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10. Considerations for high field strength scanners (3T and above)

The use of higher field strength units in a clinical setting is increasing.  The increased SNR gained offers many advantages in clinical scanning.  However, there are also further safety considerations and employers and staff need to be fully conversant with these.  Staff will require further training in order to ensure patient safety.  Employers should conduct a comprehensive risk assessment prior to the use of a new high field strength installation and there should be an independent set of local rules created.

10.1  Implantable medical and non-medical objects

13695342-exclamation-point.jpgThere must be no assumption made that because a device or implant is safe at 1.5T it will still be safe at field strengths of 3T and above.  Departments should proceed with caution and if they are unable to obtain the required information then scanning should not take place.

Departments should ensure that the implant or device has been tested up to the required field strength.  For those listed as conditional at field strengths of 3T, departments should be fully aware of the conditions under which the device or implant can be safely scanned in liaison with the MR safety advisor and manufacturer.  A record of all implants and their MR safety status should be included within the local rules alongside processes for scanning patients with MR conditional implants.

10.2  Projectile and attractive forces

The attractive force on a ferro-magnetic object is proportional to the spatial gradient of the magnetic field.  This is normally steeper at higher fields particularly since active shielding is used to reduce the extent of the stray fields around the scanner.  Therefore, objects including implants that have been found to be safe to use in the presence of 1.5T systems may not be so with 3T systems.4

13695342-exclamation-point.jpg Departments will need to reassess all equipment for use in 3T units that has been proved to be safe in 1.5T units.  In particular, any equipment that is found to be conditionally safe at 3T must be carefully assessed, as the distance that an object can be safely kept from the magnet will alter at the higher field strength.

Equipment should be clearly labelled as detailed in section 3.5

10.3  Specific Absorption Rate (SAR)

SAR is the measurement of energy deposited by an RF field in a given mass of tissue.  SAR increases quadratically with field strengths.  When using RF-intensive pulse sequences such as FSE, EPI and FLAIR then SAR limits can quickly be reached. Extra precautions should therefore  be taken to ensure patient safety. Manufacturers have developed many SAR reducing sequences and staff should be fully conversant with their use.  See also section 5.1.1.

10.4  Peripheral Nerve Stimulation (PNS)

This effect is increased at 3T and the radiographer should be aware of methods to reduce this, both by means of patient positioning, sequence selection and parameter manipulation if required.

10.5  Acoustic Noise

The noise levels generated in a 3T system approaches twice those generated by a 1.5T system and can be in excess of 130 dBA. Higher gradient performance at 3T scanning also causes higher sound pressure levels.

10.6  Pregnancy

Please also refer to section 9.

10.6.1   Scanning of pregnant patients at 3T

Departments need to carefully assess the risks and benefits prior to making the decision to scan at the higher field strength; they should consider whether scanning at a lower field strength such as 1.5T would adequately answer the clinical question.  The decision to scan a pregnant patient at 3T should be clearly documented and the risks and benefits discussed with the patient. 

10.6.2   Pregnant staff in high field strength units

Please refer to section 9.6.  Employers must conduct a risk assessment as detailed in section 9.6.1.  This should include detailed field plots to fully assess the static field exposure.

10.7  Design and planning of a high field strength unit

Where a department has several MRI scanners of different field strengths then the design and the layout of the unit should reflect the caution needed when imaging a patient who has an implant or device that is safe to scan only at the lower field strength, in order to avoid the possibility of the patient entering the higher field strength unit.  Accurate and clear labelling of distinct areas should be employed and local rules should reflect the need for a strict pattern of working that avoids patients being inadvertently scanned on the higher field strength scanner.

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