2. Introduction
Computed Tomography (CT) was introduced into
clinical practice in 1972 and revolutionized X Ray
imaging by providing high quality images which
reproduced transverse cross sections of the body.
Tissues are therefore not superimposed on the
image as they are in conventional projections
The technique offered in particular improved low
contrast resolution for better visualization of soft
tissue, but with relatively high absorbed radiation
dose
3. Introduction (contd.)
Computed tomography (CT), originally known as
computed axial tomography (CAT or CT scan) is a
medical imaging method employing tomography
where digital geometry processing is used to generate
a three-dimensional image of the internal structures of
an object from a large series of two-dimensional X-ray
images taken around a single axis of rotation.
The word "tomography" is derived from the Greek tomos
(slice) and graphia (describing). CT produces a volume
of data which can be manipulated, through a process
known as windowing, in order to demonstrate various
structures based on their ability to block the x-ray
beam.
4. Evolution of CT
X-Ray image formation – 2D with super imposition of
tissues
Conventional Tomography – due to blurring of non
focused tissues
Computed axial tomography-Images of exquisite
clarity, no superimposition
5. Definition & Types
ECAT TCAT
• CTCT is a process of creating a cross -sectionalis a process of creating a cross -sectional
tomo- graphic plane or slice of any part of the bodytomo- graphic plane or slice of any part of the body
in which computer is used to make a mathematicalin which computer is used to make a mathematical
reconstruction of a tomo-graph (CT image).reconstruction of a tomo-graph (CT image).
• CT is mainly two typesCT is mainly two types
6. ECAT (EMISSION TYPE)
It needs Gamma Camera
After administration of
radionuclide to patient,
patient becomes temporary
source of emitted radiation,
so it is called ECAT
7. TCAT (Transmission)
In this type of CT x-ray
emitted from x-ray tube and
passes through the body of
a patient to a sensitive
recorder so here patient is
acts as transmitter.
Is generally called CT Scan
In this x-ray examination
depends upon attenuation
of x-ray beam
8. BASIC PRINCIPLE:-The Internal Structures of An
Object Can Be Reconstructed From Multiple
Projections Of The Object
A narrow beam of X ray scans across a
patient in synchrony with a radiation
detector on the opposite side of the
patient.
Sufficient no. of transmission
measurements are taken at different
orientation of X ray source & detectors,
the distribution of attenuation
coefficients within the layer may be
determined.
By assigning different levels to different
attenuation coefficients, an image
can be reconstructed with aid of
computer that represent various
structures with diff attenuation
properties.
9.
10. EVOLUTION OF CT SCAN
Various generations
• First generation - One detector, translation-
rotation Pencil-beam
• Second generation - Multiple detectors, translation-
rotation Small fan-beam
• Third generation - Multiple detectors, rotation-
rotation Large fan-beam
• Fourth generation - Detector ring, source-rotation
Large fan-beam
• Spiral / Helical scanning - Cone-beam geometry
11. The first generation of CT scanners employed a
rotate / translate, pencil beam system
FIRST GENERATION
The x-ray tube and a single
detector (per CT slice) translate
across the field of view,
producing a series of parallel
rays. The system then rotates
slightly and translates back
across the field of view,
producing ray measurements at a
different angle. This process is
repeated at 1-degree intervals
over 180 degrees, resulting in the
complete CT data set.
ROTATE/TRANSLATE, PENCIL
12. First Generation
(Brain Scanner)
Head was enclosed in water bath b/w X
ray tube & a pair of detectors below .
A third reference detector intercepted
a portion of the beam before it reached
the patient.
Patient remain stationary & Gantry
moves through two types of motion: one
is linear & other rotary
Beam- narrow pencil beam, filtered
with 6mm Al eq.
Tube - oil cooled stationary anode, focal
spot 2.25 x12mm operated at
120kvp & 33mA
Each slice of 180 degree rotation took 5
min so total time for clinical study was
approx 25-30 min
13. The first CT scanner, an
EMI Mark 1, produced
images with 80 x 80 pixel
resolution (3-mm pixels),
and each pair of slices
required approximately
4.5 min-of scan time and
1.5 minutes of
reconstruction time.
The First CT Scanner
14. Advantages:
It employed pencil beam geometry which allowed very
efficient scatter reduction.
Limitations
The detector suffered from significant
amount of “afterglow,”
It took 4.5 to 5.5 minutes to complete one
scan resulting to limited patient
throughput.
Only head scan possible.
Advantages & Limitations
15. The next incremental
improvement to the
CT scanner was the
incorporation of a
linear array of 30
detectors.
A relatively narrow
fan angle of 10
degrees was used
SECOND GENERATION
Rotate/Translate, Narrow Fan
16. Second Generation
(ROTATE-TRANSLATE)
A fan beam with 20-30 degree
divergence.
Number of detectors were increased i.e.
up to 30.
Rotary movement was in arc of 30º &
linear movements were 6 as compare to
EMI scanner
Scan time for head 10-90 sec. Body
scanning was also possible
Advantage
The shortest scan time with a second-
generation scanner was 18 seconds per
slice, 15 times faster than with the first-
generation system.
Limitations
more scattered radiation detected than
the pencil beam used in first-generation
CT.
17.
18. Third Generation:
Rotate/Rotate, Wide Fan Beam
The mechanically joined x-ray
tube and detector array
rotate together around the
patient without translation.
The detector array is long
enough so that the fan angle
encompasses the entire width
of the patient.
The translational motion of first- and second-
generation CT scanners was a fundamental
impediment to fast scanning.
Multiple detectors,
rotate-rotate, Large fan-
beam
19. Advantages
The early third-generation scanners could deliver scan times
shorter than 5 seconds.
Newer systems have scan times of ½ second.
Limitations
Third-generation scanners suffered from the significant problem of
ring artifacts.
Detectors and the associated electronics are expensive, this led
to more expensive CT scanners.
Advantages & Limitations
20. Fourth-generation CT scanners were designed to
overcome the problem of ring artifacts.
Fourth Generation
Detector ring, Source-
rotation, Large fan-beam
Based on Rotate-fixed systemBased on Rotate-fixed system
i.e. tube rotates through 360i.e. tube rotates through 360ºº &&
detectors stationarydetectors stationary
A ring of detectors (1000-2000)A ring of detectors (1000-2000)
surrounds the patient.surrounds the patient.
Fan shaped beamFan shaped beam
Scan time very short i.e. 1sec.Scan time very short i.e. 1sec.
DisadvantagesDisadvantages
High cost because more no ofHigh cost because more no of
detector usedetector use
More scatter radiationMore scatter radiation
21. MILLISECOND SCANNER SYSTEM
Multiple X ray Tubes(5th
Gen.)
First used by Mayo Clinic’s
They used 28 X-ray tubes position around a
semicircular gantry, aligned with 28 light amplifiers &
TV cameras that are placed behind a single curved
fluorescent screen
Gantry rotates about 15 revolution per sec
Data can be acquired in 16 ms.
Disadvantages
High cost
Heavy structure mechanical motion difficult
22. Developed by Imatron Inc. which was a result of of work by Dr. Douglas &
colleagues during late 1980s
Commonly referred to CVCT Scanner
Basic components-
An electron gun 320cm long with its focusing & deflecting coils (electron are
accelerated at 130keV
4 Tungsten targets rings180cm in dia.
A ring of detectors arranged in an arc of 210 degree
The transmitted X ray photons are measured by integrated crystals photo-diode
detector system and digitized by an acquisition system.
Scan time very less 50-100 msec. because there is no mechanical rotation of
the X ray source and gantry
Fifth Generation:Fifth Generation:
E-Beam CT Stationary/StationaryE-Beam CT Stationary/Stationary
23. The gantry had to be stopped after each
slice was acquired, because the detectors
(in third-generation scanners) and the x-ray
tube (in third- and fourth-generation
machines) had to be connected by wires to
the stationary scanner electronics.
The ribbon cable used to connect the third-
generation detectors with the electronics had to
be carefully rolled out from a cable spool as the
gantry rotated, and then as the gantry stopped
and began to rotate in the opposite direction
the ribbon cable had to be retracted.
LIMITATIONS OF THIRD AND FOURTH
GENERATION CT SCANNERS
24. HELICAL/SPIRAL CT SCANNER
Introduced in 1989 by Dr. Kalender
Spiral CT is made possible by the use of slip ring
technology. Slips rings are an electromechanical
devices that conduct electricity and electrical
signals through ring & brushes from a rotating surface
onto fixed surface & vice-versa.
Composite brushes are made up of conductive
material (silver graphite)
Brushes are to be replaced every yr. or during
preventive maintenance.
3 kind of slip rings are used
-1st provide high & low voltage to X ray tube
-2nd
provide low voltage to control system on gantry
-3rd
transfer signal from rotating detectors array to
DAS
25. In the early 1990s, the design
of third- and fourth-generation
scanners evolved to
incorporate slip ring
technology.
A slip ring is a circular contact
with sliding brushes that allows
the gantry to rotate
continually, untethered by
wires.
The use of slip-ring technology
eliminated the inertial
limitations at the end of each
slice acquisition, and the
rotating gantry was free to
rotate continuously
throughout the entire patient
examination.
EVOLUTION OF SLIP RING TECHNOLOGY
26. Helical CT
Helical CT (also called spiral CT) scanners
acquire data while the table is moving;
As a result, the x-ray source moves in a helical
pattern around the patient being scanned.
27.
28. Helical CT (Contd.)
Helical CT scanners use
either third- or fourth-
generation slip-ring designs.
the total scan time required to
image the patient is much
shorter (e.g., 30 seconds for
the entire abdomen).
helical scanning allows the
use of less contrast agent and
increased patient throughput.
Entire scan can be performed
within a single breath-hold of
the patient, avoiding
inconsistent levels of
inspiration.
29. Helical CT (Contd.)
The advent of helical
scanning has
introduced many
different considerations
for data acquisition.
In order to produce
reconstructions of planar
sections of the patient,
the raw data from the
helical data set are
interpolated to
approximate the
acquisition of planar
reconstruction data.
30. Advantages of spiral CT
Advantages
No motion artifacts
Improved lesion
detection.
Reduced partial volume
Multiplanar Imaging
Improved Pt throughput
Optimized IV contrast
How
Removes respiratory
misregistration.
Reconstruction at
arbitrary intervals.
Allow reconstruction at
overlapping intervals.
Scanning time is
reduced.
Data obtained during
peak of contrast
enhancement.
31. When multiple detector arrays are used, the
collimator spacing is wider and therefore more
of the x-rays that are produced by the x-ray
tube are used in producing image data.
With conventional, single detector array scanners,
opening up the collimator increases the slice
thickness, which is good for improving the utilization
of the x-ray beam but reduces spatial resolution in
the slice thickness dimension.
With the introduction of multiple detector arrays. the
slice thickness is determined by the detector size and
not by the collimator.
This represents a major shift in CT technology.
MULTI DETECTOR CT
32. DEVELOPMENTS IN MULTI DETECTOR CT
Multi-detector CTs debuted in
1992 when Elscint introduced its
CT Twin, the first dual-slice
scanner.
The first four-slice scanners were
presented in 1998, followed by
16-slice systems in 2001; 32- and
40-slice scanners followed within
a short period. A 64-slice
scanner was unveiled during
the 2005 annual Radiological
Society of North America
scientific meeting,
128- and 256-detector scanners
appear to be on the horizon.
33. x-ray tube/generator systems.x-ray tube/generator systems.
x-ray detectors,x-ray detectors,
computer hardware,computer hardware,
motor control systems,motor control systems,
sophisticated reconstruction algorithms.sophisticated reconstruction algorithms.
CT Scanners represent a marriage of diverseCT Scanners represent a marriage of diverse
technologies comprising:technologies comprising:
34. X-Ray Generators for CT (Contd.)
In X-ray generators of the ct scanners, low
voltage low frequency alternating current
from the main power supply is converted into
high voltage, high frequency (500- 25000 Hz),
direct current of almost constant potential
supply to X-Ray tube.
the voltage ripples is less than 1% compared
to 4% from a three phase 12 pulse generator.
Current CT generators have maximum power
rating of about 60KW that allows KV in the
range of 80-140KVps and tube current in the
range of 100mA-400mA.
35. COLLIMATORS
Beam collimation at 2 points,
one close to X ray tube &
other at detectors
Collimators regulates the
slice thickness
Each detector has its own
collimators
In some volume scanner the
beam is collimated through
multiple slits to reduced
scatter produced before
striking the patient, known as
Multi-slit Multi-slice CT
scanner.
36. DETECTOR TECHNOLOGY
CT detectors capture
the radiation beam
from the patient,
convert it into
electrical signals,
which are
subsequently
converted into
binary coded
information for
onward transmission
to computer system
for further processing
37. TYPES OF DETECTORS
Three types of detection systems are available for
CT machines:
Multiple scintillation detectors with photo multiplier tubes
Multiple scintillation detectors with photo-diodes
A single multi chamber inert gas (xenon) detectors.
38. IMAGE RECONSTRUCTION
In computed tomography, a cross
sectional layer of the body is divided into
tiny blocks
Each blocks is assigned a no.
proportional to the degree that block
attenuated the X ray beam.
This block individually called voxel
The linear attenuation coefficient is used
to quantitative
attenuation
N = Nºe-µx
If the block of material with different
attenuation coefficient placed in the path
then,
N = Nºe-[(µ1+µ2+……+µn)x ]
The values of µ1,µ2,….µn can not be
39. IMAGE DISPLAY
A CT imaged displayed is
consist of a matrix of picture
elements called ‘pixels’
Each pixel represent the linear
attenuation values of X ray at
the point of body .
Pixel is a 2D display of a voxel
Matrix used are
*256x256(over 65000 pixels)
*512x512(over 260000 pixels)
*1024x1024(app.1050000
pixels)
40. CT NUMBER
It is defined as a relative
comparison of x-ray
attenuation of each voxel
of tissue with an equal
volume of water.
CT no=k(µρ - µω)
µω
To honour Hounsfield CT no.
based on magnification
constant of 1000 are also
called HU (Hounsfield unit)
41. Windowing is a system where the CT no. range of
interest is spread cover the full grey scale
available on the display system
WINDOW WIDTH –Means total range of CT no.
values selected for gray scale interpretation. It
corresponds to contrast of the image.
WINDOW LEVEL– represents the CT no. selected
for the centre of the range of the no.
displayed on the image. It corresponds to
brightness of image .
WindowingWindowing
43. ADVANCEMENTS
Detector miniaturization, faster gantry rotation
and enhanced computerization.
Number of detectors has increased, so has
rotational speed (presently 0.33 s per rotation.).
Dual-slice scanners permitted either resolution,
speed, volume or power enhancements but
scanners with a minimum of 16/64 slices allow
unlimited improvement in all four areas.
Applications also includes Cardiac CT, 3DCT, CT
Angiography, CT Fluoroscopy, Virtual
endoscopy and traditional CT.
44. Advancements of CT
19721972 19801980 19901990 20002000
Minimum scan timeMinimum scan time 300 s300 s 5-10 s5-10 s 1-2 s1-2 s 0.3-1s0.3-1s
Data acquired per 360°Data acquired per 360° 57.6 kB57.6 kB 1 MB1 MB 2MB2MB 42 MB42 MB
Data per spiral sequenceData per spiral sequence -- -- 24-48 MB24-48 MB 200-500 MB200-500 MB
Image matrixImage matrix 808022
25625622
51251222
51251222
Power (generator)Power (generator) 2 kW2 kW 10 kW10 kW 40 kW40 kW 60 kW60 kW
Slice thicknessSlice thickness 13 mm13 mm 2-10 mm2-10 mm 1-10 mm1-10 mm 0.5-5 mm0.5-5 mm
45. Dual Energy CT Methods
Dual Source-Siemens
Energy discriminating Detectors –Philips
kVp Switching-GE
46. CONCLUSION
During the coming years, cone-beam CT with large-area
detectors may allow coverage of entire organs in a single
axial scan
In the meantime, 64-detector systems are the best available
technology, and some believe a critical point has been
reached:
The best study obtainable may not be necessary. Thus,
protocols are designed to reasonably bridge the possible
and the necessary.
Even so, more advanced systems are fast deluging
physicians with incredibly high volumes of CT images.
The respective technical developments in CT detectors will
have to be reassessed constantly in the future, whereas the
development of detector systems which is equally suited
both for radiography and CT is the need of the day.
47. 47
Introduction
MRI is a computer based cross sectional imaging modality
Which can provide both anatomic as well as physiological
Information non invasively, without the use of ionizing
Radiation .
Definition : MRI is a diagnostic imaging modality in which a
Magnetic resonance , MR active nuclei ,RF pulses and
computer are used to generate the MR images in
transverse, coronal and saggital planes for diagnostic
purpose.
MRI PRINCIPLE and PHYSICS
48. 48Basic of MRI
Atomic structure
Atom : matter is composed of atoms, which are composed
proton, neutron and electron. having the central nucleus
and orbital electrons
Atomic number: Sum of the protons and neutron in the
nucleus.
Mass number: Sum of the proton and neutron in the
nucleus.
Proton spinning on their own axis.
Electron orbiting the nucleus, spin but very less in
comparison to protons.
Nucleus itself spins about its own axis.
49. 49MR active nuclei
MR active nuclei are characterized by their tendency to
align their axis of rotation to an externally applied
magnetic field.
According to law of quantum mechanics nuclei with odd
number of protons have a total magnetic moment.
Some important MR active nuclei.
HYDROGEN 1
CARBON 13
NITROGEN 15
OXYGEN 17
FLUORINE 19
SODIUM 23
PHOSPORUS 31
50. 50HYDROGEN NUCLEUS
Biological tissues are predominantly made
of
12
C ,16
O , 1
H, and 14
N.
Hydrogen is the major species that is MR
sensitive.
Hydrogen is most abundant atom in body.
The majority of hydrogen is in water (H2O).
51. 2) MR PROTON ALIGNMENT
Hydrogen is the most abundant element in the human
body. Hydrogen protons align with the magnetic field
when the human body is placed in an MR magnet.
In a magnetic field, the protons line up in the direction of
the magnetic field, similar to the way a compass lines up
in the earth’s magnetic field.
51
Nucleus of an atom has magnetic properties. When nucleus has an odd
number of protons (or neutrons), there is a magnetization. E.g.
Hydrogen-1 Proton
Nucleus behaves like a dipole magnet
52. 52
A Single Proton
There is electric chargeThere is electric charge
on the surface of theon the surface of the
proton, thus creating aproton, thus creating a
small current loop andsmall current loop and
generating magneticgenerating magnetic
momentmoment µµ..
Thus proton “magnet” differs from the magnetic bar in that itThus proton “magnet” differs from the magnetic bar in that it
also possesses angular momentum caused by spinning.also possesses angular momentum caused by spinning.
53. MR PROTON ALIGNMENT
All the protons pointing in the direction of
the magnetic filed act together to
produce a net magnetization, as if they
were combined into one larger magnet.
When a patient’s body is placed in a
magnetic field, the hydrogen protons line
up in the direction of that magnetic field.
53
No external magnetic field External magnetic field B0
54. NET MAGNETIZATION VECTOR
NET MAGNETIZATION VECTOR
An excess of hydrogen nuclei will line up parallel to B0
and create the NMV of the patient. NMV or
longitudinal magnetization along
external magnetic field cannot be measured directly.
for measurement it has to be transverse.
54
55. Transverse magnetization
After forming the longitudinal magnetization R .F pulse is sent.
Precessing protons pick up some energy from R F pulse.
Some protons go to higher level and starts precessing anti-parallel.
This results in the reduction of magnitude of longitudinal
magnetization.
Forces of protons add up to form a new magnetic vector (x-y)
plane
this is called transverse magnetization.
55
57. 4) PRECESSION
A spinning top, which is hit, performs a wobbling type of
motion .
Protons in a strong magnetic field also show this type of
motion, which is called precession.
The precession actually goes very fast, the precession
frequency for hydrogen protons is somewhere around 42.3
MHz in a magnetic field strength of 1 Tesla.
57
58. 58
Larmor equation.
Precession speed of proton can be measured as a
precessional frequency.
It depends upon on magnetic field strength.
According Larmor frequency.
W = r Χ Bo.
Where.
W= PF in MHz.
Bo= strength of M G in Tesla.
r = gyro magnetic ratio.
59. 3) RESONANCE
Resonance is the absorption or emission of energy only at certain
specific frequencies.
Exchange of energy between two systems at a specific frequency is
called resonance. Magnetic resonance corresponds to the
energetic interaction between spins and electromagnetic
radiofrequency (RF).
The resonance frequency, called Larmor frequency (ω0) or
precessional frequency, is proportional to the main magnetic field
strength: ω0 = γ B0.
59
60. PRECESSIONAL FREQUENCY OF
HYDROGEN AT DIFFERENT MAGNETIC
FIELD STRENGTH
At 0.5 Tesla -21.28 MHz.
At 1.0 Tesla -42.57 MHz.
At 1.5 Tesla -64 MHz.
At 3 Tesla - 127.71
60
61. Prerequisite for resonance
Protons resonate if energy delivered by RF waves is-
Delivered at exactly its precessional frequency of
proton.
and 90 degree NMV and B0
61
62. How to measure longitudinal
magnetization
for the measurement longitudinal magnetization it
has to be transverse.
How it can be transverse.
It can be transverse with the application of R F waves .
62
63. What are radio frequency waves
Radio waves are a type
of electromagnetic radiation with
wavelengths in the electromagnetic
spectrum longer than infrared light.
Naturally-occurring radio waves are
produced by lightning, or
by astronomical objects.
63
64. Effects of radio frequency waves on
protons
R F pulse is sent .
When R F pulse and proton have same frequency protons
pick up some energy from RF pulse and starts wobble this
is called resonance.
64
65. 65
RF Pulse
Radio- wave :Radio- wave :
It is an electro magnetic wave and a short burst ofIt is an electro magnetic wave and a short burst of
pulse which is called RF Pulse. We need a specialpulse which is called RF Pulse. We need a special
(RF) pulse that can exchange energy with protons.(RF) pulse that can exchange energy with protons.
Energy exchange is possible when protons and RFEnergy exchange is possible when protons and RF
wave have same frequency. So energy transformationwave have same frequency. So energy transformation
is possibleis possible
Apply RF pulse at resonance frequency
Protons absorb energy
Protons ‘jump’ to a
higher state
67. Flip angle
The NMV (net magnetization vector)
moves out of
alignment away from BO is called flip
angle .
Magnitude of the flip angle is depends
upon.
Amplitude and duration of RF pulse.
67
69. 69
Phases of magnetic movement
Phase :Phase : NMV move into phase with each other. The phase isNMV move into phase with each other. The phase is
the position of each magnetic moment as the precessionalthe position of each magnetic moment as the precessional
path around Bo. Two types 1) Out of phase 2 ) In phasepath around Bo. Two types 1) Out of phase 2 ) In phase
Out of phase
In phase
70. 70
MR Signal : If a receiver coil is placed in the transverse
plane, a voltage is induced in the receiver coil. When in phase.
Magnetization cuts across the coil and produces Magnetic field
Fluctuation inside the coil. Therefore NMV precess at the
Larmor frequency in transverse plane, a voltage is induced in
the coil. It Constitutes MR Signal
The MR Signal
71. 71
FID : When the RF pulse is switched off the NMV is again
Influenced by Bo and, it tries to realign it. In order to do so
NMV must loss the energy given to it by the RF pulse, as
relaxation occurs, the NMV returns to realign with Bo
Relaxation : During relaxation the NMV gives up absorbed
RF energy and return to Bo. The magnetic movements of
NMV loss transverse magnetization due to dephasing.
Relaxation results :
- The recovery of LM is caused by a process termed T1
recovery
- The decay of TM is caused by a process termed T2 deacy
Result of resonance Contd..Result of resonance Contd..
72. Free induction decay
As the magnitude of the transverse
magnetization decreases .so does the
magnitude of the voltage induced in the
receiver coil. This decrease is called
free induction decay.
72
74. Relaxation
Relaxation – it means recovery of protons
back towards equilibrium after been
disturbed by RF pulse.
Type of relaxation
Longitudinal relaxation or spin lattice
relaxation.
Transverse relaxation or spin-spin relaxation
74
78. Longitudinal relaxation (T1)
When RF pulse is switched off ,protons
starts losing their
energy.
They transfer their energy to surrounding
or lattice hence
it is called spin-lattice relaxation.
78
79. 5) T1 RELAXATION TIMES
The time it takes for a proton to process back into
alignment with the external magnetic field is called the T1
relaxation time.
Differences in T1 relaxation times depend on binding of
the proton in different tissues.
Protons in different types of tissues have different
relaxation times because their elasticity and chemical
bonds are different.
79
81. T2 RELAXATION TIMES
T2 relaxation time of a tissue is the time it takes for the
protons to lose their phase.
The T2 relaxation time of a tissue is always shorter
than its T1 relaxation time.
Protons in a magnetic field also have a second
relaxation time called T2 relaxation time depends on
interactions between the protons in small volume of
tissue.
81
83. T1 AND T2 VALUES FOR VARIOUS ORGANS AT 1T
MAGNETIC FIELD STRENGTH
OrganOrgan T1 (ms)T1 (ms) T2 (ms)T2 (ms)
FatFat
LiverLiver
SpleenSpleen
MuscleMuscle
White matterWhite matter
Gray matterGray matter
CSFCSF
BloodBlood
WaterWater
220220
440440
460460
600600
700700
820820
20002000
800800
25002500
9090
5050
8080
4040
9090
100100
300300
180180
25002500
83
84. PULSE TIMING PARAMETER
A pulse sequence is a combination of RF
pulses, signals intervening periods of recovery.
A pulse sequence consists of several
components two most important are.
the time of repetition (TR).
the time of echo (TE).
84
85. 85
Repetition time-TR
Time interval between
application of two RF pulses
Measured in ms
Determine amount of
relaxation allowed occur
between two RF pulses
Determine T1 relaxatation
86. 86
Echo time - TE
Time between application
of RF pulse to peak of signal
induced.
Measured in ms
Determine amount of
transverse magnetization
decay allowed to occur
before signal is read.
TE controls amount of T2
relaxation.
88. 88
TR & TE - applications
T1T1 PDPD T2T2
TRTR SHORTSHORT LONGLONG LONGLONG
TETE SHORTSHORT SHORTSHORT LONGLONG
Long TR 2000 ms+
Short TR 250-700 ms
Long TE 60 ms+
Short TE 10-25 ms
89. 89
T1 vs. T2
T1 weighted images
Short TR & TE
Black fluid (csf, urine etc)
White fat
Anatomical detail
High SNR
CE T1 for pathology
T2 weighted images
Long TR & TE
White fluid
Relatively Black fat
Detect of pathology
↑ H2O - ↑ signal
90. CONTRAST MECHANISM
Image obtain contrast mainly through the
mechanism of:
TI recovery
T2 decay
Proton or spin density (no of proton per unit
volume of
tissue)
90
91. 91
T1 Recovery in FAT
Occurs due to the nuclei giving up their energy to
surrounding. Slow molecular tumbling in fat allows the
recovery process in FAT is rapid. T1 time of Fat is short
T1 Recovery in fat
92. 92
T1 recovery in water
Occurs due to nuclei giving up the energy acquired
from
the RF excitation to the surrounding lattice. In water
molecular mobility is high. Resulting in less efficient T1
recoverey.T1 recovery of water is longer so T1 time of
water is long
T1 recovery in water
93. 93
T1 contrast : T1 time of fat is shorter than water fat
vector
realigns with Bo faster than water. Longitudinal
components of magnetization of fat is larger than
water
T2 decay in FAT : It occurs as the result of magnetic
fields
of the nuclei interacting each other. Energy
exchange is
more efficient in the hydrogen in fat. The T2 time of fat
is
short (80ms)
T2 decay in fat
94. 94
T2 decay in water :T2 decay in water : Energy exchange in water is less efficientEnergy exchange in water is less efficient
than in fat, T2 time of hydrogen in water is long. T2 time of waterthan in fat, T2 time of hydrogen in water is long. T2 time of water
(200ms)(200ms)
T2 Contrast :T2 Contrast : T2 time of Fat is shorter than that of water.T2 time of Fat is shorter than that of water.
Transverse components of magnetization of fat decays fasterTransverse components of magnetization of fat decays faster
water has high signal, appear bright. on T2 contrast image fatwater has high signal, appear bright. on T2 contrast image fat
has low signal and appears darkhas low signal and appears dark
on T2 contrast imageson T2 contrast images
T2 decay in water
95. 95
Proton density contrast
It is the difference in signal intensity between
tissues of their relative number of protons per
unit
volume. High signal (on brain tissue) Bright on
proton density contrast. Tissue on low proton
density have low signal e.g. Cortical bone –
dark
on proton density image. Proton density is the
basic of MRI contrast
97. MRI EQUIPMENT
The component of MRI system.
Magnet.
Radio Frequency source.
Image processor.
computer.
Types of magnetism
Para magnetic
Diamagnetic.
Ferromagnetic.
97
99. Advantages of MRI
MRI does not use ionizing radiation, and is thus
preferred over CT in children and patients requiring
multiple imaging examinations
MRI has a much greater range of available soft tissue
contrast, depicts anatomy in greater detail, and is
more sensitive and specific for abnormalities within
the brain itself
MRI scanning can be performed in any imaging
plane without having to physically move the patient
MRI contrast agents have a considerably smaller risk
of causing potentially lethal allergic reaction
MRI allows the evaluation of structures that may be
obscured by artifacts from bone in CT images
99
