MRI HARDWARE AND INSTRUMENTATION - POONAM RIJAL.pptx

radiology topic MRI

1. MRI HARDWARE AND INSTRUMENTATION PRESENTED BY :POONAM RIJAL Bsc.MIT 2 ND YEAR ROLL NO :154 MMC , IOM

2. CONTENTS Introduction Scanner configuration MRI magnets Gradient coils Shim coils RF coils Specification Summary References

3. INTRODUCTION Magnetic Resonance Imaging (MRI) is a sophisticated imaging technique that relies on strong magnetic fields, radio waves, and computer technology to produce detailed images of the inside of the human body . MRI relies on the interaction between RF energy and the natural magnetic properties of hydrogen atoms within the body tissues It works on the principle of Nuclear Magnetic Resonance(NMR ). The hardware and instrumentation involved in MRI are critical to its functionality and performance

4. COMPONENTS OF MRI Scanner Patient transport system Computers and Recording hardware

5. SCANNER A powerful magnet to create magnetic field A shim system to improve the homogeneity of the magnetic field A gradient system to create linear slopes in field strength in any direction An RF transmission system to generate and transmit pulses of electromagnetic radiation A set of RF receiver coils to detect signal from the patient

6. BASIC MRI HARDWARE A patient transport system to transfer the patient inside the MRI bore . A computer system to allow input of parameters and displaying images A computer subsystem capable of coordinating the application of RF pulses and gradients and reconstructing the acquired data into images and storing them.

7. SCANNER CONFIGURATION There are currently three main types of scanner configuration in clinical use : - Closed-bore systems Open systems Extremity systems

8. CLOSED BORE SYSTEM the most popular type of MRI scanner worldwide . familiar tunnel-shaped magnet bore resemble like CT machine. Longitudinal table movement allows the patient to be positioned with the region of interest lying at the center of the magnet bore. This encloses the patient to the front and back but still allows limited access . generate the main magnetic field using toroidal superconducting solenoid electromagnets positioned in circumference to the cylindrical bore. can generate very high magnetic field strengths, typically between (1-3T) for clinical use and up to 8 T ( and above) for research studies.

9. OPEN SYSTEM have a different design, whereby the patient is positioned on a wider imaging table that is maneuvered between two magnetic poles that are located above and below the imaging volume leaving a relatively unobstructed view from all sides . advantageous when scanning obese patient , claustrophobic patients who may find the open access more tolerable . The design facilitates clinicians side access to the patient when undertaking interventions such as biopsies. also permit a degree of sideways table movement useful when imaging lateral body structures such as the shoulder or elbow Open scanners use large permanent magnets or superconducting solenoids to generate the main magnetic field . The maximum currently available field strength for an open superconducting MRI system is 1.2 T.

10. UPRIGHT OPEN MRI SYSTEM Ideal for those who are either unable to lie flat or are not able to comfortably fit in traditional MRI tunnel scanner due to size of mobility issues Useful for claustrophobic patient or those patient that need to be observed during the MRI procedure Scanning in flexion, extension, rotation, sitting is possible that aids to a more accurate diagnosis

11. EXTREMITY SYSTEM are designed to scan limbs and are smaller in size than their whole-body counterparts . Magnetic field is generally generated by permanent magnets therefore restricted to below 1T Smaller in size thus reduced magnetic fringe field, space efficient, cheaper to purchase and low running cost

12. MRI SYSTEM REQUIREMENT There are six main requirements, each of which presents technological challenges :- - The field strength (flux density) must be high, typically between 1.0 and 8.0 T. - The fringe field having a strength of 0.5 mT (5 G) or greater must not extend outside of safety - Zones III and IV , should ideally be contained within the magnet room. - The field must be spatially homogenous to a very high degree.

13. MRI SYSTEM REQUIREMENT - The homogeneity must extend over a large spherical imaging volume (40 cm) to accommodate the required anatomical FOVs. - The field must be temporally stable. This means that the flux density must not vary over time . - The weight and bulk of the magnet must be kept at a level that does not pose any problems with installation in a normal imaging department.

14. 1) MAGNET SYSTEM Three methods to generate static magnetic fields Permanent magnet Resistive magnet Superconducting magnet

15. a) PERMANENT MAGNET Equipped with large discs of ferromagnetic alloy such as neodymium, boron, and iron or ALNICO- (aluminum , nickel and cobalt) Ferromagnetic disc/ pole shoes are mounted on the yoke that position them directly above and below the imaging volume The magnetic field is created by the inherent ferromagnetism of the alloy, namely the combined force of unpaired electrons in the atom of the metal that create a macroscopic magnetic filed The flux lines of the static field run vertically in this type of scanner

16. Characteristics of Permanent Magnet Remains magnetized permanently Low field strength ( 0.2T to 0.45T) Magnetic field inhomogeneity – 10-20ppm Extremely heavy (up to 80 tons ) Cheapest to run and maintain Cannot be shut down Power consumption~ 20 kW Flux lines run vertically Small fringe fields <1m (0.5mT)

17. ELECTROMAGENT Phenomenon of electromagnetism was discovered by Hans Christian oersted in 1820’s . He observed that a direct current flowing through a conductive wire induced a magnetic field around that conductor . Together with Michael faraday’s law of electromagnetic induction, oersted’s law provided the mechanism of electromagnets as those use in MRI. The direction of the induced lines of magnetic flux is visualized by using “right hand grip rule”

18. Right Hand Grip Rule This analogy suppose that a conductor such as a length of wire is gripped into the right hand - thumb indicates the direction of current flow along the wire - direction of fingers as they curl indicates the direction of induced magnetic field This model can be adopted for solenoid magnet where, fingers represent a direction of current flow through the winding of solenoids and the thumb indicates the direction of induced magnetic field

19. b) RESISTIVE ELECTROMAGNET Employ copper wound solenoid that operate just below normal temp Field strength can be adjusted and can be switched off safely after use. However, it takes 15-30 minutes to re-establish the magnetic field . Excessively high current ~ 10kW is required to attain maximum field density of around 0.4T Cheapest and smallest The resistivity in the windings produces significant heat thus water cooling is required to prevent damage to the system achieved by sitting the solenoid magnets in a water filled vessel .

20. Characteristics of Resistive Magnet Field strength up to 0.5T Magnetic field inhomogeneity -10-50 ppm Power consumption – 50-100KW Weight of magnet -4 tons Field can be switched off immediately Flux lines runs horizontally Modest fringe field ~ 2m(0.5mT)

21. c) SUPERCONDUCTING MAGNET Are electromagnet made up of superconductiong wires . Superconducting wires has a resistance nearly equal to zero . Creates magnetic field as same way as resistive magnet However the winding of the solenoid are spun from a type of metal alloy that is - niobium/titanium Were introduced to avoid resistivity issue because niobium/titanium exhibits superconductivity below a certain critical temperature(10K) these devices uses a cryogens to reduce the temperature of the windings to within 4k ( -269°C)

22. CRYOSTAT thermal vaccum flask containing liquid helium Liquid helium is the cryogen of choice that keeps the coil of wire at a temperature of 4.2k The coil and the liquid helium is kept in a large dewar . The typical volume of liquid helium in a MR magnet is about 1700 litres .

23. CRYOSTAT The dewar is surrounded by liquid nitrogen at 77.4k (-169°C) which acts as a thermal buffer between room temperature (293k) and liquid helium . Vaccum is present between each compartment that acts as a temperature shield . The expensive liquid helium is replenished periodically as the cryogens boil off due to heat leaks.

24. SUPERCONDUCTING MAGNET as long as the critical temperature is not exceeded, current will flow through the magnet solenoid indefinitely , yielding an extremely stable magnetic field . However if temperature exceeds the wire becomes resistive thus the energy stored in the magnetic field will then dissipate accompanied by rapid heating and damaging the magnetic coil If not controlled it can lead to vapourization of the liquid helium in the cooling bath . This undesirable phenomenon is called Quenching . Superconducting magnet using helium less than 10Litres . Examples include GE's  Freelium , Siemens  DryCool , and Philips'  BlueSeal  magnets . high temperature superconductor “Magnesium Diboride ” (MgB2) with Tc = 39k (-234°C)

25. COLD HEAD Under the hood of the MRI scanner we have a cold head that consist of a compressor for compressing the helium gas. Compressed gas is then passed through a series of heat exchangers to cool down significantly. The cooled gas is then fed to an expansion valve which reduces the pressure of the gas causing it to expand and significantly drops the temperature to cryogenic temperatures. The coolent head operates in a closed loop system continuously cycling the gas through these component. During cooling process chirping sound is produced .

26. SAFETY FROM QUENCHING Quenching causes loss of magnetic superconductivity with sudden boiling off of liquid helium Liquid helium in large quantity can completely displace oxygen and cause unconciousness within 10 sec when inhaled Thus the patient and staff must be immediately evacuated from the scanner room if quenching occurs. All superconducting magnets have a quench button that can be turned off within the few seconds .

27. CHARACTERISTICS OF SUPERMAGNETS resemble Ideal characteristics of the magent used in MRI :- High field strength 0.37 to 4T (can range higher for research ) Magnetic field inhomogenicity 0.1-5 ppm Flux lines runs horizontally Large fringe fields 10 m (0.5 mT ) Weight of magnet ranges up to 10 tons Power consumption nearly 20 kW Expensive to buy and run and difficulty to maintain

28. FRINGE FIELD The stray magnetic field outside the bore of the magnet. 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.

29. MAGNET SHIELDING Shielding means preventing individuals from the effect of magnetic field by keeping barriers between them . Reducing fringe field that potentially can interfare with other mechanically and magnetically activated devices. Potential safety hazards for the individuals if they gain access to a field strength of 5G. Shielding can be achieved by two ways :- Passive shielding Active shielding

30. PASSIVE SHIELDING Large steel plates incorporated either around the scanner or in the wall of the Magnet room. Lines of magnetic flux travel through ferromagnetic media in preference to air Passive shielding reroutes the fringe field away from the outside environment and back toward the scanner Passive shielding has several major disadvantages:- -The iron cladding can weigh over 20 tons, it is very expensive -the proximity of ferromagnetic metal can adversely affect the homogeneity of the scanner that it is intended to shield.

31. ACTIVE SHIELDING In addition to the main magnet solenoids, two larger diameter solenoids known as bucking coils positioned at each end of the bobbin They are designed and oriented so that the electrical current in the coil produces a magnetic field that reduces and opposes the main magnetic field . To do this, the bucking coils carry a current flowing in the opposite direction to the main magnet windings, reversing the flux.

32. 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. The uniformity should be in the range of 5ppm

33. SHIM COILS Extra loop of coils located around the circumference of the inner wall of the cryostat. Ensures the homogenicity of the magnetic field within the imaging volume (10ppm) . Shim coils requires a power supply separate from other power supplies within the system . We need homogenous magnetic field because Any distortion in magnetic field can cause distortion of the image Excitation of hydrogen nuclei is frequency dependent

34. SHIMMING Process by which main magnetic field is made homogenous . There are three mains type of shimming :- Passive shimming Active shimming G radient offset (dynamic) shimming

35. PASSIVE SHIMMING Field is corrected by using a ferromagnetic material , typically iron or steel . Ferromagnetic material are placed in a regular pattern at a specific location along the inner bore of the magnet

36. ACTIVE SHIMMING Uses electromagnets instead of ferromagnetic shims which is used in conjunction to passive shimming In modern scanners, active shimming is performed by additional superconducting solenoids inside the cryostat Current is passed through these ‘shim coils’ which generates small magnetic fields gradients superimposed on the main magnetic field and remove the field non-uniformities

37. G RADIENT OFFSET (DYNAMIC) SHIMMING Designed to manipulate the magnetic field during image acquisition Uses gradient set, another electromagnet that can achieve a homogeneity of better than 10ppm for good image quality Dynamic shimming can help to correct inhomogeneities caused by a large patient which can change homogeneity upto 9ppm by diamagnetic repulsion

38. GRADIENT SYSTEM Gradient system is an essential component of MRI system determining scan time, slice location, spatial encoding It creates the linear magnetic field gradient along all three orthogonal axes of imaging volume These mutually perpendicular axes are labelled x,y and z according to Cartesian geometry .

39. GRADIENT COIL set of wires generally a copper plated cylinder with the conductive elements etched into the surface of the metal plating Enables us to create magnetic field which are in way superimposed over main magnetic field Produce deliberate variation in the main magnetic field most widely manufactured superconducting scanners utilize distributed windings in a "fingerprint" pattern consisting of multiple thin metallic strips or large copper sheets etched into complex patterns and applied to the cylinder.   3 sets of wires each can create magnetic field in a specific direction (X,Y,Z) The variation in the frequency created by the gradient permits localization of image slice as well as the phase encoding and frequency encoding .

40. GRADIENT AMPLIFIERS A gradient coil requires a power supply g enerated by three gradient amplifiers one for each orthogonal gradient direction. They are typically housed in a cabinet remote from the scanner . Their function is to supply the current required by the gradient coils during image acquisition . Older models of MRI scanner used linear analog amplifiers whose major drawback was power losses as the higher the percentage of time that a gradient spends at maximum the more heating occurs

41. GRADIENT AMPLIFIERS Recent models are equipped with a different type of amplifier known as a pulse width modulated (PWM) design PWM amplifiers help reduce this power loss by applying voltage to the coil in short closely spaced discrete bursts. This technique improves the efficiency of the gradient coil, and, if the pulses are of sufficiently high frequency (closely spaced in time), the coil interprets the supply as smooth and continuous.

42. GRADIENT CHARACTERISTICS The ideal gradient set should be powerful, capable of high-amplitude gradient slopes, and rapid to reduce scan time Gradient amplitude: Gradient amplitude defines the power of the gradient, specifically how steep a gradient slope can be achieved when the gradient coil element is activated. Unit is mT /m or G/m Gradient Rise time: Gradient rise time is defined as the time taken for the magnetic field gradient to reach the required amplitude It is measured in microseconds ( μs ) and values are typically below 1000  μs

43. GRADIENT CHARACTERISTICS Gradient slew rate Gradient slew rate describes how quickly the gradient magnetic field can be applied at a given amplitude and over a given distance measured in meters The units used for measurement are T/m/s Slew rate is determined by dividing the amplitude of the gradient by the rise time Gradient power duty cycle: a more meaningful measure of gradient performance which is defined as the time that the gradient coil is operating at the required maximum amplitude and is expressed as a percentage of the total acquisition time. An efficient duty cycle ideally requires gradient amplifiers employing high-amplitude gradient pulses with very short spacing between pulses.

44. RADIOFREQUENCY SYSTEM Radiofrequency system transmits and receive electromagnetic radiation signals during image acquisition. RF used in MRI are called B1 being a secondary magnetic field . RF transmit system- RF transmit coils RF receive system- RF receive coils

45. Filter/ Receive amplifier Computer / control system RF Transmit coil Amplifier RF Pulse generator Computer/ control system RF Receive coil Preamplifier DAC ADC Rx switch Tx switch

46. RF SHIELDING RF range used in MRI 1-200MHz Radio ,TV and other communication system works on 1-100MHz. RF shielding is accomplished by lining the walls, floor, ceiling, and door with any conductive metal Copper was the original material of choice, due to its excellent conductivity, but it is comparatively expensive and heavy Aluminum panels are now favored by many manufacturers, as they are easier to handle absorb external radiofrequency such design is called Faraday Cage .

47. RF COILS Most important component of MRI . Coils to transmit the RF pulse and receive the RF signal . Radiofrequency coils can be divided on the basis of signal transmitted/received , anatomy covered and the type of polarization On the basis of signal transmitted or received :- Transmitter coil Receiver coil Transceiver coil

48. RF COILS On the basis of anatomy covered Surface coil Volume coil Phased array coil On the basis of polarization Linear polarized coil Circular polarized coil

49. RF TRANSMITTER COIL Generates the RF pulse that produces the magnetic field perpendicular to the main magnetic field and oscillate at resonant frequency Transmitter coil are usually built in in the MR gantry called as body coil( birdcage design ) located around the inner circumference of the magnet bore . Are positioned close to the patient must enclose entire region of interest surrounding the anatomical area .

50. RF RECEIVER COIL Coil is positioned within the imaging volume and connected to the receiver circuitry that is used to detect the MR signal from the patient Thus they optimize the signal coming from a given region of the body. To achieve satisfactory fill factor the structure and appearance of the coil vary greatly to match the shape and size of the anatomy under investigation .

51. SURFACE COILS It is a loop of wire, either circular or rectangular that is placed over the region of interest for increased magnetic sensitivity Receive-only coils Have a great SNR for tissues placed near the coil therefore the further the tissue is from coil, the less sensitive it is. Signals can be obtained from the depth which are 50-75% of the diameter of the coil. Are not typically used as RF transmitters because of their poor RF homogeneity. Generally used for shoulders , spines and TMJ .

52. VOLUME COILS Offer a comparatively homogenous RF field over a large imaging volume. This means that volume coils can be used to image anatomy at any location within the region of interest, at any depth. Signal from any anatomical structure enclosed by the coil is received with equal sensitivity All anatomy to be covered should be inside the coil . Volume coils are now offered with phased array capability. Better depth coverage but has low SNR Used for imaging head ( transmit/receive ) and knee (only receive)

53. VOLUME COILS Common types of volume coils are: Paired saddle coil Birdcage coil Helmholtz coil

54. ARRAY COIL Have advantage of both high SNR as well as of large field of view . Are the collection of small surface coils whose signals are combined and fed into an independent receiver circuitry Phased array – group of overlapping surface coils linked into a common output Parallel array – groups of non overlapping surface coils optimized for parallel imaging The number of coil elements and channels available in MRI coil varies between 2 to 128 depending on the purpose of coil.

55. LINEAR POLARIZED COILS Transmission and reception of RF pulse takes place in a single axis . I n the transmit mode LP coils are inefficient in that half their power is wasted. In the receiver mode they are incapable of extracting full phase information from the the MR signal.  Most surface coils are linearly polarized . .

56. CIRCULARY POLARIZED OR QUADRATURE COIL The coil is connected to two power supplies in which the supply from second terminal is delayed by 90˚ with respect to the first Creates a pure rotating B1 field . Better SNR as compared with linear polarized coils . Nowadays most volume coils are circularly polarized . Most common RF transmit quadrature coil is birdcage coil .

57. 2) PATIENT TRANSPORT SYSTEM a large non-ferromagnetic patient couch Facilitates patient access by lowering or raising the table height and driving horizontally into the magnetic bore. The main aim of couch movement is to position the center of the region of interest at the true iso -center of the imaging volume where the magnetic homogeneity is greatest The front cone of scanner is equipped with laser positioning lights Offers a call button and patient microphone to allow two way communication between the patient and staffs at operating console patient cooling fan and headset may also be present. patient table

58. 3) COMPUTER SYSTEM AND GRAPHICAL USER INTERFACE the entire process of MRI acquisition is arranged by host computer The graphical user interface (GUI) of the system is used for patient identification that prepare the system to calculate the amplitude of the RF pulses and level the physical orientation of anatomy shown on images Pulse generator module (PGM), an additional computer that sends instructions relating to timing, amplitude and the shape of the transmitted RF pulses and the timing and duration of sampling window Host computer is connected to the server for image archiving

59. SPECIFICATION OF MRI USED IN OUR DEPARTMENT Model: S iemens Healthineers Magnetom Amira Superconducting magnet of magnetic strength 1.5T Closed type Bore size: 60cm Magnet weight: 3200 kg Magnet length: 155cm Maximum Gradient strength: 33mT/m Slew rate: 125 mT /m/ ms Active and passive shimming Fringe field – 4 × 2.5 m ( 0.5 mT ) Maximum helium capacity -1300 litres

60. RF COIL WITH NUMBER OF CHANNELS IN MAGNETOM AMIRA 1.5T Head/Neck coil: 16 Phased array spine coil: 18 Shoulder coil 16 Hand/Wrist coil 16 Body coil 13 Foot/Ankle coil 16 Breast coil 18

61. SUMMARY MR scanner is a coil within a coil within a coil within a coil (Static magnetic coil – shim coil – gradient coil – radiofrequency coil) Most of the MR scanner used today are of closed bore superconducting type with niobium titanium coil in a helium bath . shim coil ensures the homogenicity of the magnetic field within the imaging volume. Gradient coil creates linear magnetic field gradient along all three orthogonal axes of imaging volume.

62. SUMMARY Radiofrequency coil are used to transmit RF pulse and receive RF signal Patient transport system facilitates patient access by lowering or raising the table height and driving horizontally into the magnetic bore. the entire process of MRI acquisition is arranged by host computer

63. REFERENCES MRI in practice by Catherine Westbrook and John Talbot 5 th edition The essential physics of medical imaging by Jerrold T. Bushburg Mriquestions.com Chat.openai.com www.slideshare.net Previous presentations

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