Technology of MRI Scanners
INTRODUCTION
Magnetic Instrumentations are the hardware in the system component of MRI that very significant in the production of Magnetic Resonance (MR) images. It act together with the software program such as pulse sequence and image formation program to complete the process including nuclear alignment, Radio Frequency (RF) excitation, spatial encoding and image formation. The example of hardware in MR imaging are magnet, RF source, magnetic field gradient system, computer system, and image processor.
MRI INSTRUMENTATION
1. Define gauss, tesla, and electromagnetic spectrum:
1.1 Gauss
Gauss (G) is the older unit of flux density or the unit of low magnetic field strength. 1 Gauss can be define as 1 line or flux per cm2 (Willis, 2009).
1.2 Tesla
Tesla unit is the preferred SI unit in larger magnetic field and it can be define as the field strength of 1 Weber per m2. 1 Tesla is equal to 10 000 Gauss (Willis, 2009).
1.3 Electromagnetic spectrum
Electromagnetic spectrum is a continuum of all electromagnetic waves or energy arrange according to frequency and wavelength. In MRI, radiofrequency (RF) refers to that portion electromagnetic spectrum which can be generated by alternating current fed to an antenna (Hardiman, 2005).
2. Define paramagnetic, Diamagnetic and Super magnetic:
2.1 Paramagnetic
Paramagnetic material such as Ferum, Magnesium and Gadolinium have unpaired electron resulting in positive magnetic susceptibility which induced small magnetic momen. Paramagnetic ion induced large fluctuating magnetic field in external magnetic field. Commonly, Gadolinium (Gd) is used as a MR contrast agent (Ballinger, 1998).
2.2 Diamagnetic
Diamagnetic materials have paired electrons such as water, wood, glass, and gold which show no net magnetic moment with the absent of magnetic field. Non magnetic material quickly repels the field when placed in magnetic field, resulting in small negative magnetic susceptibility which contribute to the loss of signal in MRI (Ballinger, 1998).
2.3 Super magnetic
Superparamagnetic materials such as iron oxide can be used as t2* as it has magnetic susceptibility slightly higher than paramagnetic (low positive) and lower than ferromagnetic material (high positive). It causes large magnetic moment in the presence of external magnetic field but no remnant magnetic moments when the field is zero (Ballinger, 1998).
3. Describe 3 types of magnet and give advantages and disadvantages of each:
3.1 Superconducting magnet
Superconducting magnet have high field strength up to 3.0 T in clinical scanner while 9.4 T in research facility. It is also high in field homogeneity over largest volume. As it provide high field strength, it cause high signal to noise ratio and also fast scanning. It gives low power consumption as it always produce a magnetic field and electric current without primary source (Blink, 2004).
Superconducting magnet is high in capital cost and cryogenic cost. It also produces acoustic noise, motion artifact and technical complexity as susceptibility effect is increased when imaging moves to higher field strength (Blink, 2004).
3.2 Resistive magnet
The resistive magnets commonly have open designs which make it light weight and overcome claustrophobia among the patient. It been shut off when not in used to conserve the power (Blink, 2004).
However, it has limited field strength which below than 0.2 T and low overall volume of homogenous field. It needs high power consumption and has large fringe field (Blink, 2004). Water cooling is required because large amount of heat are generated to produce the magnetic field by an electromagnet (Wang, n.d).
3.3 Permanent magnet
Permanent magnets have low power consumption and low operating cost. It also cause small fringe field and does not use cryogen (Wang, n.d).
Permanent magnets have limited field strength which is below than 0.3T and give no quench possibility. It is quite heavy and may require reinforced flooring to site the system, particularly if the magnet is not located at ground level (Blink, 2004).
4. Discuss field strength in relation to image contrast:
In general, image quality is based on contrast resolution and noise which influence by the field strength of magnet. High field strength provides more signal-to-noise ratio than low field strength. It allows fast imaging that reduces patient motion so that better spatial resolution and image contrast (Thomas Magee, 2003).
5. Discuss purpose of gradient in MRI:
Mainly, the gradient is used to either dephase or rephase the magnetic moment of nuclei (Guang Cao, n.d). It also used for slice selection, gradient refocusing, gradient echoes, and gradient moment nulling. It also can spatially locating (encoding) signal along the long axis of the anatomy called Frequency Encoding as well as spatially locating (encoding) signal along the short axis of the anatomy called Phase Encoding (ASRT, 2008).
6. Discuss the purpose of cryogen:
Cryogen is used in MRI to supercool the electrical conductor in superconductive magnet. It is because a quench cause a rapid loss of static magnetic field in MRI (Blink, 2004). So, the cryogen usually liquid helium and some liquid nitrogen is used to ensure the temperature as low as -269Esc (-452Esc) are achieve (C.L. Dennis, 2009).
7. Discuss the type and purpose of shielding and shimming:
In MRI, RF shielding use copper shielding or Faraday cage to reduce the transmission of electric or magnetic fields from one space to another. Meanwhile, magnetic shielding used to reduce the level of RF radiation that enter or leaves the shielded room (Hipskind, 2009). Passive magnetic shielding use a steel plate while active magnetic shielding employ additional solenoid electromagnet to reduce the area affected by the fringe field (ETS.LINDGREN, 2009).
Shimming is used to remove small inhomogeneities which present in the magnetic field (Z. Ren, 2009). It uses metal discs or plates in passive shimming to get magnetic field to a particular level of homogeneity and additional solenoid magnet in active shimming to optimize for each patient examination (D. Tomasi, 2009).
8. Discuss construction, use and selection of linear, quadrature, phase array, multichannel, transmits or receive coil and receive coil only:
Linear or surface coil consist of single or double loop of copper wire that use to improve SNR when examine the structure near the skin surface such as temporo-mandibular joint, orbit or shoulder (Blink, 2004).
Quadrature or circularly polarized coils contain at least two loops of wire which commonly used today that produce v2 more signal than single loop coil (Zhou, n.d).
Phased array coils consist of multiple surface coils which have the highest SNR but limited sensitive area (Zhou, n.d).
Multichannel or Helmholtz coil have pair of circular coil that widely used in MRI because of its fairly uniform magnetic field (Zhou, n.d).
Transmit/receive coil transmit RF then change to a receive mode to receive the MR signal. It allows acquisition of more slices and reduce artifact. It has complex design and decrease uniformity over volume of interest (Spring, 2005).
Receive only coil design only receive MR signal using body coil as a transmitter and they include surface and phased array coil. It has simple design which used together with the transmit body coil to provide uniform excitation over the entire volume of interest (Spring, 2005).
9. Explain the principle of magnetism, the specific of MR instrumentation, and hardware required for MR imaging:
9.1 Principle of magnetism
Magnetism not just occurs in ferromagnetic substances like iron. In an external magnetic field, magnetization can occur in tissue but it disappears when the field is removes (Michael N.Hoff, n.d).
9.2 Specific of MR instrumentation
Ancillary equipment needs an additional instrumentation for scanning such as ECG leads and respiratory bellows as well as the power injector. It also supplies patient monitoring like ECG, pulse oximetry and fiber optic while patient transportation provide a wheelchairs, stretchers, patient table and step stool (ASRT, 2008).
9.3 Hardware required for MR imaging
Other than magnet, gradient and radio frequency system, the hardware required for MR imaging is the computer. It controls all the system and has the pulse control unit as well as array processor for fourier transform and 2D and 3D imaging. Computer also stored the data in the hard drive as well as processing and handling it (ASRT, 2008).
10. Explain MRI imaging chain:
Diagram1: Schematic diagram of MRI imaging chain (Anonymos, n.d).
Mainly, the central computer controls the scanning operation. It specifies the shape of gradient and RF waveform as well as the timing to be used. Then, the information is passes to the waveform generator before the signal is passes to be amplified and sent to the coils. Once NMR signal has been phase, it is sensitively detected and turned to a digital signal by analogue to digital converter. The image is displayed on a monitor after digital signal sent to image processor for Fourier transformation (Anonymos, n.d).
The signal before Fourier transformation called raw data is stored to enable the application of corrections in the post processing. Matrix sizes of 2n are usually used to allow the use of fast Fourier transformation (Anonymos, n.d).
11. Describe MRI quality assurance program:
The evaluation of all MRI system should be done prior to and following installation. It must be monitor at least annually to ensure proper functioning and high-quality diagnostic images are produced (ASRT, 2008).
For the performance evaluation, it should include the tests phase stability, magnetic field homogeneity, calibration of all RF coils, image signal-to-noise ratio and uniformity for all coils, inter-slice RF interference, artifact evaluation, hardcopy and softcopy fidelity, spatial resolution and low contrast object detectability, magnetic field gradient calibration, intensity for all volume coil, film processor quality control, physical and mechanical inspection, and evaluation of MRI safety (Geoffrey D. Clarke, 2002).
In quality control program, the physicist/MR scientist should assist annually on the RF calibration and image SNR and uniformity for the head coil, film processor QC, physical and mechanical inspection, magnetic field gradient calibration, and hardcopy and softcopy fidelity. The corrective action is taken if the parameter falls outside control limit (Geoffrey D. Clarke, 2002).
The protocol of acceptance testing include the evaluation of the coil and follow-up procedure or written survey report from the physicist/ MR scientist to the physicians and to the responsible professional (ASRT, 2008).
CONCLUSION
In MRI system, a magnet is required for nuclear alignment, radio frequency source for excitation, magnetic field gradient system for spatial encoding, computer system for image formation process as well as the user interface, and image processor to convert signal into images. This hardware plays important role along the software program like pulse sequence and image formation program to produce the MR images. However, to ensure proper functioning and high-quality diagnostic images produce, the MRI system should be evaluated through Quality Assurance (QA) program because if there are parameter falls outside control limit, immediate corrective action is taken.
