Transcript Medical Imaging

MEDICAL
IMAGING
Created By : Shale Olagbegi
DOD TELEHEALTH RESEARCH TOPIC
7/17/2015
What Is Medical Imaging?
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Medical imaging is the process by which physicians
evaluate an area of the subject's body that is not
normally visible. This process could be clinical or
research motivated and can also have scientific and
industrial applications.
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Origin of Medical Imaging
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In its most primitive form, imaging can refer to the
physician simply feeling an area of the body in order to
visualize the condition of internal organs.
It remains an important step today in making initial
assessments of potential problems, although additional
steps are often used to confirm a diagnosis.
The primary drawback of this approach is that findings
are subject to interpretation, and while a recorded image
can be produced manually, in practice this is often not
done.
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Modern Imaging Techniques
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Radiographs
Computed Tomography
Magnetic Resonance Imaging
Ultrasound
Mammography
Microwave Imaging
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Radiography
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This is the creation of radiographs,
photographs made by exposing a photographic film
or other image receptor to X-rays.
Since X-rays penetrate solid objects, but are slightly
attenuated by them, the picture resulting from the
exposure reveals the internal structure of the object.
The most common use of radiography is in the
medical field (where it is known as medical
imaging).
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Theory of Radiography
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The type of electromagnetic radiation of most interest to radiography is x-ray and gamma radiation. This radiation is much more energetic than the
more familiar types such as radio waves and visible light. It is this relatively high energy, which makes gamma rays useful in radiography but potentially
hazardous to living organisms.
They are produced by X-ray tubes, high energy X-ray equipment or natural radioactive elements, such as Radium and Radon, and artificially produced
radioactive isotopes of elements, such as Cobalt 60 and Iridium 192. Electromagnetic radiation consists of oscillating electric and magnetic fields. It is
generally pictured as a single sinusoidal wave.
It is characterized by its wavelength (the distance from a point on one cycle to the point on the next cycle) or its frequency (the number of oscillations
per second). All electromagnetic waves travel at the same speed, the speed of light (c). The wavelength (W) and the frequency (ν) are all related by
the equation:
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Wν = c
This is true for all electromagnetic radiation.
Electromagnetic radiation is known by various names, depending on its energy . The energy of these waves is related to the frequency and the
wavelength by the relationship:
E = hν = hc / W
Where h is a constant known as Planck's Constant.
Gamma rays are indirectly ionizing radiation. A gamma ray passes through matter until it undergoes an interaction with an atomic particle, usually an
electron. During this interaction, energy is transferred from the gamma ray to the electron, which is a directly ionizing particle. As a result of this energy
transfer, the electron is liberated from the atom and proceeds to ionize matter by colliding with other electrons along its path.
For the range of energies commonly used in radiography, the interaction between gamma rays and electrons occurs in two ways. One effect takes
place where all the gamma ray's energy is transmitted to an entire atom. The gamma ray no longer exists and an electron emerges from the atom
with kinetic (motion in relation to force) energy almost equal to the gamma energy. This effect is predominant at low gamma energies and is known as
the photoelectric effect. The other major effect occurs when a gamma ray interacts with an atomic electron, freeing it from the atom and imparting to it
only a fraction of the gamma ray's kinetic energy. A secondary gamma ray with less energy (hence lower frequency) also emerges from the interaction.
This effect predominates at higher gamma energies and is known as the Compton effect.
In both of these effects the emergent electrons lose their kinetic energy by ionizing surrounding atoms. The density of ions so generated is a measure
of the energy delivered to the material by the gamma rays.
The most common means of measuring the variations in a beam of radiation is by utilizing its effects onto a photographic film. This effect is the same
as that of light, and the more intense the radiation is, it will produce a darker film, or a more exposed film. Other methods are in use, such as the
ionizing effect measured electronically, its ability to discharge an electro statically charged plate or to cause certain chemicals to fluoresce as in
fluoroscopy.
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What's an X-Ray?
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X-rays are basically the same thing as visible light rays.
Both are wavelike forms of electromagnetic energy
carried by particles called photons.
The difference between X-rays and visible light rays is
the energy level of the individual photons. This is also
expressed as the wavelength of the rays.
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Theory of X-ray
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X rays were discovered in 1895 by W. C. Roentgen,
who called them X rays because their nature was at first
unknown; they are sometimes also called Roentgen, or
Röntgen, rays. X-ray line spectra were used by H. G. J.
Moseley in his important work on atomic numbers
(1913) and also provided further confirmation of the
quantum theory of atomic structure.
Also important historically is the discovery of X-ray
diffraction by Max von Laue (1912) and its subsequent
application by W. H. and W. L. Bragg to the study of
crystal structure.
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Production of X Rays
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An important source of X rays is synchrotron radiation. X rays are also produced in a
highly evacuated glass bulb, called an X-ray tube, that contains essentially two
electrodes—an anode made of platinum, tungsten, or another heavy metal of high
melting point, and a cathode. When a high voltage is applied between the
electrodes, streams of electrons (cathode rays) are accelerated from the cathode to
the anode and produce X rays as they strike the anode.
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Two different processes give rise to radiation of X-ray frequency. In one process
radiation is emitted by the high-speed electrons themselves as they are slowed or
even stopped in passing near the positively charged nuclei of the anode material.
This radiation is often called brehmsstrahlung [Ger.,=braking radiation]. In a second
process radiation is emitted by the electrons of the anode atoms when incoming
electrons from the cathode knock electrons near the nuclei out of orbit and they are
replaced by other electrons from outer orbits. The spectrum of frequencies given off
with any particular anode material thus consists of a continuous range of frequencies
emitted in the first process, and superimposed on it a number of sharp peaks of
intensity corresponding to discrete frequencies at which X rays are emitted in the
second process. The sharp peaks constitute the X-ray line spectrum for the anode
material and will differ for different materials.
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Applications of X Rays
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Most applications of X rays are based on their ability to pass through matter. This ability varies
with different substances; e.g., wood and flesh are easily penetrated, but denser substances
such as lead and bone are more opaque. The penetrating power of X rays also depends on their
energy. The more penetrating X rays, known as hard X rays, are of higher frequency and are thus
more energetic, while the less penetrating X rays, called soft X rays, have lower energies. X rays
that have passed through a body provide a visual image of its interior structure when they strike a
photographic plate or a fluorescent screen; the darkness of the shadows produced on the plate
or screen depends on the relative opacity of different parts of the body.
Photographs made with X rays are known as radiographs or ski graphs. Radiography has
applications in both medicine and industry, where it is valuable for diagnosis and nondestructive
testing of products for defects. Fluoroscopy is based on the same techniques, with the
photographic plate replaced by a fluorescent screen (see fluorescence; fluoroscope ); its
advantages over radiography in time and cost are balanced by some loss in sharpness of the
image. X rays are also used with computers in CAT (computerized axial tomography) scans to
produce cross-sectional images of the inside of the body.
Another use of radiography is in the examination and analysis of paintings, where studies can
reveal such details as the age of a painting and underlying brushstroke techniques that help to
identify or verify the artist. X rays are used in several techniques that can provide enlarged
images of the structure of opaque objects. These techniques, collectively referred to as X-ray
microscopy or microradiograph, can also be used in the quantitative analysis of many materials.
One of the dangers in the use of X rays is that they can destroy living tissue and can cause
severe skin burns on human flesh exposed for too long a time. This destructive power is used in
X-ray therapy to destroy diseased cells.
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Medical uses
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X-rays have been developed for their use in medical
imaging.
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Radiology is a specialized field of medicine that employs
radiography and other techniques for diagnostic
imaging.
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The use of X-rays are especially useful in the detection
of pathology of the skeletal system, but are also useful
for detecting some disease processes in soft tissue.
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X-ray, which can be used to identify lung diseases such
as pneumonia, lung cancer or pulmonary oedema.
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Other X-Ray Uses
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The most important contributions of X-ray technology
have been in the world of medicine, but X-rays have
played a crucial role in a number of other areas as well.
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X-rays have been pivotal in research involving quantum
mechanics theory, crystallography and cosmology.
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In the industrial world, X-ray scanners are often used to
detect minute flaws in heavy metal equipment.
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And X-ray scanners have become standard equipment
in airport security, of course.
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Cat Scan
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(CT), also known as computed axial tomography or
computer-assisted tomography (CAT) and body section
roentgenography, is medical imaging method employing
tomography where digital processing is used to generate a
three-dimensional image of the internals of an object from a
large series of two-dimensional X-ray images taken around a
single axis of rotation.
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The word "tomography" is derived from the Greek tomos
(slice) and graphia (describing).
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Although most common in healthcare, CT is also used in
other fields, e.g. nondestructive materials testing
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History of Cat Scan
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The CT system was invented in 1972 by Godfrey Newbold
Hounsfield of EMI Central Research Laboratories; owned by
Creative Technology.) using X-rays.
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Allan McLeod Cormack of Tufts University independently
invented the same process and they shared a Nobel Prize in
Medicine in 1979. The first scanner took several hours to
acquire the raw data and several days to produce the
images. Modern multi-detector CT systems can complete a
scan of the chest in less time than it takes for a single breath
and display the computed images in a few seconds.
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Principles of Cat Scan
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X-ray slice data is generated using an X-ray source that rotates
around the object; X-ray sensors are positioned on the opposite
side of the circle from the X-ray source. Many data scans are
progressively taken as the object is gradually passed through the
gantry. They are combined together by the mathematical
procedure known as tomographic reconstruction.
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Newer machines with faster computer systems and newer
software strategies can process not only individual cross sections
but continuously changing cross sections as the gantry, with the
object to be imaged, is slowly and smoothly slid through the X-ray
circle. These are called helical or spiral CT machines. Their
computer systems integrate the data of the moving individual
slices to generate three dimensional volumetric information, in turn
viewable from multiple different perspectives on attached CT
workstation monitors.
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Principles of Cat Scan (cont’d)
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EBT Machine
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In conventional CT machines, an X-ray tube is physically rotated behind a
circular shroud in the less used electron beam tomography (EBT)
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The data stream representing the varying radiographic intensity sensed
reaching the detectors on the opposite side of the circle during each
sweep—360 degree in conventional machines, 220 degree in EBT—is
then computer processed to calculate cross-sectional estimations of the
radiographic density, expressed in Hounsfield units.
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CT is used in medicine as a diagnostic tool and as a guide for
interventional procedures. Sometimes contrast materials such as
intravenous iodinated contrast is used. This is useful to highlight
structures such as blood vessels that otherwise would be difficult to
delineate from their surroundings. Using contrast material can also help to
obtain functional information about tissues.
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Principles of Cat Scan (cont’d)
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Pixels in an image obtained by CT scanning are displayed in terms of
relative radio-density. The pixel itself is displayed according to the mean
attenuation of the tissue that it corresponds to on a scale from −1024 to
+3071 on the Hounsfield scale. Water has an attenuation of 0 Hounsfield
units (HU) while air is −1000 HU, bone is typically +400 HU or greater and
metallic implants are usually +1000 HU.
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Improvements in CT technology have meant that the overall radiation
dose has decreased, scan times have decreased and the ability to
reconstruct images (for example, to look at the same location from a
different angle) has increased over time. Still, the radiation dose from CT
scans is several times higher than conventional X-ray scans.
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Presently, the cost of an average CT scanner is US$1.3 million.
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Diagnostic use of Cat Scan
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Since its introduction in the 1970’s , CT has become an important tool in
medical imaging to supplement X – rays and medical ultrasonography.
Although it is still quite expensive, it is the gold standard in the diagnosis of
a large number of different disease entities.
Cranial CT
Diagnosis of cerebra vascular accidents and interracial hemorrhage is the
most frequent reason for a "head CT" or "CT brain". Scanning is done
without intravenous contrast agents (contrast may resemble a bleed). CT
generally does not exclude infarct in the acute stage, but is useful to
exclude a bleed (so anticoagulant medication can be commenced safely).
For detection of tumors, CT scanning with IV contrast is occasionally used
but is less sensitive than (MRI).
CT can also be used to detect increases in intracranial pressure, e.g.
before lumbar puncture or to evaluate the functioning of a
ventriculoperitoneal shunt.
CT is also useful in the setting of trauma for evaluating facial and skull
fractures.
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Diagnostic use of Cat Scan (cont’d)
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Chest CT
CT is excellent for detecting both acute and chronic changes in the lung
parenchyma. For detection of airspace disease or cancer, ordinary noncontrast scans are adequate.
For evaluation of chronic interstitial processes. For evaluation of the
mediastinum and hilar regions for lymphadenopathy, IV contrast is
administered.
CT angiography of the chest (CTPA) is also becoming the primary method
for detecting pulmonary embolism (PE) and aortic dissection, and requires
accurately timed rapid injections of contrast and high-speed helical
scanners. CT is the standard method of evaluating abnormalities seen on
chest X-ray and of following findings of uncertain acute significance.
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Diagnostic use of Cat Scan (cont’d)
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Cardiac CT
With the advent of sub second rotation combined with multi-slice CT (up to
64 slices), high resolution and high speed can be obtained at the same
time, allowing excellent imaging of the coronary arteries. It is uncertain
whether this modality will replace the invasive coronary catheterization.
Abdominal and pelvic CT
Many abdominal disease processes require CT for proper diagnosis. CT
has limited application in the evaluation of the pelvis. For the female pelvis
in particular, ultrasound is the imaging modality of choice. Nevertheless, it
may be part of abdominal scanning (e.g. for tumors), and has uses is
assessing fractures.
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Extremities of Cat Scan
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CT is often used to image complex fractures, especially
ones around joints, because of the ability to reconstruct
the area of interest in multiple planes
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Magnetic resonance imaging
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(MRI) - also called magnetic resonance tomography
(MRT) - is a method of creating images of the inside of
opaque organs in living organisms as well as detecting
the amount of bound water in geological structures.
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MRI Machine
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MRI
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To understand how MRI works, let's start by focusing on the
"magnetic" in MRI. 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 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.
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MRI (cont’d)_
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Numbers like that help provide an intellectual understanding
of the magnetic strength, but everyday examples are also
helpful.
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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Purpose Of MRI
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To obtain two-dimensional views of an internal organ or
structure, especially the brain and spinal cord.
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To assess response to treatment, especially cancer
chemotherapy or radiation therapy.
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To assess sports-related injury to bones and joints.
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How it works
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MRI uses a powerful magnetic field and radio waves to
alter the natural alignment of hydrogen atoms within the
body.
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Computers record the activity of the hydrogen atoms
and translate that into images.
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Preparation
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All jewelry, hair clips, and other metal objects must be
removed.
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Some facilities ask patients to disrobe and put on a hospital
gown; others allow patients to wear clothing so long as it
doesn't have metal parts.
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A contrast medium may be injected before some studies
(e.g., gadolinium may be injected before an MRI study of the
brain); people who are claustrophobic or have difficulty lying
still may be given a sedative. Otherwise, no special
preparation is required.
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Test procedure
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You will be instructed to lie as still as possible on a
narrow table that slides into a tubelike structure that
holds the magnet (see figure).
A loud thumping or hammering noise will be heard
during the test; you may request earplugs or listen to
music with earphones to reduce the noise level.
At certain points during the test, the noise will stop and
you will be able to hear instructions from the doctor or
technician administering the test.
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FIGURE Magnetic Resonance
Imaging
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Variations: Echoplanar MRI is a new technique that allows for
rapid accumulation of data such as cardiac motion.
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After the test: You can resume your pretest activities immediately.
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Factors affecting results: Movement, extreme obesity, and the
presence of metal objects can all affect results.
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Interpretation: A radiologist or other medical specialist interprets
the results.
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FIGURE Magnetic Resonance
Imaging (cont’d)
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Advantages
MRI offers increased-contrast resolution, enabling better
visualization of soft tissues. Also, it allows for multiplanar imaging,
as opposed to CT, which is usually only axial.
It provides highly detailed information without exposing the body to
radiation. In many instances, it provides more useful images than
CT scanning and ultrasound.
Disadvantages
It is an expensive procedure and not available in many small
hospitals and rural areas.
It also cannot be used for patients with implanted pacemakers and
certain other metal objects.
MRI systems are very, very expensive to purchase, and therefore
the exams are also very expensive.
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Ultrasound
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This is a technique that uses sound waves to study and
treat hard-to-reach body areas. In scanning with
ultrasound, high-frequency sound waves are transmitted
to the area of interest and the returning echoes
recorded.
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Ultrasound equipment and test
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What is an Ultrasound Test?
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An ultrasound test is a radiology technique, which uses highfrequency sound waves to produce images of the organs and
structures of the body. The sound waves are sent through body
tissues with a device called a transducer. The transducer is placed
directly on top of the skin, which has a gel applied to the surface.
The sound waves that are sent by the transducer through the body
are then reflected by internal structures as "echoes." These echoes
return to the transducer and are transmitted electrically onto a
viewing monitor. The echo images are then recorded on a plane
film and can also be recorded on videotape. After the ultrasound,
the gel is easily wiped off.
The technical term for ultrasound testing and recording is
"sonography." Ultrasound testing is painless and harmless.
Ultrasound tests involve no radiation and studies have not revealed
any adverse effects.
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Major Uses of Ultrasound
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Ultrasound has been used in a variety of clinical
settings, including obstetrics and gynecology, cardiology
and cancer detection.
The main advantage of ultrasound is that certain
structures can be observed without using radiation.
Ultrasound can also be done much faster than X-rays or
other radiographic techniques.
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Here is a short list of some uses for
ultrasound:
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Obstetrics and Gynecology
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Cardiology
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measuring the size of the fetus to determine the due date
determining the position of the fetus to see if it is in the normal head down position or breech
checking the position of the placenta to see if it is improperly developing over the opening to the uterus (cervix)
seeing the number of fetuses in the uterus
checking the sex of the baby (if the genital area can be clearly seen)
checking the fetus's growth rate by making many measurements over time
detecting ectopic pregnancy, the life-threatening situation in which the baby is implanted in the mother's Fallopian
tubes instead of in the uterus
determining whether there is an appropriate amount of amniotic fluid cushioning the baby
monitoring the baby during specialized procedures - ultrasound has been helpful in seeing and avoiding the baby
during amniocentesis (sampling of the amniotic fluid with a needle for genetic testing). Years ago, doctors use to
perform this procedure blindly; however, with accompanying use of ultrasound, the risks of this procedure have
dropped dramatically.
seeing tumors of the ovary and breast
seeing the inside of the heart to identify abnormal structures or functions
measuring blood flow through the heart and major blood vessels
Urology
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measuring blood flow through the kidney
seeing kidney stones
detecting prostate cancer early
For what purposes are ultrasounds
performed?
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Ultrasound examinations can be used in various
areas of the body for a variety of purposes. These
purposes include examination of the chest, abdomen,
blood vessels (such as to detect blood clots in leg
veins) and the evaluation of pregnancy.
In the chest, ultrasound can be used to obtain
detailed images of the size and function of the heart.
Ultrasound can detect abnormalities of the heart
valves, such as mistral valve prolapse, aortic stenosis,
and infection.
Ultrasound is commonly used to guide fluid
withdrawal aspiration) from the chest, lungs, or
around the heart.
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For what purposes are ultrasounds
performed? contd
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Ultrasounds also commonly used to examine internal
structures of the abdomen. Ultrasound can detect fluid,
cysts, tumors or abscess in the abdomen or liver. Impaired
blood flow from clots or arteriosclerosis in the legs can be
detected by ultrasound. Aneurysms of the aorta can also be
seen. Ultrasound is also commonly used to evaluate the
structure of the thyroid gland in the neck.
During pregnancy, an ultrasound can be used to evaluate
the size, gender, movement, and position of the growing
baby. The baby's heart is usually visible early, and as the
baby ages, body motion becomes more apparent. The baby
can often be visualized by the mother during the ultrasound,
and the gender of the baby is sometimes detectable.
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How do patients prepare for an
ultrasound?
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Preparation for ultrasound is minimal. Generally, if internal
organs such as the gallbladder are to be examined, patients
are requested to avoid eating and drinking with the exception
of water for six to eight hours prior to the examination. This is
because food causes gallbladder contraction, minimizing the
size, which would be visible during the ultrasound.
In preparation for examination of the baby and womb during
pregnancy, it is recommended that mothers drink at least four
to six glasses of water approximately one to two hours prior
to the examination for the purpose of filling the bladder. The
extra fluid in the bladder moves air-filled bowel loops away
from the womb so that the baby and womb are more visible
during the ultrasound test.
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What is Mammography
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This is a specific type of imaging that uses a low-dose xray system for examining the breasts.
The images of the breasts can be viewed on film at a
view box or as soft copy on a digital mammography
work station.
Most medical experts agree that successful treatment of
breast cancer often is linked to early diagnosis.
Mammography plays a central part in early detection of
breast cancers because it can show changes in the
breast up to two years before a patient or physician can
feel them.
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A mammography unit
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Procedures involved
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A mammography unit is a rectangular box that houses the
tube in which x-rays are produced. The unit is a dedicated
equipment because it is used exclusively for x-ray exam of
the breast, with special accessories that allow only the breast
to be exposed to the x-rays. Attached to the unit is a device
that holds and compresses the breast and positions it so
images can be obtained at different angles.
The breast is exposed to a small dose of radiation to produce
an image of internal breast tissue. The image of the breast is
produced as a result of some of the x-rays being absorbed
(attenuation) while others pass through the breast to expose
either a film (conventional mammography) or digital image
receptor (digital mammography). The exposed film is either
placed in a developing machine—producing images much
like the negatives from a 35mm camera—or images are
digitally stored on computer
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Uses of Mammography
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The detection of breast cancer is X-ray imaging of the
breasts.
When mammography screening is combined with a
follow-up ultrasonic examination of those women whose
mammographies show signs of possible cancer
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Other common uses of the procedure
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Mammography is used to aid in the diagnosis of breast
diseases in women. Screening mammography can assist
your physician in the detection of disease even if you have no
complaints or symptoms.
Initial mammographic images themselves are not always
enough to determine the existence of a benign or malignant
disease with certainty. If a finding or spot seems suspicious,
your radiologist may recommend further diagnostic studies.
Diagnostic mammography is used to evaluate a patient with
abnormal clinical findings, such as a breast lump or lumps,
that have been found by the woman or her doctor. Diagnostic
mammography may also be done after an abnormal
screening mammography in order to determine the cause of
the area of concern on the screening exam
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Screening mammography
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Imaging examination of the breast by means of x-rays,
of individuals usually without symptoms to detect those
with a high probability of having breast disease.
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Microwave Imaging
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What is Microwave Imaging
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The term microwave imaging covers all processes in
which measurements of electromagnetic fields in the
microwave region from 300 MHz to 30 GHz are used for
creating images.
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Processes involved in microwave
imaging
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To create images from microwave measurements, it is necessary to
construct a microwave camera, which is able to transmit microwaves and
measure the scattered waves at one or more antennas. Different types of
microwave cameras are currently being used for imaging in such areas as
ground penetrating radar and remote sensing. Depending on the items to
be imaged, different types of microwave cameras are needed. These range
from small antennas used for near field measurements in ground
penetrating radar to the large airborne systems used in remote sensing.
There are two key issues to address when designing a microwave
cameras. One is the increase of the signal to noise ratio in the system and
the other is to assure that the system has a large dynamic range. The
importance of both of these is closely related to the fact that the scattered
signal is often very weak in comparison to the transmitted signal. This
implies that any noise in the system will have a large impact on the image
quality and that the system must be able to distinguish even small
differences in the received signals. To obtain the maximum amount of
information from the microwave measurements, inverse scattering
techniques must be applied.
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Techniques involved in MI
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Inverse scattering is the technique in which the images
are created by inverting a model of the scattering
mechanisms derived from Maxwell's equations.
The quality of the images when using inverse scattering
for microwave imaging are determined by:
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The accuracy of the forward model
The accuracy of the inversion algorithm.
By using Maxwell equations, an exact solution to the
forward scattering problem can be determined.
7/17/2015
References
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www.answers.com
www.colorado.edu/physics
www.reference.dictionary.com
www.medicalimaging.org
7/17/2015