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Biosignal Analysis and Medical Imaging

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Title: Biosignal Analysis and Medical Imaging


1
Biosignal Analysis and Medical Imaging
  • Nazife Baykal
  • Informatics Institute, METU

2
Biosignal Analysis and Medical Imaging
  • Biosignal Analysis
  • Medical Imaging

3
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?

4
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.

5
Classes of Biosignals
6
Importance of Biosignals
  • diagnosis
  • patient monitoring
  • biomedical research

7
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
8
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

9
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

10
Stages of Biosignal Processing
Biological process
Interpreted signal
11
Stages of Biosignal Processing
signal in electrical form
digitized signal
pressure, temperature, pH level, etc.
isolated and amplified signal
12
Stages of Biosignal Processing
Signal acquisition and sampling
Sampling frequency should be twice the frequency
of the original signal
13
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

14
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.

15
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

16
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

17
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

18
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.

19
Biosignal Processing
4 different types of processing
20
Medical Imaging
  • Ultrasound
  • Radiology
  • MRI
  • Nuclear Medicine

21
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

22
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

23
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
24
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.

25
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.

26
Ultrasound
  • Visualization of reflections can be performed in
    several ways
  • Amplitude mode (A-Mode)
  • Motion mode (M-Mode)
  • Compound scan (C-Scan)
  • Doppler effect

27
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

28
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
29
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

30
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.

31
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.

32
Radiology
33
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

34
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.

35
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.

36
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.

37
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.

38
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.

39
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

40
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

41
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

42
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.

43
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.
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