Nuclear magnetic resonance imaging
The mobile MRI unit at Northwick Park Hospital
Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to investigate the anatomy and physiology of the body in both health and disease. MRI scanners use strong magnetic fields and radio waves to form images of the body. The technique is widely used in hospitals for medical diagnosis, staging of disease and for follow-up without exposure to ionizing radiation.
MRI produces images that are 2 D slices through the body and they have excellent spatial resolution (i.e. you can see very small details in the images), making it an important tool for doctors.
MRI is a way of looking inside the body and is especially good at producing images of soft tissues such as muscle, fat, cartilage and the brain. It does this by producing a map which depends on the density of hydrogen in the body.
MRI uses a very strong superconducting magnet with a magnetic field strength of around 40 000 times that of the Earth. The nucleus of a hydrogen atom is a single proton, and acts very like a little bar magnet.
When a person is lying in the magnetic field of the MRI scanner the nuclei of the hydrogen atoms in their body line up, like compass needles in the Earth’s magnetic field, either pointing in the direction of the field or opposite to it.
The hydrogen nuclei (protons) don’t stay still though, but move like a spinning top around the direction of the magnetic field.
A radiofrequency field, an alternating magnetic field that has the same frequency as radio waves, is then applied. This flips some of the protons round and makes them all move round together. This produces a changing magnetic field at right angles to the large magnetic field, which can induce a voltage in a coil of wire. This signal can be used to produce an image which, which depends on the number of protons and how tightly they are held by surrounding molecules.
A third magnetic field has a gradient so it is stronger at one end than the other. This allows the scanner to select a slice of the body to look at, by selecting the required field strength. The gradient fields change rapidly and make the scanner very noisy.
The absorption of radiation by a proton in a magnetic field
Imagine a proton (of spin 1/2) in a magnetic field. This proton is in the lower energy level (i.e. its magnetic moment does not oppose the applied field). The nucleus is spinning on its axis. In the presence of a magnetic field, this axis of rotation will precess around the magnetic field;
Precession is a change in the orientation of the rotational axis of a rotating body or the slow movement of the axis of a spinning body around another axis due to a twisting force
If energy is absorbed by the proton, then the angle of precession, q, will change. For a proton of spin 1/2, absorption of radiation “flips” the magnetic moment so that it opposes the applied field (the higher energy state).
Only a small proportion of “target” protons are in the lower energy state (and can absorb radiation). There is the possibility that by exciting these protons, the populations of the higher and lower energy levels will become equal. If this occurs, then there will be no further absorption of radiation. The spin system is saturated. The possibility of saturation means that we must be aware of the relaxation processes which return nuclei to the lower energy state.
How do protons in the higher energy state return to the lower state? Emission of radiation is insignificant because the probability of re-emission of photons varies with the cube of the frequency. At radio frequencies, re-emission is negligible. We must focus on non-radiative relaxation processes (thermodynamics!).
When the proton is in a magnetic field, the initial populations of the energy levels are determined by thermodynamics, as described by the Boltzmann distribution. This is very important, and it means that the lower energy level will contain slightly more protons than the higher level. It is possible to excite these protons into the higher level with electromagnetic radiation. The frequency of radiation needed is determined by the difference in energy between the energy levels.
Ideally, the NMR spectroscopist would like relaxation rates to be fast – but not too fast. If the relaxation rate is fast, then saturation is reduced. If the relaxation rate is too fast, line-broadening in the resultant NMR spectrum is observed.
There are two major relaxation processes;
Spin – lattice (longitudinal) relaxation
Spin – spin (transverse) relaxation
Spin – lattice relaxation
Protons in an NMR experiment are in a sample. The sample in which the protons are held is called the lattice. Protons in the lattice are in vibrational and rotational motion, which creates a complex magnetic field. The magnetic field caused by motion of protons within the lattice is called the lattice field. This lattice field has many components. Some of these components will be equal in frequency and phase to the Larmor frequency of the protons of interest. These components of the lattice field can interact with protons in the higher energy state, and cause them to lose energy (returning to the lower state). The energy that a proton loses increases the amount of vibration and rotation within the lattice (resulting in a tiny rise in the temperature of the sample).
The relaxation time, T1 (the average lifetime of proton in the higher energy state) is dependent on the magnetogyric ratio (ratio of its magnetic dipole moment to its angular momentum) of the proton and the mobility of the lattice. As mobility increases, the vibrational and rotational frequencies increase, making it more likely for a component of the lattice field to be able to interact with excited protons. However, at extremely high mobilities, the probability of a component of the lattice field being able to interact with excited proton decreases.
Spin – spin relaxation
Spin – spin relaxation describes the interaction between neighbouring protons with identical precessional frequencies but differing magnetic quantum states. In this situation, the protons can exchange quantum states; a proton in the lower energy level will be excited, while the excited proton relaxes to the lower energy state. There is no net change in the populations of the energy states, but the average lifetime of a proton in the excited state will decrease.
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation. This energy is at a specific resonance frequency which depends on the strength of the magnetic field and the magnetic properties of the isotope of the atoms; in practical applications, the frequency is similar to VHF and UHF television broadcasts (60–1000 MHz). NMR allows the observation of specific quantum mechanical magnetic properties of the atomic nucleus. Many scientific techniques exploit NMR phenomena to study molecular physics, crystals, and non-crystalline materials through NMR spectroscopy. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging (MRI).
A key feature of NMR is that the resonance frequency of a particular substance is directly proportional to the strength of the applied magnetic field. It is this feature that is exploited in imaging techniques; if a sample is placed in a non-uniform magnetic field then the resonance frequencies of the sample’s nuclei depend on where in the field they are located. Since the resolution of the imaging technique depends on the magnitude of magnetic field gradient, many efforts are made to develop increased field strength, often using superconductors. The effectiveness of NMR can also be improved using hyperpolarization, and/or using two-dimensional, three-dimensional and higher-dimensional multi-frequency techniques.
Resonance occurs when the driving frequency acting on the nuclei is equal to the natural frequency of the nuclei. At this point you will get a greater signal.
Modern 3 tesla clinical MRI scanner
To perform a study, the patient is positioned within an MRI scanner which forms a strong magnetic field around the area to be imaged.
The human body is largely composed of water molecules, which each contain two hydrogen nuclei, or protons. When a person goes inside the powerful magnetic field (B0) of the scanner, the magnetic moments of these protons align with the direction of the field.
An oscillating resonant radio frequency electromagnetic field is then briefly turned on, causing the protons to alter their magnetisation alignment relative to the field applying energy to the patient. When this field is turned off, the protons return to the original magnetisation alignment, and these changes in magnetisation alignment cause a changing magnetic flux, which yields a changing voltage in receiver coils to give the signal. The frequency at which a proton or group of protons in a voxel (A voxel represents a value on a regular grid in three-dimensional space) resonates depends on the strength of the local magnetic field around the proton or group of protons. By applying additional magnetic fields (gradients) that vary linearly over space, specific slices to be imaged can be selected, and an image is obtained by taking the 2-D Fourier transform of the spatial frequencies of the signal (a.k.a., k-space). Due to the magnetic Lorentz force from B0 on the current moving through the gradient coils, the gradient coils will try to move. The knocking sounds heard during an MRI scan are the result of the gradient coils trying to move against the constraint of the concrete or epoxy in which they are secured.
Diseased tissue, such as tumours, can be detected because the protons in different tissues return to their equilibrium state at different rates (i.e., they have different T1 times). By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.
Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumours or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MRI images of joints. Unlike CT, MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants, cochlear implants, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radio frequency pulses.
MRI is used to image every part of the body, and is particularly useful for neurological conditions, for disorders of the muscles and joints, for evaluating tumours, and for showing abnormalities in the heart and blood vessels.
Below is a schematic to help in the visualisation of the imaging process. It is presumed that there are two regions of the sample which contain enough hydrogens to produce a strong NMR signal. The top sketch visualizes an NMR process with a constant magnetic field applied to the entire sample. The hydrogen spin-flip frequency is then the same for all parts of the sample. Once excited by the RF signal, the hydrogens will tend to return to their lower state in a process called “relaxation” and will re-emit RF radiation at their Larmor frequency. This signal is detected as a function of time, and then is converted to signal strength as a function of frequency by means of a Fourier transformation. Since the protons in each of the active areas of the sample are subjected to the same magnetic field, they will produce the same frequency of radiation and the Fourier transform of the detected signal will have only one peak. This one peak demonstrates the presence of hydrogen atoms, but gives no information to locate them in the sample.
The relative signal intensity (brightness) of tissues in an MRI image is determined by factors such as:
The radiofrequency pulse and gradient waveforms used to obtain the image
Intrinsic T1 and T2 characteristics of different tissues
The proton density of different tissues
By controlling the radiofrequency pulse and gradient waveforms, computer programs produce specific pulse sequences that determine how an image is obtained (weighted) and how various tissues appear. Images can be:
In nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) the term relaxation describes how signals change with time. In general signals deteriorate with time, becoming weaker and broader. The deterioration reflects the fact that the NMR signal, which results from nuclear magnetization, arises from the over-population of an excited state. Relaxation is the conversion of this non-equilibrium population to a normal population. In other words, relaxation describes how quickly spins “forget” the direction in which they are oriented. The rates of this spin relaxation can be measured in both spectroscopy and imaging applications.
The deterioration of an NMR signal is analysed in terms of two separate processes, each with their own time constants. One process, associated with T1, is responsible for the loss of signal intensity. The other process, associated with T2, is responsible for the broadening of the signal. Stated more formally, T1 is the time constant for the physical processes responsible for the relaxation of the components of the nuclear spin magnetization vector M parallel to the external magnetic field, B0 (which is conventionally oriented along the z axis). T2 relaxation affects the components of M perpendicular to B0. In conventional NMR spectroscopy T1 determines the recycle time, the rate at which an NMR spectrum can be acquired. Values of T1 range from milliseconds to several seconds.
T1 is spin–lattice relaxation time and T2 the spin-spin relaxation time
T2 relaxation generally proceeds more rapidly than T1 recovery, and different samples and different biological tissues have different T2. For example, fluids have the longest T2 (on the order of seconds for protons), and water based tissues are in the 40–200 ms range, while fat based tissues are in the 10–100 ms range. Amorphous solids have T2 in the range of milliseconds, while the transverse magnetization of crystalline samples decays in around 1/20 ms.
Effects of TR (repetition time) and TE (echo time) on MR signal.
Contrast resolution is the ability to distinguish between differences in intensity in an image. The measure is used in medical imaging to quantify the quality of acquired images. It is a difficult quantity to define, because it depends on the human observer as much as the quality of the actual image. For example, the size of a feature affects how easily it is detected by the observer.
Image contrast is created by differences in the strength of the NMR signal recovered from different locations within the sample. This depends upon the relative density of excited nuclei (usually water protons), on differences in relaxation times of those nuclei after the pulse sequence, and often on other parameters discussed under specialized MR scans. Contrast in most MR images is actually a mixture of all these effects, but careful design of the imaging pulse sequence allows one contrast mechanism to be emphasized while the others are minimized. The ability to choose different contrast mechanisms gives MRI tremendous flexibility. In the brain, T1-weighting causes the nerve connections of white matter to appear white, and the congregations of neurons of grey matter to appear gray, while cerebrospinal fluid (CSF) appears dark. The contrast of white matter, grey matter and cerebrospinal fluid is reversed using T2 imaging, whereas proton-density-weighted imaging provides little contrast in healthy subjects. Additionally, functional parameters such as cerebral blood flow (CBF), cerebral blood volume (CBV) or blood oxygenation can affect T1 and T2 and so can be encoded with suitable pulse sequences.
Examples of T1 weighted, T2 weighted MRI scans.
In MRI, determining contrast is of high importance for calibration because the operator has a high degree of control of how the signal intensities of various structures vary in the images by using different MRI methods and imaging parameters. Unlike most other imaging techniques, such as x-ray CT in which the Hounsfield units value for water is set to zero, there is no standard reference signal for MRI. Thus the contrast-to-noise ratio is often employed as an index for contrast because this metric does not require a reference signal.
Contrast resolution or contrast-detail is an approach to describing the image quality in terms of both the image contrast and resolution.
Contrast resolution is usually measured by generating a pattern from a test object that depicts how image contrast changes as the structures being imaged get smaller and closer together.
Resolving power is the ability of an imaging device to separate (i.e. to see as distinct) points of an object that are located at a small angular distance or it is the power of an optical instrument to separate far away objects that are close together into individual images. The term resolution or minimum resolvable distance is the minimum distance between distinguishable objects in an image, although the term is loosely used by many users of microscopes and telescopes to describe resolving power. In scientific analysis, in general, the term “resolution” is used to describe the precision with which any instrument measures and records (in an image or spectrum) any variable in the specimen or sample under study.
In some situations it is not possible to generate enough image contrast to adequately show the anatomy or pathology of interest by adjusting the imaging parameters alone, in which case a contrast agent may be administered. This can be as simple as water, taken orally, for imaging the stomach and small bowel. However, most contrast agents used in MRI are selected for their specific magnetic properties. Most commonly, a paramagnetic contrast agent (usually a gadolinium compound) is given. Gadolinium-enhanced tissues and fluids appear extremely bright on T1-weighted images. This provides high sensitivity for detection of vascular tissues (e.g., tumours) and permits assessment of brain perfusion (e.g., in stroke). There have been concerns raised recently regarding the toxicity of gadolinium-based contrast agents and their impact on persons with impaired kidney function.
More recently, superparamagnetic contrast agents, e.g., iron oxide nanoparticles, have become available. These agents appear very dark on T2-weighted images and may be used for liver imaging, as normal liver tissue retains the agent, but abnormal areas (e.g., scars, tumours) do not. They can also be taken orally, to improve visualization of the gastrointestinal tract, and to prevent water in the gastrointestinal tract from obscuring other organs (e.g., the pancreas). Diamagnetic agents such as barium sulphate have also been studied for potential use in the gastrointestinal tract, but are less frequently used.
Instrumentation and equipment
The MRI equipment consists of following components:
The magnet generates the magnetic field.
Shim coils (Coils carrying a relatively small current that are used to provide auxiliary magnetic fields in order to compensate for inhomogeneities in the main magnetic field of a MR system) make the magnetic field homogeneous (spread out evenly).
Radio frequency coils (large coil inductors with considerable dimensions and a defined wavelength, commonly used in configurations for MR imaging) transmit the radio signal into the body part being imaged. Generally they are transmit Receive Coil, receive Only Coil and transmit Only Coil (they generate the pulses of RF waves to energise the molecular spins)
Receiver coils (A coil, or antenna, positioned within the imaging volume and connected to the receiver circuitry that is used to detect or receive the MR signal from the patient as the disturbed spins relax back into their equilibrium distribution) detect the returning radio signals.
Gradient coils (current carrying coils designed to produce a desired magnetic field gradient so that the magnetic field will be stronger in some locations than others) provide spatial localisation of the signals.
Shielding coils (confines the region of strong magnetic field surrounding a magnet) produce a magnetic field that cancels the field from primary coils in regions where it is not desired.
The main superconducting electromagnet producing a primary field strength of 1.5 Tesla
Surrounded by a bath of liquid helium to keep the coils of wire very very cold – at Absolute Zero to be exact. This is so that resistance in the wire doesn’t cause heat to build up in the wire which in turn would cause the magnetic field to dissipate.
Gradient coils (3) in X,Y, and Z planes : data plotting
Active shielding-additional superconducting coils which work against the primary field to help confine the effects of the magnetic field to a certain area so that the effects on patients/ staff/ other equipment can be controlled within that area.
Clinical magnets come in a variety of shapes, sizes and field strengths.
Surface coils are loops of wire that detect MR signal and are designed to fit specific body parts
Top left is for the neck, Top middle is for body flex- abdominal/ pelvic work mostly and top right is for an extremity/ knee coil
The computer reconstructs the signals into the image.
The MRI scanner room is shielded by a faraday shield (in MRI, one use of the Faraday shield is the shielding of the scanning room, to block incoming radio frequency (RF) signals which would contaminate the send and received signals of the MRI scanner, and it suppresses RF signals, which would else pollute the environment around).
Different cooling systems cool the magnet, the scanner room and the technique room.
MRI applications and safety
In an MRI dept, the safety of staff + patients is always of the utmost importance.
MRI is 3000 times stronger than the Earth’s magnetic field. A magnet of that strength, in the wrong hands, could be very dangerous, and for lots of different reasons.
Firstly, as a superconducting magnet – it cannot be switched off. Whether it is scanning or not, the magnet is always on, so the area around it has to be closely monitored and access controlled.
Secondly, there is no ionising radiation, but when scanning there are radiofrequency waves present and those can have biological effects.
But that’s not all ………
The static magnetic field has a projectile effect. The force is proportional to the size of the object.
The static magnetic field can induce electrical currents in the patient causing biological side effects such as nausea or vertigo.
Therefore as operators of the equipment we are governed by operating guidelines to limit exposure to these effects, but generally speaking there are no serious adverse effects when operating at current clinical field strengths.
Twitching sensations in hands an feet can occur due to peripheral nerve stimulation
Rapid switching of gradient coils is noisy but patients are given ear protection.
The pulses of radiofrequency waves that are used to excite the spins produce heat which is transferred to the patients who get hot
Again users operate under specific guidelines to reduce the risk of overheating patients. Scanners are calibrated to ensure that the energy delivered to the patient does not raise their body temperature by more than 1 degree per KG. This is measured buy the specific absorption rate (SAR)
Overheating can cause serious effects in the form of BURNS……….
The presence of any quantity of Liquid helium in a hospital is a risk in itself.
It can cause asphyxiation (from displacement of oxygen), hypothermia and frostbite
So MRI departments are carefully designed to include warnings and adequate extration systems to cope with any leakage of helium gas as it boils off during scanner use
There is an oxygen alarm, external Vent Pipes and an outward opening doo to the scanner room.
Now, as if all of that wasnt scary enough.
Electronic devices in or on the patients can malfunction and have serious medical consequences.
And may even be fatal!!
All of these potentially serious risks need monitored and managed at all times in an MRI unit.
Therefore there are strict safety procedures.
Disadvantages of MRI:
MR Safety issues – Projectile effect
Acoustic noise levels
Patient Issues – Claustrophobia
Time consuming procedure
Unsuitable for trauma patients
Advantages of MRI:
No Ionising radiation involved
Excellent anatomical detail
Choice of any anatomical plane
Versatility in tissue differentiation
MRI is used for diagnosing many problems. It can be used to identify tumours, diagnose multiple sclerosis (MS) and is often used on sportspeople to see problems with ligaments inside joints like the knee and ankle. It can also be used to show the anatomy of the brain and how it works.
MRI vs CT (advantages)
Anatomical soft tissue detail –Brain + CNS – Spine + Joints
Various planes available
No ionising radiation (this is very important when examining children)
MRI vs CT (disadvantages):
Safety issues – not for all patients – serious risks
Patient co-operation is paramount
MRI vs CT – but which one?
Spinal Injury – CT shows up a bony injury but MR shows up a spinal cord injury
Cancer – CT is faster if the disease has spread throughout body but MR is better for investigating a specific tumour
Stroke – CT on A+E admission provides diagnosis – MR within 48 hours indicates prognosis
Multiple Sclerosis – MR only modality that can visualise the disease
Joints – CT can show up a bony injury but MR can show up muscles and ligaments
Spine – CT can show up a bony injury that MR can’t
Magnetic resonance imaging can be used to investigate abnormal muscle water distribution by measurement T2 relaxation times and the magnetisation transfer (MT) ratio, which may precede fatty infiltration and potentially provide a sensitive biomarker
Magnetic resonance imaging (MRI) of the nervous system (including the brain) produces high quality two- or three-dimensional images of nervous system structures without use of ionising radiation.
One advantage of MRI of the brain over computed tomography of the head is better tissue contrast, and it has fewer artifacts than CT when viewing the brainstem. MRI is also superior for pituitary imaging. It may however be less effective at identifying early cerebritis.
MRI is used in the assessment of the function and structure of the cardiovascular system.
MRI is used in the investigation of the musculoskeletal system include spinal imaging, assessment of joint disease and soft tissue tumors.
Hepatobiliary MR is used to detect and characterise lesions of the liver, pancreas and bile ducts. Focal or diffuse disorders of the liver may be evaluated using diffusion-weighted, opposed-phase imaging and dynamic contrast enhancement sequences. Extracellular contrast agents are widely used in liver MRI and newer hepatobiliary contrast agents also provide the opportunity to perform functional biliary imaging. Anatomical imaging of the bile ducts is achieved by using a heavily T2-weighted sequence in magnetic resonance cholangiopancreatography (MRCP). Functional imaging of the pancreas is performed following administration of secretin. MR enterography provides non-invasive assessment of inflammatory bowel disease and small bowel tumors. MR-colonography can play a role in the detection of large polyps in patients at increased risk of colorectal cancer.
Functional MRI (fMRI) is used to understand how different parts of the brain respond to external stimuli.
MRI is very good at investigating what is going on in the abdomen. Most doctors prefer to use CT scans to look at the stomach. But an MRI may sometimes provide more information.
MR enteroclysis in a 21-year-old man with active Crohn’s disease. Coronal true-FISP (a) and Haste (b) images show mucosal irregularity (arrows) as thin lines of high signal intensity, longitudinally, or transversely (fissure ulcers) orientated within the thickened in the terminal ileum consistent with diffuse ulcerations in Crohn’s ileitis. Axial true-FISP sequence (c) detects wall thickening of terminal ileum as well as the cecal wall (arrows). Axial fat-suppressed T2 Haste sequence (d) MR image shows high signal intensity bowel wall (arrows) and fluid surrounding the distal ileum (small arrow). Coronal (e) and axial (f) contrasts GRE T1 with fat saturated images show marked contrast enhancement, with avid enhancement of the mucosa of the terminal ileum and cecal walls. Note the high signal intensity linear structure due to increased vascularity (small arrows in (e)) close to the mesenteric border of the involved small bowel segment, the so-called comb sign. These MR findings are indicative of active Crohn’s disease.
Miscellaneous Implants and Devices
Many miscellaneous implants, materials, devices, and objects have been tested with regard to MRI procedures and the MRI environment.
For example, various types of firearms have been tested in the MRI environment. These firearms exhibited strong ferromagnetism. In fact, two of the six firearms evaluated discharged in a reproducible manner while in the MR system room. Of course in the UK very few people should be carrying a gun anyway..
MRI-guided biopsy, therapeutic, and minimally invasive surgical procedures are important clinical applications that are performed on conventional, open-architecture, or “double-donut” MR systems specially designed for this work. These procedures present challenges with regard to the instruments and devices that are needed to support these interventions.
Metallic surgical instruments and other devices potentially pose hazards (e.g., missile effects) or other problems (i.e., image distortion that can obscure the area of interest) that must be addressed to apply MRI-guided techniques effectively.
Various manufacturers have used “weakly” ferromagnetic, nonferromagnetic or nonmetallic materials to make special instruments for interventional MRI procedures.
Other medical products and devices have been developed with metallic components that are either entirely nonferromagnetic and non-conducting or made from metals that have a low magnetic susceptibility (e.g., titanium, non-magnetic types of stainless steel, etc.) that are acceptable for use in the MR environment.
Patients who have any metallic materials within the body must notify their doctor prior to the examination or inform the MRI staff. Metallic chips, materials, surgical clips, or foreign material (artificial joints, metallic bone plates, or prosthetic devices, etc.) can significantly distort the images obtained by the MRI scanner. Patients who have heart pacemakers, metal implants, or metal chips or clips in or around the eyeballs cannot be scanned with an MRI because of the risk that the magnet may move the metal in these areas. Similarly, patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should not have MRI scanning.
During the MRI scan, the patient lies in a closed area inside the magnetic tube for more than 20 minutes without moving (movement would blur the image). Some patients can experience a claustrophobic sensation during the procedure. Therefore, patients with any history of claustrophobia should relate this to the practitioner who is requesting the test, as well as the radiology staff. A mild sedative can be given prior to the MRI scan to help alleviate this feeling. It is customary that the MRI staff will be nearby during MRI scan. Furthermore, there is usually a means of communication with the staff (such as a buzzer held by the patient) which can be used for contact if the patient cannot tolerate the scan.
MRI machines work by generating a very large magnetic field using a super conducting magnet and many coils of wires through which a current is passed. Maintaining a large magnetic field needs a lot of energy, and this is accomplished using superconductivity, which involves trying to reduce the resistance in the wires to almost zero. This is done by bathing the wires in a continuous supply of liquid helium at -269.1C. A typical MRI scanner uses 1,700 litres of liquid helium, which needs to be topped up periodically.
Quenching is when the wire in an electromagnet stops being superconducting and starts to generate a lot of heat. At this point, any liquid helium around the magnet rapidly boils off and escapes from the vessel housing the magnet. For MRI scanners using liquid helium special ventilation facilities are needed.
A bigger problem than the helium boiling off is that we are running out of helium and some companies are working on ways of cooling the magnets to close to 0K using a very small quantity of liquid helium.
MRI images of the brain
This is a functional MRI image which shows both the structure of the brain, and those parts of the brain which are active when the patient moves his or her left finger.