MRI 1.5T and 3T What is the difference?

The strength of the MR scanner is represented in Tesla (T). Tesla is the SI unit of magnetic flux density. The cgs unit of measurement of magnetic field is gauss (G). One Tesla is equal to 10,000 gauss. It is the measure of the main magnetic field generated by the MR equipment. The main magnetic field of a 1.5T scanner is about 30,000 times stronger than earth's magnetic field. The main magnetic field of 3T is 60,000 times the strength of earth's magnetic field. 1.5T and 3T magnets are common in clinical use. MR signal generated by the scanner increases with the strength of magnetic field . The signal strength is almost double in 3T compared to 1.5T scanner. The signal to noise ratio (SNR) increases with magnetic field strength. The extra signal generated in a 3T scanner can be utilised to reduce the scan time or to increase the spatial resolution of the picture. Other than the physical strength of the magnet there are some differences in the image appearance between 3T and 1.5T.

Chemical shift

Chemical shift is due to the difference in resonance frequencies of water and fat protons in the tissue.Chemical shift effect is prominent in 3T compared to 1.5T. This phenomenon is advantageous in MR spectroscopy imaging due to high spectral resolution. Spectral resolution is the ability to visualise the changes in metabolite peaks. Water and fat peaks are distinct and which will helps in good fat saturation. The increased chemical shift is a disadvantage for muculoskeletal imaging where the cartilage and bone interface show shading effect. Increasing the bandwidth can reduce chemical shift effect.

Susceptibility effect

Magnetic susceptibility is more prominent in 3T compared to 1.5T. It is the ability of a material or tissue to be magnetised in a given magnetic field. The higher susceptibility is beneficial in functional MRI (fMRI). Metal implants show higher susceptibility at 3T fields and susceptibility artifacts are more prominent in 3T. Tissue air interface in the base of skull shows susceptibility artifacts in Diffusion and gradient images.

Dielectric effect

Dielectric artifacts appear due to increased RF inhomogeneity with areas of non uniformity or shading in MRI images. These artifacts are caused by the shortening of RF wavelengths inside the body and exist at all field strengths. Dielectric effects are increased at 3T and the effects can be more with multichannel or phased array coils. Dielectric effect can be reduced by placing a dielectric pad between the patient and coil. Dielectric pad is a high conductivity pad made of a dilute manganese chloride solution. The new generation 3T scanners are equipped with multi drive RF transmission technology to reduce the dielectric effect.

Specific Absorption Rate (SAR)

Specific Absorption Rate (SAR) is the measure of heat energy deposited by Radio frequency field in a given mass of tissue. As per the instruction of FDA* and IEC* the SAR should not exceed 8 watts per kg (W/kg) for any 5 minutes period or 4W/kg for whole body averaged over a period of 15 minutes. The precessional frequency of hydrogen proton is high in 3T and flipping the net magnetization vector requires more RF energy compared to 1.5T. Increasing the field strength from 1.5T to 3T will lead 4 times increase in the SAR value. Most of the current 3T scanners are equipped with real time SAR monitors which helps to keep RF exposure within safety limits.

MR safety

The 3T field is twice the strength of 1.5T field and even small objects can become a deadly projectile and hazardous to patients. The extended fringe field is another area of concern. MR siting requires special attention to contain 5 Gauss exclusion zone within the magnet room or it can interfere with nearby electronic equipments or other imaging modalities. Another area of concern in the MRI environment is the safe usage of medical devices and implants. The implants which is safe in 1.5T environment may not be safe or may be conditional in 3T field. The medical devices such as monitors,ventilators,infusion pump which is safe in 1.5T may cause interference with 3T if the instructions of the manufacturer on the usage of the equipment is not followed.

*FDA - Food and Drug Administration

*IEC- International Electro technical Commission

Please note: This information is provided for the educational purpose only.You should contact the referring physician, Radiologist or MRI technologist for specific information on MRI safety to patients. Reference Manual for Magnetic Resonance Safety,Implants and Devices can be available from Biomedical Research publishing group Los Angels,CA, by Frank G.Shellock Ph.D

http://www.magneticresonancesafetytesting.com/