Factors for MRI Image Quality

Factors for MRI Image Quality Title: MRI Image Quality TABLE OF CONTENTS 1.0 INTRODUCTION 2.0 SIGNAL TO NOISE RATIO (SNR) Figure 1 3.0 CONTRAST TO NOISE RATIO (CNR) 4.0 RESOLUTION AND SCAN TIME 5.0 THE RELATIONSHIP BETWEEN MRI PARAMETERS AND IMAGE QUALITY Table 1: MRI parameters trade-offs (Proprofs.com, 2015) 6.0 CONCLUSION REFERENCES 1.0 INTRODUCTION Image quality is the most important element in imaging radiography. According to Courses Washington Education, (2015) image quality must be assessed on the basis of average performance of some task of interest by some observer or decision maker. It was stated that image quality must be defined in terms of a task on what information to be retrieved from an image; and the observer on how the information will be extracted from the image. Since MRI image is a digital image, it is greatly depends on image contrast and its spatial characteristics. Nevertheless, one of the disadvantages of this flexibility is a greater difficulty in terms of the choice of scanning parameters. In general scan times are not negligible and there is a certain tendency towards artefact. However, the most fundamental limitation in MRI is the signal-to noise ratio (SNR) which is dependent upon the hardware, mostly the main field strength and radiofrequency (RF) coils, upon the relaxation properties of tissue and upon the choice of sequence parameters. Good image quality depends upon making good scanning parameter choices (McRobbie, 2007). 2.0 SIGNAL TO NOISE RATIO (SNR) The signal to noise ratio can be defined as the ratio of the amplitude of the signal received to the average amplitude of the noise whereas the signal is the voltage induced in the receiver coil by the precession of the net magnetic vector in the transverse plane (Westbrook et al. 2011, pp.104). Noise can be defined as an undesirable backgroundinterferenceor disturbance that affects image quality (Mr-tip.com, 2015). According to Weber (2015), noise is caused by two factors which are the electromagnetic noise in the body due to movement of charged particles; and small anomalies in the measurement electronics, which depends on the size of the RF coil and the bandwidth of the pulse sequence. In addition, noise occurs at all frequencies and is also random in time and space. On the other hand, the signal is cumulative, and occurs at time TE, depends on many factors and it can be altered. The signal is therefore can be changed in relation to the noise. Increasing the signal increases the SNR, while decreasing the signal decreases the SNR. Therefore, any factor that affects the signal amplitude in gives effect to the SNR. The factors that affect the SNR include magnetic field strength, the proton density, voxel volume, repetition time (TR), echo time (TE), flip angle, number of excitations (NEX), receive bandwidth and coil type (Westbrook et al. 2011, pp.104). According to McRobbie in his book, MRI from Picture to Proton (2007), images with a poor SNR will appear fuzzy. An important aspect of image optimization is to certify that there is a high enough SNR for the images to be diagnostically valuable yet low SNR may result in losing small details or the obscuring of subtle contrast changes. Therefore, contrast to noise ratio (CNR) is always taken into consideration in the aspect of image quality. Figure 1: Increasing the basic resolution will increase the image quality. However, increasing the resolution more than the acceptable range will produce grains in the image due to low SNR and reducing it will produce a blurry image due to high SNR. Increasing basic resolution will result in prolonged time. (Image adapted from Mrimaster.com, 2015) 3.0 CONTRAST TO NOISE RATIO (CNR) CNR can be defined as a measure to assess the ability of an imaging procedure to generate clinically useful image contrast. However, the image contrast itself is not precise enough to qualify an image, because in a noisy image it is uncertain where the contrast originates. It depends on two factors either due true tissue contrast, or it may be due to noise fluctuations. The human ability to distinguish between objects is proportional to contrast, and it decreases linearly with noise (KTH, 2015). By improving CNR the perception of the distinct differences between two clinical areas of interest will be increased. In a simple word, acontrasttonoiseratio is a summary ofSNRandcontrast. It is the difference inSNRbetween two relevant tissue types (Mr-tip.com, 2015). CNR is controlled by the same factors that affect SNR. However, it is considered as the most critical factor affecting image quality (scrsl.weebly.com, 2015). 4.0 RESOLUTION AND SCAN TIME In MRI imaging, the scan time is advisable to be as short as possible. This is because the longer the patient lies on the table; the more likely it is that they will move. Moreover, if the patient has moved during the scan, the image produced will have a great SNR (Westbrook et al. 2011). The minimum scan time in MRI imaging is affected by TR, matrix size and NEX while the spatial resolution is determined by matrix size, FOV and slice thickness. By increasing matrix size or decreasing FOV and slice thickness increases spatial resolution at the expense of either decreased signal-to-noise or increased scan time. In order to obtain images of high resolution with high signal-to-noise requires longer scan times. All of the scan parameters affect signal-to-noise ratio. However, the signal within an image can be enhanced either by increasing TR, FOV, slice thickness and NEX or by decreasing TE and matrix size. The most direct way to increase signal is by increasing NEX, but increasing NEX from two to four which doubles the scan time, increases the signal by only the square root of two. Lastly, TE does not affect scan time; however, it does determine the maximum number of slices in multi-slice mode. Increasing the TE or shortening TR decreases the number of slices that can be obtained with one pulse sequence (Spinwarp.ucsd.edu, 2015). 5.0 THE RELATIONSHIP BETWEEN MRI PARAMETERS AND IMAGE QUALITY An image that is obtained in a short scan time, with a good spatial resolution and high SNR is preferable yet is hardly to achieve as increasing one factor certainly reduces one or both of the other two (Westbrook et. al, 2011). Trade-offsexists when changing imaging parameters to obtain the best images possible. For instance, the SNR, resolution, and acquisition time, are all interconnected. Changing one will affect the others. It is important to decide what factors are more important for an examination of a particular body part, patient and suspected abnormality. For example, when looking at the pituitary or cranial nerves, some SNR may need to be less considered or use longer acquisition time to improve the spatial resolution. However, in a claustrophobic of patient in pain who may be moving around, both resolution and SNR for the shortest possible examination time need to be considered to produce better image quality and preventing motion artefact (Ballinger, 2015). Thetable below summarizes the trade-offs in MRI between SNR, resolution, time, maximum number of slices and distance covered. Table 1: MRI parameters trade-offs (Proprofs.com, 2015) 6.0CONCLUSION In conclusion, the quality of an MR image depends on several factors which include the spatial resolution and image contrast, SNR and CNR and also artefacts. An MR examination is cooperation between scan time and image quality and its sequence parameters will have to be optimized in function of the organs and pathology. Moreover, the signal intensities and contrast are determined by the timing parameters TR and TE and also the flip angle. Besides, to produce a good image in MRI the scan time should always be as short as possible to avoid patient movement by using the shortest TR possible, select the coarse matrix possible and reduce the NEX to a minimum. REFERENCES Ballinger, J. (2015).Trade offs | Radiology Reference Article | Radiopaedia.org. [online] Radiopaedia.org. Available at: http://radiopaedia.org/articles/trade-offs [Accessed 3 May 2015]. Barrett, H. and Myers, K. (2004).Foundations of image science. Hoboken, NJ: Wiley-Interscience. Courses Washington Education, (2015).Image Quality. [online] Available at: http://courses.washington.edu/bioen508/Lecture1_partC.pdf [Accessed 30 Apr. 2015]. KTH, (2015).Point Spread Function. [online] Available at: http://medim.sth.kth.se/6l2872/F/F9.pdf [Accessed 30 Apr. 2015]. McRobbie, D. (2007).MRI from picture to proton. Cambridge, UK: Cambridge University Press. Mrimaster.com, (2015). MRI resolution and image quality | how to manipulate mri scan parameters. [online] Available at: https://mrimaster.com/index.4.html [Accessed 4 May 2015]. Mr-tip.com, (2015).MRI Database : Contrast to Noise Ratio. [online] Available at: http://www.mr-tip.com/serv1.php?type=db1dbs=Contrast+to+Noise+Ratio [Accessed 30 Apr. 2015]. Mr-tip.com, (2015).MRI Database : Magnetic Resonance Imaging MRI. [online] Available at: http://www.mr-tip.com/serv1.php?type=db1dbs=Magnetic Resonance Imaging MRI [Accessed 28 Apr. 2015]. Mr-tip.com, (2015).MRI Database : Noise. [online] Available at: http://www.mr-tip.com/serv1.php?type=db1dbs=Noise [Accessed 30 Apr. 2015]. Nde-ed.org, (2015).Image Considerations. [online] Available at: https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/TechCalibrations/imageconsiderations.htm [Accessed 30 Apr. 2015]. Proprofs.com, (2015).Flashcards Table on MRIImaging Parameters. [online] Available at: http://www.proprofs.com/flashcards/tableview.php?title=mriimaging-parameters [Accessed 3 May 2015]. scrsl.weebly.com, (2015).Parameters and Trade-offs. [online] Available at: http://scrsl.weebly.com/uploads/5/1/3/0/5130772/magnetic_resonance_imaging_6parameters__trade_offs.ppt [Accessed 30 Apr. 2015]. Spinwarp.ucsd.edu, (2015). [online] Available at: http://spinwarp.ucsd.edu/neuroweb/Text/br-100.htm [Accessed 3 May 2015]. Spinwarp.ucsd.edu, (2015). [online] Available at: http://spinwarp.ucsd.edu/neuroweb/Text/br-100.htm [Accessed 3 May 2015]. UChicago, (2015).How does MRI work?. [online] Available at: http://wiki.ci.uchicago.edu/pub/HNL/DifficultQuestion/howdoesmriwork.pdf [Accessed 30 Apr. 2015]. University of Bergen, (2015).Contrast (CNR) in MRI. [online] Available at: https://wikihost.uib.no/mriwiki/images/5/5c/MRCourseLectures_Oct2011_ContrastTheory.pdf [Accessed 30 Apr. 2015]. Weber, D. (2015).MRI Quality. [online] Psdlw.users.sourceforge.net. Available at: http://psdlw.users.sourceforge.net/career/dweber_docs/mri_quality.html [Accessed 30 Apr. 2015]. Westbrook, C. and Kaut-Roth, C. (2011).MRI in practice. Oxford: Blackwell Science. 1

Global Challenges in Economics

â€Å"Take a look at this,† a coworker of mine said, tossing a pamphlet on my desk. It was the first thing in the morning. Sipping my coffee, I logged on to the network and leafed through the Excellence in Government publication. A few moments later the man returned and said, â€Å"Did you look that over? I think you†ll be perfect for it; give it a shot!† Already an employee with the Department of Veterans Affairs and a retired United States Army officer, I didn†t give the Excellence in Government Fellow program was for me. However, about a year ago I discovered that the Department of Veterans Affairs was suffering financially and structurally. Anyone who has worked within a governmental organization knows that effective management skills are requisite: such skills can often best be acquired through interfacing the knowledge of the business world with the particular needs of government organizations. Moreover, as our public service bureaucracies increasingly take on the character of corporations, departments like the Department of Veterans Affairs must be restructured and analyzed so that they can meet the standards of a demanding economic and political environment. Having recently become keenly aware of and concerned about the changes in our government services and to their fundamental structures, I am seeking a challenging professional position with the American government. As global economic and political affairs become more complex, the United States government will need new leaders and visionaries to rise to the fore. Like me, many of these new leaders will have dedicated their time and energy in service to the United States Army. Also like me, many aspiring new leaders will have worked in volunteer or professional capacities for the government. I offer years of varied and dedicated service for both the United States Army and the Department of Veteran†s Affairs. Therefore, I have already proven that I possess the skills and talents to be an effective public servant. Years of experience working in service to the government illustrates my level of passion, commitment, and drive. However, in order to participate fully, I will need the networking and experiential opportunities I can acquire through the Excellence in Government Fellow program. Thank you for your consideration.

Telling Victim from Villain at Duke

Although the infamous Duke University lacrosse scandal is long gone and mostly forgotten, there are still people interested in it and people ready to use this interest. Among them – William D. Cohan, the author of a new and very thorough retelling (it cannot be called investigation per se) of this story, â€Å"The Price of Silence†. Back in 2006, the public was immediately attracted to the case’s unpleasant nature: an African American student of North Carolina Central University who also worked as a stripper, dancer and escort, accused three white Duke lacrosse players of raping her at a party organized by the team’s captains. The case was quickly labeled as a hate crime, but later it was revealed that accusations were, in fact, false – the investigation uncovered a lot of inconsistencies in the accuser’s interpretations of events, and the defendants were found innocent. William D. Cohan didn’t uncover any new evidence and doesn’t make any allegations in this respect. But this, however, doesn’t make the story he tells any less fascinating. First of all, he introduced a lot of new details into it, and although these details don’t change the overall picture, they let the reader get a better understanding of what happened in Duke back in 2006. This allows to flesh out the personalities of the people involved in the case, thus making them look like real individuals to the reader. However, although Cohan’s account makes for a rather fascinating reading and the amount of work he did calls for due respect, in some aspects the book can be found somewhat wanting. For example, the multitude of new details adds to the case, but the author didn’t seem to make a lot of work structuring and organizing them. Sometimes it looks as if he tried to include everything he managed to find without taking into account whether this particular bit of information was all that relevant. For example, he informs us about the nickname of Mike Nifong’s (the initial prosecutor) father, the classes that Richard Broadhead, Duke’s president, took during his study at Yale, and so forth. Needless to say, all these things have from little to nothing to do with the subject matter, and needlessly clutter the text. Another peculiar aspect of Cohan’s book is that he largely avoids making his own assumptions and performing his own analysis. He gives you a lot of information, some of which cannot be found anywhere but in this book, and allows you to draw your own conclusions. Some will be refreshed by the author’s not trying to impose his point of view, but there certainly will be a number of readers to find this approach disappointing. In other words: if you are interested in the lacrosse scandal case, it is a thing for you to read; just don’t expect particular revelations.

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