MRI Shim Coils

Shim Coils

 

Most modern MRI techniques (e.g. EPI, chemical shift imaging) require magnetic fields homogeneous to less than 3.5 parts per million (ppm) over the imaging volume. The raw field produced by a superconducting magnet is approximately 1000 ppm or worse, thus the magnetic field has to be corrected or shimmed. Usually this is accomplished by a combination of current loops (active or dynamic shims) and ferromagnetic material (passive or fixed shims). Gradient coils are used to provide a first-order shim. The patient distorts the magnetic field when put into the scanner and so an active shim correction must be made before scanning. The operator will perform active shimming to improve the homogeneity on an individual patient basis

MRI Quenching

Quenching

A quench is basically a sudden loss of the main magnetic field and may occur intentionally or unintentionally. Stored electrical energy in the NbTi winding will be dissipated as heat if the superconducting process fails. This heat will cause other parts of the windings to rise above their maximum rated temperature and this will propagate the effect throughout the magnet. This will result in a sudden loss of the B₀ field and the loss of the cryogen. A safety feature of any MRI system is the presence of quench-pipes that are vented outside the building and this prevents cold burns and suffocation in the event of a quench .

MRI Fringe Fields and Shielding

Fringe Fields and Shielding

Walls, floors and ceilings cannot contain static magnetic fields. The stray magnetic field outside the bore of the magnet is known as the fringe field. All magnets have a fringe field to some extent and these fields must be taken into account when installing a magnet.

Fringe fields can be compensated for by the use of magnetic field shielding which may be active or passive. Passive shielding is the more expensive alternative using iron plates to restrict the field lines. Some manufacturers offer actively shielded magnets that reduce the fringe field to about 30m². Active shielding partially cancels the field outside the main magnet coils thus reducing the magnitude of the fringe field. The 0.5mT isomagnetic line is taken as the critical cut-off limit.

MRI Room Planning Example

MRI Room Planning Example

Elevation A-A

 Elevation B-B

MRI Space Planning

MRI Space Planning

1. MRI Rooms Space Planning

Room	Technical Finished-Room Size	Technical Finished-Room High
Control room	min. 150 x 190 cm	min. 210 cm
Equipment room	min. 230 x 165 cm	min. 220 cm
Examination Room     •  Full Body        Mobile patient table	min. 351 x 662 cm	min. 240 cm

2. Area requirements and floor loading for the magnet  

The system has to be installed on a solid underground with sufficient carrying capacity,

such as, e.g. concrete. The load bearing capacity has to be checked by a stress analyst.

The floor in the vicinity of the magnet must be levelled to within max. ± 2 mm.

External vibration or shocks affecting the magnet may degrade image quality. In the 3 spatial orientations the building vibration must not exceed the following specification:

 

2.1 Building vibration specification:

a _max = - 80 dB(g) in the frequency range from 0 to 100Hz.

The requirement for a _max is frequency dependent.

3. Weights to be consider

 

Magnet:	As per Manufacturer,~  5500 kg
Patient table:	~250kg
Electronic cabinets	~1850 kg
UPS to support the MRI system	~2500 kg
Iron Shielding	As per static calculation
RF Cabin	Depends on the material and size

4. MRI Environmental Requirements

MRI	Examination room	Equipment room	Control room/ Evaluation
Room temperature   Temperature gradient	18 to 22 °C   n.a.	15 to 30 °C   </= 1.0 K/5min	15 to 30 °C   n.a.
Relative humidity	40 to 60 %	40 to 80 %	40 to 60 %
Absolute humidity	< 11 g / kg	< 11 g / kg	< 11 g / kg

5. Room lighting in the MRI room

 

The magnetic field adversely affects the operating life of light bulb located in the

immediate vicinity of the magnet. The filament in the light bulb oscillates with the frequency of the power supply.

It is therefore recommended to connect light fixtures in the vicinity of the magnet to

a DC voltage supply. If room lighting is supplied with DC voltage, correct polarity of
the sockets should be ensured during their installation.
Residual AC ripple should be £ 5 %.

Fluorescent lighting must not be installed inside the examination room. Only lamps without phase angle control should be used. Do not use energy-saving lamps.

6. Noise Emission

If required, noise reduction should be realized based on the noise emission values as specified.

MRI	Examination room	Control room	Equipment room
Average values across 8 hours	≤ 80 dB(A)	≤ 54 dB(A)	≤ 67 dB(A)

 

7. RF Shielding

An RF shielding (Faraday cage) is required for the MR examination room. This shielding

protects the environment from RF interference and conversely protects the MR system

from external interference.

 

Required attenuation: >90 (Co-siting 100) dB over the frequency range 15 to 128 MHz.

These values must be verified by measurement before the MR system is installed.

RF shielding components (doors, windows, interfaces) and complete modular RF cabins

can be supplied from different sources.

 

8. Quench Pipe

A quench pipe (thermally insulated tube) made of non-magnetic metal must be fitted

from the super-conducting magnet to the outside of the building in order to vent the
vaporising helium gas.

9. Magnetic field level warning signs in the control zone ³ 0.5 mT

If the magnetic flux density in a given area exceeds 0.5 mT, it is necessary to display

warning signs and restrict access in accordance with local regulations.

 

10. Water connection

Either you connect a chiller recommended by the MRI manufacturer or you can connect it to hospital chiller system.

11. Power Supply:

The Superconductive magnet normally required  3Phase/N/PE, AC 380V, Connections Value ~120 KVA.

12. Guidelines for max. permissible flux density  (mT)

Max. permissible flux density		mT
Servoventilator, Defibrillator		20
Filter		10
Small motors, watches, photographic equipment, magnetic data storage devices (short term)		3
Computers, disc drivers, oscilloscopes, host PC, imager PC		1
B/W monitors, X-ray tubes, storage of magnetic data carriers, cardiac pacemakers, insulin pumps. (safety limit for unrestricted open access)		0.5
CT-systems		0.2
Colour monitors (CRT)		0.15
Linear accelerators		0.1
Image intensifiers, gamma cameras linear accelerators		0.05
Laser cameras / digital cameras	see manual of the manufacturer

The magnetic stray field is present in all three dimensions around the magnet and can
be reduced by a magnetic shielding. Typical lines of constant magnetic flux density are
shown in the drawing.

 

Superconducting Magnet

Superconducting Magnet

The main components of an MRI system are the superconducting magnet, the gradient system, the RF system and the computer system. The magnet produces a strong, static field and the radiofrequency transmit and receive coils excite and detect the MR signal. The magnetic field gradients localise the MR signal and the computer system facilitates scanner control, image display and archiving. This chapter will describe each of these components in turn. Figure 1-1 below shows the general layout of an MRI system.

 

MRI Magnet Types

Magnet Types

The magnet is the main component of any MR system and there are four different types of magnets capable of MRI:

·         Superconducting magnets

·         Air-cored resistive magnets

·         Iron-cored electromagnets

·         Permanent magnets

 
Superconducting magnets are by far the most common type and the three main manufacturers are Siemens, GE and Philips.