6 The MR Scanner
6 The MR Scanner
All major components of an MRI system have now been mentioned. They are (▶ Fig. 27):
– A strong magnet to generate the static magnetic field (B0).
– A gradient system consisting of three coils to produce linear field distortions in the x-, y-, and z-directions and the corresponding amplifiers.
– A radiofrequency (RF) transmitter with a transmit coil built into the scanner.
– A highly sensitive RF receiver to pick up and amplify the MR signal.
Alternatively, imagers may use a single RF coil switched between the transmit and receive modes.
– Additional coils, either receive coils or transmit/receive coils.
– Various computers for controlling the scanner and the gradients (control computer), for creation of the MR images (array processor), and for coordinating all processes (main or host computer, to which are con- nected the operator’s console and image archives).
– Other peripheral devices such as a control for the patient table, electro- cardiography (ECG) equipment and respiration monitors to trigger specialized MR sequences, a cooling system for the magnet, a second operator’s console (e.g. for image processing), a device for film expo- sure, or a PACS (picture archiving and communications system).
Fig. 27. The major components of an MR scanner
6.1 The Magnet
The main magnetic field generated by the magnet must have the following features:
– An adequate strength, which typically ranges from 0.1 to 3.0 T in medical MR imaging.
– A high stability without fluctuations in field strength.
– The best homogeneity possible with a uniform strength throughout the entire field and without “holes”. Field homogeneity is usually expressed in ppm relative to the main field over a certain distance. Inhomogenei- ties throughout the scan volume should be below 5 ppm (0.0005%).
6 The MR Scanner
Three types of magnets are distinguished:
– Resistive magnets are conventional electromagnets that depend on a high and constant power supply to create a magnetic field. The maximum field strength generated by resistive magnets is about 0.3 T. Their major disadvantages are the high operating costs due to the large amounts of power required and a field homogeneity that is often poor. An advan- tage is the safety of the system as the field can be turned off instantly in an emergency.
– Permanent magnets consist of ferromagnetic substances and create a magnetic field that is maintained without an external power supply.
However, permanent magnets are very heavy, can generate a field with a maximum strength of only 0.5 T, and rely on a constant external temperature.
– Superconducting magnets consist of a coil made of a niobium-titanium (Nb-Ti) alloy whose resistance to current flow is virtually eliminated when cooled to near absolute zero (about 4°Kelvin or –269°C). In this superconducting state, which is achieved using coolants known as cryogens (usually liquid helium), a current once induced flows practi- cally forever. Once the magnetic field has been established, it is main- tained without additional power input. Very strong and highly homoge- neous magnetic fields of up to 18 T can be generated using super- conducting magnets. However, liquid helium evaporates and must be resupplied regularly. In an emergency it is not possible to simply switch off the magnet. About 95% of all MR systems used today have super- conducting magnets. A quench refers to a magnet’s sudden loss of superconductivity with subsequent breakdown of the magnetic field and may be induced by very minute movements of the coil. Due to the frictional energy released by this process, the coil temperature rises above the superconductivity threshold and the coils suddenly develop resistance. The current passing through an area of elevated coil re- sistance creates heat, which causes a sudden boiloff of cryogens. The risk of quenches is reduced by insulation of the Nb-Ti with an extra copper winding. Magnetic quenches are serious events but have become rare with the use of state-of-the-art magnet technology.
Magnetic field homogeneity is a primary consideration in medical MRI, regardless of the magnet used. To achieve an optimal homogeneity, it is often necessary to make adjustments known as shimming. This is done either pas- sively by placing pieces of sheet metal at certain locations within the magnet bore and on the outer surface of the scanner or actively by the activation of specialized coils of which over 20 may be present in a scanner.
Another important aspect is shielding of the magnet, which serves to control the fringe fields external to the magnet. In the past, fringe fields were contained mainly by incorporating large amounts of iron into the walls and the ceiling of the scanner room (10–20 tons!). Because of weight and expense, this form of shielding is increasingly being abandoned and mag- nets with integrated or active shielding are used instead. Actively shielded magnets have a double set of windings of which the inner one creates the field while the outer one provides return paths for the magnetic field lines.
6.2 The Gradient System
Magnetic field gradients are applied for slice selection and spatial encod- ing (▶ Chapter 4). A set of three separate gradient coils, each with its own amplifier, is needed to alter the magnetic field strength along the x-, y-, and z-axes. These are switched on separately or in combination, e.g. to define an oblique slice. The isocenter is the geometric center of the main magnetic field, where the field strength is not affected by any of the three gradients.
The gradient coils generate magnetic fields that are small compared with the main field but nevertheless need a current of several hundred amperes.
The changing magnetic fields generated when the gradients are switched lead to the typical banging sound heard during an MR scan. Similar to a loudspeaker, which is nothing but a coil inside a magnetic field, the gradient coils “try to move” when the current is switched on and off, which causes a noisy clanging.
Despite the high currents, the gradient fields must be extremely stable in order to prevent image distortions. Moreover, it has been shown for gradi- ent coils as well that actively shielded coils (▶ Chapter 6.1) are superior to the simpler versions: with smaller fringe fields, there is less external RF in- terference (induction of so-called eddy currents, ▶ Chapter 13.7).
Gradient performance is measured by three parameters:
– Maximum gradient strength (in units of mT/m) – Rise time – time to maximum gradient amplitude – Slew rate – maximum gradient amplitude/rise time
6 The MR Scanner
6.3 The Radiofrequency System
The radiofrequency (RF) system comprises a powerful RF generator (the Larmor frequency at 1.5 T is 63.8 MHz, which is in the range of FM trans- mitters) and a highly sensitive receiver. The stability of these two compo- nents is crucial: as both the frequency and the phase of the signal are needed for spatial encoding, any distortions, e.g. by phase rotation introduced by the receiver, would result in a blurred image. Moreover, to adequately detect the weak MR signal, effective RF shielding of the scanner room is necessary to prevent interference from external sources. This can be achieved by hous- ing the magnet in a closed conductive structure known as a Faraday cage.
The RF subsystem also includes the transmit and receive coils. These may be combined coils acting as both transmitters and receivers such as the body coil which is integrated into the scanner. It is not visible from the outside and consists of a “cage” of copper windings encircling the patient.
The RF transmitter serves to deliver pulses that correspond to the resonant frequency of hydrogen atoms.
As discussed in ▶ Chapter 5, the SNR can be modulated by employing coils other than the body coil. Careful coil selection according to the anat- omy being imaged is important for optimizing image quality.
6.4 The Computer System
The computers of an MRI system control and coordinate many processes ranging from turning on and off gradients and the RF coils to data handling and image processing.
References
McFall JR (1997) Hardware and coils for MR imaging. In: Riederer SJ, Wood ML (eds) Categorical course in physics: The basic physics of MR imaging. RSNA Publi- cations no 41, Oak Brook
1.
