Biosignal Analysis and Medical Imaging

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Biosignal Analysis and Medical Imaging

• Nazife Baykal
• Informatics Institute, METU

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Biosignal Analysis and Medical Imaging

• Biosignal Analysis
• Medical Imaging

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Biosignal Analysis

• What is a Biosignal?
• What is the Importance of Biosignals?
• What are the Characteristics of Biosignals?
• What is the Aim of Biosignal Processing?
• What are the Stages of Biosignal Processing?
• What are the Application Areas of Biosignal
  Analysis?

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What is a Biosignal?

• electrical, such as the depolarization of a nerve
  or muscle cell membrane,
• mechanical, such as the sound generated by
  opening and closing of heart valves or
• chemical, such as pressure values of blood gases,
  PO2 and PCO2.

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Classes of Biosignals
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Importance of Biosignals

• diagnosis
• patient monitoring
• biomedical research

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Characteristics of Biosignals

• often hidden in a background of other signals and
  noise components
• generated by highly complex and dynamic
  biological processes with parameters usually
  more than a few and varying continuously

cardiac cycle
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Aim of Biosignal Processing

• In order to derive the required information from
  the biosignals
• disturbance should be filtered out
• the amount of data should be reduced by
  discriminating only the most significant ones
  related with the required information

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Stages of Biosignal Processing

• At least four stages
• Signal acquisition
• Transformation and reduction of the signals
• Computation of signal parameters that are
  diagnostically significant
• Interpretation or classification of the signals

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Stages of Biosignal Processing
Biological process
Interpreted signal
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Stages of Biosignal Processing
signal in electrical form
digitized signal
pressure, temperature, pH level, etc.
isolated and amplified signal
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Stages of Biosignal Processing
Signal acquisition and sampling
Sampling frequency should be twice the frequency
of the original signal
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Stages of Biosignal Processing

• Signal transformation
• noise component
• due to the electronics in the measuring device,
• artifacts related to the patients movements, or
• other background signals recorded simultaneously
• more data than actually needed to derive
  parameters offering semantic information

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Stages of Biosignal Processing

• Parameter selection
• Usually, relevant information is not the direct
  result of a sample or recording of a signal.
• Parameters bearing resemblance to the signs and
  symptoms that are used to make diagnosis are
  extracted from the signal.

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Stages of Biosignal Processing

• Signal classification
• the interpretation stage
• derived features of selected relevant parameters
  used for human or computer-assisted decision
  making by means of decision support methods

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Application Areas of Biosignal Analysis

• particularly in the areas of screening,
  functional explorations, and intensive care
• analysis of ECGs
• waveforms may be learned,
• stored and then compared to
• the forms being observed
• analysis of EEGs
• studying the cortexs responses
• diagnosing sensorial deficits

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Application Areas of Biosignal Analysis

• in ICUs
• integrating signals from multiple sources
• presenting information in the most appropriate
  form
• interpreting variations over prolonged time
  periods
• learning and recognizing profiles
• triggering intelligent alarms

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Application Areas of Biosignal Analysis

• Biosignals offer parameters that support medical
  decision making and trend analysis.
• Biosignal analysis techniques help to extract
  these parameters accurately, analyze and
  interpret them objectively.

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Biosignal Processing
4 different types of processing
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Medical Imaging

• Ultrasound
• Radiology
• MRI
• Nuclear Medicine

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Medical Imaging

• Two broad types of medical images
• physical, optical or electromagnetic images,
  which are analog and continuous by nature
• mathematical images, which are digital by nature

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Medical Imaging

• Computers are applied in medical imaging to
• construct an image from measurements.
• identify quantitative parameters of clinical
  interest such as certain distances, densities,
  etc.
• improve image quality by image processing,
  compensate for imperfections in the
  image-generating system, and reduce noise
• store and retrieve images
• reduce the amount of storage required and the
  transmission time via image compression
  techniques
• indirectly improve patient care

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Ultrasound

• based on an acoustic probe that emits and
  captures ultrasonic waves
• probes piezzoelectric crystals transforms
  electrical energy to acoustic echoes and vice
  versa

blood flow through umbilical cord
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Ultrasound

• The crystal can be
• pulsed to transmit a short burst of ultrasonic
  energy as a miniature loudspeaker
• and then switched to act as a microphone to
  receive the reflected signals from various tissue
  interfaces.
• The time delay between the transmitted pulse and
  its echo is a measure of the depth of the tissue
  interface.

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Ultrasound

• When the depth information is suitably displayed,
  anatomical images of investigated area can be
  obtained.
• In order to display, the signals which come from
  several unidirectional paths are digitized and
  processed.

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Ultrasound

• Visualization of reflections can be performed in
  several ways
• Amplitude mode (A-Mode)
• Motion mode (M-Mode)
• Compound scan (C-Scan)
• Doppler effect

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Radiology

• Using ionizing radiation from an x-ray source has
  increasingly been used for diagnostic purposes
• Conventional radiology
• Computed radiology
• Digital subtraction angiography
• Computed tomography

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Radiology

• As the radiation passes through the object ,
• a portion is scattered,
• most is absorbed,
• 1-4 is transmitted to the detector.
• Transmitted beams are detected via a fluoroscopic
  screen
• that produces visible light that is exposed on a
  film,
• or that can be viewed directly, or with an image
  intensifier.

A simple x-ray system
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Radiology

• The absorption of x-ray beam depends on the type
  of the tissue and the amount of tissue traversed
• e.g. Lung tissue absorbs x-rays much less than
  bone

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Radiology

• Problem with x-ray
• intensifier screens
• Residual images
• Afterimages
• Storage-phosphor screens can be used to reduce
  this effect.
• The latent image stored on the phosphor plate can
  be read out by a laser scanner and digital x-ray
  images can be obtained.

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Radiology

• Contrast is not good enough.
• Blood vessels can not be discriminated on images.
  
• injecting contrast agents containing
  x-ray-absorbing iodine into blood vessels
• Still the contrast generated by contrast agent
  injection is not enough to distinguish vessels in
  the presence of bone structures.
• Digital Subtraction Angiography (DSA) is used to
  display the underlying vessels by subtracting
  undesirable structures from the images.

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Radiology
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Radiology

• In conventional x-ray image, a 3-D information is
  projected in 2-D.
• To obtain a 3-D impression, the only way is to
  expose radiographs from every possible direction.
• But this is prohibited by limits on the dose of
  radiation the patient can safely receive

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Radiology

• In CT, anatomical
• information is digitally reconstructed from
  x-ray transmission data obtained by scanning an
  area from many directions.
• Since its introduction, the scanning time and and
  resolution quality have been rather improved
  today.

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Magnetic Resonance Imaging

• Charged atomic particles have a magnetic moment
  due to their spinning motion. They behave like
  small magnets and, in an external magnetic field,
  they tend to align themselves parallel to the
  field.
• Body tissues are rich in water and hydrogen
  nuclei ionized in water form a huge amount of
  spinning charged particles. In an external
  magnetic field, these aligned particles generate
  a magnetization parallel to the direction of this
  field and proportional both to the strength of
  the field and the density of spinning charged
  particles in the tissue.

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Magnetic Resonance Imaging

• The axes of spinning particles precess in the
  magnetic field with a Larmer frequency.
• While the nuclei in a tissue are under the
  influence of the external magnetic field, if
  pulses of EM radiation are beamed into this
  tissue, the magnetic component of the EM
  radiation exerts a force on the nuclei. When the
  magnetic component of the EM radiation has a
  direction ? to the external magnetic field, it
  may cause the magnetization to precess around the
  direction of external field. This absorption of
  energy-response occurs if and only if the EM
  radiation is at the Larmor frequency.

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Magnetic Resonance Imaging

• After the pulse, particles return to
• equilibrium and the magnetization
• gradually precess back to the
• external field direction.
• Lenzs Law a changing magnetic
• field flux induces a current in a
• coil.
• Amplitude of this current is proportional to the
  of resonating nuclei in that volume, and the
  frequency of the current will be equal to Larmer
  frequency.

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Magnetic Resonance Imaging

• Patient placed in the magnetic field and the EM
  wave pulses at RF applied ? to the magnetic axis
• After RF pulse emission, amplitudes of induced
  currents on coils are measured.
• These data are analyzed by the computer and
  images that depend on the characteristics of the
  tissue are provided.

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Magnetic Resonance Imaging

• no ionizing radiation is used
• no measurable biological aftereffects have been
  seen
• repeated images of a tissue can be obtained
  without harm or concern for exposure and
  information about the stages of a metabolism can
  be derived

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Nuclear Medicine

• makes use of radioactive material injection for
  the diagnosis of the disease or for the
  examination of the patient
• the isotope emits radiation that is captured by a
  radiation-sensitive camera
• the distribution of radioactivity inside the body
  of the patient is measured

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Nuclear Medicine

• making use of radioactive markers, it can be used
  to image organ function as opposed to simple
  organ morphology
• ?radioactivity distribution represents the
  metabolic activity distribution over an organ
• source of radiation is not external but rather
  within the patient

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Nuclear Medicine

• For performing dynamic studies, several images
  should be received consecutively.
• Image obtained with a gamma camera is just the
  2-D projection of the 3-D radioactivity
  distribution.

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Nuclear Medicine

• Two techniques used to visualize 3-D activity
  distributions
• single photon emission computed tomography
  (SPECT)
• positron emission tomography (PET)
• In both techniques, several planes or slices of
  activity obtained from a large number of angles
  should be constructed at the same time.