MRI INSTRUMENTATION dome by Poojah Sah from IOM

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1. MRI INSTRUMENTATION PRESENTER:POOJA SHAH ROLL NO:73 15 th Shrawan,2071 MMC,IOM

2. INTRODUCTION Basics of MRI: Excitation,detection and acquistion

3. WHY MRI? No ionizing radiation Contrast resolution is high Direct multiplanar imaging Best for neuro muscular study No bone artifacts MRV and MRA performed without CM Gadolinium more safer than iodinated CM

4. LIMITATIONS OF MRI Long time imaging Patients with metal implant or electronic implant are at risk in MRI Small calcifications oftenly cannot be detected High costs Emergency case not possible Claustrophobia

5. COMPONENTS OF MRI A Magnet Different coils A radio frequency source An image processor A computer system Pulse control Patient transportation system Operator interface

6. MAGNET Magnetism:susceptibility Ideal characteristics High field strength Field homogeneity Less fringe field Less operating and maintenance cost

7. MAGNET Types of magnet: Permanent magnets Electromagnets Superconducting magnets

8. MAGNET Permanent magnet: Require no power supply The main advantage is that it prevent from claustrophobia Field strengths:0.2-0.3T Small fringe field - (0.5mT at 1m ) Less operating and maintenance cost Cannot be shut down,long scan time

9. MAGNET ELECTROMAGNET(EM): Follows laws of electromagnetism The amt of current moving in the wire is proportional to the strength of the resultant magnetic field.

10. MAGNET SOLENOIDAL EM: Many loops of wire forms the spring like and act they are parallel to each other These are evenly spaced so the magnetic field in this is considerably uniform Solenoidal electromagnet is often said to be resistive magnet as the efficiency of current is governed by th resistance of the coil

11. MAGNET RESISTIVE EM: The magnetic field is generated by a current, which runs through loops of wire.(electro magnet),winding is thick. Lighter than permanent, switch off High power consumption , large heat production (water cooling)and high operating cost Limited field strength (0.3 T) Relatively poor homogeneity of the field (30 to 50 ppm in a 10 cm3 volume ). Relatively safe but this type of magnet does create significant stray magnetic fields

12. MAGNET

13. MAGNET

14. MAGNET Superconducting magnet: Superconducting magnet is made up of long coils niobium-titanium alloy maintained at temperature of 4k. Once current is made to flow , it flows until the temperature is maintained. No power needed to maintain the magnetic field. Initially,current is passed through the loops of wire to create the magnetic field then the wires are super cooled with substances known as cryogens-cryogen bath After achieving preferable current ,it is stopped High magnetic field strenths with low power requirements(0.5-4T) and may upto 9T

15. Magnetic homogeneity is best of all system- 1 ppm in 40 cc volume .

16. CUT SECTION OF SUPERCONDUCTING MAGNET

17. MAGNET Disadavantages High initial capital Cryogens used are very expensive Extensive fringe field Cannot be shut off in emergency Higher temperature- resistance increases- loss of superconductivity- loss of magnetic field ( quenching) Increase in resistance- increase in temperature- helium vaporizes- displaces oxygen- creates asphyxiating environment

18. FINAL COMPARISION PERMANENT MAGNET ELECTROMAGNET SUPERCONDUCTING MAGNET No power supply High power No or less power Magnetized permanently No permanent magnetism Magnetic strength remains constant Heavy Comparably lighter Lighter than PM heavier than EM Low field strength Low field strength High field strength Small fringe fields Small fringe fields Large fringe fields No cooling Water Helium

19. MAGNETIC FIELD STRENGTH With the use of high magnetic field strength,there is higher longitudinal field strength. MR signal is proportional to square of magnetic field strength Improved SNR can be used for high spatial resolution

20. LIMITATIONS OF HIGH MAGNETIC STRENGTH T1 time increases with field strength. If the same pulse sequence with identical parameters (TR, TE, etc ) is used at low field and high field , T1 contrast would be less pronounced in the high-field image To achieve similar T1 contrast in GRE sequence , TR should be increased, which increases imaging time. SAR(the amount of RF power deposited per kg of body weight) increases with increase in field strength.

21. MAGNETIC HOMOGENEITY Uniformity of the magnetic field Protons resonate at the same frequency in a coherent manner and thus induce the maximum possible signal Absence of uniformity is inhomogeneity Usually expressed in units of ppm relative to the main field over a certain distance . Field in homogeneity of 10 ppm in a field of 1.5 T means maximum field variation is 0.15 gauss . Inhomogeneity can destroy the linear dependence of the larmour frequecy and spatial locations.

22. SHIM COILS Inhomogeneity are corrected by shim coils Either by metal or by loops of current carrying wire Need to be embedded in the gradient coil Expensive

23. FRINGE FIELD The stray magnetic field that extends all directions away from the center of the magnet near the magnet in the z direction and depends on bore size and magnet configuration .

24. FRINGE FIELD Two distances are of concern regarding the fringe field. The 0.5 mT (5 G)distance is considered the minimum safe distance for persons with pacemakers. The 0.1 mT (1 G) distance, the nominal distance for other equipment that uses video monitors.

25. MAGNETIC SHIELDING The goal of magnetic shielding is to protect the environment from MRI magnetic field. Two types: Active shielding Passive shielding

26. GRADIENT COIL Sets of wire in the magnet,creates additional magnetic field which superimposes with main magnetic field Three sets of gradient coils Gradient coils for z-axis are anti helmoltz coil and for x and y-axis are paired saddle coils Gradient coil provide linear gradation or slope of the magnetic field strength from one end of the solenoid to the other end . The gradient are applied by passing current through the gradient coil in certain direction This either increases or decreases the magnetic field on either side of iso centre .

27. MRI Instrumentation

28. In a cylindrical bore superconducting magnet, the gradient coils are mounted on a cylindrical structure, which is often made of epoxy resin .

29. FUNTIONS OF GRADIENT COILS Used for encoding For rewinding,spoiling and rephase Scan field is dependent upon performance of the gradient coil High gradient strength coil used for faster imaging,for good spatial resolution,also faster switching can capable of faster scanning

30. AMPLIFIERS AND PULSE CONTROL UNIT Gradient coil amplifier supply to the gradient n pulse control unit allow the gradient coils to be switched on and off very rapidly by coordinating the function of gradient amplifier and coils

31. Quality of Gradient system Gradient strength Amplitude of gradient , variation of magnetic field per unit length ( mT /m) Ranges from 5 to 50 mT /m Depend on magnitude of current supplied Direction of gradient depend on direction of current Limited due to power consumption and heat produced in windings.

32. RISE TIME SLEW TIME:- Affected by eddy current Actively shielded gradient coils and compensation circuits reduce the problems introduced by eddy current

33. Disadvantage of gradient Gradient induces currents in the human body that can cause painful peripheral nerve stimulation and cardiac stimulation if the amplitude is higher Gradient is the cause of noise (Noise is due to flexing and torque experienced by gradient coil due to rapid change in magnetic field.)

34. RF COILS Radiofrequency coils used to excite the protons and detect the MR signal. In order to produce an image,RF must be first be transmitted at the resonating frequency of hydrogen so that the resonance can occur. The transverse component of magnetisation must then be detected by RF transmitters.

35. RF COILS TRANSMIT COIL: Energy transmitted in form of short intense burst of RF Pulse produced is called 90*RF pulse Created by an oscillating secondary magnetic field To accomplish excitation,the secondary magnetic field should be perpendicular to the main magnetic field The RF transmitter contains four main components: a frequency synthesizer, the digital envelope of RF frequencies, a high-power amplifier, and a coil or antenna

36. The actual amount of power required from the amplifier to rotate the protons from equilibrium depends on the field strength, coil transmission efficiency, transmitter pulse duration & desired excitation angle.

37. RF COILS RECEIVER COIL: Consists of coil,pre amplifier and signal processing system If a loop of wire is exposed to oscillating field,a current is induced in the loop Receiver coil and transmitter coil may be same or different Receiver coils must be placed properly in order to adequately detect the MR signal

38. TYPES OF RF COILS The configuration of RF transmitters and receiver coils directly affects the quality of MR signal. Volume coil or bird cage coil Surface coil Phased array coil Solenoidal coil Helmoltz coil

39. RF COILS VOLUME COIL: Both transmit and receiver Encompasses the entire anatomy Yield uniform SNR over entire imaging volume Less SNR

40. Volume coils[both R/T] Head coil Knee coil

41. RF COILS QUADRATURE COILS

42. Surface coil Placed close to the area under examination Designed for small area. The size of area extends to the circumference of the coil and at depth into the patient equal to the radius of coil Example of surface coil are flex coils that can be wrapped around knee shoulder and ankle. Endoluminal coil for prostate imaging are the other example of local coil. The coil consists of a single or double loop of copper wire. Advantage of these coils.- high SNR and high resolution. Limitation – small FOV and loss of signal intensity from the centre to the periphery.

43. Shoulder coil Neck coil Spine coil

44. Phased Array Coil They consist of numerous small coils. Signal of individual coil is separately received and processed to form single larger FOV MRI systems with 32 independent receiver channels are available at which up to 76 coil elements can be positioned simultaneously. These coils takes advantage of both surface coil as well as volume coils. Increased SNR and resolution and more coverage area.

45. C Spine Neck Array Coil

46. Spine array coil

47. COIL SAFETY Coils are connected to the system by cables that transmit RF energy to the coil and signal transmitted to image processor Heat may burn to the patient So cables properly not looped,no touching to patient and bore of magnet Regular inspection

48. Optimization of the RF channel The exact Larmor frequency is set, this being slightly modified by the patient’s presence in the magnetic field Transmission power is adjusted according to the weight of the patient and the transmit coil, to obtain the desired flip angles The receiver gain is adjusted to avoid signal saturation or conversely, weak amplification resulting in a deteriorated SNR 

49. RF shielding RF signals in environment can propagate to MRI antennas and interfere with the MRI signal producing artifact, which can be prevented using RF shielding. RF shielding can be made of virtually any type of metal; however, the most prominent types used for MRI shielding are copper, galvanized steel, and aluminum. MRI room is lined by copper sheet or copper wire mesh (faraday’s cage)

50. Faraday’s cage

51. Pan type shielding

52. RF Filters & Waveguides The RF shield forms a complete box around the MRI system RF filters are mounted on the RF shield and create an penetration point for electrical power for lighting or powered outlets within the MRI room. RF filters also accommodate data cable are penetrations in the RF shield that allow a fluid flow into the MRI room. Air conditioning, water and medical gases all provide a fluid flow and require a waveguide.

53. If there are electrical wires, filter is needed If it is fluid (air or liquid),waveguide is needed

54. PATIENT TRANSPORTATION SYSTEM Al systems are mechanically and hydraulically driven couch to lift the patient up to the level of the bore and to slide them into the magnet Buttons and pedals Table comfortables Some systems enable couch to be undocked from the magnet

55. Operating console MR computer system varies with manufacturer It consist of A mini computer with expansion capabilities. An array processor for fourier transformation. An image processor that takes data from array processor to form an image. Hard disc drives for storage o raw data and pulse sequence parameters. A power distribution mechanism to distribute and filter the alternating and D.C.

56. In addition to data acquisition and viewing the recently the acquired images, the operator console also allows various image manipulation technique. It includes viewing several images at the same time. viewing images in a cine loop. reformatting 3D volume images.

57. MR sitting consideration Site should be large enough, air conditioned Should not be easily accessible to people Lifts and power cables should not be near as they may cause RF interference & distort the image by Zipper artifact. Wall of the room should incorporate wire mesh (faraday cage)to screen the imager from external RF interference. Sound level & its impact on surrounding should be considered

58. Specification of our MR system Manufacturer:Hitachi Medical Corporation Equipment model:AIRIS Vento Permanent magnet, 0.3 Tesla field strength Two-pillar design ( 210° opening to the front, 70° opening to the rear) Total weight 11.2 tons 22mT/m gradient strength 55mT/m/ ms slew rate Additional shim coil Fully motorized patient table Lateral table movement ±100 mm

59. CONCLUSION MRI has a synonym “COMPROMISE”. Equipments used in MRI are day by day having new development with different technology. To know about how MRI works detail knowledge of MRI instruments is to be known

60. References Catherine Westbrook,Carolyn Kaut.MRI in practice.2 nd edition.Blackwell Science Ltd;1993 and 1998 Evert J Blink.MRI Physics.Netherlands;2004 Prof . Dr. Hans H. Schild,Lt . Oberarzt in Institute of Klinische,Strahlenkunde des Klinikums der,Johann-Gutenberg-University.MRI made easy.Germany;Nationales Druckhaus Berlin;1990

61. THANK YOU

62. ???????? List the different components of MRI? What are the differences of superconducting magnets and permanent magnets? What are shim coils? What is fringe field? What are the two types of magnetic field shielding? What is rise time and slew time? 7. What is the use of pulse control unit?