Magnetic resonance imaging

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Chapter 19 Magnetic resonance imaging

Juliet C. Semple

CHAPTER CONTENTS

Introduction 237
Types of magnet 238
MRI scanning unit 238
The physics bit 239
The coils 240
MRI pulse sequences 240
Image weighting 242
Signal-to-noise ratio 244
Spatial resolution 247
Artefacts 247
MRI contrast agents 250
MRI safety 250
Implants 252
Quenching of the magnet 253
Quality assurance 254
Advanced techniques 254
Advantages and disadvantages of MRI 254

KEY POINTS

image MRI works because of the Larmor equation which states that

image

where ω is the precessional frequency of a proton, γ is the gyromagnetic ratio and B0 is the strength of the magnetic field.

image The three main types of MRI magnet are superconducting, resistive and permanent.
image A radiofrequency (RF) coil picks up the MRI signal generated.
image The two main types of MRI pulse sequence are spin echoes and gradient echoes
image The main sorts of image weighting are called T1, T2 and proton density.
image The quality of the MRI images produced depends a lot on the signal-to-noise ratio and spatial resolution.
image MRI images are prone to many different types of artefact.
image The contrast agent used in MRI is called gadolinium.
image Safety is paramount in an MRI unit.
image MRI is continually developing and there are many new advanced techniques.

INTRODUCTION

An MRI (magnetic resonance imaging) scanner is found in most hospitals today. It is an extremely valuable investigative tool and is an essentialpiece of equipment in a modern radiology department. The MRI scanner can produce high quality diagnostic images of almost any part of the human body.

In June 1970 a medical doctor called Raymond Damadian used the technique of nuclear magnetic resonance (now called MRI) to differentiate healthy from cancerous tissue in mice. In March 1973, Paul Lauterbur, a professor of Chemistry at the State University of New York at Stony Brook published a paper in Nature showing that by using magnetic resonance (MR) it was possible to build up two dimensional pictures of structures that could not be visualised with other methods. Professor Sir Peter Mansfield from the department of physics at Nottingham University showed how MR signals could be mathematically analysed, which made it possible to develop a useful imaging technique. In the late 1970s and early 1980s a number of groups in the USA and UK, together with manufacturers, showed promising results of MRI in vivo. The first commercial MR scanner in Europe (from Picker Ltd.) was installed in 1983 in Manchester.

TYPES OF MAGNET

There are many types of MRI magnet and a multitude of manufacturers who make them.

T (or tesla) is the SI unit of magnetic flux density or magnetic induction.

This unit is used to describe the magnetic field strength of the MRI scanner. Another term you may hear is gauss. There are 10 000 gauss in 1 tesla. To give you an idea of how strong the magnetic field is in an MRI scanner, the Earth’s magnetic field is 0.5 gauss, whereas an MRI scanner in a hospital is commonly 15 000 gauss.

There are two types of magnet used in an MRI scanner: electromagnets and permanent magnets. Electromagnets can be either superconducting or resistive.

SUPERCONDUCTING MAGNETS

Superconducting magnets are the most frequently used type in hospitals. This type of magnet has a field strength of 0.5 T or upwards (1.5 T MRI scanners are normally used in a modern MRI department although 3 T scanners are becoming more common). These superconducting magnets need liquid helium to cool them. The magnetic field is always present with these scanners (24 hours a day, 7 days a week, and 365 days a year); in other words, it is present when the scanner is not scanning. The only way to stop the magnetic field is to remove the liquid helium from the system. In an emergency situation this can be done by ‘quenching’ the magnet – at the press of a button the helium is released from the system through a venting pipe to the outside world, resulting in a loss of the magnetic field.

RESISTIVE MAGNETS

Resistive magnets are electrically powered and therefore can be turned off. They have a field strength of up to 0.6 T.

PERMANENT MAGNETS

Permanent magnets have a field strength up to 0.3 T and also have a permanent magnetic field.

MRI SCANNING UNIT

The MRI scanning unit may well be a separate unit to the radiology department. It will consist of the MRI scanning room which houses the MRI scanner, the MRI control room where the console that controls the scanner is located and the MRI plant room which contains the electronics and equipment that make the scanner work. The entire unit will be behind locked doors as safety is paramount in an MRI environment.

TYPES OF SCANNER

Scanners can be closed or open. Closed systems are the most commonly used (Fig. 19.1). These scanners are tube shaped and, although called a closed system, have an opening at each end. This tube is called the bore. On a modern 1.5 T system the bore is likely to have a diameter in the region of 50–60 cm.

image

Figure 19.1 A closed MRI system. With permission of GE Healthcare.

Open systems are not tube shaped and tend to be more open, as the name implies (Fig. 19.2). They use two doughnut shaped magnets that are placed above and below the patient.

image

Figure 19.2 An open MRI system. With permission of GE Healthcare.

THE PHYSICS BIT

If a piece of card is placed over a bar magnet and then sprinkled with some iron filings, the filings line up with the magnetic field lines running from the north pole of the magnet to the south pole of the magnet (Fig. 19.3). The MRI scanner is just the same (however, the magnet is a lot stronger). The magnetic field lines run down the centre of the bore and extend around the sides of the scanner in the same pattern as the iron filings described above (Fig. 19.4).

image

Figure 19.3 Magnetic field with iron filings.

image

Figure 19.4 Scanner flux lines.

When the patient is placed into the scanner the magnetic field aligns all the hydrogen protons in his body in the same direction as the main magnetic field and they precess like a spinning top. The human body is made up of approximately 72% water, and as water is H2O there are a lot of hydrogen protons in the body. This is what makes MRI such a good diagnostic tool.

A radiofrequency or an additional magnetic field/gradient is applied to flip these protons from the longitudinal plane to the transverse plane. Then the protons are allowed to relax backto the longitudinal plane. When they relax back they emit a small radio signal. This signal is picked up in an antenna or coil. Different tissues emit different intensities of signal and, with some clever computational analysis, we can differentiate these different intensities and their position and therefore produce a picture. This follows the Larmor equation.

The Larmor equation states that

image

where

ω is the precessional frequency of a proton

γ is the gyromagnetic ratio

B0 is the strength of the magnetic field.

The frequency is measured in MHz (megahertz). The gyromagnetic ratio is a constant for each proton and the strength of the magnetic field is measured in tesla. So if the gyromagnetic ratio of hydrogen is 42.56 MHz, and if we were using a 1.5 T scanner, we could work out the precessional frequency of hydrogen as follows:

image

image

image

THE COILS

As mentioned previously, when the protons relax back they emit a signal that we pick up in a coil. These coils are called radiofrequency coils. The biggest coil of all is the MRI scanner itself. The scanner is made up of thousands of niobium–titanium coils wrapped around the bore. These coils produce the main magnetic field.

It is possible to do an MRI scan using the main magnetic field/coil but in practice smaller coils are used that plug into the scanner. There are coils for most body parts; a brain/head coil, a spine coil, a flex coil (for wrapping around small body areas), a torso coil, and a knee coil. There are also many specialised coils, like breast coils, neurovascular coils, TMJ (temporomandibular joint) coils and shoulder coils. If the MRI scanner is based in a specialist hospital, the coils will be specialised for the scanner’s workload; moreover, each coil differs functionally:

• Receive only coils – these use the main magnetic field to excite the protons and then the receive coil, as its name suggests, receives the signal.
• Transmit/receive coils – these transmit a radiofrequency (RF) pulse and then receive the signal back.
• Phased array coils – these have an array of elements that can pick up the signal. MRI radiographers can select which elements they wish to use to get the best pictures possible. It also means that the entire spine can be scanned without putting the patient on a separate coil.

MRI PULSE SEQUENCES

In MRI scanning there are different types of pulse sequence. The two main types are:

• spin echo sequences
• gradient echo sequences.

There are then many types of sequence within the two main types. Some of the more common spin echo sequences are fast/turbo spin echo (FSE/TSE), fluid attenuated inversion recovery (FLAIR) and short TI inversion recovery (STIR). Gradient echo sequences include steady state free precession (SSFP), spoiled gradient echo and coherent gradient echo. All of these pulse sequences take minutes to achieve. In MRI there is also a very fast sequence (a matter of seconds) called echo planer imaging or EPI. EPI scans can be spin echo or gradient echo sequences.

During a scan the hydrogen protons are flipped over and allowed to relax back. The resulting signal from this relaxation is picked up in the coil and generates a picture. The way these protons flip over varies from sequence to sequence. A variety of different radiofrequency (RF) pulses and magnetic gradients are applied at various points in time and at different angles to create the pulse sequence. The basic principles of each sequence will be discussed below.

SPIN ECHO (SE) SEQUENCE

A 180 ° pulse follows a 90 ° pulse and then the signal is generated (Fig. 19.5).

image

Figure 19.5 Spin echo.

FAST SPIN ECHO (FSE/TSE) SEQUENCE

A 90 ° pulse is followed by a series of pulses (normally a 180 ° pulse), with a signal being generated after each 180 ° pulse (Fig. 19.6). The number of 180 ° pulses is called the ‘echo train length’ (ETL).

image

Figure 19.6 Fast spin echo.

FLUID ATTENUATED INVERSION RECOVERY (FLAIR) AND SHORT TI INVERSION RECOVERY (STIR) SEQUENCES

A 180 ° inverting pulse is applied before the 90 ° pulse (Fig. 19.7).

image

Figure 19.7 Inversion recovery.

GRADIENT ECHO (GE) SEQUENCE

Gradient echo scans are very different to a spin echo sequence. A smaller RF pulse produces a small flip angle of less than 90 ° and, instead of another RF pulse, a magnetic gradient is then applied (Fig. 19.8).

image

Figure 19.8 Gradient echo.

PULSE SEQUENCE FACTORS

• TE (or echo time) is the time from the 90 ° pulse to the peak signal being generated in the coil. It is measured in milliseconds (ms).
• TR (or repetition time) is the time from the application of one RF pulse to the application of the next RF pulse. It is also measured in ms.
• TI (or inversion time) is the time between the 90 ° inversion pulse to the 90 ° pulse and is measured in ms.
• Flip angle is a small RF pulse.

These times are input into the scanner to create the pulse sequence. By changing the times for these factors we can produce images with different ‘weightings’. Weighting of an image determines the signal intensity of different tissues.

IMAGE WEIGHTING

The main image weightings you will use in MRI are T1, T2 and proton density (PD). Spin echo and gradient echo sequences can be T1, T2 or PD.

T1 WEIGHTED SPIN ECHO OVERVIEW

T1 weighted scans use a short TE and a short TR. Short TE is defined as less than 35 ms and a short TR as less than 800 ms. These figures vary between scanners and field strengths but the figures quoted are a good rule of thumb.

A T1 SE sequence, for example, could be achieved with a TE of 15 ms and a TR of 600 ms.

T2 WEIGHTED SPIN ECHO OVERVIEW

T2 weighted scans use a long TE and a long TR. A long TE is defined as more than 80 ms and a long TR as over 2000 ms.

So a T2 SE sequence, for example, could be achieved with a TE of 102 ms and a TR of 3000 ms.

PD WEIGHTED SPIN ECHO OVERVIEW

PD weighted scans use a short TE and a long TR. So an example of these parameters would be a TE of 20 ms and a TR of 3000 ms.

T1 WEIGHTED GRADIENT ECHO OVERVIEW

In the T1 GE sequence, flip angles are used as well as setting the TE and TR. To obtain a T1 weighted GE scan, a large flip angle in the order of 70–110 ° is used, plus a short TE of 5–10 ms and a short TR of less than 750 ms.

T2 WEIGHTED GRADIENT ECHO OVERVIEW

When discussing a T2 weighted GE scan it is referred to as T2* not T2. A T2* GE scan uses a small flip angle of 5–20 °, a long TE of 15–25 ms and a long TR of more than 100 ms.

PD WEIGHTED GRADIENT ECHO OVERVIEW

To obtain a PD weighted GE scan, a small flip angle in the order of 5–20 ° is used, plus a short TE of 5–10 ms and a long TR of more than 100 ms.

INTERPRETING IMAGES

Different tissues relax back at different speeds (fat for example is a lot quicker than water) so, depending on when in time the echo/signal is observed, different densities will appear on an image for the different tissues.

For example, on a T1 SE weighted scan, fat looks bright white and water dark black – so the fat in the scalp is seen as a bright area and the ventricles of the brain, which contain cerebrospinal fluid (CSF), look dark because CSF is a fluid like water (Fig. 19.9). On the T2 FSE weighted scan the opposite is seen – the fat is now darker and the fluid bright (Fig. 19.10). A PD scan looks at just the density of hydrogen protons and produces a very grey image that effectively is a proton density map (Fig. 19.11).

image

Figure 19.9 T1 brain.

image

Figure 19.10 T2 brain.

image

Figure 19.11 PD brain.

By knowing what different tissues are made up of and how they look on different image weightings a diagnosis can be made. For example, a scan of a patient’s brain may demonstrate a lump or possibly a tumour, but how does the radiologist know what the lump is? Well, what does it look like on T1 and T2?

The pathology shown in Figure 19.12 looks bright on T2 and dark on T1 so implies the lesion is made primarily of fluid. This is the case and the radiologist can suggest an arachnoid cyst as the likely diagnosis. So the radiologist can make accurate diagnoses knowing how differenttissues and pathologies look on different image weightings.

image

Figure 19.12 A T2 weighted image. B T1 weighted image.

SIGNAL-TO-NOISE RATIO

Signal-to-noise ratio (SNR) is the ratio of the amplitude of the signal received to the average amplitude of the noise. SNR is affected by the scanner hardware, the field strength of the scanner, the pulse sequences employed, the choice of radiofrequency coil and the parameters set by the operator. Good image quality is determined by good parameter selection, utilisation of the most appropriate coil and utilising the scanner capabilities. MRI scanning is one big parameter juggling and trade-off exercise. Parameters can be set that would produce a scan of exceptional quality but would take 20 minutes to run, by which time the patient would probably have moved. Parameters can also be set that ask too much of the system and create poor or, even worse, undiagnostic images.

FACTORS AFFECTING THE SNR

• Field of view (FOV) – the larger the FOV, the more signal generated, so increasing the SNR (Fig. 19.13).
• Matrix size – scans can be performed with varying matrix sizes. The matrix is the frequency encoding multiplied by the phase encoding. Examples could be 256×256 or 512×512. The 512×512 matrix will have fewer protons in each voxel (the 3D equivalent of a pixel) than the 256×256 matrix scan so therefore has less SNR (Fig. 19.14).
• Slice width – the smaller or thinner the slice, the smaller the signal generated, thereby reducing the SNR (Fig. 19.15).
• NEX – NEX means number of excitations (how many times the protons are excited in a slice). The more times the protons are excited the better the SNR (Fig. 19.16).
• Coil choice – the most suitable coil for the body part under examination needs to be selected (Fig. 19.17). Ideally, it needs to be as close to the area of interest as possible and cover the area under examination. Most modern MRI scanners now use multichannel coils, which increase the SNR.
• Sequence parameters – TR, TE and flip angle affect SNR. Spin echo sequences tend to have a better SNR than gradient echo sequences. Increasing the TR increases the SNR. Decreasing the TE increases the SNR.
• Bandwidth – the receive bandwidth is the range of frequencies sampled by the readout gradient (Fig. 19.18). As bandwidth increases, SNR decreases.
image

Figure 19.13 A Large field of view. B Small field of view.

image

Figure 19.14 A High matrix. B Low matrix.

image

Figure 19.15 A Thick slice width. B Thin slice width.

image

Figure 19.16 A High number of excitations (NEX). B Low NEX.

image

Figure 19.17 A 8-channel brain coil. B Body coil (the main magnetic field).

image

Figure 19.18 Bandwidth.

SPATIAL RESOLUTION

Spatial resolution is the ability to differentiate between two points as separate and distinct in the image. Good spatial resolution is needed to produce good quality images. Thin slices, high matrices and small FOVs are required in order to achieve good spatial resolution.

ARTEFACTS

As with most imaging modalities MRI is not exempt from the problem of artefacts, and probably has more to contend with than other modalities.

PHASE WRAP

Phase wrap is when an area of anatomy outside of the field of view is mapped (or wraps) inside the image. This occurs along the phase encoding direction. Changing the phase encoding direction or using the ‘no phase wrap’ option on thescanner can remedy this. In Figure 19.19 the nose is wrapped around the image and is now on the back of the head, not the front.

image

Figure 19.19 Phase wrap.

PHASE MISMAPPING

Phase mismapping is when an area of anatomy moves along a gradient during the scan. Examples would be eyes moving during a brain scan, swallowing during a neck scan and the chest wall moving during a scan of the thorax. The artefact produced is called ghosting and always occurs along the phase encoding direction. Selecting the correct axis for the phase encoding can, however, reduce it.

Another remedy is to use a saturation band to nullify the signal from the moving anatomy. For example, when imaging the lumbar spine an artefact can occur across the spinal cord from the moving contents of the abdomen. If a presaturation pulse/band is placed in front of the spinal column covering the abdominal contents, the artefact on the image is reduced (Fig. 19.20).

image

Figure 19.20 A With saturation band. B Without saturation band.

CHEMICAL SHIFT ARTEFACT

Chemical shift artefact is caused by the different precessional frequencies of fat and water. Precessional frequency is proportional to the strength of the main magnetic field. At low field strengths the difference between the precessional frequencies is not great, but at high field strengths the difference is enough to cause chemical shift artefact. An example is shown in Figure 19.21.

image

Figure 19.21 Chemical shift.

To reduce this artefact the biggest receive bandwidth should be used in conjunction with the smallest field of view possible.

CROSS-TALK

Cross-talk, or cross-excitation, occurs when the RF excitation pulse in an adjacent slice excites protons in the slice being scanned. To reduce the effects of this artefact it is best to keep a gap between slices of about 10% and try not to cross slices. An example of cross-talk is shown in Figure 19.22; the dark band across the image is where the cross-excitation has occurred.

image

Figure 19.22 Cross-talk.

MAGNETIC SUSCEPTIBILITY ARTEFACT

Magnetic susceptibility artefact is caused by the ability of a substance to become magnetised. The degree to which it becomes magnetised results in a difference in phase and precessional frequency and there is signal loss or void on the image. This primarily happens with metal. A patient who has a metal implant in the area being scanned will have images with magnetic susceptibility artefact. If at all possible the metal should be removed, but in the case of a metal prosthesis fixed within the body the only option is to choose pulse sequences that are less susceptible to this type of artefact. In general, SE sequences are less susceptible than GE sequences. An example of signal void or magnetic susceptibility is shown in Figure 19.23; there are black signal voids where this patient’s hips should be due to his bilateral hip replacements causing magnetic susceptibility artefact.

image

Figure 19.23 Signal void.

ZIPPER ARTEFACT

Zipper artefact is caused by extraneous RF entering the scan room. It results in a line across the image that looks like a zip, hence the name. It is caused by a gap in the RF cage that shields the MRI scan room. As long as the RF cage is intact this artefact should not be seen.

MRI CONTRAST AGENTS

In common with most imaging modalities, extra information can be gained by the administration of a contrast agent. In MRI a contrast media called gadolinium is used. Gadolinium is one of the ‘rare earth’ metals known as lanthanoids. The gadolinium is bound to a chelate called DTPA and Gd-DTPA or gadopentetate is formed. This is a water-soluble contrast agent that is commonly used in MRI. It is relatively safe to use, with a low anaphylactic risk and few other side effects, but as with all contrast media the radiographer should be aware of potential adverse affects and have the appropriate training to deal with these if they were to occur.

The administration of gadolinium affects the T1 recovery of tissue and shortens its T1 recovery time. On T1 weighted scans areas of contrast enhancement will appear bright white on the image. Contrast agents are taken up by tissues with an enriched blood supply (e.g. tumours and sites of infection). Some pathology has very characteristic patterns of contrast enhancement; for example, brain abscesses ring enhance and meningiomas tend to have uniform enhancement. The administration of contrast can therefore aid the radiologist with making a diagnosis. Because some tissues appear bright on a T1 weighted scan without the administration of gadolinium (e.g. fat) a T1 weighted scan is performed pre and post the administration of the contrast. The pre contrast image can then be compared to the post contrast image to see which tissues are really enhancing (Fig. 19.24).

image

Figure 19.24 A Pre-administration of gadolinium. B Post-administration of gadolinium.

MRI SAFETY

The most important issue when undertaking MRI scanning is MRI safety. This point cannot be emphasised enough. To reiterate, the magnetic field is present 24 hours a day, 7 days a week, 365 days a year, even when the scanner is not performing a scan and is silent. All personnel and patients entering the MRI scan area should complete an MRI safety questionnaire. Every MRI unit will have their own safety form but it will cover the same main safety issues (Fig. 19.25). Most MRI safety issues relate to the main magnetic field and the time varying magnetic field (the switching of gradients and RF pulses during scanning).

image

Figure 19.25 MRI safety form.

PROJECTILE EFFECT

On a modern 1.5 T MRI system the strength of the magnet will accelerate a ferromagnetic object to 40 mph. Just think of the damage even a 1 p coin could make if it hit an individual at 40 mph; the result of an oxygen cylinder hitting at 40 mph would probably be fatal. This is called the projectile or missile effect.

METAL OBJECT REMOVAL

All metal objects need to be removed from everybody (including radiographers, patients anddoctors) before entering the MRI scan room. This includes coins, keys, scissors, stethoscopes, hair clips, cigarette lighters, etc. The list is endless, so the safest thing is to remove everything; some units even require all patients to wear only a hospital gown to stop anything metal being accidentally overlooked. Other objects like bankcards or mobile phones need to be removed because although they might not be attracted to the magnet the magnetic strip on the cards will be wiped and the components of the phone ruined.

IMPLANTS

Implants that should not be exposed to a high magnetic field

• Pacemaker
• Cochlear implant
• Aneurysm clip
• Some heart valves
• Neurostimulators

Anyone (patient or staff) who has any of the preceding implants should never enter the MRI scan room. Death could result due to the failure of the implant.

INTRAOCCULAR FOREIGN BODIES

It is also necessary before entering the scan room to check for intraoccular foreign bodies, usually caused from high speed welding, grinding or metalwork. Even a small amount of metal in the eye can be pulled by the magnetic field and result in damage to the eye or, even worse, blindness. Patients who need to undergo an MRI and have a previous history of metal going into their eye can be screened with an orbital X-ray; if the X-ray is clear then an MRI scan can be performed. It is also necessary to check for shrapnel or bullets. These are not always a contraindication for MRI but the extent and site of the injury need to be fully assessed before entering the scan room.

OTHER IMPLANTS

Most MRI centres will also check for other metal implants/prostheses. Anything that can be removed should be, even if it is not magnetic. Metal can heat up sufficiently during an MRI scan (because of the current induced in it) to cause burns. Jewellery should be removed before undertaking a scan. Also, if the metal is in the area of interest it will cause signal void/magnetic susceptibility artefact on the images. It is safe to have a scan with most orthopaedic implants (e.g. a hip replacement) but most MRI centres tend to wait 8 weeks after surgery before scanning.

PREGNANCY AND BREASTFEEDING

MRI does not use ionising radiation and essentially there are no biological affects. Routinely, most centres will not scan patients in the first trimester of pregnancy; however, if the gain outweighs the very low risks a scan will be performed. The pregnant patient should make her own decision. Most centres would not administer gadolinium to the pregnant patient. It is worth noting that gadolinium passes across the breast into breast milk, so if a patient who is breast-feeding requires a scan they need to express breast milk prior to the examination and refrain from breast-feeding for at least 24 hours.

IMPLANT SAFETY SHEETS

The main advice if there is uncertainty as to whether someone or something is safe to go into an MRI scan room is DON’T go into the scanner without seeking further advice. Most manufacturers of implants produce safety sheets that state their suitability for MRI scanning. The ultimate responsibility regarding who and what goes into the MRI scan room rests with the MRI radiographer.

NOISE

Having an MRI scan is a very noisy procedure. The magnetic field itself is silent, and when the scanner is not scanning a soft ‘chirping’ sound in the scan room is heard, which is the compressor pumping the liquid helium around the scanner. However, when the scanner is performing a scan the noise is literally deafening. The noise is the magnetic gradients switching very quickly. All patients and personnel who remain in the scan room have to wear ear protection. Earplugs are perfectly adequate for the job but most modern scanners have a stereo system attached to some MRI compatible headphones so the patient can listen to music whilst also protecting their ears.

SPECIFIC ABSORPTION RATE

RF can heat tissues within the patient. There are limits to the heating of tissues. The specific absorption rate (SAR) is measured in watts kg−1 and is how much heat the tissue can dissipate. Levels are set to protect the patient, and most modern scanners will not let these levels be exceeded. However, the scanner can only work this out by the patient’s weight so it is important that an accurate weight is recorded when registering patient details into the scanner.

INDUCING CURRENT IN LOOPS

Any looped wire or lead in a moving/switching magnetic field can have current induced in it. This current is enough to make the wire or lead heat up and burn the patient. It is important that leads or wires (e.g. MRI compatible ECG leads) do not touch the patients’ skin directly and are not coiled up. A small piece of gauze or a foam pad can be placed between the lead and the skin to avoid contact.

OTHER SAFETY CONSIDERATIONS

MRI compatible trolleys and wheelchairs that are not magnetic and are suitable for use in the scan room can be purchased. Also, most modern scanners have either undockable tables or tabletops that allow you to transfer the patient from a non-compatible bed or chair onto the scanner table outside the scan room and then move them into the scan room. A sick patient may need to be monitored or have drugs administered during the scan. Remember, equipment cannot go into the scan room unless it is MRI compatible. Various companies make compatible anaesthetic and monitoring equipment but it is very expensive. Some units prefer to use conventional equipment outside the scan room with extra long leads and connections. General anaesthetic scans are possible in the MRI scanner but are not to be undertaken lightly. Experienced personnel, the correct equipment and strict policies should be in place to ensure patient safety at all times. MRI scanners can be very isolated departments and the patient is remote to the radiographers and doctors.

The MRI scan unit should be locked and only trained personnel given either the key or code to the door. Anyone else entering the unit should be screened fully before being allowed to enter and should be supervised at all times.

QUENCHING OF THE MAGNET

If the worst happens and a metal object does enter the scan room, is stuck to the magnet and cannot be pulled off, the only way to extricate it is to remove the main magnetic field. To do this the magnet has to be quenched. There is a button in every MRI scanner to quench it. It is usually underneath a protective cover so that it does not get pressed by accident. When the quench button is pressed it very quickly releases all the helium in the scanner. It should be vented through a pipe from the scanner to the outside world. The magnetic field will then collapse so the object can be removed from the scanner. The release of the helium can also cause problems; for example, if there is a blockage in the pipe, the helium will enter the scan room, suffocating anyone inside – all MRI units should be fitted with an oxygen sensor that will set off an alarm if such an event happens. On hearing the alarm all personnel should be evacuated from the scan room. Quenching is something that should only be done in anemergency or life-threatening situation because the magnet can be damaged due to the heat generated in the coils and the cost of replacing tens of thousands of litres of liquid helium is expensive.

QUALITY ASSURANCE

MRI scanners, like all pieces of radiology equipment, need to have Quality Assurance or QA tests performed on them regularly. Different units will perform different tests specific to their unit but the most common test is an SNR test. This can be performed on a phantom, which is a plastic ball filled with fluid. Ideally this test should be done weekly on each coil the unit possesses.

ADVANCED TECHNIQUES

There are many advanced techniques that can be performed on an MRI scanner. A few of these are

• diffusion
• perfusion
• angiography (contrast enhanced and non-contrast enhanced)
• functional MRI (or fMRI)
• spectroscopy.

ADVANTAGES AND DISADVANTAGES OF MRI

ADVANTAGES

• Excellent soft tissue detail.
• Does not use ionising radiation.
• Multiplaner (can scan in all planes without moving the patient).

DISADVANTAGES

• Long scan times.
• Expensive.
• Not safe/suitable for all patients.
• Claustrophobic for some patients.

Bibliography

Marieb EN. Human anatomy and physiology, 3rd edn. California: Benjamin/Cummings, 1995.

Moses KP, et al. Atlas of clinical gross anatomy. Philadelphia: Elsevier Mosby, 2005.

Thibodeau GA, Patton KT. Anatomy and physiology, 5th edn. Missouri: Elsevier Mosby, 2003.

Van Wynsberghe D, Noback CR, Carola R. Human anatomy and physiology, 3rd edn. New York: McGraw-Hill, 1995.

Waugh A, Grant A, Ross, Wilson. Anatomy and physiology in health and illness, 9th edn. Edinburgh: Elsevier, 2002.

Recommended reading

Westbrook C, Kaut Roth C, Talbot J. MRI in practice, 3rd edn. Oxford: Blackwell Publishing, 2005.

This text provides a clear introduction to MRI. The text is well laid out with clear diagrams..

Westbrook C. MRI at a glance. Oxford: Blackwell Science, 2002.

This provides clear, concise information on MRI physics..

McRobbie DW, Moore E, Graves M, Prince M. MRI from picture to proton, 2nd edn. Cambridge: Cambridge University Press, 2007.

This text approaches the theory as it is encountered when working in an MRI department from the image initially through to the physics at the very end..

Waugh A, Grant A, Ross, Wilson. Anatomy and physiology in health and illness, 9th edn. Edinburgh: Elsevier, 2002.

This text is currently used by first-year students on our radiography programme, as the overall content provides just the right depth in detail, and it has a good reader-friendly structure with clear illustrations..

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