Cryogenic Gases

With high-field systems, correct dimensioning or upgrading of ventilation and helium venting pipes in relation to the type of magnet being installed and room size should be addressed at the design stage, given that 700 l of gas is produced from 1 l of liquid helium. The cryostat of a 3.0 T superconducting magnet contains approximately 3 m3 helium, i.e. roughly three times the content of a 1.0 T magnet. In normal boil-off, i.e. gradual, controlled evaporation conditions, the gas is channelled outside through a dedicated exhaust pipe. In case of a quench, i.e. a sudden increase in coil temperature and violent, uncontrolled evaporation of the helium in which the coils are immersed, the oxygen concentration in the scan room may drop to levels likely to cause asphyxia. In case of an emergency, the forced ventilation plant therefore needs to guarantee at least 20 air changes/h to restore safety conditions in the room as quickly as possible. In particular, the design of the ventilation plant inlet and outlet points must ensure against a short circuit of the air flow, and inlet and outlet fluxes need to be proportionate.

To ensure helium outflow in normal as well as emergency conditions, the quench pipe diameter must be correctly dimensioned in relation to pipe length and the number of bends. Finally, the oxygen monitor, which activates the ventilation plant in case of an emergency, needs to be placed at an appropriate height in the room (since helium is lighter than air and tends to rise) and close to the likeliest point of leakage, which is the limiting pressure valve fitted on the quench pipe.

To avoid introducing cryogenic gas containers into the hospital, an insulated pipe allowing the helium to be refilled from the outside should be envisaged.

Acoustic Noise

In an MR unit, field gradients are the main source of acoustic noise, which is produced by the rapid current changes inside the coils. In the presence of a strong magnetic field, these currents are subject to the Lorentz force acting on the coils. Changes in gradient amplitude and steepness connected with different sequences may affect noise levels, which tend to increase with decreasing slice thickness, field of view, TR and TE [16].

There is growing interest in high-field functional MR using fast gradients exceeding 30 mT/m; these, however, involve a noise level close to 120 dB(A), which varies in relation to the sequences performed but is consistently higher than the noise produced by a 1.5 T unit.

At present, the simplest and least expensive means to reduce noise is passive patient protection using earplugs or earphones, even though this impairs communication with the operator and may cause discomfort.

„Silent" coils made from new materials with new assembly techniques or active noise cancellation systems are being developed. In addition, optimization of the choice of imaging parameters should enable less noisy sequences to be obtained [16].

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