Basic MRI physics and some MRCP case discussion in hepato-biliary surgery

1. Basic MRI in
Hepatobiliary surgery
DR. YASNA KIBRIA
MD RESIDENT , PHASE A
Department of RADIOLOGY and IMAGING
BSMMU

7. Indications of MRI:
•In Liver:
1. Detection of focal lesions
2. Preoperative planning
3. Monitoring and detecting recurrence
4. Suspected liver metastasis
•In Biliary Tree:
1. Congenital variants
2. Cystic diseases of bile duct
3. Choledocholithiasis
4. Primary sclerosing cholangitis
5. Cholangiocarcinoma
6. Post surgical biliary complications
•In Pancreas:
1. Pancreatic divisum
2. Chronic pancreatitis
3. Pancreatic carcinoma

9. BASIC PRINCIPLES of MRI
Four basic steps are involved in getting an MR image-
1. Placing the patient in the magnet
2. Sending radiofrequency (RF) pulse by coil
3. Receiving signals from the patient by coil
4. Transformation of signals into image by complex processing in the
computers.

12. WE are made up of ELEMENTS
• Human body is built of about 26 elements.
• Oxygen, hydrogen ,carbon ,nitrogen etc. constitute 96% of human
body mass.
• Most of the mass of the human body is oxygen and most of the
atoms in the human body are hydrogen atoms.
• An average 70 kg adult human body contains approximately 3x 10^27
atoms of which 67% are hydrogen atoms.

13. Why HYDROGEN? Why PROTON?
• Hydrogen is the Simplest element with
atomic number of 1 and atomic weight
of 1.
• When in ionic state ( H+ ), it is nothing
but a proton.
• Hydrogen ions are present in
abundance in body water and H+ gives
best and most intense signal among all
nuclei.
• Proton is not only positively charged ,
but also has magnetic spin (wobble) !
• MRI utilizes this magnetic spin property
of protons of hydrogen to elicit images.
• Essentially all MRI are hydrogen or
proton imaging.

15. WE ARE MAGNETS !! REALLY !!?
But why we can’t act like magnets??
• The protons ( Hydrogen
ions) in the body are
spinning in a haphazard
fashion and cancel all the
magnetism. That is our
natural state.

16. PRECESSION
• Normally , alignment of the proton
magnets is random.
• But when an external magnetic field
is applied ,these randomly moving
protons align ( their magnetic
moments align ) and spin in the
direction of external magnetic field (
as the compass aligns in presence of
earth’s magnetic field ).
• Some of them align parallel and
others anti-parallel to external
magnetic field.
• When a proton aligns along external
magnetic fields , not only it rotates
around itself ( called SPIN) ,but also
its axis of rotation moves forming a
“cone”-this movement of axis of
rotation of a proton is called
PRECESSION

17. LONGITUDINAL MAGNETIZATION
• External magnetic field is directed along Z axis which is the long axis of the
patient as well as bore of the magnet.
• Proton align parallel or anti-parallel to external magnetic field ,i.e. along
positive or negative sides of Z axis.Forces of protons on negative and
positive sides cancel each other out.
• However, there are always more protons spinning on positive side or
parallel to Z axis than negative side as it requires less energy to do so.
• After cancelling each others forces there are few protons on positive side
that retain their forces and these forces add up together to form a
magnetic vector along the Z axis.This is called net longitudinal
magnetization.
• But this formed longitudinal magnetization we can’t measure directly as it
is along the external magnetic field.

22. TRANSVERSE MAGNETIZATION
• In order to measure the net magnetization ,we need to flip it towards
transverse plane by sending a radiofrequency pulse (RF pulse ).
• The precessing protons pick up some energy from the RF pulse and go
to higher energy level and start precessing antiparallel to Z axis.
• This imbalance results in tilting of magnetization into transverse (X-Y)
plane.
• This is called transverse magnetization.

24. RF PULSE and RESONANCE
• Radiofrequency pulse is the short burst of electromagnetic wave in
the radiofrequency range , used in combination with magnetic
gradients to generate a magnetic resonance imaging.
• For the exchange of energy , frequency of protons and RF pulse have
to be same . (Larmor frequency )
• When RF pulse and protons have same frequency ,protons of low
energy state can pick up some energy and can go to higher energy
state-this phenomena is known as RESONANCE –the R in MRI.
• RF pulse not only causes protons to go to higher energy level but also
makes them precess in step ,in phase or synchronously.

27. MR SIGNAL
• Transverse magnetization vector
constantly rotate at Larmur
frequency in transverse plane and
induces a electric current.
• The receiver coil receives this
current as MR signal.
• The strength of the signal is
proportional to the magnitude of
the transverse magnetization and
this signals are transformed into
MR image by computers using
mathematical methods.

28. RELAXATION : it means recovery of protons back towards
equilibrium after been disturbed by RF excitation.
WHAT happens when RF Pulse is switched off?
protons starts doing two things simultaneously –
Losing energy and returning to spin-up : longitudinal magnetization starts
increasing along Z axis.
Dephasing : transverse magnetization starts decreasing in transverse plane.

29. LONGITUDINAL RELAXATION
• When RF pule is switched off ,spinning
protons start losing their energy and start to
spin up along the positive side of Z axis.so
there is gradual increase in the magnitude (
recovery )of longitudinal magnetization.
• The energy released by protons is
transferred to surrounding (the crystalline
lattice of molecules)-hence the longitudinal
is also called as “spin-lattice” relaxation.
• The time taken by LM to recover its original
value after RF pulse is switched off is called
longitudinal relaxation time or T1.

30. TRANSVERSE RELAXATION
• The transverse magnetization represents composition of magnetic forces of
protons precessing at same frequency.These protons are constantly exposed to
static or slowly fluctuating local magnetic fields.
• So when RF pulse is switched off they start loosing phase and results in gradual
decrease in magnitude of transverse magnetization and is termed as Transversal
relaxation.
• Since dephasing is related to fluctuating local magnetic fields caused by adjacent
spins (protons ), transverse relaxation is also called ‘spin-spin’ relaxation.
• The time taken by TM to reduce its original value is transverse relaxation time or
T2.

32. T1 CURVE T2 CURVE

36. TR and TE
• TR : Time to REPEAT
is the time interval between start of
one RF pulse and start of next RF pulse.
• TE : Time to ECHO
is the time interval between start of
RF pulse and reception of the signal
(echo).
**TR is always higher than TE.
Short TR + short TE = T1 WI
Long TR + long TE = T2 WI
Long TR + Short TE = PD WI

37. Typical TR and TE values in milliseconds

38. • HOW does one make images T1 weighted?
This is done by keeping the TR SHORT.
• How does one make images T2 weighted?
This is done by keeping the TE longer.

41. MR Protocol in Liver :
• A standard MR
examination of liver is
composed of seven
main series:
1. T1WI ( pre contrast)
2. In-phase
3. Out-of-phase
4. T2WI
5. Diffusion WI
6. MRCP images
7. Post contrast T1
images

42. T1 Weighted Imaging:
• The term T1WI refers to an imaging series that demonstrates low
signal for water molecule- Dark. In contrast materials have high
intrinsic T1 signal are T1 bright or hyperintense ( compare to the
paraspinal musculatures).
• T1WI are excellent for delineation of anatomy.
• A normal liver should demonstrate uniform T1 signal similar or
isointense to the paraspinal muscles and slightly hyperintense to the
spleen.

45. In-phase and Out-of-phase
• When water and fat signals within a
voxel are additive the image is known
as In-phase image. When these signals
in a voxel are in opposite direction and
cancel each other out the image is
known as Out-of-phase.
• These images are used to identify fat
in the liver or within a liver lesions.
• For example: in diffuse hepatic
steatosis the entire liver loses signal
intensity on Out-of-phase compared
to In-phase image.

46. T2 Weighting Imaging:
• On routine T2WI fluid, edema, fat and some hemorrhagic products are
bright.
• T2WI are generally obtained with fat suppression which increases contrast
between a lesion and the liver.
• Solid hepatic mases are typically isointense in T2WI .
• T2WI are excellent for detection of liver lesion due to high contrast.
• T2WI are useful in characterization of benign lesions as cysts and
hemangiomas. These masses will maintain their hyperintensity as the T2
weighting are increased.
• By contrast solid metastases loose their hyperintensity as T2 weighting is
increased.

49. Magnetic Resonance
Cholangiopancreatography (MRCP)
• It is an MRI technique , has got a widespread clinical acceptance & has almost
replaced diagnostic ERCP.
• MRCP visualizes intra and extra-hepatic biliary tree & pancreatic ductal system
, non-invasively without use of any contrast injection or radiation.
PRINCIPLES :
• Heavily T2 weighted images are used to visualize static fluid or bile in the
pancreatobiliary tree.
• The images are made heavily T2 weighted by using longer echo times (TE ) in
the range of 600-1200 ms.
• At this long TE only fluid or tissues with high T2 relaxation time will retain
signal.
• Background tissues with shorter T2 don’t retain sufficient signal at longer TEs
and are suppressed.

50. Technique and protocols
• Fasting for 8-12 hours prior to the examination is required to reduce
gastroduodenal secretions, reduce bowel peristalsis (and related
motion artifact) and to promote distension of gall bladder.
• If fluid still present in the stomach it can be suppressed by giving
barium ,blueberry or pineapple juice.
• MRCP is performed on a 1.5 T or superior MRI system, using a phased-
array body coil.
Sequences used in MRCP :
Two main sequences are used- 3D FSE & Single-shot FSE sequences.

53. CHOLELITHIASIS and CHOLEDOCHOLITHIASIS

54. Caroli’s disease

55. CHOLEDOCHAL CYST

56. Primary Sclerosing Cholangitis

57. PANCREATIC DIVISUM

58. BILIARY STRICTURES

59. Carcinoma of
head of pancreas

60. Chronic
pancreatitis

61. •Secretin Stimulated MRCP/ S-MRCP:
1. Secretin is given intravenously(1 unit/kg) and heavily T2w
images are acquired every 30 sec for 10 min.
2. It distends pancreatic duct upto 3mm.
3. Peak response occurs at 3-5 min and response completely
vanes by 10 min.
4. Improve visualization of pancreatic side branches.
5. Limitation – high cost of secretin.

63. Diffusion Weighted Imaging :
• A subtype of T2WI.
• Provide information about Brownian movement of
water molecules in a voxel.
• Background liver has low signal intensity in DWI
and image parameters are modified to cancel
signal from bile duct and vessels.
• Highly sensitive modality for detection of focal
hepatic lesion.
• Water molecules that freely move within a voxel
are termed “ unrestricted” and result in low signal
in DWI. Water molecules that don’t move freely
are termed “ restricted” and demonstrate high
signal in DWI.
• The degree of diffusion restriction can be
quantified by ADC map constructed from DWI
dataset.

68. Contrast Enhanced MRI
Extracellular agent : non-specific gadolinium
chelates –
Gadobenate,Gadoxetate
Reticuloendothelial / kupffer cell agent :
SPIO (Super Paramagnetic Iron
Oxide)- ferumoxide , ferucarbotran
Hepatobiliary agent :
• Protein bound Gadolinium chelates
• Manganese based agent- Mangafodifir

71. Hepatocyte agents :
Mangafodifir trisodium
• Taken up by hepatocytes.
• Results in increased signal intensity of
normal liver parenchyma.

73. Understanding the Phases :
Arterial phase :
20-40sec after injection.
Refers to images acquired when contrast
first opacifies the early portal veins.
Hypervascular tumors enhance via the
hepatic artery, when normal liver
parenchyma does not yet enhance, because
contrast is not yet in the portal venous
system.
Hypervascular tumors enhance optimally at
35 sec after contrast injection.
Hypervascular lesions
• Benign:
 Hemangioma
 Adenoma
 FNH
• Malignant:
 HCC
Metastases(RCC,carcinoid,thyroid
ca,NET,sarcoma)

74.  Portal venous phase
Contrast completely opacifies the portal veins and liver parenchyma
enhances homogeneously relative to hepatic arteries.
To detect hypovascular tumors(more common, majorities are metastases).
Scanning is done at about 75 seconds.
 Delayed/equilibrium/washout phase
Begins at about 3-4minutes after contrast injection &imaging is best done at
10 minutes.
Valuable for washout of contrast (HCC), retention of contrast in blood pool
(hemangioma) & retention of contrast in fibrous tissue (capsule of HCC,
central scar of FNH).

76. Focal liver lesions :
HEMANGIOMA
• T1WI: Hypo-intense relative to liver
parenchyma.
• T2WI: Significantly hyperintense –producing
light bulb appearance.
• T1+C (Gd) :
Discontinuous, nodular, peripheral
enhancement starting at arterial phase &
gradual central filling in.
Retention of contrast in delayed phase.
Enhancement must match blood pool in each
phase(similar to aorta in arterial phase ,
portal vein in portal venous phase).

77. Focal Nodular Hyperplasia
• T1WI: Iso-intense to normal liver
parenchyma.
• T2WI: Iso to slightly hyper-intense.
• Central scar is hypointense inT1WI &
hyperintense in T2WI.
• T1C+(GD): lesion enhance markedly &
uniformly in arterial phase with exception
of central scar.
• Isointense to normal liver parenchyma in
PVP.
• Contrast accumulates within the central
scar in delayed phase.

78. Hepatic adenoma
• T1WI: mildly increased signal intensity(
fat & hemorrhage).
• T2WI: heterogeneous with iso, hypo &
hyperintense areas.
• Capsule-hypointense rim.
• T1C+(GD) : : early peripheral with
centripetal enhancement, no retention of
contrast later phases because of AV
shunting

79. Hepatocellular Carcinoma
• T1WI : variable (fatty change,
internal fibrosis,hge)
• T2WI : hyperintense
• Capsule : hypo in T1 &T2WI
T1+C :non necrotic area enhances
strongly in arterial phase & early
washout in subsequent phases.
Enhancing rim around the mass
indicate capsule.
Detection of venous invasion
(portal,hepatic veins,IVC).

80. METASTASES
• Liver is the most common site of metastases.
• Usually multiple.
• Majorities are hypovascular (GI tract,lung ,breast , head &neck
tumour, lymphoma).
• Hypervascular metastasis are less .(NET, RCC, carcinoid, sarcoma,
melanoma).
• Calcified metastases are uncommon( colon, stomach,
breast,melanoma).
• Cystic metastases occur from mucinous ca of ovary, colon, sarcoma,
melanoma.

81. MRI features of metastases :
• Variable but usually most metastatic nodules are hypointense on T1W
& hyperintense on T2WI.
• High signal intensity in T1WI- mets from melanoma, ca colon.
• Higher signal on T2WI- mets with liquifective necrosis.
• CEMRI: variable.

83. THANK
YOU