The Emerging Technique of Using Diffusion Tensor Imaging to Perform Muscle Fiber Tracking - Assignment Example

1-Discuss the emerging technique of using diffusion tensor imaging (DTI) to perform muscle fiber tracking Review of Brownian movement Magnetic Resonance Imaging (MRI) is based on the molecules’ random movements that allow probing of tissue structure more in depth that the usual image resolution. The Brownian motion results from the thermal energy carried by these molecules, and causes bouncing or crossing of metabolite particles (typically water) with the tissue components. The use of many metabolites as a suitable reference particle is because all molecules exhibit Brownian movement at temperatures higher than absolute zero. These interactions present as three-dimensional distribution of molecules, and each of the cell membranes, extracellular matrices, and other tissue components has their own unique set of distribution, thus providing clues to the structure and relative positioning of these components. This is because each cellular structure present with a unique set of obstacles that limit molecular movement in some directions (Le Bihan, 2001). Limitation of diffusion coefficient MRI is one of the most powerful imaging techniques in the medical field as it is currently the only tool by which the process observed above can be done non-invasively without interfering with the metabolite-particle-tissue component interactions. The usual plain diffusion MRI describes the metabolite particle distribution using spin-echo MRI signal attenuation dependent on a single, scalar parameter, the diffusion coefficient b. Thus, this parameter directly reflects the physical properties of histologic structures. In addition, diffusion coefficients measured at different times in a particular patient or among different patients can be compared without any need for normalization (Le Bihan, 2001). Diffusion Tensor Imaging However, because of the three-dimensional characteristic of particle movements, diffusion should similarly be defined by multiple factors. Diffusion tensor imaging (DTI) provides better details on tissue structure, as it allows full extraction, characterization and exploitation of the diffusion MRI data. This is because it is a quantitative method that allows the measurement of tensor, D, which provides a better description of the direction and correlation of molecular mobility of each particle, aside from the measurement of diffusion coefficient. D, in particular, is a sensitive test in determining changes in the extracellular/intracellular volume ratio. In addition, the anisotropy of diffusion allows differentiation of histologic structures (Le Bihan, 2001). It measures the restriction on the Brownian movement of water molecules. Normally, water molecules move isotropically, unrestricted in all three dimensions. However, water movement is blocked by cellular membranes and structures, diffusing preferentially along the length of defined tissue planes. Aside from the image enhancement it provides, DTI also avoids the health risks associated with the use of contrast dye gadolinium (Borrero, Mountz and Mountz, 2011). In this effect, it can be used to show subtle abnormalities in order to differentiate many diseases. In theory, diffusion in fibers running in the direction of the gradient run faster than those moving in perpendicular direction (Le Bihan, 2001). Muscle fiber tracking The structure of muscles is unique because its fibers are strictly parallel to each other, meaning they do not diverge, cross or come in contact with one another. In addition, muscles characteristically originate from and end to a definite point. These structural characteristics can be modified by intrinsic fat infiltration and extrinsic MRI-caused artifacts, causing early termination or incorrect fiber tract (Heemskerk et al. 2009). Pennation is particularly important because it is a better measure of muscle force as compared to anatomical cross-sectional area (Lansdown et al. 2007). DTI can be used to reconstruct muscle structure, and measure muscle architectural parameters such as pennation angle, fiber length and physiological cross-sectional area without the need of studying cadaveric specimens that fail to represent muscles in real life. Figures 1 and 2 show some of the information that can be obtained from the imaging procedure. DTI-MRI also allows study of changes in the muscles as influenced by age and health status of an individual. Because of the distinct structure of muscle fibers, it allows the measurement and reconstruction of fiber tracks. Since diffusion of water is longer along the length of a muscle fiber than perpendicular to it, measurement of diffusion coefficient from at least six non-collinear directions can be used to derive the main direction of diffusion. In addition, the definite start and end points of fibers allow quantification of the fiber tracks (Heemskerk et al. 2009). 2- Discuss current and emerging MRI techniques used in the imaging of cartilage. Suggest a grading system that could be used to describe cartilage Review of cartilage The connective tissue, cartilage, is a mostly proteinaceous macromolecular network that contains fluid to buffer any mechanical force applied on it. It contains extracellular matrix-secreting chondrocytes, collage and proteoglycan aggregates. Collagen needs to follow a specific orientation to be able to perform optimally its function. Proteoglycans contribute to the stability of the cartilage structure. In osteoarthritis, these molecules are gradually decreasing (Borrero, Mountz and Mountz, 2011). Various grading systems Quantitative MRI techniques, such as dGEMRIC and three-dimensional MRI measurements for cartilage morphometry, are being developed for determining cartilage lesions and extent of the disease. However, they are time-consuming enough to be applied in the clinics Hannila et al. 2007). This makes proper pairing of diagnosis and management much more critical. Figure 3 shows the different MRI techniques that can detect cartilage pathology. Facilitating this coupling is a proper grading system of cartilage lesions, with each grade having a management plan catering to it. 1. Grading based on appearance More commonly, cartilage lesions are described based on their appearance in MRI. Grade 1 describes non-articular foci or early mucinous degeneration of the cartilage resulting from asymptomatic and unnoticed mechanical load. Despite this sign of degradation, Grade 1 lesions are clinically insignificant. Grade 2 lesions, on the other hand, are horizontal MR signal in the intra-substance region. Although it is extensive enough to reach the capsular periphery, it still does involve the articular surface. The mucinous degradation is now coupled with microscopic areas of collagen fragmentation. The worst condition, grade 3, is the extension of the increased signal at the or beyond the articular surface (McMahon, 2012). In addition, three T2 patterns also describe pathologic cartilages. These are 1) focally increased T2 signal seen in the radial zone, 2) heterogeneously increased T2 signaling that extends to the articular surface, and 3) macroscopic fissuring of cartilages (Borrero, Mountz and Mountz, 2011). 2. Grading based on relaxation time T2 relaxation mapping exploits this differential movement of water across cellular structures. In the assessment of cartilage pathology, differential loss of free water Brownian movement signifies loss or disorganization of cartilage. Hannila et al. (2007) have suggested another way of grading cartilage pathology using T2 relaxation time, as measured by T2 relaxation mapping. In their study, it was found that normal human cartilage at 1.5T range from 20 to 40 ms, and most lesions have a relaxation time of 40-60 ms, graded as second T2 grade. On the other hand, long T2 relaxation time (60-80 ms) were observed from synovial fluid-containing cartilage defects, resulting from increased water mobility. Lesions with two different grading can be classified according to the higher grade detected. Both in vitro and in vivo studies show that prolonged T2 relaxation time is a manifestation of extracellular matrix degeneration, specifically collagen denaturation, as is observed in osteoarthritis (Hannila et al. 2007). This grading system has been used to describe pathologic conditions. Patients with juvenile RA were found to have a longer average T2 relaxation time, due to the increased water mobility (Borrero, Mountz and Mountz, 2011). However, T2 is influenced by topographical location, depth of tissue, age, and selected sequence design, making the comparison of T2 values invalid until reference values are established. As well, increased T2 relaxation times in the middle, transitional zone also describe age-related, asymptomatic changes. Still, this tool must be explored further as because it can detect early cartilage changes. In fact, when used to diagnoses knee injury, T2 mapping was able to detect lesions that seemed normal in standard MRI, as seen in figure 4. Abnormal areas were also more conspicuous in T2 maps than in MRI images. Finally, T2 mapping can be performed using commercially available equipment (Hannila et al. 2007). T1 mapping can also be used in describing cartilage lesions. In measuring the relaxation time after the spread of intravenous administration of negatively-charged paramagnetic contrast agent gadopentetic acid, it was found that increased distribution of the dye implies decrease in proteoglycan concentration. In contrast, dye levels are low among healthy subjects. T1 signal is thus a specific marker for GAG, providing a quantitative measure of cartilage stability (Borrero, Mountz and Mountz, 2011). 3. Time-intensity curves Inflammation of joints is also assessed by comparing time-intensity curves (TICs).Type 1 shows no enhancement, type 2 is characterized by slow enhancement, types 3-5 all describe fast enhancement, and type 6 is depicted by arterial enhancement. Particularly, type 3 is fast enhancement with subsequent plateau, type 4 is followed by washout phase, and type 5 is characterized by a continuous enhancement. It was found that patients with rheumatoid arthritis have an increased frequency of type 4 TICs, indicating increased vascular permeability and density (Borrero, Mountz and Mountz, 2011). References Borrero, C. G., Mountz, J. M. and Mountz, J. D., 2011. Emerging MRI methods in rheumatoid arthritis. Nature Reviews Rheumatology, 7, pp. 85-95. Hannila, I. et al., 2007. Patellar cartilage lesions: comparison of magnetic resonance imaging and T2 relaxation-time mapping. Acta Radiologica, 48, pp. 444-448. Heemskerk, A. M. et al. 2009. Quantitative Assessment of DTI-Based Muscle Fiber Tracking and Optimal Tracking Parameters. Magnetic Resonance in Medicine, 61, pp. 467-472 Lansdown, D. A. et al. 2007. Quantitative diffusion tensor MRI-based fiber tracking of human skeletal muscle. Journal of Applied Physiology, 103, pp. 673-681. Le Bihan, D. et al., 2001. Diffusion Tensor Imaging: Concepts and Applications. Journal of Magnetic Resonance Imaging, 13, pp. 534-546. McMahon, K. 2012. MRES7015. Fundamental Musculoskeletal MRI. Queensland, Australia: University of Queensland.   Read More