The Basic Principles of MRI Image Production - Assignment Example

The basic principles of MRI image production: Introduction: Magnetic Resonance Imaging (MRI) is presently one of the most preferred diagnostic tools in healthcare. Most hospitals are equipped with this technology because of its advantages and potentials in the efficient delivery of health care services. (Meriles 2006). Magnetic Resonance Imaging is based on Nuclear Magnetic Resonance where there is an “absorption and emission of energy by atomic nuclei in the presence of an externally applied magnetic field.” (McKie and Brittenden 2005, 13-14). Magnetic Resonance Imaging is a special radiology technology designed to image internal structures of the body using magnetism, radio waves, and a computer to produce the images of body structures. (Consiglio 2006). A combination of the above tools makes this technology possible. Here we will discuss what an MRI and which principles does it need to use. Major parts of MRI: MRI scanner has five major parts. These are: magnet, antenna, transmitter, receiver, and computer. MRI magnet works best with a magnet that has high field strength which would help in achieving optimum signal-to-noise ratio SNR. A transmitter function is sending out radiofrequency transmissions to an antenna, which then sends out RF to the patient. The antenna then receives returning transmissions. The coil is usually placed directly on the patient’s skin depending on the part of the body from which information is being gathered. The receiver would then amplify the signal picked up from the coils. Information gathered from the coils is then processed by the computer. MRI principles and MRI Process: The MRI is one of the major diagnostic examinations available in the healthcare industry. MRI, “depends on the magnetic spin properties of hydrogen nuclei in tissues and how these nuclei recover after excitation with radio frequency electromagnetic waves”. MRI has greater sensitivity to small and subtle differences in the tissue types. (Faerber 1995, 1) discusses that the MRI process starts with the patient lying in a strong magnetic field. This magnetic field ranges from 0.3 to 2.0 Tesla. Tesla (T) is the unit of the strength of the magnetic field. The magnetic field then seeks to align the hydrogen nuclei of the body into two equilibrium states. Radiofrequency waves are then sent into the body in order to excite this equilibrium state. After the radiofrequency waves return to their normal state, the excitation energy is released into the body as radiofrequency energy. The energy is detected by special energy coils making it possible to detect the course from excitation to relaxation. Relaxation times are usually measured in the T1 and T2 parameters. These two parameters are largely dependent on the tissue involved and on other elements like water, tumor, cyst, and other such characteristics in body tissues. Thus, images can be shaped with appropriate spatial localization of the signals, and made diagnostic decisions to the lesions. (Faerber 1995, 1-3). The figure below shows the basic MRI process. It shows the magnet, the antenna, and the gradient. The different parts of the body have different coils. The examined part can be scanned only with the help of the machine, shown in the picture below. Figure 1: MRI process (Short n.d.). MRI Parameters (T1, T2, PD weighted): Jacobson (2008) expands on Faerber’s explanation by mentioning that the impact and rate of energy release which manifest as protons resume the T1 relaxation alignment; as they wobble or process during the procedure, they are recorded by a coil as spatially localized signal intensities. The computer then analyzes these signals in order to project the anatomic images. The brightness of the tissues in the MRI image is firm by radiofrequency pulse and gradient wavelengths which obtain images, tissue characteristics, and tissue density. When the radiofrequency pulse and waveforms are controlled, the computer program creates specific pulses which later resolve into images. These images can be weighted based on T1, T2, or proton density. Body tissues manifest in different ways. Jacobson (2008) discusses that fat appears bright with a high signal intensity when the T1 weight is used, but they appear dark on T2 images. Water and other body fluids appear dark when the T1 weight is used, and appear bright on the T2 weight. Because of that, the contrast media is not done in T2. In some cases, contrast agents are used in order to show up vascular structures and to help characterize inflammation and tumors. Both T1 and T2 weighted images are actually used to provide complementary information; hence they are both important elements in detecting abnormalities in the body (Jacobson 2008). The figures below compare the T1, T2, and PD images respectively. Figure 3 ( Frahm et al. 2003). Types of magnets in MRI systems: There are three types of magnets which are used in the magnetic resonance systems. They are the permanent, the resistive, and the superconductive magnets. The magnetic field is actually measured in the unit known as Tesla or ‘T’. This magnetic field is also important in establishing a net magnetization of hydrogen protons which are needed for the MRI (Consiglio 2006, 5). The static magnetic force – torque and translational force – all act concurrently on fixed magnetic objects and these forces can be dangerous for patients with implanted devices or magnetic foreign bodies. These forces can cause bullet effects and then cause the object to gain speed and kinetic energy on its path to the magnet. This projectile effect can cause severe injury and can also be fatal to people within the path (Consiglo 2006, 5). MRI mechanism: The hydrogen proton nuclei are most concentrated in the body’s water and fat molecules. When the patient is entered in the machine, energy is released in the form of radiofrequency pulses which are then matched to the frequency of hydrogen atoms – atoms that absorb energy pulses. When radiofrequencies are applied, hydrogen atoms then re-release this energy as magnetic resonance; this resonance then stimulates a small voltage in a receiver coil near the patient. (McKie & Brittenden 2005, 13-14). Robert-Jan, et.al. (1999, 149) discuss that quantum mechanics plays a large role in the imaging process. Nuclei spins around their axes and work like small magnets. Apart from hydrogen, other nuclei like sodium, carbon, and phosphorus also play a role in clinical imaging. These tiny magnets are distributed in space and normally cancel each other out. However, when the patient is also submitted to a strong external magnetic field, the nuclei would either adapt an orientation parallel or anti-parallel to the external field. (Robert-Jan et.al. 1999, 149). In order to achieve successful imaging, high resolution images, large field of views, and insensitivity to field in homogeneities and motion, parallel orientation is preferred. These are the elements which make MR imaging a challenging feat to undertake. Motion is a challenge because it actually creates artifacts from other sources of movement like the cardiac wall, blood flow through blood vessels, respiratory movements, and even peristalsis. However, advancements have recently been made in order to address these problems and in order to ensure successful image acquisition. (Constable 2003). Jacobson (2008) discusses that the MRI has different variations. The diffusion-weighted MRI is seen when the intensity of the signal is related to the diffusion of water in the molecules or in the tissues. This is used to detect ischemia and infarctions. The echo planar imaging is a very fast technique and is used in the functional imaging of the brain and the heart. The gradient echo imaging is conducted through pulse sequence, and is used to obtain images of moving blood and CSF. The perfusion MRI is used to assess relative cerebral blood flow and to detect ischemia during imaging for stroke and infarctions. The functional MRI is blood-oxygenation-level dependent BOLD, and is the most “commonly employed method for non-invasive localization of activity in the brain in cognitive psychology investigations.” (Sutton et.al. 2008, 33). Functional MRIs (fMRI) are used to measure the ‘function’ and the activity of the brain. Through the BOLD, the fMRI now evaluates differences in magnetic susceptibility between oxygenated and deoxygenated haemoglobin in microcirculation through the brain pathways. Depending on changes in metabolism, perfusion and blood velocity affect neuronal function; and such images can be detected through fMRI based on blood oxygenation and magnetic susceptibility. (Marzola et.al. 2003, 165). Magnetic field gradients and Water diffusion : Magnetic Resonance Imaging is sensitive to water self-diffusion, magnetic field gradients undergo phase shifts based on loss of coherence and on decrease in the intensity of signal. Through DW-MRI, there is now a unique form of MR contrast that enables the diffusion motion of water molecules to be measured and, as a consequence of the interactions between tissue water and cellular structures, provides information about size, shape, orientation and geometry of the brain structure. (Marzola et.al. 2003, 165, 970). This sensitivity to water now allows assessments on neuron-degeneration in dementia as exhibited by altered diffusion coefficient. Sensitivity can also be used to explain pathways in the brain based on the assumption that same sensitivity and algorithms are relevant to segment specific tracts. Consiglio (2006, 6) discusses that the gradient fields of the MRI function by localizing tissue signals. Three gradients are found in the system, and each field functions through the flow of electrical current through separate loops of wire mounted in one gradient coil. During an MRI, the gradient fields send out electrical currents in the nerves and muscles, these currents later act as conductors in the patient’s body. Because of these gradients, patient is bound to feel mild coetaneous sensations, muscle contractions, and even cardiac arrhythmias. The noises heard during MRI are a result of vibrations and forces in the gradient system. As electrical currents are changed, forces and vibrations are produced. Vibrations then generate the acoustic noise heard during MRI. (Consiglio 2006, 5). Figure 4: Diffusion weighted images corresponding to three different diffusion weighting gradients. (b-factors indicated expressed in s:mm2) ( Frahm et al. 2003). Conclusion: Magnetic resonance imaging functions through a special radiology technology which allows further diagnosis when the other modalities remain unable to do it. The principles behind the MRI process rely on the magnetic spin properties of the hydrogen nuclei “H” in the body. The MRI has 5 major parts which work together. The magnet realigns the proton nuclei of the body tissues. The transmitter sends radiofrequency transmissions to the antenna which in turn sends frequencies into the patient. The antenna also returns the transmissions. The coil helps to gather the information needed, and the receiver amplifies the signals gathered. To finish, the computer processes the information gathered from the 4 previous elements. This complex process afterwards allows for the transmission and protuberance of accurate information about the body part. Although there have been numerous advances in MRI modality, the basic principles still concern. The series used are adapted to yield the maximum amount of information in the shortest time. Also the close MRI machine makes loud, banging noises. The new generation of MRI solves some of the late disadvantages. The future of the Medical imaging department will rely heavily on the MRI especially for women and young patients. References: Faerber, E. N. 1995. CNS Magnetic Resonance Imaging in Infants and Children. London: Mac Keith Press. Frahm, J., P. Dechent, J. Baukewig, and K. Merboldt. 2003. Advances in functional MRI of the human brain. Progress in Nuclear Magnetic Resonance Spectroscopy 44(1-2): 1–32. Geuns, R. J. M., P. A. Wielopolski, H. G. de Bruin, B. J. Rensing, P. M. A. Ooijen, M. Hulsoff, M. Oudkerk, and P. J. Feyter. 1999. Basic Principles of Magnetic Resonance Imaging. Progress in Cardiovascular Diseases 42(2): 149-156. Jacobson, J. 2008. Magnetic Resonance Imaging. http://www.merck.com/mmpe/sec22/ch329/ch329d.html (accessed September 3, 2010). Marzola, P., O. Francesco, and A. Sbarbati. 2003. High field MRI in preclinical research. European Journal of Radiology 48(2): 165-170. McKie, S. and J. Brittenden. 2005. Basic science: magnetic resonance imaging. Current Orthopaedics 19(1): 13-19. Richerche C. N. 2006. MRI and Patient Safety. Canadian Journal of Medical Radiation Technology 37(2): 5-9. Short, N. M. n.d. Technical and Historical Perspectives of Remote Sensing. http://www.fas.org/irp/imint/docs/rst/Intro/Part2_26c.html (accessed September 18, 2010). Sutton P. B., C. Ouyang, D. C. Karampinos, and G. Miller. 2009. Current trends and challenges in MRI acquisitions to investigate brain function. International Journal of Psychophysiology 73(1): 33-42. Todd, C. 2003. MR physics of body MR imaging. Radiologic Clinics of North America. 41(1): 1-15. Read More