TwinTree Insert


Chapter Three
MR Imaging Instrumentation

03-01 The 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 (Figures 03-01 and 03-02).

Figure 03-01:
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.

Figure 03-02:
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.

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

spaceholder redThe central part of the MR machine is the magnet. Its 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.

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.

03-01-01 The MR Machine

Any MR imaging equipment includes the following elements:

spaceholder darkbluea magnet large enough to house the sample to be examined (mouse or patient);

spaceholder darkbluegradient coils and electronics;

spaceholder darkblueRF-pulse transmitter and RF receiver;

spaceholder darkbluepower supplies and cooling systems;

spaceholder darkbluea data acquisition and processing system, including a powerful computer;

spaceholder darkblueoperation and evaluation console(s).

A typical layout of an imaging system is depicted in Figures 03-03 and 03-04; in this case a mobile MR imaging unit is shown.

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

Figure 03-04:
Complete superconducting magnetic resonance imaging system (in a trailer). All necessary sys­tems and sub­units have been accommodated in limited space.

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).

Table 03-01:
Definition of field strength, set by EMRF in 1989.
Up to 2 Tesla, there are only minor side effects of the magnetic field.

Theoretically, one could perform MR studies at the earth's magnetic field — which has been proposed and done [⇒ Béné 1972]. 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- and mT-range (ULF) [⇒ Inglis 2013, ⇒ Kraus 2014, ⇒ Sarracanie 2015]. 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. Such ul­tra­low equipment (below 0.1 T) is hardly used any more. Most clinical ma­chi­nes ope­ra­te at medium and high fields. For specialized applied research there is a trend towards ultrahigh field (UHF) whole-body ma­chi­nes, operating between 3 T and 14 T. Research machines with small bores for animal studies operate at even higher fields [⇒ Rinck 2021].

There is no optimum field strength for MR imaging for clinical diagnostics. 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.

There is an increased incidence of physiological hazards in the red area of Table 03-01. For further details on possible hazards and side effects of magnetic fields, espe­cial­ly at field strengths beyond 2 T, refer to Chapter 18.

spaceholder red

An old topic in MR imaging, yet always fashionable:

inkpot The field-strength war

spaceholder red