MRI physics basics II

1. D R . A R C H A N A K O S H Y
MRI PHYSICS II

2. OVERVIEW
1. MRI Instrumentation
2. Sequences
3. Artefacts
4. MR safety

5. RADIOFREQUENCY COIL
• RF coils are used to transmit RF pulse into the patient
and to receive the signals from the patient .
• Energy is transmitted in the form of short intense bursts of
radiofrequencies known as RF PULSES .
• Causes phase coherence and flip some of the protons
from a high energy to a low energy state .
• Rotating TM vector induces current in the receiver coil
,which forms signal .

6. SURFACE COILS
• Surface coils are the simplest design of coil.
• A loop of wire, either circular or rectangular, that is placed over the
region of interest
• The depth of the image of a surface coil is generally limited to about
one radius.
• Commonly used for spines, shoulders, temporomandibular joints,
and other relatively small body parts.

7. PAIRED SADDLE COIL
• Commonly used for imaging of the knee.
• Provide better homogeneity of the RF in the area of interest and are used
as volume coils, unlike surface coils.
• Paired saddle coils are also used for the x and y gradient coils.
• By having current flow in opposite directions in the two halves of the
gradient coil, the magnetic field is made stronger near one and weaker
near the other.

8. • The Helmholtz pair coils consist of two circular coils parallel to each
other. They are used as z gradient coils in MRI scanners.
• They are also used occasionally as RF coils for pelvis imaging and
cervical spine imaging.
HELMHOLTZ COIL

9. BIRDCAGE COIL
• Provides the best RF homogeneity of all the RF coils.
• Has the appearance of a birdcage.
• This coil is commonly used as a transceiver coil for imaging of the
head.
• This type of coil is also used occasionally for imaging of the
extremities, such as the knees.

11. MAGNETS
• Magnetic susceptibility Is the ability of the substance to get
affected by external magnetic field .
1. Paramagnetism –Unpaired electrons within the atom
-Results in a small magnetic field around them called magnetic
moment .
-Gadolinum , Oxygen,Melanin
2. Diamagnetism- React in opposite way when external
magnetic field is applied .
3. Ferromagnetism- Strongly attracted to a magnetic field .
(Fe,Co,Ni)
-Used to make permanent magnets

12. PERMANENT MAGNETS
• Do not require power supply and are of low cost .
• Magnetic field is directed vertically .
• Open MRI is possible with permanent magnets , useful for claustrophobic
patients .
• Magnetic field strength achievable with permanent magnet is low in the
range of 0.2-0.5 Tesla .
• Low Spatial noise resolution hence higher applications like spectroscopy
cannot be done on them .

14. ELECTROMAGNETS
• Based on the principle of
electromagnetism –Moving electric
charge induces a magnetic field around
it .
• When a wire is looped like a spring coil
, the magnetic field generated is
directed along the long axis of the coil.
• Capital cost is low ,operational cost is
high for electromagnets because
enormous power is required .
• Easy to install and can be readily turned
on and off inexpensively.

15. SUPERCONDUCTING MAGNETS
• Higher field strength is achieved by eliminating resistance.
• Once the wires/coils are energized, the current continues in a loop
as long as it is maintained below the critical temperature.
• No power loss
• Continuous power supply is not required to maintain a magnetic
field .

16. SEQUENCES

17. SPIN ECHO
The MR scanner can only detect signal in the transverse plane.
• There are two disadvantages of this approach:
(1) The signal decays very rapidly, requiring an extremely fast scanner to
detect it before it dies out
(2) The signal depends on T2*, not T2, so it is very susceptible to local
magnetic field inhomogeneity.
In order to combat both of these challenges, we can add another pulse to
generate an "echo" that occurs later in time and happens to reverse the
T2* effects to leave us with T2 only. This is referred to as the spin
echo.

18. • 3 main phases
(1) 90-degree pulse followed by free induction decay;
(2) 180-degree pulse followed by rephasing
(3) The echo that occurs at TE, when the scanner acquires
the signal -Readout.
- We need to repeat this sequence a number of times to
acquire the entire image.
- The time between each repetition of the sequence is
referred to as the repetition time or TR

19. FORMATION OF A SPIN ECHO
• In terms of k-space representation of the spin-echo sequence, the application of
phase encoding and read dephase gradients results in movement from the centre of
k-space A to position B.
• The 180º pulse reverses the k-space position in both phase and frequency directions,
resulting in movement from B to C.
• This is followed by frequency encoding from C to D via the centre of k-space.
• Each line of data is Fourier transformed to extract frequency information from the
signal and the process is repeated for different phase encode steps.

21. GRADIENT COILS
• Induce small linear changes in magnetic field along one or more
dimensions.
• Consists of three sets of coils that produce field with changing
strength in X,Y,Z directions .
• Produces two types of spatial encoding referred to as Frequency
and Phase Encoding.
• Three basic differences between Spin echo and gradient echo
sequence imaging :
1. There is no 180 degree pulse in GRE.
-Rephasing of TM in GRE is done by gradients, particularly by
reversal of frequency encoding gradient .
2. Flip angle in GRE is smaller,usually less than 90 degree .
21

22. GRADIENT ECHO SEQUENCE
• Consists of a series of excitation pulses, each separated
by a repetition time TR.
• Data is acquired at some characteristic time after the
application of the excitation pulses and this is defined as
the echo time TE.-Time between the mid-point of the
excitation pulse and the mid-point of the data acquisition.

23. INVERSION RECOVERY SEQUENCE
• A variant of a SE sequence in that it begins with a 180º
inverting pulse.
• This inverts the longitudinal magnetisation vector
through 180º.
• When the inverting pulse is removed, the magnetisation
vector begins to relax back to B0.

24. Uses of Inversion Recovery Sequence
Contrast is based on T1 recovery curves following the 180º inversion pulse.
Inversion recovery is used to produce heavily T1 weighted images to demonstrate
anatomy.
The 180º inverting pulse can produce a large contrast difference between fat and water
because full saturation of the fat or water vectors can be achieved by utilising the
appropriate TI.

25. STIR (SHORT T1 INVERSION RECOVERY )
• The inversion recovery sequences are spin echo sequences with a 180°
preparation pulse to flip the longitudinal magnetization into the opposite
direction (i.e. spins are flipped to the 180° position).
• To generate an MR signal, the longitudinal magnetization is then
converted to transverse magnetization through the application of a 90°
pulse.
• The interval between the 180° pulse and the 90° stimulation pulse is
known as inversion time (TI).
• As the spins relax back to their equilibrium configuration the signal for
each spin group will evolve from a negative signal which is zero (null
point) to a positive signal. This rate is determined by the T1 of the spin
group.
• Since at 1.5 T, T1 fat= 260 ms and for most other tissues T1 > 500
ms,there fore the null point for the fat signal will occur sooner than the
other tissues.
• Therefore an inversion recovery sequence with a short inversion time
(TI) of 130-150 is used for fat suppression.

27. FAT SATURATION
• Normal MR imaging methods visualize protons from both water
and fat molecules within the tissue.
• Fat and water have a chemical shift difference of approximately
3.5 ppm in their resonant frequencies.
•
• Fat saturation uses a narrow-bandwidth rf pulse centered at the
fat resonant frequency applied in the absence of a gradient .
• The resulting transverse magnetization is then dephased by
spoiler gradients.
• A standard imaging sequence may then be performed that
produces images from the water protons within the slice.

28. • TWO main advantages over STIR imaging for fat
suppression.
1. It may be incorporated into any type of imaging
sequence.
2. T1 fat saturation sequences may also be used with
gadolinium based T1 contrast agents since the contrast
agent shortens the T1 relaxation times of only the water
protons
3. Enables the enhanced tissues to generate significant
signals while the fat signal remains minimal in the
presence or absence of the contrast agent

30. PROTON DENSITY IMAGE
• Tissues with the higher concentration or density of protons
produce the strongest signals and appear the brightest on the
image.
• Proton density weighted sequence produces contrast mainly
by minimizing the impact of T1 and T2 differences with long
TR (2000-5000ms) and short TE (10-20).

31. DIFFUSION WEIGHTED IMAGING (DWI)
• MR imaging based upon measuring the random Brownian motion of
water molecules within a voxel of tissue.
• Densely cellular tissues or those with cellular swelling exhibit lower
diffusion coefficients
• Particularly useful in tumour characterisation and cerebral
ischaemia
• The motion of these water molecules can be restricted by the
presence of barriers, principally cell membranes.
• The degree of diffusion restriction can be quantified by a diffusion
coefficient, which reflects the average distance a particle will move
in a second

32. • Typical DWI sequences are spin echo sequences, with 90-
and 180-degree pulses.
• The diffusion gradients are turned on before and after the
180-degree pulse.
• DWI sequences need to be extremely fast in order to
eliminate any motion within the body part .
• The TR is long in order to reduce T1 effects and improve
signal.
• The TE is kept as short as possible, but the insertion of the
diffusion gradient after the 180 pulse necessitates a longer
TE.
• Therefore, DWI images are also T2 weighted .

33. FLAIR ( FLUID ATTENUATED INVERSION
RECOVERY )
• Another variation of the inversion recovery sequence.
• The signal from fluid e.g. cerebrospinal fluid (CSF) is nulled
by selecting a TI corresponding to the time of recovery of CSF
from 180º inversion to the transverse plane.
• The signal from CSF is nullified and FLAIR is used to
suppress the high CSF signal in T2 and proton density
weighted images so that pathology adjacent to the CSF is
seen more clearly.

34. USES :
1. To assess the extent of
perilesional edema
2. Infarcts are better
appreciated
3. Bright lesions of Multiple
Sclerosis
4. Increased signal in mesial
temporal sclerosis
5. Fast FLAIR – subarachnoid
haemorrhage

35. ARTEFACTS

36. 1. MR HARDWARE AND
ROOM SHIELDING
• Zipper artifact
• Zebra stripes
• Moire fringes
• Central point artifact
• RF overflow artifacts
• Inhomogeneity artifacts
• Shading artifact
2. MR SOFTWARE
• Slice-overlap artifact- cross-
talk artifact
• Cross excitation
3. PATIENT AND
PHYSIOLOGIC MOTION
• Phase-encoded motion artifact
4. TISSUE HETEROGENEITY
AND FOREIGN BODIES
• Black boundary artifact
• Magic angle effect
• Susceptibility artifact/magnetic
susceptibility artifact
• Chemical shift artifact
5. FOURIER TRANSFORM
AND NYQUIST SAMPLING
THEOREM
• Gibbs artifact/truncation artifact
• Aliasing/wrap around artefact

37. GHOSTS
• Replica of a structure in the image .
• Produced by anatomy moving along a
gradient during pulse sequence resulting
into phase mismapping .
• Can originate from any structure moving
during the acquisition of data .
• Always occur along the phase encoding
axis
• Can be caused by mechanical vibration
,temporal variation or receiver coil
sensitivity ,fluctuations of magnetic field
and simulated echoes .

38. Appearance of ghosting on final clinical image depends on
where in k-space such phase errors occur:
• If along the x-axis of k-space : Frequency encoding direction
• If along y-axis : Phase encoding direction
• If in middle of k-space : Smearing appearance
• If phase errors are periodic (as in pulsatile motion)-Periodic
ghosting.
Relatively more common in the phase encoding direction.

40. ALIASING/WRAPAROUND
• In aliasing, anatomy that
exists outside the field of
vision appear in an image .
• Produces a signal if it is in
close proximity to the receiver
coil
• During signal encoding
,signals from this outside field
of view structures are also
allocated pixel positions .
• Aliasing along frequency
encoding axis is called
frequency wrap
• Along phase encoding axis is
called phase wrap

41. CHEMICAL SHIFT RELATED ARTEFACTS
• Due to the different chemical environments , protons in water
and fat precess at different frequencies –CHEMICAL SHIFT
• Frequency of water protons is about 3.5 ppm greater than that
of fat protons .
• Chemical shift forms the basis of MR spectroscopy ,however
some chemical shifts become source of artefacts in MR
imaging .
(i) Chemical shift misregistration artefact
(ii) Interference from chemical shift (in phase/out phase)

43. TRUNCATION ARTEFACTS
• Produce low intensity bands running through a high
intensity area .
• Caused by under sampling of the data so that interfaces of
high and low signals are incorrectly represented on the
image .
• Can be misleading in long narrow structures, such as
spinal cord or intervertebral disc .
GIBBS’ ARTEFACT : In T1 weighted sagittal image of
the cervical spine,CSF in central canal appear dark as
compared to spinal cord
May be misinterpreted as syringomyelia

45. MAGNETIC SUSCEPTIBILITY ARTEFACT
• These artefacts results from local magnetic field
inhomogeneities introduced by a metallic object into the
otherwise homogeneous external magnetic field B0.
• These local magnetic field inhomogeneities are known as
magnetic susceptibility and are a property of the object being
imaged
• More prominent in Gradient echo sequence than Spin echo
sequence .
• Corrected by removing all metals and use of soin echo
sequence .

47. ZIPPER ARTIFACTS
• Lines with alternating bright and dark pixels propogating
along the frequency encoding direction .
• Can be eliminated by spoiler gradients in special pattern to
remove simulated echoes .
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48. CROSS-EXCITATION ARTIFACTS
• The imperfect shape of RF slice profiles leads to the unintended
excitation of adjacent tissue.
• This excitation results in the saturation of such tissue
• Manifest as decreased signal intensity and decreased contrast that
can hinder lesion detection.
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49. SPIKE ARTIFACT
• Caused by one defective data point in k-space.
• The resulting image show diagonal lines throughout the
image.
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50. ZEBRA STRIPES
• Observed along the periphery of
gradient-echo images (abrupt
transition in magnetization at the
air-tissue interface)
• Increased by aliasing that results
from the use of a relatively small
field of view.
• May also occur when pt. touches
the coil or a result of phase wrap.
CORRECTIVE MEASURES
• Expanding the FOV, using SE
pulse sequences.
• Using oversampling techniques to
reduce aliasing.
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51. RF OVERFLOW ARTIFACTS (CLIPPING)
Causes a nonuniform, washed-out appearance to an image.
Occurs when the signal received from the amplifier exceeds
the dynamic range the analog-to-digital converter causing
clipping.
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52. MOIRE FRINGES
An interference pattern most commonly seen when doing gradient
echo images.
One cause is aliasing of one side of the body to the other results
in superimposition of signals of different phases that add and
cancel.
Can also be caused by receiver picking up a stimulated echo.
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53. MRI SAFETY

54. FORCES IN THE MR ENVIRONMENT
• There are two types of effects the magnet will
have on Ferromagnetic substances
• Translation: The “Missile Effect”
• Rotation/Torque: The “Rotational Effect”

55. TRANSLATIONAL FORCE: THE
MISSILE EFFECT
• Also referred to as the “Projectile Effect” and is used to
describe the attraction of the object to the center of the
magnetic field
• This transforms objects into projectiles as they accelerate
toward the magnet
• These items can become airborne, accelerating at speeds
of up to 40 miles per hour.
• This effect causes accidents jeopardizing the safety of
patients and staff, as well as the MRI equipment itself.

57. ROTATIONAL/TORQUE FORCE:
ROTATIONAL EFFECT
• This force relates to the North and South pole orientation of
the scanner’s magnetic field
• Ferrous objects will attempt to align their long axes with this
orientation
• This force will rotate objects until they are aligned with the
magnetic field.
• Translational and Rotational Force happen at the same time
making objects projectiles and dangerous!

58. METAL OBJECTS BECOMING
PROJECTILES

59. • All persons that have reason to enter the MR suite area
should be trained in MR safety procedures. These include
but are not limited to:
• MR technologists, researchers, research assistants
• Research volunteers and test subjects
• Maintenance and janitorial personnel
• Public safety forces (i.e. law and fire personnel) that would
respond to the MR suite for an emergency must also know
the potential hazards of the MR equipment.

60. ITEMS THAT CAN BE DAMAGED BY THE
MAGNET
• The magnetic field can seriously damage or impair the
following personal items:
• Cameras
• Watches
• Credit /Bank cards
• Hearing Aids
• Hair Accessories, Belt Buckles, Shoes
• I-pods

61. NOISE
• The MR scanner can produce very high acoustic noise
levels.
• Some patients may experience discomfort from the
associated noise of the scanner.
• Prior to scanning ear plugs and a head set is provided to
the patient to reduce the noise level.

62. ABSOLUTE CONTRAINDICATIONS
• Cardiac Pacemakers
• Cochlear (inner ear) implants
• Ferromagnetic or unidentifiable aneurysm clips of the brain
• Implanted neuro stimulators
• Metal or unidentifiable foreign bodies in the eyes
• Implanted pumps to deliver medicine that cannot be removed
• Full mouth braces or retainers that cannot be removed

63. This could be you!

65. FUNCTIONAL MRI
• Hemoglobin has different
magnetic properties when
bound to oxygen, that can be
distinguished by fMRI.
• Areas of brain activity have a
surge of oxygenated blood.
• fMRI can identify areas of the
brain with high
oxyhemoglobin content,
which correlates to areas of
heightened brain activity.