The Arrangement of Fibers in a Muscle - Assignment Example

SPECIALISED NURSING Name: Course: Professor: Institution: May 16, 2012 Qs 1.Diffusion tensor imaging (DTI) in muscle fiber tracking Background of Diffusion tensor imaging (DTI) The arrangement of fibers in a muscle is in such a way that if affects the force produced and shortens the values of velocity. This is more evident in pinnate muscles. In the mentioned muscles fibers are inserted at an oblique angle to the aponeurosis. This arrangement is what causes an increase in sarcomeres that lie parallel and consequently the force production. This increase in production of force causes the velocity to shorten as a shortening component is produced orthogonally to the muscle axis (Heemskerk et. al, 2006, p. 21). For quite a good number of years there was only pennation measurement technique which was used for measuring cadaver specimens. However, this method had so many errors due to fixation artifacts. As a result ultrasound was introduced for measuring pennation angles. Again, this method was challenged by some of the pennation angles of some muscles are heterogeneous. Ultrasound was also found to use 2D imaging. It was due to these inefficiencies that a more accurate method that would provide 3D fiber reconstruction was found. Diffusion tensor imaging technique (DTI) Diffusion tensor imaging (DTI) is based on the association between water diffusion direction, cellular geometry of tissues, white matter of the CNS and the cardiac muscles. Trajectories of local fiber can be reconstructed by measuring diffusion in six non collinear directions. A tensor model can be used to describe this type of diffusion. Tensor has three Eigen values. These values describe the diffusion coefficient magnitude in three directions that are all orthogonal. The coefficient gradient is also defined in three eigenvectors and these specify the directions (Heemskerk et al, 2005, p. 1336). Fiber trajectories are reconstructed by following the greatest diffusion route and summing up points at regular intervals. In addition the eigenvector that is correspondent to the largest eigenvalue is considered to be a coincident to the longitudinal axis. Measurements of angle pennation that are obtained using DTI fiber tracking are in agreement to those done using direct anatomical inspection. Any change in these measurements is in order as they change with the change in foot angle.DTI is also feasible in human muscle. It can also be used in studies of muscle micro architecture and muscle injury. The use of DTI helps us understand the structure of muscles and their function in relation to age and health. It is also useful in studying the heterogeneous patterns of muscle architecture and its impact on muscle mechanics. Nevertheless, a number of practical complications are encountered in these studies. Some of the complications include a short transverse relaxation time (T2) in the muscles. This causes a rapid delay in signal that consequently lowers signal to noise ratio. This short transverse relaxation time limits the amount of diffusion weighing that would be obtained in echo time (Heemskerk et. al, 2006 p. 279). It also limits the radio frequency coil length and the size of static magnetic field. In a simpler way, DTI has the capability of collecting information concerning diffusion weighed images. It then puts together all the information concerning where water has the possibility of diffusing. Diffusion tensor imaging uses an ellipsoid to show where water can diffuse. A long thin ellipsoid is a representation of good diffusion in the axis of the ellipsoid. A sphere indicates that water has diffused uniformly in all directions. A tensor is a description of an ellipsoid for every voxel (Heemskerk et al, 2005, p. 1336). Qs 2 Osteoarthritis is a major cause of disability today in the society. This is due to degeneration or acute injury to the cartilage.MRI is a method of assessing the damage on the cartilage and the adjacent bone as well as measuring the treatment effectiveness.MRI offers an excellent soft tissue contrast, and it is so far the best method that is in use of assessing the articular cartilage. Conventional MRI Methods One advantage as to why MRI is the best method of imaging the soft tissue is its ability to highlight the different tissues. This is due to its ability to manipulate the contrast. Contrast mechanisms that are mostly used in MRI are 2D , proton density, T1 or T2 weighed images that can either be with water suppression or not. Imaging parameters and relaxation time of tissues are critical determinants of the contrast between fluid and cartilage. Contrast in tissues that are lipids containing and those that are not, is increased by lipid suppression. Fat saturation is the most common type of lipid suppression. In this the fat spins are excited and before imaging they are then dephased (Mithoefer, 2006, p. 1413). Alternatively, water spins can be excited alone by spectral spartial excitation. In areas where magnetic fields are inhomogeneous inversion recovery can be used for lipid suppression at the expense of signal to noise ratio. Cartilage imaging has a contrast that is important for the visibility of the cartilage itself and also the visibility of lesions. Surface defects are well visible when there is a high signal from fluid. Such scans are normally 2D and sometimes they might leave gaps that making it possible to hide some areas of cartilage that could be damaged. Spoiled gradient-recalled echo (SPGR) is a three dimensional method and it has the possibility of producing high cartilage signal. However, it has low signal from the adjacent fluid of the joint and due to this it cannot highlight the surface defects adequately. In addition it also cannot give a thorough evaluation of other structures like ligaments. 2D Fast Spin-Echo Imaging (FSE) FSE are excellent in diagnosing cartilage lesions. These techniques are good in providing contrast between tissues and also signal to noise ratio. Since these methods utilize 2D they can be challenging with small structures or those that are oblique. Due to these 3D techniques are more appealing (Wang, 2006, p. 79). FSE image showing cartilage damage 3D Gradient-Echo Techniques MRI techniques that utilize 3D have the ability acquire information of isotropic sizes. However, compared to spin echo methods they do not provide good contrast. SPGR which is three dimensional has more accuracy while detecting cartilage lesions.3D gradient echo technique has two main disadvantages. First it does not have contrast between fluid and cartilage and it also has long imaging times. SPGR utilizes gradient and radiofrequency spoiling that minimize artifacts and are also able to provide T1 weighing. This has its own disadvantage in that it reduces the signal compared to other techniques (Wang, 2006, p. 79). Gradient echo and SPGR techniques can be combined with other methods that are water and fat separating to produce water and fat images that are of high resolutions. Gradient echo imaging accentuates signal from joint fluid while SPGR suppresses the signal. A number of newer techniques are in the process of being established as the current 3D imaging sequences do not provide an optimal resolution and have a number of challenging artifacts (Galban, 2004, p. 256) SPGR showing dark synovial fluid 3D gradient-echo images, showing a dark synovial fluid New MRI techniques Driven Equilibrium Fourier Transform Imaging (DEFT) DEFT is a traditional method of enhancing signal in spectroscopy. This technique utilizes 900 pulse to make magnetization go back to the Z-axis making the signals from tissue increase with long T1.DEFT is different from conventional methods in that its T1 and T2 weighed images their contrast is dependent on the ration of the two in a given tissue. While imaging musculoskeletal DEFT produces sequence by improving the signal from synovial fluid. This gives a synovial fluid that is bright at short TRs. During short TRs DEFT method has a higher cartilage to fluid contrast compared to SPGR and FSE (Galban, 2004, p. 257) Balanced SSFP Imaging This method has a high signal and it obtains 3D images. Due to advances in technology, SSFP is not easily affected by off resonance and banding artifacts .However, these artifacts are still there when TR increases. Due to the capabilities with this technique image resolution is limited by keeping TR at milliseconds. There are a number of methods that have been put forward to give fat suppression in combination with SSFP imaging. A good example is the use of conventional method of fat suppression when the TR is short and the magnetic field is homogenous (Coull et al, 2003, p. 552). A combination of SSFP and fluctuating equilibrium (FEMR) utilize the differences of fat and water frequencies while separating the two. The most ideal method utilizes multiple acquisitions while separating water and fat and this does not depend on the frequencies of water and fat to limit the TR.SSFP utilizes 3D and can be used for imaging the internal derangements of structures such as ligaments (Hardy & Weil, 2010 pp 607). Water image using SSFP fat image using SSFP NB: In the first image the joint fluid is bright than in the second figure. Vastly Interpolated Projection Reconstruction Imaging (VIPR) This techniques was first developed for time resolved CE-MRA. It was later adopted for musculoskeletal system using SSFP imaging. There has been witnessed a lot of advantages with using a combination of 3D k-space acquisition and SSFP. The radial acquisition permits a k space trajectory that has two radial lines for every TR. In this it does not waste time for both the dephasing and rephrasing frequency gradients (Jacob, et al.2010 pp 848). The origin of one radial line is at the origin of the k space origin and the other one is from a different path. This allows acquisition to be in the entire TR.VIPR is a three dimensional imaging that is based on SSFP and this allows the joint fluid to be bright. In this an excellent contrast is provided which provides a good diagnosis for ligament injuries and cartilage damage (Coull et al, 2003, p. 551). Separating water and fat using a linear combination gives a clear contrast between cartilage and bone. There is an alternative method where water and fat are separated by use of a single pass which uses the progression of water and fat and the water spins in the echoes that are produced in each TR. 3D FSE Imaging An FSE that is two dimensional is powerful and can give excellent results but it has anisotropic voxels and effects of partial volume. 3D FSE was used many years ago with a flip angle so as to avoid blurring (Galban, 2004, p. 257). They also reduce parallel imaging and this greatly decreases the imaging time.3D FSE is similar to where multiple planes of 2D in FSE are combined to diagnose cartilage defects or diagnosis (Johnston. et al, 2009 pp 149). Cartilage damage is a common problem and it presents itself in different degrees. The international cartilage repair society has given a grading system for these damages. A normal and health cartilage without any damage is said to be in grade 0.Grade 1 cartilage is the one with blisters or soft spots. For those cartilages that have minor tears that are visible on the surface they are categorized as grade 2.Other cartilages could be damaged to the extent of having crevices that go beyond 50% of the cartilage’s layer are said to be in grade 3 (Galban, 2004, p. 257).There are other cartilages that could be damaged to an extent of exposing the tear which is under the bone, these are classified as grade 4. MRI is an excellent tool that helps us understand the cartilages. There are a lot of improvements which have been done in morphological imaging, by way of resolution, acquisition time and contrast. These improvements have allowed the cartilages details like thickness and volume to be established. The choice on the methods of cartilage imaging to be used is dependent on the patient factors. For those patients that may be suffering from internal derangement, 3D SPGR or even standard FSE may be sufficient. Patients that require surgical therapy require a more detailed technique (Galban, 2004, p. 256). The new methods mentioned here like VIPR, and 3D FSE are able to provide isotropic resolution for the whole joint. Isotropic data obtained can then be reformatted to oblique planes to improve the signal to noise ration. Combination of water- fat separation methods with these methods provides combined images of water and fat. Isotropic imaging reformations can be time saving (Kainz, 2007 pp 450). There is a need for further studies to prove the sensitivity of these isotropic methods to joint pathology like meniscal tears. High resolution can be limited by trade-off between resolution and SNR. The long imaging time involved make the patient have some motions and this can as well limit the high resolution of images. Bibliography Coull R, Raffiq, T., James LE, Stephens, MM. (2003) Open treatment of anterior impingement of the ankle. 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