MEDICAL IMAGING

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MEDICAL IMAGING

• Created By Shale Olagbegi
• DOD TELEHEALTH RESEARCH TOPIC

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What Is Medical Imaging?

• 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

• 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

• Radiographs
• Computed Tomography
• Magnetic Resonance Imaging
• Ultrasound
• Mammography
• Microwave Imaging

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Radiography

• 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

• 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
  (?) are all related by the equation
• W? 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 h? 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?

• 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

• 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

• 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 electrodesan 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.
• 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

• 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

• X-rays have been developed for their use in
  medical imaging.
• Radiology is a specialized field of medicine that
  employs radiography and other techniques for
  diagnostic imaging.
• 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.
• 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

• 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.
• X-rays have been pivotal in research involving
  quantum mechanics theory, crystallography and
  cosmology.
• In the industrial world, X-ray scanners are often
  used to detect minute flaws in heavy metal
  equipment.
• And X-ray scanners have become standard equipment
  in airport security, of course.

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Cat Scan

• (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.
• The word "tomography" is derived from the Greek
  tomos (slice) and graphia (describing).
• 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

• The CT system was invented in 1972 by Godfrey
  Newbold Hounsfield of EMI Central Research
  Laboratories owned by Creative Technology.)
  using X-rays.
• 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

• 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.
• 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 (contd)

• EBT Machine
• In conventional CT machines, an X-ray tube is
  physically rotated behind a circular shroud in
  the less used electron beam tomography (EBT)
• The data stream representing the varying
  radiographic intensity sensed reaching the
  detectors on the opposite side of the circle
  during each sweep360 degree in conventional
  machines, 220 degree in EBTis then computer
  processed to calculate cross-sectional
  estimations of the radiographic density,
  expressed in Hounsfield units.
• 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 (contd)

• 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.
• 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.
• Presently, the cost of an average CT scanner is
  US1.3 million.

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Diagnostic use of Cat Scan

• Since its introduction in the 1970s , 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 (contd)

• Chest CT
• CT is excellent for detecting both acute and
  chronic changes in the lung parenchyma. For
  detection of airspace disease or cancer, ordinary
  non-contrast 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 (contd)

• 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

• 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

• (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

• 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 (contd)_

• 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

• To obtain two-dimensional views of an internal
  organ or structure, especially the brain and
  spinal cord.
• To assess response to treatment, especially
  cancer chemotherapy or radiation therapy.
• To assess sports-related injury to bones and
  joints.

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How it works

• MRI uses a powerful magnetic field and radio
  waves to alter the natural alignment of hydrogen
  atoms within the body.
• Computers record the activity of the hydrogen
  atoms and translate that into images.

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Preparation

• All jewelry, hair clips, and other metal objects
  must be removed.
• 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.
• 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

• 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

• Variations Echoplanar MRI is a new technique
  that allows for rapid accumulation of data such
  as cardiac motion.
• After the test You can resume your pretest
  activities immediately.
• Factors affecting results Movement, extreme
  obesity, and the presence of metal objects can
  all affect results.
• Interpretation A radiologist or other medical
  specialist interprets the results.

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FIGURE Magnetic Resonance Imaging (contd)

• 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

• 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?

• An ultrasound test is a radiology technique,
  which uses high-frequency 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

• 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

• Obstetrics and Gynecology
• 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
• Cardiology
• seeing the inside of the heart to identify
  abnormal structures or functions
• measuring blood flow through the heart and major
  blood vessels
• Urology
• measuring blood flow through the kidney
• seeing kidney stones
• detecting prostate cancer early

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For what purposes are ultrasounds performed?

• 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
For what purposes are ultrasounds performed?

• 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?

• 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

• This is a specific type of imaging that uses a
  low-dose x-ray 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

• 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 machineproducing images much
  like the negatives from a 35mm cameraor images
  are digitally stored on computer

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Uses of Mammography

• 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

• 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

• 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

• 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

• 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

• 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
• 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.
  

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References

• www.answers.com
• www.colorado.edu/physics
• www.reference.dictionary.com
• www.medicalimaging.org