Basic Introduction to IR sequence

1. DAW KHIN THE NU AYE
B.Med.Tech, M.Med.Tech (Medical Imaging Technology)
Demonstrator
Department of Medical Imaging Technology
University of Medical Technology, Yangon

2.  Magnetic Resonance Imaging
 Nuclear Magnetic Resonance Imaging (NMRI)
 Magnetic Resonance Tomography (MRT)
MRI - an advanced medical imaging
technique using a powerful magnetic field, radio
frequency pulses and a computer to produce detailed
pictures of organs, soft tissues, bone and virtually all
other internal body structures.

3. Siemens: Magnetom Skyra
Transforming 3T Productivity
Siemens: Magnetom Verio
Proven 3T clinical imaging

5.  Magnets field strength:
Imaging - 0.2T to 2 .0T
Spectroscopy- 2.0T to 7.0T
Low field - 0.2 to 0.5T
Intermediate 0.5 to 1.5T
High Field - 1.5 to 4.0T
Ultrahigh - >4.0T
Earth’s Magnetic Field = 5 x 10-5 Tesla

6.  A typical clinical scanner has a magnetic field
of 1.5 Tesla.
 To put this into perspective, the Earth’s field is
approximately 0.5 Gauss (1 T = 10000 G)
 Meaning the scanners is thirty thousand times
stronger
(Ref: Liney- MRI in clinical practice)

7.  A pulse sequence is defined as a series of RF pulses,
gradients applications and intervening time periods.
 They enable control of the way in which the system
applies RF pulses and gradients.
 By selecting the intervening time periods, image
weighting is controlled.
 Pulse sequences are required because without a
mechanism of refocusing spins, there is insufficient
signal to produce an image.

8.  Spin Echo
Conventional spin echo
Fast Spin Echo
 Inversion Recovery
 Gradient Echo
Coherent
Incoherent
 Steady state free precession
 Ultra-fast imaging

9.  Pre Saturation Pulses/Bands
 Fat Suppression FATSAT
 Magnetisation Transfer
 FSE Optimisation
 Flow Compensation (Gradient Moment Nulling)

10.  Inversion recovery is a pulse sequence that begins with
a 180 degree inverting pulse.
 This inverts the NMV through 180°
 The TR is the time between successive 180° inverting
pulses.
 When the pulse is removed the NMV begins to relax
back to Bo.
 A 90° pulse is then applied at time interval TI (Time
from Inversion) after the 180° inverting pulse.

11.  A further 180° RF pulse is applied which rephases
spins in the transverse plane and produces an echo
at time TE after the excitation pulse.
 To nullify the magnetic field inhomogeneties
 IR sequences were initially designed to produce
very heavy T1 weighting.
 However, at present, they are mainly used in
conjunction with a FSE sequence to produce T2
weighted images.

12. Ref: MRI – Basics principles and applications (Third Edition)

13. (a) Fat signal as double headed arrow
(b) Inversion pulse (180˚ flip angle)
(c) Fat and water signal along the z-
direction
(d) The two signals recover along the z-
direction, fat covering more quickly
owing to a shorter T1
(e) At a null point, the fat
magnetization has a zero
component in this direction
(f) 90˚ pulse to flip the spins into the
transverse plane, water as
measured signal which then
proceeds to dephase as normal
(Ref: Liney- MRI in clinical practice)
Fig: Short T1 Inversion Recovery for Fat Suppression

14. A 90˚ excitation pulse is then applied after a time from the 180˚ inverting pulse known as the
TI.
The contrast of the resultant image depends primarily on the length of the TI as well as the
TR and TE.
The contrast in the image primarily depends on the magnitude of the longitudinal
magnetisation (as in SE) following the chosen delay time TI.

15. Schematic T1 Weighted Inversion Recovery Diagram for Fat and Water Protons

16.  TE – controls the amount of decay
 TR - long enough to allow full longitudinal recovery
of magnetization before each inverting pulse
NMV has passed
through the
transverse plane
90° pulse is
applied
heavy saturation
and T1 weighting
results

17. T1 weighting
- Medium TI 400 – 800 ms
- Short TE 10- 20 ms
- Long TR 2000 ms+
- Average scan time 5 -15 min
Proton Density Weighting
- Long TI 1800 ms
- Short TE 10 -20 ms
- Long TR 2000 ms+
- Average scan time 5 -15 min
Pathology Weighting
- Medium TI 400 – 800 ms
- Long TE 70 ms+
- Long TR 2000 ms+
- Average scan time 5 – 15 min

19.  Supression of the fat signal in a T1 weighted image
 It takes Fat to recover from full inversion to the
transverse plane so that there is no longitudinal
magnetisation corresponding to fat.
 When the 90º excitation pulse is applied after the
delay time TI, the signal from fat is nullified.
 TI - 150 to 175ms for fat supression
 This value varies at different field strengths (140ms
for 1.5T scanner)

20. Fig: STIR
No fat vector when 90˚ is applied.
Parameters
Short TI 150 – 175 ms
Short TE 10 – 30 ms
Long TR 2000 ms+
Average Scan Time 5 – 15 min

21. Ref: revisemri.com

22. Ref: neuroradiologycases.com
Ref: musculoskeletalmri.blogspot.com

23.  Variation of the inversion recovery sequence
 TI corresponding to the time of recovery of CSF from
180˚ inversion to the transverse plane
 The signal from fluid e.g. cerebrospinal fluid (CSF) is
nulled
 Used to suppress the high CSF signal in T2 and
proton density weighted images
 Pathology adjacent to the CSF is seen more clearly
 TI - approximately 2000 ms at 1.5T

24. Ref: aboutcancer.com Ref: aboutcancer.com
Ref: library.med.utah.edu

25. Parameters
Long TI 1700 – 2200 ms
Short or Long TE depending on weighting required
Long TR 6000 ms+
Average scan time 13-20 mins
Ref: medscape.com

26. Uses
Mainly in CNS (T1 and FLAIR)
FLAIR - periventricular lesions
- lesions in cervical and thoracic cord
-reduces image degradation from partial volume effects & motion
artefacts
Musculoskeletal systems (STIR)
STIR - also called “search and destroy”
- bone lesions conspicuity
(by nulling the signal from normal marrow)
- sensitive to inflammation without the help of Gd contrast

27. Benefits Drawbacks
 Versatile
 Very Good SNR as the TR is
long
 Excellent T1 Contrast and Good
Image Quality
 Sensitive to Pathology
 Delineation of lesions
 Reduction of flow-related
Artifacts
 Long scan time unless used in
conjunction with fast spin echo

28. Parameter Benefit Limitation
TR increased Increased SNR Increased scan time
Decreased T1 weighting
TR decreased Decreased scan time
Increased T1 weighting
Decreased SNR
TE increased Increased T2 weighting Decreased SNR
TE increased Increased SNR Decreased T2 weighting
NEX increased Increased SNR
More signal averaging
Direct proportional increase in
scan time
Slice thickness increased Increased SNR
Increased coverage of anatomy
Decreased spatial resolution
More partial voluming
Slice thickness decreased Increased spatial resolution
Reduced partial voluming
Decreased SNR
Decreased coverage of anatomy

29. Parameter Benefit Limitation
FOV increased Increased SNR
Increased coverage of anatomy
Decreased spatial resolution
Decreased likelihood of
aliasing
FOV decreased Increased spatial resolution
Increased likelihood of aliasing
Decreased SNR
Decreased coverage of
anatomy
Matrix increased Increased spatial resolution Increased scan time
Decreased SNR if pixel is
small
Matrix decreased Decreased scan time
Increased SNR if pixel is large
Decreased spatial
resolution

30. Parameter Benefit Limitation
Receive bandwidth
increased
Decrease in chemical
shift
Decrease in minimum
TE
Decreased SNR
Receive bandwidth
decreased
Increased SNR Increase in chemical shift
Increase in minimum TE
Large coil Increased area of
received signal
Lower SNR
Sensitive to artefacts
Aliasing with small FOV
Small coil Increased SNR
Less sensitive to artefacts
Less prone to aliasing
with a small FOV
Decreased area of
received signal

31.  Basics of MRI (Professor Sir Michael Brady FRS FREng, Department of
Engineering Science,Oxford University)
 MRI in Practice 2nd Edition (Catherine Westbrook & Carolyn Kaut)
 MRI at a Glance (Philip I.Aaronson & Jeremery P.T. Ward)
 MRI Study Guide (Australian Institute of Radiography)
 Clinical MR Imaging- A Practical Approach (R.Reimer, Paul M.Parizel, F.-
A.Stichnoth)
 MRI Basic Principle and Applications 3rd Edition (Mark A. Brown and Richard
C.Semelka)
 www.en.wikipedia.org
 www.mritutor.org
 www.webmd.com

32.  www.musculoskeletalmri.blogspot.com
 www.neuroradiologycases.com
 www.revisemri.com
 www.medscape.com
 www.rochester.com
 www.lecb.physics.lsa.umich.edu
 www.aboutcancer.com
 www.mritutor.org
 www.ajnr.org
 www.medicinenet.com
 www.radiologyinfo.org
 www.beyonddiscovery.org