UTE MRI of the Osteochondral Junction

Magnetic resonance bone imaging: applications to vertebral lesions

MR bone imaging is a recently introduced technique, that allows visualization of bony structures in good contrast against adjacent structures, like CT. Although CT has long been considered the modality of choice for bone imaging, MR bone imaging allows visualization of the bone without radiation exposure while simultaneously allowing conventional MR images to be obtained. Accordingly, MR bone imaging is expected as a new imaging technique for the diagnosis of miscellaneous spinal diseases. This review presents several sequences used in MR bone imaging including black bone imaging, ultrashort/zero echo time (UTE/ZTE) sequences, and T1-weighted 3D gradient-echo sequence. We also illustrate clinical cases in which spinal lesions could be effectively demonstrated on MR bone imaging, performed in most cases using a 3D gradient-echo sequence at our institution. The lesions presented herein include degenerative diseases, tumors and similar diseases, fractures, infectious diseases, and hemangioma. Finally, we discuss the differences between MR bone imaging and previously reported techniques, and the limitations and future perspectives of MR bone imaging.

Clinical application of ultrashort echo time (UTE) and zero echo time (ZTE) magnetic resonance (MR) imaging in the evaluation of osteoarthritis

Novel compositional magnetic resonance (MR) imaging techniques have allowed for both the qualitative and quantitative assessments of tissue changes in osteoarthritis, many of which are difficult to characterize on conventional MR imaging. Ultrashort echo time (UTE) and zero echo time (ZTE) MR imaging have not been broadly implemented clinically but have several applications that leverage contrast mechanisms for morphologic evaluation of bone and soft tissue, as well as biochemical assessment in various stages of osteoarthritis progression. Many of the musculoskeletal tissues implicated in the initiation and progression of osteoarthritis are short T2 in nature, appearing dark as signal has already decayed to its minimum when image sampling starts. UTE and ZTE MR imaging allow for the qualitative and quantitative assessments of these short T2 tissues (bone, tendon, calcified cartilage, meniscus, and ligament) with both structural and functional reference standards described in the literature [1-3]. This review will describe applications of UTE and ZTE MR imaging in musculoskeletal tissues focusing on its role in knee osteoarthritis. While the review will address tissue-specific applications of these sequences, it is understood that osteoarthritis is a whole joint process with involvement and interdependence of all tissues. KEY POINTS: • UTE MR imaging allows for the qualitative and quantitative evaluation of short T2 tissues (bone, calcified cartilage, and meniscus), enabling identification of both early degenerative changes and subclinical injuries that may predispose to osteoarthritis. • ZTE MR imaging allows for the detection of signal from bone, which has some of the shortest T2 values, and generates tissue contrast similar to CT, potentially obviating the need for CT in the assessment of osseous features of osteoarthritis.

Deep-Learning-Aided Evaluation of Spondylolysis Imaged with Ultrashort Echo Time Magnetic Resonance Imaging

Isthmic spondylolysis results in fracture of pars interarticularis of the lumbar spine, found in as many as half of adolescent athletes with persistent low back pain. While computed tomography (CT) is the gold standard for the diagnosis of spondylolysis, the use of ionizing radiation near reproductive organs in young subjects is undesirable. While magnetic resonance imaging (MRI) is preferable, it has lowered sensitivity for detecting the condition. Recently, it has been shown that ultrashort echo time (UTE) MRI can provide markedly improved bone contrast compared to conventional MRI. To take UTE MRI further, we developed supervised deep learning tools to generate (1) CT-like images and (2) saliency maps of fracture probability from UTE MRI, using ex vivo preparation of cadaveric spines. We further compared quantitative metrics of the contrast-to-noise ratio (CNR), mean squared error (MSE), peak signal-to-noise ratio (PSNR), and structural similarity index (SSIM) between UTE MRI (inverted to make the appearance similar to CT) and CT and between CT-like images and CT. Qualitative results demonstrated the feasibility of successfully generating CT-like images from UTE MRI to provide easier interpretability for bone fractures thanks to improved image contrast and CNR. Quantitatively, the mean CNR of bone against defect-filled tissue was 35, 97, and 146 for UTE MRI, CT-like, and CT images, respectively, being significantly higher for CT-like than UTE MRI images. For the image similarity metrics using the CT image as the reference, CT-like images provided a significantly lower mean MSE (0.038 vs. 0.0528), higher mean PSNR (28.6 vs. 16.5), and higher SSIM (0.73 vs. 0.68) compared to UTE MRI images. Additionally, the saliency maps enabled quick detection of the location with probable pars fracture by providing visual cues to the reader. This proof-of-concept study is limited to the data from ex vivo samples, and additional work in human subjects with spondylolysis would be necessary to refine the models for clinical use. Nonetheless, this study shows that the utilization of UTE MRI and deep learning tools could be highly useful for the evaluation of isthmic spondylolysis.

Ultrashort time-to-echo MR morphology of cartilaginous endplate correlates with disc degeneration in the lumbar spine

PURPOSE

Using ultrashort echo time (UTE) MRI, we determined prevalence of abnormal cartilaginous endplate (CEP), and the relationship between CEP and disc degeneration in human lumbar spines.

MATERIALS AND METHODS

Lumbar spines from 71 cadavers (age 14-74 years) were imaged at 3 T using sagittal UTE and spin echo T2 map sequences. On UTE images, CEP morphology was defined as "normal" with linear high signal intensity or "abnormal" with focal signal loss and/or irregularity. On spin echo images, disc grade and T2 values of the nucleus pulposus (NP) and annulus fibrosus (AF) were determined. 547 CEPs and 284 discs were analysed. Effects of age, sex, and level on CEP morphology, disc grade, and T2 values were determined. Effects of CEP abnormality on disc grade, T2 of NP, and T2 of AF were also determined.

RESULTS

Overall prevalence of CEP abnormality was 33% and it tended to increase with older ages (p = 0.08) and at lower spinal levels of L5 than L2 or L3 (p = 0.001). Disc grades were higher and T2 values of the NP were lower in older spines (p < 0.001) and at lower disc level of L4-5 (p < 0.05). We found significant association between CEP and disc degeneration; discs adjacent to abnormal CEPs had high grades (p < 0.01) and lower T2 values of the NP (p < 0.05).

CONCLUSION

These results suggest that abnormal CEPs are frequently found, and it associates significantly with disc degeneration, suggesting an insight into pathoetiology of disc degeneration.

Ultra-short echo time MR imaging in assessing cartilage endplate damage and relationship between its lesion and disc degeneration for chronic low back pain patients

OBJECTIVE

To investigate the feasibility of ultra-short echo time (UTE) magnetic resonance imaging (MRI) in the assessment of cartilage endplate (CEP) damage and further evaluate the relationship between total endplate score (TEPS) and lumbar intervertebral disc (IVD) degeneration for chronic low back pain patients.

MATERIALS AND METHODS

IVD were measured in 35 patients using UTE imaging at 3T MR. Subtracted UTE images between short and long TEs were obtained to depict anatomy of CEP. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated to assess the image quality quantitatively. A new grading criterion for endplate damage evaluation was developed based on Rajasekaran.S grading system in this study. Two radiologists were employed to evaluate CEP and bony vertebral endplates (VEP) using this new grading criterion and assess TEPS, independently. Cohen's kappa analysis was applied to evaluate the inter-observer agreement of endplate damage assessment between two radiologists, and the Kendall's TAU-B analysis was employed to determine the relationship between TEPS and IVD degeneration evaluated with Pfirrmann grading.

RESULTS

Well structural CEP was depicted on subtracted UTE images and confirmed by high SNR (33.06±2.92) and CNR values (9.4±2.08). Qualified subtracted UTE images were used by two radiologists to evaluate the degree of CEP and VEP damage. Excellent inter-observer agreement was confirmed by high value in Cohen's kappa test (0.839, P < 0.001). Ensured by this, 138 endplates from 69 IVDs of 35 patients were classified into six grades based on the new grading criterion and TEPS of each endplate was calculated. In addition, the degeneration degree of IVDs were classified into five grades. Finally, using Kendall's TAU-B analysis, significant relationship was obtained between endplate damage related TEPS and IVD degeneration (r = 0.864, P < 0.001).

CONCLUSION

Ensured by high image quality, UTE imaging might be considered an effective tool to assess CEP damage. Additionally, further calculated TEPS has shown strong positive association with IVD degeneration, suggesting that the severity of endplate damage is highly linked with the degree of IVD degeneration.

The value of magnetic resonance ultrashort echo time imaging to evaluate non-calcified cartilage of the knee joint and its damage

Objectives

To image knee osteochondral specimens using magnetic resonance (MR) ultrashort echo time imaging with pointwise encoding time reduction with radial acquisition combined fat suppression (PETRA-FS) sequence to determine whether it can reveal non-calcified cartilage, including the deep radial layer, and to assess its effectiveness in cartilage damage diagnosis.

Materials and methods

PETRA-FS imaging was performed on 58 osteochondral specimens of the lower femur and upper tibia to observe depth of cartilage damage, combined with histological results to observe signal intensity composition. Sensitivity, specificity, and reliability of PETRA-FS sequence for diagnosing cartilage damage were evaluated using histological results as the gold standard. Diagnostic efficacy was assessed using receiver operating characteristic (ROC) curve.

Results

MR ultrashort echo time imaging PETRA-FS sequence showed non-calcified cartilage, including tangential, transitional, and radial layers, which showed a high signal. PETRA-FS sequence showed 37 cases of cartilage damage and 21 cases of no damage among 58 specimens, kappa value of 0.75. Histological analysis of the 58 osteochondral specimens revealed 38 cases of cartilage injury and 20 cases of undamaged cartilage. Using histological results as the gold standard, PETRA-FS sequence had a sensitivity of 87.00%, specificity of 80.00%, kappa value of 0.81, and an area under the ROC curve (AUC) of 0.83 for cartilage injury diagnosis.

Conclusion

MR ultrashort echo time imaging PETRA-FS sequence can show non-calcified cartilage, including the deep radial layer (which cannot be shown by conventional MR), by exhibiting a high signal in knee osteo-chondral specimens. Thus, PETRA-FS sequences may have important diagnostic value for cartilage injury diagnosis.

Ultrashort echo time magnetic resonance imaging of the osteochondral junction

Osteoarthritis is a common chronic degenerative disease that causes pain and disability with increasing incidence worldwide. The osteochondral junction is a dynamic region of the joint that is associated with the early development and progression of osteoarthritis. Despite the substantial advances achieved in the imaging of cartilage and application to osteoarthritis in recent years, the osteochondral junction has received limited attention. This is primarily related to technical limitations encountered with conventional MR sequences that are relatively insensitive to short T2 tissues and the rapid signal decay that characterizes these tissues. MR sequences with ultrashort echo time (UTE) are of great interest because they can provide images of high resolution and contrast in this region. Here, we briefly review the anatomy and function of cartilage, focusing on the osteochondral junction. We also review basic concepts and recent applications of UTE MR sequences focusing on the osteochondral junction.

Articular Cartilage Assessment Using Ultrashort Echo Time MRI: A Review

Articular cartilage is a major component of the human knee joint which may be affected by a variety of degenerative mechanisms associated with joint pathologies and/or the aging process. Ultrashort echo time (UTE) sequences with a TE less than 100 µs are capable of detecting signals from both fast- and slow-relaxing water protons in cartilage. This allows comprehensive evaluation of all the cartilage layers, especially for the short T₂ layers which include the deep and calcified zones. Several ultrashort echo time (UTE) techniques have recently been developed for both morphological imaging and quantitative cartilage assessment. This review article summarizes the current catalog techniques based on UTE Magnetic Resonance Imaging (MRI) that have been utilized for such purposes in the human knee joint, such as T₁, T2∗ , T_1ρ, magnetization transfer (MT), double echo steady state (DESS), quantitative susceptibility mapping (QSM) and inversion recovery (IR). The contrast mechanisms as well as the advantages and disadvantages of these techniques are discussed.

The Value of Ultrashort Echo Time MR in Depiction of the Eustachian Tube at 3 Tesla

OBJECTIVES

To investigate the feasibility of ultrashort echo time (UTE) imaging in the visualization of Eustachian tubes (ETs).

METHODS

The local institutional review board approved the study protocol. Twenty volunteers were involved in this study. The scanning scheme consisted of T2-sampling perfection with application-optimized contrasts by using different flip angle evolutions (T2-SPACE) MR imaging with water excitation, a UTE scan with a prototype sequence and a CT scan. The UTE images were compared with both the T2-SPACE and CT images. The quality of the images was rated by two radiologists blindly. Interobserver agreement was assessed using the kappa statistic. Statistical analysis was performed using SPSS software (version 17, SPSS Inc.). A value of p < 0.05 was considered statistically significant.

RESULTS

For all subjects, the T2-SPACE images successfully displayed the cartilaginous part of the ET (Fig. 1). The CT images fully showed the bony part of the ET for all the subjects (Fig. 2). The UTE images successfully displayed both the cartilaginous and bony parts of the ET (Fig. 3). However, the UTE images showed the cartilaginous and bony portions of the ET as a whole.

CONCLUSIONS

By analyzing the depiction of the ET from UTE images and comparing it with that from MR and CT images, we found that UTE images could display not only the cartilaginous structure of the ET that cannot be seen by conventional MR sequences, but also the bony structure that previously could only be seen on CT images. This imaging modality could help provide a convenient and new method to display the overall shape of the ET.

The interface region between articular cartilage and bone by μMRI and PLM at microscopic resolutions

This dual-modality microscopic imaging study quantifies the interface region between the noncalcified cartilage and the subchondral bone plate, which includes the deep portion of the noncalcified articular cartilage and the zone of calcified cartilage (ZCC). This interface region is typically not visible in routine MRI but becomes visible in MRI with the application of an ultra-short echo time (UTE) sequence. A number of cartilage-bone blocks from a well-documented canine humeral head were harvested for imaging by microscopic MRI (μMRI) and PLM (polarized light microscopy). In μMRI, T2 anisotropic images were acquired by 2D gradient-echo, magnetization-prepared spin-echo and UTE sequences at the 0° and 55° (the magic angle) orientations at 11.7 μm/pixel resolution. In PLM, quantitative optical retardation (nm) and collagen orientation (°) were mapped from the thin sections from the same μMRI specimens at 0.5-2 μm pixel resolutions. The orientational and organizational architecture of the collagen matrix in this interface region was quantified and correlated between the complementary imaging. The magic angle effect as seen in the noncalcified cartilage was statistically confirmed in ZCC in μMRI, which was further supported by quantitative PLM. With an enhanced understanding of the tissue properties in this important interface region, it will potentially be possible to monitor the changes of this tissue region which is instrumental to the initiation and development of osteoarthritis and other joint diseases.

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.

High-contrast osteochondral junction imaging using a 3D dual adiabatic inversion recovery-prepared ultrashort echo time cones sequence

While conventional MRI sequences cannot visualize tissues from the osteochondral junction (OCJ) due to these tissues' short transverse T₂ /T₂ * relaxations, ultrashort echo time (UTE) sequences can overcome this limitation. A 2D UTE sequence with a dual adiabatic inversion recovery preparation (DIR-UTE) for selective imaging of short T₂ tissues with high contrast has previously been developed, but high sensitivity to eddy currents and aliased out-of-slice excitation make it difficult to image the thin layer of the OCJ in vivo. Here, we combine the DIR scheme with a 3D UTE cones sequence for volumetric imaging of OCJ tissues in vivo, aiming to generate higher OCJ contrast compared with a recently developed single IR-prepared UTE sequence with a fat saturation module (IR-FS-UTE). All sequences were implemented on a 3-T clinical scanner. The DIR-UTE cones sequence combined a 3D UTE cones sequence with two narrow-band adiabatic IR preparation pulses centered on water and fat spectrum frequencies, respectively. The 3D DIR-UTE cones sequence was first applied to a phantom, then to the knees of four healthy volunteers and four patients diagnosed with osteoarthritis and compared with the IR-FS-UTE sequence. In both phantom and volunteer studies, the proposed DIR-UTE cones sequence showed much higher contrast for OCJ imaging than the IR-FS-UTE sequence did. The 3D DIR-UTE cones sequence showed a significantly higher contrast-to-noise ratio between the OCJ and subchondral bone fat (mean, standard deviation [SD]: 25.7 ± 2.3) and between the OCJ and superficial layers of cartilage (mean, SD: 22.2 ± 3.5) compared with the IR-FS-UTE sequence (mean, SD: 10.8 ± 2.5 and 16.3 ± 2.6, respectively). The 3D DIR-UTE cones sequence is feasible for imaging of the OCJ region of the knee in vivo and produces both high resolution and high contrast.

Bright ultrashort echo time SWIFT MRI signal at the osteochondral junction is not located in the calcified cartilage

In this study, we aimed to precisely localize the hyperintense signal that is generated at the osteochondral junction when using ultrashort echo time magnetic resonance imaging (MRI) and to investigate the osteochondral junction using sweep imaging with Fourier transformation (SWIFT) MRI. Furthermore, we seek to evaluate what compositional properties of the osteochondral junction are the sources of this signal. In the study, we obtained eight samples from a tibial plateau dissected from a 68-year-old male donor, and one additional osteochondral sample of bovine origin. The samples were imaged using high-resolution ultrashort echo time SWIFT MRI and microcomputed tomography (μCT) scans. Localization of the bright signal in the osteochondral junction was performed using coregistered data sets. Potential sources of the signal feature were examined by imaging the bovine specimen with variable receiver bandwidths and by performing variable flip angle T1 relaxation time mapping. The results of the study showed that the hyperintense signal was found to be located entirely in the deep noncalcified articular cartilage. The intensity of this signal at the interface varied between the specimens. Further tests with bovine specimens indicated that the imaging bandwidth and T1 relaxation affect the properties of the signal. Based on the present results, the calcified cartilage has low signal intensity even in SWIFT imaging. Concomitantly, it appears that the bright signal seen in ultrashort echo time imaging resides within the noncalcified cartilage. Furthermore, the most likely sources of this signal are the rapid T1 relaxation of the deep cartilage and the susceptibility-induced effects arising from the calcified tissues.

Assessment of mechanical properties of articular cartilage with quantitative three-dimensional ultrashort echo time (UTE) cones magnetic resonance imaging

Conventional magnetic resonance imaging (MRI) is not capable of detecting signal from the deep cartilage due to its short transverse relaxation time (T2). Moreover, several quantitative MRI techniques are significantly influenced by the magic angle effect. The combinations of ultrashort echo time (UTE) MRI with magnetization transfer (UTE-MT) and Adiabatic T_1ρ (UTE-AdiabT_1ρ) imaging allow magic angle-insensitive assessments of all regions of articular cartilage. The purpose of this study was to investigate the correlations between quantitative three-dimensional UTE MRI biomarkers and mechanical properties of human tibiofemoral cartilage specimens. In total, 40 human tibiofemoral cartilage specimens were harvested from three male and four female donors (64 ± 18 years old). Cartilage samples were scanned using a series of quantitative 3D UTE Cones T₂* (UTE-T₂*), T₁ (UTE-T₁), UTE-AdiabT_1ρ, and UTE-MT sequences in a standard knee coil on a clinical 3T scanner. UTE-MT data were acquired with a series of MT powers and frequency offsets to calculate magnetization transfer ratio (MTR), as well as macromolecular fraction (MMF) and macromolecular T₂ (T₂mm) through modeling. Cartilage stiffness and Hayes elastic modulus were measured using indentation tests. Correlations of 3D UTE Cones MRI measurements in the superficial layer, deep layer, and global regions of interest (ROIs) with mechanical properties were investigated. Cartilage mechanical properties demonstrated highest correlations with UTE measures of the superficial layer of cartilage. AdiabT_1ρ, MTR, and MMF in superficial layer ROIs showed significant correlations with Hayes elastic modulus (p < 0.05, R = -0.54, 0.49, and 0.66, respectively). These UTE measures in global ROIs showed significant, though slightly lower, correlations with Hayes elastic modulus (p < 0.05, R = -0.37, 0.52, and 0.60, respectively). Correlations between other UTE MRI measurements (T₂*, T₁, and T₂mm) and mechanical properties were non-significant. The 3D UTE-AdiabT_1ρ and UTE-MT sequences were highlighted as promising surrogates for non-invasive assessment of cartilage mechanical properties. MMF from UTE-MT modeling showed the highest correlations with cartilage mechanics.

Knee osteochondral junction imaging using a fast 3D T1-weighted ultrashort echo time cones sequence at 3T

The osteochondral junction (OCJ) of the knee joint is comprised of multiple tissue components, including a portion of the deep layer cartilage, calcified cartilage, and subchondral bone. The OCJ is of increasing radiological interest as it may be relevant in the early pathogenesis of osteoarthritis (OA). Due to its short transverse relaxation, the OCJ is invisible to clinical MR sequences. The purpose of this study was to develop a fast 3D T₁-weighted ultrashort echo time cones sequence with fat saturation (FS-UTE-Cones) for high resolution and high contrast imaging of the OCJ on a clinical 3T scanner. First, numerical simulations were performed to investigate how the flip angle affected the signal intensities and contrasts of both short and long T₁ tissues. The results from these simulations demonstrated that higher short T₁ contrast could be achieved with higher flip angle. Next, T₁ relaxation was measured for the different layers of a human patellar cartilage sample, and the results showed that the deepest layer had a significantly shorter T₁ value than other layers. Finally, a healthy knee joint was scanned with different flip angles and the OCJ was highlighted in the T₁-weighted FS-UTE-Cones sequence using a flip angle greater than 20°. The clinical T₂-weighted and proton density-weighted FSE sequences were also included for comparison, revealing a dark OCJ region. Representative T₁-weighted FS-UTE-Cones images of the whole knee of a healthy volunteer showed high signal intensity bands in the OCJ regions of the patella, femur, and tibia. On the other hand, T₁-weighted FS-UTE-Cones imaging of the knee joints of OA patients revealed regions with reduction or loss of these high signal intensity bands in the OCJ regions, indicating abnormal OCJ tissue composition. The proposed 3D T₁-weighted FS-UTE-Cones sequence with a 3-min scan time may be very useful for demonstrating the involvement of the OCJ regions in early OA.

Quantitative MRI UTE-T2* and T2* Show Progressive and Continued Graft Maturation Over 2 Years in Human Patients After Anterior Cruciate Ligament Reconstruction

Background:

Noninvasive quantitative magnetic resonance imaging (MRI) measures to assess anterior cruciate ligament (ACL) graft maturity are needed to help inform return to high-demand activities and to evaluate the effectiveness of new treatments to accelerate ACL graft maturation. Quantitative MRI ultrashort echo time T2* (UTE-T2*) and T2* mapping captures short T2 signals arising from collagen-associated water in dense regular connective tissues, such as tendon, ligament, and maturing grafts, which are invisible to conventional MRI.

Hypothesis:

Quantitative MRI UTE-T2* and T2* mapping is sensitive to ACL graft changes over the first 2 years after ACL reconstruction (ACLR).

Study Design:

Case series; Level of evidence, 4.

Methods:

A total of 32 patients (18 men; mean ± SD age, 30 ± 9 years) undergoing unilateral ACLR and 30 uninjured age-matched controls (18 men; age, 30 ± 9 years) underwent 3-T MRI examination. Patients who underwent ACLR were imaged at 6 weeks, 6 months, and 1 and 2 years postoperatively. Two separate ACLR cohorts were scanned with 2 MRI platforms at 2 institutions. Twelve ACLR knees were scanned with a 3-dimensional acquisition-weighted stack of spirals UTE sequence on a Siemens scanner, and 20 ACLR knees were scanned with a 3-dimensional Cones UTE sequence on a GE scanner. UTE-T2* or T2* maps were calculated for the intra-articular portion of the ACL graft.

Results:

Mean ACL graft UTE-T2* and T2* decreased from 1 to 2 years after ACLR. ACL graft T2* increased 25% to 30% during the first 6 months (P < .013) to a level not different from that of uninjured native ACL (P > .4), stabilized between 6 months and 1 year (P ≥ .999), and then decreased 19% between 1 and 2 years after ACLR (P = .027). At 6-month follow-up, ACL graft UTE-T2* differed from that of tendon (P < .02) but not uninjured native ACL (P > .7) and showed the greatest variability among patients.

Conclusion:

UTE-T2* mapping suggested substantial changes within the graft during the first 6 months postsurgery. T2* and UTE-T2* mapping showed relatively stable graft composition from 6 months to 1 year, consistent with remodeling, followed by decreases from 1 to 2 years, suggestive of continuing maturation. MRI UTE-T2* and T2* mapping demonstrated potential clinical utility as noninvasive quantitative imaging metrics for evaluation of human ACL grafts.

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.

Axial loading during MRI induces significant T2 value changes in vertebral endplates-a feasibility study on patients with low back pain

BACKGROUND

The function of the endplate (EP) is the most important factor influencing nutritional supply to the avascular intervertebral disc (IVD). It is desired to have a non-invasive method to assess functional EP characteristics in vivo. Assessment of functional EP characteristics is important in order to understand its relation to IVD degeneration, which in turn might deepen the understanding of the pathophysiology behind low back pain (LBP). It was hypothesized that, by comparing quantitative MRI of EPs performed with conventional supine MRI (unloaded MRI) with axial loading during MRI (alMRI), dynamical properties of the EP can be displayed. The aim was therefore to investigate the feasibility of axial loading during MRI (alMRI) to instantaneously induce quantitative EP changes.

METHODS

T2 mapping of 55 vertebral EPs (L1-S1) in five LBP patients was performed during conventional supine MRI (unloaded MRI) and subsequent alMRI. With T2 mapping, the cartilaginous EP and bony EP cannot be separated; hence, the visualized EP was termed EP zone (EPZ). Each EPZ was segmented at multiple midsagittal views, generating volumetric regions of interest. EPZs demonstrating signal inhomogeneity and/or adjacent Modic changes (MC) were termed abnormal EPZs. EPZ mean T2 values were compared between unloaded MRI and alMRI, and their relationship with abnormal EPZs was determined.

RESULTS

alMRI induced significantly higher (p = 0.01) EPZ mean T2 values compared with unloaded MRI. Significantly higher mean T2 values were seen in inferior EPZs compared with superior EPZs, both with unloaded MRI (35%, p < 0.001) and with alMRI (26%, p = 0.04). Significant difference between unloaded MRI and alMRI was seen in normal (p = 0.02), but not in abnormal EPZs (p = 0.5; n = 12).

CONCLUSIONS

alMRI induces changes in human EPZ characteristics in vivo. The T2 value significantly increased in normal EPZs, with lack of such in abnormal EPZs. Combining T2 mapping with alMRI provides a clinical feasible, non-invasive method with potential to reveal biochemical behavioral patterns, thus adding another dimension of the EPZs characteristics compared with information obtained with solely unloaded MRI.

New Techniques in MR Imaging of the Ankle and Foot

Foot and ankle disorders are common in everyday clinical practice. MR imaging is frequently required for diagnosis given the variety and complexity of foot and ankle anatomy. Although conventional MR imaging plays a significant role in diagnosis, contemporary management increasingly relies on advanced imaging for monitoring therapeutic response. There is an expanding need for identification of biomarkers for musculoskeletal tissues. Advanced imaging techniques capable of imaging these tissue substrates will be increasingly used in routine clinical practice. Radiologists should therefore become familiar with these innovative MR techniques. Many such techniques are already widely used in other organ systems.

Prestructural cartilage assessment using MRI

Cartilage loss is irreversible, and to date, no effective pharmacotherapies are available to protect or regenerate cartilage. Quantitative prestructural/compositional MR imaging techniques have been developed to characterize the cartilage matrix quality at a stage where abnormal findings are early and potentially reversible, allowing intervention to halt disease progression. The goal of this article is to critically review currently available technologies, present the basic concept behind these techniques, but also to investigate their suitability as imaging biomarkers including their validity, reproducibility, risk prediction and monitoring of therapy. Moreover, we highlighted important clinical applications. This review article focuses on the currently most relevant and clinically applicable technologies, such as T2 mapping, T2*, T1ρ, delayed gadolinium enhanced MRI of cartilage (dGEMRIC), sodium imaging and glycosaminoglycan chemical exchange saturation transfer (gagCEST). To date, most information is available for T2 and T1ρ mapping. dGEMRIC has also been used in multiple clinical studies, although it requires Gd contrast administration. Sodium imaging and gagCEST are promising technologies but are dependent on high field strength and sophisticated software and hardware.

LEVEL OF EVIDENCE

5 J. Magn. Reson. Imaging 2017;45:949-965.

Quantitative magnetic resonance imaging of the lumbar intervertebral discs

Human lumbar spine is composed of multiple tissue components that serve to provide structural stability and proper nutrition. Conventional magnetic resonance (MR) imaging techniques have been useful for evaluation of IVD, but inadequate at imaging the discovertebral junction and ligamentous tissues due primarily to their short T2 nature. Ultrashort time to echo (UTE) MR techniques acquire sufficient MR signal from these short T2 tissues, thereby allowing direct and quantitative evaluation. This article discusses the anatomy of the lumbar spine, MR techniques available for morphologic and quantitative MR evaluation of long and short T2 tissues of the lumbar spine, considerations for T2 relaxation modeling and fitting, and existing and new techniques for spine image post-processing, focusing on segmentation. This article will be of interest to radiologic and orthopaedic researchers performing lumbar spine imaging.

Ultrashort time to echo magnetic resonance techniques for the musculoskeletal system

Magnetic resonance (MR) imaging has been widely implemented as a non-invasive modality to investigate musculoskeletal (MSK) tissue disease, injury, and pathology. Advancements in MR sequences provide not only enhanced morphologic contrast for soft tissues, but also quantitative biochemical evaluation. Ultrashort time to echo (UTE) sequence, in particular, enables novel morphologic and quantitative evaluation of previously unseen MSK tissues. By using short minimum echo times (TE) below 1 msec, the UTE sequence can unveil short T2 properties of tissues including the deepest layers of the articular cartilage, cartilaginous endplate at the discovertebral junction, the meniscus, and the cortical bone. This article will discuss the application of UTE to evaluate these MSK tissues, starting with tissue structure, MR imaging appearance on standard versus short and ultrashort TE sequences, and provide the range of quantitative MR values found in literature.

Evaluation of the disco-vertebral junction using ultrashort time-to-echo magnetic resonance imaging: inter-reader agreement and association with vertebral endplate lesions

OBJECTIVE

To evaluate ultrashort time to echo (UTE) magnetic resonance (MR) morphology of the cartilaginous endplates (CEP) in cadaveric lumbar spines with bony vertebral endplate (VEP) lesions, to determine inter-reader agreement as well as associations between the CEP morphology and VEP lesions as well as other abnormalities.

MATERIALS AND METHODS

MR imaging of cadaveric lumbar spines from 10 donors was performed at 3T using a UTE MR sequence. Two musculoskeletal radiologists identified the location of vertebral endplate lesions in consensus. The morphology of the CEP overlying the lesions and in the adjacent normal regions was assessed individually. A total of 55 vertebral lesions and 55 normal regions were assessed. The presence of osteophytosis, morphological changes of the anterior and posterior longitudinal ligament, and intervertebral disc signal and morphology was also assessed. Agreement between observers was determined using Cohen's kappa analysis, and association between CEP and vertebral endplate lesions was determined using the chi square test.

RESULTS

Fifty-five vertebral endplate lesions were identified and the morphology of CEP evaluated by two readers was in substantial agreement with Cohen's kappa of 0.78. The presence of vertebral endplate abnormality was associated with the presence of osteophytes (39 out of 55 levels), altered morphology and signal of the anterior longitudinal ligament (23 out of 55 levels) and intervertebral discs (30 out of 55 levels).

CONCLUSION

UTE MRI enables evaluation of the CEP with substantial inter-reader agreement. Abnormal changes of the CEP may facilitate formation of lesions of vertebral endplate over time and are associated with degenerative changes of the lumbar spine.

Quantitative rotating frame relaxometry methods in MRI

Macromolecular degeneration and biochemical changes in tissue can be quantified using rotating frame relaxometry in MRI. It has been shown in several studies that the rotating frame longitudinal relaxation rate constant (R1ρ ) and the rotating frame transverse relaxation rate constant (R2ρ ) are sensitive biomarkers of phenomena at the cellular level. In this comprehensive review, existing MRI methods for probing the biophysical mechanisms that affect the rotating frame relaxation rates of the tissue (i.e. R1ρ and R2ρ ) are presented. Long acquisition times and high radiofrequency (RF) energy deposition into tissue during the process of spin-locking in rotating frame relaxometry are the major barriers to the establishment of these relaxation contrasts at high magnetic fields. Therefore, clinical applications of R1ρ and R2ρ MRI using on- or off-resonance RF excitation methods remain challenging. Accordingly, this review describes the theoretical and experimental approaches to the design of hard RF pulse cluster- and adiabatic RF pulse-based excitation schemes for accurate and precise measurements of R1ρ and R2ρ . The merits and drawbacks of different MRI acquisition strategies for quantitative relaxation rate measurement in the rotating frame regime are reviewed. In addition, this review summarizes current clinical applications of rotating frame MRI sequences. Copyright © 2016 John Wiley & Sons, Ltd.

Ultrashort TE evaluation of the osteochondral junction in vivo: a feasibility study

OBJECTIVE

To determine whether differences in the osteochondral junction (OCJ) in two different age groups could be detected with ultrashort time-to-echo (UTE) MRI in vivo.

METHODS

10 healthy controls aged 20-30 years (Group 1) and 10 patients aged 40-50 years with knee pain but no evidence of osteoarthritis (OA) (Group 2) underwent 3-T MRI of the knee using a two-dimensional UTE sequence. Evaluation of the presence/absence of a high-signal-intensity (SI) line at the medial tibial/lateral tibial (MT/LT) OCJ was performed. Regions of interest (ROI) were created at the MT OCJ and LT OCJ. The peak OCJ SI ratio was calculated by measuring peak OCJ SI using averaged craniocaudal SI profiling, then dividing by the mean SI of a background ROI to standardize between studies. Groups were compared using χ(2) and Mann-Whitney U tests.

RESULTS

A high SI line was present in significantly more Group 1 subjects at both MT (p < 0.001) and LT (p = 0.002). There was a significant difference (p < 0.001) in MT peak OCJ SI ratio between Group 1 (mean ± SD = 0.85 ± 0.10) and Group 2 (0.77 ± 0.12). There was no significant difference (p = 0.19) in LT peak OCJ SI ratio between Group 1 (0.81 ± 0.23) and Group 2 (0.80 ± 0.13).

CONCLUSION

Significant differences in the UTE MR characteristics of the OCJ were demonstrated between the two age groups.

ADVANCES IN KNOWLEDGE

Alterations in the UTE appearances of the tibial plateau OCJ in the older group with knee pain compared with a younger, healthy control cohort may reflect the very early stages of OA.

Morphological MR imaging of the articular cartilage of the knee at 3 T-comparison of standard and novel 3D sequences

Objectives

This study sought to compare various 3D cartilage sequences and to evaluate the usefulness of ultrashort TE (UTE) imaging, a new technique to isolate signal from the osteochondral junction.

Methods

Twenty knees were examined at 3 T with 3D spoiled GRE (FLASH), double-echo steady-state (DESS), balanced SSFP, 3D turbo spin-echo (TSE), and a prototype UTE sequence. Two radiologists independently evaluated all images. Consensus readings of all sequences were the reference standard. Statistical analysis included Friedman and pairwise Wilcoxon tests. Retrospective analysis of UTE morphology of osteochondral tissue in normal and abnormal cartilage seen at conventional MR was also performed.

Results

Three-dimensional TSE was superior to other sequences for detecting cartilage lesions. FLASH and DESS performed best in the subjective quality analysis. On UTE images, normal cartilage exhibited a high-intensity linear signal near the osteochondral junction. Retrospective analysis revealed abnormal UTE morphology of the osteochondral junction in 50 % of cartilage lesions diagnosed at conventional MR.

Conclusions

Cartilage imaging of the knee at 3 T can be reliably performed using 3D TSE, showing high accuracy when compared to standard sequences. Although UTE depicts signal from the deep cartilage layer, further studies are needed to establish its role for assessment of cartilage.

Main Messages

• MRI is the best available imaging method for assessment of knee cartilage.

• Cartilage imaging can be reliably performed using 3D TSE.

• UTE cannot be used as a single sequence to assess cartilage.