Basic Principles. MRI scans work as an imaging method due to the unique make-up of the human body. We are comprised entirely of cells which all contain water – principally made of hydrogen ions (H 2 O). The magnet embedded within the MRI scanner can act on these positively charged hydrogen ions...
What makes MRI an useful technique?
The following are examples in which an MRI scanner would be used:
- anomalies of the brain and spinal cord
- tumors, cysts, and other anomalies in various parts of the body
- breast cancer screening for women who face a high risk of breast cancer
- injuries or abnormalities of the joints, such as the back and knee
- certain types of heart problems
- diseases of the liver and other abdominal organs
What does a MRI do exactly?
Typical responsibilities include:
- Operating, adjusting, and maintaining imaging equipment
- Interviewing patients to get their medical history
- Answering questions and explaining the process to prepare patients for the procedure
- Positioning the patient in the scanner and taking the images per physician instructions
What are MRIs not useful for?
mri not useful for whiplash injuries. AS Brett, reviewing Ronnen HR et al. Radiology 1996 Oct Patients with whiplash injuries frequently report persistent symptoms for weeks or even months.
How to prepare for your MRI?
What to Expect Prior to Your MRI Procedure:
- You will be able to take your daily medications and eat normally unless your doctor has instructed otherwise.
- We recommend that you arrive at least 15-30 minutes prior to your exam to complete any required forms and check-in with our staff.
- You will need to remove any metal on your body, such as jewelry, body piercings, keys, and coins.
What are the basic principle of MRI?
MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field. When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field.
What are the three principal components of an MRI system?
The principal components of an MRI system are shown in a block diagram in Figure 9-1. Magnet, operating console, and system electronics are the three principal components of an MRI system.
What is the principle of MRI Class 10?
The principle used in MRI is that the protons in different types of human tissues have their own specific magnetic field environment.
What physical principle is MRI based upon?
MRI is based on the principles of nuc1ear magnetic resonance (NMR) and started out as a tomographic imaging technique, that is it produced an image of the NMR signal in a thin slice through the human body. The human body is primarily fat and water. Fat and water have many hydrogen atoms.
What are the 5 components of an MRI scanner?
An MRI system consists of four major components: a main magnet formed by superconducting coils, gradient coils, radiofrequency (RF) coils, and computer systems. Each component has safety considerations.
What are the types of MRI?
There are two main types of MRI machines: closed bore and open. While closed bore MRI machines take the highest quality images, open MRI machines may provide more comfort during the imaging due to the lack of an enclosed space.
What is the full name of MRI?
Magnetic resonance imagingMagnetic resonance imaging / Full nameMagnetic resonance imaging (MRI) of the abdomen. The patient lies on a table that slides into the MRI machine, which takes pictures of the inside of the body.
What is MRI and its application?
Magnetic resonance imaging (MRI) is a medical imaging technique that uses a magnetic field and computer-generated radio waves to create detailed images of the organs and tissues in your body. Most MRI machines are large, tube-shaped magnets.
What are the uses of MRI?
Magnetic resonance imaging (MRI) uses a large magnet and radio waves to look at organs and structures inside your body. Health care professionals use MRI scans to diagnose a variety of conditions, from torn ligaments to tumors. MRIs are very useful for examining the brain and spinal cord.
What is the main magnetic field in MRI?
The B o in MRI refers to the main magnetic field and is measured in Tesla (T). The majority of MRI systems in clinical use are between 1.5T and 3T. Altering the field strength will affect the Larmour frequency at which the protons precess.
How is the image formed in MRI?
MRI uses magnetic fields and radio waves to measures how much water is in different tissues of the body, maps the location of the water and then uses this information to generate a detailed image. The images are so detailed because our bodies are made up of around 65% water, so we have lots of signal to measure.
What are the advantages of MRI?
MRI provides better soft tissue contrast than CT and can differentiate better between fat, water, muscle, and other soft tissue than CT (CT is usually better at imaging bones). These images provide information to physicians and can be useful in diagnosing a wide variety of diseases and conditions.
What are the components of an MRI machine?
An MRI scanner is made up of four components: the magnet, gradient coils, r.f. transmitter and receiver, and the computer. In this section the general design and construction of these components is discussed.
What is the most important component in the MRI?
The most important component of an MRI system is the magnet. The horizontal tube in which the patient enters, known as the bore, contains the strong magnet from front to back. The entire system proves incredibly strong with the ability of producing a large, stable magnetic field.
How many types of MRI coils are there?
Types. RF coils for MRI can be grouped into two different classes: volume coils and surface coils.
What is T1 and T2 image MRI?
The most common MRI sequences are T1-weighted and T2-weighted scans. T1-weighted images are produced by using short TE and TR times. The contrast and brightness of the image are predominately determined by T1 properties of tissue. Conversely, T2-weighted images are produced by using longer TE and TR times.
What is the purpose of MRI?
The brain, spinal cord and nerves, as well as muscles, ligaments, and tendons are seen much more clearly with MRI than with regular x-rays and CT; for this reason MRI is often used to image knee and shoulder injuries.
How does an MRI work?
How does MRI work? MRI of a knee. MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field. When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field.
Why do you give contrast agents before an MRI?
Contrast agents (often containing the element Gadolinium) may be given to a patient intravenously before or during the MRI to increase the speed at which protons realign with the magnetic field. The faster the protons realign, the brighter the image.
What is MRI scanner?
MRI scanners are particularly well suited to image the non-bony parts or soft tissues of the body. They differ from computed tomography (CT), in that they do not use the damaging ionizing radiation of x-rays. The brain, spinal cord and nerves, as well as muscles, ligaments, and tendons are seen much more clearly with MRI than with regular x-rays and CT; for this reason MRI is often used to image knee and shoulder injuries.
What is MRI used for?
It is often used for disease detection, diagnosis, and treatment monitoring. It is based on sophisticated technology that excites and detects the change in the direction of the rotational axis of protons found in the water that makes up living tissues.
What is a specialized MRI?
One kind of specialized MRI is functional Magnetic Resonance Imaging (fMRI.) This is used to observe brain structures and determine which areas of the brain “activate” (consume more oxygen) during various cognitive tasks. It is used to advance the understanding of brain organization and offers a potential new standard for assessing neurological status and neurosurgical risk.
What are some coping mechanisms for MRI?
Additional coping mechanisms include listening to music or watching a video or movie, closing or covering the eyes, and holding a panic button. The open MRI is a machine that is open on the sides rather than a tube closed at one end, so it does not fully surround the patient.
What are the three main views obtained when MRI scanning?
Fig 2 – The three main views obtained when MRI scanning. Left to right: Sagittal, coronal and axial.
How does MRI work?
MRI scans work as an imaging method due to the unique make-up of the human body. We are comprised entirely of cells which all contain water – principally made of hydrogen ions (H 2 O).
What is the best way to see the human body?
Magnetic resonance imaging can produce highly sophisticated and highly detailed images of the human body. Generally speaking, MRI scanning is excellent for visualising soft tissue - and so it is often used in the detection of tumours, strokes and bleeds. It also can be used to visualise the functionality of suspected masses and tumours through IV, gadolinium-based agents.
What is the thecal sac on MRI?
The figure below shows a T2 weighted, sagittal MRI of the lumbar spine. The thecal sac is easily visible as the 1cm thick white band running posterior to the vertebral bodies. This is interupted at the L4/L5 level by a small round dark area, which is the herniation of the intervertebral disc into the central canal.
What is MRI scanning based on?
Fig 1 – MRI scanning is based on the excitation and relaxation of protons.
How long does it take to get an MRI?
However, they are time consuming – averaging approximately 35-45 minutes to complete. This limits their use in trauma and emergency situations, where CT scanning is often preferred. They are also by far the most expensive of all the imaging modalities available.
What is the most advanced imaging method?
Magnetic resonance imaging (MRI) is arguably the most sophisticated imaging method used in clinical medicine. In recent years, MRI scans have become increasingly common, as costs decrease.
What is a 3T MRI?
A modern whole-body 3T MRI clinical scanner – the cylindrical bore houses the superconducting coil, which produces the main magnetic field ( B 0 ), the gradient coils, as well as the body RF coil which excites the spins. MRI signal reception is performed using a torso phased-array coil designed for abdominal imaging and wrapped around the patient as shown (Image courtesy: Ann Sawyer, Stanford University)
How is the magnetic field generated in an MRI?
The main components of an MRI system are shown in Fig. 4.4. The main magnetic field B 0 is created by winding a superconducting material around a cylindrical bore, which also houses the patient table. The superconducting material enables the sustenance of high currents, which are needed for producing strong magnetic fields (1–3 T), typically used in modern whole-body MRI systems. The RF coil that produces the excitation RF B 1 field is also integrated into the bore. The MRI signal is typically sensed using RF coils with differing geometries, tailored to the body part being imaged. A torso phased-array receive coil for abdominal imaging is shown here. The spatially varying magnetic field gradients are also integrated into the bore and produced by winding wires around a core. These gradients are situated in a different layer in the bore than the superconducting windings, which produce B 0. The signal detected by the RF coils are filtered, amplified, digitized, and then processed to generate the final MRI image.
What are the elements of a typical MRU exam?
Elements of a typical MRU exam illustrating the key steps of patient preparation, data acquisition (imaging), image processing, and modeling. The image-processing step is used to segment the kidney into medulla, cortex, and collecting system ( color coded here) to generate separate cortical and medullary GFR measurements
What is static MRU?
Static MRU is very useful for characterization of fluid-filled and dilated structures such as hydronephrosis, megaureters, cysts, ureteroceles, and similar pathologies. While it is very useful for obtaining anatomic information in patients with dilated or obstructed collecting systems, it is not very useful for eliciting functional information, especially in non-dilated systems. Because gadolinium contrast agents cause concomitant reduction of T 2 relaxation times in addition to T 1, static MRU is performed prior to injection of gadolinium-based contrast agents. In static MRU, the long T 2 relaxation time of urine is exploited for achieving T 2 -weighting [ 7, 8 ]. A pulse sequence with a long echo time (TE) is used to suppress signal from background tissue, which have shorter T 2 relaxation times. Fat suppression can also be used to further reduce background signal and improve conspicuity of structures of interest such as the ureters. In addition to thin section multi-slice 2D imaging, thick slab 2D imaging or even 3D T 2 -weighted imaging sequences have been reported for use in static fluid MRU.
What is the magnetic field of a proton?
A proton can be thought of as a charged sphere spinning about its axis , which produces a magnetic field. These protons are most commonly referred to as spins in MRI. In the absence of any external magnetic field, the spins are randomly oriented in all directions, resulting in no net macroscopic magnetic field. When placed in a constant external magnetic field B 0, the spins align themselves parallel or antiparallel to B 0. There is a slight excess of spins aligned parallel to the B 0 field, resulting in a net magnetic moment M along B 0. This phenomenon is called polarization. Spins exhibit another phenomenon called precession. While spinning about their own axis, they also wobble around the B 0 field, analogous to the wobbling motion of a spinning top in a gravitational field. This precession frequency is called the Larmor frequency which is the product of the external magnetic field B 0 and the gyromagnetic ratio γ, a constant for each nucleus. At 1.5 T, the Larmor frequency for protons is 63 MHz. Physically, this corresponds to a two-state split in energy levels when spins are placed in a magnetic field, with the energy difference between the states given by the Larmor frequency. When spins make transitions between the two energy levels, it results in absorption or emission of electromagnetic energy (photons) at the Larmor (resonance) frequency. This phenomenon is called nuclear magnetic resonance (NMR).
What is echo time in MRI?
The time between the application of a B 1 RF pulse and center of data acquisition is called echo time (TE). This parameter along with the sequence repetition time (TR) is a strong determinant of MRI image contrast. The spin–lattice and spin-spin relaxation times T 1 and T 2 along with the spin (or proton) density are different for protons in different molecules (e.g., fat, gray matter, white matter) and environments, generating soft tissue contrast. In the human body, typical values of T 2 are 10–50 ms and T 1 100–1,500 ms. By changing the TE and TR, differences in T 1, T 2, or proton density (PD) can be highlighted, giving rise to T 1 -weighted, T 2 -weighted, or PD-weighted images, the three most common types of images acquired in MRI. T 2 -weighted sequences are characterized by long TR and long TEs, while T1-weighted sequences usually employ short TRs and short TEs. Proton-density weighting can be elicited using long TRs and short TEs, minimizing both T 1 and T 2 effects.
What is a half fourier MRI?
In order to minimize scan times and reduce motion artifacts from breathing and peristalsis, a half-Fourier single-shot imaging technique (called single-shot fast spin echo (SSFSE), rapid acquisition with relaxation enhancement (RARE), or single-shot turbo spin echo (TSE) by the main MRI vendors) is most commonly used in conjunction with breath-holding [ 8, 9 ]. Sometimes, respiratory gating is used to gate the acquisition to the quiescent period of the respiratory cycle and obviate the need for breath-holding. The use of respiratory-gated 3D spin echo sequences has also been reported but at the cost of increased scan times [ 10 ]. The use of variable refocusing flip angle 3D fast spin echo sequences (called Cube, SPACE, or VISTA by three of the main MRI vendors) can enable the use of long echo trains, which significantly reduces scan times. Three-dimensional imaging sequences enable the acquisition of volumes with thin section thicknesses, which can then be post-processed to yield maximum intensity projection (MIP) or volume rendered images for depiction of the entire urinary tract.
How does parallel imaging work?
Parallel imaging is an MR technique designed to reduce scan time. Sensitivity encoding (SENSE™, Philips) and simultaneous acquisition of spatial harmonics (SMASH) are two such examples. SENSE works through under-sampling of the MR data and by collecting data simultaneously from multiple imaging coils. Reconstruction of the data requires an accurate knowledge of the individual coil sensitivities prior to the acquisition of the data. Therefore, a reference scan acquiring low resolution individual coil data is acquired prior to the main imaging sequence. Thus, a SENSE factor of 2 may reduce imaging time by up to 50%. However, with higher SENSE factors there may be a diminishing amount of MR signal that is recorded.5
What is magnetic susceptibility?
Magnetic susceptibility is the degree of magnetization that a tissue or material exhibits in response to a magnetic field. This may have either a beneficial or deleterious effect on the overall image quality. Magnetic susceptibility artifacts are more prominent at 3 T compared to 1.5 T. The phenomenon may be beneficial in functional or diffusion MRI by improving tissue contrasts, but disadvantageous by producing signal voids at air/tissue interfaces in diffusion sequences. An eddy current is an induced current generated due to the interaction between the rapidly changing magnet field and the conducting structures within the MRI scanner. Eddy currents may lead to perturbations in the gradient field, reducing resolution of the subsequent MR Image.7
What is T1 in physics?
T1is also known as “spin-lattice relaxation ”, whereby the “lattice” is the surrounding nucleus environment. As longitudinal relaxation occurs, energy is dissipated into the lattice. T1is the length of time taken for the system to return 63% toward thermal equilibrium following an RF pulse as an exponential function of time. T1can be manipulated by varying the times between RF pulses, the repetition time (TR). Water and cerebrospinal fluid (CSF) have long T1values (3000–5000 ms), and thus they appear dark on T1-weighted images, while fat has a short T1value (260 ms) and appears bright on T1-weighted images.5
What is the magnetic field of a nucleus?
Application of a strong, external magnetic field (B0) aligns the nucleus either in parallel with or perpendicular to the external field. A liquid solution containing many nuclear spins, placed within the B0field, will contain nuclear spins in one of two energy states: a low-energy state (oriented parallel to the magnetic field) or a high-energy state (orientated perpendicular to the magnetic field direction). In solids or liquids, there would tend to be an excess of spins in the same direction as B0. Although a bar magnet would orientate completely parallel or antiparallel to the field, the nucleus has an angular momentum due to its rotation, so it will rotate, or precess, around the B0axis (Figure 2). This behavior is often compared to the wobbling motion of a gyroscope under the influence of the Earth's magnetic field and explains the use of “spin” to explain what is in reality a quantum mechanical phenomenon. The velocity of rotation around the field direction is the Larmor frequency. This is proportional to the field strength, and is described by the Larmor equation (Figure 3).5
How to localize MR signal?
Localizing the MR signal spatially to a region of interest requires the use of gradients. These are additional spatially linear variations in the static field strength. Gradients can be applied in any orthogonal direction using the three sets of gradient coils, Gx, Gy, and Gz, within the MR system. Faster or slower precession is detected as higher or lower MR signal. Thus, the frequency measurements can be used to distinguish MR signals at different positions in space and enable image reconstruction in three dimensions (Figure 6).5
When was NMR first used?
The nuclear magnetic resonance (NMR) phenomenon was first described experimentally by both Bloch and Purcell in 1946 , for which they were both awarded the Nobel Prize for Physics in 1952.1, 2The technique has rapidly evolved since then, following the introduction of wide-bore superconducting magnets (approximately 30 years ago), allowing development of clinical applications. The first clinical magnetic resonance images were produced in Nottingham and Aberdeen in 1980, and magnetic resonance imaging (MRI) is now a widely available, powerful clinical tool.3, 4This article covers a brief synopsis of basic principles in MRI, followed by an overview of current applications in medical practice.
Is MRI used in medical research?
The development of magnetic resonance imaging (MRI) for use in medical investigation has provided a huge forward leap in the field of diagnosis, particularly with avoidance of exposure to potentially dangerous ionizing radiation. With decreasing costs and better availability, the use of MRI is becoming ever more pervasive throughout clinical practice. Understanding the principles underlying this imaging modality and its multiple applications can be used to appreciate the benefits and limitations of its use, further informing clinical decision-making.
What is the advantage of MRI?
MRI has the advantage that it does not use ionising radiation and is non-invasive. In addition, it has a high soft tissue resolution and imaging capabilities in multiple planes.
What is the best imaging modality for musculoskeletal pathologies?
Magnetic resonance imaging (MRI) is the imaging modality of choice for the evaluation of a variety of musculoskeletal pathologies. MRI has the advantage that it does not use ionising radiation and is non-invasive. In addition, it has a high soft tissue resolution and imaging capabilities in multiple planes.
