The cartilage black line sign: an unexpected MRI appearance of deep cartilage fissuring in three patients

Super-resolution synthetic MRI using deep learning reconstruction for accurate diagnosis of knee osteoarthritis

OBJECTIVE

To assess the accuracy of deep learning reconstruction (DLR) technique on synthetic MRI (SyMRI) including T2 measurements and diagnostic performance of DLR synthetic MRI (SyMRIDL) in patients with knee osteoarthritis (KOA) using conventional MRI as standard reference.

MATERIALS AND METHODS

This prospective study recruited 36 volunteers and 70 patients with suspected KOA from May to October 2023. DLR and non-DLR synthetic T2 measurements (T2-SyMRIDL, T2-SyMRI) for phantom and in vivo knee cartilage were compared with multi-echo fast-spin-echo (MESE) sequence acquired standard T2 values (T2MESE). The inter-reader agreement on qualitative evaluation of SyMRIDL and the positive percent agreement (PPA) and negative percentage agreement (NPA) were analyzed using routine images as standard diagnosis.

RESULTS

DLR significantly narrowed the quantitative differences between T2-SyMRIDL and T2MESE for 0.8 ms with 95% LOA [-5.5, 7.1]. The subjective assessment between DLR synthetic MR images and conventional MRI was comparable (all p > 0.05); Inter-reader agreement for SyMRIDL and conventional MRI was substantial to almost perfect with values between 0.62 and 0.88. SyMRIDL MOAKS had substantial inter-reader agreement and high PPA/NPA values (95%/99%) using conventional MRI as standard reference. Moreover, T2-SyMRIDL measurements instead of non-DLR ones significantly differentiated normal-appearing from injury-visible cartilages.

CONCLUSION

DLR synthetic knee MRI provided both weighted images for clinical diagnosis and accurate T2 measurements for more confidently identifying early cartilage degeneration from normal-appearing cartilages.

CRITICAL RELEVANCE STATEMENT

One-acquisition synthetic MRI based on deep learning reconstruction provided an accurate quantitative T2 map and morphologic images in relatively short scan time for more confidently identifying early cartilage degeneration from normal-appearing cartilages compared to the conventional morphologic knee sequences.

KEY POINTS

Deep learning reconstruction (DLR) synthetic knee cartilage T2 values showed no difference from conventional ones. DLR synthetic T1-, proton density-, STIR-weighted images had high positive percent agreement and negative percentage agreement using MRI OA Knee Score features. DLR synthetic T2 measurements could identify early cartilage degeneration from normal-appearing ones.

Dark Cartilage Lesions in the Knee: MRI Appearance and Clinical Significance

Early investigations into the magnetic resonance imaging (MRI) appearance of articular cartilage imaging relied on assessment of the morphology, with subsequent investigators reporting identifying increased T2 signal intensity, bright signal, in degenerated cartilage. The cartilage "black line sign" is a finding that has recently been described in the radiology literature to characterize cartilage pathology. This sign refers to a focal linear hypointense signal within articular cartilage that is oriented perpendicular to the subchondral bone on T2-weighted MRI. The diagnostic significance and clinical relevance of this sign is debated. Since its first description, several papers have further delineated the etiology, prevalence, and clinical relevance of these and other dark cartilage abnormalities. The intent of this article is to summarize these findings, with hopes of bringing to light the importance of dark cartilage lesions and their clinical implication in the world of knee surgery. We will briefly discuss the most probable etiologies of dark cartilage abnormalities and the major factors determining the unique signal intensity. The described anatomical patterns of this finding, the clinical importance, potential mimics, and current treatment recommendations will be reviewed.

Imaging update on cartilage

In this update article, we present a review of the literature regarding the physiology of the articular cartilage, role of MR imaging in cartilage assessment, MRI sequences and protocols for cartilage imaging, brief overview of classifications and nomenclature for chondral and osteochondral lesions, MR imaging following cartilage repair and degenerative osteoarthritis.

Knee Cartilage Imaging

Articular cartilage injury and degeneration represent common causes of knee pain, which can be evaluated accurately and noninvasively using MRI. This review describes the structure of cartilage focusing on its histologic appearance to emphasize that structure will dictate patterns of tissue failure as well as MR appearance. In addition to identifying cartilage loss, MRI can demonstrate signal changes that correspond to intrinsic structural abnormalities which place the cartilage at risk for subsequent more serious injury or premature degeneration, allowing for earlier intervention and treatment of important causes of pain and morbidity.

Hypointense signal lesions of the articular cartilage: a review of current concepts

Discussion of articular cartilage disease detection by MRI usually focuses on the presence of bright signal on T2-weighted sequences, such as in Grade 1 chondromalacia and cartilage fissures containing fluid. Less emphasis has been placed on how cartilage disease may be manifested by dark signal on T2-weighted sequences. The appearance of the recently described "cartilage black line sign" of the femoral trochlea highlights these lesions and further raises the question of their etiology. We illustrate various hypointense signal lesions that are not restricted to the femoral trochlea of the knee joint and discuss the possible etiologies for these lesions.

The trochlear cleft: the “black line” of the trochlear trough

OBJECTIVE

The “cartilage black line sign” is a recently described T2 dark cartilage lesion that we have identified appearing as a cleft in the trochlear trough. The purpose of our study was to define the MR imaging characteristics of a trochlear cleft, determine its incidence, and correlate the MR findings with arthroscopy.

MATERIALS AND METHODS

A total of 1,300 consecutive MR examinations of the knee were retrospectively reviewed by consensus of two fellowship-trained musculoskeletal radiologists. The MR imaging characteristics and location of a trochlear cleft were determined. Imaging results were compared to arthroscopy when available. Patient age and gender were compared to 25 randomly selected control patients without trochlear clefts.

RESULTS

A total of 25 (1.9%) individuals (11 females and 14 males; age range 19–45 years; mean age 28 years) were diagnosed with a trochlear cleft. The control group consisted of 11 females and 14 males; age range 19–83 years; mean age 46 years. Mean cleft length was 7 mm (range 6–12 mm); cleft location was consistently in the lower trochlear trough. No full-thickness cartilage defects were identified in the eight individuals in whom arthroscopic correlation was available. A grade 2 cartilage lesion was identified in a single individual; another progressed from grade 0 to a full-thickness trochlear lesion over an 8-month interval. Eight individuals were athletes. No significant difference in gender was noted between the two groups, however, the study group was significantly younger p<0.0001.

CONCLUSIONS

A trochlear cleft is a rare finding in young active individuals. It most likely indicates an incomplete cartilage fissure which may rarely progress to a full-thickness defect.

MR imaging of the articular cartilage of the knee and ankle

Cartilage abnormalities in the knee and ankle are a common source of pain and are often difficult to diagnose clinically or radiographically. MR imaging is a valuable tool for diagnosing and characterizing cartilage lesions of both the knee and ankle. An understanding of the appearance of cartilage, and an understanding of how and when to report cartilage injury in the knee and ankle based on current grading systems allows the radiologist to provide the most helpful reports to referring clinicians. This article presents the range of cartilage pathologies in the knee and ankle and provides clinically relevant guidelines.

Cartilage magnetic resonance imaging techniques at 3 T: current status and future directions

Magnetic resonance imaging (MRI) remains the imaging modality of choice for morphological and compositional evaluation of the articular cartilage. Accurate detection and characterization of cartilage lesions are necessary to guide the medical and surgical therapy and are also critical for longitudinal studies of the cartilage. Recent work using 3.0-T MRI systems shows promise in improving detection and characterization of the cartilage lesions, particularly with increasing use of high-resolution and high-contrast 3-dimensional sequences, which allow detailed morphological assessment of cartilage in arbitrary imaging planes. In addition, implementation of biochemical sequences in clinically feasible scan times has a potential in the early detection of cartilage lesions before they become morphologically apparent. This article discusses relative advantages and disadvantages of various commonly used as well as experimental MRI techniques to directly assess the morphology and indirectly evaluate the biochemical composition of the articular cartilage.