Technology of MRI Scanners

Technology of magnetized resonance imaging ScannersINTRODUCTIONMagnetic Instrumentations are the hardware in the system parcel of magnetized resonance imaging that very significant in the production of Magnetic tintinnabulation (MR) images. It act in concert with the software schedule such(prenominal) as measure sequence and image formation program to complete the crop including thermonuclear alignment, Radio Frequency (RF) excitation, spatial encoding and image formation. The example of hardware in MR imaging are magnet, RF source, magnetised domain of a function of honor incline system, computer system, and image processor.MRI INSTRUMENTATION1. Define gauss, tesla, and electromagnetic spectrum1.1 GaussGauss (G) is the onetime(a) social unit of flux density or the unit of low magnetic matter of operations of battle potency. 1 Gauss can be line as 1 line or flux per cm2 (Willis, 2009).1.2 TeslaTesla unit is the preferred SI unit in larger magnetic country and it can be define as the field strength of 1 Weber per m2. 1 Tesla is qualified to 10 000 Gauss (Willis, 2009).1.3 Electromagnetic spectrumElectromagnetic spectrum is a continuum of every(prenominal) 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 flowing fed to an forward pass (Hardiman, 2005).2. Define paramagnetic, Diamagnetic and Super magnetic2.1 ParamagneticParamagnetic substantive such as Ferum, Magnesium and Gadolinium perplex unpaired negatron resulting in positive magnetic susceptibility which induced small magnetic momen. Paramagnetic ion induced large fluctuating magnetic field in impertinent magnetic field. Commonly, Gadolinium (Gd) is expenditured as a MR business line agent (Ballinger, 1998).2.2 DiamagneticDiamagnetic materials flummox paired electrons such as water, wood, glass, and gold which show no net magnetic moment with the deficient of magnetic field. Non magnetic material quickly repels the field when placed in magnetic field, resulting in small negative magnetic susceptibility which work to the loss of sign of the zodiac in MRI (Ballinger, 1998).2.3 Super magneticSuperparamagnetic materials such as iron oxide can be exercised as t2* as it has magnetic susceptibility slightly higher than paramagnetic (low positive) and lower than ferromagnetic material (high positive). It reachs 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 each3.1 Superconducting magnetSuperconducting magnet have high field strength up to 3.0 T in clinical scanner while 9.4 T in look for facility. It is too high in field homogeneity over largest al-Quran. As it provide high field strength, it cause high communicate to hitch ratio and in like man ner truehearted examine. It gives low power consumption as it always capture a magnetic field and galvanizing current without primary source (Blink, 2004).Superconducting magnet is high in capital speak to and cryogenic cost. It in like manner promotes acoustic noise, motion artifact and technical complexity as susceptibility effect is increased when imaging moves to higher field strength (Blink, 2004).3.2 Resistive magnetThe resistive magnets commonly have aerofoil designs which make it light weight and overcome claustrophobia among the unhurried. 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 ask high power consumption and has large fringe field (Blink, 2004). urine cooling is infallible because large amount of heat are generated to arise the magnetic field by an electromagnet (Wang, n.d).3.3 Permanent magnetPermanent magnets have low po wer 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 decimate possibility. It is quite heavy and may require reinforced flooring to billet the system, particularly if the magnet is not located at ground take (Blink, 2004).4. debate field strength in relation to image line of workIn general, image quality is based on contrast proclamation and noise which influence by the field strength of magnet. High field strength provides more(prenominal) signal/noise ratio ratio than low field strength. It allows degraded imaging that reduces patient motion so that better spatial colony and image contrast (Thomas Magee, 2003).5. Discuss purpose of side in MRIMainly, the incline is used to either de leg or rephase the magnetic moment of nuclei (Guang Cao, n.d). It also used for slice selection, gradient refocusing, gradient echoes, and gradient mom ent 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 material body Encoding (ASRT, 2008).6. Discuss the purpose of cryogenCryogen is used in MRI to supercool the galvanic conductor in superconductive magnet. It is because a quench cause a speedy loss of static magnetic field in MRI (Blink, 2004). So, the cryogen ordinarily liquid helium and some liquid nitrogen is used to date the temperature as low as -269c (-452c) are achieve (C.L. Dennis, 2009).7. Discuss the type and purpose of shield and shimmingIn 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 act ive magnetic shielding engage 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 armament, multichannel, transmits or buzz off scroll and receive gyre onlyLinear or surface axial motion consist of virtuoso or double loop of copper wire that use to rectify 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 2 more signal than single loop coil (Zhou, n.d).Phased a rray coils consist of multiple surface coils which have the highest SNR but limited handsome 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 hence 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 sender 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 imaging9.1 Principle of magnetismMagnetism not and occurs in ferromagnetic substances like iron. In an external magnetic field, magne tization can occur in tissue but it disappears when the field is removes (Michael N.Hoff, n.d).9.2 item of MR instrumentationAncillary 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 persona optic while patient transportation provide a wheelchairs, stretchers, patient table and step stool (ASRT, 2008).9.3 Hardware required for MR imaging some other than magnet, gradient and radio frequency system, the hardware required for MR imaging is the computer. It manoeuvres 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 chainDiagram1 conventional 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 observe and turned to a digital signal by analogue to digital substituteer. The image is displayed on a monitor after digital signal sent to image processor for Fourier vicissitude (Anonymos, n.d).The signal before Fourier transformation called raw data is stored to enable the application of corrections in the side processing. Matrix sizes of 2n are usually used to allow the use of fast Fourier transformation (Anonymos, n.d).11. Describe MRI quality assurance programThe paygrade of all MRI system should be done prior to and sideline installation. It must be monitor at least annually to image proper functioning and high-quality diagnostic images are produced (ASRT, 2008).For the performance evaluation, it should include t he 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 proclamation and low contrast object detectability, magnetic field gradient calibration, posture for all volume coil, film processor quality control, physical and mechanized inspection, and evaluation of MRI safety (Geoffrey D. Clarke, 2002).In quality control program, the physicist/MR scientist should look annually on the RF calibration and image SNR and uniformity for the headman coil, film processor QC, physical and mechanical inspection, magnetic field gradient calibration, and hardcopy and softcopy fidelity. The corrective action is taken if the argument 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 answerable professional (ASRT, 2008).CONCLUSIONIn 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.