch grading of glenoid bone loss (κ = 1, κ = 0.81, respectively).

CONCLUSIONS

3D-MRI is comparable to 3D-CT with respect to axial glenoid bone loss, as measured by glenoid version. However, for coronal bone loss estimation, measured by glenoid inclination, 3D-CT remains the gold standard. Thus, 3D-MR can be used as an alternative for preoperative assessment of glenoid version in arthritic shoulders.

Noninvasive shape-fitting method quantifies cam morphology in femoroacetabular impingement syndrome: Implications for diagnosis and surgical planning

There are considerable limitations associated with the standard 2D imaging currently used for the diagnosis and surgical planning of cam-type femoroacetabular impingement syndrome (FAIS). The aim of this study was to determine the accuracy of a new patient-specific shape-fitting method that quantifies cam morphology in 3D based solely on preoperative MRI imaging. Preoperative and postoperative 1.5T MRI scans were performed on n = 15 patients to generate 3D models of the proximal femur, in turn used to create the actual and the virtual cam. The actual cams were reconstructed by subtracting the postoperative from the preoperative 3D model and used as reference, while the virtual cams were generated by subtracting the preoperative 3D model from the virtual shape template produced with the shape-fitting method based solely on preoperative MRI scans. The accuracy of the shape-fitting method was tested on all patients by evaluating the agreement between the metrics of height, surface area, and volume that quantified virtual and actual cams. Accuracy of the shape-fitting method was demonstrated obtaining a 97.8% average level of agreement between these metrics. In conclusion, the shape-fitting technique is a noninvasive and patient-specific tool for the quantification and localization of cam morphology. Future studies will include the implementation of the technique within a clinically based software for diagnosis and surgical planning for cam-type FAIS.

The Modified Longitudinal Capsulotomy by Outside-In Approach in Hip Arthroscopy for Femoroplasty and Acetabular Labrum Repair-A Cohort Study

Hip arthroscopy is difficult to perform due to the limited arthroscopic view. To solve this problem, the capsulotomy is an important technique. However, the existing capsulotomy approaches were not perfect in the surgical practice. Thus, this study aimed to propose a modified longitudinal capsulotomy by outside-in approach and demonstrate its feasibility and efficacy in arthroscopic femoroplasty and acetabular labrum repair. A retrospective cohort study was performed and twenty-two postoperative patients who underwent hip arthroscopy in our hospital from January 2019 to December 2021 were involved in this study. The patients (14 females and 8 males) had a mean age of 38.26 ± 12.82 years old. All patients were diagnosed cam deformity and labrum tear in the operation and underwent arthroscopic femoroplasty and labrum repair by the modified longitudinal capsulotomy. The mean follow-up time was 10.4 months with a range of 6−12 months. There were no major complications, including infection, neurapraxias, hip instability or revision in any patients. The average mHHS were 74.4 ± 15.2, 78.2 ± 13.7 and 85.7 ± 14.5 in 3 months, 6 months and 12 months after surgery, respectively, which were all better than that before surgery (44.9 ± 8.6) (p < 0.05). The average VAS were 2.8 ± 1.2, 1.5 ± 0.6 and 1.2 ± 0.7 in 3 months, 6 months and 12 months after surgery, respectively, which were all lower than that before surgery (5.5 ± 2.0) (p < 0.05). The modified longitudinal capsulotomy by outside-in approach is proved to be a safe and feasible method for hip arthroscopy considering to the feasibility, efficacy and security. The arthroscopic femoroplasty and labrum repair can be performed conveniently by this approach and the patient reported outcomes after surgery were better that before surgery in short-term follow-up. This new method is promising and suggested to be widely used clinically.

Three-Dimensional Quantification of Cam Resection Using MRI Bone Models: A Comparison of 2 Techniques

Background:

The current clinical standard for the evaluation of cam deformity in femoroacetabular impingement syndrome is based on radiographic measurements, which limit the ability to quantify the complex 3-dimensional (3D) morphology of the proximal femur.

Purpose:

To compare magnetic resonance imaging (MRI)–based metrics for the quantification of cam resection as derived using a best-fit sphere alpha angle (BFS-AA) method and using 3D preoperative-postoperative surface model subtraction (PP-SMS).

Study Design:

Descriptive laboratory study.

Methods:

Seven cadaveric hemipelvises underwent 1.5-T MRI before and after arthroscopic femoral osteochondroplasty, and 3D bone models of the proximal femur were reconstructed from the MRI scans. The alpha angles were measured radially along clockfaces using a BFS-AA method from the literature and plotted as continuous curves for the pre- and postoperative models. The difference between the areas under the curve for the pre- and postoperative models was then introduced in the current study as the BFS-AA–based metric to quantify the cam resection. The cam resection was also quantified using a 3D PP-SMS method, previously described in the literature using the metrics of surface area (FSA), volume (FV), and height (maximum [FH_max] and mean [FH_mean]). Bivariate correlation analyses were performed to compare the metrics quantifying the cam resection as derived from the BFS-AA and PP-SMS methods.

Results:

The mean ± standard deviation maximum pre- and postoperative alpha angle measurements were 59.73° ± 15.38° and 48.02° ± 13.14°, respectively. The mean for each metric quantifying the cam resection with the PP-SMS method was as follows: FSA, 540.9 ± 150.7 mm²; FV, 1019.2 ± 486.2 mm³; FH_max, 3.6 ± 1.0 mm; and FH_mean, 1.8 ± 0.5 mm. Bivariate correlations between the BFS-AA–based and PP-SMS–based metrics were strong: FSA (r = 0.817, P = .012), FV (r = 0.888, P = .004), FH_max (r = 0.786, P = .018), and FH_mean (r = 0.679, P = .047).

Conclusion:

Strong positive correlations were appreciated between the BFS-AA and PP-SMS methods quantifying the cam resection.

Clinical Relevance:

The utility of the BFS-AA technique is primarily during preoperative planning. The utility of the PP-SMS technique is in the postoperative setting when evaluating the adequacy of resection or in patients with persistent hip pain with suspected residual impingement. In combination, the techniques allow surgeons to develop a planned resection while providing a means to evaluate the depth of resection postoperatively.

MRI-- and CT--based metrics for the quantification of arthroscopic bone resections in femoroacetabular impingement syndrome

The purpose of this in vitro study was to quantify the bone resected from the proximal femur during hip arthroscopy using metrics generated from magnetic resonance imaging (MRI) and computed tomography (CT) reconstructed three-dimensional (3D) bone models. Seven cadaveric hemi-pelvises underwent both a 1.5 T MRI and CT scan before and following an arthroscopic proximal femoral osteochondroplasty. The images from MRI and CT were segmented to generate 3D proximal femoral surface models. A validated 3D--3D registration method was used to compare surface--to--surface distances between the 3D models before and following surgery. The new metrics of maximum height, mean height, surface area and volume, were computed to quantify bone resected during osteochondroplasty. Stability of the metrics across imaging modalities was established through paired sample t--tests and bivariate correlation. Bivariate correlation analyses indicated strong correlations between all metrics (r = 0.728--0.878) computed from MRI and CT derived models. There were no differences in the MRI- and CT-based metrics used to quantify bone resected during femoral osteochondroplasty. Preoperative- and postoperative MRI and CT derived 3D bone models can be used to quantify bone resected during femoral osteochondroplasty, without significant differences between the imaging modalities.

MRI-based synthetic CT shows equivalence to conventional CT for the morphological assessment of the hip joint

This study evaluated the accuracy of synthetic computed tomography (sCT), as compared to CT, for the 3D assessment of the hip morphology. Thirty male patients with asymptomatic hips, referred for magnetic resonance (MR) imaging and CT, were included in this retrospective study. sCT images were generated from three-dimensional radiofrequency-spoiled T1-weighted multi-echo gradient-echo MR images using a commercially available deep learning-enabled software and were compared with CT images through mean error and surface distance computation and by means of eight clinical morphometric parameters relevant for hip care. Parameters included center-edge angle (CEA), sharp angle, acetabular index, extrusion index, femoral head center-to-midline distance, acetabular version (AV), and anterior and posterior acetabular sector angles. They were measured by two senior orthopedic surgeons and a radiologist in-training on CT and sCT images. The reliability and agreement of CT- and sCT-based measurements were assessed using intraclass correlation coefficients (ICCs) for absolute agreement, Bland-Altman plots, and two one-sided tests for equivalence. The surface distance between CT- and sCT-based bone models were on average submillimeter. CT- and sCT-based measurements showed moderate to excellent interobserver and intraobserver correlation (0.56 < ICC < 0.99). In particular, the inter/intraobserver agreements were good for AV (ICC > 0.75). For CEA, the intraobserver agreement was good (ICC > 0.75) and the interobserver agreement was moderate (ICC > 0.69). Limits of agreements were similar between intraobserver CT and intermodal measurements. All measurements were found statistically equivalent, with average intermodal differences within the intraobserver limits of agreement. In conclusion, sCT and CT were equivalent for the assessment of the hip joint bone morphology.

Magnetic Resonance Imaging Is Not Reliable in Classifying Anterior Inferior Iliac Spine Morphology Compared to 3-Dimensional Computerized Tomography

PURPOSE

The purpose of this study is to determine the reliability of standard magnetic resonance imaging (MRI) evaluation of AIIS morphology compared with three-dimensional (3D) computerized tomography (CT) (reference standard).

METHODS

Sixty hips in 30 patients met the inclusion criteria. The images were reviewed and classified by two fellowship-trained orthopedic surgeons. A second imaging viewing session was conducted in the same manner for validation of AIIS evaluation. The agreement and accuracy indices between the two raters were calculated for each imaging modality (inter-rater agreement) as well the agreement across the imaging modality for each rater (intermethod agreement).

RESULTS

The inter-rater agreement for the morphological evaluation of the AIIS for the first session according to 3D CT was .553 (P < .05) and by means of MRI was .0163 (P < .05). The inter-rater agreement for the second session by means of 3D CT was .449 (P < .05) and according to MRI was 0 (P < .05). The inter-method agreement for rater 1 for the first session was .04 (P < 0.05), while the agreement for rater 2 was .016 (P < .05). The intermethod agreement for rater 1 on the second session was .35 (P < 0.05), while that of rater 2 was .297(P < .05). The overall accuracy of MRI compared to 3D CT for rater 1 for the first session was .531, .490, and .959 for types I, II, III respectively and .551, .531, and .980 for the second session for types I, II, and III respectively. The overall accuracy of MRI compared to 3D CT for rater 2 for the first session was .551, .469, and .918 for types I, II, III respectively and .633, .592, and .918 for the second session for types I, II, and III, respectively.

CONCLUSION

MRI evaluations and subsequent classifications of AIIS morphology demonstrated a poor to slight correlation compared with that of the reference standard of 3D CT.

LEVEL OF EVIDENCE

Level II, retrospective diagnostic study.

Editorial Commentary: Low-Radiation Dose 3-Dimensional Computed Tomography Scan Reconstruction Is the Best Way to Visualize the Anterior Inferior Iliac Spine-For Now

Femoroacetabular impingement comes in several anatomic variations that may coexist, and subspine impingement is a commonly discussed cause of indirect extra-articular hip pathology. Although a classification system to identify and understand anterior inferior iliac spine morphology has been in place for some time, attempts have been made to visualize and understand the anatomy based on other imaging modalities. Standard radiographs are a common part of the initial patient evaluation pathway, along with thorough history taking and physical examination findings. Magnetic resonance imaging scans are obtained typically to evaluate the soft tissue, muscle, ligaments, articular cartilage, and labrum for pathology. For many hip preservation surgeons, a computed tomography scan with 3-dimensional reconstruction is standard protocol for patients who progress along the treatment pathway toward a surgical procedure because understanding the complex hip anatomy is key to successful surgical treatment. Many hip arthroscopy patients are in their young adult years, and we always attempt to reduce the amount of radiation exposure. Eliminating this computed tomography scan and using standard-of-care magnetic resonance imaging to simplify patient care, reduce radiation, and reduce health care costs would certainly be beneficial to our hip preservation patients.

Magnetic Resonance Imaging Versus Computed Tomography for Three-Dimensional Bone Imaging of Musculoskeletal Pathologies: A Review

Magnetic resonance imaging (MRI) is increasingly utilized as a radiation‐free alternative to computed tomography (CT) for the diagnosis and treatment planning of musculoskeletal pathologies. MR imaging of hard tissues such as cortical bone remains challenging due to their low proton density and short transverse relaxation times, rendering bone tissues as nonspecific low signal structures on MR images obtained from most sequences. Developments in MR image acquisition and post‐processing have opened the path for enhanced MR‐based bone visualization aiming to provide a CT‐like contrast and, as such, ease clinical interpretation. The purpose of this review is to provide an overview of studies comparing MR and CT imaging for diagnostic and treatment planning purposes in orthopedic care, with a special focus on selective bone visualization, bone segmentation, and three‐dimensional (3D) modeling. This review discusses conventional gradient‐echo derived techniques as well as dedicated short echo time acquisition techniques and post‐processing techniques, including the generation of synthetic CT, in the context of 3D and specific bone visualization. Based on the reviewed literature, it may be concluded that the recent developments in MRI‐based bone visualization are promising. MRI alone provides valuable information on both bone and soft tissues for a broad range of applications including diagnostics, 3D modeling, and treatment planning in multiple anatomical regions, including the skull, spine, shoulder, pelvis, and long bones.

MR Imaging of the Pelvic Bones: The Current and Cutting-Edge Techniques

This review presents an overview of the spectrum of the current and cutting-edge MRI techniques for pelvic bone imaging in clinical practice. The current MRI sequences and their advantages, disadvantages and usefullness in the imaging of this complex anatomical region are addressed. Finally, cutting-edge techniques are discussed, including susceptibility weighted MRI, ultrashort echo time MRI, zero echo time MRI and a deep learning-based multiparametric MRI technique named 'synthetic CT,' creating CT-like images without ionizing radiaton.

MAIN POINTS

GRE, SWI, UTE, ZTE MRI and synthetic CT sequences depict the cortical outline of the bones better in comparison to conventional MR images.MRI-based synthetic CT can create HU maps and allows for automated segmentation of pelvic bones.The current and cutting-edge MR techniques for bone imaging are complementary in the characterization of a variety of musculoskeletal disorders.

Three-Dimensional Measures of Bony Resection During Femoral Osteochondroplasty Are Related to Alpha Angle Measures: A Cadaveric Study

Purpose

Methods

Results

Conclusions

Clinical Relevance

Automated Morphometric Analysis of the Hip Joint on MRI from the German National Cohort Study

Purpose

To develop and validate an automated morphometric analysis framework for the quantitative analysis of geometric hip joint parameters in MR images from the German National Cohort (GNC) study.

Materials and Methods

A secondary analysis on 40 participants (mean age, 51 years; age range, 30-67 years; 25 women) from the prospective GNC MRI study (2015-2016) was performed. Based on a proton density-weighted three-dimensional fast spin-echo sequence, a morphometric analysis approach was developed, including deep learning-based landmark localization, bone segmentation of the femora and pelvis, and a shape model for annotation transfer. The centrum-collum-diaphyseal, center-edge (CE), three alpha angles, head-neck offset (HNO), and HNO ratio along with the acetabular depth, inclination, and anteversion were derived. Quantitative validation was provided by comparison with average manual assessments of radiologists in a cross-validation format. Paired-sample t tests with a Bonferroni-corrected significance level of .005 were employed alongside mean differences and 10th/90th percentiles, median absolute deviations (MADs), and intraclass correlation coefficients (ICCs).

Results

High agreement in mean Dice similarity coefficients was achieved (average of 97.52% ± 0.46 [standard deviation]). The subsequent morphometric analysis produced results with low mean MAD values, with the highest values of 3.34° (alpha 03:00 o'clock position) and 0.87 mm (HNO) and ICC values ranging between 0.288 (HNO ratio) and 0.858 (CE) compared with manual assessments. These values were in line with interreader agreements, which at most had MAD values of 4.02° (alpha 12:00 o'clock position) and 1.07 mm (HNO) and ICC values ranging between 0.218 (HNO ratio) and 0.777 (CE).

Conclusion

Automatic extraction of geometric hip parameters from MRI is feasible using a morphometric analysis approach with deep learning.Keywords: Computer-Aided Diagnosis (CAD), Interventional-MSK, MR-Imaging, Neural Networks, Skeletal-Appendicular, Hip, Anatomy, Computer Applications-3D, Segmentation, Vision, Application Domain, Quantification Supplemental material is available for this article. © RSNA, 2021.

Correlation of the Imaging Features of Femoroacetabular Impingement Syndrome With Clinical Findings and Patient Functional Scores

The relationship among the severity of the imaging features of femoroacetabular impingement syndrome (FAIS), patient symptoms, and function has not been elucidated. Understanding this relationship helps to improve the prognostic value of imaging. The goal of this study was to examine the correlation of clinical findings, patient pain, and function with severity, as measured with radiographic and 3-dimensional magnetic resonance imaging (3D-MRI). Data collected prospectively through a longitudinally maintained hip database were reviewed, and 37 hips from 31 patients were included. All patients were examined by an experienced orthopedic surgeon, and preoperative radiographs and 3D-MRI were obtained. Preoperatively, the patients completed validated patient-reported outcome measures (PROMs). Mean±SD alpha angles were 69.4°±10.3°, 70.0°±10.3°, 70.6°±8.4°, and 74.8°±9.2° at 12 o'clock, 1 o'clock, 2 o'clock, and 3 o'clock, respectively. Mean lateral center edge angle was 30.1°±5.3°. The authors did not observe a statistically significant correlation between PROMs and the features measured by radiographs and 3D-MRI (P>.05). In this subset of prospectively imaged patients with FAIS, the authors did not find a correlation between the severity of symptoms measured by PROMs and features on radiographs and 3D-MRI. The severity of dysfunction is multifactorial, and anatomic severity, as measured radiographically and with 3D-MRI, may not correlate with symptoms. Further investigation is necessary to address the sources of patient pain. [Orthopedics. 2021;44(4):e577-e582.].

3D MRI Models of the Musculoskeletal System

Computed tomography (CT) is most commonly used to produce three-dimensional (3D) models for evaluating bone and joint morphology in clinical practice. However, 3D models created from magnetic resonance imaging (MRI) data can be equally effective for comprehensive and accurate assessment of osseous and soft tissue structure morphology and pathology. The quality of 3D MRI models has steadily increased over time, with growing potential to replace 3D CT models in various musculoskeletal (MSK) applications. In practice, a single MRI examination for two-dimensional and 3D assessments can increase the value of MRI and simplify the pre- and postoperative imaging work-up. Multiple studies have shown excellent performance of 3D MRI models in shoulder injuries, in the hip in the setting of femoroacetabular impingement, and in the knee for the creation of bone surface models. Therefore, the utility of 3D MRI postprocessed models is expected to continue to rise and broaden in applications. Computer-based and artificial intelligence-assisted postprocessing techniques have tremendous potential to improve the efficiency of 3D model creation, opening many research avenues to validate the applicability of 3D MRI and establish 3D-specific quantitative assessment criteria. We provide a practice-focused overview of 3D MRI acquisition strategies, postprocessing techniques for 3D model creation, MSK applications of 3D MRI models, and an illustration of cases from our daily clinical practice.

3D MRI with CT-like bone contrast - An overview of current approaches and practical clinical implementation

CT is the imaging modality of choice for assessment of 3D bony morphology but incurs the penalty of ionizing radiation. Improving the ability of 3D MRI to provide high-resolution images of cortical bone with CT-like bone contrast has been a focus of recent research. The ability of 3D MRI to deliver cortical bone information with similar diagnostic performance to CT would complement assessment of soft tissues and medullary bone from a single MRI examination, simplifying evaluation and obviating radiation exposure from additional CT. This article presents an overview of current 3D MRI approaches for imaging cortical bone with CT-like bone contrast including ultrashort echo time, zero echo time, T1-weighted gradient recalled echo, susceptibility-weighted imaging and deep learning techniques. We also discuss clinical implementation of an optimized stack-of-stars 3D gradient recalled echo pulse sequence (3D-Bone) on commercially available MRI scanners for rendering 3D MRI with CT-like bone contrast in our institutional practice.

Consistency of 3D femoral torsion measurement from MRI compared to CT gold standard

Background

Several hip and knee pathologies are associated with aberrant femoral torsion. Diagnostic workup includes computed tomography (CT) and magnetic resonance imaging (MRI). For three-dimensional (3D) analysis of complex deformities it would be desirable to measure femoral torsion from MRI data to avoid ionizing radiation of CT in a young patient population. 3D measurement of femoral torsion from MRI has not yet been compared to measurements from CT images. We hypothesize that agreement will exist between MRI and CT 3D measurements of femoral torsion.

Methods

CT and MRI data from 29 hips of 15 patients with routine diagnostic workup for suspected femoroacetabular impingement (FAI) were used to generate 3D bone models. 3D measurement of femoral torsion was performed by two independent readers using the method of Kim et al. which is validated for CT. Inter-modalitiy and inter-reader intraclass correlation coefficients (ICC) were calculated.

Results

Between MRI and CT 3D measurements an ICC of 0.950 (0.898; 0.976) (reader 1) respectively 0.950 (0.897; 0.976) (Reader 2) was found. The ICC (95% CI) expressing the inter-reader reliability for both modalities was 0.945 (0.886; 0.973) for MRI and 0.957 (0.910; 0.979) for CT, respectively. Mean difference between CT and MRI measurement was 0.42° (MRI – CT, SD: 2.77°, p = 0.253).

Conclusions

There was consistency between 3D measurements of femoral torsion between computer rendered MRI images compared to measurements with the “gold standard” of CT images. ICC for inter-modality and inter-reader consistency indicate excellent reliability. Accurate, reliable and reproducible 3D measurement of femoral torsion is possible from MRI images.

Sex differences in the prevalence of radiographic findings of structural hip deformities in patients with symptomatic femoroacetabular impingement

The purpose of this study was to determine the sex differences in the overall prevalence of radiographic femoroacetabular impingement (FAI) deformity patients presenting with hip pain and to identify the most common radiographic findings in male and female patients. A geographic database was used to identify patients between the age of 14 and 50 years with hip pain from 2000 to 2016. A chart and radiographic review was performed to identify patients with cam, pincer and mixed-type FAI. A total of 374 (449 hips) out of 612 (695 hips) male patients and 771 (922 hips) out of 1281 (1447 hips) female patients had radiographic features consistent with FAI. Ninety-four male hips (20.9%) and 45 female hips (4.9%) had cam type, 20 male hips (4.5%) and 225 female hips (24.4%) had pincer type and 335 male hips (74.6%) and 652 female hips (70.7%) had mixed type. The overall prevalence of radiographic findings consistent with FAI in male and female patients with hip pain was 61.1% and 60.2%, respectively. Mixed type was the most prevalent. The most common radiographic finding for cam-type FAI was an alpha angle >55°, and the most common radiographic finding for pincer-type FAI was a crossover sign. Male patients were found to have a higher prevalence of cam-type deformities, whereas female patients were found to have a higher prevalence of pincer-type deformities.

Differences between 3D isovoxel fat suppression VIBE MRI and CT models of proximal femur osseous anatomy: A preliminary study for bone tumor resection planning

Purpose

To evaluate the osseous anatomy of the proximal femur extracted from a 3D-MRI volumetric interpolated breath-hold (VIBE) sequence using either a Dixon or water excitation (WE) fat suppression method, and to measure the overall difference using CT as a reference standard.

Material and methods

This retrospective study reviewed imaging of adult patients with hip pain who underwent 3D hip MRI and CT. A semi-automatically segmented CT model served as the reference standard, and MRI segmentation was performed manually for each unilateral hip joint. The differences between Dixon-VIBE-3D-MRI vs. CT, and WE-VIBE-3D-MRI vs. CT, were measured. Equivalence tests between Dixon-VIBE and WE-VIBE models were performed with a threshold of 0.1 mm. Bland–Altman plots and Lin’s concordance-correlation coefficient were used to analyze the agreement between WE and Dixon sequences. Subgroup analyses were performed for the femoral head/neck, intertrochanteric, and femoral shaft areas.

Results

The mean and maximum differences between Dixon-VIBE-3D-MRI vs. CT were 0.2917 and 3.4908 mm, respectively, whereas for WE-VIBE-3D-MRI vs. CT they were 0.3162 and 3.1599 mm. The mean differences of the WE and Dixon methods were equivalent (P = 0.0292). However, the maximum difference was not equivalent between the two methods and it was higher in WE method. Lin’s concordance-correlation coefficient showed poor agreement between Dixon and WE methods. The mean differences between the CT and 3D-MRI models were significantly higher in the femoral shaft area (P = 0.0004 for WE and P = 0.0015 for Dixon) than in the other areas. The maximum difference was greatest in the intertrochanteric area for both techniques.

Conclusion

The difference between 3D-MR and CT models were acceptable with a maximal difference below 3.5mm. WE and Dixon fat suppression methods were equivalent. The mean difference was highest at the femoral shaft area, which was off-center from the magnetization field.

The Lisbon Agreement on Femoroacetabular Impingement Imaging-part 3: imaging techniques

OBJECTIVES

Imaging diagnosis of femoroacetabular impingement (FAI) remains controversial due to a lack of high-level evidence, leading to significant variability in patient management. Optimizing protocols and technical details is essential in FAI imaging, although challenging in clinical practice. The purpose of this agreement is to establish expert-based statements on FAI imaging, using formal consensus techniques driven by relevant literature review. Recommendations on the selection and use of imaging techniques for FAI assessment, as well as guidance on relevant radiographic and MRI classifications, are provided.

METHODS

The Delphi method was used to assess agreement and derive consensus among 30 panel members (musculoskeletal radiologists and orthopedic surgeons). Forty-four questions were agreed on and classified into five major topics and recent relevant literature was circulated, in order to produce answering statements. The level of evidence was assessed for all statements and panel members scored their level of agreement with each statement during 4 Delphi rounds. Either "group consensus," "group agreement," or "no agreement" was achieved.

RESULTS

Forty-seven statements were generated and group consensus was reached for 45. Twenty-two statements pertaining to "Imaging techniques" were generated. Eight statements on "Radiographic assessment" and 12 statements on "MRI evaluation" gained consensus. No agreement was reached for the 2 "Ultrasound" related statements.

CONCLUSION

The first international consensus on FAI imaging was developed. Researchers and clinicians working with FAI and hip-related pain may use these recommendations to guide, develop, and implement comprehensive, evidence-based imaging protocols and classifications.

KEY POINTS

• Radiographic evaluation is recommended for the initial assessment of FAI, while MRI with a dedicated protocol is the gold standard imaging technique for the comprehensive evaluation of this condition. • The MRI protocol for FAI evaluation should include unilateral small FOV with radial imaging, femoral torsion assessment, and a fluid sensitive sequence covering the whole pelvis. • The definite role of other imaging methods in FAI, such as ultrasound or CT, is still not well defined.

Use of a 3D virtual dynamic hip model to quantify the amount of osteoplasty required in femoroacetabular impingement patients

OBJECTIVE

Compare required osteoplasty predicted by a 3D virtual dynamic hip model in femoroacetabular impingement patients to actual osteoplasty performed.

MATERIALS AND METHODS

Retrospective study on 20 consecutive FAI patients with a preoperative CT who underwent arthroscopy from October 2016 to September 2017. A 3D virtual dynamic hip model was created from the CT. The model displayed virtual osteoplasty depth required to restore physiologic range of motion on an osteoplasty map. Depths of virtual osteoplasty and actual osteoplasty at surgery were compared and correlated with alpha angle, lateral center edge angle, femoral version, and acetabular version.

RESULTS

Actual femoroplasty depth correlated with alpha angle (r = 0.85, p ≤ 0.001) and actual acetabuloplasty depth correlated with lateral center edge angle (r = 0.83, p < 0.001). Virtual osteoplasty depth did not correlate with alpha angle (p = 0.25), lateral center edge angle (p = 0.50), femoral version (p = 0.09), or acetabular version (p = 0.09). The 3D model predicted a mean virtual osteoplasty of 6.2 ± 0.3 mm compared to mean actual osteoplasty of 5.9 ± 1.1 mm. There was no significant difference between the two means (p = 0.26), though there was a significant difference in variance (p = 0.001). There was poor test reliability between virtual osteoplasty compared with actual osteoplasty (ICC = 0.30).

CONCLUSION

3D model predicted virtual osteoplasty depths varied with actual osteoplasty and was independent of 2D measurements.

Three-dimensional MRI Bone Models of the Glenohumeral Joint Using Deep Learning: Evaluation of Normal Anatomy and Glenoid Bone Loss

Purpose

To use convolutional neural networks (CNNs) for fully automated MRI segmentation of the glenohumeral joint and evaluate the accuracy of three-dimensional (3D) MRI models created with this method.

Materials and Methods

Shoulder MR images of 100 patients (average age, 44 years; range, 14-80 years; 60 men) were retrospectively collected from September 2013 to August 2018. CNNs were used to develop a fully automated segmentation model for proton density-weighted images. Shoulder MR images from an additional 50 patients (mean age, 33 years; range, 16-65 years; 35 men) were retrospectively collected from May 2014 to April 2019 to create 3D MRI glenohumeral models by transfer learning using Dixon-based sequences. Two musculoskeletal radiologists performed measurements on fully and semiautomated segmented 3D MRI models to assess glenohumeral anatomy, glenoid bone loss (GBL), and their impact on treatment selection. Performance of the CNNs was evaluated using Dice similarity coefficient (DSC), sensitivity, precision, and surface-based distance measurements. Measurements were compared using matched-pairs Wilcoxon signed rank test.

Results

The two-dimensional CNN model for the humerus and glenoid achieved a DSC of 0.95 and 0.86, a precision of 95.5% and 87.5%, an average precision of 98.6% and 92.3%, and a sensitivity of 94.8% and 86.1%, respectively. The 3D CNN model, for the humerus and glenoid, achieved a DSC of 0.95 and 0.86, precision of 95.1% and 87.1%, an average precision of 98.7% and 91.9%, and a sensitivity of 94.9% and 85.6%, respectively. There was no difference between glenoid and humeral head width fully and semiautomated 3D model measurements (P value range, .097-.99).

Conclusion

CNNs could potentially be used in clinical practice to provide rapid and accurate 3D MRI glenohumeral bone models and GBL measurements. Supplemental material is available for this article. © RSNA, 2020.

Advances in FAI Imaging: a Focused Review

PURPOSE OF REVIEW

Femoroacetabular impingement (FAI) is one of the main causes of hip pain in young adults and poses clinical challenges which have placed it at the forefront of imaging and orthopedics. Diagnostic hip imaging has dramatically changed in the past years, with the arrival of new imaging techniques and the development of magnetic resonance imaging (MRI). This article reviews the current state-of-the-art clinical routine of individuals with suspected FAI, limitations, and future directions that show promise in the field of musculoskeletal research and are likely to reshape hip imaging in the coming years.

RECENT FINDINGS

The largely unknown natural disease course, especially in hips with FAI syndrome and those with asymptomatic abnormal morphologies, continues to be a problem as far as diagnosis, treatment, and prognosis are concerned. There has been a paradigm shift in recent years from bone and soft tissue morphological analysis towards the tentative development of quantitative approaches, biochemical cartilage evaluation, dynamic assessment techniques and, finally, integration of artificial intelligence (AI)/deep learning systems. Imaging, AI, and hip preserving care will continue to evolve with new problems and greater challenges. The increasing number of analytic parameters describing the hip joint, as well as new sophisticated MRI and imaging analysis, have carried practitioners beyond simplistic classifications. Reliable evidence-based guidelines, beyond differentiation into pure instability or impingement, are paramount to refine the diagnostic algorithm and define treatment indications and prognosis. Nevertheless, the boundaries of morphological, functional, and AI-aided hip assessment are gradually being pushed to new frontiers as the role of musculoskeletal imaging is rapidly evolving.

Correlations and Reproducibility Between Radiographic and Radial Alpha Angles in the Evaluation of Cam Morphology

Background:

The alpha angle used to evaluate cam morphology can be determined on different imaging views; however, 2-dimensional (2D) imaging can present limitations in terms of the reproducibility of the radial alpha angle. Recent developments in 3-dimensional (3D) high-resolution magnetic resonance imaging (MRI) have allowed detailed evaluations of the radial alpha angle.

Purpose:

To determine whether there are any correlations or discrepancies between the 2D alpha angle on plain radiography and the maximum radial alpha angle on 3D MRI.

Study Design:

Cohort study (diagnosis); Level of evidence, 2.

Methods:

A total of 42 hips from 39 patients (19 males, 20 females) were analyzed, including 22 hips with femoroacetabular impingement (FAI; mean age, 41 years) and 20 hips with borderline developmental dysplasia of the hip (BDDH; mean age, 43 years). Radial images were reconstructed from 3D multiple echo recombined gradient echo (MERGE) MRI. Differences in the maximum radial alpha angle on MRI between hips with FAI and BDDH were evaluated. Correlations and discrepancies between the maximum radial alpha angle on MRI and alpha angles on the anteroposterior, cross-table lateral, and 45° Dunn views of radiography were also evaluated.

Results:

The maximum radial alpha angle was significantly higher for hips with FAI than for hips with BDDH. On average, the greatest alpha angle on radial MRI was higher than the alpha angle on each of the 3 radiographic views for both FAI and BDDH. The 45° Dunn view revealed the smallest discrepancy for both FAI (P = .005) and BDDH (P = .002). The cross-table lateral view had the highest correlation with the maximum radial alpha angle for BDDH (P < .001).

Conclusion:

We reconfirmed the utility of the 45° Dunn view, with it presenting the best reproducibility for the maximum radial alpha angle in the evaluation of cam morphology, while the cross-table lateral view revealed the best correlation with the maximum radial alpha angle, particularly for hips with BDDH.

The Lisbon Agreement on Femoroacetabular Impingement Imaging-part 1: overview

OBJECTIVES

Imaging assessment for the clinical management of femoroacetabular impingement (FAI) syndrome remains controversial because of a paucity of evidence-based guidance and notable variability in clinical practice, ultimately requiring expert consensus. The purpose of this agreement is to establish expert-based statements on FAI imaging, using formal techniques of consensus building.

METHODS

A validated Delphi method and peer-reviewed literature were used to formally derive consensus among 30 panel members (21 musculoskeletal radiologists and 9 orthopaedic surgeons) from 13 countries. Forty-four questions were agreed on, and recent relevant seminal literature was circulated and classified in five major topics ('General issues', 'Parameters and reporting', 'Radiographic assessment', 'MRI' and 'Ultrasound') in order to produce answering statements. The level of evidence was noted for all statements, and panel members were asked to score their level of agreement with each statement (0 to 10) during iterative rounds. Either 'consensus', 'agreement' or 'no agreement' was achieved.

RESULTS

Forty-seven statements were generated, and group consensus was reached for 45 (95.7%). Seventeen of these statements were selected as most important for dissemination in advance. There was no agreement for the two statements pertaining to 'Ultrasound'.

CONCLUSION

Radiographic evaluation is the cornerstone of hip evaluation. An anteroposterior pelvis radiograph and a Dunn 45° view are recommended for the initial assessment of FAI although MRI with a dedicated protocol is the gold standard imaging technique in this setting. The resulting consensus can serve as a tool to reduce variability in clinical practices and guide further research for the clinical management of FAI.

KEY POINTS

• FAI imaging literature is extensive although often of low level of evidence. • Radiographic evaluation with a reproducible technique is the cornerstone of hip imaging assessment. • MRI with a dedicated protocol is the gold standard imaging technique for FAI assessment.

Interchangeability of CT and 3D "pseudo-CT" MRI for preoperative planning in patients with femoroacetabular impingement

OBJECTIVE

To determine whether a 3D magnetic resonance imaging (MRI) sequence with postprocessing applied to simulate computed tomography (CT) ("pseudo-CT") images can be used instead of CT to measure acetabular version and alpha angles and to plan for surgery in patients with femoroacetabular impingement (FAI).

MATERIALS AND METHODS

Four readers retrospectively measured acetabular version and alpha angles on MRI and CT images of 40 hips from 20 consecutive patients (9 female patients, 11 male patients; mean age, 26.0 ± 6.5 years) with FAI. 3D models created from MRI and CT images were assessed by 2 orthopedic surgeons to determine the need for femoroplasty and/or acetabuloplasty. Interchangeability of MRI with CT was tested by comparing agreement between 2 readers using CT (intramodality) with agreement between 1 reader using CT and 1 using MRI (intermodality).

RESULTS

Intramodality and intermodality agreement values were nearly identical for acetabular version and alpha angle measurements and for surgical planning. Increases in inter-reader disagreement for acetabular version angle, alpha angle, and surgical planning when MRI was substituted for CT were - 2.1% (95% confidence interval [CI], - 7.7 to + 3.5%; p = 0.459), - 0.6% (95% CI, - 8.6 to + 7.3%; p = 0.878), and 0% (95% CI, - 15.1 to + 15.1%; p = 1.0), respectively, when an agreement criterion ≤ 5° was used for angle measurements.

CONCLUSION

Pseudo-CT MRI was interchangeable with CT for measuring acetabular version and highly favorable for interchangeability for measuring alpha angle and for surgical planning, suggesting that MRI could replace CT in assessing patients with FAI.

Evaluation of Osseous Morphology of the Hip Using Zero Echo Time Magnetic Resonance Imaging

BACKGROUND

Femoroacetabular impingement syndrome (FAIS) is a common disorder of the hip resulting in groin pain and ultimately osteoarthritis. Radiologic assessment of FAI morphologies, which may present with overlapping radiologic features of hip dysplasia, often requires the use of computed tomography (CT) for evaluation of osseous abnormality, owing to the difficulty of direct visualization of cortical and subchondral bone with conventional magnetic resonance imaging (MRI). The use of a zero echo time (ZTE) MRI pulse sequence may obviate the need for CT by rendering bone directly from MRI.

PURPOSE/HYPOTHESIS

The purpose was to explore the application of ZTE MRI to the assessment of osseous FAI and dysplasia morphologies of the hip. It was hypothesized that angular measurements from ZTE images would show significant agreement with measurements obtained from CT images.

STUDY DESIGN

Cohort study (diagnosis); Level of evidence, 2.

METHODS

Thirty-eight hips from 23 patients were imaged with ZTE MRI and CT. Clinically relevant angular measurements of hip morphology were made in both modalities and compared to assess agreement. Measurements included coronal and sagittal center-edge angles, femoral neck-shaft angle, acetabular version (at 1-, 2-, and 3-o'clock positions), Tönnis angle, alpha angle, and modified-beta angle. Interrater agreement was assessed for a subset of 10 hips by 2 raters. Intermodal agreement was assessed on the complete cohort and a single rater.

RESULTS

Interrater agreement was demonstrated in both CT and ZTE, with intraclass correlation coefficient values ranging from 0.636 to 0.990 for ZTE and 0.747 to 0.983 for CT, indicating "good" to "excellent" agreement. Intermodal agreement was also shown to be significant, with intraclass correlation coefficients ranging from 0.618 to 0.904.

CONCLUSION

Significant agreement of angular measurements for hip morphology exists between ZTE MRI and CT imaging. ZTE MRI may be an effective method to quantitatively evaluate osseous hip morphology.

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.