Medical Imaging

1
Medical Imaging

• Dr. Der-Ming Liou
• Health Informatics Group
• Institute of Public Health
• National Yang-Ming University

2
Medical Image

• Generated by means of radiation
• electromagnetic (EM)
• ultrasound
• electrons
• Displayed for interpretation
• the original image carrier
• a processed film on which the image was formed
• other carrier
• photograph or computer display monitor

3
Computer Applied to Medical Image

• Construct an image from measurements
• Obtain an image reconstructed for optimal
  extraction of a particular feature from an image
• Present images
• Improve image quality by image processing
• Store and retrieve images

4
Using Medical Image

• Carriers
• Modulation (by human body)
• Detection
• Demodulation
• Display
• Further Manipulations
• Applications

5
Overview of the Use of Electromagnetic Radiation
6
Ultrasound

• Produced by piezoelectric crystals
• transform electrical energy to acoustic energy
• vibrate with 2 10 MHz (audible sound 20 KHz)
• pulsed sound waves are emitted
• the energies and the arrival times of the
  received echo caused by the reflections are
  measured

short
l
2l
accuracy
Source
reflector
reflector
at least
2l
echo
distance
7
Echo Scanner (1)

• Sound pulse
• generated with a few MHz
• absorbed, scattered, or reflected in the patient

different density
small
1020l
different velocity of sound
crystal
(diffuse)
different acoustic impedance
Reflected interface
water? soft tissue
bone
Soft tissue
air
small
scatter
object
8
Echo Scanner (2)

• The resolution of echo scanner
• the degree with which details located close
  together can still be distinguished
• determined by the l of the sound wave and the
  duration of the emitted pulse
• the smaller l the better the resolution
• The attenuation coefficient a
• sound frequency for soft tissue
• square of the frequency for other types of tissue
• the more the beam is attenuated, the more
  difficult it is to measure the reflections of
  deeper structures

9
Echo Scanner (3)

• Resolution 8 penetration path
• deeper structure can only be visualized with
  relative low frequency and with low resolution
• Type of tissue influences the amount of
  absorption of the beam
• air and bone are strong absorbers
• muscle tissue and water hardly attenuate the beam
• f 3 MHz (l 0.5 mm) depth up to 10 cm
• f 513 MHz (l 0.250.075 mm) for eye
  examination

10
A-Mode

• Amplitude mode
• the energy of each echo is displayed as a
  function of the time interval between the pulse
  and the echo
• vertical axis amplitude of the echo
• horizontal axis time
• the time interval between the pulse and the echo
  corresponds to the distance between the
  transducer and the reflecting tissue boundary
• Provide one-dimensional information about the
  location of the reflecting boundaries

11
M-Mode

• Motion mode (brightness mode)
• similar to A-mode
• one-dimensional information
• the amplitude of each echo is represented as the
  brightness of the point located along the time
  axis
• other axis is now used to display the echoes of
  subsequent pulses
• possible to visualize the movement of objects
  that are crossed by the ultrasonic beam

12
C-Scan

• Boundary not perpendicular to the beam axis
• produce weak echoes
• A-, B-, and M-mode have fixed beam directions
• do not display all boundaries equally well
• C-scan (compound-scan)
• the crystal can be moved
• the direction of the beam is changed
• the crystal is connected to a flexible arm
• linked to a fixed point of reference
• disadvantage taking a few seconds to build up
  the image impossible to follow moving structures

13
Sector Scan

• Parallel scan
• the movement of the transducer can be sped up by
  mechanical means
• each successive beam makes a small angle with the
  previous one
• two dimensional images are obtained
• to use a stationary probe containing a linear
  array of about 100 crystals
• electrically activated one after one
• distance between certain boundaries can be
  obtained

14
Sector Scan by Ultrasound of part of the Aorta
15
Dopper Effect

• The measurement of flow velocities
• f ? when the target approach to the sound source
• the shift in frequency is a the frequency of the
  incident beam and to the velocity of the target
• everything that moves inside the beam contributes
  to the Doppler signal
• by repositioning the sample volume of the
  transducer is a systematic way, a two dimensional
  velocity distribution can be determined
• the flow map can be superimposed on the echo image

Target
sound
wave
velocity
transducer
16
Advantages of Ultrasound

• No harmful side effect
• for examining pregnant women and young children
• Applications
• Determination of heart function
• examination of the brain
• obstetric examinations
• eye examinations
• determination of the perfusion of tissues
• detection of tumors and cysts

17
Radiology

• X-rays
• discovery in 1895
• simple applications
• computed radiology
• digital subtraction angiography (DSA)
• complex applications
• computed tomography (CT)
• Magnetic resonance image (MRI)

18
Principle of an X-ray
19
Principle of DSA

• Blood vessels often absorb as much as radiation
  as the surrounding tissues
• cant be discerned on X-ray image
• Injecting contrast agents into blood vessels
• iodine
• Due to the presence of bone
• small contrast medium in the blood vessels are
  difficult to distinguish
• the eye is not able to detect contrast
  differences of less than 3

20
Procedure of DSA

• An initial X-ray shadow image is obtained with
  the help of an image intensifier
• signal coming from the television camera is
  digitized and stored in the computer
• The contrast medium is injected into the veins
• The precontrast image (mask) is subtracted from
  the subsequent images
• the resulting images only contain the information
  about the location of the contrast medium
• since the contrast medium fills the vessels, the
  resulting images only show the vessels

21
Procedure of DSA (cont.)

• bone structures that disturb the image have been
  removed by the subtraction
• The differences in intensity can be amplified in
  such a way that the eye is able to perceive the
  blood vessels in the image
• The quality of the image deteriorates when
  patient move
• corrected by using suitable software
• the contrast medium is injected in the form of
  bolus
• the proximal part of vessels is visible on the
  first image
• the distal part is visible on the later
  subtraction images

22
Principle of Contrast Enhancement
23
Example DSA of the Bifurcation of the Aorta
24
Two Dimension Image of X-ray

• X-ray image are projection (shadow) images
• can not reveal the real geometric distribution of
  organs
• organ are situated behind each other
• superimposed in the image so that a three
  dimensional volume is projected in two dimension
• To obtain a three-dimension impression
• obtain a number of images from different angles

25
Computed Tomography (1)

• A cross section with a thickness about 1mm
• divide the cross section into a large number of
  small squares
• each with an area of about 1 mm2
• when a narrow X-ray beam (pencil beam) pass
  through the slice (the beam attenuate)
• the attenuation is determined by the molecular
  composition and the density of the tissue present
  in the square
• an X-ray tube and a detector can be both shifted
  along a line and rotate

26
Computed Tomography (2)

• To obtain an anatomical image
• by displaying the attenuation coefficients
• to determine the attenuation coefficients of each
  square
• determination of the attenuation of each square
  in the cross-section is the purpose of the
  procedure
• Translate the beam over a distance width (row)
• take into account the attenuation coefficient of
  the square of the squares located on a neighbor
  row
• repeated by translating the beam and measuring
  the transmitted intensity
• until cover the total cross section

27
Computed Tomography (3)

• Its not possible to get determine the individual
  attenuation coefficient of each square
• an intensity profile of the transmitted beam as
  a function of the position of the beam
• a measure of total attenuation
• each point in the profile indicates how strongly
  the incident beam was attenuated by the row of
  squares that was passed by the beam in that
  position
• Repeat the procedure outline above for various
  angles of the beam
• possible to compute the attenuation per square

28
Principle of the CT
29
Intensity Profile
30
Example of Cross-sections
31
Back Projection (1)

• Used in practice to obtain the attenuation
  coefficients mi
• Can be used when intensity profiles that cover
  the total cross section under various angles are
  available
• In an individual profile
• each point represents the amount of attenuation
  by the pixels transmitted by the beam
• Each profile show a dip at the location where the
  beam passed

32
Back Projection (2)
33
Back Projection (3)

• If we have only one intensity profile
• we cant determine where on the path the pixel
  was located
• we cant even decide whether the absorption was
  due to a single pixel or was due to the
  attenuating medium that was present over the
  whole path
• the only inference is that the attenuating medium
  was present only along one line in the cross
  section
• the intensity profile showed a dip at only one
  point
• The back-projection method assume
• the absorption medium is uniformly distributed
  over the line

34
Back Projection (4)

• The error from the assumption can be corrected
• if we have several intensity profiles obtained at
  different angles
• The reconstruction has a star-like distribution
• The intensity in the center will increase much
  faster than the intensity at the periphery
• With the use of more angles
• the back-projection image become similar to the
  actual one only it is less sharp
• instead of an image showing one attenuating
  pixel, the neighboring pixels are visible in the
  reconstructed image as well

35
Back Projection (5)

• This blurring effect can be corrected to a
  certain extent by using appropriate filtering
  techniques
• A real cross section can be considered a union of
  cross section with each one containing only one
  attenuation pixel
• The back projection technique can be used
• MRI (magnetic resonance imaging)
• SPECT (single photon emission computed tomography)

36
Attenuation Coefficients
37
CT Equipment

• Second generation scanners
• the single detector is replaced by a frame of
  multiple detectors
• the X-ray tube produces a fan-shaped beam
• translation and rotation remain necessary
• Third generation
• the detector array is so large
• transmission through the complete cross section
  of the patient can be measured simultaneously
• translation is no longer necessary, rotation is
  necessary
• Fourth generation
• a stationary detector array covers 360o
• only X-ray tube rotate

38
Principle of MRI (1)

• Certain atomic nuclei behave like a spinning top
• behave like small magnets
• Under normal circumstances
• the body is not magnetic
• the hydrogen nuclei within the body point into
  all directions randomly
• the net magnetic field strength (magnetization)
  0
• When we place an ensemble of nuclei with spin in
  a strong magnetic field
• the nuclei tend to align themselves with the
  magnetic field

39
Principle of MRI (2)

• This alignment occurs
• the nuclei prefer to be in a state with the
  lowest energy
• 00 K ?all nuclei align themselves to the external
  magnetic field
• At room temperature
• the nuclei also possess thermal energy
• external magnetic field
• 0.1 tesla excess 1/106
• 1 ml H20 3 x 1022 molecules 1017 hydrogen
  atoms aligning parallel to the magnetic field

40
Spin Alignment
41
EM Radiation

• While the nuclei are under influence of the
  external magnetic field
• pulse of electromagnetic radiation are beamed
  into the tissue
• EM radiation is characterized by
• an electric and a magnetic component
• the magnetic component of the EM radiation exerts
  a force on the magnetic nuclei
• When the magnetic component of the EM radiation
  has a direction perpendicular to the external
  magnetic field
• cause the magnetization to precess around the
  direction of external field

42
Larmor Frequency

• in such a way
• the angle between the direction of the
  magnetization and the external field will
  increase linearly with time
• only happen when the EM radiation has a certain
  frequency
• the frequency is proportional to the strength of
  the external magnetic field
• gyromagnetic ratio
• characteristic for the element (isotope)
• the range of radio frequencies 2 to 50 MHz

43
Precession of Magnetization
44
Principle of Gamma Camera
45
A Scintigram of the Lungs
46
Principle of ECG-gated Scintigraphy
47
Rotating Gamma Camera