When medical professionals refer to magnetic resonance (MR) scanners, they sometimes say the scanner is a 1.5T or 3.0T scanner. This is because scanners are frequently identified by their magnetic field strength. In terms of MR, T stands for tesla, a unit of measurement.¹ Tesla is the unit of measurement to define the magnetic flux density. This is a unit of measurement on the International System of Units, which is the metric system. One tesla is the same as one weber (the representation of magnetic flux) per square meter. One tesla is equal to 10,000 gauss. With higher tesla scanners, the magnet is stronger, both in general and within the bore of the machine. The magnet and its magnetic field is arguably the most important aspect of an MRI scanner. Across the MR industry, most scanners are 1.5T or 3.0T, however there are varying strengths below 1.5T and more recently, up to 7.0T.

The importance of the MRI magnet

Magnetic resonance imaging (MRI), as the name suggests, wouldn't exist without the magnet. Similarly, the magnetic field used by the scanner wouldn't exist without the magnet. The magnet field produced by Earth is 0.5 gauss.² The magnet field produced by the magnet in a 1.5T MRI machine is 15,000 gauss, meaning the magnet in a 1.5T scanner is 30,000 times stronger than that produced by the Earth. The scanner uses this strength to align the hydrogen nuclei and produce the images for a MRI exam. The scanner uses the magnet to generate a magnetic field, which causes the signal produced by a patient's body. The strength of the magnet directly affects the strength of this signal.

1.5T and 3.0T scanners

1.5T MRI is the standard imaging method for most routine scans. In some cases, the increased magnet strength of a 3.0T scanner is necessary. This is especially true in MRI of the prostate, MR spectroscopy, functional MRI and arterial spin labeling.³ Longer sequences at 1.5T can greatly improve the quality of images, while 3.0T provides clarity and better detail.⁴ 3.0T MR is more likely to have artifacts caused by noise.⁵ 1.5T requires longer scans to create clear images, while 3.0T takes a shorter amount of time because of the increased signal strength. 3.0T allows more patients to be scanned in the same amount of time for one scan on a 1.5T scanner.

The advantage of stronger magnets

Even stronger magnets than 3.0T could bring additional benefits, such as creating detailed images, as well as increasing T1 dispersion and chemical shift. At this point, 7.0T scanners are mostly used in research settings and have yet to make a big splash in the clinical setting. The 3.0T scanners provide similar advantages over 1.5T.

The signal-to-noise ratio (SNR) is incredibly important in MR. Higher SNR means higher image quality.⁶ The signal is what comes from the patients body during and MR which is received by the coils placed near the body part being imaged. The noise is caused by those coils as the vibrate in response to the magnetic field. With the higher magnetic field, the signal being read by the coils and transmitted to the computer is increased. This allows for a better image, because there is less obstruction due to the noise. Significantly stronger magnets could provide this increased SNR after spatial resolution correction.⁷

T1 dispersion refers to the variation in T1 magnetic field strengths which probes the behavior of macro-molecules in tissue. and is used as a contrast medium.⁸ This replaces the typical contrast medium for MRI which is gadolinium-based. T1 dispersion contrast is a form of magnetization transfer contrast (MTC). It is measured throughout a MR angiography (MRA), or a test that studies the blood vessels in a specific area of the body.⁹ MRA can be used to detect abnormalities and diagnose blood disorders. The increased T1 dispersion provided by 7.0T MRA may produce better information and quality about the studied blood vessels.

MR spectroscopy (MRS) is used to measure the difference in the resonance frequency of a nucleus within its chemical environment and the shift in that frequency caused by the magnetic fields.¹⁰ Traditionally, MRS has studied protons, because they are naturally abundant and highly sensitive to shifts in the magnetic field. MRS is used to analyze and diagnose abnormalities in the brain and with the central nervous systems. Utilizing stronger magnetic fields causes an increased chemical shift to study and, in turn, highlights these abnormalities more effectively.

1.5T, 3.0T and 7.0T MR scanners each have their own place in the medical imaging field. 1.5T continues to provide most routine exams with enough accuracy to help to diagnose and monitor diseases. When more detailed scans are needed, 3.0T provides these better images in less time. 7.0T, though still new, may be useful with its high SNR, better spatial resolution, and increased T1 dispersion and chemical shifts.

For more information, see "1.5T Compared to 3.0T MRI Scanners".

References

1. Rohit Sharma, et al. "Tesla (SI unit)." Radiopaedia. Web. 12 December 2018. <https://radiopaedia.org/articles/tesla-si-unit>.

2. "How Magnetic Resonance Imaging works explained simply." howequipmentworks.com. Web. 13 December 2018. <https://www.howequipmentworks.com/mri_basics/>.

3. William A. Faulkner. "1.5 T Versus 3 T." Web. 12 December 2018. <http://www.medtronic.com/mrisurescan-us/pdf/UC201405147a_EN_1_5T_Versus_3T_MRI.pdf>.

4. Eric Evans. "The Pros and Cons of 1.5T V. 3T MRI: One Size Does Not Fit All. Linkedin. 14 February 2018. Web. 12 December 2018. <https://www.linkedin.com/pulse/pros-cons-15t-v-3t-mri-one-size-does-fit-all-eric-evans/>.

5. Vikki Harmonay. "3T MRI vs. 1.5T MRI - Do You Know the Difference?" atlantisworldwide.com. 18 October 2016. Web. 12 December 2018. <https://info.atlantisworldwide.com/blog/3t-mri-vs-1.5t-mri>.

6. Daniel J Bell, et al. "Signal-to-noise ratio." Radiopaedia. Web. 12 December 2018. <https://radiopaedia.org/articles/signal-to-noise-ratio-1>.

7. Elisabeth Springer, et al. "Comparison of Routine Brain Imaging at 3 T and 7 T." Invest Radiol. August 2016; 51(8): 469-482. Web. 12 December 2018. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5704893/>.

8. Sharon E. Ungersma, et al. "Magnetic resonance imaging with T₁ dispersion contrast." Magnetic Resonance in Medicine. 3 May 2006. Web. 12 December 2018. <https://onlinelibrary.wiley.com/doi/full/10.1002/mrm.20910>.

9. "MR Angiography (MRA)." RadiologyInfo.org. 1 April 2017. Web. 12 December 2018. <https://www.radiologyinfo.org/en/info.cfm?pg=angiomr>.

10. Mauricio Castillo, Lester Kwock, and Suresh K. Mukherji. "Clinical Applications of Proton MR Spectroscopy." AJNR. January 1996; 17: 1-5. Web. 12 December 2018. <http://www.ajnr.org/content/ajnr/17/1/1.full.pdf>.