Diffusion tensor imaging (DTI) based fibre tracking in skeletal muscles has been applied to quantify and reconstruct the architecture of muscles (Heemskerk, Sinha, Wilson, Ding, & Damon, 2010). Additionally, (DTI) is increasingly applied in the characterization and detection of skeletal muscles. This promising method has generated high expectations and aroused much enthusiasm, because it is capable of revealing particular information regarding skeletal muscles that cannot be detected non-invasively by other imaging techniques. DTI is also able to evaluate the apparent fractional anisotropy (FA), and diffusion coefficient (ADC) which are closely associated with the muscle pathology and physiology. For numerous reasons, skeletal muscles are suitable for validation studies of in vivo diffusion tensor-MRI. First of all, it is easy to access skeletal muscles by dissection. Secondly, in vivo imaging of skeletal muscles while at rest is technically easier than the heart. Lastly, the skeletal muscles are composed of extended muscle fibres that are structured in a very ordered and regular manner. The extremely ordered muscle fibre arrangement results in diffusion anisotropy (Scheel, Roth, Winkler, & Arampatzis, 2013).
How DTI can be obtained
It can be measured by evaluating the diffusion in at least six directions. These directional data can be joined with nearby pixels to enable the fibre trajectory to be reconstructed. To analysis the accurateness of reconstructed fibres, the fibre arrangements inside the muscles make it a perfect target. The alignment of muscle fibres is comparable to each other and they are not crossing, kissing or diverging as seen in Fig1 , in the way that white matter tracts do. Additionally, the fibres run from the origin point to the insertion point within some muscles. For instance, the bipennate tibialis anterior (TA) muscle fibres arise from the superficial fascia of the anterior compartment or the tibia to the middle of aponeurosis. The knowledge of the starting point and the ending point of fibres allows a perfect valuation of fibre tracking quantitatively (Hasan, Walimuni, Abid, & Hahn, 2011).
Human skeletal muscles can be divided into two main types of fibres, having important differences in terms of microstructure and function. Type-1 fibres, which contract slowly and are capable of major strength owing to their oxidative metabolism, myoglobin, mitochondria and capillaries. By contrast, type-2 fibres contract rapidly, display higher maximum muscle performance and, having fewer capillaries and less myoglobin, depend on anaerobic metabolism that causes exhaustion more quickly. Due to the difference between fibre types in both microstructural intracellular makeup and average fibre diameter, there is an assumption that these variances cause different diffusion amounts that are large enough to be evaluated by DTI (Scheel et al., 2013).
Low anisotropy is observed at fibre crossings, and eigenvector directions do not match the directions where both tracts cross. The ellipsoidal tensor representation is also a limitation for directional diffusion coefficient distribution. The signal to noise ratio also affects fibre tracking quality, as there is greater noise sensitivity than for other techniques. In clinical practice, it becomes a trade-off between data acquisition time and data quality .It is essential to use imaging techniques with movement compensation, such as the technique of navigator echo, to reduce patient motion
artifacts caused by long acquisition times. Moreover, it is preferable to apply imaging techniques, such as PROPELLER or line scans with less distortion, than to use correction of image distortion. Development of imaging hardware and imaging techniques are important in addressing all these limitations. Validation is another vital issue for tractography. By contrasting with other visualizing techniques, tractography is attended by segmentation process for fibre tracking. Fibre tract segmentation is more challenging than it is with vascular structures. In this regard, symbolic display methods or segmentation free image-based visualization techniques have slight advantages. The establishment of a validation scheme for tractography is the main reason for its wider acceptance in clinical use (Hasan et al., 2011).
Higher field strengths, when available, must be selected given the increases in SNR it allows. Nonetheless, fibre tracking and DTI are feasible with a 1.5 T image. The sequence of diffusion is generally a spin echo based echo planar imaging (EPI) pulse sequence. The b value should be carefully selected once it is the main parameter controlling sensitivity in a diffusion weighted sequence. Its increase results in a decline in signal to noise ratio. In skeletal muscles assessment, the described b value is typically smaller than in contrast to noise ration studies, it rang between 400 and 1000 s/mm. The TE must be low. Sensitivity encoding (SENSE) methods must be applied. To improve fibre tracking accuracy, diffusion gradients might be used in more than six directions as in Fig2 that help in problem evaluation. The acquisition numbers for each encoding direction might be improved at the cost of a longer investigation period and potential motion artifacts (Khalil et al., 2010).
Introduction and Morphology
With the increase in the range of treatment options available, the field of articular cartilage imaging has improved .In the human body, the articular surfaces of joints are covered by hyaline cartilage. Force distribution, preventing wear and reducing friction represent the functions of articular cartilage (Buckwalter & Mankin, 1997).
Hyaline cartilage is consist of approximately 80% water, with the remainder composed of ground substance, cellular substance and intercellular matrix, which are essentially responsible for providing stability to the cartilage. The absence of, lymphatics blood vessels and nerve endings does not enable its reinforcement. Once a joint is crushed, water can be distributed again through the cushioning of the cartilage the compressive force absorb, whereas the whole volume of cartilage might remain unchanged (Rubenstein, Kim, Morova-Protzner, Stanchev, & Henkelman, 1993).
Normal Articular Cartilage in MRI Appearance
It is challengable procedure to image the thiner cartilage, which, at its greatest thickness is not more than 4 mm with smooth curved contours. MRI holds the greatest promise for the investigation and evaluation of articular cartilage. The advantages of MRI are its superb soft tissue contrast and its high spatial resolution. MRI sensitivity is reduced because of the effects of partial volume averaging that might usually cover early chondral injuries like cartilage flaps and fissures. To determine the effectiveness of MR imaging to make images with high level of quality , it is important to improve the spatial resolution for a suitable signal to noise ratio (SNR). High field strength magnets accessibilities together with dedicated pulse sequences and dedicated surface coils have improved articular cartilage imaging to a high standard as seen in Fig3 (Mosher, Dardzinski, & Smith, 2000).
New Pulse Sequences
Diffusion-weighted imaging (DWI)
There is a modification of the magnetic field gradients due to water movement. This motion has been considered as a b-factor, which has TE illustrating the T2 weighting degree and is represented pixel by pixel. (DWI) has been determined as a possible technique for measuring cartilage collapse in vivo and observing its healing after surgery (Mamisch et al., 2008).
Apparent diffusion coefficient (ADC)
(ADC) defines the interaction among water molecules relative to the adjacent situation. This measurement has been calculated in articular cartilage, recommending tissue morphology integrity. An enlarged diffusivity proposes structural degeneration of the ECM, therefore, it can be readily used as a possible procedure in post cartilage imaging reparation with bigger diffusion related to the ECM structural mortification (Kornaat et al., 2005).
Diffusion tensor imaging (DTI)
Collagen fibrils Orientation in the ECM of the articular cartilage has been calculated applying (DTI). The collagen network interruption is mapped on DTI. But negative features are the longer duration of image acquisition and intricate data analysis. DTI has been applied to report compression effects on the cartilage collagen system (De Visser, Crawford, & Pope, 2008).
Dual Echo Steady State (DESS)
DESS scanning is valuable to evaluate cartilage image and 3-mm slices can be obtained in almost 6 minutes. Original morphology analyses of cartilage were conducted using DEFT imaging as shown in Fig4,whereas this method has not been accepted to be more to 2 dimention methods.
Figure 3: wrist sagittal water selective 3D DESS imaging ,there is the highest signal intensity with highest CNR of cartilage (Gold, Fuller, Hargreaves, Stevens, & Beaulieu, 2005)
Balanced SSFP Imaging
This is an effective 3D MR sequence acquired with fast acquisition times and high signals, using steady state precision as in Fig6. High resolution is achieved by multiple acquisitions at the expense of increased scanning duration. Because of the 3D nature of the acquisition and the bright synovial fluid, bSSFP is also useful for imaging of internal damage of other structures with menisci and ligaments (Gold et al., 2005).
Figure 6: Sagittal (bSSFP) image of knee joint displays superficial partial thickness cartilage thinning (arrow) on anterior weight bearing surface of lateral femoral condyle (Gold et al., 2005)
Vastly Interpolated Projection Reconstruction Imaging (VIPR)
(VIPR), the intial established to resolve the time CE-MRA, was later modified for bSSFP musculoskeletal system imaging. The radial acquisition permits a very effective k-space trajectory that gathers two radial lines for each TR with no time wasting on frequency rephasing and dephasing gradients. One radial line starts at the k-space basis ,whereas the other is obtained along a various return route to the original, permitting acquisition to happen during nearly the whole TR. The optimal
TR is required for the most efficient application of linear mixtures of bSSFP to conduct water-fat separation at 1.5 Tesla, while still having time for sufficient spatial encoding. Applying VIPR to the knee provides isotropic 0.5- to 0.7-mm 3D imaging that enables reformations in random planes. Because this technique depends on SSFP, joint fluid is bright, providing brilliant contrast for cartilage damage diagnosis. Contrast among the bone and cartilage is created by spliting water and fat with linear SSFP combinations, as presented in Fig.7. The Scan duration for the isotropic acquisition is 5m. Another single-pass technique separates water and fat by abusing the various phase progressions of water and fat spins among the double echoes obtained for each TR. (Kijowski et al., 2009).
Figure 7: (VIPR) (SSFP) imaging of knee. This SSFP-based method produces 0.4-mm isotropic resolution, allowing? reformations in any imaging plane. (A) Coronal image (B) Sagittal reformation (C) Axial reformation (D) VIPR k-space trajectory. (Kijowski et al., 2009)
Driven Equilibrium Fourier Transform Imaging (DEFT)
(DEFT) imaging has been used in spectroscopy as a technique of signal enhancement in the past. The sequence apply a 90° pulse to return longitudinal magnetization plane, rising signal from tissues with longer T1 relaxation time like synovial fluid. The contrast in DEFT imaging is depend on T1/T2 ratio of a given tissue. For musculoskeletal imaging, the DEFT sequence makes contrast by increasing the signal from synovial fluid instead of reducing the signal from cartilage as in T2-weighted
sequences. This causes bright synovial fluid at short TRs. At small TRs, DEFT reveals greater contrast of cartilage-to-fluid than proton density, FSE, or T2-weighted FSE and SPGR. DEFT imaging has been joined with a 3D echo-planar readout to make it an effective 3D cartilage imaging method. Different with T2-weighted FSE, cartilage signal is conserved because of the short TE with the DEFT sequence (Woertler, Rummeny, & Settles, 2005).
It is a possibly sensitive MRI method to identify early proteoglycans reduction in the extracellular matrix (ECM). It detects the changes in the spins relaxation in a magnetic field. This method is varied from T2 mapping that determines the modifications in the collagen architecture; furthermore, the T1rho value differences are unusually larger in osteoarthrosis cases, present a wider spectrum of disease detection as seen in Fig8(Pedersen et al., 2011)
Figure 8: Color map of T1rho relaxation time in tibial cartilage at 3 T. T1rho mapping is sensitive to the status of proteoglycan in articular cartilage (Zuo, Li, Banerjee, Han, & Majumdar, 2007).
Delayed Gadolinium-enhanced MRI of the Cartilage(dGEMRIC)
There is a direct relationship of glycosaminoglycan (GAG) concentration affected by patient’s body mass index and the contrast-enhanced T1. There is an efficient contrast penetration into cartilage area, which has interrupted GAG concentration. A normal articular cartilage has a low (gadolinium diethylenetriaminepentaacetic) acid (GdDTPA) concentration, while damaged articular cartilage areas enable contrast material to dribble in delayed hyperenhancement of articular cartilage areas. Imaging wise, a twice amount of the Gd contrast substantial is added inside the vein and the
images is obtained after 2 hours, allowing enough period for contrasting material to infiltrate into damaged collagen areas. The GAG concentration is proportional to Gd contrast concentration on T1-weighted image inversly. A T1 map is designed depending on contrast concentration indirectly measuring GAG in cartilage. (dGEMRIC) has a important promise to evaluate biochemical modifications in cartilage morphology (Mlynárik, Sulzbacher, Bittšanský, Fuiko, & Trattnig, 2003).
3D FSE Imaging
2D FSE is a influential clinical instrument, however this technique suffers from slice gaps, partial volume effects and anisotropic voxels . 3D FSE, with parallel imaging to decrease imaging time, and flip angle modulation to decrease blurring, has made isotropic scanning with spin echo contrast. Images from this technique show isotropic resolution with the capability to obtain high quality multiplanar improvements Fig.9 New studies in patients with arthroscopic correlation indicated that 3D FSE was equivalent to a mixture of cartilage defects and multiple planes of 2D FSE in the diagnosis of menisci, ligament (Busse et al., 2008).
Figure 9 3D FSE imaging using parallel imaging and flip angle modulation (A)Coronal image(B) Sagittal reformation (C)Axial reformation (Busse et al., 2008).
Articular cartilage has a high intensity of sodium. Because of this, sodium MRI has been used for cartilage imaging. The proteoglycan depletion areas are characterized as areas of high concentration of sodium as in Fig10. This method is considered an alternative technique to dGEMRIC to evaluate focal cortical deficiency. Triple- quantum-filtered sodium in cartilage imaging could be even more sensitive to early variations than sodium imaging (Busse et al., 2008)
Figure10: 3D cones acquisition of sodium MRI in at 3 T. Sodium is marker for proteoglycan in articular cartilage (Mellon et al., 2009)
Some centres currently have 7Tesla human MRI systems. Though these systems experience high power deposition and radiofrequency penetration problems, they have a significant SNR improvement over lower field strength systems. These systems are capable to reach a maximum resolution in a short period of time and might be useful for viewing cartilage ultrastructure. Fig.11 presents a characteristic data set at 7 Tesla applying SPGR acquisition.
Figure 11: Sagittal images at 7 Tesla applying 3D SPGRecho method. (A) Image obtained with no parallel imaging. (B) two parallel acceleration factor.
Cartilage damage grading
Grade 0 The cartilage is intact and undamaged.
Grade 1 The cartilage has some softening and blistering.
Grade 2 There are minor tears in the surface of the cartilage or a minor defect (fewer than 50% of the total thickness) in the cartilage.
Grade 3 The defect is deeper in the cartilage (more than 50%)
Grade 4 All of the cartilage thickness has been lost, leaving joint bones uncovered.
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