1. MRI in Obstetrics PART II BASIC PHYSICS
( ALSO PL SEE PART 1 which contains BASIC PHYSICS OF MRI FOR
COMPLETION )
Dr Shivamurthy H M Prof
Dr Ayesha Sultana Rajgoli, Resident
Dept of OBG
SNMC , Bagalkot
Karnataka , India .
2. What is Resonance ?
Also called REVERBERANCE
The reinforcement or propagation of
sound from an object to a neighbouring
object in air media
The same applys to Magnetic field also
The sound waves sent from TF1 will cause TF2
to vibrate with same frequency
3. Definition - MRI
It is an imaging modality
A procedure that uses Magnetism, Radio waves, and a
Computer , to create pictures of areas inside the body on a
Monitor screen.
5. Principles in M R I
Four basic steps are involved
1) Placing the patient in the magnetic field.
2) Transmitting Radio frequency pulse by coil on to the subject.
3) Receiving signals from the patient by coil.
4) Transformation of signals into image by complex processing in the
computers
7. (2) Propagation MRI Pulse (produced by the external
magnetic Source) which delivered to pt body
โข These Pulses from external magnetic coil are also called Resonance
Frequency (R F) Pulses
โข The Protons in pts body will pick up energy in stepwise manner from
RF source and precess and they upgrade in their own energy also.
8. โข The movement of protons in the patients body are under the
influence of external magnetic field .
โข Under this external magnetic influence , all randomly moving
protons in patients body will align and precess along the external
magnetic field.(z axis)
9. โข The magnetic field in the patient body will process and
generates current .
โข This current is received as signal by Radio frequency-coil.
(3) MR Signals are received
10. (4) Formationof image from the signlas
MR signals which are received by the coil which is below the
patient , are transformed into image by computers by complex
processing.
11. Comparison of MRI with USG
USG MRI
Less soft tissue resolution Superior spatial and soft tissue resolution
Small field of view Large field of view
Multiplanar view difficult Multiplanar view achieved
Poor quality image in oligohydramnios
and obesity
Better visualization
Polyhydramnios better image Problem due to excessive fetal movement
Cheaper Costly
Widely available Limited availability
No claustrophobia Claustrophobia
Less time required Time consuming
12. (a) High cost
(b) Maternal obesity
(c) Polyhydromnios
(d) Claustrophobia- It is a form of anxiety disorder, in which an
Irrational fear of having no escape or being closed in can lead to
panic attack.
Limitations OF MRI
13. Indications
MATERNAL
a) Cerebral vascular flow study and detection of thrombosis.
b) Assessment of CPD during third trimester and in labour ( ? )
c) placenta previa work up
d) Evaluation of maternal tumours
e) Evaluation of placenta previa and accreta
f) Evaluation of intra-abdominal malignancies like lymphoma and
renal cell carcinoma.
14. ๏ฌ Evaluation of retroperitoneal space, for eg: detection of adrenal
tumours like pheochromocytoma.
๏ฌ It is also used for the characterisation of uterine and ovarian
masses.
15. Ventriculomegaly
Agenesis of corpus callosum
Cavum septum pellucidum abnormalities
Holoprosencephaly
Posterior fossa abnormalities
Cerebral cortical malformation
Cephalocoele
Hydranencephaly
Monochorionic twin pregnancy complications
Fetal indications
(1) BRAIN AND SPINE
16. โข Fetal brain is well observed on T2 weighted sequences because of
contrast between CSF and brain tissue.
โข After 17wks advantages of MRI in diagnosis of both developmental
and acquired intracranial abnormalities have been well established,
however diagnosis of some abnormalities may not be possible until
after 24wks.
โข Additional sequences such as T1 weighted and diffusion weighted
imaging provide information about brain development, cell
density,myelination.
โข In case of septo - optic dysplasia, MR imaging may confirm absence
of cavum septum pellucidum
BASIC MRI KNOWLADGE OF CENTRAL NERVOUS SYSTEM
18. (2) SKULL FACE AND NECK
Venolymphatic malformations
Hemangiomas
Goitre
Facial clefts
Teratomas
Other abnormalities with potential airway obstruction
19. Congenital cystic adenomatoid malformation
Extralobar pulmonary sequestration
Bronchogenic cyst congenital lobar overinflation
Diaphragmatic hernia
Effusions
Mediastinal masses
Assessment for esophageal atresia
Evaluation of pulmonary hypoplasia secondary to diaphragmatic
hernia
oligohydromnios, chest mass, or skeletal dysplasia
(3) THORAX
20. a) Abdomino pelvic cystic mass evaluation
b) Tumour evaluation(Sacrococcygeal teratoma,suprarenal or renal
masses)
c) Complex genitourinary anamolies (bladder outlet obstruction
syndromes
d) Bladder exstrophy, cloacal exstrophy)
5) RETROPERITONEUM
Diagnose Anamolies(Anorectal malformations,complex obstructions)
6) COMPLICATIONS OF MONOCHORIONIC TWINS
Determine vascular anatomy prior to laser treatment
Evaluate co-joined twins
4) ABDOMEN AND PELVIS
21. โข Fetal brain anatomy before and after surgical intervention
โข Anamolies for which fetal surgery is planned
โข FETAL SURGERY ASSESSMENT
22. (a) MRI during an early viable pregnancy first trimester
(b) Internal cardiac pacemaker
(c) Implanted defibrillator
(d) Implants or other metals in the body.
(e) Renal failure.
(f) skin patches and tattoos.
Contraindications for Obstetric MRI study
23. ๏ฌ MRI is a 2D plane in the body is
excited and imaged.
๏ฌ Each RF pulse excites a narrow
slice.
๏ฌ Each RF pulse excites the entire
imaging volume and encoding is
used to discriminate spatially.
How 2DMRI is different from 3D MRI ?
๏ฌ In 3D MRI, a whole slab is excited and
encoded.
๏ฌ Each RF pulse excites the entire
imaging volume and encoding is used
to discriminate spatially.
๏ฌ 3D imaging takes longer time due to
the need for slice direction encoding
but has a higher signal to noise ratio
since all the spins inside the slab are
contributing to signal
2D MRI 3D MRI
24. Effects of gestational age and radiation dose on radiation induced teratogenesis
Gestational period Effects Estimated
threshold dose
Before implantation
( 0-2 wks after fertilization)
Death of embryo or no consequence 50-100mgy
Organogenesis
( 2-8 wks after fertilization)
CONGENITAL anomalies
(skeletal,eyes,genitalia)
200mgy
Fetal period Effects Estimated threshold dose
8-15 wks Intellectual disability microcephaly(high risk) 60-310mgy
200mgy
16-25 wks Intellectual disability
(Low risk)
250-280mgy
S, patel S J , Reedee D L , Katz D et al Radigraphics 2007,27:1705 -22
25. ACOG Recommendations 2017 regarding diagnostic imaging
procedures during pregnancy and lactation:
USG and MRI are not associated with risk and are the imaging techniques of choice for
the pregnancy.
They should be used prudently and only when it provides medical benefit to the patient.
With few exceptions, radiation exposure through Xry CT scan, or Nuclear medicine
imaging
is at a dose much lower than the exposure associated with fetal harm.
If these techniques are necessary in addition to USG or MRI or are more readily
available for the diagnosis , they should not be withheld in pregnancy.
The use of gadolinium contrast with MRI may be used as a contrast agent in a
pregnant woman only if it significantly improves diagnostic performance and is expected
to improve fetal or maternal outcome.
Breastfeeding should not be interrupted after Gadolinium administration.
27. Coronal section of brain shows
(a) Cavum septum pellucidum
(b) Anterior horn of lateral ventricle
(c) communicating the dilated lateral ventricles
(a) Gross dilatation of lateral and third ventricles
(b) Septum pelluicidum is absent
(c) The optic chiasma and proximal portion of
optic nerves are small
Cavum Septum Pellucidum
MRI DIAGRAM
1 Contrast image
2 Axial and coronal T1weighted images at
28. AGENESIS OF CORPUS CALLOSUM
Absence of corpus callosum
Poorly formed corpus callosum
Axial T2 Image
36. Coronal T2W image
CERVICAL TERATOMA
Mass involving the anterior neck with
hyperextension of the head
Large mixed signal intensity mass
within the soft tissue of the fetal neck.
The head is being deviated to the side
37. Congenital diaphragmatic hernia
It shows the stomach bowel
loops and liver
A hole in the diaphragm
allows abdominal organs
to move into the chest
Saggital view non contrast
38. Saggital and coronal
PULMONARY ADENOMATOID MALFORMATION
Cyst or mass in the lung
Complex hyperintense lesion with cyst
nside in the right lungs
39. Polycystic Kidneys with Oligohydramnios
Coronal T2 image
Coronal T2 showing the enlarged
kidneys
41. Placenta deeply in the uterine wallPLACENTA ACCRETA
Thin strips of tissue that is slightly
hyperintense.
Thinned myometrium in LUS
42. Placenta invades the uterine
wall.
T2 w
a)Abnormal f placental tissue abutting
the bladder wall
b)Appears less conspicuous in the
same area
43. ๏ฌ The main role of MRI is to confirm or exclude lesions suspected on
routine USG as well as to define their extent and demonstrate
associated abnormalities if any.
๏ฌ Fetal MRI Scores over USG due to its higher spatial resolution,
Larger field of view, and ability to visualize fetal anatomy well,
despite scanty liquor or maternal obesity.
Conclusion
44. ๏ฌ Fetal MRI is now increasingly used in clinical practice, partly
because of the increasing interest in fetal medicine and fetal
surgery.
๏ฌ With accurate diagnosis of abnormality fetal MRI helps in
facilitating surgical planning and intervention.
๏ฌ It also helps in predicting postnatal outcome as well as
management and genetic counselling.
45. ๏ฌ Sequences such as DWI, apparent diffusion coefficient, MRI
spectroscopy,Functional imaging and volumetric data acquisition
are under research.
๏ฌ Although US remains the primary and predominant modality for
disorders related to pregnancy, MRI is now being increasingly used
for specific obstetric indications.
46. Wider the bandwidth thicker is the slice.
(2) Phase encoding( X axis)
These gradient are used to localize the point in a slice from where signal is
coming.
They are applied perpendicular to each other and perpendicular to the slice
selection gradient.
Typically, for transverse or axial sections following are axes and gradients
applied even though X and Y axes are varied
47. ๏ฌ In a usual sequence, slice selection gradient is turned on
at the time of RF pulse.
๏ฌ Phase coding gradient is turned on for a short time after
slice selection gradient.
๏ฌ Frequency encoding(y axis) or read out gradient is on in
the end at the time of signal reception.
๏ฌ Information from all three axes is sent to the computers to
get the particular point in that slice from which the signal is
coming
48. ๏ฌ Short TR and short TE gives T1 weighted images.
๏ฌ Long TR and short TE gives T2 weighted images.
