Introduction to Magnetic Resonance Imaging (MRI) Physics
Magnetic resonance imaging was discovered simultaneously by two physicists in 1947 named Felix Bloch and Edward Mills Purcell. The first clinical images were obtained in 1977 by Paul Lauterbur, Peter Mansfield and Raymond Damadian.
MRI uses magnetic fields and radiofrequencies rather than ionizing radiation used in XRay and CT. The magnetic field strength of an MRI machine is measured in Tesla (T). The majority of MRI systems in clinical practice are 1.5-3T. These produce an extremely strong magnetic field, up to 50,000 times that of the Earth's magnetic field (0.00003T). An electromagnet of similar strength would be able to pick up a car.
The body is made up of 70% water, which is composed of hydrogen and oxygen atoms. MRI relies on the magnetic properties of hydrogen atoms to produce images. The hydrogen nucleus is composed of a single proton with no neutrons. As a spinning charged particle, this produces a magnetic field called a magnetic moment. Normally the protons are orientated randomly, so there is no overall magnetic field.
The components of the MRI system include the primary magnet, gradient magnets, radiofrequency (RF) coils and the computer system. MRI differs from imaging such as radiography and CT scanning as it uses magnetic fields and radiofrequencies rather than ionizing radiation.
The primary magnetic field refers to the strength of the static permanent field at 1.5-3T. Hydrogen atoms align parallel or antiparallel to the primary field (Bo). This is called longitudinal magnetization in the long axis of the magnetic field. A greater proportion of the hydrogen protons align in the direction parallel to the primary magnetic field (or low energy state)...
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...r is the sum of longitudinal and transverse magnetization. It spirals around the z axis with net precession. The changing magnetic moment of the net magnetic vector results in free induction decay (FID). This induces an electrical signal which is received by the RF coil in the transverse plane and reduces as the net magnetic vector moves to the long or z axis.
The computer system receives the RF signal and performs an analog to digital conversion. The digital signal representing the imaged body part is stored in the temporary image space, or K-Space. The K-Space stores digitized MR signals during data acquisition. The digital signal is then sent to an image processor where a mathematical formula called furiay transformation is applied. And the image of the MRI scan is displayed on a monitor. Then these images can be used by physicians to diagnose their patients.
My interest in MRI started when I first read the book “MRI, The Basics” written by the author Ray Hashemi. By the time I successfully finished my MRI clinical placement in Tehran University of Medical Sciences, I knew for sure that MRI would be the field I would be choosing to take on. What attracts me most about MRI is how beautifully scientist could create a technology that can take advantage of the magnetic moments of human body for imaging it without any harms of ionizing radiation. Although there are drawbacks to MRI, combining it with other modalities would be a more effective approach to an accurate diagnosis.
All MRI examinations were performed with a 1.5-T MRI system (Achieva; Philips Medical Systems, Best, the Netherlands) using body coil (a phased-array coil).T1, T2 and fat suppressed T2-weighted imaging as well as diffusion weighted imaging were performed during the same MRI examination in all patients. Images will acquired with the patient is in supine position with head pointing to the magnet (head first supine; HFS). The body coil was securely tightened using straps to prevent respiratory artifacts. The center the laser beam localizer will placed over symphysis pubis. Chloral hydrate syrup at a dose of 1 ml/kg body weight was needed for children less than 5 years for sedation during MRI examination.
Other testing procedures that are commonly employed, in order to gain a better visual image of the excitatory activity in the brain are the PET scan and the MRI. According to Kalat (2004), these methods are non-invasive, meaning that they don’t require the insertion of objects into the brain, yet they yield results that allow researchers to record brain activity. The PET scan (positron emission tomography) involves the researcher injecting a radioactive chemical into the patient’s body, which is then absorbed mainly by the brain’s most active cells. With the use of radioactive detectors, placed around the patient’s head, a map is produced that shows which areas of the brain are most active.
by the internal computers of the instrument, to create an image of internal body tissues. These images were then displayed on the screen for the user,
One of the most recently new advances in radiology is the use of magnetic resonance imaging (MRI). MRI has been around for the past century. It was at first called Nuclear Magnetic Resonance (NMR) and then it changed to MRI once there was an available image. Walter Gerlach and Otto Stern were the first scientists to start experimenting with the magnetic imaging. Their very first experiment was looking at the magnetic moments of silver by using some type of x-ray beam. The scientists then discovered this was by realizing that the magnetic force in the equipment and in the object itself. In 1975, the first image was finally created using and MRI machine. The scientists used a Fourier Transformation machine to reconstruct images into 2D. The first images ever use diagnostically was in 1980. This is when hospitals began to use them. At first the images took hours to develop and were only used on the patients that needed it most. Even though MRI has been around for a long time, it has advanced and has been one of the best imaging modalities recently (Geva, 2006).
As a starting point in CT diagnostic imaging the form of radiation used to provide an image are x-rays photons , this can also be called an external radiation dose which detect a pathological condition of an organ or tissue and therefore it is more organ specific. However the physics process can be described as the radiation passes through the body it is received by a detector and then integrated by a computer to obtain a cross-sectional image (axial). In this case the ability of a CT scanner is to create only axial two dimensional images using a mathematical algorithm for image reconstruction. In contrast in RNI the main property for producing a diagnostic image involves the administration of small amounts of radiotracers or usually called radiopharmaceutical drugs to the patient by injection or oral. Radio meaning the emitted of gamma rays and pharmaceutical represents the compound to which a nuclide is bounded or attached. Unlike CT has the ability to give information about the physiological function of a body system. The radiopharmaceutical often referred to as a nuclide has the ability to emit ga...
Garcia, Kimberly. Wilhelm Roentgen and the Discovery of X Rays. Bear: Mitchell Lane, 2003. Print.
Magnetic resonance angiography (MRA), similar to CT, uses a magnetic field and pulses of radio wave energy to provide pictures of blood vessels inside the body. A dye is often used during the procedure to make blood vessels appear clearer. Lastly, a cerebral angiogram may be done. This is an x-ray test, where a catheter is inserted into a blood vessel, usually in the groin or arm, and moved from the vessel into the brain. A dye is also injected.
In 1971, scientists were motivated to use magnetic resonance for detection of diseases after discovering the difference in nuclear magnetic relaxation times of tissues and tumors (Dr. J. Hornak, n.d). In 1973, Lauterbur and Mansfield defined the use of magnetic field gradients for spatial localization of NMR signals, laying the foundation for MRI in the future. Two years later, Richard Ernst used Fourier Transform (an algorithm for the analysis of heat transfer between solid bodies) to encode and reconstruct 2D images, which underpins MRI today (Geva, 2006).
MRI is a procedure, in wide use since the 80s, to see the anatomy of the internal organs of the body. It is based on the phenomenon of nuclear magnetic resonance (NMR), first described in landmark papers over fifty years ago (Rabi et al. 1938; Rabi, Millman, and Kusch 1939; Purcell et al. 1945; Bloch, Hansen, and Packard 1946) (4 ). . The MRI is a valuable diagnostic and research tool with also practical applications for surgical planning and conquering diseases. This imaging procedure is painless and non-invasive although sometimes discomforting as the patient lies down in a body tube that surrounds them. For many years, closed MRI units have been the standard in helping physicians make a diagnosis. These closed MRI units featured a long tube that the patient would be placed inside during their procedure. This was often uncomfortable for many patients due to the "closed in" feeling and was especially stressful for patients who suffer from claustrophobia. The newest generation of MRI units is now open on all four sides which completely alleviates the "closed in" feeling, while still providing the physician with the most accurate information possible to aid in diagnosis (2).. A patient does not see or feel anything. A faint knocking sound may be heard as the machine processes information. Patients may choose to listen to music -- even having the option of bringing their own CDs to listen to. Most MRI procedures take less than an hour. MRI technology is based on three things: magnetism, radiofrequency and computers. The magnetic resonance machine, is a big and strong magnet. When the body is inside, every proton of the body is oriented in the same way (for instance, with the positive pole up). Water ...
During the late 1970's, the world of diagnostic imaging changed drastically due to the introduction of Magnetic Resonance Imaging, also known as MRI. For over 30 years, they have grown to become one of the most significant imaging modalities found in the hospitals and clinics ("EDUCATIONAL OBJECTIVES AND FACULTY INFORMATION"). During its ancient days, these machines were referred to as NMRI machines or, “Nuclear Magnetic Resonance Imaging.” The term “nuclear” comes from the fact that the machine has the capability of imaging an atom's nucleus. Eventually, the term was dropped and replaced with just MRI, because “nuclear” did not sit well with the public view ("EDUCATIONAL OBJECTIVES AND FACULTY INFORMATION"). Many people interpreted the machine to produce an excess amount of radiation in comparison to the traditional X-ray machine. What many of them were unaware of, MRI does not disperse a single ounce of ionizing radiation making it one of the safest diagnostic imaging machine available to this date. MRI machines actually use strong magnetic fields and radio waves to produce high quality images consisting of precise details that cannot be seen on CT (Computed Tomography) or X-ray. The MRI magnet is capable of fabricating large and stable magnetic fields making it the most important and biggest component of MRI. The magnet in an MRI machine is measured on a unit called Tesla. While regular magnets commonly use a unit called gauss (1 Tesla = 10,000 gauss). Compared to Earth's magnetic field (0.5 gauss), the magnet in MRI is about 0.5 to 3.0 tesla range meaning it is immensely strong. The powerful magnetic fields of the machine has the ability to pull on any iron-containing objects and may cause them to abruptly move with great for...
Magnetoencephalography (MEG) is a non invasive technique for studing neuronal activity in the brain. Unlike electrophysiological methods that depend on volume currents, MEG depends on the primary current (2, 8).
CTscans stands for “Computed Tomography”. It is a way of looking inside your body using a special camera. It is an advanced scanning x-ray and computer system that makes detailed pictures of horizontal cross-sections of the body, or the part of the body that is x-rayed. A CT scan is a diagnostic test that combines the use of x-ray with computer technology. A series of x-beams from many different angles are used to get these cross-sectional images of the patient’s body. In a computer, these pictures are assembled into a 3-dimentianal picture that can display organs, tissues, bones, and any such thing. It can even show ducts, blood vessels and tumors. One of the advantages of CT is that it clearly shows soft tissue structures (such as brain), as well as dense tissue structure (such as bone). The pictures of a Ctscanner are a lot more detailed than the pictures of a regular X-ray machine. It can make pictures of areas protected or surrounded by bones, which a regular X-ray machine can not. Because of this, a CT scanner is said to be 100 times as affective and clever as an ordinary X-ray, and can therefore diagnose some diseases a lot earlier and quicker. It is recent technology that has made it possible to accurately scan objects into a computer in three dimensions, even though the machines and ideas were developed in the 1970s. In the 70s doctors started to use this new type of machine that could give detailed pictures of organs that the older type of x-ray, machine could not give.
Images of human anatomy have been around for more than 500 years now. From the sketches created by Leonardo da Vinci, to the modern day Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) scan, images have played a great role in medicine. Evolution in medical imaging brought together people from various disciplines such as Biology, Physics, Chemistry and Mathematics, a collaboration which has further contributed to healthcare as a whole. Modern day imaging improves medical workflows by facilitating a non-invasive insight into human body, accurate and timely diagnostics, and persistence of an analysis.
A magnet is a solid object, usually made of metal iron, which has the ability to attract other materials (e.g., iron, steel, cobalt and nickel) within a magnetic field.