Low-Field MR Imaging — Development in Finland

Abstract

The development project for application of MR imaging to diagnosis of internal hemorrhages was initiated by the Instrumentarium Corporation in 1978. The goal was to develop a diagnostic tool for emergency clinics. Due to the rapid development of imaging technology, the goal was changed to a cost-effective MR unit. During the past 16 years, several generations of low-field units have been introduced. Consequently, a vast amount of clinical and technical knowledge about low-field MR has been gained. The interest in low-field units is rapidly increasing. A part of this may be explained by the pressure to reduce the cost of health care. There are some features which make the low-field approach clinically interesting. These include the feasibility of open magnet configurations, and the availability of unique contrast parameters such as magnetization transfer and T1p. One important aspect is the inherent safety of a low-field MR unit. This article reviews the methods and devices introduced through the development of low-field technology in Finland.

Utilization of Low-Field MR Scanners

“…The signal-to-noise ratio also depends on the properties of the object to be imaged and the scanning instrumentation. 1 The higher signal-to-noise ratio of a high-ˆeld system results in improved image resolution. Highˆeld strength results in images with higher spatial resolution [high-resolution images with a smaller FOV (ˆeld of view) and thinner slices] and higher temporal resolution (high-speed multiple sub-second images).…”

“…26 RF irradiation involves much smaller risks during low-ˆeld MR imaging than during high-ˆeld MR imaging. 1 Similarly, acoustic noise is a less-serious problem at lowˆeld strength than at highˆeld strength. 1 The motion of large nonferromagnetic metallic implants within the magneticˆeld of MR scanners can induce voltages and currents within the implant of such magnitude and direction as to oppose the primary motion vector.…”

“…1 Similarly, acoustic noise is a less-serious problem at lowˆeld strength than at highˆeld strength. 1 The motion of large nonferromagnetic metallic implants within the magneticˆeld of MR scanners can induce voltages and currents within the implant of such magnitude and direction as to oppose the primary motion vector. In other words, during the motion of even nonferromagnetic metallic implants, forces may be generated that oppose the motion of the implant within the static magnetiĉ eld of the MR scanner.…”

“…The borders between the categories are somewhat vague. 1 In this paper, lowˆeld strength is deˆned as less than 0.5T; middleˆeld strength as between 0.5 and 1.0T; and highˆeld strength as more than 1.0T.…”

Mutual benefit achieved by combining ultralow-field magnetic resonance and hyperpolarizing techniques

“…Therefore, low field (<0.5 T) or ultralow-field (ULF, <10 mT) MRI and MRS experiments have been performed for over 20 years. 4 Besides the cost advantage, ULF MRI offers very interesting prospects like the implementation of hybrid systems that allow interleaved MRI and magnetoencephalography (MEG) a) Author to whom correspondence should be addressed: kai.buckenmaier@ tuebingen.mpg.de measurements, [5][6][7][8] imaging in the vicinity of metals 9 or drastically greater differences of the longitudinal relaxation time constants (T 1 ) of different tissue types, 10,11 thus enhancing contrast. For ULF MRI, the resolution limiting noise level of the system is determined by sensor noise instead of the sample noise as is the case for high field MRI.…”