MRI – The Super Massive Magnets (An overview)

What is MRI?

MRI stands for Magnetic Resonance Imaging; it is a technique that has been used over the last 35 years for medical diagnostics. It involves the interaction of the atoms in the body with electromagnetic fields – we rely on the nuclear spin of an atom to create the images.

The human body is largely composed of water molecules (H2O). These molecules contain 2 hydrogen nuclei, which is 2 protons. When protons come under the effect of a magnetic field, the magnetic moments of these protons align with the direction of the field.
Essentially all MRI is hydrogen (proton) imaging.

Physics of MRI

Nuclear Magnetism Resonance (NMR)

Subatomic particles have quantum properties of a “spin”. (A spin is a form of angular momentum). Some nuclei, such a protons, have a non-zero spin, and therefore a magnetic moment –  the nuclei produces a tiny magnetic field around itself. When these spins are placed in a strong external magnetic field, they precess around an axis along the direction of the field.

The best way to think of this effect is to consider each atom as a tiny bar magnet – because we have a system of tiny magnets, by applying a bigger magnetic field, we can manipulate their direction. It is worth noting that because their coupling is very weak, this does not change their physical or chemical properties and that other than radicals, molecules tend to gave zero magnetic moment.

If we apply an external magnetic field B0 to our system of tiny magnets, we can cause some of the to line up with it resulting in a non zero bulk magnetisation M. This effect is called nuclear spin polarisation and B0 is often called the polarising field.

Resonance and Relaxation

Magnetisation can be manipulated by changing the magnetic field environment (static, gradient and RF fields). RF waves are used to manipulate the magnetization of protons; externally applied RF waves perturb magnetisation into different axis (transverse axis). Only transverse magnetisation produces signal. When perturbed protons return to their original state, they emit RF signals that are detected by coils in the MRI machine.
When RF pulses are stopped, the high energy gained by the protons are re-transmitted, and the protons relax by two mechanisms:

• T1 (spin lattice relaxation) – by which original magnetisation begins to recover.
• T2 (spin spin relaxation) – by which magnetisation in x-y plane decays towards zero in an exponential fashion. This is due to incoherence of protons.

T1 Relaxation

After protons are excited with an RF pulse, they move out of alignment with B0 (the polarising field). Once the RF is stopped, they realign after some time – this is called T1 relaxation. T1 is defined as the time it takes to recover 63% of it longitudinal magnetisation.

T2 Relaxation

When the tipped spins of the protons are precessing, they de-phase; this is die to them not spinning at precisely the same speed. As they get out of phase, the magnetisation is no longer coherent, and the signal delays. T2 relaxation is the time for 63% of the protons to become dephased owing to interactions among nearby protons.

TR (echo time) & TR (repetition time)

TE – time interval in which signals are measured after RF excitation.

TR – the time between two excitation is called repetition time.

Construction of an MRI scanner

In a clinical environment, magnetic fields of above 0.5 Teslas are mostly desired; superconducting magnets are therefore required in the scanners as they produce very large magnetic fields that are exceptionally stable.

Magnets

Superconducting magnets are used for MRI scanning; they are solenoid-shaped coil made of alloys surrounded by copper. 

Maintaining large magnetic fields require a large amount of energy which is accomplished by superconductivity, or reducing the resistance in the wire to almost zero. To do this, the wires are continually bathed in liquid helium, at around 10 Kelvin. This cold is then insulated by a vacuum.

Gradient Coils

Gradient coils are used to spatially encode the positions of the protons by varying the magnetic field linearly across the imaging volume. They produce deliberate variations in the magnetic field B0. There are 3 sets of gradient coils – one for each direction. The variation in the magnetic field permits localisation of image slices, as well as phase encoding and frequency encoding.

Radio frequency coil

These coils broadcast the RF signal to the patient, and also receives the returning signal. Different coils can be used when imaging different parts of the body:

• Birdcage coil – commonly used for imaging of the head
• Paired saddle coil – commonly used for imaging of the knee. These coils are used as volume coils, and provide better homogeneity of the RF in areas of interest.
• Surface coils – used for spines, shoulders, joints, and small body parts.
• Helmholtz pair coil – these are two circular coils parallel to each other. They are used as Z gradient coils, and sometimes as RF coils for pelvis and cervical spine imaging.

Imaging Techniques

Different tissues have different relaxation times. These relaxation time differences are used to generate image contrast.

Contrast enhanced imaging

Image contrast is created by differences in the strength of the NMR signal, recovered from different locations within the sample (eg. fat and muscle). This depends on the relative density of the excited nuclei (protons), on difference in relaxation times (T1, T2 & T2*) of the nuclei after the pulse sequence. 

Contrast in most MR images is actually a mixture of all these effects, but by carefully planning the design of the imaging pulse sequence allows one contrast mechanism to be emphasized, whilst the others are minimised. This ability to choose different contrast mechanisms gives MRI tremendous flexibility in imaging.

In some situations, it is not possible to generate enough image contrast to adequately show the anatomy or pathology of interest by adjusting the pulse sequence – in this case, a contrast agent would be administered. 

Echo-planar imaging

In brain scanning, or in cases where images are needed rapidly, echo-planar imaging (EPI) is used. In each case, each RF excitation is followed by a train of gradient echoes with different spatial coding. 

Benefits:

• Reduced imaging time
• Decreased motion artifact
• Ability to image rapid physiologic processes of the human body

Drawbacks:

• Sensitive to susceptibility effects
• Sensitive to main magnetic field inhomogeneity
• Long gradient echo train causes greater T2* weighting
• Requires high-performance gradients

MRI Sequences

Spin echo sequences

T1 weighted image (T1)

This technique measures the spin-lattice relaxation by using a short repetition time (TR) and echo time (TE).

Main significance:

• Lower signal for more water content (highlights tumors, inflammation, infection, hemorrhage etc.)
• High signal for fat
• High signal for paramagnetic substances such as MRI contrasts.

T2 weighted image (T2)

This technique measures the spin-spin relaxation by using longer TR and TE times.

Main significance:

• Higher signal for more water content
• Low signal for fat
• Low signal for paramagnetic substances

Both T1 and T2 are used as standard foundations and comparisons for other sequences.

Inversion recovery sequences

Short tau inversion recovery (STIR)

This technique suppresses fat by setting an inversion time where the signal of fat is zero.

Main significance:

• Gives high signal for edema, such as in more severe stress fractures.

Fluid attenuated inversion recovery (FLAIR)

This technique suppresses fluid by setting an inversion time that nulls fluids. By choosing a careful inversion time (TI), the signal from any particular tissue can be nulled. The TI depends on the tissue from the formula: TI = ln(2)*T1.

Main significance:

• It can be used in brain imaging to suppress cerebrospinal fluid effects on the image
• Gives high signal in cases such as multiple sclerosis plaques, subarachnoid haemorrhages, and meningitis.

Double inversion recovery (DIR)

This technique simultaneously suppresses cerebrospinal fluid and white matter by two inversion times.

Main significance:

• Gives high signal of multiple sclerosis plaques.

Gradient echo sequences

Steady-state free precession imaging (SSFP)

This technique maintains a steady residual transverse magnetisation over successive cycles. States of SSFP is a kind of steady states of magnetisation achieved by a series of RF irradiation and natural relaxation behaviors of spins. The influencing factors include, the flip angles of RF pulses, repetition time (TR) of pulse repeats, and the relaxation time constants (T1 & T2).

Main significance:

• Creations of cardiac MRI videos

Commercial names for this sequence include FLASH: fast imaging using low angle shot (Siemens), SPGR (GE), and T1 FFE: T1 fast field echo (Phillips)

Diffusion weighted imaging (DWI)

DWI uses specific MRI sequences as well as a software that generates images from the resulting data, that uses the diffusion of water molecules to generate contrast in MR images.

Conventional (DWI)

This technique measure the Brownian motion of water molecules.

Main significance:

• Produces high signal within cerebral infarction (an area of tissue in the brain resulting from a blockage/narrowing in the arteries).

Apparent diffusion coefficient imaging (ADC)

Reduces T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion.

Main significance:

• Gives low signal minutes after cerebral infarction

Diffusion tensor imaging (DTI)

Produces tractography (3D model to visualise neural tracts), by an overall greater Brownian motion of water molecules in the directions of the nerve fibers.

Main significance:

• Evaluates white matter deformation by tumors

Perfusion-weighted imaging (PWI)

PWI sequence postprocesses data to obtain perfusion maps with different parameters, such as BV (blood volume), BF (blood flow), MTT (mean transit time) and TTP (time to peak).

Main significance of PWI:

• In cerebral infarction, the infarcted core and the penumbra have decreased perfusion (as show in picture).

Dynamic susceptibility (DSC)

Gadolinium contrast in injected, and rapid repeated imaging (generally gradient-echo echo-planar T2 weighted) quantifies susceptibility-induced signal loss.

Dynamic contrast enhance (DCE)

Measured the shortening of the spin-lattice relaxation (T1) induced by a gadolinium contrast bolus.

Arterial Spin labelling (ASL)

Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest. It does not need gadolinium contrast.

Functional MRI (fMRI)

Blood-oxygen-level dependent imaging (BOLD)

This techniques recognises the changes in oxygen saturation-dependent magnetism of hemoglobin reflects tissue activity.

Main significance:

• Localises highly active brain areas before surgery.

Magnetic resonance angiography (MRA) and venography

Time-of-flight (TOF)

Blood entering the imaged area is not yet magnetically saturated, giving it a much higher signal when using short echo time and flow compensation.

Main significance:

• Detects aneurysms, stenosis or dissections.

Phase-contrast MRA (PC-MRA)

PC-MRA determines flow velocitites. Two gradients with equal magnitude but opposite direction are used to encode a phase shift, which is proportional to the velocity of spins.

Main significance:

• Detects aneurysms, stenosis or dissections (as pictured).

Susceptibility weighted imaging (SWI)

This sequence is sensitive for blood and calcium, by a fully flow compensated, long echo, gradient recalled echo (GRE) pulse sequence to exploit magnetic susceptibility difference between tissues.

Main significance:

• Detects small amounts of hemorrhage or calcium.

Safety of MRI applications

Strength of magnetic field

A magnetic field of 1.5T is about 30,000 times the strength of the Earth’s magnetic field. It is so strong that it can pull stretchers, beds and oxygen cylinders across the room – this can create flying projectiles – deaths have occurred from traumas as a results of this!

The strong field can also effect devices such as pacemakers; patients who have a pacemaker are not allowed to undergo an MRI scan. Intracranial aneurysm clips are ferromagnetic, and as a result experience a torque. Several cases of a fatal hemorrhage have been reported when a patient with an clip has entered a magnetic field.

Gradient fields

Changing the magnetic fields induces electrical currents in conductors – patient with metal within their body have the potential for electrical currents to be induced in the metal, with subsequent heating. This may occur to metal foreign bodies or some surgical implants.

Quenching

Quenching is rapid expulsion of the liquid cryogen used to maintain the MR magnet in a superconducting state.

In the result of the emergency stop button being pressed or there is an equipment fault, the liquid helium boils off rapidly and a loud bang is given off. This gas should be safely expelled via a dedicated venting system. If somewhat the system fails and the gas is emitted into the room, there is a risk of asphyxiation and frost bite.

Being a Medical Physicist for the week..

Two months back I underdid a weeks worth of work experience at the Royal Stoke University Hospital, Stoke-on-Trent, within the area of Medical Physics. Since my GCSEs I knew Medical Physics was the area I wanted to specialise in, and so I decided to see for myself what the area consisted in. I had previously done a placement in the x-rays area during my first year at sixth form, but this time I wanted to branch out into the other areas.

So what exactly is Medical Physics?

Medical Physics is the applications of physics in medicine/healthcare. They help to ensure and develop equipment that was safe for use in both diagnostic and therapeutic healthcare, as well as developing new techniques, to help save patients’ lives. To become a Medical Physicist, you need a First or Upper Second Class Degree in Undergraduate Physics, and then to complete 3 years training through the NHS Scientist Training Programme (STP).
Medical Physics is usually split into 4/5 categories:
– Radiotherapy Physics
– Nuclear Medicine/Clinical Pharmaceutical Science
– Ionising Imaging
– Non-ionising Imaging
– Radiation Safety
A Physicist would specialise in one of these categories, in which they would of studied during their STP. Currently so far, I am leaning towards specialising in Radiotherapy or Nuclear Medicine, for reasons I shall mention later.
On my placement I was lucky enough to be shown around all these areas, and so I’m now going to explain what each area is about, and what I learnt from it.

Radiotherapy Physics

This aspect of Physics is the area of treatment for cancer using high energy radiation such as X-rays and Gamma Rays. The machines used for these treatments (as pictured) are called Linear Accelerators (LINAC). It is designed to deliver the high energy rays to the region of the patient’s tumour, in a way that destroys the cancerous cells without damaging the surrounding normal cells. The equipment works in such a way that electrons are accelerated close to the speed of light and to then collide with a heavy metal target. As a result of this collision, high energy rays are produced. These rays are shaped to as to when they exit the machines, it is directed towards the patient’s tumour. The machine rotates around the patient so the high energy treatment is given at different angles.

A Medical Physicist would be responsible for creating treatment plans for the patient, the precision and accuracy of treatment by determining how to deliver the prescribed dosage and calculate the amount of time it will take for the LINAC to deliver that dose. This is planned by using advanced computer  calculations (as seen in the image below).

 

Referring to the image, you can see how the 3 different angle of rays are used to treat the cancer. In this case the tumour is located in the lung and can be show within the red outline. There are 3 different rays shown; these are different angles the treatment will be administrated. The colours shown shows a spectrum – the purple/blue end show the area where less amounts of radiation if effected; the red/orange end show where large amounts of radiation are affected.

From working in this department I got shown around the planning and treatment areas. The planning starts in the office with the Physicists; when planning they work very closely with the Doctors and Radiotherapists.
When asked what they found which type of cancer is hardest to plan, I found that lung cancers can be the trickiest to plan/administrate. This is due to the fact the lungs are always moving and so therefore the tumour does not stay in one place. To overcome this, the patient’s breathing rate is monitored and bursts of radiation is given when the lungs are empty (the longest time the lungs are most still).
Other than the planning, the Physicists also weekly check the LINACs are working properly. This includes making sure the lasers are aligned, the radiation beam is aligned, testing the radiation dosage given matches the important, and the temperature given out.

I feel after my experience in radiotherapy, this is the area I would like to specialise in as there is a range of activities within the job, and no planning would be the same – there are different cancers in different places in different parts of the body.

Nuclear Medicine

This is an area of medical imaging where small amounts of radioactive material with relative short half-lives are used. They are put inside the patient and then a gamma camera records the radiation emitting from within the body, unlike x-rays that uses external sources.
Depending on the type of examination, the radioactive tracer is either injected into the body, swallowed, or inhaled as a gas; It then eventually accumulates in the organ or area of the body being examined. The gamma rays emitted from the radioactive tracer is then detected by a gamma camera that produces pictures of the area and molecular information.

 

 

 

A physicist once again has a range of jobs – this includes regular quality control of equipment, research and development of new techniques (to which physicists have a role of teaching and training to other members of the hospital staff), radiation protection work (to which I noticed this area in the hospital was the most I’d seen of thick lead equipment), and finally in some hospitals (such as Royal Stoke) physicists have a role to play in the radiopharmacy or the administration of radioisotopes to the patients before their scan. When speaking to the physicist, she stated that she enjoyed working in this area because she liked the patient interacting and explaining her knowledge of safety and the treatment being given to them. Another role is the advise what to do with the radioactive waste, and the quality standards of the radioactive material.

 

Radiation safety plays a large part in this area as it is very easy to be exposed to radiation as there is are radioactive elements there (as oppose to other areas where machines create the x-rays). As well as the thick lead equipment, gloves must be worn, lead aprons are worn, and everyone who has been in a room with a radioactive substance must scan themselves with a Geiger counter afterwards.

Another area that is similar to this area is working of a PET (positron emission tomography)scanner. This is fairly new to the Royal Stoke so the facilities were state of the art. I was explained how the treatment rooms were specially designed so that were facing in different directions. This is for the safety of the hospital staff and not so much the patients, as at the angles, the radiation is bounced at different directions and spread out, so staff walking past would not constantly be building up counts of radiation day in day out.

Imaging with Ionising Radiation

Imaging with radiation that is ionising means that the radiation has enough energy that it can interact with the atoms in your body. The types of machines/equipment is hospitals that use this type of radiation is:
– General X-rays
– CT Scans
– Fluoroscopy

General X-rays are used from anything from diagnosing broken bones, to being used in theatres to help with surgery. The rays used for medical purposes in x-rays are safer than in comparison to radiotherapy, because the dose of radiation is very small. The strength of radiation in relation to long-term risk is measured using units called millisieverts (mSv). Some examples of typical exposures are:

• chest X-ray – 0.02 mSv
• a year’s worth of medical tests – 0.4 mSv
• average annual exposure to natural radiation – 2.2 mSv

In the UK, 20 mSv is the maximum that someone who works with radiation is allowed to be exposed to in any given year. Radiographers and Medical Physicists wear film badge dosimeters to show what their exposure is/has been. Whilst on placement, I was lucky enough to go into Interventional Radiology, dress up in scrubs, and watch a couple of operations taking place. Despite medical physicists not working in this area, it was an interesting insight into seeing how x-rays are used in theatre.

CT (computed tomography) scans produce an image of a 2D image “slice” through the body. An x-ray beam of the same wavelength (monochromatic) is rotated around the body and picked up by the many detectors. This type of scan requires a higher does of radiation, however the benefits out way this.

Fluoroscopy shows a moving image of the body using a fluorescent screen and an image intensifier. This is useful for showing the organs as they work. Fluoroscopy can also be used in interventional radiology, for assistance when the surgeons are inserting catheters or other various tubes such as stents.

In this area, a Physicists role is to check the machines are working correctly – for example, the energy given out matches the value input. Using specialised equipment, you would also measure and calculate the doses of radiation received by patients during treatment. and by the staff delivering it. They will survey the working environment and monitor the performance of equipment to ensure that it is complying with the regulations.

Imaging with non Ionising Radiation

This is the area where the radiation is not harmful to humans. The main machines/devices used in this area for imaging are:
– MRI Scans
– Ultrasound

MRI (magnetic resonance imaging), uses strong magnetic fields and radio waves to produce the images of inside the body. It is used to create detailed examinations of practically any part of the body including the brain, spinal cords, heart & blood vessels, internal organs, and the joints & bones. The machine works via the interaction of the matter of the body, and the electromagnetic fields. Our bodies are mainly composed of the water, containing two hydrogen nuclei (H₂O), and whilst the scan is operating, these protons align with the direction of the field due to the magnetic moment. A radio frequency pulsation is applied, cause the protons to alter their magnetization alignment relative to the field.
MRI scans, however, are not suitable for people who with pacemakers, ICDs, nerve stimulators, metal implants, surgical clips, IUDs, and artificial joints. This is due to the strong magnets affecting the metal implants or fragments.

An ultrasound scan is when a small device is used, called a probe, that gives off high-frequency sound waves. This creates the sound waves, which is done by applying an electric current through piezoelectric crystals, causing them to change shape and vibrate, hence producing sound waves. The probe also receives the echoes which gives the picture of the area in scan. Ultrasound scans are harmless to us, so therefore are used for maternity scans to check up on the growing fetus.

For me, this is the area with the least interest for me. This is most probably because I am very interested in working with ionising radiation, as that is the area I enjoy learning about the most.