Echo-Planar Imaging in Magnetic Resonance Imaging - Essay Example

?EPI in MRI Introduction MRI, or magnetic resonance imaging, fabricates information for a picture from a sequence of separate samples of signals. Echo planar imaging, or EPI, is a method that can form an absolute image from a single data sample. This provides significant advantages, as far as the speed factor is concerned (Cohen, 2000, p.2). “EPI is fundamentally just a trick of spatial encoding” (Cohen, 2000, p.3). EPI is a speedy, elastic method of imaging. It has high-quality contrast and resolution capabilities, and also possesses many prospective applications in clinical imaging, including functional MRI and rapid whole brain imaging. However, EPI can be very intolerant and the system needs careful selection of parameters. Echo-planar imaging is the fastest and most supple means of MR imaging nowadays. It offers substantial autonomy in the assortment of the parameters needed for contrast and resolution. However, according to the process, the system of formation of the images works close to its confines of performance depending on amplitude of the gradients and the number of times it rises, the stability in the structure and general stature of the noise formed and, thus, proves to be a difficult method. Even so, the advantages of EPI in “functional neuroimaging” have increased its demand and provide technological development (Cohen, 2000, p.15). The present study focuses on the concept of EPI in MRI and discusses the method, its working, the issues of image ghosting, its disadvantages, its sensitivity, as well as clinical benefits. BLIP EPI Method Echo-planar imaging (EPI) is skilled to considerably cut down the times of magnetic resonance (MR) imaging. It allows getting hold of representations within a timeframe of only 20–100 msec. This particular resolution of time enables successful elimination of motion-related relics. Consequently, it becomes possible to achieve imaging of rapidly changing physiologic processes. In order to understand the basic principles and working of the method, the understanding of k space theory is necessary. K space is a realistic medium of data where the MR imaging is in a digitized form and represents the picture before “Fourier transform analysis”. All points in k space contain data from all locations within an MR image. The Fourier transform of k space is the image” (Poustchi-Amin et al, 2001, pp.767-779). While studying the working of echo-planar imaging, it is helpful to put the usual spin-echo (SE) imaging side by side. In pulse progression of a SE, a single line of the data of representation, which is actually a single line in k space or a single stride of encoding a phase, is composed in every time of repetition (TR) period. Then the series of the pulse is continued for numerous times of TR in anticipation of all the steps of encoding the phase, collection and filling the k space. As a result, the time of imaging becomes equal to the creation of the TR, as well as the number of phase-encoding steps (Poustchi-Amin et al, 2001, pp.767-779). As compared to Fast Spin Echo (FSE), the gradient refocused echo in EPI adds a single line in the area of k-space. The direction in which line is read is altered by the positive and negative read gradients. A shift in k-space occurs through the presence of the phase blip that occurs between the echoes. This is the method known as blip EPI. The spins are excited once in the process of EPI and the pulses between the echoes do not involve any 180 degrees RF. If there is a spin echo EPI sequence, a large positive phase may be used that encodes the gradient if it does not reach the 180° degree pulse. Gradients are used by EPI, having both a negative and positive polarity, such that both odd and even echoes may be produced. The contrast in EPI can be restricted by varying the preliminary preparation of the echo from a spin echo to a gradient echo, or by the use of inversion recovery (Module 1, pp.7-16). In the process of echo-planar imaging, several lines of information related to the picture are achieved following a particular stimulation of RF. Analogous to the progression of conventional SE, the series of a SE echo-planar imaging also begins with pulses that are at RF of degrees 90° and 180°. On the other hand, subsequent to the pulsation of the 180° RF, the frequency-encoding slope fluctuates at a fast pace from a positive amplitude to negative amplitude. A chain of echoes of the gradient gets formed in this manner. Each echo represents a phase encoded in different ways by blips of encoding of the phase present on the axis of the phase-encoding medium. Every alternation of the gradient of encoding the frequency communicates to a single line of information of representation in k space, “and each blip corresponds to a transition from one line to the next in k space. This technique is called blipped echo-planar imaging” (Poustchi-Amin et al, 2001, pp.767-779). If the original echo-planar imaging method is considered, it might be learnt that the gradient of encoding the phase used to be set aside on weakly but incessantly throughout the complete acquirement process. Nowadays, there are several variants of the echo-planar imaging. Asymmetric echo-planar imaging is one such variant where information is accumulated only at some point for the duration of the positive gradient of encoding the frequency. On the other hand, the gradient lobe of the negative encoding of frequency is exploited to pass through to an opposite area of k space (Poustchi-Amin et al, 2001, pp.767-779). Blipped echo-planar single-plus method, or BEST, is a progression that scrutinizes the k-space in a form that is rectangular in shape. Modulus BEST, or MBEST, is customized translation of BEST (Ramalho, Borges & Filho, 2001). Figure 1: Timing Diagram of MBEST (Ramalho, Borges & Filho, 2001). Here, the magnetization is revolved to the region of the slice through selective slices representing pulses at 90° RF and is evoked Np times by irregular read gradients Gx. A single stimulation scrutinizes the entire network in the k-space. Every echo is encoded in phases by blipping Gy before collecting the data of each echo. An additional characteristic of BEST and MBEST cycles is the exchange of rapid gradients. The periods of rising and diminishing of these gradients “are of the order of 100ms, which confer to them a profile similar to trapezes and not a perfect square form” (Ramalho, Borges & Filho, 2001). Nyquist Ghosting: Nyquist ghost is a characteristic object of the EPI that is capable of degrading the superiority in general of the time series of fMRI. Studies reveal that the power of Nyquist ghost and its sequential variation in the time sequences of EPI-fMRI have the capability to vary appreciably depending on the bandwidth of the readout process and the spacing of the echos. The particular model of such variation may be dependent on every individual system of MR scanner as far as the characteristics of the gradients are concerned, along with the calibrations of EPI sequences and the functional design of radiofrequency coil. The use of low values of bandwidth may condense the force and sequential deviation of the Nyquist ghost of the time series of EPI-FMRI. The employment of a minimum value of ES might on the other hand not prove to be advantageous to the extent desired, particularly when the gradients characterize the scanner of MR with low performances and calibrations of EPI sequences being suboptimal. The fMRI time series may be greatly affected by the Nyquist ghost at particular values of ES as an effect of potential resonance (Giannelli et al, 2010). EPI’s Sensitivity to Off-Resonance Effects EPI is highly susceptible to off-resonance effects and need to primarily deal with off-resonance results arising from spins of fat, and off-resonance consequences from spins of water. Spins are learnt to build up a shift in segment under the gradient of the readout of EPI. Fat spins have a frequency of 220 Hz away from the spins of water and they build up phase related to a chemical shift that equals 220 Hz multiplied with the time of ETL, where the ETL time refers to the time required for the complete train of the echo to participate in a single TR. EPI also needs to deal with the shifts from water protons, that is to say, the off-resonance effects of the water spins. Regions close to the interfaces of tissue-air generally experience minute “inhomogeneities in the local magnetic field” (Schrack, 1996). On the other hand, regions with tissues that are highly magnetized and located adjoin to regions having lower magnetization power cause disturbances in the local field. These areas contain water protons that build up a phase shift. It is not possible to suppress the water signal like the fat one could be done. This is so because if both water and fat are concealed, nothing would generate the signals needed in the system (Schrack, 1996). Clinical Benefits of EPI The velocity, the multiplicity of difference, and the elasticity in terms of obtainable resolution are some factors that are considered regarding the clinical applications of EPI. The various clinical functions where EPI may be applicable include “functional brain imaging, perfusion imaging, diffusion imaging, abdominal imaging, cardiac imaging, lung function, and musculoskeletal function” (Module 8, pp.4-11). Functional magnetic resonance imaging (fMRI) of the brain makes use of the paramagnetic deoxyhaemoglobin as an agent of contrast, or which is the BOLD effect (blood oxygenation level dependent). When it is a non-stimulated state, the incidence of deoxyhaemoglobin causes occurrence of local field gradients and a faster time of T2* decay (Module 8, pp.4-11). Functional brain imaging can be utilized for a wide range of purposes. It incorporates the psychological investigations into the brain functions. Additionally, it can be used to collect additional information for surgical/therapeutic purposes. Through perfusion imaging of the brain, the delivery of blood to a vascular bed can be assessed. EPI can be successfully used for this purpose owing to its speed (it provides good temporal resolution while it follows the contrast) along with its potential of T2*-weighted imaging. Perfusion maps can find out the occurrence and level of prospective tissue within the ischemic penumbra. The identification of at risk tissue reflects on EPI being used for coherent choice of patients for acute interventional stroke treatment. Diffusion involves random molecular thermal motion (Brownian motion) and participates significantly in cerebrovascular accidents. Sensitization of Spin Echo EPI to the motion of the water molecules diffused through a tissue compartment through inclusion of well-built, motion-sensitizing pulses of gradient before and after the ?-pulse (Module 8, pp.14-16). EPI also contributes in abdominal imaging of patients. A multislice multishot EPI test can be completed in 18 seconds, or single shot EPI can be used, where complete exposure can be done in 3-5 seconds. The “gold standard of cardiac imaging” (Module 8, pp.19-21) is a firm state complimentary precession or coherent gradient echo method known as true-FISP. It can be used to obtain images of the heart at a number of time points from systole to diastole. Single slice multi-phase information can be produced in a minimum time of 16 seconds, through segmented k-space collection and short repetition times. EPI can also play a vital role to whole heart breath-hold contrast uptake reading to image myocardial perfusion. It also lets for the imaging of coronary arteries without any cardiac motion artifacts. The rapid imaging capacity of EPI can be used to track the dynamic circulation of hyperpolarised 3He in the course of the lung during breathing. EPI can also be utilized to pursue the impulsive, incessant, slow opening and closing of the temporomandibular joint. It has been obtained that it gives good approach into the relative motion connecting the condyle and disc, and the twist of the disc during motion (Module 8, pp.19-21). EPI also provides adequate spatial and sequential resolution to differentiate the patterns of perfusion in the “subendocardial, middle, and subepicardial zones of the left ventricular myocardium. MR imaging of the coronary arteries is currently an area of great interest. Echo-planar imaging has been used for this purpose and will most likely play a role in further development of the technique.  In the pediatric population, cardiac echo-planar imaging offers the potential to image congenital heart disease without the need for sedation” (Poustchi-Amin, 2001, p.767-779). The findings of Echoplanar Imaging Thrombolytic Evaluation Trial (EPITHET) do not modify the existing clinical practices (Davis et al, 2008, pp.299-309). The EPITHET study experienced whether intravenous thrombolysis given after three to six hours of the onset of the stroke upholds reperfusion and satisfies infarct development in patients who have results of “potentially salvageable brain tissue (mismatch) based on perfusion- and diffusion-weighted MRI examination. In 101 patients randomized to thrombolysis or placebo (86% of whom had mismatch), reperfusion was significantly increased by thrombolytic therapy and was strongly (but non-significantly) associated with infarct growth attenuation, good neurological outcome, and good functional outcome” (Davis et al, 2008, pp.299-309). Conclusion From the above study, it can be concluded that the MRI function has been modified and enhanced significantly with the incorporation of EPI. EPI, along with functional MRI, has proven to benefit the clinical practices to great extents, although there are still much of the developments that are necessary for the process to be understood and adopted. Certain disadvantages of the system have also been determined, particularly with regard to its sensitivity that needs to be considered such that measures may be adopted to handle it successfully. The clinical benefits of EPI in MRI are proven and, hence, medical practices need to consider further research and experiments to successfully make use of this technique in medical treatments of patients. References Cohen, M.S. (2000). Echo-planar imaging (EPI) and functional MRI. [Online] Available at: http://www.uib.no/med/avd/miapr/arvid/MOD3_2002/Diffusjon/EPI_fMRI_Cohen.pdf [Accessed on September 1, 2012] Davis, S.M. et al (2008). Effects of alteplase beyond 3 h after stroke in the Echoplanar Imaging Thrombolytic Evaluation Trial (EPITHET): a placebo-controlled randomized trial. Lancet Neurol, Vol.7, No.4, pp.299-309, [Online] Available at: http://f1000.com/1103191 [Accessed on September 4, 2012] Giannelli, M. et al (2010). Characterization of Nyquist ghost in EPI-fMRI acquisition sequences implemented on two clinical 1.5 T MR scanner systems: effect of readout bandwidth and echo spacing. Journal of Applied Clinical Medical Physics, Vol.11, No.4, [Online] Available at: http://www.jacmp.org/index.php/jacmp/article/view/3237/2035 [Accessed on September 3, 2012] Module 1 Module 8 Poustchi-Amin, M. et al (2001). Principles and Applications of Echo-planar Imaging: A Review for the General Radiologist. Radiographics, Vol.21, pp.767-779, [Online] Available at: http://radiographics.rsna.org/content/21/3/767.long [Accessed on September 2, 2012] Ramalho, S.S., Borges, N.M. & W.W. Filho (2001). Images by Nuclear Magnetic Resonance. A Modified Version of the EPI Method. Brazilian Journal of Physics, Vol.31, No.2, [Online] Available at: http://www.scielo.br/scielo.php?pid=S0103-97332001000200025&script=sci_arttext [Accessed on September 2, 2012] Schrack, T. (1996). Echo Planar Imaging. [Online] Available at: http://mr.imaging-ks.nu/docs/EPI_APPS.PDF [Accessed on September 3, 2012] Read More