MR diffusion, perfusion, and artifacts are discussed. Diffusion imaging measures water mobility and is used to characterize tissues and pathology. Perfusion imaging measures blood flow and is used to assess diseases like stroke and tumors. Common artifacts include ghosts from motion, aliasing from small fields of view, truncation artifacts at tissue interfaces, and magnetic susceptibility artifacts at tissue boundaries. Clinical applications of diffusion, perfusion, and other sequences are also outlined.

1. MR DIFFUSION ,
PERFUSION and
ARTIFACTS
DR. YASNA KIBRIA
MD RESIDENT ,PHASE-A
Department of RADIOLOGY and IMAGING
BSMMU

2. DIFFUSION
• Diffusion means random movement
of the water protons.
• The process by which water protons
diffuse randomly in the space is
called BROWNIAN MOTION.
• The difference in the mobility of
water molecules between tissues
gives the contrast in DWI and helps
to characterize tissues and
pathology.

5. ISOTROPIC DIFFUSION
• Possibility of water protons moving in
any one particular direction is equal to
the probability that it will move in any
other direction.
• Isotropy= uniformity in all direction .
• Isotropic diffusion forms the basis for
routine DWI.

6. ANISOTROPIC DIFFUSION
• In anisotropic diffusion , water
molecules have preferred
direction of movement.
• Water protons move more easily
in some direction than other.
• Anisotropic diffusion forms the
basis for DTI or Tactography.

8. HOW do we acquire DWIs?
• The “stejskal-Tanner pulsed gradient
spin echo sequence “ –was the first
experimental sequence described for
acquisition of DWI.
• It is a T2-w spin echo sequence with
diffusion gradients applied before
and after the 180 degree pulse.

16. THE b VALUE
• The b value indicates the magnitude of diffusion weighting provided by the
diffusion gradient. It also indicates sensitivity of the sequence to the diffusion.
• Expressed in second/mm2.
• The b value increases with diffusion gradient strength, duration and time
between application of the two gradients.
• As the b value increases the signal from water molecule reduces.
• At high b value(b=1000) only tissues with very high T2 relaxation time or those
with restricted diffusion will have high signal.

28. DIFFUSION TRACE
• Isotropic diffusion is the basis of DWI.
• However, there are some anisotropy
of water molecules in the tissues .
• To reduce this anisotropy ,the image
with higher b value like b=1000 is
acquired in three directions along X,Y,Z
axes.
• Diffusion changes along all 3 axes are
then averaged out to get a “TRACE “
diffusion image.

31. CLINICAL APPLICATIONS OF DWI
NEUROIMAGING applications :
1. Stroke
2. Epidermoid vs arachnoid cyst
3. Abscess vs simple cystic lesion
4. DWI in brain tumors
DWI in BODY IMAGING:
• Relatively new
• Lower b value is used
• Mainly focused on tumor imaging & assessing treatment response
• Staging of the tumors & lymphomas

38. DIFFUSION TENSOR IMAGING
• DTI is based on the
anisotropic diffusion of water
molecules.
• Tensor Is the mathematical
formalism used to model
anisotropic diffusion.

39. TECHNIQUE OF DTI :
• MR scanner axes X ,Y, Z are never perfectly parallel to white matter tracts at
every point in the image.
• In DTI, images are acquired in at least 6 , usually 12-24 directions instead of 3
in the usual.

41. • Various maps used to
indicate orientation of
fibres :
FA ( Fractional Anisotropy)
RA (Regional Anisotropy )
VA (Volume ratio )

42. USES
• Diffusion Tensor measures
the magnitude of the ADC in
preferred direction of water
and also perpendicular to
the direction.
• The resultant image show
white matter tracts very well
• Hence, this technique is also
called as TRACTOGRAPHY.

43. CLINICAL applications
• assess the deformation of white matter by tumors - deviation,
infiltration, destruction of white matter
• delineate the anatomy of immature brains
• pre-surgical planning
• Alzheimer disease - detection of early disease
• schizophrenia

44. • Basic colors can tell the observer how the
fibers are oriented in a 3D coordinate
system, this is termed an "anisotropic
map". The software could encode the
colors in this way:
• Red indicates directions in the X axis: right
to left or left to right.
• Green indicates directions in
the Y axis: posterior to anterior or
from anterior to posterior.
• Blue indicates directions in the Z axis: foot-
to-head direction or vice versa.

47. MR PERFUSION
• PERFUSION :
Refers to the passage of blood from an
arterial supply to venous drainage
through the microcirculation.
Perfusion is necessary for the nutritive
supply to the tissues & for clearance of
products of metabolism.
It can be affected by various diseases.so
measuring changes in perfusion can help
in diagnosis of certain diseases.

49. PRINCIPLES
• Paramagnetic substance like Gadolinium causes
shortening of both T1 & T2.
• T1 shortening results into increased signal
intensity.
• T2 shortening results into signal drop or
blackening.
• Gd based contrast agent passes through
microvasculature in high concentration
decrease in signal in surrounding tissues from
magnetic susceptibility induced shortening of
T2* relaxation time.
• This signal drop is proportional to the
perfusion.
• More the number of microvasculature/small
vessels per voxel-more will be the signal drop.

50. Technique of MR PERFUSION with exogenous
contrast agent (DSC ):
• A dose of 0.1 mmol/kg of Gd based
CONTRAST AGENT is injected
intravenously using power injector at
the rate of 5 ml/sec .
• Fast T2* weighted EPI sequence is run
to catch first pass of the contrast
through microcirculation.
• This sequence typically acquire 15-20
slices covering entire brain in 1-2
seconds.
• From raw data images various color
maps are constructed using
software.
• These maps include :
1. rCBV: relative Cerebral Blood
Volume
2. CBF:Cerebral Blood Flow
3. TTP: Time to Peak
4. MTT: Mean Transit Time

52. Routine contrast
enhancement
Perfusion imaging
1. Sequence T1 weighted imaging T2* weighted EPI sequence
2. Signal change Increase in signal intensity Drop in signal intensity
3. Mechanism Gd causes reduction in T1
relaxation time
Gd causes reduction in T2 or
T2* relaxation time &
magnetic susceptibility
4. Detects Break in the BBB leading to
leakage of Gd
Gd in the microvasculature
(capillaries).
Thus gives information about
number of small vessels
(vascularity ) & perfusion of
the tissue.

53. PERMEABILITY OR LEAKINESS
• Increased permeability or leakiness
because of break in BBB results in
accumulation of Gd based contrast in
extravascular space.
• T1 enhancing effects of this extravascular
Gd may predominate to counteract the T2
signal lowering effect of intravascular Gd,
resulting in falsely low rCBV values.
• To reduce permeability induced effects on
rCBV include mathematical calculation of
PERMEABILITY or K2 maps.

56. MR PERFUSION in STROKE
• DWI & PWI together are very
effective in detection of early
ischemia.
• The mismatch between PW &
DW represents potentially
salvageable tissue(PENUMBRA).
• Small mismatch has a good
outcome.

58. MR PERFUSION in brain TUMORs
• MR perfusion can be useful in-
1. Grading tumors like gliomas
2. In guiding biopsies
3. Differentiating between therapy induced necrosis & recurrent /residual
tumors.
OTHER clinical uses: CNS vasculitis

62. GHOSTS /MOTION ARTIFACTS
• Ghosts are replica of something in the image.
• Ghosts are produced by body part moving
along a gradient during pulse sequence
resulting into phase mismapping.
AXIS :almost always along PHASE encoding
gradient.
CORRECTION :
1. Phase encoding axis swap
2. Saturation band
3. Respiratory compensation
4. ECG gating for cardiac motion

63. ALIASING /WRAPAROUND
• In aliasing, anatomy that exists outside FOV
appears within the image & on the opposite side.
• When FOV is smaller than the anatomy being
imaged , aliasing occurs.
AXIS :can occur along any axis-frequency ,phase,
slice selection gradient.
CORRECTION:
1. Frequency wrap :low pass filters.
2. Phase wrap : increasing FOV along phase
encoding gradient.

64. CHEMICAL SHIFT related Artifacts :
Because of different chemical environment protons in water & fat
precess at different frequencies.

65. TRUNCATION ARTIFACTS (edge ,
Gibbs’ & ringing artifacts )
• Truncation artifacts produce low intensity
band running through high intensity area.
• The artifacts caused by under sampling of
data so that interfaces of high & low signal
are incorrectly presented on the image.
AXIS : Phase encoding
CORRECTION :
Increase the number of phase encoding steps.
Ex- 256x256 matrix instead of 256 x 128.

66. MAGNETIC SUSCEPTIBILITY ARTIFACTS
• Some tissues magnetize to different
degree than other , resulting into
differences in precessional frequency
& phase.
• This causes dephasing at the interface
of these tissues & signal loss.
AXIS : frequency & phase encoding.
CORRECTION :
1. Use of SE sequence
2. Remove all metals

67. GOOD effects of Magnetic
Susceptibility Artifacts:
1. Used to diagnose haemorrhage ,
hemosiderin deposition &
calcification.
2. Forms the basis of post-contrast
T2* weighted MR PERFUSION studies.
3. Used to quantify myocardial &
liver iron overload.

68. ZIPPER ARTIFACTS
• Is a line with alternating bright &
dark pixels propagating along
frequency encoding gradient.
• Caused by stimulated echo that
have missed phase encoding.
CORRECTION :
Site of the leak should be located
& corrected.

69. Straight LINES
• Regularly spaced straight lines
through MR image is caused by
spike in K-space (bad data point
in K-space)
• Spike can be result from loose
electrical connectivity &
breakdown of interconnections
in RF coils.

70. SHADING ARTIFACTS
• In shading artifacts , image has
uneven contrast with loss of signal
intensity in one part of image.
• Uneven excitation of the nuclei due
to RF pulses applied at flip angle
other than 90 & 180 degree ,
abnormal loading of the
coil,inhomogeneity of the magnetic
field.
CORRECTION :
1. Load the coil correctly
2. Shimming to reduce magnetic field
inhomogeneity.

71. CLINICAL importance of OTHER MR
SEQUENCES:
 T1 WEIGHTED IMAGES :
Subacute haemorrhage
Fat containing structures
Anatomical details
 T2 WEIGHTED IMAGES:
Edema
Demyelination
Infarction
Chronic haemorrhage
 STIR :
Bone marrow imaging
Orbital imaging
SI joint imaging
 FLAIR :
Peri-lesional edema
Brain infarction
Sub-arachnoid haemorrhage
Syrinx /cysts in spinal cord
PD weighted images:
Detection of joint & muscle diseases &
injuries.
Well differentiation of GM & WM in brain.