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Medical imaging
Medical imaging uses electromagnetic radiation,
sound or ingestion of radioactive substances
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Ultrasound Imaging
Transducer
Reflector
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Use high-frequency sound (ultrasound)
Not audible (typically above 1MHZ)
Piezoelectric crystal creates sound waves
Aimed at a specific area of the body
Density changes reflect sound waves
Echoes are recorded
Delay of reflected signal determines the
position
Works for both still and moving objects
Safe! No known tissue damages
Very cheap
Commonly used to examine fetuses in uterus
wrt size, position, or abnormality
Also for heart, liver, kidneys, gall bladder,
breast, eye, and major blood vessels
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X Rays
On 8 November 1895, Wilhelm
Conrad Roentgen (1845-1923)
Was experimenting on emissions
generated by discharging
electrical current in evacuated
glass tubes. To Roentgen's
surprise, an object across the
room began to glow. This proved
to be a barium platinocyanide-coated
screen too far away to be reacting
to the cathode rays as he
understood them.
First x ray picture
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X Rays for everyone
In New York City in 1898, for example, Roentgen's
discovery could be seen in 'one portrait" studios,
coin operated "amusement machines“
("See the bones of your hand for a nickel!!"),
at meetings of the Brooklyn Boys X-Ray Club,
and in countless electrical and photographic companies.
This early X-ray studio, run by
electricians, made "bone portraits" for
interested clients, as well as attending
to the radiographic needs of nearby hospitals.
Some authors, like F.T. Addyman, entered
into agreements with manufacturers so that
a reader could order delivered to his or her
home a full set of the apparatus described in
the text.
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X ray images
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Modern X ray imaging
Electrons are accelerated by a high
voltage and collide with a target (just like a
TV tube. As the electrons decelerate they
produce X rays and also they knock inner
electrons out of the target atoms and
when the atom returns to it’s ground state
X rays are produced of specific energies.
The image produced in an X ray is an
absorption image, for example, bones
absorb more X rays. The main advances in
100 years is to be able to get images with
much smaller doses,
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CAT scan
A CAT scan uses X rays to image
a thin slice of the human body.
Images are taken of successive
slices and computers are used
to produce 3 D rotatable images
The x-ray tube is 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 the patient
as the data is gathered.
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CAT images
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Magnetic resonance imaging
MRI is an interplay of magnetism and resonance.
The body is made up mostly of water and fat .
Both of these contain a large number of Hydrogen atoms.
These atoms have a spin and act like a magnet
If we put the human body inside a large magnet,
which has a strength much larger than the magnetic
field of earth then the tiny spins of the hydrogen atoms
align themselves with the magnetic field, with either N-S
and S-N configuration or N-N and S-S configuration.
The N-S/S-N configuration takes less energy to align
so it is slightly more abundant than N-N/S-S orientation.
This gives a net spin vector along the magnetic field (Bo)
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Spin alignment
In a magnetic field the
atoms act like little magnets
and line up with the field.
M
There are more parallel to
the field because this is a
lower energy configuration
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N
N
Medical imaging
N
N
N
N
N
N
N
N
N
N
B0
N
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Precession (like a top)
In a magnetic field the nuclei do not actually
line up with the external field but the direction
of the nuclear magnet precesses around the
direction of the external field like a top. The
frequency of rotation depends on the external
field. At 1.0 T the protons spin at approx. 42
million times a second At 1.5T, they spin at
approx. 63 million times a second
An essential feature of MRI is that
there is a magnetic field gradient
across the patient so that the precession
frequency is position dependent
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B0
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Levitated daisy
Most objects behave
aa weak magnets. In
this picture a daisy is
levitated with the
magnet force upwards
balancing gravity
downwards
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MRI in practice
The precession frequency depends on the magnetic field and a
critical element in MRI is that the magnetic field has a different
value at each point in the patient so the precession frequency is
different. An RF pulse is sent in with a specific frequency and the
protons with that frequency absorb energy. When the pulse stops (
~ few milliseconds) the protons relax and re emit the energy. This
energy is detected as a current in a coil and the characteristics of
that current determine the density of protons at that scan point.
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MRI images of the brain
sagittal
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Axial
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coronal
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CT and MRI Images
CT image of human lungs
MRI image of a human head
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Compare CT and MRI
CT
MRI
Spatial resolutions Slice spacing: 0.5-10 mm Slice spacing: 0-10 mm
xy-plane: 0.1 mm
xy-plane: 0.1 mm
Measurement
mechanism
X-ray projection
Nuclear magnetic
resonance
Health risk
High
Low
Measurement cost
Low
High
Suitable materials
Bones, teeth, lungs, or
tissues with injected
trace
Soft tissues containing
water
Noise level
Relative high
Low
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Nuclear Medicine
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Nuclear medicine studies document organ and function and structure, in
contrast to conventional radiology, which creates images based upon anatomy.
Many of the nuclear medicine studies can measure the degree of function
present in an organ, often times eliminating the need for surgery. Moreover,
nuclear medicine procedures often provide important information that allows
the physician to detect and treat a disease early in its course when there may
be more success. It is nuclear medicine that can best be used to study the
function of a damaged heart or restriction of blood flow to parts of the brain.
The liver, kidneys, thyroid gland, and many other organs are similarly imaged.
What is Nuclear Imaging ?
The process involves injecting into the body a small amount of chemical substance
tagged with a short lived radioactive tracer. Depending on the chemical substance
used, the radiopharmaceutical concentrates in the part of the body being
investigated and gives off gamma rays. A gamma camera then detects the source
of the radiation to build a picture. These are called scans.
Nuclear Imaging Scans
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Brain Scans These investigate blood circulation and diseases of the brain
such as infection, stroke or tumor. Technetium is injected into the blood so
the image is that of blood patterns.
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Thyroid Uptakes and Scans These are used to diagnose disorders of the
thyroid gland. Iodine 131 is given orally , usually as sodium iodide solution.
It is absorbed into the blood through the digestive system and collected in
the thyroid.
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Lung Scans These are used to detect blood clots in the lungs. Albumen,
which is part of human plasma, can be coagulated, suspended in saline and
tagged with technetium.
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Cardiac Scans These are used to study blood flow to the heart and can
indicate conditions that could lead to a heart attack. Imaging of the heart
can be synchronised with the patient's ECG allowing assessment of wall
motion and cardiac function.
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Bone Scans These are used to detect areas of bone growth, fractures,
tumors, infection of the bone etc. A complex phosphate molecule is labeled
with technetium. If cancer has produced secondary deposits in the bone,
these show up as increased uptake or hot spots.
Radioisotopes Used in Nuclear Medicine
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For imaging Technetium is used extensively, as it has a short physical half
life of 6 hours. However, as the body is continually eliminating products the
biological half life may be shorter. Thus the amount of radioactive exposure
is limited.
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Technetium is a gamma emitter. This is important as the rays need to
penetrate the body so the camera can detect them.
•
It has such a short half life, it cannot be stored for very long because it will
have decayed. It is generated by a molybdenum source (parent host) which
has a much greater half life and the Tc extracted on the day it is required.
The molybdenum is obtained from a nuclear reactor and imported. For
treatment of therapy, beta emitters are often used because they are
absorbed locally.
Radioactive elements
The half life needs to be short so that an image
can be obtained in a reasonable time and also that the
radioactive dose be non hazardous. Isotopes have to be made
close to point of use.
Labeling agent
carbon-11
oxygen-15
fluorine-18
bromine-75
Half-life
20.3 minutes
2.03 minutes
109.8 minutes
98.0 minutes
The different radioactive elements provide different images.For
example oxygen-15 provides an image of how oxygen is being
absorbed. Flourine is used to study brain metabolism and carbon
for blood flow.
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HOW IS TECHETIUM USED FOR A
HEART SCAN
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The test is usually performed at least 12 hours after a suspected heart attack, but it
can also be done during triage of a patient who goes to a hospital emergency room
with chest pain but does not appear to have had a heart attack. Recent clinical
studies demonstrate that technetium heart scans are very accurate in detecting heart
attacks while the patient is experiencing chest pain. They are far more accurate than
electrocardiogram findings.
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The technetium heart scan is usually performed in a hospital's nuclear medicine
department but it can be done at the patient's bedside during a heart attack if the
equipment is available. The scan is done two to three hours after the technetium is
injected. Scans are usually done with the patient in several positions, with each scan
taking 10 minutes. The entire test takes about 30 minutes to an hour. The scan is
usually repeated over several weeks to determine if any further damage has been
done to the heart. The test is also called technetium 99m pyrophosphate
scintigraphy, hot-spot myocardial imaging, infarct avid imaging, or myocardial
infarction scan.
The Gamma Camera
The modern gamma camera
consists of:
- multihole collimator
- large area (e.g 5 cm )
NaI(Tl) (Sodium Iodide Thallium activated)
scintillation crystal
- light guide for optical
coupling array (commonly
hexagonal) of photo-multiplier
tubes
- lead shield to minimize
background radiation
most gamma cameras use thallium-activated NaI (NaI(Tl))
NaI(Tl) emits blue-green light at about 415 nm
the spectral output matches well the response of
standard bialkali photomultipliers (e.g SbK2Cs )
the linear attenuation coefficient of NaI(Tl) at 150 KeV is about 2.2 1/cm .
Therefore about 90% of all photons are absorbed within about 10 mm
NaI(Tl) is hydroscopic and therefore requires hermetic encapsulation
PET scan
Positron – electron tomography
requires ingesting a radioactive
element that decays with positron
emission. The positron annihilates
with an electron and produces
a pair of back to back photons.
Since there are many decays in a small
volume one can intersect many photon
pairs to find the number of photons/unit
volume.
PET scans are used, for example, to
image blood flow in the Brain.
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PET Scan for Cancer
PET can help doctors locate the presence of cancer/infection
anywhere in the body. Because cancers are multiplying and require
energy for growth, the PET scan is designed to detect any mass that
is growing fast. The PET scan involves the use of radioactive glucose
which is injected into the body. The glucose is taken up by the cancer
cells and this activity can be monitored by the PET scan. PET scan
has the ability to identify tumors in their very early phase. The PET
scan can also detect the spread of cancer in other parts of the body.
PET scanning is the most sensitive test for detecting cancer
and its location. When used in conjunction with other
radiological tests like Ultrasound, CT Scans and/or MRI, it is
very effective in the detection of cancer or its spread.
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PET images
Resting
Music
Visual
Thinking
PET can be used to study changes in the
brain due to different stimuli
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SPECT
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SPECT: Single Photon Emission
Computerized Tomography
Inject a radioactive chemical
Tracer emits gamma rays
Capture the emission
with a gamma camera
Measure metabolic rate (time)
Cheap
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Summary
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CT / MRI show that you have a brain
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PET / SPECT show that you use it!
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Medical Isotopes
Reactor-produced radioisotopes
Chromium-51
Used to label red blood cells and quantify gastro-intestinal protein loss.
Copper-64
Used to study genetic disease affecting copper metabolism.
Iodine-131
Used to diagnose and treat various diseases associated with the human thyroid. Also used in diagnosis of the adrenal
medullary and for imaging suspected neural crest and other endocrine tumours.
Iridium-192
Supplied in wire form for use as an internal radiotherapy source.
Molybdenum-99
Used as the ‘parent’ in a generator to produce technetium-99m, the most widely used isotope in nuclear medicine.
Phosphorus-32
Used in the treatment of excess red blood cells.
Samarium-153
Used with Ethylene Diamine Tetr4amethylene Phosphonate* (Quadramet) to reduce the pain associated with bony
metastases of primary tumours.
Technetium-99m
Used to image the brain, thyroid, lungs, liver, spleen, kidney, gall bladder, skeleton, blood pool, bone marrow, salivary and
lacrimal glands and heart blood pool and to detect infection.
Yttruim-90
Used for liver cancer therapy.
Cyclotron-produced radioisotopes
Gallium-67
Used in imaging to detect tumours and infections.
Iodine-123
Used in imaging to monitor thyroid function and detect adrenal dysfunction.
Thallium-201
Used in imaging to detect the location of damaged heart muscle.
Carbon-11, Nitrogen-13,
Oxygen-14 and
Fluorine-18
These are used in Positron Emission Tomography (PET) to study brain physiology and pathology, for detecting the location
of epileptic foci and in dementia psychiatry and neuropharmacology studies. They are also used to detect heart
problems and diagnose certain types of cancer.
*Only the more commonly used radioisotopes are listed here.
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Industrial isotopes
Naturally occurring radioisotopes
Carbon-14
Used to measure the age of organic material that is up to 50,000 years old.
Chlorint-36
Used to measure sources of chloride and the age of water that is up to 2 million years old.
Lead-210
Used to date layers of sand and soil laid down up to 80 years ago.
Tritium
Used to measure the age of ‘young’ groundwater (up to 30 years old).
Artificially produced radioisotopes
Americium 241/ beryllium
Used in neutron gauging.
Cobalt-60*
Used in radiography and gauging.
Caesium-137
Used in radiotracing to identify sources of soil erosion and depositing; also for thickness gauging.
Gadolinium*
Used in x-ray fluorescence.
Gold-198*
Used to trace factory waste causing ocean pollution, and to trace sand movement in river beds and on ocean floors.
Gold-198* and
technetium-99m*
Used to study sewage and liquid waste movements.
Iridium-192*
Used in radiography.
Iridium-192*, gold-198*
and chromium-51*
Used to trace sand to study coastal erosion.
Tritiated water
Used as a tracer to study sewage and liquid wastes.
Ytterbium-169*
Used in radiography.
Zinc-65* and manganese54*
Used to predict the behaviour of heavy metal components in effluents from mining waste water.
*Manufactured by ANSTO
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Industrial Applications
X raying luggage
Developing techniques to image containers
using neutrons
Measuring thickness, controlling fabrication
of Plastic film or thin steel
. The film runs at high speed between a radioactive
source and a detector and the detector signal strength
is used to control the thickness of the plastic film.
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Gamma Knife
A Gamma Knife contains 201 cobalt-60 sources of
approximately 30 curies (1.1 TBq), in a circular array
in a heavily shielded assembly. The device aims
gamma radiation through a target point . The patient
wears a helmet that is fixed to the skull. Gamma
Knife therapy, radiation to kill cancer cells and
shrink tumors. Gamma Knife radiosurgery is able to
accurately focus many beams of high-intensity
gamma radiation to converge on one or more
tumors. Each individual beam is of relatively low
intensity, so the radiation has little effect on
intervening brain tissue and is concentrated only at
the tumor itself.
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