small presentation on mri strength and its limitations

1. MAAJID MOHI UD DIN MALIK
LECTURER COPMS AU,
BATHINDA PUNJAB

2.  The biggest and most important
component in an MRI system is
the magnet. The magnet in an MRI
system is rated using a unit of measure
known as a Tesla. Another unit of
measure commonly used with magnets is
the gauss (1 Tesla = 10,000 gauss). The
magnets in use today in MRI are in the
0.5-Tesla to 3.0-Tesla range, or 5,000 to
30,000 gauss.

3.  Extremely powerful magnets -- up to 60
Tesla -- are used in research. Compared
with the Earth's 0.5-gauss magnetic
field, you can see how incredibly
powerful these magnets are.
 Because of the power of these magnets,
the MRI suite can be a very dangerous
place if strict precautions are not
observed. Metal objects can become
dangerous projectiles if they are taken
into the scan room.

4.  For example, paperclips, pens, keys,
scissors, hemostats, stethoscopes and
any other small objects can be pulled
out of pockets and off the body without
warning, at which point they fly toward
the opening of the magnet (where the
patient is placed) at very high speeds,
posing a threat to everyone in the
room. Credit cards, bank cards and
anything else with magnetic encoding
will be erased by most MRI systems.

6. The magnetic force exerted on an object
increases exponentially as it nears the magnet.
Imagine standing 15 feet (4.6 m) away from the
magnet with a large pipe wrench in your hand.
You might feel a slight pull. Take a couple of
steps closer and that pull is much stronger.
When you get to within 3 feet (1 meter) of the
magnet, the wrench likely is pulled from your
grasp. The more mass an object has, the more
dangerous it can be -- the force with which it is
attracted to the magnet is much stronger.

7.  Mop buckets, vacuum cleaners, IV poles,
oxygen tanks, patient stretchers, heart
monitors and countless other objects have all
been pulled into the magnetic fields of MRI
machines. Smaller objects can usually be
pulled free of the magnet by hand. Large ones
may have to be pulled away with a winch, or
the magnetic field may even have to be shut
down.

8.  Prior to allowing a patient or support staff
member into the scan room, he or she is
thoroughly screened for metal objects -- and
not just external objects. Often, patients
have implants inside them that make it very
dangerous for them to be in the presence of a
strong magnetic field. Metallic fragments in
the eye are very dangerous because moving
those fragments could cause eye damage or
blindness. People with pacemakers cannot be
scanned or even go near the scanner because
the magnet can cause the pacemaker to
malfunction.

9.  Aneurysm clips in the brain can be very
dangerous as the magnet can move them,
causing them to tear the very artery they
were placed on to repair.
 MRI magnetic fields are incredibly
strong. A watch flying off an arm and
into the MRI machine is entirely possible.

10.  The main advantages of magnetic resonance
imaging (MRI) scans are that:
 They do not involve exposure to radiation, so
they can be safely used in people who might be
particularly vulnerable to the effects of
radiation, such as pregnant women and babies,
 They are particularly useful for showing soft
tissue structures, such as ligaments and
cartilage, and organs such as the brain, heart,
and eyes

11.  They can provide information about how the
blood moves through certain organs and
blood vessels, allowing problems with blood
circulation, such as blockages, to be
identified.
 MRI scans have specific advantages over X-
rays, as they can:
 show swelling and inflammation
 show both three-dimensional and cross-section
images of the body

12.  The main disadvantages of magnetic resonance
imaging (MRI) scans are listed below.
 MRI scanners are very expensive; a single scanner
can cost over a million pounds. This means that
the number of scanners that a Primary Care Trust
(PCT) can afford to fund is limited. Therefore, if
your condition is non-urgent, you may have to
wait several months to have a MRI scan.
 The combination of being put in an enclosed space
and the loud noises that are made by the magnets
can make some people feel claustrophobic while
they are having a MRI scan.

13.  MRI scanners can be affected by movement,
making them unsuitable for investigating
problems such as mouth tumors because
coughing, or swallowing, can make the images
that are produced less clear.
 Bone and calcium do not show up on an MRI
scan. This means that tissue calcification, a
feature of a number of diseases such as
osteoporosis, cannot be detected using MRI
scanning.

14.  Put subject in big magnetic field
 2) Transmit radio waves into
subject [2~10 ms]
 3) Turn off radio wave transmitter
 4) Receive radio waves re-
transmitted by subject
 5) Convert measured RF data to
image

15.  Quantum properties of nuclear spins
 Radio frequency (RF) excitation
properties
 Tissue relaxation properties
 Magnetic field strength and gradients
 Timing of gradients, RF pulses, and
signal detection

16.  Nucleus needs to have 2 properties:
 Spin
 charge
 Nuclei are made of protons and neutrons
 Both have spin ½
 Protons have charge
 Pairs of spins tend to cancel, so only atoms
with an odd number of protons or
neutrons have spin
 Good MR nuclei are 1H, 13C, 19F, 23Na, 31P

17.  Biological tissues are predominantly 12C, 16O,
1H, and 14N
 Hydrogen atom is the only major species that is
MR sensitive
 Hydrogen is the most abundant atom in the
body
 The majority of hydrogen is in water (H2O)
 Essentially all MRI is hydrogen (proton)
imaging

19.  Moving (spinning) charged particle
generates its own little magnetic field
 Such particles will tend to line up with
external magnetic field lines (think of iron
filings around a magnet)
 Spinning particles with mass have
angular momentum
 Angular momentum resists attempts to
change the spin orientation (think of a
gyroscope)

20. Note: Resonance at 1.5T = Larmor frequency X 1.5

21.  NMR measures the net magnetization of atomic
nuclei in the presence of magnetic fields
 Magnetization can be manipulated by changing the
magnetic field environment (static, gradient, and RF
fields)
 Static magnetic fields don’t change (< 0.1 ppm / hr.):
The main field is static and (nearly) homogeneous
 RF (radio frequency) fields are electromagnetic fields
that oscillate at radio frequencies (tens of millions of
times per second)
 Gradient magnetic fields change gradually over space
and can change quickly over time (thousands of times
per second)

22. • RF electromagnetic fields are used to manipulate
the magnetization of specific types of atoms
• This is because some atomic nuclei are sensitive
to magnetic fields and their magnetic properties
are tuned to particular RF frequencies
• Externally applied RF waves can be transmitted
into a subject to perturb those nuclei
• Perturbed nuclei will generate RF signals at the
same frequency – these can be detected coming
out of the subject