9848 Lecture Notes - Lecture 10: Ionizing Radiation, Faraday Cage, Mammography
19 Jun 2018
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Intro to MRI – lecture
What is an MRI? - Uses magnestic fields and radiowaves
Basics
- An MRI is a medical imaging device which uses magnetic fields and radio waves to get a
clinical picture.
The body part being scanned is placed in
the centre of the magnetic field, often also
within a coil which receives signal from
that part.
Takes between 15 minutes and 1.5 hours
depending on the anatomy and pathology.
Does not use any x-ray radiation.
Makes a lot of acoustic noise.
View from inside the bore
Many common MRI scanners have a 70cm
bore diameter, some have 60cm bores.
This can change the patient experience
significantly
Components of MRI suite room
- Suite Design – Zones 1-4 = Zones 1 & 2 are
public access. Zones 3 & 4 are restricted
access, with secure doors at the start of each
zone
- Suite doors = Lockable Doors - very limited access, Large enough to allow an MRI safe
trolley with ancillary equipment entry
- Faraday Cage & Scan room Door = Copper, or copper mesh, covering all four walls,
ceiling, floor, door and windows. • Designed to exclude electric fields, in this case radio-
frequencies. • Protects images from interference – has no effect on the magnetic field
- Scanner Bore and Table =
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- Coils (Dedicated and Multi Use) = wehre put body part being exaimoned in, receiving
Antenor . eg head coil
- Console – computer , wher you control the machinery , desk
- Equipment room= Primarily used by engineers, though often also a storage/locker
room
Why do an MRI?
Advantages
- No ionising radiation – no radiation dose.
- Excellent soft tissue contrast.
- Less invasive – less examinations require an intravenous injection.
- May provide both anatomical and physiological information. (Structure and function).
Disadvantages
- Claustrophobia
- Physical size of the patient/body part
- Not all patients can safely be scanned
- Availability - Limited numbers of MRI scanners in Australia . Even more limited
licenced scanners
- Cost - Medicare eligibility/ Gap , Private scans , Multi-region scan requests , Additional
costs for IV contrast/sedation etc.
Alternatives
- X-ray (including Mammography) Quick, cheap, common, x-ray dose, gold standard?
- CT scan Quite quick, relatively cheap, common, x-ray dose.
- Ultrasound No x-ray dose, fairly quick, price variable, can get functional information.
- Nuclear Medicine (including PET) Injection of Isotopes, ionising radiation, less available,
price variable, primarily gets physiological information.
How do we get an image?
Protons
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Main Magnetic Field
RF Pulse
Relaxation
Different Pulse Types, and Diagrams
Principles of MRI
Whole body filled with water – therefore hydrogen
The hydrogen nucleus is a single, positively charged proton.
Protons have a property called spin which makes them magnetic.
•Because of this they will ”line up” when placed in a static magnetic field such as an MRI.
(The main magnetic field is called B0).
This generates a net magnetisation along the bore of the scanner, in the z-axis.
When you put someone
in mri look for the elft
overs.
•The MRI scanner then
applys a radiofrequency
field (RF-pulse) at the Lamour frequency to the body in the scanner. Throw radioplus which gives
them more energy – makes them flkip to another dircetions – all start to spin at same time in same
direction. Then get chilled- release energy they recived (FID)
This added energy causes the protons to ’flip’ to a plane perpendicular to
the scanner’s magnetic field – so we get a net magnetisation in the x-y
plane.
The protons also start to precess (spin) in phase with each other.
Proto
ns which have been ’flipped’ into a higher energy state naturally want to
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Document Summary
An mri is a medical imaging device which uses magnetic fields and radio waves to get a clinical picture. The body part being scanned is placed in the centre of the magnetic field, often also within a coil which receives signal from that part. Takes between 15 minutes and 1. 5 hours depending on the anatomy and pathology. Many common mri scanners have a 70cm bore diameter, some have 60cm bores. Suite design zones 1-4 = zones 1 & 2 are public access. Zones 3 & 4 are restricted access, with secure doors at the start of each zone. Suite doors = lockable doors - very limited access, large enough to allow an mri safe trolley with ancillary equipment entry. Coils (dedicated and multi use) = wehre put body part being exaimoned in, receiving. Console computer , wher you control the machinery , desk. Equipment room= primarily used by engineers, though often also a storage/locker room.
