Magnetic Resonance Imaging Mri Principles And Applications
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MRI is known as Magnetic resonance imaging, or nuclear magnetic resonance imaging. It is a commonly used medical imaging technology in recent years. In this article, the history of this technology, the physical principles behind this technology, its applications, developments, and its advantages and disadvantages, will be introduced.
With the rapid development of global technology, many imaging technologies have been invented, such as computed tomography (CT) , radiography, MRI, fMRI, and recently developed photoacoustic imaging. Medical imaging is a technology that being used to acquire the images of human's body for medical purpose, such as diagnosing or examining diseases. It is also being used for medical scientific researches, for example, by using MRI technology, scientists had successfully obtained the 3D image of human's brain to help them study the functions and structure of this most important organ of human's body.
MRI is one of the most commonly used imaging technologies although it is a relatively new technology. The first article about this technology is published in 1973. One year after that, the first cross-sectional image of a living mouse was obtained by P. C. Lauterbur . The technology was first used on human body in 1977, and on the other hand, the first human X-ray image was obtained in 1895. One aspect that can reflect the importance of MRI is that, Paul Lauterbur and Sir Peter Mansfield were awarded 2003 Nobel Prize in Physiology or Medicine for their outstanding contribution to magnetic resonance imaging.
The emerging of MRI technology is due to the great development of nuclear magnetic resonance. That is why in the early years, this technology was called nuclear magnetic resonance imaging (NMRI). However, one important thing about MRI that should be mentioned is that, this technology is not as dangerous as its sound. In the following chapter, the operating principles of MRI device will be explained.
Physical principles behind MRI
It is known that over 70% of human's body was made up by water molecules of which each contains two hydrogen nuclei or protons. That means almost every human's organs and tissues contain a large number of water molecules. Meanwhile, the scientists have found out that, the magnetic moments of some of the protons in water molecules align with the direction of the field when it was been placed inside a strong magnetic field. This raised possibility that this phenomenon can be used to develop a new advanced medical imaging technology, that is why MRI was invented. Of course, to get the imaging of human's body, other devices and technologies should also be used.
First of all, a very powerful magnetic field needs to be generated. To generate this strong magnetic field, we need a radio frequency transmitter. The function of this device is to produce an electromagnetic field. When large number of electrons flowing the metal ring(which is called "solenoids", also known as "gradient coils") around the MRI device, a strong magnetic field will be generated, as shown in Figure 1. In simple terms, the photons of this field have just the right energy, known as the resonance frequency, to flip the spin of the aligned protons. The more powerful and longer duration the field has, the more aligned spins will be affected. The protons will begin to decay to original spin-down state, and during this procedure, photons will be released. It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. For different parts of human's body, additional magnetic field may be applied providing a straightforward method to control where the protons are activated by radio photons. One thing should be mentioned is that, when the gradient coils were producing the powerful magnetic field, there will be large noise during the operation. Therefore, some efforts should be made to reduce this noise, otherwise it can reach approximate 130 decibels (the human pain threshold) , that will be very harmful to human's body.
Fig 1 Flow of electrons along a wire and the induced magnetic field
The principle why images can be constructed is because that, different organs or tissues inside human's body have different amount of water molecules, thus different positions of human's body will return to their equilibrium state at different rates. By using computer to do the calculation, the images of organs and tissues can be obtained. Sometimes, a method of injection which is called contrast agents, could be used for MRI imaging. Contrast agents could be injected intravenously or directly into a joint. The former method could help to enhance the appearance of blood vessels, tumors or inflammation. The later one is used in terms of arthrograms. MRI is widely used to get the images of most parts of human's body, such as brain, muscle tissue and tumors, it is particularly useful for tissues with many hydrogen nuclei and little density contrast.
Applications of MRI
In medical field, MRI technology is used to detect tissues which have pathological trends, for example, tumors. By using this technology, normal tissues and pathologic tissues can be distinguished easily because MRI has better contrast resolution(the ability to distinguish the differences between two arbitrarily similar but not identical tissues) than CT. One other important reason why MRI is used is that, not like CT scans and traditional X-ray, MRI uses strong magnetic fields and non-ionizing radiation, and there are no solid evidence that can prove this technology can bring any damage to human's health.
There are two different kinds of MRI scan: Basic MRI scans and Specialized MRI scans. The basic MRI has four kinds of different weights:
T1 weighted MRI
T1 weighted scans is one basic type of MR contrast, which uses gradient echo sequence with short TE and short TR. A gradient echo, GRE,(also known as field-echo or gradient-recalled echo) is produced when ever the amplitude of a gradient is reversed. T1 weighted scan is widely used for clinical scan. By using an inversion pulse, the T1 weighted MRI can improve the contrast. Moreover, this scan can be very fast because the repetition time (TR) is short. Due to this, 3D images could be captured. The advantage of T1 weighted MRI is that, it can create good gray matter and white matter contrast. One example of T1 weighted MRI image is shown in Figure 2.
Fig 2 Sagittal T1 weighted image of spine 
T2 weighted MRI
Being different form T1 weighted scan, T2 weighted MRI used a spin echo (SE) sequence with long TE and long TR. Although TE and TR are longer than T1 weighted MRI, T2 weighted MRI is less
susceptible to inhomogeneities in the magnetic field, that's why it has long been used for clinical purpose. As T2 weighted scan is sensitive to water content, it is very useful for detecting edema. An T2 weighted image is shown in Figure 3.
Fig 3 Sagittal T2 weighted image through the spine in the same patient seen in Fig 1 
T^2 weighted MRI
T^2 weighted MRI is much more like the combination of T1 weighted MRI and T2 weighted MRI, because it uses a spin echo, but with long TE and long TR.
Spin density weighted MRI
Spin density weight scan have no contrast from T2 or T1 decay, instead, the contrast comes from the different amount of available spins. It uses a spin echo or gradient echo sequence with short TE and long TR.
The specialized MRI scans includes Diffusion MRI, Magnetization Transfer MRI, Fluid attenuated inversion recovery (FLAIR), magnetic resonance angiography, magnetic resonance gated intracranial CSF dynamics (MR-GILD), magnetic resonance spectroscopy and Functional MRI. By using different specialized MRI scans, the doctors and scientists can analyze and study one particularly part of human's body and acquire the data and information they need. For example, Functional MRI's are used to determine the different function of different parts of human brain. This specialized MRI technology are widely used for determining motor imagery, speech portions of the brain, and diagnosing which parts of the brain may be affected by a tumor.
MRIs have the special advantage that it can take internal images of human's body without making any incisions. Although MRI technology is somewhat expensive for ordinary clinic to use for their daily operation, but the non-intrusive procedures of MRI is very effective ,which makes it attractive. MRI technology is particularly useful for the following diseases:
"â-†Inflammation or infection in an organ;
â-†other irregularities that exist in tissue or organs in their body."
MRI versus CT
Questions frequently arise regarding whether computer tomography (CT) or MRI is the best way to obtain diagnostic outcome for the patient. Before MRI's emerging, CT scanner is widely used for clinical purpose. CT uses ionizing radiation (X-rays in this case) to obtain images of human's body. It is effective for capturing the images of tissues with high density, such as bones and calcifications (calcium based). However, using X-rays for human body examining is harmful to human's health, so it is not a very ideal clinical technology. But the situation has been changed after MRI came out. According the illustration of Thumb MRI Center, the applications of MRI and its advantages are:
"Brain for tumor, mass, stroke, multiple sclerosis, etc;
All spinal cord, spine, and vertebral applications;
For older patients CT may be substitute, as degenerative change is more of a factor than disc protrusion;
All orbit, sensoryneural hearing loss, pituitary, and cranial nerve applications;
All joints: from fingers to toes, include soft tissue tumors and injury."
Since MRI uses radio frequency (RF) light, MRI does not have any known health risks to patients. However, MRI also has its inherent flaws.
As mentioned before, MRI requires a large mechanical equipment to generate powerful magnetic field. Not only delicate components are needed, but also huge amount of power will be consumed, not mention complicated installation procedure of MRI units. A picture of MRI instrument is shown in Figure 3. The price of the MRI machine is the biggest part of the cost. The average MRI machine cost is over $1 million. According to the investigation of the internet website "comparemricost.com", MRI cost can range between $400 to $3,500 depending upon which MRI procedure is performed(example: brain MRI vs. shoulder MRI) and where the MRI test performed is performed.
Fig 3 A conventional fMRI scanner
Demanding Strict Operating Circumstance
Since MRI equipment will generate strong magnetic field(normally hundreds times of global magnetic), the operating circumstance require no metal device in the examining room, and the patients should not have any kind of metal implants within their body, such as pacemaker. As strong magnetic field will be generated during the operation of the equipment and significant electricity power will be consumed, the institution who own the MRI machine should draft a detailed evacuate plan in case accident occurs. Warning labels should also be placed on a noticeable place in the MRI scanning room, as shown in Figure 4. MRI also requires a specialist team to operate and maintain the equipment.
Fig 4 Example safety labels
Although MRI's benefits are numerous, there are still inherent potential hazards existing in MRI technology. For example, patients with metallic implants or ferromagnetic foreign bodies are strictly forbidden to approach the MRI instrument. Because the powerful magnetic field could enormously affect metal objects. Several cases of arrhythmia or death have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions . Although some highly specialized protocols have been developed to permit scanning of select pacing devices, pacemaker is still one of the absolute forbidden objects regarding of the MRI scanning . There are other medical implants which are contraindicated for MRI scans, such as vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, etc. Nowadays, most of the MRI centers require their patients to take a X-ray scanning to detect the metal objects on or in their body, in case of potential hazard.
According to the "American College of Radiology's White Paper on MR safety", there are some other safety issues, such as, projectile or missile effect, radio frequency energy, peripheral nerve stimulation(PNS), acoustic noise, cryogens, contrast agents, claustrophobia and discomfort, etc.
Every technology has its own advantages and flaws, after all, MRI is a very mature and safe technology comparing with other existing medical imaging techniques , and it has also been proved that MRI is an advanced medical method for detecting patients diseases. In the following chapter, the new improvements and additional technology of MRI will be introduced.
New development of MRI
Since pulse sequences have evolved rapidly in recent years, it becomes more convenient to obtain high quality human body images with MRI, particularly structure undergoing small physiological motion. However, this always take a long time for scanning and examinations, it is time consuming. Therefore, rapid imaging sequences have emerged to conquer this problem. Fast spin echo and gradient echo are two examples. They provide shorter scan times and images with even better images. Moreover, the recent technical developments in system hardware and software have allowed for ultra-fast imaging sequences in the order of milliseconds . Ultra-fast imaging sequence is a new and advanced pulse sequence for MRI technology, it provides a wide range of applications that conventional MR imaging sequences cannot imaging. Such applications include:
Functional brain imaging,
Perfusion and diffusion imaging,
Real-time imaging of cardiac motion and perfusion,
Dast abdominal imaging,
Improved MR angiography,
Real-time monitoring of interventional procedures."
The following content of this article will describe and discuss several recent developments in MRI technology and their applications.
High speed gradient systems
Gradient switching is one of the biggest factors that affect the timing of pulse sequence. The three gradients (X,Y and Z) will switch on and off many time during the sequence, that is for spatial encoding and signal refocusing, shown in Fig 1. Every time when a gradient is switched on, gradient will absorb power until it reaches its peak. Then the gradient will stay on for some time before it reverse for the same period of time. For each time of the gradient switching, millisecond of wasted time will be accumulated. As capturing a pulse sequence will accompany with many times of gradient switching, the wasted time will be multiplied. High speed gradients system is a modified system which can save significant time for MRI scanning. The engineering details behind high speed gradients system will not be discussed in this article.
The inherent flaws of high speed gradients system is the safety and power considerations. Firstly, fast gradient switching can bring peripheral nerve stimulation for patients. For this reason, ultra-fast gradient systems should operate under the stimulation threshold, which is 6 T/s for all gradients, and 20T/s for axial gradients . Secondly, high speed gradients switching requires about 1000kW electrical power. Although it has higher power consumption, the benefits it brings to imaging technique are greater.
This development of MRI technology brought many benefits, such as shorter imaging times, more slices and higher resolution matrices than in conventional imaging .
Development in fast spin echo
Fast spin echo pulse sequence captures data for several lines of K space per TR. Since this technique can provide high quality images in shorter time comparing with traditional pulse sequences, it has been used for an alternative for T2 weighted spin echo acquisitions . However, fast spin echo also has its own flaws, such as high costs, image blurring and high signal from fat. Therefore, some of the institution still use conventional spin echo for long TR/short TE proton density weight . Studies have shown that spin echo with fat saturation techniques yield much higher contrast to noise than either gradient echo echo planar imaging(GE EPI) or spin echo echo planar imaging(SE EPI).
Developments in inversion recovery
The modification of TI changes image contrast in conventional spin echo inversion recovery sequences . Normally, inversion recovery need a long TR and short TE, thus result in long imaging times and few slice locations. However, fast inversion recovery can save the time wasted by tradition inversion recovery for obtaining more slices.
Developments in gradient echo
Other than using RF pulses, the gradient echo pulse sequences use gradient pulses to refocus echoes, this brings shorter TRs and shorter imaging times. By modifying the flip angle, contrast can be maintained in short TR gradient echo sequences. By shortening the flip angle, less saturation occurs and less T1 contrast is present.
There are still several trade-offs of gradient echo sequences. Because using gradient echo pulse but not RF pulse, susceptibility and chemical shift usually occur. But these affects can be minimized.
Applications of echo planar imaging(EPI)
EPI acquiring several lines of K space in one TR, that makes it capturing images than most of the existing imaging technology. EPI sequences can be modified in several different ways: A train of gradient echoes(GE EPI), spin echoes(SE EPI), single shot EPI, and multiple shot EPI.
GE EPI is acquired with a RF pulse followed by a number of gradient blips creating a train of gradient echoes, as shown in Figure 5 . As gradient echoes are not so intensive as spin echoes, GE EPI is faster than SE EPI. However, GE EPI has the same flaws as conventional gradient echo, which are detrimental artefacts.
Fig 5 Gradient echo EPI
To avoid the problem GE EPI has, SE EPI uses a different application of refocusing pulse. That is applying a RF refocusing pulse after the initial excitation pulse. By using SE EPI, some of the artefacts caused by magnetic field inhomogeneities and chemical shift can be cleaned up .
Diffusion is a word to describe the movement of molecules caused by thermal motion. Many diseases have something to do with the diffusion of molecules inside cells, such as stroke. The imaging sequence of diffusion imaging is sensitized to motion on a molecular level by using a bipolar gradient scheme with very high amplitudes . To some extent, spin echo imaging is structured to observe and reflect diffusion in tissues. Spins with different frequencies are refocused during spin echo acquisitions. Diffusion weighted images can be more effectively acquired by combining EPI with two large gradient pulses applied after excitation . The applications of diffusion are mainly focused on stroke diagnosing.
Perfusion is the regional blood flow in tissue, it is also a measure of the quality of vascular supply to a tissue and, since vascular supply and metabolism are usually related, perfusion can also be used to measure tissue activity . By tagging the water in arterial blood, perfusion imaging acquires images of tissues. The water can be tagged by using a bolus injection of exogenous contrast agent, or by saturating the protons in arterial blood with RF inversion or saturation pulses . This technology is widely used for evaluating ischaemic disease or metabolism at rest or during exercise.
Functional imaging (fMRI)
Functional MR imaging (fMRI) is a type of specialized MRI scan. It mainly focus on capturing the images of human brain, measures the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. Since its invention in the early 1990s, functional magnetic resonance imaging has rapidly assumed a leading role among the techniques used to localize brain activity.In the early days, contrast agents were used to observe the blood flow, nowadays, the blood is used as an internal contrast without injecting contrast agents. fMRI is one of the most recently developed and advanced neuroimaging technology. There is no doubt that this technology will help the scientists and doctors to explore more information of the function of brain.
Magnetic Resonance Imaging(MRI) uses RF waves and magnetic induced by electrical power, to capture clear and detailed images of human's and animal's organs and tissues. It is a relatively advanced technology. Many clinical applications have been applied based on MRI technology, furthermore, with the rapid development of imaging and signal processing technology, MRI is becoming more superior and safer technology.
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