The Larmor equation shows precessional frequency increases with increased static magnetic field strength. How does it impact on image quality between a 3T and 1.5T MRI scanner? Essay Example
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Table of Contents
3The Larmor Equation. 2.
High image quality with а 3Т as compared with a 1.5Т MRI scanner 4
Lower Image Quality with 3T MRI Scanners 5
Example of 3T and 1.5T imaging 6
Larmor equation determines Radio frequency which is used by Magnetic Resonance Imaging (MRI) to create images. Radio frequency determines the magnetic field strength of MRI. Magnetic field strength is responsible in ensuring that Larmor frequencies is maintained at a certain level by controlling protons on the lower energy level for an image to be produced(Bright, 2012). According to Westbrook (2011) Magnetic field strength is affected by SNR (Signal- to- Noise Ratio) thus affecting the larmor equation. Images scanned by a 3T or a 1.5MRI scanner their quality is affected by larmor equation. This is because MRI scanners have more protons that face upward than downward which results in the increase magnetic vector that improves the image quality (Mrimaster 2013). The paper aims to discuss how Larmor equation impacts on the quality of images scanned by 3T and 1.5T MRI scanners.
The Larmor Equation
The following is the Larmor equation which is used to calculate larmor frequencies.
Where presents an angular frequency or Larmor frequency (MHz) of proton precession in external magnetic field. is the gyromagnetic ratio in MHz/T and is known as a constant of proportionality.B0 is external magnetic field strength presented as T(Schoenberg, 2007). This equation has both positive and negative impacts on image quality produced because of the Larmor frequency the MRI scanner have. The Larmor frequency which is a 1.5T or a 3T MRI scanner is calculated as below;
For 1.5T MRI
= = 63.772MHz
= = 127.544MHz
The calculation implies that 3T MRI scanner (127.544MHz) has stronger magnetic fields as compared to 1.5T MRI scanner (63.772MHz). Stronger magnetic fields require stronger Radio Frequencies (RF) to generate MR images (Westbrook, 2008).
High image quality with а 3Т as compared with a 1.5Т MRI scanner
There are many advantages of using a 3T MRI as compared to 1.5T MRI scanner in producing better quality images. 3Т MRI scanner causes SNR to increase uunlike 1.5Т MRI scanner because SNR has an inverse relationship with noise. A 3T MRI scanner creates double SNR of those created by 1.5T scanners. When the SNR is high, the clarity of the image and diagnostic strength also get better because scanning time is reduced thus 3T imaging as enhanced image quality comparable to 1.5T imaging. In addition, 3T imaging develops a better spatial resolution and quality images due to the fact that it reduces slice thickness or have increased plane resolution (Westbrook- et al 2011). 3T MRI scanner allows a person to make small field of views as well as thin slices because double SNR which cannot be done with 1.5T imaging (Bright, 2011).
3T imaging has high Specific Absorption Rate which is the amount of energy released by a tissue of radiology group field. According to Schoenberg (2007), the average energy released by MRI scanner should be a maximum of 8w per kilogram of tissue within a period of 5 minutes. This ensures that the body temperature remains at a recommended rate of change of 1 degree Celsius. Usually radio frequency energy dissipation in the body causes tissue heating which can lead to increased body temperature. People who have the problem to remain still for some time will have their images taken by 3T MRIs canner which has reduced scan time and producing better quality imaging as compared to a 1.5 T MRI scanner which does not have the ability have to produce quality images at reduced time (Schoenberg, 2007).
3T MRI scanner has the ability to create improved Spectroscopy imaging due to the fact that it has doubled chemical shift during scanning as opposed to 1.5T MRI scanner. This leads to a better spectral resolution as well as enhanced visibility of metabolic changes. This means that unlike 1.5T MRI scanner, 3T MRI scanner can be used to capture images of the abdomen, and still afford to produce better quality image (Burghart and Finn, 2011). It also suppresses fat and water in tissues or muscles thus leading to clarity of images(Westbrook, 2008).
3T MRI have other advantages such as diffusion tensor imaging, “Diffusion-Weighted Imaging (DWI) and Blood Oxygen Level Dependent (BOLD) imaging. Unlike 1.5T, 3T MRI has a high CNR for background suppression of tissue resulting to improved Time-Of-Flight (TOF) imaging (Westbrook, 2013).
Lower Image Quality with 3T MRI Scanners
At times 3T MRI scanner can produce poor quality images as opposed to 1.5T MRI scanner especially when taking images using Radio frequency intensive pulse sequence. When there are increased SAR in 3T, radio frequency limits may be exceeded affecting image quality because dissipation image will be high. This can be sorted out by reducing slice thickness and increased time to have a better quality image. This is possible in RF-intensive pulse sequences such as Fast Fluid- Attenuated Inversion Recovery (FLAIR) and echo planar imaging (Burghart and Finn. 2011).
Example of 3T and 1.5T imaging
The diagram below shows an example of original image which has been done by 3T AND 1.5T imaging (Butler, 2012).
From the above diagram, it is clear that high frequency diagram has a greater image quality as compared to other image. The image produced by 1.5T is blurry, and its distorted if it involves abdominal. Looking at the contrast between the images produced by 1.5T and 3T MRI scanners, the quality difference will be noted (Westbrook, RothandTalbot 2011).
The above high frequency component image has average pixels as compared to second image which may lead one to believe that they are from the same sources (Westbrook 2008). This confirms that the two MRI scanners are not similar in producing a quality image as they allowed different frequency features to be captured. In the case of 3T MRI scanner, the image is not very much affected by colour, inversion, opacity and saturation. (Mriprotocol 2012). This kind of scanner can be used to improve the quality of an image which has shadows much light affects the edges of the image. Looking at the1.5T MRI image, there is a contrast between the original image and the filtered image, which can be distinguished by a difference in the colour of the image and blurriness. There is also a departure in terms of the shadow of the image (Mrimaster 2013).
The paper intended to highlight the impacts of Larmor Equation on image quality as scanned by 3T and 1.5T MRI scanners. The paper has shown that 3T MRI scanner has stronger magnetic fields by having Larmor frequency of 127.544MHz which double of that of 1.5T MRI scanner (127.544MHz). It also achieves better spatial resolution, consistency in the quality of images and speed which would be better than 1.5T MRI systems. This gives 3T MRI scanners produce better quality images than 1.5T MRI scanners (Burghart and Finn. 2011). At times 3T imaging has poor quality image when there is increased SAR in 3T leading to increased radio frequency to limits impacting image quality(Westbrook,2013).
Bright, Ann. 2011. Planning and Positioning in MRI. London: Churchill Livingstone
Burghart, Geraldine & Carol Ann Finn. 2011. Handbook of MRI Scanning. New York: Mosby
Butler, Renée. 2012. Foundations of Medical Image Quality. <http://courses.washington.edu/radxphys/course_content/r2/medical_img_quality/class2012/Image%20Quality_20130620.pdf>
Mrimaster 2013. Resolution and Image Quality . <http://mrimaster.com/index.4.html>
Mriprotocol , 2012. Signal-to-noise ratio (SNR), imaging time, and resolution: Three factors of MRI image quality. < http://mriprotocol.blogspot.com/2012/10/signal-to-noise-ratio-snr-imaging-time.html>
Schoenberg SO, Zech CJ, Panteleon A, et al.2007. New perspectives and challenges in abdominal 3T MRimaging. Appl Radiol. 2007; 36 (suppl). Available at: http://www.appliedradiology.com/articles/Article.asp?ID=1330&IssueID=170&ThreadID=. Accessed May 29, 2008.
Westbrook, Catherine. 2013. MRI at a Glance. New York: John Wiley & Sons.
Westbrook, Catherine. 2008. Handbook of MRI technique. Oxford: Wiley-Blackwell.
Westbrook, Carolyn, Kaut Roth & John Talbot. 2011. MRI in practice. Oxford: Wiley-Blackwell.
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