MR-Imaging in Osteoarthritis: Current Standard of Practice and Future Outlook

Osteoarthritis (OA) is a common degenerative joint disease that affects millions of people worldwide. Magnetic resonance imaging (MRI) has emerged as a powerful tool for the evaluation and monitoring of OA due to its ability to visualize soft tissues and bone with high resolution. This review aims to provide an overview of the current state of MRI in OA, with a special focus on the knee, including protocol recommendations for clinical and research settings. Furthermore, new developments in the field of musculoskeletal MRI are highlighted in this review. These include compositional MRI techniques, such as T2 mapping and T1rho imaging, which can provide additional important information about the biochemical composition of cartilage and other joint tissues. In addition, this review discusses semiquantitative joint assessment based on MRI findings, which is a widely used method for evaluating OA severity and progression in the knee. We analyze the most common scoring methods and discuss potential benefits. Techniques to reduce acquisition times and the potential impact of deep learning in MR imaging for OA are also discussed, as these technological advances may impact clinical routine in the future.

3D MRI of Articular Cartilage

Osteoarthritis, characterized by the breakdown of articular cartilage and other joint structures, is one of the most prevalent and disabling chronic diseases in the United States. Magnetic resonance imaging is a commonly used imaging modality to evaluate patients with joint pain. Both two-dimensional fast spin-echo sequences (2D-FSE) and three-dimensional (3D) sequences are used in clinical practice to evaluate articular cartilage. The 3D sequences have many advantages compared with 2D-FSE sequences, such as their high in-plane spatial resolution, thin continuous slices that reduce the effects of partial volume averaging, and ability to create multiplanar reformat images following a single acquisition. This article reviews the different 3D imaging techniques available for evaluating cartilage morphology, illustrates the strengths and weaknesses of 3D approaches compared with 2D-FSE approaches for cartilage imaging, and summarizes the diagnostic performance of 2D-FSE and 3D sequences for detecting cartilage lesions within the knee and hip joints.

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.

Significant Association between the T2 Values of Vertebral Cartilage Endplates and Pfirrmann Grading

Objective

The T2 value of lumbar cartilage endplates was measured using the T2 mapping imaging technique, aiming to explore the correlation between the T2 value and Pfirrmann grading of intervertebral discs.

Methods

A total of 130 patients with lumbar spine MR examination due to persistent low back pain were enrolled, including 71 men and 59 women (age: 21–63 years). Lumbar Modic changes and Schmorl nodules were recognized by conventional T1WI and T2WI images in 49 patients, and these patients were excluded from the study. A total of 81 patients were enrolled in this study, including 45 men (45.16 ± 12.20 years) and 36 women (43.33 ± 11.27 years). Pfirrmann (Pm) grading of each lumbar disc was performed based on conventional T2WI median sagittal images and the position of cartilage endplates (CEP) was determined by IDEAL‐SPGR images. Meanwhile, the T2 mapping technique was used to obtain T2 values of cartilage endplates. The T2 values of CEP corresponding to different Pm grade discs were compared, and the correlation between the T2 value and the Pm grade of intervertebral discs was analyzed.

Results

The T2 values of cephalic and caudal CEP of L_1–2 in Pm grades I–II, Pm grades III, and Pm grades IV–V were 61.96 ± 5.89 ms, 54.45 ± 3.29 ms, 42.47 ± 3.69 ms and 64.35 ± 5.93 ms, 55.28 ± 3.97 ms, 44.75 ± 2.12 ms, respectively. For cephalic and caudal CEP of L_2–3, the T2 values in Pm grades I–II, Pm grades III, and Pm grades IV–V were 62.96 ± 6.93 ms, 55.19 ± 4.02 ms, 48.67 ± 4.56 ms and 65.51 ± 6.49 ms, 57.16 ± 5.55 ms, 52.05 ± 4.20 ms, respectively. The T2 values of cephalic and caudal CEP from L_3–4 to L₅–S₁ in Pm grades I–II, Pm grades III, and Pm grades IV–V were (63.72 ± 5.76 ms, 53.96 ± 6.52 ms, 48.05 ± 5.00 ms), (65.46 ± 6.37 ms, 55.70 ± 7.50 ms, 48.10 ± 3.27 ms); (66.34 ± 7.68 ms, 56.76 ± 9.48 ms, 47.80 ± 4.33 ms), (64.44 ± 4.65 ms, 59.30 ± 8.80 ms, 47.30 ± 5.78 ms), (65.32 ± 5.11 ms, 55.33 ± 6.65 ms, 48.18 ± 5.37 ms), and (63.47 ± 4.92 ms, 50.32 ± 8.86 ms, 44.77 ± 4.69 ms), respectively. There were significant differences in T2 values of cartilage endplates between the Pm grades I–II, III, and IV–V of intervertebral discs (P = 0.000). T2 values corresponding to Pm I–II grades were higher than those in Pm III grade, while T2 values in Pm grades IV–V were lowest. The T2 value of the L_4–5, L₅–S₁ segment endplates was highly correlated with the Pm grades (r = −0.711, −0.721, −0.796, −0.745; P = 0.000) and that of L_1–2, L_2–3 endplates were moderately correlated (r = −0.542, −0.562, −0.637, −0.612; P = 0.000).

Conclusion

The T2 values of cartilage endplates revealed varying degrees of degeneration of intervertebral discs, and more severe degeneration corresponded to lower T2 values. Measurement of changes in the T2 value through cartilage endplates can be useful for the diagnosis of early intervertebral disc degeneration and the prevention of disc degeneration.

Accuracy of magnetic resonance imaging to detect cartilage loss in severe osteoarthritis of the first carpometacarpal joint: comparison with histological evaluation

Background

Magnetic resonance imaging (MRI) is increasingly used for research in hand osteoarthritis, but imaging the thin cartilage layers in the hand joints remains challenging. We therefore assessed the accuracy of MRI in detecting cartilage loss in patients with symptomatic osteoarthritis of the first carpometacarpal (CMC1) joint.

Methods

Twelve patients scheduled for trapeziectomy to treat severe symptomatic osteoarthritis of the CMC1 joint underwent a preoperative high resolution 3D spoiled gradient (SPGR) MRI scan. Subsequently, the resected trapezium was evaluated histologically. The sections were scored for cartilage damage severity (Osteoarthritis Research Society International (OARSI) score), and extent of damage (percentage surface area). Each MRI scan was scored for the area of normal cartilage, partial cartilage loss and full cartilage loss. The percentages of the total surface area with any cartilage loss and full-thickness cartilage loss were calculated using MRI and histological evaluation.

Results

MRI and histological evaluation both identified large areas of overall cartilage loss. The median (IQR) surface area of any cartilage loss on MRI was 98% (82–100%), and on histological assessment 96% (87–98%). However, MRI underestimated the extent of full-thickness cartilage loss. The median (IQR) surface area of full-thickness cartilage loss on MRI was 43% (22–70%), and on histological evaluation 79% (67–85%). The difference was caused by a thin layer of high signal on the articulating surface, which was interpreted as damaged cartilage on MRI but which was not identified on histological evaluation.

Conclusions

Three-dimensional SPGR MRI of the CMC1 joint demonstrates overall cartilage damage, but underestimates full-thickness cartilage loss in patients with advanced osteoarthritis.

IDEAL 3D spoiled gradient echo of the articular cartilage of the knee on 3.0 T MRI: a comparison with conventional 3.0 T fast spin-echo T2 fat saturation image

BACKGROUND

Many two-dimensional (2D) morphologic cartilage imaging sequences have disadvantages such as long acquisition time, inadequate spatial resolution, suboptimal tissue contrast, and image degradation secondary to artifacts. IDEAL imaging can overcome these disadvantages.

PURPOSE

To compare sound-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and quality of two different methods of imaging that include IDEAL 3D SPGR and 3.0-T FSE T2 fat saturation (FS) imaging and to evaluate the utility of IDEAL 3D SPGR for knee joint imaging.

MATERIAL AND METHODS

SNR and CNR of the patellar and femoral cartilages were measured and calculated. Two radiologists performed subjective scoring of all images for three measures: general image quality, FS, and cartilage evaluation. SNR and CNR values were compared by paired Student's t-tests.

RESULTS

Mean SNRs of patellar and femoral cartilages were 90% and 66% higher, respectively, for IDEAL 3D SPGR. CNRs of patellar cartilages and joint fluids were 2.4 times higher for FSE T2 FS, and CNR between the femoral cartilage and joint fluid was 2.2 times higher for FSE T2 FS. General image quality and FS were superior using FSE T2 FS compared to those of IDEAL 3D SPGR imaging according to both readers, while cartilage evaluation was superior using IDEAL 3D SPGR. Additionally, cartilage injuries were more prominent in IDEAL 3D SPGR than in FSE T2FS according to both readers.

CONCLUSION

IDEAL 3D SPGR images show excellent visualization of patellar and femoral cartilages in 3.0 T and can compensate for the weaknesses of FSE T2 FS in the evaluation of cartilage injuries.

OARSI Clinical Trials Recommendations: Knee imaging in clinical trials in osteoarthritis

Significant advances have occurred in our understanding of the pathogenesis of knee osteoarthritis (OA) and some recent trials have demonstrated the potential for modification of the disease course. The purpose of this expert opinion, consensus driven exercise is to provide detail on how one might use and apply knee imaging in knee OA trials. It includes information on acquisition methods/techniques (including guidance on positioning for radiography, sequence/protocol recommendations/hardware for magnetic resonance imaging (MRI)); commonly encountered problems (including positioning, hardware and coil failures, sequences artifacts); quality assurance (QA)/control procedures; measurement methods; measurement performance (reliability, responsiveness, validity); recommendations for trials; and research recommendations.

Classification of histologically scored human knee osteochondral plugs by quantitative analysis of magnetic resonance images at 3T

This work evaluates the ability of quantitative MRI to discriminate between normal and pathological human osteochondral plugs characterized by the Osteoarthritis Research Society International (OARSI) histological system. Normal and osteoarthritic human osteochondral plugs were scored using the OARSI histological system and imaged at 3 T using MRI sequences producing T1 and T2 contrast and measuring T1, T2, and T2* relaxation times, magnetization transfer, and diffusion. The classification accuracies of quantitative MRI parameters and corresponding weighted image intensities were evaluated. Classification models based on the Mahalanobis distance metric for each MRI measurement were trained and validated using leave-one-out cross-validation with plugs grouped according to OARSI histological grade and score. MRI measurements used for classification were performed using a region-of-interest analysis which included superficial, deep, and full-thickness cartilage. The best classifiers based on OARSI grade and score were T1- and T2-weighted image intensities, which yielded accuracies of 0.68 and 0.75, respectively. Classification accuracies using OARSI score-based group membership were generally higher when compared with grade-based group membership. MRI-based classification--either using quantitative MRI parameters or weighted image intensities--is able to detect early osteoarthritic tissue changes as classified by the OARSI histological system. These findings suggest the benefit of incorporating quantitative MRI acquisitions in a comprehensive clinical evaluation of OA.

MR imaging of articular cartilage at 1.5T and 3.0T: comparison of IDEAL 2D FSE and 3D SPGR with fat-saturated 2D FSE and 3D SPGR in a porcine model

BACKGROUND

IDEAL technique is a robust fat-water separation method which is potentially useful in cartilage imaging with significant improvement in the cartilage signal-to-noise ratio (SNR).

PURPOSE

To identify whether iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) improved diagnostic performance for IDEAL 2D fast spin echo (FSE) and 3D spoiled gradient-recalled echo (SPGR) versus fat-saturated (FS) protocols at 1.5 and 3.0 T in the evaluation of patella-femoral cartilage lesions.

MATERIAL AND METHODS

Forty-six artificial cartilage lesions were created in femoro-patellar articular cartilages of 11 porcine knees. All knees underwent MR examination at 1.5 and 3.0 T with MR protocol consisting of sagittal FS 2D FSE and 3D SPGR and IDEAL 2D FSE and IDEAL 3D SPGR, respectively. Qualitative assessment was performed to compare the diagnostic performance between 1.5- and 3.0-T protocols and between IDEAL and FS protocols.

RESULTS

IDEAL 3D SPGR had a significantly higher accuracy for detecting partial thickness cartilage lesions (P < 0.01) than FS SPGR protocols, whereas there was no significant difference in diagnostic performance between IDEAL and FS 2D FSE except for one cartilage lesion. For all imaging sequences, no significant difference was observed in the diagnostic performance between 1.5- and 3.0-T imaging protocols (P = 0.42-0.91).

CONCLUSION

Compared with conventional FS SPGR imaging, IDEAL 3D SPGR provided a better diagnostic performance for evaluation of porcine knee articular cartilage lesions in the knee joints at 1.5 and 3.0 T. IDEAL 3D SPGR may therefore be useful for detecting partial-thickness cartilage lesions in patients with degenerative osteoarthritis.

Usefulness of IDEAL T2-weighted FSE and SPGR imaging in reducing metallic artifacts in the postoperative ankles with metallic hardware

PURPOSE

The aim of this work is to prospectively compare the effectiveness of iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL), T2-weighted fast spin-echo (FSE), and spoiled gradient-echo (SPGR) MR imaging to frequency selective fat suppression (FSFS) protocols for minimizing metallic artifacts in postoperative ankles with metallic hardware.

MATERIALS AND METHODS

The T2-weighted and SPGR imaging with IDEAL and FSFS were performed on 21 ankles of 21 patients with metallic hardware. Two musculoskeletal radiologists independently analyzed techniques for visualization of ankle ligaments and articular cartilage, uniformity of fat saturation, and relative size of the metallic artifacts. A paired t test was used for statistical comparisons of MR images between IDEAL and FSFS groups.

RESULTS

IDEAL T2-weighted FSE and SPGR images enabled significantly improved visualization of articular cartilage (p < 0.05), the size of metallic artifact (p < 0.05), and the uniformity of fat saturation (p < 0.05). However, no significant improvement was found in the visibility of ligaments.

CONCLUSIONS

IDEAL T2-weighted FSE and SPGR imaging effectively reduces the degree of tissue-obscuring artifacts produced by fixation hardware in ankle joints and improves image quality compared to FSFS T2-weighted FSE and SPGR imaging. However, visibility of ligaments was not improved using IDEAL imaging.

Gradient echo imaging

Magnetic resonance imaging (MRI) based on gradient echoes is used in a wide variety of imaging techniques and clinical applications. Gradient echo sequences form the basis for an essential group of imaging methods that find widespread use in clinical practice, particularly when fast imaging is important, as for example in cardiac MRI or contrast-enhanced MR angiography. However, the term "gradient echo sequence" is somewhat unspecific, as even images acquired with the most common sequences employing the gradient echo for data acquisition can significantly differ in signal, contrast, artifact behavior, and sensitivity to, eg, flow. This is due to the different use of sequence timing and basic sequence building blocks such as spoiler gradients or specific radiofrequency (RF) pulse phase patterns. In this article the basic principles of gradient echo formation compared to spin echo imaging are reviewed and the properties of gradient echo imaging in its simplest form (TR ≫ T(2)) are described. Further, the most common three variants of fast gradient echo sequences (TR < T(2)), namely, unbalanced gradient echo, RF spoiled gradient echo, and balanced steady state free precession; are discussed. For each gradient echo sequence type, examples of applications exploiting the specific properties of the individual technique are presented.

Advanced MRI of articular cartilage

Musculoskeletal MRI is advancing rapidly, with innovative technology and significant potential for immediate clinical impact. In particular, cartilage imaging has become a topic of increasing interest as our aging population develops diseases such as osteoarthritis. Advances in MRI hardware and software have led to increased image quality and tissue contrast. Additional developments have allowed the assessment of cartilage macromolecular content, which may be crucial to the early detection of musculoskeletal diseases. This comprehensive article considers current morphological and physiological cartilage imaging techniques, their clinical applications, and their potential to contribute to future improvements in the imaging of cartilage.

Routine 3D magnetic resonance imaging of joints

Due to its high spatial resolution and excellent tissue contrast, magnetic resonance imaging (MRI) has become the most commonly used imaging method to evaluate joints. Most musculoskeletal MRI is performed using 2D fast spin-echo sequences. However, 3D sequences have also been used for joint imaging and have the advantage of acquiring thin continuous slices through joints, which reduces the effects of partial volume averaging. With recent advances in MR technology, 3D sequences with isotropic resolution have been developed. These sequences allow high-quality multiplanar reformat images to be obtained following a single acquisition, thereby eliminating the need to repeat sequences with identical tissue contrast in different planes. Preliminary results on the diagnostic performance of 3D isotropic resolution sequences are encouraging. However, additional studies are needed to determine whether these sequences can replace currently used 2D fast spin-echo sequences for providing comprehensive joint assessment in clinical practice.

Articular cartilage in the knee: current MR imaging techniques and applications in clinical practice and research

Magnetic resonance (MR) imaging is the most important imaging modality for the evaluation of traumatic or degenerative cartilaginous lesions in the knee. It is a powerful noninvasive tool for detecting such lesions and monitoring the effects of pharmacologic and surgical therapy. The specific MR imaging techniques used for these purposes can be divided into two broad categories according to their usefulness for morphologic or compositional evaluation. To assess the structure of knee cartilage, standard spin-echo (SE) and gradient-recalled echo (GRE) sequences, fast SE sequences, and three-dimensional SE and GRE sequences are available. These techniques allow the detection of morphologic defects in the articular cartilage of the knee and are commonly used in research for semiquantitative and quantitative assessments of cartilage. To evaluate the collagen network and proteoglycan content in the knee cartilage matrix, compositional assessment techniques such as T2 mapping, delayed gadolinium-enhanced MR imaging of cartilage (or dGEMRIC), T1ρ imaging, sodium imaging, and diffusion-weighted imaging are available. These techniques may be used in various combinations and at various magnetic field strengths in clinical and research settings to improve the characterization of changes in cartilage.

Three-dimensional magnetic resonance imaging of joints

Magnetic resonance (MR) imaging is one of the most commonly used imaging modality for evaluating patients with joint pain. Musculoskeletal MR protocols at most institutions consist of 2-dimensional fast spin echo (FSE) sequences repeated in multiple planes. Three-dimensional sequences have also been used to evaluate the musculoskeletal system and have many potential advantages over 2-dimensional FSE sequences. Three-dimensional sequences acquire thin continuous slices through joints with high in-plane spatial resolution, which minimize the effects of partial volume averaging. Newly developed 3-dimensional isotropic resolution sequences can also be used to create high-quality multiplanar reformat images that allow joints to be evaluated in any orientation after a single acquisition. Preliminary results on the use of 3-dimensional isotropic resolution sequences for evaluating the musculoskeletal system are encouraging. However, additional studies are needed to document the advantages of 3-dimensional sequences before they can replace currently used 2-dimensional FSE sequences for evaluating the musculoskeletal system in clinical practice.

Clinical cartilage imaging of the knee and hip joints

OBJECTIVE

MRI is commonly used to evaluate the articular cartilage of the knee and hip joints in clinical practice. This article will discuss the advantages and limitations of currently available MRI techniques for evaluating articular cartilage.

CONCLUSION

Because of its high spatial resolution, multiplanar capability, and excellent tissue contrast, MRI is the imaging technique of choice for evaluating the articular cartilage of the knee and hip joints.

Isotropic 3-dimensional fast spin echo imaging versus standard 2-dimensional imaging at 3.0 T of the knee: artificial cartilage and meniscal lesions in a porcine model

PURPOSE

To compare different fat-saturated (FS) 3-dimensional (3D) intermediate-weighted (IM-w) fast spin echo (FSE) sequences with a standard FS 2-dimensional (2D) IM-w FSE sequence using a porcine in vitro model with artificially created cartilage and meniscus lesions.

METHODS

Using a ceramic scalpel, cartilage lesions with different depths and sizes were created in porcine knee specimens at the patella as well as the medial and lateral femoral and tibial cartilage. In addition, lateral and medial meniscal lesions were produced. Magnetic resonance imaging was performed at 3.0 T in sagittal plane using an 8-channel knee coil. A standard FS 2D IM-w FSE sequence and 3 newly developed isotropic 3D FSE sequences: (i) non-FS echo train length (ETL): 78, (ii) FS ETL: 44, and (iii) FS ETL: 44, were used. The images were independently analyzed by 4 radiologists concerning image quality (1 = optimal image quality, 4 = substantially limited quality) and absence or presence of lesions using a 5-level confidence score (1 = definite no presence of abnormality, 5 = definite presence of abnormality). Radiologists were also asked to measure diameter and categorize the depth of cartilage lesions using a modified Noyes classification. Average scores for image quality, confidence of diagnosis, and sensitivity, specificity, and accuracy were calculated. In addition, contrast-to-noise ratios were calculated.

RESULTS

Image quality was significantly (P < 0.05) lower on the 3D FSE images than on the 2D FSE images [3D (i): 1.6 (SD, 0.43); 3D (ii): 2.35 (SD, 0.7); 3D (iii): 2.35 (SD, 0.5); 2D: 1.3 (SD, 0.35)]. No significant differences in diagnostic performance were found between 3D (i) and 2D FSE sequences. However, 16% fewer lesions were correctly detected with the 3D (ii) and (iii) sequences. Sensitivity was highest for the 2D sequence, and specificity was highest for the 3D (i) sequence. Confidence scores were higher for the 3D (i) sequence than for the 2D sequence. A significant increase (P < 0.05) in correctly measured cartilage lesions size and depth was found for the 3D (i) sequence over the standard 2D FSE sequence.

CONCLUSIONS

Although the 3D FSE sequence performed better in depiction and characterization of cartilage abnormalities than the standard 2D FSE sequence, we currently do not recommend to use it as substitute. For the diagnosis of meniscal defects, however, no significant improvement was found.

Improved time-of-flight magnetic resonance angiography with IDEAL water-fat separation

PURPOSE

To implement IDEAL (iterative decomposition of water and fat using echo asymmetry and least squares estimation) water-fat separation with 3D time-of-flight (TOF) magnetic resonance angiography (MRA) of intracranial vessels for improved background suppression by providing uniform and robust separation of fat signal that appears bright on conventional TOF-MRA.

MATERIALS AND METHODS

IDEAL TOF-MRA and conventional TOF-MRA were performed in volunteers and patients undergoing routine brain MRI/MRA on a 3T magnet. Images were reviewed by two radiologists and graded based on vessel visibility and image quality.

RESULTS

IDEAL TOF-MRA demonstrated statistically significant improvement in vessel visibility when compared to conventional TOF-MRA in both volunteer and clinical patients using an image quality grading system. Overall image quality was 3.87 (out of 4) for IDEAL versus 3.55 for conventional TOF imaging (P = 0.02). Visualization of the ophthalmic artery was 3.53 for IDEAL versus 1.97 for conventional TOF imaging (P < 0.00005) and visualization of the superficial temporal artery was 3.92 for IDEAL imaging versus 1.97 for conventional TOF imaging (P < 0.00005).

CONCLUSION

By providing uniform suppression of fat, IDEAL TOF-MRA provides improved background suppression with improved image quality when compared to conventional TOF-MRA methods.

Multiecho IDEAL gradient-echo water-fat separation for rapid assessment of cartilage volume at 1.5 T: initial experience

Institutional review board approval and informed consent were obtained for this HIPAA-compliant study. The purpose was to prospectively compare multiecho iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) gradient-echo (GRE) magnetic resonance (MR) imaging with three-dimensional fat-suppressed (FS) spoiled GRE (SPGR) MR imaging to evaluate the articular cartilage of the knee. Six healthy volunteer and 10 cadaver knees were imaged at 1.5 T. Signal-to-noise ratio (SNR), SNR efficiency, and cartilage volume were measured. SNR and SNR efficiency were significantly higher with multiecho IDEAL GRE than with FS SPGR imaging (P < .031). Both methods produced equivalent cartilage volumes (overall concordance correlation coefficient, 0.998) with high precision and accuracy. The use of a cartilage phantom confirmed high accuracy in volume measurements and high reproducibility for both methods. Multiecho IDEAL GRE provides high signal intensity in cartilage and synovial fluid and is a promising technique for imaging articular cartilage of the knee.

Turboprop IDEAL: a motion-resistant fat-water separation technique

Suppression of the fat signal in MRI is very important for many clinical applications. Multi-point water-fat separation methods, such as IDEAL (Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation), can robustly separate water and fat signal, but inevitably increase scan time, making separated images more easily affected by patient motions. PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) and Turboprop techniques offer an effective approach to correct for motion artifacts. By combining these techniques together, we demonstrate that the new TP-IDEAL method can provide reliable water-fat separation with robust motion correction. The Turboprop sequence was modified to acquire source images, and motion correction algorithms were adjusted to assure the registration between different echo images. Theoretical calculations were performed to predict the optimal shift and spacing of the gradient echoes. Phantom images were acquired, and results were compared with regular FSE-IDEAL. Both T1- and T2-weighted images of the human brain were used to demonstrate the effectiveness of motion correction. TP-IDEAL images were also acquired for pelvis, knee, and foot, showing great potential of this technique for general clinical applications.

Isotropic 3D fast spin-echo imaging versus standard 2D imaging at 3.0 T of the knee--image quality and diagnostic performance

The objective of this study was to compare a newly developed fat-saturated intermediate-weighted (IM-w) 3D fast spin-echo (FSE) sequence with standard 2D IM-w FSE sequences regarding image quality and diagnostic performance in assessing abnormal findings of the knee. MR imaging was performed at 3.0 T in 50 patients. Images were assessed independently by three radiologists. Image quality was rated significantly higher (p < 0.05) for the 2D versus the 3D FSE sequences. Sensitivity for cartilage lesions was slightly higher for the 3D sequence, but specificity was lower. Low contrast objects were better visualized with 2D sequences, while high contrast objects were better shown with the 3D sequence. Confidence scores were higher for 2D than for 3D sequences, but differences were not significant. In conclusion, isotropic 3D FSE IM-w imaging may enhance standard knee MRI by increased visualization of high contrast lesions; however, 3D FSE image quality was lower.

Cartilage imaging at 3.0T with gradient refocused acquisition in the steady-state (GRASS) and IDEAL fat-water separation

PURPOSE

To demonstrate the feasibility of evaluating the articular cartilage of the knee joint at 3.0T using gradient refocused acquisition in the steady-state (GRASS) and iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) fat-water separation.

MATERIALS AND METHODS

Bloch equation simulations and a clinical pilot study (n = 10 knees) were performed to determine the influence of flip angle of the IDEAL-GRASS sequence on the signal-to-noise ratio (SNR) of cartilage and synovial fluid and the contrast-to-noise ratio (CNR) between cartilage and synovial fluid at 3.0T. The optimized IDEAL-GRASS sequence was then performed on 30 symptomatic patients as part of the routine 3.0T knee MRI examination at our institution.

RESULTS

The optimal flip angle was 50 degrees for IDEAL-GRASS cartilage imaging, which maximized contrast between cartilage and synovial fluid. The IDEAL-GRASS sequence consistently produced high-quality fat- and water-separated images of the knee with bright synovial fluid and 0.39 x 0.67 x 1.0 mm resolution in 5 minutes. IDEAL-GRASS images had high cartilage SNR and high contrast between cartilage and adjacent joint structures. The IDEAL-GRASS sequence provided excellent visualization of cartilage lesions in all patients.

CONCLUSION

The IDEAL-GRASS sequence shows promise for use as a morphologic cartilage imaging sequence at 3.0T.

Generalized k-space decomposition with chemical shift correction for non-Cartesian water-fat imaging

Chemical-shift artifacts associated with non-Cartesian imaging are more complex to model and less clinically acceptable than the bulk fat shift that occurs with conventional spin-warp Cartesian imaging. A novel k-space based iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) approach is introduced that decomposes multiple species while simultaneously correcting distortion of off-resonant species. The new signal model accounts for the additional phase accumulated by off-resonant spins at each point in the k-space acquisition trajectory. This phase can then be corrected by adjusting the decomposition matrix for each k-space point during the final IDEAL processing step with little increase in reconstruction time. The technique is demonstrated with water-fat decomposition using projection reconstruction (PR)/radial, spiral, and Cartesian spin-warp imaging of phantoms and human subjects, in each case achieving substantial correction of chemical-shift artifacts. Simulations of the point-spread-function (PSF) for off-resonant spins are examined to show the nature of the chemical-shift distortion for each acquisition. Also introduced is an approach to improve the signal model for species which have multiple resonant peaks. Many chemical species, including fat, have multiple resonant peaks, although such species are often approximated as a single peak. The improved multipeak decomposition is demonstrated with water-fat imaging, showing a substantial improvement in water-fat separation.