MRI utilizes the magnetic spin property of protons in hydrogen atoms to produce images. When an external magnetic field is applied, protons in the body align in one direction. RF waves are used to manipulate the magnetization of hydrogen nuclei. As the nuclei relax, they emit RF signals that are detected to produce images. T1-weighted images highlight tissues based on the time it takes hydrogen nuclei to recover longitudinal magnetization, while T2-weighted images highlight tissues based on the time for hydrogen nuclei to become dephased. Flair imaging uses a 180 degree pulse to null CSF signal, highlighting lesions adjacent to CSF. MRI is useful for imaging soft tissues and has advantages over CT such as no ionizing radiation and ability to use contrasts,

1. Dr.Ankit Choudhary
PGT M.S. ENT
IPGMER

2. Still a mystery
I keep six honest serving-men
(They taught me all I knew);
Their names are What and Why and When
And How and Where and Who.

3.  MRI is based on Nuclear Magnetic Resonance (NMR)
 Two basic principles of NMR
1. Atoms with an odd number of protons or neutrons have spin
2. A moving electric charge, be it positive or negative, produces a magnetic
field
 Body has many such atoms (1H, 13C, 19F, 23Na)
 Hydrogen nuclei is one of them which is not only positively charged,
but also has magnetic spin
 MRI utilizes this magnetic spin property of protons of hydrogen to
elicit images
 Thus basically all MRI are Proton(Hydrogen) Imaging

4. • In our natural state Hydrogen ions in
body are spinning in a haphazard
fashion, and cancel all the magnetism.
• When an external magnetic field is
applied protons in the body align in one
direction. (As the compass aligns in the
presence of earth’s magnetic field)
 Magnetic field strength: 0.3 – 7 T (2500
times more than earth’s magnetic field).
Average field strength – 1.5 T
 Half of the protons align along the
magnetic field and rest are aligned
opposite with ratio of antiparallel versus
parallel Protons roughly 100,000 to
100,006 per Tesla of B0
 These extra protons produce net
magnetization vector(M) which
depends on B0 and temperature

5.  RF waves are used to manipulate the magnetization of H nuclei
 Externally applied RF waves perturb magnetization into different axis
(transverse axis).
 When RF pulse is stopped hydrogen nuclei relax and higher energy
gained by proton is retransmitted by two mechanisms emiting RF
signals which can be detected with the help of receiving coils
 The original magnetization begins to recover (T1)
 The excessive spin begins to dephase (T2)
 TE (echo time) : time interval in which signals are measured after RF
excitation
 TR (repetition time) : the time between two excitations is called repetition
time
 By varying the TR and TE one can obtain T1WI and T2WI
 In general a short TR (<1000ms) and short TE (<45 ms) scan is T1WI
 Long TR (>2000ms) and long TE (>45ms) scan is T2WI
 Long TR (>2000ms) and short TE (<45ms) scan is proton density image

6. T1-weighted
Time it takes for the
hydrogen nucleus to
recover 63% of its
longitudinal
magnetization
T2-weighted
Time for 63% of the
protons to become
dephased

7. IMAGING
CT SCAN CSF Edema White
Matter
Gray
Matter
Blood Bone
MRI T1 CSF
Air
Edema Gray
Matter
White
Matter
Cartilage Fat
MRI T2 Cartilage
Air
Fat White
Matter
Gray
Matter
Edema CSF
MRI T2
Flair
CSF Cartilage Fat White
Matter
Gray
Matter
Edema

9. CT SCAN
MRI T1 Weighted
MRI T2 Weighted
MRI T2 Flair

10.  180° preparatory pulse is applied to flip the net magnetization
vector 180° and null the signal from a particular entity (eg,
water in tissue).
 In contrast to real image reconstruction, negative signals are
recorded as positive signals of the same strength so that the
nulled tissue remains dark and all other tissues have higher
signal intensities.
 FLAIR images are heavily T2-weighted with CSF signal
suppression, highlights hyperintense lesions and improves their
conspicuity and detection, especially when located adjacent to
CSF containing spaces
 In STIR sequences, an inversion-recovery pulse is used to null
the signal from fat

11.  Pregnancy is a relative contraindication, as we will
never be able to tell with 100% certainty that MRI is
100% safe during pregnancy
 No mobiles, no credit cards, please!
 Tatoo marks
 Known potential safety concerns due to large static
magnetic field:
 Internal cardiac pacemakers
 Steel cerebral aneurysm clips (ferromagnetic)
 Small steel slivers embedded in eye
 Life-support equipment with magnetic steel
 Cochlear implants
 Stents anywhere in the body

12.  Faster
 Less expensive
 Less sensitive to patient
movements
 Easier in claustrophobics
 Acute haemorrhage
 Calcification
 Bone details
 Foreign body
 No ionising radiation
 Greater details, hence more
sensitive and more specific
 Any plane scanning
 Contrast less allergic
 Contrast can be used in
Renal diseases/faliure
CT

13.  Otology
 Gd enhanced MRI for CP angle tumour
 MRV in Glomus Juglare
 Rhinology
 Pitutary Tumours
 Nasopharyngeal CA involving base of skull
 CSF Rhinorrheoa
 Orbit
 Laryngology
 Dynamic MRI in OSA
 Laryngeal CA
 Oral Malignancy
…………………..nd many more

33.  MRI has limitations:
 Bone
 Air
 Time consuming
 Poor spatial resolution
 Expertise!
 Claustrophobia
 Necrotising Systemic Fibrosis

34. Ask a specific question
Get a specific answer
Because the sequences can be tailored accordingly.