Breaking the dogma of magnetic field strength in MRI

Quietly, without much fuss, it seems that the high-field nerds will wake up in a downward trend. After several decades, low (<0.5 Tesla) and medium (0.5T - <1.5T) Polish MR machines1 they are pushing upwards in the markets.

The issue of field strength has divided the MR community since the early 1980s. At that time, all MR machines were working in low fields; many prototypes of the time had a power of approximately 0.15 tesla. Their image quality was poor. It improved to 0.5 tesla and 0.7 tesla.

Then some manufacturers, encouraged by their researchers and marketing people, took an MRI of up to 1.5 tesla with a high-field superconducting magnet system: “Increase the field strength and you’ll have nicer images.”

These systems were and still are huge machines like dinosaurs. They were expensive, difficult to manufacture, bulky to install, and expensive to maintain, but the image quality suddenly became better and more patients could be examined daily. Faster recording became one of the slogans.

At the time, Derek Shaw was one of the leading MR scientists in Europe. He has worked for several major MR manufacturers, including GE Medical Systems. In 1996, he wrote in the book’s chapter, “Without the pressure on the high field, MRI systems today could be quite different, probably lower on the cost / performance scale.”2

For manufacturers, health insurance companies and owners of magnetic resonance imaging, the high field meant higher profits, which is a constant topic not only in medical technology. After the introduction of 1.5-tesla machines, competition between different companies brought clinical 3-tesla MRI equipment.

High and extremely high Polish dogma was born and established. In some countries, small and medium terrain equipment has even been banned by ticklish regulations imposed by reimbursement agencies, although there has been strong evidence that low and medium field systems have some major advantages.3.4

The antithesis of the middle field

Meanwhile, it seems, the American, European and Asian money markets to go are creamy. Times have changed. The competition is tough. Complicated and complicated equipment no longer necessarily has to find uncritical users. New customers must be found. As far as everyone knows, new demands must be created, even if they need to go back from extravagance to frugality – and use common sense.

Low- and medium-field scanners had the disadvantage of lower image quality, which in the meantime was overcome by improvements in software and hardware in general and, for example, noise reduction. Here, phase-series coils and parallel recording helped to achieve a significant shift.

Since the relaxation of T1 is longer in higher fields (e.g., the T1 gray matter at 3-tesla is more than three times longer than at 0.3 tesla), averaging the data to increase signal strength is feasible in low and medium fields.

The inherent advantages of small and medium field machines include ease of installation and handling, as well as general patient acceptability. Low and medium fields are ideal for open MRI systems, which drastically reduces claustrophobia.

Open systems are suitable for interventional MRI. Moreover, there is minimal gradient switching noise compared to high and ultra high field devices (no danger of hearing damage to patients) and no perturbation of the vestibular apparatus leading to dizziness.

Since there is hardly a magnetic boundary field, a heavy shield is not needed to protect the environment from the magnetic field radiating from the system.

These systems are also less prone to artifacts: There are fewer metal artifacts and chemical shifts, reduced sensitivity, and dielectric effects. Penetration into tissues is better, and there is less deposition of radiofrequency power.

On the financial side, the prices of equipment with small and medium terrain are more appropriate than the prices for devices for large and ultra high terrains. Maintenance and energy costs are also lower. With the latest technology, helium replacement is unnecessary, eliminating the need to purchase and refill liquid helium at a permanently higher cost.

MgB2 superconducting wires and coils

The main step towards achieving superior diagnostic quality in small and medium fields was the invention of wires and coils using magnesium dibordide (MgB2). They have been created commercially for several years, eliminating the need for liquid helium and possible extinguishing.5 MgB2 machines need one liter of helium to keep their superconducting magnet cool, compared to hundreds of liters in old-field high-field machines.

They enable, for example, the production of superconducting open MR systems with easy access that work on 0.5 tesla with recording performance equal to high-field equipment. This development is a major challenge for existing high-field equipment, especially because the diagnostic quality of mid-range systems has already been described as competing with high-field and pre-introduction high-temperature superconducting coils.

The largest Italian manufacturer of magnesium dibordide wires has started to produce its own MRI equipment that can take images in any position, lying, standing, sitting, bending. The picture quality at 0.5 tesla is astonishing. The German company offers a mid-range donut machine for high-ground equipment. Other manufacturers will follow soon. The features of the new state-of-the-art mid-field technology have clear benefits for patients.

The science behind the contrast of images in different magnetic fields

As the commercial battles of the field power flared up in the 1980s, one of the most sophisticated research projects on tissue relaxation behavior by creating a nuclear magnetic resonance (NMRD) dispersion was carried out. This huge scientific effort remained unique, coordination and logistics were complex, no one ever repeated them.

The results did not overlap with commercial ideas and were deliberately overlooked in the race for higher fields.

It was an interdisciplinary project involving several universities and lasted more than two years, using the IBM Off-Road Cycling Spectrometer, a machine of which IBM’s research lab in New York State built only a few. This machine can change the strength of the magnetic field within seconds between ultra low and high fields to measure the relaxation times T1 that change with the strength of the field.

For brain studies, for example, samples of gray and white matter of the normal human brain from different anatomical locations of the brain were excised, within 24 hours after death, from patients who died from other neurological causes. Tissue samples weighing between 200 and 600 mg were transferred to sample tubes immediately after dissection, quickly deep-frozen, transported to the NMR laboratory on dry ice (-78.5 ° C) and stored in a deep freezer until rapid inspection. Samples were thawed at room temperature immediately before measurement.

Measurements of up to 1.5 tesla were performed on an NMRD relaxometer. Advantages of relaxation measurements ex vivo samples are extremely high measurement accuracy, selection of tissue that looks homogeneous with the ability to reject mixed tissue samples, and detailed histology available after measurement. Compared to NMRD data, calculating T1 using an MRI system is a rough estimate.

Relaxometry made it possible to determine the rates of longitudinal relaxation of numerous tissues and chemical compounds. The resulting dispersion profiles of nuclear magnetic relaxation made it possible to predict tissue contrast and contrast media efficiency at any field strength.6-11

T1 tissue does not show a monotonic increase with field strength. Characteristic data of transverse decomposition and longitudinal dispersion of relaxation for the main components of the human brain, ie gray and white matter, were observed. The white matter shows a dispersion not found in any other tissue. This is most likely caused by an additional relaxing process that occurs in myelin and which involves MR-invisible membrane lipids themselves. Due to this fact, the pure T1 contrast of normal brain tissue and pathological lesions (multiple sclerosis, astrocyte) increases from low field strength to a maximum between 0.3 tesla and 0.5 tesla MHz and then decreases.

Thus, the optimal T1 contrast for brain scans with a decent signal / noise can best be achieved around 0.5 tesla. As we wrote in a publication more than 30 years ago, “The consequences of this particular behavior are thought to be important for neurological MRI, adding a new element to the sometimes controversial issue of optimal field strength.”

Suddenly, these scientific results seem to make commercial sense as well.

References

1. EMRF (European Magnetic Resonance Imaging Foundation). Definition of field strength. In: Rinck PA. Introduction to magnetic resonance imaging in medicine. 2nd ed. New York: Thieme Medical Publishers. 1990. 12. | Rinck PA. Magnetic field strength. In: Rinck PA. Magnetic resonance imaging in medicine. Critical introduction. 12th Edition Board of Directors, Norderstedt, Germany. 2018. ISBN 978-3-7460-9518-9. 38.

2. Shaw D. From a 5 mm tube to a man. The subjects studied by NMR continue to grow. In: Grant DM, Harris RK. Encyclopedia of Nuclear Magnetic Resonance Imaging. Volume 1, Historical Perspective. Chichester: John Wiley and Sons. 1996, 623-624.

3. Hoult DI, Chen CN, Sank VJ. Dependence of MRI II on the field. Arguments concerning optimal field strength. Magn Reson Med 1986; 3: 730-746.

4. Posin JP, Arakawa M, Crooks LE, Feinberg DA, Hoenninger JC, Watts JC, Mills CM, Kaufman L. Hydrogen MR head imaging at 0.35 T and 0.7 T: magnetic field strength effects. Radiology. 1985; 157: 679-83.

5. Bertora L. MG magnets based on MgB2. in: Flückiger R, editor (s). MgB2 superconducting wires. Basics and applications. Hackensack, NJ, USA: World Scientific Publishing. 2016. 485-536.

6. Rinck PA, Muller RN, Fischer H. Contrast dependence on field and temperature in magnetic resonance imaging. RöFo Fortschr Röntgenstr 1987; 147: 200-206 (in German).

7. Rinck PA, Fischer HW, Vander Elst L, Van Haverbeke Y, Muller RN. Relaxometry of off-road cycling: medical applications. Radiology 1988; 168: 843-849.

8. Fischer HW, Van Haverbeke Y, Rinck PA, Schmitz-Feuerhake I, Muller RN. The effect of aging and storage conditions on excised tissue followed by longitudinal relaxation dispersion profiles. Magn Reson Med 1989; 9: 315-324.

9. Fischer HW, Rinck PA, Van Haverbeke Y, Muller RN. Nuclear relaxation of gray and white matter of the human brain: an analysis of field dependence and implications on MRI. Magn Reson Med 1990; 16: 317-334.

10. Muller RN, Vander Elst L, Rinck PA, Vallet P, Maton F, Fischer H, Roch A, Van Haverbeke Y. Significance of nuclear magnetic relaxation dispersion (NMRD) profile in the development of MRI contrast media. Invest Radiol 1988; 23, Suppl 1: S229-231.

11. Rinck PA, Muller RN. Field strength and dose dependence on contrast enhancement using gadolinium-based MR contrast agents. Eur Radiol 1999; 9: 998-1004.

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