The Basic Principles Of MR Image Production - Essay Example

THE BASIC PRINCIPLES OF MRI INTRODUCTION Magnetic resonance imaging (MRI) has been an incredibly successful imaging modality due to its ability to noninvasively acquire high resolution images of the internal structure of subjects without the use of ionizing radiation (Luypaert et al, 2001; Rodr´ıguez, 2003). Unlike other imaging methods, MRI generates its signal source from within the image subject itself and the creation of the signal is based on the phenomenon of nuclear magnetic resonance (NMR). The NMR phenomenon occurs for certain types of nuclei when they are inserted into a magnetic field and exposed to electromagnetic radiation of the proper frequency. MRI is the safest imaging clinical imaging technique that is used for a variety of medical purposes like differentiating between normal and pathological tissues for diagnosis and tracking dynamic changes in tissue properties over time. 2. BASICS OF MRI A. Magnetism Magnetism is physical phenomenon in which materials and moving charged particles can attract or repel other materials or moving charged particles (Ballinger, Intro to MRI, 1998). Magnetism results from moving electric charges or intrinsic spin moments of electrons or nucleis. Spin is a quantum mechanical property. This means that the available spin energy levels are constrained to specific, discrete values. A spin 1/2 particle has only two possible spin states: spin up (+ 1/2) or spin down (- 1/2). The magnetic moment is aligned with the spin. The spin-up and spin-down states are described as being equal in energy, or degenerate. However, if another magnetic field is introduced, the spin-up and spin-down states will be no longer equal in energy. The energy difference introduced by applying the external magnetic field is known as the Zeeman splitting (van Geuns, 1999). This effect is very important in such applications as magnetic resonance imaging. Magnetism can be classified as paramagnetism, diamagnetism, ferromagnetism, and antiferromagnetism (Ballinger, Intro to MRI, 1998). B. Resonance and RF Having microscopic magnetization, protons within a magnetic field produce wobbling as they spin. The rate of this wobbling or precession constitutes resonance or Larmor frequency (Intro to MRI). The application of a radio frequency pulse at the Larmor frequency causes a change in the distribution of spins with respect to their energy state and precessional phase coherence (Rodr´ıguez, 2003). Practically, it means that If individual nuclei is exposed to RF radiation at the Larmor frequency, nuclei in the lower energy state jumps to the higher energy state (Intro to MRI). Upon cessation of the RF pulse, a coil can be used to measure an induced signal by the precessing net magnetic moment. The measured signal contains information about the relaxation rates and precessional frequencies of the sample. C. Relaxation Upon removal of the RF pulse, the excited spins relax back into the lower energy state. The rate of spin relaxation is exponential with time constant, T1. When an excited spin population relaxes back to thermal equilibrium, the absorbed energy must also be released (Ballinger, Intro to MRI, 1998). The protons found in different biological tissues will often have very different values of T1 (typical values are in the range of .1s to 2s). This fact can be used to obtain contrast between tissues through appropriate timing of an MRI acquisition. T2 relaxation also known as traverse or spin-spin relaxation occurs when spins in the high and low energy state transmit energy to each other but do not abandon energy to the surrounding lattice (Ballinger, Intro to MRI, 1998). The time that elapses between excitation and the point of maximum spin phase coherence in the data acquisition is known as the echo-time (TE). For a given TE, different tissues will accrue different amounts of phase incoherence, and thus signal loss, dependent upon their T2 values, which are shorter than T1 values. The large variations in the magnetic field result in large differences in the phase of the spins within the boundary regions. The signal in these regions decays with a time constant known as T2*. The T2* rate is much higher than T2 and produces large signal loss during data acquisition (Ballinger, 1998). D. Proton Density. The image is divided in small homogenous volumes, called voxels that are comprised of spins containing one signal value (Secca, Basic Principles of MRI and F-MRI in Neurosciences). The signal assigned to any voxel constitutes an average of the signals of all spins within the voxel. Due to the fact that voxels cover different tissues, quantity of spins in a single voxel varies (Secca, Basic Principles of MRI and F-MRI in Neurosciences). Thus, proton density, a term referring to the number of protons contributing to signal occurring within a voxel, determines the difference of voxels. E. Repetition Time The term repetition time (TR) refers to the time adjusted between repetitions of the basic sequence of the imaging sequence (Secca, Basic Principles of MRI and F-MRI in Neurosciences). For instance, if the imaging should be improved due to noise, the sequence is repeated. TR comprises one of the image parameters, because along with total number of phase encoding steps it determines the time required to obtain an image. 3. INSTRUMENTATION A. Magnet Magnet in MRI, usually referred as the primary magnet, constitutes the main part of the machine and its main function is to produce a magnetic field to create MRI images. Practically, primary magnet is formed by directing a current through electrical wire coil, which forms a magnetic field of 1.5 to 3 Tesla within the coil center (Rodr´ıguez, 2003). Primary magnet is complimented with gradient magnets, or supplementary magnets, which are responsible for targeting specific parts of the body. B. RF and Gradient Coils The primary purpose of RF coils is to transmit the RF signal to the body and receive the returning signal (Ballinger, Intro to MRI, 1998; Rodr´ıguez, 2003). There are various architectures of RF coils, depending on what part of the body is targeted: pair saddle coils for knees MRI, Helmholtz pair coil for pelvis MRI, bird cage coil for head, and surface coils for spine and other parts. Gradient coils aim to provide localization of MR images varying magnetic field in different axes, namely z (Helmholtz coil), y and x (pair saddle coil). C. Electronics and Data Procession Figure 1 illustrates a digital information flow in a typical MRI machine. The computer starts the process of MRI sending the gradient amplifier and RF transmitter requests for particular pulse sequences. Consequently, RF receiver transmits the signal obtained by the antenna (RF coils) from the body to the A-D converter that relays digital signal into computer, which eventually form the final image (Ballinger, Intro to MRI, 1998). Figure 1. Electronics and Data Procession 4. FUNCTIONAL MRI Functional MRI is one of the most popular techniques used in neuroimaging research. By acquiring series of brain MR images after presenting subjects with stimuli, neuropsychological and cognitive principles underlying brain function can be explored. Functional MRI using different contrasts have been developed over the years and modern functional MRI techniques exploit the use of susceptibility based contrast (e.g., Blood Oxygen Level Dependent, BOLD), perfusion based contrast (e.g., Arterial Spin Labeling, ASL) and others for study of neural function (Bandettini et al. 1992; Secca, Basic Principles of MRI and F-MRI in Neurosciences; Mayfield Clinic and Spine Institute, 2002). Today BOLD functional MRI is the most widely used technique due to higher signal-to-noise ratio (SNR), better sensitivity and higher temporal resolution that allows detection of activation from single stimuli. During neural activation, there is increase in metabolic demand in brain regions where neurons that are active. The cerebral blood flow (CBF), cerebral blood volume (CBV) increased in the affected region lead to increased blood oxygenation level (Bandettini et al. 1992). It is known that there is difference in magnetic susceptibility between oxyhemoglobin and deoxyhemoglobin. Thus magnetic field around firing neurons has better field uniformity than inactive brain regions. 5. THE BASIC PHYSICS OF MRI MRI is based on the principles of Nuclear Magnetic Resonance. Nuclear Magnetic Resonance occurs when magnetic field impacts atomic nuclei, forcing them to emit or absorb energy (McKie and Brittenden, 2005). In order to produce images, MRI utilizes properties of hydrogen proton nuclei due to its abundance in human body. During MRI procedure, a data measurement occurs through series of radiofrequency pulses, produced to match the resonant frequency of hydrogen atoms (McKie and Brittenden, 2005). Hydrogen atoms emit energy back as magnetic resonance signal, which is transmitted through receiver coil located near patient. Between each pulse, hydrogen population is allowed to relax back to equilibrium through T1 and T2 relaxation mechanisms. Practically, human tissues such as muscles, bones, ligaments, and fluids have different T1 and T2 relaxation times. Therefore, for particular TE and TR, different tissues will recover different magnetization, allowing for contrast formation. For instance, elevating time to echo and pulse repetition time produces T2 weighting that in turn reveals abnormal fluids such as tumors or oedema (McKie and Brittenden, 2005). 6. TYPES OF SCANNERS From the practical perspective, the MRI scanners are differentiated in terms of strength and architecture. The strength is one of most important aspects in MRI functioning, because strength (1.5 Tesla for optimal images) can either compromise image or make it clear. Different MRI architectural designs exist to provide more comfort and usability for particular types of patients: MRI with open magnet, short bore magnets, standing and sitting MRIs, etc (McKie and Brittenden, 2005; Mayfield Clinic and Spine Institute, 2002). 7. MAJOR COMPONENTS OF MRI In order to produce MRI images, some technological procedures and processes must be conducted, particularly nuclear alignment, RF excitation, spatial encoding, and image formation. There are five major components in any MRI system: magnet, gradient systems, RF coil system, receiver, and computer system. A. Magnet. For medical purposes, not only a magnet should be able to generate a high magnetic field, but also generated field should be uniform. In the majority of clinical MRI systems magnetic field formed by a magnet varies from 0.05 to 3.0 Tesla (Rodr´ıguez, 2003). B. Gradient Coils. In order to provide localization of MR images, the magnetic field uniformity is changed in different axes (z, y and x) using gradient coils. The change is achieved by passing currents through specifically located coils of wire, surrounding the imaging target. C. RF Coil System. RF coil system transmits the signal to the imaging object, detects the returning signal coming from the spins and transmits it further on the MRI system (Ballinger, Intro to MRI, 1998; Rodr´ıguez, 2003). Because of needs for localization in MR imaging, RF coils can architecturally vary, depending on the target positioning. For instance, bird-cage coil (volume coil) has typical cylindrical-shaped structure, while surface coils can be single-loop and array coils. D. Receiver System. MRI system employs receiver system to convert RF signal coming from RF coil into identifiable data for computer processing system, which forms an image. E. Computer System. Practically, MRI computer system represents an interface enabling responsible personnel to manage MRI process, including system tests, scanning, and images retrieval. Computer system decodes the signal using Fourier transform algorithms, which are employed for production of two and three dimension images. Figure 3. MRI system and its major components (Rodr´ıguez, 2003). 8. MRI SEQUENCES T1 weighted scan utilizes a gradient echo sequence with short TE and short TR, and because of short repetition time the sequence allows collection of high resolution images at the very fast rate. T2 scan utilized a spin echo sequence with long TE and ling TR and is widely used to capture liquid on the images. T2* weighted scan utilizes a gradient echo sequence, with long TE and long TR, and is effective in contrast increase for particular tissues like venous blood. Proton density weighted scan utilizes either spin echo sequence or a gradient echo sequence with short TE and long TR (McKie and Brittenden, 1992). STIR/SPIR sequence allows to accurately detecting soft tissue and marrow pathology, because it reduces the signal intensity from fat, while dramatically increases the signal from fluid (McKie and Brittenden, 1992). Fast spin echo sequence represents an accelerated method of receiving T2 and proton density images for the purpose of reducing the acquisition time (McKie and Brittenden, 1992). 9. HOW DOES MRI WORK. As discussed, MRI is based on the principles of nuclear magnetic resonance. Hydrogen atoms in human body are susceptible to magnetism, therefore when body is located within magnetic field, atoms align with it. Further, emitted radio wave impacts atoms and disrupts their polarity (Mayfield Clinic and Spine Institute, 2002). When atoms return to their original position, computer system calculates the difference and produces a very detailed image, which enables medical specialists to see every abnormality occurred in targeted area. In some instances, where images should be enhanced, specific contrast agent, such as gadolinium, is injected into patient’s bloodstream and absorbed in tissues providing additional contrast and sharpness to the images (Mayfield Clinic and Spine Institute, 2002). 10. DIFFUSION AND PERFUSION MRI Both diffusion and perfusion MR imaging received wide application in neuro-vascular clinical applications (Luypaert et al, 2001). Diffusion is a transport process which results in mixing or mass transport without requiring bulk motion. The diffusive movement resulting from the collision between water molecules and the surrounding medium is a fraction of the structure and geometry of the tissue at a microscopic level (Luypaert et al, 2001). As a result, diffusion reflects microscopic characteristics of the medium. Diffusion weighted images are generated by applying a pair of large gradient pulses. The diffusion properties are relatively independent of orientation or isotropic in the gray-matter of the human brain. However, in brain white matter, which is fibrous tissue, the diffusivity will fluctuate with orientation. Perfusion weighted imaging utilizes the methods of contrast placement in cerebral blood. T2* perfusion imaging uses multi or single shot techniques to transmit signal across a brain when injected contrast passes through the capillary system (Luypaert et al, 2001). Computation of the perfusion of the targeted region occurs at signal decrease. 11. CONCLUSION Magnetic Resonance Imaging (MRI) is a state of the art imaging technology that has shown many advantages in medical imaging such as noninvasive measurement, absence of side effects, remarkable soft-tissue contrast, and an ability to acquire images in any plane. One of the major advantages of MRI is its rich numbers of image contrast and method to quantify various physiologic information. For instance, T1 and T2* MRI are routinely used. This particular paper examined the main principles of MRI, its major components, MRI techniques and sequences. REFERENCES Ballinger R. Introduction to MRI. 1998 Retrieved from Sept 30, 2010 Bandettini, P.A., et al. Time course EPI of human brain function during task activation. Magnetic Resonance Medicine, (1992), 25(2), 390-7. R. Luypaert, S. Boujraf, S. Sourbron, M. Osteaux. Diffusion and perfusion MRI: basic physics, European Journal of Radiology 38 (2001) 19–27 Magnetic Resonance Imaging and MR Antiography. Mayfield Clinic and Spine Institute, 2002 S. McKie, J. Brittenden. Basic science: magnetic resonance imaging, Current Orthopaedics (2005) 19, 13–19 A.O. Rodr´ıguez. Principles of magnetic resonance imaging. Revista Mexicana De Fisica (2003), 50 (3) 272–286 Secca M.F. Basic Principles of MRI and F-MRI in Neurosciences Robert-Jan M. van Geuns, Piotr A. Wielopolski, Hein G. de Bruin, Benno J. Rensing, Peter M.A. van Ooijen, Marc Hulshoff, Matthijs Oudkerk, and Pim J. de Feyter. Basic Principles of Magnetic Resonance Imaging. W.B. Saunders Company, 1999 Read More