Current and Emerging MRI Techniques - Essay Example

Q Muscle fibre infrastructure is complex. Functionality of the muscle is entirely depending upon its internal makeup. Technique such as Diffusion tensor imaging (DTI) helps to study relationship between the architecture of muscle fibre and functionality. DTI is used especially for in vivo analysis. It has been an important technique for muscle fibre studies so far. It is a non-invasive method for viewing musculature of vital organs such as brain, spinal cord, kidney, heart and skeletal muscles. DTI has been a key test while investigating white matter affections (Damon, Ding, Anderson, Freyer & Gore, 2002, pp.97-104). Diffusion tensor imaging (DTI) is a imaging technique that helps to understand fibrous structures of the body in details such as muscles and white matter. (Villanova, Zhang, Kindlmann & Laidlaw, 2005, pp 1-38). Theory DTI is based on the physiology of the cell membranes and protein molecules of the muscular tissue that often limit and rate the diffusion of water molecules in single particular direction (Damon et al., 2002, pp.97-104). However, water diffusion rate is faster along the longitudinal axis than its transverse axis. DTI defines the infrastructure of the cells by calculating diffusion rate of the water molecules. Tensors measure this independent diffusion of water molecules corresponding with the principle eigenvalue of long axis of muscle fibre. As per Villanova et al, “It was the first imaging modality of its kind to measure water diffusion in vivo” (pp 1-38). Discussion and benefits of DTI method DTI is based on the method that eigenvector is associated parallel to the longitudinal cell axis. However it posses ‘principal eigenvalue ‘that has been verified in terms of quality and quantity in vivo (Damon et al., 2002, pp.97-104). DTI technique focuses on the orientation dependence of the diffusion. “A diffusion tensor describes the orientation dependence of diffusion assuming free diffusion in a uniform anisotropic medium (Gaussian diffusion)” (Villanova et al.,2005, pp 1-38).Hence, combination of the data of local water molecule direction and the data based on voxel results along with the fibre-tracking algorithm together form overview of the muscle fibre. Diffusion tensor imaging technique plays an important role in viewing muscle fibre structure in vivo studies. This method is able to provide following details about the following (Mori, 2005, pp.468-480): Characteristics of water diffusion Extension of diffusion Diffusion orientation Limitations of DTI DTI analysis demands special skills. Sometimes errors are unavoidable. Noise production comes up as a constant issue during the processing. “Fibre tracking methods relate values spatially by following major coherent fibre structures, but are prone to error due to, for example, partial volume effects, noise, and numerical integration inaccuracies. Fibre tracking methods usually reduce the dimensionality of the tensor from 6 D to 3 D, based in the assumption that linear structures are the most interesting to study” (Villanova et al., 2005, pp 1-38). DTI process often produces noise while 3D vector is in use. Hence, planned vector direction often differs from the actual deviation of fibre. The 3 D vector field obtained from DTI contains noise and therefore the calculated vector direction may deviate from the real fibre orientation.This is one of the strong error caused during DTI. It can be minimised by manipulating tensor fields (Mori, 2005, pp.468-480). DTI is prone to have minor errors that are acceptable up to certain limit. At times visualization is difficult due to multivalued tensors. There is a great research work going on to develop different methods to obtain better images. “Progress is frequently reported on resolution improvement and reductions in imaging time, noise, and distortion” (Villanova et al., 2005, pp.1-38). DTI is still a fresh technique providing information about the structure and function relationship in human muscles in vivo. Moreover, it will certainly need further research for its development and maximum scope. Q-2 Diagnoses of cartilage diseases are difficult with x ray. Stress induced damages are not evident in X ray due to slow and peculiar response of cartilage to injury. MRI is a great non-invasive method to visualise complex cartilage structure. They require some modifications in the projections of field strength and resolution with maximum sequences for best viewing. MRI focuses on cartilage morphology including fissures, abrasion and clefts as well as damage to surrounding structures. It gives the better picture of stress areas of cartilage and indicates the area of recovery too. Need for accuracy Cartilage repair has limited scope. Cartilage diseases progress into whole area pathology. Hence, early diagnosis is essential for better prognosis.MR Spectroscopy is a recent technique that complements MRI of cartilage. In addition, spectroscopy provides conclusive data about the overall makeup of cartilage focusing on fat, cell markers and metabolic outcomes. MRI provides data with the bunch of images to show exact condition of the cartilage. MRI helps to study every injury to cartilage (MRI is key to understanding cartilage health, 2007). Cartilages are prone to pressure from surrounding structures. MRI is the best imaging technique for early interventions. Advantage of MRI MRI has a contrast adjusting capacity in order to view early degeneration signs. It is useful in research field due to its ability to give details about the response of the cartilage after pharmacological and surgical intervention (Link, pp.49-66).MRI cartilage demands high spatial resolution. Common processes are; Standard spin echo, gradient –recalled echo sequences, fast SE sequences, 3 dimensional SE and GRE. These methods are used on regular basis with the purpose of examination of cartilage for any defect. They are useful in providing quantitative and semi quantitative data for research purposes. Researchers suggest that, MRI is immensely helpful to understand the effect of drugs at each level (Crema, Roemer, Marra, Burstein & Gold, 2011, pp.37-62). “For the compositional evaluation of the cartilage few other MRI test are significant such as T2 mapping, delayed gadolinium-enhanced MR imaging of cartilage (dGEMRIC), T1p mapping, sodium technique and diffusion weighted imaging” (Crema et al., 2011, pp.37-62). Magnetic strengths are adjusted in such a manner to get maximum information regarding collagen network of cartilage. It also speaks about the protoglycan part of cartilage. All these techniques have their pros and corns. Research analyst decide about which technique to be used and the decision differs from case to case (Crema et al., 2011, pp.37-62). Current and emerging techniques Latest MRI techniques not only give details about the morphology but also help to understand what is going on inside the cartilage and its respond to the treatment. They give details about the composition, cartilage segmentation and margins. Modern techniques comprise of imaging with higher fields. Some technologies that are involved in morphology assessment help to localise any irregularity on the surface area whereas composition assessment detect biochemical changes in the tissue (Chu, Williams & Schreiber, 2010, pp.91-98). MRI for cartilage morphology 1. 2D fast SE- This is a standard techniques used for diagnostic and research purpose.T1, T2 sequences, proton density sequences provide very clear demarcation between cartilage and fluid. This is extremely useful method for contrast variations in the target tissue along with the desired description in the imaging plain. However, oblique or small tissue structures are not captured clearly in this method while proton density images are often blurred (Link, pp.49-66). This serves the purpose of examining menisci and ligaments and bone marrow. Problem is the anisotropic voxel and partial volume effects (Crema et al., 2011, pp.37-62). 2. 3D fast SE- Sections are thin. Cartilage demonstrates low to moderate signal intensity whereas fluid from the joint space or degenerative cartilage tissue gives high intensity signals. Cartilage and fluid show classic contrast. However, volume artefact and unsuitability to low contrast lesion is the limitation of this technique (Chu, Williams & Schreiber, 2010, pp.91-98; Link, pp.49-66). 3. 3D SPGR (spoiled gradient-recalled echo) - This method is more sensitive than 2D fast SE method. This has a decrease incidence of volume artifact. Isotropic voxel helps to adjust multiple plains together (Crema et al., 2011, pp.37-62). 4. 3D DESS (dual-echo steady state) - Two or more gradient echoes and pulse sequence reset in each pair offer reconstruction of image together. Greater flip angles are needed sometimes. High SNR and distinct cartilage and fluid contrast are its assets. Rest of the characteristic are same as 3D SE (Crema et al., 2011, pp.37-62). 5. 3D bSSFP (balanced steady state free precession) - This technique has similar characteristics and benefits as 3D DESS. However, incidence of banding artifact is higher due to long TR leads (Crema et al., 2011, pp.37-62). 6. 3D DEFT (driven equilibrium Fourier transform) - This process enhances signal intensity of fluid over cartilage. This technique offer information comparable data to the other standard technique such as 2D fast SE and SPGR. However, it is time consuming with deficient fat saturation at times (Crema et al., 2011, pp.37-62). 7. 3D fast SE SPACE – Good SNR. Isotropic voxel help to restructure multiple planes. Various turbo factors are being used along with the flip angle to maintain the steady state. Prolong processing time is its drawback (Crema et al., 2011, pp.37-62). MR imaging techniques for composition of cartilage: 1. T2 mapping- This technique is very helpful. It is used to access the collagen network and fluid content of the affected cartilage portion (Crema et al., 2011, pp.37-62). 2. dGEMRIC- Very useful technique to measure Glycosaminoglycans content of the cartilage. Well utilised in clinical research too. Intravenous contrast is applied during the procedure (Crema et al., 2011, pp.37-62). 3. T1p imaging- This technique is used to determine Collagen network and Glycosaminoglycans. This technique has many positive aspects such as no need of contrast agent and high accuracy in detecting early degeneration signs (Crema et al., 2011, pp.37-62). 4. Sodium imaging-This technique is advised for glycosaminoglycans findings. The measurements have direct co relation with glycosaminoglycans with no need of contrast agent. This technique has limitations such as low signal to noise ratio and spatial resolution (Crema et al., 2011, pp.37-62). 5. Diffusion weighted imaging- This technique provides data about glycosaminoglycans and collagen structure (Crema et al., 2011, pp.37-62). Grading of Cartilaginous lesions- Cartilaginous lesions are graded on MRI on ‘Modified Noyes scale’ with respect to arthroscopic grading. This grading is based on shape and structure of the cartilage lesion (Gold, Chen & Koo, 2009, pp.91-98). Modified Noyes score MRI finding 0 No abnormality detected. Normal appearance. 1 Slight Signal deviation. It often shows Increase in T2 2 Extent of damage to cartilage- less than 50% 3 Extent of cartilage damage is more than 50% 4 Extent of cartilage damage is almost 100% Following pictures are to demonstrate: Axial fast spin-echo (FSE) images with fat saturation of cartilage damage. Pictures (Gold, Chen & Koo, 2009, pp.91-98). A. Axial intermediate-weighted FSE image shows grade1 (increased signal) cartilage damage in lateral facet. B.Grade2 cartilage loss of less than 50% (arrow) over medial facet. C- Full-thickness defect (grade 4) over trochlea with delamination and fragment (arrow) MRI has become a handy tool for all sorts of diagnostic and research purposes. In vivo analysis of muscle fibres and early diagnosis of the cartilage degeneration have become possible due to recent developments in MRI technology. Ultimately, all these discoveries are helping the humankind. References Anonymous (2007). MRI is key to understanding cartilage health. [Online]. Available http://www.sciencedaily.com/releases/2007/06/070614093101.htm [Accessed: May 11, 2012]. Chu, C.R., Williams, A. & Schreiber, V.M. (2010). MRI of the patellofemoral articular cartilage. Patellofemoral pain, instability, and arthritis. DOI:, pp. 91-98. Crema, M.D., Roemer, F.W. & Marra, M.D. (2011).Articular cartilage in the knee: Current MR imaging techniques and applications in clinical practice and research. Radiographics. pp 37-62. [Online].Available from: http://radiographics.rsna.org/content/31/1/37.full.pdf [Accessed: May 10, 2012]. Damon, B.M., Ding, Z. & Anderson, A.W. (2002).Validation of diffusion tensor MRI-based muscle fibre tracking. Magnetic Resonance in Medicine, vol. 48, no. pp. 97-104. Gold, G.E., Chen, C.A. & Koo, S. (2009). Recent advances in MRI of articular cartilage. Musculoskeletal Imaging- Review, 193. [Online]. Available from: Department of Radiology, http://www.ajronline.org/content/193/3/628.full.pdf [Accessed: May 11, 2012]. Link, T.M. MRI of cartilage: Standard techniques. Cartilage Imaging: Significance, techniques and new developments., pp. 49-66. Mori, S. (2002). Fibre tracking: Principles and strategies –A technical review. NMR Biomed. vol.15. [Online]. Available from http://cs.unc.edu/Research/MIDAG/defmreps/styner_www/public/DTI_tutorial/3%20NMR%20in%20biomedicine%202002%20Mori.pdf [Accessed: May 11, 2012]. Villanova, A., Zhang, S. & Kindlmann, G. An introduction to visualization of diffusion tensor imaging and its applications. pp. 1-38. [Online].Available from: Department of Biomedical Engineering: http://vis.cs.brown.edu/docs/pdf/Vilanova-2005-IVD.pdf [Accessed: May 10, 2012]. Read More