Magnetic resonance imaging

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Chapter 19 Magnetic resonance imaging

Juliet C. Semple

MRI works because of the Larmor equation which states that

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

The three main types of MRI magnet are superconducting, resistive and permanent.

A radiofrequency (RF) coil picks up the MRI signal generated.

The two main types of MRI pulse sequence are spin echoes and gradient echoes

The main sorts of image weighting are called T1, T2 and proton density.

The quality of the MRI images produced depends a lot on the signal-to-noise ratio and spatial resolution.

MRI images are prone to many different types of artefact.

The contrast agent used in MRI is called gadolinium.

Safety is paramount in an MRI unit.

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 essential piece 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.

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.

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).

Figure 19.3 Magnetic field with iron filings.

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 back to 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

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:

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).

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).

Figure 19.6 Fast spin echo.

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

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