Benefits and Limitations of Higher Field Imaging in Clinical Settings - Essay Example

Benefits and Limitations of Higher Field Imaging in Clinical Settings Name: Course: Professor: Institution: City & State: Date: Abstract The Magnetic Resonance Imaging (MRI) technology is evolving daily. The 3 Tesla (3T) MRI is a higher field strength increasingly replacing the previous 1.5T in a clinical setting. It has several benefits and limitations as well. This study discusses some of them including improved image quality, as indicated by the Signal to noise ratio (SNR), contrast and resolution. With the approval of the use of the higher field MRI in clinical settings, what has been the effect on the Signal to noise ratio, contrast and resolution, and what are the challenges posed by these technological advancements. The potential drawbacks and challenges include increased susceptibility to the patients and the safety concerns due to quadrupling of radiofrequency (RF) power deposition and longer T1- and shorter T2-relaxation time as compared with 1.5T MRI (Futterer & Barentsz, 2009, p. 1). Introduction Magnetic resonance imaging (MRI) is a procedure that makes use of radio waves and a magnetic field in body imaging, differing from X-ray in that it does not use iodizing radiation in producing images. “MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.” (Duggan, 2008, p. 56). The spinning property or rotation of nuclei at different speeds due to a strong magnetic field gradients helps to obtain 3-D spatial information of the tissues. The advancement of the imaging field is having the radiologists changing from traditional machines to modern ones with greater field strength. The magnetic field strength is measured in Tesla (T), using clinical MRI scanners. Currently the scanners are operating at strength between 0.35T, which is the low field, to a high field of 1-1.5T, or ultra-high field of greater than or equal to 3T, which until recently, has had its use in research only (Duggan, 2008). At higher field strengths there is higher susceptibility which can contribute to signal loss or even pose safety hazards. However, magnetic resonance (MR) spectroscopists are on highly ambitious continued research for higher field strength to increase responsiveness and improved capabilities of MRI. For instance even the ultra-high 7T MRI system seems to be picking ground in some places, according to Robitaille and Berliner (p. 60). The specific absorption rate (SAR) of a certain tissue is directly proportional to the square of the angle of flip and the square of the strength of the magnetic field (Bo). It is also directly proportional to the imaged patient’s volume (V) and the duty cycle (D). The SAR for clinical imaging of the abdomen is limited by the Electrochemical commission (IEC) to either 4 W/kg over a 15 minutes period or 8 W/kg over a 5 minutes period. This and other measures are undertaken to prevent the soft tissues heating by more than one degree Celsius (Chang & Kamel, 2010). The general clarity and quality of the MRI produced image is reliant on a number of parameters (Duggan, 2008). The image qualities discussed below including Signal to Noise Ratio (SNR), contrast and resolution help to analyze the advantages and the challenges of the higher field scanners in medical field. The 3-tesla MRI has been a major technical upgrade, enabling both clinicians and radiologists to enhance patient care through better imaging with quality and clarity due to several of its benefits. Advantages of 3Tesla MRI Clinical Scanners Increase in Signal to noise Ratio (SNR) Imaging at higher field strengths implies an increased number of protons aligned with the stronger static magnetic field. This contributes to a much greater signal and higher signal-to-noise ratio (SNR) .3T MRI scanners have been approved for the brain and whole body in general, with several potential benefits. SNR is a measurement that describes the quality of the images or procedures produced by an MRI system. The Signal to noise ratio (SNR) of 3T MRI has increased tremendously in comparison with the 1.5T MRI, to an almost double, putting into consideration other factors including physiological noise, inadequate scanner hardware, and others which slightly can limit the SNR; the higher the SNR of a system the better the quality of images or procedures that can be produced. Figure 1 High Field SNR: 7Tesla and 3Tesla (Sandrick, 2010). At 7T, the white matter SNR equals 65T. The white matter SNR equals 26.3T The SNR value is given by the SI divided with STD measurements: SNR=SI/STD noise, and according to Futterer and Barentsz (2009), noise remains almost unchanged, while its increase is almost linear with the magnetic field strength. According to Schaller (2008), a comparison was done on both 3 T and 1.5T by obtaining for each a maximum intensity projection (MIP), which was then evaluated by a radiologist (p. 35). With the image quality at both considered acceptable for clinical diagnosis, the results, he explains, revealed the doubling of SNR at 3T compared with 1.5T. The background suppression and vessel conspicuity were also higher at 3T and also, ‘despite drop-off in SNR associated with parallel imaging, vessel edges are still well defined at 3T.’ (p. 35) The result of an increased SNR has been the shortening of the acquisition or examination time or to increase the potential resolution of the images. The SNR –starved techniques and diffusion tensor imaging are some of the beneficiaries of increased field strength. Others are newer advancements for time of speeded imaging including GRAPPA, SMASH, and SENSE, which are supported by amplification of SNR at higher field strength owing to their intrinsic tradeoff between imaging speed and SNR (Robitaille & Berliner, 2006, p. 60). Decreased scan times when selected in 3T MRI can assist in reduction of data artifacts involving the motion of patients in instances of difficulties in holding still through the MRI procedure. Improvement in Resolution The capacity to envisage adjustments in enhanced spectral resolution or peaks metabolites is noted, as a result of a doubling chemical transfer with 3T MRI, since the chemical transfer outcome linearly increases with the central magnetic field strength. This occurs as a result of superior MR spectroscopy (MRS) imaging in comparison to MRS imaging at 1.5T (Dugan, 2008). The diagnostic strength and clarity of the image is improved because of an increased SNR on top of a higher spatial resolution. With 3T MRI, this is achievable. One is able to reduce slice thickness or raise in-plane resolution. On the other hand, the presence of thinner slices and smaller FOVs is attributed to an approximately doubled SNR in comparison to 1.5T MRI, which leads to an increase in temporal and spatial resolution. The intrinsic increase of spectral resolution at 3T offers an increased potential for spectroscopic imaging (Futterer & Barentsz, 2009). For instance, a 3T scanning for a head scan provides sections of 3mm and 4 mm in thickness, as compared to about 5mm for the 1.5 T. Therefore a better work is done with less time than with the 1.5T system. The 3T field strength would also allow visualization of body structures like the trabecular structures of spongious bone due to a better resolution than with the 1.5T field strength. Contrast Increased field strength leads to a prolonged T1 relaxation time. Use of similar imaging parameters at 3T and 1.5T will lead to a decrease significantly in relative soft-tissue contrast at 3T. This would mean a longer pulse repetition time (TR) in generating a similar degree of soft tissue contrast (Chang & Kamel, 2010). On the other hand, for effects of 3T on T2 relaxation times are not as predictable, and for most soft tissues, T2 relaxation time decrease slightly, whereas for fat, T2 relaxation times increase slightly. The overall increase in SNR at 3T on T2- weighted imaging is more pronounced in comparison with T-1 weighted imaging. The higher field strength comes with increased development having paramagnetic contrast agents as a result of pulse sequences of gradient-echo. As a result, the spin-echo T1 imaging at 3T is not similar to 1.5T owing to the long time of relaxation. For that reason, it is feasible to prune slice thickness and take images in each of the three planes using volume acquisition. It implies the possibility of detecting more lesions at 3T than with 1.5T. A transfer to high bandwidth, from spin-echo to a moderate echo-train has the added advantage of trimming down the susceptibility artifact. This is particularly valuable to patients who have had metal implants and surgery, in addition to being sensitive to chemical shift. Additionally, the tissues’ longer T1 at 3T adds to an increased conspicuity of improvement that involves a higher contrast-to-background ratio. 3T experiences greater sensitivity to tissue mineralization and deposition of blood products (Tanenbaum, 2005). Fundamental physics (nuclear spin property) of 3T MRI Similar to mass or electrical charge, spin is also a basic property of nature. Spin values are in multiples of 1/2 and can either be positive or negative. Protons, neutrons and electrons have spin (Faulkner, 1996). In NMR, unpaired nuclear spins are very important. When the total number of protons or the mass number (sum of protons and neutrons) is odd, a nucleus develops angular momentum, and consequently, spins (Medscape, 2006). When a nucleus spins, a magnetic field that has magnetic moment is generated. Nuclear Spin System, also referred to as spin packet, is a group of identical nuclei found in a particular sample of matter (Medscape, 2006). A nucleus’ spin orientations are unsystematic and they cancel one another when there is no external magnetic field. However, when the nucleus is placed inside an external magnetic field, the spins align themselves in line with the field, and there is production of a net magnetization that is in line with the field (Faulkner, 1996). High field imaging Limitations Increase in Susceptibility When 3T MRI is employed, the chemical shift doubles in comparison to using 1.5T MRI. While this enhances the spectral resolution, it is a shortcoming for imaging bone interfaces and cartilage of musculoskeletal regions. Doubling the bandwidth can overcome this. Figure 2 Dielectric effect – A was affected by dielectric effect, B used dielectric pads, hence lacks an artifact. (Dietrich et al, 2008) Furthermore, there is a noteworthy increase in homogeneity as a result of the impact of dielectric artifacts due to local eddy currents brought about by increased body tissue conductivity. More often than not, this is prominent in larger anatomic areas such as abdomen, resulting into loss of image clarity, like in liver imaging. Similarly standing wave or dielectric effects are more pronounced at 3T than at lower field strengths. This is a major contributing factor in signal intensity variations across an image. Safety issues The energy deposited in a given mass of tissue due to an increased Specific absorption rate (SAR) in 3T MRI causes increased tissue heating as the magnetic field strength increases. This can be a cause of alarm, and especially so for patients with various types of implants which might not have been identified before the scan is done. The International electrochemical commission (IEC) guidelines limit the SAR in MRI to not exceeding 8W/kg of tissue. In addition, the higher field strength comes with an increase in gradient noise which is not safe for the patient. This is due to the levels of sound pressure that increase with field strength. The levels of noise at 3T can be in excess of 130 dBA, while the limit permissible by the IEC stands at 99 dBA. There is need routinely, thus for use of earplugs or other active noise cancellation devices (Duggan, 2008). The field strength also scales the chemical shift effect providing a boost in metabolite peak separation, and makes RF fat suppression more robust at 3T. This could be a significant limiting factor in routine anatomic imaging. However, the fortunate thing is that late-generation multichannel coils with higher bandwidths for spin echo (SE) and FSE imaging has reduced chemical shift effects in a range similar to the one of 1.5T (Medscape, 2006). Other limitations of Magnetic Resonance Imaging include the need for more complicated pieces of instrumentation, as well as the process takes longer to get a scan than the CT process, and thus, it is more vulnerable to movement of a patient. Since most MRI instruments are advanced, they call for the use of Niobium tin in order to produce fields above this range (Stafford, n.d). However, the substance is very brittle, and thus the exclusive use of the substance is extremely difficult. Conclusion There are a lot of advantages accrued to the Magnetic resonance technology advancement to the higher field, but an almost equal number of challenges to be overcome first in this evolution as have been successfully covered in this discussion. A superior spatial resolution, improved and consistent image quality and speed are just but to mention of the few advantages of 3T imaging as compared to the 1.5T systems mostly in use today. However, the challenges coming with the 3T imaging poses specific threats and dangers which should be addressed using various available techniques like utilizing thinner slices, increasing bandwidth or spatial resolution at higher field. The radiologists are also in the best position to advice and select the most appropriate MRI system depending on the clinical problem to be solved. References Chang, K. and Kamel, I., 2010. Abdominal imaging at 3T: Challenges and solutions. Applied Radiology, 9 (5), pp. 5-56. Dietrich, O. et al. 2008. Artifacts in 3-Tesla MRI: Physical Background and Reduction Strategies. Eur J Radiolo, 65 (1), pp. 29-35. Duggan, T., 2008. The Evolution of Magnetic Resonance Imaging: 3T MRI in Clinical Applications. Eradimaging, 2 (5), pp. 6-89. Faulkner, Wm. 1996. Basic Principles of MRI. OutSource, Inc. Futterer, J. and Barentsz, J., 2009. 3T MRI of prostate cancer’. Applied Radiology, 1 Jan, p. 1. Medscape, A., 2006. Clinical 3T MRI: Mastering the challenges. London: Anderson Publishing. Robitaille, P. and Berliner, L., 2006. Ultra high field magnetic resonance imaging. New York: Springer Science Business Media. Sandrick, K., 2010. 3T MR - Extra Power at 3T Gives Users a Choice. UBM Medica. Schaller, B., 2008. Neuroscience Imaging Research Trends. New York: Nova Science Publishers. Stafford, R.J., n.d. High Field MRI: Technology, Applications, Safety, and Limitations. Retrieved from http://www.aapm.org/meetings/05am/pdf/18-2826-94182-387.pdf [Accessed 22 October 2011]. 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