Medical Imaging

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Medical Imaging

• X-rays
• CT or CAT scan
• PET scan
• MRI

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X-rays
Electrons emitted from a heated cathode, bombard
the anode
Bremsstrahlung does not depend on target
material. Continuous spectrum. The free electron
is attracted to the atom nucleus in the anode. As
the electron speeds past, the nucleus alters its
course. The electron loses energy, which it
releases as an X-ray photon.
Characteristic X-rays The free electron collides
with an atom in the anode, knocking an electron
out of a lower orbital. A higher orbital electron
fills the empty position, releasing its excess
energy as a photon.
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How X-rays interact with the tissues in your body

• The atoms that make up your body tissue absorb
  visible light photons very well. The energy level
  of the photon fits with various energy
  differences between electron positions. Radio
  waves don't have enough energy to move electrons
  between orbitals, so they pass through most
  stuff. X-rays also pass through most things, but
  for the opposite reason They have too much
  energy.
• They can, however, knock an electron away from an
  atom altogether. Some of the energy from the
  X-ray photon works to separate the electron from
  the atom, and the rest sends the electron flying
  through space. A larger atom is more likely to
  absorb an X-ray photon in this way, because
  larger atoms have greater energy differences
  between orbitals -- the energy level more closely
  matches the energy of the photon. Smaller atoms,
  where the electron orbitals are separated by
  relatively low jumps in energy, are less likely
  to absorb X-ray photons.
• The soft tissue in your body is composed of
  smaller atoms, and so does not absorb X-ray
  photons particularly well. The Ca atoms that make
  up your bones are much larger, so they are better
  at absorbing X-ray photons.
• When you pass X-rays through the body, different
  attenuation will be encountered from different
  tissues gt an image occurs

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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

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The CT machine
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 the patient as
the data is gathered.
Outside view of modern CT system showing the
patient table and CT scanning aperture
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CT schematic view

• Diagram showing relationship of x-ray tube,
  patient, detector, and image reconstruction
  computer and display monitor

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How CT works

• Inside the covers of the CT scanner is a rotating
  frame which has an x-ray tube mounted on one side
  and the banana shaped detector mounted on the
  opposite side. A fan beam of x-ray is created as
  the rotating frame spins the x-ray tube and
  detector around the patient. Each time the x-ray
  tube and detector make a 360o rotation, an image
  or "slice" has been acquired. This "slice" is
  collimated (focused) to a thickness between 1 mm
  and 10 mm using lead shutters in front of the
  x-ray tube and x-ray detector.
• As the x-ray tube and detector make this 360o
  rotation, the detector takes numerous snapshots
  (called profiles) of the attenuated x-ray beam.
  Typically, in one 360o lap, about 1,000 profiles
  are sampled. Each profile is subdivided spatially
  (divided into partitions) by the detectors and
  fed into about 700 individual channels. Each
  profile is then backwards reconstructed (or "back
  projected") by a dedicated computer into a
  two-dimensional image of the "slice" that was
  scanned.

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How to see soft tissues with CAT scans

• 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.

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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.

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PET scans
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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 DecayWhen a nucleus undergoes positron
  decay, the result is a new nuclide with 1 fewer
  proton and 1 more neutron, as well as the
  emission of a positron and a neutrino
• The radionuclides that decay via positron
  emission are proton-rich and move closer to the
  line of stability while giving off a positive
  charge. The neutrino is very light, if it has any
  mass at all, and interacts only very weakly with
  other particles. It is therefore not directly
  relevant to nuclear medicine. However, its
  presence in the positron decay makes the energy
  of the positron variable, as opposed to gamma
  emissions, which are of a fixed energy for a
  given radionuclide.

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Positron Annihilation

• As positrons pass through matter and lose
  energy through ionization and excitation of
  nearby atoms and molecules.
• After losing enough energy, and having traveled
  a distance in the neighborhood of 1 mm (depending
  on the initial positron energy), the positron
  will annihilate with a nearby electron

• 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 Ptor 0 in the final state

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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

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Some commonly used nuclides (b emitters)
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How to trace the location of the positron emission
Abnormal lymph nodes (cancer) from a PET scan
image

• PET scanner and shows in fine detail the
  metabolism of glucose, by tracing the positron
  emission from 18F.
• Cancerous tissue uses more glucose, so they
  produce stronger signals.

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MRI ( used to be NMR), but people are afraid of
the word nuclear .
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The MRI machine

• The basic design used in most MRI scanners is a
  giant cube. The cube in a typical system might be
  7 feet tall by 7 feet wide by 10 feet long (2 m
  by 2 m by 3 m), although new models are rapidly
  shrinking
• There is horizontal tube running through the
  magnet from front to back.
• The patient, lying on his or her back, slides
  into the magnet on a special table. Whether or
  not the patient goes in head first or feet first,
  as well as how far in the magnet they will go, is
  determined by the type of exam to be performed.
• MRI scanners vary in size and shape, and newer
  models have some degree of openness around the
  sides, but the basic design is the same.

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Nuclei have spin! MRI is tracing the water in
your body.

• 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 ( its in the RF range) and
  move between energy levels ( flip its spin.

• Without the external magnetic field the energy
  levels with different projections of j are
  degenerate.
• The protons can spin around any axis

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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.

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Magnets

• The biggest and most important component in an
  MRI system is the magnet. The magnets in use
  today in MRI are in the 0.5-tesla to 2.0-tesla
  range, or 5,000 to 20,000 gauss. Magnetic fields
  greater than 2 tesla have not been approved for
  use in medical imaging, though much more powerful
  magnets -- up to 60 tesla -- are used in
  research. Compared with the Earth's 0.5-gauss
  magnetic field, you can see how incredibly
  powerful these magnets are.
• The MRI suite can be a very dangerous place if
  strict precautions are not observed. Metal
  objects can become dangerous projectiles if they
  are taken into the scan room. For example,
  paperclips, pens, keys, scissors, hemostats,
  stethoscopes and any other small objects can be
  pulled out of pockets and off the body without
  warning, at which point they fly toward the
  opening of the magnet (where the patient is
  placed) at very high speeds, posing a threat to
  everyone in the room. Credit cards, bank cards
  and anything else with magnetic encoding will be
  erased by most MRI systems.

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Magnets can be dangerous

• In this photograph, you can see a fully loaded
  pallet jack that has been sucked into the bore of
  an MRI system.

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The resonance

• The MRI machine applies an RF (radio frequency)
  pulse that is specific only to hydrogen. The
  system directs the pulse toward the area of the
  body we want to examine. The pulse causes the
  protons in that area to absorb the energy
  required to make them go to an energy level with
  different magnetic quantum number. This is the
  "resonance" part of MRI. The specific frequency
  of resonance is different for different types of
  tissue.
• These RF pulses are usually applied through a
  coil. MRI machines come with many different coils
  designed for different parts of the body knees,
  shoulders, wrists, heads, necks and so on. These
  coils usually conform to the contour of the body
  part being imaged, or at least reside very close
  to it during the exam.

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Spin relaxation the signal is recorded

• Next step spin relaxation back to the ground
  state. The signal is recorded by a detector of RF
  radiation. But how do we know where the signal is
  coming from ?

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MRI position information

• Applying a gradient field ( position dependent
  field strength) which alters the energy level
  splitting in nuclei which are in different parts
  of the body.
• When you record the signal every part of the body
  plays a different note

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The gradient magnets

• Every MRI system has a gradient magnet in
  addition to the main magnet. There are three
  gradient magnets inside the MRI machine. These
  magnets are very, very low strength compared to
  the main magnetic field they may range in
  strength from 180 gauss to 270 gauss, or 18 to 27
  millitesla (thousandths of a tesla). These
  magnetic fields are needed to provide position
  information in the image.
• At approximately the same time, the three
  gradient magnets jump into the act. They are
  arranged in such a manner inside the main magnet
  that when they are turned on and off very rapidly
  in a specific manner, they alter the main
  magnetic field on a very local level. What this
  means is that we can pick exactly which area we
  want a picture of. In MRI we speak of "slices."

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MRI images

• MRI provides an unparalleled view inside the
  human body. The level of detail we can see is
  extraordinary compared with any other imaging
  modality. MRI is the method of choice for the
  diagnosis of many types of injuries and
  conditions because of the incredible ability to
  tailor the exam to the particular medical
  question being asked. By changing exam
  parameters, the MRI system can cause tissues in
  the body to take on different appearances. This
  is very helpful to the radiologist (who reads the
  MRI) in determining if something seen is normal
  or not. We know that when we do "A," normal
  tissue will look like "B" -- if it doesn't, there
  might be an abnormality.

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Visualization

• 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 abnormalities and disease
  processes better than we could without the
  contrast.

This MRI scan shows the upper torso in side view
so that the bones of the spine are evident
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Summary How MRI works

• When in the MRI scanner
• 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.