[MRI of the knee joint: first results of a comparison of 0,2-T specialized system and 1,5-T high field strength magnet]

An evolution of low-field strength MRI

The paper describes the evolution of low-field MRI from the very early pioneering days in the late 70 s until today. It is not meant to give a comprehensive historical account of the development of MRI, but rather to highlight the different research environments then and now. In the early 90 s, when low-field systems below 1.5 T essentially vanished, there were just no reasonable means available to make up for the factor of roughly three in signal-to-noise-ratio (SNR) between 0.5 and 1.5 T. This has drastically changed. Improvements in hardware-closed Helium-free magnets, RF receiver systems and especially much faster gradients, much more flexible sampling schemes including parallel imaging and compressed sensing and especially the use of AI at all stages of the imaging process have made low-field MRI a clinically viable supplement to conventional MRI. Ultralow-field MRI with magnets around 0.05 T are also back and constitute a bold and courageous endeavor to bring MRI to communities, which have neither the means nor the infrastructure to sustain a current standard of care MRI.

[Low-field magnetic resonance imaging : Just less expensive or completely different?]

Over the years the development of field strength in magnetic resonance imaging (MRI) has continued to increase from the low-field systems in the early years (0.2-0.5 T) to 1.5 T to 3 T to 7 T and more. In the last 2 years, there has been a renewed interest in MRI at lower fields, mainly driven by the development of "dry" superconductive magnets. The following article demonstrates that this renewed interest for lower fields is not a déjà vu purely driven by economic needs. The field strength appears to be from yesterday, but the combination with the tremendous improvements and innovations of all relevant components-gradients, radiofrequency system and especially new algorithms including the use of artificial intelligence (AI)-allow the realization of diagnostically adequate MRI without compromise in patient throughput and efficiency. In addition to the lower field, there are also some inherent advantages, e.g., for MRI of the lung and of metallic implants and especially for interventional MRI. It has already been shown that many of the devices used for interventional procedures (catheters, biopsy needles) can be used at lower fields without costly modifications. In addition, low-field MRI also allows the robust use of highly efficient sampling methods like spiral MRI. It is therefore safe to predict that low-field MRI is not only a cost-efficient compromise, but has the potential to open up new fields of application.

Arthroscopic and low-field MRI (0.25 T) evaluation of meniscus and ligaments of painful knee

Objective:

Magnetic resonance imaging (MRI) is an accurate, non-invasive, cost-effective technique for examination of the soft tissue and osseous structures of the knee. The purpose of this study was to evaluate the accuracy of low-field MRI by comparing the results with subsequent arthroscopy.

Materials and Methods:

MR imaging study of 146 patients was done using 0.25 T ESTOATE G-SCAN and the sequence used were SE, FSE and GRE in all the three planes. The comparison was based on five parameters: accuracy, sensitivity, specificity, positive predictive value, and negative predictive value.

Result:

Our study showed high accuracy (98.08%) and negative predictive value (98.62%) for MRI in comparison with arthroscopy.

Conclusion:

Low-field MRI alleviates the need of arthroscopy for detection of meniscus tears and ligament tears.

Segmenting articular cartilage automatically using a voxel classification approach

We present a fully automatic method for articular cartilage segmentation from magnetic resonance imaging (MRI) which we use as the foundation of a quantitative cartilage assessment. We evaluate our method by comparisons to manual segmentations by a radiologist and by examining the interscan reproducibility of the volume and area estimates. Training and evaluation of the method is performed on a data set consisting of 139 scans of knees with a status ranging from healthy to severely osteoarthritic. This is, to our knowledge, the only fully automatic cartilage segmentation method that has good agreement with manual segmentations, an interscan reproducibility as good as that of a human expert, and enables the separation between healthy and osteoarthritic populations. While high-field scanners offer high-quality imaging from which the articular cartilage have been evaluated extensively using manual and automated image analysis techniques, low-field scanners on the other hand produce lower quality images but to a fraction of the cost of their high-field counterpart. For low-field MRI, there is no well-established accuracy validation for quantitative cartilage estimates, but we show that differences between healthy and osteoarthritic populations are statistically significant using our cartilage volume and surface area estimates, which suggests that low-field MRI analysis can become a useful, affordable tool in clinical studies.

Acute knee trauma: value of a short dedicated extremity MR imaging examination for prediction of subsequent treatment

PURPOSE

To assess the predictive value of a short magnetic resonance (MR) imaging examination, in addition to or instead of radiography, performed in patients with acute knee trauma to identify those who require additional treatment versus those who do not and can be discharged without further follow-up.

MATERIALS AND METHODS

The randomized controlled trial and use of collected data for prediction modeling were approved by the institutional review board; informed consent was obtained. Patients with recent knee injury were included in the trial if radiography was ordered. They were randomized into a group undergoing only radiography and a group undergoing radiography plus immediate MR imaging. A 0.2-T dedicated extremity MR imager and four short pulse sequences were used. Univariable and multivariable logistic regression analysis was used to evaluate patient characteristics, trauma mechanism, and findings at radiography and MR imaging for their value in prediction of need for subsequent treatment within the 6-month follow-up.

RESULTS

Data in 189 patients (123 male patients, 66 female patients; mean age, 33.4 years), 109 of whom underwent treatment after their initial visit, were analyzed. Age of 30 years or older, indirect trauma mechanism, radiographic results, and MR imaging results were significant predictors of need for treatment in univariable and multivariable analyses (P < .05). In the multivariable analysis, only abnormal MR imaging results were significantly predictive of need for treatment, and only when MR imaging replaced radiography (odds ratio, 2.61; 95% confidence interval: 1.12, 6.06).

CONCLUSION

Implementation of a dedicated extremity MR imaging examination, in addition to or instead of radiography, performed in patients with traumatic knee injury improves prediction of the need for additional treatment but does not significantly aid in identification of patients who can be discharged without further follow-up. Value of a short MR imaging examination in the initial stage after knee trauma is limited.

Effectiveness of MR imaging in selection of patients for arthroscopy of the knee

PURPOSE

To determine the effectiveness of magnetic resonance (MR) imaging in the appropriate identification of those patients with a high clinical suspicion of internal derangements of the knee who require arthroscopic therapy.

MATERIALS AND METHODS

In a prospective multicenter study, MR imaging was performed at 0.5 T in 430 consecutive patients. The sensitivity and specificity of MR imaging in the patients who underwent arthroscopy and the corrected sensitivity and specificity of MR in all the study patients were calculated. For this correction, patients with negative MR and arthroscopic results were considered representative of the patients with negative MR results who were conservatively treated, and the number of the former was doubled. The standard errors of the corrected values were adjusted with the delta method.

RESULTS

At MR imaging, arthroscopy was indicated in 221 patients, 200 of whom underwent arthroscopy. Two hundred nine patients with negative MR imaging results were randomized for arthroscopic (105 patients) or for conservative treatment (104 patients). Of the 105 patients randomized for arthroscopy, 93 actually underwent arthroscopy. Arthroscopic treatment was necessary in 13 of 93 patients with a negative diagnosis at MR imaging. Arthroscopic treatment was necessary in 179 of 200 patients with a positive diagnosis at MR (sensitivity, 93.2%; specificity, 79.2%). Sensitivity and specificity corrected for randomization were 87.3% and 88.4%. Sensitivity and specificity corrected for randomization, respectively, were 84.1% and 94.2% for the diagnosis of medial meniscal tears and 69.5% and 94.5% for the diagnosis of lateral meniscal tears at MR.

CONCLUSION

MR imaging is an effective tool in the selection of patients for arthroscopy from among a general population.

Detection of articular cartilage lesions: experimental evaluation of low- and high-field-strength MR imaging at 0.18 and 1.0 T

The objective of this study was to compare the diagnostic performance of a dedicated orthopedic magnetic resonance (MR) imaging system (0.18 T) and a conventional MR imaging system (1.0 T) in the detection of articular cartilage lesions. Fifty knee joint specimens of pigs with artificially created articular cartilage lesions of different diameters, grades (2-3), and localizations, as well as 50 joints with intact articular cartilage, were imaged at 0. 18 and 1.0 T. Diagnostic performance was determined by means of receiver operating characteristics (ROC) analysis with three independent observers. For none of the pulse sequences used at 0.18 T or 1.0 T areas under ROC curves (A(z)) showed significant differences between the three observers. A(z) values from averaged data were as follows: a) 0.18 T: T1-weighted spin echo (SE): 0.70, proton-density-weighted SE: 0.59, T2-weighted SE: 0.61, two-dimensional (2D) gradient-echo (GRE): 0.73, 3D GRE: 0.75; and b) 1.0 T: T1-weighted SE: 0.73, fat-suppressed T2-weighted turbo-SE: 0. 79, 2D fast low-angle shot (FLASH): 0.79, fat-suppressed 3D FLASH: 0. 96, and water-excited 3D double-echo steady state (DESS): 0.96. With the use of 3D pulse sequences, the high-field system demonstrated a significantly better diagnostic performance than the low-field system in the detection of grades 2 and 3 articular cartilage lesions (P < 0.001).

MR imaging of the knee at 0.2 and 1.5 T: correlation with surgery

OBJECTIVE

The purpose of this study was to compare the diagnostic efficacy of low- and high-field-strength MR imagers in the diagnosis of anterior cruciate ligament tears and meniscus tears.

SUBJECTS AND METHODS

In 219 patients with suspected internal derangement of the knee, MR imaging at 0.2 and 1.5 T was performed with similar sequences. Only patients with surgically confirmed diagnosis (n = 90) were included in the statistical analysis. Radiologists were unaware of diagnosis and field strength. Sensitivity, specificity, diagnostic accuracy, and inter- and intraobserver variability were determined.

RESULTS

There was excellent correlation between the field strengths in accuracy, sensitivity, and specificity for anterior cruciate ligament and meniscus tears. Accuracy for medial meniscus, lateral meniscus, and anterior cruciate ligament tears was 91-93%, 88-90%, and 93-96%, respectively, at 0.2 T and 91-94%, 91-93%, and 97-98%, respectively, at 1.5 T. Inter- and intraobserver variability values showed excellent correlation (kappa > 0.8).

CONCLUSION

The level of diagnostic accuracy in anterior cruciate ligament tears and meniscus tears is comparable for low- and high-field-strength MR imagers.

MR arthrography of the shoulder: comparison of low-field (0.2 T) vs high-field (1.5 T) imaging

The objective of this study was to compare the image quality, sensitivity, specificity, and diagnostic accuracy of an open low-field MR system (0.2 T) with a standard high-field MR system (1.5 T) after arthrography of the shoulder. Thirty-eight patients either with suspected chronic instability (n = 12) or rotator cuff abnormalities (n = 26) were examined. Intra-articular injection of diluted Gd-DTPA was followed in randomized order either first by imaging on an open 0.2-T system or on a 1.5-T system. The image material was evaluated independently by two radiologists in a blinded fashion with respect to overall image quality and the detection of rotator cuff as well as capsular and labral abnormalities. Surgical correlation was available in 27 (71%) of 38 patients. For both systems, sensitivity and specificity for rotator cuff tears were 100% each, and for labrum pathologies, these values were 100 and 93%, respectively. The agreement for detection of labral pathologies between low-field and high-field examinations was good (kappa = 0.69, kappa = 0.61). For the detection of full-thickness tears of the rotator cuff, the agreement between the low-field and high-field MR examinations was very good and significant (kappa = 0.94, kappa = 1, p < 0.001). Overall image quality was rated good in 17 (45%) and fair in 21 (55%) of 38 cases on the 0.2-T MR system, and good in 32 (84%) and fair in 6 (16%) of 38 cases on the 1.5-T system. Motion artifacts were considered low in 24 (63%) and moderate in 14 (37%) of 38 cases for the 0.2-T system and low in 34 (89%) and moderate in 4 (11%) for 1.5-T system. Based on our results, low-field MR compares favorably to high-field MR in the detection of major abnormalities of the glenohumeral joint, at least when MR arthrography is used. Disadvantages are the duration of the examination and thus the risk of reduced image quality caused by motion artifacts.

Comparison of low field (0.2T) and high field (1.5T) MR imaging in the differentiation of torned from intact menisci

PURPOSE

To evaluate the usefulness of a low field MRI system (0.2T; Esaote, Biomedica) for the evaluation of meniscal tears with regard to anatomic site, and to compare the results with findings from a high field unit (1.5T; Siemens, Erlangen).

MATERIAL AND METHODS

MRI was performed in 25 patients in a low field (0.2T; Esaote, Biomedica), and a high field (1.5T; Siemens, Erlangen) MRI unit. The images were analyzed for the presence or absence of meniscal tears and the confidence of decision making. Results were further analyzed for the number of identical and unidentical findings on both imaging modalities. In seven patients, arthroscopy was performed and the findings compared with the results from MR imaging. Statistical analysis was performed by chi 2-test, Wilcoxon test and Friedman analysis.

RESULTS

Qualitative evaluation of the level of confidence in decision making was significantly superior on high field strength images. When comparing the evaluations from both image modalities in 21 of 25 patients (84%), the diagnosis concerning the presence or absence of meniscal tears was identical.

CONCLUSION

Although low field MR imaging might offer diagnostic potential concerning the presence or absence of meniscal tears, the level of confidence in decision making is significantly superior with high field strength imaging, probably reflecting the higher conspicuity of lesions from high field strength units.

Low-field magnetic resonance imaging (0.2 T) of tendons with sonographic and histologic correlation. Cadaveric study

RATIONALE AND OBJECTIVES

The authors evaluate the role of low-field strength magnetic resonance imaging (MRI) compared with sonography in the evaluation of degenerative changes of tendons, with histologic correlation, based on investigations of horse cadavers.

METHODS

Low-field MRI and sonography was performed in 42 hours specimens for the evaluation of tendons and ligaments. Magnetic resonance imaging included sagittal and axial T1-weighted, T2-weighted, and gradient echo images. Sonography and MR images were evaluated for degenerative changes or tears and the findings were correlated with the histologic results.

RESULTS

Using histologic findings as a gold standard, the accuracy for the sonographic evaluation was 65.9%, sensitivity was 16.7%, and specificity was 100%. The corresponding data for low-field MR imaging were 70.5% accuracy, 44.4% sensitivity, and 88.5% specificity.

CONCLUSIONS

Low-field MRI investigation allows more accurate staging of tendinous changes than sonography. It is more reproducible and potentially includes the advantages of the combined evaluation of bones, ligaments, and soft tissue.

MRI of the foot and ankle: diagnostic performance and patient acceptance of a dedicated low field MR scanner

The objective of this study was to compare image quality and patient acceptance of a dedicated .2-T MR system and a 1.0-T whole body system. Forty-one consecutive patients referred for MRI of the foot or ankle were prospectively examined with a dedicated .2-T low field system and a 1.0-T whole body system. Images were evaluated qualitatively by two observers and quantitatively using signal-difference-to-noise ratios. The patients were interviewed with respect to positioning, examination time, noise, claustrophobia, confidence in the diagnosis, and willingness to repeat the examination, using a questionnaire. The qualitative score was significantly higher for the 1.0-T system (2.6 vs 2.2 for reader 1 [P = .008] and 2.6 vs 1.7 for reader 2 [P < .0001]), respectively). The signal-difference-to-noise ratios were also superior for the 1.0-T MR system (2.96 vs .88, P < .0001). However, 96% of the lesions visualized at 1.0 T were also detected with the low field system. Patient acceptance was significantly better for the 1.0-T MR scanner (48.6 vs 43.9, P = .007). Image quality of the dedicated low field system was inferior to the 1.0-T system using objective parameters, and patients did not prefer the low field system. Although only 4% of lesions were missed in this series, the low field MR system can only be recommended when funding is limited and the available space is limited.

MRI preferable to diagnostic arthroscopy in knee joint injuries. A double-blind comparison of 47 patients

We compared the findings of low-field MRI of the knee with those of subsequent arthroscopy. In a double-blind set-up, 47 patients with knee joint injuries were enrolled. Two radiologists independently interpreted the MRI examinations and consensus was obtained in case of discrepancy. Arthroscopy was performed without knowledge of the MRI findings. The accuracy rates of MRI for evaluating the medial meniscus, lateral meniscus and anterior cruciate ligament were 77%, 91% and 96%, respectively, when arthroscopy was considered the "golden standard". When MRI was considered the standard, the figures for arthroscopy were 74%, 91% and 96%. MRI found the indication for treatment in 18 of 21 patients who were treated at the arthroscopy. In 17 patients, neither MRI nor arthroscopy detected any lesion. In the remaining 9 patients, MRI demonstrated a lesion, but no lesion was found at the subsequent arthroscopy. Our conclusion is that low-field MRI can be used as a first-line diagnostic examination in patients with suspected meniscus or cruciate ligament injuries and thus a substantial number of negative diagnostic arthroscopies can be avoided.

Accuracy of imaging the menisci on an in-office, dedicated, magnetic resonance imaging extremity system

Magnetic resonance imaging effectively defines and characterizes musculoskeletal pathologic lesions, particularly meniscal tears. Most studies comparing the efficacy of magnetic resonance imaging and arthroscopic evaluation have been performed on high-field (1.5-T) systems. The effectiveness of a low-field (0.2-T), dedicated, extremity magnetic resonance imaging device in diagnosing meniscal tears was studied prospectively on 35 patients with knee symptoms who subsequently had arthroscopic evaluation. Magnetic resonance imaging examinations were performed before surgery and were read by an experienced radiologist who was blinded to the results of the arthroscopic evaluations. Specificity was 100% for both the medial and lateral menisci. Sensitivity was 86% for the medial menisci, 89% for the lateral menisci, and 87% for both. Accuracy was 91% for the medial menisci, 97% for the lateral menisci, and 94% overall. The positive predictive values were 100% for the medial menisci, 100% for the lateral menisci, and 100% for both. The negative predictive values were 81% for the medial menisci, 96% for the lateral menisci, and 91% for both. The low-field magnetic resonance imaging system provided specificity and sensitivity that were equal to or better than previous reports with high-field systems. In particular, this low-field system eliminated the problem of false-positive results that has been found in some studies using high-field systems.