Transcript Document

Medical Imaging
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X-rays
CT or CAT scan
PET scan
MRI
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X-rays
Electrons emitted from a heated
cathode, bombard the anode:
Bremsstrahlung: does not depend on
Characteristic X-rays: The free
target material. Continuous spectrum.
electron collides with an atom in the
The free electron is attracted to the
anode, knocking an electron out of a
atom nucleus in the anode. As the
lower orbital. A higher orbital electron
electron speeds past, the nucleus
fills the empty position, releasing its
alters its course. The electron loses
excess
energy as a photon.
energy, which it releases as an X-ray
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photon.
How X-rays interact with the tissues in your body
The atoms that make up your body tissue absorb visible light
photons very well. The energy level of the photon fits with various
energy differences between electron positions. Radio waves don't
have enough energy to move electrons between orbitals, so they
pass through most stuff. X-rays also pass through most things,
but for the opposite reason: They have too much energy.
 They can, however, knock an electron away from an atom
altogether. Some of the energy from the X-ray photon works to
separate the electron from the atom, and the rest sends the
electron flying through space. A larger atom is more likely to
absorb an X-ray photon in this way, because larger atoms have
greater energy differences between orbitals -- the energy level
more closely matches the energy of the photon. Smaller atoms,
where the electron orbitals are separated by relatively low jumps
in energy, are less likely to absorb X-ray photons.
 The soft tissue in your body is composed of smaller atoms, and
so does not absorb X-ray photons particularly well. The Ca atoms
that make up your bones are much larger, so they are better at
absorbing X-ray photons.
 When you pass X-rays through the body, different attenuation will
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encountered from different
tissues => an image occurs
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Computed Tomography Imaging (CT
Scan, CAT Scan)
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Same principal in X-ray pictures and in CAT scan:
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x-rays pass through the body they are absorbed or
attenuated (weakened) at differing levels
The picture contains a “shadow” of the dense tissues in
your body
If you want a 3D view
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replace the film by a banana shaped detector which
measures the x-ray profile.
Take pictures from different angles
A Computer reconstructs the image
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The CT machine
Outside view of modern CT
system showing the patient
table and CT scanning
aperture
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Inside view of modern CT system, the
x-ray tube is on the top at the 1 o'clock
position and the arc-shaped CT
detector is on the bottom at the 7
o'clock position. The frame holding the
x-ray tube and detector rotate around
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CT – schematic view
Diagram showing relationship of x-ray tube, patient, detector,
and image reconstruction computer and display monitor
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How CT works
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Inside the covers of the CT scanner is a rotating frame which has
an x-ray tube mounted on one side and the banana shaped
detector mounted on the opposite side. A fan beam of x-ray is
created as the rotating frame spins the x-ray tube and detector
around the patient. Each time the x-ray tube and detector make a
360o rotation, an image or "slice" has been acquired. This "slice"
is collimated (focused) to a thickness between 1 mm and 10 mm
using lead shutters in front of the x-ray tube and x-ray detector.
As the x-ray tube and detector make this 360o rotation, the
detector takes numerous snapshots (called profiles) of the
attenuated x-ray beam. Typically, in one 360o lap, about 1,000
profiles are sampled. Each profile is subdivided spatially (divided
into partitions) by the detectors and fed into about 700 individual
channels. Each profile is then backwards reconstructed (or "back
projected") by a dedicated computer into a two-dimensional
image of the "slice" that was scanned.
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How to see soft tissues with CAT scans
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In a normal X-ray picture, most soft tissue doesn't
show up clearly. To focus in on organs, or to
examine the blood vessels that make up the
circulatory system, doctors must introduce contrast
media into the body.
Contrast media are liquids that absorb X-rays more
effectively than the surrounding tissue.
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To bring organs in the digestive and endocrine systems into
focus, a patient will swallow a contrast media mixture,
typically a barium compound.
If the doctors want to examine blood vessels or other
elements in the circulatory system, they will inject contrast
media into the patient's bloodstream.
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Some
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scan images
"CAT scanning" (Computed Axial Tomography), was developed in the early to mid 1970s and is
now available at over 30,000 locations throughout the world. CT is fast, patient friendly and has
the unique ability to image a combination of soft tissue, bone, and blood vessels.
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PET scans
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PET scans
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PET imaging relies on the nature of the positron and b+ decay. The
positron was first conceived by Paul Dirac in the late 1920s, in his
theory combining quantum mechanics and special relativity. It was
experimentally discovered in 1932, the same year as the neutron. The
positron is the antimatter counterpart to the electron, and therefore has
the same mass as the electron but the opposite charge.
Beta+ Decay
When a nucleus undergoes positron decay, the result is a new nuclide
with 1 fewer proton and 1 more neutron, as well as the emission of a
positron and a neutrino:
The radionuclides that decay via positron emission are proton-rich and
move closer to the line of stability while giving off a positive charge.
The neutrino is very light, if it has any mass at all, and interacts only
very weakly with other particles. It is therefore not directly relevant to
nuclear medicine. However, its presence in the positron decay makes
the energy of the positron variable, as opposed to gamma emissions,
which are of a fixed energy for a given radionuclide.
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Positron Annihilation
 As positrons pass through matter and lose energy through ionization
and excitation of nearby atoms and molecules.
 After losing enough energy, and having traveled a distance in the
neighborhood of 1 mm (depending on the initial positron energy), the
positron will annihilate with a nearby electron
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Conserve energy and momentum in this reaction:
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Initial energy comes from the mass of the electron and
positron
Final energy : kinetic energy of 2 photons (511 KeV each)
Why 2 photons ?
Well we need to conserve momentum. In the initial state
– electron and positron at rest Ptot =0 . Since a photon
can not exist at rest ( it moves with the speed of light),
you need 2 photons back-to-back to get Ptor = 0 in the
final state
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A Coincidence Event
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The simultaneous emission of the 2
photons in opposite directions is the basis
of coincidence detection and coincidence
imaging. The line along which the
photons are emitted can be pointed in
any direction, but if you measure enough
lines, they will cross in some region of the
body where most of the emissions
happened.
A ring of radiation detectors surrounds the
patient in whom a positron emission and
subsequent annihilation has occurred.
The simultaneous detection of 2 photons
is referred to as a "coincidence". This
meaning is very different from the
common usage of the term "coincidence"
to mean that 2 events happened without
common cause. More coincidence events
along a line means more radiation from
this part of the body
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Some commonly used nuclides (b+ emitters)
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Nuclide
Half life
11C
20.3 min
13N
9.97 min
15O
124 sec
18F
110 min
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How to trace the location of the positron
emission
Abnormal lymph nodes (cancer)
from a PET scan image
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PET scanner and shows in fine detail the metabolism of glucose, by tracing
the positron emission from 18F.
Cancerous tissue uses more glucose, so they produce stronger signals.
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MRI ( used to be NMR), but
people are afraid of the word
nuclear ….
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The MRI machine
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The basic design used in most MRI
scanners is a giant cube. The cube in a
typical system might be 7 feet tall by 7
feet wide by 10 feet long (2 m by 2 m by
3 m), although new models are rapidly
shrinking:
There is horizontal tube running through
the magnet from front to back.
The patient, lying on his or her back,
slides into the magnet on a special table.
Whether or not the patient goes in head
first or feet first, as well as how far in the
magnet they will go, is determined by the
type of exam to be performed.
MRI scanners vary in size and shape,
and newer models have some degree of
openness around the sides, but the basic
design is the same.
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Nuclei have spin! MRI is tracing the water in your body.
•Without the external
magnetic field the energy
levels with different
projections of j are
degenerate.
•The protons can “spin”
around any axis
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•Aligning the spins in external magnetic field.
•Now the z-axis is defined.
•In addition, the levels with different
magnetic quantum number are split in
energy (the degeneracy is lifted). Thus then
proton can absorb radiation ( it’s in the RF
range) and move between energy levels (
flip its spin.
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Why do we need the magnetic field ?
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A very uniform, or homogeneous, magnetic
field of incredible strength and stability is
critical for high-quality imaging. You can form
this field using a solenoidal coil. It forms the
main magnetic field. It is needed to split the
energy levels in the nuclei.
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Magnets
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The biggest and most important component in an MRI system is the
magnet. The magnets in use today in MRI are in the 0.5-tesla to
2.0-tesla range, or 5,000 to 20,000 gauss. Magnetic fields greater
than 2 tesla have not been approved for use in medical imaging,
though much more 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.
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. 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.
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Magnets can be dangerous
In this photograph, you can see a fully loaded pallet jack that has been sucked into
the bore of an MRI system.
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The resonance
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The MRI machine applies an RF (radio frequency) pulse that is
specific only to hydrogen. The system directs the pulse toward
the area of the body we want to examine. The pulse causes the
protons in that area to absorb the energy required to make
them go to an energy level with different magnetic quantum
number. This is the "resonance" part of MRI. The specific
frequency of resonance is different for different types of tissue.
These RF pulses are usually applied through a coil. MRI
machines come with many different coils designed for different
parts of the body: knees, shoulders, wrists, heads, necks and so
on. These coils usually conform to the contour of the body part
being imaged, or at least reside very close to it during the exam.
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Spin relaxation: the signal is recorded
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Next step: spin relaxation back to the ground
state. The signal is recorded by a detector of
RF radiation. But how do we know where the
signal is coming from ?
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MRI position information
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Applying a gradient
field ( position
dependent field
strength) which alters
the energy level
splitting in nuclei which
are in different parts of
the body.
When you record the
signal every part of the
body “plays a different
note”
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The gradient magnets
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Every MRI system has a gradient magnet in addition to the
main magnet. There are three gradient magnets inside the MRI
machine. These magnets are very, very low strength compared
to the main magnetic field; they may range in strength from 180
gauss to 270 gauss, or 18 to 27 millitesla (thousandths of a
tesla). These magnetic fields are needed to provide position
information in the image.
At approximately the same time, the three gradient magnets
jump into the act. They are arranged in such a manner inside the
main magnet that when they are turned on and off very rapidly in
a specific manner, they alter the main magnetic field on a very
local level. What this means is that we can pick exactly which
area we want a picture of. In MRI we speak of "slices."
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MRI images
MRI provides an unparalleled view inside the human body. The level of detail we
can see is extraordinary compared with any other imaging modality. MRI is the
method of choice for the diagnosis of many types of injuries and conditions
because of the incredible ability to tailor the exam to the particular medical
question being asked. By changing exam parameters, the MRI system can cause
tissues in the body to take on different appearances. This is very helpful to the
radiologist (who reads the MRI) in determining if something seen is normal or not.
We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there
might be an abnormality.
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Visualization
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In X-ray and CT scan you would use
injectable contrast, or dyes to alter the X-ray
intensity from different regions of the body.
The contrast used in MRI is fundamentally
different.
MRI contrast works by altering the local
magnetic field in the tissue being examined.
Normal and abnormal tissue will respond
differently to this slight alteration, giving us
differing signals. These varied signals are
transferred to the images, allowing us to
visualize many different types of tissue
This MRI scan shows the
upper torso in side view
abnormalities and disease processes better
so that the bones of the
than we could without the contrast.
spine are evident
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Summary: How MRI works
When in the MRI scanner:
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The nuclei of a patient's hydrogen atoms align with the scanner's
magnetic field.
Pulses of radio waves are sent into the scanner that make the hydrogen
nuclei flip their spin and jump into a higher energy level (precess around
a different axis)
After the radio wave pulsing stops, the nuclei realign their spin with the
external magnetic field
During the realignment process, the nuclei emit photons of radio
frequency. These signals are captured by the computer system that
analyzes and translates them into an image of the body part being
scanned.
A gradient in the magnetic field makes the energy level splitting different
at different locations in the body – thus analyzing to frequency of the
emitted radio waves, we get position information. Different tissues have
different resonance frequency – thus you get contrast in the image.
The image appears on a viewing monitor and then is sent to a camera to
be developed on several large sheets of film.
Radiologists interpret the images on film or directly from a viewing
station. They dictate a report of the findings which is sent to the patients
referring physician.
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