One of the toughest and mind boggling topic explained in simple and interesting manner . FOR YOUNG RADIOLOGY RESIDENT AND TECHNOLOGIST. MRI MADE EASY

1. MRI
INSTRUMENTATION
DR MOHAMMAD MASOOD PG 3RD YEAR
MODERATOR : PROF. FEROZE SHAHEEN
S/R INCHARGE : DR HASEEB
DR WAMIQ
DEPARTMENT OF RADIODIAGNOSIS AND IMAGING
,SKIMS

2. .
Nikola Tesla, 1890 (age 34)

3. • When Nikola Tesla first described the rotating magnetic field in 1882, he could
hardly have imagined what it would lead to.
• Just 130 years later, with the assistance of a paper napkin and a thin grad
student, that discovery has become the basis for the highly popular tool of
magnetic resonance imaging.

5. • In 1937, Isidor Rabi, a physics professor at Columbia University, developed a
method for measuring the movements of atomic nuclei — a state he decided to
call nuclear magnetic resonance (NMR), for which efforts he was awarded the
1944 Nobel Prize in Physics.
• used mainly to analyze the structures of chemical substances

6. • In 1937, Isidor Rabi, a physics professor at Columbia University, developed a
method for measuring the movements of atomic nuclei — a state he decided to
call nuclear magnetic resonance (NMR), for which efforts he was awarded the
1944 Nobel Prize in Physics.
• Around 1960s, a doctor named Raymond Damadian began to wonder if the
same methods could be used on living organisms.

7. FEATURED HISTORY: MAGNETIC RESONANCE
IMAGING
• In 1937, Isidor Rabi, a physics professor at Columbia University, developed a
method for measuring the movements of atomic nuclei — a state he decided to
call nuclear magnetic resonance (NMR), for which efforts he was awarded the
1944 Nobel Prize in Physics.
• Around 1960s, a doctor named Raymond Damadian began to wonder if the
same methods could be used on living organisms.
• In 1971, he concluded that since cancerous tissue contained more water than
healthy tissue, it could be detected by scanners that bathed a part of the human
body in radio waves and measured the emissions from the local hydrogen atoms.

8. Raymond Damadian's "Apparatus and method for detecting cancer in
tissue

9. FEATURED HISTORY: MAGNETIC RESONANCE
IMAGING
• Around the same time, a chemist named Paul Lauterbur was hard at work using
NMR to create images — first of vials of water, then of clams and green peppers.
When he read Damadian’s 1971 findings, he realized that his work could have
biomedical applications. He was the first to realize that a gradient magnetic field
would allow observers to take two-dimensional images of an object, which could
then be stacked to create a three-dimensional view — an idea he first sketched
out on a paper napkin between bites of a Big Boy hamburger.

10. • Meanwhile, in England, physicist Peter Mansfield was tackling the question of
time, trying to find a way to complete scans in minutes rather than hours. By
abandoning the usual “sensitive point scan” method and adopting a new
technique he called “line scan imaging”, Mansfield was able to capture images of
his grad student Andrew Maudsley’s finger in only 15–23 minutes per section,
marking the first time that a human body part had been successfully scanned with
NMR technology.

11. FEATURED HISTORY: MAGNETIC RESONANCE
IMAGING
• Damadian was busy constructing: a 1.5-ton machine called Indomitable, which
leaked $2,000 worth of helium coolant per week. On May 11, 1977, Damadian
donned the cardboard-backed antenna coil, slid onto Indomitable’s moving
platform, and lay there while his assistants booted up the imaging systems …
with no resulting image. After hours of adjusting the device, eventually someone
suggested that perhaps Damadian’s body-fat content was too high. A thinner grad
student, Larry Minkoff, volunteered to be the next guinea pig. He observed
Damadian closely for side effects, and after seven weeks of seeing none, he
climbed into Indomitable. Nearly five hours later, the team was rewarded with a
two-dimensional image of Minkoff’s chest.

13. • Eventually, Damadian’s methods
were rejected as too slow for routine
clinical use, and Lauterbur and
Mansfield’s methods were adopted
instead. For this success, Lauterbur
and Mansfield were awarded the
2003 Nobel Prize in Medicine

14. • early marketers decided to drop both Lauterbur’s preferred term
“zeugmatography” (derived from the Greek word zeugma, or “yoke”) and the
word “nuclear” from “nuclear magnetic resonance,” reasoning that this would
allay people’s fears about radiation.

15. MR INSTRUMENTATIONS
• The main constituents of an MRI scanner is the magnet subsystem which
produces a spatial and temporally constant magnetic field B0, the gradient
subsystem to produce varying magnetic field gradients for spatial encoding, RF
subsystem to transmit and receive radiofrequency B1 and the computer and
microprocessors to specify and control the pulse sequence ,calculate, process,
display, store and transfer the resultant
clinical image.

16. • There is viewing console for operator input of control parameters and display of
imaging.
• In addition, magnetic and RF shielding is required.
• Patient table, hardware for physiological monitoring of patient (ECG
and respiratory gating) and monitoring equipment is also needed.

20. MAGNETS
• the key component of the MR scanner.
• determines the appearance, cost and capacity of the device.
• There are three types of magnets used in MRI— permanent magnets, resistive
magnets and superconducting magnets.

21. PERMANENT MAGNETS
• Permanent magnet is made up of ferromagnetic substances.
• Usually MR magnets are made up of alnico, which is alloy of aluminium, nickel
and cobalt.
• Do not require power supply and are of low cost.
• Magnetic field of permanent magnet is directed vertically.
• Magnetic field strength achievable with permanent magnet is low in the range,
typically 0.2 to 0.5 Tesla. Hence they have low SNR and low resolution.
• Open MRI is possible with permanent magnet, which are useful in claustrophobic
patients

22. ELECTROMAGNETS
• based on principle of electromagnetism. Law of electromagnetism states that
moving electric charge induces magnetic field around it.
• If a current is passed through a wire, a magnetic field is created around that wire,
strength of the resultant magnetic field is proportional to the amount of current
moving through the wire.
• When a wire is looped like a coil and current is passed through it, the magnetic
field generated is directed along the long axis of the coil. Magnets made of such
coils are called solenoid or resistive electromagnets.

23. • The field strength obtained with electromagnets is limited to 0.2 to 0.3 Tesla
because of continuous power and cooling requirements.
• Even though capital cost is low operational cost is high for electromagnets
because enormous power requirement.

24. SUPERCONDUCTING MAGNETS
• Some metals like mercury or Niobium-Titanium alloy lose their electric resistance
at very low temperature and become superconductors.
• Therefore in superconducting magnet higher field strength is achieved by
completely eliminating the resistance to the flow of current
• .Moreover, once superconductor wires or coils are energized the current
continues in the loop as long as the superconducting wire is maintained below the
critical temperature.
• no power loss and continuous power supply is not required to maintain magnetic
field

25. THE STRUCTURE OF THE SUPERCONDUCTING MAGNET

26. IMPORTANT COMPONENTS ARE AS FOLLOWS
• 1. Superconducting Wires: Magnetic field is produced when current is passed
through the superconducting wires. These wires are made up of Nb/Ti alloy. This
alloy becomes superconducting at10K (kelvin). A wire containing filaments of
Nb/Ti alloy embedded in a copper matrix, is wound tightly and precisely on
insulated aluminium bore tube.
• 2. Helium: The coil made of superconducting material is cooled to 4K (-269
degree Celsius) by cryogens like liquid helium, which surrounds the coil all
around. Because of smaller heat leaks into the system, helium steadily boils off.
This boil off is reduced by much cheaper liquid nitrogen.

27. • 3. Liquid Nitrogen and Radiation Shield: The can of liquid helium is surrounded by
cooled liquid nitrogen and radiation shields. This prevents any heat exchange
between helium and the surrounding. Nitrogen boils at 80 degree K and is much
cheaper than the helium.

28. STARTING THE MAGNET
• When magnet is started first time, it is done in a particular way or sequence.
• First, superconducting coil is cooled to -269 degree Celsius by helium and liquid nitrogen
• Then the magnet is energized by delivering current from external power source to the
superconducting coil.
• Once desirable level of current is achieved, power supply is cut off.
• The current continues to circulate through the coil as long as the temperature is maintained
below -269 degrees Celsius.
• The current and the magnetic field produced remain constant and subjected only to minor
changes.

29. MAGNET GEOMETRY
Tunnel systems:
These have a tunnel bore with horizontal magnet field, best magnetic field
homogeneity and are seen only with superconducting systems. Wide-bore
designs have superior homogeneity for large FOV’s, while short-bore magnets
provide relief from claustrophobia
Open systems
Have a vertical magnetic field; thus perpendicular to head-foot axis of
patient. can be permanent or superconducting magnet

31. • Specialty magnets are specially designed for a particular type of examination usually for
musculoskeletal applications.
• Have a superconducting/resistive magnet. 70 Mt/m of power and a slew rate of 300 T/m is the
available gradient strength with one vendor.10 Dedicated RF coils (80–180 mmin diameter) are
available for a range of anatomies. There is a chair/recliner so that the patient is seated
comfortably

33. • Magnetic field should be uniform all over its extent to get correct information
(signal) from the patient. Even though magnetic field is more or less uniform there
might be minor inhomogeneity.
• THE PROCESS OF MAKING THE MAGNETIC FIELD HOMOGENEOUS IS
CALLED AS “SHIMMING
• This process is necessary as the magnetic field is prone to become
inhomogeneous because of the difficulty of winding a perfect coil and presence of
metal within the environment.

34. SHIM COILS
• The magnetic field must be stable and not affected by temperature.
• The homogeneity of a magnet is specified as the maximum deviation of the
field in points per million(ppm) over a spherical volume of a given diameter
(dsv).
• uniformity should be in the range of 5 ppm.
• Imperfections in manufacturing of the coils lead to harmonic error field. The
process of ‘shimming’ is required to overcome these errors.

35. • Two methods are available for shimming
• Passive shimming: It is achieved by placing metal pieces, called shim
plates, in the field to oppose the inhomogeneity.
• Active shimming: A set of coils is used and current is passed through
these ‘shim coils’.generates small magnetic fields gradients
superimposed on the main magnetic field and remove the field
nonuniformities.

36. • Homogeneity of 10 ppm may be sufficient for routine spin echo imaging. However,
for
proton spectroscopy a highly homogeneous field of 0.1 ppm is required to be able
to detect metabolites with smaller chemical shift differences.

38. SHIELDING
• The stray magnetic field outside the bore of the magnet is known as fringe field.
• This stray field can cross conventional walls, floors or ceilings and can potentially
harmful to patients with pacemakers, for monitoring devices, and other
magnetically activated devices.
• Passive shielding is done by lining the wall of MR room by steel or copper. This
shielded chamber is called as ‘Faraday cage’.
• Active shielding uses additional solenoid magnet outside the cryogen bath that
restricts the Bo lines to an acceptable location.

40. GRADIENT COILS
• Gradients or gradient coils are used to vary magnetic field strength over
the extent of magnetic field.(Provide gradient magnetic field)

41. GRADIENT COILS
• Gradients or gradient coils are used to vary magnetic field strength over
the extent of magnetic field.(Provide gradient magnetic field)
• Gradient system consists of three sets of coils ,namely X, Y, and Z gradients
carrying direct current
• All coils are connected to amplifiers, which control the rise and maximum
value of the gradient.
• Current in the two coils flow in opposite directions creating a magnetic
field gradient between the two coils.

43. GRADIENT COILS
• The three gradients applied along X, Y and Z axes are perpendicular to
each other and are used for slice selection, phase encoding and
frequency encoding.

45. • The Z gradient produces , varying magnetic field in CRANIO CAUDAL direction
(Helmholtz coil).
• The field is lower at head side and higher at toe side. So protons process slower
at head end and faster at toe end.
• When RF is applied ,only spins having frequency equal to that of RF , will
undergo resonance and give MR signal .Thus this gradient helps to select the
thickness of tissue…………..hence called SLIDE SELECTION GRADIENT .

47. • The Y gradient produces varying magnetic field from front to backside of human
body. Spins in front precess slowly than at back.
• There is phase difference between spins , depends upon their position.
• The MR thus generated with gradient will have phase variation.
• Hence this gradient is called PHASE ENCODING GRADIENT.

49. • The X gradient is applied from side to side of patient protons of left side precess
slowly than of right side .
• This MR signal has frequency variation
• Hence is called FREQUENCY GRADIENT

52. • Fast switching gradients can lead to induction of eddy currents due to the
changing magnetic fields produced. These degrade imaging sequences by
producing geometric distortion, blurred images and artifacts like ghosting .

53. “Eddy current’ artifact—inhomogeneities in the
gradient cause distortion in the image

54. RADIOFREQUENCY COILS
• Radiofrequency coils (RF coils) are the "antennae" of the MRI system,
broadcasting the RF signal to the patient and/or receiving the return signal. RF
coils can be receive-only, in which case the body coil is used as a transmitter;
or transmit and receive (transceiver).

55. RADIOFREQUENCY COILS
• Radiofrequency coils (RF coils) are the "antennae" of the MRI system,
broadcasting the RF signal to the patient and/or receiving the return signal. RF
coils can be receive-only, in which case the body coil is used as a transmitter;
or transmit and receive (transceiver).
• Energy is transmitted in the form of short intense bursts of radiofrequencies
known as radiofrequency pulses. These RF pulses cause phase coherence and
flip some of the protons from a low energy states to high energy states.

56. • Rotating transverse magnetization (TM) induces current in the receiver coil, which
forms the MR signal.

57. • Based on design, RF coils can be divided as volume coils , surface coils and
phased-array coil.
• A volume coil typically surrounds either the whole body or a specific region.
• It provides a homogeneous B1 field (RF field perpendicular to main field
B0) in the imaging volume circumscribed by the coil.
• However, the SNR of the images obtained with volume coil is usually less
than that obtained with surface or phased-array coil.

58. • Head coils are example of volume coils.
• The main coil of the magnet,also called the body coil, is a volume coil. It is
located in the magnet bore as the inner most ring.
• Some configurations of the volume coil include solenoid, saddle and bird
cage coils

59. Circularly polarized head coil, volume coil—both
transmit and receive

60. SURFACE COIL
• Surface coils are usually receiving coils with a limited area of sensitivity from
which they receive signal.
• Surface coils fit closely over specific anatomic region.
• The sensitivity of the coil is related to the radius of the coil.
• These include circular coil configuration for orbits and TM joints, rectangular
coils for lumbar spine or irregular shapes for shoulder, cervical spine.

61. • Surface coils are advantageous as they increase the SNR.
Surface loop coil—receive only

62. • For a solenoidal magnet the transmitter and receiver coils are saddle shaped and
have the volume of the greatest B1 field homogeneity along the linear portion of
the coil, the ends being inhomogeneous.
Linearly polarized body coil comprising flat conductors
configured in a hollow tube

63. • An alternate design called a bird cage coil or resonator has improved B1
homogeneity
and has higher sensitivity. These are volume coils consisting of two circular or
elliptical conducting end rings joined by conducting rings or legs.

64. BIRD CAGE RESONATOR CIRCULARLY POLARIZED BODY COIL SHOWING 2 RF
FIELDS WITH A PHASE SHIFT

65. PHASED ARRAY COILS:
• Arrays of surface coil are used to extend the effective FOV of the receiver coil
while maintaining the improved SNR characteristics of a limited FOV of a single
coil.
• The coils are electromagnetically decoupled from one another by selecting the
overlap between neighboring coils, so that mutual inductances disappear.
• Phased array coils have to be positioned perpendicular to the main magnetic
field.

66. Phased array coil—multiple coil elements, increasing
SNR and FOV

67. COMPUTERS
• The command center of the MRI system.
 shapes and times the RF pulses
 turns the gradients on and off
 controls the RF receiver to collect data
 required for manipulation, storage, retrieval

68. DATA PROCESSING AND IMAGE
RECONSTRUCTION
• The two primary dedicated digital control systems are
• PULSE GENERATOR and the DATA ACQUISITION SYSTEM
• The pulse generator synchronizes the gradients and RF pulses after selection at
operator console. Digital synchronization is provided for the receive channel
analog to digital convertor, such that the detected signal correlate with the applied
gradient and RF frequency.
• The temporal positional accuracy and repeatability (TPAR) is an important
specification of the system.

69. • Data is collected from one or more receiver channels.
• This may be in analog or preferable digital domain.
• In conventional MR system, multiple analog coaxial cables are required.
• In digital broadband system digitization is inside the RF receive coil, the number
of RF channels is now determined by the coils, rather than the system.
• There is a single broadband fiberoptic cable that is independent of number of
elements/channels in an RF coil. A gain of 40% in SNR is claimed

70. • Data processing from multiple channels in the digital domain allows for correction
of the inhomogeneous sensitivity of the coil and produces an acceptable image
with a required FOV.

71. • The complex MR signal is sampled and computer analyzed into a spectrum of
component frequencies using a mathematical process called Fourier analysis.
• The data from every signal in a selected slice are stored in k space. k-space
refers to a data matrix containing the raw MRI data.
• This data is subjected to mathematical function or formula called a Fourier
transform to generate the final image.

72. • A single slice corresponds to a k-space plane acquired in real-time. Each point on
the k-space contains specific frequency, phase (x ,y coordinates) and signal
intensity information (brightness).
• Each pixel in the resultant image is the weighted sum of all the individual points
in the k-space.
• Disruption of any point in the k-space translates into some form of final image
distortion,

74. • k space has to be completely filled with the data from the imaging sequence
before the signal is analysed and processed into the image.
• The data once acquired, is stored in a large memory array after being filtered
• After the image is reconstructed, may be displaced for instantaneous viewing or
stored in a database for review.

75. ADVANCED MR APPLICATIONS
AND HARDWARE
• Magnetic Resonance-High-Density Focussed Ultrasound (or MRgFUS)
• Magnetic Resonance Elastogram.
• MR Surgical Suite: Interventional MRI
• Magnetic Resonance–Positron Emission Tomography

76. • Magnetic Resonance-High-Density Focussed Ultrasound (or MRgFUS)
• A noninvasive method using high-intensity focused ultrasound to heat and
destroy targeted tissue And MRI for precise visualization , guidance and real
time treatment control .Clinical uses….FIBROIDS,BONE METASTASIS AND
PROSTRATE ETC.

77. ADVANCED MR APPLICATIONS
AND HARDWARE
• Magnetic Resonance-High-Density Focussed Ultrasound (or MRgFUS)
• Magnetic Resonance Elastogram.
• An imaging technique used to measure the elasticity of tissue by gently shaking
the tissue in a magnetic resonance imaging (MRI) machine.
• The technique employs standard MRI equipment with a few modifications and a
vibrating metal plate placed on the skin.
• Finally the data generated is processed to generate elastograms—color coded
anatomic images that depict the relative stiffness of tissue in the cross section of
interest
• This technique is used to assess liver fibrosis.

79. ADVANCED MR APPLICATIONS
AND HARDWARE
• Magnetic Resonance-High-Density Focussed Ultrasound (or MRgFUS)
• Magnetic Resonance Elastogram.
• MR Surgical Suite: Interventional MRI
• Interventional MRI is the use of MR techniques for guidance of both diagnostic and minimally
invasive therapeutic interventions.
• The magnet is of the ‘open’ type to allow access to the patient.
• Use of new fast gradient echo pulse sequences allows continuous MR images (0.3–7 seconds
per image).
• especially useful in areas of complex anatomy like skull base, retropharynx, etc.

81. ADVANCED MR APPLICATIONS
AND HARDWARE
• Magnetic Resonance-High-Density Focussed Ultrasound (or MRgFUS)
• Magnetic Resonance Elastogram.
• MR Surgical Suite: Interventional MRI
• Magnetic Resonance–Positron Emission Tomography
• Fully integrated magnetic resonance–positron emission tomography (MR–
PET) suite has been introduced. Has integrated cooling feature with
specialized shielding to eliminate magnetic field interference in PET data
processing.