Chapter Three
Magnetic Resonance Instrumentation

03-01 Essentials

ost pioneers in the field of magnetic resonance either built their ma­chi­nes themselves or modified existing equipment. In the 1950s, Erik Ode­blad made his ground-breaking NMR measurements of tissues with a specially adapted spectrometer and in the 1970s Paul C. Lauterbur de­ve­lo­ped the idea of a whole-body imaging system with a design of his own (Figure 03-01a).

Figure 03-01a: A sketch of a possible magnet con­fi­gu­ra­tion for medical zeugmatography. Graphic depiction from 1978 of what would become the first whole-body MR apparatus at Paul C. Lauterbur’s laboratory [⇒ Lai, Hou­se, Lauterbur]. The magnetic field was to be created by a resistive magnet system.

There is a wide variety of MR imaging systems and technologies. The ex­ten­sive range of MR systems can be confusing for the potential buyer. There­fore, they should identify their specific needs.

Analytical NMR and MR imaging systems are very similar in their basic com­ponents. However, imaging machines additionally require gradient coils and Fa­ra­day shielding which protects the equipment against undesirable interference by radio waves from broadcasting stations transmitting on, or close to, the re­so­nance fre­quen­cy (Figure 03-01b).

Figure 03-01b: One of the first commercial prototypes of an MR machine in 1984, based on a re­sis­tive magnet with a Faraday cage around the patient table. André Luiten, one of the early MR scientists at Philips, is standing next to it.

03-01-02 Components of an MR Machine

Any MR imaging equipment includes the following elements (Figures 03-02 and 03-03):

 a magnet large enough to accommodate the sample to be
  examined (mouse or patient);
 gradient coils and electronics;
 RF-pulse transmitter and RF receiver;
 power supplies and cooling systems;
 a data acquisition and processing system, including a
  powerful computer;
 operation and evaluation console(s).

A typical layout of an imaging system is depicted in Figure 03-03; in this case a mobile MR imaging unit is shown.

Figure 03-02:
The main components of an MR imaging system.

Figure 03-03:
Complete superconducting magnetic resonance imaging system (in a trailer). All necessary sys­tems and subunits have been accommodated in limited space.

The central part of the MR machine is the magnet whose quality depends on three main criteria – it should create a static, stable, and homogeneous magnetic field.

A static field does not vary over time. The earth's magnetic field is a static field, as is the field around a bar magnet. Both fields are also stable, which is a condition also required of a magnet used for MR imaging. The static magnetic field at one end of the sample to be studied must be exactly the same as at the other end: the field must be homogeneous.

03-01-02 Magnetic Field Strength

MR imaging systems are generally classified according to their magnetic field strength. The field strength can differ by several hundred percent according to the purpose of the equipment.

Table 03-01 gives an overview of the classification.

Table 03-01: Definition of field strength. Definitions set by EMRF in 1989. Definitions in publications on MR topics and in sales brochures change according to commercial and scientific marketing schemes and prevailing scientific dogma.

An old topic in magnetic resonance imaging, still always fashionable:
The field-strength war.

Theoretically, one could perform MR studies at the earth's magnetic field – which has been proposed and done [⇒ Béné]. Of course, the performance of equip­ment used at such low magnetic field is poor. More sophisticated ap­pro­a­ches have been published by groups working at ultralow fields in the μT-range (ULFMRI) [⇒ Inglis, ⇒ Kraus]. ULFMRI does not yet have sufficient spatial and temporal resolution.

Lauterbur's first whole body system operated at a field strength of 0.09 T. Ul­tra­low equipment (below 0.1 T) is hardly used any more, most clinical ma­chi­nes ope­ra­te at medium and high fields, but there is a trend towards ultrahigh re­search ma­chi­nes, operating between 3 T and 14 T.

There is no optimum field strength for MR imaging or other techniques such as functional imaging. The diverse nature of applications requires different sys­tems operating at an appropriate field; a single perfect or ideal field strength for all cli­ni­cal indications and/or research questions cannot be set.

For further details on possible hazards and side effects of magnetic fields, espe­cial­ly at field strengths beyond 2 T, refer to Chapter 18.