Transcript slides - Phenix at Vanderbilt

Medical Imaging




X-rays
CT or CAT scan
PET scan
MRI
Apr 18,2008
PHYS117B.02
1
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
atomic 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
Apr 18,2008
2
PHYS117B.02
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 levels
more closely match 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
Apr be
18,2008
3
PHYS117B.02
encountered from different
tissues => an image occurs

Computed Tomography Imaging (CT
Scan, CAT Scan)

Same principal in X-ray pictures and in CAT scan:



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



replace the film by a banana shaped detector which
measures the x-ray profile.
Take pictures from different angles
A Computer reconstructs the image
Apr 18,2008
PHYS117B.02
4
The CT machine
Outside view of modern CT
system showing the patient
table and CT scanning
aperture
Apr 18,2008
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
PHYS117B.02 the patient as the data is gathered.5
CT – schematic view
Diagram showing relationship of x-ray tube, patient, detector,
and image reconstruction computer and display monitor
Apr 18,2008
PHYS117B.02
6
How CT works


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.
Apr 18,2008
PHYS117B.02
7
Apr 18,2008
PHYS117B.02
8
How to see soft tissues with CAT scans


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.


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.
Apr 18,2008
PHYS117B.02
9
Some

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.
Apr 18,2008
PHYS117B.02
10
PET scans
Apr 18,2008
PHYS117B.02
11
PET scans



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, and interacts only very weakly with other
particles. Thus it escapes the body undetected and is 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.
Apr 18,2008
PHYS117B.02
12
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

Conserve energy and momentum in this reaction:




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 Ptot = 0 in the
final state
Apr 18,2008
PHYS117B.02
13
A Coincidence Event



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
Apr 18,2008
PHYS117B.02
14
Some commonly used nuclides (b+ emitters)
Apr 18,2008
Nuclide
Half life
11C
20.3 min
13N
9.97 min
15O
124 sec
18F
110 min
PHYS117B.02
15
How to trace the location of the positron
emission
Abnormal lymph nodes (cancer)
from a PET scan image


PET scanner shows in fine detail the metabolism of glucose, by tracing the
positron emission from 18F.
Cancerous tissue uses more glucose, so it produces stronger signals.
Apr 18,2008
PHYS117B.02
16
MRI ( used to be NMR), but
people are afraid of the word
nuclear ….
Apr 18,2008
PHYS117B.02
17
The MRI machine




Apr 18,2008
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.
PHYS117B.02
18
Nuclei have spin! MRI is tracing the water in your body (
i.e. – the location of hydrogen …or protons).
•Without the external
magnetic field the energy
levels with different
projections of j are
degenerate.
•The protons can “spin”
around any axis
Apr 18,2008
•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.
PHYS117B.02
19
Why do we need the magnetic field ?

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.
Apr 18,2008
PHYS117B.02
20
Magnets: most important component in MRI
machine


The magnets in use today in MRI are in the 0.5 Tesla to 2.0
Tesla range. 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 about 30 microTesla 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.
Apr 18,2008
PHYS117B.02
21
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.
Apr 18,2008
PHYS117B.02
22
The resonance


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.
Apr 18,2008
PHYS117B.02
23
Spin relaxation: the signal is recorded

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 ?
Apr 18,2008
PHYS117B.02
24
MRI position information


Apr 18,2008
PHYS117B.02
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”
25
The gradient magnets


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 18 to 27 milliTesla. 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. Thus we take a picture of a "slice."
Apr 18,2008
PHYS117B.02
26
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.
Apr 18,2008
PHYS117B.02
27
Visualization



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 material.
spine are evident
Apr 18,2008
PHYS117B.02
28
Summary: How MRI works







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.
Apr 18,2008
PHYS117B.02
29